RFC 3031
Multiprotocol Label Switching Architecture
4. Some Applications of MPLS
4.1. MPLS and Hop by Hop Routed Traffic
A number of uses of MPLS require that packets with a certain label be forwarded along the same hop-by-hop routed path that would be used for forwarding a packet with a specified address in its network layer destination address field.4.1.1. Labels for Address Prefixes
In general, router R determines the next hop for packet P by finding the address prefix X in its routing table which is the longest match for P's destination address. That is, the packets in a given FEC are just those packets which match a given address prefix in R's routing table. In this case, a FEC can be identified with an address prefix. Note that a packet P may be assigned to FEC F, and FEC F may be identified with address prefix X, even if P's destination address does not match X.4.1.2. Distributing Labels for Address Prefixes
4.1.2.1. Label Distribution Peers for an Address Prefix
LSRs R1 and R2 are considered to be label distribution peers for address prefix X if and only if one of the following conditions holds: 1. R1's route to X is a route which it learned about via a particular instance of a particular IGP, and R2 is a neighbor of R1 in that instance of that IGP 2. R1's route to X is a route which it learned about by some instance of routing algorithm A1, and that route is redistributed into an instance of routing algorithm A2, and R2 is a neighbor of R1 in that instance of A2
3. R1 is the receive endpoint of an LSP Tunnel that is within
another LSP, and R2 is a transmit endpoint of that tunnel, and
R1 and R2 are participants in a common instance of an IGP, and
are in the same IGP area (if the IGP in question has areas),
and R1's route to X was learned via that IGP instance, or is
redistributed by R1 into that IGP instance
4. R1's route to X is a route which it learned about via BGP, and
R2 is a BGP peer of R1
In general, these rules ensure that if the route to a particular
address prefix is distributed via an IGP, the label distribution
peers for that address prefix are the IGP neighbors. If the route to
a particular address prefix is distributed via BGP, the label
distribution peers for that address prefix are the BGP peers. In
other cases of LSP tunneling, the tunnel endpoints are label
distribution peers.
4.1.2.2. Distributing Labels
In order to use MPLS for the forwarding of packets according to the
hop-by-hop route corresponding to any address prefix, each LSR MUST:
1. bind one or more labels to each address prefix that appears in
its routing table;
2. for each such address prefix X, use a label distribution
protocol to distribute the binding of a label to X to each of
its label distribution peers for X.
There is also one circumstance in which an LSR must distribute a
label binding for an address prefix, even if it is not the LSR which
bound that label to that address prefix:
3. If R1 uses BGP to distribute a route to X, naming some other
LSR R2 as the BGP Next Hop to X, and if R1 knows that R2 has
assigned label L to X, then R1 must distribute the binding
between L and X to any BGP peer to which it distributes that
route.
These rules ensure that labels corresponding to address prefixes
which correspond to BGP routes are distributed to IGP neighbors if
and only if the BGP routes are distributed into the IGP. Otherwise,
the labels bound to BGP routes are distributed only to the other BGP
speakers.
These rules are intended only to indicate which label bindings must
be distributed by a given LSR to which other LSRs.
4.1.3. Using the Hop by Hop path as the LSP
If the hop-by-hop path that packet P needs to follow is <R1, ..., Rn>, then <R1, ..., Rn> can be an LSP as long as: 1. there is a single address prefix X, such that, for all i, 1<=i<n, X is the longest match in Ri's routing table for P's destination address; 2. for all i, 1<i<n, Ri has assigned a label to X and distributed that label to R[i-1]. Note that a packet's LSP can extend only until it encounters a router whose forwarding tables have a longer best match address prefix for the packet's destination address. At that point, the LSP must end and the best match algorithm must be performed again. Suppose, for example, that packet P, with destination address 10.2.153.178 needs to go from R1 to R2 to R3. Suppose also that R2 advertises address prefix 10.2/16 to R1, but R3 advertises 10.2.153/23, 10.2.154/23, and 10.2/16 to R2. That is, R2 is advertising an "aggregated route" to R1. In this situation, packet P can be label Switched until it reaches R2, but since R2 has performed route aggregation, it must execute the best match algorithm to find P's FEC.4.1.4. LSP Egress and LSP Proxy Egress
An LSR R is considered to be an "LSP Egress" LSR for address prefix X if and only if one of the following conditions holds: 1. R has an address Y, such that X is the address prefix in R's routing table which is the longest match for Y, or 2. R contains in its routing tables one or more address prefixes Y such that X is a proper initial substring of Y, but R's "LSP previous hops" for X do not contain any such address prefixes Y; that is, R is a "deaggregation point" for address prefix X. An LSR R1 is considered to be an "LSP Proxy Egress" LSR for address prefix X if and only if: 1. R1's next hop for X is R2, and R1 and R2 are not label distribution peers with respect to X (perhaps because R2 does not support MPLS), or 2. R1 has been configured to act as an LSP Proxy Egress for X
