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Versions: 00 01 02 03                                                   
                 RMT Working Group                              Brian Whetten/Consultant
                 Internet Engineering Task Force          Dah Ming Chiu/Sun Microsystems
                 Internet Draft                         Miriam Kadansky/Sun Microsystems
                 draft-ietf-rmt-bb-tree-config-03.txt            Seok Joo Koh/ETRI/KOREA
                 18 November, 2002                                  Gursel Taskale/Tibco
                 Expires 18 May, 2003                            Brian Neil Levine/Umass
                 Reliable Multicast Transport Building Block: Tree Auto-Configuration
                 Status of this Memo
                    This document is an Internet-Draft and is in full conformance with
                    all provisions of Section 10 of RFC2026.
                    Internet-Drafts are valid for a maximum of six months and may be
                    updated, replaced, or obsoleted by other documents at any time.  It
                    is inappropriate to use Internet-Drafts as reference material or to
                    cite them other than as a "work in progress".
                    The list of current Internet-Drafts can be accessed at
                    The list of Internet-Draft Shadow Directories can be accessed at
                    The Reliable Multicast Transport Working Group has been chartered
                    to standardize multicast transport services. This working group
                    expects to initially standardize three protocol instantiations.
                    This draft is concerned with the requirements of the tree-based
                    ACK protocol. In particular, it is concerned with defining the
                    building block for auto-configuration of the logical ACK-tree.
                    According to the charter, a building block is "a coarse-grained
                    modular component that is common to multiple protocols along with
                    abstract APIs that define a building block's access methods and
                    their arguments."  For more information, see the Reliable
                    Multicast Transport Building Blocks and Reliable Multicast Design
                    Space documents [WLKHFL01][HWKFV00].
                 Table of Contents
                     1. Introduction
                        1.1 Terminology
                        1.2 Assumptions
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                        1.3 Requirements
                        1.4 Applicability Statement
                     2. Overview
                     3. Session Announcement
                     4. Service Node Discovery and Selection
                        4.1 Service Node Discovery Algorithms
                            4.1.1 Static
                            4.1.2 ERS
                            4.1.3 Mesh
                        4.2 Distance Metrics
                            4.2.1 TTL Hop-Count
                            4.2.2 Number of Levels
                            4.2.3 Delay
                            4.2.4 Address
                            4.2.5 Static
                            4.2.6 GRA
                        4.3 Service Node Selection
                     5. Tree Formation
                     6. Tree Maintenance
                        6.1 Monitoring Parents and Children
                        6.2 Optimizing the Tree
                     7. Messages
                     8. External Interfaces
                        8.1 Interfaces to the BB
                            8.1.1 start
                            8.1.2 end
                            8.1.3 incomingMessage
                            8.1.4 getStatistics
                            8.1.5 getSNs
                            8.1.6 setSN
                            8.1.7 acceptChild
                            8.1.8 removeChild
                            8.1.9 treeLevelUpdate
                            8.1.10 lostSN
                            8.1.11 setOptimization
                            8.1.12 recordSNs
                        8.2 Interfaces from the BB
                            8.2.1 outgoingMessage
                            8.2.2 SNlist
                     9. References
                     Authors' Addresses
                 1. Introduction
                    The Reliable Multicast Transport (RMT) working group has been
                    chartered to standardize IP multicast transport services. This
                    draft is concerned with the requirements of the tree-based ACK
                    protocol [WCPKT00]. In particular, this draft defines a building
                    block for auto-configuration of a tree comprised of a single
                    Sender, Service Nodes, and Receivers into a tree (called a Session
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                    Tree in this document). The design goals of this draft are
                    motivated by the needs of TRACK-based protocols, however the trees
                    it constructs are useful for other services.
                    This building block can be interfaced to any other BB or PI
                    wishing to use a tree structure.  To actually use this BB's
                    features, the PI needs to includes the messages described in this
                    BB in its packets. An example of how to use this BB can be found
                    in the TRACK PI[WCPKT01].
                    The process of session tree construction is difficult for IP
                    multicast. The best session trees match the underlying multicast
                    routing tree topology [LPG98], however the multicast service model
                    [DEE89] does not provide explicit support for discovering routing
                    tree topology.
                    Furthermore, deployed multicast architectures can vary; for
                    example, hosts may be restricted to multicast reception and not
                    transmission with Source-Specific multicast routing [HC00]; and
                    routers may provide special extended routing services with Generic
                    Router Assist [CST00].  The RMT charter does not restrict the use
                    of any particular network service in constructing the tree. It
                    only suggests preferred scenarios.  Accordingly, there are several
                    viable solutions for constructing a tree, depending on network
                    The optimality of a tree may also depend on other factors, such as
                    the need for load balancing, and the need to minimize the depth
                    when used for collecting feedback information. The goal of this
                    building block is to specify a distributed procedure for
                    automatically constructing a tree that is loop-free and as
                    efficient as possible given the information available.
                    This draft describes a unified solution for tree construction in
                    the presence of different multicast service models and routing
                    protocols. In particular, it specifies a single procedure which
                    may be used with various techniques for service node discovery and
                    distance measurements, several of which are specified within this
                    document. The difference in these techniques primarily affects the
                    optimality of the tree.  The unified algorithm ensures that
                    different implementations can interoperate and construct a loop-
                    free tree.
                    In order to accommodate various multicast deployments, this
                    document divides the tree building process into the following
                    major components:
                       1. Several techniques for discovering neighboring service nodes.
                          In particular: Static, Expanding Ring Search, and Mesh.
                          Discovering neighboring service nodes is a necessary
                          condition for getting connected, so each node in the session
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                          must implement at least one of the above techniques.
                       2. Several algorithms for selecting neighboring service nodes.
                          In particular, the measurement and use of neighbor distance
                          and sender distance are described.  These service node
                          selection algorithms help produce a good tree.
