TSGWG J. Touch
Internet Draft USC/ISI
Intended status: Experimental July 21, 2015
Expires: January 2016
Transport Options for UDP
draft-touch-tsvwg-udp-options-01.txt
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Abstract
Transport protocols are extended through the use of transport header
options. This document experimentally extends UDP to provide a
location, syntax, and semantics for transport layer options.
Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................2
3. Background.....................................................3
4. The UDP Option Area............................................3
5. UDP options vs. UDP-Lite.......................................5
6. Options in a Stateless, Unreliable Transport Protocol..........5
7. Security Considerations........................................6
8. IANA Considerations............................................6
9. References.....................................................6
9.1. Normative References......................................6
9.2. Informative References....................................6
10. Acknowledgments...............................................7
1. Introduction
Transport protocols use options as a way to extend their
capabilities. TCP [RFC793], SCTP [RFC4960], and DCCP [RFC4340]
include space for these options but UDP [RFC768] currently does not.
This document defines an experimental extension to UDP that provides
space for transport options including their generic syntax and
semantics for their use in UDP's stateless, unreliable message
protocol.
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. Lowercase uses of these words are not to be
interpreted as carrying significance described in RFC 2119.
In this document, the characters ">>" preceding an indented line(s)
indicates a statement using the key words listed above. This
convention aids reviewers in quickly identifying or finding the
portions of this RFC covered by these key words.
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3. Background
Many protocols include a default header and an area for header
options. These options enable the protocol to be extended for use in
particular environments or in ways unforeseen by the original
designers. Examples include TCP's Maximum Segment Size, Window
Scale, Timestamp, and Authentication Options
[RFC793][RFC5925][RFC7323].
These options are used both in stateful (connection-oriented, e.g.,
TCP [RFC793], SCTP [RFC4960], DCCP [RFC4340]) and stateless
(connectionless, e.g., IPv4 [RFC791], IPv6 [RFC2460] protocols. In
stateful protocols they can help extend the way in which state is
managed. In stateless protocols their effect is often limited to
individual packets, but they can have an aggregate effect on a
sequence as well. One example of such uses is Substrate Protocol for
User Datagrams (SPUD) [Tr15], and this document is intended to
provide an out-of-band option area as an alternative to the in-band
mechanism currently proposed [Hi15].
UDP is one of the most popular protocols that lacks space for
options [RFC768]. The UDP header was intended to be a minimal
addition to IP, providing only ports and a data checksum for
protection. This document experimentally extends UDP to provide a
trailer area for options located after the UDP data payload.
4. The UDP Option Area
The UDP transport header includes demultiplexing and service
identification (port numbers), a checksum, and a field that
indicates the payload length. This length field is typically
redundant with total IP datagram length and header length.
For IPv4, the total datagram length (including IP header) is the
"Total Length" field and the header and its options are 4*IHL
("Internet Header Length") [RFC791]. For IPv6, the last IP option
with "Next Header" = UDP (i.e., 17) indicates the size of the
transport payload as its "Payload Length" directly [RFC2460]. In
both cases, the space available for the UDP transport protocol data
unit is indicated by IP
As a result of this redundancy, the UDP length field can be used in
other ways. UDP-Lite uses this field to indicate UDP checksum
coverage. This document uses this field to create a place for UDP
transport options.
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The UDP option area is defined as the location between the end of
the UDP payload (as indicated by UDP length) and the end of the IP
datagram (as indicated by the IP length and IP header length), i.e.,
as a trailing options area. This area can occur at any valid byte
offset, i.e., it need not be 16-bit or 32-bit aligned. In effect,
this document redefines the UDP "Length" field as a "trailer
offset".
UDP options are defined using a syntax similar to that of TCP
[RFC793]. They are typically a minimum of two bytes in length as
shown in Figure 1, excepting only the one byte options "No
Operation" (NOP) and "End of Options List" (EOL) described below.
+--------+--------+
| Kind | Length |
+--------+--------+
Figure 1 UDP option default format
>> UDP options MAY occur at any UDP length offset.
>> The UDP length MUST be at least as large as the UDP header (8)
and no larger than the payload of the IP datagram (IPlen -
IPhdrlen). Values outside this range MUST be silently discarded as
invalid and logged where rate-limiting permits.
>> UDP options MUST be interpreted in the order in which they occur
in the UDP option area.
