Furquan Ansari
   Internet Draft                                          Lucent Tech.
   Document: draft-ansari-forces-discovery-01.txt      Hormuzd Khosravi
   Expires: April 18, 2006                                  Intel Corp.
   Working Group: ForCES                               Jamal Hadi Salim
                                                          Znyx Networks
                                                        Joel M. Halpern
                                                        Megisto Systems
                                                       October 19, 2005

                    ForCES Intra-NE Topology Discovery


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  Conventions used in this document

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


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   This  document  describes  a  mechanism  for  discovering  inter-FE
   topology and topology maintenance. Such a mechanism is essential for
   all these elements in the set to behave as a single Network Element,
   as required by the ForCES architecture as well as to perform certain
   optimizations at the FE by making use of the topology. The discovery
   mechanism only operates during post-association phase of ForCES

Table of Contents
1. Definitions........................................................2
2. Introduction.......................................................3
   2.1. Motivation....................................................3
3. Topology Discovery Mechanism.......................................5
   3.1. Minimum requirements..........................................6
   3.2. Protocol Details..............................................6
      3.2.1. Neighbor Finite State Machine............................7
      3.2.2. Topology Discovery and Maintenance.......................8
      3.2.3.  Full  topology  computation  at  the  CE  from  partial
   3.3. Protocol and Message Headers..................................9
      3.3.1. TLV definitions.........................................10
   3.4. Inter-FE Topology Discovery Examples.........................10
      3.4.1. Forwarding Elements connected in a daisy chain..........12
      3.4.2. Forwarding Elements connected in a ring.................13
4. Security Considerations...........................................14
5. References........................................................14
   5.1. Normative....................................................14
   5.2. Informative..................................................14
6. Authors' Addresses................................................14
7. IANA Considerations...............................................15
8. Full Copyright Notice.............................................15
9. Acknowledgements..................................................16

1. Definitions

   Inter-FE topology discovery: Topology discovery relates to how the
   FEs are interconnected with each other with respect to packet
   forwarding. This is the complete view of the intra-NE network as
   seen by the CE.

  Inter-FE topology maintenance: Once the inter-FE topology has been
  discovered, it has to be continuously monitored to ensure that any
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  changes to the topology are reported to the corresponding CE. This
  represents the steady state and final phase of the protocol.

2. Introduction

   The ForCES framework document [RFC 3746] describes how a set of
   control elements (CEs) and forwarding elements (FEs) interact with
   each other to form a single network element (NE). It describes the
   ForCES  post-association  phase  protocol  working  across  the  Fp
   reference point between CE and FE. This document describes an
   important aspect of the ForCES operational infrastructure- that of
   discovering the layout of the different elements within an NE.

   The Inter-FE/Intra-NE topology discovery protocol module may be
   implemented as a separate LFB on the FE. The protocol runs in an
   ongoing discovery and maintenance mode wherein the LFB maintains
   information about the known adjacencies per interface it is operated
   on. Each FE simply maintains its own adjacency tables and notifies
   the CE of any changes to the adjacency table based on the ForCES
   notification mechanism or if the CE explicitly requests an update.
   It is up to the CE to construct the full topology based on the
   information received from individual FEs within the NE. Given that
   the CE can request and the FEs should report back the topology
   updates using ForCES protocol, it is implicit that the topology
   discovery and maintenance operation occurs in the ForCES post-
   association phase.

   The proposed discovery mechanism is required to scale to a very
   large number of forwarding elements in the NE, with minimal impact
   on the resources. The following list provides some of the features
   and goals of the discovery mechanism.

   . Determine connectivity between elements
   . React to changes in link connectivity
   . Construct topology information from the collected partial topology
   . Tolerant to protocol message losses
   . Applicable to all inter-FE network topologies such as ring, mesh,
     star etc.
   . Cause minimal overhead
   . Agnostic of the network interconnect technology

