Internet DRAFT - draft-pdutta-mpls-tldp-hello-reduce
draft-pdutta-mpls-tldp-hello-reduce
Network Working Group P. Dutta
Internet-Draft Alcatel-Lucent
Intended status: Standards Track G. Heron
Expires: March 5, 2013 Cisco Systems
T. Nadeau
Juniper Networks
September 01, 2012
Targeted LDP Hello Reduction
draft-pdutta-mpls-tldp-hello-reduce-04
Abstract
Targeted LDP (t-LDP) Hellos are used for establishing adjacencies
with non-directly connected peers. After an LDP session is
established to a Targeted Peer, there are deployment scenerios where
it is not necessary to send Targeted LDP Hellos at the configured
intervals. This document proposes a mechanism to turn off or reduce
the rate of exchange of Targeted LDP Hellos after LDP session is
established to a peer.
Requirements Language
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 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 5, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Targeted LDP Hello Reduction Procedure . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Operational Considerations . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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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. There are
certain deployment scenerios where 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. Moreover additional mechanisms such as
centralized BFD [RFC5880] may be used to track liveliness of ldp
sessions.
When an LSR maintains a large number of LDP sessions (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 or in Seamless MPLS
[I-D.ietf-mpls-seamless-mpls] , there can be very large volume of LDP
sessions (e.g 10,000) with targeted LDP adjacencies to each base
station (or last mile in a MPLS domain).
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
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throttling spurious adjacencies but such methods may not be available
in low cost MPLS devices deployed in access. So it is important to
avoid dependency on Targeted LDP hellos on t-LDP adjacency
maintenance as far as possible. Reduction of Hellos provide
probabilistically better resilience on maintenance of hello
adjacencies during sporadic hello attacks.
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]. The solutions
described in this document may not be applicable in scenerios where
Session Keepalives or BFD may not act as substitute for Targeted LDP
Hellos. Refer to section 6 for operational considerations while
deploying the solution described in this document.
2. Terminology
This document uses the terminology defined in [RFC3031] and
[RFC5036].
3. 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.
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
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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
start proposing "relaxed" hold time (higher than configured) in
Common Hello Parameters TLV in the subsequent Hello Messages.
Each LSR increases the advertised hold time by some factor after
sending a set of Hellos (let's say 5) advertising consistent hold
time. As the process of relaxing the advertised hold time continues,
after a certain period of time an LSR reaches the maximum holdtime
value of 0xffff. Thus after the session is ESTABLISHED, the hello
hold time between the LSRs gets negotiated to infinite. Note that
the Targeted Hello Adjacency continue to exist and only the adjacency
hold times are now infinite.
4. If there are any changes in any parameters associated with a
t-LDP Hello adjacency (e.g Configuration Sequence Number etc) then an
updated Hello MUST be sent immediately without any changes to the
"current" hold time (e.g inifinite) that was advertised in the last
Hello Message. Since hellos are not reliable, after any parameter
change an implementation may send a set of hellos (let's say 5) at
configured intervals (or faster) to reflect the change. But those
hellos would continue to advertise infinite hold time and would fall
back to reduced transmission rate after those 5 packets are sent.
5. 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. This also applies when
LDP session restarts gracefully [RFC3478] when peering LSRs are
graceful restart capable. Thus the reduction procedures allow an
operator to configure very aggressive Targeted LDP Hello Holdtime to
expedite bringing up a large number of LDP sessions in the event of
failure but reduces the overhead of hello adjacency maintenance by
manifold when sessions are ESTABLISHED. It is desirable to configure
aggressive hold times in order to tear down spurious hello
adjacencies sooner.
6. When a t-LDP adjacency with a remote LSRs has negotiated to
infinite hold time and then remote LSR decides to tear down the
adjacency without impacting the established LDP Session then local
LSR would not be able to detect that remote node is no longer
accepting hellos. It is RECOMMENDED that when a LSR that implements
the Hello Reduction procedures send one or a set of contiguous hellos
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(let's say 3) advertising hold time of 1 second while bringing down
t-LDP Hello adjacency. This graceful closure procedure would cause
the hello holdtimes at receiving LSR to be renegotiated to 1 second,
which would eventually lead to tear down of the adjacency (due to
timeout) by receiving LSR.
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.
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. However, it is RECOMMENDED
to reach Infinite holdtime after session setup to derive maximum
advantage from the procedure described above.
The Hello Reduction procedures does not apply to Basic Discovery
(Link LDP Hellos) as Link LDP Hellos need to be sent over an interval
continually in order to discover and set up sessions with new peers,
especially over a multi-access interface.
4. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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. Operational Considerations
The method proposed in the document reduces significant burden on an
LDP LSR that maintains Targeted LDP sessions to a large number (in
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thousands) of peers. Further, if BFD [RFC5880] [RFC5883] is used for
tracking connectivity to peers it is desirable to turn off Targeted
LDP hellos after the LDP session is setup. However there are
scenerios where tunring off Targeted LDP Hellos may not be desirable.
Such scenerios are as follows:
1. When transport address of the LDP session is different from the
IP addresses used to exchange t-LDP Hellos then Session Keepalives
are not substitute for reachability or liveliness of adjacency. It
is possible to use BFD to track the reachability of IP addresses used
for t-LDP Hellos in which case t-LDP Hellos may be redundant.
However if an implementation/deployment uses t-LDP hellos for
purposes other than liveliness tracking then it is not recommended to
turn on t-LDP hello reduction procedures.
2. While t-LDP Hello Reduction Procedures are deployed, it may be
possible that t-LDP Hellos are disabled at remote LSR without
bringing down the LDP session. If the remote LSR does not implement
the procedure for graceful teardown of hello adjacency as described
in step 6 in section 3 then it is possible that local LSR may not be
able detect that remote LSR is no longer accepting Hellos and thus
Hello adjacency would continue to exist in local LSR. It is also
possible that the hello(s) sent during graceful cloure of adjacency
may get lost (since LDP Hellos are not reliable) and thus local LSR
may not detect the loss of adjacency with remote LSR.
7. Acknowledgements
The authors would like acknowledge the detailed review and the
comments, suggestions from Markus Jork, Thomas Beckhaus, Lizhong Zin
and Eric Rosen.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
8.2. Informative References
[I-D.ietf-mpls-seamless-mpls]
Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
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M., and D. Steinberg, "Seamless MPLS Architecture",
draft-ietf-mpls-seamless-mpls-01 (work in progress),
March 2012.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3478] Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
Restart Mechanism for Label Distribution Protocol",
RFC 3478, February 2003.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, June 2010.
Authors' Addresses
Pranjal Kumar Dutta
Alcatel-Lucent
701 E Middlefield Road
Mountain View, CA 94043
USA
Email: pranjal.dutta@alcatel-lucent.com
Giles Heron
Cisco Systems
9-11 New Square
Bedfont Lakes, Feltham, Middlesex TW14 8HA
United Kingdom
Email: giheron@cisco.com
Thomas Nadeau
Juniper Networks
Email: tnadeau@juniper.net
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