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Versions: 00 01 draft-ietf-isis-udl

Networking Working Group                                     L. Ginsberg
Internet-Draft                                              S. Mirtorabi
Intended status: Standards Track                              S. Previdi
Expires: January 14, 2014                                         A. Roy
                                                           Cisco Systems
                                                           July 13, 2013


                 IS-IS Support for Unidirectional Links
                     draft-ginsberg-isis-udl-01.txt

Abstract

   This document defines support for the operation of IS-IS over
   Unidirectional Links without the use of tunnels or encapsulation of
   IS-IS Protocol Data Units.  Adjacency establishment when the return
   path from the router at the receive end of a unidirectional link to
   the router at the transmit end of the unidirectional link is via
   another unidirectional link is supported.  The extensions defined
   here are backwards compatible - only the routers directly connected
   to a unidirectional link need to be upgraded.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 14, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Encoding Extensions  . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  UDL LSPs and the UDL-TLV . . . . . . . . . . . . . . . . .  4
     2.2.  UDL Intermediate System Neighbors sub-TLV  . . . . . . . .  5
       2.2.1.  UDL Point-to-Point Intermediate System Neighbor
               Sub-TLV  . . . . . . . . . . . . . . . . . . . . . . .  5
       2.2.2.  UDL LAN Intermediate System Neighbor Sub-TLV . . . . .  6
     2.3.  UDL LSP Range sub-TLV  . . . . . . . . . . . . . . . . . .  7
     2.4.  UDL LSP Entry sub-TLV  . . . . . . . . . . . . . . . . . .  7
     2.5.  UDL Manual Area Addresses sub-TLV  . . . . . . . . . . . .  8
   3.  Adjacency Establishment  . . . . . . . . . . . . . . . . . . .  9
     3.1.  Adjacency Establishment in Point-to-Point Mode . . . . . .  9
     3.2.  Adjacency Establishment in Broadcast Mode  . . . . . . . . 10
     3.3.  UDL link metric configuration  . . . . . . . . . . . . . . 10
   4.  Adjacency Maintenance  . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Adjacency Maintenance by IS-T  . . . . . . . . . . . . . . 11
     4.2.  Adjacency Maintenance by IS-R  . . . . . . . . . . . . . . 12
     4.3.  Use of BFD . . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Operation of the Update Process on a UDL . . . . . . . . . . . 13
   6.  Support for UDL on the Return Path . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informational References . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17






















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1.  Introduction

   Operation of IS-IS depends upon two-way connectivity.  Adjacencies
   are formed by exchanging hellos on a link, flooding of the link state
   database is made reliable by exchanges between neighbors on a link,
   etc.  However, there are deployments where operation of the protocol
   is desired over links which are unidirectional i.e., one end of the
   link can only send Protocol Data Units (PDUs) and one end of the link
   can only receive PDUs.  Traditional methods of supporting
   Unidirectional Links (UDLs) have involved establishing a tunnel from
   the Intermediate System (IS) at the receive end of the UDL to the IS
   at the transmit end of the UDL, encapsulating/decapsulating the IS-IS
   PDUs as they enter/exit the tunnel, and associating the PDUs received
   via the tunnel with the UDL at the transmit end.  This typically
   requires static configuration and may introduce Maximum Transmission
   Unit (MTU) issues due to the required encapsulation.

   This specification defines extensions to the protocol which support
   correct and reliable operation of IS-IS over UDLs without the need
   for tunnels or any form of encapsulation.


2.   Encoding Extensions

   Although the IS at the transmit end of a UDL link (IS-T) can send
   IS-IS PDUs normally on the link, the IS at the receive end of a UDL
   link (IS-R) requires assistance from other ISs in the network to pass
   the information it would normally send directly to IS-T.  The Update
   Process as defined in [IS-IS] allows information generated by one IS
   in the network to be reliably flooded to all other ISs in the network
   using Link State PDUs (LSPs).  The extensions defined here utilize
   LSPs to allow IS-R to send information normally sent in hellos (IIHs)
   or sequence number PDUs (SNPs) to IS-T in LSPs.  As LSPs are flooded
   to all ISs in an area/sub-domain, care is taken to minimize the LSP
   churn necessary to support adjacency establishment and maintenance
   between IS-T and IS-R.

