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rfc:rfc5420

Network Working Group A. Farrel, Ed. Request for Comments: 5420 Old Dog Consulting Obsoletes: 4420 D. Papadimitriou Updates: 3209, 3473 Alcatel Category: Standards Track JP. Vasseur

                                                   Cisco Systems, Inc.
                                                           A. Ayyangar
                                                      Juniper Networks
                                                         February 2009
      Encoding of Attributes for MPLS LSP Establishment Using
    Resource Reservation Protocol Traffic Engineering (RSVP-TE)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 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.

Farrel, et al. Standards Track [Page 1] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

Abstract

 Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) may
 be established using the Resource Reservation Protocol Traffic
 Engineering (RSVP-TE) extensions.  This protocol includes an object
 (the SESSION_ATTRIBUTE object) that carries a Flags field used to
 indicate options and attributes of the LSP.  That Flags field has
 eight bits, allowing for eight options to be set.  Recent proposals
 in many documents that extend RSVP-TE have suggested uses for each of
 the previously unused bits.
 This document defines a new object for RSVP-TE messages that allows
 the signaling of further attribute bits and also the carriage of
 arbitrary attribute parameters to make RSVP-TE easily extensible to
 support new requirements.  Additionally, this document defines a way
 to record the attributes applied to the LSP on a hop-by-hop basis.
 The object mechanisms defined in this document are equally applicable
 to Generalized MPLS (GMPLS) Packet Switch Capable (PSC) LSPs and to
 GMPLS non-PSC LSPs.
 This document replaces and obsoletes the previous version of this
 work, published as RFC 4420.  The only change is in the encoding of
 the Type-Length-Variable (TLV) data structures.

Farrel, et al. Standards Track [Page 2] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

Table of Contents

 1. Introduction and Problem Statement ..............................4
    1.1. Applicability to Generalized MPLS ..........................5
    1.2. A Rejected Alternate Solution ..............................5
 2. Terminology .....................................................6
 3. Attributes TLVs .................................................6
    3.1. Attribute Flags TLV ........................................7
 4. LSP_ATTRIBUTES Object ...........................................8
    4.1. Format .....................................................9
    4.2. Generic Processing Rules for Path Messages .................9
    4.3. Generic Processing Rules for Resv Messages .................9
 5. LSP_REQUIRED_ATTRIBUTES Object .................................10
    5.1. Format ....................................................11
    5.2. Generic Processing Rules ..................................11
 6. Inheritance Rules ..............................................11
 7. Recording Attributes Per LSP ...................................12
    7.1. Requirements ..............................................12
    7.2. RRO Attributes Subobject ..................................12
    7.3. Procedures ................................................13
         7.3.1. Subobject Presence Rules ...........................13
         7.3.2. Reporting Compliance with LSP Attributes ...........14
         7.3.3. Reporting Per-Hop Attributes .......................14
         7.3.4. Default Behavior ...................................14
 8. Summary of Attribute Bit Allocation ............................14
 9. Message Formats ................................................15
 10. Guidance for Key Application Scenarios ........................16
    10.1. Communicating to Egress LSRs .............................16
    10.2. Communicating to Key Transit LSRs ........................17
    10.3. Communicating to All LSRs ................................17
 11. IANA Considerations ...........................................17
    11.1. New RSVP C-Nums and C-Types ..............................17
    11.2. New TLV Space ............................................18
    11.3. Attribute Flags ..........................................19
    11.4. New Error Codes ..........................................19
    11.5. New Record Route Subobject Identifier ....................19
 12. Security Considerations .......................................20
 13. Acknowledgements ..............................................20
 14. Changes from RFC 4420 to RFC 5420 .............................20
 15. Normative References ..........................................21
 16. Informative References ........................................21

Farrel, et al. Standards Track [Page 3] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