                       3. A single distributed procedure for construction and
                           maintenance of loop-free Session Trees.
                 1.1 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.
                    A session is used to distribute data over a multicast address.  A
                    Session Tree is used to provide reliability and feedback services
                    for a session.
                    The single sender of data on a multicast session. The Sender is the
                    root of the Session Tree.
                    A receiver receives data from the sender via the multicast session.
                 Session Identifier
                    A fixed-size number, chosen either by the application that creates
                    the session or by the transport.  Senders and Receivers use the
                    Session Identifier to distinguish sessions.  The size of this
                    number is specified by the Protocol Instantiation (PI).
                 Service Node (SN)
                    A node within the tree which receives and retransmits data, and
                    aggregates and forwards control information toward the Sender.  The
                    Sender operates as the root Service Node in any session tree.
                    Service Nodes MAY be dedicated servers within a network designed to
                    participate in multiple Sessions and support multiple trees, or
                    they MAY be Receivers participating in an individual session.  SNs
                    MAY limit the number of Children they choose to service, and MAY
                    also make other restrictions on the characteristics of each Child
                    (distance, location, etc.).  An SN that has accepted Children for a
                    session is called a Parent.
                    In other documents, Service Node is sometimes referred to as Repair
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                    Head (RH).
                 Session Tree (ST)
                    The Session Tree is a tree spanning all receivers of a multicast
                    session.  It is rooted at the Sender, consisting of zero of more
                    Service Nodes as interior nodes, and zero or more receivers as leaf
                    nodes.  An ST is constructed for the forwarding of control
                    information back to the Sender as well as for the resending of
                    missed data to the Receivers. The ST for a particular session may
                    change over the course of the session.
                    A Parent is an SN or Receiver's predecessor in the ST on the path
                    toward the Sender.  Every SN or Receiver on the tree except the
                    Sender itself has a parent. Each Parent communicates with its
                    children using either an assigned multicast address or through
                    unicast.  If a multicast address is used, this may be the same
                    address used by the session, or one specifically assigned to the
                    The set of Receivers and SNs for which an SN or the Sender is
                    providing repair and feedback services.
                 Tree Level
                    A number indicating the number of "generations" a node is from the
                    root. The sender is at TL=0.  Those that use the sender as their
                    parent are at TL=1 and so on. When a receiver is not connected to
                    the tree yet, it has a tree level value greater or equal to 128.
                    The reason for reserving part of the space (of tree levels) for
                    indicating "off-tree" is so that special measures can be used to
                    prevent forming loops.  The largest value is 255, so the range of
                    off-tree levels are in the range 128 - 255.  Initially, all
                    receivers have a TL value of 128.  Once a Node joins the tree, its
                    Tree Level is updated to be one more than its Parent's level.
                 Distance Metric
                    There are several techniques to quantify distances between nodes
                    (Receivers, SNs, and the Sender) in a session.  Each type of
                    quantification is called a distance metric.  Several Distance
                    Metrics are described in this draft.
                 Sender Distance
                    The distance from a node to the Sender.
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                 Neighbor Distance
                    The distance from a node to a Neighbor.
                    A node's (Receiver or SN) Neighbors are SNs that are close to the
                    node, according to the Distance Metric(s) used by the node.
                 1.2 Assumptions
                    This document describes how to build trees under the following
                      a. The multicast group has only a single sender.
                      b. A single SN can serve multiple sessions.
                      c. Sessions can take advantage of a pre-deployed infrastructure
                         of SNs (ones that are not necessarily aware of a session
                         before the receivers), or recruit Receivers to be SNs.
                      d. Generic Router Assist[CST00] and Expanding Ring Search[YGS95]
                         are not required of the network infrastructure, but if
                         available they should be able to be utilized.
                 1.3 Requirements
                    The following are specifically required:
                      a. While tree-building may take advantage of information from the
                         routing layer, the mechanisms described are independent of the
                         routing protocol(s) used by the underlying multicast tree.
                      b. All trees constructed must be loop-free
                      c. These mechanisms must support late joiners and tree
                 1.4 Applicability Statement
                    The authors recognize that automatic tree construction is a very
                    difficult problem.  Nonetheless, complete reliance on manual
                    configuration is very user unfriendly and error prone as well.
                    This building block describes a procedure for constructing loop-
                    free trees when there is minimal manual configured information
                    This is analogous to providing a system with default configurations
                    that allow the system to work correctly, but not necessarily
                    There are many possible criteria for tree optimality.  This BB does
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                    not attempt to define a single optimality criterion, nor does it
                    try to produce an optimal tree.  It is, however, the goal of the
                    BB to construct better trees as more configuration and measurement
                    data are introduced to the procedure.
                    This BB describes only a subset of the possible parameters for
                    constructing optimal trees, in particular sender distance and
                    neighbor distance.  There are many techniques for measuring these
                    distances.  Some of the techniques may not be applicable globally.
                    Expanding ring search (ERS) is an effective technique in a local
                    subnet or intranet (especially when the IP multicast routing
                    protocol is dense-mode based).  On the other hand, it is not
                    practical in a multi-domain network; it is not effective when the
                    routing protocol is sparse-mode based; and it can add significant
                    control traffic overhead.
                    Generic Router Assist (GRA) can provide measurement hooks to
                    determine SNs that are located along the path for multicast data
                    distribution. However, such facilities may not be available in all
                    The tree construction procedure does allow manual configuration and
                    various distance measurement techniques to be selectively and
                    independently applied for different subgroups of receivers and SNs,
                    to achieve incremental improvement to the quality of the tree.
                    There are many other criteria for tree-building than what is
                    described in this document, for instance, methods based on load
                    balancing and minimizing feedback latency.
