



SAVNET                                                             D. Li
Internet-Draft                                                     J. Wu
Intended status: Informational                       Tsinghua University
Expires: 20 September 2026                                        L. Qin
                                                                M. Huang
                                                 Zhongguancun Laboratory
                                                                 N. Geng
                                                                  Huawei
                                                           19 March 2026


Source Address Validation in Intra-domain Networks Gap Analysis, Problem
                      Statement, and Requirements
          draft-ietf-savnet-intra-domain-problem-statement-22

Abstract

   This document provides a gap analysis of existing intra-domain source
   address validation mechanisms, describes the fundamental problems,
   and defines the basic requirements for technical improvements.

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
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   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 20 September 2026.

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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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Current Operational Intra-domain SAV Mechanisms . . . . . . .   5
   3.  Gap Analysis  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Asymmetric Routing Scenario . . . . . . . . . . . . . . .   6
     3.2.  Hidden Prefix Scenario  . . . . . . . . . . . . . . . . .   8
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Requirements for New SAV Mechanisms . . . . . . . . . . . . .   9
     5.1.  Accurate Validation . . . . . . . . . . . . . . . . . . .  10
     5.2.  Automatic Updates . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Incremental Deployment Support  . . . . . . . . . . . . .  10
     5.4.  Fast Convergence  . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Authentication of Information Used for SAV  . . . . . . .  11
     5.6.  Vulnerability Prevention  . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Source Address Validation (SAV) defends against source address
   spoofing.  Network operators can enforce SAV at the following levels
   (see [RFC5210]):

   *  Within the access network

   *  Within the domain (i.e., the autonomous system)

   *  Between domains (i.e., autonomous systems)

   Some access networks have already deployed SAV mechanisms.  These
   mechanisms typically are deployed on switches and prevent hosts from
   using the source address of another host on the Internet.  Mechanisms
   include:

   *  Source Address Validation Improvement (SAVI) Solution for DHCP
      [RFC7513]



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   *  IP Source Guard (IPSG) based on DHCP snooping [IPSG]

   *  Cable Source-Verify [cable-verify]

   However, access-network SAV mechanisms are not universally deployed.
   Therefore, intra-domain (i.e., intra-AS) SAV or/and inter-domain
   (i.e., inter-AS) SAV are required.

   This document provides a gap analysis of the current operational
   intra-domain SAV mechanisms, identifies key problems to solve, and
   proposes basic requirements for any new intra-domain SAV solutions.

   In this document, intra-domain SAV refers to SAV at external
   interfaces that do not carry external BGP (eBGP) sessions (i.e.,
   external non-BGP interfaces).  SAV at internal interfaces or eBGP
   interfaces is considered out of scope.  Within a domain, as
   illustrated in Figure 1, an external non-BGP interface may connect to
   a set of hosts, a non-BGP customer network, or a non-BGP Internet
   Service Provider (ISP) network.  The goal of intra-domain SAV at such
   interfaces is to prevent traffic using unauthorized source addresses
   from entering the domain.






























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         +-----------------+         +---------------+
         |   Non-BGP ISP   |         | eBGP Neighbor |
         +-----------------+         +---------------+
                 |                           |
                 |                           |
                 |                           |
   +-------------|---------------------------|---------+
   |Domain       \/                          |         |
   |         +---#------+               +----------+   |
   |         | Router 3 +---------------+ Router 4 |   |
   |         +----------+               +----------+   |
   |          /        \                     |         |
   |         /          \                    |         |
   |        /            \                   |         |
   | +----------+     +----------+      +----------+   |
   | | Router 1 |     | Router 2 +------+ Router 5 |   |
   | +------*---+     +--*-------+      +----X-----+   |
   |        /\           /\                  /\        |
   |         \           /                   |         |
   +----------\---------/--------------------|---------+
          +----------------+         +---------------+
          |Non-BGP Customer|         |   A Set of    |
          |   Network      |         |     Hosts     |
          |     (P1)       |         |     (P2)      |
          +----------------+         +---------------+

     This document focuses on SAV at external non-BGP
     interfaces including Interfaces  'X', '*', and '#'.

