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Network Working Group                                          M. Bhatia
Internet-Draft                                            Alcatel-Lucent
Intended status: Standards Track                               V. Manral
Expires: October 19, 2014                            Hewlett-Packard Co.
                                                                D. Zhang
                                                                  Huawei
                                                         M. Jethanandani
                                                       Ciena Corporation
                                                          April 17, 2014


                BFD Generic Cryptographic Authentication
                 draft-ietf-bfd-generic-crypto-auth-06

Abstract

   This document proposes an extension to Bidirectional Forwarding
   Detection (BFD) to allow the use of arbitrary cryptographic
   authentication algorithms in addition to the already-documented
   authentication schemes described in the base specification.  This
   document adds the basic infrastructure that is required for
   supporting algorithm and key agility for BFD.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 October 19, 2014.






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Copyright Notice

   Copyright (c) 2014 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
   (http://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 extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  BFD Security Association  . . . . . . . . . . . . . . . . . .   4
   3.  Authentication Procedures . . . . . . . . . . . . . . . . . .   5
     3.1.  Authentication Types  . . . . . . . . . . . . . . . . . .   5
     3.2.  Authentication Section Format . . . . . . . . . . . . . .   5
     3.3.  Procedures at the Sending Side  . . . . . . . . . . . . .   6
     3.4.  Procedure at the Receiving Side . . . . . . . . . . . . .   7
     3.5.  Key Selection for BFD Packet Transmission . . . . . . . .   8
     3.6.  Replay Protection using Extended Sequence Numbers . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The base specification of Bidirectional Forwarding Detection
   [RFC5880] defines five authentication schemes: Simple Password, Keyed
   MD5, Meticulous Keyed MD5, Keyed SHA-1, and Meticulous SHA-1.  In
   Simple Password, passwords are transferred in plain text.  An
   attacker with physical access to the network can easily eavesdrop on
   the password and compromise the security of the BFD packet exchanges.
   In Keyed MD5 and Meticulous Keyed MD5, the BFD devices on the both
   sides of a BFD session share a secret key which is used to generate a
   keyed MD5 digest for each packet, and a monotonically increasing
   sequence number scheme is used to prevent replay attacks.  Keyed
   SHA-1 and Meticulous SHA-1 modes are similar to MD5, and it uses
   SHA-1 instead of MD5 to generate a digest for each packet.



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   The security strength of the cryptographic algorithms adopted in the
   authentication schemes are relatively weak.  Both the MD5 algorithm
   and the SHA-1 algorithm are known to be vulnerable to collision
   attacks.  In MD5-attack [MD5-attack] and Dobb96a [Dobb96a], Dobb96b
   [Dobb96b], several methods of generating hash collisions for some
   applications of MD5 are proposed.  Similar security vulnerabilities
   of SHA-1 are introduced in SHA-1-attack1 [SHA-1-attack1] and
   SHA-1-attack2 [SHA-1-attack2].  It is therefore desired that BFD must
   support newer algorithms that have not yet been broken.
   Additionally, the transition mechanism from one algorithm to the
   other must be seamless.

   The other issue with the existing authentication schemes is the
   vulnerability to replay attacks.  In non-meticulous authentication
   schemes, sequence numbers are only increased occasionally.  This
   behavior can be taken advantage of by an attacker to perform intra-
   session replay attacks.  In meticulous authentication schemes,
   sequence numbers are required to monotonically increase with each
   successive packet, which eliminates the possibility of intra-session
   replay attacks.

   BFD session timers are often defined with the granularity of
   microseconds.  Although in practice BFD devices send packets at
   millisecond intervals, they can potentially send packets at a much
   higher rate.  Since the cryptographic sequence number space is only
   32 bits, when using Meticulous Authentication, a sequence number used
   in a BFD session can reach its maximum value and roll over within a
   short period.  For instance, if the value of a sequence number is
   increased by one every millisecond, then it will reach its maximum in
   less than 8 weeks.  This can potentially be exploited to launch
   inter-session replay attacks.

