[Serusers] More on load balancing: LVS + SRVs, SER + Asterisk

[Pietro Ravasio]

Hi all,

I've read different approaches towards load balancing, expecially in these
two threads:
http://lists.iptel.org/pipermail/serusers/2005-May/019107.html
http://lists.iptel.org/pipermail/serusers/2005-April/018058.html

Unfortunately it seems that at the moment there's not a "state of the art"
solution, so here is my "two cent" idea.

Project goal: to develop a near-linearly scalable platform with NAT
traversal support and call control.

Proposed solution: multiple instances of SER will be reacheable via an
unique virtual IP handled by LVS (with udp persistence), all SERs will use
the same MySQL backend, an array of Asterisk servers will act as SBCs
(Session Border Controller).

Every SER will have a "third level" SIP domain associated to it, for
instance UA_A will be registered as UA_A at ser1.domain.com and UA_B will be
registered as UAC_B at ser2.domain.com. UAC_A will be able to call UA_B as "B"
or as "B at domain.com"; SRV records for "domain.com" will be LVS virtual IP,
SRV records for third level domain will be internal, not visible to Uas and
associated to IP addresses:ports of every SER instance.

To achieve this, we will need to store a domain for any UA in the "location"
table, any instance of SER will do this, UAs don't need to know anything
about "internal" domains. (Is this to be considered an "hack"?)

## Without call control, there would be only one call leg:
When UA_A calls UA_B, its INVITE will reach the same SER it has previously
registered to (SER_1, thanks to LVS persistence), SER_1 will then look for
UA_B and will notice that it is registered as UA_B at ser2.domain.com, DNS will
be queried and SER_2 will be contacted in order to handle INVITE and send it
to UA_B through already opened UDP channel (resolving NAT).

## Instead, we want to be able to control the call process and to handle RTP
nat too, so let's use Asterisk:
-First Leg (left): UA_A calls UA_B, INVITE reaches SER_1 (LVS persistence),
SER_1 finds out that UA_B is @ domain ser2.domain.com but its dialplan tells
him to send the INVITE to one of the Asterisk servers (one of them). SER
modifies the INVITE  sent to * with Contact: (ip:port) of SER_2.
-Second Leg (right): Asterisk (after obtaining authorization and call
duration from AAA server) will look for UA_B at SER_2 and send the INVITE to
SER_2 with SDP modified for the RTP, SER_2 will then forward INVITE (solving
NAT problems) to UA_B ip:port (parameters taken from memory or DB).

This kind of platform would permit call forking too: if UA_B were registered
through two different proxies SER_1 would fork the call to two different
"call instances" of Asterisk(s). 

Note that we want Asterisk to mangle SDP only when both UA_A and UA_B have
private IP addresses

Have you got any suggestion/comment? (expecially regarding:
- more SERs using the same MySQL backend
- binding a third level domain at the proxy

Thanks and kind regards!
Pietro Ravasio


--
Ing. Pietro Ravasio - pietro.ravasio at abbeynet.it
Enterprise Market Manager
Abbeynet S.p.A. - http://www.abbeynet.it
TEL: +390702109044
GSM: +393282172055
FAX: +390702109055