Re: [NANOG] Microsoft.com PMTUD black hole?

[Iljitsch van Beijnum <iljitsch () muada com>]

On 6 mei 2008, at 23:48, Valdis.Kletnieks () vt edu wrote:

> > A more common approach is to rewrite the MSS option in all TCP SYNs
> > with a smaller value so there won't be TCP segments large enough to
> > trigger the problem. AFAIK, all boxes that do PPPoE do this.

> And just the other day, you were saying:

> > Very few people out there use an MTU significantly below 1500  
> > bytes. A
> > 1500-byte MTU will give you an _average_ packet size of ~1000 on  
> > long-
> > lived TCP flows because there is one tiny ACK for every two full size
> > data segments.

Right. Why is that noteworthy?

I have a lot more to say about MTU issues in this draft about  
negotating MTUs between two hosts/routers on a subnet so jumboframes  
can be deployed without manual configuration:

http://www.ietf.org/internet-drafts/draft-van-beijnum-multi-mtu-02.txt

> Apparently, there's a *reason* why RFC1122, section 3.3.3 says:

>         It is generally desirable to avoid local fragmentation and to
>         choose EMTU_S low enough to avoid fragmentation in any gateway
>         along the path.  In the absence of actual knowledge of the
>         minimum MTU along the path, the IP layer SHOULD use
>         EMTU_S <= 576 whenever the destination address is not on a
>         connected network, and otherwise use the connected network's
>         MTU.

Tell it to Microsoft and their ICMP-filtering friends...

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