1. Introduction and Problem Statement

 This document replaces and obsoletes the previous version of this
 work, published as RFC 4420 [RFC4420].  The only change is in the
 encoding of the Type-Length-Variable (TLV) data structures presented
 in Section 3.  See Section 14 for a summary of changes.
 Traffic-Engineered Multiprotocol Label Switching (MPLS) Label
 Switched Paths (LSPs) [RFC3031] may be set up using the Path message
 of the RSVP-TE signaling protocol [RFC3209].  The Path message
 includes the SESSION_ATTRIBUTE object, which carries a Flags field
 used to indicate desired options and attributes of the LSP.
 The Flags field in the SESSION_ATTRIBUTE object has eight bits.  Just
 three of those bits are assigned in [RFC3209].  A further two bits
 are assigned in [RFC4090] for fast re-reroute functionality, leaving
 only three bits available.  Several recent proposals and Internet
 Drafts have demonstrated that there is a high demand for the use of
 the other three bits.  Some, if not all, of those proposals are
 likely to go forward as RFCs, resulting in depletion or near
 depletion of the Flags field and a consequent difficulty in signaling
 new options and attributes that may be developed in the future.
 This document defines a new object for RSVP-TE messages that allows
 the signaling of further attributes bits.  The new object is
 constructed from TLVs, and a new TLV is defined to carry a variable
 number of attributes bits.
 The new RSVP-TE message object is quite flexible, due to the use of
 the TLV format and allows:
  1. future specification of bit flags
  2. additional options and attribute parameters carried in TLV format
 Note that the LSP Attributes defined in this document are
 specifically scoped to an LSP.  They may be set differently on
 separate LSPs with the same Tunnel ID between the same source and
 destination (that is, within the same session).
 It is noted that some options and attributes do not need to be acted
 on by all Label Switched Routers (LSRs) along the path of the LSP.
 In particular, these options and attributes may apply only to key
 LSRs on the path, such as the ingress LSR and egress LSR.  Special
 transit LSRs, such as Area or Autonomous System Border Routers (ABRs
 or ASBRs), may also fall into this category.  This means that the new
 options and attributes should be signaled transparently, and only
 examined at those points that need to act on them.

Farrel, et al. Standards Track [Page 4] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 On the other hand, other options and attributes may require action at
 all transit LSRs along the path of the LSP.  Inability to support the
 required attributes by one of those transit LSRs may require the LSR
 to refuse the establishment of the LSP.
 These considerations are particularly important in the context of
 backward compatibility.  In general, it should be possible to provide
 new MPLS services across a legacy network without upgrading those
 LSRs that do not need to participate actively in the new services.
 Moreover, some features just require action on specific intermediate
 hops, not on every visited LSR.
 Note that options already specified for the SESSION_ATTRIBUTE object
 in preexisting RFCs are not migrated to the new mechanisms described
 in this document.
 RSVP includes a way for unrecognized objects to be transparently
 forwarded by transit nodes without them refusing the incoming
 protocol messages and without the objects being stripped from the
 outgoing protocol message (see [RFC2205], Section 3.10).  This
 capability extends to RSVP-TE and provides a good way to ensure that
 only those LSRs that understand a particular object examine it.
 This document distinguishes between options and attributes that are
 only required at key LSRs along the path of the LSP, and those that
 must be acted on by every LSR along the LSP.  Two LSP Attributes
 objects are defined in this document; using the C-Num definition
 rules inherited from [RFC2205], the first is passed transparently by
 LSRs that do not recognize it, and the second causes LSP setup
 failure with the generation of a PathErr message with an appropriate
 Error Code if an LSR does not recognize it.

1.1. Applicability to Generalized MPLS

 The RSVP-TE signaling protocol also forms the basis of a signaling
 protocol for Generalized MPLS (GMPLS) as described in [RFC3471] and
 [RFC3473].  The extensions described in this document are equally
 applicable to MPLS and GMPLS.

1.2. A Rejected Alternate Solution

 A rejected alternate solution was to define a new C-Type for the
 existing SESSION_ATTRIBUTE object.  This new C-Type could allow a
 larger Flags field and address the immediate problem.

Farrel, et al. Standards Track [Page 5] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 This solution was rejected because:
  1. A new C-Type is not backward compatible with deployed

implementations that expect to see a C-Type of 1 or 7. It is

   important that any solution be capable of carrying new attributes
   transparently across legacy LSRs if those LSRs are not required to
   act on the attributes.
  1. Support for arbitrary attributes parameters through TLVs would have

meant a significant change of substance to the existing object.

2. Terminology

 This document uses terminology from the MPLS architecture document
 [RFC3031] and from the RSVP-TE protocol specification [RFC3209],
 which inherits from the RSVP specification [RFC2205].  It also makes
 use of the Generalized MPLS RSVP-TE terminology introduced in
 [RFC3471] and [RFC3473].
 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].

3. Attributes TLVs

 Attributes carried by the new objects defined in this document are
 encoded within TLVs.  One or more TLVs may be present in each object.
 There are no ordering rules for TLVs, and no interpretation should be
 placed on the order in which TLVs are received.
 Each TLV is encoded as follows.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Type              |           Length              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 //                            Value                            //
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type
       The identifier of the TLV.

