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RFC 7356

Proposed STD
Pages: 23
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IS-IS Flooding Scope Link State PDUs (LSPs)

 


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Internet Engineering Task Force (IETF)                       L. Ginsberg
Request for Comments: 7356                                    S. Previdi
Category: Standards Track                                        Y. Yang
ISSN: 2070-1721                                            Cisco Systems
                                                          September 2014


              IS-IS Flooding Scope Link State PDUs (LSPs)

Abstract

   Intermediate System to Intermediate System (IS-IS) provides efficient
   and reliable flooding of information to its peers; however, the
   current flooding scopes are limited to either area scope or domain
   scope.  There are existing use cases where support of other flooding
   scopes is desirable.  This document defines new Protocol Data Units
   (PDUs) that provide support for new flooding scopes as well as
   additional space for advertising information targeted for the
   currently supported flooding scopes.  This document also defines
   extended Type-Length-Values (TLVs) and sub-TLVs that are encoded
   using 16-bit fields for Type and Length.

   The protocol extensions defined in this document are not backwards
   compatible with existing implementations and so must be deployed with
   care.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7356.

Page 2 
Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Extended TLVs . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Use of Extended TLVs and Extended Sub-TLVs  . . . . . . .   5
     2.2.  Use of Standard Code Points in Extended TLVs and Extended
           Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.   Definition of New PDUs . . . . . . . . . . . . . . . . . . .   6
     3.1.  Flooding Scoped LSP Format  . . . . . . . . . . . . . . .   7
     3.2.  Flooding Scoped CSNP Format . . . . . . . . . . . . . . .  10
     3.3.  Flooding Scope PSNP Format  . . . . . . . . . . . . . . .  12
   4.  Flooding Scope Update Process Operation . . . . . . . . . . .  13
     4.1.  Scope Types . . . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Operation on Point-to-Point Circuits  . . . . . . . . . .  14
     4.3.  Operation on Broadcast Circuits . . . . . . . . . . . . .  14
     4.4.  Use of Authentication . . . . . . . . . . . . . . . . . .  15
     4.5.  Priority Flooding . . . . . . . . . . . . . . . . . . . .  15
   5.  Deployment Considerations . . . . . . . . . . . . . . . . . .  15
   6.  Graceful Restart Interactions . . . . . . . . . . . . . . . .  16
   7.  Multi-instance Interactions . . . . . . . . . . . . . . . . .  16
   8.  Circuit Scope Flooding  . . . . . . . . . . . . . . . . . . .  16
   9.  Extending LSP Set Capacity  . . . . . . . . . . . . . . . . .  17
   10. Domain Scope Flooding . . . . . . . . . . . . . . . . . . . .  18
   11. Announcing Support for Flooding Scopes  . . . . . . . . . . .  19
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     15.2.  Informative References . . . . . . . . . . . . . . . . .  22

1.  Introduction

   The Update Process, as defined by [IS-IS], provides reliable and
   efficient flooding of information to all routers in a given flooding
   scope.  Currently, the protocol supports two flooding scopes and
   associated PDUs.  Level 1 (L1) Link State PDUs (LSPs) are flooded to
   all routers in an area.  Level 2 (L2) LSPs are flooded to all routers
   in the Level 2 subdomain.  The basic operation of the Update Process
   can be applied to any subset of the routers in a given topology so
   long as that topology is not partitioned.  It is, therefore, possible
   to introduce new PDUs in support of other flooding scopes and utilize
   the same Update Process machinery to provide the same reliability and
   efficiency that the Update Process currently provides for L1 and L2
   scopes.  This document defines these new PDUs and the modified Update
   Process rules that are to be used in supporting new flooding scopes.

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   New deployment cases have introduced the need for reliable and
   efficient circuit scope flooding.  For example, Appointed Forwarder
   information, as defined in [RFC7176], needs to be flooded reliably
   and efficiently to all Routing Bridges (RBridges) on a broadcast
   circuit.  Currently, only IS-IS Hellos (IIHs) have the matching scope
   -- but IIHs are unreliable, i.e., individual IIHs may be lost without
   affecting correct operation of the protocol.  To provide reliability
   in cases where the set of information to be flooded exceeds the
   carrying capacity of a single PDU requires sending the information
   periodically even when no changes in the content have occurred.  When
   the information content is large, this is inefficient and still does
   not provide a guarantee of reliability.  This document defines
   circuit scope flooding in order to provide a solution for such cases.

