Information Technology Reference
In-Depth Information
for that connection. Thus, a node can have several TELNET connections with the same port
number but each connection will have a different socket number. A port number can be any
value but there is a standard convention that most systems adopt. Table 24
.1
defines some of
the most common values. Standard applications normally use port values from 0 to 255,
while unspecified applications can use values above 255. Section 24.12 outlines the main
ports.
Table 24.1
Typical TCP port numbers
Port
Process name
Notes
20
FTP-DATA
File transfer protocol (data)
21
FTP
File transfer protocol (control)
23
TELNET
Telnet
25
SMTP
Simple mail transfer protocol
49
LOGIN
Login protocol
53
DOMAIN
Domain name server
79
FINGER
Finger
161
SNMP
Simple network management protocol
24.2.2 TCP header format
The sender's TCP layer communicates with the receiver's TCP layer using the TCP protocol
data unit. It defines parameters such as the source port, destination port, and so on, and is
illustrated in Figure 24.
1.
The fields are:
•
Source and destination port number - 16-bit values that identify the local port number
(source number and destination port number or destination port).
•
Sequence number - identifies the current sequence number of the data segment. This al-
lows the receiver to keep track of the data segments received. Any segments that are
missing can be easily identified. The sequence number of the first data byte in this seg-
ment (except when SYN is present). If SYN is present the sequence number is the initial
sequence number (ISN) and the first data octet is ISN+1.
•
Acknowledgement number - when the ACK bit is set, it contains the value of the next
sequence number the sender of the packet is expecting to receive. This is always set after
the connection is made.
•
Data offset - a 32-bit value that identifies the start of the data. It is defined as the number
of 32-bit words in the header (as the TCP header always has a multiple number of 32-bit
words).
•
Flags - the flag field is defined as
UAPRSF
, where
U
is the urgent flag (URG),
A
the ac-
knowledgement flag (ACK),
P
the push function (PSH),
R
the reset flag (RST),
S
the se-
quence synchronise flag (SYN) and
F
the end-of-transmission flag (FIN).
•
Window - a 16-bit value that gives the number of data bytes that the receiving host can
accept at a time, beginning with the one indicated in the acknowledgement field of this
segment.
•
Checksum - a 16-bit checksum for the data and header. It is the 1's complement of all the
1's complement sum of all the 16-bit words in the TCP header and text. The checksum is
assumed to be a zero when calculating the checksum.
•
UrgPtr - the urgent pointer used to identify an important area of data (most systems do
not support this facility). It is only used when the URG bit is set. This field communicates
the current value of the urgent pointer as a positive offset from the sequence number in
this segment.
