计网 传输层chapter6-transportlayer.ppt

The flags say what kind of segment it is, e.g., SYN for a connection establishment (synchronize) segment. The ack. number is a cumulative acknowledgement that means all data up to that number has been received. After the fixed part can come options. * One slight difference from before: the receiver echoes ACK+1 rather than simply ACK to show that it received and understood the connection request. Terminology: connection request is a SYN, for synchronize, disconnect request is a FIN for finish. * Exercise: using two instances of this, follow the path of a normal connection from a three-way handshake (both sides should move from closed to established using socket calls and packet exchanges) to teardown (both sides should move to closed at bottom). Ignore packet loss as it is not covered here. Then try a simultaneous open, etc. * To see why the figure is correct about where the data lies in the buffer note that it is a circular buffer. Data is put in from left to right then starts at the left edge again (bottom). Similarly, data is read out left to right (second from bottom). * These sort of things problems crop up in practice as being part of the mechanisms we have described so far. They usually call for modified sliding window / flow control rules in special cases. Solution for silly window syndrome is that receiver should not send window updates for tiny buffer openings like “one byte available”. It should wait until a whole segment can be buffered to avoid this. * RTT is round-trip time, a measure of delay that can be easily computed at one end without synchronized clocks. Average is computed with an EWMA, exponentially weighted moving average that is very simple: new value = alpha x old value + (1-alpha) x sample. Alpha is 7/8, say. Similarly for estimating the variance. TCP actually computes the deviation from the mean with another EWMA. * It’s only slow compared to jumping to a large window immediately, which was how it used to work when slow-start was in