Internet Engineering Task Force (IETF)                        J. Maenpaa
Request for Comments: 7374                                  G. Camarillo
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                             October 2014

Service Discovery Usage for REsource LOcation And Discovery (RELOAD)

Abstract

   REsource LOcation And Discovery (RELOAD) does not define a generic
   service discovery mechanism as a part of the base protocol (RFC
   6940).  This document defines how the Recursive Distributed
   Rendezvous (ReDiR) service discovery mechanism can be applied to
   RELOAD overlays to provide a generic service discovery mechanism.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7374.

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.

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Introduction to ReDiR ...........................................5
   4. Using ReDiR in a RELOAD Overlay Instance ........................8
      4.1. Data Structure .............................................8
      4.2. Selecting the Starting Level ...............................9
      4.3. Service Provider Registration ..............................9
      4.4. Refreshing Registrations ..................................10
      4.5. Service Lookups ...........................................11
      4.6. Removing Registrations ....................................13
   5. Access Control Rules ...........................................13
   6. REDIR Kind Definition ..........................................13
   7. Examples .......................................................14
      7.1. Service Registration ......................................14
      7.2. Service Lookup ............................................16
   8. Overlay Configuration Document Extension .......................16
   9. Security Considerations ........................................17
   10. IANA Considerations ...........................................17
      10.1. Access Control Policies ..................................17
      10.2. A New IETF XML Registry ..................................17
      10.3. Data Kind-ID .............................................18
      10.4. RELOAD Services Registry .................................18
   11. References ....................................................19
      11.1. Normative References .....................................19
      11.2. Informative Reference ....................................19
   Acknowledgments ...................................................19
   Authors' Addresses ................................................20

1. Introduction

   REsource LOcation And Discovery (RELOAD) [RFC6940] 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 service discovery mechanism specific to Traversal
   Using Relays around Network Address Translation (TURN), it does not
   define a generic service discovery mechanism as a part of the base
   protocol.  This document defines how the Recursive Distributed
   Rendezvous (ReDiR) service discovery mechanism specified in [Redir]
   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 with 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
   associated with providing a given service is distributed evenly among
   the nodes providing the service.

   It should be noted that a simple service discovery mechanism such as
   the one mentioned in the previous paragraph might be an appropriate
   solution in a very small overlay network consisting of perhaps tens
   of nodes.  The ReDiR-based service discovery mechanism described in
   this document is suitable for use even in overlay networks where the
   number of end users that may make service discovery requests can be
   very high (e.g., tens of thousands of nodes or even higher) and where
   a large fraction of the peers (e.g., on the order of one out of ten
   or more) can offer the service.

   ReDiR implements service discovery by building a tree structure of
   the service providers that provide a particular service.  The tree
   structure is stored 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].

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].

   DHT:  Distributed Hash Tables (DHTs) are a class of decentralized
      distributed systems that provide a lookup service similar to a
      regular hash table.  Given a key, any peer participating in the
      system 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):  Refers to a hash function (e.g., SHA-1 [RFC3174]) calculated
      over the value x.

   H(x,y,z):  Refers to a hash function calculated over a concatenated
      string consisting of x, y, and z, where x, y, and z can be both
      strings and integers.  The network byte order is used.

   I(lvl,k):  An interval at level lvl in the ReDiR tree that encloses
      key k.  As an example, I(5,10) refers to an interval at level 5 in
      the ReDiR tree within whose range key 10 falls.

   n.id:  Refers to the RELOAD Node-ID of node n.

   Namespace:  An arbitrary identifier that identifies a service
      provided in the RELOAD Overlay Instance.  Examples of potential
      namespaces include 'voice-mail' and 'turn-server'.  The namespace
      is a UTF-8-encoded [RFC3629] text string.

   numBitsInNodeId:  Refers to the number of bits in a RELOAD Node-ID.
      This value is used in the equations for calculating the ranges of
      intervals that ReDiR tree nodes are responsible for.