             Figure 1: Deployment locations of intra-domain SAV

1.1.  Terminology

   Non-BGP Customer Network: A stub network (i.e., a network that only
   originates traffic) connected to the local domain for Internet
   connectivity and does not participate in eBGP peering with the local
   domain.

   Non-BGP Internet Service Provider (ISP) Network: A network that
   forwards traffic from the local domain to the Internet and does not
   participate in eBGP peering with the local domain.

   SAV Rule: The rule in a router that describes the mapping
   relationship between a source address (prefix) and the valid incoming
   interface(s).  It is used by a router to make SAV decisions.






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   Improper Block: The validation results that the packets with
   legitimate source addresses are blocked improperly due to inaccurate
   SAV rules.

   Improper Permit: The validation results that the packets with spoofed
   source addresses are permitted improperly due to inaccurate SAV
   rules.

   SAV-specific Information: The information specialized for SAV rule
   generation.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Current Operational Intra-domain SAV Mechanisms

   Although BCP 38 [RFC2827] and BCP 84 [RFC3704] specify several
   ingress filtering methods primarily intended for inter-domain SAV,
   some of these methods have also been applied to intra-domain SAV in
   operational practice.  This section describes the mechanisms
   currently used to implement intra-domain SAV.

   *  Access Control Lists (ACLs) can be used as SAV filters [RFC2827]
      to check the source address of each packet against a set of
      permitted or prohibited prefixes.  When applied on a router
      interface, packets that do not match the ACL entries are blocked.
      Since ACLs are configured and updated manually, timely updates are
      essential whenever the set of permitted or prohibited prefixes
      changes.

   *  Strict uRPF [RFC3704] provides an automated SAV filter by
      validating the source address of each packet against the router’s
      local Forwarding Information Base (FIB).  A packet is accepted
      only if (i) the FIB contains a prefix covering the source address,
      and (ii) the FIB entry’s outgoing interface matches the packet’s
      incoming interface.  Otherwise, the packet is discarded.

   *  Loose uRPF [RFC3704] also relies on the local FIB for validation,
      but only checks for the presence of a covering prefix.  A packet
      is accepted if the FIB contains a prefix that covers the source
      address, regardless of the incoming interface.





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   *  Enhanced Feasible Path uRPF (EFP-uRPF) [RFC8704] is an advanced
      SAV mechanism specifically designed for inter-domain SAV.  It
      enforces SAV on eBGP interfaces facing a customer AS by leveraging
      BGP data received from external ASes.  EFP-uRPF is not analyzed in
      this document, as it is outside the scope of intra-domain SAV.

3.  Gap Analysis

   This section analyzes the gaps and key challenges of the current
   operational intra-domain SAV mechanisms.

   ACL-based SAV can be deployed on interfaces facing a non-BGP customer
   network or a set of hosts, permitting only packets with authorized
   source addresses.  Such mechanism can also be applied on interfaces
   facing a non-BGP ISP network to block packets with prohibited source
   addresses, including internal-use-only addresses, unallocated
   addresses, and addresses single-homed to the local domain (e.g., P1
   and P2 in Figure 1).  The main drawback of ACL-based SAV is that it
   requires manual maintenance.  Operators must update them promptly to
   reflect changes in prefixes or topology.  Failure to do so may result
   in outdated ACLs that inadvertently block legitimate traffic.

   As noted in Section 2.4 of [RFC3704], loose uRPF sacrifices
   directionality, so its effectiveness in mitigating source address
   spoofing is very limited, and improper permit problems may occur.

   With strict uRPF, it may drop legitimate packets in scenarios such as
   asymmetric routing or hidden prefixes.  The following subsections
   describe two specific gap scenarios that arise when using strict uRPF
   for intra-domain SAV.

3.1.  Asymmetric Routing Scenario

   Asymmetric routing means a packet traverses from a source to a
   destination in one path and takes a different path when it returns to
   the source.  Asymmetric routing can occur within an AS due to routing
   policy, traffic engineering, etc.

