Internet DRAFT - draft-heinanen-diffserv-mpls
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Internet Engineering Task Force Juha Heinanen
INTERNET DRAFT Telia Finland
Expires December 1999 June, 1999
Differentiated Services in MPLS Networks
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Distribution of this memo is unlimited.
This document discusses how Differentiated Services can be provided
in MPLS networks according to diffserv's native per-hop model, where
aggregate forwarding resources are allocated in each LSRs for each
supported diffserv forwarding class.
Differentiated Services are based on a per-hop model where aggregate
forwarding resources (buffer space, bandwidth, scheduling policy) are
allocated in each LSR for each diffserv forwarding class (for example
AFi or EF). This is in contrast with the traditional circuit
switching model, where resources are allocated individually for each
circuit on an end-to-end basis.
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In order to support the per-hop model, an MPLS network operator needs
to allocate a set of aggregate forwarding resources for each
supported diffserv forwarding class in each LSR. In addition, the
LSP setup message needs to indicate which diffserv forwarding
class(es) an LSP belongs to. This information can be is used as a
constraint in the LSP route selection process and also in verifying
that packets sent along an LSP belong to correct forwarding classes.
2. Single or Multi-Class LSPs
If all MPLS packets that are sent along an LSP always belong to a
single diffserv forwarding class, the LSP is a Single-Class LSP. In
this case there is no need to indicate in the Exp field of the MPLS
packet header the forwarding class of each packet, because it can be
derived from the label information. The MPLS packet header may,
however, indicate the drop precedence (if any) of the packet. All
packets of such a Single-Class LSP are at each hop assigned
forwarding resources from the aggregate allocated to that particular
If MPLS packets that are sent along an LSP may belong to more than
one diffserv forwarding class, the LSP is a Multi-Class LSP. In this
case the service class of each packet (along with the possible drop
precedence) needs to be indicated in the Exp field of the MPLS packet
header. Each MPLS packet of such a Multi-Class LSP is at each hop
assigned forwarding resources from the aggregate that corresponds to
the service class information in the MPLS packet header. If the
forwarding class of an MPLS packet is not among those listed in the
corresponding LSP setup message, the packet is discarded.
Each MPLS network decides which diffserv forwarding classes it wants
to support and to how many forwarding classes a single LSP may belong
to. It may be possible to map at an MPLS network or at an underlying
subnetwork boundary a Multi-Class LSP to more than one Single or
Multi- -Class LSP and vice versa, but details of such mappings are
outside the scope of this document.
3. Mapping MPLS Packets to Forwarding Classes
When an LSR receives an MPLS packet, it needs to be able to associate
the packet with a particular diffserv forwarding class and/or drop
precedence. For that purpose, each LSR needs to be configured for
each LSP with a list of supported
Exp field value => (FCI value, DPI value)
mappings, where the FCI (Forwarding Class Indicator) value indicates
a diffserv forwarding class and the DPI (Drop Precedence Indicator)
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value indicates a level of drop precedence. The Exp field values are
likely to be MPLS network specific, whereas the FCI and DPI values
could be either globally unique or MPLS network specific.
4. Signaling Requirements
As indicated above, signaling support for Multi-Class LSPs requires
that LSP setup messages contain a list of diffserv FCIs. If the FCI
values are MPLS network specific, they need to be mapped at MPLS
5. Security Concerns
Security concerns are not discussed in this version of the document.
6. Author Addresses
Telia Finland, Inc.
01600 Vantaa, Finland
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