RFC7046: A Common API for Transparent Hybrid Multicast
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Related keywords:
(adaptive multicast)
(multicast naming)
(multicast addressing)
(Peer-to-Peer (P2P))
Internet Research Task Force (IRTF) M. Waehlisch
Request for Comments: 7046 link-lab & FU Berlin
Category: Experimental T. Schmidt
ISSN: 2070-1721 HAW Hamburg
S. Venaas
Cisco Systems
December 2013
A Common API for Transparent Hybrid Multicast
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 that grants
access to the different flavors of multicast. It proposes an
abstract naming scheme that uses multicast URIs, and it discusses
mapping mechanisms between different namespaces and distribution
technologies. Additionally, this document describes the application
of this API for building gateways that interconnect current Multicast
Domains throughout the Internet. It reports on an implementation of
the programming Interface, including service middleware. This
document is a product of the Scalable Adaptive Multicast (SAM)
Research Group.
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Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Research Task
Force (IRTF). The IRTF publishes the results of Internet-related
research and development activities. These results might not be
suitable for deployment. This RFC represents the consensus of the
Scalable Adaptive Multicast Research Group of the Internet Research
Task Force (IRTF). Documents approved for publication by the IRSG
are not a candidate for any level of Internet Standard; see Section 2
of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7046.
Copyright Notice
Copyright (c) 2013 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.
Table of Contents
1. Introduction ....................................................4
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 .......................................................10
3.1. Objectives and Reference Scenarios ........................10
3.2. Group Communication API and Protocol Stack ................12
3.3. Naming and Addressing .....................................14
3.4. Namespaces ................................................15
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3.5. Name-to-Address Mapping ...................................15
3.5.1. Canonical Mapping ..................................16
3.5.2. Mapping at End Points ..............................16
3.5.3. Mapping at Inter-Domain Multicast Gateways .........16
3.6. A Note on Explicit Multicast (Xcast) ......................16
3.7. MTU Handling ..............................................17
4. Common Multicast API ...........................................18
4.1. Notation ..................................................18
4.2. URI Scheme Definition .....................................18
4.2.1. Syntax .............................................18
4.2.2. Semantic ...........................................19
4.2.3. Generic Namespaces .................................20
4.2.4. Application-Centric Namespaces .....................20
4.2.5. Future Namespaces ..................................20
4.3. Additional Abstract Data Types ............................21
4.3.1. Interface ..........................................21
4.3.2. Membership Events ..................................21
4.4. Group Management Calls ....................................22
4.4.1. Create .............................................22
4.4.2. Delete .............................................22
4.4.3. Join ...............................................22
4.4.4. Leave ..............................................23
4.4.5. Source Register ....................................23
4.4.6. Source Deregister ..................................23
4.5. Send and Receive Calls ....................................24
4.5.1. Send ...............................................24
4.5.2. Receive ............................................24
4.6. Socket Options ............................................25
4.6.1. Get Interfaces .....................................25
4.6.2. Add Interface ......................................25
4.6.3. Delete Interface ...................................26
4.6.4. Set TTL ............................................26
4.6.5. Get TTL ............................................26
4.6.6. Atomic Message Size ................................27
4.7. Service Calls .............................................27
4.7.1. Group Set ..........................................27
4.7.2. Neighbor Set .......................................28
4.7.3. Children Set .......................................28
4.7.4. Parent Set .........................................28
4.7.5. Designated Host ....................................29
4.7.6. Enable Membership Events ...........................29
4.7.7. Disable Membership Events ..........................30
4.7.8. Maximum Message Size ...............................30
5. Implementation .................................................30
6. IANA Considerations ............................................30
7. Security Considerations ........................................31
8. Acknowledgements ...............................................31
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9. References .....................................................32
9.1. Normative References ......................................32
9.2. Informative References ....................................33
Appendix A. C Signatures ..........................................35
Appendix B. Use Case for the API ..................................37
Appendix C. Deployment Use Cases for Hybrid Multicast .............38
C.1. DVMRP ......................................................38
C.2. PIM-SM .....................................................38
C.3. PIM-SSM ....................................................39
C.4. BIDIR-PIM ..................................................40
1. Introduction
Currently, group application programmers need to choose 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 the use of DNS for Group Addresses been established. The
standard multicast socket options [RFC3493] [RFC3678] are bound to an
IP version by not distinguishing between the naming and addressing of
multicast identifiers. Group communication, however,
o is commonly implemented in different flavors, such as any-source
multicast (ASM) vs. source-specific multicast (SSM),
o is commonly implemented on different layers (e.g., IP vs.