   Another existing limitation of [IS-IS] is the carrying capacity of an
   LSP set.  It has been noted in [RFC5311] that the set of LSPs that
   may be originated by a system at each level is limited to 256 LSPs,
   and the maximum size of each LSP is limited by the minimum Maximum
   Transmission Unit (MTU) of any link used to flood LSPs.  [RFC5311]
   has defined a backwards-compatible protocol extension that can be
   used to overcome this limitation if needed.  While the [RFC5311]
   solution is viable, in order to be interoperable with routers that do
   not support the extension, it imposes some restrictions on what can/
   cannot be advertised in the Extended LSPs and requires allocation of
   multiple unique system IDs to a given router.  A more flexible and
   less constraining solution is possible if interoperability with
   legacy routers is not a requirement.  By definition, the introduction
   of new PDUs required to support new flooding scopes is not
   interoperable with legacy routers.  It is, therefore, possible to
   simultaneously introduce an alternative solution to the limited LSP
   set carrying capacity of Level 1 and Level 2 LSPs as part of the
   extensions defined in this document.  This capability is also defined
   in this document.

   Standard IS-IS TLVs are encoded using an 8-bit type and an 8-bit
   length.  In cases where the set of information about a single object
   exceeds 255 octets, multiple TLVs are required to encode all of the
   relevant information.  This document introduces extended TLVs and
   extended sub-TLVs that use a 16-bit Type field and a 16-bit Length
   field.

   The PDU Type field in the common header for all IS-IS PDUs is a 5-bit
   field.  Therefore, possible PDU types supported by the protocol are
   limited to a maximum of 32.  In order to minimize the need to
   introduce additional PDU types in the future, the new PDUs introduced
   in this document are defined so as to allow multiple flooding scopes
   to be associated with the same PDU type.  This means if new flooding
   scopes are required in the future, the same PDU type can be used.

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1.1.  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 [RFC2119].

2.  Extended TLVs

   Standard TLVs as defined in [IS-IS] as well as standard sub-TLVs
   (first introduced in [RFC5305]) have an 8-bit Type field and an
   eight-bit Length field.  This constrains the information included in
   a single TLV or sub-TLV to 255 octets.  With the increasing use of
   sub-TLVs, it becomes more likely that the amount of information about
   a single object that needs to be advertised may exceed 255 octets.
   In such cases, the information is encoded in multiple TLVs.  This
   leads to less efficient encoding since the information that uniquely
   identifies the object must be repeated in each TLV and requires
   additional implementation complexity when receiving the information
   to ensure that all information about the object is correctly
   collected from the multiple TLVs.

   This document introduces extended TLVs and extended sub-TLVs.  These
   are encoded using a 16-bit Type field and a 16-bit Length field.

2.1.  Use of Extended TLVs and Extended Sub-TLVs

   The following restrictions apply to the use of extended TLVs and
   extended sub-TLVs:

   o  Extended TLVs and extended sub-TLVs are permitted only in Flooding
      Scope PDUs that have a flooding scope designated for their use
      (defined later in this document)

   o  A given flooding scope supports either the use of standard TLVs
      and standard sub-TLVs or the use of extended TLVs and extended
      sub-TLVs, but not both

   o  Extended TLVs and extended sub-TLVs MUST be used together, i.e.,
      using Standard sub-TLVs within an Extended TLV or using Extended
      sub-TLVs within a Standard TLV is invalid

   o  If additional levels of TLVs (e.g., sub-sub-TLVs) are introduced
      in the future, then the size of the Type and Length fields in
      these new sub-types MUST match the size used in the parent

   o  The 16-bit Type and Length fields are encoded in network byte
      order

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   o  Use of extended TLVs and extended sub-TLVs does not alter in any
      way the maximum size of PDUs that may sent or received

2.2.  Use of Standard Code Points in Extended TLVs and Extended Sub-TLVs

   Standard TLV and standard sub-TLV code points as defined in the IANA
   "IS-IS TLV Codepoints" registry MAY be used in extended TLVs and
   extended sub-TLVs.  Encoding is as specified for each of the standard
   TLVs and standard sub-TLVs with the following differences:

   o  The 8-bit Type field is encoded as an unsigned 16-bit integer
      where the 8 most significant bits (MSBs) are all 0

   o  The 8-bit Length field is replaced by the 16-bit Length field

   o  The length MAY take on values greater than 255

3.  Definition of New PDUs

   In support of new flooding scopes, the following new PDUs are
   required:

   o  Flooding Scope LSPs (FS-LSPs)

   o  Flooding Scope Complete Sequence Number PDUs (FS-CSNPs)

   o  Flooding Scope Partial Sequence Number PDUs (FS-PSNPs)

   Each of these PDUs is intentionally defined with a header as similar
   in format as possible to the corresponding PDU types currently
   defined in [IS-IS].  Although it might have been possible to
   eliminate or redefine PDU header fields in a new way, the existing
   formats are retained in order to allow maximum reuse of existing PDU
   processing logic in an implementation.

