Generic Tunnel MTU Determination
draft-generic-v6ops-tunmtu-01.txt

Abstract

The tunnel MTU for popular IP-in-IP tunneling mechanisms is currently recommended to be set to 1500 (or less) minus the length of the encapsulation headers when static MTU determination is used. This is to avoid IP fragmentation within the tunnel, but requires the tunnel ingress to either fragment any IP packet larger than the MTU or drop the packet and return an ICMP 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. Fortunately, the "Internet cell size" is 1500 bytes, i.e., the minimum MTU configured by the vast majority of links in the Internet. We also note that these same considerations apply to the encapsulation of any combination of IP-within-IP protocol versions. This document therefore presents a method to boost the tunnel MTU to larger values.

Status of This Memo

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This Internet-Draft will expire on November 23, 2012.

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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Problem Statement
• 3. Tunnel MTU and Fragmentation
• 4. Setting the Identification Field
• 5. Applicability
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Author's Address

1. Introduction

The tunnel MTU for popular IP-in-IP tunneling mechanisms is currently recommended to be set to 1500 (or less) minus the length of the encapsulation headers when static MTU determination is used. This is to avoid IP fragmentation within the tunnel [RFC0791][RFC2460], but requires the tunnel ingress to either fragment any IP packet larger than the MTU or drop the packet and return an ICMP Packet Too Big (PTB) message. 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. Fortunately, the "Internet cell size" is 1500 bytes, i.e., the minimum MTU configured by the vast majority of links in the Internet. We also note that these same considerations apply to the encapsulation of any combination of IP-within-IP protocol versions. This document therefore presents a method to boost the tunnel MTU to larger values.

Pushing the tunnel MTU to 1500 bytes or beyond is met with the challenge that the addition of the IP encapsulation header would cause an inner IP packet that is slightly less than 1500 bytes to appear as a 1501 byte or larger outer IP packet on the wire. This can result in the packet being either fragmented or dropped by a router that configures a 1500 byte link. Using the approach outlined in this document, the tunnel ingress avoids this issue by performing IP fragmentation on the inner packet before outer IP encapsulation. The approach is outlined in the following sections.

2. Problem Statement

When an IP tunnel configures a smaller MTU than 1500 bytes, packets that are small enough to traverse earlier links in the path toward the final destination can suddenly be dropped at the tunnel ingress with an ICMP PTB message returned to the original source. However, operational experience has shown that the PTB messages can be lost in the network due to filtering in which case the source does not receive notification of the loss. It is therefore highly desirable that the tunnel configure an MTU of at least 1500 bytes.

One possibility is to use IP fragmentation of the outer IP layer protocol so that inner packets up to 1500 bytes are delivered even if the tunnel encapsulation causes the outer packet to be larger than 1500 bytes. However, IPv4 fragmentation has been shown to be dangerous at high data rates due to the 16-bit Identification field wrapping while reassemblies are still active. Also, if outer IP fragmentation were used the tunnel egress would need to do the reassembly which can be an onerous burden when the egress is located on a router.

A second possibility is to enable IP PMTUD on the outer packet. However, the PTB messages that may result could either be lost on the return path to the tunnel ingress or may not contain enough information for translation into an inner packet PTB for delivery to the original source. Still another possibility is for the tunnel ingress to maintain state about MTU sizes for various tunnel egresses, but this becomes unwieldy when the number of egresses is large.

In short, PMTUD for both IPv4 and IPv6 is a mess and new approaches are needed. Preferably, PMTUD can be avoided through operational arrangements, as described in the following sections.

3. Tunnel MTU and Fragmentation

Section 3.2 of [RFC4213] presents both static and dynamic MTU determination algorithms. These algorithms have been shown to be problematic in many instances, as discussed in Section 2. This document therefore proposes a new and generic tunnel MTU and fragmentation method via the following algorithm:

   1. set the tunnel ingress MTU to "infinity"
   2. for IP packets to be admitted into the tunnel:
      a) if the packet is 1501 or more, admit it into the
         tunnel if it is an IPv6 packet or an IPv4 packet
         with DF=1. Otherwise (i.e., for an IPv4 packet
         with DF=0) break it into N pieces, where each
         piece is a random length between 512-1024 bytes.
      b) if the packet is between 1281 - 1500:
         - admit the packet into the tunnel, subject to
           rate limiting (see Section 3).
         - break the packet into 2 pieces, where the first
           piece is a random length between 512-1024 bytes
         - insert a fragment header on both pieces and set
           the Identification as specified in Section 4.
         - encapsulate both pieces in an IP header and
           send them to the tunnel far end.
      c) if the packet is 1280 or less:
         - admit the packet into the tunnel
  3. the IP destination gets to reassemble if necessary

