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Versions: (draft-waehlisch-sam-common-api) 00 01 02 03 04 05 06 07 08 09 10 RFC 7046

SAM Research Group                                          M. Waehlisch
Internet-Draft                                      link-lab & FU Berlin
Intended status: Experimental                               T C. Schmidt
Expires: February 4, 2013                                    HAW Hamburg
                                                               S. Venaas
                                                           cisco Systems
                                                          August 3, 2012


             A Common API for Transparent Hybrid Multicast
                     draft-irtf-samrg-common-api-06

Abstract

   Group communication services exist in a large variety of flavors, and
   technical implementations at different protocol layers.  Multicast
   data distribution is most efficiently performed on the lowest
   available layer, but a heterogeneous deployment status of multicast
   technologies throughout the Internet requires an adaptive service
   binding at runtime.  Today, it is difficult to write an application
   that runs everywhere and at the same time makes use of the most
   efficient multicast service available in the network.  Facing
   robustness requirements, developers are frequently forced to use a
   stable, upper layer protocol provided by the application itself.
   This document describes a common multicast API that is suitable for
   transparent communication in underlay and overlay, and grants access
   to the different multicast flavors.  It proposes an abstract naming
   by multicast URIs and discusses mapping mechanisms between different
   namespaces and distribution technologies.  Additionally, it describes
   the application of this API for building gateways that interconnect
   current multicast domains throughout the Internet, and reports on an
   implementation of the programming interface including a service
   middleware.  This document is a product of the Scalable Adaptive
   Multicast Research Group (SAM) Research Group.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 4, 2013.

Copyright Notice

   Copyright (c) 2012 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.

































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Use Cases for the Common API . . . . . . . . . . . . . . .  6
     1.2.  Illustrative Examples  . . . . . . . . . . . . . . . . . .  7
       1.2.1.  Support of Multiple Underlying Technologies  . . . . .  7
       1.2.2.  Support of Multi-Resolution Multicast  . . . . . . . .  9
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Objectives and Reference Scenarios . . . . . . . . . . . . 11
     3.2.  Group Communication API and Protocol Stack . . . . . . . . 12
     3.3.  Naming and Addressing  . . . . . . . . . . . . . . . . . . 14
     3.4.  Namespaces . . . . . . . . . . . . . . . . . . . . . . . . 15
       3.4.1.  Generic Namespaces . . . . . . . . . . . . . . . . . . 15
       3.4.2.  Application-centric Namespaces . . . . . . . . . . . . 15
     3.5.  Name-to-Address Mapping  . . . . . . . . . . . . . . . . . 16
       3.5.1.  Canonical Mapping  . . . . . . . . . . . . . . . . . . 16
       3.5.2.  Mapping at End Points  . . . . . . . . . . . . . . . . 17
       3.5.3.  Mapping at Inter-domain Multicast Gateways . . . . . . 17
     3.6.  A Note on Explicit Multicast (XCAST) . . . . . . . . . . . 17
     3.7.  MTU Handling . . . . . . . . . . . . . . . . . . . . . . . 17
   4.  Common Multicast API . . . . . . . . . . . . . . . . . . . . . 18
     4.1.  Notation . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.2.  Abstract Data Types  . . . . . . . . . . . . . . . . . . . 19
       4.2.1.  Multicast URI  . . . . . . . . . . . . . . . . . . . . 19
       4.2.2.  Interface  . . . . . . . . . . . . . . . . . . . . . . 19
       4.2.3.  Membership Events  . . . . . . . . . . . . . . . . . . 20
     4.3.  Group Management Calls . . . . . . . . . . . . . . . . . . 20
       4.3.1.  Create . . . . . . . . . . . . . . . . . . . . . . . . 20
       4.3.2.  Delete . . . . . . . . . . . . . . . . . . . . . . . . 21
       4.3.3.  Join . . . . . . . . . . . . . . . . . . . . . . . . . 21
       4.3.4.  Leave  . . . . . . . . . . . . . . . . . . . . . . . . 21
       4.3.5.  Source Register  . . . . . . . . . . . . . . . . . . . 22
       4.3.6.  Source Deregister  . . . . . . . . . . . . . . . . . . 22
     4.4.  Send and Receive Calls . . . . . . . . . . . . . . . . . . 22
       4.4.1.  Send . . . . . . . . . . . . . . . . . . . . . . . . . 23
       4.4.2.  Receive  . . . . . . . . . . . . . . . . . . . . . . . 23
     4.5.  Socket Options . . . . . . . . . . . . . . . . . . . . . . 24
       4.5.1.  Get Interfaces . . . . . . . . . . . . . . . . . . . . 24
       4.5.2.  Add Interface  . . . . . . . . . . . . . . . . . . . . 24
       4.5.3.  Delete Interface . . . . . . . . . . . . . . . . . . . 24
       4.5.4.  Set TTL  . . . . . . . . . . . . . . . . . . . . . . . 25
       4.5.5.  Get TTL  . . . . . . . . . . . . . . . . . . . . . . . 25
       4.5.6.  Atomic Message Size  . . . . . . . . . . . . . . . . . 25
     4.6.  Service Calls  . . . . . . . . . . . . . . . . . . . . . . 26
       4.6.1.  Group Set  . . . . . . . . . . . . . . . . . . . . . . 26
       4.6.2.  Neighbor Set . . . . . . . . . . . . . . . . . . . . . 26
       4.6.3.  Children Set . . . . . . . . . . . . . . . . . . . . . 27



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       4.6.4.  Parent Set . . . . . . . . . . . . . . . . . . . . . . 27
       4.6.5.  Designated Host  . . . . . . . . . . . . . . . . . . . 27
       4.6.6.  Enable Membership Events . . . . . . . . . . . . . . . 28
       4.6.7.  Disable Membership Events  . . . . . . . . . . . . . . 28
       4.6.8.  Maximum Message Size . . . . . . . . . . . . . . . . . 28
   5.  Implementation . . . . . . . . . . . . . . . . . . . . . . . . 29
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 30
   Appendix A.  C Signatures  . . . . . . . . . . . . . . . . . . . . 32
   Appendix B.  Practical Example of the API  . . . . . . . . . . . . 34
   Appendix C.  Deployment Use Cases for Hybrid Multicast . . . . . . 35
     C.1.  DVMRP  . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     C.2.  PIM-SM . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     C.3.  PIM-SSM  . . . . . . . . . . . . . . . . . . . . . . . . . 37
     C.4.  BIDIR-PIM  . . . . . . . . . . . . . . . . . . . . . . . . 37
   Appendix D.  Change Log  . . . . . . . . . . . . . . . . . . . . . 38
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
































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1.  Introduction

   Currently, group application programmers need to make the choice of
   the distribution technology that the application will require at
   runtime.  There is no common communication interface that abstracts
   multicast transmission and subscriptions from the deployment state at
   runtime, nor has been the use of DNS for group addresses established.
   The standard multicast socket options [RFC3493], [RFC3678] are bound
   to an IP version by not distinguishing between naming and addressing
   of multicast identifiers.  Group communication, however, is commonly
   implemented in different flavors such as any source (ASM) vs. source
   specific multicast (SSM), on different layers (e.g., IP vs.
   application layer multicast), and may be based on different
   technologies on the same tier as with IPv4 vs. IPv6.  It is the
   objective of this document to provide for programmers a universal
   access to group services.

   Multicast application development should be decoupled of
   technological deployment throughout the infrastructure.  It requires
   a common multicast API that offers calls to transmit and receive
   multicast data independent of the supporting layer and the underlying
   technological details.  For inter-technology transmissions, a
   consistent view on multicast states is needed, as well.  This
   document describes an abstract group communication API and core
   functions necessary for transparent operations.  Specific
   implementation guidelines with respect to operating systems or
   programming languages are out of scope of this document.

