IPv6 Path MTU Updates
draft-generic-6man-tunfrag-03.txt

Abstract

IPv6 intentionally deprecates fragmentation by routers in the network. Instead, links with restricting MTUs must either drop each too-large packet and return an ICMP Packet Too Big message or perform link-specific fragmentation (also known as "link adaptation") at a layer below IPv6. This latter category of links is often performance-challenged to accommodate steady-state link-specific fragmentation and reassembly to the point that it would be highly desirable to push the workload back to the IPv6 communication endpoints. A common case that exhibits these link characteristics is seen for IPv6-within-IP tunnels. Additionally, IPv6 nodes could better adapt to path MTUs of various sizes by using either a small amount of host-based fragmentation and/or path probing as necessary.This document therefore proposes an update to the base IPv6 specification to better accommodate path MTU issues.

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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This Internet-Draft will expire on January 09, 2013.

Copyright Notice

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. Considerations for Small MTU Paths
• 4. Considerations for Medium MTU Paths
• 5. Considerations for Large MTU Paths
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Author's Address

1. Introduction

IPv6 intentionally deprecates fragmentation by routers in the network. Instead, links with restricting MTUs must either drop each too-large packet and return an ICMP Packet Too Big message or perform link-specific fragmentation (also known as "link adaptation") at a layer below IPv6. This latter category of links is often performance-challenged to accommodate steady-state link-specific fragmentation and reassembly to the point that it would be highly desirable to push the workload back to the IPv6 communication endpoints. A common case that exhibits these link characteristics is seen for IPv6-within-IP tunnels [I-D.generic-v6ops-tunmtu]. Additionally, IPv6 nodes could better adapt to path MTUs of various sizes by using either a small amount of host-based fragmentation and/or path probing as necessary. This document therefore proposes an update to the base IPv6 specification to better accommodate path MTU issues.

2. Problem Statement

The current "Internet cell size" is effectively 1500 bytes, i.e., the minimum MTU configured by the vast majority of links in the Internet. However, due to issues with Path MTU Discovery (PMTUD) this size can often only be accommodated when links with smaller link-layer segment sizes are permitted to perform link adaptation. A common example of such links is seen for IPv6-within-IP tunnels. For those links, the tunnel ingress can perform fragmentation on the outer packet following encapsulation and can instead (or in addition) perform "tunnel fragmentation" via an encapsulation mid-layer inserted between the inner and outer header. In both cases reassembly would be performed by the tunnel egress.

Unfortunately, link-layer fragmentation can present a significant burden to the link endpoints, i.e., especially when the link supports high data rates and/or is located nearer the "middle" of the network instead of nearer the "edge". The third alternative therefore is to ask the original IPv6 source to perform fragmentation on the packet before sending it out, in which case reassembly would be performed by the final destination. This document therefore updates the IPv6 protocol specification [RFC2460] to better accommodate paths with various MTUs as described in the following sections.

3. Considerations for Small MTU Paths

Section 5 of [RFC2460] states:

"IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6."

This document does not propose to change this requirement, but notes that link-specific fragmentation can be burdensome for some links (e.g., IPv6-within-IP tunnels), to the point that it would be highly desirable for the fragmentation and reassembly to be offloaded to the communication endpoints. In order to accommodate this, when the router at the link ingress performs link adaptation on a packet it should also send an advisory ICMPv6 Packet Too Big (PTB) message back to the original source (subject to rate limiting). This document therefore proposes to add the following specification as a new final paragraph to the end of Section 5:

"In response to an IPv6 packet that is sent to an IPv6 destination located beyond a link that must perform link-specific fragmentation, the originating IPv6 node may receive an ICMP Packet Too Big message reporting a Next-Hop MTU less than 1280. In that case, the IPv6 node should perform IPv6 fragmentation on any subsequent packets that are larger than this MTU value but no larger than 1500 bytes. Note that these Packet Too Big messages are advisory in nature and do not necessarily indicate packet loss."

An example tunnel protocol that invokes this new clause appears in:[I-D.templin-intarea-seal].

4. Considerations for Medium MTU Paths

Regardless of whether there is a link that performs link adaptation in the path, when an original source receives a PTB message reporting a Next-Hop MTU value no smaller than 1280 but smaller than 1500, the source need not reduce the size of the packets it sends but may instead invoke fragmentation for packets no larger than 1500 bytes with a maximum fragment size set to 1280 bytes. These fragments will be reassembled by the final destination, which is required by [RFC2460] to configure a minimum reassembly buffer size of 1500 bytes.

5. Considerations for Large MTU Paths

An original source connected to a link that supports an MTU no smaller than 1500 bytes is permitted to send packets no larger than this MTU without fragmentation, but for packet sizes larger than 1500 bytes should implement [RFC4821] to ensure that the large packets are reaching the final destination.

6. IANA Considerations

There are no IANA considerations for this document.

7. Security Considerations

The security considerations for [RFC2460] apply also to this document.

8. Acknowledgments

This method was inspired through discussion on the IETF v6ops and NANOG mailing lists in the May/June 2012 timeframe.

9. References

9.1. Normative References

9.2. Informative References

Author's Address