Nortel Networks NN10265-111 - Manual (Page 115)

MCS 5100 RTP Media Portal Basics
115 Copyright © Nortel Networks Limited 2006
The following steps provide more detail about the call flow:
1. Client A initiates a SIP session to Client B by sending an Invite
message to the SIP Application Module. Within the Invite message,
Client A includes SDP information that identifies the called address
(IP and port of B). The Invite passes through the firewall/NAT A.
2. The SIP Application Module requests resources to support the half
call (connection) between Client B and the RTP Media Portal. The
SIP Application Module knows that Client B is obscured, so a
specific media address for Client B is not provided in the resource
request sent to the RTP Media Portal.
3. The RTP Media Portal allocates resources (an IP address and port)
for Client B. The new resources (A’) are returned to the SIP
Application Module in the response.
4. The SIP Application Module forwards the original invite to client B,
substituting the address and port of Client A with the A’ resources
allocated by the RTP Media Portal. The invite passes through
firewall B.
5. Client B answers the incoming call. The SIP message sent to the
SIP Application Module contains the address and port of Client B.
However, because Client B is obscured, the actual media address
(the address from which media packets will be sent) is not yet
known.
6. The SIP Application Module requests resources to support the half
call (connection) between Client A and the RTP Media Portal.
Again, because Client A is obscured, the specific media address
and port of Client A are not included in the request.
7. The RTP Media Portal allocates resources (an IP address and port)
for Client A. The new resources (B’) are returned to the SIP
Application Module in the response.
8. The SIP Application Module replaces the SDP information
identifying the terminating address (B) with information identifying
the terminating NAPT address (B’) and forwards on the OK
message to Client A. At this point, both clients have the correct
addresses to establish the media connection. However, there is no
complete media path through the Portal at this time, because the
Portal does not yet know the actual media address of either client.
9. Client A sends the first media packet to address B’. At this point, the
Portal examines the packet and extracts the source address, thus
“discovering” the media address used by Client A. At this point, the
connection between Client A and the Portal is complete. However,
the packet cannot yet be forwarded to Client B since the media
address of Client B is not yet known. The packet is dropped by the
Portal.