Farrel, et al. Standards Track [Page 6] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

    Length
       Indicates the total length of the TLV in octets.  That is, the
       combined length of the Type, Length, and Value fields, i.e.,
       four plus the length of the Value field in octets.
       The entire TLV MUST be padded with between zero and three
       trailing zeros to make it four-octet aligned.  The Length field
       does not count any padding.
    Value
       The data carried in the TLV.

3.1. Attribute Flags TLV

 This document defines only one TLV type value.  Type 1 indicates the
 Attribute Flags TLV.  Other TLV types may be defined in the future
 with type values assigned by IANA (see Section 11.2).
 The Attribute Flags TLV may be present in an LSP_ATTRIBUTES object
 and/or an LSP_REQUIRED_ATTRIBUTES object, defined in Sections 4 and
 5.  The bits in the TLV represent the same attributes regardless of
 which object carries the TLV.  Documents that define individual bits
 MUST specify whether the bit may be set in one object, the other, or
 both.  It is not expected that a bit will be set in both objects on a
 single Path message at the same time, but this is not ruled out by
 this document.
 The Attribute Flags TLV Value field is an array of units of 32 flags
 numbered from the most significant bit as bit zero.  The Length field
 for this TLV is therefore always a multiple of four bytes, regardless
 of the number of bits carried, and no padding is required.
 Unassigned bits are considered as reserved and MUST be set to zero on
 transmission by the originator of the object.  Bits not contained in
 the TLV MUST be assumed to be set to zero.  If the TLV is absent
 either because it is not contained in the LSP_ATTRIBUTES or
 LSP_REQUIRED_ATTRIBUTES object, or because those objects are
 themselves absent, all processing MUST be performed as though the
 bits were present and set to zero.  That is to say, assigned bits
 that are not present either because the TLV is deliberately
 foreshortened or because the TLV is not included MUST be treated as
 though they are present and are set to zero.
 No bits are defined in this document.  The assignment of bits is
 managed by IANA (see Section 11.3).

Farrel, et al. Standards Track [Page 7] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

4. LSP_ATTRIBUTES Object

 The LSP_ATTRIBUTES object is used to signal attributes required in
 support of an LSP, or to indicate the nature or use of an LSP where
 that information is not required to be acted on by all transit LSRs.
 Specifically, if an LSR does not support the object, it forwards it
 unexamined and unchanged.  This facilitates the exchange of
 attributes across legacy networks that do not support this new
 object.
 This object effectively extends the Flags field in the
 SESSION_ATTRIBUTE object and allows for the future inclusion of more
 complex objects through TLVs.
 Note that some function may require an LSR to inspect both the
 SESSION_ATTRIBUTE object and the LSP_ATTRIBUTES or
 LSP_REQUIRED_ATTRIBUTES object.
 The LSP_ATTRIBUTES object may also be used to report LSP operational
 state on a Resv message even when no LSP_ATTRIBUTES or
 LSP_REQUIRED_ATTRIBUTES object was carried on the corresponding Path
 message.  The object is added or updated by LSRs that support the
 object.  LSRs that do not understand the object or have nothing to
 report do not add the object and forward it unchanged on Resv
 messages that they generate.
 The LSP_ATTRIBUTES object class is 197 of the form 11bbbbbb.  This
 C-Num value (see [RFC2205], Section 3.10) ensures that LSRs that do
 not recognize the object pass it on transparently.
 One C-Type is defined, C-Type = 1 for LSP Attributes.
 This object is optional and may be placed on Path messages to convey
 additional information about the desired attributes of the LSP, and
 on Resv messages to report operational state.

Farrel, et al. Standards Track [Page 8] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

4.1. Format

 LSP_ATTRIBUTES class = 197, C-Type = 1
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 //                       Attributes TLVs                       //
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Attributes TLVs are encoded as described in Section 3.

4.2. Generic Processing Rules for Path Messages

 An LSR that does not support this object is required to pass it on
 unaltered, as indicated by the C-Num and the rules defined in
 [RFC2205].
 An LSR that does support this object but does not recognize a TLV
 type code carried in this object MUST pass the TLV on unaltered in
 the LSP_ATTRIBUTES object that it places in the Path message that it
 sends downstream.
 An LSR that does support this object and recognizes a TLV but does
 not support the attribute defined by the TLV MUST act as specified in
 the document that defines the TLV.
 An LSR that supports the Attribute Flags TLV but does not recognize a
 bit set in the Attribute Flags TLV MUST forward the TLV unchanged.
 An LSR that supports the Attribute Flags TLV and recognizes a bit
 that is set but does not support the indicated attribute MUST act as
 specified in the document that defines the bit.