The definition of LSP allows for the LSP Egress to be a node which does not support MPLS; in this case the penultimate node in the LSP is the Proxy Egress.4.1.5. The Implicit NULL Label
The Implicit NULL label is a label with special semantics which an LSR can bind to an address prefix. If LSR Ru, by consulting its ILM, sees that labeled packet P must be forwarded next to Rd, but that Rd has distributed a binding of Implicit NULL to the corresponding address prefix, then instead of replacing the value of the label on top of the label stack, Ru pops the label stack, and then forwards the resulting packet to Rd. LSR Rd distributes a binding between Implicit NULL and an address prefix X to LSR Ru if and only if: 1. the rules of Section 4.1.2 indicate that Rd distributes to Ru a label binding for X, and 2. Rd knows that Ru can support the Implicit NULL label (i.e., that it can pop the label stack), and 3. Rd is an LSP Egress (not proxy egress) for X. This causes the penultimate LSR on a LSP to pop the label stack. This is quite appropriate; if the LSP Egress is an MPLS Egress for X, then if the penultimate LSR does not pop the label stack, the LSP Egress will need to look up the label, pop the label stack, and then look up the next label (or look up the L3 address, if no more labels are present). By having the penultimate LSR pop the label stack, the LSP Egress is saved the work of having to look up two labels in order to make its forwarding decision. However, if the penultimate LSR is an ATM switch, it may not have the capability to pop the label stack. Hence a binding of Implicit NULL may be distributed only to LSRs which can support that function. If the penultimate LSR in an LSP for address prefix X is an LSP Proxy Egress, it acts just as if the LSP Egress had distributed a binding of Implicit NULL for X.4.1.6. Option: Egress-Targeted Label Assignment
There are situations in which an LSP Ingress, Ri, knows that packets of several different FECs must all follow the same LSP, terminating at, say, LSP Egress Re. In this case, proper routing can be achieved
by using a single label for all such FECs; it is not necessary to
have a distinct label for each FEC. If (and only if) the following
conditions hold:
1. the address of LSR Re is itself in the routing table as a "host
route", and
2. there is some way for Ri to determine that Re is the LSP egress
for all packets in a particular set of FECs
Then Ri may bind a single label to all FECS in the set. This is
known as "Egress-Targeted Label Assignment."
How can LSR Ri determine that an LSR Re is the LSP Egress for all
packets in a particular FEC? There are a number of possible ways:
- If the network is running a link state routing algorithm, and
all nodes in the area support MPLS, then the routing algorithm
provides Ri with enough information to determine the routers
through which packets in that FEC must leave the routing domain
or area.
- If the network is running BGP, Ri may be able to determine that
the packets in a particular FEC must leave the network via some
particular router which is the "BGP Next Hop" for that FEC.
- It is possible to use the label distribution protocol to pass
information about which address prefixes are "attached" to
which egress LSRs. This method has the advantage of not
depending on the presence of link state routing.
If egress-targeted label assignment is used, the number of labels
that need to be supported throughout the network may be greatly
reduced. This may be significant if one is using legacy switching
hardware to do MPLS, and the switching hardware can support only a
limited number of labels.
One possible approach would be to configure the network to use
egress-targeted label assignment by default, but to configure
particular LSRs to NOT use egress-targeted label assignment for one
or more of the address prefixes for which it is an LSP egress. We
impose the following rule:
- If a particular LSR is NOT an LSP Egress for some set of
address prefixes, then it should assign labels to the address
prefixes in the same way as is done by its LSP next hop for
those address prefixes. That is, suppose Rd is Ru's LSP next
hop for address prefixes X1 and X2. If Rd assigns the same
label to X1 and X2, Ru should as well. If Rd assigns different
labels to X1 and X2, then Ru should as well.
For example, suppose one wants to make egress-targeted label
assignment the default, but to assign distinct labels to those
address prefixes for which there are multiple possible LSP egresses
(i.e., for those address prefixes which are multi-homed.) One can
configure all LSRs to use egress-targeted label assignment, and then
configure a handful of LSRs to assign distinct labels to those
address prefixes which are multi-homed. For a particular multi-homed
address prefix X, one would only need to configure this in LSRs which
are either LSP Egresses or LSP Proxy Egresses for X.
It is important to note that if Ru and Rd are adjacent LSRs in an LSP
for X1 and X2, forwarding will still be done correctly if Ru assigns
distinct labels to X1 and X2 while Rd assigns just one label to the
both of them. This just means that R1 will map different incoming
labels to the same outgoing label, an ordinary occurrence.
Similarly, if Rd assigns distinct labels to X1 and X2, but Ru assigns
to them both the label corresponding to the address of their LSP
Egress or Proxy Egress, forwarding will still be done correctly. Ru
will just map the incoming label to the label which Rd has assigned
to the address of that LSP Egress.