   Note that in the case of all FS PDUs, the Maximum Area Addresses
   field in the header of the corresponding standard PDU has been
   replaced with a Scope field.  Therefore, maximum area addresses
   checks specified in [IS-IS] are not performed on FS PDUs.

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3.1.  Flooding Scoped LSP Format

   An FS-LSP has the following format:

                                            No. of octets
                 +-------------------------+
                 | Intradomain Routeing    |     1
                 | Protocol Discriminator  |
                 +-------------------------+
                 | Length Indicator        |     1
                 +-------------------------+
                 | Version/Protocol ID     |     1
                 | Extension               |
                 +-------------------------+
                 | ID Length               |     1
                 +-------------------------+
                 |R|R|R| PDU Type          |     1
                 +-------------------------+
                 |  Version                |     1
                 +-------------------------+
                 |  Reserved               |     1
                 +-------------------------+
                 |P|  Scope                |     1
                 +-------------------------+
                 |  PDU Length             |     2
                 +-------------------------+
                 |  Remaining Lifetime     |     2
                 +-------------------------+
                 |   FS LSP ID             |     ID Length + 2
                 +-------------------------+
                 | Sequence Number         |     4
                 +-------------------------+
                 | Checksum                |     2
                 +-------------------------+
                 |Reserved|LSPDBOL|IS Type |     1
                 +-------------------------+
                 : Variable-Length Fields  :     Variable
                 +-------------------------+


      Intradomain Routeing Protocol Discriminator: 0x83 (as defined in
      [IS-IS]).

      Length Indicator: Length of the fixed header in octets.

      Version/Protocol ID Extension: 1

      ID Length: As defined in [IS-IS].

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      PDU Type: 10 - Format as defined in [IS-IS].

      Version: 1

      Reserved: Transmitted as zero, ignored on receipt.

      Scope: Bits 1-7 define the flooding scope.

         The value 0 is reserved and MUST NOT be used.  Received FS-LSPs
         with a scope of 0 MUST be ignored and MUST NOT be flooded.

         P: Bit 8 - Priority Bit. If set to 1, this LSP SHOULD be
         flooded at high priority.

         Scopes (1 - 63) are reserved for use with standard TLVs and
         standard sub-TLVs.

         Scopes (64 - 127) are reserved for use with extended TLVs and
         extended sub-TLVs.

      PDU Length: Entire length of this PDU, in octets, including the
      header.

      Remaining Lifetime: Number of seconds before this FS-LSP is
      considered expired.

      FS LSP ID: The system ID of the source of the FS-LSP.  One of the
      following two formats is used:

        FS LSP ID Standard Format

                 +-------------------------+
                 |   Source ID             |     ID Length
                 +-------------------------+
                 | Pseudonode ID           |     1
                 +-------------------------+
                 | FS LSP Number           |     1
                 +-------------------------+


        FS LSP ID Extended Format

                 +-------------------------+
                 |   Source ID             |     ID Length
                 +-------------------------+
                 | Extended FS LSP Number  |     2
                 +-------------------------+

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      Which format is used is specific to the scope and MUST be defined
      when the specific flooding scope is defined.

      Sequence Number: Sequence number of this FS-LSP.

      Checksum: Checksum of contents of FS-LSP from the Source ID to the
      end.  Checksum is computed as defined in [IS-IS].

      Reserved/LSPDBOL/IS Type

         Bits 4-8 are reserved, which means they are transmitted as 0
         and ignored on receipt.

         LSPDBOL: Bit 3 - A value of 0 indicates no FS-LSP Database
         Overload and a value of 1 indicates that the FS-LSP Database is
         overloaded.  The overload condition is specific to FS-LSPs with
         the scope specified in the Scope field.

         IS Type: Bits 1 and 2.  The type of Intermediate System as
         defined in [IS-IS].

      Variable-length fields that are allowed in an FS-LSP are specific
      to the defined scope.