2.1.  UDL LSPs and the UDL-TLV

   Routers on the receive end of a UDL MUST reserve at least one LSP
   (for each level supported on the UDL) to advertise the UDL
   information described below.  Such LSPs are referred to as UDL-LSPs
   although the only distinction between a UDL-LSP and other LSPs is in
   the TLV information which is present in such an LSP.  LSP #0 MUST NOT
   be used to send UDL information.  UDL-LSPs have the following special
   characteristics:





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   1.  The only TLV which may be advertised in UDL-LSPs is the UDL TLV
       described below and (optionally) an Authentication TLV and/or
       Purge Originator Identification TLV [RFC6232] .  This requirement
       is enforced by the originator of the UDL-LSP but is not checked
       by receiving systems i.e., other TLVs which are included in a
       UDL-LSP are processed normally.  The reason for the restriction
       is to minimize the number of LSPs which have UDL information
       content.

   2.  Routers on the transmit side of a UDL flood UDL-LSPs regardless
       of the existence of an adjacency in the UP state on that circuit.
       Flooding of UDL-LSPs on circuits other than a UDL is as specified
       in [IS-IS] i.e., no special handling.

   A new TLV is defined in which UDL specific information appears.  All
   information in a UDL-TLV is encoded in sub-TLVs.  UDL sub-TLVs are
   formatted as specified in [RFC5305].  The format of the UDL-TLV is
   therefore:

                                             No. of octets
                 +---------------------------+
                 | Type (11)                 |     1
                 | (To be assigned by IANA)  |
                 +---------------------------+
                 | Length                    |     1
                 +---------------------------+
                 | Sub-TLVs                  |     3 - 255
                 :                           :
                 +---------------------------+



2.2.  UDL Intermediate System Neighbors sub-TLV

   UDL links may operate in Point-to-Point mode or in broadcast mode
   (assuming the subnetwork is a broadcast subnetwork).  There are
   therefore two types of Intermediate System Neighbors sub-TLVs
   defined.  A UDL-TLV MUST NOT contain more than one Intermediate
   System Neighbors sub-TLV.  If multiple Intermediate System Neighbors
   sub-TLVs appear in a UDL-TLV all information in that UDL-TLV MUST be
   ignored.

2.2.1.  UDL Point-to-Point Intermediate System Neighbor Sub-TLV

   The UDL Point-to-Point Intermediate System Neighbor Sub-TLV describes
   an adjacency on a UDL which is operating in Point-to-Point mode i.e.
   either a Point-to-Point subnetwork or a LAN subnetwork operating in
   Point-to-Point mode as described in [RFC5309].  The information



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   encoded follows the format for the Point-to-Point Three-Way Adjacency
   TLV as defined in [RFC5303] but may also include the local LAN
   address when the underlying subnetwork is a LAN.

                                            No. of octets
                 +---------------------------+
                 | Type (240)                |     1
                 | (To be assigned by IANA)  |
                 +---------------------------+
                 | Length (9 + ID Length)    |     1
                 | to (15 + ID Length)       |
                 +---------------------------+
                 | Adjacency 3-way state     |     1
                 +---------------------------+
                 | Extended Local Circuit ID |     4
                 +---------------------------+
                 | Neighbor System ID        |     ID Length
                 +---------------------------+
                 | Neighbor Extended Local   |     4
                 | Circuit ID                |
                 +---------------------------+
                 | Local LAN Address         |     6
                 +---------------------------+



2.2.2.  UDL LAN Intermediate System Neighbor Sub-TLV

   The UDL LAN Intermediate System Neighbor sub-TLV describes an
   adjacency on a UDL operating in broadcast mode on a LAN subnetwork.

