INTERNET DRAFT Advertising Generic Information in IS-IS Feb 2007
Network Working Group L. Ginsberg
Internet Draft S. Previdi
Expiration Date: Aug 2007 M. Shand
Cisco Systems
Feb 2007
Advertising Generic Information in IS-IS
draft-ginsberg-isis-genapp-00.txt
Status of this Memo
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Abstract
This draft describes the manner in which generic application
information (i.e. information not directly related to the operation
of the IS-IS protocol) SHOULD be advertised in IS-IS LSPs and
defines guidelines which SHOULD be used when flooding such
information.
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Table of Contents
1. Conventions used in this document..........................2
2. Overview...................................................2
3. Encoding Format for GENINFO................................3
3.1 GENINFO TLV .............................................3
3.2 Use of subTLVs in GENINFO TLV............................5
4. GENINFO Flooding Procedures................................6
4.1 Leaking Procedures ......................................6
4.2 Minimizing Update Confusion..............................7
4.3 Interpreting Attribute Information ......................7
5. Security Considerations....................................8
6. IANA Considerations........................................8
7. References.................................................8
7.1 Normative References.....................................8
7.2 Informative References...................................9
8. Acknowledgments............................................9
9. Authors' Addresses.........................................9
10. Full Copyright Statement..................................9
1. 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 [BCP14].
2. Overview
[IS-IS] defines the format of TLVs which may be sent in IS-IS PDUs.
The first octet of a TLV encodes the "type" or "codepoint" which
provides a scope for the information and information format which
follows. The protocol is therefore limited to 256 different
codepoints which may be assigned. This number has proved generous as
regards the information required for correct operation of the IS-IS
protocol. However, the increasing use of IS-IS LSPs for
advertisement of generic information (GENINFO) not directly related
to the operation of the IS-IS protocol places additional demands on
the TLV encoding space which has the potential to consume a
significant number of TLV codepoints. This document therefore
defines an encoding format for GENINFO which minimizes the
consumption of TLV codepoints and also maximizes the flexibility of
the formats which can be used to represent GENINFO.
This document also discusses optimal behavior associated with the
advertisement and flooding of LSPs containing GENINFO in order to
avoid the advertisement of stale information and minimize the
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presence of duplicate or conflicting information when advertisements
are updated.
The manner in which the information contained in GENINFO TLVs is
exchanged between an instance of the IS-IS protocol and the
application which generates/consumes the GENINFO is outside the
scope of this specification.
The use of the IS-IS flooding mechanism as a means of reliably and
efficiently propagating information is understandably attractive.
However, it is prudent to remember that the primary purpose of this
mechanism is to flood information necessary for the correct
operation of the IS-IS protocol. Use of this mechanism to propagate
information not required for operation of the IS-IS protocol
decreases the efficiency of the protocol and therefore may
negatively impact routing. One of the most egregious oversights is a
failure to appropriately dampen changes in the information to be
advertised, which can lead to flooding storms and possibly
destabilize routing. Documents which specify the use of the
mechanisms defined here MUST define the expected rate of change of
the information to be advertised.
3. Encoding Format for GENINFO
The encoding format defined below has the following goals regarding
the advertisement of GENINFO in IS-IS LSPs:
o Minimize the number of codepoints required
o Minimize the depth of subTLV levels required
In order to support these goals, a new IANA registry is required.
This registry is required to manage the assignment of IS-IS GENINFO
Application Identifiers. These numbers are unsigned 16 bit numbers
ranging in value from 1 to 65535. The registry is also required to
manage the assignment of application specific subTLV codepoints.
These numbers are unsigned 8 bit numbers ranging in value from 0 to
255. The assignment of the subTLV codepoints is scoped by the
Application Identifier.
3.1 GENINFO TLV
The GENINFO TLV supports the advertisement of application specific
information which is not directly related to the operation of the
IS-IS protocol.
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Type 251
Length # of octets in the value field (3 to 255)
Value
No. of octets
+-----------------------+
| Flags | 1
+-----------------------+
| Application ID | 2
+-----------------------+
| Application |
| IP Address Info | 0 to 20
+-----------------------+
| Additional Application| 0 to (252 -
| Specific Information | len of IP Address info)
+-----------------------+
Flags
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Rsvd |V|I|D|S|
+-+-+-+-+-+-+-+-+
The following bit flags are defined.