                 2. Overview
                    The tree building process described within this document builds
                    logical trees which consist of:
                        1. A root node (the Sender)
                        2. Intermediate nodes (Service Nodes or SNs) which may be
                            either Receivers or nodes specifically allocated to the
                           task of repair and aggregation
                        3. Leaf nodes which are Receivers only
                    Session trees are spanning trees rooted at the Sender. Session
                    trees can be used for the forwarding of control information (i.e.
                    ACKs) towards the root, or for forwarding of data (i.e. repairs)
                    towards the leaf nodes.
                    Session trees are constructed per Sender; each node wishing to join
                    the tree discovers its neighboring SNs and then selects its best
                    parent based on locally available information, such as the relative
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                    sender distances and neighbor distances.  This document specifies
                    several techniques for measuring distances.
                    It is important to note that SNs may be actual Receivers (e.g.
                    Receivers willing and able to also function as SNs) or pre-deployed
                    "specialized" servers that are signaled to join the tree by
                    Receivers.  We use the term Service Node to refer to either a
                    Receiver or "server" which is participating as part of the logical
                    tree formation.
                    Tree construction, regardless of SN discovery and selection
                    algorithm, proceeds generically as follows.
                       1. Session Announcement
                         Receivers of a session use standard out-of-band mechanisms
                         for discovery of a session's existence (e.g. Session
                         Advertisement ([HPW00], URL, etc).  In this way, a Receiver
                         discovers the multicast group address, the Sender's address,
                         and other information necessary for logical tree construction.
                         Sessions may be announced in two parts, the first part
                         containing generic information about the session, such as the
                         multicast address, and the second part, announced on the
                         multicast address, containing additional information.
                       2. Measurements to the Sender (optional)
                         All SNs and Receivers that know about the session optionally
                         determine their distance to the Sender.
                       3. Neighbor Discovery
                         Meanwhile, each Receiver discovers nearby SNs (candidate
                         parents) for the Session using the neighbor discovery
                       4. Service Node Selection
                         Once a Receiver (or SN needing to join the tree) discovers a
                         nearby SN, it obtains the SN's distance to the Sender as well
                         as the SN's distance to the Receiver, tree level, and other
                         suitability values, if available. After discovery is complete,
                         the best SN is selected.
                       5. Binding to Service Node
                         The Receiver or SN then binds to the chosen SN.  If a bind is
                          unsuccessful, the Receiver or SN retries with another nearby
                         SN, or starts the discovery process all over again.   Once an
                         SN receives a bind from a child, that SN must then also join
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                         the tree if it has not already, discovering an SN of its own,
                         possibly using a different method than leaf Receivers.
                       6. Optimization (optional) and Fault Recovery
                         During a session, a Receiver or SN may change to a different
                         SN for a number of reasons described below, including fault
                         tolerance.  The Session Tree is maintained and optimized over
                    This building block provides mechanisms for maintaining and
                    optimizing the tree, as well as tearing it down when the Sender is
                    done with it.  In the rest of this  document, the term 'Node'
                    denotes a Receiver or SN.
                                   |    1. Session      |
                                   |   Advertisement    |
                                             | Node receives tree-building parameters
                                   |  2. Measurements   |<-------------------------|
                                   |   to the Sender    |                          |
                                   |     (optional)     |                          |
                                   +--------------------+                          |
                                             |                                     |
                                             |                                     |
                                             V                                     |
                                   +--------------------+                          |
                                   |    3. Neighbor     |                          |
                                   |      Discovery     |                          |
                                   +--------------------+                          |
                                             |                                     |
                                             |                                     |
                                             V                                     |
                                   +--------------------+                          |
                                   |  4. Service Node   |                          |
                                   |     Selection      |                          |
                                   +--------------------+                          |
                                             |                                     |
                                             | Node picks best Neighbor            |
                                             V                                     |
                                   +--------------------+                          |
                                   |   5. Binding to    |---------------------------
                                   |    Service Node    |   6. Optimization (optional)
                                   |                    |      and Fault Recovery
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                 3. Session Announcement
                    The first step in the tree-building process is for a node to be
                    informed of the session's existence.  This can be done using some
                    out-of-band method, such as Session Advertisement [HPW00], URL, e-
                    mail, etc.
                    SNs do not necessarily receive these advertisements.  If an SN is
                    not a Receiver, it obtains the advertisement information once it
                    is contacted by a Receiver.
                    The advertisement includes the multicast address being used, the
                    Sender's address, the Session Identifier, any specific port numbers
                    to be used, and any global information useful for tree construction.
                    The advertisement may also contain information about one or more
                    Service Node Discovery options (such as Static, ERS, and Mesh)
                    that can possibly be used by Receivers in the session.
                 4. Service Node Discovery and Selection
                    Discovery is the process by which a node determines a suitable
                    Service Node.  During the discovery process, suitable neighbors are
                    found, Sender distances are optionally exchanged, and the best SN
                    is selected.
                 4.1 Service Node Discovery Algorithms
                    This draft describes three algorithms for discovering SNs: Static,
                    Expanding Ring Search (ERS), and Mesh.
                    Multiple algorithms may be used within a single session.  For
                    example, SNs may use the Mesh algorithm, while the receivers use
                    static configuration to discover the SNs; alternatively, some
                    Receivers may use static configuration while other Receivers depend
                    on ERS (in an intranet where ERS is available). Each Receiver may
                    pre-configure which algorithm to use before it starts.
                    The transport protocols request the following information from this
                    BB using the getSNs interface.
                    Service Nodes:
                       1. ParentAddress:  the address and port of the parent node to
                                          which the node should connect
                       2. UDPListenPort:  the number of the port on which the node will
                                          listen for its children's control messages
                       3. RepairAddr:     the multicast address, UDP port, and TTL on
                                          which this node sends control messages to its
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                       1. ParentAddress.
                       1. UDPListenPort
                       2. RepairAddr
                    After the above information is obtained from auto-tree-config, the
                    transport protocol may perform the necessary Bind operations for
                    participating in the Session Tree.