   In order to address the issues mentioned above, this document
   proposes two new authentication types that can be used to secure the
   BFD packets.  The two authentication types are - Cryptographic
   Authentication (CRYPTO_AUTH) and Meticulous Cryptographic
   Authentication (MET_ CRYPTO_AUTH).  Unlike earlier authentication
   types that were defined in BFD, the proposed authentication types are
   not tied to any particular authentication algorithm or construct.
   These can use different authentication algorithms and constructs like
   MD5, SHA-1, SHA-2, HMAC-SHA1, HMAC-SHA2, etc. to provide
   authentication and data integrity protection for BFD control packets.

   The packet replay mechanism has also been enhanced to improve its
   capability in handling inter and intra-session replay attacks.

   It should be noted that this document attempts to fix the security
   issues raised by the manual key management procedure that currently



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   exists within BFD, as part of the Phase One described in KARP Design
   Guidelines [RFC6518].  Therefore, only the pre-shared keys is
   considered in this document.  However, the solution described in this
   document is generic and does not preclude the possibility of
   supporting keys derived from an automated key management protocol.

2.  BFD Security Association

   The BFD protocol does not include an in-band mechanism to create or
   manage BFD Security Associations (BFD SA).  A BFD SA contains a set
   of shared parameters between any two legitimate BFD devices.

   The parameters associated with a BFD SA are listed as follows:

   o  Authentication Algorithm : This indicates the authentication
      algorithm to be used with the BFD SA.  This information SHOULD
      never be sent in plain text over the wire.

   o  Authentication Key : This indicates the cryptographic key
      associated with this BFD SA.  The length of this key is variable
      and depends upon the authentication algorithm specified by the BFD
      SA.  Operators MUST ensure that this is never sent over the
      network in clear-text via any protocol.  Care should also be taken
      to ensure that the selected key is unpredictable, avoiding any
      keys known to be weak for the algorithm in use.  Randomness
      Requirements for Security [RFC4086] contains helpful information
      on both key generation techniques and cryptographic randomness.

   o  Authentication Key Identifier (Key ID) : This is a two octet
      unsigned integer used to uniquely identify the BFD SA.  This ID
      could be manually configured by the network operator (or, in the
      future, possibly by some key management protocol specified by the
      IETF).  The receiver determines the active SA by looking at this
      field in the incoming packet.  The sender puts this Key ID in the
      BFD packet based on the active configuration.  Using Key IDs makes
      changing keys while maintaining protocol operation convenient.
      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.  A key ID indicates a tuple
      of an authentication key and an associated authentication
      algorithm.  If a key is expected to be applied with different
      algorithms, different Key IDs must be used to identify the
      associations of the key with its authentication algorithms
      respectively.  However, the application of a key for different
      purposes must be very careful, since it may make an adversary
      easier to collect more material to compromise the key.




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   o  Not Before Time : The time point before which the key should not
      be used.

   o  Not After Time : The time point after which the key should not be
      used.

3.  Authentication Procedures

   In the proposed authentication extension, an optional authentication
   section (Generic Authentication Section) and two authentication types
   (Generic Cryptographic Authentication and Generic Meticulous
   Cryptographic Authentication) are specified.

3.1.  Authentication Types

   The Authentication section is only present in a BFD packet if the
   Authentication Present (A) bit is set in the packet header.  The Auth
   Type in the Authentication section is set to TBD1 when Generic
   Cryptographic Authentication is in use, while it is set to TBD2 when
   Generic Meticulous Cryptographic Authentication is in use.

   Both the authentication types use a monotonically increasing sequence
   number to protect the BFD session against reply attacks.  The only
   difference between the two types is that the sequence number is
   occasionally incremented in the Cryptographic Authentication mode, as
   against the Meticulous Cryptographic Authentication mode, where it is
   incremented on every packet.

   As a result of this, in the Cryptographic Authentication scheme, a
   replay attack is possible till the next sequence number is sent out.

3.2.  Authentication Section Format

   A new authentication type, TBD1 or TBD2, indicating the generic
   cryptographic authentication mechanism in use, is inserted in the
   first octet of Authentication Section of the BFD control packet.