   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.  Each tree node in
      the ReDiR tree belongs to some level in the tree.  The root node
      of the ReDiR tree is located at level 0 of the ReDiR tree.  The
      child nodes of the root node are located at level 1.  The children
      of the tree nodes at level 1 are located at level 2, and so forth.
      The ReDiR tree has a branching factor b.  At every level lvl in
      the ReDiR tree, there is room for a maximum of b^lvl tree nodes.
      Each tree node in the ReDiR tree is uniquely identified by a pair
      (lvl,j), where lvl is a level in the ReDiR tree and j is the
      position of the tree node (from the left) at that level.

   Successor:  The successor of identifier k in namespace ns is the node
      belonging to the namespace ns whose identifier most immediately
      follows the identifier k.

3. Introduction to ReDiR

   Recursive Distributed Rendezvous (ReDiR) [Redir] does not require new
   functionality from the RELOAD base protocol [RFC6940].  This is
   possible since ReDiR interacts with the RELOAD Overlay Instance by
   simply storing and fetching data, that is, using RELOAD Store and
   Fetch requests.  ReDiR creates a tree structure of the service
   providers of a particular service and stores it into the RELOAD
   Overlay Instance using the Store and Fetch requests.  ReDiR service
   lookups require a logarithmic number of Fetch operations.  Further,
   if information from past service lookups is used to determine the
   optimal level in the ReDiR tree from which to start new service
   lookups, an average service lookup can typically finish after 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
   identifier, called the namespace.  All service providers of a given
   service store their information under the namespace of that service.
   Peers wishing to use a service perform lookups within the namespace
   of the service.  The result of a ReDiR lookup for an identifier k in
   namespace ns is a RedirServiceProvider structure (see Section 4.1) of
   a service provider that belongs to ns and whose Node-ID is the
   closest successor of identifier k in the namespace.

   Each tree node in the ReDiR tree contains a dictionary of
   RedirServiceProvider entries of peers providing a particular service.
   Each tree node in the ReDiR tree also belongs to some level in the
   tree.  The root node of the ReDiR tree is located at level 0.  The
   child nodes of the root node are located at level 1 of the ReDiR
   tree.  The children of the tree nodes at level 1 are located at

   level 2, 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 branching-factor.  The RELOAD overlay
   configuration document is defined in the RELOAD base protocol
   [RFC6940].  At every level lvl in the ReDiR tree, there is room for a
   maximum of branching-factor^lvl tree nodes.  As an example, in a tree
   whose branching-factor is 2, the second level can contain up to four
   tree nodes (note that a given level may contain less than the maximum
   number of tree nodes since empty tree nodes are not stored).  Each
   tree node in the ReDiR tree is uniquely identified by a pair (lvl,j),
   where lvl is a level in the ReDiR tree and j is the position of the
   tree node (from the left) at that level.  As an example, the pair
   (2,3) identifies the third tree node from the left at level 2.

   The ReDiR tree is stored into the RELOAD Overlay Instance tree node
   by tree node, by storing the values of tree node (level,j) under a
   key created by taking a hash over the concatenation of the namespace,
   level, and j, that is, as H(namespace,level,j).  As an example, the
   root of the tree for a voice mail service is stored at H("voice-
   mail",0,0).  Each node (level,j) in the ReDiR tree contains b
   intervals of the DHT's identifier space as follows:

           [2^numBitsInNodeId*b^(-level)*(j+(b'/b)),
            2^numBitsInNodeId*b^(-level)*(j+((b'+1)/b))), for 0<=b'<b,

   where b is the branching-factor and b' refers to the number of an
   interval within the ReDiR tree node j.

   Figure 1 shows an example of a ReDiR tree whose branching factor
   is 2.  In the figure, the size of the identifier space of the overlay
   is 16.  Each tree node in the ReDiR tree is shown as two horizontal
   lines separated by a vertical bar ('|') in the middle.  The
   horizontal 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
   of the RELOAD Overlay Instance.  At level 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 eighth of
   the identifier space.

