Re: [tsvwg] Status of ECN encapsulation drafts (i.e., stuck)

Bob Briscoe <> Sun, 15 March 2020 11:12 UTC

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To: Sebastian Moeller <>
Cc: Jonathan Morton <>, "" <>
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From: Bob Briscoe <>
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Date: Sun, 15 Mar 2020 11:12:08 +0000
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Subject: Re: [tsvwg] Status of ECN encapsulation drafts (i.e., stuck)
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Snipped all the comments you're agreeing with. Two responses below...

On 15/03/2020 00:03, Sebastian Moeller wrote:
>> [BB] In the absence of the IP layer solving this problem, Gorry's group found that about 20% of end-systems are just clamping the IPv4 TCP max segment size lower than necessary. And for IPv6 most are just using the min segment size. See:
>> Again only for TCP, middleboxes can edit the MSS advertised in each packet to fool the hosts into using a smaller max segment size.
> 	[SM] Yes, except given the issues with otherwise signaling too large packets a long a path, that might still be the cleanest solution...

[BB] I should add that PLPMTUD and clamping the max packet size 
complement each other.

PLPMTUD without clamping would kill latency.
Clamping avoids triggering fragmentation, but smaller than max packet 
sizes are inefficient.
Adding PLPMTUD eventually bumps up the MTU for each path to its max, 
removing the inefficiency.

>> PLPMTUD solves this problem end-to-end (see [RFC4821] for TCP etc. and [draft-ietf-tsvwg-datagram-plpmtud] not yet published for UDP). However, where a tunnel already solves the problem (sub-optimally) for IPv4 using fragmentation, I can't imagine that anyone would disable it, because it is still necessary and it still works.
> 	[SM] Well transient fragmentation (where the tunnel endpoints create the appearance of no fragmentation) comes at a considerable processing cost, and carrying fragments (especially in degenerate cases like your caida example above) skews the payload/overhead ratio badly (by artificially blowing up the packet rate), so I am confident tunnel operators would be happy to be able to drop that crutch in a heart-beat, IF they could be sure it would not be required anymore (or in a belts and suspender fashion, would keep the mechanism operational but would use PLPMTUD to make it unlikely that fragmentation actually is triggered)
[BB] Just need to point out that the /operator/ cannot use PLPMTUD to 
make triggering fragmentation unlikely. That has to be done at the 
origin sender. That's precisely why operators don't drop this crutch - 
because they can't be sure the sender has their own crutch.


Bob Briscoe