   For a BFD packet, if the Authentication Present (A) bit is set in the
   header and the Authentication Type field is TBD1 (Generic
   Cryptographic Authentication) or TBD2 (Generic Meticulous
   Cryptographic Authentication), the Authentication Section has the
   following format:









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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Auth Type   |   Auth Len    |         Auth Key ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Sequence Number (High Order 32 Bits)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Sequence Number (Low Order 32 Bits)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                  Authentication Data (Variable)               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Auth Type: The Authentication Type, which in this case is TBD1
      (Cryptographic Authentication) or TBD2 (Meticulous Cryptographic
      Authentication).

   o  Auth Len: The length of the Authentication Section.

   o  Auth Key ID: The Key ID of the authentication key used for this
      packet, enabling multiple keys to be active simultaneously.

   o  Sequence Number: A 64-bit sequence number that is used to prevent
      replay attacks.  For Cryptographic Authentication this value is
      incremented occasionally.  For Meticulous Cryptographic
      Authentication, this value is incremented for each successive
      packet transmitted for a session.

   o  Authentication Data: This field carries the digest computed by
      whatever Cryptographic Authentication algorithm is being used to
      authenticate the BFD control packet.

3.3.  Procedures at the Sending Side

   Before a BFD device sends a BFD packet out, the device needs to
   select an appropriate BFD SA from its local key database if a keyed
   digest for the packet is required.  If no appropriate SA is
   available, the BFD packet MUST be discarded.

   If an appropriate SA is available, the device then derives the key
   and the associated authentication algorithm from the SA.

   The device sets the Authentication Present (A) bit in the packet
   header.






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   The device MUST fill the Auth Type, the Auth Len fields and the
   Sequence Number field to bfd.XmitAuthSeq before the authentication
   data is computed.

   The Auth Len field in the Authentication section is set as per the
   authentication algorithm that is being used.

   The Key ID field is filled.

   The computation of the digest is performed.  The computing process
   can be various when different algorithms are adopted and is out of
   the scope of this document.

   The generated digest is placed in the Authentication Data field.

3.4.  Procedure at the Receiving Side

   When a BFD Control packet is received, the following procedure MUST
   be followed, in the order specified.

   If the Authentication Present (A) bit is set in the packet header and
   the Auth Type is TBD1 or TBD2, the receiver is to find an appropriate
   BFD SA in its local key table to process the packet.  The BFD SA is
   identified by the Key ID field in the Authentication Section of the
   incoming BFD packet.

   If the Auth Key ID field does not match the ID of any configured
   authentication key or the associated key is not in its valid period,
   the received packet MUST be discarded.

   If bfd.AuthSeqKnown is 1, examine the Sequence Number field.  For
   Cryptographic Authentication, if the Sequence Number lies outside of
   the range of bfd.RcvAuthSeq to bfd.RcvAuthSeq+(3*Detect Mult)
   inclusive (when treated as an unsigned 64 bit circular number space),
   the received packet MUST be discarded.  For Meticulous Cryptographic
   Authentication, if the Sequence Number lies outside of the range of
   bfd.RcvAuthSeq+1 to bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when
   treated as an unsigned 64 bit circular number space, the received
   packet MUST be discarded.

   The device then prepares for generating a digest of the packet.
   First of all, the authentication data in the Authentication Value
   field needs to be saved somewhere else.  Then the Authentication
   Value field is set with a pre-specified value (which may be various
   in different security algorithms) according the authentication
   algorithm indicated in the SA.  After this, the device starts
   performing the digest generating operations.  The work of defining




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   actual digest generating operations is out of the scope of this
   document.

   The calculated data is compared with the received authentication data
   in the packet and the packet MUST be discarded if the two do not
   match.  In such a case, an error event SHOULD be logged.

   An implementation MAY have a transition mode where it includes
   CRYPTO_AUTH or the MET_CRYPTO_AUTH information in the packets but
   does not verify this information.  This is provided as a transition
   aid for networks in the process of migrating to the new CRYPTO_AUTH
   and MET_CRYPTO_AUTH based authentication schemes.

