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IETF RFC 4874
Exclude Routes - Extension to Resource ReserVation Protocol-Traffic Engineering (RSVP-TE)
Last modified on Wednesday, April 4th, 2007
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Network Working Group CY. Lee
Request for Comments: 4874 A. Farrel
Updates: 3209, 3473 Old Dog Consulting
Category: Standards Track S. De Cnodder
Alcatel-Lucent
April 2007
Exclude Routes - Extension to
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 © The IETF Trust (2007).
Abstract
This document specifies ways to communicate route exclusions during
path setup using Resource ReserVation Protocol-Traffic Engineering
(RSVP-TE).
The RSVP-TE specification, "RSVP-TE: Extensions to RSVP for LSP
Tunnels" (RFC 3209) and GMPLS extensions to RSVP-TE, "Generalized
Multi-Protocol Label Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions" (RFC 3473) allow
abstract nodes and resources to be explicitly included in a path
setup, but not to be explicitly excluded.
In some networks where precise explicit paths are not computed at the
head end, it may be useful to specify and signal abstract nodes and
resources that are to be explicitly excluded from routes. These
exclusions may apply to the whole path, or to parts of a path between
two abstract nodes specified in an explicit path. How Shared Risk
Link Groups (SRLGs) can be excluded is also specified in this
document.
Lee, et al. Standards Track PAGE 1 
RFC 4874 Exclude Routes - Extension to RSVP-TE April 2007
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Notation ......................................4
1.2. Scope of Exclude Routes ....................................4
1.3. Relationship to MPLS TE MIB ................................5
2. Shared Risk Link Groups .........................................6
2.1. SRLG Subobject .............................................6
3. Exclude Route List ..............................................7
3.1. EXCLUDE_ROUTE Object (XRO) .................................7
3.1.1. IPv4 Prefix Subobject ...............................8
3.1.2. IPv6 Prefix Subobject ...............................9
3.1.3. Unnumbered Interface ID Subobject ..................10
3.1.4. Autonomous System Number Subobject .................10
3.1.5. SRLG Subobject .....................................11
3.2. Processing Rules for the EXCLUDE_ROUTE Object (XRO) .......11
4. Explicit Exclusion Route .......................................13
4.1. Explicit Exclusion Route Subobject (EXRS) .................13
4.2. Processing Rules for the Explicit Exclusion Route
Subobject (EXRS) ..........................................15
5. Processing of XRO together with EXRS ...........................16
6. Minimum Compliance .............................................16
7. Security Considerations ........................................16
8. IANA Considerations ............................................17
8.1. New ERO Subobject Type ....................................17
8.2. New RSVP-TE Class Numbers .................................18
8.3. New Error Codes ...........................................18
9. Acknowledgments ................................................19
10. References ....................................................19
10.1. Normative References .....................................19
10.2. Informative References ...................................19
Appendix A. Applications ..........................................21
A.1. Inter-Area LSP Protection .................................21
A.2. Inter-AS LSP Protection ...................................22
A.3. Protection in the GMPLS Overlay Model .....................24
A.4. LSP Protection inside a Single Area .......................25
Lee, et al. Standards Track PAGE 2
RFC 4874 Exclude Routes - Extension to RSVP-TE April 2007
1. Introduction
The RSVP-TE specification [RFC 3209] and GMPLS extensions [RFC 3473]
allow abstract nodes and resources to be explicitly included in a
path setup, using the Explicit Route Object (ERO).
In some systems, it may be useful to specify and signal abstract
nodes and resources that are to be explicitly excluded from routes.
This may be because loose hops or abstract nodes need to be prevented
from selecting a route through a specific resource. This is a
special case of distributed path calculation in the network.
For example, route exclusion could be used in the case where two
non-overlapping Label Switched Paths (LSPs) are required. In this
case, one option might be to set up one path and collect its route
using route recording, and then to exclude the routers on that first
path from the setup for the second path. Another option might be to
set up two parallel backbones, dual home the provider edge (PE)
routers to both backbones, and then exclude the local router on
backbone A the first time that you set up an LSP (to a particular
distant PE), and exclude the local router on backbone B the second
time that you set up an LSP.
Two types of exclusions are required:
1. Exclusion of certain abstract nodes or resources on the whole
path. This set of abstract nodes is referred to as the Exclude
Route list.
2. Exclusion of certain abstract nodes or resources between a
specific pair of abstract nodes present in an ERO. Such specific
exclusions are referred to as Explicit Exclusion Route.
To convey these constructs within the signaling protocol, a new RSVP
object and a new ERO subobject are introduced respectively.
- A new RSVP-TE object is introduced to convey the Exclude Route
list. This object is the EXCLUDE_ROUTE object (XRO).
- The second type of exclusion is achieved through a modification to
the existing ERO. A new ERO subobject type the Explicit Exclusion
Route Subobject (EXRS) is introduced to indicate an exclusion
between a pair of included abstract nodes.
The knowledge of SRLGs, as defined in [RFC 4216], may be used to
compute diverse paths that can be used for protection. In systems
where it is useful to signal exclusions, it may be useful to signal
SRLGs to indicate groups of resources that should be excluded on the
Lee, et al. Standards Track PAGE 3
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whole path or between two abstract nodes specified in an explicit
path.
