Re: [v6ops] new draft: draft-generic-v6ops-tunmtu

[Joe Touch <[e-mail]>]

On 6/25/2012 9:20 AM, Templin, Fred L wrote:
> Hi Fred,
>
>> -----Original Message-----
>> From: Fred Baker (fred) [mailto:[e-mail]]
...
>> If you want to go there, there are some 20-year-old discussions I need for
>> you to look at. Options proposed include at least
>>    - someone (a router, a host) fragmenting traffic should make all chunks
>> the same size
>>    - the first N-1 chunks should be approximately MTU-sized and the last a
>> caboose
>>    - the last N-1 chunks should be approximately MTU-sized and the first a
>> caboose
>
> I guess you are talking about p.45 of RFC1812? I've looked
> at that many times and have always wondered about the option
> to lead with a caboose as the first fragment. As a receiver,
> I'd like to be able to look at the first fragment as a way
> of measuring the path MTU, but I don't get that option when
> the first fragment is tiny.

Since there is no requirement that the first fragment be "as large as 
possible", it is inappropriate to infer anything from its size anyway.

>> There have been passionate discussions, each approach having proponents
>> with believed-by-them-to-be-good arguments for their position. And I'm
>> pretty sure there were other positions.
>>
>> The one thing I'll take serious exception to is a proposal that a router
>> reassemble packets and then re-fragment some other way in the normal
>> course of doing business.
>
> Per RFC1812, I agree for packets that are just passing
> through the router, but the router has to reassemble packets
> that are explicitly addressed to itself.

I don't think there's any disagreement there. There are two kinds of 
packets "router boxes" process:
	forwarded packets (when the box acts like a router)
	packets originating/terminating at the router (when the box acts
		like a host)

Note that tunnel ingress/egress processing falls into the latter 
category for the outer packet, and the former for the inner packet.

> RFC1122 also says
> that the router only needs to reassemble 576 at minimum
> so that is all that the tunnel near end can assume if the
> tunnel far end is (or might be) a router.

RFC1122 applies only to hosts (and routers when they act as a host, 
e.g., as a source or destination of packets). And yes, 576 is the 
largest reassembled MTU for IPv4 (the largest MTU, e.g., that you can 
assume for fragments, is 64).

Joe