4.2. MPLS and Explicitly Routed LSPs
There are a number of reasons why it may be desirable to use explicit
routing instead of hop by hop routing. For example, this allows
routes to be based on administrative policies, and allows the routes
that LSPs take to be carefully designed to allow traffic engineering
[MPLS-TRFENG].
4.2.1. Explicitly Routed LSP Tunnels
In some situations, the network administrators may desire to forward
certain classes of traffic along certain pre-specified paths, where
these paths differ from the Hop-by-hop path that the traffic would
ordinarily follow. This can be done in support of policy routing, or
in support of traffic engineering. The explicit route may be a
configured one, or it may be determined dynamically by some means,
e.g., by constraint-based routing.
MPLS allows this to be easily done by means of Explicitly Routed LSP
Tunnels. All that is needed is:
1. A means of selecting the packets that are to be sent into the
Explicitly Routed LSP Tunnel;
2. A means of setting up the Explicitly Routed LSP Tunnel;
3. A means of ensuring that packets sent into the Tunnel will not
loop from the receive endpoint back to the transmit endpoint.
If the transmit endpoint of the tunnel wishes to put a labeled packet
into the tunnel, it must first replace the label value at the top of
the stack with a label value that was distributed to it by the
tunnel's receive endpoint. Then it must push on the label which
corresponds to the tunnel itself, as distributed to it by the next
hop along the tunnel. To allow this, the tunnel endpoints should be
explicit label distribution peers. The label bindings they need to
exchange are of no interest to the LSRs along the tunnel.
4.3. Label Stacks and Implicit Peering
Suppose a particular LSR Re is an LSP proxy egress for 10 address
prefixes, and it reaches each address prefix through a distinct
interface.
One could assign a single label to all 10 address prefixes. Then Re
is an LSP egress for all 10 address prefixes. This ensures that
packets for all 10 address prefixes get delivered to Re. However, Re
would then have to look up the network layer address of each such
packet in order to choose the proper interface to send the packet on.
Alternatively, one could assign a distinct label to each interface.
Then Re is an LSP proxy egress for the 10 address prefixes. This
eliminates the need for Re to look up the network layer addresses in
order to forward the packets. However, it can result in the use of a
large number of labels.
An alternative would be to bind all 10 address prefixes to the same
level 1 label (which is also bound to the address of the LSR itself),
and then to bind each address prefix to a distinct level 2 label.
The level 2 label would be treated as an attribute of the level 1
label binding, which we call the "Stack Attribute". We impose the
following rules:
- When LSR Ru initially labels a hitherto unlabeled packet, if
the longest match for the packet's destination address is X,
and Ru's LSP next hop for X is Rd, and Rd has distributed to Ru
a binding of label L1 to X, along with a stack attribute of L2,
then
1. Ru must push L2 and then L1 onto the packet's label stack,
and then forward the packet to Rd;
2. When Ru distributes label bindings for X to its label
distribution peers, it must include L2 as the stack
attribute.
3. Whenever the stack attribute changes (possibly as a result
of a change in Ru's LSP next hop for X), Ru must distribute
the new stack attribute.
Note that although the label value bound to X may be different at
each hop along the LSP, the stack attribute value is passed
unchanged, and is set by the LSP proxy egress.
Thus the LSP proxy egress for X becomes an "implicit peer" with each
other LSR in the routing area or domain. In this case, explicit
peering would be too unwieldy, because the number of peers would
become too large.
4.4. MPLS and Multi-Path Routing
If an LSR supports multiple routes for a particular stream, then it
may assign multiple labels to the stream, one for each route. Thus
the reception of a second label binding from a particular neighbor
for a particular address prefix should be taken as meaning that
either label can be used to represent that address prefix.
If multiple label bindings for a particular address prefix are
specified, they may have distinct attributes.
4.5. LSP Trees as Multipoint-to-Point Entities
Consider the case of packets P1 and P2, each of which has a
destination address whose longest match, throughout a particular
routing domain, is address prefix X. Suppose that the Hop-by-hop
path for P1 is <R1, R2, R3>, and the Hop-by-hop path for P2 is <R4,
R2, R3>. Let's suppose that R3 binds label L3 to X, and distributes
this binding to R2. R2 binds label L2 to X, and distributes this
binding to both R1 and R4. When R2 receives packet P1, its incoming
label will be L2. R2 will overwrite L2 with L3, and send P1 to R3.
When R2 receives packet P2, its incoming label will also be L2. R2
again overwrites L2 with L3, and send P2 on to R3.
Note then that when P1 and P2 are traveling from R2 to R3, they carry
the same label, and as far as MPLS is concerned, they cannot be
distinguished. Thus instead of talking about two distinct LSPs, <R1,
R2, R3> and <R4, R2, R3>, we might talk of a single "Multipoint-to-
Point LSP Tree", which we might denote as <{R1, R4}, R2, R3>.
