Network Working Group R. Aggarwal
Internet Draft Juniper Networks
Expiration Date: January 2009
J. L. Le Roux
France Telecom
July 8, 2008
MPLS Upstream Label Assignment for LDP
draft-ietf-mpls-ldp-upstream-03.txt
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Abstract
This document describes procedures for distributing upstream-assigned
labels for Label Distribution Protocol (LDP). It also describes how
these procedures can be used for avoiding branch LSR traffic
replication on a LAN for LDP point-to-multipoint (P2MP)LSPs.
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Table of Contents
1 Specification of requirements ......................... 2
2 Introduction .......................................... 2
3 LDP Upstream Label Assignment Capability .............. 3
4 Distributing Upstream-Assigned Labels in LDP .......... 4
4.1 Procedures ............................................ 4
5 LDP Tunnel Identifier Exchange ........................ 5
6 LDP Point-to-Multipoint LSPs on a LAN ................. 6
7 IANA Considerations ................................... 8
8 Acknowledgements ...................................... 8
9 References ............................................ 9
9.1 Normative References .................................. 9
9.2 Informative References ................................ 9
10 Author's Address ...................................... 10
11 Intellectual Property Statement ....................... 10
12 Full Copyright Statement .............................. 11
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 [RFC2119].
2. Introduction
This document describes procedures for distributing upstream-assigned
labels [MPLS-UPSTREAM] for Label Distribution Protocol (LDP). These
procedures follow the architecture for MPLS Upstream Label Assignment
described in [MPLS-UPSTREAM].
This document describes extensions to LDP that a LSR can use to
advertise to its neighboring LSRs whether the LSR supports upstream
label assignment.
This document also describes extensions to LDP to distribute
upstream-assigned labels.
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The usage of MPLS upstream label assignment using LDP for avoiding
branch LSR traffic replication on a LAN for LDP P2MP LSPs [MLDP] is
also described.
3. LDP Upstream Label Assignment Capability
According to [MPLS-UPSTREAM], upstream-assigned label bindings MUST
NOT be used unless it is known that a downstream LSR supports them.
This implies that there MUST be a mechanism to enable a LSR to
advertise to its LDP neighbor LSR(s) its support of upstream-assigned
labels.
A new Capability Parameter, the LDP Upstream Label Assignment
Capability, is introduced to allow an LDP peer to exchange with its
peers, its support of upstream label assignment. This parameter
follows the format and procedures for exchanging Capability
Parameters defined in [LDP-CAP].
Following is the format of the LDP Upstream Label Assignment
Capability Parameter:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| Upstream Lbl Ass Cap(IANA)| Length (= 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Reserved |
+-+-+-+-+-+-+-+-+
If a LSR includes the Upstream Label Assignment Capability in LDP
Initialization Messages it implies that the LSR is capable of both
distributing upstream-assigned label bindings and receiving upstream-
assigned label bindings. The reserved bits MUST be set to zero on
transmission and ignored on receipt. The Upstream Label Assignment
Capability Parameter can be exchanged only in LDP initialization
messages.
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4. Distributing Upstream-Assigned Labels in LDP
An optional LDP TLV, Upstream-Assigned Label Request TLV, is
introduced. This TLV MUST be carried in a Label Request message if
an upstream-assigned label is being requested.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Upstream Ass Lbl Req (TBD)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An optional LDP TLV, Upstream-Assigned Label TLV is introduced to
signal an upstream-assigned label. Upstream-Assigned Label TLVs are
carried by the messages used to advertise, release and withdraw
upstream assigned label mappings.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Upstream Ass Label (TBD) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Label
This is a 20-bit label value as specified in [RFC3032] represented as
a 20-bit number in a 4 octet field.
4.1. Procedures
Procedures for Label Mapping, Label Request, Label Abort, Label
Withdraw and Label Release follow [RFC3036] other than the
modifications pointed out in this section.
A LDP LSR MUST NOT distribute the Upstream Assigned Label TLV to a
neighboring LSR if the neighboring LSR had not previously advertised
the Upstream Label Assignment Capability in its LDP Initialization
messages. A LDP LSR MUST NOT send the Upstream Assigned Label
Request TLV to a neighboring LSR if the neighboring LSR had not
previously advertised the Upstream Label Assignment Capability in its
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LDP Initialization messages.
As described in [MPLS-UPSTREAM] the distribution of upstream-assigned
labels is similar to either ordered LSP control or independent LSP
control of the downstream assigned labels.
When the label distributed in a Label Mapping message is an upstream-
assigned label, the Upstream Assigned Label TLV MUST be included in
the Label Mapping message. When a LSR receives a Label Mapping
message with an Upstream Assigned Label TLV and it does not recognize
the TLV, it MUST generate a Notification message with a status code
of "Unknown TLV" [RFC3036]. If it does recognize the TLV but is
unable to process the upstream label, it MUST generate a Notification
message with a status code of "No Label Resources". If the Label
Mapping message was generated in response to a Label Request message,
the Label Request message MUST contain an Upstream Assigned Label
Request TLV. A LSR that generates an upstream assigned label request
to a neighbor LSR, for a given FEC, MUST NOT send a downstream label
mapping to the neighbor LSR for that FEC unless it withdraws the
upstream-assigned label binding. Similarly if a LSR generates a
downstream assigned label request to a neighbor LSR, for a given FEC,
it MUST NOT send an upstream label mapping to that LSR for that FEC,
unless it aborts the downstream assigned label request.