S bit (0x01): If the S bit is set(1), the GENINFO TLV
MUST be flooded across the entire routing domain. If
the S bit is not set(0), the TLV MUST NOT be leaked
between levels. This bit MUST NOT be altered during the
TLV leaking.
D bit (0x02): When the GENINFO TLV is leaked from
level-2 to level-1, the D bit MUST be set. Otherwise
this bit MUST be clear. GENINFO TLVs with the D bit set
MUST NOT be leaked from level-1 to level-2. This is to
prevent TLV looping.
I bit (0x04): When the I bit is set the 4 octet IPv4
address associated with the application immediately
follows the Application ID.
V bit (0x08): When the V bit is set, the 16 octet IPv6
address associated with the application immediately
follows either the Application ID (if I bit is clear)
or the IPv4 address (if I bit is set).
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Application ID
An identifier assigned to this application via the
GENINFO-REG.
Application IPv4 Address Info
The IPv4 address associated with the application. This
is not necessarily an address of a router running the
IS-IS protocol.
Application IPv6 Address Info
The IPv6 address associated with the application. This
is not necessarily an address of a router running the
IS-IS protocol.
Additional Application Specific Information
Each application may define additional information to
be encoded in a GENINFO TLV following the fixed
information. Definition of such information is beyond
the scope of this document.
The Application ID in combination with the Application IPv4/IPv6
Address Information uniquely identifies the GENINFO Application
Context (GENINFO-CTX).
3.2 Use of subTLVs in GENINFO TLV
[RFC3784] introduced the definition and use of subTLVs. One of the
advantages of using subTLVs rather than fixed encoding of
information inside a TLV is to allow for the addition of new
information in a backwards compatible manner i.e. just as with TLVs,
implementations are required to ignore subTLVs which they do not
understand.
GENINFO TLVs MAY include subTLVs in the application specific
information as deemed necessary and appropriate for each
application. The scope of the codepoints used in such subTLVs is
defined by the GENINFO TLV codepoint AND the Application ID i.e. the
subTLV codepoints are private to the application. Such subTLVs are
referred to as APPSUBTLVs and MUST be assigned via the GENINFO-REG
IANA registry.
Additional levels of APPSUBTLVs may be required when there is
variable information which is scoped by a specific APPSUBTLV. These
"nested" subTLVs MUST be encoded in the same manner as subTLVs i.e.
with a one-octet Type field, a one-octet Length field, and zero or
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more octets of Value. These types MUST also be assigned via the
GENINFO-REG IANA registry.
The use of additional levels of subTLVs is discouraged due to the
inherent inefficiency in encoding introduced because the parent
subTLV must encode the nested subTLV length. While this inefficiency
is small (one additional octet), it may be sufficient to extend the
total information about a single application object beyond the
carrying capacity of a single GENINFO TLV. Given that each
Application ID can utilize the full range of subTLV codepoints (0 to
255) without conflict with any other application, the need to be
frugal in the use of APPSUBTLV codepoints is greatly reduced.
4. GENINFO Flooding Procedures
This section describes procedures which apply to the propagation of
LSPs which contain GENINFO TLVs. These procedures have been
previously discussed in [IS-IS-CAP]. This section is intended to
serve as a reference specification for future documents which define
the use of GENINFO TLV(s) for a specific application - eliminating
the need to repeat the definition of these procedures in the
application specific documents.
Each GENINFO TLV contains information regarding exactly one
application instance as identified by the GENINFO-CTX. When it is
necessary to advertise sets of information with the same GENINFO-CTX
which have different flooding scopes, a router MUST originate a
minimum of one GENINFO TLV for each required flooding scope. GENINFO
TLVs which contain information having area/level scope will have the
S bit clear. These TLVs MUST NOT be leaked into another level.
GENINFO TLVs which contain information which has domain scope will
have the S bit set. These TLVs MUST be leaked into other IS-IS
levels. When a TLV is leaked from level-2 to level-1, the D bit MUST
be set in the level-1 LSP advertisement.
4.1 Leaking Procedures
When leaking GENINFO TLVs downward from Level-2 into Level-1, if the
originator of the TLV is a Level-1 router in another area, it is
possible that multiple copies of the same TLV may be received from
multiple L2 routers in the originating area. A router performing
downward leaking MUST check for such duplication by comparing the
contents of the TLVs. The set of LSPs generated by a router for a
given level MUST NOT contain two or more copies of the same GENTLV.