   For example, a non-BGP customer network connected to multiple routers
   of the AS may need to perform load balancing on incoming traffic,
   thereby resulting in asymmetric routing.  Figure 2 illustrates an
   example of asymmetric routing.  The non-BGP customer network owns
   prefix 2001:db8::/55 [RFC6890] and connects to two routers of the AS,
   Router 1 and Router 2.  Router 1, Router 2, and Router 3 exchange
   routing information via the intra-domain routing protocol.  To
   achieve load balancing for inbound traffic, the non-BGP customer
   network expects traffic destined for 2001:db8:0::/56 to enter through
   Router 1, and traffic destined for 2001:db8:0:100::/56 to enter



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   through Router 2.  To this end, Router 1 advertises 2001:db8:0::/56
   and Router 2 advertises 2001:db8:0:100::/56 through the intra-domain
   routing protocol.  Figure 2 also shows the corresponding FIB entries
   of Router 1 and Router 2 for the two prefixes.

    +----------------------------------------------------------+
    |Domain                                                    |
    |                      +----------+                        |
    |                      | Router 3 |                        |
    |                      +----------+                        |
    |                       /       \                          |
    |                      /         \                         |
    |                     /           \                        |
    |            +----------+       +----------+               |
    |            | Router 1 |       | Router 2 |               |
    |            +-----*----+       +----------+               |
    |                  /\               /                      |
    |                   \              /                       |
    +--------------------\------------/------------------------+
     Traffic with         \          / Traffic with
     source IP addresses   \        /  destination IP addresses
     of 2001:db8:0:100::/56 \      \/  of 2001:db8:0:100::/56
                       +----------------+
                       |Non-BGP Customer|
                       |    Network     |
                       |(2001:db8::/55) |
                       +----------------+

    FIB of Router 1                FIB of Router 2
    Dest                Next_hop   Dest                Next_hop
    2001:db8:0::/56     Non-BGP    2001:db8:0:100::/56 Non-BGP
                        Customer                       Customer
                        Nestwork                       Network
    2001:db8:0:100::/56 Router 3   2001:db8:0::/56     Router 3

    The legitimate traffic originated from non-BGP customer network
    with source addresses in 2001:db8:0:100::/56 will be improperly
    blocked by strict uRPF on Router 1.

                 Figure 2: An example of asymmetric routing

   Although the non-BGP customer network does not expect to receive
   inbound traffic for 2001:db8:0:100::/56 via Router 1, it can send
   outbound traffic with source addresses in that prefix through Router
   1.  As a result, data packets between the non-BGP customer network
   and Router 1 may follow asymmetric paths.  Arrows in the figure
   indicate the direction of traffic flow.




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   If Router 1 enforces strict uRPF by checking the FIB entry for the
   prefix 2001:db8:0:100::/56, the corresponding SAV rule would only
   allow packets with a source address from 2001:db8:0:100::/56 that
   arrive via Router 3.  Consequently, when the non-BGP customer network
   sends packets with a source address in 2001:db8:0:100::/56 to Router
   1, strict uRPF would incorrectly drop these legitimate packets.
   Similarly, if Router 2 enforces strict uRPF, it would incorrectly
   block legitimate packets from the non-BGP customer network that use
   source addresses within the prefix 2001:db8:0::/56.

3.2.  Hidden Prefix Scenario

   The intra-domain hidden prefix scenario refers to two situations in
   which a host or non-BGP customer legitimately originates traffic
   using source addresses that are not visible to the intra-domain
   routing protocol:

   *  A host (for example, a cloud server instance operated by a tenant)
      that originates traffic with a source address not allocated by the
      AS operator, for legitimate purposes such as Direct Server Return
      (DSR) deployments.

   *  A non-BGP customer network that originates traffic with a source
      address not advertised to the AS operator, also for valid
      operational reasons.

   For ACL-based SAV, enforcing correct filtering in these scenarios
   requires authoritative information that explicitly specifies which
   source addresses the host or non-BGP customer is authorized to use.
   In practice, such authoritative information is often missing.