                 4.1.1 Static
                    Static algotirhm relies on a functional entity, named Tree
                    Configurator (TC), which is pre-configured by Sender for tree
                    configuration in a static manner. TC may be simply implemented by
                    a program and thus installed at Sender or any other host. It is
                    not necessarily specialized infrastructure.
                    Static scheme is a typical top-down tree configuration approach. TC
                    is used to govern the tree building based on its own (session-
                    specific) tree configuration and SN(s) selection rules for the new
                    If a TC is used for tree building, its address and port MUST be
                    included with the session advertisement. Receivers and SNs will
                    realize there is a TC for the session via Session Announcement,
                    and they can contact with the TC to get a list of candidate SNs by
                    sending a unicast Query message.
                    In response to a Query message, the TC replies with a Advertise
                    message that contains a list of candidate SNs available to the new
                    joiner. The rule of determining such candidate SNs may depend on
                    the pre-configured mechanism taken by TC. For example, TC may
                    determine the candidate SN list for a Node among the possible SNs
                    (it contains at that time) by considering which SNs are in the
                    same network domain with the Node (i.e., via comparing their
                    network prefix), or by considering the load balancing for the tree
                    topology it has configured until then. For this purpose, TC
                    maintains a pool of active SNs for the session. The list of
                    candidate SNs carried by Advertise message is ordered in
                    decreasing levels of preference, in which a lower number
                    represents a higher preference.
                    When a Receiver Node receives the responding Advertise message from
                    TC, the Node MAY proceed to try to bind to a candidate SN
                    following the given order by sending a BindRequestmessage, and
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                    then waits for the responding message such as BindConfirm or
                    BindReject from TC. These binding steps will be done according to
                    the TRACK mechanism, as described in the TRACK PI [WCMKT02]
                    In the Static algorithm, SN discovery with a TC proceeds as
                        1. The node joins the multicast session and learns of the
                           location of TC (from the session announcement).
                        2. The node optionally discovers its distance from the Sender.
                           Any metric described in Section 4.2 may be used.
                        3. The node sends a Query message to the TC for a parent. The
                           request includes (optionally) the node's distance to the
                           sender and whether the node functions as an SN or not.
                        4. The TC chooses one or more candidate parents (SNs) for the
                           node from the active SNs by its own tree configuration rule.
                           The selection of candidate parents may be done by comparing
                           the network prefix or by referring to any other information
                           such as the number of currently attached children, etc.
                        5. The TC MUST responds to the Query message with an Advertise
                           message, which include the candidate parent list. In the
                           list, each entry contains the corresponding IP address and
                           port of an SN.
                           All the entries in the list SHOULD be arranged in the
                           decreasing order of preference levels.
                           In the rejection case, the Advertise message does not
                           include any candidate parent. In this case, the Node may
                           resort to the other mechanism such as ERS and Mesh.
                           In the success case, the node will be enrolled as an active
                           SN by the TC, if it functions as an SN in the session. Each
                           active SN (functioning as a parent for the Session Tree)
                           SHOULD send the TC the periodic Query messages with a flag
                           indicating that it is active over a specific time interval.
                           Based on the Query messages, the TC updates a pool of active
                           SNs in the session. In response to the Query message, the TC
                           sends a Advertise with a flag simply indicating that the
                           Query message is received.
                        6. After receiving a successful Advertise message from the TC,
                           the node will try to connect to its parent by sending
                           BindRequest messages based on the candidate parent list, as
                           described in Section 5.
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                 4.1.2 ERS
                    ERS is a typical bottom-up tree configuration approach, which can
                    be used only in the network environments where IP multicast
                    transmissions are allowed by Receivers and SNs.
                    In ERS algorithm, SN discovery with a TC proceeds as follows:
                        1. The Nodes first look for Neighbors using a multicast Query
                           message. The initial TTL value in the Query message,
                           TTLNeighborInit, is specified in the session announcement
                           and may be as large as the session TTL (TTLSession).
                           An SN that is able to handle additional Children MUST
                           respond to a Query message by multicasting an Advertise
                           If the SN is not yet a Parent, the TTL used in this response
                           is the same TTL used in the Query message. If the SN is a
                           Parent, the TTL used is the greater of the Query TTL and the
                           Parent's current Advertise TTL.
                        2. The Node listens for Advertise messages after sending the
                           Query message. If one or more Advertise messages are
                           received during a SolicitPeriod, the best SN among them is
                           selected as described in section 4.3.
                        3. If no Advertise messages are received, the Node sends
                           another multicast Query message with a TTL that is
                           incremented by TTLIncrement.  The process of sending the
                           multicast Query message with an increasing TTL value
                           continues until a response is received.
                           The TTL value is limited by a value, TTLMax, also specified
                           in the session announcement.  TTLMax defaults to the value
                           of TTLSession.
                           If the TTL value required to reach the soliciting Node is
                           greater than the TTL used to reach the SN, an Advertise
                           message may not reach the Node. However, if future Query
                           messages have increased TTL values, the TTL may eventually
                           be large enough for the Advertise message to reach the Node.
                           However, it is possible that the Node will not locate any
                           SNs using Expanding Ring Search. It is advisable that a
                           backup method, such as static, be available.
                        4. SNs MUST suppress sending Advertise messages in response to
                           Query messages if one was sent with at least the Query's TTL
                           within the last SolicitPeriod.
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                           After multicasting a Query message, a node MUST wait for an
                           interval, BetweenQuery, before sending another Query message.
                           Nodes SHOULD suppress sending Query messages for
                           BetweenQuery seconds when they first start in order to
                           collect information from Advertise messages already
                           solicited by other nodes.
                        5. Getting a successful Advertise message via ERS, the Node
                           will try to connect to the parent by sending BindRequest
                           messages, which are described in Section 5.
                    The following variables are used in the Expanding Ring Search
                    - TTLNeighborInit: This is the initial TTL value to be used by the
                      ERS if no other TTL value is specified by the algorithm.
