Network Working Group P. Mohapatra
Internet-Draft Cisco Systems
Intended status: Standards Track R. Fernando
Expires: March 31, 2009 Juniper Networks
C. Filsfils
Cisco Systems
R. Raszuk
Juniper Networks
September 27, 2008
Fast Connectivity Restoration Using BGP Add-path
draft-pmohapat-idr-fast-conn-restore-00
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Abstract
A BGP route defines an association of an address prefix with an "exit
point" from the current Autonomous System (AS). If the exit point
becomes unreachable due to a failure, the route becomes invalid.
This usually triggers an exchange of BGP control messages after which
a new BGP route for the given prefix is installed. However,
connectivity can be restored more quickly if the router maintains
precomputed BGP backup routes. It can then switch to a backup route
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immediately upon learning that an exit point is unreachable, without
needing to wait for the BGP control messages exchange. This document
specifies the procedures to be used by BGP to maintain and distribute
the precomputed backup routes. Maintaining these additional routes
is also useful in promoting load balancing, performing maintenance
without causing traffic loss, and in reducing churn in the BGP
control plane.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Basic Idea . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Design Considerations . . . . . . . . . . . . . . . . . . . . 5
3.1. Ensuring Loop-Free Path Selection in an AS . . . . . . . . 5
3.1.1. Border routers announcing single path . . . . . . . . 5
3.1.2. Border routers announcing multiple paths . . . . . . . 6
3.1.3. Confederations . . . . . . . . . . . . . . . . . . . . 6
3.2. Keeping Path Attributes Independent of Decision Process . 7
4. Border router attr_set attribute . . . . . . . . . . . . . . . 7
5. Calculation of Best and Backup Paths . . . . . . . . . . . . . 8
6. Advertising Multiple Paths . . . . . . . . . . . . . . . . . . 12
7. Deployment Considerations . . . . . . . . . . . . . . . . . . 12
8. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Fast Connectivity Restoration . . . . . . . . . . . . . . 13
8.2. Load Balancing . . . . . . . . . . . . . . . . . . . . . . 13
8.3. Churn Reduction . . . . . . . . . . . . . . . . . . . . . 14
8.3.1. Inter-domain Churn Reduction . . . . . . . . . . . . . 14
8.3.2. Intra-Domain Churn Reduction . . . . . . . . . . . . . 14
8.4. Graceful Maintenance . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
Within an autonomous system, the availability of multiple routes to a
given destination, where each of the routes has a different "exit
point" from the local AS provides the following benefits:
o Fault tolerance: Knowledge of multiple "exit points" leads to
reduction in restoration time after failure. For instance, a
border router on receiving multiple paths to the same destination
could decide to precompute a backup path and have it ready so that
when the primary path becomes invalid, it could use the backup to
quickly restore connectivity. Currently the restoration time is
dependent on BGP protocol re-convergence that includes a set of
withdraw and advertisement messages in the network before a new
best path can be learnt.
o Load balancing: The availability of multiple paths to reach the
same destination enables load balancing of traffic provided the
paths for the given destination satisfy certain constraints.
o Churn reduction: The advertisement of multiple routes, in certain
scenarios (Section 8.3.2), could lead to less churn in the network
upon a failure, since the presence of multiple paths helps contain
the failure to the local AS where the failure occurs.
o Graceful maintenance: The availability of alternate exit points
allows one to bring down a router for maintenance without causing
significant traffic loss.
Unfortunately, the border routers in an AS do not receive multiple
paths for all prefixes. The reason is three-fold:
o The current BGP specification [RFC4271] specifies routers to
advertise only the best path for a destination to speakers. The
availability of multiple paths requires simultaneous distribution
of multiple routes for a given prefix by a BGP speaker. We refer
to this property of the network as "path diversity".
o When a router selects an IBGP learnt path as best, it does not
announce any path for that prefix to IBGP though it may have EBGP
learnt paths available. This loss of information leads to added
churn and increases convergence time if the preferred path goes
away. A mechanism to advertise the best-external path to IBGP is
proposed in [I-D.marques-idr-best-external].
o Most service providers deploy one of the scaling techniques like
route reflectors [RFC4456] or confederations [RFC5065] inside the
AS and avoid iBGP full mesh. Thus even when multiple paths exist,
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the aggregation points (route reflectors or confederation border
routers) advertise only the best path (as per the BGP base
protocol).