This creates a difficulty when we attempt to use conventional ATM
switches as LSRs. Since conventional ATM switches do not support
multipoint-to-point connections, there must be procedures to ensure
that each LSP is realized as a point-to-point VC. However, if ATM
switches which do support multipoint-to-point VCs are in use, then
the LSPs can be most efficiently realized as multipoint-to-point VCs.
Alternatively, if the SVP Multipoint Encoding (section 3.25.2) can be
used, the LSPs can be realized as multipoint-to-point SVPs.
4.6. LSP Tunneling between BGP Border Routers
Consider the case of an Autonomous System, A, which carries transit
traffic between other Autonomous Systems. Autonomous System A will
have a number of BGP Border Routers, and a mesh of BGP connections
among them, over which BGP routes are distributed. In many such
cases, it is desirable to avoid distributing the BGP routes to
routers which are not BGP Border Routers. If this can be avoided,
the "route distribution load" on those routers is significantly
reduced. However, there must be some means of ensuring that the
transit traffic will be delivered from Border Router to Border Router
by the interior routers.
This can easily be done by means of LSP Tunnels. Suppose that BGP
routes are distributed only to BGP Border Routers, and not to the
interior routers that lie along the Hop-by-hop path from Border
Router to Border Router. LSP Tunnels can then be used as follows:
1. Each BGP Border Router distributes, to every other BGP Border
Router in the same Autonomous System, a label for each address
prefix that it distributes to that router via BGP.
2. The IGP for the Autonomous System maintains a host route for
each BGP Border Router. Each interior router distributes its
labels for these host routes to each of its IGP neighbors.
3. Suppose that:
a) BGP Border Router B1 receives an unlabeled packet P,
b) address prefix X in B1's routing table is the longest match
for the destination address of P,
c) the route to X is a BGP route,
d) the BGP Next Hop for X is B2,
e) B2 has bound label L1 to X, and has distributed this binding
to B1,
f) the IGP next hop for the address of B2 is I1,
g) the address of B2 is in B1's and I1's IGP routing tables as
a host route, and
h) I1 has bound label L2 to the address of B2, and distributed
this binding to B1.
Then before sending packet P to I1, B1 must create a label
stack for P, then push on label L1, and then push on label L2.
4. Suppose that BGP Border Router B1 receives a labeled Packet P,
where the label on the top of the label stack corresponds to an
address prefix, X, to which the route is a BGP route, and that
conditions 3b, 3c, 3d, and 3e all hold. Then before sending
packet P to I1, B1 must replace the label at the top of the
label stack with L1, and then push on label L2.
With these procedures, a given packet P follows a level 1 LSP all of
whose members are BGP Border Routers, and between each pair of BGP
Border Routers in the level 1 LSP, it follows a level 2 LSP.
These procedures effectively create a Hop-by-Hop Routed LSP Tunnel
between the BGP Border Routers.
Since the BGP border routers are exchanging label bindings for
address prefixes that are not even known to the IGP routing, the BGP
routers should become explicit label distribution peers with each
other.
It is sometimes possible to create Hop-by-Hop Routed LSP Tunnels
between two BGP Border Routers, even if they are not in the same
Autonomous System. Suppose, for example, that B1 and B2 are in AS 1.
Suppose that B3 is an EBGP neighbor of B2, and is in AS2. Finally,
suppose that B2 and B3 are on some network which is common to both
Autonomous Systems (a "Demilitarized Zone"). In this case, an LSP
tunnel can be set up directly between B1 and B3 as follows:
- B3 distributes routes to B2 (using EBGP), optionally assigning
labels to address prefixes;
- B2 redistributes those routes to B1 (using IBGP), indicating
that the BGP next hop for each such route is B3. If B3 has
assigned labels to address prefixes, B2 passes these labels
along, unchanged, to B1.
- The IGP of AS1 has a host route for B3.
4.7. Other Uses of Hop-by-Hop Routed LSP Tunnels
The use of Hop-by-Hop Routed LSP Tunnels is not restricted to tunnels
between BGP Next Hops. Any situation in which one might otherwise
have used an encapsulation tunnel is one in which it is appropriate
to use a Hop-by-Hop Routed LSP Tunnel. Instead of encapsulating the
packet with a new header whose destination address is the address of
the tunnel's receive endpoint, the label corresponding to the address
prefix which is the longest match for the address of the tunnel's
receive endpoint is pushed on the packet's label stack. The packet
which is sent into the tunnel may or may not already be labeled.
If the transmit endpoint of the tunnel wishes to put a labeled packet
into the tunnel, it must first replace the label value at the top of
the stack with a label value that was distributed to it by the
tunnel's receive endpoint. Then it must push on the label which
corresponds to the tunnel itself, as distributed to it by the next
hop along the tunnel. To allow this, the tunnel endpoints should be
explicit label distribution peers. The label bindings they need to
exchange are of no interest to the LSRs along the tunnel.