   In contrast to the standard multicast socket interface, the API
   introduced in this document abstracts naming from addressing.  Using
   a multicast address in the current socket API predefines the
   corresponding routing layer.  In this specification, the multicast
   name used for joining a group denotes an application layer data
   stream that is identified by a multicast URI, independent of its
   binding to a specific distribution technology.  Such a group name can
   be mapped to variable routing identifiers.

   The aim of this common API is twofold:

   o  Enable any application programmer to implement group-oriented data
      communication independent of the underlying delivery mechanisms.
      In particular, allow for a late binding of group applications to
      multicast technologies that makes applications efficient, but
      robust with respect to deployment aspects.

   o  Allow for a flexible namespace support in group addressing, and
      thereby separate naming and addressing resp. routing schemes from
      the application design.  This abstraction does not only decouple



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      programs from specific aspects of underlying protocols, but may
      open application design to extend to specifically flavored group
      services.

   Multicast technologies may be of various peer-to-peer kinds, IPv4 or
   IPv6 network layer multicast, or implemented by some other
   application service.  Corresponding namespaces may be IP addresses or
   DNS naming, overlay hashes, or other application layer group
   identifiers like <sip:*@peanuts.org>, but also names independently
   defined by the applications.  Common namespaces are introduced later
   in this document, but follow an open concept suitable for further
   extensions.

   This document also discusses mapping mechanisms between different
   namespaces and forwarding technologies and proposes expressions of
   defaults for an intended binding.  Additionally, the multicast API
   provides internal Interfaces to access current multicast states at
   the host.  Multiple multicast protocols may run in parallel on a
   single host.  These protocols may interact to provide a gateway
   function that bridges data between different domains.  The
   application of this API at gateways operating between current
   multicast instances throughout the Internet is described, as well.
   Finally, a report is presented on an implementation of the
   programming interface including a service middleware.

   This document represents the consensus of the SAM Research Group.  It
   has been reviewed by the Research Group members active in the
   specific area of work.  In addition, this document has been
   comprehensively reviewed by people who are not "in" the Research
   Group but are expert in the area.

1.1.  Use Cases for the Common API

   The following generic use cases can be identified that require an
   abstract common API for multicast services:

   Application Programming Independent of Technologies:  Application
      programmers are provided with group primitives that remain
      independent of multicast technologies and their deployment in
      target domains.  They are thus enabled to develop programs once
      that run in every deployment scenario.  The use of Group Names in
      the form of abstract meta data types allows applications to remain
      namespace-agnostic in the sense that the resolution of namespaces
      and name-to-address mappings may be delegated to a system service
      at runtime.  Thereby, the complexity is minimized as developers
      need not care about how data is distributed in groups, while the
      system service can take advantage of extended information of the
      network environment as acquired at startup.



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   Global Identification of Groups:  Groups can be identified
      independent of technological instantiations and beyond deployment
      domains.  Taking advantage of the abstract naming, an application
      is thus enabled to match data received from different Interface
      technologies (e.g., IPv4, IPv6, or overlays) to belong to the same
      group.  This not only increases flexibility, an application may
      for instance combine heterogeneous multipath streams, but also
      simplifies the design and implementation of gateways and
      translators.

   Uniform Access to Multicast Flavors:  The URI naming scheme uniformly
      supports different flavors of group communication such as any
      source and source specific multicast, selective broadcast etc.,
      independent of their service instantiation.  The traditional SSM
      model, for instance, can experience manifold support, either by
      directly mapping the multicast URI (i.e., "group@instantiation")
      to an (S,G) state on the IP layer, or by first resolving S for a
      subsequent group address query, or by transferring this process to
      any of the various source specific overlay schemes, or by
      delegating to a plain replication server.  The application
      programmer can invoke any of these underlying mechanisms with the
      same line of code.

   Simplified Service Deployment through Generic Gateways:  The common
      multicast API allows for an implementation of abstract gateway
      functions with mappings to specific technologies residing at a
      system level.  Such generic gateways may provide a simple bridging
      service and facilitate an inter-domain deployment of multicast.

   Mobility-agnostic Group Communication:  Group naming and management
      as foreseen in the common multicast API remain independent of
      locators.  Naturally, applications stay unaware of any mobility-
      related address changes.  Handover-initiated re-addressing is
      delegated to the mapping services at the system level and may be
      designed to smoothly interact with mobility management solutions
      provided at the network or transport layer (see [RFC5757] for
      mobility-related aspects).

1.2.  Illustrative Examples

1.2.1.  Support of Multiple Underlying Technologies

   On a very high-level, the common multicast API provides the
   application programmer with one single interface to manage multicast
   content independent of the technology underneath.  Considering the
   following simple example in Figure 1: A multicast source S is
   connected via IPv4 and IPv6.  It distributes one piece of multicast
   content (e.g., a movie).  Receivers are connected via IPv4/v6 and



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   overlay multicast respectively.
    +-------+       +-------+               /         +-------+
    |   S   |       |  R1   |                        |  R3   |
    +-------+       +-------+                        +-------+
   v6|   v4|           |v4                              |OLM
     |     |          /                                 |
     |  ***| ***  ***/ **                           *** /***  ***  ***
      \*   |*   **  /**   *                        *   /*   **   **   *
      *\   \_______/_______*__v4__+-------+       *   /                *
       *\    IPv4/v6      *       |  R2   |__OLM__ *_/ Overlay Mcast  *
      *  \_________________*__v6__+-------+       *                    *
       *   **   **   **   *                        *    **   **   **  *
        ***  ***  ***  ***                          ***  ***  ***  ***

   Figure 1: Source S sends the same multicast content to all interfaces

   Using the current socket API, the application programmer needs to
   decide on the IP technologies at coding time.  Additional
   distribution techniques, such as overlay multicast, must be
   individually integrated into the application.  For each technology,
   the application programmer needs to create a separate socket and to
   initiate a dedicated join or send.  As the current socket API does
   not distinguish between group name and group address, the content
   will be delivered multiple times to the same receiver (cf., R2).
   Whenever the source distributes content via a technology that is not
   supported by the receivers or its Internet Service Provider (cf.,
   R3), a gateway is required.  Gateway functions rely on a coherent
   view of the multicast group states.

   The common multicast API simplifies programming of multicast
   applications as it abstracts content distribution from specific
   technologies.  In addition to calls which implement receiving and
   sending of multicast data, it provides service calls to grant access
   to internal multicast states at the host.

   The API described in this document defines a minimal set of
   interfaces for the system components at the host to fulfill group
   communication.  It is open to the implementation to provide
   additional convenience functions for the programmer.

   The implementation of content distribution for the example shown in
   Figure 1 may then look like:









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     //Initialize multicast socket
     MulticastSocket m = new MulticastSocket();
     //Associate all available interfaces
     m.addInterface(getInterfaces());
     //Subscribe to multicast group
     m.join(URI("opaque://news@cnn.com"));
     //Send to multicast group
     m.send(URI("opaque://news@cnn.com"),message);

            Send/receive example using the common multicast API

   The gateway function for R2 can be implemented by the service calls
   similar to:

     //Initialize multicast socket
     MulticastSocket m = new MulticastSocket();
     //Check (a) host is designated multicast node for this interface
     //      (b) receivers exist
     for all this.getInterfaces() {
       if(designatedHost(this.interface) &&
            childrenSet(this.interface,
               URI("opaque://news@cnn.com")) != NULL) {
         m.addInterface(this.interface);
       }
     }
     while(true) {
       m.send(URI("opaque://news@cnn.com"),message);
     }

              Gateway example using the common multicast API

1.2.2.  Support of Multi-Resolution Multicast

   Multi-resolution multicast adjusts the multicast stream to consider
   heterogeneous end devices.  The multicast data (e.g., available by
   different compression levels) is typically announced using multiple
   multicast addresses, which are unrelated to each other.  Using the
   common API, multi-resolution multicast can be implemented
   transparently by an operator with the help of Name-to-Address
   mapping, or by systematic naming in a subscriber-centric perspective.