                                            No. of octets
                 +---------------------------+
                 | Type (6)                  |     1
                 | (To be assigned by IANA)  |
                 +---------------------------+
                 | Length (7 + ID Length)    |     1
                 +---------------------------+
                 | Neighbor LAN ID           |     ID Length + 1
                 +---------------------------+
                 | Local LAN Address         |     6
                 +---------------------------+









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2.3.  UDL LSP Range sub-TLV

   The content of this sub-TLV describes a range of LSPs for which the
   originating router requires an update.  A UDL Intermediate System
   Neighbor sub-TLV MUST be included in any UDL-TLV where the UDL LSP
   Range sub-TLV is included.  This is necessary so that only the
   specified neighbor processes the LSP range mentioned in the sub-TLV.

                                            No. of octets
                 +---------------------------+
                 | Type (8)                  |     1
                 | (To be assigned by IANA)  |
                 +---------------------------+
                 | Length (ID Length + 2)* 2 |     1
                 +---------------------------+
                 | Start LSP ID              |     ID Length + 2
                 +---------------------------+
                 | End LSP ID                |     ID Length + 2
                 +---------------------------+



2.4.  UDL LSP Entry sub-TLV

   The content of this sub-TLV describes LSPs for which the originating
   router requires an update.  A UDL Intermediate System Neighbor sub-
   TLV MUST be included in any UDL-TLV where the UDL LSP Entry sub-TLV
   is included.  This is necessary so that only the specified neighbor
   processes the LSP entries mentioned in the sub-TLV.






















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                                            No. of octets
                 +---------------------------+
                 | Type (9)                  |     1
                 | (To be assigned by IANA)  |
                 +---------------------------+
                 | Length (10 + ID Length)*N |     1
                 +---------------------------+
                 : LSP Entries               :
                 +---------------------------+

    Each LSP Entry has the following format:

                 +---------------------------+
                 | Remaining Lifetime        |     2
                 +---------------------------+
                 | LSP ID                    |     ID Length + 2
                 +---------------------------+
                 | LSP Sequence Number       |     4
                 +---------------------------+
                 | Checksum                  |     2
                 +---------------------------+


2.5.  UDL Manual Area Addresses sub-TLV

   This sub-TLV specifies the set of manualAreaAddresses of the
   originating system.  No other sub-TLVs are allowed in a UDL-TLV which
   has this sub-TLV.  Any other sub-TLVs in such a UDL-TLV are ignored
   on receipt.

                                             No. of octets
                 +---------------------------+
                 | Type (1)                  |     1
                 | (To be assigned by IANA)  |
                 +---------------------------+
                 | Length                    |     1
                 +---------------------------+
                 : Area Address(es)          :
                 +---------------------------+

    Each Area Address has the following format:

                 +---------------------------+
                 | Address Length            |     1
                 +---------------------------+
                 | Area Address              |     Address Length
                 +---------------------------+




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3.  Adjacency Establishment

   An adjacency over a UDL link may be established over a link operating
   in Point-to-Point mode (including a LAN subnetwork configured to
   operate in Point-to-Point mode) or a link operating in broadcast
   mode.  Operation in either mode is identical except for some
   differences in the manner of adjacency establishment as specified in
   the following sub-sections.

   IS-T utilizes the set of manualAreaAddresses advertised by IS-R in a
   UDL Manual Area Address sub-TLV in combination with the UDL
   Intermediate System Neighbor sub-TLV(s) to IS-T advertised by IS-R to
   determine the level(s) associated with any adjacency to IS-R.

3.1.  Adjacency Establishment in Point-to-Point Mode

   Adjacency establishment makes use of Three Way Handshake as defined
   in [RFC5303] when operating in Point-to-Point mode.  When operating
   over a LAN subnetwork, the use of point-to-point operation over LAN
   as defined in [RFC5309] is also used.

   IS-T initiates adjacency establishment by sending Point-to-Point IIHs
   over the UDL as normal i.e., including Three-Way Handshake TLV.  Note
   that the local circuit ID specified by IS-T need only be unique among
   the set of Point-to-Point UDL links supported by IS-T on which IS-T
   is at the transmit end.

   Upon receipt of a Point-to-Point IIH IS-R creates an adjacency in the
   INIT state with IS-T and advertises the existence of the adjacency in
   its UDL-LSP(s) utilizing the UDL Point-to-Point Intermediate System
   Neighbor sub-TLV.  The Local LAN address is included if the link is a
   LAN subnetwork operating in Point-to-Point mode.  UDL-LSPs of the
   appropriate level(s) are generated according to the type of the
   adjacency with IS-T.

