TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Introduced in 1985, RFC 948 – Two Methods for the Transmission of IP
Datagrams over IEEE 802.3 Networks specifies two possibilities:
– The Ethernet compatible method: The frames are sent on a real IEEE
802.3 network in the same fashion as on an Ethernet network, that is,
using the IEEE 802.3 data-length field as the Ethernet type field, thereby
violating the IEEE 802.3 rules, but compatible with an Ethernet network.
– IEEE 802.2/802.3 LLC type 1 format: Using 802.2 LSAP header with IP
using the value 6 for the SSAP and DSAP fields.
The RFC indicates clearly that the IEEE 802.2/802.3 method is the preferred
method, that is, that all future IP implementations on IEEE 802.3 networks are
supposed to use the second method.
► Introduced in 1987, RFC 1010 – Assigned Numbers (now obsoleted by RFC
3232, dated 2002) notes that as a result of IEEE 802.2 evolution and the need
for more Internet protocol numbers, a new approach was developed based on
practical experiences exchanged during the August 1986 TCP Vendors
Workshop. It states, in an almost completely overlooked part of this RFC, that
all IEEE 802.3, 802.4, and 802.5 implementations should use the Subnetwork
Access Protocol (SNAP) form of the IEEE 802.2 LLC, with the DSAP and
SSAP fields set to 170 (indicating the use of SNAP), with SNAP assigned as
follows:
– 0 (zero) as organization code
– EtherType field:
2048 (hex 0800), for IP datagrams
2054 (hex 0806), for ARP datagrams
32821 (hex 8035), for RARP datagrams
These are the same values used in the Ethernet type field.
► In1988, RFC 1042 – Standard for the Transmission of IP Datagrams over
IEEE 802 Networks was introduced. Because this new approach (very
important for implementations) passed almost unnoticed in a little note of an
unrelated RFC, it became quite confusing. As such, in February of 1988 it
was repeated in an RFC on its own, RFC 1042, which now obsoletes RFC
948.
► In 1998, RFC 2464 – Transmission of IPv6 Packets over Ethernet Networks
was introduced. This extended Ethernet’s frame format to allow IPv6 packets
to traverse Ethernet networks.
The relevant IBM TCP/IP products implement RFC 894 for DIX Ethernet and
RFC 3232 for IEEE 802.3 networks. However, in practical situations, there are
still TCP/IP implementations that use the older LSAP method (RFC 948 or 1042).
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