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Versions: 00 draft-ietf-p2psip-service-discovery

P2PSIP Working Group                                          J. Maenpaa
Internet-Draft                                              G. Camarillo
Intended status: Standards Track                                Ericsson
Expires: April 22, 2010                                 October 19, 2009


  Service Discovery Usage for REsource LOcation And Discovery (RELOAD)
             draft-maenpaa-p2psip-service-discovery-00.txt

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   REsource LOcation and Discovery (RELOAD) does not define a generic
   service discovery mechanism as part of the base protocol.  This
   document defines how the Recursive Distributed Rendezvous (ReDiR)



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   service discovery mechanism used in OpenDHT can be applied to RELOAD
   overlays to provide a generic service discovery mechanism.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Introduction to ReDiR  . . . . . . . . . . . . . . . . . . . .  4
   4.  Using ReDiR in a RELOAD Overlay Instance . . . . . . . . . . .  6
     4.1.  Data Structure . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Selecting the Starting Level . . . . . . . . . . . . . . .  7
     4.3.  Service Provider Registration  . . . . . . . . . . . . . .  7
     4.4.  Refreshing Registrations . . . . . . . . . . . . . . . . .  8
     4.5.  Service Lookups  . . . . . . . . . . . . . . . . . . . . .  8
     4.6.  Removing Registrations . . . . . . . . . . . . . . . . . .  9
   5.  Access Control Rules . . . . . . . . . . . . . . . . . . . . .  9
   6.  REDIR Kind Definition  . . . . . . . . . . . . . . . . . . . .  9
   7.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Overlay Configuration Document Extension . . . . . . . . . . . 11
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     10.1. Access Control Policies  . . . . . . . . . . . . . . . . . 12
     10.2. Data Kind-ID . . . . . . . . . . . . . . . . . . . . . . . 12
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     11.2. Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13























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1.  Introduction

   REsource LOcation And Discovery (RELOAD) [I-D.ietf-p2psip-base] is a
   peer-to-peer signaling protocol that can be used to maintain an
   overlay network, and to store data in and retrieve data from the
   overlay.  Although RELOAD defines a Traversal Using Relays around
   Network Address Translation (TURN) specific service discovery
   mechanism, it does not define a generic service discovery mechanism
   as part of the base protocol.  This document defines how the
   Recursive Distributed Rendezvous (ReDiR) service discovery mechanism
   [Redir] used in OpenDHT can be applied to RELOAD overlays.

   In a Peer-to-Peer (P2P) overlay network such as a RELOAD Overlay
   Instance, the peers forming the overlay share their resources in
   order to provide the service the system has been designed to provide.
   The peers in the overlay both provide services to other peers and
   request services from other peers.  Examples of possible services
   peers in a RELOAD Overlay Instance can offer to each other include a
   TURN relay service, a voice mail service, a gateway location service,
   and a transcoding service.  Typically, only a small subset of the
   peers participating in the system are providers of a given service.
   A peer that wishes to use a particular service faces the problem of
   finding peers that are providing that service from the Overlay
   Instance.

   A naive way to perform service discovery is to store the Node-IDs of
   all nodes providing a particular service under a well-known key k.
   The limitation of this approach is that it scales linearly in the
   number of nodes that provide the service.  The problem is two-fold:
   the node n that is responsible for service s identified by key k may
   end up storing a large number of Node-IDs and most importantly, may
   also become overloaded since all service lookup requests for service
   s will need to be answered by node n.  An efficient service discovery
   mechanism does not overload the nodes storing pointers to service
   providers.  In addition, the mechanism must ensure that the load of
   providing a given service is distributed evenly among the nodes
   providing the service.

   ReDiR implements service discovery by building a tree structure of
   the nodes that provide a particular service and embedding it into the
   RELOAD Overlay Instance using RELOAD Store and Fetch requests.  Each
   service provided in the Overlay Instance has its own tree.  The nodes
   in a ReDiR tree contain pointers to service providers.  During
   service discovery, a peer wishing to use a given service fetches
   ReDiR tree nodes one-by-one from the RELOAD Overlay Instance until it
   finds a service provider responsible for its Node-ID.  It has been
   proved that ReDiR can find a service provider using only a constant
   number of fetch operations [Redir].



