Network Working Group                                           L. Zheng
Internet-Draft                                                   M. Chen
Intended status: Standards Track                     Huawei Technologies
Expires: September 30, November 5, 2014                                      M. Bhatia
                                                          March 29,
                                                             May 4, 2014

                 LDP Hello Cryptographic Authentication


   This document introduces a new optional Cryptographic Authentication
   TLV that LDP can use to secure its Hello messages.  It secures the
   Hello messages against spoofing attacks and some well known attacks
   against the IP header.  This document describes a mechanism to secure
   the LDP Hello messages using National Institute of Standards and
   Technology (NIST) Secure Hash Standard family of algorithms.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 30, November 5, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4   2
   2.  Cryptographic Authentication TLV  . . . . . . . . . . . . . . .  6   4
     2.1.  Optional Parameter for Hello Message  . . . . . . . . . . .  6   4
     2.2.  LDP Security Association  . . . . . . . . . . . . . . . . .  6   4
     2.3.  Cryptographic Authentication TLV Encoding . . . . . . . .  8   6
     2.4.  Sequence Number Wrap  . . . . . . . . . . . . . . . . . . .  9   8
   3.  Cryptographic Authentication Procedure  . . . . . . . . . . . . 10   8
   4.  Cross Protocol Attack Mitigation  . . . . . . . . . . . . . . . 11   8
   5.  Cryptographic Aspects . . . . . . . . . . . . . . . . . . . . 12   8
     5.1.  Preparing the Cryptographic Key . . . . . . . . . . . . . 12   9
     5.2.  Computing the Hash  . . . . . . . . . . . . . . . . . . . . 13  10
     5.3.  Result  . . . . . . . . . . . . . . . . . . . . . . . . . . 13  10
   6.  Processing Hello Message Using Cryptographic Authentication . 14  10
     6.1.  Transmission Using Cryptographic Authentication . . . . . 14  10
     6.2.  Receipt Using Cryptographic Authentication  . . . . . . . . 14  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . 16  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 18  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 19  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . . . 19  13
     10.2.  Informative References . . . . . . . . . . . . . . . . . . 19  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 20  14

1.  Introduction

   The Label Distribution Protocol (LDP) [RFC5036] sets up LDP sessions
   that run between LDP peers.  The peers could either be directly
   connected at the link level or could be multiple hops away.  An LDP
   Label Switching Router (LSR) could either be configured with the
   identity of its peers or could discover them using LDP Hello
   messages.  These messages are sent encapsulated in UDP addressed to
   "all routers on this subnet" or to a specific IP address.  Periodic
   Hello messages are also used to maintain the relationship between LDP
   peers necessary to keep the LDP session active.

   Since the Hello messages are sent using UDP and not TCP, these
   messages cannot use the security mechanisms defined for TCP
   [RFC5926].  While some configuration guidance is given in [RFC5036]
   to help protect against false discovery messages, it does not provide
   an explicit security mechanism to protect the Hello messages messages.

   Spoofing a Hello packet for an existing adjacency can cause the valid
   adjacency to time out and in turn can result in termination of the
   associated session.  This can occur when the spoofed Hello specifies
   a smaller Hold Time, causing the receiver to expect Hellos within
   this smaller interval, while the true neighbor continues sending
   Hellos at the previously agreed lower frequency.  Spoofing a Hello
   packet can also cause the LDP session to be terminated directly,
   which can occur when the spoofed Hello specifies a different
   Transport Address, other than the previously agreed one between
   neighbors.  Spoofed Hello messages have been observed and reported as
   a real problem in production networks [RFC6952].

   For Link Hello, [RFC5036] states that the threat of spoofed Hellos
   can be reduced by accepting Hellos only on interfaces to which LSRs
   that can be trusted are directly connected, and ignoring Hellos not
   addressed to the "all routers on this subnet" multicast group.  The
   Generalized TTL Security Mechanism (GTSM) provides a simple and
   reasonably robust defense mechanism for Link Hello [RFC6720], but it
   does not secure against packet spoofing attack or replay

   Spoofing attacks via Targeted Hellos are a potentially more serious
   threat.  An  [RFC5036] states that an LSR can reduce the threat of
   spoofed Targeted Hellos by filtering them and accepting only those
   originating at sources permitted by an access list.  However,
   filtering using access lists requires LSR resource, and does not
   prevent IP-address spoofing.