4.3. Generic Processing Rules for Resv Messages

 An LSR that wishes to report operational status of an LSP may include
 this object in a Resv message, or update the object that is already
 carried in a Resv message.
 Note that this usage reports the state of the entire LSP and not the
 state of the LSP at an individual LSR.  This latter function is
 achieved using the LSP Attributes subobject of the Record Route
 object (RRO) as described in Section 7.

Farrel, et al. Standards Track [Page 9] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 The bits in the Attributes TLV may be used to report operational
 status for the whole LSP.  For example, an egress LSR may report a
 particular status by setting a bit.  LSRs within the network that
 determine that this status has not been achieved may clear the bit as
 they forward the Resv message.
 Observe that LSRs that do not support the object or do not support
 the function characterized by a particular bit in the Attributes TLV
 will not clear the bit when forwarding the Resv.  Thus, care must be
 taken in defining the usage of this object on a Resv.  The usage of
 an individual bit in the Attributes TLV of the LSP_ATTRIBUTES object
 on a Resv must be fully defined in the document that defines the bit.
 Additional TLVs may also be defined to be carried in this object on a
 Resv.
 An LSR that does not support this object will pass it on unaltered
 because of the C-Num.

5. LSP_REQUIRED_ATTRIBUTES Object

 The LSP_REQUIRED_ATTRIBUTES object is used to signal attributes
 required in support of an LSP, or to indicate the nature or use of an
 LSP where that information MUST be inspected at each transit LSR.
 Specifically, each transit LSR MUST examine the attributes in the
 LSP_REQUIRED_ATTRIBUTES object and MUST NOT forward the object
 without acting on its contents.
 This object effectively extends the Flags field in the
 SESSION_ATTRIBUTE object and allows for the future inclusion of more
 complex objects through TLVs.  It complements the LSP_ATTRIBUTES
 object.
 The LSP_REQUIRED_ATTRIBUTES object class is 67 of the form 0bbbbbbb.
 This C-Num value ensures that LSRs that do not recognize the object
 reject the LSP setup, effectively saying that they do not support the
 attributes requested.  This means that this object SHOULD only be
 used for attributes that require support at some transit LSRs and so
 require examination at all transit LSRs.  See Section 4 for how end-
 to-end and selective attributes are signaled.
 One C-Type is defined, C-Type = 1 for LSP Required Attributes.
 This object is optional and may be placed on Path messages to convey
 additional information about the desired attributes of the LSP.

Farrel, et al. Standards Track [Page 10] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

5.1. Format

 LSP_REQUIRED_ATTRIBUTES class = 67, C-Type = 1
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 //                      Attributes TLVs                        //
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Attributes TLVs are encoded as described in Section 3.

5.2. Generic Processing Rules

 An LSR that does not support this object will use a PathErr to reject
 the Path message based on the C-Num using the Error Code "Unknown
 Object Class".
 An LSR that does not recognize a TLV type code carried in this object
 MUST reject the Path message using a PathErr with Error Code "Unknown
 Attributes TLV" and Error Value set to the value of the unknown TLV
 type code.
 An LSR that does not recognize a bit set in the Attribute Flags TLV
 MUST reject the Path message using a PathErr with Error Code "Unknown
 Attributes Bit" and Error Value set to the bit number of the unknown
 bit in the Attribute Flags.
 An LSR that recognizes an attribute (however encoded) but does not
 support that attribute MUST act according to the behavior specified
 in the document that defines that specific attribute.
 Note that this object is not used on a Resv.  In order to report the
 status of an LSP, either the LSP_ATTRIBUTES object on a Resv or the
 Attributes subobject in the Record Route object (see Section 7) must
 be used.

6. Inheritance Rules

 In certain circumstances, when reaching an LSP region boundary, a
 forwarding adjacency LSP (FA-LSP; see [RFC4206]) is initially set up
 to allow the establishment of the LSP carrying the LSP_ATTRIBUTES
 and/or LSP_REQUIRED_ATTRIBUTES objects.  In this case, when the

Farrel, et al. Standards Track [Page 11] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 boundary LSR supports LSP_ATTRIBUTES and LSP_REQUIRED_ATTRIBUTES
 processing, the FA-LSP MAY, upon local policy, inherit a subset of
 the Attributes TLVs, in particular when the FA-LSP belongs to the
 same switching capability class as the triggering LSP.
 When these conditions are met, the LSP_ATTRIBUTES and/or
 LSP_REQUIRED_ATTRIBUTES objects are simply copied with the inherited
 Attributes TLVs in the Path message used to establish the FA-LSP.  By
 default (and in order to simplify deployment), none of the incoming
 LSP Attributes TLVs are considered as inheritable.  Note that when
 the FA-LSP establishment itself requires one or more Attributes TLVs,
 an 'OR' operation is performed with the inherited set of values.
 Documents that define individual bits for the LSP Attribute Flags TLV
 MUST specify whether or not these bits MAY be inherited (including
 the condition to be met in order for this inheritance to occur).  The
 same applies for any other TLV that will be defined following the
 rules specified in Section 3.