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2.  Terminology

   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].

   This document uses the terminology and definitions from the Concepts
   and Terminology for Peer to Peer SIP [I-D.ietf-p2psip-concepts]
   draft.


   DHT:  Distributed Hash Tables (DHTs) are a class of decentralized
      distributed systems that provide a lookup service similar to a
      hash table.  Given a key, any participating peer can retrieve the
      value associated with that key.  The responsibility for
      maintaining the mapping from keys to values is distributed among
      the peers.

   H(x):  Hash calculated over x.

   I(l,k):  The unique interval at level l in the ReDiR tree that
      encloses key k.

   n.id:  Node-ID of node n.

   Namespace:  An arbitrary identifier that identifies a service
      provided in the RELOAD Overlay Instance.  An example of a
      namespace is "voice-mail".

   numBitsInNodeId:  Number of bits in a Node-ID.

   ReDiR tree:  A tree structure of the nodes that provide a particular
      service.  The nodes embed the ReDiR tree into the RELOAD Overlay
      Instance using RELOAD Store and Fetch requests.

   Successor:  The successor of identifier k in namespace ns is the node
      that has joined ns whose identifier most immediately follows k.



3.  Introduction to ReDiR

   Recursive Distributed Rendezvous (ReDiR) [Redir] does not require new
   functionality from the RELOAD base protocol.  This is possible since
   ReDiR interacts with the RELOAD overlay through a put/get API using
   RELOAD Store and Fetch requests.  ReDiR builds a tree structure of
   the nodes that provide a particular service and embeds it into the
   RELOAD Overlay Instance using the Store and Fetch requests.  ReDiR



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   performs lookup in a logarithmic number of Fetch operations with high
   probability.  Further, if the tree's height is estimated based on
   past lookups, the average lookup can be reduced to a constant number
   of fetch operations assuming that Node-IDs are distributed uniformly
   at random.

   In ReDiR, each service provided in the overlay is identified by an
   arbitrary identifier, called its namespace.  All service providers
   join a namespace and peers wishing to use a service perform lookups
   within the namespace of the service.  A ReDiR lookup for identifier k
   in namespace ns returns a node that has joined ns whose identifier is
   the closest successor of k.

   Each tree node in the ReDiR tree contains a list of Node-IDs of peers
   providing a particular service.  Each node in the tree has a level.
   The root is at level 0, the immediate children of the root are at
   level 1, and so forth.  The ReDiR tree has a branching factor, whose
   value is determined by a new element in the RELOAD overlay
   configuration document, called redirBranchingFactor.  At every level
   i in the tree, there are at most redirBranchingFactor^i nodes.  The
   nodes at any level are labeled from left to right, such that a pair
   (i,j) uniquely identifies the jth node from the left at level i.
   This tree is embedded into the RELOAD Overlay Instance node by node,
   by storing the values of node (i,j) at key H(namespace,i,j).  As an
   example, the root of the tree for a voice mail service is stored at
   H("voice-mail",0,0).  Each node (i,j) in the tree is associated with
   redirBranchingFactor intervals of the DHT keyspace as follows:

             [2^numBitsInNodeID*b^(-i)*(j+(b'/b)),
              2^numBitsInNodeID*b^(-i)*(j+((b'+1)/b))], for 0<=b'<b,

   where b is the redirBranchingFactor.

   Figure 1 shows an example of a ReDiR tree with a branching factor of
   2.  Each node is shown as two horizontal lines separated by a
   vertical bar.  The lines represent the two intervals each node is
   responsible for.  At level 0, there is only one node, (0,0)
   responsible for two intervals that together cover the entire
   identifier space of the RELOAD Overlay Instance.  At level 1, there
   are two nodes, (1,0) and (1,1), each of which is responsible for half
   of the identifier space.  At leavel 2, there are four nodes.  Each of
   them owns one fourth of the identifier space.  At level 3, there are
   eight nodes each of which is responsible for one eight of the
   identifier space.