   Operator-Centric:  An operator deploys a domain-specific mapping.  In
      this case, any multicast receiver (e.g., mobile or DSL user)
      subscribes to the same multicast name, which will be resolved
      locally to different multicast addresses.  Then, each Group
      Address describes a different level of data quality.





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   Subscriber-Centric:  In a subscriber-centric example, the multicast
      receiver chooses the quality in advance based on a predefined
      naming syntax.  Consider a layered video stream "blockbuster"
      available at different qualities Q_i, each of which consists of
      the base layer plus the sum of EL_j, j <= i enhancement layers.
      Each individual layer may then be accessible by a name
      "EL_j.Q_i.blockbuster", j <= i, while a specific quality
      aggregates the corresponding layers to "Q_i.blockbuster", and the
      full-size movie may be just called "blockbuster".


2.  Terminology

   This document uses the terminology as defined for the multicast
   protocols [RFC2710],[RFC3376],[RFC3810],[RFC4601],[RFC4604].  In
   addition, the following terms will be used.

   Group Address:  A Group Address is a routing identifier.  It
      represents a technological specifier and thus reflects the
      distribution technology in use.  Multicast packet forwarding is
      based on this address.

   Group Name:  A Group Name is an application identifier that is used
      by applications to manage communication in a multicast group
      (e.g., join/leave and send/receive).  The Group Name does not
      predefine any distribution technologies.  Even if it syntactically
      corresponds to an address, it solely represents a logical
      identifier.

   Multicast Namespace:  A Multicast Namespace is a collection of
      designators (i.e., names or addresses) for groups that share a
      common syntax.  Typical instances of namespaces are IPv4 or IPv6
      multicast addresses, overlay group IDs, group names defined on the
      application layer (e.g., SIP or Email), or some human readable
      strings.

   Interface:  An Interface is a forwarding instance of a distribution
      technology on a given node.  For example, the IP Interface
      192.168.1.1 at an IPv4 host.

   Multicast Domain:  A Multicast Domain hosts nodes and routers of a
      common, single multicast forwarding technology and is bound to a
      single namespace.

   Inter-domain Multicast Gateway (IMG):  An Inter-domain Multicast
      Gateway (IMG) is an entity that interconnects different Multicast
      Domains.  Its objective is to forward data between these domains,
      e.g., between an IP layer and overlay multicast.



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3.  Overview

3.1.  Objectives and Reference Scenarios

   The default use case addressed in this document targets at
   applications that participate in a group by using some common
   identifier taken from some common namespace.  This Group Name is
   typically learned at runtime from user interaction like the selection
   of an IPTV channel, from dynamic session negotiations like in the
   Session Initiation Protocol (SIP) [RFC3261], but may as well have
   been predefined for an application as a common Group Name.
   Technology-specific system functions then transparently map the Group
   Name to Group Addresses such that

   o  programmers are enabled to process group names in their programs
      without the need to consider technological mappings to designated
      deployments in target domains;

   o  applications are enabled to identify packets that belong to a
      logically named group, independent of the Interface technology
      used for sending and receiving packets.  The latter shall also
      hold for multicast gateways.

   This document considers two reference scenarios that cover the
   following hybrid deployment cases displayed in Figure 2:

   1.  Multicast Domains running the same multicast technology but
       remaining isolated, possibly only connected by network layer
       unicast.

   2.  Multicast Domains running different multicast technologies but
       hosting nodes that are members of the same multicast group.



















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                                        +-------+         +-------+
                                        | Member|         | Member|
                                        |  Foo  |         |   G   |
                                        +-------+         +-------+
                                              \            /
                                            ***  ***  ***  ***
                                           *   **   **   **   *
                                          *                    *
                                           *   MCast Tec A    *
                                          *                    *
                                           *   **   **   **   *
                                            ***  ***  ***  ***
   +-------+          +-------+                      |
   | Member|          | Member|                  +-------+
   |   G   |          |  Foo  |                  |  IMG  |
   +-------+          +-------+                  +-------+
       |                |                            |
       ***  ***  ***  ***                  ***  ***  ***  ***
      *   **   **   **   *                *   **   **   **   *
     *                    *  +-------+   *                    *
      *   MCast Tec A    * --|  IMG  |--  *   MCast Tec B    *    +-------+
     *                    *  +-------+   *                    * - | Member|
      *   **   **   **   *                *   **   **   **   *    |   G   |
       ***  ***  ***  ***                  ***  ***  ***  ***     +-------+


    Figure 2: Reference scenarios for hybrid multicast, interconnecting
    group members from isolated homogeneous and heterogeneous domains.

3.2.  Group Communication API and Protocol Stack

   The group communication API consists of four parts.  Two parts
   combine the essential communication functions, while the remaining
   two offer optional extensions for an enhanced monitoring and
   management:

   Group Management Calls  provide the minimal API to instantiate a
      multicast socket and to manage group membership.

   Send/Receive Calls  provide the minimal API to send and receive
      multicast data in a technology-transparent fashion.

   Socket Options  provide extension calls for an explicit configuration
      of the multicast socket such as setting hop limits or associated
      Interfaces.






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   Service Calls  provide extension calls that grant access to internal
      multicast states of an Interface such as the multicast groups
      under subscription or the multicast forwarding information base.

   Multicast applications that use the common API require assistance by
   a group communication stack.  This protocol stack serves two needs:

   o  It provides system-level support to transfer the abstract
      functions of the common API, including namespace support, into
      protocol operations at Interfaces.

   o  It provides group communication services across different
      multicast technologies at the local host.

   A general initiation of a multicast communication in this setting
   proceeds as follows:

   1.  An application opens an abstract multicast socket.

   2.  The application subscribes/leaves/(de)registers to a group using
       a Group Name.

   3.  An intrinsic function of the stack maps the logical group ID
       (Group Name) to a technical group ID (Group Address).  This
       function may make use of deployment-specific knowledge such as
       available technologies and group address management in its
       domain.

   4.  Packet distribution proceeds to and from one or several
       multicast-enabled Interfaces.

   The abstract multicast socket describes a group communication channel
   composed of one or multiple Interfaces.  A socket may be created
   without explicit Interface association by the application, which
   leaves the choice of the underlying forwarding technology to the
   group communication stack.  However, an application may also bind the
   socket to one or multiple dedicated Interfaces, which predefines the
   forwarding technology and the Multicast Namespace(s) of the Group
   Address(es).

   Applications are not required to maintain mapping states for Group
   Addresses.  The group communication stack accounts for the mapping of
   the Group Name to the Group Address(es) and vice versa.  Multicast
   data passed to the application will be augmented by the corresponding
   Group Name.  Multiple multicast subscriptions thus can be conducted
   on a single multicast socket without the need for Group Name encoding
   at the application side.




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   Hosts may support several multicast protocols.  The group
   communication stack discovers available multicast-enabled Interfaces.
   It provides a minimal hybrid function that bridges data between
   different Interfaces and Multicast Domains.  Details of service
   discovery are out of scope of this document.

   The extended multicast functions can be implemented by a middleware
   as conceptually visualized in Figure 3.

