RFC 3031 - 2. Introduction to MPLS
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Introduction to MPLS
This document specifies the architecture for Multiprotocol Label Switching (MPLS).
Note that the use of MPLS for multicast is left for further study.
2.1. Overview
As a packet of a connectionless network layer protocol travels from one router to the next, each router makes an independent forwarding decision for that packet. That is, each router analyzes the packet's header, and each router runs a network layer routing algorithm. Each router independently chooses a next hop for the packet, based on its analysis of the packet's header and the results of running the routing algorithm.
Packet headers contain considerably more information than is needed simply to choose the next hop. Choosing the next hop can therefore be thought of as the composition of two functions. The first function partitions the entire set of possible packets into a set of "Forwarding Equivalence Classes (FECs)". The second maps each FEC to a next hop. Insofar as the forwarding decision is concerned, different packets which get mapped into the same FEC are indistinguishable. All packets which belong to a particular FEC and which travel from a particular node will follow the same path (or if certain kinds of multi-path routing are in use, they will all follow one of a set of paths associated with the FEC).
In conventional IP forwarding, a particular router will typically consider two packets to be in the same FEC if there is some address prefix X in that router's routing tables such that X is the "longest match" for each packet's destination address. As the packet traverses the network, each hop in turn reexamines the packet and assigns it to a FEC.
In MPLS, the assignment of a particular packet to a particular FEC is done just once, as the packet enters the network. The FEC to which the packet is assigned is encoded as a short fixed length value known as a "label". When a packet is forwarded to its next hop, the label is sent along with it; that is, the packets are "labeled" before they are forwarded.
At subsequent hops, there is no further analysis of the packet's network layer header. Rather, the label is used as an index into a table which specifies the next hop, and a new label. The old label is replaced with the new label, and the packet is forwarded to its next hop.
In the MPLS forwarding paradigm, once a packet is assigned to a FEC, no further header analysis is done by subsequent routers; all forwarding is driven by the labels. This has a number of advantages over conventional network layer forwarding.
- MPLS forwarding can be done by switches which are capable of
doing label lookup and replacement, but are either not capable
of analyzing the network layer headers, or are not capable of
analyzing the network layer headers at adequate speed.
- Since a packet is assigned to a FEC when it enters the network,
the ingress router may use, in determining the assignment, any
information it has about the packet, even if that information
cannot be gleaned from the network layer header. For example,
packets arriving on different ports may be assigned to
different FECs. Conventional forwarding, on the other hand,
can only consider information which travels with the packet in
the packet header.
- A packet that enters the network at a particular router can be
labeled differently than the same packet entering the network
at a different router, and as a result forwarding decisions
that depend on the ingress router can be easily made. This
cannot be done with conventional forwarding, since the identity
of a packet's ingress router does not travel with the packet.
- The considerations that determine how a packet is assigned to a
FEC can become ever more and more complicated, without any
impact at all on the routers that merely forward labeled
packets.
- Sometimes it is desirable to force a packet to follow a
particular route which is explicitly chosen at or before the
time the packet enters the network, rather than being chosen by
the normal dynamic routing algorithm as the packet travels
through the network. This may be done as a matter of policy,
or to support traffic engineering. In conventional forwarding,
this requires the packet to carry an encoding of its route
along with it ("source routing"). In MPLS, a label can be used
to represent the route, so that the identity of the explicit
route need not be carried with the packet.
Some routers analyze a packet's network layer header not merely to choose the packet's next hop, but also to determine a packet's "precedence" or "class of service". They may then apply different discard thresholds or scheduling disciplines to different packets. MPLS allows (but does not require) the precedence or class of service to be fully or partially inferred from the label. In this case, one may say that the label represents the combination of a FEC and a precedence or class of service.
MPLS stands for "Multiprotocol" Label Switching, multiprotocol because its techniques are applicable to ANY network layer protocol. In this document, however, we focus on the use of IP as the network layer protocol.
A router which supports MPLS is known as a "Label Switching Router", or LSR.
2.2. Terminology
This section gives a general conceptual overview of the terms used in this document. Some of these terms are more precisely defined in later sections of the document.