As an effect of this behavior, the ingress border routers to an AS do
not receive additional paths necessary to provide the benefits cited
above: e.g. perform a local recovery during network failures or
achieve load balancing in steady state across multiple exit points.
The mechanism to extend BGP to allow a given BGP speaker to advertise
multiple paths simultaneously for a destination is defined in
[I-D.walton-bgp-add-paths]. The current draft describes the use of
this generic technique and certain additional procedures and
implementation guidelines to enable the above applications.
More specifically, this document describes extensions to BGP decision
process to select backup paths in a manner that ensures the important
property of consistent route selection within an AS. It also
introduces a new BGP attribute, attr_set, that border routers should
use to advertise multiple EBGP learnt paths for a given destination.
To aid with better description of the applications, the draft
illustrates certain use case scenarios for each.
One implication of multiple path advertisement is the associated
cost, namely the performance overhead of processing and memory
overhead of storing additional paths. It is anticipated that the
benefits listed above outweigh the cost in most scenarios. Be that
as it may, it is also expected that there will be configuration knobs
provided to limit the number of additional paths propagated within an
AS.
1.1. Requirements Language
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. Basic Idea
This document proposes two main additions to the BGP procedures:
1. The decision process is modified to determine backup paths along
with the best path selection when multiple paths for a
destination are available.
2. In addition to using these backup paths for fast connectivity
restoration locally, BGP speakers also advertise these paths to
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IBGP to increase the overall path diversity.
As alluded to in Section 1, BGP speakers that are the aggregation
points (router reflectors or confederation border routers) need to
announce backup paths to increase the path diversity at the
ingress routers of an IBGP network (see Figure 2). It may also be
useful, in certain cases, for the border routers to advertise
multiple paths received via EBGP for a destination when it is
redundantly connected and is transparently passing the NEXT_HOP
field unchanged instead of setting it to self (see Figure 4). To
this end, the draft defines a new attribute, attr_set, that the
border routers should advertise to ensure path selection
consistency.
The following sections elaborate on these points.
3. Design Considerations
3.1. Ensuring Loop-Free Path Selection in an AS
It is critical that BGP speakers within an AS have an eventual
consistent routing view of destinations and do not make conflicting
decisions regarding best path selection that would otherwise cause
forwarding loops. The current BGP protocol ensures this property by
defining a decision process that takes the attributes of paths as
input and determines a degree of preference of the paths by applying
a constant function. A consistent view of attributes is disseminated
through IBGP. Thus each BGP speaker within the AS determines the
same degree of preference of the paths after applying the constant
function independently. (The one exception is where IGP metric plays
the tie breaking role. In this case, different routers may choose
different next hops that are closer to them; but loop freedom is
guaranteed.).
When the above mechanism is extended to select backup paths for the
applications cited in this document, it is equally important to
maintain the same consistency property for the backup paths, i.e.
there should be no loops created when routers use the backup path in
forwarding. The rest of the document goes into the details of this
for various scenarios.
3.1.1. Border routers announcing single path
In scenarios where all border routers advertise a single external
path (their best path or best-external path) into IBGP, a consistent
routing view of best path and backup paths can be created across the
AS with the current BGP selection rules.
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3.1.2. Border routers announcing multiple paths
There are scenarios where border routers need to advertise the best
and backup EBGP learnt paths with NEXT_HOP unchanged to IBGP. If the
border router sets next hop to self, the paths become
indistinguishable and hence advertisement of only the best path is
sufficient. An example scenario is depicted in Figure 4.
By using the add-path ([I-D.walton-bgp-add-paths]) extensions, the
border routers could advertise multiple such EBGP-learnt paths. But
doing so can potentially create an inconsistency between the paths
that the sending and receiving routers select for forwarding. In
other words, the routers in the IBGP mesh can make independent and
separate decisions on the route selection since some of the values
that play a role in the tie breaking steps of the decision process at
the sender are not available to the rest of the BGP speakers of the
AS. These are mainly (1) the interior cost, i.e. the metric to reach
the external next hop, (2) BGP identifier of the peer, (3) the peer
IP address. Due to this reduction in information, there can be
inconsistency in the routing view within an AS.
Additionally, [RFC5004] proposes an extension to avoid best path
transitions at the border router between external paths based on a
temporal order of receiving the paths. This can also create an
inconsistency across the BGP speakers in the path selection.