7. Recording Attributes Per LSP

7.1. Requirements

 In some circumstances, it is useful to determine which of the
 requested LSP attributes have been applied at which LSRs along the
 path of the LSP.  For example, an attribute may be requested in the
 LSP_ATTRIBUTES object such that LSRs that do not support the object
 are not required to support the attribute or provide the requested
 function.  In this case, it may be useful to the ingress LSR to know
 which LSRs acted on the request and which ignored it.
 Additionally, there may be other qualities that need to be reported
 on a hop-by-hop basis.  These are currently indicated in the Flags
 field of RRO subobjects.  Since there are only eight bits available
 in this field, and since some are already assigned and there is also
 likely to be an increase in allocations in new documents, there is a
 need for some other method to report per-hop attributes.

7.2. RRO Attributes Subobject

 The RRO Attributes subobject may be carried in the RECORD_ROUTE
 object if it is present.  The subobject uses the standard format of
 an RRO subobject.
 The length is variable, as for the Attribute Flags TLV.  The content
 is the same as the Attribute Flags TLV -- that is, it is a series of
 bit flags.

Farrel, et al. Standards Track [Page 12] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 There is a one-to-one correspondence between bits in the Attribute
 Flags TLV and the RRO Attributes subobject.  If a bit is only
 required in one of the two places, it is reserved in the other place.
 See the procedures sections, below, for more information.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |     Length    |           Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 //                       Attribute Flags                       //
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type
       0x05
    Length
       The Length contains the total length of the subobject in bytes,
       including the Type and Length fields.  This length must be a
       multiple of four and must be at least eight.
    Attribute Flags
       The attribute flags recorded for the specific hop.

7.3. Procedures

7.3.1. Subobject Presence Rules

 As will be clear from [RFC3209], the RECORD_ROUTE object is managed
 as a "stack", with each LSR adding subobjects to the start of the
 object.  The Attributes subobject is pushed onto the RECORD_ROUTE
 object immediately prior to pushing the node's IP address or link
 identifier.  Thus, if label recording is being used, the Attributes
 subobject SHOULD be pushed onto the RECORD_ROUTE object after the
 Record Label subobject(s).
 A node MUST NOT push an Attributes subobject onto the RECORD_ROUTE
 object without also pushing an IPv4, IPv6, or Unnumbered Interface ID
 subobject.
 This means that an Attributes subobject is bound to the LSR
 identified by the subobject found in the RRO immediately before the
 Attributes subobject.

Farrel, et al. Standards Track [Page 13] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 If the new subobject causes the RRO to be too big to fit in a Path
 (or Resv) message, the processing MUST be as described in Section
 4.4.3 of [RFC3209].
 If more than one Attributes subobject is found between a pair of
 subobjects that identify LSRs, only the first one found (that is, the
 nearest to the top of the stack) SHALL have any meaning within the
 context of this document.  All such subobjects MUST be forwarded
 unmodified by transit LSRs.

7.3.2. Reporting Compliance with LSP Attributes

 To report compliance with an attribute requested in the Attribute
 Flags TLV, an LSR MAY set the corresponding bit (see Section 8) in
 the Attributes subobject.  To report non-compliance, an LSR MAY clear
 the corresponding bit in the Attributes subobject.
 The requirement to report compliance MUST be specified in the
 document that defines the usage of any bit.  This will reduce to a
 statement of whether hop-by-hop acknowledgement is required.

7.3.3. Reporting Per-Hop Attributes

 To report a per-hop attribute, an LSR sets the appropriate bit in the
 Attributes subobject.
 The requirement to report a per-hop attribute MUST be specified in
 the document that defines the usage of the bit.

7.3.4. Default Behavior

 By default, all bits in an Attributes subobject SHOULD be set to
 zero.
 If a received Attributes subobject is not long enough to include a
 specific numbered bit, that bit MUST be treated as though present and
 as if set to zero.
 If the RRO subobject is not present for a hop in the LSP, all bits
 MUST be assumed to be set to zero.

8. Summary of Attribute Bit Allocation

 This document defines two uses of per-LSP attribute flag bit fields.
 The bit numbering in the Attribute Flags TLV and the RRO Attributes
 subobject is identical.  That is, the same attribute is indicated by
 the same bit in both places.  This means that only a single registry
 of bits is maintained.

