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Re: [RRG] Fwd: Tunnel MTU



I must confess that I don't like loading up tunnel end-points
with this kind of complexity. Architecturally, I much prefer
the e2e RFC 4821 approach. That isn't exclusive to TCP.
In fact there's a fairly simple approach to MTU discovery for
UDP based protocols that have any kind of feedback from destination
to source (i.e. start small and increase the packet size until
the far end detects fragmentation or loss). 4821 does discuss this
briefly.

One could also think about a generic non-ICMP UDP echo process
for PLPMTUD, although that does raise DoS and firewall issues.

Regards
   Brian Carpenter
   University of Auckland


On 2007-09-21 09:50, Templin, Fred L wrote:
(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
fred.l.templin@boeing.com

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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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