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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
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
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.
- Spring '12