Internet Core Protocols: The Definitive Guide

Terminology

Most network managers refer to data that is sent across a network using generic terms such as “packet” or “datagram.” However, as TCP/IP has evolved, a variety of terms have been used to describe the units of data that are transmitted by specific protocols. RFC 1122 brought all of these terms together and defined the usage for each term according to specific protocols. These terms are used throughout this book in the same manner.

Frame

A frame is the unit of data that is sent across a network using the link-layer protocol appropriate for that network. This includes link-layer encapsulation technologies such as Ethernet II frames, 802.3 Ethernet frames, or Token Ring frames.

IP datagram

An IP datagram is the unit of data that is managed by the Internet Protocol, including whatever data is being transmitted, as well as the IP headers associated with that data. In essence, an IP datagram is the unit of data that IP works with explicitly.

IP packet

An IP packet is another term for IP datagrams, although this term is most often used to refer to the datagram portion of a frame, rather than referring to the datagram itself. For example, a sending and receiving system will look at an IP datagram as a single entity, while that datagram may have been split into multiple IP packets for transmission across a set of intermediary networks. Typically speaking, hosts deal with IP datagrams, while routers deal with IP packets.

Message

A message is the unit of data sent from one of the upper-layer protocols (such as UDP or TCP), including the data being transferred and the related transport-specific headers associated with that data. Although most of the time the message data will be generated by an application-specific protocol, ICMP and IGMP also communicate with IP directly and will therefore also generate message data. Messages eventually become the data portion of an IP datagram.

TCP segment

Although the headers and data that are generated by TCP are considered to be messages, TCP messages can be spread across multiple messages. In this scenario, the messages are typically referred to as segments.