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Multiprotocol Encapsulation over AAL5 (RFC 2684)
Multiprotocol encapsulation over AAL5 is specified in RFC 2684. RFC 2684 routed encapsulation
allows IP to run over ATM in native mode, as described previously in CIA (RFC 2225). This
method of encapsulation requires separate VCs for each high-level protocol and RFC 2684
routed encapsulation.
In VC-based protocol multiplexing, the carried protocol is identified implicitly by the VC
connecting the two ATM stations. Each protocol must be carried over a separate VC. There is
no need to include explicit multiplexing information in the payload of the AAL5 common part
convergence sublayer protocol data unit (CPCS-PDU). This results in minimal bandwidth and
processing overhead.
In routed LLC encapsulation, the protocol of the routed PDU is identified by prefixing the PDU
with an IEEE 802.2 LLC header, which might be followed by an IEEE 802.1a SNAP header.
The LLC header consists of three octets, the destination subnetwork attachment point (DSAP)
address, source subnetwork attachment point (SSAP) address, and the Control field.
In LLC encapsulation for routed protocols, the Control field is always hexadecimal 03. Hexa-
decimal 03 specifies an unnumbered information command PDU.
RFC 2684 also provides a method for carrying connectionless network traffic as bridged PDUs
over ATM. This method of encapsulation allows multiplexing of multiple protocols over a
single ATM VC. By prefixing the PDU with an IEEE 802.2 LLC header, the protocol of the
PDU is identified. This is the method that you use for LANE.
In LLC encapsulation of bridged PDUs, the type of bridged media is identified in the SNAP
header. The SNAP header contains a three-octet Organizationally Unique Identifier (OUI) that
identifies the organization, which administers the meaning of the following two octets. For
example, the OUI value hexadecimal 00-00-00 specifies that the following Protocol Identifier
(PID) contains an Ethernet type field.
The next two octets are the PID. Together, the OUI and the PID identify a distinct bridged
protocol. Figure 5-29 shows a bridged Ethernet media LLC/SNAP encapsulation.
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