Fernando, > -----Original Message----- > From: Int-area [mailto:[email protected]] On Behalf Of Fernando Gont > Sent: Monday, September 09, 2019 1:47 PM > To: Joe Touch <[email protected]>; Bob Hinden <[email protected]> > Cc: [email protected]; [email protected]; IESG > <[email protected]>; Suresh Krishnan <[email protected]> > Subject: Re: [Int-area] Discussion about Section 6.1 in > draft-ietf-intarea-frag-fragile > > Hi, Joe, > > Just one nit: > > On 7/9/19 20:35, Joe Touch wrote: > > FWIW, in general: > > > > With all the concern not detecting when frag fails, I’d like to point out > > that it’s equally impossible to detect when it works, e.g., when > it happens on tunnels that start more than one hop away or more than one > layer of intermediate headers. > > > > E.g, PLPMTUD turns of frag *on the connected interface*. There’s no way to > > disable source fragmentation that happens later in the > network (as it would at tunnel ingresses) or deeper in the stack (when what > you think is your interface is locally tunneled over a layer > you don’t even know about). > > > > So *all* systems that try to backoff and use smaller MTUs are actually > > *already* testing whether fragmentation already works in > those cases. Even if your app sends a 1-byte packet you have no idea that > some set of layers inflates the headers (e.g., with > signatures or key exchanges) beyond the MTU somewhere. > > This would seem to be incorrect. IP has a minimum MTU of 68 bytes, and > IPv6 has a minimum MTU of 1280. Hence if you send packets smaller than > or equal to the minimum MTU, the packets should go through.
Even if the original source uses the IPv6 minimum MTU of 1280, a tunnel somewhere further down the path could add encapsulations that would cause the (encapsulated) packet to exceed 1280 bytes. The tunnel therefore has to apply fragmentation. Fred > -- > Fernando Gont > SI6 Networks > e-mail: [email protected] > PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492 > > > > > _______________________________________________ > Int-area mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/int-area _______________________________________________ Int-area mailing list [email protected] https://www.ietf.org/mailman/listinfo/int-area