   This document introduces a new Cryptographic Authentication TLV which
   is used in LDP Hello messages as an optional parameter.  It enhances
   the authentication mechanism for LDP by securing the Hello message
   against spoofing attack.  It also introduces a cryptographic sequence
   number carried in the Hello messages that can be used to protect
   against replay attacks.  The LSRs could be configured to only accept
   Hello messages from specific peers when authentication is in use.

   Using this Cryptographic Authentication TLV, one or more secret keys
   (with corresponding key Security Association (SA) IDs) are configured in
   each system.  For each LDP Hello message, the key is used to generate
   and verify a HMAC Hash that is stored in the LDP Hello message.  For
   cryptographic hash function, this document proposes to use SHA-1,
   SHA-256, SHA-384, and SHA-512 defined in US NIST Secure Hash Standard
   (SHS) [FIPS-180-3].  The HMAC authentication mode defined in NIST
   FIPS 198 is used [FIPS-198].  Of the above, implementations MUST
   include support for at least HMAC-SHA-256 and SHOULD include support
   for HMAC-SHA-1 and MAY include support for either of HMAC-SHA-384 or

2.  Cryptographic Authentication TLV

2.1.  Optional Parameter for Hello Message

   [RFC5036] defines the encoding for the Hello message.  Each Hello
   message contains zero or more Optional Parameters, each encoded as a
   TLV.  Three Optional Parameters are defined by [RFC5036].  This
   document defines a new Optional Parameter: the Cryptographic
   Authentication parameter.

   Optional Parameter               Type
   -------------------------------  --------
   IPv4 Transport Address           0x0401 (RFC5036)
   Configuration Sequence Number    0x0402 (RFC5036)
   IPv6 Transport Address           0x0403 (RFC5036)
   Cryptographic Authentication     0x0404 (this document, TBD TBD1 by IANA)

   The Cryptographic Authentication TLV Encoding is described in section

2.2.  LDP Security Association

   An LDP Security Association (SA) contains a set of parameters shared
   between any two legitimate LDP speakers.

   Parameters associated with an LDP SA are as follows:

   o  Security Association Identifier (SA ID)

      This is a 32-bit unsigned integer used to uniquely identify an LDP
      SA between two LDP peers, as manually configured by the network operator.
      operator (or, in the future, possibly by some key management
      protocol specified by the IETF) .

      The receiver determines the active SA by looking at the SA ID
      field in the incoming Hello message.

      The sender, based on the active configuration, selects an SA to
      use and puts the correct Key SA ID value associated with the SA in the
      LDP Hello message.  If multiple valid and active LDP SAs exist for
      a given interface, the sender may use any of those SAs to protect
      the packet.

      Using SA IDs makes changing keys while maintaining protocol
      operation convenient.  Each SA ID specifies two independent parts,
      the authentication algorithm and the authentication key, as
      explained below.

      Normally, an implementation would allow the network operator to
      configure a set of keys in a key chain, with each key in the chain
      having fixed lifetime.  The actual operation of these mechanisms
      is outside the scope of this document.

      Note that each SA ID can indicate a key with a different
      authentication algorithm.  This allows the introduction of new
      authentication mechanisms without disrupting existing LDP

   o  Authentication Algorithm

      This signifies the authentication algorithm to be used with the
      LDP SA.  This information is never sent in clear text over the
      wire.  Because this information is not sent on the wire, the
      implementer chooses an implementation specific representation for
      this information.

      Currently, the following algorithms are supported:

      HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512.

   o  Authentication Key

      This value denotes the cryptographic authentication key associated
      with the LDP SA.  The length of this key is variable and depends
      upon the authentication algorithm specified by the LDP SA.

   o  KeyStartAccept

      The time that this OSPFv3 LDP router will accept packets that have been
      created with this OSPFv3 LDP Security Association.

   o  KeyStartGenerate

      The time that this LDP router will begin using this LDP Security
      Association for LDP Hello message generation.

   o  KeyStopGenerate
      The time that this LDP router will stop using this LDP Security
      Association for LDP Hello message generation.

   o  KeyStopAccept

      The time that this LDP router will stop accepting packets
      generated with this LDP Security Association.

   In order to achieve smooth key transition, KeyStartAccept SHOULD be
   less than KeyStartGenerate and KeyStopGenerate SHOULD be less than
   KeyStopAccept.  If KeyStartGenerate or KeyStartAccept are left
   unspecified, the time will default to 0 and the key will be used
   immediately.  If KeyStopGenerate or KeyStopAccept are left
   unspecified, the time will default to infinity and the key's lifetime
   will be infinite.  When a new key replaces an old, the
   KeyStartGenerate time for the new key MUST be less than or equal to
   the KeyStopGenerate time of the old key.

