Hi Chris,
-----Original Message----- From: Chris Woodfield [mailto:rekoil@semihuman.com] Sent: Monday, October 29, 2012 4:40 PM To: Templin, Fred L Cc: William Herrin; Ray Soucy; NANOG list Subject: Re: IP tunnel MTU
True, but it could be used as an alternative PMTUD algorithm - raise the segment size and wait for the "I got this as fragments" option to show up...
Yes; it is a very attractive option on the surface. Steve Deering called it "Report Fragmentation (RF)" when he first proposed it back in 1988, but it didn't gain sufficient traction and what we got instead was RFC1191. As I mentioned, SEAL does this already but in a "best effort" fashion. SEAL will work over paths that don't conform well to the RF model, but will derive some useful benefit from paths that do.
Of course, this only works for IPv4. IPv6 users are SOL if something in the middle is dropping ICMPv6.
Sad, but true. Thanks - Fred fred.l.templin@boeing.com
-C
On Oct 29, 2012, at 4:02 PM, Templin, Fred L wrote:
Hi Bill,
Maybe something as simple as clearing the don't fragment flag and adding a TCP option to report receipt of a fragmented packet along with the fragment sizes back to the sender so he can adjust his mss to avoid fragmentation.
That is in fact what SEAL is doing, but there is no guarantee that the size of the largest fragment is going to be an accurate reflection of the true path MTU. RFC1812 made sure of that when it more or less gave IPv4 routers permission to fragment packets pretty much any way they want.
Thanks - Fred fred.l.templin@boeing.com