One document matched: draft-ietf-grow-bgp-gshut-02.xml
<?xml version="1.0"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<?rfc compact="yes"?>
<rfc ipr='pre5378Trust200902' docName='draft-ietf-grow-bgp-gshut-02' category="info">
<front>
<date month='October' year='2010' day='25'/>
<title>Graceful BGP session shutdown</title>
<author surname="Pierre Francois" name ="Francois" fullname="Pierre Francois">
<organization>Universite catholique de Louvain</organization>
<address>
<postal>
<street>Place Ste Barbe, 2</street>
<city>Louvain-la-Neuve</city> <code>1348</code>
<country>BE</country>
</postal>
<uri>http://inl.info.ucl.ac.be/pfr</uri>
<email>pierre.francois@uclouvain.be</email>
</address>
</author>
<author surname="Bruno Decraene" name ="Decraene" fullname="Bruno Decraene">
<organization>France Telecom</organization>
<address>
<postal>
<street>38-40 rue du General Leclerc</street>
<city>92794 Issi Moulineaux cedex 9</city> <code></code>
<country>FR</country>
</postal>
<email>bruno.decraene@orange-ftgroup.com</email>
</address>
</author>
<author surname="Cristel Pelsser" name = "Pelsser" fullname="Cristel Pelsser">
<organization>Internet Initiative Japan</organization>
<address>
<postal>
<street>Jinbocho Mitsui Bldg.</street>
<street>1-105 Kanda Jinbo-cho</street>
<city>Tokyo</city> <code>101-0051</code>
<country>JP</country>
</postal>
<email>pelsser.cristel@iij.ad.jp</email>
</address>
</author>
<author surname="Keyur Patel" name = "Patel" fullname="Keyur Patel">
<organization>Cisco Systems</organization>
<address>
<postal>
<street> 170 West Tasman Dr</street>
<city>San Jose, CA</city> <code>95134</code>
<country>US</country>
</postal>
<email>keyupate@cisco.com</email>
</address>
</author>
<author surname="Clarence Filsfils" name = "Filsfils" fullname="Clarence Filsfils">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>De kleetlaan 6a </street>
<city>Diegem</city> <code>1831</code>
<country>BE</country>
</postal>
<email>cfilsfil@cisco.com</email>
</address>
</author>
<area>General</area>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t> 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).</t>
<t> 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.</t>
<t> 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.</t>
<t> The goal of the document is to meet the requirements
described in <xref target="REQS"/> at best, without
changing the BGP protocol or BGP implementations.</t>
<t> 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. </t>
<t>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 <xref target="RFC2119"/>.</t>
</section>
<section title = "Terminology">
<t> g-shut initiator : a router on which the session shutdown
is performed for the maintenance. </t>
<t> g-shut neighbor : a router that peers with the g-shut
initiator via (one of) the session(s) to be shut down.</t>
<t> Note that for the link-up case, we will refer to these
nodes as g-no-shut initiator, and g-no-shut neighbor.</t>
<t> Initiator AS : the Autonomous System of the g-shut initiator.</t>
<t> Neighbor AS : the Autonomous System of the g-shut neighbor.</t>
<t> 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.</t>
<!--
<t> Nominal / pre-convergence path : A path that is rendered
invalid due to maintenance. In the case of the maintenance of
an eBGP peering link, an affected path is one via the peering
link being shut down. In the case of the maintenance of an
iBGP peering link, an affected path is one learned via the
session being shut down. </t>-->
<t> Affected prefix : a prefix initially reached via an affected path.</t>
<t> Affected router : a router having an affected prefix.</t>
<t> Backup / alternate / post-convergence path : a path
towards 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.</t>
<t> 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. </t>
<t> Loss of Connectivity (LoC) : the state when a router has
no path towards an affected prefix.</t>
</section>
<section title="Packet loss upon manual eBGP session shutdown" anchor="sec.loss">
<t>Packets can be lost during a manual shutdown of an eBGP
session for two reasons. </t>
<t>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. </t>
<t>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.</t>
<t>This document only addresses the first reason. </t>
</section>
<section title = "Practices to avoid packet losses" anchor = "sec.practices">
<t>This section describes means for an ISP to reduce the
transient loss of packets upon a manual shutdown of a BGP
session. </t>
<section title = "Improving availability of alternate paths">
<t>All solutions that increase the availability of alternate
BGP paths at routers performing packet lookups in BGP tables
<xref target="BestExternal"/> <xref target="AddPath"/> help
in reducing the LoC bound with manual shutdown of eBGP
sessions.</t>
