[Asterisk-Users] MTU and Voice Delay (latency??)

[Rich Adamson]

>> Absolutely not. The MTU is the Maximum Transmission Unit, and sip
>> packets are about 214 bytes in size (including all pkt headers). Way
>> smaller then the MTU.
>>
> 
> If the only thing on my network are these Cisco Phones, would lowering the
> MTU encourage more efficient transfer of data as per here:

No. The reason is that "if" the phones are the only thing on this, the 
size of the sip packets will never be greater then 214 bytes.  The mtu 
value would need to be set to something smaller then 214 bytes in order 
for the parameter to have any impact whatsoever. If you selected 150 
bytes (for example only), the sip/rtp packets would be fragmented into 
smaller pieces forcing the destination device to reassemble those 
fragments. (I'm not sure, but I'd have to guess the Cisco's don't even 
support udp fragmentation.) Fragmentation is generally considered a 
high-cost overhead and should be avoided when possible.

Given your table below, there "are" other devices on your network and 6% 
  of those are sending packets of in the 512 to 1023 byte range. If you 
set an mtu of 512, then _those_ devices would be forced to fragment 
larger packets, giving the sip/rtp packets a greater chance of getting 
onto the external network. If your sdsl circuit is running 256kb/s, an 
mtu value of 512 would generate "at best" a 16 millisecond benefit 
roughly 6% of the time. At all other times it would generate no 
improvement whatsoever. If the sdsl circuit is operating at a speed 
greater then 256k, the best case improvement would be smaller (eg, 8 ms 
for a 512k circuit).

> http://www.voiptroubleshooter.com/problems/mtu.html
> http://www.opalsoft.net/qos/VoIP.htm
> 
> 
> Here is the breakdown of packets on the port that has the SDSL modem:
> 
> SDSL Router
> 				Amount	% of Whole
> 64 BytePkts			0		0%
> 65-127 BytePkts		20257		1%
> 128-255 BytePkts		3623518	91%
> 256-511 BytePkts		237087	6%
> 512-1023 BytePkts		112900	3%
> 1024-1522 BytePkts	48		0%
> Total				3993810	100%
> 
> The larger packets I'm sure are the bootloader stuff and config file
> downloads, etc.

That's very possible.

> I do feel like I am reaching for straws here!

You're reaching for _air_ by even thinking about mtu as an issue. In 20+ 
years of professional network management and performance assessments, 
the only time mtu is ever brought up, its generally by some first-year 
technical type that doesn't have a clue (that's not directed to you 
either). (FWIW, mtu adjustments use to be fairly popular for those 
attempting to balance response time vs throughput on 28kb dialup circuits.)

If you don't believe any of the above, go ahead and change the mtu and 
see for yourself. Bet the phones don't even work at all.

> Just as an update, the users used to be on two 2mb down/512 up ADSL lines
> (PPPoE) (4 users on each) and they never reported a problem. Now that they
> are on one SDSL (PPPoA) line (2mb) is when they report the issues.

There is something else going on besides adsl vs sdsl, and mtu is not 
even close to being the root cause.

Have you tried the previous suggestion relative to two simultaneous ftp 
sessions?

What city/state are you located in?