IPv6 Maintenance J. Massar
Internet-Draft Massar Networking
Updates: 3647 (if approved) November 11, 2014
Intended status: Standards Track
Expires: May 15, 2015

The IPv6 MTU Label


This document redefines the use of the IPv6 Flow Label field to allow specification of the lowest MTU on a path that the packet travels, thus in most cases avoiding the need for performing Path MTU Discovery and the round-trip penalty that that occurs for processing the ICMPv6 PTB and retransmitting the packets involved.

This specification allows graceful decrease of MTU so that large and non-standard MTU sizes can safely be used on the Internet.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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Table of Contents

1. Requirements notation

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].

Fields and numbers specified in this document are in network byte order (Big Endian).

2. Introduction

When a packet that is being sent does not fit the MTU of the next link the IPv6 protocol specifies that an ICMPv6 Packet Too Big (PTB) [RFC4443] error must be sent back to the originator of the packet. The original sending host then receives this packet and based on the MTU provided resends that packet but then fitting the MTU indicated in the ICMPv6 PTB. This process is called Path MTU Discovery [RFC1191].

Unfortunately there are broken networks that filter ICMPv6 altogether, even though this is against the IPv6 specification. This breaks especially TCP [RFC0793] as those packets go missing altogether and then a timer has to fire till the packet is sent again, but as that packet is also too large to fit the link it won't arrive at the destination either and thus such a connection becomes stuck and times out.

In addition for various load-balancing implementations it is apparently a heavy task to correlate incoming ICMPV6 PTBs to the original packets (which are for most part included in the ICMPv6 PTB) and then deliver it to the backend node that originally transmitted that packet so that that node can retransmit it again in smaller chunks.

The extra round-trip of the ICMPv6 PTB and the need for having the load-balancer needing to figure out where to forward the packet is a problem for large hosting providers who want to minimize latency and maximize throughput (https://tools.ietf.org/html/draft-v6ops-pmtud-ecmp-problem-00)

This specificiation mitigates these problems, in part, by allowing routers to include the lowest common MTU on the path in the IPv6 packet's Flow Label field.

Alternative techniques to solve this problem are described in: Packetization Layer Path MTU Discovery [RFC4821].

3. IPv6 MTU Label Format

The IPv6 Flow Label field consists of 20 bits [RFC3647].

The IPv6 Flow Label Field in the IPv6 Packet Header.

 0                   1                  
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
|                                       |

Hosts not supporting this specificiation will treat the IPv6 Flow Label as an opaque number as defined in [RFC3647].

When the first four bits of the IPv6 Flow Label field are set to 1, the Flow Label Field is considered to be a "IPv6 MTU Label" or for short "MTU Label".

Format of the Flow Label field in "IPv6 MTU Label" mode.

 0                   1                  
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
|1 1 1 1|  Maximum Transmission Unit    |

The Flow Label ID is 0xf0000.

This specification thus allows including the MTU (1280 - 65534) in the header.

When the MTU is < 1280, it is considered invalid due to IPv6 minimum MTU requirement.

When the MTU is set to 65535 (0xffff), it indicates a Jumbo MTU Label. [Q1]JM: The actual Jumbo MTU size should then be located in an extension header?

Following are a few examples of common MTU Labels.

MTU Label with an MTU of 1280 (0x0500)

 0                   1                  
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
|1 1 1 1|0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0|

The Flow Label ID is 0xf0500.

MTU Label with an MTU of 1500 (0x05dc)

 0                   1                  
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
|1 1 1 1|0 0 0 0 0 1 0 1 1 1 0 1 1 1 0 0|

The Flow Label ID is 0xf05dc.

MTU Label with an MTU of 9000 (0x2328)

 0                   1                  
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
|1 1 1 1|0 0 1 0 0 0 1 1 0 0 1 0 1 0 0 0|

The Flow Label ID is 0xf2328.

4. Node Requirements

If no MTU Label is present (first 4 bits not '1'), and the label is 0, the node MUST replace the label and create a MTU Label instead providing the correct MTU for the links involved. This allows backwards compatibility for nodes that do fill in the Flow Label field by not disturbing them.

Each node MUST verify that the given MTU Label is valid (>= 1280). When an MTU Label in the range of 0 - 1279 is encountered it MUST be considered invalid and overwritten by the router to the knowledge it has of the proper MTU. [Q2]JM: Or we could send an ICMPv6 Parameter problem, but this allows a form of backward compatibility when the first four bits are all set

When a Jumbo MTU Label (= 65535) is received and the node does not support Jumbo MTU it should logically update the MTU Label to be the highest MTU it does support itself. [Q3]JM: It might thus happen that a Jumbo MTU Label Extension header is left hanging.

