Generic Metric for the AIGP attribute
draft-ssangli-idr-bgp-generic-metric-aigp-07
| Document | Type | Active Internet-Draft (candidate for idr WG) | |
|---|---|---|---|
| Authors | Srihari R. Sangli , Shraddha Hegde , Reshma Das , Bruno Decraene , Bin Wen , Marcin Kozak , Jie Dong , Luay Jalil , Ketan Talaulikar | ||
| Last updated | 2023-11-09 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Call For Adoption By WG Issued | |
| Document shepherd | (None) | ||
| Shepherd write-up | Show Last changed 2023-10-26 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ssangli-idr-bgp-generic-metric-aigp-07
IDR S. Sangli
Internet-Draft S. Hegde
Intended status: Standards Track R. Das
Expires: 12 May 2024 Juniper Networks Inc.
B. Decraene
Orange
B. Wen
M. Kozak
Comcast
J. Dong
Huawei
L. Jalil
Verizon
K. Talaulikar
Cisco
9 November 2023
Generic Metric for the AIGP attribute
draft-ssangli-idr-bgp-generic-metric-aigp-07
Abstract
This document defines extensions to the AIGP attribute to carry
Generic Metric sub-types. This is applicable when multiple domains
exchange BGP routing information. The extension will aid in intent-
based end-to-end path selection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 May 2024.
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Copyright Notice
Copyright (c) 2023 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 (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Multiple Metric types . . . . . . . . . . . . . . . . . . . . 4
4. Issues with RFC7311 . . . . . . . . . . . . . . . . . . . . . 5
5. Generic Metric TLV . . . . . . . . . . . . . . . . . . . . . 6
6. Usage of Generic-Metric TLV . . . . . . . . . . . . . . . . . 7
7. Updates to Decision Procedure . . . . . . . . . . . . . . . . 9
8. Use-case: Different Metrics across Domains . . . . . . . . . 10
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 12
10. Contiguity Compliance . . . . . . . . . . . . . . . . . . . . 13
11. Backward Compatibility . . . . . . . . . . . . . . . . . . . 14
12. Security Considerations . . . . . . . . . . . . . . . . . . . 14
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
15.1. Normative References . . . . . . . . . . . . . . . . . . 14
15.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
Large Networks belonging to an enterprise may consist of nodes in the
order of thousands and may span across multiple IGP domains where
each domain can run separate IGPs or levels/areas. BGP may be used
to interconnect such IGP domains, with one or more IGP domains within
an Autonomous System. The enterprise network can have multiple
Autonomous Systems and BGP may be employed to provide connectivity
between these domains. Furthermore, BGP can be used to provide
routing over a large number of such independent administrative
domains.
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The traffic types have evolved over years and operators have resorted
to defining different metric types within a IGP domain (ISIS or OSPF)
for IGP path computation. An operator may want to create an end-to-
end path that satisfy certain intent. The intent could be to create
end-to-end path that minimizes one of the metric-types. Some metrics
can be assigned administratively by an operator and they are
described in the base ISIS, OSPF specifications. Other metrics, for
example, are the Traffic Engineering Default Metric defined in
[RFC5305] and [RFC3630] , Min Unidirectional delay metric defined in
[RFC8570] and [RFC7471] . There may be other metrics such as jitter,
reliability, fiscal cost, etc. that an operator may wish to express
as the cost of a link. The procedures mentioned in the above
specifications describe the IGP path computation within IGP domains.
With the advent of 5G applications and Network Slicing applications,
an operator may wish to provision end-to-end paths across multiple
domains to cater to traffic constraints. This is also known as
intent-based inter-domain routing. The problem space and
requirements are described in [I-D.draft-hr-spring-intentaware-
routing-using-color]
The Clasful Transport Planes as described in [I-D.draft-ietf-idr-bgp-
ct] and and Color-Based Routing as described in [I-D.draft-ietf-idr-
bgp-car] describe how end-to-end intent-based paths can be
established. The proposal described in this document can be used in
conjunction with such architectures.
When multiple domains are interconnected via BGP, protocol extensions
for advertising best-external path and/or ADDPATH as described in
[RFC7911] are employed to take advantage of network connectivity thus
providing alternate paths. The Color-Based Routing and Classful
Transport Planes routing proposals describe approaches that result in
alternate paths for a reaching one destination. During the BGP best
path computation, the step(e) as per section 9.1.2.2 of [RFC4271] ,
the interior cost of a route as determined via the IGP metric value
can be used to break the tie. In a network spanning multiple IGP
domains, the AIGP TLV encoded within the AIGP attribute described in
[RFC7311] can be used to compute the AIGP-enhanced interior cost to
be used in the decision process for selecting the best path as
documented in section 2 of [RFC7311] . The [RFC7311] specifies how
AIGP TLV can carry the accumulated IGP metric value.
