6 Replies Latest reply: Mar 21, 2015 3:52 AM by Fabiola RSS

    Ethernet frame field

    umewannabe

      Ethernet frame field

       

      Length Field: For any IEEE 802.3 standard earlier than 1997 the Length field defines the exact length of the frame's data field. This is used later as part of the FCS to ensure that the message was received properly.

       

      Otherwise the purpose of the field is to describe which higher-layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal or 1536 decimal, then the contents of the Data field are decoded according to the EtherType protocol indicated. Whereas if the value is equal to or less than 0x05DC hexadecimal or 1500 decimal then the Length field is being used to indicate the use of the IEEE 802.3 frame format. This is how Ethernet II and 802.3 frames are differentiated.

       

      Hi all. Does the above mean,  the type/lenght field in the ethernet frame can either be use for either type or length. If the size is equal or greater than 1536 then the field is being used  as a type protocol (used to indicate which higher protocol is present). If the size is less or equal to 1500 then length field is being used which is used by fcs for error detection?

        • 1. Re: Ethernet frame field
          PCreager

          Yes that's what it meant at one time, but no that is no longer an issue.  The confusion you're describing was only an issue during the transition from 802.3 to Ethernet II, but they took steps to avoid ambiguity.

           

          The original (circa 1985) Ethernet frame specification had only a Length field.  This was in the original proposal, which was eventualy standardized in IEEE 802.3.

           

          But by 1997 they realized it would be more useful to have a way to identify frame types, so they repurposed the 2-byte Length field to be used as a Type field instead.  This became the RFC 894 ("Ethernet II") standard frame we use today.

           

          To avoid any confusion they purposefully made the Ethernet Type codes exceed that value, so you will never have a value lower than 0x0600 (1536 decimal) in that field nowadays:
          http://standards.ieee.org/develop/regauth/ethertype/eth.txt

           

          For example, capture packets with something like Wireshark and most commonly you'll see Ethernet Type 0x0800, the IPv4 protocol, which is 2048 decimal.

          • 2. Re: Ethernet frame field
            Deben

            When an Ethernet receiver receives a frame, how does it know what kind of header it is? And the answer to this question lies in the value of the Type/Length field. For a receiver to discern the type of Ethernet header it’s receiving it needs to look at the value of the Type/Length field. If the Type/Length field signifies the presence of 802.2, then the receiver will know if a SNAP header is present by looking at the contents of the 802.2 DSAP field.

             

            • If the Type/Length field has a value of 1536 or higher than the frame is Ethernet V2 and the Type/Length field represents the Type of data to immediately follow in the Data field.
            • If the Type/Length field has a value of less than or equal to 1500 then it represents a Length and we can assume the field immediately following is an 802.2 DSAP. If we find that a DSAP field follows the Type/Length field, next we need to find out if this frame contains a SNAP header immediately following the 802.2 Control field. The presence of a SNAP header is signified by the DSAP field having a value of 0xAA hex. If DSAP has a value of 0xAA then we know a SNAP header immediately follows the 802.2 Control field.

             

             

            Refer https://learningnetwork.cisco.com/docs/DOC-26282 if helps ..

            • 3. Re: Ethernet frame field
              PCreager

              Hi Deben,

              You seem to imply that the older, pre-1997 formats are still in use.  If so do you know where and by whom?

               

              My understanding is that all vendors have adopted the Ethernet II framing standard.

              • 4. Re: Ethernet frame field
                Deben

                Yes now all vendors adopted and support EthernetV2 framing but think of old devices which do not support EthernetV2 framing , only support pre-1997 and still running in production network. I wanted to imply the interoperability between old and new.

                • 5. Re: Ethernet frame field
                  umewannabe

                  tks for the helpful info guys

                  • 6. Re: Ethernet frame field
                    Fabiola

                    Hi Umewannabe and all,

                    Check the article

                    http://en.wikipedia.org/wiki/EtherType

                    About current implementations and difference with previous versions.

                    Good luck!