<t>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.</t>
<t>Increasing diversity with <xref target="AddPath"/> might
lead to the respect of this property, depending on the path
propagation decision process that add-path compliant routers
would use. </t>
<t>Using advertise-best-external <xref
target="BestExternal"/> on ASBRs and RRs helps in avoiding
lack of alternate paths in route reflectors upon a
convergence. Hence it reduces the LoC duration for the
outbound traffic of the ISP upon an eBGP Session shutdown by
reducing the iBGP path hunting.</t>
<t>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 procedure described in <xref target =
"sec.inbound"/> should thus be applied upon the maintenance,
even if the procedure described in <xref target
="sec.outbound"/> is not applied.</t>
</section><!--Improving availability of alternate paths-->
<section title = "Graceful shutdown procedures for eBGP sessions">
<t> This section aims at describing a procedure to be
applied to reduce the LoC with readily available BGP
features, and without assuming a particular iBGP design
in the Initiator and Neighbor ASes.</t>
<section title = "Outbound traffic" anchor = "sec.outbound">
<t>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.</t>
<!-- <section title = "Local Preference tweaking" anchor = "sec.loc-pref">-->
<t>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.</t>
<t>This operation can be performed by
reconfiguring the out-filters
associated with the iBGP sessions
established by the g-shut initiator.
</t>
<t> The modification of the filters
MUST supplant any other rule affecting
the local-pref value of the old
paths.</t>
<t> 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 modify the
state of its dataplane, and will not
withdraw the affected paths over its
iBGP sessions when it receives
alternate paths. It will however
modify the local-pref of the affected
paths so that upstream routers will
switch to alternate ones.</t>
<t> When the actual shutdown of the
session is performed, the g-shut
initiator will itself switch to the
alternate paths.</t>
<t> In cases some BGP speakers in the
AS override the local-pref attribute
of paths received over iBGP sessions,
the procedure described above will not
work. In such cases, the recommended
procedure is to tag the paths sent
over the iBGP sessions of the g-shut
initiator with an AS specific
community. This AS specific community
should lead to the setting of the
lowest local-pref value. To be
effective, the configuration related
to this community MUST supplant or be
applied after the already configured
local-pref overriding.
</t>
<t>An operator may decide to follow a
simplified procedure and directly
apply an in-filter reducing the local
preference of the paths received over
the eBGP session being brought
down. While this procedure will be
effective in many cases, corner cases
as described in <xref target =
"sec.loc-pref.infilter"/> may happen,
which may lead to some LoC for some
affected destinations. The use of this
simplified procedure does not lead to
LoC when used in conjunction with
<xref target="BestExternal"/>.
</t>
<!--
<section title = "Outbound traffic, simplified g-shut
procedure" anchor = "sec.outbound.simplified">
<t>In an iBGP deployment where no transient lack of
alternate paths can occur during the convergence of
the BGP routers within the initiator AS, an
in-filter modification can directly be issued so
that all routers, including the g-shut initiator,
switch to the alternate paths before the data-plane
is impacted by the maintenance operation.
</t>
</section>
-->
</section><!-- outbound traffic-->
<section title = "Inbound traffic" anchor = "sec.inbound">
<t> 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.</t>
<section title = "Phone call" anchor = "sec.phone">
<t> 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.</t>
</section><!-- Phone call-->
<section title = "Community tagging" anchor = "sec.community">
<t> A community value (referred to as GSHUT
community in this document) can be agreed upon by
neighboring ASes and used to trigger the g-shut
behavior at the g-shut neighbor.</t>
<section title ="Pre-Configuration">
<t> 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.</t>
<t>This rule must supplant any other rule
affecting the local-pref value of the paths. </t>
<t>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 <xref target = "sec.outbound" /> this
approach avoids sending withdraw messages that can
lead to LoC in some cases.</t>
</section><!-- Pre-Configuration-->
<section title = "Operational action upon maintenance">
<t>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.</t>
</section><!-- Operational action upon maintenance-->
<section title = "Transitivity of the community">
<t>If the GSHUT community is an extended
community, it SHOULD be chosen non-transitive.