This document introduces a subobject to indicate an SRLG to be
signaled in either of the two exclusion methods described above.
This document does not assume or preclude any other usage for this
subobject. This subobject might also be appropriate for use within
an Explicit Route object (ERO) or Record Route object (RRO), but this
is outside the scope of this document.
1.1. Requirements Notation
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].
1.2. Scope of Exclude Routes
This document does not preclude a route exclusion from listing
arbitrary nodes or network elements to avoid. The intent is,
however, to indicate only the minimal number of subobjects to be
explicitly avoided. For instance, it may be necessary to signal only
the SRLGs (or Shared Risk Link Groups) to avoid. That is, the route
exclusion is not intended to define the actual route by listing all
of the choices to exclude at each hop, but rather to constrain the
normal route selection process where loose hops or abstract nodes are
to be expanded by listing certain elements to be avoided.
It is envisaged that most of the conventional inclusion subobjects
are specified in the signaled ERO only for the area where they are
pertinent. The number of subobjects to be avoided, specified in the
signaled XRO, may be constant throughout the whole path setup, or the
subobjects to be avoided may be removed from the XRO as they become
irrelevant in the subsequent hops of the path setup.
For example, consider an LSP that traverses multiple computation
domains. A computation domain may be an area in the administrative
or IGP sense, or may be an arbitrary division of the network for
active management and path computational purposes. Let the primary
path be (Ingress, A1, A2, AB1, B1, B2, BC1, C1, C2, Egress) where:
- Xn denotes a node in domain X, and
- XYn denotes a node on the border of domain X and domain Y.
Note that Ingress is a node in domain A, and Egress is a node in
domain C. This is shown in Figure 1 where the domains correspond
with areas.
Lee, et al. Standards Track PAGE 4
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area A area B area C
<-------------------> <----------------> <------------------>
Ingress-----A1----A2----AB1----B1----B2----BC1----C1----C2----Egress
^ \ / | \ / | \ /
| \ / | \ / | \ /
| A3----------A4--AB2--B3--------B4--BC2--C3----------C4
| ^ ^
| | |
| | |
| | ERO: (C3-strict, C4-strict,
| | Egress-strict)
| | XRO: Not needed
| |
| ERO: (B3-strict, B4-strict, BC2-strict, Egress-loose)
| XRO: (BC1, C1, C2)
|
ERO: (A3-strict, A4-strict, AB2-strict, Egress-loose)
XRO: (AB1, B1, B2, BC1, C1, C2, Egress)
Figure 1: Domains Corresponding to IGP Areas
Consider the establishment of a node-diverse protection path in the
example above. The protection path must avoid all nodes on the
primary path. The exclusions for area A are handled during
Constrained Shortest Path First (CSPF) computation at Ingress, so the
ERO and XRO signaled at Ingress could be (A3-strict, A4-strict,
AB2-strict, Egress-loose) and (AB1, B1, B2, BC1, C1, C2),
respectively. At AB2, the ERO and XRO could be (B3-strict, B4-
strict, BC2-strict, Egress-loose) and (BC1, C1, C2), respectively.
At BC2, the ERO could be (C3-strict, C4-strict, Egress-strict) and an
XRO is not needed from BC2 onwards.
In general, consideration SHOULD be given (as with explicit route) to
the size of signaled data and the impact on the signaling protocol.
1.3. Relationship to MPLS TE MIB
[RFC 3812] defines managed objects for managing and modeling MPLS-
based traffic engineering. Included in [RFC 3812] is a means to
configure explicit routes for use on specific LSPs. This
configuration allows the exclusion of certain resources.
In systems where the full explicit path is not computed at the
ingress (or at a path computation site for use at the ingress), it
may be necessary to signal those exclusions. This document offers a
means of doing this signaling.
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2. Shared Risk Link Groups
The identifier of an SRLG is defined as a 32-bit quantity in
[RFC 4202]. An SRLG subobject is introduced such that it can be used
in the exclusion methods as described in the following sections.
This document does not assume or preclude any other usage for this
subobject. This subobject might also be appropriate for use within
Explicit Route object (ERO) or Record Route object (RRO), but this is
outside the scope of this document.
2.1. SRLG Subobject
The new SRLG subobject is defined by this document as follows. Its
format is modeled on the ERO subobjects defined in [RFC 3209].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | SRLG Id (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRLG Id (continued) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L
The L bit is an attribute of the subobject. The L bit is set
if the subobject represents a loose hop in the explicit route.
If the bit is not set, the subobject represents a strict hop in
the explicit route.
For exclusions (as used by XRO and EXRS defined in this
document), the L bit SHOULD be set to zero and ignored.
Type
The type of the subobject (34)
Length
The Length contains the total length of the subobject in bytes,
including the Type and Length fields. The Length is always 8.
SRLG Id
The 32-bit identifier of the SRLG.
Reserved
This field is reserved. It SHOULD be set to zero on
transmission and MUST be ignored on receipt.