application-layer multicast), and
o may be based on different technologies on the same tier, as seen
with IPv4 vs. IPv6.
The objective of this document is to provide for programmers a
universal access to group services.
Multicast application development should be decoupled from
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 of 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 for this document.
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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 flexible namespace support in group addressing and
thereby separate naming and addressing (or routing) schemes from
the application design. This abstraction not only decouples
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 they can also be 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 usage of
this API at gateways operating between current multicast instances
throughout the Internet is described as well. Finally, a report on
an implementation of the programming Interface, including service
middleware, is presented.
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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 experts in the area.
1.1. Use Cases for the Common API
The following generic use cases can be identified; these use cases
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. Thus, for a given application, they can develop a
program that will run in every deployment scenario. The use of
Group Names in the form of abstract metadata 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. Complexity is thereby
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.
Global Identification of Groups: Groups can be identified
independent of technological instantiations and beyond deployment
domains. Taking advantage of the abstract naming, an application
can thus match data received from different Interface technologies
(e.g., IPv4, IPv6, and 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.
Uniform Access to Multicast Flavors: The URI naming scheme uniformly
supports different flavors of group communication, such as
any-source multicast and source-specific multicast, and selective
broadcast, independent of their service instantiation. The
traditional SSM model, for instance, can experience manifold
support by directly mapping the multicast URI (i.e.,
"group@instantiation") to an (S,G) state on the IP layer, by first
resolving S for a subsequent Group Address query, 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.
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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 the
system level. 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 flow of multicast
content (e.g., a movie). Receivers are connected via IPv4/v6 and
Overlay Multicast (OM), respectively.
+-------+ +-------+ +-------+
| S | | R1 | | R3 |
+-------+ +-------+ +-------+
v6| v4| |v4 |OM
| | / |
| ***| *** ***/ ** *** /*** *** ***
\* |* ** /** * * /* ** ** *
*\ \_______/_______*__v4__+-------+ * / *
*\ IPv4/v6 * | R2 |__OM__ *_/ Overlay Mcast *
* \_________________*__v6__+-------+ * *
* ** ** ** * * ** ** ** *
*** *** *** *** *** *** *** ***
Figure 1: Common Scenario: Source S Sends the Same Multicast Content
via Different Technologies
Using the current BSD 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
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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 that implement the receiving and
sending of multicast data, the API provides service calls to grant
access to internal multicast states at the host. The API description
provided in this document defines a minimal set of programming
Interfaces to the system components at the host to operate group
communication. It is left to specific implementations to provide
additional convenience functions for programmers.
The implementation of content distribution for the example shown in
Figure 1 may then look like:
//Initialize multicast socket
MulticastSocket m = new MulticastSocket();
//Associate all available Interfaces
m.addInterface(getInterfaces());
//Subscribe to Multicast Group
m.join(URI("ham:opaque:news@cnn.com"));
//Send to Multicast Group
m.send(URI("ham:opaque:news@cnn.com"),message);
Send/receive example using the common multicast API
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The gateway function for R2 can be implemented by service calls that
look like:
//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("ham:opaque:news@cnn.com")) != NULL) {
m.addInterface(this.interface);
}
}
while(true) {
m.send(URI("ham: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 that 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 from 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. In this case, each
Group Address represents a different level of data quality.
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".
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2. Terminology
This document uses the terminology as defined for the multicast
protocols discussed in [RFC2710], [RFC3376], [RFC3810], [RFC4601],
and [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 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
string.
