rfc7893.txt   rfc7893.txt 
Internet Engineering Task Force (IETF) YJ. Stein Internet Engineering Task Force (IETF) Y(J) Stein
Request for Comments: 7893 RAD Data Communications Request for Comments: 7893 RAD Data Communications
Category: Informational D. Black Category: Informational D. Black
ISSN: 2070-1721 EMC Corporation ISSN: 2070-1721 EMC Corporation
B. Briscoe B. Briscoe
BT BT
May 2016 June 2016
Pseudowire Congestion Considerations Pseudowire Congestion Considerations
Abstract Abstract
Pseudowires (PWs) have become a common mechanism for tunneling Pseudowires (PWs) have become a common mechanism for tunneling
traffic and may be found in unmanaged scenarios competing for network traffic and may be found in unmanaged scenarios competing for network
resources both with other PWs and with non-PW traffic, such as TCP/IP resources both with other PWs and with non-PW traffic, such as TCP/IP
flows. Thus, it is worthwhile specifying under what conditions such flows. Thus, it is worthwhile specifying under what conditions such
competition is acceptable, i.e., the PW traffic does not competition is acceptable, i.e., the PW traffic does not
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Status of This Memo Status of This Memo
This document is not an Internet Standards Track specification; it is This document is not an Internet Standards Track specification; it is
published for informational purposes. published for informational purposes.
This document is a product of the Internet Engineering Task Force This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has (IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741. Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7893. http://www.rfc-editor.org/info/rfc7893.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. PWs Comprising Elastic Flows . . . . . . . . . . . . . . . . 5 3. PWs Comprising Elastic Flows . . . . . . . . . . . . . . . . 6
4. PWs Comprising Inelastic Flows . . . . . . . . . . . . . . . 6 4. PWs Comprising Inelastic Flows . . . . . . . . . . . . . . . 7
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 18 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Informative References . . . . . . . . . . . . . . . . . . . 18 7. Informative References . . . . . . . . . . . . . . . . . . . 19
Appendix A. Loss Probabilities for TDM PWs . . . . . . . . . . . 20 Appendix A. Loss Probabilities for TDM PWs . . . . . . . . . . . 22
Appendix B. Effect of Packet Loss on Voice Quality for Appendix B. Effect of Packet Loss on Voice Quality for
Structure-Aware TDM PWs . . . . . . . . . . . . . . 22 Structure-Aware TDM PWs . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
A pseudowire (PW) (see [RFC3985]) is a construct for tunneling a A pseudowire (PW) (see [RFC3985]) is a construct for tunneling a
native service, such as Ethernet or TDM, over a Packet Switched native service, such as Ethernet or TDM, over a Packet Switched
Network (PSN), such as IPv4, IPv6, or MPLS. The PW packet Network (PSN), such as IPv4, IPv6, or MPLS. The PW packet
encapsulates a unit of native service information by prepending the encapsulates a unit of native service information by prepending the
headers required for transport in the particular PSN (which must headers required for transport in the particular PSN (which must
include a demultiplexer field to distinguish the different PWs) and include a demultiplexer field to distinguish the different PWs) and
preferably the 4-byte Pseudowire Emulation Edge-to-Edge (PWE3) preferably the 4-byte Pseudowire Emulation Edge-to-Edge (PWE3)
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At first glance, one may consider a PW transported over IP to be At first glance, one may consider a PW transported over IP to be
considered as a single flow, on par with a single TCP flow. Were we considered as a single flow, on par with a single TCP flow. Were we
to accept this tenet, we would require a PW to back off under to accept this tenet, we would require a PW to back off under
congestion to consume no more bandwidth than a single TCP flow under congestion to consume no more bandwidth than a single TCP flow under
such conditions (see [RFC5348]). However, since PWs may carry such conditions (see [RFC5348]). However, since PWs may carry
traffic from many users, it makes more sense to consider each PW to traffic from many users, it makes more sense to consider each PW to
be equivalent to multiple TCP flows. be equivalent to multiple TCP flows.
The following two sections consider PWs of two types: The following two sections consider PWs of two types:
Elastic Flows: Section 3 concludes that the response to congestion Elastic Flows:
of a PW carrying elastic (e.g., TCP) flows is no different Section 3 concludes that the response to congestion of a PW
from the aggregated behaviors of the individual elastic carrying elastic (e.g., TCP) flows is no different from the
flows, had they not been encapsulated within a PW. aggregated behaviors of the individual elastic flows, had they not
been encapsulated within a PW.
Inelastic Flows: Section 4 considers the case of inelastic constant Inelastic Flows:
bit rate (CBR) TDM PWs [RFC4553] [RFC5086] [RFC5087] Section 4 considers the case of inelastic constant bit rate (CBR)
competing with TCP flows. Such PWs require a preset amount TDM PWs [RFC4553] [RFC5086] [RFC5087] competing with TCP flows.
of bandwidth, that may be lower or higher than that consumed Such PWs require a preset amount of bandwidth, that may be lower
by an otherwise unconstrained TCP flow under the same network or higher than that consumed by an otherwise unconstrained TCP
conditions. In any case, such a PW is unable to respond to flow under the same network conditions. In any case, such a PW is
congestion in a TCP-like manner; although admittedly the unable to respond to congestion in a TCP-like manner; although
total bandwidth it consumes remains constant and does not admittedly the total bandwidth it consumes remains constant and
increase to consume additional bandwidth as TCP rates back does not increase to consume additional bandwidth as TCP rates
off. For TDM services, we will show that TDM service quality back off. For TDM services, we will show that TDM service quality
degradation generally occurs before the TDM PW becomes TCP- degradation generally occurs before the TDM PW becomes TCP-
unfriendly. For TDM services that do not automatically shut unfriendly. For TDM services that do not automatically shut down
down when they persistently fail to comply with acceptable when they persistently fail to comply with acceptable TDM service
TDM service criteria, a transport circuit breaker criteria, a transport circuit breaker [CIRCUIT-BREAKER] may be
[CIRCUIT-BREAKER] may be employed as a last resort to shut employed as a last resort to shut down a TDM pseudowire that can
down a TDM pseudowire that can no longer deliver acceptable no longer deliver acceptable service.
service.
Thus, in both cases, pseudowires will not inflict significant harm on Thus, in both cases, pseudowires will not inflict significant harm on
neighboring TCP flows, as in one case they respond adequately to neighboring TCP flows, as in one case they respond adequately to
congestion, and in the other they would be shut down due to being congestion, and in the other they would be shut down due to being
unable to deliver acceptable service before harming neighboring unable to deliver acceptable service before harming neighboring
flows. flows.
Note: This document contains a large number of graphs that are Note: This document contains a large number of graphs that are
necessary for its understanding, but could not be rendered in ASCII. necessary for its understanding, but could not be rendered in ASCII.
It is strongly suggested that the PDF version be consulted. It is strongly suggested that the PDF version be consulted.
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