]> Orange

bruno.decraene@orange.com

Juniper Networks

jgs@juniper.net

Nokia

wim.henderickx@nokia.com

Juniper Networks

kireeti@juniper.net

Cisco Systems

satyamoh@cisco.com

Independent Contributor

juttaro@ieee.org

Comcast

Bin_Wen@comcast.com

rtg Internet Engineering Task Force bgp rca entropy label RFC 5492 allows a BGP speaker to advertise its capabilities to its peer. When a route is propagated beyond the immediate peer, it is useful to allow certain capabilities to be conveyed further. In particular, it is useful to advertise forwarding plane features. This specification defines a new BGP transitive attribute to carry such capability information, the "Router Capabilities Attribute," or RCA. This specification also defines an RCA capability that can be used to advertise the ability to process the MPLS Entropy Label as an egress LSR for all NLRI advertised in the BGP UPDATE. It updates RFC 6790 and RFC 7447 concerning this BGP signaling.

Introduction allows a BGP speaker to advertise its capabilities to its peer. When a route is propagated beyond the immediate peer, it is useful to allow certain capabilities to be conveyed further. In particular, it may be useful to advertise forwarding plane features. This specification defines a new BGP optional transitive attribute to carry such capability information, the "Router Capabilities Attribute", or RCA. (This somewhat ponderous name is regrettable but is needed in order to be descriptive while still distinguishing it from RFC 5492 BGP Capabilities.) Since the RCA is intended chiefly for conveying information about forwarding plane features, it needs to be regenerated whenever the BGP route's next hop is changed. Since owing to the properties of BGP transitive attributes this can't be guaranteed (an intermediate router that doesn't implement this specification would be expected to propagate the RCA as opaque data), the RCA identifies itself with the next hop of its originator. If the RCA passes through a router that changes the next hop without regenerating the RCA, they will fail to match when later examined, and the recipient can act accordingly. This scheme allows RCA support to be introduced into a network incrementally. Complete details are provided in . An RCA carried in a given BGP UPDATE message conveys information that relates to all NLRI advertised in that particular UPDATE, and only to those NLRI. A different UPDATE message originated by the same source might not include an RCA, and if so, NLRI carried in that UPDATE would not be affected by the RCA. By implication, if a router wishes to use RCA to describe all NLRI it originates, it needs to include an RCA with each UPDATE it sends. In this respect, despite its similar naming, the RCA is unlike RFC 5492 BGP Capabilities. This specification also defines an RCA to advertise the ability to process the MPLS Entropy Label as an egress LSR for all NLRI advertised in the BGP UPDATE. It updates and with regard to this BGP signaling, this is further discussed in .

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 when, and only when, they appear in all capitals, as shown here.

BGP Router Capabilities Attribute

Encoding The BGP Router Capabilities attribute (RCA attribute, or just RCA) is an optional, transitive BGP path attribute with type code 39. The RCA has as its data a network layer address, representing the next hop of the route the RCA accompanies. The RCA signals potentially useful optimizations, so it is desirable to make it transitive; the next hop data is to ensure correctness if it traverses BGP speakers that do not understand the RCA. The Attribute Data field of the RCA attribute is encoded as a header portion that identifies the originator of the attribute, followed by one or more capability TLVs.

RCA Format

The meanings of the header fields (Address Family Identifier, SAFI or Subsequent Address Family Identifier, Length of Next Hop, and Network Address of Next Hop) are as given in Section 3 of . In turn, each Capability is a triple (Capability Code, Capability Length, Capability Value):

Capability TLV Format

Capability Code: a two-octet unsigned binary integer that indicates the type of Capability advertised and unambiguously identifies an individual capability. Capability Length: a two-octet unsigned binary integer that indicates the length, in octets, of the Capability Value field. A length of 0 indicates that no Capability Value field is present. Capability Value: a variable-length field. It is interpreted according to the value of the Capability Code. A BGP speaker MUST NOT include more than one instance of a capability with the same Capability Code, Capability Length, and Capability Value. Note, however, that processing multiple instances of such a capability does not require special handling, as additional instances do not change the meaning of the announced capability; thus, a BGP speaker MUST be prepared to accept such multiple instances. BGP speakers MAY include more than one instance of a capability (as identified by the Capability Code) with different Capability Value and either the same or different Capability Length. Processing of these capability instances is specific to the Capability Code and MUST be described in the document introducing the new capability. Capability TLVs MUST be placed in the RCA in increasing order of Capability Code. (In the event of multiple instances of a capability with the same Capability Code as discussed above, no further sorting order is defined here.) Although the major sorting order is mandated, an implementation MAY elect to be prepared to consume capabilities in any order, for robustness reasons.

