Internet-Draft In-band BGP looking glass capability July 2021
Jaufeerally Expires 26 January 2022 [Page]
Internet Engineering Task Force
Intended Status:
R. Jaufeerally, Ed.

An in-band BGP mechanism for looking-glass address discovery


Autonomous Systems (ASes) that peer with one another using the Border Gateway Protocol (BGP) RFC 4271 [RFC4271] do not have an automated way to tell if the prefixes announced over a particular peering link are acceped by the peer. One way in which an AS operator can verify acceptance of routes by a peer is using a looking glass which may be provided by the peer, however this introduces manual toil in the operation and turnup of peering links, and does not allow for continued validation to ensure there are no regressions. Looking glasses are often manually found by browsing the website of the peer or via a database of peering participants.

This document proposes a new in-band mechanism for transmitting administrative data between BGP peers, and defines one such use case for propagating looking glass addresses. This is done via a new Address Family Identifier (AFI) which carries a message that we define here, inside the Multi Protocol (MP_REACH and MP_UNREACH) path attribute

The looking glass that we expose via the proposed mechanism conforms to that defined in RFC 8522 [RFC8522].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 2 January 2022.

Table of Contents

1. Introduction

In this document we introduce a new Address Family Identifier (AFI) to carry an administrative message via the existing BGP update message that is defined in RFC 4271 [RFC4271]

When a peer signals support for this AFI, we define a mechanism through which an administrative message contained within BGP UPDATE path attributes can be used to propagate inforamation such as but not limited to a looking glass URL.

1.1. 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].

2. Advertising support for the administrative message

Peers that support sending and receiving administrative messages via this mechanism MUST advertise support by sending a multiprotocol capability advertisment, as defined in RFC 4760 section 8 [RFC4760]. The AFI used is that assigned by IANA (Note to editor: insert the actual value here once assigned).

A peer SHOULD only send the messages defined in this document to a peer which has advertised support for the administrative message AFI.

3. Format of the administrative message

The administrative message is transmitted through a BGP UPDATE message containing a multiprotocol reach or multiprotocol unreach path attribute with the AFI set to the AFI for this document (Note to editor: insert value here after assignment from IANA). The SAFI field SHOULD be set to 0 and MUST be ignored by the recipient, as it is unused in this specification, and is reserved for future use.

The payload format is as follows:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|            version            |              type             |
|                       payload (variable)                      |
Figure 1

Figure 1 shows the layout of the administrative message which is to be contained within the multiprotocol reahable (MP_REACH) and multiprotocol unreachable (MP_UNREACH) path attributes. Each submessage type will define its own semantics of when MP_REACH and / or MP_UNREACH is to be used.

4. Looking glass payload

The looking glass mechanism is disemminated using a administrative message with a type of 1.

The looking glass mechanism supports advertising to a BGP peer an HTTP endpoint at which looking glass operations such as but not limited to: IP prefix lookups in the RIB of the peer, ping, traceroute, etc. This endpoint, in version 1 of the looking glass protocol, MUST conform to the standard set out in RFC 8522 [RFC8522].

The looking glass address can be specified in one of two ways: (1) on the BGP peer interface itself (i.e. the looking glass can be reached on the address of the BGP peer router itself), (2) on a URL specified within the message itself.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|    version    |                      ASN                      |
|               |            URL (variable length)...           |
Figure 2: Looking glass message format

Figure 2 shows the wire format of the administrative message. The first byte is the looking glass administrative payload version number which MUST be set to 1. This may be updated in future revisions to define new functionality, so software MUST ignore paylaods with unrecognized version numbers. ASN MUST be set to the autonomous system number of the network that is operating the looking glass. That is to say, if an AS is announcing its own looking glass this should be the ASN which is announced in the BGP OPEN message. On the other hand, if this is a looking glass address propagated from another peer, this MUST be the ASN of the network originating the looking glass message. See Section 5 for details on how propagation is handled.

The URL section is a variable length field which contains the ASCII encoded HTTP(s) endpoint of the looking glass.

5. Propoagation of looking glass information

While it is useful to have a looking glass to test the connectivity and route acceptance of a direct peer, it is oftentimes desirable to know if an "upstream" peer has accepted the route as well. Consider the case of a tier-2 ISP which has several uplinks to tier-1 ISPs. In this case, the customer of a tier-2 ISP would benefit to know whether their routes have been accepted by the tier-1 upstream peers, which may have different filtering policies, and thus have a different set of accepted routes.

In the base case to advertise the looking glass of the ASN that is running the BGP speaker, the speaker sets the ASN field to the ASN of the speaker itself.

A BGP speaker SHOULD forward looking glass addresses of a peer P_a to a peer P_b when a route from P_b has been announced to P_a. This MUST only be done if P_a has advertised the looking glass address to the BGP speaker making this decision, and the forwarded URL MUST be the one which P_a has advertised.

6. Acknowledgements

The authors would like to thank the members of the GROW WG for their feedback on this draft. (Note to the editor: update this in future revisions of the draft).

7. IANA Considerations

We request IANA to allocate a new address family identifier for the administrative message, (Note to editor: fill in here after getting an assignment)

We also require a registry of types that can be contained within the administrative message type. This document allocates the first administrative message type of 1 to be used for the looking glass message. (Note to editor: add more details here.)

8. Security Considerations

This draft proposes a mechanism for providing more easily automatable access to a looking glass interface operated by a network. The scope of the dissemination of these looking glass adresses is to direct peers which are presumed to have an interest in querying the network reachability information, for example as part of debugging.

Many network operators already provide looking glass services to the general public, however these are usually not standardized in their interfaces, and moreover, are not discoverable in an automated way which makes scalability difficult, and thus this draft programatically propagates that information.

Operators MUST treat connections to the looking glass as untrusted. Operators SHOULD perform apppropriate rate-limiting and MAY deny abusive clients as per their own policy

Operators may operate the looking glass with an IP access control list in cases where access is intended only for the peer, however this is discouraged as running a public facing looking glass brings the benefit that anyone can use it to debug network issues.

9. References

9.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, , <>.
Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 4760, DOI 10.17487/RFC4760, , <>.
Stubbig, M., "Looking Glass Command Set", RFC 8522, DOI 10.17487/RFC8522, , <>.

9.2. Informative References

Author's Address

Rayhaan Jaufeerally (editor)
CH- Zurich