Subject: Re: Path MTU discovery
To: Mike O'Dell <mo@UU.NET>
From: Wolfgang Rupprecht <>
List: tech-net
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
*router requirements*?

If you read router requirements (1812) you'll see that it uses the
words "SHOULD" not "MUST". 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    <>