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<rfc category="info" docName="draft-ietf-tsvwg-transport-encrypt-21"
ipr="trust200902">
<front> <front>
<title abbrev="Transport Header Encryption">Considerations around <title abbrev="Transport Header Encryption">Considerations around
Transport Header Confidentiality, Network Operations, and the Evolution of Transport Header Confidentiality, Network Operations, and the Evolution of
Internet Transport Protocols</title> Internet Transport Protocols</title>
<seriesInfo name="RFC" value="9065"/>
<author fullname="Godred Fairhurst" initials="G." surname="Fairhurst"> <author fullname="Godred Fairhurst" initials="G." surname="Fairhurst">
<organization>University of Aberdeen</organization> <organization>University of Aberdeen</organization>
<address> <address>
<postal> <postal>
<street>Department of Engineering</street> <extaddr>Department of Engineering</extaddr>
<street>Fraser Noble Building</street> <street>Fraser Noble Building</street>
<city>Aberdeen, Scotland</city>
<city>Aberdeen</city>
<code>AB24 3UE</code> <code>AB24 3UE</code>
<country>United Kingdom</country>
<country>Scotland</country>
</postal> </postal>
<email>gorry@erg.abdn.ac.uk</email> <email>gorry@erg.abdn.ac.uk</email>
<uri>http://www.erg.abdn.ac.uk/</uri> <uri>http://www.erg.abdn.ac.uk/</uri>
</address> </address>
</author> </author>
<author fullname="Colin Perkins" initials="C." surname="Perkins">
<author fullname="Colin Perkins" initials="C.S." surname="Perkins">
<organization>University of Glasgow</organization> <organization>University of Glasgow</organization>
<address> <address>
<postal> <postal>
<street>School of Computing Science</street> <extaddr>School of Computing Science</extaddr>
<city>Glasgow, Scotland</city>
<city>Glasgow</city>
<code>G12 8QQ</code> <code>G12 8QQ</code>
<country>United Kingdom</country>
<country>Scotland</country>
</postal> </postal>
<email>csp@csperkins.org</email> <email>csp@csperkins.org</email>
<uri>https://csperkins.org/</uri> <uri>https://csperkins.org/</uri>
</address> </address>
</author> </author>
<date month="July" year="2021"/>
<date day="18" month="April" year="2021" />
<area>Transport</area> <area>Transport</area>
<workgroup>TSVWG</workgroup> <workgroup>TSVWG</workgroup>
<keyword>transport design</keyword>
<keyword>transport design, operations and management</keyword> <keyword>operations and management</keyword>
<abstract> <abstract>
<t>To protect user data and privacy, Internet transport protocols have <t>To protect user data and privacy, Internet transport protocols have
supported payload encryption and authentication for some time. Such supported payload encryption and authentication for some time. Such
encryption and authentication is now also starting to be applied to the encryption and authentication are now also starting to be applied to the
transport protocol headers. This helps avoid transport protocol transport protocol headers. This helps avoid transport protocol
ossification by middleboxes, mitigate attacks against the transport ossification by middleboxes, mitigate attacks against the transport
protocol, and protect metadata about the communication. Current protocol, and protect metadata about the communication. Current
operational practice in some networks inspect transport header operational practice in some networks inspect transport header
information within the network, but this is no longer possible when information within the network, but this is no longer possible when
those transport headers are encrypted.</t> those transport headers are encrypted.</t>
<t>This document discusses the possible impact when network traffic uses <t>This document discusses the possible impact when network traffic uses
a protocol with an encrypted transport header. It suggests issues to a protocol with an encrypted transport header. It suggests issues to
consider when designing new transport protocols or features.</t> consider when designing new transport protocols or features.</t>
</abstract> </abstract>
</front> </front>
<middle> <middle>
<section title="Introduction"> <section numbered="true" toc="default">
<name>Introduction</name>
<t>The transport layer supports the end-to-end flow of data across a <t>The transport layer supports the end-to-end flow of data across a
network path, providing features such as connection establishment, network path, providing features such as connection establishment,
reliability, framing, ordering, congestion control, flow control, etc., reliability, framing, ordering, congestion control, flow control, etc.,
as needed to support applications. One of the core functions of an as needed to support applications. One of the core functions of an
Internet transport is to discover and adapt to the characteristics of Internet transport is to discover and adapt to the characteristics of
the network path that is currently being used.</t> the network path that is currently being used.</t>
<t>For some years, it has been common for the transport-layer payload to
<t>For some years, it has been common for the transport layer payload to be protected by encryption and authentication but for the transport-layer
be protected by encryption and authentication, but for the transport headers to be sent unprotected. Examples of protocols that behave
layer headers to be sent unprotected. Examples of protocols that behave in this manner include Transport Layer Security
in this manner include <xref target="RFC8446"> Transport Layer Security (TLS) over TCP <xref target="RFC8446" format="default"/>, Datagram TLS <xr
(TLS) over TCP</xref>, Datagram TLS <xref target="RFC6347"></xref> <xref ef target="RFC6347" format="default"/> <xref target="I-D.ietf-tls-dtls13" format
target="I-D.ietf-tls-dtls13"></xref>, the <xref target="RFC3711"> Secure ="default"/>, the Secure
Real-time Transport Protocol</xref>, and <xref target="RFC8548"> Real-time Transport Protocol <xref target="RFC3711" format="default"/>, an
tcpcrypt </xref>. The use of unencrypted transport headers has led some d tcpcrypt <xref target="RFC8548" format="default"/>. The use of unencrypted tra
nsport headers has led some
network operators, researchers, and others to develop tools and network operators, researchers, and others to develop tools and
processes that rely on observations of transport headers both in processes that rely on observations of transport headers both in
aggregate and at the flow level to infer details of the network's aggregate and at the flow level to infer details of the network's
behaviour and inform operational practice.</t> behaviour and inform operational practice.</t>
<t>Transport protocols are now being developed that encrypt some or all <t>Transport protocols are now being developed that encrypt some or all
of the transport headers, in addition to the transport payload data. The of the transport headers, in addition to the transport payload data. The
QUIC transport protocol <xref target="I-D.ietf-quic-transport"></xref> QUIC transport protocol <xref target="RFC9000" format="default"/>
is an example of such a protocol. Such transport header encryption makes is an example of such a protocol. Such transport header encryption makes
it difficult to observe transport protocol behaviour from the vantage it difficult to observe transport protocol behaviour from the vantage
point of the network. This document discusses some implications of point of the network. This document discusses some implications of
transport header encryption for network operators and researchers that transport header encryption for network operators and researchers that
have previously observed transport headers, and highlights some issues have previously observed transport headers, and it highlights some issues
to consider for transport protocol designers.</t> to consider for transport protocol designers.</t>
<t>As discussed in <xref target="RFC7258" format="default"/>, the IETF has
<t>As discussed in <xref target="RFC7258"></xref>, the IETF has
concluded that Pervasive Monitoring (PM) is a technical attack that concluded that Pervasive Monitoring (PM) is a technical attack that
needs to be mitigated in the design of IETF protocols. This document needs to be mitigated in the design of IETF protocols. This document
supports that conclusion. It also recognises that RFC7258 states "Making supports that conclusion. It also recognises that <xref target="RFC7258" f
networks unmanageable to mitigate PM is not an acceptable outcome, but ormat="default"/>
states, "Making networks unmanageable to mitigate PM is not an acceptable
outcome, but
ignoring PM would go against the consensus documented here. An ignoring PM would go against the consensus documented here. An
appropriate balance will emerge over time as real instances of this appropriate balance will emerge over time as real instances of this
tension are considered". This document is written to provide input to tension are considered." This document is written to provide input to
the discussion around what is an appropriate balance, by highlighting the discussion around what is an appropriate balance by highlighting
some implications of transport header encryption.</t> some implications of transport header encryption.</t>
<t>Current uses of transport header information by network devices on <t>Current uses of transport header information by network devices on
the Internet path are explained. These uses can be beneficial or the Internet path are explained. These uses can be beneficial or
malicious. This is written to provide input to the discussion around malicious. This is written to provide input to the discussion around
what is an appropriate balance, by highlighting some implications of what is an appropriate balance by highlighting some implications of
transport header encryption.</t> transport header encryption.</t>
</section> </section>
<section anchor="Current" numbered="true" toc="default">
<section anchor="Current" <name>Current Uses of Transport Headers within the Network</name>
title="Current uses of Transport Headers within the Network"> <t>In response to pervasive surveillance <xref target="RFC7624" format="de
<t>In response to pervasive monitoring <xref target="RFC7624"></xref> fault"/>
revelations and the IETF consensus that "Pervasive Monitoring is an revelations and the IETF consensus that "Pervasive Monitoring Is an
Attack" <xref target="RFC7258"></xref>, efforts are underway to increase Attack" <xref target="RFC7258" format="default"/>, efforts are underway to
increase
encryption of Internet traffic. Applying confidentiality to transport encryption of Internet traffic. Applying confidentiality to transport
header fields can improve privacy, and can help to mitigate certain header fields can improve privacy and can help to mitigate certain
attacks or manipulation of packets by devices on the network path, but attacks or manipulation of packets by devices on the network path, but
it can also affect network operations and measurement <xref it can also affect network operations and measurement <xref target="RFC840
target="RFC8404"></xref>.</t> 4"
format="default"/>.</t>
<t>When considering what parts of the transport headers should be <t>When considering what parts of the transport headers should be
encrypted to provide confidentiality, and what parts should be visible encrypted to provide confidentiality and what parts should be visible
to network devices (including non-encrypted but authenticated headers), to network devices (including unencrypted but authenticated headers),
it is necessary to consider both the impact on network operations and it is necessary to consider both the impact on network operations and
management, and the implications for ossification and user privacy <xref management and the implications for ossification and user privacy <xref
target="Measurement"></xref>. Different parties will view the relative target="Measurement" format="default"/>. Different parties will view the r
elative
importance of these concerns differently. For some, the benefits of importance of these concerns differently. For some, the benefits of
encrypting all the transport headers outweigh the impact of doing so; encrypting all the transport headers outweigh the impact of doing so;
others might analyse the security, privacy, and ossification impacts and others might analyse the security, privacy, and ossification impacts and
arrive at a different trade-off.</t> arrive at a different trade-off.</t>
<t>This section reviews examples of the observation of transport-layer
<t>This section reviews examples of the observation of transport layer headers within the network by using devices on the network path or by usin
headers within the network by devices on the network path, or using g
information exported by an on-path device. Unencrypted transport headers information exported by an on-path device. Unencrypted transport headers
provide information that can support network operations and management, provide information that can support network operations and management,
and this section notes some ways in which this has been done. and this section notes some ways in which this has been done.
Unencrypted transport header information also contributes metadata that Unencrypted transport header information also contributes metadata that
can be exploited for purposes unrelated to network transport can be exploited for purposes unrelated to network transport
measurement, diagnostics or troubleshooting (e.g., to block or to measurement, diagnostics, or troubleshooting (e.g., to block or to
throttle traffic from a specific content provider), and this section throttle traffic from a specific content provider), and this section
also notes some threats relating to unencrypted transport headers.</t> also notes some threats relating to unencrypted transport headers.</t>
<t>Exposed transport information also provides a source of information <t>Exposed transport information also provides a source of information
that contributes to linked data sets, which could be exploited to deduce that contributes to linked data sets, which could be exploited to deduce
private information, e.g., user patterns, user location, tracking private information, e.g., user patterns, user location, tracking
behaviour, etc. This might reveal information the parties did not intend behaviour, etc. This might reveal information the parties did not intend
to be revealed. <xref target="RFC6973"></xref> aims to make designers, to be revealed. <xref target="RFC6973" format="default"/> aims to make des igners,
implementers, and users of Internet protocols aware of privacy-related implementers, and users of Internet protocols aware of privacy-related
design choices in IETF protocols.</t> design choices in IETF protocols.</t>
<t>This section does not consider intentional modification of transport <t>This section does not consider intentional modification of transport
headers by middleboxes, such as devices performing Network Address headers by middleboxes, such as devices performing Network Address
Translation (NAT) or Firewalls.</t> Translation (NAT) or firewalls.</t>
<section numbered="true" toc="default">
<section title="To Separate Flows in Network Devices"> <name>To Separate Flows in Network Devices</name>
<t>Some network layer mechanisms separate network traffic by flow, <t>Some network-layer mechanisms separate network traffic by flow
without resorting to identifying the type of traffic. Hash-based without resorting to identifying the type of traffic: hash-based
load-sharing sharing across paths (e..g., equal cost multi path, load sharing across paths (e.g., Equal-Cost Multipath
ECMP), sharing across a group of links (e.g., using a link aggregation (ECMP)); sharing across a group of links (e.g., using a Link Aggregation
group, LAG), ensuring equal access to link capacity (e.g., fair Group (LAG)); ensuring equal access to link capacity (e.g., Fair
queuing, FQ), or distributing traffic to servers (e.g., load Queuing (FQ)); or distributing traffic to servers (e.g., load
balancing). To prevent packet reordering, forwarding engines can balancing). To prevent packet reordering, forwarding engines can
consistently forward the same transport flows along the same consistently forward the same transport flows along the same
forwarding path, often achieved by calculating a hash using an n-tuple forwarding path, often achieved by calculating a hash using an n-tuple
gleaned from a combination of link header information through to gleaned from a combination of link header information through to
transport header information. This n-tuple can use the MAC address, IP transport header information. This n-tuple can use the Media Access Cont
addresses, and can include observable transport header information. rol
(MAC) address and IP
addresses and can include observable transport header information.
</t> </t>
<t>When transport header information cannot be observed, there can be <t>When transport header information cannot be observed, there can be
less information to separate flows at equipment along the path. Flow less information to separate flows at equipment along the path.
separation might not be possible when, a transport that forms traffic Flow
into an encrypted aggregate. For IPv6, the Flow Label <xref separation might not be possible when a transport forms traffic
target="RFC6437"></xref> can be used even when all transport into an encrypted aggregate. For IPv6, the Flow Label <xref target="RFC6
information is encrypted, enabling Flow Label-based ECMP <xref 437" format="default"/> can be used even when all transport
target="RFC6438"></xref> and Load-Sharing <xref information is encrypted, enabling Flow Label-based ECMP <xref target="R
target="RFC7098"></xref>.</t> FC6438" format="default"/> and load sharing <xref target="RFC7098" format="defau
lt"/>.</t>
</section> </section>
<section anchor="Current-demux" numbered="true" toc="default">
<section anchor="Current-demux" <name>To Identify Transport Protocols and Flows</name>
title="To Identify Transport Protocols and Flows"> <t>Information in exposed transport-layer headers can be used by the
<t>Information in exposed transport layer headers can be used by the network to identify transport protocols and flows <xref target="RFC8558"
network to identify transport protocols and flows <xref format="default"/>. The ability to identify transport protocols,
target="RFC8558"></xref>. The ability to identify transport protocols,
flows, and sessions is a common function performed, for example, by flows, and sessions is a common function performed, for example, by
measurement activities, Quality of Service (QoS) classifiers, and measurement activities, Quality of Service (QoS) classifiers, and
firewalls. These functions can be beneficial, and performed with the firewalls. These functions can be beneficial and performed with the
consent of, and in support of, the end user. Alternatively, the same consent of, and in support of, the end user. Alternatively, the same
mechanisms could be used to support practises that might be mechanisms could be used to support practises that might be
adversarial to the end user, including blocking, de-prioritising, and adversarial to the end user, including blocking, deprioritising, and
monitoring traffic without consent.</t> monitoring traffic without consent.</t>
<t>Observable transport header information, together with information <t>Observable transport header information, together with information
in the network header, has been used to identify flows and their in the network header, has been used to identify flows and their
connection state, together with the set of protocol options being connection state, together with the set of protocol options being
used. Transport protocols, such as TCP <xref target="RFC7414"></xref> used. Transport protocols, such as TCP <xref target="RFC7414" format="de
and the Stream Control Transport Protocol (SCTP) <xref fault"/>
target="RFC4960"></xref>, specify a standard base header that includes and the Stream Control Transmission Protocol (SCTP) <xref target="RFC496
0" format="default"/>, specify a standard base header that includes
sequence number information and other data. They also have the sequence number information and other data. They also have the
possibility to negotiate additional headers at connection setup, possibility to negotiate additional headers at connection setup,
identified by an option number in the transport header.</t> identified by an option number in the transport header.</t>
<t>In some uses, an assigned transport port (e.g., 0..49151) can <t>In some uses, an assigned transport port (e.g., 0..49151) can
identify the upper-layer protocol or service <xref identify the upper-layer protocol or service <xref target="RFC7605" form
target="RFC7605"></xref>. However, port information alone is not at="default"/>. However, port information alone is not
sufficient to guarantee identification. Applications can use arbitrary sufficient to guarantee identification. Applications can use arbitrary
ports and do not need to use assigned port numbers. The use of an ports and do not need to use assigned port numbers. The use of an
assigned port number is also not limited to the protocol for which the assigned port number is also not limited to the protocol for which the
port is intended. Multiple sessions can also be multiplexed on a port is intended. Multiple sessions can also be multiplexed on a
single port, and ports can be re-used by subsequent sessions.</t> single port, and ports can be reused by subsequent sessions.</t>
<t>Some flows can be identified by observing signalling data
<t>Some flows can be identified by observing signalling data (e.g., (e.g., see <xref target="RFC3261" format="default"/> and <xref target="R
<xref target="RFC3261"></xref>, <xref target="RFC8837"></xref>) or FC8837" format="default"/>) or
through the use of magic numbers placed in the first byte(s) of a through the use of magic numbers placed in the first byte(s) of a
datagram payload <xref target="RFC7983"></xref>.</t> datagram payload <xref target="RFC7983" format="default"/>.</t>
<t>When transport header information cannot be observed, this removes <t>When transport header information cannot be observed, this removes
information that could have been used to classify flows by passive information that could have been used to classify flows by passive
observers along the path. More ambitious ways could be used to observers along the path. More ambitious ways could be used to
collect, estimate, or infer flow information, including heuristics collect, estimate, or infer flow information, including heuristics
based on the analysis of traffic patterns, such as classification of based on the analysis of traffic patterns, such as classification of
flows relying on timing, volumes of information, and correlation flows relying on timing, volumes of information, and correlation
between multiple flows. For example, an operator that cannot access between multiple flows. For example, an operator that cannot access
the Session Description Protocol (SDP) session descriptions <xref the Session Description Protocol (SDP) session descriptions <xref target
target="RFC4566"></xref> to classify a flow as audio traffic, might ="RFC8866" format="default"/> to classify a flow as audio traffic might
instead use (possibly less-reliable) heuristics to infer that short instead use (possibly less-reliable) heuristics to infer that short
UDP packets with regular spacing carry audio traffic. Operational UDP packets with regular spacing carry audio traffic. Operational
practises aimed at inferring transport parameters are out of scope for practises aimed at inferring transport parameters are out of scope for
this document, and are only mentioned here to recognise that this document, and are only mentioned here to recognise that
encryption does not prevent operators from attempting to apply encryption does not prevent operators from attempting to apply
practises that were used with unencrypted transport headers.</t> practises that were used with unencrypted transport headers.</t>
<t>The IAB <xref target="RFC8546" format="default"/> has provided a summ
<t>The IAB <xref target="RFC8546"></xref> have provided a summary of ary of
expected implications of increased encryption on network functions expected implications of increased encryption on network functions
that use the observable headers and describe the expected benefits of that use the observable headers and describe the expected benefits of
designs that explicitly declare protocol invariant header information designs that explicitly declare protocol-invariant header information
that can be used for this purpose.</t> that can be used for this purpose.</t>
</section> </section>
<section anchor="stats" numbered="true" toc="default">
<section anchor="stats" <name>To Understand Transport Protocol Performance</name>
title="To Understand Transport Protocol Performance"> <t>This subsection describes use by the network of exposed transport-lay
<t>This subsection describes use by the network of exposed transport er headers to
layer headers to understand transport protocol performance and understand transport protocol performance and
behaviour.</t> behaviour.</t>
<section numbered="true" toc="default">
<section title="Using Information Derived from Transport Layer Headers"> <name>Using Information Derived from Transport-Layer Headers</name>
<t>Observable transport headers enable explicit measurement and <t>Observable transport headers enable explicit measurement and
analysis of protocol performance, and detection of network anomalies analysis of protocol performance and detection of network anomalies
at any point along the Internet path. Some operators use passive at any point along the Internet path. Some operators use passive
monitoring to manage their portion of the Internet by characterising monitoring to manage their portion of the Internet by characterising
the performance of link/network segments. Inferences from transport the performance of link/network segments. Inferences from transport
headers are used to derive performance metrics:</t> headers are used to derive performance metrics:</t>
<dl newline="true" spacing="normal">
<t><list style="hanging"> <dt>Traffic Rate and Volume:</dt>
<t hangText="Traffic Rate and Volume:">Per-application traffic <dd><t>Per-application traffic
rate and volume measures can be used to characterise the traffic rate and volume measures can be used to characterise the traffic
that uses a network segment or the pattern of network usage. that uses a network segment or the pattern of network usage.
