BGP-LS Extension for Inter-AS Topology Retrieval
draft-ietf-idr-bgpls-inter-as-topology-ext-21
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Aijun Wang , Huaimo Chen , Ketan Talaulikar , Shunwan Zhuang , Changwang Lin | ||
| Last updated | 2026-02-12 | ||
| Replaces | draft-wang-idr-bgpls-inter-as-topology-ext | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
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| Send notices to | (None) |
draft-ietf-idr-bgpls-inter-as-topology-ext-21
IDR Working Group A. Wang
Internet-Draft China Telecom
Intended status: Standards Track H. Chen
Expires: 17 August 2026 Individual
K. Talaulikar
Cisco Systems
S. Zhuang
Huawei Technologies
C. Lin
New H3C Technologies
13 February 2026
BGP-LS Extension for Inter-AS Topology Retrieval
draft-ietf-idr-bgpls-inter-as-topology-ext-21
Abstract
This document specifies the procedure for distributing Border Gateway
Protocol-Link State (BGP-LS) key parameters for inter-domain links
between two Autonomous Systems (ASes). It defines a new type within
the BGP-LS Network Layer Reachability Information (NLRI) for a Stub
Link, as well as three new type-length-values (TLVs) for the BGP-LS
Link descriptor. These BGP-LS extensions enable Software-Defined
Networking (SDN) controllers to automatically retrieve network
topology across diverse inter-AS environments.
These extensions and procedures allow network operators to collect
inter-domain interconnect information and automatically compute the
end-to-end network topology using information provided by the BGP-LS
protocol.
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 https://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 17 August 2026.
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Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Inter-AS Domain Scenarios . . . . . . . . . . . . . . . . . . 3
5. Stub Link NLRI . . . . . . . . . . . . . . . . . . . . . . . 4
6. IGP Information for Inter-AS Link . . . . . . . . . . . . . . 6
7. BGP-LS Extensions for Inter-AS Link . . . . . . . . . . . . . 6
7.1. Remote AS Number TLV . . . . . . . . . . . . . . . . . . 7
7.2. IPv4 Remote ASBR ID . . . . . . . . . . . . . . . . . . . 8
7.3. IPv6 Remote ASBR ID . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9.1. New BGP-LS NLRI type . . . . . . . . . . . . . . . . . . 10
9.2. New Stub Link Descriptors . . . . . . . . . . . . . . . . 10
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
BGP-LS [RFC9552] specifies the methodology for using the BGP protocol
to transfer Link-State information. This method enables SDN
controllers to automatically collect underlay network topology, but
it typically retrieves only information within a single Interior
Gateway Protocol (IGP) domain. If an operator manages multiple IGP
domains that interconnect with one another, no mechanism exists
within the current BGP-LS protocol to transfer inter-domain topology
information..
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[RFC9086] defines extensions for exporting BGP peering node topology
information (including peers, interfaces, and peering ASes) in a
manner exploitable for computing efficient BGP Peering Engineering
policies and strategies. This information can also be used to
compute interconnection topology among different IGP domains, but it
requires every border router to run the BGP-LS protocol and report
such information to SDN controllers. Given the large number of
border routers at the network boundary, this solution limits
deployment flexibility.
This document analyzes scenarios in which SDN controllers require
inter-domain topology information between different Autonomous
Systems (ASes). After describing these scenarios, this document
defines a new Stub Link type within the BGP-LS NLRI [RFC9552] to
describe inter-AS links and new TLVs for this new BGP-LS type. The
SDN controller can then automatically deduce the multi-domain
topology using information from the BGP-LS protocol.
2. 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] .
3. Terminology
The following terms are defined in this document:
* IDCs: Internet Data Centers
* MAN: Metrio-Area-Network
* SDN: Software Definition Network
4. Inter-AS Domain Scenarios
Figure 1 illustrates the multi-domain scenarios discussed in this
document. Typically, the SDN Controller can retrieve the topology of
IGP A and IGP B individually via the BGP-LS protocol, but it cannot
obtain topology connection information between these two IGP domains,
as IGP protocols are generally not run on the connected links.