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3.2.  Flooding Scoped CSNP Format

   An FS-CSNP has the following format:

                                            No. of octets
                 +-------------------------+
                 | Intradomain Routeing    |     1
                 | Protocol Discriminator  |
                 +-------------------------+
                 | Length Indicator        |     1
                 +-------------------------+
                 | Version/Protocol ID     |     1
                 | Extension               |
                 +-------------------------+
                 | ID Length               |     1
                 +-------------------------+
                 |R|R|R| PDU Type          |     1
                 +-------------------------+
                 |  Version                |     1
                 +-------------------------+
                 |  Reserved               |     1
                 +-------------------------+
                 |R|  Scope                |     1
                 +-------------------------+
                 |  PDU Length             |     2
                 +-------------------------+
                 |  Source ID              |     ID Length + 1
                 +-------------------------+
                 |  Start FS-LSP ID        |     ID Length + 2
                 +-------------------------+
                 |  End FS-LSP ID          |     ID Length + 2
                 +-------------------------+
                 : Variable-Length Fields  :     Variable
                 +-------------------------+


      Intradomain Routeing Protocol Discriminator: 0x83 (as defined in
      [IS-IS]).

      Length Indicator: Length of the fixed header in octets.

      Version/Protocol ID Extension: 1

      ID Length: As defined in [IS-IS].

      PDU Type: 11 - Format as defined in [IS-IS].

      Version: 1

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      Reserved: Transmitted as zero, ignored on receipt.

      Scope: Bits 1-7 define the flooding scope.

         The value 0 is reserved and MUST NOT be used.  Received FS-
         CSNPs with a scope of 0 MUST be ignored.

         Bit 8 is Reserved, which means it is transmitted as 0 and
         ignored on receipt.

         Scopes (1 - 63) are reserved for use with standard TLVs and
         standard sub-TLVs.

         Scopes (64 - 127) are reserved for use with extended TLV and
         extended sub-TLVs.

      PDU Length: Entire length of this PDU, in octets, including the
      header.

      Source ID: The system ID of the Intermediate System (with zero
      Circuit ID) generating this Sequence Number's PDU.

      Start FS-LSP ID: The FS-LSP ID of the first FS-LSP with the
      specified scope in the range covered by this FS-CSNP.

      End FS-LSP ID: The FS-LSP ID of the last FS-LSP with the specified
      scope in the range covered by this FS-CSNP.

      Variable-length fields that are allowed in an FS-CSNP are limited
      to those TLVs that are supported by standard CSNP.

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3.3.  Flooding Scope PSNP Format

   An FS-PSNP has the following format:

                                            No. of octets
                 +-------------------------+
                 | Intradomain Routeing    |     1
                 | Protocol Discriminator  |
                 +-------------------------+
                 | Length Indicator        |     1
                 +-------------------------+
                 | Version/Protocol ID     |     1
                 | Extension               |
                 +-------------------------+
                 | ID Length               |     1
                 +-------------------------+
                 |R|R|R| PDU Type          |     1
                 +-------------------------+
                 |  Version                |     1
                 +-------------------------+
                 |  Reserved               |     1
                 +-------------------------+
                 |U|  Scope                |     1
                 +-------------------------+
                 |  PDU Length             |     2
                 +-------------------------+
                 |  Source ID              |     ID Length + 1
                 +-------------------------+
                 : Variable-Length Fields  :     Variable
                 +-------------------------+

      Intradomain Routeing Protocol Discriminator: 0x83 (as defined in
      [IS-IS]).

      Length Indicator: Length of the fixed header in octets.

      Version/Protocol ID Extension: 1

      ID Length: As defined in [IS-IS].

      PDU Type: 12 - Format as defined in [IS-IS].

      Version: 1

      Reserved: Transmitted as zero, ignored on receipt.

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      Scope: Bits 1-7 define the flooding scope.

         The value 0 is reserved and MUST NOT be used.  Received FS-
         PSNPs with a scope of 0 MUST be ignored.

         U: Bit 8 - A value of 0 indicates that the specified flooding
         scope is supported.  A value of 1 indicates that the specified
         flooding scope is unsupported.  When U = 1, variable-length
         fields other than authentication MUST NOT be included in the
         PDU.

         Scopes (1 - 63) are reserved for use with standard TLVs and
         standard sub-TLVs.

         Scopes (64 - 127) are reserved for use with extended TLVs and
         extended sub-TLVs.

      PDU Length: Entire length of this PDU, in octets, including the
      header.

      Source ID: The system ID of the Intermediate System (with zero
      Circuit ID) generating this Sequence Number's PDU.

      Variable-length fields that are allowed in an FS-PSNP are limited
      to those TLVs that are supported by standard PSNPs.