Observing the protocol sequence number and packet size offers Observing the protocol sequence number and packet size offers
one way to measure this (e.g., measurements observing counters one way to measure this (e.g., measurements observing counters
in periodic reports such as RTCP; or measurements observing in periodic reports, such as RTCP <xref target="RFC3550"
format="default"/> <xref target="RFC3711" format="default"/> <xref
target="RFC4585" format="default"/>, or measurements observing
protocol sequence numbers in statistical samples of packet protocol sequence numbers in statistical samples of packet
flows, or specific control packets, such as those observed at flows or specific control packets, such as those observed at
the start and end of a flow).</t> the start and end of a flow).</t>
<t>Measurements can be per endpoint or for an
<t hangText="">Measurements can be per endpoint, or for an
endpoint aggregate. These could be used to assess usage or for endpoint aggregate. These could be used to assess usage or for
subscriber billing.</t> subscriber billing.</t>
<t>Such measurements can be used to trigger traffic
<t hangText="">Such measurements can be used to trigger traffic shaping and to associate QoS support within the network and
shaping, and to associate QoS support within the network and
lower layers. This can be done with consent and in support of an lower layers. This can be done with consent and in support of an
end user, to improve quality of service; or could be used by the end user to improve quality of service or could be used by the
network to de-prioritise certain flows without user consent.</t> network to deprioritise certain flows without user consent.</t>
<t>The traffic rate and volume can be determined,
<t hangText="">The traffic rate and volume can be determined
providing that the packets belonging to individual flows can be providing that the packets belonging to individual flows can be
identified, but there might be no additional information about a identified, but there might be no additional information about a
flow when the transport headers cannot be observed.</t> flow when the transport headers cannot be observed.</t>
</dd>
<t hangText="Loss Rate and Loss Pattern:">Flow loss rate can be <dt>Loss Rate and Loss Pattern:</dt>
<dd><t>Flow loss rate can be
derived (e.g., from transport sequence numbers or inferred from derived (e.g., from transport sequence numbers or inferred from
observing transport protocol interactions) and has been used as observing transport protocol interactions) and has been used as
a metric for performance assessment and to characterise a metric for performance assessment and to characterise
transport behaviour. Network operators have used the variation transport behaviour. Network operators have used the variation
in patterns to detect changes in the offered service. in patterns to detect changes in the offered service.
Understanding the location and root cause of loss can help an Understanding the location and root cause of loss can help an
operator determine whether this requires corrective action.</t> operator determine whether this requires corrective action.</t>
<t>There are various causes of loss, including: corruption of
<t>There are various causes of loss, including: corruption of link frames (e.g., due to interference on a radio link);
link frames (e.g., due to interference on a radio link),
buffering loss (e.g., overflow due to congestion, Active Queue buffering loss (e.g., overflow due to congestion, Active Queue
Management, AQM <xref target="RFC7567"></xref>, or inadequate Management (AQM) <xref target="RFC7567" format="default"/>, or ina
provision following traffic pre-emption), and policing (traffic dequate
management <xref target="RFC2475"></xref>). Understanding flow provision following traffic preemption), and policing (e.g., traff
loss rates requires maintaining per-flow state (flow ic
identification often requires transport layer information) and management <xref target="RFC2475" format="default"/>). Understandi
ng flow
loss rates requires maintaining the per-flow state (flow
identification often requires transport-layer information) and
either observing the increase in sequence numbers in the network either observing the increase in sequence numbers in the network
or transport headers, or comparing a per-flow packet counter or transport headers or comparing a per-flow packet counter
with the number of packets that the flow actually sent. Per-hop with the number of packets that the flow actually sent. Per-hop
loss can also sometimes be monitored at the interface level by loss can also sometimes be monitored at the interface level by
devices on the network path, or using in-situ methods operating devices on the network path or by using in-situ methods operating
over a network segment (see <xref over a network segment (see <xref target="other-sources" format="d
target="other-sources"></xref>).</t> efault"/>).</t>
<t>The pattern of loss can provide insight into the cause of
<t>The pattern of loss can provide insight into the cause of loss. Losses can often occur as bursts, randomly timed events,
loss. Losses can often occur as bursts, randomly-timed events,
etc. It can also be valuable to understand the conditions under etc. It can also be valuable to understand the conditions under
which loss occurs. This usually requires relating loss to the which loss occurs. This usually requires relating loss to the
traffic flowing at a network node or segment at the time of traffic flowing at a network node or segment at the time of
loss. Transport header information can help identify cases where loss. Transport header information can help identify cases where
loss could have been wrongly identified, or where the transport loss could have been wrongly identified or where the transport
did not require retransmission of a lost packet.</t> did not require retransmission of a lost packet.</t>
</dd>
<t hangText="Throughput and Goodput:">Throughput is the amount <dt>Throughput and Goodput:</dt>
<dd>Throughput is the amount
of payload data sent by a flow per time interval. Goodput (the of payload data sent by a flow per time interval. Goodput (the
subset of throughput consisting of useful traffic) <xref subset of throughput consisting of useful traffic; see <xref targe
target="RFC7928">(see Section 2.5 of </xref> and <xref t="RFC7928"
target="RFC5166"></xref>) is a measure of useful data exchanged. sectionFormat="of" section="2.5"/> and <xref target="RFC5166" forma
t="default"/>) is
a measure of useful data exchanged.
The throughput of a flow can be determined in the absence of The throughput of a flow can be determined in the absence of
transport header information, providing that the individual flow transport header information, providing that the individual flow
can be identified, and the overhead known. Goodput requires can be identified, and the overhead known. Goodput requires the
ability to differentiate loss and retransmission of packets, for ability to differentiate loss and retransmission of packets, for
example by observing packet sequence numbers in the TCP or RTP example, by observing packet sequence numbers in the TCP or RTP
headers <xref target="RFC3550"></xref>.</t> headers <xref target="RFC3550" format="default"/>.</dd>
<dt>Latency:</dt>
<t hangText="Latency:">Latency is a key performance metric that <dd><t>Latency is a key performance metric that
impacts application and user-perceived response times. It often impacts application and user-perceived response times. It often
indirectly impacts throughput and flow completion time. This indirectly impacts throughput and flow completion time. This
determines the reaction time of the transport protocol itself, determines the reaction time of the transport protocol itself,
impacting flow setup, congestion control, loss recovery, and impacting flow setup, congestion control, loss recovery, and
other transport mechanisms. The observed latency can have many other transport mechanisms. The observed latency can have many
components <xref target="Latency"></xref>. Of these, components <xref target="Latency" format="default"/>. Of these,
unnecessary/unwanted queueing in buffers of the network devices unnecessary/unwanted queueing in buffers of the network devices
on the path has often been observed as a significant factor on the path has often been observed as a significant factor
<xref target="bufferbloat"></xref>. Once the cause of unwanted <xref target="bufferbloat" format="default"/>. Once the cause of u nwanted
latency has been identified, this can often be eliminated.</t> latency has been identified, this can often be eliminated.</t>
<t>To measure latency across a part of a path, an observation
<t>To measure latency across a part of a path, an observation point <xref target="RFC7799" format="default"/> can measure the ex
point <xref target="RFC7799"></xref> can measure the experienced perienced
round trip time (RTT) using packet sequence numbers and round-trip time (RTT) by using packet sequence numbers and
acknowledgements, or by observing header timestamp information. acknowledgements or by observing header timestamp information.
Such information allows an observation point on the network path Such information allows an observation point on the network path
to determine not only the path RTT, but also allows measurement to determine not only the path RTT but also allows measurement
of the upstream and downstream contribution to the RTT. This of the upstream and downstream contribution to the RTT. This
could be used to locate a source of latency, e.g., by observing could be used to locate a source of latency, e.g., by observing
cases where the median RTT is much greater than the minimum RTT cases where the median RTT is much greater than the minimum RTT
for a part of a path.</t> for a part of a path.</t>
<t>The service offered by network operators can benefit from
<t>The service offered by network operators can benefit from
latency information to understand the impact of configuration latency information to understand the impact of configuration
changes and to tune deployed services. Latency metrics are key changes and to tune deployed services. Latency metrics are key
to evaluating and deploying AQM <xref target="RFC7567"></xref>, to evaluating and deploying AQM <xref target="RFC7567" format="def
DiffServ <xref target="RFC2474"></xref>, and Explicit Congestion ault"/>,
Notification (ECN) <xref target="RFC3168"></xref> <xref Diffserv <xref target="RFC2474" format="default"/>, and
target="RFC8087"></xref>. Measurements could identify Explicit Congestion
Notification (ECN) <xref target="RFC3168" format="default"/> <xref
target="RFC8087"
format="default"/>. Measurements could identify
excessively large buffers, indicating where to deploy or excessively large buffers, indicating where to deploy or
configure AQM. An AQM method is often deployed in combination configure AQM. An AQM method is often deployed in combination
with other techniques, such as scheduling <xref with other techniques, such as scheduling <xref target="RFC7567" f
target="RFC7567"> </xref> <xref target="RFC8290"> </xref> and ormat="default">
although parameter-less methods are desired <xref </xref> <xref target="RFC8290" format="default"> </xref>, and
target="RFC7567"> </xref>, current methods often require tuning although parameter-less methods are desired <xref target="RFC7567"
<xref target="RFC8290"></xref> <xref target="RFC8289"> </xref> format="default">
<xref target="RFC8033"> </xref> because they cannot scale across </xref>, current methods often require tuning
<xref target="RFC8290" format="default"/> <xref target="RFC8289" f
ormat="default">
</xref>
<xref target="RFC8033" format="default"> </xref> because they cann
ot scale across
all possible deployment scenarios.</t> all possible deployment scenarios.</t>
<t>Latency and round-trip time information can potentially
<t>Latency and round-trip time information can potentially
expose some information useful for approximate geolocation, as expose some information useful for approximate geolocation, as
discussed in <xref target="PAM-RTT"></xref>.</t> discussed in <xref target="PAM-RTT" format="default"/>.</t>
</dd>
<t hangText="Variation in delay:">Some network applications are <dt>Variation in Delay:</dt>
sensitive to (small) changes in packet timing (jitter). Short <dd>Some network applications are
and long-term delay variation can impact on the latency of a sensitive to (small) changes in packet timing (jitter). Short-
and long-term delay variation can impact the latency of a
flow and hence the perceived quality of applications using a flow and hence the perceived quality of applications using a
network path. For example, jitter metrics are often cited when network path. For example, jitter metrics are often cited when
characterising paths supporting real-time traffic. The expected characterising paths supporting real-time traffic. The expected
performance of such applications, can be inferred from a measure performance of such applications can be inferred from a measure
of the variation in delay observed along a portion of the path of the variation in delay observed along a portion of the path
<xref target="RFC3393"></xref> <xref target="RFC5481"></xref>. <xref target="RFC3393" format="default"/> <xref target="RFC5481" f ormat="default"/>.
The requirements resemble those for the measurement of The requirements resemble those for the measurement of
latency.</t> latency.</dd>
<dt>Flow Reordering:</dt>
<t hangText="Flow Reordering:">Significant packet reordering <dd><t>Significant packet reordering
within a flow can impact time-critical applications and can be within a flow can impact time-critical applications and can be
interpreted as loss by reliable transports. Many transport interpreted as loss by reliable transports. Many transport
protocol techniques are impacted by reordering (e.g., triggering protocol techniques are impacted by reordering (e.g., triggering
TCP retransmission or re-buffering of real-time applications). TCP retransmission or rebuffering of real-time applications).
Packet reordering can occur for many reasons, from equipment Packet reordering can occur for many reasons, e.g., from equipment
design to misconfiguration of forwarding rules. Flow design to misconfiguration of forwarding rules. Flow
identification is often required to avoid significant packet identification is often required to avoid significant packet
mis-ordering (e.g., when using ECMP, or LAG). Network tools can misordering (e.g., when using ECMP, or LAG). Network tools can
detect and measure unwanted/excessive reordering, and the impact detect and measure unwanted/excessive reordering and the impact
on transport performance.</t> on transport performance.</t>
<t>There have been initiatives in the IETF transport area to
<t>There have been initiatives in the IETF transport area to
reduce the impact of reordering within a transport flow, reduce the impact of reordering within a transport flow,
possibly leading to a reduction in the requirements for possibly leading to a reduction in the requirements for
preserving ordering. These have potential to simplify network preserving ordering. These have potential to simplify network
equipment design as well as the potential to improve robustness equipment design as well as the potential to improve robustness
of the transport service. Measurements of reordering can help of the transport service. Measurements of reordering can help
understand the present level of reordering, and inform decisions understand the present level of reordering and inform decisions
about how to progress new mechanisms.</t> about how to progress new mechanisms.</t>
<t>Techniques for measuring reordering typically observe packet
<t>Techniques for measuring reordering typically observe packet
sequence numbers. Metrics have been defined that evaluate sequence numbers. Metrics have been defined that evaluate
whether a network path has maintained packet order on a whether a network path has maintained packet order on a
packet-by-packet basis <xref target="RFC4737"></xref> <xref packet-by-packet basis <xref target="RFC4737" format="default"/> <
target="RFC5236"></xref>. Some protocols provide in-built xref
target="RFC5236" format="default"/>. Some protocols provide in-buil
t
monitoring and reporting functions. Transport fields in the RTP monitoring and reporting functions. Transport fields in the RTP
header <xref target="RFC3550"></xref> <xref header <xref target="RFC3550" format="default"/> <xref target="RFC
target="RFC4585"></xref> can be observed to derive traffic 4585"
format="default"/> can be observed to derive traffic
volume measurements and provide information on the progress and volume measurements and provide information on the progress and
quality of a session using RTP. Metadata assists in quality of a session using RTP. Metadata assists in
understanding the context under which the data was collected, understanding the context under which the data was collected,
including the time, observation point <xref including the time, observation point <xref target="RFC7799" forma
target="RFC7799"></xref>, and way in which metrics were t="default"/>, and
way in which metrics were
accumulated. The RTCP protocol directly reports some of this accumulated. The RTCP protocol directly reports some of this
information in a form that can be directly visible by devices on information in a form that can be directly visible by devices on
the network path.</t> the network path.</t>
</list></t> </dd>
</dl>
<t>In some cases, measurements could involve active injection of <t>In some cases, measurements could involve active injection of
test traffic to perform a measurement (see Section 3.4 of <xref test traffic to perform a measurement (see <xref target="RFC7799" sect
target="RFC7799"></xref>). However, most operators do not have ionFormat="of"
access to user equipment, therefore the point of test is normally section="3.4"/>). However, most operators do not have
access to user equipment; therefore, the point of test is normally
different from the transport endpoint. Injection of test traffic can different from the transport endpoint. Injection of test traffic can
incur an additional cost in running such tests (e.g., the incur an additional cost in running such tests (e.g., the
implications of capacity tests in a mobile network segment are implications of capacity tests in a mobile network segment are
obvious). Some active measurements <xref target="RFC7799"></xref> obvious). Some active measurements <xref target="RFC7799" format="defa ult"/>
(e.g., response under load or particular workloads) perturb other (e.g., response under load or particular workloads) perturb other
traffic, and could require dedicated access to the network traffic and could require dedicated access to the network
segment.</t> segment.</t>
<t>Passive measurements (see <xref target="RFC7799" sectionFormat="of"
<t>Passive measurements (see Section 3.6 of <xref section="3.6"/>)
target="RFC7799"></xref>) can have advantages in terms of can have advantages in terms of
eliminating unproductive test traffic, reducing the influence of eliminating unproductive test traffic, reducing the influence of
test traffic on the overall traffic mix, and the ability to choose test traffic on the overall traffic mix, and having the ability to cho
the point of observation (see <xref target="point"></xref>). ose
the point of observation (see <xref target="point" format="default"/>)
.
Measurements can rely on observing packet headers, which is not Measurements can rely on observing packet headers, which is not
possible if those headers are encrypted, but could utilise possible if those headers are encrypted, but could utilise
information about traffic volumes or patterns of interaction to information about traffic volumes or patterns of interaction to
deduce metrics.</t> deduce metrics.</t>
<t>Passive packet sampling techniques are also often used to scale <t>Passive packet sampling techniques are also often used to scale
the processing involved in observing packets on high rate links. the processing involved in observing packets on high-rate links.
This exports only the packet header information of (randomly) This exports only the packet header information of (randomly)
selected packets. Interpretation of the exported information relies selected packets. Interpretation of the exported information relies
on understanding of the header information. The utility of these on understanding of the header information. The utility of these
measurements depends on the type of network segment/link and number measurements depends on the type of network segment/link and number
of mechanisms used by the network devices. Simple routers are of mechanisms used by the network devices. Simple routers are
relatively easy to manage, but a device with more complexity demands relatively easy to manage, but a device with more complexity demands
understanding of the choice of many system parameters.</t> understanding of the choice of many system parameters.</t>
</section> </section>
<section anchor="tunlhf" numbered="true" toc="default">
<section anchor="tunlhf" <name>Using Information Derived from Network-Layer Header Fields</name
title="Using Information Derived from Network Layer Header Fiel >
ds">
<t>Information from the transport header can be used by a <t>Information from the transport header can be used by a
multi-field (MF) classifier as a part of policy framework. Policies multi-field (MF) classifier as a part of policy framework. Policies
are commonly used for management of the QoS or Quality of Experience are commonly used for management of the QoS or Quality of Experience
(QoE) in resource-constrained networks, or by firewalls to implement (QoE) in resource-constrained networks or by firewalls to implement
access rules (see also Section 2.2.2 of <xref access rules (see also <xref target="RFC8404" sectionFormat="of" secti
target="RFC8404"></xref>). Policies can support user on ="2.2.2"/>).
applications/services or protect against unwanted, or lower priority Policies can support user
traffic (<xref target="Implic-Unknown"></xref>).</t> applications/services or protect against unwanted or lower-priority
traffic (<xref target="Implic-Unknown" format="default"/>).</t>
<t>Transport layer information can also be explicitly carried in <t>Transport-layer information can also be explicitly carried in
network-layer header fields that are not encrypted, serving as a network-layer header fields that are not encrypted, serving as a
replacement/addition to the exposed transport header information replacement/addition to the exposed transport header information
<xref target="RFC8558"></xref>. This information can enable a <xref target="RFC8558" format="default"/>. This information can enable a
different forwarding treatment by the devices forming the network different forwarding treatment by the devices forming the network
path, even when a transport employs encryption to protect other path, even when a transport employs encryption to protect other
header information.</t> header information.</t>
<t>On the one hand, the user of a transport that multiplexes <t>On the one hand, the user of a transport that multiplexes
multiple sub-flows might want to obscure the presence and multiple subflows might want to obscure the presence and
characteristics of these sub-flows. On the other hand, an encrypted characteristics of these subflows. On the other hand, an encrypted
transport could set the network-layer information to indicate the transport could set the network-layer information to indicate the
presence of sub-flows, and to reflect the service requirements of presence of subflows and to reflect the service requirements of
individual sub-flows. There are several ways this could be done:</t> individual subflows. There are several ways this could be done:</t>
<dl newline="true" spacing="normal">
<t><list style="hanging"> <dt>IP Address:</dt>
<t hangText="IP Address:">Applications normally expose the <dd>Applications normally expose the
endpoint addresses used in the forwarding decisions in network endpoint addresses used in the forwarding decisions in network
devices. Address and other protocol information can be used by a devices. Address and other protocol information can be used by an
MF-classifier to determine how traffic is treated <xref MF classifier to determine how traffic is treated <xref target="RF
target="RFC2475"></xref>, and hence affect the quality of C2475"
format="default"/> and hence affects the quality of
experience for a flow. Common issues concerning IP address experience for a flow. Common issues concerning IP address
sharing are described in <xref target="RFC6269"></xref>.</t> sharing are described in <xref target="RFC6269" format="default"/>
.</dd>
<t hangText="Using the IPv6 Network-Layer Flow Label:">A number <dt>Using the IPv6 Network-Layer Flow Label:</dt>
of Standards Track and Best Current Practice RFCs (e.g., <xref <dd><t>A number
target="RFC8085"></xref>, <xref target="RFC6437"></xref>, <xref of Standards Track and Best Current Practice RFCs (e.g., <xref tar
target="RFC6438"></xref>) encourage endpoints to set the IPv6 get="RFC8085"
flow label field of the network-layer header. IPv6 &ldquo;source format="default"/>, <xref target="RFC6437" format="default"/>, and
nodes SHOULD assign each unrelated transport connection and <xref
application data stream to a new flow&rdquo; <xref target="RFC6438" format="default"/>) encourage endpoints to set the
target="RFC6437"></xref>. A multiplexing transport could choose IPv6
Flow Label field of the network-layer header.