3.5.  Key Selection for BFD Packet Transmission

   In [I-D.ietf-karp-crypto-key-table], a conceptual key database called
   "key table" is introduced.  A key table is located in the middle of
   key management protocols and security protocols so that a security
   protocol can derive long-term keys from the key table but does not
   have to know the details of key management.  This section describes
   how the proposed security solution selects long-lived keys from key
   tables [I-D.ietf-karp-crypto-key-table].

   Assume that a device R1 tries to send a unicast BFD packet from its
   interface I1 to the interface R2 of a remote device R2 at time T.
   Because the key should be shared by the by both R1 and R2 to protect
   the communication between I1 and I2, R1 needs to provide a protocol
   ("BFD"), an interface identifier (I1) and a peer identifier (R2) into
   the key selection function.  Any key that satisfies the following
   conditions may be selected:

   o  The Peer field includes the device ID of R2.

   o  The Protocol field matches "BFD"

   o  The PeerKeyName field is not "unknown".

   o  The Interface field includes I1 or "all".

   o  The Direction field is either "out" or "both".

   o  SendNotBefore <= current time <= SendNotAfter.

   After a set of keys are provided, a BFD implementation should support
   selection of keys based on algorithm preference.

   Upon reception of a BFD packet from R1, R2 provides the protocol
   ("BFD"), the peer identifier (R1), the key identifier derived from



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   the incoming packet (L), and the interface (I2) to the key table.
   Any key that satisfies the following conditions may be selected:

   o  The Peer field includes the device ID of R1.

   o  The Protocol field matches "BFD"

   o  The LocalKeyName is L

   o  The Interface field includes I2 or "all".

   o  The Direction field is either "out" or "both".

   o  SendNotBefore <= current time <= SendNotAfter.

3.6.  Replay Protection using Extended Sequence Numbers

   As described in Section 1, if the BFD packets in a session are
   transferred with a high frequency, a 32-bit sequence number may reach
   its maximum and have to roll back before the session finishes.  A
   attacker thus can replay the packets intercepted before the sequence
   number wrapped without being detected.  To address this problem, the
   length of the sequence number in the proposed authentication section
   has been extended to 64 bits.  After the extension, the sequence
   number space of a device will not be exhausted for half of a million
   years even if the device sends out a BFD packet in every micro-
   second.  Therefore, the replay attack risks caused by the limited
   sequence number space can be largely addressed.  However, in Generic
   Cryptographic Authentication, the sequence number is only required to
   increase occasionally.  Therefore, a replayed packet may be regarded
   as a legal one until the packet with a larger sequence number is
   received.  This type of intra-session replay attack cannot be
   addressed only by extending the length of sequence numbers.

   An anti-replay solution for BFD also needs to consider the scenarios
   where a BFD device loses its prior sequence number state (e.g.,
   system crash, loss of power).  In such cases, a BFD device has to re-
   initialize its sequence number.  Otherwise, an attacker may be able
   to replay a previously intercepted without being detected.

   To address this problem, in the proposed solution, the most
   significant 32-bit value of the sequence number is used to contain a
   boot count, and the remainder 32-bit value is used as an ordinary
   32-bit monotonically increasing sequence number.  In Generic
   Cryptographic Authentication, the remainder 32-bit value is required
   to increase occasionally, while in Generic Meticulous Cryptographic
   Authentication, the lower order 32-bit sequence number MUST be
   incremented for every BFD packet sent by a BFD device.  The BFD



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   implementations are required to retain the boot count in non-volatile
   storage for the deployment life the BFD device.  The boot count
   increases each time when the BFD device loses its prior sequence
   number state.  The SNMPv3 snmpEngineBoots variable [RFC4222] MAY be
   used for this purpose.  However, maintaining a separate boot count
   solely for BFD sequence numbers has the advantage of decoupling SNMP
   re-initialization and BFD re-initialization.  Also, in the rare event
   that the lower order 32- bit sequence number wraps, the boot count
   can be incremented to preserve the strictly increasing property of
   the aggregate sequence number.  Hence, a separate BFD boot count is
   RECOMMENDED.

