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In reality, an ALG may often take quite a bit of license with your SIP connection. It can rewrite many of the SIP fields in order
to comply with its rules, so the IP and port information getting to the service provider may actually be completely different
than those sent by the device. As long as it has the intelligence to open the proper ports, this will usually work fine.
It’s even possible that your SIP connection is being processed by more than one ALG, as in the instance of a separate router
and firewall on the connection. Of course in this scenario, the possibilities for errors compound. Sometimes it’s best to disable
unnecessary ALGs in the link. Unfortunately, diagnosing these issues require analyzing packet captures. Luckily, SIP is a well-
known protocol that can be easily deciphered by packet capture systems.
Summary
The important elements of SIP are as follows:
1
An independent connection stays open on UDP 5060 between the user and the service provider
2
Separate and multiple RTP sessions are established in each direction for calls
3
Routers and firewalls interfere with these RTP sessions by design, but ALGs built into these
devices can help.
PBXs
So far we’ve discussed SIP connections to outside or “cloud” VoIP providers. But many times, the user already has a SIP PBX
on premises, which already connects to the public telephone network by VoIP or legacy means, like analog lines or T1s. Since
most modern PBXs talk SIP to their extensions, they just need to tie a SIP-compatible device (like a codec or hybrid) to the
PBX, and allow the PBX to decide how to route calls to the device.
As mentioned before, the SIP protocol used in this scenario is the same. The device will register and maintain a SIP connection
to the PBX, and the PBX will inform the device of incoming calls. RTP channels will be created when required between the
SIP device and the PBX. This will usually be successful, since the LAN environment is less reliant on routers, subnets and
firewalls to block the RTP channels.
Summary of Contents for VH2
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