As per <xref target="RFC6437"/>, IPv6 source nodes "<bcp14>SHOULD</
bcp14> assign each
unrelated transport connection and application data stream to a
new flow."
A multiplexing transport could choose
to use multiple flow labels to allow the network to to use multiple flow labels to allow the network to
independently forward sub-flows. RFC6437 provides further independently forward subflows. <xref target="RFC6437" format="def ault"/> provides further
guidance on choosing a flow label value, stating these guidance on choosing a flow label value, stating these
&ldquo;should be chosen such that their bits exhibit a high "should be chosen such that their bits exhibit a high
degree of variability&rdquo;, and chosen so that &ldquo;third degree of variability" and chosen so that "third
parties should be unlikely to be able to guess the next value parties should be unlikely to be able to guess the next value
that a source of flow labels will choose&rdquo;.</t> that a source of flow labels will choose."</t>
<t>Once set, a flow label can provide information
<t hangText="">Once set, a flow label can provide information
that can help inform network-layer queueing and forwarding, that can help inform network-layer queueing and forwarding,
including use with IPsec, <xref target="RFC6294"></xref> and use including use with IPsec <xref target="RFC6294" format="default"/>
with Equal Cost Multi-Path routing and Link Aggregation<xref ,
target="RFC6438"> </xref>.</t> Equal-Cost Multipath routing, and Link Aggregation <xref target="R
FC6438"
<t hangText="">The choice of how to assign a flow label needs to format="default"></xref>.</t>
<t>The choice of how to assign a flow label needs to
avoid introducing linkages between flows that a network device avoid introducing linkages between flows that a network device
could not otherwise observe. Inappropriate use by the transport could not otherwise observe. Inappropriate use by the transport
can have privacy implications (e.g., assigning the same label to can have privacy implications (e.g., assigning the same label to
two independent flows that ought not to be classified the two independent flows that ought not to be classified similarly).<
same).</t> /t>
</dd>
<t <dt>Using the Network-Layer Differentiated Services Code Point:</dt>
hangText="Using the Network-Layer Differentiated Services Code Poi <dd>Applications
nt:">Applications
can expose their delivery expectations to network devices by can expose their delivery expectations to network devices by
setting the Differentiated Services Code Point (DSCP) field of setting the Differentiated Services Code Point (DSCP) field of
IPv4 and IPv6 packets <xref target="RFC2474"></xref>. For IPv4 and IPv6 packets <xref target="RFC2474" format="default"/>. F or
example, WebRTC applications identify different forwarding example, WebRTC applications identify different forwarding
treatments for individual sub-flows (audio vs. video) based on treatments for individual subflows (audio vs. video) based on
the value of the DSCP field <xref the value of the DSCP field <xref target="RFC8837" format="default
target="I-D.ietf-tsvwg-rtcweb-qos"></xref>). This provides "/>). This provides
explicit information to inform network-layer queueing and explicit information to inform network-layer queueing and
forwarding, rather than an operator inferring traffic forwarding, rather than an operator inferring traffic
requirements from transport and application headers via a requirements from transport and application headers via a
multi-field classifier. Inappropriate use by the transport can multi-field classifier. Inappropriate use by the transport can
have privacy implications (e.g., assigning a different DSCP to a have privacy implications (e.g., assigning a different DSCP to a
subflow could assist in a network device discovering the traffic subflow could assist in a network device discovering the traffic
pattern used by an application). The field is mutable, i.e., pattern used by an application). The field is mutable, i.e.,
some network devices can be expected to change this field. Since some network devices can be expected to change this field. Since
the DSCP value can impact the quality of experience for a flow, the DSCP value can impact the quality of experience for a flow,
observations of service performance have to consider this field observations of service performance have to consider this field
when a network path supports differentiated service when a network path supports differentiated service
treatment.</t> treatment.</dd>
<dt>Using Explicit Congestion Notification:</dt>
<t hangText="Using Explicit Congestion Marking:">ECN <xref <dd><t>Explicit Congestion Notification (ECN) <xref target="RFC3168"
target="RFC3168"> </xref> is a transport mechanism that uses the format="default">
</xref> is a transport mechanism that uses the
ECN field in the network-layer header. Use of ECN explicitly ECN field in the network-layer header. Use of ECN explicitly
informs the network-layer that a transport is ECN-capable, and informs the network layer that a transport is ECN capable and
requests ECN treatment of the flow. An ECN-capable transport can requests ECN treatment of the flow. An ECN-capable transport can
offer benefits when used over a path with equipment that offer benefits when used over a path with equipment that
implements an AQM method with CE marking of IP packets <xref implements an AQM method with Congestion Experienced (CE) marking
target="RFC8087"></xref>, since it can react to congestion of IP packets <xref target="RFC8087" format="default"/>, since it can react to c
ongestion
without also having to recover from lost packets.</t> without also having to recover from lost packets.</t>
<t>ECN exposes the presence of congestion. The reception of
<t>ECN exposes the presence of congestion. The reception of
CE-marked packets can be used to estimate the level of incipient CE-marked packets can be used to estimate the level of incipient
congestion on the upstream portion of the path from the point of congestion on the upstream portion of the path from the point of
observation (Section 2.5 of <xref target="RFC8087"> </xref>). observation (<xref target="RFC8087" sectionFormat="of" section="2. 5"/>).
Interpreting the marking behaviour (i.e., assessing congestion Interpreting the marking behaviour (i.e., assessing congestion
and diagnosing faults) requires context from the transport and diagnosing faults) requires context from the transport
layer, such as path RTT.</t> layer, such as path RTT.</t>
<t>AQM and ECN offer a range of algorithms and configuration
<t>AQM and ECN offer a range of algorithms and configuration
options. Tools therefore have to be available to network options. Tools therefore have to be available to network
operators and researchers to understand the implication of operators and researchers to understand the implication of
configuration choices and transport behaviour as the use of ECN configuration choices and transport behaviour as the use of ECN
increases and new methods emerge <xref target="RFC7567"> increases and new methods emerge <xref target="RFC7567" format="de fault">
</xref>.</t> </xref>.</t>
</dd>
<t hangText="Network-Layer Options">Network protocols can carry <dt>Network-Layer Options:</dt>
optional headers (see <xref target="EH"></xref>). These can <dd><t>Network protocols can carry
optional headers (see <xref target="EH" format="default"/>). These
can
explicitly expose transport header information to on-path explicitly expose transport header information to on-path
devices operating at the network layer (as discussed further in devices operating at the network layer (as discussed further in
<xref target="OAM"></xref>).</t> <xref target="OAM" format="default"/>).</t>
<t>IPv4 <xref target="RFC0791" format="default"/> has provisions
<t hangText="">IPv4 <xref target="RFC0791"></xref> has provision
for optional header fields. IP routers can examine these headers for optional header fields. IP routers can examine these headers
and are required to ignore IPv4 options that they do not and are required to ignore IPv4 options that they do not
recognise. Many current paths include network devices that recognise. Many current paths include network devices that
forward packets that carry options on a slower processing path. forward packets that carry options on a slower processing path.
Some network devices (e.g., firewalls) can be (and are) Some network devices (e.g., firewalls) can be (and are)
configured to drop these packets <xref target="RFC7126"></xref>. configured to drop these packets <xref target="RFC7126" format="de
BCP 186 <xref target="RFC7126"></xref> provides Best Current fault"/>.
Practice guidance on how operators should treat IPv4 packets BCP 186 <xref target="RFC7126" format="default"/> provides
guidance on how operators should treat IPv4 packets
that specify options.</t> that specify options.</t>
<t>IPv6 can encode optional network-layer
<t hangText="">IPv6 can encode optional network-layer
information in separate headers that may be placed between the information in separate headers that may be placed between the
IPv6 header and the upper-layer header <xref IPv6 header and the upper-layer header <xref target="RFC8200" form
target="RFC8200"></xref>. (e.g., the IPv6 Alternate Marking at="default"/>
Method <xref target="I-D.ietf-6man-ipv6-alt-mark"></xref>, which (e.g., the IPv6 Alternate Marking
Method <xref target="I-D.ietf-6man-ipv6-alt-mark" format="default"
/>, which
can be used to measure packet loss and delay metrics). The can be used to measure packet loss and delay metrics). The
Hop-by-Hop options header, when present, immediately follows the Hop-by-Hop Options header, when present, immediately follows the
IPv6 header. IPv6 permits this header to be examined by any node IPv6 header. IPv6 permits this header to be examined by any node
along the path if explicitly configured <xref along the path if explicitly configured <xref target="RFC8200"
target="RFC8200"></xref>.</t> format="default"/>.</t>
</list>Careful use of the network layer features (e.g., Extension </dd>
Headers can <xref target="EH2"></xref>) help provide similar </dl>
<t>Careful use of the network-layer features (e.g., extension
headers can; see <xref target="EH2" format="default"/>) help provide s
imilar
information in the case where the network is unable to inspect information in the case where the network is unable to inspect
transport protocol headers.</t> transport protocol headers.</t>
</section> </section>
</section> </section>
<section anchor="Measure" numbered="true" toc="default">
<section anchor="Measure" title="To Support Network Operations"> <name>To Support Network Operations</name>
<t>Some network operators make use of on-path observations of <t>Some network operators make use of on-path observations of
transport headers to analyse the service offered to the users of a transport headers to analyse the service offered to the users of a
network segment, and to inform operational practice, and can help network segment and inform operational practice and can help
detect and locate network problems. <xref target="RFC8517"></xref> detect and locate network problems. <xref target="RFC8517" format="defau
lt"/>
gives an operator's perspective about such use.</t> gives an operator's perspective about such use.</t>
<t>When observable transport header information is not available, <t>When observable transport header information is not available,
those seeking an understanding of transport behaviour and dynamics those seeking an understanding of transport behaviour and dynamics
might learn to work without that information. Alternatively, they might learn to work without that information. Alternatively, they
might use more limited measurements combined with pattern inference might use more limited measurements combined with pattern inference
and other heuristics to infer network behaviour (see Section 2.1.1 of and other heuristics to infer network behaviour (see <xref target="RFC84
<xref target="RFC8404"></xref>). Operational practises aimed at 04"
inferring transport parameters are out of scope for this document, and sectionFormat="of" section="2.1.1"/>). Operational practises aimed at
inferring transport parameters are out of scope for this document and
are only mentioned here to recognise that encryption does not are only mentioned here to recognise that encryption does not
necessarily stop operators from attempting to apply practises that necessarily stop operators from attempting to apply practises that
have been used with unencrypted transport headers.</t> have been used with unencrypted transport headers.</t>
<t>This section discusses topics concerning observation of transport <t>This section discusses topics concerning observation of transport
flows, with a focus on transport measurement.</t> flows, with a focus on transport measurement.</t>
<section anchor="point" numbered="true" toc="default">
<section anchor="point" title="Problem Location"> <name>Problem Location</name>
<t>Observations of transport header information can be used to <t>Observations of transport header information can be used to
locate the source of problems or to assess the performance of a locate the source of problems or to assess the performance of a
network segment. Often issues can only be understood in the context network segment. Often issues can only be understood in the context
of the other flows that share a particular path, particular device of the other flows that share a particular path, particular device
configuration, interface port, etc. A simple example is monitoring configuration, interface port, etc. A simple example is monitoring
of a network device that uses a scheduler or active queue management of a network device that uses a scheduler or active queue management
technique <xref target="RFC7567"></xref>, where it could be technique <xref target="RFC7567" format="default"/>, where it could be
desirable to understand whether the algorithms are correctly desirable to understand whether the algorithms are correctly
controlling latency, or if overload protection is working. This controlling latency or if overload protection is working. This
implies knowledge of how traffic is assigned to any sub-queues used implies knowledge of how traffic is assigned to any subqueues used
for flow scheduling, but can require information about how the for flow scheduling but can require information about how the
traffic dynamics impact active queue management, starvation traffic dynamics impact active queue management, starvation
prevention mechanisms, and circuit-breakers.</t> prevention mechanisms, and circuit breakers.</t>
<t>Sometimes correlating observations of headers at multiple points <t>Sometimes correlating observations of headers at multiple points
along the path (e.g., at the ingress and egress of a network along the path (e.g., at the ingress and egress of a network
segment), allows an observer to determine the contribution of a segment) allows an observer to determine the contribution of a
portion of the path to an observed metric. e.g., to locate a source portion of the path to an observed metric (e.g., to locate a source
of delay, jitter, loss, reordering, or congestion marking.</t> of delay, jitter, loss, reordering, or congestion marking).</t>
</section> </section>
<section numbered="true" toc="default">
<section title="Network Planning and Provisioning"> <name>Network Planning and Provisioning</name>
<t>Traffic rate and volume measurements are used to help plan <t>Traffic rate and volume measurements are used to help plan
deployment of new equipment and configuration in networks. Data is deployment of new equipment and configuration in networks. Data is
also valuable to equipment vendors who want to understand traffic also valuable to equipment vendors who want to understand traffic
trends and patterns of usage as inputs to decisions about planning trends and patterns of usage as inputs to decisions about planning
products and provisioning for new deployments.</t> products and provisioning for new deployments.</t>
<t>Trends in aggregate traffic can be observed and can be related to <t>Trends in aggregate traffic can be observed and can be related to
the endpoint addresses being used, but when transport header the endpoint addresses being used, but when transport header
information is not observable, it might be impossible to correlate information is not observable, it might be impossible to correlate
patterns in measurements with changes in transport protocols. This patterns in measurements with changes in transport protocols. This
increases the dependency on other indirect sources of information to increases the dependency on other indirect sources of information to
inform planning and provisioning.</t> inform planning and provisioning.</t>
</section> </section>
<section anchor="Compliance" numbered="true" toc="default">
<section anchor="Compliance" <name>Compliance with Congestion Control</name>
title="Compliance with Congestion Control">
<t>The traffic that can be observed by on-path network devices (the <t>The traffic that can be observed by on-path network devices (the
"wire image") is a function of transport protocol design/options, "wire image") is a function of transport protocol design/options,
network use, applications, and user characteristics. In general, network use, applications, and user characteristics. In general,
when only a small proportion of the traffic has a specific when only a small proportion of the traffic has a specific
(different) characteristic, such traffic seldom leads to operational (different) characteristic, such traffic seldom leads to operational
concern, although the ability to measure and monitor it is lower. concern, although the ability to measure and monitor it is lower.
The desire to understand the traffic and protocol interactions The desire to understand the traffic and protocol interactions
typically grows as the proportion of traffic increases. The typically grows as the proportion of traffic increases. The
challenges increase when multiple instances of an evolving protocol challenges increase when multiple instances of an evolving protocol
contribute to the traffic that share network capacity.</t> contribute to the traffic that share network capacity.</t>
skipping to change at line 787 skipping to change at line 626
<t>The traffic that can be observed by on-path network devices (the <t>The traffic that can be observed by on-path network devices (the
"wire image") is a function of transport protocol design/options, "wire image") is a function of transport protocol design/options,
network use, applications, and user characteristics. In general, network use, applications, and user characteristics. In general,
when only a small proportion of the traffic has a specific when only a small proportion of the traffic has a specific
(different) characteristic, such traffic seldom leads to operational (different) characteristic, such traffic seldom leads to operational
concern, although the ability to measure and monitor it is lower. concern, although the ability to measure and monitor it is lower.
The desire to understand the traffic and protocol interactions The desire to understand the traffic and protocol interactions
typically grows as the proportion of traffic increases. The typically grows as the proportion of traffic increases. The
challenges increase when multiple instances of an evolving protocol challenges increase when multiple instances of an evolving protocol
contribute to the traffic that share network capacity.</t> contribute to the traffic that share network capacity.</t>
<t>Operators can manage traffic load (e.g., when the network is <t>Operators can manage traffic load (e.g., when the network is
severely overloaded) by deploying rate-limiters, traffic shaping, or severely overloaded) by deploying rate limiters, traffic shaping, or
network transport circuit breakers <xref target="RFC8084"></xref>. network transport circuit breakers <xref target="RFC8084" format="defa
ult"/>.