4. Setting the Identification Field

The algorithm in Section 3 ignores the requirement that routers in the network must not fragment inner IPv6 packets or inner IPv4 packets with DF=1. In the case of IPv6, the use of fragmentation requires that the tunnel ingress insert an IPv6 fragment header on each fragment. In the case of IPv4 packets with DF=1, the use of fragmentation requires that the tunnel ingress rewrite the value in the Identification field. In both cases, we observe that the Identification field provides sufficient protection against accidental reassembly of fragments from different IP packets given careful operational considerations.

Specifically, the tunnel ingress must ensure that there will be no IP fragments alive in the system with duplicate Identification values. Since [RFC2460] specifies that the maximum time a node may retain an incomplete fragmented packet is 60 seconds, this means that the tunnel ingress must not allow the Identification values to be repeated within this timeframe. The tunnel ingress can therefore calculate a maximum data rate for admission of fragmented packets into the tunnel.

For IPv4, to avoid Identification value duplication the tunnel ingress must admit no more than (2^16 / 60) = 1092 IPv4 packets requiring fragmentation into the tunnel per second. In the worst case, consider that each packet is 1281 bytes (i.e., 10248 bits) in length. The tunnel ingress can then calculate the maximum data rate as (1092 * 10248) = 11190816 bits/sec, or approximately 11 Mbps. It is therefore essential that the tunnel ingress set a rate limit to no more than 11 Mbps for those IPv4 packets that will require fragmentation. This restriction can be relaxed if the tunnel ingress maintains a per-destination Identification value instead of a single Identification value for all destinations.

For IPv6, to avoid Identification value duplication the tunnel ingress must admit no more than (2^32 / 60) = 71582788 IPv6 packets requiring fragmentation into the tunnel per second. In the worst case, consider that each packet is 1281 bytes (i.e., 10248 bits) in length. The tunnel ingress can then calculate the maximum data rate as (71582788 * 10248) = 733580411424 bits/sec, or approximately 733 Gbps. It is therefore essential that the tunnel ingress set a rate limit to no more than 733 Gbps for those IPv6 packets that will require fragmentation. This restriction can be relaxed if the tunnel ingress maintains a per-destination Identification value instead of a single Identification value for all destinations.

Note that a possible conflict exists when IP fragmentation has already been performed by a source host before the fragments arrive at the tunnel ingress. In that case, there is a small possibility that the Identification values used by the source host will temporarily be in close correlation with those used by the tunnel ingress, where a "collision" may occur in which the fragments produced by the source host have the same Identification value as the fragments produced by the tunnel ingress. Two factors that mitigate such conflicts are the random length of the first fragment used by the tunnel ingress (i.e., to cause a length mismatch for colliding reassemblies) and, in even rarer instance, the use of the TCP/UDP checksum.

For that reason, the Identification values chosen for the fragmented tunneled packets are *not* assigned sequentially to avoid corrupting a block of reassemblies at a single destination host. Instead, let L be the length (in bits) of the Identification field, let N be a random value between (0, (2^L -1)) and let C be a bounded constant value (e.g., between (512, 1024)). Then, set each successive Identification value to N, then (N + C), then (N + 2C), then (N + 3C) etc. modulo 2^L until the count returns again to N. The sequence then begins at (N+1), then ((N+1) + C), then ((N+1) + 2C), etc. until all integer values within 2^L are covered. Using this method, large numbers of outstanding reassemblies can be accommodated with only very rare instances of reassembly misassociation.

5. Applicability

This approach applies to all tunneling methods that use the basic transition mechanisms, including configured tunnels [RFC4213], 6to4 [RFC3056], ISATAP [RFC5214], DSMIP [RFC5555], 6rd [RFC5969], etc.

Note that this same approach can also be applied to tunneling methods that use other than the basic transition mechanisms. These can include Teredo [RFC4380], LISP [I-D.ietf-lisp], SEAL [I-D.templin-intarea-seal], and IPv6-over-IPv6 encapsulation [RFC2473], and even IPv4-over-IPv6 encapsulation [RFC6333]

6. IANA Considerations

There are no IANA considerations for this document.

7. Security Considerations

The security considerations for basic transition mechanisms apply also to this document.

8. Acknowledgments

This method was inspired through discussion on the IETF v6ops list in the May 2012 timeframe.

9. References

9.1. Normative References

9.2. Informative References

Author's Address