BGP Next-Hop Capabilities
draft-decraene-idr-next-hop-capability-01

Abstract

RFC 5492 defines capabilities advertisement for the BGP peer. In addition, it is useful to know the capabilities of the BGP Next-Hop, in particular for forwarding plane features. RFC 5492 is not applicable because the BGP peer may be different from the BGP Next-Hop, in particular when BGP Route Reflection is used. This document defines a mechanism to advertise such BGP Next Hop Capabilities.

This document defines a new BGP non-transitive attribute to carry Next-Hop Capabilities. This attribute is deleted when the BGP Next Hop is changed.

This document also defines a Next-Hop capability to advertise the ability to handle the Entropy Label defined in RFC 6790.

Requirements Language

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 [RFC2119].

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 http://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 January 7, 2016.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. BGP Next-Hop Capabilities Attribute
• 3. BGP Next-Hop Capabilities Attribute Operation
• 4. BGP Next-Hop Capability Code Operation
• 5. BGP Next-Hop Attribute Error Handling
• 6. Entropy Label Next-Hop Capability
• 6.1. Readable Label Depth
• 6.2. Entropy Label Next-Hop Capability error handling
• 7. IANA Considerations
• 7.1. Next-Hop Capabilities Attribute
• 7.2. Next-Hop Capability registry
• 8. Security Considerations
• 9. Acknowledgement
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

[RFC5492] defines capabilities advertisement for the BGP peer. It is also useful to know the capabilities of the BGP Next-Hop, in particular for forwarding plane features. RFC 5492 is not applicable because the BGP peer may be different from the BGP Next-Hop, in particular when BGP Route Reflection is used. This document defines a mechanism to advertise such BGP Next Hop Capabilities.

This document defines a new BGP non-transitive attribute to carry Next-Hop Capabilities. This attribute is deleted when the BGP Next Hop is changed.

This document also defines a first application to advertise the capability to handle the Entropy Label defined in [RFC6790]. Note that RFC 6790 had originally defined a BGP attribute for this but it has been latter deprecated in [RFC7447]

2. BGP Next-Hop Capabilities Attribute

The BGP Next-Hop Capabilities Attribute is an optional, non-transitive BGP Attribute, of value TBD1. The attribute consists of a set of Next-Hop Capabilities.

Inclusion of a Next-Hop Capability "X" in a BGP UPDATE message, indicates that the BGP Next-Hop, encoded in either the NEXT_HOP attribute defined in [RFC4271] or the Network Address of Next Hop field of the MP_REACH_NLRI attribute defined in [RFC4760], supports the capability "X" for the NLRI advertised in this BGP UPDATE. This document does not make distinction between these two Next-Hop fields and uses the term 'BGP Next-Hop' to refer to whichever one is used in a given BGP UPDATE message.

A Next-Hop Capability is a triple (Capability Code, Capability Length, Capability Value) aka a TLV:

  
		      	A Next-Hop Capability.

             +------------------------------+
             | Capability Code (1 octet)    |
             +------------------------------+
             | Capability Length (1 octet)  |
             +------------------------------+
             | Capability Value (variable)  |
             ~                              ~
             +------------------------------+
       
						

Capability Code: a one-octet unsigned binary integer which indicates the type of "Next-Hop Capability" advertised and unambiguously identifies an individual capability.

Capability Length: a one-octet unsigned binary integer which 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 from 0 to 255 octets. It is interpreted according to the value of the Capability Code.

BGP speakers SHOULD NOT include more than one instance of a Next-Hop capability with the same Capability Code, Capability Length, and Capability Value. Note, however, that processing of multiple instances of such 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 non-zero Capability Length field, but 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.

3. BGP Next-Hop Capabilities Attribute Operation

The BGP Next-Hop Capabilities attribute being non-transitive, as per [RFC4271], a BGP speaker which does not understand it will quietly ignore it and not pass it along to other BGP peers.

A BGP speaker that understands the BGP Next-Hop Capabilities Attribute and does not change the BGP Next-Hop, SHOULD NOT change the BGP Next-Hop Capabilities Attribute and SHOULD pass the attribute unchanged along to other BGP peers.