The Upstream Assigned Label TLV may be optionally included in Label
Withdraw and Label Release messages that withdraw/release a
particular upstream assigned label binding.
5. LDP Tunnel Identifier Exchange
As described in [MPLS-UPSTREAM] an upstream LSR Ru MAY transmit a
MPLS packet, the top label of which (L) is upstream-assigned, to a
downstream LSR Rd, by encapsulating it in an IP or MPLS tunnel. In
this case the fact that L is upstream-assigned is determined by Rd by
the tunnel on which the packet is received. There must be a mechanism
for Ru to inform Rd that a particular tunnel from Ru to Rd will be
used by Ru for transmitting MPLS packets with upstream-assigned MPLS
labels.
When LDP is used for upstream label assignment, the Interface ID TLV
[RFC3472] is used for signaling the Tunnel Identifier. If Ru uses an
IP or MPLS tunnel to transmit MPLS packets with upstream assigned
labels to Rd, Ru MUST include the Interface ID TLV in the Label
Mapping messages along with the Upstream Assigned Label TLV. Three
new Interface ID TLVs are introduced to support RSVP-TE P2MP LSPs, IP
Multicast Tunnels and context labels. The TLV value acts as the
tunnel identifier.
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1. RSVP-TE P2MP LSP TLV. Type = TBD. Value of the TLV is the RSVP-TE
P2MP Session Object and optionally the P2MP Sender Template Object
[RFC4875]. The TLV value identifies the RSVP-TE P2MP LSP. It allows
Ru to tunnel an "inner" LDP P2MP LSP, the label for which is upstream
assigned, over an "outer" RSVP-TE P2MP LSP that has leaves
<Rd1...Rdn>. The P2MP LSP IF_ID TLV allows Ru to signal to
<Rd1...Rdn> the binding of the inner LDP P2MP LSP to the outer RSVP-
TE P2MP LSP. The control plane signaling between Ru and <Rd1...Rdn>
for the inner P2MP LSP uses targeted LDP signaling messages
2. IP Multicast Tunnel TLV. Type = TBD. In this case the TLV value is
a <Source Address, Multicast Group Address> tuple. Source Address is
the IP address of the root of the tunnel i.e. Ru, and Multicast Group
Address is the Multicast Group Address used by the tunnel.
3. MPLS Context Label TLV. Type = TBD. In this case the TLV value is
a <Source Address, MPLS Context Label> tuple. The Source Address
belongs to Ru and the MPLS Context Label is an upstream assigned
label, assigned by Ru. This allows Ru to tunnel an "inner" LDP P2MP
LSP, the label of which is upstream assigned, over an "outer" one-hop
MPLS LSP, where the outer one-hop LSP has the following property:
+ The label pushed by Ru for the outer MPLS LSP is an upstream
assigned context label, assigned by Ru. When <Rd1...Rdn> perform
a MPLS label lookup on this label a combination of this label and
the incoming interface MUST be sufficient for <Rd1...Rdn> to
uniquely determine Ru's context specific label space to lookup
the next label on the stack in. <Rd1...Rdn> MUST receive the data
sent by Ru with the context specific label assigned by Ru being
the top label on the label stack.
Currently the usage of the context label TLV is limited only to LDP
P2MP LSPs on a LAN as specified in the next section. The context
label TLV MUST NOT be used for any other purposes.
6. LDP Point-to-Multipoint LSPs on a LAN
This section describes one application of upstream label assignment
using LDP. Further applications are to be described in separate
documents.
[MLDP] describe how to setup P2MP LSPs using LDP. On a LAN the
solution relies on "ingress replication". A LSR on a LAN, that is a
branch LSR for a P2MP LSP, (say Ru) sends a separate copy of a packet
that it receives on the P2MP LSP to each of the downstream LSRs on
the LAN (say <Rd1...Rdn> that are adjacent to it in the P2MP LSP.
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It is desirable for Ru to send a single copy of the packet for the
LDP P2MP LSP on the LAN, when there are multiple downstream routers
on the LAN that are adjacent to Ru in that LDP P2MP LSP. This
requires that each of <Rd1...Rdn> must be able to associate the label
L, used by Ru to transmit packets for the P2MP LSP on the LAN, with
that P2MP LSP. It is possible to achieve this using LDP upstream-
assigned labels with the following procedures.
Consider a LSR Rd that receives the LDP P2MP FEC [MLDP] from its
downstream LDP peer. Further the upstream interface to reach LSR Ru
which is the next-hop to the P2MP LSP root address, Pr, in the LDP
P2MP FEC, is a LAN interface, Li. Further Rd and Ru support upstream-
assigned labels. In this case Rd instead of sending a Label Mapping
message as described in [MLDP] sends a Label Request message to Ru.
