Hi Joe, > -----Original Message----- > From: Joe Touch [mailto:[email protected]] > Sent: Tuesday, June 25, 2013 8:10 PM > To: Templin, Fred L > Cc: Carlos Pignataro (cpignata); Ronald Bonica; Internet Area > Subject: Re: [Int-area] New Version Notification for draft-bonica- > intarea-gre-mtu-00.txt > > Hi, Fred, > > On 6/25/2013 3:32 PM, Templin, Fred L wrote: > > Hi Joe, > > > > The constraint you seem to be missing is that RFC2460 only requires > > IPv6 nodes to reassemble as much as 1500 bytes. > > Right; sorry for not noticing that further. Had IPv4 on the brain.
OK, but IPv4 also has a limit of minMRU=576. So, we have: IPv4 minMTU = 576 (*) IPv4 minMRU = 576 IPv6 minMTU = 1280 IPv6 minMRU = 1500 (*) Even though the specs say that IPv4 minMTU = 68, everyone seems to be saying that for practical purposes it is now 576. > ... > > Granted, my algorithm is asking GRE egress tunnel endpoints to > > configure a reassembly buffer size of 1500 *plus* the tunnel > > encapsulation overhead, which is more than RFC2460 ensures. > > Agreed. Right. So, then the fragmentation threshold for GRE over IPv6 needs to be (1500 - HLEN) instead of 1500. > ... > > Also think about it this way. Why should the tunnel work hard to > > pass a packet that is larger than 1500 bytes and also larger than > > the minimum link MTU within the tunneled path when the source could > > instead adjust the size of the packets it is sending downward? > > That's not a good argument. I agree that all that need be satisfied is > the largest reassembled IPv6 packet, but not that we should assume the > source can or will adjust downward - if that were the case, you could > ignore the overhead of GRE and just claim that the MTU is 1280. In my lab, my implementation sends a single packet for packet sizes no larger than (1280 - HLEN). For packets that are larger than (1280 - HLEN) but no larger than 1500, it fragments the packet into two pieces and sends both pieces to the tunnel far end which then needs to reassemble. But, reassembly will be delayed by the inter-packet delay plus the time it takes to run the reassembly routine, and the delay is measurable. So, we really only want to use frag/reass when we absolutely have to. Thanks - Fred [email protected] > Joe _______________________________________________ Int-area mailing list [email protected] https://www.ietf.org/mailman/listinfo/int-area
