As per Relevance of the word distribute, we have this rfc below:
Network Working Group Y.
Request for Comments: 3107 Juniper
Category: Standards Track E.
Cisco Systems, Inc
May 2001
Carrying Label Information in BGP-4
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2001). All Rights Reserved
This document specifies the way in which the label
information for a particular route is piggybacked in the same
Gateway Protocol (BGP) Update message that is used to distribute
route itself. When BGP is used to distribute a particular route,
can be also be used to distribute a Multiprotocol Label
(MPLS) label which is mapped to that route
Table of
1 Specification of Requirements .......................... 2
2 Overview ............................................... 2
3 Carrying Label Mapping Information ..................... 3
4 Advertising Multiple Routes to a Destination ........... 4
5 Capability Advertisement ............................... 4
6 When the BGP Peers are not Directly Adjacent ........... 5
7 Security Considerations ................................ 5
8 Acknowledgments ........................................ 6
9 References ............................................. 6
10 Authors' Addresses ..................................... 7
11 Full Copyright Statement ............................... 8
Rekhter & Rosen Standards Track [Page 1]
RFC 3107 Carrying Label Information in BGP-4 May 2001
1. Specification of
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
document are to be interpreted as described in RFC 2119.
2.
When BGP is used to distribute a particular route, it can also
used to distribute an MPLS label that is mapped to that route [MPLS
ARCH]. This document specifies the way in which this is done.
label mapping information for a particular route is piggybacked
the same BGP Update message that is used to distribute the
itself
This can be useful in the following situations
- If two immediately adjacent Label Switched Routers (LSRs)
also BGP peers, then label distribution can be done without
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
LSRs, which serve only to carry traffic between exterior LSRs
Suppose that the exterior LSRs are BGP speakers. If the
speakers distribute MPLS labels to each other along with
route they distribute, then as long as the interior
support MPLS, they need not receive any of the BGP routes
the BGP 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
mapped label L to D, then A first pushes L onto the packet'
label stack. A then consults its IGP to find the next hop
B, call it C. If C has distributed to A an MPLS label for
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
[BGP-RR], then by piggybacking the label distribution on the
distribution, one is able to use the Route Reflector to
the labels as well. This improves scalability quite significantly
Note that if the Route Reflector is not in the forwarding path,
need not even be capable of forwarding MPLS packets
Label distribution can be piggybacked in the BGP Update message
using the BGP-4 Multiprotocol Extensions attribute [RFC 2283].
label is encoded into the NLRI field of the attribute, and the
Rekhter & Rosen Standards Track [Page 2]
RFC 3107 Carrying Label Information in BGP-4 May 2001
("Subsequent Address Family Identifier") field is used to
that the NLRI contains a label. A BGP speaker may not use BGP
send labels to a particular BGP peer unless that peer indicates
through BGP Capability Advertisement, that it can process
messages with the specified SAFI field
3. Carrying Label Mapping
Label mapping information is carried as part of the Network
Reachability Information (NLRI) in the Multiprotocol
attributes. The AFI indicates, as usual, the address family of
associated route. The fact that the NLRI contains a label
indicated by using SAFI value 4.
The Network Layer Reachability information is encoded as one or
triples of the form , whose fields
described below
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Label (3 octets) |
+---------------------------+
.............................
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows
a) Length
The Length field indicates the length in bits of the
prefix plus the label(s).
b) Label
The Label field carries one or more labels (that corresponds
the stack of labels [MPLS-ENCAPS]). Each label is encoded as 3
octets, where the high-order 20 bits contain the label value
and the low order bit contains "Bottom of Stack" (as defined
[MPLS-ENCAPS]).
c) Prefix
The Prefix field contains address prefixes followed by
trailing bits to make the end of the field fall on an
boundary. Note that the value of trailing bits is irrelevant
Rekhter & Rosen Standards Track [Page 3]
RFC 3107 Carrying Label Information in BGP-4 May 2001
The label(s) specified for a particular route (and associated
its address prefix) must be assigned by the LSR which is
by the value of the Next Hop attribute of the route
When a BGP speaker redistributes a route, the label(s) assigned
that route must not be changed (except by omission), unless
speaker changes the value of the Next Hop attribute of the route
A BGP speaker can withdraw a previously advertised route (as well
the binding between this route and a label) by either (a)
a new route (and a label) with the same NLRI as the
advertised route, or (b) listing the NLRI of the
advertised route in the Withdrawn Routes field of an Update message
The label information carried (as part of NLRI) in the
Routes field should be set to 0x800000. (Of course, terminating
BGP session also withdraws all the previously advertised routes.)
4. Advertising Multiple Routes to a
A BGP speaker may maintain (and advertise to its peers) more than
route to a given destination, as long as each such route has its
label(s).
