Describing TCP with Augmented Packet Header Diagrams
draft-mcquistin-augmented-tcp-example-02

Document Type Active Internet-Draft (individual)
Authors Stephen McQuistin  , Vivian Band  , Dejice Jacob  , Colin Perkins 
Last updated 2021-10-25
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Network Working Group                                       S. McQuistin
Internet-Draft                                                   V. Band
Intended status: Experimental                                   D. Jacob
Expires: 28 April 2022                                     C. S. Perkins
                                                   University of Glasgow
                                                         25 October 2021

          Describing TCP with Augmented Packet Header Diagrams
                draft-mcquistin-augmented-tcp-example-02

Abstract

   This document describes TCP, and a number of its extensions, using
   Augmented Packet Header Diagrams.  This document is an example of the
   Augmented Packet Header Diagram language: it is not intended as a
   contribution to any ongoing or future work on maintaining or
   extending TCP.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 28 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  TCP Header  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  TCP Options . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Comparison with draft-ietf-tcpm-rfc793bis . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Source code repository . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This document uses Augmented Packet Header Diagrams
   [AUGMENTED-DIAGRAMS] to describe TCP [RFC793], and is intended to
   further discussion about the design and implementation of the
   Augmented Packet Header Diagram language and tooling.  Given this
   purpose, this document is not intended as a contribution to any
   ongoing or future work on maintaining or extending TCP.  Further,
   this document does not necessarily reflect TCP, and its extensions,
   as presently standardised.

2.  TCP Header

   This document describes the TCP protocol.  The TCP protocol uses TCP
   Headers.

   A TCP Header is formatted as follows:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |       |C|E|U|A|P|R|S|F|                               |
   | Offset| Rsrvd |W|C|R|C|S|S|Y|I|         Window Size           |
   |       |       |R|E|G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          [Options]                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               :
   :                            Payload                            :
   :                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Source Port: 16 bits.  The source port number.

   Destination Port: 16 bits.  The destination port number.

   Sequence Number: 32 bits.  The sequence number of the first data
      octet in this segment (except when the SYN flag is set).  If SYN
      is set the sequence number is the initial sequence number (ISN)
      and the first data octet is ISN+1.

   Acknowledgment Number: 32 bits.  If the ACK control bit is set, this
      field contains the value of the next sequence number the sender of
      the segment is expecting to receive.  Once a connection is
      established, this is always sent.

   Data Offset (DOffset): 4 bits; DOffset >= 5.  The number of 32 bit
      words in the TCP Header.  This indicates where the data begins.
      The TCP header (even one including options) is an integral number
      of 32 bits long.

   Reserved (Rsrvd): 4 bits; Rsrvd == 0.  A set of control bits reserved
      for future use.  Must be zero in generated segments and must be
      ignored in received segments, if corresponding future features are
      unimplemented by the sending or receiving host.

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   CWR: 1 bit.  Congestion Window Reduced

   ECE: 1 bit.  ECN-Echo

   URG: 1 bit.  Urgent Pointer field significant

   ACK: 1 bit.  Acknowledgment field significant.

   PSH: 1 bit.  Push Function (see the Send Call description)

   RST: 1 bit.  Reset the connection

   SYN: 1 bit.  Synchronize sequence numbers

   FIN: 1 bit; (FIN == 0) || (SYN == 0).  No more data from sender.

   Window Size: 16 bits.  The number of data octets beginning with the
      one indicated in the acknowledgment field that the sender of this
      segment is willing to accept.

      The window size MUST be treated as an unsigned number, or else
      large window sizes will appear like negative windows and TCP will
      not work (MUST-1).  It is RECOMMENDED that implementations will
      reserve 32-bit fields for the send and receive window sizes in the
      connection record and do all window computations with 32 bits
      (REC- 1).

   Checksum: 16 bits.  The checksum field is the 16 bit one's complement
      of the one's complement sum of all 16 bit words in the header and
      text.  The checksum computation needs to ensure the 16-bit
      alignment of the data being summed.  If a segment contains an odd
      number of header and text octets, alignment can be achieved by
      padding the last octet with zeros on its right to form a 16 bit
      word for checksum purposes.  The pad is not transmitted as part of
      the segment.  While computing the checksum, the checksum field
      itself is replaced with zeros.

   Urgent Pointer: 16 bits.  This field communicates the current value
      of the urgent pointer as a positive offset from the sequence
      number in this segment.  The urgent pointer points to the sequence
      number of the octet following the urgent data.  This field is only
      be interpreted in segments with the URG control bit set.

   Options: [TCP Option]; size(Options) == (DOffset-5)*32; present
   only when DOffset > 5.  Options may occupy space at the end of the
      TCP header and are a multiple of 8 bits in length.  All options
      are included in the checksum.

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   Payload.  Payload.

3.  TCP Options

   A TCP Option is one of: a EOL Option, a NOOP Option, a Maximum
   Segment Size Option, a Window Scale Factor Option, a Timestamp
   Option, a SACK Permitted Option, or a SACK Range Option.