Farrel, et al. Standards Track [Page 14] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 The consequence is a degree of clarity in implementation and
 registration.
 Note, however, that it is not always the case that a bit will be used
 in both the Attribute Flags TLV and the RRO Attributes subobject.
 For example, an attribute may be requested using the Attribute Flags
 TLV, but there is no requirement to report the handling of the
 attribute on a hop-by-hop basis.  Conversely, there may be a
 requirement to report the attributes of an LSP on a hop-by-hop basis,
 but there is no corresponding request attribute.
 In these cases, a single bit number is still assigned for both the
 Attribute Flags TLV and the RRO Attributes subobject, even though the
 bit may be irrelevant in either the Attribute Flags or the RRO
 Attributes subobject.  The document that defines the usage of the new
 bit MUST state in which places it is used and MUST handle a default
 setting of zero.

9. Message Formats

 The LSP_ATTRIBUTES object and the LSP_REQUIRED_ATTRIBUTES object MAY
 be carried in a Path message.  The LSP_ATTRIBUTES object MAY be
 carried in a Resv message.
 The order of objects in RSVP-TE messages is recommended, but
 implementations must be capable of receiving the objects in any
 meaningful order.
 On a Path message, the LSP_ATTRIBUTES object and
 LSP_REQUIRED_ATTRIBUTES objects are RECOMMENDED to be placed
 immediately after the SESSION_ATTRIBUTE object if it is present, or
 otherwise immediately after the LABEL_REQUEST object.
 If both the LSP_ATTRIBUTES object and the LSP_REQUIRED_ATTRIBUTES
 object are present, the LSP_REQUIRED_ATTRIBUTES object is RECOMMENDED
 to be placed first.
 LSRs MUST be prepared to receive these objects in any order in any
 position within a Path message.  Subsequent instances of these
 objects within a Path message SHOULD be ignored and MUST be forwarded
 unchanged.
 On a Resv message, the LSP_ATTRIBUTES object is placed in the flow
 descriptor and is associated with the FILTER_SPEC object that
 precedes it.  It is RECOMMENDED that the LSP_ATTRIBUTES object be
 placed immediately after the LABEL object.

Farrel, et al. Standards Track [Page 15] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 LSRs MUST be prepared to receive this object in any order in any
 position within a Resv message, subject to the previous note.  Only
 one instance of the LSP_ATTRIBUTES object is meaningful within the
 context of a FILTER_SPEC object.  Subsequent instances of the object
 SHOULD be ignored and MUST be forwarded unchanged.

10. Guidance for Key Application Scenarios

 As described in the Introduction section of this document, it may be
 that requested LSP attributes need to be acted on by only the egress
 LSR of the LSP, by certain key transit points (such as ABRs and
 ASBRs), or by all LSRs along the LSP.  This section briefly describes
 how each of these scenarios is met.  This section is informational
 and does not define any new procedures.

10.1. Communicating to Egress LSRs

 When communicating LSP attributes that must be acted on only by the
 LSP egress LSR, the attributes should be communicated in the
 LSP_ATTRIBUTES object.  Because of its C-Num, this object may be
 ignored (passed onwards, untouched) by transit LSRs that do not
 understand it.  This means that the Path message will not be rejected
 by LSRs that do not understand the object.  In this way, the
 requested LSP attributes are guaranteed to reach the egress LSR.
 Attributes are set within the LSP_ATTRIBUTES object according to
 which LSP attributes are required.  Each attribute is defined in some
 RFC and is accompanied by a statement of what the expected behavior
 is.  This behavior will include whether the attribute must be acted
 on by any LSR that recognizes it, or specifically by the egress LSR.
 Thus, any attribute that must be acted on only by an egress LSR will
 be defined in this way -- any transit LSR seeing this attribute
 either will understand the semantics of the attribute and ignore it
 (forwarding it, unchanged) or will not understand the attribute and
 ignore it (forwarding it, unchanged) according to the rules of the
 LSP_ATTRIBUTES object.
 The remaining issue is how the ingress LSR can know whether the
 egress LSR has acted correctly on the required LSP attribute.
 Another part of the definition of the attribute (in the defining RFC)
 is whether reporting is required.  If reporting is required, the
 egress LSR is required to use the RRO Attributes subobject to report
 whether it has acted on the received attribute.
 If an egress LSR understands a received attribute as mandatory for an
 egress LSR but does not wish to satisfy the request, it will reject
 the Path message.  If an egress LSR understands the attribute but
 believes it to be optional and does not wish to satisfy the request,

Farrel, et al. Standards Track [Page 16] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 it will report its non-compliance in the RRO Attributes subobject.
 If the egress LSR does not understand the received attribute, it may
 report non-compliance in the RRO Attributes subobject explicitly, or
 it may omit the RRO Attributes subobject, implying that it has not
 satisfied the request.

