Making Route Servers Aware of Data Link Failures at IXPs
draft-ymbk-idr-rs-bfd-01

Abstract

When route servers are used, the data plane is not congruent with the control plane. Therefore, the peers on the Internet exchange can lose data connectivity without the control plane being aware of it, and packets are dropped on the floor. This document proposes the use of BFD between the two peering routers to detect a data plane failure, and then uses BGP next hop cost to signal the state of the data link to the route server(s).

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119] only when they appear in all upper case. They may also appear in lower or mixed case as English words, without normative meaning.

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 September 25, 2015.

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.

This document may not be modified, and derivative works of it may not be created, and it may not be published except as an Internet-Draft.

Table of Contents

• 1. Introduction
• 2. Operation
• 2.1. Mutual Discovery of Route Server Client Routers
• 2.2. Tracking Connectivity
• 3. Advertising Client Router Connectivity to the Route Server
• 4. Utilizing Next Hop Unreachablility Information at Client Routers
• 5. Recommendations for Using BFD
• 6. Bootstrapping
• 7. Other Considerations
• 8. Normative References
• Authors' Addresses

1. Introduction

In configurations (typically Internet exchanges) where EBGP routing information is exchanged between client routers through the agency of a route server [I-D.ietf-idr-ix-bgp-route-server], but traffic is exchanged directly, operational issues can arise when partial data plane connectivity exists among the route server client routers. This is because, as the data plane is not congruent with the control plane, the client routers on the Internet exchange can lose data connectivity without the control plane - the route server - being aware of it, and packets are dropped on the floor.

To remedy this, two basic problems need to be solved:

1. Client routers must have a means of verifying connectivity amongst themselves, and

2. Client routers must have a means of communicating the knowledge so gained back to the route server.

The first can be solved by application of Bidirectional Forwarding Detection [RFC5880]. The second can be solved by use of BGP NH-SAFI [I-D.ietf-idr-bgp-nh-cost]. There is a subsidiary problem that must also be solved. Since one of the key value propositions offered by a route server is that client routers need not be configured to peer with each other:

3. Client routers must have a means (other than configuration) to know of one another's existence.

This can also be solved by an application of BGP NH-SAFI.

Throughout this document, we generally assume that the route server being discussed is able to represent different RIBs towards different clients, as discussed in section 2.3.2.1. [I-D.ietf-idr-ix-bgp-route-server]. These procedures (other than the use of BFD to track next hop reachability) have limited value if this is not the case.

2. Operation

Below, we detail procedures where a route server tells its client routers about other client routers (by sending it their next hops using NH-SAFI), the client router verifies connectivity to those other client routers (using BFD) and communicates its findings back to the route server (again using NH-SAFI). The route server uses the received NH-SAFI routes as input to the route selection process it performs on behalf of the client.

2.1. Mutual Discovery of Route Server Client Routers

Strictly speaking, what is needed is not for a route server client router to know of other (control-plane) client routers, but rather to know (so that it can validate) all the next hops the route server might choose to send the client router, i.e. to know of potential forwarding plane relationships.

In effect, this requirement amounts to knowing the BGP next hops the route server is aware of in its Adj-RIBs-In. Fortunately, [I-D.ietf-idr-bgp-nh-cost] defines a construct that contains exactly this data, the "Next-Hop Information Base", or NHIB, as well as procedures for a BGP speaker to communicate its NHIB to its peer. Thus, the problem can be solved by the route server advertising its NHIB to its client router, following those procedures.

We observe that (as per NH-SAFI) the cost advertised in the route server's Adj-NHIB-Out need not reflect a "real" IGP cost, the only requirement being that the advertised costs are commensurate. A route server MAY choose to advertise any fixed cost other than all-ones (which is a reserved value in NH-SAFI). This specification does not suggest semantics be imputed to the NH-SAFI advertised by the route server and received by the client, other than "this next hop is present in the control plane, you might like to track it". The route server is not allowed to advertise a next hop as NH_UNREACHABLE.

A route server client SHOULD use BFD (or other means beyond the scope of this document) to track forwarding plane connectivity [RFC5880] to each next hop depicted in the received NH-SAFI.

2.2. Tracking Connectivity

For each next hop in the Adj-NHIB-In received from the route server, the client router SHOULD use some means to confirm that data plane connectivity does exist to that next hop.