A BGP speaker that understands the BGP Next-Hop Capabilities Attribute and changes the BGP Next-Hop, MUST remove the received BGP Next-Hop Capabilities before propagating the BGP UPDATE to other BGP peers. It MAY attach a new BGP Next-Hop Capabilities attribute describing the capabilities of the new BGP Next-Hop.

4. BGP Next-Hop Capability Code Operation

A BGP speaker receiving a BGP Next-Hop Capability Code that it supports may behave as defined in the document defining this Capability Code. A BGP speaker receiving a BGP Next-Hop Capability Code that it does not support MUST ignore this BGP Next-Hop Capability Code. In particular, this MUST NOT be handled as an error. In both cases, the BGP speaker MUST examine the remaining BGP Next-Hop Capability Code that may be present in the BGP Next-Hop Capabilities Attribute.

The BGP Next-Hop Capability Code MUST reflect the capability of the router indicated in the BGP Next-Hop. If a BGP speaker sets the BGP Next-Hop to an address of a different router (e.g. R), it MUST NOT advertise BGP Next-Hop Capabilities not supported by this router R.

The presence of a Next-Hop Capability SHOULD NOT influence route selection or route preference of an route, unless tunneling is used to reach the BGP Next-Hop or the selected route has been learnt from EBGP (i.e. the Next-Hop is in a different AS). Indeed, it is in general impossible for a node to know that all BGP routers of the Autonomous System (AS) will understand a given Next-Hop Capability; and having different routers, within an AS, use a different preference for a route, may result in forwarding loops if tunnelling is not used to reach the BGP Next-Hop.

An implementations MAY allow, by configuration, removing this attribute when advertising the routes over eBGP.

5. BGP Next-Hop Attribute Error Handling

A BGP Next-Hop Capabilities Attribute is considered malformed if the length of the Attribute is not equal to the sum of all (BGP Hop Capability Length +2) of each capability carried in this attribute. Note that "2" is the length of the fields "Type" and "Length" of each BGP Next Hop Capability.

A BGP UPDATE message with a malformed BGP Next-Hop Capabilities Attribute SHALL be handled using the approach of "attribute discard" defined in [I-D.ietf-idr-error-handling].

Unknown Next-Hop Capabilities Codes MUST be silently ignored.

A document that specifies a new Next-Hop Capability SHOULD provide specifics regarding what constitutes an error for that Next-Hop Capability.

If a Next-Hop Capability is malformed, this Next-Hop Capability Type MUST be ignored. Others Next-Hop Capabilities MUST be processed as usual.

6. Entropy Label Next-Hop Capability

The Entropy Label Next-Hop Capability has type code 1 and a length of 0 or 1 octet.

The inclusion of the "Entropy Label" Next-Hop Capability indicates that the BGP Next-Hop can be sent packets, for all routes indicated in the NRLI, with a MPLS entropy labels (ELI, EL) added immediately after the label stack advertised with the NLRI.

On the receiving side, suppose BGP speaker S has determined that packet P is to be forwarded according to BGP route R, where R is a route of one of the labeled address families. And suppose that L is the label stack embedded in the NLRI of route R. Then to forward packet P according to route R, S either replaces P's top label with L, or else pushes L onto the MPLS label stack. If the EL-Capability is advertised in the BGP UPDATE advertising this route R, S knows that it may safely place the ELI and an EL on the label stack immediately beneath L.

A BGP speaker S that sends an UPDATE with the BGP Next-Hop NH MAY include the Entropy Label Next-Hop Capability only if, for all the NLRI in the BGP UPDATE, either of the following is true:

• Egress case: NH is the egress of the LSP advertised with the NLRI and can lookup and its capable of handling the ELI.
• Transit LSR case: NH is a transit LSR for the LSP advertised with the NLRI (i.e. swap one of the label advertised in the NLRI) and next downstream BGP Next-Hop(s) has(have) advertised the Entropy Label Next-Hop Capability (or a similar capability signalled by protocol X if the route is redistributed, by NH, from X to BGP).

