OSPF Working Group A. Atlas
Internet-Draft S. Hegde
Intended status: Standards Track C. Bowers
Expires: January 1, 2018 Juniper Networks
J. Tantsura
Individual
Z. Li
Huawei Technologies
June 30, 2017
OSPF Extensions to Support Maximally Redundant Trees
draft-ietf-ospf-mrt-03
Abstract
This document specifies extensions to OSPF to support the distributed
computation of Maximally Redundant Trees (MRT). Some example uses of
the MRTs include IP/LDP Fast-Reroute and global protection or live-
live for multicast traffic. The extensions indicate what MRT
profile(s) each router supports. Different MRT profiles can be
defined to support different uses and to allow transitioning of
capabilities. An extension is introduced to flood MRT-Ineligible
links, due to administrative policy.
The need for a mechanism to allow routers to advertise a worst-case
FIB compute/install time is well understood for controlling
convergence. This specification introduces the Controlled
Convergence TLV to be carried in the Router Information LSA.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 1, 2018.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview of OSPF Extensions for MRT . . . . . . . . . . . . . 4
4.1. Supporting MRT Profiles . . . . . . . . . . . . . . . . . 4
4.2. GADAG Root Selection . . . . . . . . . . . . . . . . . . 5
4.3. Triggering an MRT Computation . . . . . . . . . . . . . . 5
5. MRT Profile TLV in Router Information LSA . . . . . . . . . . 5
6. Advertising MRT-ineligible links for MRT . . . . . . . . . . 7
6.1. MRT-Ineligible Link sub-TLV . . . . . . . . . . . . . . . 7
7. Worst-Case Network Convergence Time . . . . . . . . . . . . . 8
8. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9
8.1. Handling MRT Capability Changes . . . . . . . . . . . . . 9
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
12.1. MRT Profile and Controlled Convergence TLVs . . . . . . 10
12.2. MRT-Ineligible Link sub-TLV . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1. Normative References . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
This document describes the OSPF extensions necessary to support the
architecture that defines how IP/LDP Fast-Reroute can use MRTs
[RFC7812]. At least one common standardized algorithm (such as the
lowpoint algorithm explained and fully documented in [RFC7811]) is
required so that the routers supporting MRT computation consistently
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compute the same MRTs. MRT can also be used to protect multicast
traffic via either global protection or local
protection.[I-D.atlas-rtgwg-mrt-mc-arch]
IP/LDP Fast-Reroute using MRTs can provide 100% coverage for link and
node failures in an arbitrary network topology where the failure
doesn't split the network. It can also be deployed incrementally
inside an OSPF area; an MRT Island is formed of connected supporting
routers and the MRTs are computed inside that island.
In the default MRT profile, a supporting router both computes the
MRTs and creates the necessary transit forwarding state necessary to
provide the two additional forwarding topologies, known as MRT-Blue
and MRT-Red.
2. 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 [RFC2119]
3. Terminology
For ease of reading, some of the terminology defined in [RFC7812] is
repeated here.
network graph: A graph that reflects the network topology where all
links connect exactly two nodes and broadcast links have been
transformed into the standard pseudo-node representation.
Redundant Trees (RT): A pair of trees where the path from any node
X to the root R along the first tree is node-disjoint with the
path from the same node X to the root along the second tree.
These can be computed in 2-connected graphs.
Maximally Redundant Trees (MRT): A pair of trees where the path
from any node X to the root R along the first tree and the path
from the same node X to the root along the second tree share the
minimum number of nodes and the minimum number of links. Each
such shared node is a cut-vertex. Any shared links are cut-links.
Any RT is an MRT but many MRTs are not RTs.
MRT Island: From the computing router, the set of routers that
support a particular MRT profile and are connected via MRT-
eligible links.
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GADAG: Generalized Almost Directed Acyclic Graph - a graph that is
the combination of the ADAGs of all blocks. Transforming a
network graph into a GADAG is part of the MRT algorithm.
MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Red is the decreasing MRT where links in the
GADAG are taken in the direction from a higher topologically
ordered node to a lower one.
MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Blue is the increasing MRT where links in the
GADAG are taken in the direction from a lower topologically
ordered node to a higher one.
4. Overview of OSPF Extensions for MRT
There are two separate aspects that need to be advertised in OSPF.
Both derive from the need for all routers supporting an MRT profile
to compute the same pair of MRTs to each destination. By executing
the same algorithm on the same network graph, distributed routers
will compute the same MRTs. Convergence considerations are discussed
in [RFC7812].
The first aspect that must be advertised is which MRT profile(s) are
supported and the associated GADAG Root Selection Priority. The
second aspect that must be advertised is any links that are not
eligible, due to administrative policy, to be part of the MRTs. This
must be advertised consistently across the area so that all routers
in the MRT Island use the same network graph.