For each next hop in the Adj-NHIB-In received from the route server, the client router SHOULD setup a BFD session to it if one is not already available and track the reachability of this next hop.

For each next hop being tracked, a corresponding NH-SAFI route should be placed in the client router's own Adj-NHIB-Out to be advertised to the route server. Any next hop for which connectivity has failed should have its cost advertised as NH_UNREACHABLE. (This may also be done as a result of policy even if connectivity exists.) Any other next hop should have some feasible cost advertised. The values advertised may be all equal, or may be set according to policy or other implementation-specific means.

If the test of connectivity between one client router and another client router has failed the client router that detected this failure should perform connectivity test for a configurable amount of time (preferable 24 hours) on a regular basis (e.g. every 5 minutes). If during this time no connectivity can be restored no more testing is performed and this client router is advertised as NH_UNREACHABLE until manually changed or the client router is rebooted.

3. Advertising Client Router Connectivity to the Route Server

As discussed above, a client router will advertise its Adj-NHIB-Out to the route server. The route server should use this information as input to its own decision process when computing the Adj-RIB-Out for this peer. This peer-dependent Adj-RIB-Out is then advertised to this peer. In particular, the route server MUST exclude any routes whose next hops the client has declared to be NH_UNREACHABLE. The route server MAY also consider the advertised cost to be the "IGP cost" section 9.1 [RFC4271] when doing this computation.

4. Utilizing Next Hop Unreachablility Information at Client Routers

A client router detecting an unreachable next hop signals this information to the route server as described above. Also, it treats the routes as unresolvable as per section 9.1.2.1 [RFC4271] and proceeds with route selection as normal.

Changes in nexthop reachability via these mechanisms should receive some amount of consideration toward avoiding unnecessary route flapping. Similar mechanisms exist in IGP implementations and should be applied to this scenario.

5. Recommendations for Using BFD

The RECOMMENDED way a client router can confirm the data plane connectivity to its next hops is available, is the use of BFD in asynchronous mode. Echo mode MAY be used if both client routers running a BFD session support this. The use of authentication in BFD is OPTIONAL as there is a certain level of trust between the operators of the client routers at a particular IXP. If trust cannot be assumed, it is recommended to use pair-wise keys (how this can be achieved is outside the scope of this document). The ttl/hop limit values as described in section 5 [RFC5881] MUST be obeyed in order to secure BFD sessions from packets coming from outside the IXP.

There is interdependence between the functionality described in this document and BFD from an administrative point of view. To streamline behaviour of different implementations the following is RECOMMENDED:

• If BFD is administratively shut down by the administrator of a client router then the functionality described in this document MUST also be administratively shut down.
• If the administrator enables the functionality described in this document on a client router then BFD MUST be automatically enabled.

The following values of the BFD configuration of client routers (see section 6.8.1 [RFC5880]) are RECOMMENDED in order to allow a fast detection of lost data plane connectivity:

• DesiredMinTxInterval: 1,000,000 (microseconds)
• RequiredMinRxInterval: 1,000,000 (microseconds)
• DetectMult: 3

The configuration values above are a trade-off between fast detection of data plane connectivity and the load client routers must handle keeping up the BFD communication. Selecting smaller DesiredMinTxInterval and RequiredMinRxInterval values generates lots of BFD packets, especially at larger IXPs with many hundreds of client routers.

The configuration values above are selected in order to handle brief interrupts on the data plane. Otherwise, if a BFD session detects a brief data plane interrupt to a particular client router, it will cause to signal the route server that is should remove routes from this client router and tell it shortly afterwards to add the routes again. This is disruptive and computational expensive on the route server.

The configuration values above are also partially impacted by BGP advertisement time in reaction to events from BFD. If the configuration values are selected so that BFD detects data plane interrupts a lot faster than the BGP advertisement time, a data plane connectivity flapping could be detected by BFD but the route server is not informed about them because BGP is not able to transport this information fast enough.

As discussed, finding good configuration values is hard so a client router administrator MAY select better suited values depending on the special needs of the particular deployment.

6. Bootstrapping

If the route server starts it does not know anything about connectivity states between client routers. So, the route server assumes optimistically that all client routers are able to reach each other unless told otherwise.

7. Other Considerations

For purposes of routing stability, implementations may wish to apply hysteresis ("holddown") to next hops that have transitioned from reachable to unreachable and back.

8. Normative References

Authors' Addresses