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Network Working Group                                         M. Andrews
Internet-Draft                                                       ISC
Intended status: Informational                          October 16, 2015
Expires: April 18, 2016


                 TCP Fails To Respect IPV6_USE_MIN_MTU
                draft-andrews-tcp-and-ipv6-use-minmtu-01

Abstract

   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.  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.

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
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   This Internet-Draft will expire on April 18, 2016.

Copyright Notice

   Copyright (c) 2015 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



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   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
     1.1.  Reserved Words  . . . . . . . . . . . . . . . . . . . . .   2
   2.  MSS Negotiation . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Segment Size Calculation  . . . . . . . . . . . . . . . . . .   3
   4.  Current Usage . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   3
   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.  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 constaint 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.  Normative References

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <http://www.rfc-editor.org/info/rfc1191>.






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   [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>.

   [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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