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+-----------------+
+----+IP SDN Controller+----+
| +-----------------+ |
| |
|BGP-LS |BGP-LS
| |
+---------------+-----+ +-----+--------------+
| +--+ +-++ ++-+ +-++ +|-+ +--+|
| |S1+--------+S2+---+B1+-----------+B2+---+T1+--------+T2||
| +-++ N1 +-++ ++-+ +-++ ++++ N2 +-++|
| | | | | || | |
| | | | | || | |
| +-++ +-++ ++-+ +-++ ++++ +-++|
| |S4+--------+S3+---+B3+-----------+B4+---+T3+--------+T4||
| +--+ +--+ ++-+ +-++ ++-+ +--+|
| | | |
| | | |
| IGP A | | IGP B |
+---------------------+ +--------------------+
Figure 1: Inter-AS Domain Scenarios
5. Stub Link NLRI
[RFC9552] defines four types within the BGP Link-State NLRI (Node
NLRI, Link NLRI, IPv4 Topology Prefix NLRI, and IPv6 Topology Prefix
NLRI) for transferring topology and prefix information. For inter-AS
links, the two ends of a link reside in different IGP domains; thus,
it is not appropriate to transfer their information using the
currently defined NLRI types.
This document defines a new NLRI type 7, see__Section 9) within the
BGP Link-State (BGP-LS) NLRI, referred to as the Stub Link NLRI. The
Stub Link NLRI is encoded in the format shown in Figure 2 and is
explained below:
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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
+-+-+-+-+-+-+-+-+
| Protocol-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
| (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Local Node Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Stub Link Descriptors (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Stub Link NLRI Format
The "Protocol-ID" SHOULD be set to the value indicating the source
protocol of the stub link information, as specified in[RFC9552]
Section 5.2.
Local Node Descriptors: define the ASBRs attached to the inter-AS
stub links, and use the "Local Node Descriptor" specified in[RFC9552]
, Section 5.2.1.4. The following Node Descriptor sub-TLVs from
[RFC9552] are valid for inclusion in the local Node descriptor: AS
system, OSPF Area-ID, IGP Router-ID.
Stub Link Descriptors: define the Stub Links that have only one end
located in an IGP domain, using the "Link Descriptor definition"
specified in[RFC9552] ,Section 5.2.2 with the exceptions noted below.
The Stub Link Descriptors support the inclusion of the following sub-
TLVs:
• Link/Local Identifier (TLV 258, [RFC9552])
• IPv4 Interface Address (TLV 259, [RFC9552])
• IPv4 Neighbor Address (TLV 260, [RFC9552])
• IPv6 Interface Address (TLV 261, [RFC9552])
• IPv6 Neighbor Address (TLV 262, [RFC9552])
• Remote-AS Number (TLV 270, [This document], section Section 7.1)
• IPv4 Remote ASBR ID (TLV 271, [This document], section Section 7.2)
• IPv6 Remote ASBR ID (TLV 272, [This document], section Section 7.3)
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This newly defined NLRI can be used to describe links that have only
one end located within an IGP domain, as described in the following
sections.
To match the stub link connecting the two bordering Autonomous
Systems (ASes), the Remote AS Number TLV, the IPv4 Remote ASBR ID TLV
(if the neighboring ASBR has an IPv4 address), and/or the IPv6 Remote
ASBR ID TLV (if the neighboring ASBR has an IPv6 address) MUST be
included.
Correspondingly, the IPv4 Router-ID of Local Node TLV (1028) and/or
the IPv6 Router-ID of Local Node TLV (1029) MUST be included as Link
Attribute TLVs of the Stub Link NLRI.
The Node and Link Descriptor sub-TLVs, as well as Node and Link
attributes defined in[RFC9552] MAY be included in the NLRI if
necessary. The interface and neighbor address sub-TLVs SHOULD be
included in the Local Node Descriptors to differentiate parallel
links between two ASBRs.
