1.3.3 Terminology

This document uses the following technical terms:

Segment

A segment is the unit of end-to-end transmission in the TCP protocol. A segment consists of a TCP header followed by application data. A segment is transmitted by encapsulation inside an IP datagram.

Message

In this description of the lower-layer protocols, a message is the unit of transmission in a transport layer protocol. In particular, a TCP segment is a message. A message consists of a transport protocol header followed by application protocol data. To be transmitted end-to- end through the Internet, a message must be encapsulated inside a datagram.

IP Datagram

An IP datagram is the unit of end-to-end transmission in the IP protocol. An IP datagram consists of an IP header followed by transport layer data, i.e., of an IP header followed by a message.

In the description of the internet layer (Section 3), the unqualified term "datagram" should be understood to refer to an IP datagram.

Packet

A packet is the unit of data passed across the interface between the internet layer and the link layer. It includes an IP header and data. A packet may be a complete IP datagram or a fragment of an IP datagram.

Frame

A frame is the unit of transmission in a link layer protocol, and consists of a link-layer header followed by a packet.

Connected Network

A network to which a host is interfaced is often known as the "local network" or the "subnetwork" relative to that host. However, these terms can cause confusion, and therefore we use the term "connected network" in this document.

Multihomed

A host is said to be multihomed if it has multiple IP addresses. For a discussion of multihoming, see Section 3.3.4 below.

Physical network interface

This is a physical interface to a connected network and has a (possibly unique) link-layer address. Multiple physical network interfaces on a single host may share the same link-layer address, but the address must be unique for different hosts on the same physical network.

Logical [network] interface

We define a logical [network] interface to be a logical path, distinguished by a unique IP address, to a connected network. See Section 3.3.4.

Specific-destination address

This is the effective destination address of a datagram, even if it is broadcast or multicast; see Section 3.2.1.3.

Path

At a given moment, all the IP datagrams from a particular source host to a particular destination host will typically traverse the same sequence of gateways. We use the term "path" for this sequence. Note that a path is uni-directional; it is not unusual to have different paths in the two directions between a given host pair.

MTU

The maximum transmission unit, i.e., the size of the largest packet that can be transmitted.

The terms frame, packet, datagram, message, and segment are illustrated by the following schematic diagrams:

1. Transmission on connected network:

              _______________________________________________
             | LL hdr | IP hdr |         (data)              |
             |________|________|_____________________________|
   
              <---------- Frame ----------------------------->
                       <----------Packet -------------------->
   

2. Before IP fragmentation or after IP reassembly:

                       ______________________________________
                      | IP hdr | transport| Application Data |
                      |________|____hdr___|__________________|
   
                       <--------  Datagram ------------------>
                                <-------- Message ----------->
   

   or, for TCP:

                       ______________________________________
                      | IP hdr |  TCP hdr | Application Data |
                      |________|__________|__________________|
   
                       <--------  Datagram ------------------>
                                <-------- Segment ----------->