Internet DRAFT - draft-chopps-isis-bfd-tlv
draft-chopps-isis-bfd-tlv
IS-IS for IP Internets C. Hopps
Internet-Draft L. Ginsberg
Intended status: Standards Track Cisco Systems
Expires: May 21, 2008 November 18, 2007
IS-IS BFD Enabled TLV
draft-chopps-isis-bfd-tlv-01
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes a TLV for use in the IS-IS routing protocol
that allows for the proper use of the Bidirectional Forwarding
Detection protocol (BFD). There exist certain scenarios in which
IS-IS will not react appropriately to a BFD detected forwarding plane
failure without use of either this TLV or some other method.
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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].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Solution . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Determining Local Significance . . . . . . . . . . . . . . 4
3.2. Adjacency Establishment and Maintenance . . . . . . . . . . 4
3.3. Advertisement of Topology Specific IS Neighbors . . . . . . 5
4. Transition . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Graceful Restart . . . . . . . . . . . . . . . . . . . . . . . 6
6. The BFD Enabled TLV . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
The Bidirectional Forwarding Detection protocol [I-D.ietf-bfd-base]
is a protocol that allows for detection of a forwarding plane failure
between two routers. A router can use [I-D.ietf-bfd-base] to
validate that a peer router's forwarding ability is functioning.
One specific application of BFD as described in
[I-D.ietf-bfd-generic] is to verify the forwarding ability of an
IS-IS [RFC1195] router's adjacencies; however, the method described
in [I-D.ietf-bfd-generic] does not allow for certain failure
scenarios. We will define a TLV that will allow for proper response
to the detection of all forwarding failures where the use of BFD is
employed with IS-IS.
2. The Problem
We observe that to allow for mixed use (i.e., some routers running
BFD and some not) [I-D.ietf-bfd-generic] does not require a BFD
session be established prior to the establishment of an IS-IS
adjacency. Thus, if a router A has neighbors B and C, and B does not
support BFD, A would still form adjacencies with B and C, and would
only establish a BFD session with C.
The problem with this solution is that it assumes that the
transmission and receipt of IS-IS IIHs shares fate with forwarded
data packets. This is not a fair assumption to make given that the
primary use of BFD is to protect IPv4 (and IPv6) forwarding and IS-IS
does not utilize IPv4 or IPv6 for sending or receiving its hellos.
Thus, if we consider our previous example, and if C is currently
experiencing an IPv4 forwarding failure that allows for IS-IS IIHs to
be sent and received, when A first starts (or restarts) A will assume
that C simply does not support BFD, will form an adjacency with C,
and may incorrectly forward IPv4 traffic through C.
3. The Solution
A simple solution to this problem is for an IS-IS router to advertise
that it has BFD enabled on a given interface. It can do this through
the inclusion of a TLV in its IIHs, and indeed that is our proposal.
When sending an IIH on a BFD enabled interface, a router which
supports this extension MUST include the BFD enabled TLV in its IIH.
The contents of the TLV MUST indicate what protocols have been
enabled for BFD by including the appropriate NLPID(s).
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When sending an IIH on an interface on which BFD is NOT enabled a
router MUST NOT include the BFD enabled TLV.
3.1. Determining Local Significance
When receiving an IIH from a neighbor on an interface with BFD
enabled, if the IIH contains the BFD enabled TLV the contents of the
BFD TLV are examined to determine if they are of local significance.
The logic used to determine local significance is impacted by the
combination of topologies and NLPIDs supported on each topology by
the local system.[I-D.ietf-isis-wg-multi-topology]. We introduce the
following definitions:
NLPID_LOCAL_BFD - The set of NLPIDs for which BFD has been locally
enabled on an interface.
NLPID_LOCAL_TOPO - The set of NLPIDs supported on a given topology.
NLPID_BFD_TLV - The set of NLPIDs advertised in the BFD TLV in a
received IIH.
NLPID_BFD_TOPO - The set of NLPIDs which are common to
(NLPID_LOCAL_BFD and NLPID_LOCAL_TOPO and NLPID_BFD_TLV).
IIH_BFD_LSIG - A boolean which is TRUE when there exists at least one
topology which is supported by both the local system and the neighbor
where NLPID_BFD_TOPO is not empty.
IS-IS_BFD_TOPO_UP - A per topology boolean whose value is TRUE when
IIH_BFD_LSIG is TRUE, the topology is supported by both the local
system and the neighbor, and either BFD session state for all NLPIDs
in the corresponding NLPID_BFD_TOPO set is UP or the NLPID_BFD_TOPO
set is empty for that topology.
IS-IS_BFD_UP - A boolean whose value is TRUE when IIH_BFD_LSIG is
TRUE and there is at least one topology supported by the local system
and the neighbor which has an IS-IS_BFD_TOPO_UP value which is TRUE.
If IIH_BFD_LSIG is FALSE then the contents of the corresponding
received BFD TLV are ignored. Note that this includes the case where
BFD is not locally enabled on an interface for any NLPID.
3.2. Adjacency Establishment and Maintenance
When IIH_BFD_LSIG is TRUE, the following extensions to the rules for
adjacency establishment and maintenance MUST apply:
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o IS-IS_BFD_UP state MUST be TRUE before the adjacency can
transition from INIT to UP state
o When the IS-IS adjacency is UP and IS-IS_BFD_UP becomes FALSE the
IS-IS adjacency MUST transition to DOWN.
o On a Point-to-Point circuit whenever IS-IS_BFD_UP is FALSE, the
Three-Way adjacency state MUST be set to DOWN in the Point-to-
Point Three Way Adjacency TLV[RFC3373] in all transmitted IIHs.
o On a LAN circuit whenever IS-IS_BFD_UP is FALSE, the IS Neighbors
TLV advertising the MAC address of the neighbor MUST be omitted in
all transmitted IIHs.