6. IGP Information for Inter-AS Link
[RFC9346] and [RFC5392] define IS-IS and OSPF extensions,
respectively, to address the requirements for reporting inter-AS
links. Three sub-TLVs related to Inter-Domain Links (Remote AS
Number, IPv4 Remote ASBR ID, and IPv6 Remote ASBR ID) are defined in
these documents.
These IGP TLVs are automatically flooded within an IGP domain. This
document specifies that these MAY also be carried within the newly
defined Stub Link NLRI in the BGP-LS protocol, as descriptors for
inter-AS stub links.
7. BGP-LS Extensions for Inter-AS Link
This section proposes adding three new TLVs to be supported within
the Stub Link NLRI of the BGP-LS NLRI. These new TLVs enable BGP-LS
to transfer inter-AS information collected by the SDN controller.
The following Link Descriptor TLVs are added to the BGP-LS protocol:
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+-----------+---------------------+--------------+----------------+
| TLV Code | Description |IS-IS/OSPF TLV| Reference |
| Point | | /Sub-TLV | (RFC/Section) |
+-----------+---------------------+--------------+----------------+
| 270 |Remote AS Number | 24/21 | [RFC9346]/3.3.1|
| | | | [RFC5392]/3.3.1|
| 271 |IPv4 Remote ASBR ID | 25/22 | [RFC9346]/3.3.2|
| | | | [RFC5392]/3.3.2|
| 272 |IPv6 Remote ASBR ID | 26/24 | [RFC9346]/3.3.3|
| | | | [RFC5392]/3.3.3|
+-----------+---------------------+--------------+----------------+
Figure 3: Stub Link Descriptor TLVs
The detailed encoding of these TLVs is synchronized with the
corresponding sections in[RFC9346] and [RFC5392], which maintains
BGP-LS protocol agnosticism to the underlying protocol.
7.1. Remote AS Number TLV
A new TLV, referred to as the Remote AS Number TLV, is defined for
inclusion in the Link Descriptor when advertising inter-AS links.
The Remote AS Number TLV specifies the AS number of the neighboring
AS to which the advertised link connects.
The Remote AS Number TLV is TLV Type 270 (see Section 9 ) and is 4
octets in length. Its format is as follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote AS Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Remote AS Number TLV Format
The Remote AS Number field has 4 octets. When only 2 octets are used
for the AS number (for example, when such information is advertised
from OSPF, as in current deployments), the left (high-order) 2 octets
MUST be set to 0. The Remote AS Number TLV MUST be included when a
router advertises an inter-AS link.
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7.2. IPv4 Remote ASBR ID
A new TLV, referred to as the IPv4 Remote ASBR ID TLV, is defined for
inclusion in the Link Descriptor when advertising inter-AS links.
The IPv4 Remote ASBR ID TLV specifies the IPv4 identifier of the
remote ASBR to which the advertised inter-AS link connects. This can
be any stable, routable IPv4 address of the remote ASBR. The use of
the TE Router ID, as specified in the Traffic Engineering Router ID
TLV [RFC9346] is RECOMMENDED.
The IPv4 Remote ASBR ID TLV is TLV Type 271 (see Section 9) and is 4
octets in length. Its format is as follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: IPv4 Remote ASBR ID TLV Format
The IPv4 Remote ASBR ID TLV MUST be included if the neighboring ASBR
has an IPv4 address. If the neighboring ASBR does not have an IPv4
address (including no IPv4 TE Router ID), the IPv6 Remote ASBR ID TLV
MUST be included instead. Both an IPv4 Remote ASBR ID TLV and an
IPv6 Remote ASBR ID TLV MAY be present in an inter-AS Stub Link NLRI.
7.3. IPv6 Remote ASBR ID
A new TLV, referred to as the IPv6 Remote ASBR ID TLV, is defined for
inclusion in the Link Descriptor when advertising inter-AS links.
The IPv6 Remote ASBR ID TLV specifies the IPv6 identifier of the
remote ASBR to which the advertised inter-AS link connects. This can
be any stable, routable IPv6 address of the remote ASBR. The use of
the TE Router ID, as specified in the IPv6 Traffic Engineering Router
ID TLV [RFC9346] is RECOMMENDED.