The information provided by observing transport headers is a source The information provided by observing transport headers is a source
of data that can help to inform such mechanisms.</t> of data that can help to inform such mechanisms.</t>
<dl newline="true" spacing="normal">
<t><list style="hanging"> <dt>Congestion Control Compliance of Traffic:</dt>
<t <dd><t>Congestion control is a key transport function <xref target="
hangText="Congestion Control Compliance of Traffic:">Congestion RFC2914"
control is a key transport function <xref format="default"/>. Many network operators implicitly
target="RFC2914"></xref>. Many network operators implicitly
accept that TCP traffic complies with a behaviour that is accept that TCP traffic complies with a behaviour that is
acceptable for the shared Internet. TCP algorithms have been acceptable for the shared Internet. TCP algorithms have been
continuously improved over decades, and have reached a level of continuously improved over decades and have reached a level of
efficiency and correctness that is difficult to match in custom efficiency and correctness that is difficult to match in custom
application-layer mechanisms <xref target="RFC8085"></xref>.</t> application-layer mechanisms <xref target="RFC8085" format="defaul
t"/>.</t>
<t>A standards-compliant TCP stack provides congestion control <t>A standards-compliant TCP stack provides congestion control
that is judged safe for use across the Internet. Applications that is judged safe for use across the Internet. Applications
developed on top of well-designed transports can be expected to developed on top of well-designed transports can be expected to
appropriately control their network usage, reacting when the appropriately control their network usage, reacting when the
network experiences congestion, by back-off and reduce the load network experiences congestion, by backing off and reducing the lo ad
placed on the network. This is the normal expected behaviour for placed on the network. This is the normal expected behaviour for
IETF-specified transports (e.g., TCP and SCTP).</t> IETF-specified transports (e.g., TCP and SCTP).</t>
</dd>
<t hangText="Congestion Control Compliance for UDP traffic:">UDP <dt>Congestion Control Compliance for UDP Traffic:</dt>
<dd><t>UDP
provides a minimal message-passing datagram transport that has provides a minimal message-passing datagram transport that has
no inherent congestion control mechanisms. Because congestion no inherent congestion control mechanisms. Because congestion
control is critical to the stable operation of the Internet, control is critical to the stable operation of the Internet,
applications and other protocols that choose to use UDP as a applications and other protocols that choose to use UDP as a
transport have to employ mechanisms to prevent collapse, avoid transport have to employ mechanisms to prevent collapse, avoid
unacceptable contributions to jitter/latency, and to establish unacceptable contributions to jitter/latency, and establish
an acceptable share of capacity with concurrent traffic <xref an acceptable share of capacity with concurrent traffic <xref targ
target="RFC8085"></xref>.</t> et="RFC8085"
format="default"/>.</t>
<t>UDP flows that expose a well-known header can be observed to <t>UDP flows that expose a well-known header can be observed to
gain understanding of the dynamics of a flow and its congestion gain understanding of the dynamics of a flow and its congestion
control behaviour. For example, tools exist to monitor various control behaviour. For example, tools exist to monitor various
aspects of RTP header information and RTCP reports for real-time aspects of RTP header information and RTCP reports for real-time
flows (see <xref target="stats"></xref>). The Secure RTP and flows (see <xref target="stats" format="default"/>). The Secure RT
RTCP extensions <xref target="RFC3711"></xref> were explicitly P and
RTCP extensions <xref target="RFC3711" format="default"/> were exp
licitly
designed to expose some header information to enable such designed to expose some header information to enable such
observation, while protecting the payload data.</t> observation while protecting the payload data.</t>
<t>A network operator can observe the headers of transport
<t>A network operator can observe the headers of transport
protocols layered above UDP to understand if the datagram flows protocols layered above UDP to understand if the datagram flows
comply with congestion control expectations. This can help comply with congestion control expectations. This can help
inform a decision on whether it might be appropriate to deploy inform a decision on whether it might be appropriate to deploy
methods such as rate-limiters to enforce acceptable usage. The methods, such as rate limiters, to enforce acceptable usage. The
available information determines the level of precision with available information determines the level of precision with
which flows can be classified and the design space for which flows can be classified and the design space for
conditioning mechanisms (e.g., rate limiting, circuit breaker conditioning mechanisms (e.g., rate-limiting, circuit breaker
techniques <xref target="RFC8084"></xref>, or blocking of techniques <xref target="RFC8084" format="default"/>, or blocking
uncharacterised traffic) <xref target="RFC5218"></xref>.</t> uncharacterised traffic) <xref target="RFC5218" format="default"/>
</list></t> .</t>
</dd>
</dl>
<t>When anomalies are detected, tools can interpret the transport <t>When anomalies are detected, tools can interpret the transport
header information to help understand the impact of specific header information to help understand the impact of specific
transport protocols (or protocol mechanisms) on the other traffic transport protocols (or protocol mechanisms) on the other traffic
that shares a network. An observer on the network path can gain an that shares a network. An observer on the network path can gain an
understanding of the dynamics of a flow and its congestion control understanding of the dynamics of a flow and its congestion control
behaviour. Analysing observed flows can help to build confidence behaviour. Analysing observed flows can help to build confidence
that an application flow backs-off its share of the network load that an application flow backs off its share of the network load
under persistent congestion, and hence to understand whether the under persistent congestion and hence to understand whether the
behaviour is appropriate for sharing limited network capacity. For behaviour is appropriate for sharing limited network capacity. For
example, it is common to visualise plots of TCP sequence numbers example, it is common to visualise plots of TCP sequence numbers
versus time for a flow to understand how a flow shares available versus time for a flow to understand how a flow shares available
capacity, deduce its dynamics in response to congestion, etc.</t> capacity, deduce its dynamics in response to congestion, etc.</t>
<t>The ability to identify sources and flows that contribute to <t>The ability to identify sources and flows that contribute to
persistent congestion is important to the safe operation of network persistent congestion is important to the safe operation of network
infrastructure, and can inform configuration of network devices to infrastructure and can inform configuration of network devices to
complement the endpoint congestion avoidance mechanisms <xref complement the endpoint congestion avoidance mechanisms <xref target="
target="RFC7567"></xref> <xref target="RFC8084"></xref> to avoid a RFC7567" format="default"/> <xref target="RFC8084" format="default"/> to avoid a
portion of the network being driven into congestion collapse <xref portion of the network being driven into congestion collapse <xref tar
target="RFC2914"></xref>.</t> get="RFC2914" format="default"/>.</t>
</section> </section>
<section anchor="Implic-Unknown" numbered="true" toc="default">
<section anchor="Implic-Unknown" <name>To Characterise "Unknown" Network Traffic</name>
title="To Characterise &quot;Unknown&quot; Network Traffic">
<t>The patterns and types of traffic that share Internet capacity <t>The patterns and types of traffic that share Internet capacity
change over time as networked applications, usage patterns and change over time as networked applications, usage patterns, and
protocols continue to evolve.</t> protocols continue to evolve.</t>
<t>Encryption can increase the volume of "unknown" or <t>Encryption can increase the volume of "unknown" or
"uncharacterised" traffic seen by the network. If these traffic "uncharacterised" traffic seen by the network. If these traffic
patterns form a small part of the traffic aggregate passing through patterns form a small part of the traffic aggregate passing through
a network device or segment of the network path, the dynamics of the a network device or segment of the network path, the dynamics of the
uncharacterised traffic might not have a significant collateral uncharacterised traffic might not have a significant collateral
impact on the performance of other traffic that shares this network impact on the performance of other traffic that shares this network
segment. Once the proportion of this traffic increases, monitoring segment. Once the proportion of this traffic increases, monitoring
the traffic can determine if appropriate safety measures have to be the traffic can determine if appropriate safety measures have to be
put in place.</t> put in place.</t>
<t>Tracking the impact of new mechanisms and protocols requires <t>Tracking the impact of new mechanisms and protocols requires
traffic volume to be measured and new transport behaviours to be traffic volume to be measured and new transport behaviours to be
identified. This is especially true of protocols operating over a identified. This is especially true of protocols operating over a
UDP substrate. The level and style of encryption needs to be UDP substrate. The level and style of encryption needs to be
considered in determining how this activity is performed.</t> considered in determining how this activity is performed.</t>
<t>Traffic that cannot be classified typically receives a default <t>Traffic that cannot be classified typically receives a default
treatment. Some networks block or rate-limit traffic that cannot be treatment. Some networks block or rate-limit traffic that cannot be
classified.</t> classified.</t>
</section> </section>
<section numbered="true" toc="default">
<section title="To Support Network Security Functions"> <name>To Support Network Security Functions</name>
<t>On-path observation of the transport headers of packets can be <t>On-path observation of the transport headers of packets can be
used for various security functions. For example, Denial of Service used for various security functions. For example, Denial of Service
(DoS) and Distributed DoS (DDoS) attacks against the infrastructure (DoS) and Distributed DoS (DDoS) attacks against the infrastructure
or against an endpoint can be detected and mitigated by or against an endpoint can be detected and mitigated by
characterising anomalous traffic (see <xref characterising anomalous traffic (see <xref target="Implic-Unknown" fo
target="Implic-Unknown"></xref>) on a shorter timescale. Other uses rmat="default"/>) on a shorter timescale. Other uses
include support for security audits (e.g., verifying the compliance include support for security audits (e.g., verifying the compliance
with cipher suites), client and application fingerprinting for with cipher suites), client and application fingerprinting for
inventory, and to provide alerts for network intrusion detection and inventory, and alerts provided for network intrusion detection and
other next generation firewall functions.</t> other next generation firewall functions.</t>
<t>When using an encrypted transport, endpoints can directly provide <t>When using an encrypted transport, endpoints can directly provide
information to support these security functions. Another method, if information to support these security functions. Another method, if
the endpoints do not provide this information, is to use an on-path the endpoints do not provide this information, is to use an on-path
network device that relies on pattern inferences in the traffic, and network device that relies on pattern inferences in the traffic and
heuristics or machine learning instead of processing observed header heuristics or machine learning instead of processing observed header
information. An endpoint could also explicitly cooperate with an information. An endpoint could also explicitly cooperate with an
on-path device (e.g., a QUIC endpoint could share information about on-path device (e.g., a QUIC endpoint could share information about
current uses of connection IDs).</t> current uses of connection IDs).</t>
</section> </section>
<section anchor="Current-diag" numbered="true" toc="default">
<section anchor="Current-diag" <name>Network Diagnostics and Troubleshooting</name>
title="Network Diagnostics and Troubleshooting ">
<t>Operators monitor the health of a network segment to support a <t>Operators monitor the health of a network segment to support a
variety of operational tasks <xref target="RFC8404"></xref> variety of operational tasks <xref target="RFC8404" format="default"/>
including procedures to provide early warning and trigger action: to ,
including procedures to provide early warning and trigger action, e.g.
, to
diagnose network problems, to manage security threats (including diagnose network problems, to manage security threats (including
DoS), to evaluate equipment or protocol performance, or to respond DoS), to evaluate equipment or protocol performance, or to respond
to user performance questions. Information about transport flows can to user performance questions. Information about transport flows can
assist in setting buffer sizes, and help identify whether assist in setting buffer sizes and help identify whether
link/network tuning is effective. Information can also support link/network tuning is effective. Information can also support
debugging and diagnosis of the root causes of faults that concern a debugging and diagnosis of the root causes of faults that concern a
particular user's traffic and can support post-mortem investigation particular user's traffic and can support postmortem investigation
after an anomaly. Section 3.1.2 and Section 5 of <xref after an anomaly. Sections <xref target="RFC8404" section="3.1.2" sect
target="RFC8404"></xref> provide further examples.</t> ionFormat="bare"/>
and <xref target="RFC8404" section="5" sectionFormat="bare"/> of <xref
target="RFC8404"/> provide further examples.</t>
<t>Network segments vary in their complexity. The design trade-offs <t>Network segments vary in their complexity. The design trade-offs
for radio networks are often very different from those of wired for radio networks are often very different from those of wired
networks <xref target="RFC8462"></xref>. A radio-based network networks <xref target="RFC8462" format="default"/>. A radio-based netw ork
(e.g., cellular mobile, enterprise Wireless LAN (WLAN), satellite (e.g., cellular mobile, enterprise Wireless LAN (WLAN), satellite
access/back-haul, point-to-point radio) adds a subsystem that access/backhaul, point-to-point radio) adds a subsystem that
performs radio resource management, with impact on the available performs radio resource management, with impact on the available
capacity, and potentially loss/reordering of packets. This impact capacity and potentially loss/reordering of packets. This impact
can differ by traffic type, and can be correlated with link can differ by traffic type and can be correlated with link
propagation and interference. These can impact the cost and propagation and interference. These can impact the cost and
performance of a provided service, and is expected to increase in performance of a provided service and is expected to increase in
importance as operators bring together heterogeneous types of importance as operators bring together heterogeneous types of
network equipment and deploy opportunistic methods to access shared network equipment and deploy opportunistic methods to access a shared
radio spectrum.</t> radio spectrum.</t>
</section> </section>
<section anchor="Implic-Cost" numbered="true" toc="default">
<section anchor="Implic-Cost" title="Tooling and Network Operations"> <name>Tooling and Network Operations</name>
<t>A variety of open source and proprietary tools have been deployed <t>A variety of open source and proprietary tools have been deployed
that use the transport header information observable with widely that use the transport header information observable with widely
used protocols such as TCP or RTP/UDP/IP. Tools that dissect network used protocols, such as TCP or RTP/UDP/IP. Tools that dissect network
traffic flows can alert to potential problems that are hard to traffic flows can alert to potential problems that are hard to
derive from volume measurements, link statistics or device derive from volume measurements, link statistics, or device
measurements alone.</t> measurements alone.</t>
<t>Any introduction of a new transport protocol, protocol feature, <t>Any introduction of a new transport protocol, protocol feature,
or application might require changes to such tools, and so could or application might require changes to such tools and could
impact operational practice and policies. Such changes have impact operational practice and policies. Such changes have
associated costs that are incurred by the network operators that associated costs that are incurred by the network operators that
need to update their tooling or develop alternative practises that need to update their tooling or develop alternative practises that
work without access to the changed/removed information.</t> work without access to the changed/removed information.</t>
<t>The use of encryption has the desirable effect of preventing <t>The use of encryption has the desirable effect of preventing
unintended observation of the payload data and these tools seldom unintended observation of the payload data, and these tools seldom
seek to observe the payload, or other application details. A flow seek to observe the payload or other application details. A flow
that hides its transport header information could imply "don't that hides its transport header information could imply "don't
touch" to some operators. This might limit a trouble-shooting touch" to some operators. This might limit a trouble-shooting
response to "can't help, no trouble found".</t> response to "can't help, no trouble found".</t>
<t>An alternative that does not require access to an observable
<t>An alternative that does not require access to observable
transport headers is to access endpoint diagnostic tools or to transport headers is to access endpoint diagnostic tools or to
include user involvement in diagnosing and troubleshooting unusual include user involvement in diagnosing and troubleshooting unusual
use cases or to troubleshoot non-trivial problems. Another approach use cases or to troubleshoot nontrivial problems. Another approach
is to use traffic pattern analysis. Such tools can provide useful is to use traffic pattern analysis. Such tools can provide useful
information during network anomalies (e.g., detecting significant information during network anomalies (e.g., detecting significant
reordering, high or intermittent loss), however indirect reordering, high or intermittent loss); however, indirect
measurements need to be carefully designed to provide information measurements need to be carefully designed to provide information
for diagnostics and troubleshooting.</t> for diagnostics and troubleshooting.</t>
<t>If new protocols, or protocol extensions, are made to closely <t>If new protocols, or protocol extensions, are made to closely
resemble or match existing mechanisms, then the changes to tooling resemble or match existing mechanisms, then the changes to tooling
and the associated costs can be small. Equally, more extensive and the associated costs can be small. Equally, more extensive
changes to the transport tend to require more extensive, and more changes to the transport tend to require more extensive, and more
expensive, changes to tooling and operational practice. Protocol expensive, changes to tooling and operational practice. Protocol
designers can mitigate these costs by explicitly choosing to expose designers can mitigate these costs by explicitly choosing to expose
selected information as invariants that are guaranteed not to change selected information as invariants that are guaranteed not to change
for a particular protocol (e.g., the header invariants and the for a particular protocol (e.g., the header invariants and the
spin-bit in QUIC <xref target="I-D.ietf-quic-transport"></xref>). spin bit in QUIC <xref target="RFC9000" format="default"/>).
Specification of common log formats and development of alternative Specification of common log formats and development of alternative
approaches can also help mitigate the costs of transport approaches can also help mitigate the costs of transport
changes.</t> changes.</t>
</section> </section>
</section> </section>
<section numbered="true" toc="default">
<section title="To Mitigate the Effects of Constrained Networks"> <name>To Mitigate the Effects of Constrained Networks</name>
<t>Some link and network segments are constrained by the capacity they <t>Some link and network segments are constrained by the capacity they
can offer, by the time it takes to access capacity (e.g., due to can offer by the time it takes to access capacity (e.g., due to
under-lying radio resource management methods), or by asymmetries in underlying radio resource management methods) or by asymmetries in
the design (e.g., many link are designed so that the capacity the design (e.g., many link are designed so that the capacity
available is different in the forward and return directions; some available is different in the forward and return directions; some
radio technologies have different access methods in the forward and radio technologies have different access methods in the forward and
return directions resulting from differences in the power budget).</t> return directions resulting from differences in the power budget).</t>
<t>The impact of path constraints can be mitigated using a proxy <t>The impact of path constraints can be mitigated using a proxy
operating at or above the transport layer to use an alternate operating at or above the transport layer to use an alternate
transport protocol.</t> transport protocol.</t>
<t>In many cases, one or both endpoints are unaware of the <t>In many cases, one or both endpoints are unaware of the
characteristics of the constraining link or network segment and characteristics of the constraining link or network segment, and
mitigations are applied below the transport layer: Packet mitigations are applied below the transport layer. Packet
classification and QoS methods (described in various sections) can be classification and QoS methods (described in various sections) can be
beneficial in differentially prioritising certain traffic when there beneficial in differentially prioritising certain traffic when there
is a capacity constraint or additional delay in scheduling link is a capacity constraint or additional delay in scheduling link
transmissions. Another common mitigation is to apply header transmissions. Another common mitigation is to apply header
compression over the specific link or subnetwork (see <xref compression over the specific link or subnetwork (see <xref target="HC"
target="HC"></xref>).</t> format="default"/>).</t>
<section anchor="HC" numbered="true" toc="default">
<section anchor="HC" title="To Provide Header Compression"> <name>To Provide Header Compression</name>
<t>Header compression saves link capacity by compressing network and <t>Header compression saves link capacity by compressing network and
transport protocol headers on a per-hop basis. This has been widely transport protocol headers on a per-hop basis. This has been widely
used with low bandwidth dial-up access links, and still finds used with low bandwidth dial-up access links and still finds
application on wireless links that are subject to capacity application on wireless links that are subject to capacity
constraints. These methods are effective for bit-congestive links constraints. These methods are effective for bit-congestive links
sending small packets (e.g., reducing the cost for sending control sending small packets (e.g., reducing the cost for sending control
packets or small data packets over radio links).</t> packets or small data packets over radio links).</t>
<t>Examples of header compression include use with TCP/IP and <t>Examples of header compression include use with TCP/IP and
RTP/UDP/IP flows <xref target="RFC2507"></xref>, <xref RTP/UDP/IP flows <xref target="RFC2507" format="default"/> <xref targe
target="RFC6846"></xref>, <xref target="RFC2508"></xref>, <xref t="RFC6846" format="default"/> <xref target="RFC2508" format="default"/> <xref t
target="RFC5795"></xref>, <xref target="RFC8724"></xref>. Successful arget="RFC5795" format="default"/> <xref target="RFC8724" format="default"/>. Su
ccessful
compression depends on observing the transport headers and compression depends on observing the transport headers and
understanding of the way fields change between packets, and is hence understanding the way fields change between packets and is hence
incompatible with header encryption. Devices that compress transport incompatible with header encryption. Devices that compress transport
headers are dependent on a stable header format, implying headers are dependent on a stable header format, implying
ossification of that format.</t> ossification of that format.</t>
<t>Introducing a new transport protocol, or changing the format of <t>Introducing a new transport protocol, or changing the format of
the transport header information, will limit the effectiveness of the transport header information, will limit the effectiveness of
header compression until the network devices are updated. Encrypting header compression until the network devices are updated. Encrypting
the transport protocol headers will tend to cause the header the transport protocol headers will tend to cause the header
compression to fall back to compressing only the network layer compression to fall back to compressing only the network-layer
headers, with a significant reduction in efficiency. This can limit headers, with a significant reduction in efficiency. This can limit
connectivity if the resulting flow exceeds the link capacity, or if connectivity if the resulting flow exceeds the link capacity or if
the packets are dropped because they exceed the link MTU.</t> the packets are dropped because they exceed the link Maximum
Transmission Unit (MTU).</t>
<t>The Secure RTP (SRTP) extensions <xref target="RFC3711"></xref> <t>The Secure RTP (SRTP) extensions <xref target="RFC3711" format="def
ault"/>
were explicitly designed to leave the transport protocol headers were explicitly designed to leave the transport protocol headers
unencrypted, but authenticated, since support for header compression unencrypted, but authenticated, since support for header compression
was considered important.</t> was considered important.</t>
</section> </section>
</section> </section>
<section numbered="true" toc="default">
<section title="To Verify SLA Compliance"> <name>To Verify SLA Compliance</name>
<t>Observable transport headers coupled with published transport <t>Observable transport headers coupled with published transport
specifications allow operators and regulators to explore and verify specifications allow operators and regulators to explore and verify
compliance with Service Level Agreements (SLAs). It can also be used compliance with Service Level Agreements (SLAs). It can also be used
to understand whether a service is providing differential treatment to to understand whether a service is providing differential treatment to
certain flows.</t> certain flows.</t>
<t>When transport header information cannot be observed, other methods <t>When transport header information cannot be observed, other methods
have to be found to confirm that the traffic produced conforms to the have to be found to confirm that the traffic produced conforms to the
expectations of the operator or developer.</t> expectations of the operator or developer.</t>
<t>Independently verifiable performance metrics can be utilised to <t>Independently verifiable performance metrics can be utilised to
demonstrate regulatory compliance in some jurisdictions, and as a demonstrate regulatory compliance in some jurisdictions and as a
basis for informing design decisions. This can bring assurance to basis for informing design decisions. This can bring assurance to
those operating networks, often avoiding deployment of complex those operating networks, often avoiding deployment of complex
techniques that routinely monitor and manage Internet traffic flows techniques that routinely monitor and manage Internet traffic flows
(e.g., avoiding the capital and operational costs of deploying flow (e.g., avoiding the capital and operational costs of deploying flow
rate-limiting and network circuit-breaker methods <xref rate-limiting and network circuit breaker methods <xref target="RFC8084"
target="RFC8084"></xref>).</t> format="default"/>).</t>
</section> </section>
</section> </section>
<section anchor="Implic" numbered="true" toc="default">
<section anchor="Implic" title="Research, Development and Deployment"> <name>Research, Development, and Deployment</name>
<t>Research and development of new protocols and mechanisms need to be <t>Research and development of new protocols and mechanisms need to be
informed by measurement data (as described in the previous section). informed by measurement data (as described in the previous section).