DLCI a label used in Frame Relay networks to
identify frame relay circuits
forwarding equivalence class a group of IP packets which are
forwarded in the same manner (e.g.,
over the same path, with the same
forwarding treatment)
frame merge label merging, when it is applied to
operation over frame based media, so
that the potential problem of cell
interleave is not an issue.
label a short fixed length physically
contiguous identifier which is used to
identify a FEC, usually of local
significance.
label merging the replacement of multiple incoming
labels for a particular FEC with a
single outgoing label
label swap the basic forwarding operation
consisting of looking up an incoming
label to determine the outgoing label,
encapsulation, port, and other data
handling information.
label swapping a forwarding paradigm allowing
streamlined forwarding of data by using
labels to identify classes of data
packets which are treated
indistinguishably when forwarding.
label switched hop the hop between two MPLS nodes, on which
forwarding is done using labels.
label switched path The path through one or more LSRs at one
level of the hierarchy followed by a
packets in a particular FEC.
label switching router an MPLS node which is capable of
forwarding native L3 packets
layer 2 the protocol layer under layer 3 (which
therefore offers the services used by
layer 3). Forwarding, when done by the
swapping of short fixed length labels,
occurs at layer 2 regardless of whether
the label being examined is an ATM
VPI/VCI, a frame relay DLCI, or an MPLS
label.
layer 3 the protocol layer at which IP and its
associated routing protocols operate
link layer synonymous with layer 2
loop detection a method of dealing with loops in which
loops are allowed to be set up, and data
may be transmitted over the loop, but
the loop is later detected
loop prevention a method of dealing with loops in which
data is never transmitted over a loop
label stack an ordered set of labels
merge point a node at which label merging is done
MPLS domain a contiguous set of nodes which operate
MPLS routing and forwarding and which
are also in one Routing or
Administrative Domain
MPLS edge node an MPLS node that connects an MPLS
domain with a node which is outside of
the domain, either because it does not
run MPLS, and/or because it is in a
different domain. Note that if an LSR
has a neighboring host which is not
running MPLS, that that LSR is an MPLS
edge node.
MPLS egress node an MPLS edge node in its role in
handling traffic as it leaves an MPLS
domain
MPLS ingress node an MPLS edge node in its role in
handling traffic as it enters an MPLS
domain
MPLS label a label which is carried in a packet
header, and which represents the
packet's FEC
MPLS node a node which is running MPLS. An MPLS
node will be aware of MPLS control
protocols, will operate one or more L3
routing protocols, and will be capable
of forwarding packets based on labels.
An MPLS node may optionally be also
capable of forwarding native L3 packets.
MultiProtocol Label Switching an IETF working group and the
effort associated with the working
group
network layer synonymous with layer 3
stack synonymous with label stack
switched path synonymous with label switched path
virtual circuit a circuit used by a connection-oriented
layer 2 technology such as ATM or Frame
Relay, requiring the maintenance of
state information in layer 2 switches.
VC merge label merging where the MPLS label is
carried in the ATM VCI field (or
combined VPI/VCI field), so as to allow
multiple VCs to merge into one single VC
VP merge label merging where the MPLS label is
carried din the ATM VPI field, so as to
allow multiple VPs to be merged into one
single VP. In this case two cells would
have the same VCI value only if they
originated from the same node. This
allows cells from different sources to
be distinguished via the VCI.
VPI/VCI a label used in ATM networks to identify
circuits
2.3. Acronyms and Abbreviations
ATM Asynchronous Transfer Mode BGP Border Gateway Protocol DLCI Data Link Circuit Identifier FEC Forwarding Equivalence Class FTN FEC to NHLFE Map IGP Interior Gateway Protocol ILM Incoming Label Map IP Internet Protocol LDP Label Distribution Protocol L2 Layer 2 L3 Layer 3 LSP Label Switched Path LSR Label Switching Router MPLS MultiProtocol Label Switching NHLFE Next Hop Label Forwarding Entry SVC Switched Virtual Circuit SVP Switched Virtual Path TTL Time-To-Live VC Virtual Circuit VCI Virtual Circuit Identifier VP Virtual Path VPI Virtual Path Identifier
2.4. Acknowledgments
The ideas and text in this document have been collected from a number of sources and comments received. We would like to thank Rick Boivie, Paul Doolan, Nancy Feldman, Yakov Rekhter, Vijay Srinivasan, and George Swallow for their inputs and ideas.