The encoding described above allows a single BGP Update message
carry multiple routes, each with its own label(s).
In the case where a BGP speaker advertises multiple routes to
destination, if a route is withdrawn, and a label(s) is specified
the time of withdrawal, only the corresponding route with
corresponding label is withdrawn. If a route is withdrawn, and
label is specified at the time of withdrawal, then only
corresponding unlabeled route is withdrawn; the labeled routes
left in place
5. Capability
A BGP speaker that uses Multiprotocol Extensions to carry
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
advertise the (AFI, SAFI) pairs available on a particular connection
A BGP speaker should not advertise this capability to another
speaker unless there is a Label Switched Path (LSP) between the
speakers
Rekhter & Rosen Standards Track [Page 4]
RFC 3107 Carrying Label Information in BGP-4 May 2001
A BGP speaker that is capable of handling multiple routes to
destination (as described above) should use the Capabilities
Parameter, as defined in [BGP-CAP], to inform its peers about
capability. The value of this capability is 4.
6. When the BGP Peers are not Directly
Consider the following LSR topology: A--B--C--D. Suppose that
distributes a label L to A. In this topology, A cannot simply push
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
sends the packet to B, it must push on another label, which
distributed by B. B must replace this label with yet another label
which was distributed by C. In other words, there must be an
between A and D. If there is no such LSP, A cannot make use of
L. This is true any time labels are distributed between non-
LSRs, whether that distribution is done by BGP or by some
method
This document does NOT specify any procedure for ensuring in
time that label distribution between non-adjacent LSRs is done
when the appropriate MPLS infrastructure exists in the network
networks connecting the two LSRs. Ensuring that the
infrastructure exists is an issue for network management
operation
7. Security
When an LSR A is directly connected to an LSR B via a point-to-
interface, then when A receives packets over that interface, it
that they come from B. This makes it easy for A to discard
packets from B whose top labels are not among the labels that
distributed to B. That is, A can easily ensure that B only
those labels which it is entitled to use. This technique can be
to prevent "label spoofing", i.e., the situation in which an
imposes a label which has not been properly distributed to it
The procedures discussed in this document would commonly be used
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
processes a labeled packet, it really has no way to determine
other LSR B pushed on the top label. Hence it cannot tell
the label is one which B is entitled to use. In fact, when
Reflectors are in use, A may not even know the set of LSRs
receive its label mappings. So the previous paragraph's
for preventing label spoofing does not apply
Rekhter & Rosen Standards Track [Page 5]
RFC 3107 Carrying Label Information in BGP-4 May 2001
It is possible though to use other techniques to avoid label
problems. If, for example, one never accepts labeled packets
the network's "external" interfaces, and all the BGP-
labels are advertised via IBGP, then there is no way for an
router to put a labeled packet into the network. One can
assume that one's IBGP peers (or the IBGP peers of one's
Reflector) will not attempt label spoofing, since they are all
the control of a single administration
This condition can actually be weakened significantly. One doesn'
need to refuse to accept all labeled packets from
interfaces. One just needs to make sure that any labeled
received on an external interface has a top label which was
distributed out that interface
Then a label spoofing problem would only exist if there are
trusted and untrusted systems out the same interface. One way
avoid this problem is simply to avoid this situation
8.
Thanks to Ravi Chandra, Enke Chen, Srihari Ramachandra, Eric Gray
Liam Casey for their comments
9.
[BGP-4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[BGP-CAP] Chandra, R. and J. Scudder, "Capabilities
with BGP-4", RFC 2842, May 2000.
[BGP-MP] Bates, T., Rekhter, Y, Chandra, R. and D. Katz
"Multiprotocol Extensions for BGP-4", RFC 2858,
2000.
[BGP-RR] Bates, T. and R. Chandra, "BGP Route Reflection:
alternative to full mesh IBGP", RFC 1966, June 1996.
[MPLS-ARCH] Rosen, E., Vishwanathan, A. and R. Callon
"Multiprotocol Label Switching Architecture" RFC 3031,
January 2001.
[MPLS-ENCAPS] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T. and A. Conta, "MPLS Label
Encoding", RFC 3032, January 2001.
Rekhter & Rosen Standards Track [Page 6]
RFC 3107 Carrying Label Information in BGP-4 May 2001
10. Authors'
Yakov
Juniper
1194 N. Mathilda
Sunnyvale, CA 94089
EMail: yakov@juniper.
Eric
Cisco Systems, Inc
250 Apollo
Chelmsford, MA 01824
EMail: erosen@cisco.
Rekhter & Rosen Standards Track [Page 7]
RFC 3107 Carrying Label Information in BGP-4 May 2001
11. Full Copyright
Copyright (C) The Internet Society (2001). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Rekhter & Rosen Standards Track [Page 8]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
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