Data can also help promote acceptance of proposed standards Data can also help promote acceptance of proposed standards
specifications by the wider community (e.g., as a method to judge the specifications by the wider community (e.g., as a method to judge the
safety for Internet deployment).</t> safety for Internet deployment).</t>
<t>Observed data is important to ensure the health of the research and <t>Observed data is important to ensure the health of the research and
development communities, and provides data needed to evaluate new development communities and provides data needed to evaluate new
proposals for standardisation. Open standards motivate a desire to proposals for standardisation. Open standards motivate a desire to
include independent observation and evaluation of performance and include independent observation and evaluation of performance and
deployment data. Independent data helps compare different methods, judge deployment data. Independent data helps compare different methods, judge
the level of deployment and ensure the wider applicability of the the level of deployment, and ensure the wider applicability of the
results. This is important when considering when a protocol or mechanism results. This is important when considering when a protocol or mechanism
should be standardised for use in the general Internet. This, in turn, should be standardised for use in the general Internet. This, in turn,
demands control/understanding about where and when measurement samples demands control/understanding about where and when measurement samples
are collected. This requires consideration of the methods used to are collected. This requires consideration of the methods used to
observe information and the appropriate balance between encrypting all observe information and the appropriate balance between encrypting all
and no transport header information.</t> and no transport header information.</t>
<t>There can be performance and operational trade-offs in exposing <t>There can be performance and operational trade-offs in exposing
selected information to network tools. This section explores key selected information to network tools. This section explores key
implications of tools and procedures that observe transport protocols, implications of tools and procedures that observe transport protocols
but does not endorse or condemn any specific practises.</t> but does not endorse or condemn any specific practises.</t>
<section anchor="Implic-Independent" numbered="true" toc="default">
<section anchor="Implic-Independent" title="Independent Measurement"> <name>Independent Measurement</name>
<t>Encrypting transport header information has implications on the way <t>Encrypting transport header information has implications on the way
network data is collected and analysed. Independent observation by network data is collected and analysed. Independent observations by
multiple actors is currently used by the transport community to multiple actors is currently used by the transport community to
maintain an accurate understanding of the network within transport maintain an accurate understanding of the network within transport
area working groups, IRTF research groups, and the broader research area working groups, IRTF research groups, and the broader research
community. This is important to be able to provide accountability, and community. This is important to be able to provide accountability and
demonstrate that protocols behave as intended, although when providing demonstrate that protocols behave as intended; although, when providing
or using such information, it is important to consider the privacy of or using such information, it is important to consider the privacy of
the user and their incentive for providing accurate and detailed the user and their incentive for providing accurate and detailed
information.</t> information.</t>
<t>Protocols that expose the state of the transport protocol in their <t>Protocols that expose the state of the transport protocol in their
header (e.g., timestamps used to calculate the RTT, packet numbers header (e.g., timestamps used to calculate the RTT, packet numbers
used to assess congestion and requests for retransmission) provide an used to assess congestion, and requests for retransmission) provide an
incentive for a sending endpoint to provide consistent information, incentive for a sending endpoint to provide consistent information,
because a protocol will not work otherwise. An on-path observer can because a protocol will not work otherwise. An on-path observer can
have confidence that well-known (and ossified) transport header have confidence that well-known (and ossified) transport header
information represents the actual state of the endpoints, when this information represents the actual state of the endpoints when this
information is necessary for the protocol's correct operation.</t> information is necessary for the protocol's correct operation.</t>
<t>Encryption of transport header information could reduce the range <t>Encryption of transport header information could reduce the range
of actors that can observe useful data. This would limit the of actors that can observe useful data. This would limit the
information sources available to the Internet community to understand information sources available to the Internet community to understand
the operation of new transport protocols, reducing information to the operation of new transport protocols, reducing information to
inform design decisions and standardisation of the new protocols and inform design decisions and standardisation of the new protocols and
related operational practises. The cooperating dependence of network, related operational practises. The cooperating dependence of network,
application, and host to provide communication performance on the application, and host to provide communication performance on the
Internet is uncertain when only endpoints (i.e., at user devices and Internet is uncertain when only endpoints (i.e., at user devices and
within service platforms) can observe performance, and when within service platforms) can observe performance and when
performance cannot be independently verified by all parties.</t> performance cannot be independently verified by all parties.</t>
</section> </section>
<section anchor="Implic-design" numbered="true" toc="default">
<section anchor="Implic-design" title="Measurable Transport Protocols"> <name>Measurable Transport Protocols</name>
<t>Transport protocol evolution, and the ability to measure and <t>Transport protocol evolution and the ability to measure and
understand the impact of protocol changes, have to proceed understand the impact of protocol changes have to proceed
hand-in-hand. A transport protocol that provides observable headers hand-in-hand. A transport protocol that provides observable headers
can be used to provide open and verifiable measurement data. can be used to provide open and verifiable measurement data.
Observation of pathologies has a critical role in the design of Observation of pathologies has a critical role in the design of
transport protocol mechanisms and development of new mechanisms and transport protocol mechanisms and development of new mechanisms and
protocols, and aides understanding of the interactions between protocols and aides in understanding the interactions between
cooperating protocols and network mechanisms, the implications of cooperating protocols and network mechanisms, the implications of
sharing capacity with other traffic and the impact of different sharing capacity with other traffic, and the impact of different
patterns of usage. The ability of other stakeholders to review patterns of usage. The ability of other stakeholders to review
transport header traces helps develop insight into the performance and transport header traces helps develop insight into the performance and
the traffic contribution of specific variants of a protocol.</t> the traffic contribution of specific variants of a protocol.</t>
<t>Development of new transport protocol mechanisms has to consider <t>Development of new transport protocol mechanisms has to consider
the scale of deployment and the range of environments in which the the scale of deployment and the range of environments in which the
transport is used. Experience has shown that it is often difficult to transport is used. Experience has shown that it is often difficult to
correctly implement new mechanisms <xref target="RFC8085"></xref>, and correctly implement new mechanisms <xref target="RFC8085" format="defaul
that mechanisms often evolve as a protocol matures, or in response to t"/> and
changes in network conditions, changes in network traffic, or changes that mechanisms often evolve as a protocol matures or in response to
changes in network conditions, in network traffic, or
to application usage. Analysis is especially valuable when based on to application usage. Analysis is especially valuable when based on
the behaviour experienced across a range of topologies, vendor the behaviour experienced across a range of topologies, vendor
equipment, and traffic patterns.</t> equipment, and traffic patterns.</t>
<t>Encryption enables a transport protocol to choose which internal <t>Encryption enables a transport protocol to choose which internal
state to reveal to devices on the network path, what information to state to reveal to devices on the network path, what information to
encrypt, and what fields to grease <xref target="RFC8701"></xref>. A encrypt, and what fields to grease <xref target="RFC8701" format="defaul t"/>. A
new design can provide summary information regarding its performance, new design can provide summary information regarding its performance,
congestion control state, etc., or to make available explicit congestion control state, etc., or make explicit
measurement information. For example, <xref measurement information available. For example, <xref target="RFC9000" f
target="I-D.ietf-quic-transport"></xref> specifies a way for a QUIC ormat="default"/>
endpoint to optionally set the spin-bit to explicitly reveal the RTT specifies a way for a QUIC
endpoint to optionally set the spin bit to explicitly reveal the RTT
of an encrypted transport session to the on-path network devices. of an encrypted transport session to the on-path network devices.
There is a choice of what information to expose. For some operational There is a choice of what information to expose. For some operational
uses, the information has to contain sufficient detail to understand, uses, the information has to contain sufficient detail to understand,
and possibly reconstruct, the network traffic pattern for further and possibly reconstruct, the network traffic pattern for further
testing. The interpretation of the information needs to consider testing. The interpretation of the information needs to consider
whether this information reflects the actual transport state of the whether this information reflects the actual transport state of the
endpoints. This might require the trust of transport protocol endpoints. This might require the trust of transport protocol
implementers, to correctly reveal the desired information.</t> implementers to correctly reveal the desired information.</t>
<t>New transport protocol formats are expected to facilitate an <t>New transport protocol formats are expected to facilitate an
increased pace of transport evolution, and with it the possibility to increased pace of transport evolution and with it the possibility to
experiment with and deploy a wide range of protocol mechanisms. At the experiment with and deploy a wide range of protocol mechanisms. At the
time of writing, there has been interest in a wide range of new time of writing, there has been interest in a wide range of new
transport methods, e.g., Larger Initial Window, Proportional Rate transport methods, e.g., larger initial window, Proportional Rate
Reduction (PRR), congestion control methods based on measuring Reduction (PRR), congestion control methods based on measuring
bottleneck bandwidth and round-trip propagation time, the introduction bottleneck bandwidth and round-trip propagation time, the introduction
of AQM techniques and new forms of ECN response (e.g., Data Centre of AQM techniques, and new forms of ECN response (e.g., Data Centre
TCP, DCTCP, and methods proposed for L4S). The growth and diversity of TCP, DCTCP, and methods proposed for Low Latency Low Loss Scalable throu
ghput (L4S)). The growth and diversity of
applications and protocols using the Internet also continues to applications and protocols using the Internet also continues to
expand. For each new method or application, it is desirable to build a expand. For each new method or application, it is desirable to build a
body of data reflecting its behaviour under a wide range of deployment body of data reflecting its behaviour under a wide range of deployment
scenarios, traffic load, and interactions with other scenarios, traffic load, and interactions with other
deployed/candidate methods.</t> deployed/candidate methods.</t>
</section> </section>
<section anchor="other-sources" numbered="true" toc="default">
<section anchor="other-sources" title="Other Sources of Information"> <name>Other Sources of Information</name>
<t>Some measurements that traditionally rely on observable transport <t>Some measurements that traditionally rely on observable transport
information could be completed by utilising endpoint-based logging information could be completed by utilising endpoint-based logging
(e.g., based on <xref target="Quic-Trace">Quic-Trace</xref> and qlog (e.g., based on <xref target="Quic-Trace" format="default">QUIC trace</x
<xref target="I-D.marx-qlog-main-schema"></xref>). Such information ref> and
<xref target="I-D.ietf-quic-qlog-main-schema" format="default">qlog</xre
f>). Such information
has a diversity of uses, including developers wishing to has a diversity of uses, including developers wishing to
debug/understand the transport/application protocols with which they debug/understand the transport/application protocols with which they
work, researchers seeking to spot trends and anomalies, and to work, researchers seeking to spot trends and anomalies, and
characterise variants of protocols. A standard format for endpoint to characterise variants of protocols. A standard format for endpoint
logging could allow these to be shared (after appropriate logging could allow these to be shared (after appropriate
anonymisation) to understand performance and pathologies.</t> anonymisation) to understand performance and pathologies.</t>
<t>When measurement datasets are made available by servers or client <t>When measurement datasets are made available by servers or client
endpoints, additional metadata, such as the state of the network and endpoints, additional metadata, such as the state of the network and
conditions in which the system was observed, is often necessary to conditions in which the system was observed, is often necessary to
interpret this data to answer questions about network performance or interpret this data to answer questions about network performance or
understand a pathology. Collecting and coordinating such metadata is understand a pathology. Collecting and coordinating such metadata is
more difficult when the observation point is at a different location more difficult when the observation point is at a different location
to the bottleneck or device under evaluation <xref to the bottleneck or device under evaluation <xref target="RFC7799" form
target="RFC7799"></xref>.</t> at="default"/>.</t>
<t>Despite being applicable in some scenarios, endpoint logs do not <t>Despite being applicable in some scenarios, endpoint logs do not
provide equivalent information to on-path measurements made by devices provide equivalent information to on-path measurements made by devices
in the network. In particular, endpoint logs contain only a part of in the network. In particular, endpoint logs contain only a part of
the information to understand the operation of network devices and the information to understand the operation of network devices and
identify issues such as link performance or capacity sharing between identify issues, such as link performance or capacity sharing between
multiple flows. An analysis can require coordination between actors at multiple flows. An analysis can require coordination between actors at
different layers to successfully characterise flows and correlate the different layers to successfully characterise flows and correlate the
performance or behaviour of a specific mechanism with an equipment performance or behaviour of a specific mechanism with an equipment
configuration and traffic using operational equipment along a network configuration and traffic using operational equipment along a network
path (e.g., combining transport and network measurements to explore path (e.g., combining transport and network measurements to explore
congestion control dynamics, to understand the implications of traffic congestion control dynamics to understand the implications of traffic
on designs for active queue management or circuit breakers).</t> on designs for active queue management or circuit breakers).</t>
<t>Another source of information could arise from Operations,
<t>Another source of information could arise from operations, Administration, and Maintenance (OAM) (see <xref target="OAM" format="de
administration and management (OAM) (see <xref target="OAM"></xref>) fault"/>).
information data records could be embedded into header information at Information data records could be embedded into header information at
different layers to support functions such as performance evaluation, different layers to support functions, such as performance evaluation,
path-tracing, path verification information, classification and a path tracing, path verification information, classification, and a
diversity of other uses.</t> diversity of other uses.</t>
<t>In-situ OAM (IOAM) data fields <xref target="I-D.ietf-ippm-ioam-data"
<t>In-situ OAM (IOAM) data fields <xref format="default"/> can be encapsulated into a
target="I-D.ietf-ippm-ioam-data"></xref> can be encapsulated into a
variety of protocols to record operational and telemetry information variety of protocols to record operational and telemetry information
in an existing packet, while that packet traverses a part of the path in an existing packet while that packet traverses a part of the path
between two points in a network (e.g., within a particular IOAM between two points in a network (e.g., within a particular IOAM
management domain). The IOAM-Data-Fields are independent from the management domain). IOAM-Data-Fields are independent from the
protocols into which the IOAM-Data-Fields are encapsulated. For protocols into which IOAM-Data-Fields are encapsulated. For example, IOA
example, IOAM can provide proof that a certain traffic flow takes a M
pre-defined path, SLA verification for the live data traffic, and can provide proof that a traffic flow takes a
predefined path, SLA verification for the live data traffic, and
statistics relating to traffic distribution.</t> statistics relating to traffic distribution.</t>
</section> </section>
</section> </section>
<section anchor="Transport-encrypt" numbered="true" toc="default">
<section anchor="Transport-encrypt" <name>Encryption and Authentication of Transport Headers</name>
title="Encryption and Authentication of Transport Headers">
<t>There are several motivations for transport header encryption.</t> <t>There are several motivations for transport header encryption.</t>
<t>One motive to encrypt transport headers is to prevent network <t>One motive to encrypt transport headers is to prevent network
ossification from network devices that inspect well-known transport ossification from network devices that inspect well-known transport
headers. Once a network device observes a transport header and becomes headers. Once a network device observes a transport header and becomes
reliant upon using it, the overall use of that field can become reliant upon using it, the overall use of that field can become
ossified, preventing new versions of the protocol and mechanisms from ossified, preventing new versions of the protocol and mechanisms from
being deployed. Examples include:</t> being deployed. Examples include:</t>
<ul spacing="normal">
<t><list style="symbols"> <li>During the development of TLS 1.3 <xref target="RFC8446" format="def
<t>During the development of TLS 1.3 <xref target="RFC8446"></xref>, ault"/>,
the design needed to function in the presence of deployed the design needed to function in the presence of deployed
middleboxes that relied on the presence of certain header fields middleboxes that relied on the presence of certain header fields
exposed in TLS 1.2 <xref target="RFC5426"></xref>.</t> exposed in TLS 1.2 <xref target="RFC5426" format="default"/>.</li>
<li>The design of Multipath TCP (MPTCP) <xref target="RFC8684" format="d
<t>The design of Multipath TCP (MPTCP) <xref efault"/> had to account for middleboxes (known as
target="RFC8684"></xref> had to account for middleboxes (known as
"TCP Normalizers") that monitor the evolution of the window "TCP Normalizers") that monitor the evolution of the window
advertised in the TCP header and then reset connections when the advertised in the TCP header and then reset connections when the
window did not grow as expected.</t> window did not grow as expected.</li>
<li>TCP Fast Open <xref target="RFC7413" format="default"/> can experien
<t>TCP Fast Open <xref target="RFC7413"></xref> can experience ce
problems due to middleboxes that modify the transport header of problems due to middleboxes that modify the transport header of
packets by removing "unknown" TCP options. Segments with packets by removing "unknown" TCP options. Segments with
unrecognised TCP options can be dropped, segments that contain data unrecognised TCP options can be dropped, segments that contain data
and set the SYN bit can be dropped, and some middleboxes that and set the SYN bit can be dropped, and some middleboxes that
disrupt connections that send data before completion of the disrupt connections can send data before completion of the
three-way handshake.</t> three-way handshake.</li>
<li>Other examples of TCP ossification have included middleboxes that
<t>Other examples of TCP ossification have included middleboxes that
modify transport headers by rewriting TCP sequence and modify transport headers by rewriting TCP sequence and
acknowledgement numbers, but are unaware of the (newer) TCP acknowledgement numbers but are unaware of the (newer) TCP
selective acknowledgement (SACK) option and therefore fail to selective acknowledgement (SACK) option and therefore fail to
correctly rewrite the SACK information to match the changes made to correctly rewrite the SACK information to match the changes made to
the fixed TCP header, preventing correct SACK operation.</t> the fixed TCP header, preventing correct SACK operation.</li>
</list></t> </ul>
<t>In all these cases, middleboxes with a hard-coded, but incomplete, <t>In all these cases, middleboxes with a hard-coded, but incomplete,
understanding of a specific transport behaviour (i.e., TCP), interacted understanding of a specific transport behaviour (i.e., TCP) interacted
poorly with transport protocols after the transport behaviour was poorly with transport protocols after the transport behaviour was
changed. In some cases, the middleboxes modified or replaced information changed. In some cases, the middleboxes modified or replaced information
in the transport protocol header.</t> in the transport protocol header.</t>
<t>Transport header encryption prevents an on-path device from observing <t>Transport header encryption prevents an on-path device from observing
the transport headers, and therefore stops ossified mechanisms being the transport headers and therefore stops ossified mechanisms being
used that directly rely on or infer semantics of the transport header used that directly rely on or infer semantics of the transport header
information. This encryption is normally combined with authentication of information. This encryption is normally combined with authentication of
the protected information. RFC 8546 summarises this approach, stating the protected information. <xref target="RFC8546" format="default"/> summa
that it is "The wire image, not the protocol's specification, determines rises this
approach, stating
that "[t]he wire image, not the protocol's specification, determines
how third parties on the network paths among protocol participants will how third parties on the network paths among protocol participants will
interact with that protocol" <xref target="RFC8546">(Section 1 of interact with that protocol" (<xref target="RFC8546" sectionFormat="of"
</xref>), and it can be expected that header information that is not section="1"/>), and it can be expected that header information that is not
encrypted will become ossified.</t> encrypted will become ossified.</t>
<t>Encryption does not itself prevent ossification of the network <t>Encryption does not itself prevent ossification of the network
service. People seeking to understand or classify network traffic could service. People seeking to understand or classify network traffic could
still come to rely on pattern inferences and other heuristics or machine still come to rely on pattern inferences and other heuristics or machine
learning to derive measurement data and as the basis for network learning to derive measurement data and as the basis for network
forwarding decisions <xref target="RFC8546"></xref>. This can also forwarding decisions <xref target="RFC8546" format="default"/>. This can a
create dependencies on the transport protocol, or the patterns of lso
create dependencies on the transport protocol or the patterns of
traffic it can generate, also resulting in ossification of the traffic it can generate, also resulting in ossification of the
service.</t> service.</t>
<t>Another motivation for using transport header encryption is to <t>Another motivation for using transport header encryption is to
improve privacy and to decrease opportunities for surveillance. Users improve privacy and to decrease opportunities for surveillance. Users
value the ability to protect their identity and location, and defend value the ability to protect their identity and location and defend
against analysis of the traffic. Revelations about the use of pervasive against analysis of the traffic. Revelations about the use of pervasive
surveillance <xref target="RFC7624"></xref> have, to some extent, eroded surveillance <xref target="RFC7624" format="default"/> have, to some exten t, eroded
trust in the service offered by network operators and have led to an trust in the service offered by network operators and have led to an
increased use of encryption. Concerns have also been voiced about the increased use of encryption. Concerns have also been voiced about the
addition of metadata to packets by third parties to provide analytics, addition of metadata to packets by third parties to provide analytics,
customisation, advertising, cross-site tracking of users, to bill the customisation, advertising, cross-site tracking of users,
customer, or to selectively allow or block content.</t> customer billing, or selectively allowing or blocking content.</t>
<t>Whatever the reasons, the IETF is designing protocols that include <t>Whatever the reasons, the IETF is designing protocols that include
transport header encryption (e.g., QUIC <xref transport header encryption (e.g., QUIC <xref target="RFC9000" format="def
target="I-D.ietf-quic-transport"></xref>) to supplement the already ault"/>) to supplement the already
widespread payload encryption, and to further limit exposure of widespread payload encryption and to further limit exposure of
transport metadata to the network.</t> transport metadata to the network.</t>
<t>If a transport protocol uses header encryption, the designers have to <t>If a transport protocol uses header encryption, the designers have to
decide whether to encrypt all, or a part of, the transport layer decide whether to encrypt all or a part of the transport-layer
information. Section 4 of <xref target="RFC8558"></xref> states: information. <xref target="RFC8558" sectionFormat="of" section="4"/> state
s,
"Anything exposed to the path should be done with the intent that it be "Anything exposed to the path should be done with the intent that it be
used by the network elements on the path".</t> used by the network elements on the path."</t>
<t>Certain transport header fields can be made observable to on-path <t>Certain transport header fields can be made observable to on-path
network devices, or can define new fields designed to explicitly expose network devices or can define new fields designed to explicitly expose
observable transport layer information to the network. Where exposed observable transport-layer information to the network. Where exposed
fields are intended to be immutable (i.e., can be observed, but not fields are intended to be immutable (i.e., can be observed but not
modified by a network device), the endpoints are encouraged to use modified by a network device), the endpoints are encouraged to use
authentication to provide a cryptographic integrity check that can authentication to provide a cryptographic integrity check that can
detect if these immutable fields have been modified by network devices. detect if these immutable fields have been modified by network devices.
