| Internet-Draft | Accumulated Metric in NHC attribute | August 2024 |
| Sangli, et al. | Expires 3 March 2025 | [Page] |
RFC7311 describes mechanism for carrying accumulated IGP cost across BGP domains however it limits to IGP-metric only. There is a need to accumulate and propagate different types of metrics as it will aid in intent-based end-to-end path across BGP domains. This document defines BGP extensions for Generic Metric sub-types that enable different types of metrics to be accumulated and carried in BGP. This is applicable when multiple domains exchange BGP routing information.¶
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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 many such independent administrative domains.¶
The traffic types have evolved over years and operators have resorted to defining different types of metrics 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. These metrics can be assigned administratively by an operator. While some are described in the base ISIS, OSPF specifications, other metrics are the Traffic Engineering Default Metric defined in [RFC5305] and [RFC3630], Min Unidirectional delay metric defined in [RFC8570] and [RFC7471]. There may be other parameters such as jitter, reliability, fiscal cost, etc. that an operator may incorporate while computing the cost of a link. The procedures mentioned in the above specifications describe the IGP path computation within IGP domains.¶
For computing the best path for a BGP route in such a domain, the step(e) of the section 9.1.2.2 of [RFC4271] specifies that the interior cost of a route as determined via the IGP metric value is to be used to break the tie among multiple paths. 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. For each route that is advertised, the IGP cost of the end-to-end path is accumulated and encoded in the AIGP attribute as described in [RFC7311]. This can be used to compute the AIGP-enhanced interior cost and it will be used in the decision process for selecting the best path as documented in section 2 of [RFC7311] . The [RFC7311] specifies how AIGP attribute can carry the accumulated IGP metric value. However, [RFC7311] describes only one TLV (AIGP TLV) in the AIGP attribute to carry the IGP cost. Most of the implementations available today encode IGP-metric metric type in the AIGP TLV. See section Section 15 for different interpretations of [RFC7311] that are deployed today.¶
With the advent of 5G applications and Network Slicing applications, for catering to the various traffic constraints, an operator may wish to provision end-to-end paths across multiple domains satisfying required intents. This is also known as intent-based inter-domain routing. The description of the problem space and requirements can be found in [I-D.draft-hr-spring-intentaware-routing-using-color]. The Clasful Transport Planes as described in [I-D.draft-ietf-idr-bgp-classful-transport-planes] and and Color-Based Routing as described in [I-D.draft-ietf-idr-bgp-car] describe how intent-based end-to-end paths can be established. The proposal described in this document can be used in conjunction with such architectures.¶
If the type of metric used in a IGP domain differs from the accumulated metric type carried in BGP, the metric values should be synchronized and translated between IGP domain and BGP. The metric-type and metric-value in the AIGP TLV does not support different IGP metric-types defined in the IGP-Protocol registry for metric-types. Hence there is a need to provide a generic metric TLV template to embed the different types of IGP metrics and their values in BGP.¶
This document proposes "Accumulated Metric" TLV in the Next-Hop Dependent Capability (NHC) attribute described in [I-D.ietf-idr-entropy-label] to carry the accumulated metric value for end-to-end path, hereby referred as AMetric. The AMetric supports all the metric types defined in the IGP-Parameters metric-type registry. Additionally this document provides procedures for computation and usage of accumulated generic metric value during the BGP best path computation.¶
[RFC7311] introduces the notion that a set of ASes can be under a common administrative domain. This document borrows the same concept, "AMetric administrative domain" to refer to ASes under a common administration within which an operator wishes to establish any intent-based end-to-end path.¶
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.¶
