The TCP/IP Guide  9  Networking Fundamentals       9  Fundamental Network Characteristics

Connection-Oriented and Connectionless Protocols 1 2 Message Formatting: Headers, Payloads and Footers

Common Names For Messages

The most common terms that are used for messages are the following:

• Packet: This term is considered by many to most correctly refer to a message sent by protocols operating at the network layer of the OSI Reference Model. So, you will commonly see people refer to “IP packets”. However, this term is commonly also used to refer generically to any type of message, as I mentioned at the start of this topic.
  
  
• Datagram: This term is basically synonymous with “packet” and is also used to refer to network layer technologies. It is also often used to refer to a message that is sent at a higher level of the OSI Reference Model (more often than “packet” is).
  
  
• Frame: This term is most commonly associated with messages that travel at low levels of the OSI Reference Model. In particular, it is most commonly seen used in reference to data link layer messages. It is occasionally also used to refer to physical layer messages, when message formatting is performed by a layer one technology. A frame gets its name from the fact that it is created by taking higher-level packets or datagrams and “framing” them with additional header information needed at the lower level.
  
  
• Cell: Frames and packets, in general, can be of variable length, depending on their contents; in contrast, a cell is most often a message that is fixed in size. For example, the fixed-length, 53-byte messages sent in Asynchronous Transfer Mode (ATM) are called cells. Like frames, cells usually are used by technologies operating at the lower layers of the OSI model.
  
  
• Protocol Data Unit (PDU) and Service Data Unit (SDU): These are the formal terms used in the OSI Reference to describe protocol messages. A PDU at layer N is a message sent between protocols at layer N. It consists of layer N header information and an encapsulated message from layer N+1, which is called both the layer N SDU and the layer N+1 PDU. After you stop scratching your head, see the topic on OSI model data encapsulation for a discussion of this that may actually make sense. J

I should also point out that there are certain protocols that use unusual names to refer to their messages, which aren’t used elsewhere in the world of networking. One prominent example is the Transmission Control Protocol (TCP), which calls its messages segments.

As for this Guide and its use of these terms, I have made a specific effort not to imply anything about the nature of a message solely based on the name it uses, but I do try to follow the most common name used for a particular technology. For example, messages sent over Ethernet are almost always called Ethernet frames—they are not generally called Ethernet datagrams, for example. However, I do not structure discussions so that the type of name used for a message is the only way to determine what sort of message it is.

Connection-Oriented and Connectionless Protocols 1 2 Message Formatting: Headers, Payloads and Footers