Internet DRAFT - draft-andrews-tcp-and-ipv6-use-minmtu
draft-andrews-tcp-and-ipv6-use-minmtu
Network Working Group M. Andrews
Internet-Draft ISC
Intended status: Informational October 18, 2015
Expires: April 20, 2016
TCP Fails To Respect IPV6_USE_MIN_MTU
draft-andrews-tcp-and-ipv6-use-minmtu-04
Abstract
The IPV6_USE_MIN_MTU socket option directs the IP layer to limit the
IPv6 packet size to the minimum required supported MTU from the base
IPv6 specification, i.e. 1280 bytes. Many implementations of TCP
running over IPv6 neglect to check the IPV6_USE_MIN_MTU value when
performing MSS negotiation and when constructing a TCP segment
despite MSS being defined to be the MTU less the IP and TCP header
sizes (60 bytes for IPv6). This leads to oversized IPv6 packets
being sent resulting in unintended Path Maximum Transport Unit
Discovery (PMTUD) being performed and to fragmented IPv6 packets
being sent.
Status of This Memo
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . 2
2. MSS Negotiation . . . . . . . . . . . . . . . . . . . . . . . 3
3. Segment Size Calculation . . . . . . . . . . . . . . . . . . 3
4. Current Usage . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 3
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
8. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
The IPV6_USE_MIN_MTU [RFC3542], Section 11.1, socket option directs
the IP layer to limit the IPv6 packet size to the minimum required
supported MTU from the base IPv6 specification [RFC2460], i.e. 1280
bytes. Many implementations of TCP running over IPv6 neglect to
check the IPV6_USE_MIN_MTU value when performing MSS negotiation and
when constructing a TCP segment despite MSS being defined to be the
MTU less the IP and TCP header sizes [RFC0879] (60 bytes for IPv6).
This leads to oversized IPv6 packets being sent resulting in
unintended Path Maximum Transport Unit Discovery (PMTUD) [RFC1191]
being performed and to fragmented IPv6 packets being sent.
TCP, when running over IPv6, SHOULD check the value of
IPV6_USE_MIN_MTU when performing MSS negotiation. TCP
implementations already use learnt PMTU and interface MTU when
performing MSS negotiation. This is yet another constraint on the
MTU which SHOULD be considered.
TCP, when running over IPv6, SHOULD check the value of
IPV6_USE_MIN_MTU when calculating the segment size to send. TCP
implementations already use learnt PMTU and interface MTU when
calculating the segment size to send.
1.1. Reserved Words
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 [RFC2119].
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2. MSS Negotiation
TCP, when running over IPv6, SHOULD check the value of
IPV6_USE_MIN_MTU when performing MSS negotiation. If the value of
IPV6_USE_MIN_MTU is one (1) then the application has requested that
PMTUD not be performed on the socket and that IPv6 packets be sent at
a size no greater then the network minimum MTU of 1280 bytes. This
means that the TCP MSS negotiation size SHOULD be no bigger than 1220
(1280 - 40 - 20) to account for the IPv6 header and the TCP header
and MAY be smaller.
If this negotiation is properly performed then PMTUD of reply traffic
should not normally occur.
3. Segment Size Calculation
TCP, when running over IPv6, SHOULD check the value of
IPV6_USE_MIN_MTU when calculation the next segment to send. If the
value of IPV6_USE_MIN_MTU is one (1) them the maximum segment size
SHOULD be 1220.
If the TCP layer neglects to check the value of IPV6_USE_MIN_MTU and
it is one (1), the packet, when passed to the IPv6 layer, will be
fragmented if the resulting packet is bigger that 1280 octets. This
can result in communications failures due to intermediate nodes not
passing fragmented packets.
4. Current Usage
A example of current usage of IPV6_USE_MIN_MTU=1 and TCP is in DNS
nameservers. This is done as the TCP message streams are normally no
more than a couple of IPv6 packets so there is little benefit in
using maximum sized packet, and no real negative effects from using
smaller packets. There are lots of servers / clients that these
servers talk to and maintaining PMTU knowledge is not effective for
long enough resulting in PMTUD being repeated performed. There are
external time constraints where recovery from lost ICMPv6 PTB will
result in a elapsed transaction time that falls outside of the time
constraint window.
5. Acknowledgements
I would like to thank Havard Eidnes, Sander Steffann, John Leslie,
and Brian E Carpenter for their feedback.
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6. Security Considerations
The document makes no changes that could impact on the security of a
IPv6 stack.
7. IANA Considerations
There are no actions for IANA.
8. Normative References
[RFC0879] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
<http://www.rfc-editor.org/info/rfc879>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<http://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003,
<http://www.rfc-editor.org/info/rfc3542>.
Author's Address
M. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: marka@isc.org
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