2.1. Motivation

   The ForCES architecture defines a network element (NE) as a single
   managed entity made up of a collection of FEs and CEs and is
   indistinguishable from other network elements in the network. This
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   NE model definition leads to three types of links from the network’s
   perspective: internal (or intra-NE) links and external (or inter-NE)
   links and control links. Intra-NE links are purely internal to the
   NE and are not exposed to the external world; whereas, inter-NE (or
   external) links are exposed to the external world and over which
   routing adjacencies (such as OSPF, IS-IS, BGP etc.) can be formed.
   An NE can contain FEs that have zero or more internal/external links
   – e.g. in Fig. 1, FE3 has two internal links and no external links
   while FE1 and FE2 have two internal links and one external link
   each. Control links are those links that are used for communication
   between the CE and FE. If the CE and FE are a single Layer 3 hop
   apart as in Fig.1, the control link is typically a physical link
   e.g. link A of FE1 in the figure. Control links can be logical as
   well. It is important to note that the type definition for given for
   a link is only logical, because a given physical link may be a
   combination of more than one type - e.g. it could simultaneously be
   a control link and an internal link at the same time.

   A packet entering a ForCES NE may travel multiple FEs within the NE
   before it exits onto the output link. This requires that the packet
   be correctly forwarded from the ingress FE to the egress FE. This
   internal forwarding requires knowledge of the physical FE inter-
   connection topology so that the CE can appropriately setup internal
   LFB tables at each FE to handle packet traversal in a sane manner.
   We use a simple topology discovery mechanism that only operates on
   internal links and provides the necessary routing information to
   forward the packets from the ingress FE to the egress FE. Further,
   the  mechanism  should  be  able  to  reroute  around  internal  link
   failures, if a path exists. This makes the NE highly available and

                  NE 1
   .         -----------------          .
   .         |      CE       |          .
   .         -----------------          .            ----------
   .        A ^    B ^    C ^           .            |  NE 1  |
   .         /       |       \          .            |        |
   .        /      A v        \         .            ----------
   .       /      ------- B    \        .             ^      ^
   .      /    +->| FE3 |<-+    \       . <====>     /        \
   .     /     |C |     |  |     \      .           /          \
   .  A v      |  -------  |      v A   .          v            v
   .  -------B |           |D -------   .       --------      ---------
   .  | FE1 |<-+           +->| FE2 |   .       | NE 2 |<---->|  NE 3 |
   .  |     |<--------------->|     |   .       |      |      |       |
   .  ------- C             C -------   .       --------      ---------
   .   D^                       ^B      .
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        |                       |
        V                       v
    --------                 --------
    | NE 2 |<--------------->| NE 3 |
    |      |                 |      |
    --------                 --------

                (a)                                        (b)

   Figure 1:(a) illustrates the internal/external links and topology
   within a NE. (b) Shows the network topology as seen by external
   routing protocols

3. Topology Discovery Mechanism

   Since the topology discovery protocol described here operates in the
   ForCES post-association phase, it is independent of whether the CE
   and the FE are a single or multiple hops (layer 2 or layer 3) apart
   from each other. It is up to the ForCES association protocol to
   determine how to setup the ForCES channel between the CE and FE if
   they are multiple hops away. The topology discovery protocol is
   expected to work on all types of media and interfaces – such as
   point-to-point as well as multi-access links.

   In order to keep the discovery and maintenance mechanism as simple
   as  possible,  the  FEs  only  maintain  relationships  with  their
   respective neighbors to determine the status of the neighbors. No
   databases are exchanged between the neighbors. This implies that the
   topology view for each FE is only limited to the adjacent elements.
   This partial topology information may be reported back to the CE (or
   queried by the CE) over the ForCES protocol using the ForCES
   notification mechanism. Since the CE receives such information from
   all  the  FEs,  it  can  easily  construct  the  full  topology  from
   individual partial topologies reported by each FE. Once the CE
   constructs the full topology, such information can be passed to the
   FEs,  if  needed  (depending  on  policy).  The  FEs  may  use  such
   information for dynamic intra-NE route calculation or certain other

   Topology information is needed by a lot of LFBs and associated
   services that span multiple FEs within a NE. In the case where the
   FE aids the CE in offloading the table updates, then it makes sense
   for the FE to be topology aware. It is sometimes also helpful to
   keep full topology information at the FEs for cases such as “message
   snooping” optimizations. For example, if an FE is aware of the
   topology, it could snoop on messages sent to other FEs (e.g.
   broadcasts,  multicasts)  and  update  its  own  tables  dynamically
   without involving the CE. Another example would be FE-FE primary-
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   backup handover scenario. With each FE being fully aware of the
   complete topology, the backup FE can take over the responsibilities
   of the primary without involving the CE for such a handover.