This document proposes two modifications to ensure consistency:
a. Border routers SHOULD not apply the modification to the selection
rules as proposed in [RFC5004] to avoid best path transitions for
parallel EBGP connection scenario where the border router wishes
to transitively transmit the NEXT_HOP value unchanged.
b. To overcome the "information reduction" problem described above,
the document specifies an attribute called "border router
attr_set attribute" that encodes the properties of each path
advertised that would otherwise not be included using the normal
attributes in a BGP UPDATE message (see Section 4).
3.1.3. Confederations
When an AS employs confederations and the confederation border
routers advertise multiple paths, there is no way to distinguish the
originator (the actual egress border router originating the prefix to
the AS). To ensure consistent path selection, the confederation
border routers should create the ORIGINATOR_ID attribute as described
in [RFC4456] that carries the BGP identifier of the originator of the
route to the local AS.
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3.2. Keeping Path Attributes Independent of Decision Process
In addition to providing consistency in path selection, the solution
should satisfy the following important property: the attributes
associated with a particular path should be invariant when a
different path is advertised or withdrawn. Other things being equal,
it is best to avoid the potential churn introduced by the feedback
loops that would occur if path attributes were changed at the sender
as a result of running the decision process. Thus we do not use any
attributes with semantics like "this is my second best path", "this
is my third best path", etc. This requirement precludes use of
marking or other means of indicating path ordering from sender's
perspective since a change in the ordering requires re-advertising
most of the paths.
4. Border router attr_set attribute
Border router attr_set attribute is an optional non-transitive
attribute that is composed of a set of Type-Length-Value (TLVs)
encodings. The type code of the attribute is to be assigned by IANA.
Each TLV contains an attribute of the path from the border router
that is not otherwise sent as part of the UPDATE message. The TLV is
structured as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr Type | Length |
| (1 octet) | (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Value |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Border router Attr_set attribute fomrat
a. Attr Type (1 octet): It identifies the type of the attribute that
is encoded by the border router. Unknown types are to be ignored
and skipped upon receipt. This document defines the following
types:
* Interior Cost: Attr Type = 1
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* peer BGP Identifier: Attr Type = 2
* IPv4 Peer Address: Attr Type = 3
* IPv6 Peer Address: Attr Type = 4
b. Length (1 octet): the total number of octets of the Value field.
c. Value (variable): The value field encodes the attribute of the
corresponding type. For "Interior Cost" type, it encodes the
four octet metric value. For "BGP Identifier" type, it encodes
the four-octet router identifier of the neighbor for the path.
For "IPv4 Peering Address" type, the 4 byte BGP IPv4 peering
address is encoded. For "IPv6 Peering Address" type, the 16 byte
BGP IPv6 peering address is encoded.
A brief description of how a BGP speaker constructs the attribute is
provided in Section 6.
5. Calculation of Best and Backup Paths
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/----------------------------------------\
| +----+ IBGP |
| r1 |
| +----+ |
.
| . |
.
| +----+ |
| RR |
| . +----+ . |
. .
| . . |
+----+ +----+
| | r3 | | r4 | |
+----+ +----+
| | | |
\ | P1 | P2 /
----------------------------------------
| |
EBGP | | EBGP
...............
/ \
Destination a
\ /
...............
Figure 2: Basic RR topology
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/----------------------------------------\
| |
+----+ +----+
| | r1 |...........| r2 | |
+----+ +----+
| . . |
. AS 65502 .
| . . |
. +----+ .
| .....|CBR2|....... |
+----+
| | |
| CONFED EBGP
| +----+ |
.....|CBR1|.......
| . +----+ . |
. .
| . AS 65501 . |
. .
| +----+ +----+ |
| r3 |...........| r4 |
| +----+ +----+ |
|P1 |P2
| | | |
----------------------------------------
| |
EBGP | | EBGP
...............
/ \
Destination a
\ /
...............
Figure 3: Confederation topology
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/----------------------------------------\
| +----+ IBGP |
| r1 |
| +----+ |
.
| . |
.
| +----+ |
| RR |
| . +----+ . |
. .
| . . |
+----+ +----+
| | r3 | | r4 | |
+----+ +----+
| | | |
\ P1| |P2 /
----------------------------------------
| |
EBGP| |EBGP
...............
/ \
Destination a
\ /
...............
