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Re: [RRG] Re: [RAM] Tunneling overheads and fragmentation
On 2007-09-12 01:33, Robin Whittle wrote:
On the RAM list, Iljitsch van Beijnum wrote:
I'm still operating under the assumption that both those places are in
ISP networks. Now obviously there are lots of places in ISP networks
that only support 1500-byte packets,
Can somebody provide evidence for this statement?
but what would be a better decision
here: push out a reduced packet size EVERYWHERE which we probably won't
be able to raise any time soon, or require ISPs to either:
1. Implement path MTU discovery correctly, or:
2. Make sure that all encapsulated packets travel over paths that
support at least 1500 byte + encapsulation sized packets
All these options look impossible or ugly to me.
The first one has been eluding us for years, but the network
still works. What's the evidence on actual deployment? (Also
see below.)
The second one sounds like something that is in the ISPs'
enlightened self interest, in which case it will happen.
...
The trouble is that these proposals (Ivip etc.) all involve
tunneling and it is not clear to me how to support PMTUD in the
tunneled portion of the path without efforts which I think are
unsustainable.
For those proposals (all other than Ivip) where the tunneled packet
has an outer header Source Address (SA) which is that of the ITR,
full support of PMTUD would require extreme efforts by the ITR,
caching each recently sent packet, to match ICMP packets coming back
from the tunneled section of the path to the original packet which
was received,
Hence RFC 4821, which does need to get deployed.
Brian
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