6.1. Readable Label Depth

When stacked LSPs are used and the ingress nests LSP inside this BGP signaled LSP, it would be useful for ingress LSRs to know how many additional labels the downstream LSR may read when load-balancing based on the Entropy Label. In other words, how many labels the ingress LER may push, in addition to the BGP label(s) advertised in the Network Layer Reachability Information (NLRI) field, before pushing an entropy label that will be seen by all downstream LSRs.

This maximum number of additional labels is called the Readable Label Depth (RLD) of the LSP(s). It is related, yet different, to the RLD of an node which is defined in [I-D.ietf-mpls-spring-entropy-label]

The RLD of the LSP(s) advertised in the NLRI, may be advertised in the value field of the Entropy Label Next-Hop Capability. This value field is optional. If present, the value field is a one-octet unsigned binary integer which indicates the maximum Readable Label Depth (RLD) of the LSP(s) advertised in the NLRI. In other words, this is the maximum number of additional MPLS labels that may be pushed by the ingress, in addition to the label(s) of the NRLI advertised in the BGP UPDATE, before pushing the ELI, EL labels, if it wish that all downstream LSR be capable of performing load-balancing based on the entropy label.

S SHOULD advertise a RLD of:

• its own local RLD minus the number of labels advertised in the NLRI, if S is the egress of the LSP(s) advertised in the NLRI;
• the minimum of:
  • its own node RLD minus the number of labels advertised in the NLRI;
  • the RLD of the LSP from itself to BGP NEXT_HOP of its received route minus the number of labels received in the NLRI(if any);
  • the RLD in the received BGP route (if any).

The first term represents the limitation of the new BGP NEXT_HOP (S), the second term the contribution from the new (sent) BGP NEXT_HOP (S) toward the old (received) BGP NEXT_HOP (S'), the third term represent the contribution from the old BGP NEXT_HOP (S') toward the egress.

255 is a reserved value.

Note that the local RLD is meant as a node value. If a router has multiple line cards with different capabilities, the router SHOULD advertise the smallest one. However, a router MAY choose to only consider the line cards that may be used by the BGP routers receiving the ELC. e.g. if the ELC is advertised over an EBGP session with peer S', a router MAY consider only the line cards connected to peer S'.

6.2. Entropy Label Next-Hop Capability error handling

If the Entropy Label Next-Hop Capability is present more than once, it MUST be considered as received once with a length of 0.

If the Entropy Label Next-Hop Capability is received with a length other than 0 or 1, it is not considered malformed, but its semantics are exactly the same as if it had a length of 0. This is to allow for graceful future extension.

7. IANA Considerations

7.1. Next-Hop Capabilities Attribute

IANA is requested to allocate a new Path Attribute, called "Next-Hop Capabilities", type Code TBD1, from the "BGP Path Attributes" registry.

7.2. Next-Hop Capability registry

The IANA is requested to create and maintain a registry entitled "Next-Hop Capabilities".

The registration policies [RFC5226] for this registry are:

  
     1-63   IETF Review
   64-127   First Come First Served
  128-250   Standards Action
  251-254   Experimental Use
      255   Reserved
  
						

IANA is requested to make the following initial assignments:

  
		Registry Name: Next-Hop Capability.

 Value      Meaning                                  Reference
 ---------- ---------------------------------------- ---------
       0    Reserved                                 This document
       1    Entropy Label                            This document
   2-250    Unassigned
 251-255    Experimental                             This document
     255    Reserved                                 This document
       
						

8. Security Considerations

This document does not introduce new security vulnerabilities in BGP. Specifically, an operator who is relying on the information carried in BGP must have a transitive trust relationship back to the source of the information. Specifying the mechanism(s) to provide such a relationship is beyond the scope of this document. Please refer to the Security Considerations section of [RFC4271] for security mechanisms applicable to BGP.

9. Acknowledgement

The Entropy Label Next-Hop Capability defined in this document is based on the ELC BGP attribute defined in section 5.2 of [RFC6790].

The authors wish to thank John Scudder for the discussions on this topics and Eric Rosen for his review of this document.

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses