MPLS Working Group Kamran Raza
Internet Draft Sami Boutros
Intended status: Standards Track
Expires: October 26, 2014 Cisco Systems
April 27, 2014
Controlling State Advertisements Of Non-negotiated LDP Applications
draft-ietf-mpls-ldp-ip-pw-capability-07.txt
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Abstract
There is no capability negotiation done for Label Distribution
Protocol (LDP) applications that setup Label Switched Paths (LSPs)
for IP prefixes or that signal Point-to-point (P2P) Pseudowires
(PWs) for Layer 2 Virtual Private Networks (L2VPNs). When an LDP
session comes up, an LDP speaker may unnecessarily advertise its
local state for such LDP applications even when the peer session is
established for some other applications like Multipoint LDP (mLDP)
or Inter-Chassis Communication Protocol (ICCP). This document
defines a solution by which an LDP speaker announces to its peer its
disinterest in such non-negotiated applications, thus disabling the
unnecessary advertisement of corresponding application state, which
would have otherwise be advertised over the established LDP session.
Table of Contents
1. Introduction 3
2. Conventions used in this document 4
3. Non-negotiated LDP applications 4
3.1. Non-interesting State 5
4. Controlling State Advertisement 5
4.1. State Advertisement Control Capability 5
4.2. Capabilities Procedures 8
4.2.1. State Control Capability in an Initialization message 9
4.2.2. State Control capability in a Capability message 9
5. Operational Examples 9
5.1. Disabling Prefix-LSPs and P2P-PWs apps on an ICCP session 9
5.2. Disabling Prefix-LSPs app on a PW Targeted LDP session 10
5.3. Disabling Prefix-LSPs app dynamically on an LDP session 10
5.4. Disabling Prefix-LSPs app on an mLDP-only session 11
5.5. Disabling IPv4 or IPv6 Prefix-LSPs app on an dual-stack LSR 11
6. Security Considerations 11
7. IANA Considerations 12
8. References 12
8.1. Normative References 12
8.2. Informative References 12
9. Acknowledgments 13
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1. Introduction
LDP Capabilities specification [RFC5561] introduced a mechanism to
negotiate LDP capabilities for a given feature between peer Label
Switching Routers (LSRs). The capability mechanism insures that no
unnecessary state is exchanged between peer LSRs unless the
corresponding feature capability is successfully negotiated between
the peers.
Newly defined LDP features and applications, such as Typed Wildcard
Forwarding Equivalence Class (FEC) [RFC5918], Inter-Chassis
Communication Protocol [ICCP], mLDP [RFC6388], and L2VPN Point-to-
multipoint (P2MP) PW [P2MP-PW] make use of LDP capabilities framework
for their feature negotiation. However, the earlier LDP application
to establish LSPs for IP unicast prefixes, and application to signal
L2VPN P2P PW [RFC4447] [RFC4762] allowed LDP speakers to exchange
application state without any capability negotiation, thus causing
unnecessary state advertisement when a given application is not
enabled on one of the LDP speakers participating in a given session.
For example, when bringing up and using an LDP peer session with a
remote Provider Edge (PE) LSR for purely ICCP signaling reasons, an
LDP speaker may unnecessarily advertise labels for IP (unicast)
prefixes to this ICCP related LDP peer.
Another example of unnecessary state advertisement can be cited when
LDP is to be deployed in an IP dual-stack environment. For instance,
an LSR that is locally enabled to setup LSPs for both IPv4 and IPv6
prefixes may advertise (address and label) bindings for both IPv4 and
IPv6 address families towards an LDP peer that is interested in IPv4
bindings only. In this case, the advertisement of IPv6 bindings to
the peer is unnecessary, as well as wasteful, from the point of view
of LSR memory/CPU and network resource consumption.
To avoid this unnecessary state advertisement and exchange, currently
an operator is typically required to configure and define filtering
policies on the LSR, which introduces unnecessary operational
overhead and complexity for such deployments.
This document defines an LDP Capabilities [RFC5561] based solution by
which an LDP speaker may announce to its peer(s) its disinterest (or
non-support) for state to setup IP Prefix LSPs and/or to signal L2VPN
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P2P PW at the time of session establishment. This capability helps in
avoiding unnecessary state advertisement for such feature
applications. This document also states the mechanics to dynamically
disable or enable the state advertisement for such applications
during the session lifetime. The non-interesting state of an
application depends on the type of application and is described later
in section 3.1.
