Information Technology Reference
In-Depth Information
the TCP header. Its format is
+--------+
|00000000|
+--------+
24.2.5 No-operation (type=1)
The no-operation can be used between options. A typical application is to align the beginning
of a subsequent option, so that it is on a 32-bit word boundary. Its format is
+--------+
|00000001|
+--------+
24.2.6 Maximum segment size (type=2, length=4)
In this option the maximum receive segment size is defined, and is preceeded by a 16-bit
maximum segment size. It is only sent in an initial connection request, that is, when the
SYN control bit is set. If it is not included, then any segment size is allowed. Its format is
+--------+--------+---------+--------+
|00000010|00000100| max seg size |
+--------+--------+---------+--------+
24.3 UDP
TCP allows for a reliable connection-based transfer of data. The User datagram protocol
(UDP) is an unreliable connection-less approach, where datagrams are sent into the network
without an acknowledgement or connections. It is defined in RFC768 and uses IP as its un-
derlying protocol. It has the advantage over TCP in that it has a minimal protocol mecha-
nism, but does not guarantee delivery of any of the data. Figure 24.2 shows its format. The
fields are:
•
Source port - this is an optional field and is set to a zero if not used. It identifies the local
port number which should be used when the destination host requires to contact the
originator.
•
Destination - port to connect to on the destination.
•
Length - number of bytes in the datagram, including the UDP header and the data.
•
Checksum - it is the 16-bit 1's complement of all 1's complement sum of the IP header,
the UDP header and the data (which, if necessary, is padded with zero bytes at the end, to
make an even number of bytes).
When used with IP the UDP/IP header is shown in Figure 24.3. The protocol field is set to 17
to identify UDP.