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

                           Figure 1: ReDiR Tree

   Figure 2 illustrates how tree nodes are numbered in the ReDiR tree at
   levels 0-2.

     Level 0  ________________(0,0)________________
                      |                   |
     Level 1  ______(1,0)______   ______(1,1)______
                 |         |         |         |
     Level 2  _(2,0)_   _(2,1)_   _(2,2)_   _(2,3)_
               |   |     |   |     |   |     |   |
     Level 3  _|_ _|_   _|_ _|_   _|_ _|_   _|_ _|_

                        Figure 2: ReDiR Tree Nodes

   Figure 3 illustrates how intervals are assigned to tree nodes in the
   ReDiR tree at levels 0 and 1.  As an example, the single tree node
   (0,0) at level 0 is divided into two intervals, each of which covers
   half of the identifier space of the overlay.  These two intervals are
   [0,7] and [8,15].

     Level 0  ______[0,7]_______|_______[8,15]_____
                      |                   |
     Level 1  _[0,3]__|__[4,7]_   _[8,11]_|_[12,15]
                 |         |         |         |
     Level 2  ___|___   ___|___   ___|___   ___|___
               |   |     |   |     |   |     |   |
     Level 3  _|_ _|_   _|_ _|_   _|_ _|_   _|_ _|_

                     Figure 3: Intervals in ReDiR Tree

   Note that all of the examples above are simplified.  In a real ReDiR
   tree, the default ReDiR branching factor is 10, meaning that each
   tree node is split into 10 intervals and that each tree node has 10
   children.  In such a tree, level 1 contains 10 nodes and 100
   intervals.  Level 2 contains 100 nodes and 1000 intervals, level 3
   1000 nodes and 10000 intervals, etc.  Further, the size of the
   identifier space of a real RELOAD Overlay Instance is at the minimum
   2^128.

4. Using ReDiR in a RELOAD Overlay Instance

4.1. Data Structure

   ReDiR tree nodes are stored using the dictionary data model defined
   in the RELOAD base protocol [RFC6940].  The data stored is a
   RedirServiceProvider Resource Record:

         enum { none(0), (255) }
           RedirServiceProviderExtType;

         struct {
           RedirServiceProviderExtType   type;
           Destination                   destination_list<0..2^16-1>;
           opaque                        namespace<0..2^16-1>;
           uint16                        level;
           uint16                        node;
           uint16                        length;

           select (type) {
               /* This type may be extended */
           } extension;

         } RedirServiceProvider;

   The contents of the RedirServiceProvider Resource Record are as
   follows:

   type

      The type of an extension to the RedirServiceProvider Resource
      Record.  Unknown types are allowed.

   destination_list

      A list of IDs through which a message is to be routed to reach the
      service provider.  The destination list consists of a sequence of
      Destination values.  The contents of the Destination structure are
      as defined in the RELOAD base protocol [RFC6940].

   namespace

      An opaque UTF-8-encoded 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.

   length

      The length of the rest of the Resource Record.

   extension

      An extension value.  The RedirServiceProvider Resource Record can
      be extended to include, for instance, information specific to the
      service or service provider.

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 set to 2.  This recommendation is based on
   the findings in [Redir], which indicate that this starting level
   results in good performance.  In subsequent registrations, Lstart
   MAY, as an optimization, be set to the lowest level at which a
   registration operation has last completed.

   In the case of subsequent service lookups, nodes MAY, as an
   optimization, record the levels at which the last 16 service lookups
   completed and take Lstart to be the mode of those depths (mode, in
   statistics, is the value that appears most often in a set of data).

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 a starting level Lstart (see Section 4.2 for
       the details on selecting the starting level).  Therefore, node n
       sets the current level to level=Lstart.

   2.  Node n MUST send a RELOAD Fetch request to fetch the contents of
       the tree node responsible for I(level,n.id).  An interval I(l,k)
       is the interval at level l in the ReDiR tree that includes key k.
       The fetch MUST be a wildcard fetch.