4.8. MPLS and Multicast
Multicast routing proceeds by constructing multicast trees. The tree
along which a particular multicast packet must get forwarded depends
in general on the packet's source address and its destination
address. Whenever a particular LSR is a node in a particular
multicast tree, it binds a label to that tree. It then distributes
that binding to its parent on the multicast tree. (If the node in
question is on a LAN, and has siblings on that LAN, it must also
distribute the binding to its siblings. This allows the parent to
use a single label value when multicasting to all children on the
LAN.)
When a multicast labeled packet arrives, the NHLFE corresponding to
the label indicates the set of output interfaces for that packet, as
well as the outgoing label. If the same label encoding technique is
used on all the outgoing interfaces, the very same packet can be sent
to all the children.
5. Label Distribution Procedures (Hop-by-Hop)
In this section, we consider only label bindings that are used for
traffic to be label switched along its hop-by-hop routed path. In
these cases, the label in question will correspond to an address
prefix in the routing table.
5.1. The Procedures for Advertising and Using labels
There are a number of different procedures that may be used to distribute label bindings. Some are executed by the downstream LSR, and some by the upstream LSR. The downstream LSR must perform: - The Distribution Procedure, and - the Withdrawal Procedure. The upstream LSR must perform: - The Request Procedure, and - the NotAvailable Procedure, and - the Release Procedure, and - the labelUse Procedure. The MPLS architecture supports several variants of each procedure. However, the MPLS architecture does not support all possible combinations of all possible variants. The set of supported combinations will be described in section 5.2, where the interoperability between different combinations will also be discussed.5.1.1. Downstream LSR: Distribution Procedure
The Distribution Procedure is used by a downstream LSR to determine when it should distribute a label binding for a particular address prefix to its label distribution peers. The architecture supports four different distribution procedures. Irrespective of the particular procedure that is used, if a label binding for a particular address prefix has been distributed by a downstream LSR Rd to an upstream LSR Ru, and if at any time the attributes (as defined above) of that binding change, then Rd must inform Ru of the new attributes. If an LSR is maintaining multiple routes to a particular address prefix, it is a local matter as to whether that LSR binds multiple labels to the address prefix (one per route), and hence distributes multiple bindings.
5.1.1.1. PushUnconditional
Let Rd be an LSR. Suppose that: 1. X is an address prefix in Rd's routing table 2. Ru is a label distribution peer of Rd with respect to X Whenever these conditions hold, Rd must bind a label to X and distribute that binding to Ru. It is the responsibility of Rd to keep track of the bindings which it has distributed to Ru, and to make sure that Ru always has these bindings. This procedure would be used by LSRs which are performing unsolicited downstream label assignment in the Independent LSP Control Mode.5.1.1.2. PushConditional
Let Rd be an LSR. Suppose that: 1. X is an address prefix in Rd's routing table 2. Ru is a label distribution peer of Rd with respect to X 3. Rd is either an LSP Egress or an LSP Proxy Egress for X, or Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and Rn has bound a label to X and distributed that binding to Rd. Then as soon as these conditions all hold, Rd should bind a label to X and distribute that binding to Ru. Whereas PushUnconditional causes the distribution of label bindings for all address prefixes in the routing table, PushConditional causes the distribution of label bindings only for those address prefixes for which one has received label bindings from one's LSP next hop, or for which one does not have an MPLS-capable L3 next hop. This procedure would be used by LSRs which are performing unsolicited downstream label assignment in the Ordered LSP Control Mode.5.1.1.3. PulledUnconditional
Let Rd be an LSR. Suppose that: 1. X is an address prefix in Rd's routing table 2. Ru is a label distribution peer of Rd with respect to X
3. Ru has explicitly requested that Rd bind a label to X and
distribute the binding to Ru
Then Rd should bind a label to X and distribute that binding to Ru.
Note that if X is not in Rd's routing table, or if Rd is not a label
distribution peer of Ru with respect to X, then Rd must inform Ru
that it cannot provide a binding at this time.
If Rd has already distributed a binding for address prefix X to Ru,
and it receives a new request from Ru for a binding for address
prefix X, it will bind a second label, and distribute the new binding
to Ru. The first label binding remains in effect.
This procedure would be used by LSRs performing downstream-on-demand
label distribution using the Independent LSP Control Mode.
5.1.1.4. PulledConditional
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X
3. Ru has explicitly requested that Rd bind a label to X and
distribute the binding to Ru
4. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
Rn has bound a label to X and distributed that binding to Rd
Then as soon as these conditions all hold, Rd should bind a label to
X and distribute that binding to Ru. Note that if X is not in Rd's
routing table and a binding for X is not obtainable via Rd's next hop
for X, or if Rd is not a label distribution peer of Ru with respect
to X, then Rd must inform Ru that it cannot provide a binding at this
time.
However, if the only condition that fails to hold is that Rn has not
yet provided a label to Rd, then Rd must defer any response to Ru
until such time as it has receiving a binding from Rn.
