Header, Payload or Trailer?

About this interactive

To send a message to one machine on a hub, the sender has to add that machine's MAC address to it. Adding the extra information needed for delivery is called encapsulation, and it is not only Ethernet that does it: almost every protocol does. An encapsulated message has up to three parts. The payload, or data: what is being delivered. The header, in front: information that makes delivery work, defined by the protocol. It might carry addressing, a sequence number so that data sent in many pieces can be put back in order, or a field saying which protocol is inside the payload. The trailer, at the end, which only some protocols add: often error checking, so the receiver can tell whether anything was damaged on the way. Messages are often encapsulated more than once: a message that already has a header becomes the payload of the next protocol, which adds its own. An Ethernet frame is the whole of these together. Its header holds the destination MAC address (48 bits), which every receiving card reads first; the source MAC address (48 bits), so the receiver knows who sent it; and a 16-bit type field describing the payload. The payload is 368 to 12,000 bits (46 to 1,500 bytes). The trailer is the 32-bit frame check sequence: the sender calculates it from the frame, and the receiver calculates it again; if they differ, the frame was damaged and is thrown away. It catches accidental damage. It does not stop someone changing the frame on purpose, because they could simply calculate a new one.

What you'll learn

Sort the parts of an encapsulated message into header, payload or trailer, with the Ethernet frame as the worked case
Describe an Ethernet frame field by field: destination and source MAC addresses (48 bits each), type (16 bits), payload (368 to 12,000 bits) and frame check sequence (32 bits)

Aligned to

CompTIA Network+

1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.

Cisco CCST Networking

1.1 Identify the fundamental conceptual building blocks of networks

Cisco CCNA

1.1 Explain the role and function of network components

1.13 Describe switching concepts

Key terms

Encapsulation: The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
Header: Control information prepended to the payload during encapsulation that supports delivery functions such as addressing, sequencing, and protocol identification.
Payload: The actual data content of a packet or message, excluding headers and metadata.
Trailer: Control information appended to the payload during encapsulation, commonly used for error checking to verify data integrity during delivery.
Ethernet Frame: The Layer 2 data unit used on Ethernet networks, containing a preamble, source and destination MAC addresses, an EtherType field identifying the Layer 3 protocol, a payload (typically an IP packet), and a 4-byte FCS for error detection. Switches use the MAC addresses in Ethernet frames to make forwarding decisions.
Frame Check Sequence: A cyclic redundancy check value appended to the end of a data link frame that allows the receiver to detect transmission errors. The sending device calculates the FCS from the frame contents, and the receiving device recalculates it; a mismatch means the frame was corrupted and should be discarded.
MAC Address: A 48-bit hardware address assigned to a network interface and written as six hexadecimal octets, such as 00:1A:2B:3C:4D:5E. It identifies a device uniquely on a local network segment, and switches forward frames by looking it up in their MAC address table.

Topics

Encapsulation Header Payload Trailer Ethernet Frame Frame Check Sequence Interactive Categorize

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