[sr-dev] a bit about dialog module

[Daniel-Constantin Mierla]

Hello,

the internal dialog id - the pair h_entry,h_id - seems exposed to some 
issues. They:
- h_entry is the slot index (in the internal hash table) where the 
dialog structure will be stored
- h_id is an incremented counter for each slot

h_entry is computed as a hash function over Call-Id value, then made 
modulo nr. of slots.

The problems can appear upon restart when changing the hash_size module 
parameter:

http://kamailio.org/docs/modules/stable/modules_k/dialog.html#id2534423

If it is decremented, then some dialogs might be left outside since 
their h_entry might not be valid anymore. Also, when hash_size is 
changed, for matching on SIP dialog attributes of ongoing calls, the 
hash function over callid made modulo over the new nr. of slots may get 
a different hash table slot index, resulting in dialog structure not 
being found.

A very unlikely issue is caused by the h_id, which being incremented can 
overflow and start from 0 again. But will overlap if an dialog is still 
going on after 2^32 new dialogs on that hash table slot were initiated.

I wanted to brought all in the discussion to make a decision. I see few 
options for the moment:
1) document this issue, so a restart with change of hash_size parameter 
must have 0 active dialogs
2) replace h_entry with the value of the hash function over call id, 
doing modulo operation each time to find the slot. The unlikely issue 
with overflow of h_id is still there
3) switch over another kind of unique id (sriud.{c,h} added recently in 
lib/srutils/ can be an option). This will imply changing the database 
structure

Besides these, might be better to drop using the Record-Route header 
parameter to carry dialog id and just do SIP based dialog matching. 
Completely trusting those tokens in RR parameter might add some security 
risks and if there is a fallback to SIP-based dialog matching, using the 
shortcut with RR does not impact anymore that much.

Opinions and other suggestions?

Cheers,
Daniel

-- 
Daniel-Constantin Mierla
Kamailio Advanced Training, April 23-26, 2012, Berlin, Germany
http://www.asipto.com/index.php/kamailio-advanced-training/