127 lines
5.2 KiB
Text
127 lines
5.2 KiB
Text
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What is the problem with SIP retransmits?
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-----------------------------------------
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Sometimes you get messages in the console like these:
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- "retrans_pkt: Hanging up call XX77yy - no reply to our critical packet."
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- "retrans_pkt: Cancelling retransmit of OPTIONs"
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The SIP protocol is based on requests and replies. Both sides send
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requests and wait for replies. Some of these requests are important.
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In a TCP/IP network many things can happen with IP packets. Firewalls,
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NAT devices, Session Border Controllers and SIP Proxys are in the
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signalling path and they will affect the call.
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SIP Call setup - INVITE-200 OK - ACK
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------------------------------------
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To set up a SIP call, there's an INVITE transaction. The SIP software that
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initiates the call sends an INVITE, then wait to get a reply. When a
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reply arrives, the caller sends an ACK. This is a three-way handshake
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that is in place since a phone can ring for a very long time and
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the protocol needs to make sure that all devices are still on line
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when call setup is done and media starts to flow.
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- The first reply we're waiting for is often a "100 trying".
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This message means that some type of SIP server has received our
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request and makes sure that we will get a reply. It could be
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the other endpoint, but it could also be a SIP proxy or SBC
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that handles the request on our behalf.
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- After that, you often see a response in the 18x class, like
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"180 ringing" or "183 Session Progress". This typically means that our
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request has reached at least one endpoint and something
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is alerting the other end that there's a call coming in.
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- Finally, the other side answers and we get a positive reply,
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"200 OK". This is a positive answer. In that message, we get an
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address that goes directly to the device that answers. Remember,
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there could be multiple phones ringing. The address is specified
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by the Contact: header.
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- To confirm that we can reach the phone that answered our call,
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we now send an ACK to the Contact: address. If this ACK doesn't
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reach the phone, the call fails. If we can't send an ACK, we
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can't send anything else, not even a proper hangup. Call
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signalling will simply fail for the rest of the call and there's
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no point in keeping it alive.
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- If we get an error response to our INVITE, like "Busy" or
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"Rejected", we send the ACK to the same address as we sent the
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INVITE, to confirm that we got the response.
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In order to make sure that the whole call setup sequence works and that
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we have a call, a SIP client retransmits messages if there's too much
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delay between request and expected response. We retransmit a number of
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times while waiting for the first response. We retransmit the answer to an
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incoming INVITE while waiting for an ACK. If we get multiple answers,
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we send an ACK to each of them.
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If we don't get the ACK or don't get an answer to our INVITE,
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even after retransmissions, we will hangup the call with the first
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error message you see above.
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Other SIP requests
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------------------
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Other SIP requests are only based on request - reply. There's
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no ACK, no three-way handshake. In Asterisk we mark some of
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these as CRITICAL - they need to go through for the call to
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work as expected. Some are non-critical, we don't really care
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what happens with them, the call will go on happily regardless.
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The qualification process - OPTIONS
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-----------------------------------
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If you turn on qualify= in sip.conf for a device, Asterisk will
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send an OPTIONS request every minute to the device and check
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if it replies. Each OPTIONS request is retransmitted a number
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of times (to handle packet loss) and if we get no reply, the
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device is considered unreachable. From that moment, we will
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send a new OPTIONS request (with retransmits) every tenth
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second.
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Why does this happen?
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---------------------
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For some reason signalling doesn't work as expected between
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your Asterisk server and the other device. There could be many reasons
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why this happens.
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- A NAT device in the signalling path
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A misconfigured NAT device is in the signalling path
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and stops SIP messages.
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- A firewall that blocks messages or reroutes them wrongly
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in an attempt to assist in a too clever way.
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- A SIP middlebox (SBC) that rewrites contact: headers
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so that we can't reach the other side with our reply
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or the ACK.
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- A badly configured SIP proxy that forgets to add
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record-route headers to make sure that signalling works.
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- Packet loss. IP and UDP are unreliable transports. If
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you loose too many packets the retransmits doesn't help
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and communication is impossible. If this happens with
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signalling, media would be unusable anyway.
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What can I do?
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--------------
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Turn on SIP debug, try to understand the signalling that happens
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and see if you're missing the reply to the INVITE or if the
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ACK gets lost. When you know what happens, you've taken the
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first step to track down the problem. See the list above and
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investigate your network.
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For NAT and Firewall problems, there are many documents
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to help you. Start with reading sip.conf.sample that is
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part of your Asterisk distribution.
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The SIP signalling standard, including retransmissions
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and timers for these, is well documented in the IETF
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RFC 3261.
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Good luck sorting out your SIP issues!
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/Olle E. Johansson
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-- oej (at) edvina.net, Sweden, 2008-07-22
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-- http://www.voip-forum.com
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