| 9.1 |  | Transport Protocols |
| 9.1.1 |  | Sessions in an IP world |

| Overview The previous module provided a general overview of firewalls and firewall technologies. This overview made it clear that the primary function of any firewall is to enforce an access control policy between two or more networks. Understanding how the PIX Security Appliance processes inbound and outbound transmissions is very important. This section will look at the different inbound and outbound transmission types and illustrate how the PIX Security Appliance handles each of them.

Transport layer protocols
Before the student can gain a deeper understanding of how the Cisco PIX Security Appliance processes inbound and outbound transmissions, it is necessary to have a solid understanding of the two primary transport protocols . While both of these protocols were discussed as part of the Cisco Certified Network Associate (CCNA) curriculum, this module will review each of them .

The first transport layer protocol is TCP. TCP is a connection-oriented transport layer protocol in the TCP/IP protocol stack that provides reliable, full-duplex data transmission. TCP is utilized by applications that value reliability over speed and conciseness.

Protocols that use TCP include the following:

- FTP (File Transfer Protocol)
- HTTP (Hypertext Transport Protocol)
- SMTP (Simple Mail Transport Protocol)
- SSH (Secure Shell)
- SSL (Secure Sockets Layer)

The second transport layer protocol is UDP. UDP is a connectionless transport layer protocol in the TCP/IP protocol stack. It is a simpler protocol than TCP that does not rely on acknowledgments or guaranteed delivery when exchanging datagrams. Because UDP is unreliable, it requires other protocols to handle error processing and retransmission:

- Request-reply, or ping-pong services, such as DNS
- Flow services such as video, Voice over IP (VoIP), and Network File System (NFS)

Protocols that use UDP include the following:

- TFTP
- SNMP
- DHCP
- DNS

Now that the UDP and TCP transport layer protocols have been reviewed, their interaction with the PIX Security Appliance will be examined.

| Web Links RFC 1180 http://www.faqs.org/rfcs/rfc1180.html