One document matched: draft-francois-bgp-gshut-00.txt
Network Working Group Pierre Francois
Internet-Draft Universite catholique de Louvain
Expires: September 16, 2008 Bruno Decraene
France Telecom
Cristel Pelsser
NTT Corporation
March 15, 2008
Graceful BGP session shutdown
draft-francois-bgp-gshut-00
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Abstract
This draft describes operational procedures aimed at reducing the
amount of traffic lost during planned maintenances of routers,
involving the shutdown of BGP peering sessions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Packet loss upon manual eBGP session shutdown . . . . . . . . 4
4. Practices to avoid packet losses . . . . . . . . . . . . . . . 4
4.1. Improving availability of alternate paths . . . . . . . . 4
4.2. Graceful shutdown procedures for eBGP sessions . . . . . . 5
4.2.1. Outbound traffic . . . . . . . . . . . . . . . . . . . 5
4.2.2. Inbound traffic . . . . . . . . . . . . . . . . . . . 5
4.3. Graceful shutdown procedures for iBGP sessions . . . . . . 7
5. Forwarding modes and forwarding loops . . . . . . . . . . . . 7
6. Dealing with Internet policies . . . . . . . . . . . . . . . . 8
7. Effect of the g-shut procedure on the convergence . . . . . . 8
7.1. Maintenance of an eBGP session . . . . . . . . . . . . . . 9
7.1.1. Propagation on the other eBGP sessions of the
g-shut initiator . . . . . . . . . . . . . . . . . . . 9
7.1.2. Propagation on the other iBGP sessions of the
g-shut initiator . . . . . . . . . . . . . . . . . . . 9
7.1.3. Propagation of updates in an iBGP full-mesh . . . . . 9
7.1.4. Propagation of updates from iBGP to iBGP in a RR
hierarchy . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Maintenance of an iBGP session . . . . . . . . . . . . . . 10
7.3. Applicability of the g-shut procedures . . . . . . . . . . 10
7.4. Summary of operations . . . . . . . . . . . . . . . . . . 10
7.4.1. Pre-configuration . . . . . . . . . . . . . . . . . . 11
7.4.2. Operations at maintenance time . . . . . . . . . . . . 11
8. Techniques with limited applicability . . . . . . . . . . . . 11
8.1. In-filter reconfiguration . . . . . . . . . . . . . . . . 11
8.2. Multi Exit Discriminator tweaking . . . . . . . . . . . . 12
8.3. IGP distance Poisoning . . . . . . . . . . . . . . . . . . 12
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
Routing changes in BGP can be caused by planned, manual, 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
allow routers to keep using old paths until alternate ones are
learned, ensuring 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 or BGP
implementations.
Still, it explains why standardizing 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 configuration at maintenance time.
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.
Affected path / Nominal / pre-convergence path : a BGP path via the
peering link(s) undergoing the maintenance. This path will no longer
exist after the shutdown.
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Affected prefix : a prefix initially reached via an affected path.
Affected router : a router having an affected prefix.
Backup / alternate / post-convergence path : a path toward an
affected prefix that will be selected as the best path by an affected
router, when the link is shut down and the BGP convergence is
completed.
Transient alternate path : a path towards an affected prefix that may
be transiently selected as best by an affected router during the
convergence process but that is not a post-convergence path.
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, routers' FIB 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 BR-to-BR
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
Using advertise-best-external on ASBRs improves the availability of
alternate paths. Hence it reduces the LoC duration for the outbound
traffic of the ISP upon an eBGP Session shutdown.
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All solutions that improve the availability of paths beyond the Route
Reflectors barrier also help in reducing the LoC. These have been
discussed for years but are not yet available, probably due to their
implementation complexity.
4.2. Graceful shutdown procedures for eBGP sessions
This section aims at describing a procedure to be applied to reduce
the LoC with readily available BGP features, and without assuming
particular iBGP design for the AS performing the maintenance and in
the neighboring ASes.
4.2.1. Outbound traffic
This section discusses a mean to render the affected paths less
desirable by the BGP decision process of affected routers, still
allowing these to be used during the convergence while alternate
paths are propagated to the affected routers.
A decrease of the local-pref value of the affected paths can be
issued in order to render the affected paths less preferable, at the
highest possible level of the BGP Decision Process.
