Network Working Group Pierre Francois
Internet-Draft Institute IMDEA Networks
Intended status: Informational Bruno Decraene
Expires: August 1, 2014 Orange
Cristel Pelsser
Internet Initiative Japan
Keyur Patel
Clarence Filsfils
Cisco Systems
January 28, 2014
Graceful BGP session shutdown
draft-ietf-grow-bgp-gshut-05
Abstract
This draft describes operational procedures aimed at reducing the
amount of traffic lost during planned maintenances of routers or
links, involving the shutdown of BGP peering sessions.
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 August 1, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Packet loss upon manual eBGP session shutdown . . . . . . . . 5
4. Practices to avoid packet losses . . . . . . . . . . . . . . . 5
4.1. Improving availability of alternate paths . . . . . . . . 5
4.2. Make before break convergence: g-shut . . . . . . . . . . 6
4.2.1. eBGP g-shut . . . . . . . . . . . . . . . . . . . . . 6
4.2.2. iBGP g-shut . . . . . . . . . . . . . . . . . . . . . 7
4.2.3. Router g-shut . . . . . . . . . . . . . . . . . . . . 7
5. Forwarding modes and transient forwarding loops during
convergence . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Link Up cases . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Unreachability local to the ASBR . . . . . . . . . . . . . 8
6.2. iBGP convergence . . . . . . . . . . . . . . . . . . . . . 9
7. IANA assigned g-shut BGP community . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Alternative techniques with limited applicability . . 11
A.1. Multi Exit Discriminator tweaking . . . . . . . . . . . . 11
A.2. IGP distance Poisoning . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Routing changes in BGP can be caused by planned, maintenance
operations. This document discusses operational procedures to be
applied in order to reduce or eliminate losses of packets during the
maintenance. These losses come from the transient lack of
reachability during the BGP convergence following the shutdown of an
eBGP peering session between two Autonomous System Border Routers
(ASBR).
This document presents procedures for the cases where the forwarding
plane is impacted by the maintenance, hence when the use of Graceful
Restart does not apply.
The procedures described in this document can be applied to reduce or
avoid packet loss for outbound and inbound traffic flows initially
forwarded along the peering link to be shut down. These procedures
trigger, in both involved ASes, rerouting to the alternate path,
while allowing routers to keep using old paths until alternate ones
are learned, installed in the RIB and in the FIB. This ensures that
routers always have a valid route available during the convergence
process.
The goal of the document is to meet the requirements described in
[REQS] at best, without changing the BGP protocol.
Still, it explains why reserving a community value for the purpose of
BGP session graceful shutdown would reduce the management overhead
bound with the solution. It would also allow vendors to provide an
automatic graceful shutdown mechanism that does not require any
router reconfiguration at maintenance time.
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. Terminology
g-shut initiator: a router on which the session shutdown is performed
for the maintenance.
g-shut neighbor: a router that peers with the g-shut initiator via
(one of) the session(s) to be shut down.
Initiator AS: the Autonomous System of the g-shut initiator.
Neighbor AS: the Autonomous System of the g-shut neighbor.
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Loss of Connectivity (LoC: the state when a router has no path
towards an affected prefix.
3. Packet loss upon manual eBGP session shutdown
Packets can be lost during a manual shutdown of an eBGP session for
two reasons.
First, routers involved in the convergence process can transiently
lack of paths towards an affected prefix, and drop traffic destined
to this prefix. This is because alternate paths can be hidden by
nodes of an AS. This happens when the paths are not selected as best
by the ASBR that receive them on an eBGP session, or by Route
Reflectors that do not propagate them further in the iBGP topology
because they do not select them as best.
Second, within the AS, the FIB of routers can be transiently
inconsistent during the BGP convergence and packets towards affected
prefixes can loop and be dropped. Note that these loops only happen
when ASBR-to-ASBR encapsulation is not used within the AS.
This document only addresses the first reason.