Sending the RCA Suppose a BGP speaker S has a route R it wishes to advertise with next hop N to its peer. If S is originating R into BGP, it MAY include an RCA attribute with it, that carries capability TLVs that describe aspects of R. S MUST set the header portion of the RCA to be equal to N, using the encoding given above. If S has received R from some other BGP speaker, two possibilities exist. First, S could be propagating R without changing N. In that case, S need take no special action, it SHOULD simply propagate the RCA unchanged unless specifically configured otherwise. Indeed, we observe that this is no different from the default action a BGP speaker takes with an unrecognized optional transitive attribute -- it is treated as opaque data and propagated. Second, S could be changing R in some way, and in particular, it could be changing N. If S has changed N it MUST NOT propagate the RCA unchanged. It MAY include a newly-constructed RCA attribute with R, constructed as described above in the "originating R into BGP" case. Any given capability TLV carried by the newly-constructed RCA attribute might use information from the received RCA attribute as input to its construction; the details of this are specific to the definition of each capability. In effect, this means that a given capability TLV will only be propagated along a path where it is supported by every BGP next-hop along the path. The RCA MAY be sent by default to IBGP peers. It MUST NOT be sent by default to peers not under the administrative control of the local network administrator (so, generally, to EBGP peers). We note that due to the nature of BGP optional transitive path attributes, any BGP speaker that does not implement this specification will propagate the RCA, the requirements of this section notwithstanding. Such a speaker will not update the RCA, however.

Receiving the RCA By default, the RCA MUST NOT be accepted from peers not under the administrative control of the local network administrator (so, generally, from EBGP peers); if received it MUST be discarded without further processing, except that the contents MAY be logged. An implementation MAY enable RCA processing by default from peers under the administrative control of the local network administrator (so, generally, from IBGP peers). An implementation SHOULD provide the ability to modify these default settings by configuration. When a BGP speaker receives a BGP route that includes the RCA, it MUST compare the address given in the header portion of the RCA to the next hop of the BGP route. If the two match, the RCA may be further processed. If the two do not match, it means some intermediate BGP speaker that handled the route in transit both does not support RCA, and changed the next hop of the route. In this case, the contents of the RCA cannot be used, and the RCA MUST be discarded without further processing, except that the contents MAY be logged. In considering whether the next hop "matches", a semantic match is sought. While bit-for-bit equality is a trivial test of matching, there may be certain cases where the two are not bit-for-bit equal, but still "match". An example is when a MP_REACH Next Hop encodes both a global and a link-local IPv6 address. In that case the link-local address might be removed during IBGP propagation, the two would still be considered to match if they were equal on the global part. See Section 3 of . A BGP speaker receiving a Capability Code that it supports behaves as defined in the document defining the Capability Code. A BGP speaker receiving a Capability Code that it does not support MUST ignore that Capability Code. In particular, it MUST NOT be handled as an error. The presence of a Capability SHOULD NOT influence route selection or route preference, unless tunneling is used to reach the BGP next hop or the selected route has been learned from External BGP (that is, the next hop is in a different Autonomous System). Indeed, it is in general impossible for a node to know that all BGP routers of the Autonomous System (AS) will understand a given capability, and if different routers within an AS were to use a different preference for a route, forwarding loops could result unless tunneling is used to reach the BGP next hop.

Attribute Error Handling An RCA is considered malformed if the length of the attribute is inconsistent with the lengths of the contained capability TLVs. A BGP UPDATE message with a malformed RCA SHALL be handled using the approach of "attribute discard" defined in . Unknown Capability Codes MUST NOT be considered to be an error. An RCA that contains no capability TLVs MAY be considered malformed, although it is observed that the prescribed behavior of "attribute discard" is semantically no different from that of having no TLVs to process. A document that specifies a new RCA Capability should provide specifics regarding what constitutes an error for that RCA Capability. If a capability TLV is malformed, that capability TLV MUST be ignored and removed. Other capability TLVs MUST be processed as usual.

Network Operation Considerations In the corner case where multiple nodes use the same IP address as their BGP next hop, such as with anycast nodes as described in , a BGP speaker MUST NOT advertise a given capability unless all nodes sharing this same IP address support this capability. The network operator operating those anycast nodes is responsible for ensuring that an anycast node does not advertise a capability not supported by all nodes sharing this anycast address. The means for accomplishing this are beyond the scope of this document.

Entropy Label Capability (ELCv3) When BGP is used for distributing labeled Network Layer Reachability Information (NLRI) as described in, for example, , the route may include the ELCv3 as part of the RCA. The inclusion of this capability with a route indicates that the egress of the associated Label Switched Path (LSP) can process entropy labels as an egress Label Switched Router (LSR) for that route -- see Section 4.2 of . Below, we refer to this for brevity as being "EL-capable." For historical reasons, this capability is referred to as "ELCv3", to distinguish it from the prior Entropy Label Capability (ELC) defined in and deprecated in , and the ELCv2 described in .

