GROW Working Group B. Decraene
Internet-Draft France Telecom
Intended status: Informational P. Francois
UCL
C. Pelsser
IIJ
Z. Ahmad
Orange Business Services
A. J. Elizondo Armengol
Telefonica I+D
T. Takeda
NTT
October 22, 2010
Requirements for the graceful shutdown of BGP sessions
draft-ietf-grow-bgp-graceful-shutdown-requirements-06.txt
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Abstract
The Border Gateway Protocol(BGP) is heavily used in Service Provider
networks both for Internet and BGP/MPLS VPN services. For resiliency
purposes, redundant routers and BGP sessions can be deployed to
reduce the consequences of an AS Border Router or BGP session
breakdown on customers' or peers' traffic. However simply taking down
or even bringing up a BGP session for maintenance purposes may still
induce connectivity losses during the BGP convergence. This is not
satisfactory any more for new applications (e.g. voice over IP, on
line gaming, VPN). Therefore, a solution is required for the graceful
shutdown of a (set of) BGP session(s) in order to limit the amount of
traffic loss during a planned shutdown. This document expresses
requirements for such a solution.
Table of Contents
1. Conventions used in this document...........................3
2. Introduction................................................3
3. Problem statement...........................................4
3.1. Example of undesirable BGP routing behavior.................4
3.2. Causes of packet loss.......................................5
4. Terminology.................................................6
5. Goals and requirements......................................7
6. Reference Topologies........................................9
6.1. E-BGP topologies............................................9
6.2. I-BGP topologies...........................................11
7. Security Considerations....................................15
8. IANA Considerations........................................16
9. References.................................................16
9.1. Normative References.......................................16
9.2. Informative References.....................................16
10. Acknowledgments............................................17
11. Author's Addresses.........................................17
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1. Conventions used in this document
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. Introduction
The Border Gateway Protocol(BGP) [BGP-4] is heavily used in Service
Provider networks both for Internet and BGP/MPLS VPN services [VPN].
For resiliency purposes, redundant routers and BGP sessions can be
deployed to reduce the consequences of an AS Border Router or BGP
session breakdown on customers' or peers' traffic.
We place ourselves in the context where a Service Provider performs a
maintenance operation and needs to shut down one or multiple BGP
peering link(s) or a whole ASBR. If an alternate path is available
within the AS, the requirement is to avoid or reduce customer or peer
traffic loss during the BGP convergence. Indeed, as an alternate path
is available in the Autonomous System (AS), it should be made
possible to reroute the customer or peer traffic on this backup path
before the BGP session(s) is/are torn down, the nominal path
withdrawn and the forwarding is interrupted on the nominal path.
The requirements also cover the subsequent re-establishment of the
BGP session as even this "UP" case can currently trigger route loss
and thus traffic loss at some routers.
Currently, BGP [BGP-4] and MP-BGP [MP-BGP] do not include any
operation to gracefully advertise or withdraw a prefix while traffic
toward that prefix could still be correctly forwarded using the old
path. When a BGP session is taken down, BGP behaves as if it was a
sudden link or router failure and withdraws the prefixes learnt over
that session, which may trigger traffic loss. There is no mechanism
to advertise to its BGP peers that the prefix will soon be
unreachable, while still being reachable. When applicable, such
mechanism would reduce or prevent traffic loss. It would typically be
applicable in case of a maintenance operation requiring the shutdown
of a forwarding resource. Typical examples would be a link or line
card maintenance, replacement or upgrade. It may also be applicable
for a software upgrade as it may involve a firmware reset on the line
cards and hence forwarding interruption.
The introduction of Route Reflectors as per [RR] to solve scalability
issues bound to IBGP full-meshes has worsened the duration of routing
convergence as some route reflectors may hide the back up path. Thus
depending on RR topology more IBGP hops may be involved in the IBGP
convergence.
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Note that these planned maintenance operations cannot be addressed by
Graceful Restart extensions [GR] as GR only applies when the
forwarding is preserved during the control plane restart. On the
contrary, Graceful Shutdown applies when the forwarding is
interrupted.