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, or an overlay routing Interface.
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 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.
3. Overview
3.1. Objectives and Reference Scenarios
The default use case addressed in this document targets 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, such as the selection of an IPTV
channel, or from dynamic session negotiations as used with the
Session Initiation Protocol (SIP) [RFC3261] or Peer-to-Peer SIP
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(P2PSIP) [SIP-RELOAD], 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 can process Group Names in their programs without the
need to consider technological mappings that relate to designated
deployments in target domains;
o applications can identify packets that belong to a logically named
group, independent of the Interface technology used for sending
and receiving packets; this shall also hold true 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 Tech. A *
* *
* ** ** ** *
*** *** *** ***
+-------+ +-------+ |
| Member| | Member| +-------+
| G | | Foo | | IMG |
+-------+ +-------+ +-------+
| | |
*** *** *** *** *** *** *** ***
* ** ** ** * * ** ** ** *
* * +-------+ * *
* Mcast Tech. A * --| IMG |-- * Mcast Tech. 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 abstracts the socket concept and consists
of four parts. Two parts combine the essential communication
functions, while the remaining two offer optional extensions for
enhanced monitoring and management:
Group Management Calls: provide the minimal API to instantiate an
abstract multicast socket and 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;
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.
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Multicast applications that use the common API require assistance
from 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 to / leaves / (de)registers 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 represents 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 and therefore
predefine 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
on 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. The details of service
discovery are out of scope for this document.
The extended multicast functions can be implemented by middleware, as
conceptually presented in Figure 3.
*-------* *-------*
| App 1 | | App 2 |
*-------* *-------*
| |
*---------------------* ---|
| Middleware | |
*---------------------* |
| | |
*---------* | |
| Overlay | | \ Group Communication
*---------* | / Stack
| | |
| | |
*---------------------* |
| Underlay | |
*---------------------* ---|
Figure 3: Architecture of a Group Communication Stack with Middleware
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 symbolically identical, 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 metadata type.
Throughout this document, all Group Names follow a URI notation using
the syntax defined in Section 4.2. Examples are
ham:ip:224.1.2.3:5000 for a canonical IPv4 ASM group at UDP port 5000
and ham:sip:news@cnn.com for 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. The details of automatic namespace identification or
service handling are out of scope for this document.
3.4. Namespaces
Namespace identifiers in URIs are placed in the scheme element and
characterize syntax and semantics 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 may facilitate a default mapping and a
recovery of names from addresses. When used in addresses, they
characterize the group identifier's type.
In compliance with the URI concept, namespace schemes can be added.
Examples of schemes are generic (see Section 4.2.3) or inherited from
applications (see Section 4.2.4).
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 to thereby 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 the Interfaces of the end
points. Similarly, a mapping is needed at gateways to consistently
translate between Group Addresses from different namespaces. 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, mappings can be complex and do not need to be invertible.
A mapping can be realized by embedding smaller namespaces into larger
namespaces or selecting an arbitrary, unused ID in a smaller target
namespace. For example, it is not obvious how to map a large
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identifier space (e.g., IPv6) to a smaller, collision-prone set like
IPv4 (see [MCAST-v4v6-FRAMEWORK], [MCAST-v4v6], and [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, ham: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
received within 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 via one technology and request
that the gateway 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,
the 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 predefines the receivers. From a conceptual perspective,
Xcast is an additional distribution technology (i.e., a new
technology-specific Interface) for this API. Xcast requires
aggregated knowledge of receivers that is available at the origin of
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the distribution tree. The instantiation part of the Group Name may
refer to such a management instance and tree root, which can be the
source or some co-located processor.
An implementation of Xcast then requires a topology-dependent mapping
of the Group Name to the set of subscribers. The defining details of
this multi-destination mapping are out of scope for 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, as initiated by either the network (e.g.,
path changes) or end hosts (e.g., Interface changes due to mobility).