The following UDP options are currently defined:
Kind Length Meaning
----------------------------------------------
0 - End of Options List
1 - No operation
128-253 RESERVED
254 N(>=4) RFC 3692-style experiments
255 RESERVED
>> NOP options SHOULD be used at the beginning of the UDP options
area to achieve 32-bit alignment for active (i.e., non-NOP) options.
>> When the UDP options do not consume the entire option area, the
last non-NOP option SHOULD be EOL.
>> All bytes after EOL MUST be ignored by UDP option processing.
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Note that Kind=254 is reserved for experiments [RFC3692]. Only one
such value is reserved because it experiments are expected to
already apply the shared use approach developed for TCP experimental
options [RFC6994].
>> The length of the experimental option MUST be at least 4 to
account for the Kind, Length, and the minimum 16-bit UDP ExID
identifier (similar to TCP ExIDs [RFC6994]).
5. UDP options vs. UDP-Lite
UDP Lite provides partial checksum coverage, so that packets with
errors in some locations can be delivered to the user [RFC3828]. It
uses a different transport protocol number (136) than UDP (17) to
interpret the UDP length field as the prefix covered by the UDP
checksum.
UDP already defines the UDP length field as the limit of the UDP
checksum but that would also limit the data provided to the user
(application). A goal of UDP-Lite is to deliver data beyond that
length offset, which is why a separate transport protocol number was
required.
UDP options do not need a separate transport protocol number because
the data beyond the UDP length offset is never provided to the user.
It is interpreted exclusively within the UDP transport layer.
6. Options in a Stateless, Unreliable Transport Protocol
There are two ways to interpret options for a stateless, unreliable
protocol -- an option is either local to the message or intended to
affect a stream of messages in a soft-state manner. Either
interpretation is valid for defined UDP options.
It is impossible to know in advance whether an endpoint supports a
UDP option.
>> Options MUST allow for silent failure on first receipt.
>> Options that rely on soft-state exchange MUST allow for message
reordering and loss.
>> A UDP option MUST be silently optional until confirmed by
exchange with an endpoint.
(I'm sure there will be more here)
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7. Security Considerations
(to be addressed)
8. IANA Considerations
Upon publication, IANA is hereby requested to create a new registry
for UDP Option Kind numbers, similar to that for TCP Option Kinds.
Values in this registry are to be assigned by IESG Approval or
Standards Action [RFC5226].
Upon publication, IANA is hereby requested to create a new registry
for UDP Experimental Option Experiment Identifiers (UDP ExIDs) for
use in the same manner as [RFC6994]. Values in this registry are to
be assigned using first-come, first-served (FCFS) rules [RFC5226].
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[Hi15] Hildebrand, J., B. Trammel, "Substrate Protocol for User
Datagrams (SPUD) Prototype," draft-hildebrand-spud-
prototype-03, Mar. 2015.
[RFC768] Postel, J., "User Datagram Protocol", RFC 768, August
1980.
[RFC791] Postel, J., "Internet Protocol," RFC 791, Sept. 1981.
[RFC793] Postel, J., "Transmission Control Protocol" RFC 793,
September 1981
[RFC2460] Deering, S., R. Hinden, "Internet Protocol Version 6
(IPv6) Specification," RFC 2460, Dec. 1998.
[RFC4340] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
RFC 4960, September 2007.
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[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful," RFC 3692, Jan. 2004.
[RFC3828] Larzon, L-A., M. Degermark, S. Pink, L-E. Jonsson (Ed.),
G. Fairhurst (Ed.), "The Lightweight User Datagram
Protocol (UDP-Lite)," RFC 3828, July 2004.
[RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs," RFC 5226, May 2008.
[RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
Option," RFC 5925, June 2010.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options," RFC
6994, Aug. 2013.
[RFC7323] Borman, D., R. Braden, V. Jacobson, R. Scheffenegger
(Ed.), "TCP Extensions for High Performance," RFC 7323,
Sep. 2014.
[Tr15] Trammel, B. (Ed.), M. Kuelewind (Ed.), "Requirements for
the design of a Substrate Protocol for User Datagrams
(SPUD)," draft-trammell-spud-req-00, July 2015.
10. Acknowledgments
This work benefitted from feedback from Ken Calvert, Ted Faber, and
Gorry Fairhurst, as well as discussions on the IETF SPUD email list.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292 USA
Phone: +1 (310) 448-9151
Email: touch@isi.edu
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