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. The eBGP sessions are established between the Border Routers across domains.¶
| IBGP | EBGP | IBGP | EBGP | IBGP |
+----------+ +----------+ +----------+
| | | | | |
| ASBR1+--+ASBR2 ASBR3+--+ASBR4 |
| | | | | |
PE1+ Domain1 | | Domain2 | | Domain3 |
| | | | | |
| ASBR5+---+ASBR6 ASBR7+--+ASBR8 |
| | | | | |
+----------+ +----------+ +----------+
| ISIS1 | | ISIS2 | | ISIS3 |
An operator wishes to compute end-to-end path optimized for "delay" metric-type. Each domain will be enabled for computation of the IGP paths based on delay metric type. As a result, the intra-domain reachability will be based on delay metric. Such values should also be propagated to the adjacent domains for effective end-to-end path computation. However, the AIGP TLV in the AIGP attribute as specified in [RFC7311] supports only the default IGP-metric. As a result, with AIGP TLV, only default IGP-metric based end-to-end path can be computed and this will not address the operator requirements.¶
The [I-D.ietf-lsr-flex-algo-bw-con] proposes extension in ISIS and OSPF, 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 describes extensions in BGP to support such metric types. To compute the end-to-end path with metric other than default IGP-metric cost, the new metric TLV for NHC that this document proposes will enable different types of metrics and their values to be accumulated and propagated across the domains.¶
For determining the end-to-end path for an intent and to derive the accurate cumulative cost, all routers along the path that modify the next hop should participate in cumulating the cost. New applications may require new intents that may result in new metric types to be used in the network. It is quite possible that certain domains may have specific metric types. For example, core network may have latency metric while metro may just have IGP-cost because delay in metro regions may be insignificant. It is quite possible that one or more routers might not understand the new intent and the metric.¶
If one or more routers in a domain either border routers or any intra-domain routers that modify the next hop, do not participate in accumulating the cost when they propagate a route, it is impossible for the ingress router to determine the cost of the end-to-end path accurately. This will result in sub-optimal best path selection. Such an end-to-end path is called discontinuous path for that intent. The discontinuity can manifest in different dimensions and Section 12 provides detailed explanation.¶
This document proposes "Accumulated Metric" TLV (AMetric), in the Next Hop Dependent Capability (NHC) attribute. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Accumulated Metric Code | Accumulated Metric Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Accumulated Metric Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The AMetric data carries the metric type and metric value as defined in the IGP-Protocol registry for metric-types. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric-type | Metric-flags | Metric value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The metric-flags carry additional information about the accumulated generic metric.¶
7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|R|R|R|R|R|R|N|D|
+-+-+-+-+-+-+-+-+
Bit D : Represents discontinuity in metric accumulation for the end-to-end path. 1 indicates discontinuous, 0 indicates continuous.¶
Bit N : Represents normalization of metric in the local domain. 1 indicates metric normalization has been applied. 0 indicates no normalization has been applied.¶
Bit R : Reserved for future use. MUST be set to zero when originated, and MUST be ignored on receipt.¶
The Next Hop Dependent Capability attribute has two important characteristics that are relevant for establishing the end-to-end intent path: Transitivity and Scoping.¶
Transitivity: The NHC is an optional and transitive attribute hence according to [RFC4271] if this attribute is present in an update message, it will be propagated to all neighbors. Via the AMetric in NHC attribute, the accumulated metric value is propagated to all the routers in the network, thus making the cost computation for the end-to-end intent path possible.¶
