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Unformatted text preview: intermixed with RFC 894 packets.
3. May be able to send packets using RFC 1042 encapsulation. If the host can send both types
of packets, the type of packet sent must be configurable and the configuration option must
default to RFC 894 packets.
RFC 894 encapsulation is most commonly used. Figure 2.1 shows the two different forms of
encapsulation. The number below each box in the figure is the size of that box in bytes. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (1 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm Both frame formats use 48-bit (6-byte) destination and source addresses. (802.3 allows 16-bit
addresses to be used, but 48-bit addresses are normal.) These are what we call hardware addresses
throughout the text. The ARP and RARP protocols (Chapters 4 and 5) map between the 32-bit IP
addresses and the 48-bit hardware addresses.
The next 2 bytes are different in the two frame formats. The 802 length field says how many bytes
follow, up to but not including the CRC at the end. The Ethernet type field identifies the type of
data that follows. In the 802 frame the same type field occurs later in the SNAP (Sub-network
Access Protocol) header. Fortunately none of the valid 802 length values is the same as the
Ethernet type values, making the two frame formats distinguishable.
In the Ethernet frame the data immediately follows the type field, while in the 802 frame format 3
bytes of 802.2 LLC and 5 bytes of 802.2 SNAP follow. The DSAP (Destination Service Access
Point) and SSAP (Source Service Access Point) are both set to 0xaa. The Ctrl field is set to 3.
The next 3 bytes, the org code are all 0. Following this is the same 2-byte type field that we had
with the Ethernet frame format. (Additional type field values are given in RFC 1340 [Reynolds
and Postel 1992].)
The CRC field is a cyclic redundancy check (a checksum) th...
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