Diagnosing SIP issues: one way audio, gaps in sound, registration errors, SIP ALG

Session Initiation Protocol (SIP) can get quite complicated, especially when products from different suppliers are mixed.  SIP issues are difficult to diagnose!

A SIP call relies on DNS, NTP, routers, switches, SIP proxies, media proxies and end devices.  Each component must work as expected… There are a lot of ways that this can break!

The usual suspects for the weird failure of call setup:

  1. Network MTU and SIP UDP
  2. Router: SIP ALG
  3. DNS failure
  4. NAT / Port Forwarding

The usual suspects for call quality issues:

  1. Network dropping packets
  2. Router load
  3. Prioritising Traffic /QOS

There are a few tools which are very useful in figuring out why a call or message did not flow as expected:

  1. SIP dialogs and transactions
  2. Call flow visualization
    1. homer
  3. Commandline tools
    1. ngrep
    2. sngrep
    3. tcpdump
  4. Network issues
    1. NAT
    2. Packet loss / jitter / latency
    3. wireshark

Automated test environment: SIPP, SIP Treadmill …

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