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3. Exclude Route List
The exclude route identifies a list of abstract nodes that should not
be traversed along the path of the LSP being established. It is
RECOMMENDED that the size of the exclude route list be limited to a
value local to the node originating the exclude route list.
3.1. EXCLUDE_ROUTE Object (XRO)
Abstract nodes to be excluded from the path are specified via the
EXCLUDE_ROUTE object (XRO).
Currently, one C_Type is defined, Type 1 EXCLUDE_ROUTE. The
EXCLUDE_ROUTE object has the following format:
Class = 232, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// (Subobjects) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of an EXCLUDE_ROUTE object are a series of variable-
length data items called subobjects. This specification adapts ERO
subobjects as defined in [RFC 3209], [RFC 3473], and [RFC 3477] for use
in route exclusions. The SRLG subobject as defined in Section 2 of
this document has not been defined before. The SRLG subobject is
defined here for use with route exclusions.
The following subobject types are supported.
Type Subobject
-------------+-------------------------------
1 IPv4 prefix
2 IPv6 prefix
4 Unnumbered Interface ID
32 Autonomous system number
34 SRLG
The defined values for Type above are specified in [RFC 3209] and in
this document.
The concept of loose or strict hops has no meaning in route
exclusion. The L bit, defined for ERO subobjects in [RFC 3209], is
reused here to indicate that an abstract node MUST be excluded (value
Lee, et al. Standards Track PAGE 7
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0) or SHOULD be avoided (value 1). The distinction is that the path
of an LSP must not traverse an abstract node listed in the XRO with
the L bit clear, but may traverse one with the L bit set. A node
responsible for routing an LSP (for example, for expanding a loose
hop) should attempt to minimize the number of abstract nodes listed
in the XRO with the L bit set that are traversed by the LSP according
to local policy. A node generating XRO subobjects with the L bit set
must be prepared to accept an LSP that traverses one, some, or all of
the corresponding abstract nodes.
Subobjects 1, 2, and 4 refer to an interface or a set of interfaces.
An Attribute octet is introduced in these subobjects to indicate the
attribute (e.g., interface, node, SRLG) associated with the
interfaces that should be excluded from the path. For instance, the
attribute node allows a whole node to be excluded from the path by
specifying an interface of that node in the XRO subobject, in
contrast to the attribute interface, which allows a specific
interface (or multiple interfaces) to be excluded from the path
without excluding the whole node. The attribute SRLG allows all
SRLGs associated with an interface to be excluded from the path.
3.1.1. IPv4 Prefix Subobject
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | IPv4 address (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address (continued) | Prefix Length | Attribute |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L
0 indicates that the attribute specified MUST be excluded.
1 indicates that the attribute specified SHOULD be avoided.
Attribute
Interface attribute values
0 indicates that the interface or set of interfaces
associated with the IPv4 prefix should be excluded or
avoided.
Node attribute value
1 indicates that the node or set of nodes associated with
the IPv4 prefix should be excluded or avoided.
Lee, et al. Standards Track PAGE 8
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SRLG attribute values
2 indicates that all the SRLGs associated with the IPv4
prefix should be excluded or avoided.
The rest of the fields are as defined in [RFC 3209].
3.1.2. IPv6 Prefix Subobject
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | IPv6 address (16 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 address (continued) | Prefix Length | Attribute |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L
0 indicates that the attribute specified MUST be excluded.
1 indicates that the attribute specified SHOULD be avoided.
Attribute
Interface attribute value
0 indicates that the interface or set of interfaces
associated with the IPv6 prefix should be excluded or
avoided.
Node attribute value
1 indicates that the node or set of nodes associated with
the IPv6 prefix should be excluded or avoided.
SRLG attribute value
2 indicates that all the SRLGs associated with the IPv6
prefix should be excluded or avoided.
The rest of the fields are as defined in [RFC 3209].
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RFC 4874 Exclude Routes - Extension to RSVP-TE April 2007
3.1.3. Unnumbered Interface ID Subobject
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Reserved | Attribute |
| | | |(must be zero) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TE Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L
0 indicates that the attribute specified MUST be excluded.
1 indicates that the attribute specified SHOULD be avoided.
Attribute
Interface attribute value
0 indicates that the Interface ID specified should be
excluded or avoided.
Node attribute value
1 indicates that the node with the Router ID should be
excluded or avoided (this can be achieved using an IPv4/v6
subobject as well, but is included here because it may be
convenient to use information from subobjects of an RRO, as
defined in [RFC 3477], in specifying the exclusions).
SRLG attribute value
2 indicates that all the SRLGs associated with the interface
should be excluded or avoided.
Reserved
This field is reserved. It SHOULD be set to zero on
transmission and MUST be ignored on receipt.
The rest of the fields are as defined in [RFC 3477].
3.1.4. Autonomous System Number Subobject
The meaning of the L bit is as follows:
0 indicates that the abstract node specified MUST be excluded.
1 indicates that the abstract node specified SHOULD be avoided.
The rest of the fields are as defined in [RFC 3209]. There is no
Attribute octet defined.