There is a need to synchronize the metric-type values carried between
IGP and BGP in order to avoid operational overhead of translation
between them. The existing AIGP TLV carries a TLV type and metric-
value where TLV type does not map to IGP metric-types defined in the
IGP metric-type registry. Hence there is a need to provide a generic
metric template to embed the IGP metric-type values within the AIGP
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attribute. This document extends the AIGP attribute for carrying
Generic-Metric TLV and the well-defined sub metric types. This
document also provides procedures for handling Generic-Metric during
the BGP best path computation.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Multiple Metric types
Consider the network as shown in Figure 1. The network has multiple
domains. Each domain runs a separate IGP instance. Within each
domain iBGP sessions are established between the PE routers. eBGP
sessions are established between the Border Routers across domains.
An operator wishes to compute end-to-end path optimized for a metric-
type delay. Each domain will be enabled to compute the IGP paths
based on metric-type delay. Such values should also be propagated to
the adjacent domains for effective end-to-end path computation.
| IBGP | EBGP | IBGP | EBGP | IBGP |
+----------+ +----------+ +----------+
| | | | | |
| ASBR1+--+ASBR2 ASBR3+--+ASBR4 |
| | | | | |
PE1+ Domain1 | | Domain2 | | Domain3 |
| | | | | |
| ASBR5+---+ASBR6 ASBR7+--+ASBR8 |
| | | | | |
+----------+ +----------+ +----------+
| ISIS1 | | ISIS2 | | ISIS3 |
Figure 1: WAN Network
The AIGP TLV in the AIGP attribute as specified in [RFC7311] supports
the default IGP-metric. If all domains use default IGP-metric cost,
then one can compute the end-to-end path with shortest default IGP-
metric cost. However if an operator wishes to compute the end-to-end
path with metric other than IGP cost, we need additional extensions
to the AIGP attribute for carry the metric-types and metric values.
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The [I-D.ietf-lsr-flex-algo-bw-con] proposes a generic metric type
that can embed multiple metric types within it. It supports both
standard metric-types and user-defined metric-types. This document
leverages the generic-metric draft and proposes extensions to the
AIGP attribute to carry Generic Metric TLV as specified below.
4. Issues with RFC7311
The following procedures are not clearly described in [RFC7311] .
* The section 3 describes "When an AIGP attribute is created, it
SHOULD contain no more than one AIGP TLV. However, if it contains
more than one AIGP TLV, only the first one is used as described in
Sections 3.4 and 4. In the remainder of this document, we will
use the term value of the AIGP TLV to mean the value of the first
AIGP TLV in the AIGP attribute. Any other AIGP TLVs in the AIGP
attribute MUST be passed along unchanged if the AIGP attribute is
passed along."
* ....One MUST interpret that more than one TLV of a particular type
(i.e. AIGP TLV metric-type 1) can be present in the update and
only the first occurance MUST be analysed. All other TLVs (type 2
or type 3 etc.) MUST be passed along unchanged if AIGP attribute
is passed along.
* The section 3.2 describes "Note that an AIGP attribute MUST NOT be
considered to be malformed because it contains more than one TLV
of a given type or because it contains TLVs of unknown types."
* ....One MUST interpret that opaque TLVs (TLVs with type 2 or type
3 for example) MUST be passed along if ADVERTISE_AIGP_ATTRIBUTE
has been enabled to a neighbor.
* Section 3.3 describes "The AIGP attribute MUST NOT be sent on any
BGP session for which AIGP_SESSION is disabled."
* ....While maintaining the non-transitivity is important, it is
also important to provide accumulated cost end-to-end across
domains. If there are more than one TLVs in the AIGP attribute,
it becomes important to define the behaviour of which TLV gets
updated and sent across domains.
* The rules for route redistribution is not clearly described.
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* ....When a BGP route is redistributed, should AIGP metric-value be
used directly as the cost in IGP or should there be a policy to
modify AIGP metric-value before redistributing the route into IGP.
It is important to define the behaviour of route redistribution
metric conversion when redistribution occurs on multiple domains
along the path.