                    - TTLIncrement: This is the periodic increment for the TTL used in
                    - TTLMax: This is a configured maximum TTL value to be used by
                      either Query or Advertise messages.
                    - TTLSession: This is the session TTL value for the multicast
                    - SolicitPeriod: Each receiver MUST not send more than one QUERY
                      message per SolicitPeriod.  When SN's responds to QUERY messages,
                      it also suppresses its ADVERTISE message if one has been sent
                      less than SolicitPeriod ago.  This parameter is used to control
                      the amount of control traffic during tree construction if ERS is
                    - BetweenQuery: This is the time interval a node must wait before
                      sending successive Query messages.
                    - MaxBindAttempts: This variable is an integer used to control how
                      many times a receiver tries to bind to a SN before giving up and
                      try another one.
                    - BindPeriod: In order to prevent loops, sometimes a SN must reject
                      a BindRequest (for example, when the SN is not on the tree yet
                      and has a BindRequest outstanding itself) from a receiver.  In
                      this case, if the receiver needs to retry binding to the same SN
                      again (perhaps because the receiver does not discover any
                      alternative SN's), then it must wait for BindPeriod seconds.
                 4.1.3 Mesh
                    The mesh approach relies on a set of SN's already deployed as
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                    infrastructure servers.  These SN's are on-line, but are not
                    necessarily aware of any particular session unless informed by the
                    following mechanisms.
                    SN's in the mesh are configured to know who their neighbor SN's are,
                    and exchange reachability information with their neighbors in a way
                    analogous to routers in a network.  The actually protocol used by
                    SN's to exchange such reachability information is outside the scope
                    of this BB.  (In principle, a routing protocol such as shortest-
                    path-first, or a link-state-protocol can be adapted for this
                    purpose).  Instead, this BB specifies the following properties that
                    the mesh of SN's MUST satisfy:
                       a) Each SN knows a subset of SN's in the mesh as its immediate
                       b) Each SN has a "forwarding table", such that given any other
                          (destination) SN in the mesh, the forwarding table gives a
                          "next-hop" SN that can be used to reach the destination SN,
                          plus the distance to the destination SN from the local SN.
                       c) A given SN in the mesh can "broadcast" information to all
                          other SN's in the mesh (in the sense of having a means of
                          sending the same information to all other SN's, but not
                          necessarily simultaneously).
                       d) All potential sender and receivers of a multicast session
                          can discover a "neighboring" SN in the mesh, using the
                          neighbor discovery mechanisms described in Section 4.1.
                    The reason for running a routing-like algorithm to maintain the
                    forwarding tables in each SN is to provide fault tolerance when
                    some SN's in the mesh fail.  When that happens, the remaining SN's
                    exchange information with each other to update the forwarding
                    tables. In steady state, the mesh of SN's must still satisfy the
                    above 4 properties.
                    In the simplest form, each SN in the mesh has a forwarding table
                    that contains all other SN's in the mesh.  This is called a fully-
                    connected mesh.
                    As mentioned earlier, the Mesh scheme assumes that there is a pre-
                    deployed infrastructures of SN servers. That is, the SN-SN bindings
                    and Sender-SN (Sender's SNs) will be performed internally by the
                    provider's own policy. The only thing done by Sender is to inform
                    its SN's that a session starts. The relationships between Sender
                    and its SN's (i.e., Sender-SN bindings) MUST also be pre-
                    For example, the internal bindings between Sender and SN's MAY be
                    done as follows:
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                       1. The sender locates a neighbor SN in the mesh by a pre-
                           configured mechanism. This SN is referred to as the sender's
                       2. The sender sends the multicast session id, address and port
                         (all these can be set as a abbreviated session announcement
                         message) to the sender's SN.
                       3. The sender's SN in turn "broadcasts" the session information
                           to all SN's in the mesh; since SN's can support multiple
                           sessions simultaneously, they keep the information about
                           each session in an entry in a session table.
                       4. After the Sender-SN bindings, Sender will multicasts its
                           session announcement to the multicast receivers. Then, the
                           sender's SN binds to the sender by sending a BindRequest
                    During the internal processings of Sender-SN binding described
                    until now, any Query and Advertise messages are not used.
                    When a session starts, the bindings between SN and Receivers will
                    be done. The tree binding of SN-receiver is done with the Tree
                    Configurator (TC), which was used in the Static algorithm. The
                    main difference between Static algorithm with TC and Mesh
                    algorithm with TC is that the active SNs are considered as
                    candidate SNs in the Static scheme, while the pre-deployed
                    SNs are considered as candidates in the Mesh scheme. That is, in
                    the Mesh scheme, the TC is required to already get the information
                    on the locations of the pre-deployed SNs.
                    Then the tree buildings between receivers and SNs are done as
                       1. When a session starts, a receiver sends a Query message to
                         the TC. The receiver may optionally discovers its distance
                         from the Sender. Any metric described in Section 4.2 may be
                       2. The TC chooses one or more candidate parents SNs for the
                         receiver from the pre-deployed SNs by its own tree
                         configuration rule, as described in Section 4.1.1.
                       3. TC MUST respond to the Query message with an Advertise
                         message, which include the candidate parent list. In the list,
                         each entry contains the corresponding IP address and port of
                         the parent.
                       4. Receiving a successful Advertise message from the TC, the
                         Node will try to connect to its parent by sending BindRequest
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                         messages based on the candidate parent list, as described in
                         Section 5.
                    Once a receiver is connected to a parent SN, the SN-SN bindings
                    will also be done internally by the pre-configured provider's
                    policy. For example, each SN in the mesh tries to bind to its
                    "next-hop" SN. If the "next-hop" SN is not reachable for some
                    reason, an SN may also try to bind to any neighbor SN as a back-up
                    alternative. These procedures will be devised to ensure that the
                    loop freedom is guaranteed in section 5.