4.1. Supporting MRT Profiles
An MRT Profile defines the exact MRT Algorithm, the MRT-Red LDP MT-
ID, the MRT-Blue LDP MT-ID, and the forwarding mechanisms supported
for the transit MRT-Red and MRT-Blue forwarding topologies. Finally,
the MRT Profile defines exact behavioral rules such as:
o how reconvergence is handled,
o inter-area forwarding behavior,
A router that advertises support for an MRT Profile MUST provide the
specified forwarding mechanism for its MRT-Red and MRT-Blue
forwarding topologies. A router that advertises support for an MRT
Profile MUST implement an algorithm that produces the same set of
MRT-Red and MRT-Blue next-hops for its MRT-Red and MRT-Blue
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topologies as is provided by the algorithm specified in the MRT
Profile.
A router MAY indicate support for multiple MRT Profiles. A router
computes its local MRT Island for each separate MRT Profile that the
router supports. Supporting multiple MRT Profiles also provides a
mechanism for transitioning from one profile to another. Different
uses of MRT forwarding topologies may behave better on different MRT
profiles.
The default MRT Profile is defined in [RFC7812]. Its behavior is
intended to support IP/LDP unicast and multicast fast-reroute.
4.2. GADAG Root Selection
One aspect of the MRT algorithms is that the selection of the GADAG
root can affect the alternates and the traffic through that GADAG
root. Therefore, it is important to provide an operator with control
over which router will play the role of GADAG root. A measure of the
centrality of a node may help determine how good a choice a
particular node is.
The GADAG Root Selection Policy (defined as part of an MRT profile)
may make use of the GADAG Root Selection Priority value advertised in
the MRT Profile TLV of the Router Information LSA. For example, the
GADAG Root Selection Policy for the default MRT profile is the
following: Among the routers in the MRT Island and with the highest
priority advertised, an implementation MUST pick the router with the
highest Router ID to be the GADAG root.
4.3. Triggering an MRT Computation
When an MRT Computation is triggered, it is triggered for a given MRT
Profile in a given area. First, the associated MRT Island is
determined. Then, the GADAG Root is selected. Finally, the actual
MRT algorithm is run to compute the transit MRT-Red and MRT-Blue
topologies. Additionally, the router MAY choose to compute MRT-FRR
alternates or make other use of the MRT computation results.
Prefixes can be attached and detached and have their associated MRT-
Red and MRT-Blue next-hops computed without requiring a new MRT
computation.
5. MRT Profile TLV in Router Information LSA
A router may advertise an MRT Profile TLV to indicate support for one
or more MRT Profiles. The MRT Profile TLV is advertised within the
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OSPF router information LSA which is defined for both OSPFv2 and
OSPFv3 in [RFC7770]. The RI LSA MUST have area scope.
Note that the presence of the MRT Profile TLV indicates support for a
given MRT profile in the default topology (MT-ID = 0). The
extensions in this document do not define a method to advertise
support for MRT profiles in topologies with non-zero MT-ID.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Profile ID(1) |GADAG Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .............. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Profile ID(n) |GADAG Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TYPE: TBA-MRT-OSPF-1 (To Be Allocated by IANA)
LENGTH: 4 * (number of Profiles)
Profile ID : 1 byte
GADAG Priority: 1 byte
MRT Profile TLV in Router Information LSA
Each Profile ID listed indicates support for a given MRT Profile, as
defined in [RFC7812]. A Profile ID value of 0 corresponds to the
Default MRT profile.
The GADAG Priority is the GADAG Root Selection Priority associated
with the advertising router in the MRT Island for the associated MRT
Profile, as indicated by the Profile ID. An implementation SHOULD
send a default value of 128 for the GADAG Root Selection Priority if
another value is not explicitly configured.
The length of this TLV depends on the number of MRT profiles
supported. The ordering of the profiles inside the TLV is not
significant. Multiple appearances of this TLV is not an error.
An advertising router MUST NOT advertise the same Profile ID multiple
times in one or more TLVs. If a receiving router receives multiple
appearances of the same Profile ID for the same router, it MUST treat
the advertising router as NOT supporting the MRT Profile associated
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with that Profile ID. This is the case even if the multiple
appearances of the same Profile ID have the same GADAG Priority
values. However, other Profile IDs advertised by the same
advertising router that are not repeated should continue to be
honored by the receiving router. The receiving router SHOULD also
log an error message regarding the multiple appearances of the same
Profile ID for the same router.
