[c-nsp] ip tcp adjust-mss

[Eric A Louie]

...and, after months of doing nothing and leaving the adjust-mss command in, I actually forgot why they were there, and what the effect was, and even that it was 40 bytes smaller than the MTU.  So I ended up breaking the TCP flows that were marked DF again, and in the end, I'm taking the "high road" of raising the MTU on all my MPLS links to 9100, with the understanding that I won't be dropping any more DF packets on my backbone.  Won't have to worry about non-TCP packets in this case.  Might solve some other issues that I've been seeing recently, too.

Thanks to all who contributed to my education.





>________________________________
> From: Mack McBride <mack.mcbride at viawest.com>
>To: Randy <randy_94108 at yahoo.com> 
>Cc: "cisco-nsp at puck.nether.net" <cisco-nsp at puck.nether.net> 
>Sent: Thursday, February 14, 2013 9:44 AM
>Subject: Re: [c-nsp] ip tcp adjust-mss
> 
>
>Reality is UDP matters.
>
>LR Mack McBride
>Network Architect
>
>-----Original Message-----
>From: Randy [mailto:randy_94108 at yahoo.com] 
>Sent: Wednesday, February 13, 2013 11:14 PM
>To: Mack McBride
>Cc: cisco-nsp at puck.nether.net
>Subject: Re: [c-nsp] ip tcp adjust-mss
>
>HUH!
>
>What corner cases??
>This thread was about TCP:(ip tcp adjust-mss)not UDP!
>As, has been already pointed out:
>
>To OP:
>
>Put in the little-effort required to UP MTU's on your physical-interfaces to support your services and as already-mentioned, design/re-design your network accordingly.
>
>"ip-mtu and mtu(physical-int) are two very different things.
>./Randy
>
>
>
>
>
>--- On Tue, 2/12/13, Mack McBride <mack.mcbride at viawest.com> wrote:
>
>> From: Mack McBride <mack.mcbride at viawest.com>
>> Subject: Re: [c-nsp] ip tcp adjust-mss
>> To: "Alexander Arseniev" <ecralar at hotmail.com>, "moua0100 at umn.edu" 
>> <moua0100 at umn.edu>, "cisco-nsp at puck.nether.net" 
>> <cisco-nsp at puck.nether.net>
>> Date: Tuesday, February 12, 2013, 9:31 AM 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.
>> http://serverfault.com/questions/348399/force-forwarder-dns-requests-t
>> o-tcp-mode 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).
>> Thanks
>> Alex
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