</t>
If a regular community is used, this community
SHOULD be removed from the path by the ASBR of the
peer receiving it. If not, the GSHUT community
SHOULD be removed from the path by all the ASBRs
of the neighboring AS, before propagating the path
to other peers.
<t>If a regular community is used, this community
SHOULD be removed from the path when the path is
propagated over eBGP sessions.</t>
<t>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,
from a routing stability perspective, it helps
concealing the convergence at the maintenance
location. From a policy perspective, it prevents
malignant ASes from using the community over paths
propagated through intermediate ASes that do not
support the feature, in order to perform inbound
traffic engineering at the first AS recognizing
the community.</t>
<t>ASes which support the g-shut procedure SHOULD
remove the community value(s) that they use for
g-shut from the paths received from neighboring
ASes that do not support the procedure or to whom
the service is not provided.</t>
<t> 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 SHOULD be
removed from the paths advertised outside of the
confederation.</t>
<t> 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.</t>
</section><!-- transitivity of the community-->
<section title = "Easing the configuration for G-SHUT">
<t>From a configuration burden viewpoint, it is
much easier to use a single dedicated value for
the GSHUT community.</t>
<t>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.</t>
<t>Second, on the g-shut neighbor, a simple filter
related to g-shut can be applied to all iBGP
sessions. Additionnaly, this filter does not need
to be updated each time neighboring ASes are added
or removed.</t>
<t>The FCFS community value 0xFFFF0000 has been reserved
for this purpose <xref target ="BGPWKC"/>.</t>
</section><!-- easing gshut-->
</section><!-- community tagging -->
</section><!--inbound traffic-->
<section title = "Summary of operations" target = "sec.summary">
<t>This section summarizes the configurations and actions to
be performed to support the g-shut procedure for eBGP
peering links. </t>
<section title = "Pre-configuration" target = "sec.summary.config">
<t> On each ASBR supporting the g-shut procedure, set-up
an out-filter applied on all iBGP sessions of the ASBR,
that :</t>
<list style = "symbols" hangIndent="5">
<t> sets the local-pref of the paths tagged with the
g-shut community to a low value</t>
<t> removes the g-shut community from the paths.</t>
<t> 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 preference
attribute, this AS specific g-shut community will be
used to override other local preference changes.</t>
</list>
</section><!--Pre-configuration-->
<section title = "Operations at maintenance time" target = "sec.summary.shut">
<t> On the g-shut initiator : </t>
<list style = "symbols">
<t>Apply an out-filter on the maintained eBGP session to tag
the paths propagated over the session with the g-shut
community.</t>
<t>Apply an in-filter on the maintained eBGP session to tag the
paths received over the session with the g-shut community. </t>
<t>Wait for convergence to happen.</t>
<t>Perform a BGP session shutdown. </t>
</list>
</section><!--Operations at maintenance time-->
</section><!--Summary of operations-->
<section title = "BGP implementation support for G-Shut">
<t>A BGP router implementation MAY provide features aimed at
automating the application of the graceful shutdown procedures
described above.</t>
<t>Upon a session shutdown specified as to be graceful by the
operator, a BGP implementation supporting a g-shut feature would
</t>
<t>
<list style = "numbers" hangIndent="5">
<t>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.
