MPLS Working Group Pranjal Kumar Dutta
Internet Draft Alcatel-Lucent
Intended status: Standard
Expires: January 2012 Giles Heron
Cisco Systems
Thomas Nadeau
CA Technologies
July 3, 2011
Targeted LDP Hello Reduction
draft-pdutta-mpls-tldp-hello-reduce-02.txt
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Abstract
Targeted LDP Hellos are used for establishing adjacencies with non-
directly connected peers. After an LDP session is established to a
targeted peer, the session Keepalives are sufficient to notify the
intent of an LSR to maintain its adjacency with the peer. This
document proposes a mechanism to turn off Targeted LDP Hellos after
LDP session is established to a peer.
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Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................3
3. Terminology....................................................3
4. Targeted LDP Hello Reduction Procedure.........................3
5. Security Considerations........................................5
6. IANA Considerations............................................5
7. Conclusion.....................................................5
8. References.....................................................5
8.1. Normative References......................................5
8.2. Informative References....................................5
9. Acknowledgments................................................6
1. Introduction
LDP Hello messages are exchanged as part of the LDP discovery
mechanism [RFC5036]. There are two types of LDP discovery mechanism
described in [RFC5036] - Basic Discovery and Extended Discovery.
To engage in LDP Basic Discovery on an interface, an LSR periodically
sends LDP Link Hellos out the interface to the well-known LDP
discovery port for the "all routers on this subnet" group multicast
address. Receipt of an LDP Link Hello on an interface, identifies a
hello adjacency with a potential LDP peer reachable at the link level
on the interface. Thus an LSR may establish hello adjacency with
multiple peers discovered over a single interface and must continue
to transmit hellos at regular intervals even after hello adjacency is
established to a peer.
Extended discovery is used to support LDP sessions between non-
directly connected LSRs. An LDP Targeted Hello is sent to a specific
address rather than to the "all routers" group multicast address for
the ongoing interface. Receipt of a LDP Targeted Hello indentifies a
hello adjacency with a potential LDP peer at network level.
In Extended discovery there can be only one Targeted Hello Adjacency
between two peers. Note that throughout this document "peer" means
the LDP LSR designated by a unique LDP Identifier. Once the LDP
session is operational between two targeted LDP peers, periodic
session Keepalives are used to maintain the LDP session. After the
session is operational the periodic Targeted Hellos between the LSRs
become redundant, as session Keepalives in turn serves the intent of
each LSR to maintain its adjacency to its peer.
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When an LSR maintains a large number of LDP sessions (in thousands)
to targeted peers, it is an additional burden to send and receive
Targeted Hellos for all peers at periodic intervals. In MPLS
deployments at access or mobility backhaul, there can be very large
volume of LDP sessions with targeted LDP adjacencies to each base
station. Moreover additional mechanisms such as centralized BFD [BFD]
may be used to track liveliness of ldp sessions.
Another problem with targeted hello adjacency arises is Denial Of
Service (DoS)_attacks. It is possible that existing hello adjacencies
can get lost due to DoS attack on LDP Hello receiver by spurious
hello packets. Unlike TCP sessions it is not always possible to
provide per peer protection for UDP based hellos. Implementations can
use methods to protect existing adjacencies while throttling spurious
adjacencies but such methods may not be available in low cost MPLS
devices in access. So it is important to avoid dependency on targeted
LDP hellos on session maintenance as far as possible.
This document proposes an optional mechanism to turn off Targeted LDP
Hellos after a LDP session is established to a targeted peer, without
changes in the procedures defined in [RFC5036].
2. Conventions used in this document
INFO (REMOVE): INCLUDE THIS SECTION OR PORTIONS THEREOF IF DESIRED
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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].
3. Terminology
This document uses the terminology defined in [RFC3031] and
[RFC5036].
4. Targeted LDP Hello Reduction Procedure
The Targeted LDP Hello Reduction procedure uses the existing Common
Hello Parameters TLV defined in [RFC5036]. Figure 1. shows the
encoding of the TLV from [RFC5036] for reference.
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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| Common Hello Parms(0x0400)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |T|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Common Hello Parameters TLV.
By definition in [RFC5036], a value of 0 means use the default,
which is 45 seconds for Targeted Hellos. A value of 0xffff means
infinite.
The procedure to be followed for Targeted LDP Hello Reduction
between a pair of LSRs is as follows:
1. An LSR starts transmitting periodic targeted hellos to its peer in
order to establish the targeted hello adjacency. Each LSR proposes
its configured hello hold time in the Common Hello Parameters TLV
in its hello message to the peer. The hold time used between a
pair of LSRs is the minimum of the hold times proposed in their
Hellos.
2. If the Hello is acceptable by receiving LSR it establishes
targeted hello adjacency with the source LSR. Establishment of
Hello adjacency establishes the LDP session between peering LSRs.
3. After the LDP session is ESTABLISHED [RFC5036], each LSR MAY
advertise hello holdtime value of 0xffff in the Common Hello
Parameters TLV. Thus after the session is ESTABLISHED, the hello
hold time between the LSRs gets negotiated to infinite. An LSR MAY
implement a locally configurable "tolerance" - the number of
Targeted LDP Hellos to be advertised with infinite hold time after
the LDP session is ESTABLISHED.
4. If the LDP session between two LSRs fails leading to tearing down
of adjacency, then each LSR reverts to advertising their
configured hello hold time and repeats procedure 1 to 3.
It is RECOMMENDED that each peering LSR implements the Targeted LDP
Hello Reduction procedure; otherwise negotiated hello hold time
between the LSRs does not fall back to the infinite hold time in step
3.
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Note that it is not mandatory to advertise infinite hold time after
session is established but can be any value that is significantly
larger than configured hello hold time. It is RECOMMENDED to
advertise Inifinite holdtime after session setup to derive maximum
advantage from the procedure described above.
5. Security Considerations
- Control plane aspects
- LDP security (authentication) methods as described in [RFC5036]
is applicable here.
- Data plane aspects
- This specification does not have any impact on the MPLS
forwarding plane setup by LDP.
6. IANA Considerations
This document does not require any IANA consideration.
7. Conclusion
The method proposed in the document reduces significant burden on an
LDP LSR that maintains Targeted LDP sessions to a large number (in
thousands) of peers. Further, if BFD [BFD][BFD-MHOP] is used for
tracking connectivity to peers it is desirable to turn off Targeted
LDP hellos after the LDP session is setup.
8. References
8.1. Normative References
[RFC5036] Andersson, L., et al. "LDP Specification", RFC5036,
October 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[RFC3031] Rosen, E., et al. "Multiprotocol Label Switching
Architecture", RFC 3031, January 2001.
[BFD] Katz, D., et al. "Bidirectional Forwarding Detection",
draft-ietf-bfd-base-011.txt, January 2010.
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[BFD-MHOP] Katz, D., et al. "BFD for Multihop Paths",
draft-ietf-bfd-multihop-09.txt, January 2010.
9. Acknowledgments
The authors would like acknowledge the comments and suggestions from
Wim Henderickx, Vach Kompella, Florin Balus, Mustapha Aissaoui,
Mathew Bocci and Paul Kwok.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Pranjal Kumar Dutta
701 E Middlefield Road,
Mountain View, CA 94043.
USA.
Email: pranjal.dutta@alcatel-lucent.com
Giles Heron
Cisco Systems
Email: giheron@cisco.com
Thomas D. Nadeau
CA Technologies
273 Corporate Drive, Portsmouth, NH, USA
Email: thomas.nadeau@ca.com
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