Encoding The ELCv3 has capability code 1, capability length 0, and carries no value:

ELCv3 TLV Format

Sending the ELCv3 When a BGP speaker S has a route R it wishes to advertise with next hop N to its peer, it MUST NOT include the ELCv3 capability except if it knows that the egress of the associated LSP L is EL-capable. Specifically, this will be true if S:

• Is itself the egress, and knows itself to be EL-capable, or
• Is re-advertising a BGP route it received with a valid ELCv3 capability, and is not changing the value of N, or
• Is re-advertising a BGP route it received with a valid ELCv3 capability, and is changing the value of N, and knows (for example, through configuration) that the router represented by N is either the LSP egress and is EL-capable, or that it will simply swap labels without popping the BGP-advertised label stack and processing the label below, as with a transit LSR, or
• Is redistributing a route learned from another protocol, and that other protocol conveyed the knowledge that the egress of L was EL-capable (for example, this might be known through the LDP ELC TLV, Section 5.1 of ).

The ELCv3 MAY be advertised with routes that are labeled, such as those using SAFI 4 . It MUST NOT be advertised with unlabeled routes.

Receiving the ELCv3 (Below, we assume that "includes the ELCv3" implies that the containing RCA has passed the checks specified in . If it had not passed, then the RCA would have been discarded and the ELCv3 would be deemed not to have been included.) When a BGP speaker receives an unlabeled route that includes the ELCv3, it MUST discard the ELCv3. When a BGP speaker receives a labeled route that includes the ELCv3, that indicates the LSP supports entropy labels, which implies that the receiving BGP speaker, if acting as ingress, MAY insert an entropy label as per Section 4.2 of .

ELCv3 Error Handling The ELCv3 is considered malformed and must be disregarded if its length is other than zero.

Legacy ELC The ELCv3 functionality introduced in this document replaces the "BGP Entropy Label Capability Attribute" (ELC attribute) that was introduced by , and deprecated by . The latter RFC specifies that the ELC attribute, BGP path attribute 28, "MUST be treated as any other unrecognized optional, transitive attribute". This specification revises that requirement. As the current specification was developed, it became clear that due to incompatibilities between how the ELC attribute is processed by different fielded implementations, the most prudent handling of attribute 28 is not to propagate it as an unrecognized optional, transitive attribute, but to discard it. Therefore, this specification updates , by instead requiring that an implementation that receives the ELC attribute MUST discard any received ELC attribute.

IANA Considerations IANA has made a temporary allocation in the BGP Path Attributes registry of the Border Gateway Protocol (BGP) Parameters group. IANA is requested to make this allocation permanent.

Value	Code	Reference
39	BGP Router Capabilities (RCA)	(this doc)

IANA is requested to create a new registry called "BGP Router Capability Codes" within the Border Gateway Protocol (BGP) Parameters group. The registry's allocation policy is First Come, First Served. It is seeded with the following values:

Value	Description	Reference	Change Controller
0	reserved	(this doc)	IETF
1	ELCv3	(this doc)	IETF
65500 - 65534	reserved for experimental use	(this doc)	IETF
65535	reserved	(this doc)	IETF

Security Considerations

Considerations for the RCA The header portion of the RCA contains the next hop the attribute's originator included when sending it. This will typically be an IP address of the router in question. This may be an infrastructure address the network operator does not intend to announce beyond the border of its Autonomous System, and it may even be considered in some weak sense, confidential information. Although the desired operation of the protocol is for the attribute's propagation scope to be limited to the network operator's own Autonomous System, this can't be guaranteed in all cases -- if a border router doesn't implement this specification, the attribute, like all BGP optional transitive attributes, will propagate to neighboring Autonomous Systems. Similarly, if a border router receiving the attribute from an external Autonomous System doesn't implement this specification, it will store and propagate the attribute, the requirements of notwithstanding. So, sometimes this information could leak beyond its intended scope. (Note that it will only propagate as far as the first router that does support this specification, at which point it will be discarded per .) If the attribute leaks beyond its intended scope, capabilities within it would potentially be exposed. Specifications for individual capabilities should consider the consequences of such unintended exposure.