4.  Flooding Scope Update Process Operation

   The Update Process, as defined in [IS-IS], maintains a Link State
   Database (LSDB) for each level supported.  Each level-specific LSDB
   contains the full set of LSPs generated by all routers operating in
   that level-specific scope.  The introduction of FS-LSPs creates
   additional LSDBs (FS-LSDBs) for each additional scope supported.  The
   set of FS-LSPs in each FS-LSDB consists of all FS-LSPs generated by
   all routers operating in that scope.  Therefore, there is an
   additional instance of the Update Process for each supported flooding
   scope.

   Operation of the scope-specific Update Process follows the Update
   Process specification in [IS-IS].  The circuit(s) on which FS-LSPs
   are flooded is limited to those circuits that are participating in
   the given scope.  Similarly, the sending/receiving of FS-CSNPs and
   FS-PSNPs is limited to the circuits participating in the given scope.

   Consistent support of a given flooding scope on a circuit by all
   routers operating on that circuit is required.

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4.1.  Scope Types

   A flooding scope may be limited to a single circuit (circuit scope).
   Circuit scopes may be further limited by level (L1 Circuit Scope / L2
   Circuit Scope).

   A flooding scope may be limited to all circuits enabled for L1
   routing (area scope).

   A flooding scope may be limited to all circuits enabled for L2
   routing (L2 subdomain scope).

   Additional scopes may be defined that include all circuits enabled
   for either L1 or L2 routing (domain scope).

4.2.  Operation on Point-to-Point Circuits

   When a new adjacency is formed, synchronization of all FS-LSDBs
   supported on that circuit is required; therefore, FS-CSNPs for all
   supported scopes MUST be sent when a new adjacency reaches the UP
   state.  The Send Receive Message (SRM) bit MUST be set for all
   FS-LSPs associated with the scopes supported on that circuit.
   Receipt of an FS-PSNP with the U bit equal to 1 indicates that the
   neighbor does not support that scope (although it does support FS
   PDUs).  This MUST cause the SRM bit to be cleared for all FS-LSPs
   with the matching scope, which are currently marked for flooding on
   that circuit.

4.3.  Operation on Broadcast Circuits

   FS PDUs are sent to the same destination address(es) as standard PDUs
   for the given protocol instance.  For specification of the defined
   destination addresses, consult [IS-IS], [IEEEaq], [RFC6822], and
   [RFC6325].

   The Designated Intermediate System (DIS) for a broadcast circuit has
   the responsibility to generate periodic scope-specific FS-CSNPs for
   all supported scopes.  A scope-specific DIS is NOT elected as all
   routers on a circuit MUST support a consistent set of flooding
   scopes.

   It is possible that a scope may be defined that is not level
   specific.  In such a case, the DIS for each level enabled on a
   broadcast circuit MUST independently send FS PDUs for that scope to
   the appropriate level-specific destination address.  This may result
   in redundant flooding of FS-LSPs for that scope.

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4.4.  Use of Authentication

   Authentication TLVs MAY be included in FS PDUs.  When authentication
   is in use, the scope is first used to select the authentication
   configuration that is applicable.  The authentication check is then
   performed as normal.  Although scope-specific authentication MAY be
   used, sharing of authentication among multiple scopes and/or with the
   standard LSPs/CSNPs/PSNPs is considered sufficient.

4.5.  Priority Flooding

   When the FS LSP ID Extended format is used, the set of LSPs generated
   by an IS may be quite large.  It may be useful to identify those LSPs
   in the set that contain information of higher priority.  Such LSPs
   will have the P bit set to 1 in the Scope field in the LSP header.
   Such LSPs SHOULD be flooded at a higher priority than LSPs with the P
   bit set to 0.  This is a suggested behavior on the part of the
   originator of the LSP.  When an LSP is purged, the original state of
   the P bit MUST be preserved.

5.  Deployment Considerations

   Introduction of new PDU types is incompatible with legacy
   implementations.  Legacy implementations do not support the
   FS-specific Update process(es) and, therefore, flooding of the
   FS-LSPs throughout the defined scope is unreliable when not all
   routers in the defined scope support FS PDUs.  Further, legacy
   implementations will likely treat the reception of an FS PDU as an
   error.  Even when all routers in a given scope support FS PDUs, if
   not all routers in the flooding domain for a given scope support that
   scope, then flooding of the FS-LSPs may be compromised.  When
   deploying a new flooding scope, correct operation therefore requires
   that both FS PDUs and the new scope be supported by all routers in
   the flooding domain of the new scope.

