Operational Considerations for Tunnel Fragmentation and Reassembly

Document Type Expired Internet-Draft (individual)
Last updated 2013-09-28 (latest revision 2013-03-27)
Replaces draft-bar-v6ops-ismtu
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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.


Fred Templin (fltemplin@acm.org)

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)