Internet DRAFT - draft-ymbk-idr-rs-bfd
draft-ymbk-idr-rs-bfd
Network Working Group R. Bush
Internet-Draft Internet Initiative Japan
Intended status: Standards Track J. Haas
Expires: September 25, 2015 J. Scudder
Juniper Networks, Inc.
A. Nipper
T. King, Ed.
DE-CIX Management GmbH
March 24, 2015
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.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Mutual Discovery of Route Server Client Routers . . . . . 3
2.2. Tracking Connectivity . . . . . . . . . . . . . . . . . . 4
3. Advertising Client Router Connectivity to the Route Server . 5
4. Utilizing Next Hop Unreachablility Information at Client
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Recommendations for Using BFD . . . . . . . . . . . . . . . . 5
6. Bootstrapping . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Other Considerations . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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:
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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.
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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.
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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:
o 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.
o If the administrator enables the functionality described in this
document on a client router then BFD MUST be automatically
enabled.
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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:
o DesiredMinTxInterval: 1,000,000 (microseconds)
o RequiredMinRxInterval: 1,000,000 (microseconds)
o 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.
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8. Normative References
[I-D.ietf-idr-bgp-nh-cost]
Varlashkin, I. and R. Raszuk, "Carrying next-hop cost
information in BGP", draft-ietf-idr-bgp-nh-cost-01 (work
in progress), March 2012.
[I-D.ietf-idr-ix-bgp-route-server]
Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
"Internet Exchange Route Server", draft-ietf-idr-ix-bgp-
route-server-06 (work in progress), December 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
Flap Damping", RFC 2439, November 1998.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
Authors' Addresses
Randy Bush
Internet Initiative Japan
5147 Crystal Springs
Bainbridge Island, Washington 98110
US
Email: randy@psg.com
Jeffrey Haas
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: jhaas@juniper.net
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John G. Scudder
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: jgs@juniper.net
Arnold Nipper
DE-CIX Management GmbH
Lichtstrasse 43i
Cologne 50825
Germany
Email: arnold.nipper@de-cix.net
Thomas King (editor)
DE-CIX Management GmbH
Lichtstrasse 43i
Cologne 50825
Germany
Email: thomas.king@de-cix.net
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