</t>
<t>Lower the local preference 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
preference 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. This simplified behavior
can lead to some LoC, as described in <xref
target="sec.loc-pref.infilter"/>, if not used in conjunction
with <xref target="BestExternal"/>.</t>
<t>Optionally shut down the session after a configured
time.</t>
<t>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.</t>
</list>
</t>
<t>
</t>
</section>
</section><!--Graceful shutdown procedures for eBGP sessions-->
<!--
<section title = "Using multiple GSHUT community values">
<t>As for the outbound traffic, and as
illustrated in <xref target="sec.applicability"/>, a two step approach
may be used to avoid the LoC due to a
maintenance. The behavior described
here for inbound traffic is equivalent
to the in-filter reconfiguration step
described for the outbound traffic. If
a two step approach is required by the
peer for its outbound traffic (i.e.,
for the inbound traffic of the
maintained AS), then two community values could be used.
</t>
<t>One community value, GSHUT-out, could be
tagged to the old paths in a first
step, by the g-shut initiator. The
out-filter of the iBGP sessions of the
ASBR of the g-shut neighbor would be
configured to reduce the local-pref
value of such paths.</t>
<t>The second community value, GSHUT-in, would
lead to a local-pref decrease in the
in-filter of the g-shut neighbor, hence
applying the "in-filter" behavior as
described for outbound traffic. </t>
<t>Of course, ISPs are free to agree upon a
larger set of community values to apply
more complex maintenance policies.</t>
</section>
<section title = "Simplified g-shut procedure" anchor = "sec.inbound.simplified">
</section>
-->
<section title = "Graceful shutdown procedures for iBGP sessions">
<t>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. </t>
<t>However, in the case of a shutdown of a router, a
reconfiguration of the out-filters of the g-shut
initiator MAY 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).</t>
<t>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. </t>
</section><!--Graceful shutdown procedures for iBGP sessions-->
</section><!--Practices to avoid packet losses-->
<section title="Forwarding modes and forwarding loops" anchor="sec.forwarding">
<t>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, even if the procedure described above is
applied. Hence if zero LoC is required, encapsulation is
required between ASBRs of the AS.
</t>
<t> Using the out-filter reconfiguration avoids the forwarding
loops between the g-shut initiator and its directly connected
upstream neighboring routers. 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
initiator keeps using the affected paths via its eBGP peering
links. When all the upstream routers have switched to
alternate paths, the transition performed by the g-shut
initiator when the session is actually shut down, will be
loopfree. Transient forwarding loops between other routers
will not be avoided with this procedure. </t>
</section>
<section title = "Dealing with Internet policies" anchor = "sec.policies">
<t>A side gain of the maintenance solution is that it can also
reduce the churn implied by a shutdown of an eBGP session.</t>
<t>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. </t>
<t> For example, if an eBGP link is shut down between a
provider and one of its customers, and another 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 propagation of withdraw messages over eBGP sessions with
shared-cost peers and providers.</t>
<t> Proceeding like this reduces both BGP churn and traffic
shifting as routers will less likely switch to transient
paths.</t>
<t> In the above 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.</t>
</section><!-- Dealing with Internet Policies-->
<!-- def
<section title = "Applicability of the g-shut procedure">
<t> The applicability of the procedure described in this
draft to the cases presented in <xref target = "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. </t>
</section>
-->
<!--- <section title = "How to choose a solution" anchor = "sec.pickone"> </section>-->
<!--
<section title = "Better Future" anchor = "sec.better">
<t> A standardized community value could be used to ease the
configuration related to the graceful shutdown of eBGP
sessions. By proceeding like this, neighboring ASes would not
have to agree on a specific community value dedicated to this
task, and the value of the community would not differ from one
neighboring AS to another.</t>
<t> An automatic application of the local pref tunning could be
featured by vendors. Various possibilities, from
a configuration-free to more complex, policy-friendly,
configurations of the technique could be provided by vendors.</t>
</section>
-->
<section title = "Link Up cases">
<t>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. </t>
<section title = "Unreachability local to the ASBR">
<t>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. </t>
<t>A BGP speaker implementation could avoid such losses by
ensuring that "third party" nexthops are resolved before
installing paths using these in the RIB.</t>
<t>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.