   *-------*     *-------*
   | App 1 |     | App 2 |
   *-------*     *-------*
       |             |
   *---------------------*         ---|
   |   Middleware        |            |
   *---------------------*            |
        |          |                  |
   *---------*     |                  |
   | Overlay |     |                   \  Group Communication
   *---------*     |                   /  Stack
        |          |                  |
        |          |                  |
   *---------------------*            |
   |   Underlay          |            |
   *---------------------*         ---|

    Figure 3: A middleware for offering uniform access to multicast in
                           underlay and overlay

3.3.  Naming and Addressing

   Applications use Group Names to identify groups.  Names can uniquely
   determine a group in a global communication context and hide
   technological deployment for data distribution from the application.
   In contrast, multicast forwarding operates on Group Addresses.  Even
   though both identifiers may be identical in symbols, they carry
   different meanings.  They may also belong to different Multicast
   Namespaces.  The Namespace of a Group Address reflects a routing
   technology, while the Namespace of a Group Name represents the
   context in which the application operates.

   URIs [RFC3986] are a common way to represent Namespace-specific
   identifiers in applications in the form of an abstract meta-data
   type.  Throughout this document, all Group Names follows a URI
   notation with the syntax defined in Section 4.2.1.  Examples are,
   ip://224.1.2.3:5000 for a canonical IPv4 ASM group at UDP port 5000,
   sip://news@cnn.com for an application-specific naming with service
   instantiator and default port selection.



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   An implementation of the group communication stack can provide
   convenience functions that detect the Namespace of a Group Name or
   further optimize service instantiation.  In practice, such a library
   would provide support for high-level data types to the application,
   similar to some versions of the current socket API (e.g., InetAddress
   in Java).  Using this data type could implicitly determine the
   Namespace.  Details of automatic Namespace identification or service
   handling are out of scope of this document.

3.4.  Namespaces

   Namespace identifiers in URIs are placed in the scheme element and
   characterize syntax and semantic of the group identifier.  They
   enable the use of convenience functions and high-level data types
   while processing URIs.  When used in names, they may indicate an
   application context, or facilitate a default mapping and a recovery
   of names from addresses.  They characterize its type, when used in
   addresses.

   Compliant to the URI concept, namespace-schemes can be added.
   Examples of schemes are generic or inherited from applications.

3.4.1.  Generic Namespaces

   IP This namespace is comprised of regular IP node naming, i.e., DNS
      names and addresses taken from any version of the Internet
      Protocol.  A processor dealing with the IP namespace is required
      to determine the syntax (DNS name, IP address version) of the
      group expression.

   SHA-2  This namespace carries address strings compliant to SHA-2 hash
      digests.  A processor handling those strings is required to
      determine the length of the group expression and passes
      appropriate values directly to a corresponding overlay.

   Opaque  This namespace transparently carries strings without further
      syntactical information, meanings, or associated resolution
      mechanism.

3.4.2.  Application-centric Namespaces

   SIP  The SIP namespace is an example of an application-layer scheme
      that bears inherent group functions (conferencing).  SIP
      conference URIs may be directly exchanged and interpreted at the
      application, and mapped to group addresses on the system level to
      generate a corresponding multicast group.





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   RELOAD  This namespace covers address strings immediately valid in a
      RELOAD [I-D.ietf-p2psip-base] overlay network.  A processor
      handling those strings may pass these values directly to a
      corresponding overlay.

3.5.  Name-to-Address Mapping

   The multicast communication paradigm requires all group members to
   subscribe to the same Group Name, taken from a common Multicast
   Namespace, and thereby to identify the group in a technology-agnostic
   way.  Following this common API, a sender correspondingly registers a
   Group Name prior to transmission.

   At communication end points, Group Names require a mapping to Group
   Addresses prior to service instantiation at its Interface(s).
   Similarly, a mapping is needed at gateways to translate between Group
   Addresses from different namespaces consistently.  Two requirements
   need to be met by a mapping function that translates between
   Multicast Names and Addresses.

   a.  For a given Group Name, identify an Address that is appropriate
       for a local distribution instance.

   b.  For a given Group Address, invert the mapping to recover the
       Group Name.

   In general, mapping can be complex and need not be invertible.  A
   mapping can be realized by embedding smaller in larger namespaces or
   selecting an arbitrary, unused ID in a smaller target namespace.  For
   example, it is not obvious how to map a large identifier space (e.g.,
   IPv6) to a smaller, collision-prone set like IPv4 (see
   [I-D.venaas-behave-v4v6mc-framework][I-D.venaas-behave-mcast46],
   [RFC6219] ).  Mapping functions can be stateless in some contexts,
   but may require states in others.  The application of such functions
   depends on the cardinality of the namespaces, the structure of
   address spaces, and possible address collisions.  However, some
   namespaces facilitate a canonical, invertible transformation to
   default address spaces.

3.5.1.  Canonical Mapping

   Some Multicast Namespaces defined in Section 3.4 can express a
   canonical default mapping.  For example, ip://224.1.2.3:5000
   indicates the correspondence to 224.1.2.3 in the default IPv4
   multicast address space at port 5000.  This default mapping is bound
   to a technology and may not always be applicable, e.g., in the case
   of address collisions.  Note that under canonical mapping, the
   multicast URI can be completely recovered from any data message



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   received from this group.

3.5.2.  Mapping at End Points

   Multicast listeners or senders require a Name-to-Address conversion
   for all technologies they actively run in a group.  Even though a
   mapping applies to the local Multicast Domain only, end points may
   need to learn a valid Group Address from neighboring nodes, e.g.,
   from a gateway in the collision-prone IPv4 domain.  Once set, an end
   point will always be aware of the Name-to-Address correspondence and
   thus can autonomously invert the mapping.

3.5.3.  Mapping at Inter-domain Multicast Gateways

   Multicast data may arrive at an IMG in one technology, requesting the
   gateway to re-address packets for another distribution system.  At
   initial arrival, the IMG may not have explicit knowledge of the
   corresponding Multicast Group Name.  To perform a consistent mapping,
   the group name needs to be acquired.  It may have been distributed at
   source registration, or may have been learned from a neighboring
   node, details of which are beyond the scope of this document.

3.6.  A Note on Explicit Multicast (XCAST)

   In Explicit Multicast (XCAST) [RFC5058], the multicast source
   explicitly pre-defines the receivers.  From a conceptual perspective,
   XCAST is an additional distribution technology (i.e., a new
   technology-specific interface) for this API.  The instantiation part
   of the Group Name may refer to multiple receivers.  However, from an
   implementation perspective, this specification defines the syntax of
   the Group Name according to [RFC3986], which does not support a set
   of (sub-)identifiers.  Extending [RFC3986] is out of scope of this
   document.

   Implementing XCAST without a set representation in the Group Name
   requires a topology-dependent mapping of the Name to a set of
   subscribers.  Defining these details is out of scope of this
   document.

3.7.  MTU Handling

   This API considers a multi-technology scenario, in which different
   technologies may have different Maximum Transmission Unit (MTU)
   sizes.  Even if the MTU size between two hosts has been determined,
   it may change over time either initiated by the network (e.g., path
   changes) or by the end hosts (e.g., interface change due to
   mobility).




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   The design of this API is based on the objective of robust
   communication and easy application development.  The MTU handling and
   the placement of fragmentation is thus guided by the following
   observations.

   Application  The application programmer needs a simple way to send
      packets in a technology-agnostic fashion.  Therefor it must be
      clear at the time of coding what maximum amount of data can be
      sent per socket in one message.  A regular program flow should not
      be distracted by changing MTU sizes.  Technically, the
      configuration of the maximum message size used by the application
      programmer may change and disrupt communication, when (a)
      interfaces will be added or excluded, or (b) the path MTU changes
      during transmission and thus disables the corresponding
      interfaces.