Authentication can help to prevent attacks that rely on sending packets Authentication can help to prevent attacks that rely on sending packets
that fake exposed control signals in transport headers (e.g., TCP RST that fake exposed control signals in transport headers (e.g., TCP RST
spoofing). Making a part of a transport header observable or exposing spoofing). Making a part of a transport header observable or exposing
new header fields can lead to ossification of that part of a header as new header fields can lead to ossification of that part of a header as
network devices come to rely on observations of the exposed fields.</t> network devices come to rely on observations of the exposed fields.</t>
<t>The use of transport header authentication and encryption therefore <t>The use of transport header authentication and encryption therefore
exposes a tussle between middlebox vendors, operators, researchers, exposes a tussle between middlebox vendors, operators, researchers,
applications developers, and end-users: <list style="symbols"> applications developers, and end users: </t>
<t>On the one hand, future Internet protocols that support transport <ul spacing="normal">
<li>On the one hand, future Internet protocols that support transport
header encryption assist in the restoration of the end-to-end nature header encryption assist in the restoration of the end-to-end nature
of the Internet by returning complex processing to the endpoints. of the Internet by returning complex processing to the endpoints.
Since middleboxes cannot modify what they cannot see, the use of Since middleboxes cannot modify what they cannot see, the use of
transport header encryption can improve application and end-user transport header encryption can improve application and end-user
privacy by reducing leakage of transport metadata to operators that privacy by reducing leakage of transport metadata to operators that
deploy middleboxes.</t> deploy middleboxes.</li>
<li>On the other hand, encryption of transport-layer information has
<t>On the other hand, encryption of transport layer information has
implications for network operators and researchers seeking to implications for network operators and researchers seeking to
understand the dynamics of protocols and traffic patterns, since it understand the dynamics of protocols and traffic patterns, since it
reduces the information that is available to them.</t> reduces the information that is available to them.</li>
</list></t> </ul>
<t>The following briefly reviews some security design options for <t>The following briefly reviews some security design options for
transport protocols. A Survey of the Interaction between Security transport protocols. "A Survey of the Interaction between Security
Protocols and Transport Services <xref target="RFC8922"></xref> provides Protocols and Transport Services" <xref target="RFC8922" format="default"/
> provides
more details concerning commonly used encryption methods at the more details concerning commonly used encryption methods at the
transport layer.</t> transport layer.</t>
<t>Security work typically employs a design technique that seeks to <t>Security work typically employs a design technique that seeks to
expose only what is needed <xref target="RFC3552"></xref>. This approach expose only what is needed <xref target="RFC3552" format="default"/>. This approach
provides incentives to not reveal any information that is not necessary provides incentives to not reveal any information that is not necessary
for the end-to-end communication. The IETF has provided guidelines for for the end-to-end communication. The IETF has provided guidelines for
writing Security Considerations for IETF specifications <xref writing security considerations for IETF specifications <xref target="RFC3
target="RFC3552"></xref>.</t> 552" format="default"/>.</t>
<t>Endpoint design choices impacting privacy also need to be considered <t>Endpoint design choices impacting privacy also need to be considered
as a part of the design process <xref target="RFC6973"></xref>. The IAB as a part of the design process <xref target="RFC6973" format="default"/>.
has provided guidance for analyzing and documenting privacy The IAB
considerations within IETF specifications <xref has provided guidance for analysing and documenting privacy
target="RFC6973"></xref>.</t> considerations within IETF specifications <xref target="RFC6973" format="d
efault"/>.</t>
<t><list style="hanging"> <dl newline="true" spacing="normal">
<t <dt>Authenticating the Transport Protocol Header:</dt>
hangText="Authenticating the Transport Protocol Header:">Transport <dd><t>Transport-layer header information can be authenticated. An examp
layer header information can be authenticated. An example transport le transport
authentication mechanism is TCP-Authentication (TCP-AO) <xref authentication mechanism is TCP Authentication Option (TCP-AO) <xref t
target="RFC5925"> </xref>. This TCP option authenticates the IP arget="RFC5925" format="default"> </xref>. This TCP option authenticates the IP
pseudo header, TCP header, and TCP data. TCP-AO protects the pseudo-header, TCP header, and TCP data. TCP-AO protects the
transport layer, preventing attacks from disabling the TCP transport layer, preventing attacks from disabling the TCP
connection itself and provides replay protection. Such connection itself and provides replay protection. Such
authentication might interact with middleboxes, depending on their authentication might interact with middleboxes, depending on their
behaviour <xref target="RFC3234"> </xref>.</t> behaviour <xref target="RFC3234" format="default"> </xref>.</t>
<t>The IPsec Authentication Header (AH) <xref target="RFC4302" format="d
<t>The IPsec Authentication Header (AH) <xref target="RFC4302"> efault">
</xref> was designed to work at the network layer and authenticate </xref> was designed to work at the network layer and authenticate
the IP payload. This approach authenticates all transport headers, the IP payload. This approach authenticates all transport headers
and verifies their integrity at the receiver, preventing and verifies their integrity at the receiver, preventing
modification by network devices on the path. The IPsec Encapsulating modification by network devices on the path. The IPsec Encapsulating
Security Payload (ESP) <xref target="RFC4303"></xref> can also Security Payload (ESP) <xref target="RFC4303" format="default"/> can a lso
provide authentication and integrity without confidentiality using provide authentication and integrity without confidentiality using
the NULL encryption algorithm <xref target="RFC2410"></xref>. SRTP the NULL encryption algorithm <xref target="RFC2410" format="default"/
<xref target="RFC3711"></xref> is another example of a transport >. SRTP
<xref target="RFC3711" format="default"/> is another example of a tran
sport
protocol that allows header authentication.</t> protocol that allows header authentication.</t>
</dd>
<t hangText="Integrity Check">Transport protocols usually employ <dt>Integrity Check:</dt>
<dd>Transport protocols usually employ
integrity checks on the transport header information. Security integrity checks on the transport header information. Security
method usually employ stronger checks and can combine this with methods usually employ stronger checks and can combine this with
authentication. An integrity check that protects the immutable authentication. An integrity check that protects the immutable
transport header fields, but can still expose the transport header transport header fields, but can still expose the transport header
information in the clear, allows on-path network devices to observe information in the clear, allows on-path network devices to observe
these fields. An integrity check is not able to prevent modification these fields. An integrity check is not able to prevent modification
by network devices on the path, but can prevent a receiving endpoint by network devices on the path but can prevent a receiving endpoint
from accepting changes and avoid impact on the transport protocol from accepting changes and avoid impact on the transport protocol
operation, including some types of attack.</t> operation, including some types of attack.</dd>
<dt>Selectively Encrypting Transport Headers and Payload:</dt>
<t <dd><t>A
hangText="Selectively Encrypting Transport Headers and Payload:">A transport protocol design that encrypts selected header fields
transport protocol design that encrypts selected header fields,
allows specific transport header fields to be made observable by allows specific transport header fields to be made observable by
network devices on the path. This information is explicitly exposed network devices on the path. This information is explicitly exposed
either in a transport header field or lower layer protocol header. A either in a transport header field or lower layer protocol header. A
design that only exposes immutable fields can also perform design that only exposes immutable fields can also perform
end-to-end authentication of these fields across the path to prevent end-to-end authentication of these fields across the path to prevent
undetected modification of the immutable transport headers.</t> undetected modification of the immutable transport headers.</t>
<t>Mutable fields in the transport header provide opportunities
<t>Mutable fields in the transport header provide opportunities
where on-path network devices can modify the transport behaviour where on-path network devices can modify the transport behaviour
(e.g., the extended headers described in <xref (e.g., the extended headers described in <xref target="I-D.trammell-pl
target="I-D.trammell-plus-abstract-mech"></xref>). An example of a us-abstract-mech"
format="default"/>). An example of a
method that encrypts some, but not all, transport header information method that encrypts some, but not all, transport header information
is GRE-in-UDP <xref target="RFC8086"> </xref> when used with GRE is GRE-in-UDP <xref target="RFC8086" format="default"> </xref> when us ed with GRE
encryption.</t> encryption.</t>
</dd>
<t hangText="Optional Encryption of Header Information:">There are <dt>Optional Encryption of Header Information:</dt>
<dd>There are
implications to the use of optional header encryption in the design implications to the use of optional header encryption in the design
of a transport protocol, where support of optional mechanisms can of a transport protocol, where support of optional mechanisms can
increase the complexity of the protocol and its implementation, and increase the complexity of the protocol and its implementation and
in the management decisions that have to be made to use variable in the management decisions that have to be made to use variable
format fields. Instead, fields of a specific type ought to be sent format fields. Instead, fields of a specific type ought to be sent
with the same level of confidentiality or integrity protection.</t> with the same level of confidentiality or integrity protection.</dd>
<dt>Greasing:</dt>
<t hangText="Greasing:">Protocols often provide extensibility <dd><t>Protocols often provide extensibility
features, reserving fields or values for use by future versions of a features, reserving fields or values for use by future versions of a
specification. The specification of receivers has traditionally specification. The specification of receivers has traditionally
ignored unspecified values, however on-path network devices have ignored unspecified values; however, on-path network devices have
emerged that ossify to require a certain value in a field, or re-use emerged that ossify to require a certain value in a field or reuse
a field for another purpose. When the specification is later a field for another purpose. When the specification is later
updated, it is impossible to deploy the new use of the field, and updated, it is impossible to deploy the new use of the field and
forwarding of the protocol could even become conditional on a forwarding of the protocol could even become conditional on a
specific header field value.</t> specific header field value.</t>
<t>A protocol can intentionally vary the value, format,
<t hangText="">A protocol can intentionally vary the value, format,
and/or presence of observable transport header fields at random and/or presence of observable transport header fields at random
<xref target="RFC8701"></xref>. This prevents a network device <xref target="RFC8701" format="default"/>. This prevents a network dev ice
ossifying the use of a specific observable field and can ease future ossifying the use of a specific observable field and can ease future
deployment of new uses of the value or code-point. This is not a deployment of new uses of the value or code point. This is not a
security mechanism, although the use can be combined with an security mechanism, although the use can be combined with an
authentication mechanism.</t> authentication mechanism.</t>
</list></t> </dd>
</dl>
<t>Different transports use encryption to protect their header <t>Different transports use encryption to protect their header
information to varying degrees. The trend is towards increased information to varying degrees. The trend is towards increased
protection.</t> protection.</t>
</section> </section>
<section anchor="EH2" numbered="true" toc="default">
<section anchor="EH2" <name>Intentionally Exposing Transport Information to the Network</name>
title="Intentionally Exposing Transport Information to the Network"
>
<t>A transport protocol can choose to expose certain transport <t>A transport protocol can choose to expose certain transport
information to on-path devices operating at the network layer by sending information to on-path devices operating at the network layer by sending
observable fields. One approach is to make an explicit choice not to observable fields. One approach is to make an explicit choice not to
encrypt certain transport header fields, making this transport encrypt certain transport header fields, making this transport
information observable by an on-path network device. Another approach is information observable by an on-path network device. Another approach is
to expose transport information in a network-layer extension header (see to expose transport information in a network-layer extension header (see
<xref target="EH"></xref>). Both are examples of explicit information <xref target="EH" format="default"/>). Both are examples of explicit infor
intended to be used by network devices on the path <xref mation
target="RFC8558"></xref>.</t> intended to be used by network devices on the path <xref target="RFC8558"
format="default"/>.</t>
<t>Whatever the mechanism used to expose the information, a decision to <t>Whatever the mechanism used to expose the information, a decision to
expose only specific information places the transport endpoint in expose only specific information places the transport endpoint in
control of what to expose outside of the encrypted transport header. control of what to expose outside of the encrypted transport header.
This decision can then be made independently of the transport protocol This decision can then be made independently of the transport protocol
functionality. This can be done by exposing part of the transport header functionality. This can be done by exposing part of the transport header
or as a network layer option/extension.</t> or as a network-layer option/extension.</t>
<section anchor="EH" numbered="true" toc="default">
<section anchor="EH" <name>Exposing Transport Information in Extension Headers</name>
title="Exposing Transport Information in Extension Headers"> <t>At the network layer, packets can carry optional headers that
<t>At the network-layer, packets can carry optional headers that
explicitly expose transport header information to the on-path devices explicitly expose transport header information to the on-path devices
operating at the network layer (<xref target="tunlhf"></xref>). For operating at the network layer (<xref target="tunlhf" format="default"/>
example, an endpoint that sends an IPv6 Hop-by-Hop option <xref ). For
target="RFC8200"></xref> can provide explicit transport layer example, an endpoint that sends an IPv6 hop-by-hop option <xref target="
RFC8200"
format="default"/> can provide explicit transport-layer
information that can be observed and used by network devices on the information that can be observed and used by network devices on the
path. New hop-by-hop options are not recommended in <xref path. New hop-by-hop options are not recommended in <xref target="RFC820
target="RFC8200">RFC 8200</xref> "because nodes may be configured to 0"
format="default"/> "because nodes may be configured to
ignore the Hop-by-Hop Options header, drop packets containing a ignore the Hop-by-Hop Options header, drop packets containing a
Hop-by-Hop Options header, or assign packets containing a Hop-by-Hop Hop-by-Hop Options header, or assign packets containing a Hop-by-Hop
Options header to a slow processing path. Designers considering Options header to a slow processing path. Designers considering
defining new hop-by-hop options need to be aware of this likely defining new hop-by-hop options need to be aware of this likely
behavior."</t> behavior."</t>
<t>Network-layer optional headers explicitly indicate the information <t>Network-layer optional headers explicitly indicate the information
that is exposed, whereas use of exposed transport header information that is exposed, whereas use of exposed transport header information
first requires an observer to identify the transport protocol and its first requires an observer to identify the transport protocol and its
format. (See <xref target="Current-demux"></xref>.)</t> format. See <xref target="Current-demux" format="default"/>.</t>
<t>An arbitrary path can include one or more network devices that drop <t>An arbitrary path can include one or more network devices that drop
packets that include a specific header or option used for this purpose packets that include a specific header or option used for this purpose
(see <xref target="RFC7872"></xref>). This could impact the proper (see <xref target="RFC7872" format="default"/>). This could impact the p roper
functioning of the protocols using the path. Protocol methods can be functioning of the protocols using the path. Protocol methods can be
designed to probe to discover whether the specific option(s) can be designed to probe to discover whether the specific option(s) can be
used along the current path, enabling use on arbitrary paths.</t> used along the current path, enabling use on arbitrary paths.</t>
</section> </section>
<section numbered="true" toc="default">
<section title="Common Exposed Transport Information"> <name>Common Exposed Transport Information</name>
<t>There are opportunities for multiple transport protocols to <t>There are opportunities for multiple transport protocols to
consistently supply common observable information <xref consistently supply common observable information <xref target="RFC8558"
target="RFC8558"></xref>. A common approach can result in an open format="default"/>. A common approach can result in an open
definition of the observable fields. This has the potential that the definition of the observable fields. This has the potential that the
same information can be utilised across a range of operational and same information can be utilised across a range of operational and
analysis tools.</t> analysis tools.</t>
</section> </section>
<section anchor="exposing" numbered="true" toc="default">
<section anchor="exposing" <name>Considerations for Exposing Transport Information</name>
title="Considerations for Exposing Transport Information">
<t>Considerations concerning what information, if any, it is <t>Considerations concerning what information, if any, it is
appropriate to expose include:</t> appropriate to expose include:</t>
<ul spacing="normal">
<t><list style="symbols"> <li>On the one hand, explicitly exposing derived fields containing
<t>On the one hand, explicitly exposing derived fields containing
relevant transport information (e.g., metrics for loss, latency, relevant transport information (e.g., metrics for loss, latency,
etc) can avoid network devices needing to derive this information etc.) can avoid network devices needing to derive this information
from other header fields. This could result in development and from other header fields. This could result in development and
evolution of transport-independent tools around a common evolution of transport-independent tools around a common
observable header, and permit transport protocols to also evolve observable header and permit transport protocols to also evolve
independently of this ossified header <xref independently of this ossified header <xref target="RFC8558" format=
target="RFC8558"></xref>.</t> "default"/>.</li>
<li>On the other hand, protocols and implementations might be
<t>On the other hand, protocols and implementations might be
designed to avoid consistently exposing external information that designed to avoid consistently exposing external information that
corresponds to the actual internal information used by the corresponds to the actual internal information used by the
protocol itself. An endpoint/protocol could choose to expose protocol itself. An endpoint/protocol could choose to expose
transport header information to optimise the benefit it gets from transport header information to optimise the benefit it gets from
the network <xref target="RFC8558"></xref>. The value of this the network <xref target="RFC8558" format="default"/>. The value of this
information for analysing operation of the transport layer would information for analysing operation of the transport layer would
be enhanced if the exposed information could be verified to match be enhanced if the exposed information could be verified to match
the transport protocol's observed behavior.</t> the transport protocol's observed behavior.</li>
</list></t> </ul>
<t>The motivation to include actual transport header information and <t>The motivation to include actual transport header information and
the implications of network devices using this information has to be the implications of network devices using this information has to be
considered when proposing such a method. RFC 8558 summarises this as considered when proposing such a method. <xref target="RFC8558" format="
"When signals from endpoints to the path are independent from the default"/>
summarises this as:</t>
<blockquote>
When signals from endpoints to the path are independent from the
signals used by endpoints to manage the flow's state mechanics, they signals used by endpoints to manage the flow's state mechanics, they
may be falsified by an endpoint without affecting the peer's may be falsified by an endpoint without affecting the peer's
understanding of the flow's state. For encrypted flows, this understanding of the flow's state. For encrypted flows, this
divergence is not detectable by on-path devices <xref divergence is not detectable by on-path devices.</blockquote>
target="RFC8558"></xref>.</t>
</section> </section>
</section> </section>
<section anchor="OAM" numbered="true" toc="default">
<section anchor="OAM" <name>Addition of Transport OAM Information to Network-Layer Headers</name
title="Addition of Transport OAM Information to Network-Layer Heade >
rs">
<t>Even when the transport headers are encrypted, on-path devices can <t>Even when the transport headers are encrypted, on-path devices can
make measurements by utilising additional protocol headers carrying OAM make measurements by utilising additional protocol headers carrying OAM
information in an additional packet header. OAM information can be information in an additional packet header. OAM information can be
included with packets to perform functions such as identification of included with packets to perform functions, such as identification of
transport protocols and flows, to aide understanding of network or transport protocols and flows, to aide understanding of network or
transport performance, or to support network operations or mitigate the transport performance or to support network operations or mitigate the
effects of specific network segments.</t> effects of specific network segments.</t>
<t>Using network-layer approaches to reveal information has the <t>Using network-layer approaches to reveal information has the
potential that the same method (and hence same observation and analysis potential that the same method (and hence same observation and analysis
tools) can be consistently used by multiple transport protocols. This tools) can be consistently used by multiple transport protocols. This
approach also could be applied to methods beyond OAM (see <xref approach also could be applied to methods beyond OAM (see <xref target="EH
target="EH2"></xref>). There can also be less desirable implications 2" format="default"/>). There can also be less desirable implications
from separating the operation of the transport protocol from the from separating the operation of the transport protocol from the
measurement framework.</t> measurement framework.</t>
<section numbered="true" toc="default">
<section title="Use of OAM within a Maintenance Domain"> <name>Use of OAM within a Maintenance Domain</name>
<t>OAM information can be restricted to a maintenance domain, <t>OAM information can be restricted to a maintenance domain,
typically owned and operated by a single entity. OAM information can typically owned and operated by a single entity. OAM information can
be added at the ingress to the maintenance domain (e.g., an Ethernet be added at the ingress to the maintenance domain (e.g., an Ethernet
protocol header with timestamps and sequence number information using protocol header with timestamps and sequence number information using
a method such as 802.11ag or in-situ OAM <xref a method such as 802.11ag or in-situ OAM <xref target="I-D.ietf-ippm-ioa
target="I-D.ietf-ippm-ioam-data"></xref>, or as a part of the m-data" format="default"/> or as a part of the
encapsulation protocol). This additional header information is not encapsulation protocol). This additional header information is not
delivered to the endpoints and is typically removed at the egress of delivered to the endpoints and is typically removed at the egress of
the maintenance domain.</t> the maintenance domain.</t>
<t>Although some types of measurements are supported, this approach <t>Although some types of measurements are supported, this approach
does not cover the entire range of measurements described in this does not cover the entire range of measurements described in this
document. In some cases, it can be difficult to position measurement document. In some cases, it can be difficult to position measurement
tools at the appropriate segments/nodes and there can be challenges in tools at the appropriate segments/nodes, and there can be challenges in
correlating the downstream/upstream information when in-band OAM data correlating the downstream/upstream information when in-band OAM data
is inserted by an on-path device.</t> is inserted by an on-path device.</t>
</section> </section>
<section numbered="true" toc="default">
<section title="Use of OAM across Multiple Maintenance Domains"> <name>Use of OAM across Multiple Maintenance Domains</name>
<t>OAM information can also be added at the network layer by the <t>OAM information can also be added at the network layer by the
sender as an IPv6 extension header or an IPv4 option, or in an sender as an IPv6 extension header or an IPv4 option or in an
encapsulation/tunnel header that also includes an extension header or encapsulation/tunnel header that also includes an extension header or
option. This information can be used across multiple network segments, option. This information can be used across multiple network segments
or between the transport endpoints.</t> or between the transport endpoints.</t>
<t>One example is the IPv6 Performance and Diagnostic Metrics (PDM) <t>One example is the IPv6 Performance and Diagnostic Metrics (PDM)
destination option <xref target="RFC8250"></xref>. This allows a destination option <xref target="RFC8250" format="default"/>. This allow s a
sender to optionally include a destination option that carries header sender to optionally include a destination option that carries header
fields that can be used to observe timestamps and packet sequence fields that can be used to observe timestamps and packet sequence
numbers. This information could be authenticated by a receiving numbers. This information could be authenticated by a receiving
transport endpoint when the information is added at the sender and transport endpoint when the information is added at the sender and
visible at the receiving endpoint, although methods to do this have visible at the receiving endpoint, although methods to do this have
not currently been proposed. This needs to be explicitly enabled at not currently been proposed. This needs to be explicitly enabled at
the sender.</t> the sender.</t>
</section> </section>
</section> </section>
<section numbered="true" toc="default">
<section title="Conclusions"> <name>Conclusions</name>
<t>Header encryption and strong integrity checks are being incorporated <t>Header authentication and encryption and strong integrity checks are be
into new transport protocols and have important benefits. The pace of ing incorporated
development of transports using the WebRTC data channel, and the rapid into new transport protocols and have important benefits. The pace of the
deployment of the QUIC transport protocol, can both be attributed to development of transports using the WebRTC data channel and the rapid
deployment of the QUIC transport protocol can both be attributed to
using the combination of UDP as a substrate while providing using the combination of UDP as a substrate while providing
confidentiality and authentication of the encapsulated transport headers confidentiality and authentication of the encapsulated transport headers
and payload.</t> and payload.</t>
<t>This document has described some current practises, and the <t>This document has described some current practises, and the
implications for some stakeholders, when transport layer header implications for some stakeholders, when transport-layer header
encryption is used. It does not judge whether these practises are encryption is used. It does not judge whether these practises are
necessary, or endorse the use of any specific practise. Rather, the necessary or endorse the use of any specific practise. Rather, the
intent is to highlight operational tools and practises to consider when intent is to highlight operational tools and practises to consider when
designing and modifying transport protocols, so protocol designers can designing and modifying transport protocols, so protocol designers can
make informed choices about what transport header fields to encrypt, and make informed choices about what transport header fields to encrypt and
whether it might be beneficial to make an explicit choice to expose whether it might be beneficial to make an explicit choice to expose
certain fields to devices on the network path. In making such a certain fields to devices on the network path. In making such a
decision, it is important to balance: <list style="symbols"> decision, it is important to balance: </t>
<t>User Privacy: The less transport header information that is <dl newline="true" spacing="normal">
<dt>User Privacy:</dt>
<dd>The less transport header information that is
exposed to the network, the lower the risk of leaking metadata that exposed to the network, the lower the risk of leaking metadata that
might have user privacy implications. Transports that chose to might have user privacy implications. Transports that chose to
expose some header fields need to make a privacy assessment to expose some header fields need to make a privacy assessment to
understand the privacy cost versus benefit trade-off in making that understand the privacy cost versus benefit trade-off in making that
information available. The design of the QUIC spin bit to the information available. The design of the QUIC spin bit to the
network is an example of such considered analysis.</t> network is an example of such considered analysis.</dd>
<dt>Transport Ossification:</dt>
<t>Transport Ossification: Unencrypted transport header fields are <dd>Unencrypted transport header fields are
likely to ossify rapidly, as network devices come to rely on their likely to ossify rapidly, as network devices come to rely on their
presence, making it difficult to change the transport in future. presence, making it difficult to change the transport in future.
