Subject: [RRG] Fwd: Tunnel MTU

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 20 Sep 2007 14:50:35 -0700

(Fwd'd from 'int-area'):
Please see below for a proposal that addresses the MTU issues
for *-in-IPv4 tunnels. It also addresses the multi-mtu subnet
issue, since it does not rely on ICMP "packet too big" messages
from the last-hop router. Elements of the proposal include:

  - trailing "footer" and data as part of encapsulation
  - tiny echo requests wrapped in encapsulation headers
    and trailing padding - used as probes to elicit tiny
    echo replies
  - tunnel endpoint discovers tunnel far end EMTU_R and
    reassemby timeout values in initial probes=20
  - periodic probing to discover the tunnel path MTU,
    plus EMTU_R; reassembly timeout fluctuations
  - inner fragmentation to create inner packets no larger
    than EMTU_R
  - outer fragmentation to create outer packets no larger
    than the tunnel path MTU
  - coservative use of fragmentation to avoid packet loss
    within the tunnel
  - TE drops unfragmentable packets larger than EMTU_R
    and sends ICMP PTB w/o wasting tunnel resources
  - protection against fragment misassociations
  - protection against off-path attacks
  - protection against wrapped ip_id
  - backwards compatibility
  - incremental deployment
  - NAT traversal

Please review and send comments.

Thanks - Fred
[email-masked]

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A New Internet-Draft is available from the on-line Internet-Drafts=20
directories.

	Title		: Packetization Layer Path MTU Discovery for=20
                          IP/*/IPv4 Tunnels
	Author(s)	: F. Templin
	Filename	: draft-templin-inetmtu-00.txt
	Pages		: 20
	Date		: 2007-9-18

   The nominal Maximum Transmission Unit (MTU) MTU of the Internet has
   become 1500 bytes, but existing IP/*/IPv4 tunneling mechanisms impose
   an encapsulation overhead that can reduce the effective path MTU to
   smaller values.  Additionally, existing IP/*/IPv4 tunneling
   mechanisms are limited in their ability to discover and utilize
   larger MTUs.  This document specifies new mechanisms for conveying
   packets over IP/*/IPv4 tunnels that address these issues.

A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-templin-inetmtu-00.txt

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