This operation can be performed by reconfiguring the out-filters
associated with the iBGP sessions established by the g-shut
initiator.
The modification of the filters MUST supplant any other rule
affecting the local-pref value of the old paths.
Compared to using an in-filter of the eBGP session to be shut down,
the modification of the out-filters will not let the g-shut initiator
switch to another path, as the input to the BGP decision process of
that router does not change. As a consequence, the g-shut initiator
will not send a withdraw message over its iBGP sessions when it
receives an alternate path over an iBGP session. It will however
modify the local-pref of the affected paths so that upstream routers
will switch to alternate ones.
When the actual shutdown of the session is performed, the g-shut
initiator will itself switch to the alternate paths.
4.2.2. Inbound traffic
The solution described for the outbound traffic can be applied at the
neighbor AS. This can be done either "manually" or by using a
community value dedicated to this task.
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4.2.2.1. Phone call
The operator performing the maintenance of the eBGP session can
contact the operator at the other side of the peering link, and let
him apply the procedure described above for its own outbound traffic.
4.2.2.2. Community tagging
A community value (referred to as GSHUT community in this document)
can be agreed upon by neighboring ASes. A path tagged with this
community must be considered as soon to be affected by a maintenance
operation.
4.2.2.2.1. Pre-Configuration
A g-shut neighbor is pre-configured to set a low local-pref value for
the paths received over eBGP sessions which are tagged with the GSHUT
community.
This rule must supplant any other rule affecting the local-pref value
of the paths.
This local-pref reconfiguration SHOULD be performed at the out-
filters of the iBGP sessions of the g-shut neighbor. That is, the
g-shut neighbor does not take into account this low local-pref in its
own BGP best path selection. As described in Section 4.2.1 this
avoids sending the withdraw messages that can lead to LoC.
4.2.2.2.2. Operational action upon maintenance
Upon the manual shutdown, the output filter associated with the
maintained eBGP session will be modified on the g-shut initiator so
as to tag all the paths advertised over the session with the GSHUT
community.
4.2.2.2.3. Transitivity of the community
If the GSHUT community is an extended community, it SHOULD be set non
transitive.
If a normal community is used, this community SHOULD be removed from
the path by the ASBR of the peer receiving it. If not, the GSHUT
community MAY be removed from the path by all the ASBRs of the
neighboring AS, before propagating the path to other peers.
Not propagating the community further in the Internet reduces the
amount of BGP churn and avoids rerouting in distant ASes that would
also recognize this community value. In other words, it helps
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concealing the convergence at the maintenance location.
There are cases where an interdomain exploration is to be performed
to recover the reachability, e.g., in the case of a shutdown in
confederations where the alternate paths will be found in another AS
of the confederation. In such scenarios, the community value SHOULD
be allowed to transit through the confederation but MAY be removed
from the paths advertised outside of the confederation.
When the local-pref value of a path is conserved upon its propagation
from one AS of the confederation to the other, there is no need to
have the GSHUT community be propagated throughout that confederation.
4.2.2.2.4. Easing the configuration for G-SHUT
From a configuration burden viewpoint, it would be much easier to
have the GSHUT community value be standardized.
First, on the g-shut initiator, an operator would have a single
configuration rule to be applied at the maintenance time, which would
not depend on the identity of its peer. This would make the
maintenance operations less error prone.
Second, on the g-shut neighbor, the same filter related to g-shut can
be applied to all iBGP sessions, at the g-shut neighbor.
4.3. Graceful shutdown procedures for iBGP sessions
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 affected prefixes after the convergence, then the shutdown of an
iBGP session does not lead to transient unreachability.
However, in the case of a shutdown of a router, a reconfiguration of
the out-filters 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 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 forwarding loops
If the AS applying the solution does not rely on encapsulation to
forward packets from the Ingress Border Router to the Egress Border
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Router, then transient forwarding loops and consequent packet losses
can occur during the convergence process, even if the procedure
described above is applied.
Using the out-filter reconfiguration avoids the forwarding loops
between the g-shut initiator and its upstream neighbors. Indeed,
when this reconfiguration is applied, the g-shut initiator keeps
using its own external path and lets the upstream routers converge to
the alternate ones. During this phase, no forwarding loops can occur
between the g-shut initiator and its upstream neighbors as the g-shut
neighbor keeps using the affected paths via its eBGP peering links.
