TCPM WG J. Touch Internet Draft USC/ISI Intended status: Standards Track Wes Eddy Expires: October 2014 MTI Systems April 18, 2014 TCP Extended Data Offset Option draft-touch-tcpm-tcp-edo-00.txt Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on October 18, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Touch Expires October 18, 2014 [Page 1] Internet-Draft TCP Extended Data Offset Option April 2014 Abstract TCP segments include a Data Offset field to indicate space for TCP options, but the size of the field can limit the space available for complex options that have evolved. This document specifies an optional extension to that space and explains why such extensions (including this) cannot be used in the initial SYN. Table of Contents 1. Introduction...................................................2 2. Conventions used in this document..............................3 3. Requirements for Extending TCP's Data Offset...................3 4. SYN Data Offset Cannot be Extended.............................3 5. The TCP EDO Option.............................................4 6. TCP EDO Interaction with TCP...................................5 6.1. TCP User Interface........................................5 6.2. TCP States and Transitions................................5 6.3. TCP Segment Processing....................................6 6.4. Impact on TCP Header Size.................................6 6.5. Connectionless Resets.....................................6 6.6. ICMP Handling.............................................7 7. Interactions with Middleboxes..................................7 8. Security Considerations........................................7 9. IANA Considerations............................................8 10. References....................................................8 10.1. Normative References.....................................8 10.2. Informative References...................................8 11. Acknowledgments...............................................9 1. Introduction TCP's Data Offset is a 4-bit field, which indicates the number of 32-bit words of the entire TCP header. This limits the current total header size to 60 bytes, of which the basic header occupies 20, leaving 40 bytes for options. These 40 bytes are increasingly becoming a limitation to the development of advanced capabilities. This document specifies the TCP Extended Data Offset (EDO) option, and is independent of (and thus compatible with) IPv4 and IPv6. EDO extends the space available for TCP options, except for the initial SYN segment. This document also explains why the option space of that SYN cannot be extended without severe impact on TCP's initial handshake. Touch Expires October 18, 2014 [Page 2] Internet-Draft TCP Extended Data Offset Option April 2014 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying RFC-2119 significance. In this document, the characters ">>" preceding an indented line(s) indicates a compliance requirement statement using the key words listed above. This convention aids reviewers in quickly identifying or finding the explicit compliance requirements of this RFC. 3. Requirements for Extending TCP's Data Offset The primary goal of extending the TCP Data Offset field is to increase the space available for TCP options. An important requirement of any such extension is that it not impact legacy endpoints. I.e., endpoints seeking to use this new option should not incur additional delay or segment exchanges to connect to either new endpoints supporting this option or legacy endpoints without this option. We call this a "backward downgrade" capability. 4. SYN Data Offset Cannot be Extended There have been a number of previous attempts to extend the space available for TCP options [Al06][Ed08][Ko04][Ra12][Yo11]. The key difficulty with most previous proposals is the desire to extend the option space in all TCP segments, including the initial SYN. There are three basic ways in which this has been attempted: 1. Use of a TCP option. 2. Redefinition of the existing TCP header fields. 3. Use of option space in multiple TCP segments (split-space). A new TCP option cannot extend the Data Offset of a TCP SYN. A SYN may include user data in the payload data. Legacy endpoints that ignore the new option would process the payload contents as user data and send an ACK. Once ACK'd, this data cannot be removed from the user stream. Touch Expires October 18, 2014 [Page 3] Internet-Draft TCP Extended Data Offset Option April 2014 The six reserved TCP header bits cannot be redefined easily, because the original specification did not require their contents to be ignored. Legacy endpoints are required to drop TCP segments where those bits are not zero, defeating the goal of backward downgrade. The same is true for redefinition of other TCP fields, which are all in active, current use. Option space cannot be extended across multiple SYN segments. A legacy endpoint would continue the connection with incomplete option information. As a result, EDO does not attempt to extend the space available for options in TCP SYNs. It does extend that space in all other segments, which has always been trivially possible once an option is defined. 5. The TCP EDO Option The TCP EDO option is organized as indicated in Figure 1 and Figure 2. The EDO length option consists of the required Kind and option Length fields followed by the length of the entire TCP header in bytes. For initial SYN segments (i.e., those whose ACK bit is not set), the EDO request option omits the Header_length field and is thus two bytes shorter. The codepoint value of the EDO Kind is EDO- OPT in both cases. The Header_length is in network-standard byte order. +--------+--------+ | Kind | Length | +--------+--------+ Figure 1 TCP EDO request option +--------+--------+--------+--------+ | Kind | Length | Header_length | +--------+--------+--------+--------+ Figure 2 TCP EDO length option EDO support is determined in both directions using the same exchange. An endpoint seeking to enable EDO support includes the EDO request option in the initial SYN. >> Connections using EDO MUST negotiate its availability during the initial three-way handshake. Touch Expires October 18, 2014 [Page 4] Internet-Draft TCP Extended Data Offset Option April 2014 >> An endpoint confirming EDO support MAY respond with EDO length option in its SYN-ACK. >> Once negotiated on a connection, EDO MAY be present as needed on other segments in either direction. The EDO option SHOULD NOT be used if the total option space needed can be accommodated by the existing Data Offset field. >> Because it overrides an existing TCP header field when present, the EDO length option MUST occur within the length of the TCP Data Offset. The EDO length option SHOULD occur as early as possible, either first or just after any authentication or encryption. >> The EDO length option indicates the total length of the header. The EDO Header_length field MUST NOT exceed that of the total segment size (i.e., TCP Length). The EDO Header_length SHOULD be a multiple of 4 to simplify processing. >> The EDO request option SHOULD be aligned on a 16-bit boundary and the EDO length option SHOULD be aligned on a 32-bit boundary, in both cases for simpler processing. Other options are generally handled in the same manner as when the EDO option is not active. >> Options that depend on other options, such as TCP-AO (which may include or exclude options in MAC calculations) MUST also be augmented to interpret the EDO length option to operate correctly. 