What is the difference between Ethernet II and 802.3 Ethernet?

Question

Can anyone explain to me what is the difference between Ethernet, Ethernet-II and 802.3 Ethernet.

I referred to a lot of books, they are in high level language. Can anyone explain to me in a simple manner?

Answer 1

802.3 (which uses 802.2 LLC format) has a Length field in the same place that Ethernet II has a Type field.

• IEEE 802.3 with 802.2 LLC (used by Spanning-Tree, ISIS) use the highlighted bytes for a Length field. 802.3 Upper-layer protocols are decoded via the 802.2 LLC Header / SNAP bytes. The SNAP bytes are used to decode protocols using traditional ethertype values; SNAP is only included when the 802.2 LLC DSAP / SSAP = 0xAAAA.

      +----+----+------+------+------+------+-----+
      | DA | SA | Len  | LLC  | SNAP | Data | FCS |
      +----+----+------+------+------+------+-----+
                ^^^^^^^^
  
      DA      Destination MAC Address (6 bytes)
      SA      Source MAC Address      (6 bytes)
      Len     Length of Data field    (2 bytes: <= 0x05DC or 1500 decimal)  <---
      LLC     802.2 LLC Header        (3 bytes)
      SNAP                            (5 bytes)
      Data    Protocol Data           (46 - 1500 bytes)
      FCS     Frame Checksum          (4 bytes)
  

• RFC 894 (commonly known as Ethernet II frames) use these bytes for Type. Upper-layer protocols are decoded via the Type field

      +----+----+------+------+-----+
      | DA | SA | Type | Data | FCS |
      +----+----+------+------+-----+
                ^^^^^^^^
  
      DA      Destination MAC Address (6 bytes)
      SA      Source MAC Address      (6 bytes)
      Type    Protocol Type           (2 bytes: >= 0x0600 or 1536 decimal)  <---
      Data    Protocol Data           (46 - 1500 bytes)
      FCS     Frame Checksum          (4 bytes)
  

Answer 2

History is messy.

Ethernet was originally designed by a consortium of three companies, Digital, Intel and Xerox. The consortium published two versions of the Ethernet standard themselves, before working to Standardise Ethernet with the IEEE as 802.3. Version ii of the Ethernet standard was released in 1982 with the IEEE standard 802.3 arriving in 1983.

During the IEEE standardisation process the frame format was changed. Presumably in an effort to keep a consistent frame format across different sub-standards of IEEE 802. This frame format had more overhead than the format defined in Ethernet ii.

Some protocols did adopt the 802.3 frame format, but the Ethernet ii frame format remained in use. For added fun, Novell used a frame format known as "raw 802.3" based on an early draft of the 802.3 design work but not entirely compliant with it.

In particular IP over Ethernet deployments almost universally continued to use the Ethernet ii frame format. A standard for IP over 802.3 was written in 1985 as RFC 948, but as far as I can tell this standard was stillborn. A later standard for IP over IEEE 802 family protocols was written in 1988 as RFC 1042, but it was not widely used on Ethernet.

So the result was a situation where three different frame formats, one from an international standards body, one from a vendor consortium and one from a single vendor coexisted on the same Ethernet networks.

Eventually in 1997 the IEEE added the "Ethernet ii" frame format to the 802.3 standard, presumably recognising that it was not going to go away. Indeed the opposite was happening, IP and hence the Ethernet ii frame format were coming to dominate the Ethernet world.

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So the meaning of the terms depends on the context, "Ethernet ii" could refer to the old version of the Ethernet standard, but more likely it refers to the frame format defined by that standard. Similarly 802.3 might refer to the IEEE standard, or it might refer to the frame format that originated from that standard.