Scalable voip mobility intedration and deployment- P10: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | Voice Over Ethernet 89 TCP needs to keep track of flow state in order to provide the appearance of a stream. The TCP stream is a two-way channel symmetric in the sense that no one side is favored over the other. Each side keeps sender state and receiver state. A part of that state is that every byte in the TCP stream since the stream began is given an increasing sequence number. As packets are pushed out to the network by the sender the sender keeps copies of those packets. When the receiver gets a packet it acknowledges it by sending a packet with the ACK flag set and the Acknowledgment field to the sequence number of the highest byte it has received before a break in the sequence occurs. This acknowledgment can come back as a part of the next return-direction data packet but if none are queued then ACKs are generated in their own otherwise empty packets every 200ms. This process is called delayed acknowledgment. If the acknowledgment is never received by the sender the sender has to assume that either the original data packet or the acknowledgment itself got lost. The sender will then retry the packet some time later. Once a packet has an acknowledgment received for it the sender will finally free up the packet. On the receive side the receiver cannot send information back to the application unless there is a contiguous run of bytes at the head of the reassembly buffer. If not the buffer holds onto the bytes and advertises the hole in the next acknowledgment. TCP uses sophisticated flow control techniques to prevent the sender from sending too much. The basic flow control technique is that the sender cannot have any more outstanding packets sent than the window size it hears on any given TCP packet in return. This prevents the receive buffer from being overrun. On top of that however TCP engages in congestion control. The sender specifically tries to measure the round-trip time of the network and its loss rate. Because TCP is a handshaking protocol and the sender
