TCP IP Illustrated

many implementations send this window update if the

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Unformatted text preview: Bulk Data Flow Another point to note in Figure 20.7 concerns the three consecutive ACKs, segments 14, 15, and 16. We saw two consecutive ACKs in Figure 20.3, but that was because the receiver had advertised a window of 0 (stopping the sender) so when the window opened up, another ACK was required, with the nonzero window, to restart the sender. In Figure 20.7, however, the window never reaches 0. Nevertheless, when the size of the window increases by 2048 bytes, another ACK is sent (segments 15 and 16) to provide this window update to the other end. (These two window updates in segments 15 and 16 are not needed, since the FIN has been received from the other end, preventing it from sending any more data.) Many implementations send this window update if the window increases by either two maximum sized segments (2048 bytes in this example, with an MSS of 1024) or 50% of the maximum possible window (2048 bytes in this example, with a maximum window of 4096). We'll see this again in Section 22.3 when we examine the silly window syndrome in detail. As another example of the PUSH flag, look again at Figure 20.3. The reason we see the flag on for the first four data segments (4-7) is because each one caused a segment to be generated by TCP and passed to the IP layer. But then TCP had to stop, waiting for an ACK to move the 4096-byte window. While waiting for the ACK, TCP takes the final 4096 bytes of data from the application. When the window opens up (segment 9) the sending TCP knows it has four segments that it can send immediately, so it only turns on the PUSH flag for the final segment (13). 20.6 Slow Start In all the examples we've seen so far in this chapter, the sender starts off by injecting multiple segments into the network, up to the window size advertised by the receiver. While this is OK when the two hosts are on the same LAN, if there are routers and slower links between the sender and the receiver, problems can arise. Some intermediate router must queue the packets, and it's possible for that router to run out of space. [Jacobson 1988] shows how this naive approach can red...
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