Subject: Re: Comments on draft-ietf-tcpimpl-pmtud-01.txt

From: Vernon Schryver <[email-masked]>
Date: Thu, 27 May 1999 19:11:31 -0600 (MDT)

> To: [email-masked]
> Subject: Comments on draft-ietf-tcpimpl-pmtud-01.txt
> From: Geert Jan de Groot <[email-masked]>

> >3.3.   Determining MSS from PMTU
> ...
> >     The MSS advertised at the start of a connection should be based on
>       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >     the MTU of the interfaces on the system.  Some systems use PMTUD
>       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >     determined values to determine the MSS to advertise.
> >     This results in an advertised MSS that is smaller than the largest
> >     MTU the system can receive.
>
> I would like to change the marked sentence with:
> The MSS advertised at the start of a connection should be based on
> the largest MTU of all interfaces on the system.
>
> Rationale: assume a TCP connect happens over an ethernet interface.
> While the TCP session is alive, routing changes and the session is
> now routed over an FDDI interface. If mss would have been 1460 (ethernet),
> we would see micrograms on the FDDI interface unneccesary.
> I just want to make the original text more explicit, that's all.
> ...

I disagree.  In real life, the initial interface is almost always the only
interface used during the life of a TCP connection.  With the proposal,
almost every TCP connection to a multi-homed host using and an interface
with a smaller MTU would suffer the overhead PMTU probing throughout the
life of the connection.  That is because people generally try to arrange
routing so that their big MTU interfaces are prefered.  Since most TCP
connections are local and high speed, the proposal would have eggregious
effects on performance in the common case.  The proposal would solve a
rare problem at the cost of significant harm to the common case

(Yes, of course, both rare and common modulo the rarity of mult-homed hosts).

The better solution is to do what most people already do, and arrange to
have uniform-MTU interfaces on a system, or arranging routing (e.g. by
router discovery) to prefer routes though big-MTU interfaces.

Vernon Schryver    [email-masked]

---

From: Vernon Schryver <[email-masked]>
Date: Thu, 27 May 1999 22:44:17 -0600 (MDT)

A universal 64K MSS?  wow.

Does no one else see the contradictions in simultaneously

  - proposing something that would significantly increase the use of
     PTMU Discovery and in cases where it is almost always a useless
     almost always a performance hit, 

  - in a document that is largely concerned with problems in PMTU Discovery
      that have no pleasant solutions

  - and also arguing against one of the not entirely pleasing hacks to
      deal with PMTU Discovery problems but in favor of manual diagnostic
      techniques

  - while adding get another stanza to the litany of real life problems of
      PMTU Discovery.

Perhaps I don't understand what's being said, but if I do, it seems to me
people ought to step back and look at the whole picture before fomenting
TCP standards changes.

I think PMTU Discovery is a Good Thing(tm), but too much of a Good Thing(tm)
is often a Bad Thing(tm).