Huawei U-SYS MRS6100 Technical Manual page 68

Technical Manual
U-SYS MRS6100 Media Resource Server
CSeq
CSeq refers to command sequence. A CSeq field in a request contains a command
name and a single decimal sequence number. Request client defines the sequence
number, which is unique inside the Call-ID. The initial value of the sequence number
is arbitrary. The subsequent values share the same Call-ID. For a request with a
different command name and a different message body, the CSeq sequence number
must be increased by one. A retransmitted request contains the same sequence
number. The server replicates the CSeq value from the request to the response
message to correlate the request with the response it triggers.
The CSeq value (decimal sequence number) of an ACK or a CANCEL request is the
same as that of the corresponding INVITE request. The CSeq sequence number of a
BYE request must be greater than that of the corresponding INVITE request. The
server must remember the highest sequence number of an INVITE request having the
same Call-ID. Upon receipt of an INVITE request with a lower sequence number, the
server sends a response and discards the INVITE request.
Several requests concurrently delivered by the proxy server have the same CSeq
value. Strictly speaking, CSeq is required for any request that is cancelled by a BYE or
CANCEL request and also for continuous requests with the same Call-ID sent by the
client.
An example of CSeq field:
CSeq: 1 INVITE
ViaF
The Via field indicates the path taken by the request. The field can avoid loops during
the transport of the request and ensure the same path taken by the responses, for
example, in firewall occasions.
The client originating a request must add its host name or network address in the Via
field of the request. When the client does not use the default port, it must add the used
port number in the field. During the forwarding of a request, the proxy server must add
its address in a new Via field that is put before the existing Via. If the proxy server
receives a request containing its address in a Via field, the proxy server returns a
response indicating a loop detection.
When a request is passing a network address translation (NAT) entity (firewall for
example), the requested source address and the port number may be changed and
thus the Via field cannot be the base for routing responses. To avoid that, the proxy
server must check the top Via field. If the value of the top Via is different from the
previous hop's address that is detected by the proxy server, the server add the
receive parameter in the Via which is thus called the Via field tagged by the receiver.
For example,
Via:SIP/2.0/UDP softx3000.bell-telephone.com:5060
Via:SIP/2.0/UDP 10.0.0.1:5060;received=182.20.12.30
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Chapter 5 MGCP and SIP