Network Working Group F. L. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational March 28, 2013
Expires: September 29, 2013
Operational Considerations for Tunnel Fragmentation and Reassembly
draft-generic-v6ops-tunmtu-13.txt
Abstract
The Maximum Transmission Unit (MTU) for popular IP-in-IP tunnels is
currently recommended to be set to 1500 (or less) minus the length of
the encapsulation headers when static MTU determination is used.
This 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 Path MTU
Discovery (PMTUD) point to the possibility of MTU-related black holes
when a packet is dropped due to an MTU restriction. The current
"Internet cell size" is effectively 1500 bytes (i.e., the minimum MTU
configured by the vast majority of links in the Internet) and should
therefore also be the minimum MTU assigned to tunnels, but this has
proven to be problematic in common operational practice. This
document therefore discusses operational considerations for tunnel
fragmentation and reassembly necessary to accommodate this Internet
cell size.
Status of This Memo
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
Templin Expires September 29, 2013 [Page 1]
Internet-Draft Tunnel MTU Issues March 2013
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Tunnel Fragmentation and Reassembly . . . . . . . . . . . . . 3
3. Jumbo Packet Accommodation . . . . . . . . . . . . . . . . . 5
4. Common Tunneling Mechanisms . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Maximum Transmission Unit (MTU) for popular IP-in-IP tunnels is
currently recommended to be set to 1500 (or less) minus the length of
the encapsulation headers when static MTU determination is used.
This 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 [RFC0791][RFC2460]. Concerns for operational
issues with Path MTU Discovery (PMTUD) [RFC1191][RFC1981] point to
the possibility of MTU-related black holes when a packet is dropped
due to an MTU restriction. The current "Internet cell size" is
effectively 1500 bytes (i.e., the minimum MTU configured by the vast
majority of links in the Internet) and should therefore also be the
minimum MTU assigned to tunnels, but this has proven to be
problematic in common operational practice.
[RFC4459] discusses "MTU and Fragmentation Issues with In-the-Network
Tunneling" and provides a comprehensive study of the various
datatracker.ietf.org