   3.  Node n MUST send a RELOAD Store request to add its
       RedirServiceProvider entry to the dictionary stored in the tree
       node responsible for I(level,n.id).  Note that node n always
       stores its RedirServiceProvider entry, regardless of the contents
       of the dictionary.

   4.  If node n's Node-ID (n.id) is the lowest or highest Node-ID
       stored in the tree node responsible for I(Lstart,n.id), node n
       MUST reduce the current level by one (i.e., set level=level-1)
       and continue up the ReDiR tree towards the root level (level 0),
       repeating steps 2 and 3 above.  Node n MUST continue in this way
       until it reaches either the root of the tree or a level at which
       n.id is not the lowest or highest Node-ID in the interval
       I(level,n.id).

   5.  Node n MUST also perform a downward walk in the ReDiR tree,
       during which it goes through the tree nodes responsible for
       intervals I(Lstart,n.id), I(Lstart+1,n.id), I(Lstart+2,n.id),
       etc.  At each step, node n MUST fetch the responsible tree node
       and store its RedirServiceProvider record in that tree node if
       n.id is the lowest or highest Node-ID in its interval.  Node n
       MUST end this downward walk as soon as it reaches a level l at
       which it is the only service provider in its interval I(l,n.id).

   Note that above, when we refer to 'the tree node responsible for
   I(l,k)', we mean the entire tree node (that is, all the intervals
   within the tree node) responsible for interval I(l,k).  In contrast,
   I(l,k) refers to a specific interval within a tree node.

4.4. Refreshing Registrations

   All state in the ReDiR tree is soft.  Therefore, a service provider
   needs to periodically repeat the registration process to refresh its
   RedirServiceProvider Resource Record.  If a record expires, it MUST
   be dropped from the dictionary by the peer storing the tree node.
   Deciding an appropriate lifetime for the RedirServiceProvider
   Resource Records is up to each service provider.  However, a default
   value of 10 minutes is RECOMMENDED as this is a good trade-off
   between keeping the amount of ReDiR traffic in the overlay at a
   reasonable level and ensuring that stale information is removed
   quickly enough.  Every service provider MUST repeat the entire
   registration process periodically until it leaves the RELOAD Overlay
   Instance.  The service provider SHOULD initiate each refresh process
   slightly earlier (e.g., when 90% of the refresh interval has passed)
   than the expiry time of the Resource Record.

   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 for identifier k in namespace ns is
   to find the node that is a part of ns and whose identifier most
   immediately follows (i.e., is the closest successor of) the
   identifier k.

   A service lookup operation resembles the service registration
   operation described in Section 4.3.  Service lookups start from a
   given starting level level=Lstart in the ReDiR tree (see Section 4.2
   for the details on selecting the starting level).  At each step, a
   node n wishing to discover a service provider MUST fetch the tree
   node responsible for the interval I(level,n.id) that encloses the
   search key n.id at the current level using a RELOAD Fetch request.
   Having fetched the tree node, node n MUST determine the next action
   to carry out as follows:

   Condition 1

      If there is no successor of node n present in the just-fetched
      ReDiR tree node (note: within the entire tree node and not only
      within the current interval) responsible for I(level,n.id), then
      the successor of node n must be present in a larger segment of the
      identifier space (i.e., further up in the ReDiR tree where each
      interval and tree node covers a larger range of the identifier
      space).  Therefore, node n MUST reduce the current level by one to
      level=level-1 and carry out a new Fetch operation for the tree
      node responsible for n.id at that level.  The fetched tree node is
      then analyzed and the next action determined by checking
      Conditions 1-3.

   Condition 2

      If n.id is neither the lowest nor the highest Node-ID within the
      interval (note: within the interval, not within the entire tree
      node) I(level,n.id), n MUST next check the tree node responsible
      for n.id at the next level down the tree.  Thus, node n MUST
      increase the level by one to level=level+1 and carry out a new
      Fetch operation at that level.  The fetched tree node is then
      analyzed and the next action determined by checking the conditions
      listed here, starting at Condition 1.

