[SIPForum-discussion] SIP-rport & Received parameter in via

[NK]

Hi Alex,

Please find below, hope that will help you.

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.

Regards,
Nitin Kapoor


On Wed, Apr 15, 2015 at 11:55 PM, Alex Voip <alex24voip at gmail.com> wrote:

> Hello All,
>
> I am learning SIP protocol and I have couple of questions. Could you
> please take a look at these questions?
>
> 1. How c-seq count is used by UAS to determine out of sequence request
> 2. How registration can be removed with a Retry-After header
> 3. Use of rport and received parameter used in the via header field
> 4. Difference between loose routing and strict routing
> 5. Use of q parameter in the contact header field
>
> Thanks in advance!!
>
> - Alex
>
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