</t>
</section><!--Unreachability local to the ASBR-->
<section title = "iBGP convergence">
<t> Similar corner cases as described in <xref
target="sec.loc-pref.infilter"/> for the link down case, can occur
during an eBGP link up event.</t>
<t>A typical example for such transient unreachability for a given
prefix is the following :</t>
<t><list style="empty" hangIndent="5" >
<t>1. A Route Reflector, 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. Another route reflector of the full-mesh, RR2,
knows only that path towards the prefix.
</t>
<t>2. A third Route Reflector of the RR full-mesh, RR3
receives a new best path orginated 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.
</t>
<t>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.
</t>
<t>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. </t>
</list>
</t>
<t> The use of <xref target="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.</t>
<t> 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.</t>
</section><!--iBGP convergence-->
</section><!--Link up cases-->
<section title = "IANA considerations">
<t>Applying the g-shut procedure is rendered much easier with a
reserved g-shut community value. The community value 0xFFFF0000 has been reserved
from the FCFS community pool for this purpose. </t>
</section><!--IANA-->
<section title = "Security Considerations">
<t> 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. </t>
<t> 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.</t>
<t>If this behavior is not tolerated by the ISP, it SHOULD monitor the use
of the g-shut community by this neighbor.</t>
<t>ASes which support the g-shut procedure SHOULD remove the
community value(s) that they use for g-shut from the paths
received from neighboring ASes that do not support the
procedure or to whom the service is not provided. Doing so
prevents malignant ASes from using the community through
intermediate ASes that do not support the feature, in order to
perform inbound traffic engineering.</t>
</section><!-- Security Considerations-->
<section title = "Acknowledgments">
<t>The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra
for their useful comments on this work.</t>
</section>
</middle>
<back>
<references>
<reference anchor = "AddPath">
<front>
<title>Advertisement of Multiple Paths in BGP</title>
<author initials ="" surname = "D. Walton" fullname="D. Walton"></author>
<author initials = "" surname = "A. Retana" fullname = "A. Retana"></author>
<author initials ="" surname = "E. Chen" fullname="E. Chen"></author>
<!--<date month = "July" year = "2007"></date>-->
</front>
<seriesInfo name ="Internet-Draft" value = "draft-walton-bgp-add-paths-06.txt"/>
</reference>
<reference anchor = "BestExternal">
<front>
<title>Advertisement of the best-external route to IBGP</title>
<author initials ="P." surname = "Marques" fullname="P. Marques"></author>
<author initials ="R." surname = "Fernando" fullname="R. Fernando"></author>
<author initials ="E." surname = "Chen" fullname="E. Chen"></author>
<author initials ="P." surname = "Mohapatra" fullname="P. Mohapatra"></author>
<date month = "May" year="2009"> </date>
</front>
<seriesInfo name="" value ="draft-ietf-idr-best-external-00.txt"/>
</reference>
<reference anchor = "REQS">
<front>
<title>Requirements for the graceful shutdown of BGP sessions</title>
<author initials ="B." surname = "Decraene" fullname="B. Decraene"></author>
<author initials ="P." surname = "Francois" fullname="P. Francois"></author>
<author initials ="C." surname = "Pelsser" fullname="C. Pelsser"></author>
<author initials ="Z." surname = "Ahmad" fullname="Z. Ahmad"></author>
<author initials ="A." surname = "Armengol" fullname="A. J. Elizondo Armengol"></author>
<author initials ="T." surname = "Takeda" fullname="T. Takeda"></author>
<date month = "October" year="2010"> </date>
</front>
<seriesInfo name="" value ="draft-ietf-grow-bgp-graceful-shutdown-requirements-06.txt"/>
</reference>
<reference anchor='RFC4360'>
<front>
<title>BGP Extended Communities Attribute</title>
<author initials='S.' surname='Sangli' fullname='S. Sangli'>
<organization /></author>
<author initials='D.' surname='Tappan' fullname='D. Tappan'>
<organization /></author>
<author initials='Y.' surname='Rekhter' fullname='Y. Rekhter'>
<organization /></author>
<date year='2006' month='February' />
<abstract>
<t>This document describes the "extended community" BGP-4 attribute. This attribute provides a mechanism for labeling information carried in BGP-4. These labels can be used to control the distribution of this information, or for other applications. [STANDARDS TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='4360' />