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     Level 0  __________________|__________________
                      |                   |
     Level 1  ________|________   ________|________
                 |         |         |         |
     Level 2  ___|___   ___|___   ___|___   ___|___
               |   |     |   |     |   |     |   |
     Level 3  _|_ _|_   _|_ _|_   _|_ _|_   _|_ _|_


                           Figure 1: ReDiR tree


4.  Using ReDiR in a RELOAD Overlay Instance

4.1.  Data Structure

   ReDiR tree nodes are stored using the dictionary data model defined
   in RELOAD base [I-D.ietf-p2psip-base].  The data stored is a
   RedirServiceProvider structure:

               struct {
                 NodeId                   serviceProvider;
                 opaque                   namespace<0..2^16-1>;
                 uint16                   level;
                 uint16                   node;

                 /* This type can be extended */

               } RedirServiceProviderData;

               struct {
                 uint16                   length;
                 RedirServiceProviderData data;
               } RedirServiceProvider;


   The contents of the RedirServiceProvider structure are as follows:

   length
      The length of the rest of the structure.

   data
      The service provider data.


   The contents of the RedirServiceProviderData structure are as
   follows:




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   serviceProvider
      The Node-ID of a service provider.

   namespace
      An opaque string containing the namespace.

   level
      The level in the ReDiR tree.

   node
      The position of the node storing this RedirServiceProvider record
      at the current level in the ReDiR tree.


   The RedirServiceProviderData structure can be extended to include for
   instance service or service provider specific information.

4.2.  Selecting the Starting Level

   Before registering as a service provider or performing a service
   lookup, a peer needs to determine the starting level Lstart for the
   registration or lookup operation in the ReDiR tree.  It is
   RECOMMENDED that Lstart is initially set to 2.  In subsequent
   registrations, Lstart MUST be set to the lowest level at which
   registration last completed.

   In the case of service lookups, nodes MUST record the levels at which
   the last 16 service lookups completed and take Lstart to be the mode
   of those depths.

4.3.  Service Provider Registration

   A node MUST use the following procedure to register as a service
   provider in the RELOAD Overlay Instance:

   1.  A node n with Node-ID n.id wishing to register as a service
       provider starts from level Lstart (see Section 4.2 for the
       details on selecting the starting level).  First, node n MUST
       send a RELOAD Store request to add its RedirServiceProvider entry
       to the dictionary stored in the tree node associated with
       I(Lstart,n.id).  An interval I(l,k) is defined as the unique
       interval at level l in the ReDiR tree that encloses key k
   2.  Node n MUST send a RELOAD Fetch request to obtain the contents of
       the tree node associated with I(Lstart,n.id).  The fetch MUST be
       a wildcard fetch.
   3.  If node n's Node-ID is the numerically lowest or highest among
       the Node-IDs stored in the tree node, node n MUST continue up the
       tree towards the root (level 0), repeating steps 1 and 2.  Node n



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       MUST continue this until it reaches either the root or a level at
       which n.id is not the lowest or highest Node-ID in the interval.
   4.  Node n MUST also walk down the tree through tree nodes associated
       with the intervals I(Lstart,n.id),I(Lstart+1,n.id),..., at each
       step fetching the current contents of the tree node, and storing
       its redirServiceProvider record if n.id is the lowest or highest
       Node-ID in the interval.  Node n MUST end this downward walk when
       it reaches a level at which it is the only service provider in
       the interval.

4.4.  Refreshing Registrations

   All state in the ReDiR tree is soft.  If an entry (Node-ID) is not
   refreshed for redirRefreshPeriod seconds, it MUST be dropped from the
   dictionary by the peer storing the tree node. redirRefreshPeriod is a
   new element in the RELOAD overlay configuration document (see
   Section 8).  Every service provider MUST repeat the entire
   registration process periodically until it leaves the RELOAD Overlay
   Instance.

   Note that no new mechanisms are needed to keep track of the remaining
   lifetime of RedirServiceProvider records.  The 'storage_time' and
   'lifetime' fields of RELOAD's StoredData structure can be used for
   this purpose in the usual way.

4.5.  Service Lookups

   The purpose of a service lookup on identifier k in namespace ns is to
   find the node that has joined ns whose identifier most immediately
   follows identifier k.

