6rd Tunnel MTU
draft-foo-v6ops-6rdmtu-02.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

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This Internet-Draft will expire on August 03, 2014.

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Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. 6rd Provider Edge (PE) Router MTU Mitigations
• 3. 6rd Provider Edge (CE) Router MTU Mitigations
• 4. Discussion
• 5. IANA Considerations
• 6. Security Considerations
• 7. Acknowledgments
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Author's Address

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:

   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:

   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 [RFC4821] to attempt to increase the MSS during the course of the TCP session and thereby take advantage of larger packet sizes when avaialble.

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] 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

8.2. Informative References

Author's Address