Note also that some protocols are already considering such graceful
shutdown procedure (e.g. GMPLS in [RFC5817]).
A successful approach of such mechanism should minimize the loss of
traffic in most foreseen maintenance situations.
3. Problem statement
As per [BGP-4], when one (or many) BGP session(s) are shut down, a
BGP NOTIFICATION message is sent to the peer and the session is then
closed. A protocol convergence is then triggered both by the local
router and by the peer. Alternate paths to the destination are
selected, if known. If those alternates paths are not known prior to
the BGP session shutdown, additional BGP convergence steps are
required in each AS to search for an alternate path.
This behavior is not satisfactory in a maintenance situation because
the traffic that was directed towards the removed next-hops may be
lost until the end of the BGP convergence. As it is a planned
operation, a make before break solution should be made possible.
As maintenance operations are frequent in large networks
[Reliability], the global availability of the network is
significantly impaired by this BGP maintenance issue.
3.1. Example of undesirable BGP routing behavior
To illustrate these problems, let us consider the following simple
example where one customer router "CUST" is dual-attached to two SP
routers "ASBR1" and "ASBR2".
ASBR1 and ASBR2 are in the same AS and owned by the same service
provider. Both are IBGP client of the route reflector R1.
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'
AS1 ' AS2
'
/-----------ASBR1---
/ \
/ \
CUST R1
\ /
Z/z \ /
\-----------ASBR2---
'
AS1 ' AS2
'
Figure 1. Dual attached customer
Before the maintenance, packets for destination Z/z use the ASBR1-
CUST link because R1 selects ASBR1's route based on the IGP cost.
Let's assume the service provider wants to shutdown the ASBR1-CUST
link for maintenance purposes. Currently, when the shutdown is
performed on ASBR1, the following steps are performed:
1. ASBR1 sends a withdraw to its route reflector R1 for the prefix
Z/z.
2. R1 runs its decision process, selects the route from ASBR2 and
advertises the new path to ASBR1.
3. ASBR1 runs its decision process and recovers the reachability of
Z/z.
Traffic is lost between step 1 when ASBR1 looses its route and step 3
when it discovers a new path.
Note that this is a simplified description for illustrative purpose.
In a bigger AS, multiple steps of BGP convergence may be required to
find and select the best alternate path (e.g. ASBR1 is chosen based
on a higher local pref, hierarchical route reflectors are used...).
When multiple BGP routers are involved and plenty of prefixes are
affected, the recovery process can take longer than applications
requirements.
3.2. Causes of packet loss
The loss of packets during the maintenance has two main causes:
- lack of an alternate path on some routers,
- transient routing inconsistency.
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Some routers may lack an alternate path because another router is
hiding the backup path. This router can be:
- a route reflector only propagating its best path;
- the backup ASBR not advertising the backup path because it prefers
the nominal path.
This lack of knowledge of the alternate path is the first target of
this requirement draft.
Transient routing inconsistencies happen during IBGP convergence
because all routers are not updating their RIB and FIB at the same
time. This can lead to forwarding loops and then packet drops. The
duration of these transient micro-loops may depend on the IBGP
topology (e.g. number of Route Reflectors between ingress and egress
ASBR), implementation differences among router platforms (e.g. speed
to update the RIB and FIB, possibly the order in which prefixes are
modified), forwarding mode (hop by hop IP forwarding versus
tunneling).
Transient forwarding loops can be avoided by performing only one IP
lookup on BGP routes in each AS and by using tunnels (e.g. MPLS LSP)
to send packets between ASBRs. As such, BGP/MPLS VPNs should be
immune to such micro forwarding loops.
4. Terminology
g-shut: Graceful SHUTdown. A method for explicitly notifying the BGP
routers that a BGP session (and hence the prefixes learnt over that
session) is going to be disabled.
g-noshut: Graceful NO SHUTdown. A method for explicitly notifying
the BGP routers that a BGP session (and hence the prefixes learnt
over that session) is going to be enabled.
g-shut initiator: the router on which the session(s) shutdown is
(are) performed for the maintenance.
g-shut neighbor: a router that peers with the g-shut initiator
via (one of) the session(s) undergoing maintenance.
