Frames
The data-link PDU: header, payload, trailer. Ethernet's 14-byte header is the one you will draw.
Level: beginner
What is it?
A frame is a data-link protocol data unit containing fields such as destination/source addressing, a payload, control metadata, and sometimes error-detection information. Exact formats depend on the link technology; Ethernet is the common LAN example.
Why does it matter?
Framing lets a receiver identify packet boundaries and apply local-link delivery rules. The frame header changes on each routed link even when the IP packet continues end-to-end.
Mental model
The frame is an envelope for one link hop. It carries a payload that may itself contain an IP packet; the next router typically unwraps the incoming frame and builds a different frame for the next link.
How it works
- An Ethernet frame begins with destination and source MAC addresses followed by a type/length field and payload, with a frame check sequence at the end.
- Preamble and start-frame delimiter assist reception on Ethernet but are often handled below ordinary packet captures.
- Minimum and maximum payload/frame sizes are constrained by the Ethernet standard and extensions such as jumbo frames are implementation-dependent.
- Frames on a link may be dropped when their check sequence fails.
- Routers replace link-layer headers while forwarding the IP packet to a next hop.
Steps
- The IP packet is prepared by the network layer.
- The link layer adds its own source/destination addressing and control fields.
- The physical layer transmits the encoded frame.
- The receiver verifies framing/error-detection fields and passes the payload up if accepted.
Real-world usage
- Reading packet captures and identifying Ethernet fields.
- Tracing encapsulation from HTTP to TCP to IP to Ethernet.
Common mistakes
- Confusing frame boundaries with TCP message boundaries; TCP is a byte stream.
- Assuming the same Ethernet frame crosses all routers unchanged.
- Treating every link technology as using Ethernet's exact fields.
Interview questions
Why is a frame rebuilt at a router?
The outgoing link has a new next-hop destination and link-layer format, while the IP packet is generally forwarded inside it with TTL/Hop Limit decremented.
What does an Ethernet FCS help with?
It detects many accidental frame corruptions using a cyclic redundancy check; it does not provide cryptographic authentication.