If Rd has distributed a label binding for address prefix X to Ru, and
at some later time, any attribute of the label binding changes, then
Rd must redistribute the label binding to Ru, with the new attribute.
It must do this even though Ru does not issue a new Request.
This procedure would be used by LSRs that are performing downstream- on-demand label allocation in the Ordered LSP Control Mode. In section 5.2, we will discuss how to choose the particular procedure to be used at any given time, and how to ensure interoperability among LSRs that choose different procedures.5.1.2. Upstream LSR: Request Procedure
The Request Procedure is used by the upstream LSR for an address prefix to determine when to explicitly request that the downstream LSR bind a label to that prefix and distribute the binding. There are three possible procedures that can be used.5.1.2.1. RequestNever
Never make a request. This is useful if the downstream LSR uses the PushConditional procedure or the PushUnconditional procedure, but is not useful if the downstream LSR uses the PulledUnconditional procedure or the the PulledConditional procedures. This procedure would be used by an LSR when unsolicited downstream label distribution and Liberal Label Retention Mode are being used.5.1.2.2. RequestWhenNeeded
Make a request whenever the L3 next hop to the address prefix changes, or when a new address prefix is learned, and one doesn't already have a label binding from that next hop for the given address prefix. This procedure would be used by an LSR whenever Conservative Label Retention Mode is being used.5.1.2.3. RequestOnRequest
Issue a request whenever a request is received, in addition to issuing a request when needed (as described in section 5.1.2.2). If Ru is not capable of being an LSP ingress, it may issue a request only when it receives a request from upstream. If Rd receives such a request from Ru, for an address prefix for which Rd has already distributed Ru a label, Rd shall assign a new (distinct) label, bind it to X, and distribute that binding. (Whether Rd can distribute this binding to Ru immediately or not depends on the Distribution Procedure being used.)
This procedure would be used by an LSR which is doing downstream-on- demand label distribution, but is not doing label merging, e.g., an ATM-LSR which is not capable of VC merge.5.1.3. Upstream LSR: NotAvailable Procedure
If Ru and Rd are respectively upstream and downstream label distribution peers for address prefix X, and Rd is Ru's L3 next hop for X, and Ru requests a binding for X from Rd, but Rd replies that it cannot provide a binding at this time, because it has no next hop for X, then the NotAvailable procedure determines how Ru responds. There are two possible procedures governing Ru's behavior:5.1.3.1. RequestRetry
Ru should issue the request again at a later time. That is, the requester is responsible for trying again later to obtain the needed binding. This procedure would be used when downstream-on-demand label distribution is used.5.1.3.2. RequestNoRetry
Ru should never reissue the request, instead assuming that Rd will provide the binding automatically when it is available. This is useful if Rd uses the PushUnconditional procedure or the PushConditional procedure, i.e., if unsolicited downstream label distribution is used. Note that if Rd replies that it cannot provide a binding to Ru, because of some error condition, rather than because Rd has no next hop, the behavior of Ru will be governed by the error recovery conditions of the label distribution protocol, rather than by the NotAvailable procedure.5.1.4. Upstream LSR: Release Procedure
Suppose that Rd is an LSR which has bound a label to address prefix X, and has distributed that binding to LSR Ru. If Rd does not happen to be Ru's L3 next hop for address prefix X, or has ceased to be Ru's L3 next hop for address prefix X, then Ru will not be using the label. The Release Procedure determines how Ru acts in this case. There are two possible procedures governing Ru's behavior:5.1.4.1. ReleaseOnChange
Ru should release the binding, and inform Rd that it has done so. This procedure would be used to implement Conservative Label Retention Mode.
5.1.4.2. NoReleaseOnChange
Ru should maintain the binding, so that it can use it again immediately if Rd later becomes Ru's L3 next hop for X. This procedure would be used to implement Liberal Label Retention Mode.5.1.5. Upstream LSR: labelUse Procedure
Suppose Ru is an LSR which has received label binding L for address prefix X from LSR Rd, and Ru is upstream of Rd with respect to X, and in fact Rd is Ru's L3 next hop for X. Ru will make use of the binding if Rd is Ru's L3 next hop for X. If, at the time the binding is received by Ru, Rd is NOT Ru's L3 next hop for X, Ru does not make any use of the binding at that time. Ru may however start using the binding at some later time, if Rd becomes Ru's L3 next hop for X. The labelUse Procedure determines just how Ru makes use of Rd's binding. There are two procedures which Ru may use:5.1.5.1. UseImmediate
Ru may put the binding into use immediately. At any time when Ru has a binding for X from Rd, and Rd is Ru's L3 next hop for X, Rd will also be Ru's LSP next hop for X. This procedure is used when loop detection is not in use.5.1.5.2. UseIfLoopNotDetected
This procedure is the same as UseImmediate, unless Ru has detected a loop in the LSP. If a loop has been detected, Ru will discontinue the use of label L for forwarding packets to Rd. This procedure is used when loop detection is in use. This will continue until the next hop for X changes, or until the loop is no longer detected.5.1.6. Downstream LSR: Withdraw Procedure
In this case, there is only a single procedure. When LSR Rd decides to break the binding between label L and address prefix X, then this unbinding must be distributed to all LSRs to which the binding was distributed.