Affected prefixes: a prefix initially reached via the peering
link(s) undergoing maintenance.
Affected router: a router reaching an affected prefix via a
peering link undergoing maintenance.
Initiator AS: the autonomous system of the g-shut initiator
router.
Neighbor AS(es): the autonomous system(s) of the g-shut neighbor
router(s).
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5. Goals and requirements
When a BGP session of the router under maintenance is shut down, the
router removes the routes and then triggers the BGP convergence on
its BGP peers. The goal of BGP graceful shutdown is to initiate the
BGP convergence to find the alternate paths before the nominal paths
are removed. As a result, before the nominal BGP session is shut
down, all routers learn and use the alternate paths. Then the nominal
BGP session can be shut down.
As a result, provided an alternate path with enough remaining
capacity is available in the AS, the packets are rerouted before the
BGP session termination and fewer packets (possibly none) are lost
during the BGP convergence process since at any time, all routers
have a valid path.
Another goal is to minimize packet loss when the BGP session is re-
established following the maintenance.
From the above goals we can derive the following requirements:
a) A mechanism to advertise the maintenance action to all affected
routers is REQUIRED. Such mechanism may be either implicit or
explicit. Note that affected routers can be located both in the local
AS and in neighboring ASes. Note also that the maintenance action can
either be the shutdown of a BGP session or the establishment of a BGP
session.
The mechanism SHOULD allow BGP routers to minimize packet loss when a
path is removed or advertised. In particular, it SHOULD be ensured
that the old path is not removed from the routing tables of the
affected routers before the new path is known.
The solution mechanism MUST reduce packet loss but MAY provide only a
reduction rather than full minimization, in order to trade off with
simplicity of implementation and operation as shown in some of the
following requirements.
b) An Internet wide convergence is OPTIONAL. However if the
initiator AS and the neighbor AS(es) have a backup path, they SHOULD
be able to gracefully converge before the nominal path is shut down.
c) The proposed solution SHOULD be applicable to any kind of BGP
sessions (EBGP, IBGP, IBGP route reflector client, EBGP
confederations, EBGP multi hop, MultiProtocol BGP extension...) and
any address family. If a BGP implementation allows closing or
enabling a sub-set of AFIs carried in a MP-BGP session, this
mechanism MAY be applicable to this sub-set of AFIs.
Depending on the kind of session, there may be some variations in the
proposed solution in order to fulfill the requirements.
The following cases should be handled in priority:
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- The shutdown of an inter-AS link and therefore the shutdown of an
eBGP session;
- The shutdown of an AS Border Router and therefore the shutdown of
all its BGP sessions.
Service Providers and platforms implementing a graceful shutdown
solution should note that in BGP/MPLS VPN as per [VPN], the PE-CE
routing can be performed by other protocols than BGP (e.g. static
routes, RIPv2, OSPF, IS-IS). This is out of scope of this document.
d) The proposed solution SHOULD NOT change the BGP convergence
behavior for the ASes exterior to the maintenance process, namely
ASes other than the initiator AS and it(s) neighbor AS(es).
e) An incremental deployment on a per AS or per BGP session basis
MUST be made possible. In case of partial deployment the proposed
solution SHOULD incrementally improve the maintenance process.
It should be noted that in an inter domain relation, one AS may have
more incentive to use graceful shutdown than the other. Similarly, in
a BGP/MPLS VPN environment, it's much easier to upgrade the PE
routers than the CE mainly because there is at least an order of
magnitude more CE and CE locations than PE and PE locations. As a
consequence, when splitting the cost of the solution between the g-
shut initiator and the g-shut neighbour the solution SHOULD favour a
low cost solution on the neighbour AS side in order to reduce the
impact on the g-shut neighbour. Impact should be understood as a
generic term which includes first hardware, then software, then
configuration upgrade..
f) Redistribution or advertisement of (static) IP routes into BGP
SHOULD also be covered.
g) The proposed solution MAY be designed in order to avoid
transient forwarding loops. Indeed, forwarding loops increase packet
transit delay and may lead to link saturation.
h) The specific procedure SHOULD end when the BGP session is closed
following the g-shut and once the BGP session is gracefully opened
following the g-noshut. In the end, once the planned maintenance is
finished the nominal BGP routing MUST be reestablished.
