[RRG] Critique of Sprite-MTU: PMTUD, fragmentation etc.

[Robin Whittle <rw@firstpr.com.au>]

I have a revised critique of Fred Templin's proposal:

  http://tools.ietf.org/html/draft-templin-inetmtu-05

at my IPTM (ITR Probes Tunnel MTU) page:

  http://www.firstpr.com.au/ip/ivip/pmtud-frag/#sprite_critique

The three elements of the critique apply to the use of Sprite-MTU in
an ITR-ETR system (at least for IPv4).  The third element may be
applicable to the many other uses which Sprite-MTU is intended to
help with.

In brief (please read the full critique before responding), they are:

  1 - The ITR has to set up state even when a single short
      (too short to be a problem for MTU limits) packet is
      tunneled to an ETR.

  2 - The ITR and ETR have to continually engage in Sprite-MTU
      communications as long as packets are tunneled to the ETR -
      and this should arguably start with the first packet.

Both these place a huge load on the ITR, for just a single packet -
which doesn't even need any MTU management.  My proposal avoids such
overheads, by being triggered only by longer packets.

  3 - The initial response of the ITR to a longer packet is to
      send back a Packet To Big message to the sending host and
      to tunnel the packet.  The sending host will then resend
      the packet (and probably subsequent packets) in multiple
      small fragments - but the original packet may arrive at
      the destination host and be acknowledged.  This contradictory
      response is likely to cause trouble for sending hosts,
      including those implementing RFC 4821, as well as being a
      source of extra packets, confusion etc.


This tunneling, MTU, fragmentation, PMTUD stuff is really challenging.

So far, Fred's proposal and my own IPTM (ITR Probes Tunnel MTU) are
the only two which are applicable to the ITR-ETR schemes we are
discussing on this list.

I would really appreciate some critical assessment of IPTM, and
likewise some confirmation or otherwise of my critique of Sprite-MTU.

  - Robin

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