Internet Draft
Network Working Group Yakov Rekhter
Internet Draft Eric C. Rosen
Expiration Date: July 2000 Cisco Systems, Inc.
January 2000
Carrying Label Information in BGP-4
draft-ietf-mpls-bgp4-mpls-04.txt
Status of this Memo
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Abstract
When BGP is used to distribute a particular route, it can be also be
used to distribute an MPLS label which is mapped to that route
[MPLS-ARCH]. This document specifies the way in which this is done.
The label mapping information for a particular route is piggybacked
in the same BGP Update message that is used to distribute the route
itself.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Overview ............................................... 2
3 Carrying Label Mapping Information ..................... 3
4 Advertising Multiple Routes to a Destination ........... 4
5 Capability Negotiation ................................. 5
6 When the BGP Peers are not Directly Adjacent ........... 5
7 Security Considerations ................................ 6
8 Acknowledgments ........................................ 7
9 References ............................................. 7
10 Author Information ..................................... 7
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
2. Overview
When BGP is used to distribute a particular route, it can also be
used to distribute an MPLS label that is mapped to that route [MPLS-
ARCH]. This document specifies the way in which this is done. The
label mapping information for a particular route is piggybacked in
the same BGP Update message that is used to distribute the route
itself.
This can be useful in the following situations:
- If two immediately adjacent Label Switched Routers (LSRs) are
also BGP peers, then label distribution can be done without the
need for any other label distribution protocol.
- Suppose one's network consists of two "classes" of LSR: exterior
LSRs, which interface to other networks, and interior LSRs, which
serve only to carry traffic between exterior LSRs. Suppose that
the exterior LSRs are BGP speakers. If the BGP speakers
distribute MPLS labels to each other along with each route they
distribute, then as long as the interior routers support MPLS,
they need not receive any of the BGP routes from the BGP
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speakers.
If exterior router A needs to send a packet to destination D, and
A's BGP next hop for D is exterior router B, and B has mapped
label L to D, then A first pushes L onto the packet's label
stack. A then consults its IGP to find the next hop to B, call
it C. If C has distributed to A an MPLS label for the route to
B, A can push this label on the packet's label stack, and then
send the packet to C.
If a set of BGP speakers are exchanging routes via a Route Reflector
[BGP-RR], then by piggybacking the label distribution on the route
distribution, one is able to use the Route Reflector to distribute
the labels as well. This improves scalability quite significantly.
Note that if the Route Reflector is not in the forwarding path, it
need not even be capable of forwarding MPLS packets.
Label distribution can be piggybacked in the BGP Update message by
using the BGP-4 Multiprotocol Extensions attribute [RFC 2283]. The
label is encoded into the NLRI field of the attribute, and the SAFI
("Subsequent Address Family Identifier") field is used to indicate
that the NLRI contains a label. A BGP speaker may not use BGP to
send labels to a particular BGP peer unless that peer indicates,
through BGP Capability Negotiation, that it can process Update
messages with the specified SAFI field.
3. Carrying Label Mapping Information
Label mapping information is carried as part of the Network Layer
Reachability Information (NLRI) in the Multiprotocol Extensions
attributes. The AFI indicates, as usual, the address family of the
associated route. The fact that the NLRI contains a label is
indicated by using SAFI value 4 [assignment to be confirmed by IANA].
The Network Layer Reachability information is encoded as one or more
triples of the form , whose fields are
described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Label (3 octets) |
+---------------------------+
.............................
+---------------------------+
| Prefix (variable) |
+---------------------------+
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The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the address
prefix plus the label(s).
b) Label:
The Label field carries one or more labels (that corresponds to
the stack of labels [MPLS-ENCAPS]). Each label is encoded as 3
octets, where the high-order bit contains "Bottom of Stack" (as
defined in [MPLS-ENCAPS]). The following high-order three bits
must be zero. The remaining 20 bits contain the label value.
c) Prefix:
The Prefix field contains address prefixes followed by enough
trailing bits to make the end of the field fall on an octet
boundary. Note that the value of trailing bits is irrelevant.
The label(s) specified for a particular route (and associated with it
address prefix) must be assigned by the LSR which is identified by
the value of the Next Hop attribute of the route.
When a BGP speaker redistributes a route, the label(s) assigned to
that route must not be changed (except by omission), unless the
speaker changes the value of the Next Hop attribute of the route.
A BGP speaker can withdraw a previously advertised route (as well as
the binding between this route and a label) by either (a) advertising
a new route (and a label) with the same NLRI as the previously
advertised route, or (b) listing the NLRI of the previously
advertised route in the Withdrawn Routes field of an Update message.
The label information carried (as part of NLRI) in the Withdrawn
Routes field should be set to 0x800000.
4. Advertising Multiple Routes to a Destination
A BGP speaker may maintain (and advertise to its peers) more than one
route to a given destination, as long as each such route has its own
label(s).