5. Generic Metric TLV
This document proposes a new TLV : Generic-Metric TLV in the AIGP
attribute. This will carry the metric type and metric value used in
the network. The format is shown below.
0 1 2 3
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | metric-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| metric-flags| metric-value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+..........................
Figure 2: Generic-Metric TLV
Generic-Metric TLV Type (1 octet): Code point to be assigned by
IANA
Generic-Metric TLV Length (2 octets): Value 10
Generic-Metric TLV Value (10 octets): 3 sub-fields as shown below:
1. metric-type (1 octet): Value of metric-types from IGP Protocol
registry.
2. metric-flags (1 octet): Bits defined below.
3. metric-value (8 octets): Value range (0 - 0xffffffffffffffff)
The metric-flags carry additional information about the Generic-
Metric.
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|R|R|R|R|R|R|N|I|
+-+-+-+-+-+-+-+-+
Figure 3: Generic-Metric Flags
Bit I : Represents incomplete/discontinuous metric accumulation
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for the end-to-end path. 1 indicates discontinuous, 0 indicates
continuous.
Bit N : Represents normalization. 1 indicates metric normalization
has been applied. 0 indicates no normalization has been applied.
Bit R : Reserved for future use. Reset to zero by the sender and
ignored by the receiver.
6. Usage of Generic-Metric TLV
1. When a BGP speaker wishes to generate AIGP attribute with
Generic-Metric TLV for a prefix, it MUST perform the following
procedures.
- The procedures specified in [RFC7311] section 3.4 should be
followed that describes creation of attribute, modifications by
the originator and non-originator of the route in addition to
the following procedures.
- The domain can adopt more than one metric type to represent the
intent, hence the originator BGP speaker can encode more than
one Generic-Metric TLV, each TLV carrying different metric type
as defined in the IGP Protocol Registry.
- The type of metric used in the local domain and as specified in
the IGP Protocol registry must be encoded in the metric-type
sub-field. The value of the metric or cost to reach the prefix
being advertised must be encoded in the metric-value sub-field,
normalized if required. This is the cost or the distance to
the destination prefix from the advertising BGP speaker which
sets itself as the next hop as described in section 3.4 of
[RFC7311].
- Repeated metric changes may cause large number of BGP updates
to get generated and be propagated throughout the network. In
order to avoid that, a configurable threshold is defined. If
the difference between the new metric-value and the advertised
metric-value is less than the configured threshold, the update
MAY be suppressed. For each of type of metric used in the
domain, if the new metric-value encoded in Generic-Metric TLV
is above the configured threshold, a new BGP update containing
the new set of metric-values SHOULD be advertised.
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- The "I" bit of the metric-flags MUST be reset to zero if the
BGP speaker is the originator of the AIGP attribute. If the
IGP cost to reach the next hop is normalized to the type of the
metric in the metric-type sub-field, the "N" bit of the metric-
flags sub-field MUST be set to 1, else it MUST be reset to
zero.
- Procedures for defining the cost to reach a next hop for
various metric-types is outside the scope of this document.
2. When a BGP speaker wishes to send a BGP update attaching the
AIGP attribute, it must validate if that session has been enabled
for sending the AIGP attribute as per procedures mentioned in
[RFC7311] .
3. When a BGP speaker receives a BGP update that has a route to T
with next hop N and has the AIGP attribute with Generic-Metric TLV
it MUST perform the following procedures.
- It must validate if that session has been enabled to receive
the AIGP attribute as per rules mentioned in [RFC7311] .
- There can be more than one Generic-Metric TLV, each carrying
different metric types. The BGP speaker must process every
Generic-Metric TLV.
- For each of the Generic-Metric TLVs present in the AIGP
attribute, if the BGP speaker recognizes the type of the metric
encoded in the metric-type sub-field, it must process the
metric-value and metric-field sub-fields of the Generic-Metric
TLV.
- If the BGP speaker does not recognize the type of metric
encoded in metric-type subfield of the TLV, then it must set
the "I" bit in the metric-flags to 1 before propagating to
other BGP speakers and must continue to process the next
Generic-Metric TLV if present. If the BGP speaker does not
recognize any metric-type in the Generic-Metric TLVs, it must
follow the BGP decision procedure as specified in [RFC7311].
- If the type of the metric for resolving the next hop N matches
with the metric-type of Generic-Metric TLV of the AIGP
attribute, then the metric-value sub-field must be used in the
AIGP-enhanced interior cost computation as specified in the
next section.
