I may have missed this, but how are we defining "jumbogram"? Is it anything larger than 1500bytes? Will it still be the same value going forward into the future?
Thanks - Fred [EMAIL PROTECTED] > -----Original Message----- > From: Fred Baker [mailto:[EMAIL PROTECTED] > Sent: Tuesday, July 31, 2007 2:34 PM > To: John Heffner > Cc: Internet Area > Subject: Re: [Int-area] Larger MTUs > > personally, I would detect the speed (Macs do that) and set to 1500 > for 10/100 and 9K for 1000 (and 10000). It might be of value to try > sending a jumbogram to a neighbor, perhaps in the context of an ARP/ > ND request (send a 9K packet containing an ARP request or ND > solicitation and see if you get a response) before doing so. > > On Jul 31, 2007, at 8:47 PM, John Heffner wrote: > > > Fred Baker wrote: > >> On Jul 28, 2007, at 2:05 AM, John Heffner wrote: > >>> The difficult problem is the router's behavior. If a subnet is > >>> running a mixed MTU, it's not clear what that router's interface > >>> MTU to that subnet should be. It would be nice for it to > forward > >>> the largest packets that it can support; however, to be > compliant > >>> with the spec it must generate ICMP PTB messages for any packets > >>> that are too large to be delivered. > >> here's an interesting gotcha. Macs run a 1000/100/10 Ethernet > >> interface and run a 1500 byte MTU/MRU regardless. The router can > >> correctly observe that it is 1 GBPS and send the jumbogram, the > >> switch can support the jumbogram, and have the Mac not accept the > >> packet. > >> Hence, there are variations of this that are beyond the router's > >> knowledge. > > > > Exactly. > > > > > >> I would suggest that the router respond with the ICMP when the > >> packet is too big for the configured interface MTU and not try to > >> predict the end system's or the switch's behavior. 4821 > encourages > >> the sender to use the largest segment size that it can verify > >> delivery of. Leave that to the end system. > > > > The question then is how to configure the router's MTU. The only > > safe way that won't break 1122/1981 for an Ethernet > interface would > > be to set it to 1500 bytes. But then it won't forward larger > > packets so jumbo-capable devices don't get the benefit of jumbo > > frames beyond their local subnet. Setting the router MTU larger > > will allow 4821 probing to work for jumbo-capable hosts, but then > > you risk breaking connectivity to hosts on your subnet with > smaller > > MTUs, from outside hosts/protocols with larger MTUs that rely on > > classical PMTUD (and aren't protected by the TCP MSS option). > > > > -John > > > _______________________________________________ > Int-area mailing list > [email protected] > https://www1.ietf.org/mailman/listinfo/int-area > _______________________________________________ Int-area mailing list [email protected] https://www1.ietf.org/mailman/listinfo/int-area