When the first step is finished, all the upstream routers have
switched to alternate paths and the transition performed by the
g-shut initiator will be loopfree. Transient forwarding loops
between other routers will not be avoided with this procedure.
6. Dealing with Internet policies
A side gain of the maintenance solution is that it can be used to
reduce the churn implied by a shutdown of an eBGP session.
For this, it is recommended to apply the filters modifying the local-
pref value of the paths to values strictly lower but as close as
possible to the local-pref values of the post-convergence paths.
For example, if a peering link is shut down between a provider and
one of its customers, and another peering link with this customer
remains active, then the value of the local-pref of the old paths
SHOULD be decreased to the smallest possible value of the 'customer'
local_pref range, minus 1. Thus, routers will not transiently switch
to paths received from shared-cost peers or providers, which could
lead to the sending of withdraw messages over eBGP sessions with
shared-cost peers and providers.
Proceeding like this reduces both BGP churn and traffic shifting as
routers will less likely switch to transient paths.
In our example, it also prevents transient unreachabilities in the
neighboring AS that are due to the sending of "abrupt" withdraw
messages to shared-cost peers and providers.
7. Effect of the g-shut procedure on the convergence
This section describes the effect of applying the solution.
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7.1. Maintenance of an eBGP session
This section describes the effect of applying the solution for the
shutdown of an eBGP session.
7.1.1. Propagation on the other eBGP sessions of the g-shut initiator
Nothing is propagated on the other eBGP sessions when the out-filters
reconfiguration step is applied. The reconfiguration is indeed only
defined for its iBGP sessions.
The reconfiguration of the iBGP out-filters will trigger the
reception of alternate paths at the g-shut initiator. As the eBGP
in-filters have not been modified at that step, the old paths are
still preferred by the g-shut initiator.
7.1.2. Propagation on the other iBGP sessions of the g-shut initiator
During the out-filter reconfiguration, path updates are propagated
with a reduced local-pref value for the affected paths. As a
consequence, Route Reflectors and distant ASBRs select and propagate
alternate paths through the iBGP topology as they no longer select
the old paths as best.
When the shut-down is performed, for each affected prefix, the g-shut
initiator propagates on its iBGP sessions:
. The alternate path, if the best path was received over an eBGP
sessions.
. A withdraw, if the best path was received over an iBGP sessions.
7.1.3. Propagation of updates in an iBGP full-mesh
No transient LoC can occur if a reconfiguration of the iBGP out-
filters on the g-shut initiator is performed.
7.1.4. Propagation of updates from iBGP to iBGP in a RR hierarchy
Upon the reception of the update of a primary path with a lower
local-pref value, Route Reflectors will either propagate the update,
or select an alternate path and propagate it within their RR iBGP
full-mesh, to their own Route Reflectors in the case of a multiple
level Route Reflector hierarchy, and to their clients.
During the convergence process controlled with the described
procedure, some corner case timings can trigger transient
unreachabilities.
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A typical example for such transient unreachability for a given
prefix is the following :
1. A Route Reflector RR1 only knew about the primary path upon
the shutdown.
2. A member of its RR full-mesh, RR2, propagates an update of
the old path with a lower local-pref.
3. Another member of its RR full-mesh, RR3 processes the
update, selects an alternate path, and propagates an update in
the mesh.
4. RR2 receives the alternate path, selects it as best, and
hence withdraws the updated old path on the iBGP session of the
mesh.
5. If for any reason, RR1 receives and processes the withdraw
generated in step 4 before processing the update generated in
step 3, RR1 transiently suffers from unreachability for the
affected prefix.
In such corner cases, the solution improves the iBGP convergence
behavior/LoC but does not ensure 0 packet loss, as we cannot define a
simple solution relying only on a reconfiguration of the filters of
the g-shut initiator. Improving the availability of alternate paths
in Route Reflectors, using advertise best external, or any flavor of
add-path [AddPath], seems to be the most pragmatic solution to these
corner cases.
7.2. Maintenance of an iBGP session
If the shutdown does not temper with the viability of the iBGP
topology, the described procedure is sufficient to avoid LoC.