The encoding described above allows a single BGP Update message to
carry multiple routes, each with its own label(s).
In the case where a BGP speaker advertises multiple routes to a
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destination, if a route is withdrawn, and a label(s) is specified at
the time of withdrawal, only the corresponding route with the
corresponding label is withdrawn. If a route is withdrawn, and no
label is specified at the time of withdrawal, then only the
corresponding unlabeled route is withdrawn; the labeled routes are
left in place.
5. Capability Negotiation
A BGP speaker that uses Multiprotocol Extensions to carry label
mapping information should use the Capabilities Optional Parameter,
as defined in [BGP-CAP], to inform its peers about this capability.
The MP_EXT Capability Code, as defined in [BGP-MP], is used to
negotiate the (AFI, SAFI) pairs available on a particular connection.
A BGP speaker should not advertise this capability to another BGP
speaker unless there is a Label Switched Path (LSP) between the two
speakers.
A BGP speaker that is capable of handling multiple routes to a
destination (as described above) should use the Capabilities Optional
Parameter, as defined in [BGP-CAP], to inform its peers about this
capability. The value of this capability is TBD.
6. When the BGP Peers are not Directly Adjacent
Consider the following LSR topology: A--B--C--D. Suppose that D
distributes a label L to A. In this topology, A cannot simply push L
onto a packet's label stack, and then send the resulting packet to B.
D must be the only LSR that sees L at the top of the stack. Before A
sends the packet to B, it must push on another label, which was
distributed by B. B must replace this label with yet another label,
which was distributed by C. In other words, there must be an LSP
between A and D. If there is no such LSP, A cannot make use of label
L. This is true any time labels are distributed between non-adjacent
LSRs, whether that distribution is done by BGP or by some other
method.
This document does NOT specify any procedure for ensuring in real
time that label distribution between non-adjacent LSRs is done only
when the appropriate MPLS infrastructure exists in the network or
networks connecting the two LSRs. Ensuring that the proper
infrastructure exists is an issue for network management and
operation.
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7. Security Considerations
When an LSR A is directly connected to an LSR B via a point-to-point
interface, then when A receives packets over that interface, it knows
that they come from B. This makes it easy for A to discard any
packets from B whose top labels are not among the labels that A
distributed to B. That is, A can easily ensure that B only uses
those labels which it is entitled to use. This technique can be used
to prevent "label spoofing", i.e., the situation in which an LSR
imposes a label which has not been properly distributed to it.
The procedures discussed in this document would commonly be used when
the label distribution peers are separated not merely by a point-to-
point link, but by an MPLS network. This means that when an LSR A
processes a labelled packet, it really has no way to determine which
other LSR B pushed on the top label. Hence it cannot tell whether
the label is one which B is entitled to use. In fact, when Route
Reflectors are in use, A may not even know the set of LSRs which
receive its label mappings. So the previous paragraph's technique
for preventing label spoofing does not apply.
It is possible though to use other techniques to avoid label spoofing
problems. If, for example, one never accepts labeled packets from
the network's "external" interfaces, and all the BGP-distributed
labels are advertised via IBGP, then there is no way for an untrusted
router to put a labeled packet into the network. One can generally
assume that one's IBGP peers (or the IBGP peers of one's Route
Reflector) will not attempt label spoofing, since they are all under
the control of a single administration.
This condition can actually be weakened significantly. One doesn't
need to refuse to accept all labeled packets from external
interfaces. One just needs to make sure that any labeled packet
received on an external interface has a top label which was actually
distributed out that interface.
Then a label spoofing problem would only exist if there are both
trusted and untrusted systems out the same interface. One way to
avoid this problem is simply to avoid this situation.
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8. Acknowledgments
Thanks to Ravi Chandra, Enke Chen, Srihari Ramachandra, Eric Gray and
Liam Casey for their comments.
9. References
[BGP-4] RFC 1771, "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter,
T. Li, 3/95
[BGP-CAP] "Capabilities Negotiation with BGP-4", R. Chandra, J.
, draft-ietf-idr-bgp4-cap-neg-04.txt, 9/99
[BGP-MP] RFC 2283, "Multiprotocol Extensions for BGP-4", T. Bates, R.
Chandra, D.Katz, Y. Rekhter, 2/98
[BGP-RR] RFC 1966, "BGP Route Reflection: An alternative to full mesh
IBGP", T. Bates, R. Chandra, 6/96.
[MPLS-ARCH] "Multiprotocol Label Switching Architecture"A Proposed
Architecture for MPLS", E. Rosen, A. Vishwanathan, R. Callon, draft-
ietf-mpls-arch-06.txt, 8/99.
[MPLS-ENCAPS] "MPLS Label Stack Encoding", E. Rosen, et al, draft-
ietf-mpls-label-encaps-07.txt, 9/99
10. Author Information
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: yakov@cisco.com
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
email: erosen@cisco.com
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