Subject: Re: Path MTU discovery
To: Mike O'Dell <mo@UU.NET>
From: Wolfgang Rupprecht <firstname.lastname@example.org>
Date: 07/15/1997 14:03:32
Mike O'Dell writes:
> if you read Host Requirements, MTU Discovery is required
> for a TCP implentaion, no matter how much time it takes.
I see nothing in Host Requirements rfc (1122, 1123 or even 1127) about
MTU discovery being required. Are you by chance talking about the
If you read router requirements (1812) you'll see that it uses the
words "SHOULD" not "MUST".
188.8.131.52 Path MTU Discovery
To eliminate fragmentation or minimize it, it is desirable to know
what is the path MTU along the path from the source to destination.
The path MTU is the minimum of the MTUs of each hop in the path.
[INTERNET:14] describes a technique for dynamically discovering the
maximum transmission unit (MTU) of an arbitrary internet path. For a
path that passes through a router that does not support
[INTERNET:14], this technique might not discover the correct Path
MTU, but it will always choose a Path MTU as accurate as, and in many
cases more accurate than, the Path MTU that would be chosen by older
techniques or the current practice.
When a router is originating an IP datagram, it SHOULD use the scheme
described in [INTERNET:14] to limit the datagram's size. If the
router's route to the datagram's destination was learned from a
routing protocol that provides Path MTU information, the scheme
described in [INTERNET:14] is still used, but the Path MTU
information from the routing protocol SHOULD be used as the initial
guess as to the Path MTU and also as an upper bound on the Path MTU.
You SHOULD eat your vegetables.
You MUST pay for your meal before leaving.
Wolfgang Rupprecht <email@example.com> http://www.wsrcc.com/