   The U bit in FS-PSNPs provides a means to suppress retransmissions of
   unsupported scopes.  Routers that support FS PDUs SHOULD support the
   sending of PSNPs with the U bit equal to 1 when an FS-LSP is received
   with a scope that is unsupported.  Routers that support FS PDUs
   SHOULD trigger management notifications when FS PDUs are received for
   unsupported scopes and when PSNPs with the U bit equal to 1 are
   received.

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6.  Graceful Restart Interactions

   [RFC5306] defines protocol extensions in support of graceful restart
   of a routing instance.  Synchronization of all supported FS-LSDBs is
   required in order for database synchronization to be complete.  This
   involves the use of additional T2 timers.  Receipt of a PSNP with the
   U bit equal to 1 will cause FS-LSDB synchronization with that
   neighbor to be considered complete for that scope.  See [RFC5306] for
   further details.

7.  Multi-instance Interactions

   In cases where FS-PDUs are associated with a non-zero instance, the
   use of Instance Identifier TLVs (IID-TLVs) in FS-PDUs follows the
   rules for use in LSPs, CSNPs, and PSNPs as defined in [RFC6822].

8.  Circuit Scope Flooding

   This document defines four circuit scope flooding identifiers:

   o  Level 1 Circuit Scope (L1CS) -- this uses standard TLVs and
      standard sub-TLVs

   o  Level 2 Circuit Scope (L2CS) -- this uses standard TLVs and
      standard sub-TLVs

   o  Extended Level 1 Circuit Scope (E-L1CS) -- this uses extended TLVs
      and extended sub-TLVs

   o  Extended Level 2 Circuit Scope (E-L2CS) -- this uses extended TLVs
      and extended sub-TLVs

   FS-LSPs with the Scope field set to one of these values contain
   information specific to the circuit on which they are flooded.  When
   received, such FS-LSPs MUST NOT be flooded on any other circuit.  The
   FS LSP ID Extended format is used in these PDUs.  The FS-LSDB
   associated with circuit scope FS-LSPs consists of the set of FS-LSPs
   that both have matching circuit scopes and are transmitted (locally
   generated) or received on a specific circuit.

   The set of TLVs that may be included in such FS-LSPs is specific to
   the given use case and is outside the scope of this document.

Top      ToC       Page 17 
9.  Extending LSP Set Capacity

   The need for additional space in the set of LSPs generated by a
   single IS has been articulated in [RFC5311].  When legacy
   interoperability is not a requirement, the use of FS-LSPs meets that
   need without requiring the assignment of alias system-ids to a single
   IS.  Four flooding scopes are defined for this purpose:

   o  Level 1 Flooding Scope (L1FS) -- this uses standard TLVs and
      standard sub-TLVs

   o  Level 2 Flooding Scope (L2FS) -- this uses standard TLVs and
      standard sub-TLVs

   o  Extended Level 1 Flooding Scope (E-L1FS) -- this uses extended
      TLVs and extended sub-TLVs

   o  Extended Level 2 Flooding Scope (E-L2FS) -- this uses extended
      TLVs and extended sub-TLVs

   L1FS and E-L1FS LSPs are flooded on all L1 circuits.  L2FS and E-L2FS
   LSPs are flooded on all L2 circuits.

   The FS LSP ID Extended format is used in these PDUs.  This provides
   64 K of additional LSPs that may be generated by a single system at
   each level.

   LxFS and E-LxFS LSPs are used by the level-specific Decision Process
   (defined in [IS-IS]) in the same manner as standard LSPs (i.e., as
   additional information sourced by the same IS) subject to the
   following restrictions:

   o  A valid version of standard LSP #0 from the same IS at the
      corresponding level MUST be present in the LSDB in order for the
      LxFS/E-LxFS set to be usable.

   o  Information in an LxFS of E-LxFS LSP (e.g., IS-Neighbor
      information) that supports using the originating IS as a transit
      node MUST NOT be used when the Overload bit is set in the
      corresponding standard LSP #0.

   o  TLVs that are restricted to standard LSP #0 MUST NOT appear in
      LxFS LSPs.

   There are no further restrictions as to what TLVs may be advertised
   in FS-LSPs.