10.2. Communicating to Key Transit LSRs

 Processing for key transit LSRs (such as ABRs and ASBRs) follows
 exactly as for egress LSR.  The only difference is that the
 definition of the LSP attribute in the defining RFC will state that
 the attribute must be acted on by these transit LSRs.

10.3. Communicating to All LSRs

 In order to force all LSRs to examine the LSP attributes, the
 LSP_REQUIRED_ATTRIBUTES object is used.  The C-Num of this object is
 such that any LSR that does not recognize the object must reject a
 received Path message containing the object.
 An LSR that recognizes the LSP_REQUIRED_ATTRIBUTES object but does
 not recognize an attribute will reject the Path message.
 An LSR that recognizes an attribute but does not wish to support the
 attribute reacts according to the definition of the attribute in the
 defining RFC.  This may allow the LSR to ignore the attribute and
 forward it unchanged, or may require it to fail the LSP setup.  The
 LSR may additionally be required to report whether it supports the
 attribute using the RRO Attributes subobject.

11. IANA Considerations

 The IANA allocations made for RFC 4420 [RFC4420] now apply to this
 document and are listed here for completeness.
 IANA has updated the registry entries created for RFC 4420 to
 reference this document, which is now the normative reference for
 those entries.  This document makes no further requests for IANA
 action.

11.1. New RSVP C-Nums and C-Types

 Two new RSVP C-Nums are defined in this document and have been
 assigned by IANA.

Farrel, et al. Standards Track [Page 17] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 o LSP_ATTRIBUTES object
   The C-Num (value 197) is of the form 11bbbbbb so that LSRs that do
   not recognize the object will ignore the object but forward it,
   unexamined and unmodified, in all messages resulting from this
   message.
   One C-Type is defined for this object and has been assigned by
   IANA.
   o LSP Attributes TLVs
     C-Type value 1.
 o LSP_REQUIRED_ATTRIBUTES object
   The C-Num (value 67) is of the form 0bbbbbbb so that LSRs that do
   not recognize the object will reject the message that carries it
   with an "Unknown Object Class" error.
   One C-Type is defined for this object and has been assigned by
   IANA.
   o LSP Required Attributes TLVs
     C-Type value 1.

11.2. New TLV Space

 The two new objects referenced above are constructed from TLVs.  Each
 TLV includes a 16-bit type identifier (the T-field).  The same
 T-field values are applicable to both objects.
 The IANA has created a new registry and will manage TLV type
 identifiers as follows:
  1. TLV Type (T-field value)
  2. TLV Name
  3. Whether allowed on LSP_ATTRIBUTES object
  4. Whether allowed on LSP_REQUIRED_ATTRIBUTES object
 This document defines one TLV type as follows:
  1. TLV Type = 1
  2. TLV Name = Attribute Flags TLV
  3. allowed on LSP_ATTRIBUTES object
  4. allowed on LSP_REQUIRED_ATTRIBUTES object

Farrel, et al. Standards Track [Page 18] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 New TLV type values may be allocated only by an IETF Consensus
 action.

11.3. Attribute Flags

 This document provides new attributes bit flags for use in other
 documents that specify new RSVP-TE attributes.  These flags are
 present in the Attribute Flags TLV referenced in the previous
 section.
 The IANA has created a new registry and will manage the space of
 attributes bit flags, numbering them in the usual IETF notation:
 starting at zero and continuing at least through 31.
 New bit numbers may be allocated only by an IETF Consensus action.
 Each bit should be tracked with the following qualities:
  1. Bit number
  2. Defining RFC
  3. Name of bit
  4. Whether there is meaning in the Attribute Flags TLV on a Path
  5. Whether there is meaning in the Attribute Flags TLV on a Resv
  6. Whether there is meaning in the RRO Attributes subobject
 Note that this means that all bits in the Attribute Flags TLV and the
 RRO Attributes subobject use the same bit number, regardless of
 whether they are used in one or both places.  Thus, only one list of
 bits is required to be maintained.  (It would be meaningless in the
 context of this document for a bit to have no meaning in either the
 Attribute Flags TLV or the RRO Attributes subobject.)

11.4. New Error Codes

 This document defines the following new Error Codes and Error Values.
 Numeric values have been assigned by IANA.
 Error Code                     Error Value
 29 "Unknown Attributes TLV"    Identifies the unknown TLV type code.
 30 "Unknown Attributes Bit"    Identifies the unknown Attribute Bit.

