My “What Is Layer-2 and Why Do You Need It?” blog post generated numerous replies, including this one:

Pretend you are a device receiving a stream of bits. After you receive some inter-frame spacing bits, whatever comes next is the 2nd layer; whether that is Ethernet, native IP, CLNS/CLNP, whatever.

Not exactly. IP (or CLNS or CLNP) is always a layer-3 protocol regardless of where in the frame it happens to be, and some layer-2 protocols have no header (apart from inter-frame spacing and start-of-frame indicator).

The Headerless Layer-2 Protocol

I have to admit it was hard to find a layer-2 protocol that has no header – it must have been designed for a strict point-to-point scenario (alleviating the need for layer-2 addresses) and carrying a single layer-3 protocol.

Most layer-2 protocols were designed for multi-protocol use (today that would be IPv4 and IPv6, as almost all other protocols died out), so they need a field in the layer-2 header indicating the type of layer-3 payload – Ethertype in Ethernet header, or NLPID in Frame Relay or PPP header.

The only protocol I found that met both requirements is SLIP (IP over asynchronous serial connection, RFC 1055), which has no header, and only two special characters: end-of-frame and escape. So it’s possible to have no layer-2 header on a link, but pretty unlikely.

Fibre Channel might be another very similar case, which confuses some people in claiming it’s a layer-2 protocol (even though FC standard explicitly mentions routing functionality in FC forwarders).

Almost There

Several layer-2 protocols have very short headers, including:

• PPP, which has only the layer-3 protocol ID and two bytes of historic junk (SDLC address and control bytes);
• Frame Relay header, which contains DLCI (virtual circuit #) and a few flags.

Most other layer-2 protocols originated in multi-access environment (shared cable like Ethernet or multidrop leased lines like SDLC) and thus have at least some layer-2 address information.

Anything Else?

Have I missed another interesting protocol with no (or very small) layer-2 header? Please write a comment!

Want to know more? Read these blog posts

• What is layer-2 and why do we need it?
• Bridging and routing – is there a difference?
• Bridging and routing – part II
• Transparent bridging scalability issues
• Layer-2 network is a single failure domain
• Why is TRILL not routing at layer-2?
• Is layer-3 switch more than a router?
• Is Fibre Channel switching bridging or routing?