6. Advertising MRT-ineligible links for MRT
Due to administrative policy, some otherwise eligible links in the
network topology may need to be excluded from the network graph upon
which the MRT algorithm is run. Since the same network graph must be
used across the area, it is critical for OSPF to flood which links to
exclude from the MRT calculation. This is done by introducing a new
MRT-Ineligible Link sub-TLV. For OSPFv2, this sub-TLV is carried in
the Extended Link TLV defined in [I-D.ietf-ospf-prefix-link-attr].
For OSPFv3, this sub-TLV is carried in the Router-Link TLV defined in
[I-D.ietf-ospf-ospfv3-lsa-extend].
If a link is marked by administrative policy as MRT-Ineligible, then
a router MUST flood the OSPFv2 Extended Link TLV (or OSPFv3 Router-
Link TLV) for that link, including the MRT-Ineligible Link sub-TLV.
The OSPVv2 Extended Link TLV and OSPFv3 Router-Link TLV have area
wide scope.
Note that a router that advertises support for MRT with the MRT
Profile TLV MUST also support receipt of the MRT-Ineligible Link sub-
TLVs. This ensures that all routers participating in a given MRT
Island have the same view of the links included in the MRT Island.
6.1. MRT-Ineligible Link sub-TLV
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TYPE: TBA-MRT-OSPF-2 in OSPFv2 Extended Link TLV
TBA-MRT-OSPF-3 in OSPFv3 Router-Link TLV
(To Be Allocated by IANA)
LENGTH: 0
MRT-Ineligible Link sub-TLV
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This zero-length sub-TLV can appear in the OSPFv2 Extended Link TLV
or the OSPFv3 Router-Link TLV. Its presence indicates that the
associated link MUST NOT be used in the MRT calculation for all
profiles.
7. Worst-Case Network Convergence Time
As part of converging the network after a single failure,
Section 12.2 of [RFC7812] describes the need to wait for a configured
or advertised period for all routers to be using their new SPTs.
Similarly, some proposals to avoid micro-forwarding loops during
convergence[RFC5715] require determining the maximum among all
routers in the area of the worst-case route computation and FIB
installation time. More details on the specific reasoning and need
for flooding it are given in [I-D.atlas-bryant-shand-lf-timers].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | FIB compute/install time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TYPE: TBA-MRT-OSPF-4 (To Be Allocated by IANA)
LENGTH: 4
FIB compute/install time: This is the worst-case time the router
may take to compute and install all OSPF routes in the area
after a change to a stable network. The value is
in milliseconds.
Controlled Convergence TLV in Router Information LSA
The Controlled Convergence TLV is carried in the Router Information
LSA and flooded with area-wide scope. The FIB compute/install time
value sent by a router SHOULD be an estimate taking into account
network scale or real-time measurements, or both. Advertisements
SHOULD be dampened to avoid frequent communication of small changes
in the FIB compute/install time.
A router receiving the Controlled Convergence TLV SHOULD estimate the
network convergence time as the maximum of the FIB compute/install
times advertised by the routers in an area, including itself. In
order to account for routers that do not advertise the Controlled
Convergence TLV, a router MAY use a locally configured minimum
network convergence time as a lower bound on the computed network
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convergence time. A router MAY use a locally configured maximum
network convergence time as an upper bound on the computed network
convergence time.
8. Backwards Compatibility
The MRT Profile TLV, the MRT-Ineligible Link sub-TLV, the OSPFv3 MRT-
Ineligible Link sub-TLV, and the Controlled Convergence TLV are
defined in this document. A router that does not understand the MRT
Profile TLV will ignore it. A router that does not advertise an MRT
Profile TLV with a Profile ID may do so either because it doesn't
understand the MRT Profile TLV, or because it understands these
extensions, but chooses not to advertise support for any MRT profile.
Routers that support the MRT Profile TLV will treat either case in
the same manner, by excluding the router not advertising the MRT
Profile from the particular MRT Island.
The MRT-Ineligible Link sub-TLVs will be ignored by a router that
doesn't understand MRT, and a router supporting MRT must support
receipt of the MRT-Ineligible Link sub-TLVs.
Finally, applications that utilize the Controlled Convergence TLV can
use local configuration to account for routers that do not understand
the Controlled Convergence TLV.
8.1. Handling MRT Capability Changes
When a router that is running a version of software supporting MRT is
downgraded to software that does not support MRT, it is important
that the routers still running MRT do not continue to use the Router
Information LSA (RI LSA) containing the MRT Profile TLV advertised by
the downgraded router before the downgrade. As long as the
downgraded router supports Opaque LSAs, the downgraded router will
purge the old RI LSA containing the MRT Profile TLV that it
originated before the downgrade. This will occur when the downgraded
router receives the self-originated RI LSA after restarting, as
described in Section 13.4 and 14.1 of [RFC2328]. This behavior is
clearly required when the downgraded router supports the RI LSA.
