Describing TCP with Augmented Packet Header Diagrams
draft-mcquistin-augmented-tcp-example-00
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| Authors | Stephen McQuistin , Vivian Band , Dejice Jacob , Colin Perkins | ||
| Last updated | 2020-11-02 | ||
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draft-mcquistin-augmented-tcp-example-00
Network Working Group S. McQuistin
Internet-Draft V. Band
Intended status: Experimental D. Jacob
Expires: 6 May 2021 C. S. Perkins
University of Glasgow
2 November 2020
Describing TCP with Augmented Packet Header Diagrams
draft-mcquistin-augmented-tcp-example-00
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 6 May 2021.
Copyright Notice
Copyright (c) 2020 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 . . . . . . . . . . 7
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. 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. 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. 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]; Options#Size == (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, or a SACK Permitted Option.
An EOL 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 Factor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 Factor: 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
4. Comparison with draft-ietf-tcpm-rfc793bis
While 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, it is instructive to draw
comparisons with documents that describe the same protocol. In
particular, the descriptions of the TCP header (Section 2) and TCP
options (Section 3) from this document can be compared with
Section 3.1 of [draft-ietf-tcpm-rfc793bis]. We have adopted the same
field descriptions to help with this comparison.
Beyond the broad similarities, there are a number of elements where
this document describes the protocol differently. For example, the
TCP header as defined in [draft-ietf-tcpm-rfc793bis] contains a
variable-length padding field which is not present in the description
contained in this document. The Padding field is used to pad the TCP
header to a 32-bit boundary, and to ensure that the Payload begins on
a 32-bit boundary. The length of the Padding field is the difference
between the value of the Data Offset field and the length of the
Options field. However, the length of the Options field is itself
dependent on the value of the Data Offset field and the particular
set of options contained within an instance of the TCP header. The
Augmented Packet Header Diagram format cannot express this logic: the
lengths of both the Padding and Payload fields would be variable. To
overcome this, as shown in Section 2, the description of TCP given in
this document relies upon End of List options being syntactically
identical to Padding (i.e., being all zeroes). This highlights the
limitations of the Augmented Packet Header Diagram format: prose text
is much more expressive.
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However, the precision required by the Augmented Packet Header
Diagram can be beneficial. For example, the description of the
Options field in this document sets out an explicit length and
presence constraint, where [draft-ietf-tcpm-rfc793bis] indicates that
it is "variable". Similar constraints exist in the description given
in [draft-ietf-tcpm-rfc793bis], but they are contained with the prose
description of the field.
In summary, the most striking comparison to be made is that the
description of TCP given in this document is largely similar to that
given in [draft-ietf-tcpm-rfc793bis]. By retaining the overall
structure of a packet header diagram and an accommpanying list of
descriptions for each field, the Augmented Packet Header Diagram
format is close to that already in use. This is encouraging: it
indicates that the Augmented Packet Header Diagram language can be
used to describe realistic protocols, without introducing a format
that is unfamiliar or difficult to write.
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-07, 2 November 2020,
<http://www.ietf.org/internet-drafts/draft-mcquistin-
augmented-ascii-diagrams-07.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-19, 27 October 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-tcpm-
rfc793bis-19.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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