4. Practices to avoid packet losses
This section describes means for an ISP to reduce the transient loss
of packets upon a manual shutdown of a BGP session.
4.1. Improving availability of alternate paths
All solutions that increase the availability of alternate BGP paths
at routers performing packet lookups in BGP tables such as
[BestExternal] and [AddPath] help in reducing the LoC bound with
manual shutdown of eBGP sessions.
One of such solutions increasing diversity in such a way that, at any
single step of the convergence process following the eBGP session
shutdown, a BGP router does not receive a message withdrawing the
only path it currently knows for a given NLRI, allows for a
simplified g-shut procedure.
Note that the LoC for the inbound traffic of the maintained router,
induced by a lack of alternate path propagation within the iBGP
topology of a neighboring AS is not under the control of the operator
performing the maintenance. The part of the procedure aimed at
avoiding LoC for incoming paths can thus be applied even if no LoC
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are expected for the outgoing paths.
4.2. Make before break convergence: g-shut
This section describes configurations and actions to be performed for
the graceful shutdown of eBGP peering links.
The goal of this procedure is to let the paths being shutdown
visible, but with a lower LOCAL_PREF value, while alternate paths
spread through the iBGP topology. Instead of withdrawing the path,
routers of an AS will keep on using it until they become aware of
alternate paths.
4.2.1. eBGP g-shut
4.2.1.1. Pre-configuration
On each ASBR supporting the g-shut procedure, an outbound BGP route
policy is applied on all iBGP sessions of the ASBR, that:
o matches the g-shut community
o sets the LOCAL_PREF attribute of the paths tagged with the
g-shut community to a low value
o removes the g-shut community from the paths.
o optionally, adds an AS specific g-shut community on these paths
to indicate that these are to be withdrawn soon. If some
ingress ASBRs reset the LOCAL_PREF attribute, this AS specific
g-shut community will be used to override other LOCAL_PREF
preference changes.
Note that in the case where an AS is aggregating multiple routes
under a covering prefix, it is recommended to filter out the g-shut
community from the resulting aggregate BGP route. By doing so, the
setting of the g-shut community on one of the aggregated routes will
not let the entire aggregate inherit the community. Not doing so
would let the entire aggregate undergo the g-shut behavior.
4.2.1.2. Operations at maintenance time
On the g-shut initiator, upon maintenance time, it is required to:
o apply an outbound BGP route policy on the maintained eBGP session
to tag the paths propagated over the session with the g-shut
community. This will trigger the BGP implementation to re-
advertise all active routes previously advertised, and tag them
with the g-shut community.
o apply an inbound BGP route policy on the maintained eBGP session
to tag the paths received over the session with the g-shut
community.
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o wait for convergence to happen.
o perform a BGP session shutdown.
4.2.1.3. BGP implementation support for G-Shut
A BGP router implementation MAY provide features aimed at automating
the application of the graceful shutdown procedures described above.
Upon a session shutdown specified as graceful by the operator, a BGP
implementation supporting a g-shut feature SHOULD:
1. On the eBGP side, update all the paths propagated over the
corresponding eBGP session, tagging the GSHUT community to them.
Any subsequent update sent to the session being gracefully shut
down would be tagged with the GSHUT community.
2. On the iBGP side, lower the LOCAL_PREF value of the paths
received over the eBGP session being shut down, upon their
propagation over iBGP sessions. Optionally, also tag these
paths with an AS specific g-shut community. Note that
alternatively, the LOCAL_PREF of the paths received over the
eBGP session can be lowered on the g-shut initiator itself,
instead of only when propagating over its iBGP sessions.
3. Optionally shut down the session after a configured time.
4. Prevent the GSHUT community from being inherited by a path that
would aggregate some paths tagged with the GSHUT community.
This behavior avoids the GSHUT procedure to be applied to the
aggregate upon the graceful shutdown of one of its covered
prefixes.
A BGP implementation supporting a g-shut feature SHOULD also
automatically install the BGP policies that are supposed to be
configured, as decribed in Section 4.2.1.1 for sessions over which
g-shut is to be supported.
