IP to link layer encapsulation
The requirements for IP to link layer encapsulation for hosts on a Ethernet network are:
l All hosts must be able to send and receive packets defined by RFC 894.
l All hosts should be able to receive a mix of packets defined by RFC 894 and RFC 1042.
l All hosts may be able to send RDC 1042 defined packets.
Hosts that support both must provide a means to configure the type of packet sent and the default must be
packets defined by RFC 894.
Ethernet and IEEE 802 Encapsulation formats
Ethernet (RFC 894) message format consists of:
1. 6 bytes of destination address.
2. 6 bytes of source address.
3. 2 bytes of message type which indicates the type of data being sent.
4. 46 to 1500 bytes of data.
5. 4 bytes of cyclic redundancy check (CRC) information.
IEEE 802 (RFC 1042) Message format consists of 3 sections plus data and CRC as follows:
1. 802.3 Media Access Control section used to coordinate the sending of data between computers.
1. 6 bytes of destination address.
2. 6 bytes of source address.
3. 2 bytes of length - The number of bytes that follow not including the CRC.
2. 802.2 Logical Link control establishes service access points (SAPs) between computers.
1. 1 byte destination service access point (DSAP).
2. 1 byte source service access point (SSAP).
3. 1 byte of control.
3. Sub Network Access Protocol (SNAP).
1. 3 bytes of org code.
2. 2 bytes of message type which indicates the type of data being sent.
4. 38 to 1492 bytes of data.
5. 4 bytes of cyclic redundancy check (CRC) information.
Some ethernet message types include:
l 0800 - IP datagram with length of 38 to 1492 bytes.
l 0806 - ARP request or reply with 28 bytes and pad bytes that are used to make the frame long
enough for the minimum length.
l 8035 - RARP request or reply of 28 bytes and pad bytes that are used to make the frame long
enough for the minimum length.
These message types are the same for both formats above with the exception of the pad bytes. The pad
bytes for the RFC 894 and RFC 1042 datagrams are of different lengths between the two message
formats because the RFC 894 minimum message length is 46 bytes and the RFC 1042 minimum message
length is 38 bytes. Also the two message formats above are distinguishable from each other. This is
because the RFC 894 possible length values are exclusive of RFC 1042 possible type values.