Top      ToC       Page 18 
10.  Domain Scope Flooding

   Existing support for flooding information throughout a domain (i.e.,
   to L1 routers in all areas as well as to routers in the Level 2
   subdomain) requires the use of leaking procedures between levels.
   For further details, see [RFC4971].  This is sufficient when the data
   being flooded throughout the domain consists of individual TLVs.  If
   it is desired to retain the identity of the originating IS for the
   complete contents of a PDU, then support for flooding the unchanged
   PDU is desirable.  This document, therefore, defines two flooding
   scopes in support of domain flooding.  FS-LSPs with this scope MUST
   be flooded on all circuits regardless of what level(s) is supported
   on that circuit.

   o  Domain Flooding Scope (DFS) -- this uses standard TLVs and
      standard sub-TLVs

   o  Extended Domain Flooding Scope (E-DFS) -- this uses extended TLVs
      and extended sub-TLVs

   The FS LSP ID Extended format is used in these PDUs.

   Use of information in FS-LSPs for a given scope depends on
   determining the reachability to the IS originating the FS-LSP.  This
   presents challenges for FS-LSPs with domain scopes because no single
   IS has the full view of the topology across all areas.  It is,
   therefore, necessary for the originator of domain scope DSFS and
   E-DSFS LSPs to advertise an identifier that will allow an IS who
   receives such an FS-LSP to determine whether the source of the FS-LSP
   is currently reachable.  The identifier required depends on what
   "address-families" are being advertised.

   When IS-IS is deployed in support of Layer 3 routing for IPv4 and/or
   IPv6, then FS-LSP #0 with domain scope MUST include at least one of
   the following TLVs:

   o  IPv4 Traffic Engineering Router ID (TLV 134)

   o  IPv6 Traffic Engineering Router ID (TLV 140)

   When IS-IS is deployed in support of Layer 2 routing, current
   standards (e.g., [RFC6325]) only support a single area.  Therefore,
   domain scope is not yet applicable.  When the Layer 2 standards are
   updated to include multi-area support, the identifiers that can be
   used to support inter-area reachability will be defined -- at which
   point the use of domain scope for Layer 2 can be fully defined.

Top      ToC       Page 19 
11.  Announcing Support for Flooding Scopes

   Announcements of support for flooding scope may be useful in
   validating that full support has been deployed and/or in isolating
   the reasons for incomplete flooding of FS-LSPs for a given scope.

   ISs supporting FS-PDUs MAY announce supported scopes in IIH PDUs.  To
   do so, a new TLV is defined.

   Scope Flooding Support
   Type:   243
   Length: 1 - 127
   Value
                                    No. of octets
          +----------------------+
          |R| Supported Scope    |   1
          +----------------------+
          :                      :
          +----------------------+
          |R| Supported Scope    |   1
          +----------------------+

       A list of the circuit scopes supported on this circuit and other
       non-circuit-flooding scopes supported.

       R bit MUST be 0 and is ignored on receipt.

       In a Point-to-Point IIH, L1, L2, domain, and all circuit scopes
       MAY be advertised.

       In Level 1 LAN IIHs, L1, domain, and L1 Circuit Scopes MAY be
       advertised.  L2 Scopes and L2 Circuit Scopes MUST NOT be
       advertised.

       In Level 2 LAN IIHs, L2, domain, and L2 Circuit Scopes MAY be
       advertised.  L1 Scopes and L1 Circuit Scopes MUST NOT be
       advertised.

   Information in this TLV MUST NOT be considered in adjacency
   formation.

   Whether information in this TLV is used to determine when FS-LSPs
   associated with a locally supported scope are flooded is an
   implementation choice.

Top      ToC       Page 20 
12.  IANA Considerations

   This document includes the definition of three new PDU types that are
   reflected in the "IS-IS PDU Registry".

    Value  Description
    ----  ---------------------
     10    FS-LSP
     11    FS-CSNP
     12    FS-PSNP

   A new IANA registry has been created to control the assignment of
   scope identifiers in FS-PDUs.  The registration procedure is "Expert
   Review" as defined in [RFC5226].  The registry name is "LSP Flooding
   Scope Identifier Registry".  A scope identifier is a number from
   1-127, inclusive.  Values 1 - 63 are reserved for PDUs that use
   standard TLVs and standard sub-TLVs.  Values 64 - 127 are reserved
   for PDUs that use extended TLVs and extended sub-TLVs.  The list of
   Hello PDUs in which support for a given scope MAY be announced (using
   Scope Flooding Support TLV) is specified for each defined scope.

   The following scope identifiers are defined by this document.