Scoping: The NHC provides an ability to perform Next Hop scoping. The originating router encodes the next hop address in the "Network Address of Next Hop" field in the NHC attribute before advertising the route. If the non-originator router upon learning such a route, does not modify the next hop, it will advertise the route along with capabilities in NHC without modification. On the other hand, if the non-originator router upon learning such a route, modifies the next hop, it will perform two actions: Encode the next hop address (it is going to advertise in the update), in "Network Address of Next Hop" field of the NHC attribute; and recompute the capabilities and encode them in the NHC attribute, before advertising the route. With this support, the receiving BGP speaker can make specific decisions for the route by comparing the advertised next hop present in the update message with the "Network Address of Next Hop" field of the NHC attribute. If the next hop in the update message does not match with the "Network Address of Next Hop" field in the NHC attribute, it can be concluded that the advertising router did not understand the NHC attribute and as a result, the capabilities have not been updated at the advertising router.¶
The above mechanism is leveraged to determine if the end-to-end path for an intent (represented by a metric in AMetric) is discontinuous or not. The document [I-D.ietf-idr-entropy-label] provides detailed explanation on the NHC procedures.¶
The AIGP TLV described in [RFC7311] carries IGP metric type only. The AIGP TLV is encoded in AIGP attribute, an optional and non-transitive attribute. The BGP speaker S may attach AIGP attribute with AIGP TLV in it, to a route R and advertise to its peers. When such a route propagates across domains, the metric value in AIGP TLV is accumulated thus providing end-to-end IGP cost.¶
The AMetric is like AIGP TLV. The AMetric can carry not just the IGP metric type but also other types of metrics. The AMetric will be encoded in the NHC attribute. The BGP speaker S may attach NHC attribute with AMetric in it to a route R and advertise R to its peers. When such a route propagates across domain, the metric value will be accumulated. This provides the end-to-end cost for desired intent as represented by one or more metric types.¶
The section 3.4 of [RFC7311] describe procedures for creating and modifying the AIGP TLV in the AIGP attribute and these are also applicable for creating and modifying the AMetric in the NHC attribute.¶
This section follows the approach as laid out in [RFC7311] to select the best path when the route has NHC attribute with AMetric. The domain that the BGP speaker S belongs to, may support different intent-based paths represented via different types of metric to reach next hop N. The following procedures MUST be followed in addition to the general procedure described in section 4 of [RFC7311].¶
Consider a BGP speaker S receiving a route R with next hop N. The route R is attached with NHC attribute having AMetric. For each AMetric, the BGP speaker S MUST perform following procedures.¶
If the metric-type sub-field of AMetric matches with the type of the metric used in the local domain for resolving the next hop N, the AIGP-enhanced interior cost should be computed as below.¶
If the metric-type sub-field of the AMetric does not match with the type of the metric used in the local domain for resolving the next hop N, 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).¶
The AIGP-enhanced interior cost computation as described below will be used in the decision process as described in [RFC7311].¶
A path with IGP metric-type in AMetric of NHC attribute and a path with IGP metric-type in AIGP TLV of AIGP attribute can be compared. However, a path with AMetric carrying different metric-type and a path with AIGP TLV carrying IGP metric-type cannot be compared. To enable end-to-end path selection based on intent, the path with AMetric in NHC attribute may be chosen over path with AIGP TLV in AIGP attribute. The implementation should allow a local policy to specify these preferences.¶
A path with AMetric of metric-type 'a' cannot be compared with a path with AMetric of metric-type 'b'. The path with lower metric-type MAY be chosen as best between two such paths and implemented consistently across AIGP domain.¶
+--------------+
| Domain2 |
| |
......+ASBR21 ASBR22+.....
. | | .
+------------+ . | IGP-metric | . +--------------+
| Domain1 | . +--------------+ . | Domain4 |
| | . . | |
| ASBR11+... ...+ASBR41 |
+PE1 | | PE2+
| ASBR12+... ...+ASBR42 |
| | . . | |
| IGP-metric | . . | delay-metric |
+------------+ . . +--------------+
. +--------------+ .
. | Domain3 | .
. | | .
......+ASBR31 ASBR32+.....
