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508Chapter 11  Secure Network Architecture and Components

Transport Layer Protocols

When a connection is established via the Transport layer, it is done using ports. Since port numbers are 16-digit binary numbers, the total number of ports is 2^16, or 65,536, numbered from 0 through 65,535. Ports allow a single IP address to be able to support multiple simultaneous communications, each using a different port number (i.e., multiplexing over IP). The combination of an IP address and a port number is known as a socket.

The first 1,024 of these ports (0–1,023) are called the well-known ports or the service ports. These ports are reserved for use exclusively by servers.

Ports 1,024 to 49,151 are known as the registered software ports. These are ports that have one or more networking software products specifically registered with the International Assigned Numbers Authority (IANA) at iana.org.

Ports 49,152 to 65,535 are known as the random, dynamic, or ephemeral ports because they are often used randomly and temporarily by clients as a source port. However, most OSs allow for any port from 1,024 to be used as a dynamic client source port as long as it is not already in use on that local system.

The two primary Transport layer protocols of TCP/IP are TCP and UDP. Transmission Control Protocol (TCP) is a full-duplex connection-oriented protocol, whereas User Datagram Protocol (UDP) is a simplex connectionless protocol.

Transmission Control Protocol (TCP) supports full-duplex communications, is connection oriented, and employs reliable sessions. TCP is connection oriented because it employs a handshake process between two systems to establish a communication session. The three-way handshake process (Figure 11.6) is as follows:

1.The client sends a SYN (synchronize) flagged packet to the server.

2.The server responds with a SYN/ACK (synchronize and acknowledge) flagged packet back to the client.

3.The client responds with an ACK (acknowledge) flagged packet back to the server.

FIGURE 11. 6   TheTCP three-way handshake

SYN

SYN/ACK

C S

ACK

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