3.3. Advertisement of Topology Specific IS Neighbors
When IIH_BFD_LSIG is TRUE for a given neighbor, the advertisement of
a topology specific IS-neighbor (as well as the use of the neighbor
in the topology specific decision process) is determined by the value
of IS-IS_BFD_TOPO_UP for each topology. If IS-IS_BFD_TOPO_UP is TRUE
then the topology specific neighbor is advertised. If IS-
IS_BFD_TOPO_UP is FALSE then the topology specific neighbor is NOT
advertised.
4. Transition
To allow for a non-disruptive transition to the use of BFD some
amount of time should be allowed before bringing down an UP adjacency
on a BFD enabled interface when the value of IIH_BFD_LSIG becomes
TRUE as a result of the introduction of the BFD TLV or the
modification (by adding a new supported NLPID) of an existing BFD TLV
in a neighbor's IIH. A simple way to do this is to not update the
adjacency hold-time when receiving such an IIH from a neighbor with
whom we have an UP adjacency until IS-IS_BFD_UP becomes TRUE.
If the value of IIH_BFD_LSIG becomes FALSE as a result of the removal
the BFD TLV or the modification (by removing a supported NLPID) of an
existing BFD TLV in a neighbor's IIH then BFD session establishment
is no longer required to maintain the adjacency in or transition the
adjacency to the UP state.
If a BFD session is administratively shut down [I-D.ietf-bfd-base]
and the BFD session state change would impact the value of IS-
IS_BFD_UP, then IS-IS SHOULD allow time for the corresponding NLPID
to be removed from the neighbor's BFD TLV by not updating the
adjacency hold time until IIH_BFD_LSIG becomes FALSE. Note that
while this allows a non-disruptive transition, it still enforces
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consistency between the administrative state of the BFD session and
the NLPID(s) advertised in the BFD TLV. This is necessary to provide
consistent behavior regardless of whether the BFD AdminDown state is
introduced before or after an IS-IS adjacency UP state has been
achieved.
5. Graceful Restart
It is worth considering what if anything should be done when IS-IS is
gracefully restarting [RFC3847].
In cases where BFD shares fate with the control plane, it can be
expected that BFD session failure may occur in conjunction with the
control plane restart. In such cases premature abort of IS-IS
graceful restart as a result of BFD session failure is undesirable.
Therefore, some mechanism to ignore the BFD session failure for a
limited period of time would be beneficial. How this is implemented
is beyond the scope of this document. Consult [I-D.ietf-bfd-generic]
for further details.
6. The BFD Enabled TLV
The BFD enabled TLV is formatted as shown below. The TLV SHALL only
be included in an IS-IS IIH PDU and only when BFD is enabled for one
or more supported protocols on the interface over which the IIH is
being sent. The NLPIDs encoded in the TLV are defined in [ISO9577]
Type 139 (suggested - to be assigned by IANA)
Length # of octets in the value field (1 to 255)
Value one octet NLPID for each data protocol
for which BFD support is enabled
No. of octets
+-----------------------+
| NLPID | 1
+-----------------------+
: :
: :
+-----------------------+
| NLPID | 1
+-----------------------+
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7. Security Considerations
The TLV defined within this document describes an addition to the
IS-IS Hello protocol and does not impact the security mechanism of
the IS-IS protocol.
8. IANA Considerations
The following IS-IS TLV type is defined by this draft.
Name Value IIH LSP SNP
---------------------- ----- --- --- ---
BFD Enabled TLV 139 y n n
Please update the IS-IS TLV Codepoint Registry accordingly.
Note to RFC Editor: this section may be removed on publication as an
RFC.
9. Acknowledgements
The authors wish to thank Matthew Jones, Dave Katz, Jonathan Moon,
Stefano Previdi, Michael Shiplett and David Ward, for various input
on this document.
10. References
10.1. Normative References
[ISO9577] International Organization for Standardization, "Protocol
identification in the network layer(ISO/IEC 9577)", ISO/
IEC 9577:1999, Fourth Edition, Dec 1999.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3373] Katz, D. and R. Saluja, "Three-Way Handshake for
Intermediate System to Intermediate System (IS-IS) Point-
to-Point Adjacencies", RFC 3373, September 2002.
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10.2. Informative References
[I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-06 (work in progress),
March 2007.
[I-D.ietf-bfd-generic]
Katz, D. and D. Ward, "Generic Application of BFD",
draft-ietf-bfd-generic-03 (work in progress), March 2007.
[I-D.ietf-bfd-v4v6-1hop]
Katz, D. and D. Ward, "BFD for IPv4 and IPv6 (Single
Hop)", draft-ietf-bfd-v4v6-1hop-06 (work in progress),
March 2007.
[I-D.ietf-isis-wg-multi-topology]
Przygienda, T., "M-ISIS: Multi Topology (MT) Routing in
IS-IS", draft-ietf-isis-wg-multi-topology-12 (work in
progress), November 2007.
[RFC3847] Shand, M. and L. Ginsberg, "Restart Signaling for
Intermediate System to Intermediate System (IS-IS)",
RFC 3847, July 2004.
Authors' Addresses
Christian E. Hopps
Cisco Systems
170 W. Tasman Dr.
San Jose, California 95134
USA
Email: chopps@cisco.com
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035
USA
Email: ginsberg@cisco.com
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