11.5. New Record Route Subobject Identifier

 A new subobject is defined for inclusion in the RECORD_ROUTE object.
 The RRO Attributes subobject is identified by a Type value of 5.

Farrel, et al. Standards Track [Page 19] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

12. Security Considerations

 This document adds two new objects to the RSVP Path message as used
 in MPLS and GMPLS signaling, and a new subobject to the RECORD_ROUTE
 object carried on many RSVP messages.  It does not introduce any new
 direct security issues, and the reader is referred to the security
 considerations expressed in [RFC2205], [RFC3209], and [RFC3473].
 It is of passing note that any signaling request that indicates the
 functional preferences or attributes of an MPLS LSP may provide
 anyone with unauthorized access to the contents of the message with
 information about the LSP that an administrator may wish to keep
 secret.  Although this document adds new objects for signaling
 desired LSP attributes, it does not contribute to this issue, which
 can only be satisfactorily handled by encrypting the content of the
 signaling message.
 Similarly, the addition of attribute-recording information to the RRO
 may reveal information about the status of the LSP and the
 capabilities of individual LSRs that operators wish to keep secret.
 The same strategy that applies to other RRO subobjects also applies
 here.  Note, however, that there is a tension between notifying the
 head end of the LSP status at transit LSRs, and hiding the existence
 or identity of the transit LSRs.

13. Acknowledgements

 Credit to the OSPF Working Group for inspiration from their solution
 to a similar problem.  Thanks to Rahul Aggarwal for his careful
 review and support of this work.  Thanks also to Raymond Zhang,
 Kireeti Kompella, Philip Matthews, Jim Gibson, and Alan Kullberg for
 their input.  As so often, thanks to John Drake for useful offline
 discussions.  Thanks to Mike Shand for providing the Routing
 Directorate review and to Joel Halpern for the General Area review --
 both picked up on some unclarities.
 Thanks to the OIF for noticing the discrepancy in RFC 4420 that is
 fixed in this document.  Alfred Hoenes noted several typographical
 errors.

14. Changes from RFC 4420 to RFC 5420

 This document obsoletes RFC 4420 [RFC4420].  The only change is in
 Section 3.  Section 3 describes the semantic of the Length field of
 the Attributes TLV.

Farrel, et al. Standards Track [Page 20] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 Prior to the change, the Length field indicated the length of the
 Value field only.  After the change, as described in Section 3, the
 Length field indicates the length of the whole TLV.  This change
 means that this document is consistent with the subobject format
 defined in [RFC3209] and the TLV format defined in [RFC3471].
 In addition, the RFC Editor made many editorial changes to improve
 the text and readability. These changes can be observed by comparing
 the text of this document with that of [RFC4420].

15. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2205]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
            Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
            Functional Specification", RFC 2205, September 1997.
 [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
            and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
            Tunnels", RFC 3209, December 2001.
 [RFC3471]  Berger, L., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Signaling Functional Description", RFC
            3471, January 2003.
 [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Signaling Resource ReserVation
            Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
            3473, January 2003.

16. Informative References

 [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
            Label Switching Architecture", RFC 3031, January 2001.
 [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
            Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
            May 2005.
 [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
            Hierarchy with Generalized Multi-Protocol Label Switching
            (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

Farrel, et al. Standards Track [Page 21] RFC 5420 Attributes for MPLS LSPs Using RSVP-TE February 2009

 [RFC4420]  Farrel, A., Ed., Papadimitriou, D., Vasseur, J.-P., and A.
            Ayyangar, "Encoding of Attributes for Multiprotocol Label
            Switching (MPLS) Label Switched Path (LSP) Establishment
            Using Resource ReserVation Protocol-Traffic Engineering
            (RSVP-TE)", RFC 4420, February 2006.

Authors' Addresses

 Adrian Farrel
 Old Dog Consulting
 Phone:  +44 (0) 1978 860944
 EMail:  adrian@olddog.co.uk
 Dimitri Papadimitriou
 Alcatel
 Fr. Wellesplein 1,
 B-2018 Antwerpen, Belgium
 Phone:  +32 3 240-8491
 EMail:  dimitri.papadimitriou@alcatel.be
 Jean Philippe Vasseur
 Cisco Systems, Inc.
 1414 Massachusetts Avenue
 Boxborough, MA - 01719
 USA
 EMail: jpv@cisco.com
 Arthi Ayyangar
 Juniper Networks, Inc.
 1194 N.Mathilda Ave
 Sunnyvale, CA 94089
 USA
 EMail: arthi@juniper.net

Farrel, et al. Standards Track [Page 22]

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