The design of this API is based on the objective of robust
communication and easy application development. MTU handling and the
implementation of fragmentation are thus guided by the following
observations:
Application: Application programmers need a simple way to transmit
packets in a technology-agnostic fashion. For this, it is
convenient at the time of coding to rely on a transparent maximum
amount of data that can be sent in one message from a socket. A
regular program flow should not be distracted by querying and
changing MTU sizes. Technically, the configuration of the maximum
message size used by the application programmer may change and
disrupt communication when (a) Interfaces are added or excluded or
(b) the path MTU changes during transmission and thus disables the
corresponding Interfaces.
Middleware: Middleware situated between application and technology
Interfaces ensures a general packet-handling capability, which in
turn prevents the application programmer from implementing
fragmentation. A uniform maximum message size that cannot be
changed during runtime shall be guaranteed by the group
communication stack (e.g., middleware). Otherwise, this would
conflict with a technology-agnostic application.
Technology Interfaces: Fragmentation requirements depend on the
technology in use. Hence, the (technology-bound) Interfaces need
to cope with MTU sizes that may vary among Interfaces and along
different paths.
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The concept of this API also aims at guaranteeing a maximum message
size for the application programmer, to thereby 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 uniform 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 for this
document.
4. Common Multicast API
4.1. Notation
The following description of the common multicast API is expressed in
pseudo-syntax. Variables that are passed to function calls are
declared by "in", and return values are declared by "out". A list of
elements is denoted by "<>". The pseudo-syntax assumes that lists
include an attribute that represents the number of elements.
The corresponding C signatures are defined in Appendix A.
4.2. URI Scheme Definition
Multicast Names and Multicast Addresses used in this API are
represented by a URI scheme that is specified in the following
subsections. A corresponding ham-URI denotes a multicast channel and
may be dereferenced to retrieve data published to that channel.
4.2.1. Syntax
The syntax of the multicast URI is specified using the Augmented
Backus-Naur Form (ABNF) [RFC5234] and is defined as follows:
ham-URI = ham-scheme ":" namespace ":" group [ "@" instantiation ]
[ ":" port ] [ "/" sec-credentials ]
ham-scheme = "ham" ; hybrid adaptive multicast
namespace = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
group = "*" / 1*unreserved ; unreserved per [RFC3986]
instantiation = 1*unreserved ; unreserved per [RFC3986]
port = 1*DIGIT
sec-credentials = alg ";" val
alg = 1*unreserved ; unreserved per [RFC3986]
val = 1*unreserved ; unreserved per [RFC3986]
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Percent-encoding is applied to distinguish between reserved and
unreserved assignments of the same character in the same ham-URI
component (cf. [RFC3986]).
4.2.2. Semantic
The semantic of the different parts of the URI is defined as follows:
ham-scheme: refers to the specification of the assigned identifier
"ham".
namespace: takes the role of the Multicast Namespace. It defines
the syntax of the group and instantiation part of the ham-URI. A
basic syntax for these elements is specified in Section 4.2.1.
The namespace may further restrict the syntax of designators.
Example namespaces are described in Sections 4.2.3 and 4.2.4.
group: uniquely identifies the group within the Multicast Namespace
given in the namespace. The literal "*" represents all members of
the Multicast Group.
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 semantics as defined by the
namespace. 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 security mechanisms (e.g., to
authorize Multicast Group access or authenticate multicast
operations). This parameter is optional. "alg" represents the
security algorithm in use. "val" represents the actual value for
Authentication, Authorization, and Accounting (AAA). 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.
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4.2.3. 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. The syntax of the group and instantiation follows the
"host" definition in [RFC3986], Section 3.2.2. A processor
dealing with the IP namespace is required to determine the syntax
(DNS name, IP address, version) of the group and instantiation
expression.
SHA-2: This namespace carries address strings compliant with SHA-2
hash digests. The syntax of the group and instantiation follows
the "val" definition in [RFC6920], Section 3. A processor
handling those strings is required to determine the length of the
expressions and passes appropriate values directly to a
corresponding overlay.
