TCP IP Illustrated

In section 217 well see how duplicate acknowledgments

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Unformatted text preview: a stream. For example, if bytes 1-1024 are received OK, and the next segment contains bytes 2049-3072, the receiver cannot acknowledge this new segment. All it can send is an ACK with 1025 as the acknowledgment number. There is no means for negatively acknowledging a segment. For example, if the segment with bytes 1025-2048 did arrive, but had a checksum error, all the receiving TCP can send is an ACK with 1025 as the acknowledgment number. In Section 21.7 we'll see how duplicate acknowledgments can help determine that packets have been lost. The header length gives the length of the header in 32-bit words. This is required because the length of the options field is variable. With a 4-bit field, TCP is limited to a 60-byte header. Without options, however, the normal size is 20 bytes. There are six flag bits in the TCP header. One or more of them can be turned on at the same file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/tcp_tran.htm (4 of 6) [12/09/2001 14.47.10] Chapter 17. TCP: Transmission Control Protocol time. We briefly mention their use here and discuss each flag in more detail in later chapters. URG The urgent pointer is valid (Section 20.8). ACK The acknowledgment number is valid. The receiver should pass this data to the application as soon as possible (Section 20.5). PSH RST Reset the connection (Section 18.7). SYN Synchronize sequence numbers to initiate a connection. This flag and the next are described in Chapter 18. FIN The sender is finished sending data. TCP's flow control is provided by each end advertising a window size. This is the number of bytes, starting with the one specified by the acknowledgment number field, that the receiver is willing to accept. This is a 16-bit field, limiting the window to 65535 bytes. In Section 24.4 we'll look at the new window scale option that allows this value to be scaled, providing larger windows. The checksum covers the TCP segment: the TCP header and the TCP data. This i...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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