[Freeswitch-users] Presence Sanity Check

Ira Tessler ira at connectmevoice.com
Sat Apr 6 17:28:48 MSD 2013

I just need a little guidance with the way presence works. Forgive me if I
am asking novice questions.

Background (simple version)
We run Freeswitch in a hosted/cloud environment in a data center. We have
IP phones in our office on our LAN.

That way I am understanding how Presence works, I am just learning this, is
that when a BLF button is programmed on a phone, that phone will send a
"Subscribe" message to Freeswitch. The subscriptions are stored in the
sip_subscriptions table (i think) in the sofia database for the sip
profile. When calls come in for that subscription, Freeswitch will send out
a NOTIFY message to the phone that subscribed in order to change the state
of the BLF Light.

He is my questions/issue/confusion.
All our phones use UDP which has a maximum packet size of 1500 bytes. When
doing a sofia global siptrace on, I notice that most of the NOTIFY messages
are greater then 1500 bytes. That will cause packet fragmentation. So if
the NOTIFY message is fragmented, will it get to the phone correctly? (all
the time, some of the time, never??)

If the the answer is other then ("all the time"), how do I fix this? The
only solution I can come up with is having my phones use TCP instead of
UDP. Is that the correctly solution? Did anyone else out there run into
this issue and if so, what is the "best practice" solution (if there is

Thank you in advance!

Ira Tessler
Lead Software Engineer
(732) 490-9007 x2
ira at connectmevoice.com
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