802.3: Length interpretation and payload type

As you say, the sender and receiver could in principle make whatever agreements they want. But the more standards-oriented answer would probably be that 802.3 only describes Layer 1. And that if you want to go up to Layer 3 and beyond, you first need to obsess over layer 2. Layer 2 is covered in 802.2, where LLC is.

The way I look at it, the original Xerox Ethernet spec sort of glossed over the stricter layer orientation the IEEE worked with. The "length" format is the original IEEE take on what Ethernet should look like. Check out the older 802.3 documents and you won't even find the "type" format at all.

So, like all other IEEE standards, the original 802.3 assumed that you'd layer 802.2 on top of the physical layer and the MAC layer, as all the other 802 nets did. But instead, with Ethernet, they had to deal with legacy. And the legacy was stronger than the new-fangled

Bert

Albert Manfredi wrote: (snip)

So the confusing thing is that the type field is described in layer 1, but LLC isn't?

Well, Xerox was there first, right? And type works well enough for most of us, so why change?

-- glen

In a sense, the Type field *is* the link layer, IMO. The Type field is in fact a Protocol ID field, identifying (among other possibilties) the Layer 3 protocol carried in that Ethernet frame. The MAC layer, instead, is usually considered to be the upper half of the Physical layer. So that leaves the Type field as Layer 2.

Agreed. And also, the DSAP and LSAP fields ended up being way too short to be useful. So in fact that meant that those who wanted to use LLC also ended up having to use the SNAP header, with its own 16-bit Protocol ID field. So, given that the Xerox format already had that nice 16-bit Type field, why bother with all that overhead?

(Actually, I still like to use SNAP, because it allows me to invent protocols without having to go to someone else to get them registered.)

Bert

I've got a similar impression. The modern text of 802.3 suggests it was patched once to match both DIX and IEEE 802 realities when IEEE decided to recognize both branches of Ethernet. E.g., at the very top of Section 4, Media Access Control, we can read:

The MAC sublayer defines a medium-independent facility, built on the medium-dependent physical facility provided by the Physical Layer, and under the access-layer-independent LAN LLC sublayer (or other MAC client).

Such parenthesised references to "other MAC client" can be found all over 802.3; clearly, they were added later. However, the patch wasn't ideal and created options that shouldn't have been there, like the one in question. That's my conclusion.

Yar

No. The MAC sublayer (that's the proper IEEE 802 term) is the lower part of the Data Link Layer. Ethernet/IEEE 802.3 comprises both Physical and Data Link entities.

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Sorry, you're right. My confusion. The MAC layer is often called Layer

Bert