   Existing uRPF-based mechanisms (strict uRPF or loose uRPF) also fail
   in hidden prefix scenarios.  They will drop packets from hidden
   prefixes because the source addresses are absent from the router's
   FIB or are received from unexpected interfaces.

4.  Problem Statement

   As discussed above, current operational intra-domain SAV mechanisms
   have significant limitations with respect to automatic updates and
   accurate validation:

   *  High operational overhead of ACL-based SAV.  ACL-based SAV relies
      entirely on manual maintenance, resulting in high operational
      overhead in dynamic networks.  To ensure the accuracy of ACL-based
      SAV, AS operators must manually update ACL rules whenever prefixes
      or topology change; otherwise, packets may be improperly blocked
      or permitted.



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   *  Improper block prblem of strict uRPF.  Strict uRPF can
      automatically update SAV rules based on the local FIB information,
      but it may block legitimate traffic in the asymmetric routing or
      hidden prefix scenarios.  Strict uRPF may mistakenly consider a
      valid incoming interface as invalid, resulting in legitimate
      packets being blocked (i.e., an improper block problem).

   *  Improper permit problem of loose uRPF.  Loose uRPF also
      automatically updates SAV rules based on the local FIB
      information, but its rules are overly permissive.  Any spoofed
      packet with a source address present in the FIB may be permitted
      by loose uRPF (i.e., an improper permit problem).

   The fundamental reason these limitations have persisted is the
   absence of SAV-specific, authoritative information that can be
   consumed automatically.  Current automated uRPF-based mechanisms
   derive their SAV rules solely from routing or forwarding information.
   However, routing information is designed to express reachability
   rather than authorization to use a source address.  As a result,
   uRPF-based mechanisms cannot reliably validate source addresses in
   scenarios such as asymmetric routing or hidden prefixes.  While ACL-
   based SAV can accurately encode source address authorization, it
   relies on manual configuration and ongoing operator intervention.
   Such manual maintenance does not scale in dynamic networks.
   Consequently, addressing these gaps requires the introduction of SAV-
   specific, authoritative information and the design of automated
   mechanisms that can consume this information directly, rather than
   relying only on routing or forwarding state.

   Another consideration is that uRPF-based mechanisms rely on routing
   information to make SAV decisions, assuming that the routing
   information in the local FIB is correct.  If the routing information
   is incorrect, SAV decisions may also be incorrect, potentially
   resulting in improper blocking or permitting.  Ensuring the
   correctness of routing information is the responsibility of
   mechanisms or operational processes outside the scope of SAV.
   However, if SAV relies on routing information or other contextual
   information, it is highly recommended that such information be
   validated before being used for SAV.

5.  Requirements for New SAV Mechanisms

   This section outlines five general requirements for technical
   improvements that should be considered when designing future intra-
   domain SAV architectures and solutions.  These informational
   requirements can not be used to initiate standards-track protocol
   changes.




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5.1.  Accurate Validation

   Any new intra-domain SAV mechanism MUST improve the accuracy of
   source address validation compared to existing uRPF-based mechanisms.
   In particular, it MUST reduce the occurrence of improper blocks
   (i.e., blocking legitimate traffic), improper permits (i.e., allowing
   spoofed traffic), or both.  Specifically, it MUST satisfy the
   following conditions:

   *  result in fewer improper blocks than strict uRPF, particularly in
      scenarios involving asymmetric routes or hidden prefixes;

   *  result in fewer improper permits than loose uRPF.

   To achieve higher SAV accuracy, additional information beyond the
   local FIB (e.g., SAV-specific information) may be needed to make
   validation decisions.  By integrating such information, routers may
   have the ability to account for asymmetric routes and hidden
   prefixes, resulting in more accurate SAV rules.

5.2.  Automatic Updates

   Any new intra-domain SAV mechanism MUST be capable of automatically
   generating and updating SAV rules on routers, rather than relying
   entirely on manual updates as in ACL-based SAV.  Automation helps
   reduce operational complexity and maintenance overhead, while
   allowing some initial configuration to improve SAV accuracy.  This
   ensures the mechanism is deployable in practical networks without
   introducing excessive management burden.