Figure 4: Border router with parallel eBGP links
The decision process as described in [RFC4271] is followed to
determine the overall best path for a destination. In addition, the
following rule SHOULD be inserted into the tie breaking rules of the
BGP decision process after step f) (Sect. 9.1.2.2: [RFC4271]) and
after the CLUSTER_LIST length check step (Sect. 9: [RFC4456]): a BGP
speaker SHOULD apply the tie breaking steps (steps (e), (f), and (g)
as defined in [RFC4271]) with the values encoded in the "border
router attr_set" attribute.
Note that the above step effectively compares multiple paths that are
advertised by the same egress border router (since the BGP Identifier
comparison step earlier would have eliminated paths from different
egress border routers).
Consider the network in Figure 4. r3 learns two paths P1 and P2 for
destination a and wishes to advertise both to the iBGP mesh with
NEXT_HOP value unchanged. We need to ensure that both r3 and the
other ingress routers in the network (r1, r4) make a consistent route
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selection for the best and the backup paths for destination a. The
current tie breaking rules [step f) comparison of router ID or
ORIGINATOR_ID and step g) comparision of peering ID] are insufficient
since at the ingress routers, both the paths will be received with
same values for each of the above parameters. Hence an additional
tie breaking rule comparing the original values that the border
router itself used to tie break the paths is required.
Once the best path is chosen, eliminate that path and all paths that
have the same BGP Identifier or NEXT_HOP as the choosen best path.
Note that as specified in [RFC4456], if the path carries the
ORIGINATOR_ID attribute, that should be treated as the BGP
Identifier. Then rerun the best path procedure to choose the backup
path. The Tie Breaking rules of the BGP decision process for second
best path selection are also modified as described above.
This mechanism can be recursively used to calculate multiple backup
paths if desired.
6. Advertising Multiple Paths
The technique outlined in [I-D.walton-bgp-add-paths] is used to
advertise best and backup paths selected with the rules described in
Section 5. For the purposes of the applications cited in this
document, the "Path Identifier" is always treated as an opaque value
with no semantics.
When an egress border router chooses to advertise multiple paths
learnt via EBGP to IBGP, it SHOULD include the attr_set attribute as
defined in Section 4 for each of the paths. The attribute is
constructed by encoding the following properties of the path in TLV
format:
o The interior cost to reach the NEXT_HOP of the path, encoded with
type 1.
o The BGP identifier of the EBGP peer from which it received the
path, encoded with type 2.
o The peer address of the EBGP peer from which it received the path,
encoded either with type 3 or 4.
7. Deployment Considerations
To ensure consistency in path selection process across all the
routers in an AS, the deployment considerations from the individual
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scaling technology employed in the network should be inherited/
applied. For example, as specified in [RFC4456], the intra-cluster
IGP metric values should be better than the inter-cluster IGP metric
values. Similar considerations as specified in [RFC5065] should be
designed.
8. Applications
8.1. Fast Connectivity Restoration
Consider the network in Figure 2. All 4 routers indicated are part
of a single AS. r3 and r4 are the border routers. Suppose r3 and r4
receive paths P1 and P2 for the same prefix. Also assume that P1 is
the preferred exit.
There are two scenarios to consider:
o case 1: P1 is the preferred exit for all routers within the AS
(including r4). In this case, if r4 follows [RFC4271], r4
withdraws P2 from the IBGP cloud.
o case 2: P2 is preferred exit by r4. In this case, if RR follows
[RFC4271], RR gets both paths, chooses one and sends it to r1.
In both the cases above, 'r1' holds only a single path and only after
a failure that makes P1 unavailable, it receives the alternate path
(P2).
However, if both paths were available to 'r1' and all other border
routers in the network, then they could precompute backup paths and
keep them ready to restore connectivity upon being notified of a
failure. The failure notification could be triggered due to a link
failure between 'r3' and its EBGP neighbor. This failure could be
propagated to other routers in r3's AS either via IGP or BGP,
resulting in invalidating on all these routers their primary paths
that were advertised by that neighbor to r3 (and that r3 subsequently
re-advertised into IBGP). Once these paths are invalidated, all
these routers could switch to the precomputed backup paths, without
waiting for any additional BGP advertisements.
8.2. Load Balancing
In the above network, not only can the additional path be used as a
standby best, but can also be used in steady state to load balance
traffic across the two exit points.
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8.3. Churn Reduction
There are two aspects to reducing churn - Inter-domain and Intra-
domain.
8.3.1. Inter-domain Churn Reduction
Consider the network diagram in Figure 5.
