8. MPLS Support of Diff-Serv over LC-ATM Interfaces

This section describes the specific operations required for MPLS support of Diff-Serv over label switching controlled ATM (LC-ATM) interfaces.

This document allows any number of L-LSPs per FEC within an MPLS ATM Diff-Serv domain. E-LSPs are not supported over LC-ATM interfaces.

8.1. Use of ATM Traffic Classes and Traffic Management mechanisms

The use of the "ATM service categories" specified by the ATM Forum, of the "ATM Transfer Capabilities" specified by the ITU-T or of vendor specific ATM traffic classes is outside of the scope of this specification. The only requirement for compliant implementation is that the forwarding behavior experienced by a Behavior Aggregate forwarded over an L-LSP by the ATM LSR MUST be compliant with the corresponding Diff-Serv PHB specifications.

Since there is only one bit (CLP) for encoding the PHB drop precedence value over ATM links, only two different drop precedence levels are supported in ATM LSRs. Sections 4.2.2 and 4.4.2 define how the three drop precedence levels of the AFn Ordered Aggregates are mapped to these two ATM drop precedence levels. This mapping is in accordance with the requirements specified in [DIFF_AF] for the case when only two drop precedence levels are supported.

To avoid discarding parts of the packets, frame discard mechanisms, such as Early Packet Discard (EPD) (see [ATMF_TM]) SHOULD be enabled in the ATM-LSRs for all PHBs described in this document.

8.2. LSR Implementation With LC-ATM Interfaces

A Diff-Serv capable LSR MUST support L-LSPs over LC-ATM interfaces. This specification assumes that Edge-LSRs of the ATM-LSR domain use the "shim header" encapsulation method defined in [MPLS_ATM]. Operations without the "shim header" encapsulation are outside the scope of this specification.