   Key storage SHOULD persist across a system restart, warm or cold, to
   avoid operational issues.  In the event that the last key associated
   with an interface expires, it is unacceptable to revert to an
   unauthenticated condition, and not advisable to disrupt routing.
   Therefore, the router SHOULD send a "last Authentication Key
   expiration" notification to the network manager and treat the key as
   having an infinite lifetime until the lifetime is extended, the key
   is deleted by network management, or a new key is configured

2.3.  Cryptographic Authentication TLV Encoding

       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
      |0|0|        Auth (TBD) (TBD1)        |             Length            |
      |                  Security Association ID                      |
      |       Cryptographic Sequence Number (High Order 32 Bits)      |
      |       Cryptographic Sequence Number (Low Order 32 Bits)       |
      |                                                               |
      |                Authentication Data (Variable)                 |
      ~                                                               ~
      |                                                               |
      |                                                               |

   - Type: TBD, TBD1, Cryptographic Authentication
   - Length: Specifying the length in octets of the value field.

   - Security Association ID: 32 bit field that maps to the
   authentication algorithm and the secret key used to create the
   message digest carried in LDP payload.

   Though the SA ID implies the algorithm, the HMAC output size should
   not be used by implementers as an implicit hint, because additional
   algorithms may be defined in the future that have the same output

   - Cryptographic Sequence Number: 64-bit strictly increasing sequence
   number that is used to guard against replay attacks.  The 64-bit
   sequence number MUST be incremented for every LDP Hello packet sent
   by the LDP router.  Upon reception, the sequence number MUST be
   greater than the sequence number in the last LDP Hello packet
   accepted from the sending LDP neighbor.  Otherwise, the LDP packet is
   considered a replayed packet and dropped.

   LDP routers implementing this specification MUST use existing
   mechanisms to preserve the sequence number's strictly increasing
   property for the deployed life of the LDP router (including cold
   restarts).  One mechanism for accomplishing this could be to use the
   high-order 32 bits of the sequence number as a boot count that is
   incremented anytime the LDP router loses its sequence number state.
   Techniques such as sequence number space partitioning described above
   or non-volatile storage preservation can be used but are beyond the
   scope of this specification.  Sequence number wrap is described in
   Section 2.4.

   - Authentication Data:

   This field carries the digest computed by the Cryptographic
   Authentication algorithm in use.  The length of the Authentication
   Data varies based on the cryptographic algorithm in use, which is
   shown as below:

   Auth type        Length
   ---------------  ----------
   HMAC-SHA1        20 bytes
   HMAC-SHA-256     32 bytes
   HMAC-SHA-384     48 bytes
   HMAC-SHA-512     64 bytes

2.4.  Sequence Number Wrap

   When incrementing the sequence number for each transmitted LDP
   packet, the sequence number should be treated as an unsigned 64-bit
   value.  If the lower order 32-bit value wraps, the higher order 32-
   32-bit value should be incremented and saved in non-volatile storage.
   If the LDP router is deployed long enough that the 64-bit sequence
   number wraps, all keys, independent of key distribution mechanism
   MUST be reset.  This is done to avoid the possibility of replay
   attacks.  Once the keys have been changed, the higher order sequence
   number can be reset to 0 and saved to non-volatile storage.

3.  Cryptographic Authentication Procedure

   As noted earlier, the Security Association ID maps to the
   authentication algorithm and the secret key used to generate and
   verify the message digest.  This specification discusses the
   computation of LDP Cryptographic Authentication data when any of the
   NIST SHS family of algorithms is used in the Hashed Message
   Authentication Code (HMAC) mode.

   The currently valid algorithms (including mode) for LDP Cryptographic
   Authentication include:

   HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384 and HMAC-SHA-512

   Of the above, implementations of this specification MUST include
   support for at least HMAC-SHA-256 and SHOULD include support for
   HMAC-SHA-1 and MAY also include support for HMAC-SHA-384 and HMAC-

   Implementations of this standard MUST use HMAC-SHA-256 as the default
   authentication algorithm.

4.  Cross Protocol Attack Mitigation

   In order to prevent cross protocol replay attacks for protocols
   sharing common keys, the two octet LDP Cryptographic Protocol ID is
   appended to the authentication key prior to use. use (refer to Section 8).
   Other protocols using cryptographic authentication as specified herein MUST the common key similarly append their respective own
   Cryptographic Protocol IDs to their keys in
   this step.  Refer prior to IANA Considerations (Section 8). use thus ensuring
   that a different key value is used for each protocol.