The IPv6 Remote ASBR ID TLV is TLV Type 272 (see Section 9) and is 16
octets in length. Its format is as follows:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: IPv6 Remote ASBR ID TLV Format
The IPv6 Remote ASBR ID TLV MUST be included if the neighboring ASBR
has an IPv6 address. If the neighboring ASBR does not have an IPv6
address, the IPv4 Remote ASBR ID TLV MUST be included instead. Both
an IPv4 Remote ASBR ID TLV and an IPv6 Remote ASBR ID TLV MAY be
present in an inter-AS Stub Link NLRI.
8. Security Considerations
BGP-LS security is specified in [RFC9552]. This extension to BGP-LS
focuses on scenarios where a single entity-operated network includes
multiple IGP domains composed of its backbone network, several Metro-
Area Networks (MANs), and Internet Data Centers (IDCs). The
configuration of these networks, operated by a single administrative
entity, creates a "walled garden". Within this single administrative
domain, the network operator needs to monitor and engineer traffic
flows traversing a network that spans multiple Autonomous Systems
(ASes). The network operator can obtain this inter-AS topology
information via the procedure described in this document.
A single administrative domain consisting of two ASes that passes
information about Stub Link characteristics does not cause issues
within a "walled garden". However, the Stub Link NLRI and its
characteristics (Link/Local Identifier, IPv4 Interface Address, IPv4
Neighbor Address, IPv6 Interface Address, IPv6 Neighbor Address,
Multi-Topology Identifier, Remote-AS Number, IPv4 Remote ASBR ID, and
IPv6 Remote ASBR ID) constitute critical network information. As
such, operators SHOULD handle this critical information in a manner
that restricts it to the walled garden.
9. IANA Considerations
This document defines:
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* A new BGP NLRI Type: Stub Link NLRI. The codepoint is from the
"BGP-LS NLRI Types"
* Three new Link Descriptors TLV: Remote AS Number TLV, IPv4 Remote
ASBR ID, IPv6 Remote ASBR ID. The codepoint are from "BGP-LS Node
Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
TLVs" registry.
9.1. New BGP-LS NLRI type
This document defines a new value in the registry "BGP-LS NLRI
Types":
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code Point | Description | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | Stub Link NLRI| Allocation from IANA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Stub Link NLRI Codepoint
9.2. New Stub Link Descriptors
This document defines three new values in the registry "BGP-LS NLRI
and Attribute TLVs":
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code Point | Description | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 270 | Remote AS Number | Allocation from IANA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 271 | IPv4 Remote ASBR ID | Allocation from IANA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 272 | IPv6 Remote ASBR ID | Allocation from IANA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: BGP-LS Link Descriptors TLV
10. Acknowledgement
The author would like to thank Susan Hares, Acee Lindem, Jie Dong,
Shaowen Ma, Jeff Tantsura and Dhruv Dhody for their valuable comments
and suggestions.
11. References
11.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
January 2009, <https://www.rfc-editor.org/info/rfc5392>.
[RFC9346] Chen, M., Ginsberg, L., Previdi, S., and D. Xiaodong, "IS-
IS Extensions in Support of Inter-Autonomous System (AS)
MPLS and GMPLS Traffic Engineering", RFC 9346,
DOI 10.17487/RFC9346, February 2023,
<https://www.rfc-editor.org/info/rfc9346>.
[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.
11.2. Informative References
[RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
2021, <https://www.rfc-editor.org/info/rfc9086>.
Authors' Addresses
Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing
Beijing, 102209
China
Email: wangaj3@chinatelecom.cn
Huaimo Chen
Individual
Boston, MA
United States of America
Email: hchen.ietf@gmail.com
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Ketan Talaulikar
Cisco Systems
India
Email: ketant.ietf@gmail.com
Shunwan Zhuang
Huawei Technologies
Huawei Building, No.156 Beiqing Rd.
Beijing
100095
China
Email: zhuangshunwan@huawei.com
Changwang Lin
New H3C Technologies
Huawei Building, No.156 Beiqing Rd.
Beijing
China
Email: linchangwang.04414@h3c.com
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