4.2.2. iBGP g-shut
If the iBGP topology is viable after the maintenance of the session,
i.e, if all BGP speakers of the AS have an iBGP signaling path for
all prefixes advertised on this g-shut iBGP session, then the
shutdown of an iBGP session does not lead to transient
unreachability.
4.2.3. Router g-shut
In the case of a shutdown of a router, a reconfiguration of the
outbound BGP route policies of the g-shut initiator SHOULD be
performed to set a low LOCAL_PREF value for the paths originated by
the g-shut initiator (e.g, BGP aggregates redistributed from other
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protocols, including static routes).
This behavior is equivalent to the recommended behavior for paths
"redistributed" from eBGP sessions to iBGP sessions in the case of
the shutdown of an ASBR.
5. Forwarding modes and transient forwarding loops during convergence
The g-shut procedure or the solutions improving the availability of
alternate paths, do not change the fact that BGP convergence and the
subsequent FIB updates are run independently on each router of the
ASes. If the AS applying the solution does not rely on encapsulation
to forward packets from the Ingress Border Router to the Egress
Border Router, then transient forwarding loops and consequent packet
losses can occur during the convergence process. If zero LoC is
required, encapsulation is required between ASBRs of the AS.
6. Link Up cases
We identify two potential causes for transient packet losses upon an
eBGP link up event. The first one is local to the g-no-shut
initiator, the second one is due to the BGP convergence following the
injection of new best paths within the iBGP topology.
6.1. Unreachability local to the ASBR
An ASBR that selects as best a path received over a newly brought up
eBGP session may transiently drop traffic. This can typically happen
when the nexthop attribute differs from the IP address of the eBGP
peer, and the receiving ASBR has not yet resolved the MAC address
associated with the IP address of that "third party" nexthop.
A BGP speaker implementation could avoid such losses by ensuring that
"third party" nexthops are resolved before installing paths using
these in the RIB.
If the link up event corresponds to an eBGP session that is being
manually brought up, over an already up multi-access link, then the
operator can ping third party nexthops that are expected to be used
before actually bringing the session up, or ping directed broadcast
the subnet IP address of the link. By proceeding like this, the MAC
addresses associated with these third party nexthops will be resolved
by the g-no-shut initiator.
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6.2. iBGP convergence
Corner cases leading to LoC can occur during an eBGP link up event.
A typical example for such transient unreachability for a given
prefix is the following:
Let's consider 3 route reflectors RR1, RR2, RR3. There is a full
mesh of iBGP session between them.
1. RR1 is initially advertising the current best path to the
members of its iBGP RR full-mesh. It propagated that path
within its RR full-mesh. RR2 knows only that path towards the
prefix.
2. RR3 receives a new best path originated by the "g-no-shut"
initiator, being one of its RR clients. RR3 selects it as best,
and propagates an UPDATE within its RR full-mesh, i.e., to RR1
and RR2.
3. RR1 receives that path, reruns its decision process, and
picks this new path as best. As a result, RR1 withdraws its
previously announced best-path on the iBGP sessions of its RR
full-mesh.
4. If, for any reason, RR3 processes the withdraw generated in
step 3, before processing the update generated in step 2, RR3
transiently suffers from unreachability for the affected prefix.
The use of [BestExternal] among the RR of the iBGP full-mesh can
solve these corner cases by ensuring that within an AS, the
advertisement of a new route is not translated into the withdraw of a
former route.
Indeed, "best-external" ensures that an ASBR does not withdraw a
previously advertised (eBGP) path when it receives an additional,
preferred path over an iBGP session. Also, "best-intra-cluster"
ensures that a RR does not withdraw a previously advertised (iBGP)
path to its non clients (e.g. other RRs in a mesh of RR) when it
receives a new, preferred path over an iBGP session.