In order to prevent the use of stale GENINFO information, a system
MUST NOT use a GENINFO TLV present in an LSP of a system which is
not currently reachable via Level-x paths, where "x" is the level (1
or 2) associated with the LSP in which the GENINFO TLV appears. Note
that leaking a GENINFO TLV is one of the uses which is prohibited
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under these conditions. The following example illustrates what might
occur in the absence of this restriction.
Example: If Level-1 router A generates a GENINFO TLV and floods it
to two L1/L2 routers S and T, they will flood it into the Level-2
sub-domain. Now suppose the Level-1 area partitions, such that A and
S are in one partition and T is in another. IP routing will still
continue to work, but if A now issues a revised version of the
GENINFO TLV, or decides to stop advertising it, S will follow suit,
but T will continue to advertise the old version until the LSP times
out.
Routers in other areas have to choose whether to trust T's copy of
A's GENINFO TLV or S's copy of A's information and they have no
reliable way to choose. By making sure that T stops leaking A's
information, this removes the possibility that other routers will
use stale information from A.
4.2 Minimizing Update Confusion
If an update to a TLV is advertised in an LSP with a different
number than the LSP associated with the old advertisement, the
possibility exists that other systems can temporarily have either 0
copies of a particular advertisement or 2 copies of a particular
advertisement, depending on the order in which new copies of the LSP
which had the old advertisement and the LSP which has the new
advertisement arrive at other systems.
Whenever possible, an implementation SHOULD advertise the update to
a GENINFO TLV in the LSP with the same number as the advertisement
which it replaces. Where this is not possible, the two affected LSPs
SHOULD be flooded as an atomic action.
Systems which receive an update to an existing GENINFO TLV can
minimize the potential disruption associated with the update by
employing a holddown time prior to processing the update so as to
allow for the receipt of multiple LSPs associated with the same
update prior to beginning processing.
4.3 Interpreting Attribute Information
Where a receiving system has two copies of a GENINFO TLV with the
same GENINFO-CTX, attribute information in the two TLVs which does
not conflict MUST be considered additive. When information in the
two GENINFO TLVs conflicts i.e there are different settings for a
given attribute, the procedure used to choose which copy shall be
used is undefined.
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5. Security Considerations
This document raises no new security issues for IS-IS.
6. IANA Considerations
This document defines a new ISIS TLV that needs to be reflected in
the ISIS TLV code-point registry:
Type Description IIH LSP SNP
---- ----------------------------------- --- --- ---
251 Generic Information n y n
This document also defines a new registry which needs to be created.
The new registry is required to manage two types of assigned
numbers:
1)Application Identifiers which may be used in the Generic
Information TLV. These identifiers are unsigned 16 bit numbers
ranging in value from 1 to 65535.
2)Application specific subTLV codepoints which may be used in a
GENINFO TLV when a specific Application Identifier is used. These
numbers are unsigned 8 bit numbers ranging in value from 0 to 255.
7. References
7.1 Normative References
[IS-IS] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002, Second Edition.
[RFC3784] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)", RFC
3784, June 2004.
[BCP9] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[BCP26] Narten, T. and Alvestrand, H., "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26 , RFC 2434, October
1998
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[BCP79] Bradner, S. Ed., "Intellectual Property Rights in IETF
Technology ", BCP 79 , RFC 3979, March 2005
7.2 Informative References
[IS-IS-CAP] Vasseur, JP., Shen N., and Aggarwal, R., "IS-IS
extensions for advertising router information", draft-ietf-isis-
caps-06.txt, January 2006
8. Acknowledgments
The authors would like to thank JP Vasseur and David Ward for
providing the need to produce this document.
9. Authors' Addresses
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
Email: ginsberg@cisco.com
Stefano Previdi
CISCO Systems, Inc.
Via Del Serafico 200
00142 - Roma
ITALY
Email: sprevidi@cisco.com
Mike Shand
Cisco Systems
250 Longwater Avenue,
Reading,
Berkshire,
RG2 6GB
UK
Email: mshand@cisco.com
10. Full Copyright Statement
Copyright (C) The IETF Trust (2007).
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retain all their rights.
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