Technical data

1.1.1 Data Link Layer
At the base of the TCP/IP layers, the Data Link layer formats data and provides
services that directly access the physical network.
This layer also receives data that is routed from the Internet layer and transmits
the data to its destination, converting logical IP addressesto physical addresses
of the network adapter or network interface cards (NICs) using the Address
Resolution Protocol (ARP).
Some commonly used network architectures designed for the physical network are
Ethernet and its variants, Token Ring, FDDI, and ATM.
A single host computer can have multiple NICs. This configuration is termed
a multihomed host. Depending on your network setup, the Data Link layer’s
configuration may vary. For more information, see Chapter 3.
1.1.2 Internet Layer
The Internet layer moves data around the internet. The Internet Protocol uses
addressing to deliver and route packets across networks independently of the
network cabling.
At this level, the protocols are:
• Internet Protocol (IP)
• Internet Control Message Protocol (ICMP)
• Address Resolution Protocol (ARP)
The protocol also encapsulates datagram headers, sends ICMP error and control
messages, and maps ARP address conversions.
Routing protocols at this layer are:
• Routing Information Protocol (RIP) Versions 1 and 2
• Open Shortest Path First (OSPF) Version 2
• Exterior Gateway Protocol (EGP)
• Border Gateway Protocol (BGP)
• Router Discovery
For more information about these protocols, see Chapter 5.
1.1.3 Transport Layer
The Transport layer enables a flow of data between two hosts. The protocols
at this layer are either connection oriented, in which the protocol establishes
and maintains the connection between communicating hosts to prevent errors, or
connectionless, in which a one-way datagram is sent to a destination host.
The TCP/IP Services supports both transport protocols:
• The connection-oriented protocol, Transmission Control Protocol (TCP) provides
a reliable data flow between two hosts. TCP is used for complex, large packets
that have an IP address.
• The connectionless protocol, User Datagram Protocol (UDP) provides a much
simpler service to the Application layer than TCP but does not guarantee
reliability. UDP is used for simple, small packets and requests for dynamic IP
address assignment. UDP packets have a MAC address.
Introducing Compaq TCP/IP Services for OpenVMS 1–3