2. Conventions used in this document
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].
The term "IP" in this document refers to both IPv4 and IPv6 unicast
address families.
3. Non-negotiated LDP applications
For the applications that existed prior to the definition of LDP
Capabilities framework [RFC5561], an LDP speaker typically
advertises, without waiting for any capabilities exchange and
negotiation, its corresponding application state to its peers after
the session establishment. These early LDP applications include:
o IPv4/IPv6 Prefix LSPs Setup
o L2VPN P2P FEC128 and FEC129 PWs signaling
This document onward uses following shorthand terms for these
earlier LDP applications:
o "Prefix-LSPs": Refers to an application that sets up LDP LSPs
corresponding to IP routes/prefixes by advertising label
bindings for Prefix FEC (as defined in RFC-5036).
o "P2P-PWs": Refers to an application that signals FEC 128 and/or
FEC 129 L2VPN P2P Pseudowires using LDP (as defined in
RFC-4447).
To disable unnecessary state exchange for such LDP applications over
an established LDP session, a new capability is being introduced in
this document. This new capability controls the advertisement of
application state and enables an LDP speaker to notify its peer its
disinterest in the state of one or more of these "Non-negotiated"
LDP applications at the time of session establishment. Upon receipt
of such capability, the receiving LDP speaker, if supporting the
capability, disables the advertisement of the state related to the
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application towards the sender of the capability. This new
capability can also be sent later in a Capability message to either
disable a previously enabled application's state advertisement or to
enable a previously disabled application's state advertisement.
3.1. Non-interesting State
A non-interesting state of a non-negotiated LDP application:
- is the application state which is of a no interest to an LSR
and need not be advertised to the LSR;
- need not be advertised in any of the LDP protocol messages;
- is dependent on application type and specified accordingly.
For Prefix-LSPs application type, the non-interesting state refers
to any state related to IP Prefix FEC (such as FEC label bindings,
LDP Status). This document, however, does not classify IP address
bindings as a non-interesting state and allows the advertisement of
IP Address bindings to facilitate other LDP applications (such as
mLDP) that depend on learning of peer addresses over an LDP session
for their correct operation.
For P2P-PWs application type, the non-interesting state refers to
any state related to P2P PW FEC128/FEC129 (such as FEC label
bindings, MAC [address] withdrawal, and LDP PW Status).
From now onward in this document, the term "state" will mean to
refer to the "non-interesting state" for an application, as defined
in this section.
4. Controlling State Advertisement
To control advertisement of non-interesting state related to non-
negotiated LDP applications defined in section 3, a new capability
TLV is defined as follows.
4.1. State Advertisement Control Capability
The "State Advertisement Control Capability" is a new Capability
Parameter TLV defined in accordance with section 3 of LDP
Capabilities specification [RFC5561]. The format of this new TLV is
as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| State Adv. Ctrl Cap.(IANA)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved | |
+-+-+-+-+-+-+-+-+
| |
~ State Advertisement Control Element(s) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of an "State Advertisement Control Capability" TLV
The value of the U-bit for the TLV MUST be set to 1 so that a
receiver MUST silently ignore this TLV if unknown to it, and
continue processing the rest of the message. Whereas, The value of
F-bit MUST be set to 0. Once advertised, this capability cannot be
withdrawn; thus S-bit MUST be set to 1 in an Initialization and
Capability message.
The capability data associated with this State Advertisement Control
(SAC) Capability TLV is one or more State Advertisement Control
Elements, where each element indicates enabling/disabling of
advertisement of non-interesting state for a given application. The
format of a SAC Element is defined as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D| App |Unused |
+-+-+-+-+-+-+-+-+
Figure 2: Format of "State Advertisement Control Element"
Where:
D bit: Controls the advertisement of the state specified in "App"
field:
1: Disable state advertisement
0: Enable state advertisement
When sent in an Initialization message, D bit MUST be set to 1.
App: Defines the application type whose state advertisement is to be
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controlled. The value of this field is defined as follows:
1: IPv4 Prefix-LSPs (LSPs for IPv4 prefixes)
2: IPv6 Prefix-LSPs (LSPs for IPv6 prefixes)
3: FEC128 P2P-PW (L2VPN PWid FEC signaling)
4: FEC129 P2P-PW (L2VPN Generalized PWid FEC signaling)
Any other value in this field MUST be treated as an error.