This argues that the choice to expose information to the network is This argues that the choice to expose information to the network is
made deliberately and with care, since it is essentially defining a made deliberately and with care, since it is essentially defining a
stable interface between the transport and the network. Some stable interface between the transport and the network. Some
protocols will want to make that interface as limited as possible; protocols will want to make that interface as limited as possible;
other protocols might find value in exposing certain information to other protocols might find value in exposing certain information to
signal to the network, or in allowing the network to change certain signal to the network or in allowing the network to change certain
header fields as signals to the transport. The visible wire image of header fields as signals to the transport. The visible wire image of
a protocol should be explicitly designed.</t> a protocol should be explicitly designed.</dd>
<dt>Network Ossification:</dt>
<t>Network Ossification: While encryption can reduce ossification of <dd>While encryption can reduce ossification of
the transport protocol, it does not itself prevent ossification of the transport protocol, it does not itself prevent ossification of
the network service. People seeking to understand network traffic the network service. People seeking to understand network traffic
could still come to rely on pattern inferences and other heuristics could still come to rely on pattern inferences and other heuristics
or machine learning to derive measurement data and as the basis for or machine learning to derive measurement data and as the basis for
network forwarding decisions <xref target="RFC8546"></xref>. This network forwarding decisions <xref target="RFC8546" format="default"/>
creates dependencies on the transport protocol, or the patterns of . This
creates dependencies on the transport protocol or the patterns of
traffic it can generate, resulting in ossification of the traffic it can generate, resulting in ossification of the
service.</t> service.</dd>
<dt>Impact on Operational Practice:</dt>
<t>Impact on Operational Practice: The network operations community <dd>The network operations community
has long relied on being able to understand Internet traffic has long relied on being able to understand Internet traffic
patterns, both in aggregate and at the flow level, to support patterns, both in aggregate and at the flow level, to support
network management, traffic engineering, and troubleshooting. network management, traffic engineering, and troubleshooting.
Operational practice has developed based on the information Operational practice has developed based on the information
available from unencrypted transport headers. The IETF has supported available from unencrypted transport headers. The IETF has supported
this practice by developing operations and management this practice by developing operations and management specifications,
specifications, interface specifications, and associated Best interface
Current Practises. Widespread deployment of transport protocols that specifications, and associated Best
encrypt their information will impact network operations, unless Current Practices. Widespread deployment of transport protocols that
encrypt their information will impact network operations unless
operators can develop alternative practises that work without access operators can develop alternative practises that work without access
to the transport header.</t> to the transport header.</dd>
<dt>Pace of Evolution:</dt>
<t>Pace of Evolution: Removing obstacles to change can enable an <dd>Removing obstacles to change can enable an
increased pace of evolution. If a protocol changes its transport increased pace of evolution. If a protocol changes its transport
header format (wire image), or its transport behaviour, this can header format (wire image) or its transport behaviour, this can
result in the currently deployed tools and methods becoming no result in the currently deployed tools and methods becoming no
longer relevant. Where this needs to be accompanied by development longer relevant. Where this needs to be accompanied by development
of appropriate operational support functions and procedures, it can of appropriate operational support functions and procedures, it can
incur a cost in new tooling to catch-up with each change. Protocols incur a cost in new tooling to catch up with each change. Protocols
that consistently expose observable data do not require such that consistently expose observable data do not require such
development, but can suffer from ossification and need to consider development but can suffer from ossification and need to consider
if the exposed protocol metadata has privacy implications. There is if the exposed protocol metadata has privacy implications. There is
no single deployment context, and therefore designers need to no single deployment context; therefore, designers need to
consider the diversity of operational networks (ISPs, enterprises, consider the diversity of operational networks (ISPs, enterprises,
DDoS mitigation and firewall maintainers, etc.).</t> DDoS mitigation and firewall maintainers, etc.).</dd>
<!---->
<!---->
<t>Supporting Common Specifications: Common, open, transport <dt>Supporting Common Specifications:</dt>
<dd>Common, open, transport
specifications can stimulate engagement by developers, users, specifications can stimulate engagement by developers, users,
researchers, and the broader community. Increased protocol diversity researchers, and the broader community. Increased protocol diversity
can be beneficial in meeting new requirements, but the ability to can be beneficial in meeting new requirements, but the ability to
innovate without public scrutiny risks point solutions that optimise innovate without public scrutiny risks point solutions that optimise
for specific cases, and that can accidentally disrupt operations for specific cases and that can accidentally disrupt operations
of/in different parts of the network. The social contract that of/in different parts of the network. The social contract that
maintains the stability of the Internet relies on accepting common maintains the stability of the Internet relies on accepting common
transport specifications, and on it being possible to detect transport specifications and on it being possible to detect
violations. The existence of independent measurements, transparency, violations. The existence of independent measurements, transparency,
and public scrutiny of transport protocol behaviour, help the and public scrutiny of transport protocol behaviour helps the
community to enforce the social norm that protocol implementations community to enforce the social norm that protocol implementations
behave fairly and conform (at least mostly) to the specifications. behave fairly and conform (at least mostly) to the specifications.
It is important to find new ways of maintaining that community trust It is important to find new ways of maintaining that community trust
as increased use of transport header encryption limits visibility as increased use of transport header encryption limits visibility
into transport behaviour (see also <xref into transport behaviour (see also <xref target="exposing" format="def
target="exposing"></xref>).</t> ault"/>).</dd>
<dt>Impact on Benchmarking and Understanding Feature Interactions:</dt>
<t>Impact on Benchmarking and Understanding Feature Interactions: An <dd>An appropriate vantage point for observation, coupled with timing
appropriate vantage point for observation, coupled with timing
information about traffic flows, provides a valuable tool for information about traffic flows, provides a valuable tool for
benchmarking network devices, endpoint stacks, and/or benchmarking network devices, endpoint stacks, and/or
configurations. This can help understand complex feature configurations. This can help understand complex feature
interactions. An inability to observe transport header information interactions. An inability to observe transport header information
can make it harder to diagnose and explore interactions between can make it harder to diagnose and explore interactions between
features at different protocol layers, a side-effect of not allowing features at different protocol layers, a side effect of not allowing
a choice of vantage point from which this information is observed. a choice of vantage point from which this information is observed.
New approaches might have to be developed.</t> New approaches might have to be developed.</dd>
<dt>Impact on Research and Development:</dt>
<t>Impact on Research and Development: Hiding transport header <dd>Hiding transport header
information can impede independent research into new mechanisms, information can impede independent research into new mechanisms,
measurement of behaviour, and development initiatives. Experience measurements of behaviour, and development initiatives. Experience
shows that transport protocols are complicated to design and complex shows that transport protocols are complicated to design and complex
to deploy, and that individual mechanisms have to be evaluated while to deploy and that individual mechanisms have to be evaluated while
considering other mechanisms, across a broad range of network considering other mechanisms across a broad range of network
topologies and with attention to the impact on traffic sharing the topologies and with attention to the impact on traffic sharing the
capacity. If increased use of transport header encryption results in capacity. If increased use of transport header encryption results in
reduced availability of open data, it could eliminate the reduced availability of open data, it could eliminate the
independent checks to the standardisation process that have independent checks to the standardisation process that have
previously been in place from research and academic contributors previously been in place from research and academic contributors
(e.g., the role of the IRTF Internet Congestion Control Research (e.g., the role of the IRTF Internet Congestion Control Research
Group (ICCRG) and research publications in reviewing new transport Group (ICCRG) and research publications in reviewing new transport
mechanisms and assessing the impact of their deployment).</t> mechanisms and assessing the impact of their deployment).</dd>
</list></t> </dl>
<t>Observable transport header information might be useful to various <t>Observable transport header information might be useful to various
stakeholders. Other sets of stakeholders have incentives to limit what stakeholders. Other sets of stakeholders have incentives to limit what
can be observed. This document does not make recommendations about what can be observed. This document does not make recommendations about what
information ought to be exposed, to whom it ought to be observable, or information ought to be exposed, to whom it ought to be observable, or
how this will be achieved. There are also design choices about where how this will be achieved. There are also design choices about where
observable fields are placed. For example, one location could be a part observable fields are placed. For example, one location could be a part
of the transport header outside of the encryption envelope, another of the transport header outside of the encryption envelope; another
alternative is to carry the information in a network-layer option or alternative is to carry the information in a network-layer option or
extension header. New transport protocol designs ought to explicitly extension header. New transport protocol designs ought to explicitly
identify any fields that are intended to be observed, consider if there identify any fields that are intended to be observed, consider if there
are alternative ways of providing the information, and reflect on the are alternative ways of providing the information, and reflect on the
implications of observable fields being used by on-path network devices, implications of observable fields being used by on-path network devices
and how this might impact user privacy and protocol evolution when these and how this might impact user privacy and protocol evolution when these
fields become ossified.</t> fields become ossified.</t>
<t>As <xref target="RFC7258" format="default"/> notes, "Making networks
<t>As <xref target="RFC7258"></xref> notes, "Making networks unmanageable to mitigate PM is not an acceptable
unmanageable to mitigate (pervasive monitoring) is not an acceptable outcome, but ignoring PM would go against the
outcome, but ignoring (pervasive monitoring) would go against the
consensus documented here." Providing explicit information can help consensus documented here." Providing explicit information can help
avoid traffic being inappropriately classified, impacting application avoid traffic being inappropriately classified, impacting application
performance. An appropriate balance will emerge over time as real performance. An appropriate balance will emerge over time as real
instances of this tension are analysed <xref target="RFC7258"></xref>. instances of this tension are analysed <xref target="RFC7258" format="defa ult"/>.
This balance between information exposed and information hidden ought to This balance between information exposed and information hidden ought to
be carefully considered when specifying new transport protocols.</t> be carefully considered when specifying new transport protocols.</t>
</section> </section>
<section anchor="Security" numbered="true" toc="default">
<section anchor="Security" title="Security Considerations"> <name>Security Considerations</name>
<t>This document is about design and deployment considerations for <t>This document is about design and deployment considerations for
transport protocols. Issues relating to security are discussed transport protocols. Issues relating to security are discussed
throughout this document.</t> throughout this document.</t>
<t>Authentication, confidentiality protection, and integrity protection <t>Authentication, confidentiality protection, and integrity protection
are identified as Transport Features by <xref target="RFC8095"></xref>. are identified as transport features by <xref target="RFC8095" format="def ault"/>.
As currently deployed in the Internet, these features are generally As currently deployed in the Internet, these features are generally
provided by a protocol or layer on top of the transport protocol <xref provided by a protocol or layer on top of the transport protocol <xref tar
target="RFC8922"></xref>.</t> get="RFC8922" format="default"/>.</t>
<t>Confidentiality and strong integrity checks have properties that can <t>Confidentiality and strong integrity checks have properties that can
also be incorporated into the design of a transport protocol or to also be incorporated into the design of a transport protocol or to
modify an existing transport. Integrity checks can protect an endpoint modify an existing transport. Integrity checks can protect an endpoint
from undetected modification of protocol fields by on-path network from undetected modification of protocol fields by on-path network
devices, whereas encryption and obfuscation or greasing can further devices, whereas encryption and obfuscation or greasing can further
prevent these headers being utilised by network devices <xref prevent these headers being utilised by network devices <xref target="RFC8
target="RFC8701"></xref>. Preventing observation of headers provides an 701"
format="default"/>. Preventing observation of headers provides an
opportunity for greater freedom to update the protocols and can ease opportunity for greater freedom to update the protocols and can ease
experimentation with new techniques and their final deployment in experimentation with new techniques and their final deployment in
endpoints. A protocol specification needs to weigh the costs of endpoints. A protocol specification needs to weigh the costs of
ossifying common headers, versus the potential benefits of exposing ossifying common headers versus the potential benefits of exposing
specific information that could be observed along the network path to specific information that could be observed along the network path to
provide tools to manage new variants of protocols.</t> provide tools to manage new variants of protocols.</t>
<t>Header encryption can provide confidentiality of some or all of the <t>Header encryption can provide confidentiality of some or all of the
transport header information. This prevents an on-path device from transport header information. This prevents an on-path device from
gaining knowledge of the header field. It therefore prevents mechanisms gaining knowledge of the header field. It therefore prevents mechanisms
being built that directly rely on the information or seeks to infer being built that directly rely on the information or seeks to infer
semantics of an exposed header field. Reduced visibility into transport semantics of an exposed header field. Reduced visibility into transport
metadata can limit the ability to measure and characterise traffic, and metadata can limit the ability to measure and characterise traffic and
conversely can provide privacy benefits.</t> conversely can provide privacy benefits.</t>
<t>Extending the transport payload security context to also include the <t>Extending the transport payload security context to also include the
transport protocol header protects both types of information with the transport protocol header protects both types of information with the
same key. A privacy concern would arise if this key was shared with a same key. A privacy concern would arise if this key was shared with a
third party, e.g., providing access to transport header information to third party, e.g., providing access to transport header information to
debug a performance issue, would also result in exposing the transport debug a performance issue would also result in exposing the transport
payload data to the same third party. Such risks would be mitigated payload data to the same third party. Such risks would be mitigated
using a layered security design that provides one domain of protection using a layered security design that provides one domain of protection
and associated keys for the transport payload and encrypted transport and associated keys for the transport payload and encrypted transport
headers; and a separate domain of protection and associated keys for any headers and a separate domain of protection and associated keys for any
observable transport header fields.</t> observable transport header fields.</t>
<t>Exposed transport headers are sometimes utilised as a part of the <t>Exposed transport headers are sometimes utilised as a part of the
information to detect anomalies in network traffic. "While PM is an information to detect anomalies in network traffic. As stated in <xref tar
get="RFC7258"
format="default"/>, "While PM is an
attack, other forms of monitoring that might fit the definition of PM attack, other forms of monitoring that might fit the definition of PM
can be beneficial and not part of any attack, e.g., network management can be beneficial and not part of any attack, e.g., network management
functions monitor packets or flows and anti-spam mechanisms need to see functions monitor packets or flows and anti-spam mechanisms need to see
mail message content." <xref target="RFC7258"></xref>. This can be used mail message content." This can be used
as the first line of defence to identify potential threats from DoS or as the first line of defence to identify potential threats from DoS or
malware and redirect suspect traffic to dedicated nodes responsible for malware and redirect suspect traffic to dedicated nodes responsible for
DoS analysis, malware detection, or to perform packet "scrubbing" (the DoS analysis, for malware detection, or to perform packet "scrubbing" (the
normalisation of packets so that there are no ambiguities in normalisation of packets so that there are no ambiguities in
interpretation by the ultimate destination of the packet). These interpretation by the ultimate destination of the packet). These
techniques are currently used by some operators to also defend from techniques are currently used by some operators to also defend from
distributed DoS attacks.</t> distributed DoS attacks.</t>
<t>Exposed transport header fields can also form a part of the <t>Exposed transport header fields can also form a part of the
information used by the receiver of a transport protocol to protect the information used by the receiver of a transport protocol to protect the
transport layer from data injection by an attacker. In evaluating this transport layer from data injection by an attacker. In evaluating this
use of exposed header information, it is important to consider whether use of exposed header information, it is important to consider whether
it introduces a significant DoS threat. For example, an attacker could it introduces a significant DoS threat. For example, an attacker could
construct a DoS attack by sending packets with a sequence number that construct a DoS attack by sending packets with a sequence number that
falls within the currently accepted range of sequence numbers at the falls within the currently accepted range of sequence numbers at the
receiving endpoint. This would then introduce additional work at the receiving endpoint. This would then introduce additional work at the
receiving endpoint, even though the data in the attacking packet might receiving endpoint, even though the data in the attacking packet might
not finally be delivered by the transport layer. This is sometimes known not finally be delivered by the transport layer. This is sometimes known
skipping to change at line 1830 skipping to change at line 1576
<t>Exposed transport header fields can also form a part of the <t>Exposed transport header fields can also form a part of the
information used by the receiver of a transport protocol to protect the information used by the receiver of a transport protocol to protect the
transport layer from data injection by an attacker. In evaluating this transport layer from data injection by an attacker. In evaluating this
use of exposed header information, it is important to consider whether use of exposed header information, it is important to consider whether
it introduces a significant DoS threat. For example, an attacker could it introduces a significant DoS threat. For example, an attacker could
construct a DoS attack by sending packets with a sequence number that construct a DoS attack by sending packets with a sequence number that
falls within the currently accepted range of sequence numbers at the falls within the currently accepted range of sequence numbers at the
receiving endpoint. This would then introduce additional work at the receiving endpoint. This would then introduce additional work at the
receiving endpoint, even though the data in the attacking packet might receiving endpoint, even though the data in the attacking packet might