                 4.2. Distance Measurement
                    Different techniques can be used to determine distances between
                    nodes in a session.  The distances of interest are:
                       Sender distance - the distance from a SN to sender
                       Neighbor distance - the distance from a receiver to a
                                            neighboring SN
                    These distances can be used in selecting a Service Node ifseveral
                    are discovered.
                    These techniques quantify distance differently.  Each specific way
                    of quantifying distance is called a metric.  Different Metrics are
                    not necessarily comparable.  For example, if distance between A
                    and B is X using metric m1, and distance between A and C is Y
                    using metric m2, then X > Y does not necessarily imply B is
                    farther from A than C.
                    Only distances of the same metric should be compared and ranked.
                    Therefore, a receiver SHOULD only rank two SN's based on their
                    respective sender distance if those distances are based on the same
                    On the other hand, it is not necessary for all receivers to use the
                    same metric to select theirneighboring SN to connect to.  Suppose
                    receivers use neighbor distance as a selection criterion.  One
                    receiver may determine neighbor distances to SN's based on hop
                    count, whereas another receiver may determine neighbor distances
                    to its neighboring SN's based on delay.
                 4.2.1 TTL Hop-Count
                    If this metric is used, a node periodically sends a Beacon message
                    on the Session's multicast address.  The Beacon message includes
                    the original time-to-live value set by the node.  The distance to
                    the node is then calculated as
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                         Beacon's original TTL  -  Beacon's current TTL
                    One node is closer than another if its distance is a lower number.
                    Note that the TTL value may not be available in some implementation
                 4.2.2 Number of Levels
                    One metric a receiver can use for SN selection is the number of
                    levels the SN is from the sender.   For example, given two SN's in
                    close proximity to a receiver, if one SN is n levels from the
                    sender and the other is m levels, where m<n, the receiver SHOULD
                    select the SN with m levels.  This is because a shallower tree
                    allows faster propagation of feedback information to the sender.
                    (Note, we assumed the choice is between two SN's equally close to
                    the receiver.  The receiver to SN distance is another
                    The number of levels metric is not generally available, as the tree
                    may be constructed bottom up.  If the mesh approach is used,
                    however, the distance in the SN's forwarding tables can be
                    implemented as an estimate of the number of levels from the sender.
                 4.2.3 Delay
                    Another metric is the delay from an SN to the sender.  If the SN is
                    directly connected to the sender, then the delay would simply be
                    the time to send feedback from the SN to the sender.  If the SN is
                    several levels down from the sender, then the delay would be the
                    sum of the delays for each level (with some jitter time added in
                    each level).  For example, given two SN's in close proximity to a
                    receiver, the receiver SHOULD select the SN with a smaller delay to
                    the sender.  This is again for the purpose of minimizing the
                    feedback time.
                    If the tree is built bottom up, this metric cannot be used.  If the
                    mesh approach is used, this metric can be implemented, although it
                    requires the SN's in the mesh to exchange distance information
                    based on the delay metric.
                 4.2.4 Address
                    For IPv6 addresses[HOD98], distance can be approximately determined
                    by the number of aggregation levels one address has in common with
                    another.  For this metric, one node is closer than another if its
                    address has more aggregation levels in common with the querying
                    node's address.
                 4.2.5 Static
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                    The node's distance to other nodes may be made available in some
                    well-known location.  One node's is closer than another if its
                    distance is a lower number.
                 4.2.6 GRA
                    The node's distance to the sender may be determined with help of a
                    GRA message that lists the set of GRA routers on the path from the
                 4.3 Service Node Selection
                    Once Neighbors have been discovered, a node selects the best one
                    using whatever distance information is available.
                    If there is no sender distance information to compare, the best SN
                    is simply the one that is closest to the node, without a loop
                    being formed by binding the node to the SN.
                    If sender distances are available, there are two cases:
                       Leaf nodes: For leaf nodes, the goal is to use the closest SN
                       SNs: For SNs joining the tree, it is important to pick an SN
                             that is closer to the sender; neighbor distance is a
                             secondary factor.
                    Once an SN has been selected, the node tries to bind to it as
                    described in Section 5.  Loop prevention is done during the bind
                    process using only Tree Level information.
                    This algorithm is recommended because it assigns each node the
                    closest SN, and does not require all nodes to measure their sender
                    distance at the start of the session.  Depending on the selected
                    metric, multiple nodes measuring sender distance could cause
                    message implosion, and delay tree construction.  On the other hand,
                    the SNs selected may actually be further from the Sender than
                    their children are.  However, it may be necessary to assign nodes
                    to non-optimal SNs in order to get them on the tree, since it is
                    possible that no SN closer to the Sender can accept any more
                    Alternatively, nodes may be required to measure sender distance
                    before selecting an SN in order to ensure that each parent is
                    closer to the Sender than its children.  Presumably, this results
                    in a tree in which parents detect message loss before their
                    children, minimizing repair requests.
                 5. Tree Formation
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                    The following is a detailed description of the tree formation
                    process. All tree construction follows this pattern. The ONLY
                    differences between instantiations of this building block lie in
                    how nodes discover and select neighbors.
                    Once an SN has been selected, the Node sends a BindRequest message
                    to the SN. If the SN has an outstanding request to bind to another
                    SN, it must refuse the incoming bind request in case it would form
                    a loop.
                    Otherwise, it MAY accept the Node as a child as long as selecting
                    it would not cause a loop in the tree.  Loop freedom is guaranteed
                    by these two rules:
                    1.  If the requesting node does not have children, the SN can
                        accept it as a child as long as the SN has no outstanding
                        bind requests.  If it does have an outstanding bind request,
                        the SN can accept the node as a child if its address is less
                        than the child's address.