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3.1.5. SRLG Subobject
The meaning of the L bit is as follows:
0 indicates that the SRLG specified MUST be excluded
1 indicates that the SRLG specified SHOULD be avoided
The Attribute octet is not present. The rest of the fields are as
defined in the "SRLG Subobject" section of this document.
3.2. Processing Rules for the EXCLUDE_ROUTE Object (XRO)
The exclude route list is encoded as a series of subobjects contained
in an EXCLUDE_ROUTE object. Each subobject identifies an abstract
node in the exclude route list.
Each abstract node may be a precisely specified IP address belonging
to a node, or an IP address with prefix identifying interfaces of a
group of nodes, an Autonomous System, or an SRLG.
The Explicit Route and routing processing is unchanged from the
description in [RFC 3209] with the following additions:
1. When a Path message is received at a node, the node MUST check
that it is not a member of any of the abstract nodes in the XRO if
it is present in the Path message. If the node is a member of any
of the abstract nodes in the XRO with the L-flag set to "exclude",
it SHOULD return a PathErr with the error code "Routing Problem"
and error value of "Local node in Exclude Route". If there are
SRLGs in the XRO, the node SHOULD check that the resources the
node uses are not part of any SRLG with the L-flag set to
"exclude" that is specified in the XRO. If it is, it SHOULD
return a PathErr with error code "Routing Problem" and error value
of "Local node in Exclude Route".
2. Each subobject MUST be consistent. If a subobject is not
consistent then the node SHOULD return a PathErr with error code
"Routing Problem" and error value "Inconsistent Subobject". An
example of an inconsistent subobject is an IPv4 Prefix subobject
containing the IP address of a node and the attribute field is set
to "interface" or "SRLG".
3. The subobjects in the ERO and XRO SHOULD NOT contradict each
other. If a Path message is received that contains contradicting
ERO and XRO subobjects, then:
- Subobjects in the XRO with the L flag not set (zero) MUST take
precedence over the subobjects in the ERO -- that is, a
mandatory exclusion expressed in the XRO MUST be honored and an
Lee, et al. Standards Track PAGE 11
RFC 4874 Exclude Routes - Extension to RSVP-TE April 2007
implementation MUST reject such a Path message. This means that
a PathErr with error code "Routing Problem" and error value of
"Route blocked by Exclude Route" is returned.
- Subobjects in the XRO with the L flag set do not take precedence
over ERO subobjects -- that is, an implementation MAY choose to
reject a Path message because of such a contradiction, but MAY
continue and set up the LSP (ignoring the XRO subobjects that
contradict the ERO subobjects).
4. When choosing a next hop or expanding an explicit route to include
additional subobjects, a node:
a. MUST NOT introduce an explicit node or an abstract node that
equals or is a member of any abstract node that is specified in
the EXCLUDE_ROUTE object with the L-flag set to "exclude". The
number of introduced explicit nodes or abstract nodes with the
L flag set to "avoid", which indicates that it is not mandatory
to be excluded but that it is less preferred, SHOULD be
minimized in the computed path.
b. MUST NOT introduce links, nodes, or resources identified by the
SRLG Id specified in the SRLG subobjects(s). The number of
introduced SRLGs with the L flag set to "avoid" SHOULD be
minimized.
If these rules preclude further forwarding of the Path message,
the node SHOULD return a PathErr with the error code "Routing
Problem" and error value of "Route blocked by Exclude Route".
Note that the subobjects in the XRO is an unordered list of
subobjects.
A node receiving a Path message carrying an XRO MAY reject the
message if the XRO is too large or complicated for the local
implementation or the rules of local policy. In this case, the node
MUST send a PathErr message with the error code "Routing Error" and
error value "XRO Too Complex". An ingress LSR receiving this error
code/value combination MAY reduce the complexity of the XRO or route
around the node that rejected the XRO.
The XRO Class-Num is of the form 11bbbbbb so that nodes that do not
support the XRO forward it uninspected and do not apply the
extensions to ERO processing described above. This approach is
chosen to allow route exclusion to traverse parts of the network that
are not capable of parsing or handling the new function. Note that
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Record Route may be used to allow computing nodes to observe
violations of route exclusion and attempt to re-route the LSP
accordingly.
If a node supports the XRO, but not a particular subobject or part of
that subobject, then that particular subobject is ignored. Examples
of a part of a subobject that can be supported are: (1) only prefix
32 of the IPv4 prefix subobject could be supported, or (2) a
particular subobject is supported but not the particular attribute
field.
When a node forwards a Path message, it can do the following three
operations related to XRO besides the processing rules mentioned
above:
1. If no XRO was present, an XRO may be included.
2. If an XRO was present, it may remove the XRO if it is sure that
the next nodes do not need this information anymore. An example
is where a node can expand the ERO to a full strict path towards
the destination. See Figure 1 where BC2 is removing the XRO from
the Path message.
3. If an XRO was present, the content of the XRO can be modified.
Subobjects can be added or removed. See Figure 1 for an example
where AB2 is stripping off some subobjects.
In any case, a node MUST NOT introduce any explicit or abstract node
in the XRO (irrespective of the value of the L flag) that it also has
introduced in the ERO.
