Nortel Networks WEB OS 212777 - Manual (Page 306)

Web OS 10.0 Application Guide
306 n Chapter 12: Global Server Load Balancing
212777-A, February 2002
The following procedure explains the three-way handshake between the two sites and the cli-
ent for a non-HTTP application (POP3). When POP3 processes at Site 1 terminate because of
operator error, the following events occur to allow POP3 requests to be fulfilled:
1. A user POP3 TCP SYN request is received by the virtual server at Site 1. The switch at
that site determines that there are no local resources to handle the request.
2. The Site 1 switch rewrites the request such that it now contains a client proxy IP address
as the source IP address, and the virtual server IP address at Site 2 as the destination IP
address.
3. The switch at Site 2 receives the POP3 TCP SYN request to its virtual server. The request
looks like a normal SYN frame, so it performs normal local load-balancing.
4. Internally at Site 2, the switch and the real servers exchange information. The TCP SYN
ACK from Site 2’s local real server is sent back to the IP address specified by the proxy IP
address.
5. The Site 2 switch sends the TCP SYN ACK frame to Site 1, with Site 2’s virtual server IP
address as the source IP address, and Site 1’s proxy IP address as the destination IP
address.
6. The switch at Site 1 receives the frame and translates it, using Site 1’s virtual server IP
address as the source IP address and the client’s IP address as the destination IP address.
This cycle continues for the remaining frames to transmit all the client’s mail, until a FIN
frame is received.