                    2.  If the requesting node has children, the SN can accept it as
                        a child if
                    a.  the SN's level is 128, i.e., it is the top of a sub-tree not
                        yet connected to the Sender, or
                    b.  the SN's level is less than 128, i.e., it is connected to the
                    The second rule prevents a node from selecting one of its own
                    children as its parent.  Two nodes at level 128 are prevented from
                    selecting each other using tie-breaking criteria described step 1
                    If the SN accepts the Node as a Child, it returns a BindConfirm
                    message.  If it does not accept the Node, it sends a BindReject
                    message.  If the Node does not receive a response after
                    MaxBindAttempts tries every BindPeriod seconds, it MAY select the
                    next best Neighbor from its cached list, or else run the Service
                    Node Discovery process again to determine an alternate SN to try.
                    The BindReject message contains a reason code.  If the code
                    indicates that the node was rejected because the SN was not yet on
                    the tree, the node MAY choose to retry that SN after BindPeriod
                    seconds, or select a different available SN.
                    The BindReject message may also include a list of alternative SNs
                    for the node to try.
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                    The BindConfirm message MUST include the Parent's current Tree
                    Level. The Node MUST set its Tree Level to one more than the
                    Parent's level.
                    The BindConfirm message also MUST also indicate the  starting
                    sequence number of the message from which data reliability is
                    assured.  This information is included in the BindConfirm message
                    to enable receivers to meet the PI's late join requirements.  If
                    nodes join the tree after the Sender has started to send data, it
                    is possible that some of the data is no longer available within
                    the tree. Nodes may need to have specific information about repair
                    availability before selecting a Parent.
                    Service Nodes MAY limit the number of children they support
                    depending on their capacity.  Once an SN has accepted its maximum
                    number of children, it stops accepting new children until a change
                    in membership causes its count of children to go below this limit.
                    If an SN limits the number of children it supports, it MUST reserve
                    at least one child slot for other SNs. This guarantees the growth
                    of the repair tree.
                 6.  Tree Maintenance
                    Tree maintenance is an ongoing process active in every Node.
                    Because the tree is based on the operation of SNs, as well as the
                    various underlying metrics that may change over time, it is
                    important that these dependencies be monitored for changes.  Nodes
                    MUST monitor Parents for liveness and changes in tree level, and
                    SHOULD continue to run the Neighbor Discovery and Selection
                    process in order to optimize their choice of SN.  Parents must
                    also monitor Children for liveness.
                 6.1 Monitoring Parents and Children
                    The upper Building Block or Protocol Instantiation is responsible
                    for monitoring parents and children.  Monitoring messages from
                    parents to children MUST contain the Parent's current Tree Level.
                    Children MUST set their Tree Level to one more than their Parent's
                    If a Child loses contact with its Parent for a period of time, it
                    MUST report it using the lostSN interface, and attempt to bind with
                    an alternate SN.
                    A Child that is leaving a session MUST send a unicast UnbindRequest
                    message to its Parent.  The Parent MUST respond with an
                    UnbindConfirm message.
                    A Parent that is leaving the session MUST send an EjectRequest
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                    message to its Children indicating that they need to bind with an
                    alternate SN.  If possible the EjectRequest message is multicasted,
                    but the EjectRequest message can also be sent via unicast to each
                    child individually.  Upon receiving an EjectRequest message from
                    its parent, a receiver sets its Tree Level to 128 again.  Using
                    the heartbeat mechanism, the Tree Level for all receivers in the
                    affected subtree will be updated (to a value higher than 128).
                    If a Parent does not hear from a Child for a period of time, or it
                    receives a UnbindRequest message from a Child, it removes that
                    Child from its list of Children, and reports the loss using the
                    removeChild interface.
                 6.2 Optimizing the Tree
                    Implementations of this building block SHOULD continue to run the
                    Neighbor Discovery and Selection process in order to optimize the
                    choice of SN.  This continuous process also keeps the distance
                    information for the current Parent up-to-date.  Whenever the
                    process returns a better SN than the current one, the Node MAY
                    bind to the new SN.  Once the new SN is bound to, the Node MUST
                    send a UnbindRequest message to the original Parent.  A Parent
                    with no Children MAY leave the session.
                 7. Messages
                    These messages are required for implementations of this building
                    block.  The list below indicates which message contents are
                    by implementations.  Implementations may also include other
                    protocol-specific information in these messages.  Note that these
                    messages are parts of packets specified in PI's that use this BB.
                 | Message Name   |       Description           |                     |
                 | m- or u-cast   |                             |     Contents        |
                 |   Query        | A message used to discover  | Sender distance     |
                 |    both        | neighbors. The unicast      | (optional),         |
                 |                | is sent to TC; the multicast| TTL (m-cast only)   |
                 |                | is used in ERS.             |                     |
                 |   Advertise    | A message used to advertise | IP address, port,   |
                 |    both        | an SN. The unicast is sent  | Sender distance     |
                 |                | from the TC; the multicast  | (optional)          |
                 |                | is sent by SNs themselves   |                     |
                 |  BindRequest   | Request to SN to join tree  | Current Tree Level  |
                 |   unicast      |                             |                     |
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                 |  BindConfirm   | SN accepts BindRequest      | Current Tree Level  |
                 |   unicast      |                             |                     |
                 |  BindReject    | SN rejects BindRequest      | Reject reason,      |
                 |   unicast      |                             | alternate SN list   |
                 | UnbindRequest  | Child leaving Parent(u-cast)| Unbind reason       |
                 |    unicast     | Parent leaving tree (m-cast)|                     |
                 | UnbindConfirm  |  Acknowledgement of         |                     |
                 |   unicast      |  UnbindRequest message      |                     |
                 |  EjectRequest  | Parent refusing or leaving  | Eject reason        |
                 |    both        | service to Children         |                     |
                 |  EjectConfirm  |  Acknowledgement of         |                     |
                 |   unicast      |  EjectRequest message       |                     |
                 8. External Interfaces
                    This section describes external interfaces for the building block.