4. Explicit Exclusion Route
The Explicit Exclusion Route defines abstract nodes or resources
(such as links, unnumbered interfaces, or labels) that must not or
should not be used on the path between two inclusive abstract nodes
or resources in the explicit route.
4.1. Explicit Exclusion Route Subobject (EXRS)
A new ERO subobject type is defined. The Explicit Exclusion Route
Subobject (EXRS) has type 33. Although the EXRS is an ERO subobject
and the XRO is reusing the ERO subobject, an EXRS MUST NOT be present
in an XRO. An EXRS is an ERO subobject that contains one or more
subobjects of its own, called EXRS subobjects.
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The format of the EXRS is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// one or more EXRS subobjects //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L
It MUST be set to zero on transmission and MUST be ignored on
receipt. (Note: The L bit in an EXRS subobject is as defined
for the XRO subobjects.)
Type
The type of the subobject (33).
Reserved
This field is reserved. It SHOULD be set to zero on
transmission and MUST be ignored on receipt.
EXRS subobjects
An EXRS subobject indicates the abstract node or resource to be
excluded. The format of an EXRS subobject is exactly the same
as the format of a subobject in the XRO. An EXRS may include
all subobjects defined in this document for the XRO.
Thus, an EXRS for an IP hop may look 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Type | Length | IPv4 address (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 address (continued) | Prefix Length | Attribute |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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4.2. Processing Rules for the Explicit Exclusion Route Subobject (EXRS)
Each EXRS may carry multiple exclusions. The exclusion is encoded
exactly as for XRO subobjects and prefixed by an additional Type and
Length.
The scope of the exclusion is the step between the previous ERO
subobject that identifies an abstract node, and the subsequent ERO
subobject that identifies an abstract node. The processing rules of
the EXRS are the same as the processing rule of the XRO within this
scope. Multiple exclusions may be present between any pair of
abstract nodes.
Exclusions may indicate explicit nodes, abstract nodes, or Autonomous
Systems that must not be traversed on the path to the next abstract
node indicated in the ERO.
Exclusions may also indicate resources (such as unnumbered
interfaces, link ids, and labels) that must not be used on the path
to the next abstract node indicated in the ERO.
SRLGs may also be indicated for exclusion from the path to the next
abstract node in the ERO by the inclusion of an EXRS containing an
SRLG subobject. If the L bit in the SRLG subobject is zero, the
resources (nodes, links, etc.) identified by the SRLG MUST NOT be
used on the path to the next abstract node indicated in the ERO. If
the L bit is set, the resources identified by the SRLG SHOULD be
avoided.
If a node is called upon to process an EXRS and does not support
handling of exclusions it will behave as described in [RFC 3209] when
an unrecognized ERO subobject is encountered. This means that this
node will return a PathErr with error code "Routing Error" and error
value "Bad EXPLICIT_ROUTE object" with the EXPLICIT_ROUTE object
included, truncated (on the left) to the offending EXRS.
If the presence of EXRS precludes further forwarding of the Path
message, the node SHOULD return a PathErr with the error code
"Routing Problem" and error value "Route Blocked by Exclude Route".
A node MAY reject a Path message if the EXRS is too large or
complicated for the local implementation or as governed by local
policy. In this case, the node MUST send a PathErr message with the
error code "Routing Error" and error value "EXRS Too Complex". An
ingress LSR receiving this error code/value combination MAY reduce
the complexity of the EXRS or route around the node that rejected the
EXRS.
Lee, et al. Standards Track PAGE 15
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5. Processing of XRO together with EXRS
When an LSR performs ERO expansion and finds both the XRO in the Path
message and EXRS in the ERO, it MUST exclude all the SRLGs, nodes,
links, and resources listed in both places. Where some elements
appear in both lists, it MUST be handled according to the stricter
exclusion request. That is, if one list says that an SRLG, node,
link, or resource must be excluded, and the other says only that it
should be avoided, then the element MUST be excluded.
6. Minimum Compliance
An implementation MUST be at least compliant with the following:
1. The XRO MUST be supported with the following restrictions:
- The IPv4 Prefix subobject MUST be supported with a prefix length
of 32, and an attribute value of "interface" and "node". Other
prefix values and attribute values MAY be supported.
- The IPv6 Prefix subobject MUST be supported with a prefix length
of 128, and an attribute value of "interface" and "node". Other
prefix values and attribute values MAY be supported.
2. The EXRS MAY be supported. If supported, the same restrictions as
for the XRO apply. If not supported, an EXRS encountered during
normal ERO processing MUST be rejected as an unknown ERO subobject
as described in Section 4.2. Note that a node SHOULD NOT parse
ahead into an ERO, and if it does, it MUST NOT reject the ERO if
it discovers an EXRS that applies to another node.
3. If XRO or EXRS are supported, the implementation MUST be compliant
with the processing rules of the supported, not supported, or
partially supported subobjects as specified within this document.
7. Security Considerations
Security considerations for MPLS-TE and GMPLS signaling are covered
in [RFC 3209] and [RFC 3473]. This document does not introduce any new
messages or any substantive new processing, and so those security
considerations continue to apply.
