Transport Layer Concepts

Before moving to a more detailed discussion of TCP and UDP, it is worth pausing for a moment to focus on a few of the important concepts:

• Connection-oriented and connectionless protocols

• Ports and sockets

• Multiplexing

These important concepts are essential to understanding the design of the Transport layer. You'll learn about these concepts in the following sections.

Connection-Oriented and Connectionless Protocols

To provide the appropriate level of quality assurance for any given situation, developers have come up with two alternative network protocol archetypes:

• A connection-oriented protocol establishes and maintains a connection between communicating computers and monitors the state of that connection over the course of the transmission. In other words, each package of data sent across the network receives an acknowledgment, and the sending machine records status information to ensure that each package is received without errors, retransmitting the data if necessary. At the end of the transmission, the sending and receiving computers gracefully close the connection.

• A connectionless protocol sends a one-way datagram to the destination and doesn't worry about officially notifying the destination machine that data is on the way. The destination machine receives the data and doesn't worry about returning status information to the source computer.

Figure 6.1 shows two people demonstrating connection-oriented communication. Of course, they are not intended to show the true complexity of digital communications but simply to illustrate the concept of a connection-oriented protocol.

Figure 6.1. A connection-oriented protocol.

Figure 6.2 shows how the same data would be sent using a connectionless protocol.

Figure 6.2. A connectionless protocol.

Ports and Sockets

The Transport layer serves as an interface between network applications and the network and provides a method for addressing network data to particular applications. In the TCP/IP system, applications can address data through either the TCP or UDP protocol module using port numbers. A port is a predefined internal address that serves as a pathway from the application to the Transport layer or from the Transport layer to the application (see Figure 6.3). For instance, a client computer typically contacts a server's FTP application through TCP port 21.

Figure 6.3. A port address targets data to a particular application.

A closer look at the Transport layer's application-specific addressing scheme reveals that TCP and UDP data is actually addressed to what is called a socket. A socket is an address formed by concatenating the IP address and the port number. For instance, the socket number 111.121.131.141.21 refers to port 21 on the computer with the IP address 111.121.131.141.

Figure 6.4 shows how computers using TCP exchange socket information when they form a connection.

Figure 6.4. Exchanging the source and destination socket numbers.

The following is an example of how a computer accesses an application on a destination machine through a socket:

1. Computer A initiates a connection to an application on Computer B through a well-known port. A well-known port is a port number that is assigned to a specific application by ICANN. See Tables 6.1 and 6.2 for lists of some well-known TCP and UDP ports. Combined with the IP address, the well-known port becomes the destination socket address for Computer A. The request includes a data field telling Computer B which socket number to use when sending back information to Computer A. This is Computer A's source socket address.

2. Computer B receives the request from Computer A through the well-known port and directs a response to the socket listed as Computer A's source address. This socket becomes the destination address for messages sent from the application on Computer B to the application on Computer A.

You'll learn more about how to initiate a TCP connection later in this hour.

Table 6.1. Well-Known TCP Ports

Service	TCP Port Number	Brief Description
tcpmux	1	TCP port service multiplexor
compressnet	2	Management utility
compressnet	3	Compression utility
echo	7	Echo
discard	9	Discard or null
systat	11	Users
daytime	13	Daytime
netstat	15	Network status
qotd	17	Quote of the Day
chargen	19	Character generator
ftp-data	20	File Transfer Protocol data
ftp	21	File Transfer Protocol control
telnet	23	Terminal network connection
smtp	25	Simple Mail Transport Protocol
nsw-fe	27	NSW user system
time	37	Time server
name	42	Host name server
domain	53	Domain name server (DNS)
nameserver	53	Domain name server (DNS)
DHCP	67	Dynamic Host Configuration Protocol
gopher	70	Gopher service
rje	77	Remote job entry
finger	79	Finger
http	80	WWW service
link	87	TTY link
supdup	95	SUPDUP protocol
hostnames	101	sri-nic host name server
iso-tsap	102	ISO-TSAP
x400	103	X.400 mail service
x400-snd	104	X.400 mail send
pop	109	Post Office Protocol
pop2	109	Post Office Protocol 2
pop3	110	Post Office Protocol 3
portmap	111
sunrpc	111	SUN RPC service
auth	113	Authentication service
sftp	115	Secure FTP
path	117	UUCP path service
uucp-path	117	UUCP path service
nntp	119	Usenet Network News Transfer Protocol
nbsession	139	NetBIOS session service
NeWS	144	News
tcprepo	158	TCP repository

Table 6.2. Well-Known UDP Ports

Service	UDP Port Number	Description
echo	7	Echo
discard	9	Discard or null
systat	11	Users
daytime	13	Daytime
netstat	15	Network status
qotd	17	Quote of the Day
chargen	19	Character generator
time	37	Time server
name	42	Host Name server
domain	53	Domain name server (DNS)
nameserver	53	Domain name server (DNS)
bootps	67	Bootstrap protocol service/DHCP
bootpc	68	Bootstrap protocol client/DHCP
tftp	69	Trivial File Transfer Protocol
portmap	111
sunrpc	111	SUN RPC service
ntp	123	Network Time Protocol
nbname	137	NetBIOS name
nbdatagram	148	NetBIOS datagram
sgmp	153
snmp	161	Simple Network Management Protocol
snmp-trap	162	Simple Network Management Protocol trap

Multiplexing/Demultiplexing

The socket addressing system enables TCP and UDP to perform another important Transport layer task: multiplexing and demultiplexing. As described earlier, multiplexing is the act of braiding input from several sources into a single output, and demultiplexing is the act of receiving input from a single source and delivering it to multiple outputs (see Figure 6.5).

Figure 6.5. Multiplexing and demultiplexing.

Multiplexing/demultiplexing enables the lower levels of the TCP/IP stack to process data without regard to which application initiated that data. All associations with the originating application are settled at the Transport layer, and data passes to and from the Internet layer in a single, application-independent pipeline.

The key to multiplexing and demultiplexing is the socket address. Because the socket address combines the IP number with the port number, it provides a unique identifier for a specific application on a specific machine. See the Telnet server depicted in Figure 6.6. All client machines use the well-known port address TCP 23 to contact the Telnet server, but the destination socket for each of the connecting PCs is unique. Likewise, all network applications running on the Telnet server use the server's IP address, but only the Telnet service uses the socket address, consisting of the server's IP address plus TCP port 23.

Figure 6.6. The socket address uniquely identifies an application on a particular server.