                 8.1 Interfaces to this BB.
                    These may be used by a PI, or by a higher-level BB.
                 8.1.1 start(boolean SN, advertisement)
                    start notifies the BB to begin operation.  If the SN parameter is
                    set to TRUE, the BB also starts SN operation.
                 8.1.2 end()
                    end notifies the BB to end operation.
                 8.1.3 incomingMessage(message)
                    This interface is used to pass an incoming message down from the PI.
                 8.1.4 getStatistics
                    getStatistics returns current BB statistics to the upper BB or PI.
                 8.1.5 getSNs
                    getSNs instructs the BB to start the process of finding SN
                    candidates for this node.  getSNs may return immediately with a
                    list of
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                    candidate SN, or may use the SNlist interface (see section 8.2.1)
                    to return the list at a later time.
                 8.1.6 setSN(SN)
                    setSN informs the BB that the PI or upper BB has successfully bound
                    to an SN.
                 8.1.7 acceptChild(node)
                    acceptChild asks the BB to accept or reject the node as a member.
                    The BB returns a boolean value in response.
                 8.1.8 removeChild(node)
                    removeChild is called to inform the BB that the child is no longer
                    a member.
                 8.1.9 treeLevelUpdate(newLevel)
                    This interface is used to pass down any update to the node's tree
                    level that the upper BB or the PI has learned.  newLevel replaces
                    the BB's current tree level.
                 8.1.10 lostSN
                    lostSN notifies the BB that the connection to the SN was lost.
                 8.1.11 setOptimization(boolean)
                    setOptimization tells the BB to start or stop the tree optimization
                    process. The upper BB or PI may want to control when tree
                    optimization takes place.
                 8.1.12 recordSNs(destination)
                    recordSNs tells the BB to record the current SN, plus the list of
                    alternates, to the indicated destination.
                 8.2 Interfaces from this BB to the PI or a higher-level BB.
                 8.2.1 outgoingMessage(message)
                    outgoingMessage instructs the PI to send the message.
                 8.2.2 SNlist(list)
                    SNlist returns to the upper PI or BB a list of SN candidates.  The
                    list may contain additional information for each candidate, such as
                    details about the data packets that it has available for repair.
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                 9. References
                     [DEE89] Deering, S., "Host Extensions for IP Multicasting", RFC
                     [ECTP02] ITU-T Q.8/17 and ISO/IEC JTC1/SC6/WG7, "Enhanced
                    Communication Transport Protocol: Specification of Simplex
                    Multicast Transport," ITU-T Recommendation X.606 and ISO/IEC
                    14476-1, Feb., 2002.
                    [HC00] H. Holbrook, B. Cain, "Source-Specific Multicast for IP,"
                    Internet Draft, Internet Engineering Task Force, November 2000.
                    [HOD98] R. Hinden, M. O'Dell, S. Deering, "An IPv6 Aggregatable
                    Global Unicast Address Format," Request For Comments 2374, July
                    [HPW00] M. Handley, C. Perkins, E. Whelan, "Session Announcement
                    Protocol", Internet Draft, Internet Engineering Task Force, March
                    [HWKFV00] M. Handley, B. Whetten, R. Kermode, S. Floyd, L. Vicisano,
                    and M.  Luby, "The Reliable Multicast Design Space for Bulk Data
                    Transfer," RFC 2887, August 2000.
                    [K01] S. Koh, et. al., "Configuration of ACK Tress for Multicast
                    Transport Protocols," ETRI Journal, Vol. 23, No. 3, September 2001.
                    [KV02] R. Kermode, L. Vicisano, "Author Guidelines for RMT Building
                    Blocks and Protocol Instantiation Documents," RFC 3269, April 2002.
                    [LPG98] B.N. Levine, S. Paul, and J.J. Garcia-Luna-Aceves,
                    "Organizing Multicast Receivers Deterministically According to
                    Packet-Loss Correlation," Proc. Sixth ACM International Multimedia
                    Conference (ACM Multimedia 98), Bristol, UK, September 1998.
                    [PTM93] C. Partridge, T. Mendez, and W. Milliken. "Host anycasting
                    service."  Request For Comments 1546, November 1993.
                    [WCMKT02] B. Whetten, D. Chiu, M. Kadansky, S. Koh, G. Taskale,
                    "TRACK Protocol Instantiation over UDP: Tree Acknowledgement based
                    Reliable Multicast," Internet Draft, November 2002.
                    [WLKHFL01]  B. Whetten, L. Vicisano, R. Kermode, M. Handley, S.
                    Floyd, and M. Luby, "Reliable Multicast Transport Building Blocks
                    for One-to-Many Bulk-Data Transfer," RFC 3048, January 2001.
                    [YGS95] R. Yavatkar, J. Griffioen and M. Sudan, "A Reliable
                    Dissemination Protocol for Interactive Collaborative Applications",
                    University of Kentucky, 1995.
                 INTERNET DRAFT      draft-ietf-rmt-bb-track-01.txt                  25
                 INTERNET DRAFT   draft-ietf-rmt-bb-tree-config-03.txt    November 2002
                    [SV02] T. Speakman, and L. Vicisano, "Reliable Multicast Transport
                    Building Block: Generic Router Assist - Signalling Protocol
                    Specification," Internet Draft, Internet Engineering Task Force,
                    July 2002.
                 Authors' Addresses
                       Brian Whetten
                       890 Sea Island Lane
                       Foster City, CA 94404
                       Dah Ming Chiu
                       Miriam Kadansky
                       Sun Microsystems Laboratories
                       1 Network Drive
                       Burlington, MA 01803
                       Seok Joo Koh
                       161 Kajong-dong
                       Yusong-Gu, TAEJON, 305-350, KOREA
                       Gursel Taskale
                       Brian Neil Levine
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                  The limited permissions granted above are perpetual and will not be
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