5.  Cryptographic Aspects

   In the algorithm description below, the following nomenclature, which
   is consistent with [FIPS-198], is used:

   H is the specific hashing algorithm (e.g. SHA-256).

   K is the Authentication Key from the LDP security association.

   Ks is a Protocol Specific Authentication Key obtained by appending
   Authentication Key (K) with the two-octet LDP Cryptographic Protocol
   ID appended.

   Ko is the cryptographic key used with the hash algorithm.

   B is the block size of H, measured in octets rather than bits.

   Note that B is the internal block size, not the hash size.

      For SHA-1 and SHA-256: B == 64

      For SHA-384 and SHA-512: B == 128

   L is the length of the hash, measured in octets rather than bits.

   XOR is the exclusive-or operation.

   Opad is the hexadecimal value 0x5c repeated B times.

   Ipad is the hexadecimal value 0x36 repeated B times.

   Apad is a value which is the same length as the hash output or
   message digest.  In case of IPv4, the first 4 octets contain the IPv4
   source address followed by the hexadecimal value 0x878FE1F3 repeated
   (L-4)/4 times.  In case of IPv6, the first 16 octets contain the IPv6
   source address followed by the hexadecimal value 0x878FE1F3 repeated
   (L-16)/4 times.  This implies that hash output is always a length of
   at least 16 octets.

5.1.  Preparing the Cryptographic Key

   The LDP Cryptographic Protocol ID is appended to the Authentication
   Key (K) yielding a Protocol Specific Authentication Key (Ks).  In
   this application, Ko is always L octets long.  While [RFC2104]
   supports a key that is up to B octets long, this application uses L
   as the Ks length consistent with [RFC4822], [RFC5310], [RFC5709] and
   [RFC7166].  According to [FIPS-180-3], Section 3, keys greater than L
   octets do not significantly increase the function strength.  Ks is
   computed as follows:

   If the Protocol Specific Authentication Key (Ks) is L octets long,
   then Ko is equal to Ks.  If the Protocol Specific Authentication Key
   (Ks) is more than L octets long, then Ko is set to H(Ks).  If the
   Protocol Specific Authentication Key (Ks) is less than L octets long,
   then Ko is set to the Protocol Specific Authentication Key (Ks) with
   zeros appended to the end of the Protocol Specific Authentication Key
   (Ks) such that Ko is L octets long.

5.2.  Computing the Hash

   First, the Authentication Data field in the Cryptographic
   Authentication TLV is filled with the value Apad.  Then, to compute
   HMAC over the Hello message it performs:

   H(Ko XOR Opad || H(Ko XOR Ipad || (Hello Message)))

   Hello Message refers to the LDP Hello message excluding the IP

   When XORing Ko and Ipad, or XORing Ko and Opad, Ko must be padded
   with zeros to the length of Ipad and the Opad.

5.3.  Result

   The resultant Hash becomes the Authentication Data that is sent in
   the Authentication Data field of the Cryptographic Authentication
   TLV.  The length of the Authentication Data field is always identical
   to the message digest size of the specific hash function H that is
   being used.

   This also means that the use of hash functions with larger output
   sizes will also increase the size of the LDP message as transmitted
   on the wire.

6.  Processing Hello Message Using Cryptographic Authentication

6.1.  Transmission Using Cryptographic Authentication

   Prior to transmitting the Hello message, the Length in the
   Cryptographic Authentication TLV header is set as per the
   authentication algorithm that is being used.  It is set to 24 for
   HMAC-SHA-1, 36 for HMAC-SHA-256, 52 for HMAC-SHA-384 and 68 for HMAC-

   The Security Association ID field is set to the ID of the current
   authentication key.  The HMAC Hash is computed as explained in
   Section 3.  The resulting Hash is stored in the Authentication Data
   field prior to transmission.  The authentication key MUST NOT be
   carried in the packet.

6.2.  Receipt Using Cryptographic Authentication

   The receiving LSR applies acceptability criteria for received Hellos
   using cryptographic authentication.  If the Cryptographic
   Authentication TLV is unknown to the receiving LSR, the received
   packet MUST be discarded according to Section of [RFC5036].

   The receiving LSR locates the LDP SA using the Security Association
   ID field carried in the message.  If the SA is not found, or if the
   SA is not valid for reception (i.e., current time < KeyStartAccept or
   current time >= KeyStopAccept), LDP Hello message MUST be discarded. discarded,
   and an error event SHOULD be logged.