6. TCP EDO Interaction with TCP The following subsections describe how EDO interacts with the TCP specification [RFC793]. 6.1. TCP User Interface The TCP EDO option is enabled on a connection using a mechanism similar to any other per-connection option. In Unix systems, this is typically performed using the 'setsockopt' function. 6.2. TCP States and Transitions TCP EDO does not alter the existing TCP state or state transition mechanisms. Touch Expires October 18, 2014 [Page 5] Internet-Draft TCP Extended Data Offset Option April 2014 6.3. TCP Segment Processing TCP EDO alters segment processing during the TCP option processing step. Once detected, the TCP EDO length option overrides the TCP Data Offset field for all subsequent option processing. Option processing continues at the next option after the EDO length option. 6.4. Impact on TCP Header Size The TCP EDO request option increases SYN header length by a minimum of 2 bytes. Currently popular SYN options total 15 bytes, which leaves more than enough room for the EDO request: o SACK permitted (2 bytes) [RFC2018][RFC6675] o Timestamp (10 bytes) [RFC1323] o Window scale (3 bytes) [RFC1323] TCP EDO can also be negotiated in SYNs with the following options: o TCP-AO (authentication) (16 bytes) [RFC5925] o Multipath TCP (20 bytes) [RFC6824] Some combinations of the above options may not fit in the existing SYN option space, and (as noted) that space cannot be extended. The EDO option has negligible impact on other headers, because it can either come first or just after security information, and in either case the additional 4 bytes are easily accommodated within the TCP Data Offset length. Once the EDO option is processed, the entirety of the remainder of the TCP segment is available for any remaining options. 6.5. Connectionless Resets A RST may arrive during a currently active connection or may be needed to cleanup old state from an abandoned connection. The latter occurs when a new SYN is sent to an endpoint with matching existing connection state, at which point that endpoint responds with a RST and both ends remove stale information. The EDO option is not mandatory in any TCP segment, except the SYN and SYN-ACK of the three-way handshake to establish its support. Touch Expires October 18, 2014 [Page 6] Internet-Draft TCP Extended Data Offset Option April 2014 >> The EDO length option MAY occur in a RST when the endpoint has connection state that has negotiated EDO. However, unless the RST is generated by an incoming segment that includes an EDO option, the RST MUST NOT include the EDO length option. 6.6. ICMP Handling ICMP responses are intended to include the IP and the port fields of TCP and UDP headers of typical TCP/IP and UDP/IP packets [RFC792]. This includes the first 8 data bytes of the original datagram, intended to include the transport port numbers used for connection demultiplexing. Later specifications encourage returning as much of the original payload as possible [RFC1812]. In either case, legacy options or new options in the EDO extension area might or might not be included, and so options are generally not assumed to be part of ICMP processing anyway. 7. Interactions with Middleboxes Any new TCP option may be impacted by the presence of any on-path device that examines or modifies transport headers [RFC3234]. Boxes that parse or modify TCP options need to follow the same requirements of TCP endpoints in supporting EDO, or they could interfere with connections. The primary concern is so-called "transparent" rewriting proxies, which modify TCP segment boundaries and thus would mix option information with user data if they do not support EDO. Such devices interfere with many other TCP options, and their use is not common. More common are NATs, which rewrite IP address and/or transport port fields. NATs are not affected by the EDO option. 8. Security Considerations It is meaningless to have the Data Offset further exceed that position. As a result: >> When the EDO length option is present, the EDO length option SHOULD be the last non-null option covered by the TCP Data Offset, because it would be. This also helps prevent the Data Offset from being used as a covert channel. Touch Expires October 18, 2014 [Page 7] Internet-Draft TCP Extended Data Offset Option April 2014 9. IANA Considerations We request that, upon publication, this option be assigned a TCP Option codepoint by IANA, which the RFC Editor will replace EDO-OPT in this document with codepoint value. This section is to be removed prior to publication as an RFC. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. 10.2. Informative References [Al06] Allman, M., "TCPx2: Don't Fence Me In", draft-allman- tcpx2-hack-00 (work in progress), May 2006. [Ed08] Eddy, W. and A. Langley, "Extending the Space Available for TCP Options", draft-eddy-tcp-loo-04 (work in progress), July 2008. [Ko04] Kohler, E., "Extended Option Space for TCP", draft-kohler- tcpm-extopt-00 (work in progress), September 2004. [Ra12] Ramaiah, A., "TCP option space extension", draft-ananth- tcpm-tcpoptext-00 (work in progress), March 2012. [RFC792] Postel, J., "Internet Control Message Protocol", RFC 792. [RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers," RFC 1812, June 1995. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, February 2002. Touch Expires October 18, 2014 [Page 8] Internet-Draft TCP Extended Data Offset Option April 2014 [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, June 2010. [RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M., and Y.. Nishida, "A Conservative Loss Recovery Algorithm Based on Selective Acknowledgment (SACK) for TCP", RFC 6675, August 2012. [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, January 2013. [Yo11] Yourtchenko, A., "Introducing TCP Long Options by Invalid Checksum", draft-yourtchenko-tcp-loic-00 (work in progress), April 2011. 11. Acknowledgments This document was prepared using 2-Word-v2.0.template.dot. Authors' Addresses Joe Touch USC/ISI 4676 Admiralty Way Marina del Rey, CA 90292-6695 USA Phone: +1 (310) 448-9151 Email: touch@isi.edu Wesley M. Eddy MTI Systems US Email: wes@mti-systems.com Touch Expires October 18, 2014 [Page 9]