3.1. Minimum requirements

   In order for the protocol to work as described, the following
   assumptions are made.
     . Each element has been configured with their respective IDs
        (CEID, FEID)
     . Element bindings process has already taken place. In other
        words, the CE know all the FEs it wants to control and each FE
        knows which CE is allowed to control it.
     . The ForCES protocol association has already taken place between
        the CE and the FE in question.
     . The protocol is enabled on the required interfaces.

   Note that these are configuration requirements and are satisfied by
   the respective managers (CEM/FEM).

3.2. Protocol Details

   Once the ForCES protocol association has been established between a
   CE and a given FE – i.e. it is in post-association phase, the CE
   starts  sending/advertising  Hello/Probe  messages  to  the  FE’s
   neighbors such that the messages go through the given FE. In other
   words, it looks like the given FE is generating probe messages to
   the neighbor (except that these messages are coming from the CE over
   the  ForCES  protocol  first).  However,  this  functionality  of
   generating probe messages by the CE can be offloaded to the FE
   itself (to be more precise, to an FE LFB) – so that the FE can
   originate and terminate the probe messages. This provides better
   scalability of the CE and it’s resources. The CE can now simply
   query each FE’s neighbor relationship database and register for any
   events related to topology changes.

   All Hello/Probe messages travel a single PE hop and are not routed
   to other elements beyond the first hop. An on-link IP multicast
   address is used for sending all Hello packets. The packets should be
   sent with a TTL of 255 and ignored on receipt if the TTL is not 254
   (based on some of the recommendations from the generalized TTL
   security mechanism to use TTL 255 rather than TTL 1). Hello packets
   are  only  sent  on  interfaces  configured  for  topology  discovery
   protocol operation. Further, the Hello messages will be multicast on
   multicast capable links. Each FE topology LFB component maintains
   the  neighbor  relationships  as  long  as  the  Hello  messages  are
   received from the neighbor. If it does not receive Hello messages
   after a given (configured) period of time (called FE Neighbor dead
   interval), it deletes the entry from the database and reports this
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   change to the CE in the form of an event-notification message over
   the ForCES protocol. This ensures that the CE has the complete and
   up-to-date information of the underlying topology of the Inter-FE

   The Hello message contains information necessary for discovering and
   maintaining neighbor relationships. It contains the PE ID, type of
   protocol element (i.e. CE or FE), interval between any two messages,
   interval for deeming a neighbor inactive, capability information
   etc. This is, in some ways, similar to the capabilities of the OSPF
   Hello protocol.

   On receiving the Hello messages from a neighbor, the FE responds
   back with its own Hello message in a packet format similar to the
   one  received  from  the  neighbor.  Essentially,  both  sides  are
   independently sending Hello messages to each other and listing their
   neighbor table. Also, each neighbor will see itself listed on its
   neighbors Hello message. This ensures bi-directionality of the link
   between any two neighbors.

   The operation is concisely described by the following steps:
   . CE activates the topology LFB/component on the FE to initialize on
     specific ports
   . FE topology LFB/component sends neighbor probes/hellos
   . CE queries FE for its neighbors
   . FE continues to send these probes afterwards (maintenance) and
     updates asynchronously any new updates

   Note: We would like to point out here that the Hello messaging
   mechanism can very well be replaced by the BFD (Bi-Directional
   Forwarding Detection) protocol in the future since it performs
   similar  task  of  detecting  bi-directional  faults  between  two
   forwarding engines. Further, BFD protocol has the ability to be
   bootstrapped by any other protocol that automatically forms peer,
   neighbor or adjacency relationships to seed BFD endpoint discovery.

3.2.1. Neighbor Finite State Machine
   In order to obtain bi-directionality verification of the links, and
   to make the protocol more robust, a neighbor finite state machine is
   needed. It consists of the following three states:

   Neighbor-down:  This  is  the  initial  state  of  the  neighbor
   conversation. It indicates that there has been no recent information
   received from the neighbor

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   Neighbor-heard: In this state, a Hello packet was recently seen from
   the neighbor. However, bi-directional communication has not been
   fully established with the neighbor (i.e. the PE itself was not
   listed in the neighbor’s Hello packet – which is the check for bi-
   directionality). All neighbors in this state (or higher) are listed
   in the Hello packets sent from the associated interface.