+----+
| r5 |
+----+
| EBGP
-------------
|
+----+
| r1 |
+----+
. .
. .
+----+ +----+
| r3 | | r4 |
+----+ +----+
| P1 | P2
Figure 5
'r5' is an EBGP peer of 'r1'. Today, if path P1 goes away, due to
the non-availability of other paths, 'r1' sends a withdraw to r5 thus
triggering a churn in the Internet. This could be significant if
there are multiple prefixes involved. On the other hand, if r1 had
an alternate path (with identical attributes), then this churn could
be entirely avoided by r1 performing a local repair.
8.3.2. Intra-Domain Churn Reduction
Since advertising multiple paths in general increases the path
diversity at the border routers, some of the control plane churn in
terms of a stream of advertisements, withdraws, and re-advertisements
can be reduced, thus improving the stability of the network.
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AS 2
|
|
+----+ +----+
| r1 | | r2 |
+----+ +----+
. .
. .
. .
+----+
| RR |
. +----+ .
. .
. .
+----+ +----+
| r3 | | r4 |
+----+ +----+
\ /
\ / eBGP dual-homing
AS 1(a)
Figure 6
Assuming router r3's path is the best path in the AS, RR advertises
the corresponding route information to the iBGP network. If r3 goes
down (or the peering link [r3, AS1] fails and r3 didn't change the
next hop to itself), the following will be sequence of updates from
router r1 to AS 2:
o Initial update for all prefixes when r1 chooses best path,
o Withdraws for all prefixes when r1 detects failure,
o Re-advertisement of all prefixes when the RR chooses router r4's
path as the new best path and advertises to r1.
With both the paths advertised and received on router r1, the
sequence of updates reduces to:
o Initial update for all prefixes when r1 chooses best path,
o Re-advertisement of all prefixes when r1 detects failure and
chooses router r4's path as the new best path
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8.4. Graceful Maintenance
[I-D.decraene-bgp-graceful-shutdown-requirements] defines
requirements for graceful maintenance of routers in a service
provider network. Current BGP operations treat this as a sudden link
or node failure and try to reconverge that can take in the order of
seconds or minutes.
With the procedures defined in this document, since alternate paths
are available at the ingress routers, taking down egress routers from
the network does not result in a network-wide reconvergence event.
9. Acknowledgements
The authors would like to thank Enke Chen for the many discussions
resulting in this work. In addition, the authors would also like to
acknowledge valuable review and suggestions from Eric Rosen, Yakov
Rekhter, and John Scudder on this document.
10. IANA Considerations
This document defines a new BGP optional non-transitive attribute
type, called Border Router Attr_set attribute. The attribute type is
to be assigned by IANA.
This document introduces Attr TLVs within the above attribute. The
type space for these should be set up by IANA as a registry of
1-octet attr types. These should be assigned on a first-come-first-
serve basis.
This document defines the following attr types that should be
assigned in the registry:
Attr Type
--------------- -----
Interior Cost 1
Peer BGP Identifier 2
IPv4 Peer Address 3
IPv6 Peer Address 4
11. Security Considerations
There are no additional security risks introduced by this design.
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12. References
12.1. Normative References
[I-D.decraene-bgp-graceful-shutdown-requirements]
Decraene, B., Francois, P., pelsser, c., Ahmad, Z., and A.
Armengol, "Requirements for the graceful shutdown of BGP
sessions",
draft-decraene-bgp-graceful-shutdown-requirements-00 (work
in progress), February 2008.
[I-D.marques-idr-best-external]
Marques, P., Fernando, R., Chen, E., and P. Mohapatra,
"Advertisement of the best-external route to IBGP",
draft-marques-idr-best-external-00 (work in progress),
July 2008.
[I-D.walton-bgp-add-paths]
Walton, D., "Advertisement of Multiple Paths in BGP",
draft-walton-bgp-add-paths-06 (work in progress),
July 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, August 2007.
12.2. Informative References
[RFC5004] Chen, E. and S. Sangli, "Avoid BGP Best Path Transitions
from One External to Another", RFC 5004, September 2007.
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Authors' Addresses
Pradosh Mohapatra
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: pmohapat@cisco.com
Rex Fernando
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Email: rex@juniper.net
Clarence Filsfils
Cisco Systems
Brussels,
Belgium
Email: cfilsfil@cisco.com
Robert Raszuk
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Email: raszuk@juniper.net
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