   Condition 3

      If neither of the conditions above holds, meaning that there is a
      successor s of n.id present in the just-fetched ReDiR tree node
      and n.id is the highest or lowest Node-ID in its interval, the
      service lookup has finished successfully, and s must be the
      closest successor of n.id in the ReDiR tree.

   Note that above, when we refer to 'the tree node responsible for
   interval I(l,k)', we mean the entire tree node (that is, all the
   intervals within the tree node) responsible for interval I(l,k).  In
   contrast, I(l,k) refers to a specific interval within a tree node.

   Note also that there may be some cases in which no successor can be
   found from the ReDiR tree.  An example is a situation in which all of
   the service providers stored in the ReDiR tree have a Node-ID smaller
   than identifier k.  In this case, the upward walk of the service
   lookup will reach the root of the tree without encountering a
   successor.  An appropriate strategy in this case is to pick one of
   the RedirServiceProvider entries stored in the dictionary of the root
   node at random.

   Since RedirServiceProvider records are expiring and registrations are
   being refreshed periodically, there can be certain rare situations in
   which a service lookup may fail even if there is a valid successor
   present in the ReDiR tree.  An example is a case in which a ReDiR
   tree node is fetched just after a RedirServiceProvider entry of the
   only successor of k present in the tree node has expired and just
   before a Store request that has been sent to refresh the entry
   reaches the peer storing the tree node.  In this rather unlikely
   scenario, the successor that should have been present in the tree
   node is temporarily missing.  Thus, the service lookup will fail and
   needs to be carried out again.

   To recover from the kinds of situations described above, a ReDiR
   implementation MAY choose to use the optimization described next.
   The ReDiR implementation MAY implement a local temporary cache that
   is maintained for the duration of a service lookup operation in a
   RELOAD node.  The temporary cache is used to store all
   RedirServiceProvider entries that have been fetched during the upward
   and downward walks of a service lookup operation.  Should it happen
   that a service lookup operation fails due to the downward walk
   reaching a level that does not contain a successor, the cache is
   searched for successors of the search key.  If there are successors
   present in the cache, the closest one of them is selected as the
   service provider.

4.6. Removing Registrations

   Before leaving the RELOAD Overlay Instance, a service provider SHOULD
   remove the RedirServiceProvider records it has stored by storing
   exists=False values in their place, as described in [RFC6940].

5. Access Control Rules

   As specified in the RELOAD base protocol [RFC6940], every Kind that
   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 the RELOAD base protocol [RFC6940] 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 request is signed with a key associated with a
   certificate whose Node-ID is equal to the dictionary key.  In
   addition, provided that exists=True, the Node-ID MUST belong to one
   of the intervals associated with the tree node (the number of
   intervals each tree node has is determined by the branching-factor
   parameter).  Finally, provided that exists=True,
   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-ID

      The Resource Name for the REDIR Kind-ID is created by
      concatenating three pieces of information: namespace, level, and
      node number.  Namespace is an opaque UTF-8-encoded string
      identifying a service, such as 'turn-server'.  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, as 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. Examples

7.1. Service Registration

   Figure 4 shows an example of a ReDiR tree containing information
   about four different service providers whose Node-IDs are 2, 3, 4,
   and 7.  In the example, numBitsInNodeId=4.  Initially, the ReDiR tree
   is empty; Figure 4 shows the state of the tree at the point when all
   the service providers have registered.

