[sr-dev] [kamailio/kamailio] icmp failover (#2377)

[Henning Westerholt]

Well, there are some requirements in the RFC 3261 about it (see below). But they are only SHOULD requirement.

It is probably difficult to solve it in a portable and general way, e.g. 
- [1](https://josephmuia.ca/2018-04-25-connected-udp-sockets-and-icmp-errors/)
- [2](https://github.com/gnuradio/gnuradio/issues/1769)
- [3](http://web.deu.edu.tr/doc/soket/) - part 5.4

I have nothing against to tracking this as a feature request, but only in case if somebody wants to start working on it.

```

8.1 UAC behaviour
8.1.3.1 Transaction Layer Errors

   In some cases, the response returned by the transaction layer will
   not be a SIP message, but rather a transaction layer error. [..]
   If a fatal transport error is reported by the transport layer
   (generally, due to fatal ICMP errors in UDP or connection failures in
   TCP), the condition MUST be treated as a 503 (Service Unavailable)
   status code.

18. Transports

18.2.2 Sending Responses
[..]
      o  Otherwise (for unreliable unicast transports), if the top Via
         has a "received" parameter, the response MUST be sent to the
         address in the "received" parameter, [..]
         If this fails, for example, elicits an ICMP "port
         unreachable" response, the procedures of Section 5 of [4]
         SHOULD be used to determine where to send the response.

18.4 Error Handling

   Error handling is independent of whether the message was a request or
   response.

   If the transport user asks for a message to be sent over an
   unreliable transport, and the result is an ICMP error, the behavior
   depends on the type of ICMP error.  Host, network, port or protocol
   unreachable errors, or parameter problem errors SHOULD cause the
   transport layer to inform the transport user of a failure in sending.
   Source quench and TTL exceeded ICMP errors SHOULD be ignored.

   If the transport user asks for a request to be sent over a reliable
   transport, and the result is a connection failure, the transport
   layer SHOULD inform the transport user of a failure in sending.
```

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