7. IANA assigned g-shut BGP community
Applying the g-shut procedure is rendered much easier with the use of
a single g-shut community value which could be used on all eBGP
sessions, for both inbound and outbound signaling. The community
value 0xFFFF0000 has been assigned by IANA for this purpose.
For Internet routes, a non transitive extended community will be
reserved from the pool defined in [EXT_POOL]. Using such a community
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type allows for not leaking graceful signaling out of the AS
boundaries, without the need to explicitly configure filters to strip
the community off upon path propagation.
8. Security Considerations
By providing the g-shut service to a neighboring AS, an ISP provides
means to this neighbor to lower the LOCAL_PREF value assigned to the
paths received from this neighbor.
The neighbor could abuse the technique and do inbound traffic
engineering by declaring some prefixes as undergoing a maintenance so
as to switch traffic to another peering link.
If this behavior is not tolerated by the ISP, it SHOULD monitor the
use of the g-shut community by this neighbor.
ASes using the regular (transitive) g-shut community SHOULD remove
the community from neighboring ASes that do not support the g-shut
procedure. Doing so prevents malignant remote ASes from using the
community through intermediate ASes that do not support the feature,
in order to perform inbound traffic engineering. ASes using the non-
transitive extended community do not need to do this as the community
is non transitive and hence cannot be used by remote ASes.
9. Acknowledgments
The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra
for their useful comments on this work.
10. References
[AddPath] D. Walton, E. Chen, A. Retana, and J. Scudder,
"Advertisement of Multiple Paths in BGP",
draft-ietf-idr-add-paths-09.txt (work in progress).
[BestExternal]
Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
Gredler, "Advertisement of the best-external route to
IBGP", draft-ietf-idr-best-external-05.txt.
[REQS] Decraene, B., Francois, P., Pelsser, C., Ahmad, Z.,
Armengol, A., and T. Takeda, "Requirements for the
graceful shutdown of BGP sessions", RFC 6198.
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[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
[EXT_POOL]
Decraene, B. and P. Francois, "Assigned BGP extended
communities",
draft-ietf-idr-reserved-extended-communities-06.
[BGPWKC] "http://www.iana.org/assignments/
bgp-well-known-communities".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Appendix A. Alternative techniques with limited applicability
A few alternative techniques have been considered to provide g-shut
capabilities but have been rejected due to their limited
applicability. This section describe them for possible reference.
A.1. Multi Exit Discriminator tweaking
The MED attribute of the paths to be avoided can be increased so as
to force the routers in the neighboring AS to select other paths.
The solution only works if the alternate paths are as good as the
initial ones with respect to the Local-Pref value and the AS Path
Length value. In the other cases, increasing the MED value will not
have an impact on the decision process of the routers in the
neighboring AS.
A.2. IGP distance Poisoning
The distance to the BGP nexthop corresponding to the maintained
session can be increased in the IGP so that the old paths will be
less preferred during the application of the IGP distance tie-break
rule. However, this solution only works for the paths whose
alternates are as good as the old paths with respect to their Local-
Pref value, their AS Path length, and their MED value.
Also, this poisoning cannot be applied when nexthop self is used as
there is no nexthop specific to the maintained session to poison in
the IGP.
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Authors' Addresses
Pierre Francois
Institute IMDEA Networks
Avda. del Mar Mediterraneo, 22
Leganese 28918
ES
Email: pierre.francois@imdea.org
Bruno Decraene
Orange
38-40 rue du General Leclerc
92794 Issy Moulineaux cedex 9
FR
Email: bruno.decraene@orange.com
Cristel Pelsser
Internet Initiative Japan
Jinbocho Mitsui Bldg.
1-105 Kanda Jinbo-cho
Tokyo 101-0051
JP
Email: cristel@iij.ad.jp
Keyur Patel
Cisco Systems
170 West Tasman Dr
San Jose, CA 95134
US
Email: keyupate@cisco.com
Clarence Filsfils
Cisco Systems
De kleetlaan 6a
Diegem 1831
BE
Email: cfilsfil@cisco.com
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