It is required that the unbinding of L from X be distributed by Rd to a LSR Ru before Rd distributes to Ru any new binding of L to any other address prefix Y, where X != Y. If Ru were to learn of the new binding of L to Y before it learned of the unbinding of L from X, and if packets matching both X and Y were forwarded by Ru to Rd, then for a period of time, Ru would label both packets matching X and packets matching Y with label L. The distribution and withdrawal of label bindings is done via a label distribution protocol. All label distribution protocols require that a label distribution adjacency be established between two label distribution peers (except implicit peers). If LSR R1 has a label distribution adjacency to LSR R2, and has received label bindings from LSR R2 via that adjacency, then if adjacency is brought down by either peer (whether as a result of failure or as a matter of normal operation), all bindings received over that adjacency must be considered to have been withdrawn. As long as the relevant label distribution adjacency remains in place, label bindings that are withdrawn must always be withdrawn explicitly. If a second label is bound to an address prefix, the result is not to implicitly withdraw the first label, but to bind both labels; this is needed to support multi-path routing. If a second address prefix is bound to a label, the result is not to implicitly withdraw the binding of that label to the first address prefix, but to use that label for both address prefixes.5.2. MPLS Schemes: Supported Combinations of Procedures
Consider two LSRs, Ru and Rd, which are label distribution peers with respect to some set of address prefixes, where Ru is the upstream peer and Rd is the downstream peer. The MPLS scheme which governs the interaction of Ru and Rd can be described as a quintuple of procedures: <Distribution Procedure, Request Procedure, NotAvailable Procedure, Release Procedure, labelUse Procedure>. (Since there is only one Withdraw Procedure, it need not be mentioned.) A "*" appearing in one of the positions is a wild-card, meaning that any procedure in that category may be present; an "N/A" appearing in a particular position indicates that no procedure in that category is needed. Only the MPLS schemes which are specified below are supported by the MPLS Architecture. Other schemes may be added in the future, if a need for them is shown.
5.2.1. Schemes for LSRs that Support Label Merging
If Ru and Rd are label distribution peers, and both support label merging, one of the following schemes must be used: 1. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange, UseImmediate> This is unsolicited downstream label distribution with independent control, liberal label retention mode, and no loop detection. 2. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange, UseIfLoopNotDetected> This is unsolicited downstream label distribution with independent control, liberal label retention, and loop detection. 3. <PushConditional, RequestWhenNeeded, RequestNoRetry, ReleaseOnChange, *> This is unsolicited downstream label distribution with ordered control (from the egress) and conservative label retention mode. Loop detection is optional. 4. <PushConditional, RequestNever, N/A, NoReleaseOnChange, *> This is unsolicited downstream label distribution with ordered control (from the egress) and liberal label retention mode. Loop detection is optional. 5. <PulledConditional, RequestWhenNeeded, RequestRetry, ReleaseOnChange, *> This is downstream-on-demand label distribution with ordered control (initiated by the ingress), conservative label retention mode, and optional loop detection. 6. <PulledUnconditional, RequestWhenNeeded, N/A, ReleaseOnChange, UseImmediate> This is downstream-on-demand label distribution with independent control and conservative label retention mode, without loop detection.
7. <PulledUnconditional, RequestWhenNeeded, N/A, ReleaseOnChange,
UseIfLoopNotDetected>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode, with
loop detection.
5.2.2. Schemes for LSRs that do not Support Label Merging
Suppose that R1, R2, R3, and R4 are ATM switches which do not support
label merging, but are being used as LSRs. Suppose further that the
L3 hop-by-hop path for address prefix X is <R1, R2, R3, R4>, and that
packets destined for X can enter the network at any of these LSRs.
Since there is no multipoint-to-point capability, the LSPs must be
realized as point-to-point VCs, which means that there needs to be
three such VCs for address prefix X: <R1, R2, R3, R4>, <R2, R3, R4>,
and <R3, R4>.
Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is
implemented using conventional ATM switching hardware (i.e., no cell
interleave suppression), or is otherwise incapable of performing
label merging, the MPLS scheme in use between R1 and R2 must be one
of the following:
1. <PulledConditional, RequestOnRequest, RequestRetry,
ReleaseOnChange, *>
This is downstream-on-demand label distribution with ordered
control (initiated by the ingress), conservative label
retention mode, and optional loop detection.
The use of the RequestOnRequest procedure will cause R4 to
distribute three labels for X to R3; R3 will distribute 2
labels for X to R2, and R2 will distribute one label for X to
R1.
