[c-nsp] ip tcp adjust-mss

Mack McBride mack.mcbride at viawest.com
Tue Feb 12 12:31:00 EST 2013

There are always corner cases.
That's why I said most.

LR Mack McBride
Network Architect

From: Alexander Arseniev [mailto:ecralar at hotmail.com]
Sent: Tuesday, February 12, 2013 9:03 AM
To: Mack McBride; moua0100 at umn.edu; cisco-nsp at puck.nether.net
Subject: RE: [c-nsp] ip tcp adjust-mss

> From: mack.mcbride at viawest.com<mailto:mack.mcbride at viawest.com>
> To: moua0100 at umn.edu<mailto:moua0100 at umn.edu>; cisco-nsp at puck.nether.net<mailto:cisco-nsp at puck.nether.net>
> Date: Mon, 11 Feb 2013 16:19:35 -0800
> Subject: Re: [c-nsp] ip tcp adjust-mss
> Most UDP should not hit the MTU limitation.
> The common ones that come to mind are streaming audio/video and DNS.
> LR Mack McBride
> Network Architect
Wrt "streaming audio/video" - it depends on client/server combo.
I recently saw an issue with RTSP video streaming from repubblica.it website using Samsung Galaxy RSTP client.
If UDP video packets are fragmented due to too small MTU in the path, video does not play at all.
And DNS is somewhat invalid example since when DNS reply does not fit into 512 bytes, then DNS server will set a Truncated bit in the packet, forcing client to use TCP.
No one uses 512B MTU (apart from miltary who use even smaller MTU) and DNS over UDP would not be experiencing issues due to too small MTU because own DNS payload limit is smaller than smallest real MTUs out there (except military as I mentioned).

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