RFC 2684 (rfc2684) - Page 2 of 23
Multiprotocol Encapsulation over ATM Adaptation Layer 5
Alternative Format: Original Text Document
RFC 2684 Multiprotocol Over AALS September 1999
The unit of transport in ATM is a 53 octet fixed length PDU called a
cell. A cell consists of a 5 octet header and a 48 byte payload.
Variable length PDUs, including those addressed in this memo, must be
segmented by the transmitter to fit into the 48 octet ATM cell
payload, and reassembled by the receiver. This memo specifies the
use of the ATM Adaptation Layer type 5 (AAL5), as defined in ITU-T
Recommendation I.363.5 [2] for this purpose. Variable length PDUs are
carried in the Payload field of the AAL5 Common Part Convergence
Sublayer (CPCS) PDU.
This memo only describes how routed and bridged PDUs are carried
directly over the AAL5 CPCS, i.e., when the Service Specific
Convergence Sublayer (SSCS) of AAL5 is absent. If Frame Relay
Service Specific Convergence Sublayer (FR-SSCS), as defined in ITU-T
Recommendation I.365.1 [3], is used over the CPCS, then routed and
bridged PDUs are carried using the NLPID multiplexing method
described in RFC 2427 [4]. The RFC 2427 encapsulation MUST be used in
the special case that Frame Relay Network Interworking or transparent
mode Service Interworking [9] are used, but is NOT RECOMMENDED for
other applications. Appendix A (which is for information only) shows
the format of the FR-SSCS-PDU as well as how IP and CLNP PDUs are
encapsulated over FR-SSCS according to RFC 2427.
This memo also includes an optional encapsulation for use with
Virtual Private Networks that operate over an ATM subnet.
If it is desired to use the facilities which are designed for the
Point-to-Point Protocol (PPP), and there exists a point-to-point
relationship between peer systems, then RFC 2364, rather than this
memo, applies.
2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
RFC 2119 [10].
3. Selection of the Multiplexing Method
The decision as to whether to use LLC encapsulation or VC-
multiplexing depends on implementation and system requirements. In
general, LLC encapsulation tends to require fewer VCs in a
multiprotocol environment. VC multiplexing tends to reduce
fragmentation overhead (e.g., an IPV4 datagram containing a TCP
control packet with neither IP nor TCP options exactly fits into a
single cell).
Grossman & Heinanen Standards Track