   If the cryptographic sequence number in the LDP packet is less than
   or equal to the last sequence number received from the same neighbor,
   the LDP message MUST be discarded, and an error event SHOULD be

   Before the receiving LSR performs any processing, it needs to save
   the values of the Authentication Data field.  The receiving LSR then
   replaces the contents of the Authentication Data field with Apad,
   computes the Hash, using the authentication key specified by the
   received Security Association ID field, as explained in Section 3.
   If the locally computed Hash is equal to the received value of the
   Authentication Data field, the received packet is accepted for other
   normal checks and processing as described in [RFC5036].  Otherwise,
   if the locally computed Hash is not equal to the received value of
   the Authentication Data field, the received packet MUST be discarded,
   and an error event SHOULD be logged.  The foresaid logging need to be
   carefully rate limited, since while a LDP router is under attack of a
   storm of spoofed hellos, the resource taking for logging could be

   After the LDP Hello message has been successfully authenticated,
   implementations MUST store the 64-bit cryptographic sequence number
   for the Hello message received from the neighbor.  The saved
   cryptographic sequence numbers will be used for replay checking for
   subsequent packets received from the neighbor.

7.  Security Considerations

   Section 1 of this document describes the security issues arising from
   the use of unauthenticated LDP Hello messages.  In order to address
   those issues, it is RECOMMENDED that all deployments use the
   Cryptographic Authentication TLV to authenticate the Hello messages.

   The quality of the security provided by the Cryptographic
   Authentication TLV depends completely on the strength of the
   cryptographic algorithm in use, the strength of the key being used,
   and the correct implementation of the security mechanism in
   communicating LDP implementations.  Also, the level of security
   provided by the Cryptographic Authentication TLV varies based on the
   authentication type used.

   It should be noted that the authentication method described in this
   document is not being used to authenticate the specific originator of
   a packet but is rather being used to confirm that the packet has
   indeed been issued by a router that has access to the Authentication

   Deployments SHOULD use sufficiently long and random values for the
   Authentication Key so that guessing and other cryptographic attacks
   on the key are not feasible in their environments.  Furthermore, it
   is RECOMMENDED that Authentication Keys incorporate at least 128
   pseudo-random bits to minimize the risk of such attacks.  In support
   of these recommendations, management systems SHOULD support
   hexadecimal input of Authentication Keys.

   The mechanism described herein is not perfect and does not need to be
   perfect.  Instead, this mechanism represents a significant increase
   in the effort required for an adversary to successfully attack the
   LDP Hello protocol while not causing undue implementation,
   deployment, or operational complexity.

8.  IANA Considerations

   The IANA is requested to as assign a new TLV from the "Multiprotocol
   Label Switching Architecture (MPLS) Label Switched Paths (LSPs)
   Parameters - TLVs" registry, "TLVs and sub-TLVs" sub- registry.

   Value   Meaning                            Reference
   -----   --------------------------------   ---------
   TBD1     Cryptographic Authentication TLV   this document (sect 3.2) 2.3)

   The IANA is also requested to as assign value from the
   "Authentication Cryptographic Protocol ID", registry under the
   "Keying and Authentication for Routing Protocols (KARP) Parameters"

   Value   Meaning                            Reference
   -----   --------------------------------   ---------
   TBD2     LDP Cryptographic Protocol ID      this document (sect 4)

9.  Acknowledgements

   The authors would like to thank Liu Xuehu for his work on background
   and motivation for LDP Hello authentication.  The authors also would
   like to thank Adrian Farrel, Eric Rosen, Sam Hartman, Eric Gray,
   Kamran Raza and Acee Lindem for their valuable comments.

   We would also like to thank the authors of RFC 5709 and RFC 7166 from
   where we have taken most of the cryptographic computation procedures

10.  References

10.1.  Normative References

              "Secure Hash Standard (SHS), FIPS PUB 180-3", October

              "The Keyed-Hash Message Authentication Code (HMAC), FIPS
              PUB 198", March 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4822]  Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
              Authentication", RFC 4822, February 2007.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.

   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, October 2009.

   [RFC7166]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 7166, March 2014.

10.2.  Informative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [RFC5926]  Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
              for the TCP Authentication Option (TCP-AO)", RFC 5926,
              June 2010.

   [RFC6720]  Pignataro, C. and R. Asati, "The Generalized TTL Security
              Mechanism (GTSM) for the Label Distribution Protocol
              (LDP)", RFC 6720, August 2012.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, May 2013.

Authors' Addresses

   Lianshu Zheng
   Huawei Technologies


   Mach(Guoyi) Chen
   Huawei Technologies


   Manav Bhatia