                                       FS LSP ID Format/ IIH Announce
  Value Description                    TLV Format        P2P L1LAN L2LAN
  ----- ------------------------------ ----------------- ---------------
  1     Level 1 Circuit Flooding Scope Extended/Standard  Y    Y     N
  2     Level 2 Circuit Flooding Scope Extended/Standard  Y    N     Y
  3     Level 1 Flooding Scope         Extended/Standard  Y    Y     N
  4     Level 2 Flooding Scope         Extended/Standard  Y    N     Y
  5     Domain Flooding Scope          Extended/Standard  Y    Y     Y
  (6-63)Unassigned

  64    Level 1 Circuit Flooding Scope Extended/Extended  Y    Y     N
  65    Level 2 Circuit Flooding Scope Extended/Extended  Y    N     Y
  66    Level 1 Flooding Scope         Extended/Extended  Y    Y     N
  67    Level 2 Flooding Scope         Extended/Extended  Y    N     Y
  68    Domain Flooding Scope          Extended/Extended  Y    Y     Y
  (69-127) Unassigned

   The definition of a new IS-IS TLV is reflected in the "IS-IS TLV
   Codepoints" registry:

   Value  Name                       IIH LSP SNP Purge
   ----  ------------                --- --- --- -----
   243   Scope Flooding Support       Y   N   N    N

Top      ToC       Page 21 
   The IANA "IS-IS TLV Codepoints" registry has been extended to allow
   definition of codepoints less than or equal to 65535.  Codepoints
   greater than 255 can only be used in PDUs designated to support
   extended TLVs.  This registry has also been updated to point to this
   document as a reference (in addition to [RFC3563] and [RFC6233]).

13.  Security Considerations

   Security concerns for IS-IS are addressed in [IS-IS], [RFC5304], and
   [RFC5310].

   The new PDUs introduced are subject to the same security issues
   associated with their standard LSP/CSNP/PSNP counterparts.  To the
   extent that additional PDUs represent additional load for routers in
   the network, this increases the opportunity for denial-of-service
   attacks.

14.  Acknowledgements

   The authors wish to thank Ayan Banerjee, Donald Eastlake, Hannes
   Gredler, and Mike Shand for their comments.

15.  References

15.1.  Normative References

   [IEEEaq]   IEEE, "Standard for Local and metropolitan area networks
              -- Media Access Control (MAC) Bridges and Virtual Bridged
              Local Area Networks -- Amendment 20: Shortest Path
              Bridging", IEEE Std 802.1aq-2012, June 2012.

   [IS-IS]    ISO/IEC 10589:2002, Second Edition, "Information
              technology -- Telecommunications and information exchange
              between systems -- Intermediate System to Intermediate
              System intradomain routeing information exchange protocol
              for use in conjunction with the protocol for providing the
              connectionless-mode network service (ISO 8473)", 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4971]  Vasseur, JP., Shen, N., and R. Aggarwal, "Intermediate
              System to Intermediate System (IS-IS) Extensions for
              Advertising Router Information", RFC 4971, July 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

Top      ToC       Page 22 
   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, October 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

   [RFC5306]  Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",
              RFC 5306, October 2008.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.

   [RFC6822]  Previdi, S., Ginsberg, L., Shand, M., Roy, A., and D.
              Ward, "IS-IS Multi-Instance", RFC 6822, December 2012.

15.2.  Informative References

   [RFC3563]  Zinin, A., "Cooperative Agreement Between the ISOC/IETF
              and ISO/IEC Joint Technical Committee 1/Sub Committee 6
              (JTC1/SC6) on IS-IS Routing Protocol Development", RFC
              3563, July 2003.

   [RFC5311]  McPherson, D., Ginsberg, L., Previdi, S., and M. Shand,
              "Simplified Extension of Link State PDU (LSP) Space for
              IS-IS", RFC 5311, February 2009.

   [RFC6233]  Li, T. and L. Ginsberg, "IS-IS Registry Extension for
              Purges", RFC 6233, May 2011.

   [RFC6325]  Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011.

   [RFC7176]  Eastlake, D., Senevirathne, T., Ghanwani, A., Dutt, D.,
              and A. Banerjee, "Transparent Interconnection of Lots of
              Links (TRILL) Use of IS-IS", RFC 7176, May 2014.

Top      ToC       Page 23 
Authors' Addresses

   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, CA  95035
   USA

   EMail: ginsberg@cisco.com


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144
   Italy

   EMail: sprevidi@cisco.com


   Yi Yang
   Cisco Systems
   7100-9 Kit Creek Road
   Research Triangle Park, NC  27709-4987
   USA

   EMail: yiya@cisco.com