   Middleware  A middleware situated between application and technology
      interfaces ensures a general ability of packet handling, which
      prevents the application programmer to implement fragmentation.  A
      maximum message size guaranteed by the group communication stack
      (e.g., middleware) is not allowed to change during runtime, as
      this would conflict with technology-agnostic development.

   Technology Interfaces  Fragmentation depends on the technology in
      use.  The (technology-bound) interfaces, thus, need to deal with
      MTU sizes that may vary among interfaces and along different
      paths.

   The concept of this API aims at guaranteeing a maximum message size
   for the application programmer, thereby to handle fragmentation at
   the interface level, if needed.  Nevertheless, the application
   programmer should be able to determine the technology-specific atomic
   message size to optimize data distribution or for other reasons.

   The maximum message size should take realistic values (e.g.,
   following IP clients) to enable smooth and efficient services.  A
   detailed selection scheme of MTU values is out of scope of this
   document.


4.  Common Multicast API

4.1.  Notation

   The following description of the common multicast API is described in
   pseudo syntax.  Variables that are passed to function calls are
   declared by "in", return values are declared by "out".  A list of
   elements is denoted by <>.  The pseudo syntax assumes that lists



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   include an attribute which represents the number of elements.

   The corresponding C signatures are defined in Appendix A.

4.2.  Abstract Data Types

4.2.1.  Multicast URI

   Multicast Names and Multicast Addresses used in this API follow an
   URI scheme that defines a subset of the generic URI specified in
   [RFC3986] and is syntactically compliant with the guidelines given in
   [RFC4395].

   The multicast URI is defined as follows:

      scheme "://" group "@" instantiation ":" port "/" sec-credentials

   The parts of the URI are defined as follows:

   scheme  refers to the specification of the assigned identifier
      [RFC3986] which takes the role of the Multicast Namespace.

   group  identifies the group uniquely within the Namespace given in
      scheme.

   instantiation  identifies the entity that generates the instance of
      the group (e.g., a SIP domain or a source in SSM, a dedicated
      routing entity or a named processor that accounts for the group
      communication), using syntax and semantic as defined by the
      Namespace given in scheme.  This parameter is optional.  Note that
      ambiguities (e.g., identical node addresses in multiple overlay
      instances) can be distinguished by ports.

   port  identifies a specific application at an instance of a group.
      This parameter is optional.

   sec-credentials  used to implement optional security credentials
      (e.g., to authorize a multicast group access).  Note that security
      credentials may carry a distinct technical meaning w.r.t.  AAA
      schemes and may differ between group members.  Hence the sec-
      credentials are not considered part of the Group Name.

4.2.2.  Interface

   The Interface denotes the layer and instance on which the
   corresponding call will be effective.  In agreement with [RFC3493],
   we identify an Interface by an identifier, which is a positive
   integer starting at 1.



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   Properties of an Interface are stored in the following data
   structure:

       struct if_prop {
         unsigned int if_index; /* 1, 2, ... */
         char        *if_name;  /* "eth0", "eth1:1", "lo", ... */
         char        *if_addr;  /* "1.2.3.4", "abc123", ... */
         char        *if_tech;  /* "ip", "overlay", ... */
       };

   The following function retrieves all available Interfaces from the
   system:

       getInterfaces(out Interface <ifs>);

   It extends the functions for Interface Identification defined in
   Section 4 of [RFC3493] and can be implemented by:

       struct if_prop *if_prop(void);

4.2.3.  Membership Events

   A membership event is triggered by a multicast state change, which is
   observed by the current node.  It is related to a specific Group Name
   and may be receiver or source oriented.

       event_type {
               join_event;
               leave_event;
               new_source_event;
       };

       event {
              event_type event;
              Uri group_name;
              Interface if;
       };

   An event will be created by the group communication stack and passed
   to applications that have registered for events.

4.3.  Group Management Calls

4.3.1.  Create

   The create call initiates a multicast socket and provides the
   application programmer with a corresponding handle.  If no Interfaces
   will be assigned based on the call, the default Interface will be



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   selected and associated with the socket.  The call returns an error
   code in the case of failures, e.g., due to a non-operational
   communication middleware.

       createMSocket(in Interface <ifs>,
                     out Socket s);

   The ifs argument denotes a list of Interfaces (if_indexes) that will
   be associated with the multicast socket.  This parameter is optional.

   On success a multicast socket identifier is returned, otherwise NULL.

4.3.2.  Delete

   The delete call removes the multicast socket.

       deleteMSocket(in Socket s, out Int error);

   The s argument identifies the multicast socket for destruction.

   On success the out parameter error is 0, otherwise -1.

4.3.3.  Join

   The join call initiates a subscription for the given Group Name.
   Depending on the Interfaces that are associated with the socket, this
   may result in an IGMP/MLD report or overlay subscription, for
   example.

       join(in Socket s, in Uri groupName, out Int error);

   The s argument identifies the multicast socket.

   The groupName argument identifies the group.

   On success the out parameter error is 0, otherwise -1.

4.3.4.  Leave

   The leave call results in an unsubscription for the given Group Name.

       leave(in Socket s, in Uri groupName, out Int error);

   The s argument identifies the multicast socket.

   The groupName identifies the group.

   On success the out parameter error is 0, otherwise -1.



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4.3.5.  Source Register

   The srcRegister call registers a source for a Group on all active
   Interfaces of the socket s.  This call may assist group distribution
   in some technologies, for example the creation of sub-overlays or may
   facilitate a name-to-address mapping.  Likewise, it may remain
   without effect in some multicast technologies.

       srcRegister(in Socket s, in Uri groupName,
                   out Interface <ifs>, out Int error);

   The s argument identifies the multicast socket.

   The groupName argument identifies the multicast group to which a
   source intends to send data.

   The ifs argument points to the list of Interface indexes for which
   the source registration failed.  A NULL pointer is returned, if the
   list is empty.  This parameter is optional.

   If source registration succeeded for all Interfaces associated with
   the socket, the out parameter error is 0, otherwise -1.

4.3.6.  Source Deregister

   The srcDeregister indicates that a source does no longer intend to
   send data to the multicast group.  This call may remain without
   effect in some multicast technologies.

       srcDeregister(in Socket s, in Uri groupName,
                     out Interface <ifs>, out Int error);

   The s argument identifies the multicast socket.

   The group_name argument identifies the multicast group to which a
   source has stopped to send multicast data.

   The ifs argument points to the list of Interfaces for which the
   source deregistration failed.  A NULL pointer is returned, if the
   list is empty.

   If source deregistration succeeded for all Interfaces associated with
   the socket, the out parameter error is 0, otherwise -1.

4.4.  Send and Receive Calls






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4.4.1.  Send

   The send call passes multicast data destined for a Multicast Name
   from the application to the multicast socket.

   It is worth noting that it is the choice of the programmer to send
   data via one socket per group or to use a single socket for multiple
   groups.

       send(in Socket s, in Uri groupName,
            in Size msgLen, in Msg msgBuf,
            out Int error);

   The s argument identifies the multicast socket.

   The groupName argument identifies the group to which data will be
   sent.

   The msgLen argument holds the length of the message to be sent.

   The msgBuf argument passes the multicast data to the multicast
   socket.

   On success the out parameter error is 0, otherwise -1.  A message
   that is too long is indicated by an implementation-specific error
   code (e.g., EMSGSIZE in C).

4.4.2.  Receive

   The receive call passes multicast data and the corresponding Group
   Name to the application.  This may come in a blocking or non-blocking
   variant.

   It is worth noting that it is the choice of the programmer to receive
   data via one socket per group or to use a single socket for multiple
   groups.

       receive(in Socket s, out Uri groupName,
               out Size msgLen, out Msg msgBuf,
               out Int error);

   The s argument identifies the multicast socket.

   The group_name argument identifies the multicast group for which data
   was received.