   When IS-T receives the UDL-LSP(s) generated by IS-R containing the
   UDL Point-to-Point Intermediate System Neighbor sub-TLV it validates
   the 3 way information and, if valid, transitions its adjacency to UP
   state.  In subsequent Point-to-Point IIHs IS-T includes IS-R's
   circuit ID information as indicated in the UDL Point-to-Point IS
   Neighbor sub-TLV in its 3 way handshake TLV.  A complete set of CSNPs
   is sent to IS-R for the level(s) appropriate for the type of
   adjacency.  LSPs which are updated as a result of the existence of
   the adjacency to IS-R are sent to IS-R, but IS-T does NOT propagate
   its full LSP Database.  This is done to minimize the amount of
   redundant flooding.

   IS-R uses normal adjacency bring up rules based on the 3 way



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   handshake information it receives in Point-to-Point IIHs from IS-T
   and advertises its IS neighbor to IS-T in the usual manner i.e. in an
   LSP other than a UDL-LSP.  Following transition of the adjacency to
   IS-T to the UP state IS-R MAY request IS-T to flood its complete LSP
   Database by sending an LSP Range sub-TLV to IS-T in a UDL-LSP.

3.2.  Adjacency Establishment in Broadcast Mode

   IS-T initiates adjacency establishment by sending LAN IIHs of the
   appropriate level(s) over the UDL as normal.  IS-T specifies itself
   in the LAN ID field of the IIH, including a non-zero circuit ID.
   Note that the local circuit ID specified by IS-T need only be unique
   among the set of LAN UDL links supported by IS-T on which IS-T is at
   the transmit end.  This is because pseudo-node LSPs will never be
   generated for a UDL.  Operation in broadcast mode supports a UDL with
   a single IS-T and multiple IS-Rs.

   Upon receipt of a LAN IIH PDU IS-R creates an adjacency in the INIT
   state with IS-T and advertises the existence of the adjacency in its
   UDL-LSP(s) utilizing the UDL LAN Intermediate System Neighbor sub-
   TLV.  UDL-LSPs of the appropriate level(s) are generated according to
   the levels supported by IS-R and IS-T.

   When IS-T receives the UDL-LSP(s) generated by IS-R containing the
   UDL LAN Intermediate System Neighbor sub-TLV(s) it validates the
   LANID and, if valid, transitions its adjacency to UP state.  In
   subsequent LAN IIH PDUs, IS-T includes IS-R's LAN Address as
   indicated in the UDL LAN IS Neighbor info.  A complete set of CSNPs
   for the appropriate level is sent over the circuit.  LSPs which are
   updated as a result of the existence of the adjacency to IS-R are
   sent to IS-R, but IS-T does NOT propagate its full LSP Database.
   This is done to minimize the amount of redundant flooding.

   IS-R uses normal adjacency bring up rules based on the IS Neighbor
   LAN Address information it receives in LAN IIH PDUs from IS-T and
   advertises its IS neighbor to IS-T in an LSP other than a UDL-LSP.
   Note that there is no pseudo-node on a UDL LAN circuit - therefore
   both IS-T and IS-R MUST advertise an IS Neighbor TLV to each other,
   not to a pseudo-node.  This is identical to what is done on a Point-
   to-Point subnetwork.  Following transition of the adjacency to IS-T
   to the UP state IS-R MAY request IS-T to flood its complete LSP
   Database by sending an LSP Range sub-TLV to IS-T in a UDL-LSP.

3.3.  UDL link metric configuration

   What metrics are configured on a UDL depend upon the intended use of
   the UDL.  If the UDL is to be used for unicast forwarding, then IS-T
   should be configured with the value appropriate to its intended



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   preference in the network topology and IS-R should be configured with
   maximum link metric (2^24 -1) as defined in [RFC5305] (assuming wide
   metrics are in use).  If the UDL is to be used for building a
   multicast Reverse Path Forwarding tree, then IS-R should be
   configured with the value appropriate to its intended preference in
   the network topology and IS-T should be configured with maximum link
   metric (2^24 -1).If the link is to be used for both unicast
   forwarding and multicast, then it is necessary to have two different
   metric configurations and perform two different SPF calculations.
   This may be achieved through the use of multi-topology extensions as
   defined in [RFC5120].  Note that the configured link metrics have no
   bearing on adjacency establishment - they only affect the building of
   a Shortest Path Tree (SPT).