This Label Request message MUST contain an Upstream Assigned Label
Request TLV.
Ru on receiving this message sends back a Label Mapping message to Rd
with an upstream-assigned label. This message also contains a MPLS
Context Label TLV, as described in the previous section, with the
value of the MPLS label set to a value assigned by Ru on inteface Li
as specified in [MPLS-UPSTREAM]. Processing of the Label Request and
Label Mapping messages for LDP upstream-assigned labels is as
described in section 4.2. If Ru receives a Label Request for an
upstream assigned label for the same P2MP FEC from multiple
downstream LSRs on the LAN, <Rd1...Rdn>, it MUST send the same
upstream-assigned label to each of <Rd1...Rdn>.
Ru transmits the MPLS packet using the procedures defined in [MPLS-
UPSTREAM] and [MPLS-MCAST-ENCAPS]. The MPLS packet transmitted by Ru
contains as the top label the context label assigned by Ru on the LAN
interface, Li. The bottom label is the upstream label assigned by Ru
to the LDP P2MP LSP. The top label is looked up in the context of the
LAN interface, Li, [MPLS-UPSTREAM] by a downstream LSR on the LAN.
This lookup enables the downstream LSR to determine the context
specific label space to lookup the inner label in.
Note that <Rd1...Rdn> may have more than one equal cost next-hop on
the LAN to reach Pr. It MAY be desirable for all of them to send the
label request to the same upstream LSR and they MAY select one
upstream LSR using the following procedure:
1. The candidate upstream LSRs are numbered from lower to higher IP
address
2. The following hash is performed: H = (Sum Opaque value) modulo N,
where N is the number of candidate upstream LSRs. Opaque value is
defined in [MLDP] and comprises the P2MP LSP identifier.
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3. The selected upstream LSR U is the LSR that has the number H.
This allows for load balancing of a set of LSPs among a set of
candidate upstream LSRs, while ensuring that on a LAN interface a
single upstream LSR is selected. It is also to be noted that the
procedures in this section can still be used by Rd and Ru if other
LSRs on the LAN do not support upstream label assignment. Ingress
replication and downstream label assignment will continue to be used
for LSRs that do not support upstream label assignment.
7. IANA Considerations
This document defines a new LDP Upstream Label Assignment Capability
Parameter. IANA is requested to assign the value 0x0507 to this
Parameter.
This document defines a new LDP Upstream-Assigned Label Request TLV,
IANA is requested to assign the type value of this TLV.
This document defines a new LDP Upstream-Assigned Label TLV, IANA is
requested to assign the type value of this TLV.
This document defines three new Interface ID TLVs:
- RSVP-TE P2MP LSP TLV
- IP Multicast Tunnel TLV
- MPLS Context Label TLV
IANA is requested to assign the type values of these TLVs.
8. Acknowledgements
Thanks to Yakov Rekhter for his contribution. Thanks to Ina Minei and
Thomas Morin for their comments. The hashing algorithm used on LAN
interfaces is taken from [MLDP].
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9. References
9.1. Normative References
[RFC3031] "MPLS Architecture", E. Rosen, A. Viswanathan, R. Callon,
RFC 3031.
[MPLS-UPSTREAM] R. Aggarwal, Y. Rekhter, E. Rosen, "MPLS Upstream
Label Assignment and Context Specific Label Space", draft-ietf-mpls-
upstream-label-06.txt
[MPLS-MCAST-ENCAPS] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter,
draft-ietf-mpls-codepoint-10.txt
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels.", Bradner, March 1997
[RFC3472] Ashwood-Smith, P. and L. Berger, Editors, " Generalized
Multi-Protocol Label Switching (GMPLS) Signaling - Constraint-based
Routed Label Distribution Protocol (CR-LDP) Extensions", RFC 3472,
January 2003.
[RFC3471] Berger, L. Editor, "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description", RFC 3471 January
2003.
[RFC3036] L. Andersson, et. al., "LDP Specification", January 2001.
9.2. Informative References
[MVPN] E. Rosen, R. Aggarwal [Editors], "Multicast in BGP/MPLS VPNs",
draft-ietf-l3vpn-2547bis-mcast-06.txt
[RFC4875] R. Aggarwal, D. Papadimitriou, S. Yasukawa [Editors],
"Extensions to RSVP-TE for Point to Multipoint TE LSPs", RFC 4875
[MLDP] I. Minei et. al, "Label Distribution Protocol Extensions for
Point-to-Multipoint Label Switched Paths", draft-etf-mpls-ldp-
p2mp-05.txt
[LDP-CAP] B. Thomas, et. al., "LDP Capabilities", draft-thomas-mpls-
ldp-capabilities-02.txt
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10. Author's Address
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Phone: +1-408-936-2720
Email: rahul@juniper.net
Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
E-mail: jeanlouis.leroux@orange-ftgroup.com
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12. Full Copyright Statement
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