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- If the metric-type of the path used for resolving the next hop
N does not match with the metric-type of Generic-Metric TLV of
the AIGP attribute, then the BGP speaker may normalize the cost
of the path used for resolving the next hop before using it in
the AIGP-enhanced cost computation. A policy may be used to
provide the metric normalization. Additionally, the BGP
speaker must set the "N" bit to indicate that metric
normalization has been done before propagating the Generic-
Metric TLV to other BGP speakers.
- If the BGP speaker modifies the next hop it must update the
Generic-Metric TLV(s).
7. Updates to Decision Procedure
This section follows the approach as laid out in [RFC7311] to select
the best path when the route has AIGP attribute with Generic-Metric
TLV. The domain that the router R belongs to, may support different
intent based paths represented via different types of metric. The
following describes procedures in addition to the general procedure
described in section 4 of [RFC7311] .
When R receives a route T with next hop N and the AIGP attribute with
one or more Generic-Metric TLVs, for each Generic-Metric TLV the BGP
speaker MUST perform following procedures.
If the metric-type sub-field matches with the type of the metric for
the path used for resolving the next hop N, the AIGP-enhanced
interior cost should be computed as below.
Let m be the cost to reach the next hop N that IGP uses for its
path computation as described in [RFC7311] .
If the type of the metric for the path used for resolving the next
hop N does not match with the metric-type sub-field of the Generic-
Metric TLV, the cost of the path to reach next hop N may be
normalized. The normalized metric value can be zero, maximum metric
value or scaled up (multiple of a positive number).
Let m be the normalized value of the cost to reach the next hop N
that IGP uses for its path computation as described in [RFC7311] .
The AIGP-enhanced interior cost computation as described below will
be used in the decision process as described in [RFC7311] .
Let A be the value of the value of the metric-value sub-field of
the Generic-Metric TLV.
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The AIGP-enhanced interior cost will be A+m as described in
[RFC7311] .
A path with Generic-Metric TLV and a path with AIGP TLV cannot be
compared. To enable end-to-end path selection based on intent, the
path with Generic-Metric TLV MUST be chosen over path with AIGP TLV.
The implementation should allow a local policy to specify the
preference.
A path with Generic-Metric TLV of metric-type 'a' cannot be compared
with a path with Generic-Metric TLV of metric-type 'b'. The path
with lower metric-type MAY be chosen as best between two paths with
Generic-Metric TLV and implemented consistently across AIGP domain.
8. Use-case: Different Metrics across Domains
+--------------+
| Domain2 |
| |
......+ASBR21 ASBR22+.....
. | | .
+------------+ . | IGP-metric | . +--------------+
| Domain1 | . +--------------+ . | Domain4 |
| | . . | |
| ASBR11+... ...+ASBR41 |
+PE1 | | PE2+
| ASBR12+... ...+ASBR42 |
| | . . | |
| IGP-metric | . . | delay-metric |
+------------+ . . +--------------+
. +--------------+ .
. | Domain3 | .
. | | .
......+ASBR31 ASBR32+.....
| |
| delay-metric |
+--------------+
Figure 4: Different metric across network
Each domain is a separate Autonomous System. Within each domain,
ASBR and PE form iBGP peering and they may employ Route Reflectors.
The IGP within each domain uses domain specific metric. Domain3 and
Domain4 use delay as the metric while Domain1 and Domain2 use default
IGP-metric cost. ASBRs across domains form eBGP peering.
Scenario 1: Find delay-based end-to-end path from Domain1 to Domain4.
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This can be achieved by the advertising router to add the AIGP
attribute with metric type 1 that represents delay metric. In the
above network diagram, ASBR41 (and ASBR42) will advertise prefix
PE2-loopback with Generic-Metric TLV with delay as metric-type.
The metric-value sub-field of the Generic-Metric TLV will
represent the cost to reach PE2's loopback end-point from the
advertising router as they will do next hop self.
In Domain3, when ASRB32 advertises the prefix PE2-loopback within
the local domain, it may add cost to the metric-value, the value
representing the delay introduced by the DMZ link between ASRB32
to ASBR42. When ASRBR31 advertises the prefix PE2-lookback, it
will perform the following procedures.
1. Compute the delay d of the path to reach ASBR32 from which it
has chosen the best path.
2. Add the above d value to the metric-value sub-field of the
Generic-Metric TLV.
In Domain2 however, the local metric type is default IGP-metric.