4.  Adjacency Maintenance

   This section defines how adjacencies are maintained once established.
   Adjacency maintenance is defined without the need to send periodic
   UDL-LSP updates as this would be a significant burden on the entire
   network.

4.1.  Adjacency Maintenance by IS-T

   IS-T sends IIH PDUs as normal on a UDL.  As IS-R does NOT send IIH
   PDUs to IS-T, IS-T maintains the adjacency to IS-R so long as all of
   the following conditions are TRUE:

   o  IS-T has a valid UDL-LSP from IS-R which includes Point-to-Point
      UDL IS Neighbor information or LAN UDL IS Neighbor information (as
      appropriate) regarding the adjacency IS-R has with IS-T on the
      UDL.

   o  IS-T can calculate a return path rooted at IS-R to IS-T which does
      not traverse the UDL on which the adjacency is associated

   When either of the above conditions becomes FALSE, IS-T brings down
   its adjacency to IS-R.  Note that the return path calculation is only
   required when a topology change occurs in the network.  It therefore
   need only be done in conjunction with a normal event driven SPF
   calculation.

   NOTE: Immediately after the adjacency to IS-R has come up, if the
   only available return path traverses a UDL link on which the
   adjacency is still in the process of coming UP, the return path check
   will fail.  This is possible because we bypass normal flooding rules
   to allow the UDL-LSP to be flooded even when the adjacency is not UP
   on a UDL link (as described later in this document).  If IS-T



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   immediately brings the adjacency to IS-R down in this case, a
   circular dependency condition arises.  To avoid this, if the return
   path check fails immediately after the adjacency comes up, a timer Tp
   is started.  The timer is cancelled when a return path check
   succeeds.  If the timer expires, IS-T brings down the adjacency to
   IS-R.  A recommended value for the timer Tp is a small multiple
   (e.g., "twice") of the estimated time necessary to propagate LSPs
   across the entire domain.

   Although it is unorthodox to bring up an adjacency without confirmed
   two way connectivity, the extension is well grounded because the
   receipt of IS-R's UDL-LSP by IS-T is indicative of the existence of a
   return path even though it cannot yet be confirmed by examination of
   the LSP database.  This unconfirmed two way connectivity is a
   condition which we do not want to persist indefinitely - hence the
   use of timer Tp.

4.2.  Adjacency Maintenance by IS-R

   IS-R maintains its adjacency with IS-T based on receipt of IIHs from
   IS-T as normal.  So long as IS-T follows the rules for adjacency
   maintenance described in the previous section this is sufficient.

   Further protection against pathological behavior on the part of IS-T
   (e.g., failure to perform the return path calculation after a
   topology change) MAY be implemented by IS-R.  When IS-R receives a
   CSNP from IS-T which contains an SNP entry identifying an LSP which
   is not in IS-R's Link State Database (LSDB) a timer Tf is started for
   each such LSP.  This includes entries which are older than, newer
   than, or non-existent in IS-R's LSDB.The timer Tf is cancelled if:

   o  The associated LSP is received by IS-R on any circuit by normal
      operation of the Update process or

   o  A subsequent set of CSNPs received from IS-T does not include the
      LSP entry

   If any timer Tf expires IS-R brings down the adjacency with IS-T.

   In the absence of pathological behavior by IS-T the Tf extension is
   not required.  Its use is therefore optional.

4.3.  Use of BFD

   A multi-hop BFD session [RFC5883] MAY be established between IS-T and
   IS-R.  This can be used to provide fast failure detection.  If used,
   this would also make the calculation by IS-T of a return path from
   IS-R to IS-T optional.



