Internet DRAFT - draft-foo-v6ops-6rdmtu
draft-foo-v6ops-6rdmtu
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational February 14, 2014
Expires: August 18, 2014
6rd Tunnel MTU
draft-foo-v6ops-6rdmtu-03.txt
Abstract
The tunnel MTU on 6rd Provider Edge (PE) and Consumer Edge (CE)
routers is currently recommended to be set to 1480. This is to avoid
IPv4 fragmentation within the tunnel, but requires the tunnel ingress
to drop any IPv6 packet larger than 1480 bytes and return an ICMPv6
Packet Too Big (PTB) message. Concerns for operational issues with
both IPv4 and IPv6 Path MTU Discovery point to the possibility of
MTU-related black holes when a packet is dropped due to an MTU
restriction somewhere in the Internet. Fortunately, the "Internet
cell size" is 1500 bytes (i.e., the minimum MTU configured by the
vast majority of links in the Internet) so if the 6rd PE router can
set a tunnel MTU of at least 1500 bytes the MTU issues are
alleviated. This document specifies methods that can be employed to
support these larger sizes.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 18, 2014.
Copyright Notice
Copyright (c) 2014 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
Templin Expires August 18, 2014 [Page 1]
Internet-Draft SEAL February 2014
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
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. 6rd Provider Edge (PE) Router MTU Mitigations . . . . . . . . . 3
3. 6rd Provider Edge (CE) Router MTU Mitigations . . . . . . . . . 4
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Templin Expires August 18, 2014 [Page 2]
Internet-Draft SEAL February 2014
1. Introduction
The tunnel MTU on 6rd Provider Edge (PE) and Consumer Edge (CE)
routers is currently recommended to be set to 1500 bytes minus the
IPv4 header encapsulation overhead minus the encapsulation overhead
for any additional encapsulations that may occur on the path
[RFC5969]. This is to avoid IPv4 fragmentation within the tunnel
[RFC0791], but requires the tunnel ingress to drop any IPv6 packet
larger than the tunnel MTU and return an ICMPv6 Packet Too Big (PTB)
message [RFC2460]. Concerns for operational issues with both IPv4
and IPv6 Path MTU Discovery [RFC1191][RFC1981] point to the
possibility of MTU-related black holes when a packet is dropped due
to an MTU restriction somewhere in the Internet. Fortunately, the
"Internet cell size" is 1500 bytes (i.e., the minimum MTU configured
by the vast majority of links in the Internet) so if the 6rd PE
router can set a tunnel MTU of at least 1500 bytes the MTU issues are
alleviated. This document specifies methods that can be employed to
support these larger sizes.
Pushing the 6rd tunnel MTU to 1500 bytes or larger is met with the
challenge that the addition of the IPv4 encapsulation header would
cause a 1500 byte IPv6 packet to appear as a 1520 byte IPv4 packet on
the wire. This can result in the packet being either fragmented or
dropped by an IPv4 router that configures a smaller link MTU,
depending on the setting of the "Don't Fragment" (DF) bit in the IPv4
header. Therefore, this document recommends complementary mechanisms
to ensure that packets of various sizes can be delivered as long as
the underlying IPv4 network can support the larger sizes. The
following two sections present the methods used by 6rd PE and CE
routers.
2. 6rd Provider Edge (PE) Router MTU Mitigations
The 6rd PE Router employs the following MTU-handling mitigations:
Templin Expires August 18, 2014 [Page 3]
Internet-Draft SEAL February 2014
1. Set the 6rd tunnel interface MTU to the maximum of 1500 and
the MTU of the underlying IPv4 interface minus the expected
encapsulation overhead for the IPv4 header as well as any
other encapsulations that may occur on the path.
2. For each 6rd CE, maintain a RATE-LIMIT boolean variable set
to TRUE.
3. When the PE sends an IPv6 packet no larger than 1500 bytes
minus encapsulation overhead to a CE, encapsulate and set the
DF bit to 1.
4. When the PE sends an IPv6 packet larger than 1500 bytes to a
CE, encapsulate and set the DF bit to 1. Optionally cache any
IPv4 MTU values returned in ICMPv4 packet too big messages
that may result.
5. When the PE sends an IPv6 packet larger than 1500 bytes minus
the encapsulation overhead but no larger than 1500 bytes,
encapsulate and set the DF bit to 0. Send the packet to the CE
subject to rate limiting if RATE-LIMIT is TRUE. The packet may
be fragmented in the IPv4 network on the path to the CE.
6. Send a 1500 byte IPv6 probe packet to each active CE subject
to rate limiting using the neighbor reachability test procedure
specified in Section 8 of RFC5969. If the probe succeeds, set
RATE-LIMIT for the CE to FALSE.