   A service lookup is similar to the registration operation.  The
   service lookup starts from some level level=Lstart (see Section 4.2
   for the details on selecting the starting level).  At each step, the
   node n wishing to discover a service provider MUST fetch the current
   interval I(level,n.id) using a RELOAD Fetch request and MUST
   determine where to look next as follows:

   1.  If the successor of node n is not present in the tree node
       associated with I(level,n.id), then its successor must occur in a
       larger range of the keyspace.  Node n MUST set level=level-1 and
       try again.
   2.  If n.id is sandwiched between two Node-IDs in I(level,n.id), the
       successor must lie somewhere in I(level,n.id).  Node n MUST set
       level=level+1 and repeat.
   3.  Otherwise, there is a Node-ID s stored in the node associated
       with I(level,n.id) whose identifier succeeds n.id, and there are
       no Node-IDs between n.id and s.  Thus, s must be the closest



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       successor of n.id, and the lookup is done.

4.6.  Removing Registrations

   Before leaving the RELOAD Overlay Instance, a service provider MUST
   remove the redirServiceProvider records it has stored by storing
   "nonexistent" values in their place, as described in
   [I-D.ietf-p2psip-base].


5.  Access Control Rules

   As specified in RELOAD base [I-D.ietf-p2psip-base], every kind which
   is storable in an overlay must be associated with an access control
   policy.  This policy defines whether a request from a given node to
   operate on a given value should succeed or fail.  Usages can define
   any access control rules they choose, including publicly writable
   values.

   ReDiR requires an access control policy that allows multiple nodes in
   the overlay read and write access to the ReDiR tree nodes stored in
   the overlay.  Therefore, none of the access control policies
   specified in RELOAD base [I-D.ietf-p2psip-base] is sufficient.

   This document defines a new access control policy, called NODE-ID-
   MATCH.  In this policy, a given value MUST be written and overwritten
   only if the the request is signed with a key associated with a
   certificate whose Node-ID is equal to the dictionary key.  In
   addition, the Node-ID MUST belong to one of the two intervals
   associated with the tree node.  Finally, H(namespace,level,node),
   where namespace, level, and node are taken from the
   RedirServiceProvider structure being stored, MUST be equal to the
   Resource-ID for the resource.  The NODE-ID-MATCH policy may only be
   used with dictionary types.


6.  REDIR Kind Definition

   This section defines the REDIR kind.

   Name
      REDIR

   Kind IDs







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      The Resource Name for the REDIR Kind-ID is created by
      concatenating three pieces of information: service name, level,
      and node number.  Service name is a string identifying a service,
      such as "voice-mail".  Level is an integer specifying a level in
      the ReDiR tree.  Node number is an integer identifying a ReDiR
      tree node at a specific level.  The data stored is a
      RedirServiceProvider structure that was defined in Section 4.1.

   Data Model
      The data model for the REDIR Kind-ID is dictionary.  The
      dictionary key is the Node-ID of the service provider.

   Access Control
      The access control policy for the REDIR kind is the NODE-ID-MATCH
      policy that was defined in Section 5.



7.  Example

   Figure 2 shows an example of a ReDiR tree containing information
   about four different service providers whose Node-IDs are 2, 3, 4,
   and 7.  Initially, the ReDiR tree is empty.

     Level 0  ____2_3_________7_|__________________
                      |                   |
     Level 1  ____2_3_|_4_____7   ________|________
                 |         |         |         |
     Level 2  ___|2_3   4__|__7   ___|___   ___|___
               |   |     |   |     |   |     |   |
     Level 3  _|_ _|3   4|_ _|_   _|_ _|_   _|_ _|_


                     Figure 2: Example of a ReDiR tree

   First, peer 2 whose Node-ID is 2 joins the namespace.  Since this is
   the first registration peer 2 performs, peer 2 sets the starting
   level Lstart to 2, as was described in Section 4.2.  Also all other
   peers in this example will start from level 2.  Peer 2 stores a
   RedirServiceProvider record containing its Node-ID in the tree node
   associated with interval I(2,2).  Peer 2 also fetches the contents of
   that tree node from the overlay.  Since peer 2's Node-ID is the only
   Node-ID stored in the interval, peer 2 continues up the tree.  In
   fact, peer 2 continues up in the tree all the way to the root
   inserting its own Node-ID in all levels since the tree is empty.  As
   described in Section 4.3, peer 2 also walks down the tree.  The
   downward walk peer 2 does ends at level 2 since peer 2 is the only
   node in its interval at that level.