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5.  Operation of the Update Process on a UDL

   For purposes of LSP propagation IS-T views the UDL as if it were a
   broadcast subnetwork where IS-T is the Designated Intermediate System
   (DIS).  This is true regardless of the mode of operation of the
   circuit (point-to-point or broadcast).  Therefore, IS-T propagates
   new LSPs on the UDL as they arrive but after sending an LSP on the
   UDL the SRM flag for that LSP is cleared i.e. no acknowledgement for
   the LSP is required or expected.  IS-T also sends periodic CSNPs on
   the UDL.

   IS-R cannot propagate LSPs to IS-T on the UDL.  IS-R also cannot
   acknowledge LSPs received from IS-T on the UDL.  In this respect IS-R
   operates on the UDL in a manner identical to a non-DIS on a broadcast
   circuit.  If an LSP entry in a CSNP received from IS-T identifies an
   LSP which is "newer than" an LSP in IS-R's LSDB, IS-R MAY request the
   LSP from IS-T by sending a UDL-LSP with an LSP entry as described
   above.  Since IS-R's UDL-LSP(s) will be propagated throughout the
   network even though the information is only of use to IS-Ts, it is
   recommended that some small delay occur between the receipt of a CSNP
   from IS-T and the generation of a UDL-LSP with an updated LSP entry
   by IS-R so as to allow for the possible receipt of the LSP either
   from IS-T or on another link.

   If the number of LSP entries to be requested exceeds the space
   available in the UDL TLV associated with the adjacency to IS-T, IS-R
   MUST NOT generate multiple UDL TLVs associated with the same
   adjacency.  Instead it should maintain the state of SSN flags
   appropriately for the LSP entries that require updates and send
   additional LSP entries (if necessary) in a subsequent UDL-LSP after
   the previously requested updates arrive.

   Use of the LSP Range sub-TLV by IS-R allows more efficient encoding
   of a request for multiple LSPs.  This could be especially useful
   following an adjacency UP event on a UDL.  As described in Section 3,
   IS-T does NOT propagate its full LSP database following transition of
   an adjacency to IS-R to the UP state.  This is consistent with IS-T
   operating in the role of DIS on a broadcast circuit.  If IS-R has
   neighbors on other circuits it is possible that it will have received
   LSPs from other neighbors.  In such a case flooding of the full LSP
   database by IS-T would be redundant.  It is therefore left to the
   discretion of IS-R to request those portions of the LSP database
   which are not current.  This is consistent with IS-R operating as a
   non-DIS on a broadcast circuit.

   On receipt of a UDL-LSP generated by IS-R, IS-T checks the neighbor
   information in each UDL-TLV.  If the information matches an existing
   adjacency that IS-T has with IS-R then IS-T sets SRM flag on the UDL



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   for any LSPs in its LSDB which are "newer" than the corresponding
   entries IS-R sent in LSP Entry sub-TLVs in UDL TLVs.  SRM flags are
   also set on the UDL for LSPs which fall in the ranges specified in
   LSP Range sub-TLVs in UDL TLVs.  UDL-TLVs associated with adjacencies
   to routers other than IS-T are ignored by IS-T.


6.  Support for UDL on the Return Path

   If all return paths from IS-R to IS-T traverse a UDL, then in order
   to bring up the adjacency between IS-T and IS-R at least one of the
   adjacencies on a return path UDL must already be UP.  This is
   required because IS-T relies on receiving the UDL-LSP(s) generated by
   IS-R in order to bring up its adjacency.  In order to overcome a
   circular dependency in the case where multiple pairs of UDL neighbors
   are trying to bring up an adjacency at the same time, an extension to
   LSP propagation rules is required.

   When a new UDL-LSP is received by any IS which has one or more active
   UDLs on which it is operating as an IS-T, the set of neighbors other
   than the local system which are advertised in UDL-TLVs in the
   received UDL-LSP is extracted - call this UDL-LSP-ISN-SET.  A return
   path from the originating IS-R to each neighbor in the UDL-LSP-ISN-
   SET is calculated.  If there is no return path to one or more
   neighbors in this set periodic propagation of that UDL-LSP on all
   UDLs on which the local system acts as IS-T is initiated regardless
   of the state of an adjacency on that UDL.  Periodic transmission of
   that UDL-LSP continues until a return path to all neighbors in the
   UDL-LSP-ISN-SET exists.  This calculation is redone whenever the UDL-
   LSP is updated and when a topology change in the network occurs as a
   result of updates to the LSDB.  Note that periodic retransmission is
   only done on UDLs on which the local system acts as IS-T.