2. <PulledUnconditional, RequestOnRequest, N/A, ReleaseOnChange,
UseImmediate>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode,
without loop detection.
3. <PulledUnconditional, RequestOnRequest, N/A, ReleaseOnChange,
UseIfLoopNotDetected>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode, with
loop detection.
5.2.3. Interoperability Considerations
It is easy to see that certain quintuples do NOT yield viable MPLS
schemes. For example:
- <PulledUnconditional, RequestNever, *, *, *>
<PulledConditional, RequestNever, *, *, *>
In these MPLS schemes, the downstream LSR Rd distributes label
bindings to upstream LSR Ru only upon request from Ru, but Ru
never makes any such requests. Obviously, these schemes are
not viable, since they will not result in the proper
distribution of label bindings.
- <*, RequestNever, *, *, ReleaseOnChange>
In these MPLS schemes, Rd releases bindings when it isn't using
them, but it never asks for them again, even if it later has a
need for them. These schemes thus do not ensure that label
bindings get properly distributed.
In this section, we specify rules to prevent a pair of label
distribution peers from adopting procedures which lead to infeasible
MPLS Schemes. These rules require either the exchange of information
between label distribution peers during the initialization of the
label distribution adjacency, or a priori knowledge of the
information (obtained through a means outside the scope of this
document).
1. Each must state whether it supports label merging.
2. If Rd does not support label merging, Rd must choose either the
PulledUnconditional procedure or the PulledConditional
procedure. If Rd chooses PulledConditional, Ru is forced to
use the RequestRetry procedure.
That is, if the downstream LSR does not support label merging,
its preferences take priority when the MPLS scheme is chosen.
3. If Ru does not support label merging, but Rd does, Ru must
choose either the RequestRetry or RequestNoRetry procedure.
This forces Rd to use the PulledConditional or
PulledUnConditional procedure respectively.
That is, if only one of the LSRs doesn't support label merging,
its preferences take priority when the MPLS scheme is chosen.
4. If both Ru and Rd both support label merging, then the choice
between liberal and conservative label retention mode belongs
to Ru. That is, Ru gets to choose either to use
RequestWhenNeeded/ReleaseOnChange (conservative) , or to use
RequestNever/NoReleaseOnChange (liberal). However, the choice
of "push" vs. "pull" and "conditional" vs. "unconditional"
belongs to Rd. If Ru chooses liberal label retention mode, Rd
can choose either PushUnconditional or PushConditional. If Ru
chooses conservative label retention mode, Rd can choose
PushConditional, PulledConditional, or PulledUnconditional.
These choices together determine the MPLS scheme in use.
6. Security Considerations
Some routers may implement security procedures which depend on the
network layer header being in a fixed place relative to the data link
layer header. The MPLS generic encapsulation inserts a shim between
the data link layer header and the network layer header. This may
cause any such security procedures to fail.
An MPLS label has its meaning by virtue of an agreement between the
LSR that puts the label in the label stack (the "label writer"), and
the LSR that interprets that label (the "label reader"). If labeled
packets are accepted from untrusted sources, or if a particular
incoming label is accepted from an LSR to which that label has not
been distributed, then packets may be routed in an illegitimate
manner.
7. Intellectual Property
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
8. Authors' Addresses
Eric C. Rosen Cisco Systems, Inc. 250 Apollo Drive Chelmsford, MA, 01824 EMail: erosen@cisco.com Arun Viswanathan Force10 Networks, Inc. 1440 McCarthy Blvd. Milpitas, CA 95035-7438 EMail: arun@force10networks.com Ross Callon Juniper Networks, Inc. 1194 North Mathilda Avenue Sunnyvale, CA 94089 USA EMail: rcallon@juniper.net9. References
[MPLS-ATM] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y., Rosen, E., Swallow, G. and P. Doolan, "MPLS using LDP and ATM VC Switching", RFC 3035, January 2001. [MPLS-BGP] "Carrying Label Information in BGP-4", Rekhter, Rosen, Work in Progress. [MPLS-CR-LDP] "Constraint-Based LSP Setup using LDP", Jamoussi, Editor, Work in Progress. [MPLS-FRMRLY] Conta, A., Doolan, P. and A. Malis, "Use of Label Switching on Frame Relay Networks Specification", RFC 3034, January 2001. [MPLS-LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B. Thomas, "LDP Specification", RFC 3036, January 2001.
[MPLS-RSVP-TUNNELS] "Extensions to RSVP for LSP Tunnels", Awduche, Berger, Gan, Li, Swallow, Srinvasan, Work in Progress. [MPLS-SHIM] Rosen, E., Rekhter, Y., Tappan, D., Fedorkow, G., Farinacci, D. and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001. [MPLS-TRFENG] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J. McManus, "Requirements for Traffic Engineering Over MPLS", RFC 2702, September 1999.
10. Full Copyright Statement
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