It is also reasonable to expect this behavior even when the software
on the downgraded router does not understand the RI LSA. Although
this precise behavior is not explicitly described in [RFC2328] , it
is reasonable to infer from the documents. As long as the downgraded
router supports Opaque LSAs, it is required to flood link-state type
10 (area-local scope) Opaque LSAs, even those that it does not
understand [RFC5250]. So, when a restarting router receives a self-
originated link-state type 10 Opaque LSA whose Option Type it does
not recognize, it can (in principle) flood it or purge it. Purging
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an unknown self-originated Opaque LSA is the most reasonable thing to
do.
9. Implementation Status
[RFC Editor: please remove this section prior to publication.]
Please see [RFC7812] for details on implementation status.
10. Security Considerations
This OSPF extension is not believed to introduce new security
concerns. It relies upon the security architecture already provided
for Router LSAs and Router Information LSAs.
11. Acknowledgements
The authors would like to thank Anil Kumar SN for his suggestions and
review.
12. IANA Considerations
12.1. MRT Profile and Controlled Convergence TLVs
IANA is requested to allocate values for the following OSPF Router
Information TLV Types [RFC7770]: MRT Profile TLV (TBA-MRT-OSPF-1),
and Controlled Convergence TLV (TBA-MRT-OSPF-4). The requested
entries in the OSPF Router Information (RI) TLVs registry are shown
below.
Type Value Capabilities Reference
------------- ---------------------- ------------
TBA-MRT-OSPF-1 MRT Profile TLV [This draft]
TBA-MRT-OSPF-4 Controlled Convergence TLV [This draft]
12.2. MRT-Ineligible Link sub-TLV
IANA is requested to allocate a value from the OSPF Extended Link TLV
sub-TLV registry defined in [I-D.ietf-ospf-prefix-link-attr] for the
MRT-Ineligible Link sub-TLV (TBA-MRT-OSPF-2). The OSPF Extended Link
TLV sub-TLV registry after implementing the above request is shown
below.
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Value Description Reference
------------- ---------------------- ------------
0 Reserved [prefix-link-attr-draft]
TBA-MRT-OSPF-2 MRT-Ineligible Link sub-TLV [This draft]
2-32767 Unassigned [prefix-link-attr-draft]
32768-33023 Reserved for Experimental Use [prefix-link-attr-draft]
33024-65535 Reserved [prefix-link-attr-draft]
IANA is requested to allocate a value from the OSPFv3 Extended-LSA
sub-TLV registry [I-D.ietf-ospf-ospfv3-lsa-extend] for the MRT-
Ineligible Link sub-TLV (TBA-MRT-OSPF-3). The OSPFv3 Extended-LSA
sub-TLV registry has not yet been created by IANA.
13. References
13.1. Normative References
[I-D.ietf-ospf-ospfv3-lsa-extend]
Lindem, A., Roy, A., Goethals, D., Vallem, V., and F.
Baker, "OSPFv3 LSA Extendibility", draft-ietf-ospf-ospfv3-
lsa-extend-14 (work in progress), April 2017.
[I-D.ietf-ospf-prefix-link-attr]
Psenak, P., Gredler, H., rjs@rob.sh, r., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", draft-ietf-ospf-prefix-link-attr-13 (work
in progress), August 2015.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <http://www.rfc-editor.org/info/rfc5250>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <http://www.rfc-editor.org/info/rfc7770>.
[RFC7811] Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "An Algorithm for Computing IP/LDP Fast Reroute
Using Maximally Redundant Trees (MRT-FRR)", RFC 7811,
DOI 10.17487/RFC7811, June 2016,
<http://www.rfc-editor.org/info/rfc7811>.
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[RFC7812] Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for
IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-
FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016,
<http://www.rfc-editor.org/info/rfc7812>.
13.2. Informative References
[I-D.atlas-bryant-shand-lf-timers]
K, A. and S. Bryant, "Synchronisation of Loop Free Timer
Values", draft-atlas-bryant-shand-lf-timers-04 (work in
progress), February 2008.
[I-D.atlas-rtgwg-mrt-mc-arch]
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
Envedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
arch-02 (work in progress), July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <http://www.rfc-editor.org/info/rfc5715>.
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: akatlas@juniper.net
Shraddha Hegde
Juniper Networks
Embassy Business Park
Bangalore, KA 560093
India
Email: shraddha@juniper.net
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Chris Bowers
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: cbowers@juniper.net
Jeff Tantsura
Individual
USA
Email: jefftant.ietf@gmail.com
Zhenbin Li
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
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