| |
| delay-metric |
+--------------+
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.¶
It can be noted that for a path, a domain may normalize the metric-value used to resolve next hop to match with the metric-type present in the AMetric. 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. Such normalization of metric values to the match with the metric type present in the AMetric(s) can be done via policy. The 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.¶
In a domain, the cost of a path is derived from the metric of its links, summed up typically. It is important to maintain the same behavior for inter-domain path. 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.¶
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 tunneling technology viz. MPLS, SRv6, IP, etc. is adopted to reach the next hop. In a network where any tunneling technology is used, one can incrementally deploy the generic metric functionality. In a network without any tunneling technology, it is recommended that all routers MUST support Accumulated Generic-Metric based AIGP-enhanced interior cost computation. Additionally, to have consistent BGP best path in the network, all routers should use the accumulated cost during the best path computation. To ease the deployment of this generic metric based end-to-end path selection, it is recommended to enable AMetric via configuration and should be disabled by default.¶
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 the metric-value in the AMetric of the NHC attribute, optionally modified via the local policy as the IGP cost during route redistribution into IGP. The local policy should apply metric normalization or translation based on metric-type of AMetric and the metric-type adopted in the local domain.¶
The network operators would like to avoid the scenarios when the entire network has to be upgraded before enabling the new functionality. New functionality across a network is typically deployed incrementally and one cannot expect that all routers shall be capable of handling new functionality on day-one. However, for determining the end-to-end path for an intent and to derive the accurate cumulative cost, all routers along the path that modify the next hop should participate in accumulating the cost. The discontinuity can manifest in three different forms.¶
The AIGP TLV is used for accumulating the metric across domains. The AIGP attribute is optional and non-transitive. The accumulated metric is used in best path computation. The AIGP mechanism requires all the routers MUST understand the new attribute so that accumulated metric will reach all the routers for consistent best path computation. Hence, if an advertising router along the path does not understand the AIGP attribute, the accumulation will not be complete making the path discontinuous. The receiving (or ingress) router will not be able to compute the end-to-end cost and so the path will not be considered for providing end-to-end intent.¶
On the other hand, when the ingress router receives a route with AIGP attribute, the value accumulated in the TLV is guaranteed to reflect the end-to-end cost. Therefore First-Order discontinuity does not exist. The [RFC7311] introduced both AIGP attribute and AIGP TLV together, most of BGP routers support AIGP TLV along with the attribute. Hence Type-B discontinuity is less likely to happen. The [RFC7311] specifies only default IGP-metric in AIGP TLV, hence Type-C discontinuity is not applicable. If [RFC7311] was extended to support generic metric types, it will suffer from Type-C discontinuity.¶
The AMetric is part of NHC which is an optional and transitive attribute. The routes with NHC attribute and accumulated metric reaches all the routers across the domains and it will result in consistent best path computation. If any advertising router along the path does not understand the NHC attribute, it fails to update the next hop field in NHC attribute. This is the Type-A discontinuity. However, with the NHC procedures, the receiving router will detect this through next hop validation thus providing mechanism to detect the Type-A discontinuity deterministically.¶
If the advertising router along the path supports NHC attribute, but does not support AMetric, following NHC procedures, the router will not propagate the accumulated metric. This is the Type-B discontinuity but this will not result non-deterministic behaviour the receiving BGP router will not select the path for desired intent given the lack of AMetric. If the advertising router understands NHC attribute and AMetric, but does not understand the specific metric-type, by following the NHC procedures, the router will still propagate NHC attribute and AIGPv2 even though unrecognized metric is not accumulated. This is Type-C discontinuity. The procedures in this document specifies the "D" bit of the unrecognized AIGPv2 to be set to 1 by the advertising router and hence the receiving router deterministically identifies the discontinuity.¶
Even though NHC attribute is transitive, the AMetric might not be interpreted and/or updated by routers along the path. If all BGP routers that modify the next hop accumulate the cost and propagate the metric, the receiving BGP router will be assured of a correct end-to-end path for the intent and the metric. Although the three types of discontinuity can be addressed using a local policy, it is recommended that operators identify such routers and upgrade them to achieve intent-based end-to-end path for optimal results.¶
This document does not introduce any new security considerations beyond those already specified in [RFC4271], [RFC7311] and [I-D.ietf-idr-entropy-label].¶
IANA is requested to assign a code point for AMetric. The metric-type field refers to the IGP-Protocols registry for metric-type defined in [I-D.ietf-lsr-flex-algo-bw-con]¶
This section provides an overview of limitations and different interpretations of [RFC7311]. Various vendors have interpreted [RFC7311] differently and encode AIGP TLV or treat AIGP attribute differently. The following lists some of them.¶
The authors would like to thank John Scudder, Jeff Haas, Robert Raszuk, Kaliraj Vairavakkalai, and Peng Shaofu for careful review and suggestions.¶