   The msgLen argument holds the length of the received message.




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   The msgBuf argument points to the payload of the received multicast
   data.

   On success the out parameter error is 0, otherwise -1.  A message
   that is too long is indicated by an implementation-specific error
   handling (e.g., EMSGSIZE).

4.5.  Socket Options

   The following calls configure an existing multicast socket.

4.5.1.  Get Interfaces

   The getInterface call returns an array of all available multicast
   communication Interfaces associated with the multicast socket.

       getInterfaces(in Socket s,
                     out Interface <ifs>, out Int error);

   The s argument identifies the multicast socket.

   The ifs argument points to an array of Interface index identifiers.

   On success the out parameter error is 0, otherwise -1.

4.5.2.  Add Interface

   The addInterface call adds a distribution channel to the socket.
   This may be an overlay or underlay Interface, e.g., IPv6 or DHT.
   Multiple Interfaces of the same technology may be associated with the
   socket.

       addInterface(in Socket s, in Interface if,
                    out Int error);

   The s and if arguments identify a multicast socket and Interface,
   respectively.

   On success the value 0 is returned, otherwise -1.

4.5.3.  Delete Interface

   The delInterface call removes the Interface if from the multicast
   socket.







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       delInterface(in Socket s, Interface if,
                    out Int error);

   The s and if arguments identify a multicast socket and Interface,
   respectively.

   On success the out parameter error is 0, otherwise -1.

4.5.4.  Set TTL

   The setTTL call configures the maximum hop count for the socket a
   multicast message is allowed to traverse.

       setTTL(in Socket s, in Int h,
              in Interface <ifs>,
              out Int error);

   The s and h arguments identify a multicast socket and the maximum hop
   count, respectively.

   The ifs argument points to an array of Interface index identifiers.
   This parameter is optional.

   On success the out parameter error is 0, otherwise -1.

4.5.5.  Get TTL

   The getTTL call returns the maximum hop count a multicast message is
   allowed to traverse for the socket.

       getTTL(in Socket s,
              out Int h, out Int error);

   The s argument identifies a multicast socket.

   The h argument holds the maximum number of hops associated with
   socket s.

   On success the out parameter error is 0, otherwise -1.

4.5.6.  Atomic Message Size

   The getAtomicMsgSize function returns the maximum message size that
   an application is allowed to transmit per socket at once without
   fragmentation.  This value depends on the interfaces associated with
   the socket in use and thus may change during runtime.





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       getAtomicMsgSize(in Socket s,
                        out Int return);

   On success, the function returns a positive value of appropriate
   message size, otherwise -1.

4.6.  Service Calls

4.6.1.  Group Set

   The groupSet call returns all multicast groups registered at a given
   Interface.  This information can be provided by group management
   states or routing protocols.  The return values distinguish between
   sender and listener states.

       struct GroupSet {
         uri groupName; /* registered multicast group */
         int type;       /* 0 = listener state, 1 = sender state,
                            2 = sender & listener state */
       }

       groupSet(in Interface if,
                out GroupSet <groupSet>, out Int error);

   The if argument identifies the Interface for which states are
   maintained.

   The groupSet argument points to a list of group states.

   On success the out parameter error is 0, otherwise -1.

4.6.2.  Neighbor Set

   The neighborSet function returns the set of neighboring nodes for a
   given Interface as seen by the multicast routing protocol.

       neighborSet(in Interface if,
                   out Uri <neighborsAddresses>, out Int error);

   The if argument identifies the Interface for which neighbors are
   inquired.

   The neighborsAddresses argument points to a list of neighboring nodes
   on a successful return.

   On success the out parameter error is 0, otherwise -1.





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4.6.3.  Children Set

   The childrenSet function returns the set of child nodes that receive
   multicast data from a specified Interface for a given group.  For a
   common multicast router, this call retrieves the multicast forwarding
   information base per Interface.

       childrenSet(in Interface if, in Uri groupName,
                   out Uri <childrenAddresses>, out Int error);

   The if argument identifies the Interface for which children are
   inquired.

   The groupName argument defines the multicast group for which
   distribution is considered.

   The childrenAddresses argument points to a list of neighboring nodes
   on a successful return.

   On success the out parameter error is 0, otherwise -1.

4.6.4.  Parent Set

   The parentSet function returns the set of neighbors from which the
   current node receives multicast data at a given Interface for the
   specified group.

       parentSet(in Interface if, in Uri groupName,
                 out Uri <parentsAddresses>, out Int error);

   The if argument identifies the Interface for which parents are
   inquired.

   The groupName argument defines the multicast group for which
   distribution is considered.

   The parentsAddresses argument points to a list of neighboring nodes
   on a successful return.

   On success the out parameter error is 0, otherwise -1.

4.6.5.  Designated Host

   The designatedHost function inquires whether this host has the role
   of a designated forwarder resp. querier, or not.  Such an information
   is provided by almost all multicast protocols to prevent packet
   duplication, if multiple multicast instances serve on the same
   subnet.



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       designatedHost(in Interface if, in Uri groupName
                      out Int return);

   The if argument identifies the Interface for which designated
   forwarding is inquired.

   The groupName argument specifies the group for which the host may
   attain the role of designated forwarder.

   The function returns 1 if the host is a designated forwarder or
   querier, otherwise 0.  The return value -1 indicates an error.

4.6.6.  Enable Membership Events

   The enableEvents function registers an application at the group
   communication stack to receive information about group changes.
   State changes are the result of new receiver subscriptions or leaves
   as well as of source changes.  Upon receiving an event, the group
   service may obtain additional information from further service calls.

       enableEvents();

   Calling this function, the stack starts to pass membership events to
   the application.  Each event includes an event type identifier and a
   Group Name (cf., Section 4.2.3).

   The multicast protocol has not to support membership tracking to
   enable this feature.  This function can also be implemented at the
   middelware layer.

4.6.7.  Disable Membership Events

   The disableEvents function deactivates the information about group
   state changes.

       disableEvents();

   On success the stack will not pass membership events to the
   application.

4.6.8.  Maximum Message Size

   The getMaxMsgSize function returns the maximum message size that an
   application is allowed to transmit per socket at once.  This value is
   statically guaranteed by the group communication stack.






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       getMaxMsgSize(out Int return);

   On success, the function returns a positive value of allowed message
   size, otherwise -1.


5.  Implementation

   A reference implementation of the Common API for Transparent Hybrid
   Multicast is available with the HAMcast stack [hamcast-dev],
   [GC2010], [LCN2012].  This open-source software supports the
   multicast API (C++ and Java library) for group application
   development, the middleware as a userspace system service, and
   several multicast-technology modules.  The middleware is implemented
   in C++.

   This API is verified and adjusted based on the real-world experiences
   gathered in the HAMcast project, and by additional users of the
   stack.


6.  IANA Considerations

   This document makes no request of IANA.


7.  Security Considerations

   This draft does neither introduce additional messages nor novel
   protocol operations.


8.  Acknowledgements

   We would like to thank the HAMcast-team, Nora Berg, Dominik
   Charousset, Gabriel Hege, Fabian Holler, Alexander Knauf, Sebastian
   Meiling, Sebastian Woelke, and Sebastian Zagaria, at the HAW Hamburg
   for many fruitful discussions and for their continuous critical
   feedback while implementing the common multicast API and a hybrid
   multicast middleware.  We gratefully acknowledge WeeSan, Mario
   Kolberg, and John Buford for reviewing and their suggestions to
   improve the document.  We would like to thank the Name-based socket
   BoF (in particular Dave Thaler) for clarifying insights into the
   question of meta function calls.

   This work is partially supported by the German Federal Ministry of
   Education and Research within the HAMcast project (see
   http://hamcast.realmv6.org), which is part of G-Lab.



