计算机网络系统方法5e mk- chapter 5.ppt

Keeping the Pipe Full 16-bit AdvertisedWindow field must be big enough to allow the sender to keep the pipe full Clearly the receiver is free not to open the window as large as the AdvertisedWindow field allows If the receiver has enough buffer space The window needs to be opened far enough to allow a full delay × bandwidth product’s worth of data Assuming an RTT of 100 ms Keeping the Pipe Full Required window size for 100-ms RTT. Triggering Transmission How does TCP decide to transmit a segment? TCP supports a byte stream abstraction Application programs write bytes into streams It is up to TCP to decide that it has enough bytes to send a segment Triggering Transmission What factors governs this decision Ignore flow control: window is wide open, as would be the case when the connection starts TCP has three mechanism to trigger the transmission of a segment 1) TCP maintains a variable MSS and sends a segment as soon as it has collected MSS bytes from the sending process MSS is usually set to the size of the largest segment TCP can send without causing local IP to fragment. MSS: MTU of directly connected network – (TCP header + and IP header) 2) Sending process has explicitly asked TCP to send it TCP supports push operation 3) When a timer fires Resulting segment contains as many bytes as are currently buffered for transmission Silly Window Syndrome If you think of a TCP stream as a conveyer belt with “full” containers (data segments) going in one direction and empty containers (ACKs) going in the reverse direction, then MSS-sized segments correspond to large containers and 1-byte segments correspond to very small containers. If the sender aggressively fills an empty container as soon as it arrives, then any small container introduced into the system remains in the system indefinitely. That is, it is immediately filled and emptied at each end, and never coalesced with adjacent containers to create larger containers. Silly Window Syndrome Silly Window Syndrome Na