   Neighbor-adjacent: In this state, the communication between the two
   neighbors is bi-directional. This has been assured by the Hello
   protocol operation. This state corresponds to the final steady state
   of the protocol.

3.2.2. Topology Discovery and Maintenance

   Since the CE needs to maintain consistent and up-to-date view of the
   inter-FE topology, it needs to obtain real-time information of the
   status of the internal links connecting the FEs. Since the topology
   discovery and maintenance occurs during the post-association phase,
   we make use of the event-notification and query/response messages
   [ForCESP] of the ForCES protocol to provide this information to the
   CE. It is important to note that each FE only maintains partial
   topology   information   obtained   through   neighbor   relationship
   maintenance through Hello messages. The partial topology view seen
   by each FE is only the neighbor connectivity information. The CE has
   to derive the complete topology view of the interconnected FEs based
   on the partial topology information reported by each FE (or queried
   by the CE). This ensures that that only the CE maintains all the
   intelligence and the protocol operation on the FEs is very simple
   and  has  minimal  overhead.  However,  as  mentioned  above,  if
   optimizations can be performed by having the complete topology
   information available at the FEs, the CE can push such information
   to any FE interested in it (interest on the FE may be shown in the
   form of policy configuration). This is an optional feature available
   on each FE, which can be turned on or off through configuration or
   during capability exchange negotiation at setup time. Each FE vendor
   may decide to make use of this feature in different ways, so the
   capability to obtain such topology information should exist.

   The periodic Hello messages maintain PE neighbor relationships.
   Any change in the link or neighbor status causes the FE to generate
   an  asynchronous/event-driven  message  to  the  CE  indicating  this
   change. The mechanism defined in [ForCESP] is used for delivering
   event-driven messages from the FE to the CE. This involves the CE
   subscribing to such event-driven messages from the FE.

   The CE aggregates the partial topology information received from
   each FE and generates the inter-FE topology. With this complete
   knowledge of the inter-FE topology, it can now make appropriate
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   updates to the LFB tables on each FE to move packets inside the NE –
   from ingress FE to egress FE, assuming that the destination of the
   packet is not the current NE itself. Any changes in the internal
   link  states  (and  hence  the  topology)  requires  that  the  CE
   reconfigure the LFB tables on the FEs based on the most up-to-date
   information to ensure that the packets do not end up in a black hole
   or enter a loop.

3.2.3. Full topology computation at the CE from partial topologies

   The CE receives neighbor relationships information from each FE that
   it uses to construct the full topology of the internal network. Each
   FE’s neighbor relationship table contains information regarding the
   local element ID, local port connecting the neighbor, the neighbor’s
   ID, the neighbor’s port and any optional additional information.
   Note that the fact that the FE already knows the neighbor’s port
   information implies that it received the probe/hello messages from
   the neighbor on that port in response to the hello sent and was,
   therefore, able to establish bi-directionality of the link. If all
   the links in the internal network are point-to-point links, the CE
   simply  has  to  aggregate  all  the  neighbor  relationship  tables
   obtained from all the FEs to generate the full topology. If we
   assume the topology to be a graph, each edge of the graph will be
   present twice – essentially providing the same information from the
   two endpoints of the graph. After deleting all the duplicate entries
   (and thus reducing the table size by half), the CE now has accurate
   view of the full topology. Please refer section 3.4 [Fig. 3(b)] for
   more details.

   [Sub-section  on  generating  full  topology  from  partial  topology
   information  for  broadcast/multi-access,  point-to-multipoint  etc.
   type of links]

3.3. Protocol and Message Headers

   The protocol message consists of a fixed length header (16 bytes)
   followed by one or more optional TLVs. The format of the message 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
   |   Version   |    Flags      |          Packet Length        |
   |          Checksum           |          Port ID              |
   |                         PE ID                               |
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   |        TLV-Type             |          TLV-Length           |
   |                         TLV-Value     ...                   |
   |                            ...                              |

                                Figure (2)

   Version: Version number of this protocol. Currently acceptable value
   is 0x01

   Flags: These indicate whether the message is sent by a FE, (0x01) or
   CE (0x02). More options may be defined in the future.

