Huawei U-SYS MRS6100 Technical Manual page 69

Technical Manual
U-SYS MRS6100 Media Resource Server
When the request from the point 10.0.0.1 passes a NAT point with the external
address
softx3000.bell-telephone.com. Noticing the inconsistence between the previous hop's
address and the Via field address, the proxy server adds the actual sending address,
as a receiver's tag, at the end of the top Via and then adds its own address in a new
Via that is put at the topmost.
If the proxy server sends a request to multicast address, the proxy server must add
the maddr parameter in the Via to indicate that.
The proxy server or UAC complies with the following rules to process the received
Via:
Rule 1: The first Via field should indicate the proxy server or the UAC itself. If not,
the proxy server or UAC discards the message. Otherwise, the proxy server or
UAC deletes the Via field.
Rule 2: If there is no a second Via field, the response should reach the
destination. Otherwise, proceed as follows.
Rule 3: If the second Via field contains the maddr parameter, the proxy server or
UAC sends the response according to the multicast address indicated in the
parameter. The used port number is specified in the sent-by parameter. If not
specified, the proxy server or UAC uses a port number 5060. The lifetime of the
response should be specified in the ttl parameter. If not specified, the proxy
server or UAC sets it to 1.
Rule 4: If the second Via field does not contain the maddr parameter but has a
field tagged by the receiver, the proxy server or UAC sends the response to the
address specified in the received parameter.
Rule 5: If there is neither the maddr parameter nor a tag, the proxy server or
UAC sends the response to the address specified in the sent-by parameter.
The Via field has a general format:
Via: sent-protocol sent-by; hidden; ttl; maddr; received, branch
The sent-protocol is in the form of protocol-name/protocol-version/transport, in which
the default values for protocol-name and transport are SIP and UDP. The sent-by is
usually the host and port number of the sender. The hidden parameter has a key
word—hidden. If the hidden parameter exists, it indicates that the field has been
encrypted by the previous proxy for privacy purpose. For the meanings of the maddr
and received parameters, refer to earlier descriptions. The ttl and maddr parameters
are coordinated with each other. The branch parameter is used by the proxy server
concurrently delivering requests to tag the branches. If the response reaches the
destination, the proxy uses it to judge the branch from which the response comes.
An example of Via field:
Via:SIP/2.0/UDP182.20.1.116:5060;ttl=16;maddr=182.20.10.20;branch=z9hG4bkbc
427dad6
182.20.12.30, the
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request reaches
5-44
Chapter 5 MGCP and SIP
the proxy server