I think I detest fragmentation, or rather re-assembly, as much as anybody, whether designing hardware or software solutions. And of course, in a closely managed environment, you can ensure that the outer MTU is sufficient to contain the inner MTU. That isn't the point. The point is that designing a protocol proposed as a general-purpose protocol for the Internet, that might be used for a hundred years, we can't guarantee that it will always be deployed in such a managed environment. In fact, we can pretty much guarantee that it will be used in completely unmanaged (or badly managed) ways.
This isn't theory. We've seen it a lot where IPv6 has been tunneled across IPv4 islands, with lots of MTU and fragmentation failures. That's even simpler than IP in UDP in IP. Regards Brian On 01/06/2016 16:09, Xuxiaohu wrote: > > >> -----Original Message----- >> From: Brian E Carpenter [mailto:brian.e.carpen...@gmail.com] >> Sent: Wednesday, June 01, 2016 4:01 AM >> To: Xuxiaohu; Joe Touch >> Cc: joel jaeggli; Fred Baker (fred); Wassim Haddad; int-area@ietf.org >> Subject: Re: [Int-area] Call for adoption of draft-xu-intarea-ip-in-udp-03 >> >> On 31/05/2016 20:13, Xuxiaohu wrote: >>> >>> >>>> -----Original Message----- >>>> From: Brian E Carpenter [mailto:brian.e.carpen...@gmail.com] >>>> Sent: Tuesday, May 31, 2016 4:46 AM >>>> To: Joe Touch >>>> Cc: joel jaeggli; Xuxiaohu; Fred Baker (fred); Wassim Haddad; >>>> int-area@ietf.org >>>> Subject: Re: [Int-area] Call for adoption of >>>> draft-xu-intarea-ip-in-udp-03 >>>> >>>> And being pedantic... >>>> On 31/05/2016 06:12, Joe Touch wrote: >>>>> >>>>> >>>>> On 5/29/2016 4:23 PM, joel jaeggli wrote: >>>>>>>> I.e., you MUST support source fragmentation at the ingress at the >>>>>>>> outer >>>>>>>> IPv6 layer (because UDP doesn't have support for fragmentation >>>>>>>> and reassembly). If you make this requirement, you can handle >>>>>>>> IPv6 over the tunnel. >>>>>> Yeah I don't support it for this reason. getting IP fragments back >>>>>> together in the same place a reassembled is hard is in some cases >>>>>> especially when you hash. (see frag drop) given alternatives that >>>>>> better address such situations it seems hard to justify. >>>>> >>>>> If you intend to support recursive IP tunneling* and believe that IP >>>>> has a minimum MTU, then you have to accept reassembly. >>>> >>>> If you intend to support recursive datagram tunneling and believe >>>> that any path has a minimum MTU, then you have to accept reassembly. >>>> >>>> This is physics, and nothing to do with design details. >>>> >>>> (Something I discovered in about 1983, when implementing OSI/CLNP at >>>> CERN over a homebrew network with 128 byte packets.) >>> >>> Reassembly on the tunnel egress may be acceptable at that old time. However, >> due to the considerable improvement in network bandwidth capability, the >> practice acceptable at the old time may have become outdated today. >> >> I don't understand what network capacity has to do with the physical and >> mathematical fact that packets larger than N bytes will not fit into a packet >> limited to N bytes. > > This article > (http://learning.nil.com/assets/Tips-/The-Never-Ending-Story-of-IP-Fragmentation.pdf) > may be useful for you to understand why network capability is a key factor > to be considered for fragmentation and reassembly on routers (here routers > are not software routers or CPU-only routers which were dominant in the old > time). Of course, you could also have a look at the current fragmentation and > reassembly implementations of major router vendors if you believed that > article is a little bit old. > > Xiaohu > >> That was true in 1983 and will still be true in 2083. >> >> Brian >> >>> See the MAP implementation experience shared by Ole recently. >>> >>> Xiaohu >>> >>>> Brian >>>> >>>>> >>>>> Joe >>>>> >>>>> * where "recursive IP tunneling" is IP in [zero or more other >>>>> protocols] in IP. >>>>> >>>>> _______________________________________________ >>>>> Int-area mailing list >>>>> Int-area@ietf.org >>>>> https://www.ietf.org/mailman/listinfo/int-area >>>>> _______________________________________________ Int-area mailing list Int-area@ietf.org https://www.ietf.org/mailman/listinfo/int-area