   Packet  Length:  The  length  of  the  protocol  message  in  bytes,
   including the header and the following TLVs.

   Checksum: 16-bit checksum for the protocol message. The checksum
   calculation does not include the IP header.

   Port ID: This indicates the port on which this packet was sent out
   by the sender – useful for topology construction.

   PE ID: This is the 32-bit identifier of the sender. It could either
   be CE ID or FE ID, depending on the sender.

   The protocol header is followed by one or many TLVs. The following
   TLVs types are defined:

   Hello  TLV:  Indicates  the  Hello  message  as  exchanged  by  the
   neighbors. The TLV defines the common hello parameters such as the
   Hello Interval, Hold time, Uni-directional targeted Hellos, Sequence
   space number, if needed etc.

   Capabilities TLV: Provides the capabilities information - TBD

   Vendor specific TLV: TBD

3.3.1. TLV definitions

3.4. Inter-FE Topology Discovery Examples

   The following examples illustrate the topology discovery mechanism.
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   For sake of simplicity, we assume that there is only one CE per NE.
   The FEIDs of the FEs in the topologies below are FE1, FE2, FE3, and
   FE4. Each FE has port IDs labeled alphabetically. This is also the
   case with the CE.

            |      CE       |
           A ^    B ^    C ^
            /       |       \
           /      A v        \
          /      ------- B    \
         /    +->| FE3 |<-+    \
        /     |C |     |  |     \
     A v      |  -------  |      v A
     -------B |           |E -------
     | FE1 |<-+           +->| FE2 |
     |     |<--------------->|     |
     ------- C             D -------
      E ^ D|                 C ^  | B
        |  |                   |  |
        |  v                   |  v

        FE3 Control Element reachability Table
        <Dest Addr>             <local intf>
          CE                         A

        <local intf> <neighbor_FEID> <neighbor_portID>
              B            FE2                 E
              C            FE1                 B

    Figure 3. (a) Full mesh among FE1, FE2, and FE3

   During the element-binding phase, each FE sends out hello messages
   with its FEID and Port ID (as outlined earlier) to all of its
   neighbors. Since each neighboring FE also listens to such messages,
   it  receives  the  hello  message  and  adds  it  to  the  neighbor
   association table, which may look like that shown in Fig.4(a). In
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   the topology discovery phase, which is post ForCES association
   stage, the CE queries each FE about its neighbor table. The FE
   responds  back  with  the  partial  topology  information  available
   through its neighbor relationships. Both the query and the response
   are carried by the ForCES protocol. The CE collects the partial
   topology information from all the FEs in the NE and aggregates this
   information to fully construct the inter-FE topology. Any changes to
   the FE neighbor table, e.g. when a link state changes, generates a
   trigger/update message to the CE. The new information is used to
   recalculate  the  new  topology  and  subsequently  the  CE  takes
   appropriate actions based on the new topology – such as updating the
   packet forwarding tables on the FEs.

   The following examples show the neighbor association tables.

3.4.1. Forwarding Elements connected in a daisy chain

                   |     CE     |
               A  ^ ^ B    ^    ^ D
                 /  |       \    \
          /------   |        --\  -------\
       A v        A v           v A       v A
      -------B    -------B    -------B    -------
      | FE1 |<--->| FE2 |<--->| FE3 |<--->| FE4 |
      -------    E-------    E-------    D-------
      D ^  |C     D ^  |C      D^  |C      C^  |B
        |  |        |  |        |  |        |  |
        |  v        |  v        |  v        |  v

    --------------------------------    ------------------------------
   <locl intf> <nbr_FEID> <nbr_port>  <locl intf> <nbr_FEID> <nbr_port>
         B        FE2         E            E          FE1        B
                                           B          FE3        E

    --------------------------------   -----------------------------
   <locl intf> <nbr_FEID> <nbr_port>  <locl intf> <nbr_FEID> <nbr_port>
         B        FE4         D           D          FE3         B
         E        FE2         B