The duration of the g-shut procedure, and hence the time before the
BGP session is safely closed SHOULD be discussed by the solution
document. Examples of possible solutions are the use of a pre-
configured timer, of a message to signal the end of the BGP
convergence or monitoring the traffic on the g-shut interface...
i) The solution SHOULD be simple and simple to operate. Hence it
MAY only cover a subset of the cases. (As a consequence, most of the
above requirements are expressed as "SHOULD" rather than "MUST")
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The metrics to evaluate and compare the proposed solutions are, in
decreasing order of importance:
- The duration of the remaining loss of connectivity when the BGP
session is brought down or up
- The applicability to a wide range of BGP and network topologies,
especially those described in section 6;
- The simplicity;
- The duration of transient forwarding loops;
- The additional load introduced in BGP (eg BGP messages sent to peer
routers, peer ASes, the Internet).
6. Reference Topologies
In order to benchmark the proposed solutions, some typical BGP
topologies are detailed in this section. The solution documents
should state the applicability of the solutions for each of these
possible topologies.
However, solutions SHOULD be applicable to all possible BGP
topologies and not only to these below examples. Note that this
is a "SHOULD" rather than a "MUST" as a partial lightweight
solution may be preferred to a full but more complex solution.
Especially since some ISP may not be concerned by some topologies
(e.g. confederations).
6.1. EBGP topologies
We describe here some frequent EBGP topologies that SHOULD be
supported by the solution.
6.1.1. 1 ASBR in AS1 connected to two ASBRs in the neighboring AS2
In this topology we have an asymmetric protection scheme between
AS1 and AS2:
- On AS2 side, two different routers are used to connect to AS1.
- On AS1 side, one single router with two BGP sessions is used.
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'
AS1 ' AS2
'
/----------- ASBR2.1
/ '
/ '
ASBR1.1 '
\ '
\ '
\----------- ASBR2.2
'
'
AS1 ' AS2
'
Figure 2. EBGP topology with redundant ASBR in one of the AS.
The requirements of section 5 should be applicable to:
- Maintenance of one of the routers of AS2;
- Maintenance of one link between AS1 and AS2, performed either
on an AS1 or AS2 router.
Note that in case of maintenance of the whole router, all its BGP
sessions need to be gracefully shutdown at the beginning of the
maintenance and gracefully brought up at the end of the
maintenance.
6.1.2. 2 ASBRs in AS1 connected to 2 ASBRs in AS2
In this topology we have a symmetric protection scheme between
AS1 and AS2: on both sides, two different routers are used to
connect AS1 to AS2.
'
AS1 ' AS2
'
ASBR1.1----------- ASBR2.1
'
'
'
'
'
ASBR1.2----------- ASBR2.2
'
AS1 ' AS2
'
Figure 3. EBGP topology with redundant ASBR in both ASes
The requirements of section 5 should be applicable to:
- Maintenance of any of the ASBR routers (in AS1 or AS2);
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- Maintenance of one link between AS1 and AS2 performed either on
an AS1 or AS2 router.
6.1.3. 2 ASBRs in AS2 each connected to two different ASes
In this topology at least three ASes are involved. Depending on
which routes are exchanged between these ASes, some protection
for some of the traffic may be possible.
'
AS1 ' AS2
'
ASBR1.1----------- ASBR2.1
| '
| '
'''''|''''''''''
| '
| '
ASBR3.1----------- ASBR2.2
'
AS3 ' AS2
Figure 4. EBGP topology of a dual homed customer
The requirements of section 5 do not translate as easily as in
the two previous topologies because we do not require propagating
the maintenance advertisement outside of the two ASes involved in
an eBGP session.
For instance if ASBR2.2 requires a maintenance affecting ASBR3.1,
then ASBR3.1 will be notified. However we do not require for ASBR1.1
to be notified of the maintenance of the eBGP session between
ASBR3.1-ASBR2.2.
6.2. IBGP topologies
We describe here some frequent IBGP topologies that SHOULD be
supported by the solution.
