6rd Tunnel MTU

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Last updated 2014-08-18 (latest revision 2014-02-14)
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


The tunnel MTU on 6rd Provider Edge (PE) and Consumer Edge (CE) routers is currently recommended to be set to 1480. This is to avoid IPv4 fragmentation within the tunnel, but requires the tunnel ingress to drop any IPv6 packet larger than 1480 bytes and return an ICMPv6 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 somewhere in the Internet. Fortunately, the "Internet cell size" is 1500 bytes (i.e., the minimum MTU configured by the vast majority of links in the Internet) so if the 6rd PE router can set a tunnel MTU of at least 1500 bytes the MTU issues are alleviated. This document specifies methods that can be employed to support these larger sizes.


Fred Templin (fltemplin@acm.org)

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