7.3. Applicability of the g-shut procedures
The applicability of the procedure described in this draft to the
cases presented in [REQS] can be shown by combining the effects
described in this section. A complete case by case analysis will be
provided in the next versions of the draft.
7.4. Summary of operations
This section summarizes the configurations and actions to be
performed to support the g-shut procedure for eBGP peering links.
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7.4.1. Pre-configuration
On each ASBR supporting the g-shut procedure, set-up an out-filter
applied on all iBGP sessions of the ASBR, that :
. sets the local-pref of the paths tagged with the g-shut community
to a low value
. removes the g-shut community from the path.
7.4.2. Operations at maintenance time
On the g-shut initiator :
. Apply an in-filter on the maintained BGP session to tag the paths
received over the session with the g-shut community.
. Apply an out-filter on the maintained BGP session to tag the paths
propagated over the session with the g-shut community.
. Wait for convergence to happen.
. Perform a BGP session shutdown.
8. Techniques with limited applicability
8.1. In-filter reconfiguration
An In-filter reconfiguration on the eBGP session undergoing the
maintenance could be performed instead of out-filter reconfigurations
on the iBGP sessions of the g-shut initiator.
Upon the application of the maintenance procedure, if the g-shut
initiator has an alternate path in its Adj-Rib-In, it will switch to
it directly.
If this new path was advertised by an eBGP neighbor of the g-shut
initiator, the g-shut initiator will send a BGP Path Update message
over its iBGP and eBGP sessions.
If this new path was received over an iBGP session, the g-shut
initiator will select that path and directly propagate a withdraw
message over the iBGP sessions for which it is not acting as a Route
Reflector. There can be iBGP topologies where the iBGP peers of the
g-shut initiator do not know an alternate path, and hence may drop
traffic.
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Also, applying an In-filter reconfiguration on the eBGP session
undergoing the maintenance may lead to transient LoC, in full-mesh
iBGP topologies if
a. An ASBR of the initiator AS, ASBR1 did not initially select
its own external path as best, and
b. An ASBR of the initiator AS, ASBR2 propagates an Update
message along its iBGP sessions upon the reception of ASBR1's
update following the in-filter reconfiguration on the g-shut
initiator, and
c. ASBR1 receives the update message, runs its Decision Process
and hence propagates a withdraw of its external path after
having selected ASBR2's path as best, and
d. An impacted router of the AS processes the withdraw of ASBR1
before processing the update from ASBR2.
Applying a reconfiguration of the out-filters prevents such transient
unreachabilities.
Indeed, when the g-shut initiator propagates an update of the old
path first, the sending of the withdraw by ASBR2 does not trigger
unreachability in other nodes as the old path is still available.
Indeed, even though it receives alternate paths, the g-shut initiator
keeps using its old path as best as the in-filter of the maintained
eBGP session has not been modified yet.
Applying the out-filter reconfiguration also prevents packet loops
between the g-shut initiator and its direct neighbors when
encapsulation is not used between the ASBRs of the AS.
8.2. 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.
8.3. 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
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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.
9. IANA considerations
Applying the g-shut procedure is rendered much easier when the g-shut
community value is to be standardized. Hence this draft suggests to
reserve a community value, e.g., 0xFFFF0005, for this purpose.
10. 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.
11. References
[AddPath] D. Walton, A. Retana, and E. Chen, "Advertisement of
Multiple Paths in BGP", draft-walton-bgp-add-paths-05.txt
(work in progress).
[REQS] Decraene, B., Francois, P., Pelsser, C., and Z. Ahmad,
"Requirements for the graceful shutdown of BGP sessions",
draft-decraene-bgp-graceful-shutdown-requirements-00.txt
, December 2007.
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Authors' Addresses
Pierre Francois
Universite catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve 1348
BE
Email: pierre.francois@uclouvain.be
URI: http://inl.info.ucl.ac.be/pfr
Bruno Decraene
France Telecom
38-40 rue du General Leclerc
92794 Issi Moulineaux cedex 9
FR
Email: bruno.decraene@orange-ftgroup.com
Cristel Pelsser
NTT Corporation
9-11, Midori-Cho 3 Chrome
Musashino-Shi, Tokyo 180-8585
JP
Email: pelsser.cristel@lab.ntt.co.jp
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