   An EOL Option is formatted as follows:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 0.  This option code indicates
      the end of the option list.

   A NOOP Option is formatted as follows:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |       1       |
   +-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 1.  This option code can be used
      between options, for example, to align the beginning of a
      subsequent option on a word boundary.

   A Maximum Segment Size Option is formatted as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |     Length    |   Maximum Segment Size (MSS)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 2.  If this option is present,
      then it communicates the maximum receive segment size at the TCP
      endpoint that sends this segment.

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   Option Length (Length): 1 byte; Length == 4.  Option length.

   Maximum Segment Size (MSS): 2 bytes.  The maximum segment size
      allowed.

   A Window Scale Factor Option is formatted as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       3       |     Length    | Window Scale  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 3.  If present, this option
      carries the window scale factor.

   Option Length (Length): 1 byte; Length == 3.  Option length.

   Window Scale: 1 byte.  Window scale factor.

   A Timestamp Option is formatted as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Timestamp value (TSval)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Timestamp echo reply (TSecr)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 8.  If present, this option
      carries a timestamp and an echoed timestamp.

   Option Length (Length): 1 byte; Length == 10.  Option length.

   Timestamp value (TSval): 4 bytes.  TSval.

   Timestamp echo reply (TSecr): 4 bytes.  TSecr.

   A SACK Permitted Option is formatted as follows:

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    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       4       |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 4.  If present, this option
      indicates that SACK is permitted.

   Option Length (Length): 1 byte; Length == 2.  Option length.

   A SACK Range Option is formatted as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            [Blocks]                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Option Kind (Kind): 1 byte; Kind == 5.  If present, this option
      contains one or more SACK blocks.

   Option Length (Length): 1 byte.  Option length.

   Blocks: (Length-2)/8 SACK Blocks.  SACK blocks.

   A SACK Block is formatted as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Left Edge                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Right Edge                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Left Edge: 4 bytes.  This is the first sequence number of this block.

   Right Edge: 4 bytes.  This is the sequence number immediately
      following the last sequence number of this block.

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4.  Comparison with draft-ietf-tcpm-rfc793bis

   The Augmented Packet Header Diagram format used in this example has
   also been adopted within [draft-ietf-tcpm-rfc793bis].  This example
   goes beyond [draft-ietf-tcpm-rfc793bis], and includes a number of TCP
   options that are not defined as part of that document, including the
   Window Scale Factor, Timestamp, SACK permitted, and SACK block
   options.  In addition, the definition of the TCP header (Section 2)
   in this document includes a number of field constraints that are not
   specified in [draft-ietf-tcpm-rfc793bis].

   The purpose of this document is to give an example use of the
   Augmented Packet Header Diagrams, and not to contribute to ongoing or
   future TCP standardisation efforts.  We include additional
   constraints and TCP options to demonstrate the ease with which these
   can be expressed in our format, and that these are then supported by
   the generated parser code.

5.  IANA Considerations

   This document contains no actions for IANA.

6.  Security Considerations

   The security implications of the Augmented Packet Header Diagrams
   format are considered in [AUGMENTED-DIAGRAMS].

7.  Acknowledgements

   This work has received funding from the UK Engineering and Physical
   Sciences Research Council under grant EP/R04144X/1.

8.  Informative References

   [AUGMENTED-DIAGRAMS]
              McQuistin, S., Band, V., Jacob, D., and C. S. Perkins,
              "Describing Protocol Data Units with Augmented Packet
              Header Diagrams", Work in Progress, Internet-Draft, draft-
              mcquistin-augmented-ascii-diagrams-09, 25 October 2021,
              <http://www.ietf.org/internet-drafts/draft-mcquistin-
              augmented-ascii-diagrams-09.txt>.

   [RFC793]   Postel, J., "Transmission Control Protocol", RFC 793,
              September 1981, <https://www.rfc-editor.org/info/rfc793>.

   [draft-ietf-tcpm-rfc793bis]
              Eddy, W., "Transmission Control Protocol (TCP)
              Specification", Work in Progress, Internet-Draft, draft-

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              ietf-tcpm-rfc793bis-25, 7 September 2021,
              <http://www.ietf.org/internet-drafts/draft-ietf-tcpm-
              rfc793bis-25.txt>.

Appendix A.  Source code repository

   The source code for tooling that can be used to parse this document,
   and generate parser code for the protocol it describes, is available
   from https://github.com/glasgow-ipl/ips-protodesc-code.

Authors' Addresses

   Stephen McQuistin
   University of Glasgow
   School of Computing Science
   Glasgow
   G12 8QQ
   United Kingdom

   Email: sm@smcquistin.uk

   Vivian Band
   University of Glasgow
   School of Computing Science
   Glasgow
   G12 8QQ
   United Kingdom

   Email: vivianband0@gmail.com

   Dejice Jacob
   University of Glasgow
   School of Computing Science
   Glasgow
   G12 8QQ
   United Kingdom

   Email: d.jacob.1@research.gla.ac.uk

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   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow
   G12 8QQ
   United Kingdom

   Email: csp@csperkins.org

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