Network Working Group R. Bush
Internet-Draft Internet Initiative Japan
Intended status: Standards Track J. Haas
Expires: January 7, 2016 J. Scudder
Juniper Networks, Inc.
A. Nipper
T. King, Ed.
DE-CIX Management GmbH
July 6, 2015
Making Route Servers Aware of Data Link Failures at IXPs
draft-ietf-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 January 7, 2016.
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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
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
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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
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. Modelling the IXP Network using BGP Link-State . . . . . . . 5
5. Utilizing Next Hop Unreachability Information at Client
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Recommendations for Using BFD . . . . . . . . . . . . . . . . 6
7. Bootstrapping . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Capability Detection . . . . . . . . . . . . . . . . . . . . 8
9. Other Considerations . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
11. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
In configurations (typically Internet Exchange Points (IXP)) 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 IXP 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 Link-
State [I-D.ietf-idr-ls-distribution]. 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 Link-State.
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 BGP Link-State), the client router verifies connectivity to
those other client routers (using BFD) and communicates its findings
back to the route server (again using BGP Link-State). The route
server uses the received BGP Link-State 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 for the particular per-client Loc-RIB (see
section 2.3.2.1. [I-D.ietf-idr-ix-bgp-route-server]). We introduce
a new table for each client to store known next hops, their
compatibility with this proposed solution and their learned
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reachability. We call these tables per-client Next Hop Information
Base (NHIB). BGP Link-State is used to transfer the NHIBs from the
route server to route server clients.
At the route server, the NHIB for each client is populated with the
next hops from its Loc-RIB. If the BGP capabilities learned during
BGP session setup identify a next hop as compatible with this
proposal, this is reflected in the NHIB. Initially, it is assumed
that the client router is able to reach its next hops which is stored
in the NHIB.
If a next hop is added to the NHIB for a particular client, a route
SHOULD be added to the router server's Adj-NHIB-Out. This route
contains a BGP Link-State SAFI and models the next hop as node (see
section 3.2.1 [I-D.ietf-idr-ls-distribution]) and the connectivity
between the route server and the next hop as link (see section 3.2.2
[I-D.ietf-idr-ls-distribution]). If a next hop is removed from a
NHIB, the corresponding route in the Adj-NHIB-Out SHOULD be removed.
A route server client SHOULD use BFD [RFC5880] (or other means beyond
the scope of this document) to track forwarding plane connectivity to
each next hop depicted in the received BGP Link-State information.
2.2. Tracking Connectivity
For each next hop in the NHIB received from the route server (called
Adj-NHIB-In), the client router SHOULD use some means to confirm that
data plane connectivity does exist to that next hop.
The client router maintains its own NHIB in order to keep track of
its (potential) next hops, their capabilities as learned from the
route server, and their reachability. The NHIB is updated according
to the Adj-NHIB-In and client routers own tests to verify
connectivity to next hops.
For each next hop in the Adj-NHIB-In received from the route server,
the client router SHOULD evaluate the next hop's compatibility with
this proposal. If the next hop supports this proposed mechanism 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 in the Adj-NHIB-In, a corresponding BGP Link-State
SAFI containing a node NLRI route SHOULD be placed in the client
router's own Adj-NHIB-Out to be advertised to the route server. If
the next hop is not compatible with this proposal a route containing
a BGP Link-State SAFI and a link NLRI SHOULD be placed in the client
router's own Adj-NHIB-Out. The link NLRI is configured as follows:
the local node is set to the client router, the remote node if set to
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the particular next hop. Any next hop that is compatible with this
proposal and for which connectivity is in the process of verification
(in other words a BFD session is initiated) or is already verified a
route containing a BGP Link-State SAFI and a link NLRI as described
above SHOULD be placed to the client router's own Adj-NHIB-Out. For
any next hop for which connectivity has failed a route SHOULD be
placed in the client router's own Adj-NHIB-Out to withdraw the
previously advertised link from the route server. (This may also be
done as a result of policy even if connectivity exists.)
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 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 update the reachability
information of next hops in the client's NHIB table accordingly.
Furthermore, the route server SHOULD use reachability information
from the NHIB 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 not
reachable.
4. Modelling the IXP Network using BGP Link-State
This section describes how BGP Link-State is used to a) transfer the
per-client NHIB form the route server to the route server clients and
b) transfer the reachability information about next hops from the
route server client to the route server.
Each route server client and the route server are modeled as nodes
(see section 3.2.1 [I-D.ietf-idr-ls-distribution]). As node ID the
BGP identifier (see section 1.1 [RFC4271]) is used.
BGP Link-State defines as link a so-called half-way link (see section
3.2.2 [I-D.ietf-idr-ls-distribution]). To cover the bidirectional
connectivity between two nodes two link definitions are required. In
order to model the connectivity between two route server clients a
link is used.
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For both nodes and links the Protocol-ID is set to 5 to reflect the
virtual modeling. The instance identifier for nodes and links is set
to 0 as the default layer 3 routing topology is utilized.
The link descriptor TLV code points 259-262 are applied depending on
the IP protocol version used. Prefix descriptors are not applied.
A way is needed to model whether a client router is compatible the
mechanisms described in this document or not. For this, a new node
descriptor Sub-TVLs (see section 3.2.1.4
[I-D.ietf-idr-ls-distribution]) is introduced.
+--------------------+-----------------------------+--------+
| Sub-TLV Code Point | Description | Length |
+--------------------+-----------------------------+--------+
| 516 | Compatible to this document | 1 |
+--------------------+-----------------------------+--------+
Table 1: Node Descriptor Sub-TLV
The value of this Sub-TVL is set to 0 if a client router does not
support the mechanisms described in this document (of if the support
is administratively disabled). Otherwise the value is set to 1.
5. Utilizing Next Hop Unreachability 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.
6. 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.
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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.
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 it 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.
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7. 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.
8. Capability Detection
In order for two BGP speakers to follow the mechanism defined in this
document, they MUST use BGP Capabilities Advertisements [RFC5492].
This is done as specified in [RFC4760], by using capability code 1
(multiprotocol BGP), with an AFI XXX and SAFI XXX.
9. 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.
10. Acknowledgments
The authors would like to thank the authors of
[I-D.ietf-idr-bgp-nh-cost] for their work as it was a basis for this
proposal.
11. Normative References
[I-D.ietf-idr-bgp-nh-cost]
Varlashkin, I., Raszuk, R., Patel, K., Bhardwaj, M., and
S. Bayraktar, "Carrying next-hop cost information in BGP",
draft-ietf-idr-bgp-nh-cost-02 (work in progress), May
2015.
[I-D.ietf-idr-ix-bgp-route-server]
Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
"Internet Exchange BGP Route Server", draft-ietf-idr-ix-
bgp-route-server-07 (work in progress), June 2015.
[I-D.ietf-idr-ls-distribution]
Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", draft-ietf-idr-ls-distribution-11
(work in progress), June 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January
2007.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, February 2009.
[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
John G. Scudder
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: jgs@juniper.net
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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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