5.3.  Incremental Deployment Support

   Any new intra-domain SAV mechanism MUST support incremental
   deployment and provide measurable benefits even when only a subset of
   external non-BGP interfaces deploy the mechanism.

5.4.  Fast Convergence

   If any new intra-domain SAV mechanism requires disseminating SAV-
   specific information among intra-domain routers via a protocol, two
   considerations are essential.  First, such mechanism MUST allow
   routers to learn updated SAV-specific information in a timely manner.
   Second, such mechanism MUST NOT transmit excessive SAV-specific
   information via a protocol, as this could significantly increase the
   burden on the routers’ control planes and potentially degrade the
   performance of existing protocols.





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5.5.  Authentication of Information Used for SAV

   Any new intra-domain SAV mechanism SHOULD verify the authenticity and
   trustworthiness of information before using it.  Using incorrect
   information may result in the generation of incorrect SAV rules,
   potentially permitting spoofed packets or causing legitimate traffic
   to be blocked.  If any new intra-domain SAV mechanism introduces any
   new SAV-specific information, it MUST ensure that such information
   can be authenticated.

5.6.  Vulnerability Prevention

   Any new intra-domain SAV mechanism MUST NOT introduce additional
   security vulnerabilities to existing intra-domain architectures or
   protocols.  Protection against compromised or malicious intra-domain
   routers is out of scope, as such routers can compromise not only SAV
   mechanisms but also the entire intra-domain routing domain.

6.  Security Considerations

   This document discusses the limitations of existing intra-domain SAV
   practices and identifies problems and informational requirements for
   improved intra-domain SAV mechanisms.  It does not specify new
   protocols or mechanisms and, as such, does not introduce any new
   security considerations.

7.  IANA Considerations

   This document does not request any IANA allocations.

8.  Acknowledgements

   Many thanks to the valuable comments from: Jared Mauch, Joel Halpern,
   Aijun Wang, Michael Richardson, Gert Doering, Libin Liu, Li Chen,
   Tony Przygienda, Yingzhen Qu, James Guichard, Linda Dunbar, Robert
   Sparks, Stephen Farrel, Ron Bonica, Xueyan Song, etc.

9.  References

9.1.  Normative References

   [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>.






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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <https://www.rfc-editor.org/info/rfc3704>.

   [RFC5210]  Wu, J., Bi, J., Li, X., Ren, G., Xu, K., and M. Williams,
              "A Source Address Validation Architecture (SAVA) Testbed
              and Deployment Experience", RFC 5210,
              DOI 10.17487/RFC5210, June 2008,
              <https://www.rfc-editor.org/info/rfc5210>.

   [RFC8704]  Sriram, K., Montgomery, D., and J. Haas, "Enhanced
              Feasible-Path Unicast Reverse Path Forwarding", BCP 84,
              RFC 8704, DOI 10.17487/RFC8704, February 2020,
              <https://www.rfc-editor.org/info/rfc8704>.

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,
              <https://www.rfc-editor.org/info/rfc7513>.

   [cable-verify]
              "Cable Source-Verify and IP Address Security", January
              2021, <https://www.cisco.com/c/en/us/support/docs/
              broadband-cable/cable-security/20691-source-verify.html>.

   [IPSG]     "Configuring DHCP Features and IP Source Guard", January
              2016, <https://www.cisco.com/c/en/us/td/docs/switches/lan/
              catalyst2960/software/release/12-
              2_53_se/configuration/guide/2960scg/swdhcp82.html>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

Authors' Addresses




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   Dan Li
   Tsinghua University
   Beijing
   China
   Email: tolidan@tsinghua.edu.cn


   Jianping Wu
   Tsinghua University
   Beijing
   China
   Email: jianping@cernet.edu.cn


   Lancheng Qin
   Zhongguancun Laboratory
   Beijing
   China
   Email: qinlc@mail.zgclab.edu.cn


   Mingqing Huang
   Zhongguancun Laboratory
   Beijing
   China
   Email: huangmq@mail.zgclab.edu.cn


   Nan Geng
   Huawei
   Beijing
   China
   Email: gengnan@huawei.com


















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