The ASBR22 may follow the procedure similar to ASBR32 and add the
delay value corresponding to the DMZ link between ASBR22 and
ASBR41 before advertising the path internally in Domain2. When
ASBR21 computes the AIGP-enhanced interior cost, as mentioned
before, it may normalize the igp cost to reach ASBR22 and may add
the normalized value to the delay-metric. The ASBR21 will also
update metric-flags sub-field to indicate that metric value has
been normalized. In the above network example, the delay cost
from ASBR21 to ASBR22 is negligible and hence delay-metric value
will be unchanged.
The procedures for AIGP-enhanced interior cost computation at
ASBR11 (and ASBR12) will follow DMZ delay computation procedure
described above. PE1 will have two paths to reach PE2-loopback:
P1 via ASBR11 (and domain2) and P2 via ASBR12 (and domain3), each
having respective AIGP-enhanced interior cost representing end-to-
end delay. The local metric type is default IGP-metric and hence
PE1 may normalize the internal igp cost for the AIGP-enhanced
interior cost computation. The BGP decision process described in
Section 7 will result in delay optimized end-to-end path for
PE2-loopback on PE1 that can be used to resolve the service
prefixes.
Scenario 2: Provide best-effort or default IGP-metric based end-to-
end path while leveraging the domain-specific delay-based metric for
intra-domain path selection.
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All the ASBR routers will update the Generic Metric TLV for the
default IGP-metric metric-type, accumulating the cost for end-to-
end path. PE1 router will have two paths (from ASBR11 and ASBR12)
decorated with different best-effort default IGP-metric cost. The
intra-domain path to reach the domain exit can be based on domain-
specific metric-type. For example, in Domain3, ASBR31 can select
lowest delay path to reach ASBR32. The ASBR and the PE routers
may be configured to prefer one metric-type for end-to-end path
while another metric-type for intra-domain and such configuration
mechanism is outside the scope of this document.
Scenario 3: Path selection when a router along the path does not
support the new type of metric.
The Domain2 implements only default IGP-metric and does not
support delay-metric. When ASBR21 receives the route with AIGP
attribute and the Generic-Metric TLV, the metric type delay-metric
is unrecognized. The ASBR21 will update the metric-flags, setting
the "I" bit to 1 indicating that accumulation is incomplete. When
such a route reaches PE1, the PE1 router will have two paths, one
via ASBR11 with "I" bit set and another path from ASBR12 with "I"
bit reset to zero. The local policy on PE1 can provide guidance
on the preference between these two paths.
9. Deployment Considerations
It can be noted that a domain may normalize the metric-value of the
metric-type of the path used to resolve next hop to the metric-type
present in the Generic-Metric TLV. The idea is to propagate the cost
of reaching the prefix through the domain while maintaining the
metric-type chosen by the originating router and domain thereby
providing an end-to-end path for the desired intent. The
normalization of metric types to the one carried in the AIGP
attribute can be done via policy. Definition of such policies and
how they can be enforced is outside the scope of this document. In
topologies where there is a common router between adjacent domains
that do iBGP peering, the Border router can provide the
normalization.
It is important to maintain the property of IGP cost to a destination
decrease as one gets closer to the destination. The AIGP-enhanced
interior cost should not be allowed to decrease through the metric
normalization. When adjacent domains use different metric types, the
ASBR that connects two domains is better suited to pass on the metric
values by setting itself as next hop.
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All routers of a domain MUST compute the AIGP-enhanced interior cost
as described above to be used during decision process. Within a
domain, if one router R1 applies AIGP-enhanced interior cost while R2
does not, it may lead to routing loop unless some sort of tunnelling
technology viz MPLS, SRv6, IP, etc. is adopted to reach the next hop.
In a network where any tunnelling technology is used, one can
incrementally deploy the Generic-Metric functionality. In a network
without any tunnelling technology, it is recommended that all routers
MUST support Generic-Metric based AIGP-enhanced interior cost
computation.
In certain networks, routes may be redistributed between BGP and IGP,
usually controlled via a policy. When a route is propagated across
domains, a router should use AIGP metric-value of Generic-Metric TLV,
optionally modified via the local policy as the IGP cost during route
redistribution in to IGP. The local policy should apply metric
normalization or translation based on metric-type of Generic-Metric
TLV and the metric-type adopted in the IGP.
10. Contiguity Compliance
AIGP attribute is optional and non-transitive, however new TLV might
not be interpreted and/or updated by routers along the path. The
contiguity of the AIGP domain across multiple IGP or AS domains is
important to maintain end-to-end path of a certain intent. All the
BGP routers along the path that modify the next hop should accumulate
the cost and propagate the accumualated cost in the AIGP attribute.