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6.2.1. IBGP Full-Mesh
In this topology we have a full mesh of iBGP sessions:
P1 ------ P2
| \ / |
| \ / |
| \ / | AS1
| / \ |
| / \ |
ASBR1.1---ASBR1.2
\ /
\ /
''''''\''''/''''''''''''
\ / AS2
ASBR2.1
Figure 5. IBGP full mesh
When the session between ASBR1.1 and ASBR2.1 is gracefully
shutdown, it is required that all routers of AS1 reroute traffic
to ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut
down.
Symmetrically, when the session between ASBR1.1 and ASBR2.1 is
gracefully brought up, it is required that all routers of AS1
preferring ASBR1.1 over ASBR1.2 reroute traffic to ASBR1.1 before
the less preferred path trough ASBR1.2 is possibly withdrawn.
6.2.2. Route Reflector
In this topology, route reflectors are used to limit the number of
IBGP sessions. There is a single level of route reflectors and the
route reflectors are fully meshed.
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P1 RR----- P2 RR
| \ / |
| \ / |
| \ / | AS1
| \ / |
| / \ |
| / \ |
| / \ |
ASBR1.1 ASBR1.2
\ /
\ /
''''''\''''''/''''''''''''
\ /
\ / AS2
ASBR2.1
Figure 6. Route Reflector
When the session between ASBR1.1 and ASBR2.1 is gracefully
shutdown, it is required that all BGP routers of AS1 reroute
traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1
is shut down.
Symmetrically, when the session between ASBR1.1 and ASBR2.1 is
gracefully brought up, it is required that all routers of AS1
preferring ASBR1.1 over ASBR1.2 reroute traffic to ASBR1.1 before
the less preferred path trough ASBR1.2 is possibly withdrawn.
6.2.3. hierarchical Route Reflector
In this topology, hierarchical route reflectors are used to limit
the number of IBGP sessions. There could me more than levels of
route reflectors and the top level route reflectors are fully
meshed.
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P1/hRR -------- P2/hRR
| |
| |
| | AS1
| |
| |
P3/RR P4/RR
| |
| |
| | AS1
| |
| |
ASBR1.1 ASBR1.2
\ /
\ /
''''''\'''''''''/''''''''''''
\ /
\ / AS2
ASBR2.1
Figure 7. Hierarchical Route Reflector
When the session between ASBR1.1 and ASBR2.1 is gracefully
shutdown, it is required that all BGP routers of AS1 reroute
traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1
is shut down.
Symmetrically, when the session between ASBR1.1 and ASBR2.1 is
gracefully brought up, it is required that all routers of AS1
preferring ASBR1.1 over ASBR1.2 reroute traffic to ASBR1.1 before
the less preferred path trough ASBR1.2 is possibly withdrawn.
6.2.4. Confederations
In this topology, a confederation of ASs is used to limit the number
of IBGP sessions. Moreover, RRs may be present in the member ASs of
the confederation.
Confederations may be run with different sub-options. Regarding the
IGP, each member AS can run its own IGP or they can all share the
same IGP. Regarding BGP, local_pref may or may not cross the member
AS boundaries.
A solution should support the graceful shutdown and graceful bring up
of EBGP sessions between member-ASs in the confederation in addition
to the graceful shutdown and graceful bring up of EBGP sessions
between a member-AS and an AS outside of the confederation.
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ASBR1C.1 ---------- ASBR1C.2
| |
| |
| AS1C |
| |
| |
"""|"""""""""""""""""""|"""
| " |
ASBR1A.2 " ASBR1B.2
| " |
| " |
| AS1A " AS1B | AS1
| " |
| " |
ASBR1A.1 " ASBR1B.1
\ " /
\ " /
''''''\'''''''''''''/''''''''''''
\ /
\ / AS2
ASBR2.1
Figure 8. Confederation
In the above figure, member-AS AS1A, AS1B, AS1C belong to a
confederation of ASs in AS1. AS1A and AS1B are connected to AS2.