Note that any security concerns that exist with explicit routes
should be considered with regard to route exclusions. For example,
some administrative boundaries may consider explicit routes to be
security violations and may strip EROs from the Path messages that
they process. In this case, the XRO should also be considered for
removal from the Path message.
Lee, et al. Standards Track PAGE 16
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It is possible that an arbitrarily complex XRO or EXRS sequence could
be introduced as a form of denial-of-service attack since its
presence will potentially cause additional processing at each node on
the path of the LSP. It should be noted that such an attack assumes
that an otherwise trusted LSR (i.e., one that has been authenticated
by its neighbors) is misbehaving. A node that receives an XRO or
EXRS sequence that it considers too complex according to its local
policy may respond with a PathErr message carrying the error code
"Routing Error" and error value "XRO Too Complex" or "EXRS Too
Complex".
8. IANA Considerations
It might be considered that an alternative approach would be to
assign one of the bits of the ERO subobject type field (perhaps the
top bit) to identify that a subobject is intended for inclusion
rather than exclusion. However, [RFC 3209] states that the type field
(seven bits) should be assigned as 0 - 63 through IETF consensus
action, 64 - 95 as first come first served, and 96 - 127 are reserved
for private use. It would not be acceptable to disrupt existing
implementations, so the only option would be to split the IETF
consensus range leaving only 32 subobject types. It is felt that 32
would be an unacceptably small number for future expansion of the
protocol.
8.1. New ERO Subobject Type
IANA registry: RSVP PARAMETERS
Subsection: Class Names, Class Numbers, and Class Types
A new subobject has been added to the existing entry for:
20 EXPLICIT_ROUTE
The registry reads:
33 Explicit Exclusion Route subobject (EXRS)
The Explicit Exclusion Route subobject (EXRS) is defined in Section
4.1, "Explicit Exclusion Route Subobject (EXRS)". This subobject may
be present in the Explicit Route Object, but not in the Route Record
Object or in the new EXCLUDE_ROUTE object, and it should not be
listed among the subobjects for those objects.
Lee, et al. Standards Track PAGE 17
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8.2. New RSVP-TE Class Numbers
IANA registry: RSVP PARAMETERS
Subsection: Class Names, Class Numbers, and Class Types
A new class number has been added for EXCLUDE_ROUTE object (XRO) as
defined in Section 3.1, "EXCLUDE_ROUTE Object (XRO)".
EXCLUDE_ROUTE
Class-Num of type 11bbbbbb
Value: 232
Defined CType: 1 (EXCLUDE_ROUTE)
Subobjects 1, 2, 4, and 32 are as defined for Explicit Route Object.
An additional subobject has been registered as requested in Section
8.1, "New ERO Subobject Type". The text should appear as:
Sub-object type
1 IPv4 address [RFC 3209]
2 IPv6 address [RFC 3209]
4 Unnumbered Interface ID [RFC 3477]
32 Autonomous system number [RFC 3209]
33 Explicit Exclusion Route subobject (EXRS) [RFC 4874]
34 SRLG [RFC 4874]
The SRLG subobject is defined in Section 3.1.5, "SRLG Subobject".
The value 34 has been assigned.
8.3. New Error Codes
IANA registry: RSVP PARAMETERS
Subsection: Error Codes and Globally-Defined Error Value Sub-Codes
New Error Values sub-codes have been registered for the Error Code
'Routing Problem' (24).
64 = Unsupported Exclude Route Subobject Type
65 = Inconsistent Subobject
66 = Local Node in Exclude Route
67 = Route Blocked by Exclude Route
68 = XRO Too Complex
69 = EXRS Too Complex
Lee, et al. Standards Track PAGE 18
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9. Acknowledgments
This document reuses text from [RFC 3209] for the description of
EXCLUDE_ROUTE.
The authors would like to express their thanks to Lou Berger, Steffen
Brockmann, Igor Bryskin, Dimitri Papadimitriou, Cristel Pelsser, and
Richard Rabbat for their considered opinions on this document. Also
thanks to Yakov Rekhter for reminding us about SRLGs!
Thanks to Eric Gray for providing GenArt review and to Ross Callon
for his comments.
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 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 3473] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January
2003.
[RFC 3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
in Resource ReSerVation Protocol - Traffic Engineering
(RSVP-TE)", RFC 3477, January 2003.
[RFC 4202] Kompella, K. and Y. Rekhter, "Routing Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4202, October 2005.
10.2. Informative References
[CRANKBACK] Farrel, A., Satyanarayana, A., Iwata, A., Ash, G., and S.
Marshall-Unitt, "Crankback Signaling Extensions for MPLS
Signaling", Work in Progress, January 2007.
[RFC 3630] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC 3630,
September 2003.
Lee, et al. Standards Track PAGE 19
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[RFC 3784] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)",
RFC 3784, June 2004.
[RFC 3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Traffic Engineering
(TE) Management Information Base (MIB)", RFC 3812, June
2004.
[RFC 4208] Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
"Generalized Multiprotocol Label Switching (GMPLS) User-
Network Interface (UNI): Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Support for the Overlay
Model", RFC 4208, October 2005.
