On 2/21/12 3:21 PM, Oliver wrote:
> On Tuesday 21 Feb 2012 22:19:19 Etienne Champetier wrote:
>>
>> A plausible example:
>> First default route: optical fiber (MTU 1500)
>> Second default route: pppoe (MTU 1492) (failover link)
>> If the optical fiber goes down, the traffic from the router will pass
>> through the PPPoE link, but with a MSS set to 1500-40 instead of 1492-40
>>
>
> Presuming, for a moment that you are multihomed and actually have the same
> IP usable on either interface (because, if you don't, your problem isn't
> the MSS) the correct solution is for the router to drop the oversized
> packet and send back an ICMP Fragmentation Needed/ICMPv6 Packet Too Big to
> the original sender who will then reduce the transfer size and sliding
> window accordingly.
>
> TCPMSS is ONLY ever needed for cases where someone criminally braindead is
> filtering the ICMP mentioned above, as I believe the manpage states.
There are also a lot of routers out there that *still* don't do PMTU correct
('still', because I worked on the standard more than 20 years ago).
-Philip
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