Unused: MBZ on transmit and ignored on receipt.
The "Length" field of SAC Capability TLV depends on the number of
SAC Elements present in the TLV. For example, if there are two
elements present, then the Length field is set to 3 octets. A
receiver of this capability TLV can deduce number of elements
present in the TLV by using the Length field.
From now onward, this document uses the term "element" to refer to a
SAC Element.
As described earlier, SAC Capability TLV MAY be included by an LDP
speaker in an Initialization message to signal to its peer LSR that
state exchange for one or more application(s) need to be disabled on
the given peer session. This TLV can also be sent later in a
Capability message to selectively enable or disable these
applications. If there are more than one elements present in a SAC
Capability TLV, the elements MUST belong to distinct app types and
an app type MUST NOT appear more than once. If a receiver
receives such a malformed TLV, it SHOULD discard this TLV and
continue processing rest of the message. If an LSR receives a
message with a SAC capability TLV containing an element with "App"
field set to a value other than defined above, the receiver MUST
ignore and discard the element and continue processing the rest of
the TLV.
To control more than one application state, a sender LSR can either
send a single capability TLV in a message with multiple elements
present, or can send separate messages with capability TLV
specifying one or more elements. A receiving LSR, however, MUST
treat each incoming capability TLV with an element corresponding to
a given application type as an update to its existing policy for the
given type.
To understand capability updates from an example, let us consider 2
LSRs, S (LDP speaker) and P (LDP peer), both of which support all
the non-negotiated applications listed earlier. By default, these
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LSR will advertise state for these applications, as configured, to
their peer as soon as an LDP session is established. Now assume that
P receives from S a SAC capability in an Initialization message with
"IPv6 Prefix-LSPs" and "FEC129 P2P-PW" applications disabled. This
updates P's outbound policy towards S to advertise state related to
only IPv4 Prefix-LSPs and FEC128 P2P-PW applications. Later, P
receives another capability update from S via a Capability message
with "IPv6 Prefix-LSPs" enabled and "FEC128 P2P-PWs" disabled. This
results in P's outbound policy towards S to advertise both IPv4 and
IPv6 Prefix-LSPs application state, and disable both FEC128 and
FEC129 P2P-PWs signaling. Finally, P receives another update from S
via a Capability message that specifies to disable all four non-
negotiated applications state, resulting in P outbound policy
towards S to block/disable state for all these applications, and
only advertise state for any other application, as applicable.
4.2. Capabilities Procedures
The SAC capability conveys the desire of an LSR to disable the
receipt of unwanted/unnecessary state from its LDP peer. This
capability is unilateral and unidirectional in nature, and a
receiving LSR is not required to send a similar capability TLV in an
Initialization or Capability message towards the sender of this
capability. This unilateral behavior conforms to the procedures
defined in the Section 6 of LDP Capabilities [RFC5561].
After this capability is successfully negotiated (i.e. sent by an
LSR and received/understood by its peer), then the receiving LSR
MUST NOT advertise any state related to the disabled applications
towards the capability sending LSR until and unless these
application states are explicitly enabled again via a capability
update. Upon receipt of a capability update to disable an enabled
application [state] during the lifetime of a session, the receiving
LSR MUST also withdraw from the peer any previously advertised state
corresponding to the disabled application.
If a receiving LDP speaker does not understand the SAC capability
TLV, then it MUST respond to the sender with "Unsupported TLV"
notification as described in LDP Capabilities [RFC5561]. If a
receiving LDP speaker does not understand or does not support an
application specified in an application control element, it SHOULD
silently ignore/skip such an element and continue processing rest of
the TLV.
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4.2.1. State Control Capability in an Initialization message
LDP Capabilities [RFC5561] framework dictates that the S-bit of
capability parameter in an Initialization message MUST be set to 1
and SHOULD be ignored on receipt.
An LDP speaker determines (e.g. via some local configuration or
default policy) if it needs to disable Prefix-LSPs and/or P2P-PWs
applications with a peer LSR. If there is a need to disable, then
the SAC TLV needs to be included in the Initialization message with
respective SAC elements included with their D bit set to 1.
