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TCP Common Applications and Server Port Assignments
We saw in the
overview of TCP that the protocol originally
included the functions of both modern TCP and the Internet Protocol.
TCP was split into TCP and IP to allow applications that didn't need
TCP's complexity to bypass it, using the much simpler UDP
instead as a transport layer protocol instead. This was an important
step in the development of the TCP/IP protocol suite, since there are
several important protocols for which UDP is ideally suited, and even
some where TCP is more of a nuisance than a benefit.
Most commonly, however, UDP is only
used in special cases. The UDP applications topic describes the two
types of protocols that may be better suited to UDP than TCP: ones where
speed is more important than reliability, and ones that send only short
messages infrequently. The majority of TCP/IP applications do not fall
into these categories. Thus, even though the layering of TCP and IP
means most protocols aren't required to use TCP, most
of them do anyway. The majority of the protocols that use it employ
all or at least most of the features it provides. The establishment
of a persistent connection is necessary for many interactive protocols,
such as Telnet,
as well as for ones that send commands and status replies, like HTTP.
Reliability and flow control are essential for protocols that send large
files, like FTP
or the
e-mail protocols.
Table 150
shows some of the more significant application protocols that run on
TCP. I have shown for each the well-known or registered port number
reserved for that protocol's server process (clients use ephemeral
ports, not the port numbers in the table).
I have also shown the special keyword shortcut for each
port assignment, and provided brief comments on why the protocol is
well-matched to TCP.
Table 150: Common TCP Applications and Server Port Assignments
Port #
|
Keyword
|
Protocol
|
Comments
|
20 and 21
|
ftp-data / ftp
|
File Transfer
Protocol (FTP, data and control)
|
Used to send large files, so
ideally suited for TCP.
|
23
|
telnet
|
Telnet
Protocol
|
Interactive
session-based protocol. Requires the connection-based nature of TCP.
|
25
|
smtp
|
Simple Mail
Transfer Protocol (SMTP)
|
Uses an exchange of commands,
and sends possibly large files between devices.
|
53
|
domain
|
Domain
Name Server (DNS)
|
An example
of a protocol that uses both UDP and TCP. For simple requests and replies,
DNS uses UDP. For larger messages, especially zone transfers, TCP is
used.
|
70
|
gopher
|
Gopher Protocol
|
A messaging protocol that has
been largely replaced by the WWW.
|
80
|
http
|
Hypertext
Transfer Protocol (HTTP / World Wide Web)
|
The classic
example of a TCP-based messaging protocol.
|
110
|
pop3
|
Post Office
Protocol
(POP version 3)
|
E-mail message retrieval protocols;
use TCP to exchange commands and data.
|
119
|
nntp
|
Network
News Transfer Protocol (NNTP)
|
Used for transferring
NetNews (USEnet) messages, which can be lengthy.
|
139
|
netbios-ssn
|
NetBIOS Session
Service
|
A session protocol, clearly better
suited to TCP than UDP.
|
143
|
imap
|
Internet
Message Access Protocol (IMAP)
|
Another e-mail
message retrieval protocol.
|
179
|
bgp
|
Border Gateway
Protocol (BGP)
|
While interior routing protocols
like RIP and OSPF use either UDP or IP directly, BGP runs over TCP.
This allows BGP to assume reliable communication even as it sends data
over potentially long distances.
|
194
|
irc
|
Internet
Relay Chat (IRC)
|
IRC is like
Telnet in that it is an interactive protocol that is strongly based
on the notion of a persistent connection between a client and server.
|
2049
|
nfs
|
Network File
System (NFS)
|
NFS was original implemented
using UDP for performance reasons. Given that it is responsible for
large transfers of files and UDP is unreliable, this was probably not
the best idea, and TCP versions were created. The latest version of
NFS uses TCP exclusively.
|
6000
- 6063
|
TCP
|
x11
|
Used
for the X Window graphical system. Multiple ports are dedicated to allow
many sessions.
|
A couple of the protocols
in the table above use both TCP and UDP, to get the best of both
worlds. Short, simple messages can be sent with UDP, while larger
files are moved with TCP. Many of the protocols that use both TCP and
UDP are actually utility/diagnostic
protocols (such as Echo, Discard and the
Time Protocol). These are a special case, because they were designed
to use both UDP and TCP specifically to allow their use for diagnostics
on both protocols.
I haven't included an exhaustive
list of TCP applications above; see the
topic on common TCP/IP applications and port numbers,
which also contains a pointer to the full (massive) list of TCP well-known
and registered server ports.
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Version 3.0 - Version Date: September 20, 2005
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