The Linux Programming Interface

1268 Chapter 61

allow the receiving TCP to double-check that an incoming segment has arrived

at the correct destination (i.e., that IP has not wrongly accepted a datagram

that was addressed to another host or passed TCP a packet that should have

gone to another upper layer). UDP calculates the checksum in its packet headers

in a similar manner and for similar reasons. See [Stevens, 1994] for further

details on the pseudoheader.

z Urgent pointer: If the URG control bit is set, then this field indicates the location

of so-called urgent data within the stream of data being transmitted from the

sender to the receiver. We briefly discuss urgent data in Section 61.13.1.

z Options: This is a variable-length field containing options controlling the opera-

tion of the TCP connection.

z Data: This field contains the user data transmitted in this segment. This field

may be of length 0 if this segment doesn’t contain any data (e.g., if it is simply

an ACK segment).

61.6.2 TCP Sequence Numbers and Acknowledgements

Each byte that is transmitted over a TCP connection is assigned a logical sequence

number by TCP. (Each of the two streams in a connection has its own sequence num-

bering.) When a segment is transmitted, its sequence number field is set to the logical

offset of the first byte of data in the segment within the stream of data being

transmitted in this direction over the connection. This allows the receiving TCP to

assemble the received segments in the correct order, and to indicate which data

was received when sending an acknowledgement to the sender.

To implement reliable communication, TCP uses positive acknowledgements;

that is, when a segment is successfully received, an acknowledgement message (i.e.,

a segment with the ACK bit set) is sent from the receiving TCP to the sending TCP,

as shown in Figure 61-3. The acknowledgement number field of this message is set to

indicate the logical sequence number of the next byte of data that the receiver

expects to receive. (In other words, the value in the acknowledgement number

field is the sequence number of the last byte in the segment that it acknowledges,

plus 1.)

Figure 61-3: Acknowledgements in TCP

When the sending TCP transmits a segment, it sets a timer. If an acknowledgement

is not received before the timer expires, the segment is retransmitted.

segment (x bytes)

(Seq # range: N to N+x – 1

)

ACK of segment

(Ack #: N+x)

Sender Receiver

Network

Host A Host B