One document matched: draft-ietf-grow-bgp-gshut-04.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-04' category="info">
<front>
<date month='October' year='2012' day='22'/>
<title>Graceful BGP session shutdown</title>
<author surname="Pierre Francois" name ="Francois" fullname="Pierre Francois">
<organization>Institute IMDEA Networks</organization> <address>
<postal>
<street>Avda. del Mar Mediterráneo, 22</street>
<city>Leganese</city> <code>28918</code>
<country>ES</country>
</postal>
<email>pierre.francois@imdea.org</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 Issy Moulineaux cedex 9</city> <code></code>
<country>FR</country>
</postal>
<email>bruno.decraene@orange.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>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 or links,
involving the shutdown of BGP peering sessions.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t> 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).</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
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.</t>
<t> The goal of the document is to meet the requirements described in <xref
target="REQS"/> at best, without changing the BGP protocol.</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> Initiator AS: the Autonomous System of the g-shut initiator.</t>
<t> Neighbor AS: the Autonomous System of the g-shut neighbor.</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 such as
<xref target="BestExternal"/> and <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>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 are expected for the
outgoing paths.</t>
</section><!--Improving availability of alternate paths-->
<section title = "Make before break convergence: g-shut" target = "sec.summary">
<t>This section describes configurations and actions to
be performed to perform a graceful shutdown procedure for eBGP
peering links. </t>
<t>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.</t>
<section title = "eBGP g-shut">
<section title = "Pre-configuration" anchor = "sec.summary.config">
<t> On each ASBR supporting the g-shut procedure, an outbound BGP route policy is
applied on all iBGP sessions of the ASBR, that:</t>
<list style = "symbols" hangIndent="5">
<t> matches the g-shut community</t>
<t> sets the LOCAL_PREF attribute 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_PREF
attribute, this AS specific g-shut community will be
used to override other LOCAL_PREF preference changes.</t>
</list>
<t>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. </t>
</section><!--Pre-configuration-->
<section title = "Operations at maintenance time" anchor = "sec.summary.shut">
<t> On the g-shut initiator, upon maintenance time, it is required to: </t>
<list style = "symbols">
<t> 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.
</t>
<t> apply an inbound BGP route policy 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 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 graceful by the
operator, a BGP implementation supporting a g-shut feature SHOULD:
</t>
<t>
<list style = "numbers" hangIndent="5">
<t>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.
</t>
<t>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. </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> A BGP implementation supporting a g-shut feature SHOULD also automatically
install the BGP policies that are supposed to be configured, as decribed in
<xref target = "sec.summary.config"/> for sessions over which g-shut
is to be supported. </t>
</section><!--Summary of operations-->
</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 = "iBGP g-shut">
<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>
</section><!--iBGP g-shut -->
<section title = "Router g-shut">
<t>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 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>
</section>
</section>
<section title="Forwarding modes and transient forwarding loops during convergence" anchor="sec.forwarding">
<t>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 runned 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. </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>Corner cases leading to LoC 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> Let's consider 3 route reflectors RR1, RR2, RR3. There is a full mesh
of iBGP session between them. </t>
<t><list style="empty" hangIndent="5" >
<t>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.
</t>
<t>2. 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 assigned g-shut BGP community">
<t> 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. </t>
<t> For Internet routes, a non transitive extended community will be reserved
from the pool defined in <xref target = "EXT_POOL"/>. Using such a community 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.
</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 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.</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 = "E. Chen" fullname="E. Chen"></author>
<author initials = "" surname = "A. Retana" fullname = "A. Retana"></author>
<author initials = "" surname = "J. Scudder" fullname = "J. Scudder"></author>
<!--<date month = "July" year = "2007"></date>-->
</front>
<seriesInfo name ="Internet-Draft" value = "draft-ietf-idr-add-paths-07.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>
<author initials = "H." surname = "Gredler" fullname = "H. Gredler"></author>
</front>
<seriesInfo name="" value ="draft-ietf-idr-best-external-05.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>
</front>
<seriesInfo name="RFC" value ="6198"/>
</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='EXT_POOL'>
<front>
<title>Assigned BGP extended communities</title>
<author initials='B.' surname='Decraene' fullname='D. Decraene'>
<organization /></author>
<author initials='P.' surname='Francois' fullname='P. Francois'>
<organization /></author>
</front>
<seriesInfo name="" value ="draft-ietf-idr-reserved-extended-communities-03"/>
</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 = "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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