Some SIP stuff

1. How c-seq count is used by UAS to determine out of sequence request

 

# CSeq numbering is independent for requests originated by each end. Note that while the UAC is expected to use consecutive values, the UAS is expected to allow gaps in the numbering. This is needed in case messages get lost along the way.

2. How registration can be removed with a Retry-After header .

 

# This question is not clear to, however if you are asking in the terms of SIP Proxy here you go.

 

An SIP Proxy Server which is overloaded or administratively disabled MAY generate a 503 Service Unavailable response to a REGISTER request, and SHOULD include a Retry-After header value indicating how long before the SIP-PBX SHOULD re-attempt the request to the same SIP Proxy Server.  An SIP-PBX receiving such a response MUST support the Retry-After header, and MUST honor the value as follows: if the value is 32 seconds or less, it MUST wait the requested time and retry the request to the same SIP Proxy Server; if the value is larger, it MUST remember the value for that SIP Proxy Server address instance, and try any alternate SIP Proxy Server addresses it can.  If an alternate SIP Proxy Server can be successfully reached and Registration succeeds through the alternate, the SIP-PBX MAY discard the Retry-After value of the original.  Otherwise, it MUST wait to reattempt Registration to the original SIP Proxy Server for the Retry-After value in seconds. 

 

3. Use of rport and received parameter used in the via header field

 

# When a server attempts to send a response, it examines the topmost Via header field value of that response. If the "sent-protocol" component indicates an unreliable unicast transport protocol, such as UDP, and there is no "maddr" parameter, but there is both a "received" parameter and an "rport" parameter, the response MUST be sent to the IP address listed in the "received" parameter, and the port in the "rport" parameter. The response MUST be sent from the
same address and port that the corresponding request was received on. 

 

4. Difference between loose routing and strict routing

 

# Strict Routing rewrote the Request-URIii. That means the Request-URI always contained URI of the next hop (which can be either next proxy server which inserted Record-Route header field or destination user agent). Because of that it was necessary to save the original Request-URI as the last Route header field.

 

# Loose routing,  works in a little bit different way. The Request-URI is no more overwritten, it always contains URI of the destination user agent. If there are any Route header field in a message, than the message is sent to the URI from the topmost Route header field. This is significant change--Request-URI doesn't necessarily contain URI to which the request will be sent. In fact, loose routing is very similar to IP source routing.

5. Use of q parameter in the contact header field

 

# "q"  & "expire" parameters are only used when the Contact is present in a  REGISTER request or response, or in a 3xx response.  Additional
  parameters may be defined in other specifications. 

 

The "q" parameter indicates a relative preference for the particular Contact header field value compared to other bindings for this address-of-record.