Each hop, including the source, that is transmitting or forwarding the packet MUST update the MTU Label to be the correct lowest MTU for that path as it has knowledge of. This includes the MTU of the ingress and egress interface and learning about the MTU from a different path between the same two points or by having another protocol (e.g. TCP) providing these details. MSS clamping [RFC6691] thus can affect the MTU Label if an implementation has this knowledge.

The first packet being sent on a path MUST have a maximum size of 1280, while setting the MTU Label to the MTU of the egress interface.

Network equipment SHOULD have a configuration option to force overwriting of non-MTU Label Flow Labels, this to force network equipment to handle the MTU Label. This might cause issues when Flow Labels are actually used, hence is not the default.

5. Updating the MTU Label

Given below is an example network along with a table of actions per node that illustrates how a label is updated and how the knowledge learnt is used.

A typical asymmetric network as found on the Internet

            |   Host A   |
                   | MTU=1500
            |  Router 1   |
              |         ^
    MTU=1500  |         | MTU=1500
              v         |
 +--------------+     +--------------+
 |   Router 2   |     |   Router 7   |
 +--------------+     +--------------+
        |                    ^
        |                    |
        | MTU=1480           | MTU=1280
        |                    |
        |             +--------------+
        |             |   Router 6   |
        |             +--------------+
        |                    ^ 
        |                    | MTU=1500
        v                    |
 +--------------+     +--------------+
 |   Router 3   |     |   Router 5   |
 +--------------+     +--------------+
              |         ^ 
     MTU=1500 |         | MTU=1500
              v         |
            |  Router 4   |
                   | MTU=9000
            |    Host B   |

In this example network the routing protocols involved cause an asymmetric routing of packets.

When Host A sends a packet to Host B, the path is: HA, R1, R2, R3, R4, HB. The return path for a packet from Host B to Host A is: HB, R4, R5, R6, R7, R1, HA.

Given that network the following decisions are made.

How MTU gets updated
Node Decision Incoming Link MTU Outgoing Link MTU MTU Label Change
HA No knowledge, thus use outgoing link-MTU - 1500 1500
R1 Outbound not lower, don't update 1500 1500 "
R2 Outbound is lower, update (possible PTB) 1500 1480 1480
R3 Outbound is higher, don't update 1480 1500 "
R4 Outbound is higher, don't update 1500 9000 "
HB Remember A to B = 1480 9000 - "
Reply packet:
HB Learnt A-B = 1480, lower than link-MTU, use it - 9000 1480
R4 Outbound is higher, don't update 9000 1500 "
R5 Outbound is higher, don't update 1500 1500 "
R6 Outbound MTU is lower, update it (possible PTB) 1500 1280 1280
R7 Outbound is higher, don't update 1280 1500 "
R1 Outbound is higher, don't update 1500 1500 "
HA Learnt B-A is 1280 1500 1500 "

In this situation, there would have been two possible locations where a PTB is sent (R2->R3 + R6->R7). But as the first packet sent in a direction MUST be sized at a maximum of 1280, no PTB is possible. After this first packet has been passed the real MTU is learned and this, possibly higher MTU can be used.

This does demonstrate that even with this extra information, it might not always be perfect to avoid PMTU blackholes. Nor does the MTU Label avoid the need to handle PTB or retransmitting packets in a smaller way. Hence, sending, receiving, forwarding and handling ICMPv6 remains important and MUST not be filtered.

Note that in the above list does not mention any of the standard functions and checks like updating the Hop Limit that a router is supposed to do as per the IPv6 protocol.

6. Handling network changes

As the process of MTU Labeling happens per packet, new information will become available to the host continuously. When a sending host receives information that an MTU is lower than a packet it recently sent it could decide to resend that packet directly to avoid it from being blackholed in the upstream.

7. MTU Rediscovery

A host can decide that with a long-standing connection a re-probe of the MTU is needed. It can do so by ignoring the cached information and sending a packet with a maximum size of 1280 while setting the MTU Label to the largest it's egress link supports. This is similar to the situation a first packet would be in and thus restarts the process at the highest possible MTU.

8. References

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3647] Chokhani, S., Ford, W., Sabett, R., Merrill, C. and S. Wu, "Internet X.509 Public Key Infrastructure Certificate Policy and Certification Practices Framework", RFC 3647, November 2003.
[RFC4443] Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, July 2012.

Author's Address

Jeroen Massar Massar Networking Swiss Post Box 101811 Zuercherstrasse 161 Zürich, CH-8010 CH EMail: jeroen@massar.ch URI: http://jeroen.massar.ch