<format type='TXT' octets='24145' target='ftp://ftp.isi.edu/in-notes/rfc4360.txt' />
</reference>
<reference anchor = "Clarification4360">
<front>
<title>RFC 4360 Clarification Request</title>
<author initials ="B." surname = "Decraene" fullname="B. Decraene"></author>
<author initials ="L." surname = "Vanbever" fullname="L. Vanbever"></author>
<author initials ="P." surname = "Francois" fullname="P. Francois"></author>
<date month = "October" year="2009"> </date>
</front>
<seriesInfo name="" value ="draft-decraene-idr-rfc4360-clarification-00"/>
</reference>
<reference anchor='BGPWKC'>
<front>
<title>http://www.iana.org/assignments/bgp-well-known-communities</title>
</front>
</reference>
<reference anchor='RFC2119'>
<front>
<title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='Scott Bradner'>
<organization>Harvard University</organization>
<address>
<postal>
<street>1350 Mass. Ave.</street>
<street>Cambridge</street>
<street>MA 02138</street></postal>
<phone>- +1 617 495 3864</phone>
<email>sob@harvard.edu</email></address></author>
<date year='1997' month='March' />
<area>General</area>
<keyword>keyword</keyword>
<abstract>
<t>
In many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
<list>
<t>
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.
</t></list></t>
<t>
Note that the force of these words is modified by the requirement
level of the document in which they are used.
</t></abstract></front>
<seriesInfo name='BCP' value='14' />
<seriesInfo name='RFC' value='2119' />
<format type='TXT' octets='4723' target='ftp://ftp.isi.edu/in-notes/rfc2119.txt' />
<format type='HTML' octets='17491' target='http://xml.resource.org/public/rfc/html/rfc2119.html' />
<format type='XML' octets='5777' target='http://xml.resource.org/public/rfc/xml/rfc2119.xml' />
</reference>
</references>
<section title = "Alternative techniques with limited applicability">
<t>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.</t>
<section title = "In-filter reconfiguration" anchor = "sec.loc-pref.infilter">
<t>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.</t>
<t>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.</t>
<t>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 advertising the new path over its iBGP and eBGP sessions.</t>
<t>If this new path was received over an iBGP session, the g-shut
initiator will select that path and withdraw the previously
advertised path over its non-client iBGP sessions. 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.</t>
<t> Also, applying an In-filter reconfiguration on the eBGP
session undergoing the maintenance may lead to transient LoC, in
full-mesh iBGP topologies if </t>
<t>
<list style="empty" hangIndent="5" >
<t> a. An ASBR of the
initiator AS, ASBR1 did not
initially select its own
external path as best, and
</t>
<t> </t>
<t> b. An ASBR of the
initiator AS, ASBR2 advertises
a new path along its
iBGP sessions upon the
reception of ASBR1's update
following the in-filter
reconfiguration on the g-shut
initiator, and</t>
<t> </t>
<t> c. ASBR1 receives the
update message, runs its
Decision Process and hence
withdraws its
external path after having
selected ASBR2's path as best,
and</t>
<t> </t>
<t> d. An impacted router of the AS processes the
withdraw of ASBR1 before processing
the update from ASBR2.</t>
</list>
</t>
<t> Applying a reconfiguration of the out-filters prevents
such transient unreachabilities.</t>
<t> Indeed, when the g-shut initiator propagates an update of
the old path first, the withdraw from 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. </t>
<t> 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.</t>
<t>Note that applying this simplified procedure in conjunction
with <xref target="BestExternal"/> does not lead to LoC.</t>
</section><!--In-filter reconfiguration-->
<section title = "Multi Exit Discriminator tweaking" anchor = "sec.med-poison">
<t> 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. </t>
<t> 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. </t>
</section><!--MED-->
<section title = "IGP distance Poisoning" anchor = "sec.igp-poison">
<t> 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.</t>
<t> 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.</t>
</section><!--IGP distance poisoning-->
</section><!--Techniques with limited applicability-->
</back>
</rfc>
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