An LDP speaker that supports the SAC capability MUST interpret the
capability TLV in a received Initialization message such that it
disables the advertisement of the application state towards the
capability sending LSR for Prefix-LSPs and/or P2P-PWs applications
if their SAC element's D bit is set to 1.
4.2.2. State Control capability in a Capability message
If the LDP peer supports "Dynamic Announcement Capability"
[RFC5561], then an LDP speaker may send SAC capability in a
Capability message towards the peer. Once advertised, these
capabilities cannot be withdrawn and hence the S-bit of the TLV MUST
be set to 1 when sent in a Capability message.
An LDP speaker may decide to send this TLV towards an LDP peer if
one or more of its Prefix-LSPs and/or P2P-PWs applications get
disabled, or if previously disabled application gets enabled again.
In this case, the LDP speaker constructs the TLV with appropriate
SAC element(s) and sends the corresponding capability TLV in a
Capability message.
Upon receipt of this TLV in a Capability message, the receiving LDP
speaker reacts in the same manner as it reacts upon the receipt of
this TLV in an Initialization message. Additionally, the peer
withdraws/advertises the application state from/to the capability
sending LDP speaker according to the capability update.
5. Operational Examples
5.1. Disabling Prefix-LSPs and P2P-PWs applications on an ICCP session
Consider two PE routers, LSR1 and LSR2, which understand/support SAC
capability TLV, and have an established LDP session to exchange ICCP
state related to dual-homed devices connected to these LSRs. Let us
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assume that both LSRs are provisioned not to exchange any state for
Prefix-LSPs (IPv4/IPv6) and P2P-PWs (FEC128/129) application.
To indicate their disinterest in these applications, the LSRs will
include a SAC capability TLV (with 4 SAC elements corresponding to
these 4 applications with D bit set to 1 for each one) in the
Initialization message. Upon receipt of this TLV in Initialization
message, the receiving LSR will disable the advertisement of
IPv4/IPv6 label bindings, as well as P2P PW FEC128/129 signaling,
towards its peer after session establishment.
5.2. Disabling Prefix-LSPs application on a PW Targeted LDP session
Now, consider LSR1 and LSR2 have an established Targeted LDP (T-LDP)
session for P2P-PWs application to exchange label bindings for
FEC 128/129. Given that there is no need to exchange IP Prefix label
bindings amongst the PE LSRs over a PW T-LDP session in most typical
deployments, let us assume that LSRs are provisioned to disable
IPv4/IPv6 Prefix-LSPs application state on the given PW session.
To indicate their disinterest in Prefix-LSPs application over a PW
T-LDP session, the LSRs will follow/apply the same procedures as
described in previous section. As a result, only P2P-PWs related
state will be exchanged between these LSRs over this T-LDP session.
5.3. Disabling Prefix-LSPs application dynamically on an established
LDP session
Assume that LSRs from previous sections were initially provisioned
to exchange both Prefix-LSPs and P2P-PWs state over the session
between them, and also support "Dynamic Announcement" Capability
[RFC5561]. Now, assume that LSR1 is dynamically provisioned to
disable (IPv4/IPv6) Prefix-LSPs over T-LDP session with LSR2. In
this case, LSR1 will send SAC capability TLV in a Capability message
towards LSR2 with application control elements defined for IPv4 and
IPv6 Prefix-LSPs with D bit set to 1. Upon receipt of this TLV, LSR2
will disable Prefix-LSPs application state(s) towards LSR1 and
withdraw all previously advertised application state from LSR1. To
withdraw label bindings from its peer, LSR2 MAY use a single Prefix
FEC Typed Wildcard Label Withdraw message [RFC5918] if the peer
supports Typed Wildcard FEC capability.
This dynamic disability of Prefix-LSPs application does not impact
L2VPN P2P-PWs application on the given session, and both LSRs should
continue to exchange PW Signaling application related state.
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5.4. Disabling Prefix-LSPs application on an mLDP-only session
Now assume that LSR1 and LSR2 have formed an LDP session to exchange
mLDP state only. In typical deployments, LSR1 and LSR2 also exchange
bindings for IP (unicast) prefixes upon mLDP session, which is
unnecessary and wasteful for an mLDP-only LSR.