3. 6rd Provider Edge (CE) Router MTU Mitigations
The 6rd CE Router employs the following MTU-handling techhniques:
Templin Expires August 18, 2014 [Page 4]
Internet-Draft SEAL February 2014
1. Set the 6rd tunnel interface MTU to the maximum of 1280 and the
the MTU of the underlying IPv4 interface minus the expected
encapsulation overhead for the IPv4 header as well as any other
encapsulations that may occur on the path.
2. If the underlying interface has a sufficiently-large MTU, send
a 1500 byte IPv6 probe packet to the PE using the neighbor
reachability test procedure specified in Section 8 of RFC5969.
If the probe succeeds, set the IPv4 MTU for the PE to the MTU
of the underlying IPv4 interface; else, set the IPv4 MTU to
1520 minus the expected encapsulation overhead.
3. For each TCP session initiated by an IPv6 host within the CE's
LAN, rewrite the Maximum Segment Size (MSS) to 1500 minus the
TCP header length minus the IPv6 header length minus the
encapsulation overhead for (see: [RFC0879][RFC6691]). As a
result, the local IPv6 host and its remote IPv6 correspondent
will begin their TCP messages exchanges using IPv6 packets no
larger than the minimum tunnel path MTU.
4. When the CE sends an IPv6 packet to the PE, if the encapsulated
packet is larger than the IPv4 MTU for the PE drop and return
an ICMPv6 Packet Too Big. Else, set the DF bit to 1 and send
the packet.
5. For each neighboring CE, maintain a RATE-LIMIT boolean variable
set to TRUE.
6. When the CE sends an IPv6 packet no larger than 1500 bytes minus
the encapsulation overhead to a neighboring CE, encapsulate and
set the DF bit to 1.
7. When the CE sends an IPv6 packet larger than 1500 bytes to a
neighboring CE, encapsulate and set the DF bit to 1. Optionally
cache any IPv4 MTU values returned in ICMPv4 packet too big
messages that may result.
8. When the CE sends an IPv6 packet larger than 1500 bytes minus
the encapsulation overhead but no larger than 1500 bytes to a
neighboring CE, encapsulate and set the DF bit to 0. Send the
packet to the neighboring CE subject to rate limiting if
RATE-LIMIT is TRUE.
9. Send a 1500 byte IPv6 probe packet to each active neighboring
CE subject to rate limiting using the neighbor reachability
test procedure specified in Section 8 of RFC5969. If the probe
succeeds, set RATE-LIMIT for the CE to FALSE.
4. Discussion
There are several interrelated aspects to the recommended MTU
mitigations. First, the unconditional rewriting of the MSS by CE
routers ensures that the initial packets sent by IPv6 correspondents
will be no larger than the minimum tunnel path MTU following
encapsulation. The IPv6 correspondents can thereafter use
Templin Expires August 18, 2014 [Page 5]
Internet-Draft SEAL February 2014
Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821] to attempt
to increase the MSS during the course of the TCP session and thereby
take advantage of larger packet sizes when available.
However, not all transport protocols observe the TCP MSS and so the
packets of other protocols generated by IPv6 hosts may be larger than
would fit in the minimum tunnel path MTU. Since most IPv6 hosts
expect to see a minimum MTU of 1500 bytes without any ancillary MTU
assurance mitigations, the approach specified here takes special care
of packets larger than the minimum tunnel path MTU but no larger than
1500 bytes. Namely, these packets are allowed to undergo IPv4
fragmentation on the path from the PE to a CE or on the path from a
CE to another CE. Since sustained fragmentation at high data rates
is dangerous however [RFC4963][RFC6864] packets in this size range
must only be admitted into the tunnel subject to rate limiting so
that reassembly misassociations do not occur. Meanwhile, packets
larger than 1500 bytes are admitted into the tunnel unconditionally
on a "best effort" basis with the understanding that these packets
may be dropped silently.
Using these methods, CE routers may need to perform a small amount of
IPv4 reassembly. PE routers on the other hand will never be asked to
perform reassembly.
5. IANA Considerations
There are no IANA considerations for this document.
6. Security Considerations
The security considerations for 6rd apply also to this document.
7. Acknowledgments
This method was inspired through many years of discussion on IETF
lists and other forums on the topic of tunnel MTU.
8. References
8.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
Templin Expires August 18, 2014 [Page 6]
Internet-Draft SEAL February 2014
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
8.2. Informative References
[RFC0879] Postel, J., "TCP maximum segment size and related topics",
RFC 879, November 1983.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, July 2012.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, February 2013.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
Templin Expires August 18, 2014 [Page 7]