4.  IANA Considerations

   IANA is requested to assign two authentication types from the "BFD
   Authentication Types" sub-registry within the "Bidirectional
   Forwarding Detection (BFD) Parameters" registry.

   +---------+-----------------------------------------+---------------+
   | Address | BFD Authentication Type Name            | Reference     |
   +---------+-----------------------------------------+---------------+
   | TBD1    | Cryptographic Authentication            | This document |
   | TBD2    | Meticulous Cryptographic Authentication | This document |
   +---------+-----------------------------------------+---------------+

5.  Security Considerations

   The proposed sequence number extension offers most of the benefits of
   more complicated mechanisms involving challenges.  There are,
   however, a couple drawbacks to this approach.

   First, it requires the BFD implementation to be able to save its boot
   count in non-volatile storage.  If the non-volatile storage is ever
   repaired or upgraded such that the contents are lost or the BFD
   device is replaced with a model, the keys MUST be changed to prevent
   replay attacks.

   Second, if a device is taken out of service completely (either
   intentionally or due to a persistent failure), the potential exists
   for re-establishment of a BFD adjacency by replaying the entire BFD
   session establishment.  This scenario is however, extremely unlikely
   and can be easily avoided.  For instance, after recovering from a
   system failure, a BFD device has to re-establish BFD sessions.  At
   this stage, if the device randomly selects its discriminators to
   identify new BFD sessions, the possibility of re-establishing a BFD
   session by replaying the entire BFD session establishment will be
   eliminated.  For the implementations in which discriminators are not
   randomly selected, this issue can be largely mitigated by integrating



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   the boot count of the remote BFD router in the generation of the
   authentication data for outgoing BFD packets.  Of course, this attack
   could also be thwarted by changing the relevant manual keys.

   There is a transition mode suggested where devices can ignore the
   CRYPTO_AUTH or the MET_CRYPTO_AUTH information carried in the
   packets.  The operator must ensure that this mode is only used when
   migrating to the new CRYPTO_AUTH/MET_CRYPTO_AUTH based authentication
   scheme as this leaves the device vulnerable to an attack.

6.  Acknowledgements

7.  References

7.1.  Normative References

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

7.2.  Informative References

   [Dobb96a]  Dobbertin, H., "Cryptanalysis of MD5 Compress", May 1996.

   [Dobb96b]  Dobbertin, H., "The Status of MD5 After a Recent Attack",
              CryptoBytes", 1996.

   [I-D.ietf-karp-crypto-key-table]
              Housley, R., Polk, T., Hartman, S., and D. Zhang,
              "Database of Long-Lived Symmetric Cryptographic Keys",
              draft-ietf-karp-crypto-key-table-10 (work in progress),
              December 2013.

   [MD5-attack]
              Wang, X., Feng, D., Lai, X., and H. Yu, "Collisions for
              Hash Functions MD4, MD5, HAVAL-128 and RIPEMD", August
              2004.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.






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   [RFC4222]  Choudhury, G., "Prioritized Treatment of Specific OSPF
              Version 2 Packets and Congestion Avoidance", BCP 112, RFC
              4222, October 2005.

   [RFC6518]  Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Design Guidelines", RFC 6518,
              February 2012.

   [SHA-1-attack1]
              Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
              Full SHA-1", 2005.

   [SHA-1-attack2]
              Wang, X., Yao, A., and F. Yao, "New Collision Search for
              SHA-1", 2005.

Authors' Addresses

   Manav Bhatia
   Alcatel-Lucent
   Bangalore
   India

   Email: manav.bhatia@alcatel-lucent.com


   Vishwas Manral
   Hewlett-Packard Co.
   19111 Pruneridge Ave.
   Cupertino, CA  95014
   USA

   Email: vishwas.manral@hp.com


   Dacheng Zhang
   Huawei
   Beijing
   China

   Email: zhangdacheng@huawei.com










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   Mahesh Jethanandani
   Ciena Corporation
   3939 North 1st Street
   San Jose, CA  95110
   USA

   Phone: +1 (408) 904-2160
   Fax:   +1 (408) 944-9290
   Email: mjethanandani@gmail.com










































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