For calculating the end-to-end path for an intent expressed via a
type of metric, all such routers MUST support the Generic-Metric
handling for that type of metric and intent. This will assure the
correct end-to-end path for the intent and the metric.
If a router along the path did not recognize a certain type of
metric present in the Generic-Metric TLV, from the "I" bit of the
metric-flags, the receiving router can infer that metric
accumulation is not complete and appropriate decision can be taken
during the best path computation.
If a router along the path did not support Generic-Metric TLV and
yet propagated the AIGP attribute, the metric-flags would not
indicate the discontiguity. It is recommended that operators
identify such routers and upgrade them to support Generic-Metric
TLV and it would bring in determinism.
If a router along the path did not support Generic-Metric TLV and
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chose to drop the AIGP attribute, the receiving router will not be
able to compute end-to-end path for the desired intent and metric
type. Identifying such routers and upgrading them to support
Generic-Metric TLV would deliver the desired results.
11. Backward Compatibility
When a BGP speaker receives an update with the AIGP attribute it may
have Generic-Metric TLV. If the BGP speaker understands the AIGP
attribute but does not understand the Generic-Metric TLV, it will
process the AIGP attribute as per [RFC7311] . However when it needs
to advertise the prefix to its peers it will pass on the AIGP
attribute with all the TLVs including the unknown Generic-Metric TLV
as per [RFC7311] . If a BGP speaker does not understand the Generic-
Metric TLV, it may chose sub-optimal BGP path.
12. Security Considerations
This document does not introduce any new security considerations
beyond those already specified in [RFC4271], [RFC7311] .
13. IANA Considerations
IANA is requested to assign a code point for Generic Metric TLV. The
metric-type field refers to the IGP metric-type registry defined in
[I-D.ietf-lsr-flex-algo-bw-con]
14. Acknowledgements
The authors would like to thank John Scudder, Jeff Haas, Robert
Raszuk, Kaliraj Vairavakkalai, and Peng Shaofu for careful review and
suggestions.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
15.2. Informative References
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[I-D.hr-spring-intentaware-routing-using-color]
Hegde, S., Rao, D., Uttaro, J., Bogdanov, A., and L.
Jalil, "Problem statement for Inter-domain Intent-aware
Routing using Color", Work in Progress, Internet-Draft,
draft-hr-spring-intentaware-routing-using-color-03, 23
October 2023, <https://datatracker.ietf.org/doc/html/
draft-hr-spring-intentaware-routing-using-color-03>.
[I-D.ietf-idr-bgp-car]
Rao, D., Agrawal, S., and Co-authors, "BGP Color-Aware
Routing (CAR)", Work in Progress, Internet-Draft, draft-
ietf-idr-bgp-car-03, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
car-03>.
[I-D.ietf-idr-bgp-ct]
Vairavakkalai, K. and N. Venkataraman, "BGP Classful
Transport Planes", Work in Progress, Internet-Draft,
draft-ietf-idr-bgp-ct-18, 5 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ct-18>.
[I-D.ietf-lsr-flex-algo-bw-con]
Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak,
P., and T. Li, "Flexible Algorithms: Bandwidth, Delay,
Metrics and Constraints", Work in Progress, Internet-
Draft, draft-ietf-lsr-flex-algo-bw-con-07, 26 September
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
lsr-flex-algo-bw-con-07>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC7311] Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro,
"The Accumulated IGP Metric Attribute for BGP", RFC 7311,
DOI 10.17487/RFC7311, August 2014,
<https://www.rfc-editor.org/info/rfc7311>.
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[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.
Authors' Addresses
Srihari Sangli
Juniper Networks Inc.
Exora Business Park
Bangalore, KA 560103
India
Email: ssangli@juniper.net
Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore, KA 560103
India
Email: shraddha@juniper.net
Reshma Das
Juniper Networks Inc.
1133 Innovation Way
Sunnyvale, CA 94089
USA
Email: dreshma@juniper.net
Bruno Decraene
Orange
France
Email: bruno.decraene@orange.com
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Bin Wen
Comcast
USA
Email: bin_wen@comcast.com
Marcin Kozak
Comcast
USA
Email: marcin_kozak@comcast.com
Jie Dong
Huawei
China
Email: jie_dong@huawei.com
Luay Jalil
Verizon
USA
Email: luay.jalil@verizon.com
Ketan Talaulikar
Cisco
India
Email: ketant.ietf@gmail.com
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