not finally be delivered by the transport layer. This is sometimes known not finally be delivered by the transport layer. This is sometimes known
as a &ldquo;shadowing attack&rdquo;. An attack can, for example, disrupt as a "shadowing attack". An attack can, for example, disrupt
receiver processing, trigger loss and retransmission, or make a receiver processing, trigger loss and retransmission, or make a
receiving endpoint perform unproductive decryption of packets that receiving endpoint perform unproductive decryption of packets that
cannot be successfully decrypted (forcing a receiver to commit cannot be successfully decrypted (forcing a receiver to commit
decryption resources, or to update and then restore protocol state).</t> decryption resources, or to update and then restore protocol state).</t>
<t>One mitigation to off-path attacks is to deny knowledge of what header
<t>One mitigation to off-path attack is to deny knowledge of what header
information is accepted by a receiver or obfuscate the accepted header information is accepted by a receiver or obfuscate the accepted header
information, e.g., setting a non-predictable initial value for a information, e.g., setting a nonpredictable initial value for a
sequence number during a protocol handshake, as in <xref sequence number during a protocol handshake, as in <xref target="RFC3550"
target="RFC3550"></xref> and <xref target="RFC6056"></xref>, or a port format="default"/>
value that cannot be predicted (see Section 5.1 of <xref and <xref target="RFC6056" format="default"/>, or a port
target="RFC8085"></xref>). A receiver could also require additional value that cannot be predicted (see <xref target="RFC8085" sectionFormat="
of"
section="5.1"/>). A receiver could also require additional
information to be used as a part of a validation check before accepting information to be used as a part of a validation check before accepting
packets at the transport layer (e.g., utilising a part of the sequence packets at the transport layer, e.g., utilising a part of the sequence
number space that is encrypted; or by verifying an encrypted token not number space that is encrypted or by verifying an encrypted token not
visible to an attacker). This would also mitigate against on-path visible to an attacker. This would also mitigate against on-path
attacks. An additional processing cost can be incurred when decryption attacks. An additional processing cost can be incurred when decryption
is attempted before a receiver discards an injected packet.</t> is attempted before a receiver discards an injected packet.</t>
<t>The existence of open transport protocol standards and a research
<t>The existence of open transport protocol standards, and a research
and operations community with a history of independent observation and and operations community with a history of independent observation and
evaluation of performance data, encourages fairness and conformance to evaluation of performance data encourage fairness and conformance to
those standards. This suggests careful consideration will be made over those standards. This suggests careful consideration will be made over
where, and when, measurement samples are collected. An appropriate where, and when, measurement samples are collected. An appropriate
balance between encrypting some or all of the transport header balance between encrypting some or all of the transport header
information needs to be considered. Open data, and accessibility to information needs to be considered. Open data and accessibility to
tools that can help understand trends in application deployment, network tools that can help understand trends in application deployment, network
traffic and usage patterns can all contribute to understanding security traffic, and usage patterns can all contribute to understanding security
challenges.</t> challenges.</t>
<t>The security and privacy considerations in "A Framework for
<t>The Security and Privacy Considerations in the Framework for Large-Scale Measurement of Broadband Performance (LMAP)" <xref target="RFC
Large-Scale Measurement of Broadband Performance (LMAP) <xref 7594"
target="RFC7594"></xref> contain considerations for Active and Passive format="default"/> contain considerations for Active and Passive
measurement techniques and supporting material on measurement measurement techniques and supporting material on measurement
context.</t> context.</t>
<t>Addition of observable transport information to the path increases <t>Addition of observable transport information to the path increases
the information available to an observer and may, when this information the information available to an observer and may, when this information
can be linked to a node or user, reduce the privacy of the user. See the can be linked to a node or user, reduce the privacy of the user. See the
security considerations of <xref target="RFC8558"></xref>.</t> security considerations of <xref target="RFC8558" format="default"/>.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section> </section>
<section anchor="IANA" numbered="true" toc="default">
<section anchor="Acknowledgements" title="Acknowledgements"> <name>IANA Considerations</name>
<t>The authors would like to thank Mohamed Boucadair, Spencer Dawkins, <t>This document has no IANA actions.</t>
Tom Herbert, Jana Iyengar, Mirja Kuehlewind, Kyle Rose, Kathleen
Moriarty, Al Morton, Chris Seal, Joe Touch, Brian Trammell, Chris Wood,
Thomas Fossati, Mohamed Boucadair, Martin Thomson, David Black, Martin
Duke, Joel Halpern and members of TSVWG for their comments and
feedback.</t>
<t>This work has received funding from the European Union&rsquo;s
Horizon 2020 research and innovation programme under grant agreement No
688421, and the EU Stand ICT Call 4. The opinions expressed and
arguments employed reflect only the authors' view. The European
Commission is not responsible for any use that might be made of that
information.</t>
<t>This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.</t>
</section> </section>
</middle> </middle>
<back> <back>
<references title="Informative References">
&RFC4566;
&RFC8684;
&RFC5426;
&RFC0791;
&RFC2410;
&RFC2474;
&RFC2475;
&RFC2507;
&RFC2508;
&RFC2914;
&RFC3168;
&RFC3234;
&RFC3261;
&RFC3393;
&RFC3550;
&RFC3711;
&RFC4302;
&RFC4303;
&RFC4585;
&RFC4737;
&RFC4960;
&RFC5166;
&RFC5795;
&RFC5218;
&RFC5236;
&RFC8446;
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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&RFC6846; <!-- [I-D.trammell-plus-abstract-mech] IESG state Expired -->;
&RFC8701; <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.t rammell-plus-abstract-mech-00.xml"/>;
&I-D.ietf-quic-transport; <!-- [I-D.ietf-ippm-ioam-data] IESG state IESG Evaluation::Revised I-D Needed -- >
&I-D.trammell-plus-abstract-mech; <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.i etf-ippm-ioam-data-12.xml"/>;
&I-D.ietf-ippm-ioam-data; <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC .8922.xml"/>;
&RFC8922; <!-- [I-D.ietf-tsvwg-rtcweb-qos] Published as RFC 8837 -->;
&I-D.ietf-tsvwg-rtcweb-qos; <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC .8837.xml"/>;
&RFC8837; <!-- [I-D.ietf-quic-qlog-main-schema] IESG state I-D Exists -->;
&I-D.marx-qlog-main-schema; <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.i etf-quic-qlog-main-schema-00.xml"/>;
&I-D.ietf-tls-dtls13; <!-- [I-D.ietf-tls-dtls13] in MISSREF state -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.d
raft-ietf-tls-dtls13-43.xml"/>
&I-D.ietf-6man-ipv6-alt-mark; <!-- [I-D.ietf-6man-ipv6-alt-mark] IESG state I-D Exists -->;
&RFC3552; <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.i etf-6man-ipv6-alt-mark-06.xml"/>;
&RFC8724; <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
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<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC
.8724.xml"/>
<reference anchor="Measurement"> <reference anchor="Measurement">
<front> <front>
<title>Measurement-based Protocol Design, Eur. Conf. on Networks and <title>Measurement-based Protocol Design</title>
Communications, Oulu, Finland.</title> <author initials="G" surname="Fairhurst" fullname="Gorry Fairhurst"/>
<author initials="M" surname="Kuehlewind" fullname="Mirja Kuehlewind"/
<author initials="G" surname="Fairhurst"></author> >
<author initials="D" surname="Lopez" fullname="Diego Lopez"/>
<author initials="M" surname="Kuehlewind"></author> <date month="June" year="2017"/>
<author initials="D" surname="Lopez"></author>
<date month="June" year="2017" />
</front> </front>
<refcontent>European Conference on Networks and Communications, Oulu, Fin land.</refcontent>
</reference> </reference>
<reference anchor="Latency"> <reference anchor="Latency">
<front> <front>
<title>Reducing Internet Latency: A Survey of Techniques and Their <title>Reducing Internet Latency: A Survey of Techniques and Their
Merits, IEEE Comm. Surveys &amp; Tutorials. 26;18(3) Merits</title>
p2149-2196</title> <author initials="B" surname="Briscoe" fullname="Bob Briscoe"/>
<author initials="A" surname="Brunstrom" fullname="Anna Brunstrom"/>
<author initials="B" surname="Briscoe"></author> <author initials="A" surname="Petlund" fullname="Andreas Petlund"/>
<author initials="D" surname="Hayes" fullname="David Hayes"/>
<date month="November" year="2014" /> <author initials="D" surname="Ros" fullname="David Ros"/>
<author initials="I" surname="Tsang" fullname="Ing-Jyh Tsang"/>
<author initials="S" surname="Gjessing" fullname="Stein Gjessing"/>
<author initials="G" surname="Fairhurst" fullname="Gorry Fairhurst"/>
<author initials="C" surname="Griwodz" fullname="Carsten Griwodz"/>
<author initials="M" surname="Welzl" fullname="Michael Welzl"/>
<date month="November" year="2014"/>
</front> </front>
<seriesInfo name="DOI" value="10.1109/COMST.2014.2375213"/>
<refcontent>IEEE Communications Surveys &amp; Tutorials, vol. 18, no. 3,
pp. 2149-2196,
thirdquarter 2016</refcontent>
</reference> </reference>
<reference anchor="bufferbloat"> <reference anchor="bufferbloat">
<front> <front>
<title>Bufferbloat: dark buffers in the Internet. Communications of <title>Bufferbloat: Dark Buffers in the Internet</title>
the ACM, 55(1):57-65</title> <author initials="J" surname="Gettys" fullname="Jim Gettys"/>
<author initials="K" surname="Nichols" fullname="Kathleen Nichols"/>
<author initials="J" surname="Gettys"></author> <date month="January" year="2012"/>
<author initials="K" surname="Nichols"></author>
<date month="January" year="2012" />
</front> </front>
<seriesInfo name="DOI" value="10.1145/2063176.2063196"/>
<refcontent>Communications of the ACM, Vol. 55, no. 1, pp. 57-65</refcont
ent>
</reference> </reference>
<reference anchor="Quic-Trace"> <reference anchor="Quic-Trace" target="https://github.com/google/quic-trac e">
<front> <front>
<title>https:QUIC trace utilities <title>QUIC trace utilities
//github.com/google/quic-trace</title> </title>
<author> <author>
<organization></organization> <organization/>
</author> </author>
<date />
</front> </front>
</reference> </reference>
<reference anchor="PAM-RTT"> <reference anchor="PAM-RTT">
<front> <front>
<title>Revisiting the Privacy Implications of Two-Way Internet <title>Revisiting the Privacy Implications of Two-Way Internet
Latency Data (in Proc. PAM 2018)</title> Latency Data</title>
<author initials="B." surname="Trammell" fullname="Brian Trammell">
<author initials="B." surname="Trammell"> <organization/>
<organization></organization>
</author> </author>
<author initials="M." surname="Kuehlewind" fullname="Mirja Kuehlewind"
<author initials="M." surname="Kuehlewind"> >
<organization></organization> <organization/>
</author> </author>
<date month="March" year="2018"/>
<date month="March" year="2018" />
</front> </front>
<refcontent>Passive and Active Measurement</refcontent>
</reference> </reference>
</references> </references>
<section anchor="Acknowledgements" numbered="false" toc="default">
<section title="Revision information"> <name>Acknowledgements</name>
<t>-00 This is an individual draft for the IETF community.</t> <t>The authors would like to thank <contact fullname="Mohamed Boucadair"/>
, <contact
<t>-01 This draft was a result of walking away from the text for a few fullname="Spencer Dawkins"/>, <contact fullname="Tom Herbert"/>, <contact
days and then reorganising the content.</t> fullname="Jana
Iyengar"/>, <contact fullname="Mirja Kühlewind"/>, <contact fullname="Kyle
<t>-02 This draft fixes textual errors.</t> Rose"/>,
<contact fullname="Kathleen Moriarty"/>, <contact fullname="Al Morton"/>,
<t>-03 This draft follows feedback from people reading this draft.</t> <contact
fullname="Chris Seal"/>, <contact fullname="Joe Touch"/>, <contact fullnam
<t>-04 This adds an additional contributor and includes significant e="Brian
reworking to ready this for review by the wider IETF community Colin Trammell"/>, <contact fullname="Chris Wood"/>,
Perkins joined the author list.</t> <contact fullname="Thomas Fossati"/>, <contact fullname="Mohamed Boucadair
"/>, <contact
<t>Comments from the community are welcome on the text and fullname="Martin Thomson"/>, <contact fullname="David Black"/>, <contact f
recommendations.</t> ullname="Martin
Duke"/>, <contact fullname="Joel Halpern"/>, and members of TSVWG for thei
<t>-05 Corrections received and helpful inputs from Mohamed r comments and
Boucadair.</t> feedback.</t>
<t>This work has received funding from the European Union's
<t>-06 Updated following comments from Stephen Farrell, and feedback via Horizon 2020 research and innovation programme under grant agreement No
email. Added a draft conclusion section to sketch some strawman 688421 and the EU Stand ICT Call 4. The opinions expressed and
scenarios that could emerge.</t> arguments employed reflect only the authors' views. The European
Commission is not responsible for any use that might be made of that
<t>-07 Updated following comments from Al Morton, Chris Seal, and other information.</t>
feedback via email.</t> <t>This work has received funding from the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.</t>
<t>-08 Updated to address comments sent to the TSVWG mailing list by
Kathleen Moriarty (on 08/05/2018 and 17/05/2018), Joe Touch on
11/05/2018, and Spencer Dawkins.</t>
<t>-09 Updated security considerations.</t>
<t>-10 Updated references, split the Introduction, and added a paragraph
giving some examples of why ossification has been an issue.</t>
<t>-01 This resolved some reference issues. Updated section on
observation by devices on the path.</t>
<t>-02 Comments received from Kyle Rose, Spencer Dawkins and Tom
Herbert. The network-layer information has also been re-organised after
comments at IETF-103.</t>
<t>-03 Added a section on header compression and rewriting of sections
referring to RTP transport. This version contains author editorial work
and removed duplicate section.</t>
<t>-04 Revised following SecDir Review</t>
<t><list style="symbols">
<t>Added some text on TLS story (additional input sought on relevant
considerations).</t>
<t>Section 2, paragraph 8 - changed to be clearer, in particular,
added "Encryption with secure key distribution prevents"</t>
<t>Flow label description rewritten based on PS/BCP RFCs.</t>
<t>Clarify requirements from RFCs concerning the IPv6 flow label and
highlight ways it can be used with encryption. (section 3.1.3)</t>
<t>Add text on the explicit spin-bit work in the QUIC DT. Added
greasing of spin-bit. (Section 6.1)</t>
<t>Updated section 6 and added more explanation of impact on
operators.</t>
<t>Other comments addressed.</t>
</list>-05 Editorial pass and minor corrections noted on TSVWG
list.</t>
<t>-06 Updated conclusions and minor corrections. Responded to request
to add OAM discussion to Section 6.1.</t>
<t><!--
Three example scenarios illustrate different directions in which this
could evolve:
In one scenario, transport protocol designs expose the transport heade
r and do not use confidentiality to protect the transport information. Middlebox
es could utilise this information and could rely on the presence and format of a
ny exposed information to build tooling and procedures that support troubleshoot
ing, measurement and other functions. As the design evolves, these tools will ha
ve to be updated to reflect the format of the header information in updated vers
ions of the protocol. The protocol could then experience unintentional impact fr
om the middlebox dependencies either loosing functionality or requiring the midd
leboxes to be updated to track the protocol evolution. This could limit the abil
ity to deploy changes to the protocol.
In another scenario, transport protocols could be designed to intentio
nally expose information to the network as a part of the transport header. This
design fixes the invariant format of the exposed information between versions of
the protocol. Only the exposed part of the transport information can be utilise
d by an operator to support measurement and other operational procedures. Common
approaches between versions of the protocol and between different operators cou
ld emerge based on the ossified header information, enabling consistent traffic
management as the protocol evolves.
In a third scenario, a protocol that encrypts all header information p
revents tooling from directly using transport header information. This could lea
d to network operators acting independently from apps/transport developments to
extract the information to operate and manage their network. A range of approach
es could proliferate to support specific goals. For some applications, operators
could introduce on addition of a shim header to each packet in a flow as the fl
ow crosses a network segment; other operators/managers could develop heuristics
and pattern recognition to derive information that classifies flows and estimate
s quality metrics for the service being used; some could decide to rate-limit or
block traffic until new tooling is in place.
Other scenarios could also prevail, and time will tell the final impac
t on network operation and evolution of the Internet.
-->-07 Addressed feedback from Ruediger and Thomas.</t>
<t>Section 2 deserved some work to make it easier to read and avoid
repetition. This edit finally gets to this, and eliminates some
duplication. This also moves some of the material from section 2 to
reform a clearer conclusion. The scope remains focussed on the usage of
transport headers and the implications of encryption - not on proposals
for new techniques/specifications to be developed.</t>
<t>-08 Addressed feedback and completed editorial work, including
updating the text referring to RFC7872, in preparation for a WGLC.</t>
<t>-09 Updated following WGLC. In particular, thanks to Joe Touch
(specific comments and commentary on style and tone); Dimitri Tikonov
(editorial); Christian Huitema (various); David Black (various). Amended
privacy considerations based on SECDIR review. Emile Stephan (inputs on
operations measurement); Various others.</t>
<t>Added summary text and refs to key sections. Note to editors: The
section numbers are hard-linked.</t>
<t>-10 Updated following additional feedback from 1st WGLC. Comments
from David Black; Tommy Pauly; Ian Swett; Mirja Kuehlewind; Peter
Gutmann; Ekr; and many others via the TSVWG list. Some people thought
that "needed" and "need" could</t>
<t>represent requirements in the document, etc. this has been
clarified.</t>
<t>-11 Updated following additional feedback from Martin Thomson, and
corrections from other reviewers.</t>
<t>-12 Updated following additional feedback from reviewers.</t>
<t>-13 Updated following 2nd WGLC with comments from D.L.Black; T.
Herbert; Ekr; and other reviewers.</t>
<t>-14 Update to resolve feedback to rev -13. This moves the general
discussion of adding fields to transport packets to section 6, and
discusses with reference to material in RFC8558.</t>
<t>-15 Feedback from D.L. Black, T. Herbert, J. Touch, S. Dawkins and M.
Duke. Update to add reference to RFC7605. Clarify a focus on immutable
transport fields, rather than modifying middleboxes with Tom H.
Clarified Header Compression discussion only provides a list of examples
of HC methods for transport. Clarified port usage with Tom H/Joe T.
Removed some duplicated sentences, and minor edits. Added NULL-ESP.
Improved after initial feedback from Martin Duke.</t>
<t>-16 Editorial comments from Mohamed Boucadair. Added DTLS 1.3.</t>
<t>-17 Revised to satisfy ID-NITs and updates REFs to latest rev,
updated HC Refs; cited IAB guidance on security and privacy within IETF
specs.</t>
<t>-18 Revised based on AD review.</t>
<t>-19 Revised after additional AD review request, and request to
restructure.</t>
<t>-20 Revised after directorate reviews and IETF LC comments.</t>
<t>Gen-ART:</t>
<t><list style="symbols">
<t>While section 2 does include a discussion of traffic
mis-ordering, it does not include a discussion of ECMP, and the
dependence of ECMP on flow identification to avoid significant
packet mis-ordering.:: ECMP added as example.</t>
<t>Section 5.1 of this document discusses the use of Hop-by-Hop IPv6
options. It seems that it should acknowledge and discuss the
applicability of the sentence "New hop-by-hop options are not
recommended..." from section 4.8 of RFC 8200. I think a good
argument can be made in this case as to why (based on the rest of
the sentence from 8200) the recommendation does not apply to this
proposal. The document should make the argument.:: Quoted RFC
sentences directly to avoid interpretting them.</t>
<t>I found the discussion of header compression slightly confusing.
Given that the TCP / UDP header is small even compared to the IP
header, it is difficult to see why encrypting it would have a
significant impact on header compression efficacy. :: Added a
preface that explains that HC methods are most effective for
bit-congestive links.</t>
<t>The wording in section 6.2 on adding header information to an IP
packet has the drawback of seeming to imply that one could add (or
remove) such information in the network, without adding an
encapsulating header. That is not permitted by RFC 8200 (IPv6). It
would be good to clarify the first paragraph. (The example, which
talks about the sender putting in the information is, of course,
fine.) :: Unintended - added a sentence of preface.</t>
</list></t>
<t>SECDIR:: Previous revisions were updated following Early Review
comments.</t>
<t>OPSEC:: No additional changes were requested in the OPSEC review.</t>
<t>IETF LC:: Tom Herbert: Please refer to 8200 on EH :: addressed in
response to Joel above. Michael Richardson, Fernando Gont, Tom Herbert:
Continuation of discussion on domains where EH might be (or not) useful
and the tussle on what information to reveal. Unclear yet what
additional text should be changed within this ID.</t>
<t>------------</t>
<t>- 21 Revised after IESG review:</t>
<t>Revision 21 includes revised text after comments from Zahed, Erik Kline
, Rob Wilton, Eric Vyncke, Roman Danyliw, and Benjamin Kaduk.</t>
<t></t>
</section> </section>
</back> </back>
</rfc> </rfc>
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