Considerations for the ELCv3 Capability Insertion of an ELCv3 by an attacker could cause forwarding to fail. Deletion of an ELCv3 by an attacker could cause one path in the network to be overutilized and another to be underutilized. However, we note that an attacker able to accomplish either of these (below, an "on-path attacker") could equally insert or remove any other BGP path attribute or message. The former attack described above denies service for a given route, which can be accomplished by an on-path attacker in any number of ways even absent ELCv3. The latter attack defeats an optimization but nothing more; it seems dubious that an attacker would go to the trouble of doing so rather than launching some more damaging attack.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. BGP-4 Multiprotocol Extensions (BGP-MP) defines the format of two BGP attributes (MP_REACH_NLRI and MP_UNREACH_NLRI) that can be used to announce and withdraw the announcement of reachability information. This document defines how compliant systems should make use of those attributes for the purpose of conveying IPv6 routing information. [STANDARDS-TRACK] This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document obsoletes RFC 1771. [STANDARDS-TRACK] This document defines extensions to BGP-4 to enable it to carry routing information for multiple Network Layer protocols (e.g., IPv6, IPX, L3VPN, etc.). The extensions are backward compatible - a router that supports the extensions can interoperate with a router that doesn't support the extensions. [STANDARDS-TRACK] Load balancing is a powerful tool for engineering traffic across a network. This memo suggests ways of improving load balancing across MPLS networks using the concept of "entropy labels". It defines the concept, describes why entropy labels are useful, enumerates properties of entropy labels that allow maximal benefit, and shows how they can be signaled and used for various applications. This document updates RFCs 3031, 3107, 3209, and 5036. [STANDARDS-TRACK] The BGP Entropy Label Capability attribute is defined in RFC 6790. Regrettably, it has a bug: although RFC 6790 mandates that routers incapable of processing Entropy Labels must remove the attribute, fulfillment of this requirement cannot be guaranteed in practice. This specification deprecates the attribute. A forthcoming document will propose a replacement. According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable because a session reset would impact not only routes with the offending attribute but also other valid routes exchanged over the session. This document partially revises the error handling for UPDATE messages and provides guidelines for the authors of documents defining new attributes. Finally, it revises the error handling procedures for a number of existing attributes. This document updates error handling for RFCs 1997, 4271, 4360, 4456, 4760, 5543, 5701, and 6368. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Orange Juniper Networks, Inc. Nokia RFC 5492 advertises the capabilities of the BGP peer. When the BGP peer is not the same as the BGP Next-Hop, it is useful to also be able to advertise the capability of the BGP Next-Hop, in particular to advertise forwarding plane features. This document defines a mechanism to advertise such BGP Next Hop dependent Capabilities. This document defines a new BGP non-transitive attribute to carry Next-Hop Capabilities. This attribute is guaranteed to be deleted or updated when the BGP Next Hop is changed, in order to reflect the capabilities of the new BGP Next-Hop. This document also defines a Next-Hop capability to advertise the ability to process the MPLS Entropy Label as an egress LSR for all NLRI advertised in the BGP UPDATE. It updates RFC 6790 with regard to this BGP signaling. Juniper Networks Juniper Networks RFC 6790 defined the Entropy Label Capability Attribute (ELC); RFC 7447 deprecated that attribute. This specification, dubbed "Entropy Label Capability Attribute version 2" (ELCv2), was intended to be offered for standardization, to replace the ELC as a way to signal that a BGP protocol speaker is capable of processing entropy labels. Although ultimately a different specification was chosen for that purpose, at least one implementation of ELCv2 was shipped by Juniper Networks and is currently in use in service provider networks. This document is published in order to document what was implemented. As the Internet has grown, and as systems and networked services within enterprises have become more pervasive, many services with high availability requirements have emerged. These requirements have increased the demands on the reliability of the infrastructure on which those services rely. Various techniques have been employed to increase the availability of services deployed on the Internet. This document presents commentary and recommendations for distribution of services using anycast. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines an Optional Parameter, called Capabilities, that is expected to facilitate the introduction of new capabilities in the Border Gateway Protocol (BGP) by providing graceful capability advertisement without requiring that BGP peering be terminated. This document obsoletes RFC 3392. [STANDARDS-TRACK] This document specifies a set of procedures for using BGP to advertise that a specified router has bound a specified MPLS label (or a specified sequence of MPLS labels organized as a contiguous part of a label stack) to a specified address prefix. This can be done by sending a BGP UPDATE message whose Network Layer Reachability Information field contains both the prefix and the MPLS label(s) and whose Next Hop field identifies the node at which said prefix is bound to said label(s). This document obsoletes RFC 3107.

Other Means of Signaling EL Capability A router that supports this specification could also have other means to know that an egress is EL-capable, for example, it could support ELCv2 , or it could know through configuration. If a router learns through any means that an egress is EL-capable, it MAY treat the egress as EL-capable. For example, reception of a valid ELCv2 would be sufficient (even if a valid ELCv3 is not received), and similarly, reception of a valid ELCv3 would be sufficient (even if a valid ELCv2 is not received). The details of which methods are accepted for signaling EL capability are beyond the scope of this specification but SHOULD be configurable by the user.

Acknowledgements This specification derives from two earlier documents, and . included the following acknowledgements: included the following acknowledgements:

Contributors Cisco Systems

sekrier@cisco.com

Juniper Networks

kfwang@juniper.net