Using the procedures defined earlier, an LSR can indicate its
disinterest in Prefix-LSPs application state to its peer upon
session establishment time or dynamically later via LDP capabilities
update.
Reference to section 3.1, the peer disables the advertisement of any
state related to IP Prefix FECs, but still advertises IP address
bindings that are required for the correct operation of mLDP.
5.5. Disabling IPv4 or IPv6 Prefix-LSPs application on an dual-stack LSR
In IP dual-stack scenarios, LSR2 may advertise unnecessary state
(e.g. IPv6 prefix label bindings) towards peer LSR1 corresponding to
IPv6 Prefix-LSPs application once a session is established mainly
for exchanging state for IPv4. The similar scenario also applies
when advertising IPv4 Prefix-LSPs state on a session meant for IPv6.
The SAC capability and its procedures defined in this document can
help to avoid such unnecessary state advertisement.
Consider IP dual-stack environment where LSR2 is enabled for Prefix-
LSPs application for both IPv4 and IPv6, but LSR1 is enabled for (or
interested in) only IPv4 Prefix-LSPs. To avoid receiving unwanted
state advertisement for IPv6 Prefix-LSPs application from LSR2, LSR1
can send SAC capability with element for IPv6 Prefix-LSPs with D bit
set to 1 in the Initialization message towards LSR2 at the time of
session establishment. Upon receipt of this capability, LSR2 will
disable all IPv6 label binding advertisement towards LSR1. If IPv6
Prefix-LSPs application is later enabled on LSR1, LSR1 can update
the capability by sending SAC capability in a Capability message
towards LSR2 to enable this application dynamically.
6. Security Considerations
The proposal introduced in this document does not introduce any new
security considerations beyond that already apply to the base LDP
specification [RFC5036] and [RFC5920].
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7. IANA Considerations
This document defines a new LDP capability parameter TLV. IANA is
requested to assign the lowest available value after 0x0500 from
"TLV Type Name Space" in the "Label Distribution Protocol (LDP)
Parameters" registry as the new code point for the new LDP
capability TLV code point.
+-----+---------------------+---------------+-----------------------+
|Value| Description | Reference |Notes/Registration Date|
+-----+---------------------+---------------+-----------------------+
| TBA | State Advertisement | This document | |
| | Control Capability | | |
+-----+---------------------+---------------+-----------------------+
8. References
8.1. Normative References
[RFC5036] L. Andersson, I. Minei, and B. Thomas, "LDP
Specification", RFC 5036, September 2007.
[RFC5561] B. Thomas, K. Raza, S. Aggarwal, R. Aggarwal, and JL. Le
Roux, "LDP Capabilities", RFC 5561, July 2009.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
8.2. Informative References
[RFC5918] R. Asati, I. Minei, and B. Thomas, "Label Distribution
Protocol Typed Wildcard FEC", RFC 5918, August 2010.
[RFC4447] L. Martini, E. Rosen, El-Aawar, T. Smith, and G. Heron,
"Pseudowire Setup and Maintenance using the Label
Distribution Protocol", RFC 4447, April 2006.
[RFC4762] M. Lasserre, and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[P2MP-PW] Martini, L. et. al, "Signaling Root-Initiated Point-to-
Multipoint Pseudowires using LDP", draft-ietf-pwe3-p2mp-pw-
04.txt, Work in Progress, March 2012.
[ICCP] L. Martini, S. Salam, A. Sajassi, and S. Matsushima,
"Inter-Chassis Communication Protocol for L2VPN PE
Redundancy", draft-ietf-pwe3-iccp-16.txt, Work in
Progress, March 2014.
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[RFC6388] I. Minei, I. Wijnands, K. Kompella, and B. Thomas, "LDP
Extensions for P2MP and MP2MP LSPs", RFC 6388, November
2011.
[RFC5920] L. Fang, et al., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
9. Acknowledgments
The authors would like to thank Eric Rosen for his valuable input
and comments. We also acknowledge Karthik Subramanian and IJsbrand
Wijnands for bringing up mLDP use case.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Kamran Raza
Cisco Systems, Inc.,
2000 Innovation Drive,
Ottawa, ON K2K-3E8, Canada.
E-mail: skraza@cisco.com
Sami Boutros
Cisco Systems, Inc.
3750 Cisco Way,
San Jose, CA 95134, USA.
E-mail: sboutros@cisco.com
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