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9.  Informative References

   [GC2010]   Meiling, S., Charousset, D., Schmidt, T., and M.
              Waehlisch, "System-assisted Service Evolution for a Future
              Internet - The HAMcast Approach to Pervasive Multicast",
              Proc. of IEEE GLOBECOM 2010 Workshops. MCS 2010, pp. 938-
              942, Piscataway, NJ, USA: IEEE Press, December 2010.

   [I-D.ietf-mboned-auto-multicast]
              Bumgardner, G., "Automatic Multicast Tunneling",
              draft-ietf-mboned-auto-multicast-14 (work in progress),
              June 2012.

   [I-D.ietf-p2psip-base]
              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
              Base Protocol", draft-ietf-p2psip-base-22 (work in
              progress), July 2012.

   [I-D.venaas-behave-mcast46]
              Venaas, S., Asaeda, H., SUZUKI, S., and T. Fujisaki, "An
              IPv4 - IPv6 multicast translator",
              draft-venaas-behave-mcast46-02 (work in progress),
              December 2010.

   [I-D.venaas-behave-v4v6mc-framework]
              Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
              Multicast Translation",
              draft-venaas-behave-v4v6mc-framework-03 (work in
              progress), June 2011.

   [LCN2012]  Meiling, S., Schmidt, T., and M. Waehlisch, "Large-Scale
              Measurement and Analysis of One-Way Delay in Hybrid
              Multicast Networks", Proc. of 37th Annual IEEE Conference
              on Local Computer Networks (LCN 2012). Piscataway, NJ,
              USA: IEEE Press, October 2012.

   [RFC1075]  Waitzman, D., Partridge, C., and S. Deering, "Distance
              Vector Multicast Routing Protocol", RFC 1075,
              November 1988.

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

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              October 1999.




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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3678]  Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
              Extensions for Multicast Source Filters", RFC 3678,
              January 2004.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4604]  Holbrook, H., Cain, B., and B. Haberman, "Using Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Protocol Version 2 (MLDv2) for Source-
              Specific Multicast", RFC 4604, August 2006.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, October 2007.

   [RFC5058]  Boivie, R., Feldman, N., Imai, Y., Livens, W., and D.
              Ooms, "Explicit Multicast (Xcast) Concepts and Options",
              RFC 5058, November 2007.

   [RFC5757]  Schmidt, T., Waehlisch, M., and G. Fairhurst, "Multicast
              Mobility in Mobile IP Version 6 (MIPv6): Problem Statement
              and Brief Survey", RFC 5757, February 2010.



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   [RFC6219]  Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, "The
              China Education and Research Network (CERNET) IVI
              Translation Design and Deployment for the IPv4/IPv6
              Coexistence and Transition", RFC 6219, May 2011.

   [hamcast-dev]
              "HAMcast developers",
              <http://hamcast.realmv6.org/developers>.


Appendix A.  C Signatures

   This section describes the C signatures of the common multicast API,
   which are defined in Section 4.

       int createMSocket(int* result, size_t num_ifs, const uint32_t* ifs);

       int deleteMSocket(int s);

       int join(int msock, const char* group_uri);

       int leave(int msock, const char* group_uri);

       int srcRegister(int msock,
                       const char* group_uri,
                       size_t num_ifs,
                       const uint32_t *ifs);

       int srcDeregister(int msock,
                         const char* group_uri,
                         size_t num_ifs,
                         const uint32_t *ifs);

       int send(int msock,
                const char* group_uri,
                size_t buf_len,
                const void* buf);

       int receive(int msock,
                   const char* group_uri,
                   size_t buf_len,
                   void* buf);









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       int getInterfaces(int msock,
                         size_t* num_ifs,
                         uint32_t** ifs);

       int addInterface(int msock, uint32_t iface);

       int delInterface(int msock, uint32_t iface);

       int setTTL(int msock, uint8_t value,
                  size_t num_ifs, uint32_t* ifs);

       int getTTL(int msock, uint8_t* result);

       int getAtomicMsgSize(int msock);

       typedef struct {
           char* group_uri; /* registered mcast group */
           int type; /* 0: listener state,
                        1: sender state
                        2: sender and listener state */
       }
       GroupSet;

       int groupSet(uint32_t iface,
                    size_t* num_groups,
                    GroupSet** groups);

       int neighborSet(uint32_t iface,
                       const char* group_name,
                       size_t* num_neighbors,
                       char** neighbor_uris);

       int childrenSet(uint32_t iface,
                       const char* group_name,
                       size_t* num_children,
                       char** children_uris);

       int parentSet(uint32_t iface,
                     const char* group_name,
                     size_t* num_parents,
                     char** parents_uris);










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       int designatedHost(uint32_t iface,
                          const char* group_name);

       typedef void (*MembershipEventCallback)(int,          /* event type   */
                                                uint32_t,     /* interface id */
                                                const char*); /* group uri    */

       int registerEventCallback(MembershipEventCallback callback);

       int disableEvents();

          int getMaxMsgSize();


Appendix B.  Practical Example of the API




































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     -- Application above middleware:

     //Initialize multicast socket;
     //the middleware selects all available interfaces
     MulticastSocket m = new MulticastSocket();

     m.join(URI("ip://224.1.2.3:5000"));
     m.join(URI("ip://[FF02:0:0:0:0:0:0:3]:6000"));
     m.join(URI("sip://news@cnn.com"));

     -- Middleware:

     join(URI mcAddress) {
       //Select interfaces in use
       for all this.interfaces {
         switch (interface.type) {
           case "ipv6":
             //... map logical ID to routing address
             Inet6Address rtAddressIPv6 = new Inet6Address();
             mapNametoAddress(mcAddress,rtAddressIPv6);
             interface.join(rtAddressIPv6);
           case "ipv4":
             //... map logical ID to routing address
             Inet4Address rtAddressIPv4 = new Inet4Address();
             mapNametoAddress(mcAddress,rtAddressIPv4);
             interface.join(rtAddressIPv4);
           case "sip-session":
             //... map logical ID to routing address
             SIPAddress rtAddressSIP = new SIPAddress();
             mapNametoAddress(mcAddress,rtAddressSIP);
             interface.join(rtAddressSIP);
           case "dht":
             //... map logical ID to routing address
             DHTAddress rtAddressDHT = new DHTAddress();
             mapNametoAddress(mcAddress,rtAddressDHT);
             interface.join(rtAddressDHT);
            //...
         }
       }
     }



Appendix C.  Deployment Use Cases for Hybrid Multicast

   This section describes the application of the defined API to
   implement an IMG.




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C.1.  DVMRP

   The following procedure describes a transparent mapping of a DVMRP-
   based any source multicast service to another many-to-many multicast
   technology, e.g., an overlay.

   An arbitrary DVMRP [RFC1075] router will not be informed about new
   receivers, but will learn about new sources immediately.  The concept
   of DVMRP does not provide any central multicast instance.  Thus, the
   IMG can be placed anywhere inside the multicast region, but requires
   a DVMRP neighbor connectivity.  Thus the group communication stack
   used by the IMG is enhanced by a DVMRP implementation.  New sources
   in the underlay will be advertised based on the DVMRP flooding
   mechanism and received by the IMG.  Based on this, the event
   "new_source_event" is created and passed to the application.  The
   relay agent initiates a corresponding join in the native network and
   forwards the received source data towards the overlay routing
   protocol.  Depending on the group states, the data will be
   distributed to overlay peers.