     Level 0  ____2_3___4_____7_|__________________
                      |                   |
     Level 1  ____2_3_|_4_____7   ________|________
                 |         |         |         |
     Level 2  ___|2_3   4__|__7   ___|___   ___|___
               |   |     |   |     |   |     |   |
     Level 3  _|_ _|3   _|_ _|_   _|_ _|_   _|_ _|_

                     Figure 4: 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.  First, peer 2 fetches
   the contents of the tree node associated with interval I(2,2) from
   the RELOAD Overlay Instance.  This tree node is the first tree node
   from the left at level 2 since key 2 is associated with the second
   interval of the first tree node.  Peer 2 also stores its
   RedirServiceProvider record in that tree node.  Since peer 2's Node-
   ID is the only Node-ID stored in the tree node (i.e., peer 2's Node-
   ID fulfills the condition in Section 4.3 that it be the numerically
   lowest or highest among the keys stored in the node), 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 (which means that peer 2's Node-ID always fulfills the
   condition that it be the numerically lowest or highest Node-ID in the
   interval I(level, 2) during the upward walk).  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.

   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
   RedirServiceProvider entry in the same interval I(2,3) that already
   contains the RedirServiceProvider entry of peer 2.  Interval I(2,3),
   that is, the interval at level 2 enclosing key 3, is associated with
   the right-hand-side interval of the first tree node.  Since peer 3
   has the numerically highest Node-ID in the tree node associated with
   I(2,3), peer 3 continues up the tree.  Peer 3 also stores its
   RedirServiceProvider record at levels 1 and 0 since its Node-ID is
   numerically highest among the Node-IDs stored in the intervals to
   which its own Node-ID belongs.  Peer 3 also does a downward walk that
   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
   service provider 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, peer 7 also 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 (and lowest) Node-ID in its interval I(1,7)
   (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
   that interval.  Peer 7 also has the numerically highest Node-ID in
   the interval I(0,7) associated with its Node-ID at level 0.  Finally,
   peer 7 performs 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 has the numerically lowest Node-ID in the tree node
   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 which it
   stores its RedirServiceProvider record and finishes the upward walk
   since the root level was reached.  Peer 4 also does a downward walk
   starting from level 2.  The downward walk stops at level 2 because
   peer 4 is the only peer in the interval I(2,4).

7.2. Service Lookup

   This subsection gives an example of peer 5 whose Node-ID is 5
   performing a service lookup operation in the ReDiR tree shown in
   Figure 4.  This is the first service lookup peer 5 carries out, and
   thus, the service lookup starts from the default starting level 2.
   As the first action, peer 5 fetches the tree node corresponding to
   the interval I(2,5) from the starting level.  This interval maps to
   the second tree node from the left at level 2 since that tree node is
   responsible for the interval (third interval from left) to which
   Node-ID 5 falls at level 2.  Having fetched the tree node, peer 5
   checks its contents.  First, there is a successor, peer 7, present in
   the tree node.  Therefore, Condition 1 of Section 4.5 is false, and
   there is no need to perform an upward walk.  Second, Node-ID 5 is the
   highest Node-ID in its interval, so Condition 2 of Section 4.5 is
   also false, and there is no need to perform a downward walk.  Thus,
   the service lookup finishes at level 2 since Node-ID 7 is the closest
   successor of peer 5.

   Note that the service lookup procedure would be slightly different if
   peer 5 used level 3 as the starting level.  Peer 5 might use this
   starting level, for instance, if it has already carried out service
   lookups in the past and follows the heuristic in Section 4.2 to
   select the starting level.  In this case, peer 5's first action is to
   fetch the tree node at level 3 that is responsible for I(3,5).  Thus,
   peer 5 fetches the third tree node from the left.  Since this tree
   node is empty, peer 5 decreases the current level by one to 2 and
   thus continues up in the tree.  The next action peer 5 performs is
   identical to the single action in the previous example of fetching
   the node associated with I(2,5) from level 2.  Thus, the service
   lookup finishes at level 2.

8. Overlay Configuration Document Extension

   This document extends the RELOAD overlay configuration document
   defined in the RELOAD base protocol specification [RFC6940] by adding
   a new element, "branching-factor", inside the new "REDIR" kind
   element:

   redir:branching-factor:  The branching factor of the ReDiR tree.  The
      default value is 10.