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         CE Topology (Aggregate)View            CE Topology View
     --------------------------------    ------------------------------
      <Node>  <Port>  <Node>  <Port>     <Node>  <Port>  <Node>  <Port>
        FE1     B       FE2     E\         FE1      B      FE2     E
        FE2     E       FE1     B/   =>    FE2      B      FE3     E
        FE2     B       FE3     E\         FE3      B      FE4     D
        FE3     E       FE2     B/       ------------------------------
        FE3     B       FE4     D\
        FE4     D       FE3     B/

      Fig.3(b) Multiple FEs in a daisy chain

3.4.2. Forwarding Elements connected in a ring

                      ^ |
                     D| v E
                   ----------- A
                   |   FE1   |<-----------------------|
                   -----------                        |
                   C ^    ^ B                         |
                    /      \                          |
             | ^   /        \    ^ |                   V A
           B v |C v D      C v  D| v E             ----------
           ---------        ---------             D|        |
           | FE2   |        |  FE3  |<------------>|   CE   |
           ---------        ---------  A           |        |
             A ^  ^ E        ^ B                   ----------
               |   \        /                      C ^  ^ B
               |    \      /                         |  |
               |   D v   E v                          |  |
               |   ----------- A                     |  |
               |   |   FE4   |<----------------------|  |
               |   -----------                          |
               |    C |  ^ B                            |
               |      v  |                              |
               |                                        |

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    --------------------------------   -----------------------------
   <locl intf> <nbr_FEID> <nbr_port>  <locl intf> <nbr_FEID> <nbr_port>
       B           FE3        C           E          FE4        D
       C           FE2        D           D          FE1        C

    --------------------------------  -----------------------------
   <locl intf> <nbr_FEID> <nbr_port>  <locl intf> <nbr_FEID> <nbr_port>
        B         FE4         E           D          FE2         E
        C         FE1         B           E          FE3         B

           Fig. 3(c) Multiple FEs connected in a ring

4. Security Considerations

   Like all protocols, this protocol will have security issues as well.
   These issues will be researched in detail in future draft versions.

5. References

5.1. Normative

   [RFC3746]  Yang, L., Dantu, R., Anderson, T. and R. Gopal,
              "Forwarding and Control Element Separation (ForCES)
              Framework", RFC 3746, April 2004.
   [RFC3654]  Khosravi, H. and T. Anderson, "Requirements for
              Separation of IP Control and Forwarding", RFC 3654,
              November 2003.
   [ForCESP]  F P Team, "ForCES protocol specification",
              draft-ietf-forces-protocol-00.txt, Sept 2004.

5.2. Informative

   [OSPF]     J. Moy, “OSPF Version 2”, 1998, RFC 2328.
   [BGP]      Y. Rekhter, T. Li, “A Border Gateway Protocol 4 (BGP-4)”,
              1995, RFC 1771.
   [IS-IS]    R. Collela et al., “Guidelines for OSI NSAP Allocation in
              the Internet”, 1994, RFC 1629.

6. Authors' Addresses

   Furquan Ansari
   Bell Labs Research, Lucent Tech.
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Internet Draft             ForCES Discovery                  July 2004
   101 Crawfords Corner Road
   Holmdel, NJ 07733
   Phone: +1 732-949-5249
   Email: furquan@lucent.com

   Hormuzd Khosravi
   2111 N.E. 25th Avenue JF3-206
   Hillsboro, OR 97124-5961
   Phone: +1 503 264 0334
   Email: hormuzd.m.khosravi@intel.com

   Jamal Hadi Salim
   ZNYX Networks
   Ottawa, Ontario, Canada
   Email: hadi@znyx.com

   Joel M. Halpern
   Megisto systems, Inc.
   20251 Century Blvd.
   Germantown, MD, 20874-1162, USA
   Phone: +1 301 444 17
   Email: jhalpern@megisto.com

7. IANA Considerations
  There are no IANA considerations at this point since the protocol completely
  operates within an NE.

8. Full Copyright Notice

   "Copyright (C) The Internet Society (2005). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights."

   "This document and the information contained herein are provided on

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9. Acknowledgements
   We would like to thank Thyaga Nandagopal of Lucent Technologies for
   his thoughts and contributions to the initial draft.

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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