   DVMRP establishes source specific multicast trees.  Therefore, a
   graft message is only visible to DVMRP routers on the path from the
   new receiver subnet to the source, but in general not to an IMG.  To
   overcome this problem, data of multicast senders in the overlay may
   become noticable via the Source Register call, as well as by an IMG
   that initiates an an all-group join in the overlay using the
   namespace extension of the API.  Each IMG is initially required to
   forward the data received in the overlay to the underlay, independent
   of native multicast receivers.  Subsequent prunes may limit unwanted
   data distribution thereafter.

C.2.  PIM-SM

   The following procedure describes a transparent mapping of a PIM-SM-
   based any source multicast service to another many-to-many multicast
   technology, e.g., an overlay.

   The Protocol Independent Multicast Sparse Mode (PIM-SM) [RFC4601]
   establishes rendezvous points (RP).  These entities receive listener
   subscriptions and source registering of a domain.  For a continuous
   update an IMG has to be co-located with an RP.  Whenever PIM register
   messages are received, the IMG must signal internally a new multicast
   source using the event "new_source_event".  Subsequently, the IMG
   joins the group and a shared tree between the RP and the sources will
   be established, which may change to a source specific tree after PIM
   switches to phase three.  Source traffic will be forwarded to the RP
   based on the IMG join, even if there are no further receivers in the
   native multicast domain.  Designated routers of a PIM-domain send



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   receiver subscriptions towards the PIM-SM RP.  The reception of such
   messages initiates the event "join_event" at the IMG, which initiates
   a join towards the overlay routing protocol.  Overlay multicast data
   arriving at the IMG will then transparently be forwarded in the
   underlay network and distributed through the RP instance.

C.3.  PIM-SSM

   The following procedure describes a transparent mapping of a PIM-SSM-
   based source specific multicast service to another one-to-many
   multicast technology, e.g., an overlay.

   PIM Source Specific Multicast (PIM-SSM) is defined as part of PIM-SM
   and admits source specific joins (S,G) according to the source
   specific host group model [RFC4604].  A multicast distribution tree
   can be established without the assistance of a rendezvous point.

   Sources are not advertised within a PIM-SSM domain.  Consequently, an
   IMG cannot anticipate the local join inside a sender domain and
   deliver a priori the multicast data to the overlay instance.  If an
   IMG of a receiver domain initiates a group subscription via the
   overlay routing protocol, relaying multicast data fails, as data is
   not available at the overlay instance.  The IMG instance of the
   receiver domain, thus, has to locate the IMG instance of the source
   domain to trigger the corresponding join.  In agreement with the
   objectives of PIM-SSM, the signaling should not be flooded in
   underlay and overlay.

   A solution can be to intercept the subscription at both, source and
   receiver sites: To monitor multicast receiver subscriptions
   ("join_event" or "leave_event") in the underlay, the IMG is placed on
   path towards the source, e.g., at a domain border router.  This
   router intercepts join messages and extracts the unicast source
   address S, initializing an IMG specific join to S via regular
   unicast.  Multicast data arriving at the IMG of the sender domain can
   be distributed via the overlay.  Discovering the IMG of a multicast
   sender domain may be implemented analogously to AMT
   [I-D.ietf-mboned-auto-multicast] by anycast.  Consequently, the
   source address S of the group (S,G) should be built based on an
   anycast prefix.  The corresponding IMG anycast address for a source
   domain is then derived from the prefix of S.

C.4.  BIDIR-PIM

   The following procedure describes a transparent mapping of a BIDIR-
   PIM-based any source multicast service to another many-to-many
   multicast technology, e.g., an overlay.




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   Bidirectional PIM [RFC5015] is a variant of PIM-SM.  In contrast to
   PIM-SM, the protocol pre-establishes bidirectional shared trees per
   group, connecting multicast sources and receivers.  The rendezvous
   points are virtualized in BIDIR-PIM as an address to identify on-tree
   directions (up and down).  Routers with the best link towards the
   (virtualized) rendezvous point address are selected as designated
   forwarders for a link-local domain and represent the actual
   distribution tree.  The IMG is to be placed at the RP-link, where the
   rendezvous point address is located.  As source data in either cases
   will be transmitted to the rendezvous point link, the BIDIR-PIM
   instance of the IMG receives the data and can internally signal new
   senders towards the stack via the "new_source_event".  The first
   receiver subscription for a new group within a BIDIR-PIM domain needs
   to be transmitted to the RP to establish the first branching point.
   Using the "join_event", an IMG will thereby be informed about group
   requests from its domain, which are then delegated to the overlay.


Appendix D.  Change Log

   The following changes have been made from
   draft-irtf-samrg-common-api-05

   1.  Added preparations for IRSG review

   2.  Fixed error codes

   3.  Editorial improvements

   4.  Updated references

   The following changes have been made from
   draft-irtf-samrg-common-api-04

   1.  Added section "A Note on Explicit Multicast (XCAST)"

   2.  Added section "MTU Handling"

   3.  Added socket option getAtomicMSgSize

   4.  Added service call getMaxMsgSize

   The following changes have been made from
   draft-irtf-samrg-common-api-03

   1.  Added section "Illustrative Example"





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   2.  Added section "Implementation"

   3.  Minor clarifications

   The following changes have been made from
   draft-irtf-samrg-common-api-02

   1.  Added use case of multicast flavor support

   2.  Restructured Section 3

   3.  Major update on namespaces and on mapping

   4.  C signatures completed

   5.  Many clarifications and editorial improvements

   The following changes have been made from
   draft-irtf-samrg-common-api-01

   1.  Pseudo syntax for lists objects changed

   2.  Editorial improvements

   The following changes have been made from
   draft-irtf-samrg-common-api-00

   1.  Incorrect pseudo code syntax fixed

   2.  Minor editorial improvements

   The following changes have been made from
   draft-waehlisch-sam-common-api-06

   1.  no changes; draft adopted as WG document (previous
       draft-waehlisch-sam-common-api-06, now
       draft-irtf-samrg-common-api-00)

   The following changes have been made from
   draft-waehlisch-sam-common-api-05

   1.  Description of the Common API using pseudo syntax added

   2.  C signatures of the Comon API moved to appendix

   3.  updateSender() and updateListener() calls replaced by events





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   4.  Function destroyMSocket renamed as deleteMSocket.

   The following changes have been made from
   draft-waehlisch-sam-common-api-04

   1.  updateSender() added.

   The following changes have been made from
   draft-waehlisch-sam-common-api-03

   1.  Use cases added for illustration.

   2.  Service calls added for inquiring on the multicast distribution
       system.

   3.  Namespace examples added.

   4.  Clarifications and editorial improvements.

   The following changes have been made from
   draft-waehlisch-sam-common-api-02

   1.  Rename init() in createMSocket().

   2.  Added calls srcRegister()/srcDeregister().

   3.  Rephrased API calls in C-style.

   4.  Cleanup code in "Practical Example of the API".

   5.  Partial reorganization of the document.

   6.  Many editorial improvements.

   The following changes have been made from
   draft-waehlisch-sam-common-api-01

   1.  Document restructured to clarify the realm of document overview
       and specific contributions s.a. naming and addressing.

   2.  A clear separation of naming and addressing was drawn.  Multicast
       URIs have been introduced.

   3.  Clarified and adapted the API calls.

   4.  Introduced Socket Option calls.





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   5.  Deployment use cases moved to an appendix.

   6.  Simple programming example added.

   7.  Many editorial improvements.


Authors' Addresses

   Matthias Waehlisch
   link-lab & FU Berlin
   Hoenower Str. 35
   Berlin  10318
   Germany

   Email: mw@link-lab.net
   URI:   http://www.inf.fu-berlin.de/~waehl


   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   Hamburg  20099
   Germany

   Email: schmidt@informatik.haw-hamburg.de
   URI:   http://inet.cpt.haw-hamburg.de/members/schmidt


   Stig Venaas
   cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: stig@cisco.com















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