[RFC 4216] Zhang, R. and JP. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
Lee, et al. Standards Track PAGE 20
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Appendix A. Applications
This section describes some applications that can make use of the
XRO. The intention is to show that the XRO is not an application-
specific object, but that it can be used for multiple purposes. In a
few examples, other solutions might be possible for that particular
case, but the intention is to show that a single object can be used
for all the examples, hence making the XRO a rather generic object
without having to define a solution and new objects for each new
application.
A.1. Inter-Area LSP Protection
One method to establish an inter-area LSP is where the ingress router
selects an ABR, and then the ingress router computes a path towards
this selected ABR such that the configured constraints of the LSP are
fulfilled. In the example of Figure A.1, an LSP has to be
established from node A in area 1 to node C in area 2. If no loose
hops are configured, then the computed ERO at A could look as
follows: (A1-strict, A2-strict, ABR1-strict, C-loose). When the Path
message arrives at ABR1, then the ERO is (ABR1-strict, C-loose), and
it can be expanded by ABR1 to (B1-strict, ABR3-strict, C-loose).
Similarly, at ABR3 the received ERO is (ABR3-strict, C-loose), and it
can be expanded to (C1-strict, C2-strict, C-strict). If a backup LSP
also has to be established, then A takes another ABR (ABR2 in this
case) and computes a path towards this ABR that fulfills the
constraints of the LSP and that is disjoint from the path of the
primary LSP. The ERO generated by A looks as follows for this
example: (A3-strict, A4-strict, ABR2-strict, C-loose).
In order to let ABR2 expand the ERO, it also needs to know the path
of the primary LSP so that the ERO expansion is disjoint from the
path of the primary LSP. Therefore, A also includes an XRO that at
least contains (ABR1, B1, ABR3, C1, C2). Based on these constraints,
ABR2 can expand the ERO such that it is disjoint from the primary
LSP. In this example, the ERO computed by ABR2 would be (B2-strict,
ABR4-strict, C-loose), and the XRO generated by B contains at least
(ABR3, C1, C2). The latter information is needed for ABR4 to expand
the ERO so that the path is disjoint from the primary LSP in area 2.
Lee, et al. Standards Track PAGE 21
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Area 1 Area 0 Area 2
<---------------><--------------><--------------->
+---A1---A2----ABR1-----B1-----ABR3----C1---C2---+
| | | | |
| | | | |
A | | | C
| | | | |
| | | | |
+---A3---A4----ABR2-----B2-----ABR4----C3---C4---+
Figure A.1: Inter-area LSPs
In this example, a node performing the path computation first selects
an ABR and then computes a strict path towards this ABR. For the
backup LSP, all nodes of the primary LSP in the next areas have to be
put in the XRO (with the exception of the destination node if node
protection and no link protection is required). When an ABR computes
the next path segment, i.e., the path over the next area, it may
remove the nodes from the XRO that are located in that area with the
exception of the ABR where the primary LSP is exiting the area. The
latter information is still required because when the selected ABR
(ABR4 in this example) further expands the ERO, it has to exclude the
ABR on which the primary LSP is entering that area (ABR3 in this
example). This means that when ABR2 generates an XRO, it may remove
the nodes in area 0 from the XRO but not ABR3. Note that not doing
this would not cause harm in this example because there is no path
from ABR4 to C via ABR3 in area 2. If there is a link between ABR4-
ABR3 and ABR3-C, then it is required to have ABR3 in the XRO
generated by ABR2.
Discussion on the length of the XRO: When link or node protection is
requested, the length of the XRO is bounded by the length of the RRO
of the primary LSP. It can be made shorter by removing nodes by the
ingress node and the ABRs. In the example above, the RRO of the
primary LSP contains 8 subobjects, while the maximum XRO length can
be bounded by 6 subobjects (nodes A1 and A2 do not have to be in the
XRO). For SRLG protection, the XRO has to list all SRLGs that are
crossed by the primary LSP.
A.2. Inter-AS LSP Protection
When an inter-AS LSP (which has to be protected by a backup LSP to
provide link or node protection) is established, the same method as
for the inter-area LSP case can be used. The difference is when the
backup LSP is not following the same AS-path as the primary LSP
because then the XRO should always contain the full path of the
primary LSP. In case the backup LSP is following the same AS-path
Lee, et al. Standards Track PAGE 22
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(but with different ASBRs -- at least in case of node protection), it
is similar to the inter-area case: ASBRs expanding the ERO over the
next AS may remove the XRO subobjects located in that AS. Note that
this can only be done by an ingress ASBR (the ASBR where the LSP is
entering the AS).
Discussion on the length of the XRO: the XRO is bounded by the length
of the RRO of the primary LSP.
Suppose that SRLG protection is required, and the ASs crossed by the
main LSP use a consistent way of allocating SRLG-ids to the links
(i.e., the ASs use a single SRLG space). In this case, the SRLG-ids
of each link used by the main LSP can be recorded by means of the
RRO; the SRLG-ids are then used by the XRO. If the SRLG-ids are only
meaningful when local to the AS, putting SRLG-ids in the XRO crossing
many ASs makes no sense. To provide SRLG protection for inter-AS
LSPs the link IP address of the inter-AS link used by the primary LSP
can be put into the XRO of the Path message of the detour LSP or
bypass tunnel. The ASBR where the detour LSP or bypass tunnel is
entering the AS can translate this into the list of SRLG-ids known to
the local AS.
