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

Messages: Packets, Frames, Datagrams and Cells 1 Message Addressing and Transmission Methods: Unicast, Broadcast and Multicast Messages

Messages are the structures used to send information over networks. They vary greatly from one protocol or technology to the next in how they are used, and as I described in the previous topic, they are also called by many different names. Shakespeare had the right idea about names, however. The most important way that messages differ is not in what they are called but in terms of their content.

Every protocol uses a special formatting method that determines the structure of the messages it employs. Obviously, a message that is intended to connect a Web server and a Web browser is going to be quite different from one that connects two Ethernet cards at a low level. This is why I separately describe the formats of dozens of different protocol messages in various parts of this Guide.

While the format of a particular message type depends entirely on the nature of the technology that uses it, messages on the whole tend to follow a fairly uniform overall structure. In generic terms, each message contains the following three basic elements (see Figure 3):

• Header: Information that is placed before the actual data. The header normally contains a small number of bytes of control information, which is used to communicate important facts about the data that the message contains and how it is to be interpreted and used. It serves as the communication and control link between protocol elements on different devices.
  
  
• Data: The actual data to be transmitted, often called the payload of the message (metaphorically borrowing a term from the space industry!) Most messages contain some data of one form or another, but some actually contain none: they are used only for control and communication purposes. For example, these may be used to set up or terminate a logical connection before data is sent.
  
  
• Footer: Information that is placed after the data. There is no real difference between the header and the footer, as both generally contain control fields. The term trailer is also sometimes used.
  

  Figure 3: Network Message Formatting

  In the most general of terms, a message consists of a data payload to be communicated, bracketed by a set of header and footer fields. The data of any particular message sent in a networking protocol will itself contain an encapsulated higher-layer message containing a header, data, and footer. This “nesting” can occur many times as data is passed down a protocol stack. The header is found in most protocol messages; the footer only in some.

  
  

Since the header and footer can both contain control and information fields, you might rightly wonder what the point is of having a separate footer anyway. One reason is that some types of control information are calculated using the values of the data itself. In some cases, it is more efficient to perform this computation as the data payload is being sent, and then transmit the result after the payload in a footer. A good example of a field often found in a footer is redundancy data, such as a CRC code, that can be used for error detection by the receiving device. Footers are most often associated with lower-layer protocols, especially at the data link layer of the OSI Reference Model.

Generally speaking, any particular protocol is only concerned with its own header (and footer, if present). It doesn't care much about what is in the data portion of the message, just as a delivery person only worries about driving the truck and not so much on what it contains. At the beginning of that data will normally be the headers of other protocols that were used higher up in the protocol stack; this too is shown in Figure 3. In the OSI Reference Model, a message handled by a particular protocol is said to be its protocol data unit or PDU; the data it carries in its payload is its service data unit or SDU. The SDU of a lower-layer protocol is usually a PDU of a higher-layer protocol. The discussion of data encapsulation contains a full explanation of this important concept.

Messages: Packets, Frames, Datagrams and Cells 1 Message Addressing and Transmission Methods: Unicast, Broadcast and Multicast Messages