Niels Bakker wrote:
* sam_mailinglists@spacething.org (Sam Stickland) [Tue 03 Feb 2009, 13:04 CET]:
For what it's worth, TCP will negiogate MSS and will work with mismatched MTU in a single LAN segment.
No
Machine 1 -- switch with 1500 byte MTU -- switch with smaller MTU -- switch with 1500 byte MTU -- machine 2
Same situation as when you have IP routers with smaller MTUs in the path that also do not send ICMP Fragmentation Needed errors (or those are dropped on the way to you)
If you configure one of those machines with an MTU equal to or smaller than the smallest MTU in the path then yes TCP (assuming MSS option is used) won't send packets that happen to be too big, but again, same story as for routers vs on a LAN. The problem isn't that machine 1 and 2 in the above example disagree on MTU, the problem is that equipment in the path disagrees on the MTU and cannot (in the case of switches) send notifications of such, or those will not arrive (in the case of stupid firewall admins in control of networks). Sorry, I should had clarified. I meant mismatched host MTUs within a jumbo MTU supporting L2 domain.
Sam