In normal operation, for the traffic toward AS2,
. AS1A sends the traffic directly to AS2 through ASBR1A.1
. AS1B sends the traffic directly to AS2 through ASBR1B.1
. AS1C load balances the traffic between AS1A and AS1B
When the session between ASBR1A.1 and ASBR2.1 is gracefully shutdown,
it is required that all BGP routers of AS1 reroute traffic to
ASBR1B.1 before the session between ASBR1A.1 and ASBR2.1 is shut
down.
Symmetrically, when the session between ASBR1A.1 and ASBR2.1 is
gracefully brought up, it is required that all routers of AS1
preferring ASBR1A.1 over ASBR1.2 reroute traffic to ASBR1A.1 before
the less preferred path trough ASBR1.2 is possibly withdrawn.
7. Security Considerations
At the requirements stage, this graceful shutdown mechanism is
expected to not affect the security of the BGP protocol, especially
if it can be kept simple. No new sessions are required and the
additional ability to signal the graceful shutdown is not expected to
bring additional attack vector as BGP neighbors already have the
ability to send incorrect or misleading information or even shut down
the session.
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Security considerations MUST be addressed by the proposed
solutions. In particular they SHOULD address the issues of bogus
g-shut messages and how they would affect the network(s), as well
as the impact of hiding a g-shut message so that g-shut is not
performed.
The solution SHOULD NOT increase the ability for one AS to
selectively influence routing decision in the peer AS (inbound
Traffic Engineering) outside the case of the BGP session
shutdown. Otherwise, the peer AS SHOULD have means to detect such
behavior.
8. IANA Considerations
This document has no actions for IANA.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[BGP-4] Y. Rekhter, T. Li, "A Border Gateway protocol 4 (BGP)", RFC
4271, January 2006.
[MP-BGP] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC 4760 January 2007.
[RR] T. Bates, E. Chen, R. Chandra
"BGP Route Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456 April 2006.
[VPN] E. Rosen, Y. Rekhter
"BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364
February 2006.
9.2. Informative References
[RFC5817] Z. Ali, J.P. Vasseur, A. Zamfir and J. Newton
"Graceful Shutdown in MPLS and Generalized MPLS Traffic
Engineering Networks", RFC 5817 April 2010.
[GR] S. Sangli, E. Chen, R. Fernando, J. Scudder, Y. Rekhter
"Graceful Restart Mechanism for BGP", RFC 4724 January 2007.
[Reliability] Network Strategy Partners, LLC.
"Reliable IP Nodes: A prerequisite to profitable IP services",
November 2002. http://www.nspllc.com/NewPages/Reliable_IP_Nodes.pdf
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10. Acknowledgments
Authors would like to thank Nicolas Dubois, Benoit Fondeviole,
Christian Jacquenet, Olivier Bonaventure, Steve Uhlig, Xavier
Vinet, Vincent Gillet, Jean-Louis le Roux, Pierre Alain Coste and
Ronald Bonica for the useful discussions on this subject, their
review and comments.
This draft has been partly sponsored by the European project IST
AGAVE.
Authors' Addresses
Bruno Decraene
France Telecom
38-40 rue du General Leclerc
92794 Issy Moulineaux cedex 9
France
Email: bruno.decraene@orange-ftgroup.com
Pierre Francois
Universite catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve 1348
BE
Email: francois@info.ucl.ac.be
Cristel Pelsser
Internet Initiative Japan
Jinbocho Mitsui Building
1-105 Kanda jinbo-cho
Chiyoda-ku, Tokyo 101-0051
Japan
Email: cristel@iij.ad.jp
Zubair Ahmad
Orange Business Services
13775 McLearen Road, Oak Hill VA 20171
USA
Email: zubair.ahmad@orange-ftgroup.com
Antonio Jose Elizondo Armengol
Division de Analisis Tecnologicos
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Technology Analysis Division
Telefonica I+D
C/ Emilio Vargas 6
28043, Madrid
E-mail: ajea@tid.es
Tomonori Takeda
NTT Corporation
9-11, Midori-Cho 3 Chrome
Musashino-Shi, Tokyo 180-8585
Japan
Email: takeda.tomonori@lab.ntt.co.jp
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