   The RELAX NG grammar for this element is:

   namespace redir = "urn:ietf:params:xml:ns:p2p:redir"

   parameter &= element redir:branching-factor { xsd:unsignedInt }?

The 'redir' namespace is added into the <mandatory-extension> element
in the overlay configuration file.

9. Security Considerations

   This document defines a new access control policy called NODE-ID-
   MATCH (see Section 5) whose purpose is to control which nodes in the
   overlay are allowed read and write access to the ReDiR tree nodes.
   The NODE-ID-MATCH access control policy ensures that the only node in
   the overlay that can store a pointer to a specific service provider
   in the ReDiR tree is the service provider itself.  This prevents
   attacks where a malicious node inserts pointers to other nodes in the
   ReDiR tree.  Further, the NODE-ID-MATCH access control policy ensures
   that a node can only store information in locations in the ReDiR tree
   where it is entitled to do so.  In other words, a node can only store
   one RedirServiceProvider record at any given level in the ReDiR tree.
   This prevents an attack where a malicious node is trying to insert a
   high number of pointers to the ReDiR tree.

   When it comes to attacks such as a malicious node refusing to store a
   value or denying knowledge of a value it has previously accepted,
   such security concerns are already discussed in the RELOAD base
   specification [RFC6940].

10. IANA Considerations

10.1. Access Control Policies

   This document adds a new access control policy to the "RELOAD Access
   Control Policies" registry:

      NODE-ID-MATCH

   This access control policy was described in Section 5.

10.2. A New IETF XML Registry

   This document registers one new URI for the 'redir' namespace in the
   "IETF XML Registry" defined in [RFC3688].

   URI: urn:ietf:params:xml:ns:p2p:redir

   Registrant Contact: The IESG

   XML: N/A, the requested URI is an XML namespace

10.3. Data Kind-ID

   This document adds one new data Kind-ID to the "RELOAD Data Kind-ID"
   registry:

             +--------------+------------+-----------+
             | Kind         |    Kind-ID |    RFC    |
             +--------------+------------+-----------+
             | REDIR        |      0x104 | [RFC7374] |
             +--------------+------------+-----------+

   This Kind-ID was defined in Section 6.

10.4. RELOAD Services Registry

   IANA has created a new registry for ReDiR namespaces:

   Registry Name: RELOAD Services Registry

   Reference: [RFC7374]

   Registration Procedure: Specification Required

   Entries in this registry are strings denoting ReDiR namespace values.
   The initial contents of this registry are:

             +----------------+-----------+
             | Namespace      |     RFC   |
             +----------------+-----------+
             | turn-server    | [RFC7374] |
             +----------------+-----------+
             | voice-mail     | [RFC7374] |
             +----------------+-----------+

   The namespace 'turn-server' is used by nodes that wish to register as
   providers of a TURN relay service in the RELOAD overlay and by nodes
   that wish to discover providers of a TURN relay service from the
   RELOAD overlay.  In the TURN server discovery use case, the ReDiR-
   based service discovery and registration mechanism specified in this
   document can be used as an alternative to the TURN server discovery
   mechanism specified in the RELOAD base specification [RFC6940].  The
   namespace 'voice-mail' is intended for a voice mail service
   implemented on top of a RELOAD overlay.

11. References

11.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001,
              <http://www.rfc-editor.org/info/rfc3174>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003,
              <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004, <http://www.rfc-editor.org/info/rfc3688>.

   [RFC6940]  Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
              Base Protocol", RFC 6940, January 2014,
              <http://www.rfc-editor.org/info/rfc6940>.

11.2. Informative Reference

   [Redir]    Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
              Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu,
              "OpenDHT: A Public DHT Service and Its Uses", October
              2005.

Acknowledgments

   The authors would like to thank Marc Petit-Huguenin, Joscha
   Schneider, Carlos Bernardos, Spencer Dawkins, Barry Leiba, Adrian
   Farrel, Alexey Melnikov, Ted Lemon, and Stephen Farrell for their
   comments on the document.

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