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IETF RFC 5420
Encoding of Attributes for MPLS LSP Establishment Using Resource Reservation Protocol Traffic Engineering (RSVP-TE)
Last modified on Wednesday, February 4th, 2009
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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 [RFC 4420]. 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) [RFC 3031] may be set up using the Path message
of the RSVP-TE signaling protocol [RFC 3209]. 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 [RFC 3209]. A further two bits
are assigned in [RFC 4090] 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:
- future specification of bit flags
- 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 [RFC 2205], 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 [RFC 2205], 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 [RFC 3471] and
[RFC 3473]. 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:
- 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.
- 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
[RFC 3031] and from the RSVP-TE protocol specification [RFC 3209],
which inherits from the RSVP specification [RFC 2205]. It also makes
use of the Generalized MPLS RSVP-TE terminology introduced in
[RFC 3471] and [RFC 3473].
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].
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
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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
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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 [RFC 2205], 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
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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
[RFC 2205].
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
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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
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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 [RFC 4206]) 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.
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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 [RFC 3209], 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.
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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 [RFC 3209].
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.
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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.
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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 [RFC 4420] 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
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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:
- TLV Type (T-field value)
- TLV Name
- Whether allowed on LSP_ATTRIBUTES object
- Whether allowed on LSP_REQUIRED_ATTRIBUTES object
This document defines one TLV type as follows:
- TLV Type = 1
- TLV Name = Attribute Flags TLV
- allowed on LSP_ATTRIBUTES object
- allowed on LSP_REQUIRED_ATTRIBUTES object
Farrel, et al. Standards Track PAGE 18
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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:
- Bit number
- Defining RFC
- Name of bit
- Whether there is meaning in the Attribute Flags TLV on a Path
- Whether there is meaning in the Attribute Flags TLV on a Resv
- 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.
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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 [RFC 2205], [RFC 3209], and [RFC 3473].
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 [RFC 4420]. 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
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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 [RFC 3209] and the TLV format defined in [RFC 3471].
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 [RFC 4420].
15. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC 3209] 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.
[RFC 3471] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description", RFC
3471, January 2003.
[RFC 3473] 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
[RFC 3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC 4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
[RFC 4206] 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
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[RFC 4420] 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
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Encoding of Attributes for MPLS LSP Establishment Using Resource Reservation Protocol Traffic Engineering (RSVP-TE)
RFC TOTAL SIZE: 47668 bytes
PUBLICATION DATE: Wednesday, February 4th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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