   If the network is partitioned the lack of a return path from a given
   IS-R to a given IS-T may persist.  It is therefore recommended that
   the periodic retransmission employ an exponential backoff timer such
   that when the partition persists the periodic retransmission period
   is long enough so as to not represent a significant burden.  It is
   recommended that the periodic retransmission be initially set to the
   locally configured CSNP interval.  Note that periodic retransmission
   is only performed on UDL links and if an IS-R has previously received
   the same UDL-LSP it will silently ignore the retransmission since the
   UDL-LSP will already be in its LSDB.  Unnecessary reflooding of the
   retransmitted UDL-LSP beyond the UDL does not occur.

   IS-R MUST accept and propagate UDL-LSPs received on a UDL even when
   there is no adjacency in the UP state on the UDL circuit.  Flooding
   of UDL-LSPs by IS-R uses normal flooding rules.  LSPs received by



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   IS-R on the UDL which do NOT include UDL TLVs are discarded unless
   the adjacency is UP (normal processing).

   This extension allows establishment of an adjacency on a UDL even
   when the return path transits another UDL which is also in the
   process of bringing up an adjacency.  The periodic nature of the
   flooding is meant to compensate for the unreliability of the
   flooding.  After the adjacency is UP, IS-R can request LSPs from IS-T
   by putting LSP entries into UDL-LSPs - but that ability is not
   available until the adjacency is UP.


7.  IANA Considerations

   This document requires the definition of a new IS-IS TLV to be
   reflected in the "IS-IS TLV Codepoints" registry:

   Type  Description                       IIH LSP SNP Purge
   ----  ------------                      --- --- --- -----
   11   Unidirectional Link Information    N   Y   N    Y

   This document requires that a new IANA registry be created to control
   the assignment of sub-TLV code points to be advertised within a
   Unidirectional Link Information TLV.  The registration procedure is
   "Expert Review" as defined in [RFC5226].  The following sub-TLVs are
   defined by this document.  Values are suggested values subject to
   assignment by IANA.
   Value    Description
   -----    ------------------------------
   1        Manual Area Addresses
   6        LAN IS Neighbor
   9        LSP Entry
   240      Point-to-Point IS Neighbor





8.  Security Considerations

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


9.  Acknowledgements

   The idea of supporting IS-IS on UDLs without using tunnels or
   encapsulation was originally introduced in the US patent "Support of



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   unidirectional link in IS-IS without IP encapsulation and in presence
   of unidirectional return path" (patent number: 7,957,380), by Sina
   Mirtorabi, Abhay Kumar Roy, Lester Ginsberg.


10.  References

10.1.  Normative References

   [IS-IS]    "Intermediate system to Intermediate system intra-domain
              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.", Nov 2002.

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

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

   [RFC5303]  Katz, D., Saluja, R., and D. Eastlake, "Three-Way
              Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
              October 2008.

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

10.2.  Informational References

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

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

   [RFC5309]  Shen, N. and A. Zinin, "Point-to-Point Operation over LAN
              in Link State Routing Protocols", RFC 5309, 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.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, June 2010.




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   [RFC6232]  Wei, F., Qin, Y., Li, Z., Li, T., and J. Dong, "Purge
              Originator Identification TLV for IS-IS", RFC 6232,
              May 2011.


Authors' Addresses

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

   Email: ginsberg@cisco.com


   Sina Mirtorabi
   Cisco Systems
   3800 Zankar Road
   San Jose, CA  95134
   USA

   Email: smirtora@cisco.com


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144
   Italy

   Email: sprevidi@cisco.com


   Abhay Roy
   Cisco Systems
   560 McCarthy Blvd.
   Milpitas, CA  95135
   USA

   Email: akr@cisco.com










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