Discussion on the length of the XRO: the XRO only contains 1
subobject, which contains the IP address of the inter-AS link
traversed by the primary LSP (assuming that the primary LSP and
detour LSP or bypass tunnel are leaving the AS in the same area, and
that they are also entering the next AS in the same area).
Lee, et al. Standards Track PAGE 23
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A.3. Protection in the GMPLS Overlay Model
When an edge-node wants to establish an LSP towards another edge-node
over an optical core network as described in [RFC 4208] (see Figure
A.2), the XRO can be used for multiple purposes.
Overlay Overlay
Network +--------------------------------+ Network
+----------+ | | +----------+
| +----+ | | +-----+ +-----+ +-----+ | | +----+ |
| | | | | | | | | | | | | | | |
| --+ EN1+-+-----+--+ CN1 +---+ CN2 +---+ CN3 +---+-----+-+ EN3+-- |
| | | | +--+--+ | | | | +---+--+ | | | |
| +----+ | | | +--+--+ +--+--+ +--+--+ | | | +----+ |
| | | | | | | | | | |
+----------+ | | | | | | | +----------+
| | | | | | |
+----------+ | | | | | | | +----------+
| | | | +--+--+ | +--+--+ | | | |
| +----+ | | | | | +------+ | | | | +----+ |
| | +-+--+ | | CN4 +-------------+ CN5 | | +--+-+ | |
| --+ EN2+-+-----+--+ | | +---+-----+-+ EN4+-- |
| | | | | +-----+ +-----+ | | | | |
| +----+ | | | | +----+ |
| | +--------------------------------+ | |
+----------+ Core Network +----------+
Overlay Overlay
Network Network
Legend:
EN - Edge-Node
CN - Core-Node
Figure A.2
A first application is where an edge-node wants to establish multiple
LSPs towards the same destination edge-node, and these LSPs need to
have few or no SRLGs in common. In this case EN1 could establish an
LSP towards EN3, and then it can establish a second LSP listing all
links used by the first LSP with the indication to avoid the SRLGs of
these links. This information can be used by CN1 to compute a path
for the second LSP. If the core network consists of multiple areas,
then the SRLG-ids have to be listed in the XRO. The same example
applies to nodes and links.
Lee, et al. Standards Track PAGE 24
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Another application is where the edge-node wants to set up a backup
LSP that is also protecting the links between the edge-nodes and
core-nodes. For instance, when EN2 establishes an LSP to EN4, it
sends a Path message to CN4, which computes a path towards EN4 over
(for instance) CN5. When EN2 gets back the RRO of that LSP, it can
signal a new LSP to CN1 with EN4 as the destination and the XRO
computed based on the RRO of the first LSP. Based on this
information, CN1 can compute a path that has the requested diversity
properties (e.g., a path going over CN2 and CN3, and then to EN4).
It is clear that in these examples, the core-node may not alter the
RRO in a Resv message to make its only contents be the subobjects
from the egress core-node through the egress edge-node.
A.4. LSP Protection inside a Single Area
The XRO can also be used inside a single area. Take for instance a
network where the TE extensions of the IGPs as described in [RFC 3630]
and [RFC 3784] are not used. Hence, each node has to select a next-
hop and possibly crankback [CRANKBACK] has to be used when there is
no viable next-hop. In this case, when signaling a backup LSP, the
XRO can be put in the Path message to exclude the links, nodes, or
SRLGs of the primary LSP. An alternative way to provide this
functionality would be to indicate the following in the Path message
of the backup LSP: the primary LSP and which type of protection is
required. This latter solution would work for link and node
protection, but not for SRLG protection.
When link or node protection is requested, the XRO is of the same
length as the RRO of the primary LSP. For SRLG protection, the XRO
has to list all SRLGs that are crossed by the primary LSP. Note that
for SRLG protection, the link IP address to reference the SRLGs of
that link cannot be used since the TE extensions of the IGPs are not
used in this example. Hence, a node cannot translate any link IP
address located in that area to its SRLGs.
Lee, et al. Standards Track PAGE 25
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Authors' Addresses
Cheng-Yin Lee
EMail: c.yin.lee@gmail.com
Adrian Farrel
Old Dog Consulting
Phone: +44 (0) 1978 860944
EMail: adrian@olddog.co.uk
Stefaan De Cnodder
Alcatel-Lucent
Copernicuslaan 50
B-2018 Antwerp
Belgium
Phone: +32 3 240 85 15
EMail: stefaan.de_cnodder@alcatel-lucent.be
Lee, et al. Standards Track PAGE 26
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Full Copyright Statement
Copyright © The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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This document and the information contained herein are provided on an
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RFC TOTAL SIZE: 59569 bytes
PUBLICATION DATE: Wednesday, April 4th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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