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   The next peer to join the namespace is peer 3, whose Node-ID is 3.
   Peer 3 starts from level 2.  At that level, peer 3 stores its Node-ID
   in the same interval I(2,3) that already contains the Node-ID of peer
   2.  Since peer 3 has the numerically highest Node-ID in the interval,
   it continues up the tree.  Peer 3 stores its node-ID also at levels 1
   and 0 since its Node-ID is numerically highest among the Node-IDs
   stored in the tree nodes it fetches.  Peer 3 also does a downward
   walk which starts from level 2 (i.e., the starting level).  Since
   peer 3 is not the only node in interval I(2,3), it continues down the
   tree to level 3.  The downward walk ends at this level since peer 3
   is the only entry in the interval I(3,3).

   The third peer to join the namespace is peer 7, whose Node-ID is 7.
   Like the two earlier peers, also peer 7 starts from level 2 because
   this is the first registration it performs.  Peer 7 stores its
   RedirServiceProvider record at level 2.  At level 1, peer 7 has the
   numerically highest Node-ID (because it is the only node in interval
   I(1,7); peers 2 and 3 are stored in the same tree node but in a
   different interval) and therefore it stores its Node-ID in the tree
   node associated with interval I(1,7).  Peer 7 also has the
   numerically highest Node-ID in the interval I(0,7) associated with
   its Node-ID at level 0.  Peer 7 also does a downward walk, which ends
   at level 2 because peer 7 is the only node in its interval at that
   level.

   The final peer to join the ReDiR tree is peer 4, whose Node-ID is 4.
   Peer 4 starts by storing its RedirServiceProvider record at level 2.
   Since it is the only peer associated with interval I(2,4), it
   continues up in the tree to level 1.  At level 1, peer 4 stores its
   record in the tree node associated with interval I(1,4) because it
   has the numerically lowest Node-ID in that interval.  Next, peer 4
   continues to the root level.  At the root level, there are other
   peers having higher and lower Node-IDs than peer 4 in interval
   I(0,4).  Therefore, peer 4 does not store a RedirServiceProvider
   record at the tree node associated with the interval I(0,4).  Peer 4
   also does a downward walk starting from level 2.  The downward walk
   stops at level 3 because peer 4 is the only peer in the interval
   I(3,4).


8.  Overlay Configuration Document Extension

   This document extends the RELOAD overlay configuration document by
   adding two new elements inside each "configuration" element.







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   redirBranchingFactor:  The branching factor of the ReDir tree.  The
      default value is 10.

   redirRefreshPeriod:  The interval at which a service provider needs
      to repeat the ReDiR registration process.



9.  Security Considerations

   There are no new security considerations introduced in this document.
   The security considerations of RELOAD [I-D.ietf-p2psip-base] apply.


10.  IANA Considerations

10.1.  Access Control Policies

   This document introduces one additional access control policy to the
   "RELOAD Access Control Policy" Registry:

                  NODE-ID-MATCH


   This access control policy was described in Section 5.

10.2.  Data Kind-ID

   This document introduces one additional data kind-ID to the "RELOAD
   Data Kind-ID" Registry:

                  +--------------+------------+----------+
                  | Kind         |    Kind-ID |      RFC |
                  +--------------+------------+----------+
                  | REDIR        |        104 | RFC-AAAA |
                  +--------------+------------+----------+


   This kind-ID was defined in Section 6.


11.  References

11.1.  Normative References

   [I-D.ietf-p2psip-base]
              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and



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              H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
              Base Protocol", draft-ietf-p2psip-base-04 (work in
              progress), October 2009.

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

11.2.  Informative References

   [I-D.ietf-p2psip-concepts]
              Bryan, D., Matthews, P., Shim, E., Willis, D., and S.
              Dawkins, "Concepts and Terminology for Peer to Peer SIP",
              draft-ietf-p2psip-concepts-02 (work in progress),
              July 2008.

   [Redir]    Rhea, S., Godfrey, P., Karp, B., Kubiatowicz, J.,
              Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu, "Open
              DHT: A Public DHT Service and Its Uses".


Authors' Addresses

   Jouni Maenpaa
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Jouni.Maenpaa@ericsson.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com













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