Opaque: This namespace transparently carries strings without further
syntactical information, meanings, or associated resolution
mechanisms. The corresponding syntax for the group and
instantiation part of the ham-URI is defined in Section 4.2.1.
4.2.4. 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 at the system level to
generate a corresponding Multicast Group. The syntax of the group
and instantiation is represented by the "userinfo" component
[RFC3261], Section 25.1.
RELOAD: This namespace covers address strings that are valid in a
REsource LOcation And Discovery [RELOAD] overlay network. A
processor handling those strings may pass these values directly to
a corresponding overlay that may manage multicast distribution
according to [RFC7019].
4.2.5. Future Namespaces
The concept of the common multicast API allows for any namespace that
complies with the superset syntax defined in Section 4.2.1. This
document specifies a basic set of Multicast Namespaces in
Sections 4.2.3 and 4.2.4. If additional namespaces are needed in the
future, a registry for those namespaces should be created and should
be defined in a future document. All namespaces defined in such a
document should then also be assigned to the registry.
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4.3. Additional Abstract Data Types
4.3.1. Interface
The Interface denotes the layer and instance on which the
corresponding call takes effect. In agreement with [RFC3493], we
identify an Interface by an identifier, which is a positive integer
starting at 1.
Properties of an Interface are stored in the following data
structure:
struct ifProp {
UnsignedInt if_index; /* 1, 2, ... */
String *ifName; /* "eth0", "eth1:1", "lo", ... */
String *ifAddr; /* "1.2.3.4", "abc123", ... */
String *ifTech; /* "ip", "overlay", ... */
};
The following function retrieves all available Interfaces from the
system:
getInterfaces(out Interface <ifs>);
It extends the functions for Interface identification as defined in
[RFC3493], Section 4 and can be implemented by:
struct ifProp(out IfProp <ifsProps>);
4.3.2. Membership Events
A membership event is triggered by a multicast state change that is
observed by the current node. It is related to a specific Group Name
and may be receiver or source oriented.
eventType {
joinEvent;
leaveEvent;
newSourceEvent;
};
event {
EventType event;
Uri groupName;
Interface if;
};
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An event will be created by the group communication stack and passed
to applications that have registered for events.
4.4. Group Management Calls
4.4.1. 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
selected and associated with the socket. The call returns an error
code in the case of failures, e.g., due to 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, it
is NULL.
4.4.2. 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 is returned.
4.4.3. 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 / Multicast Listener Discovery (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 is returned.
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4.4.4. 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 argument identifies the group.
On success, the out parameter error is 0; otherwise, -1 is returned.
4.4.5. 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 is returned.
4.4.6. Source Deregister
The srcDeregister call indicates that a source no longer intends 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 groupName argument identifies the Multicast Group to which a
source has stopped sending multicast data.
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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 is returned.
4.5. Send and Receive Calls
4.5.1. 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 is returned.
A message that is too long is indicated by an implementation-specific
error code (e.g., EMSGSIZE in C).
4.5.2. 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.
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receive(in Socket s, out Uri groupName,
out Size msgLen, out Msg msgBuf,
out Int error);
The s argument identifies the multicast socket.
The groupName argument identifies the Multicast Group for which data
was received.
The msgLen argument holds the length of the received message.
The msgBuf argument points to the payload of the received multicast
data.
On success, the out parameter error is 0; otherwise, -1 is returned.
A message that is too long is indicated by an implementation-specific
error code (e.g., EMSGSIZE).
4.6. Socket Options
The following calls configure an existing multicast socket.
4.6.1. Get Interfaces
The getInterfaces 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 is returned.
4.6.2. Add Interface
The addInterface call adds a distribution channel to the socket.
This may be an overlay or underlay Interface, e.g., IPv6 or
Distributed Hash Table (DHT). Multiple Interfaces of the same
technology may be associated with the socket.
addInterface(in Socket s, in Interface if,
out Int error);
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The s and if arguments identify a multicast socket and Interface,
respectively.
On success, the value 0 is returned; otherwise, -1 is returned.
4.6.3. Delete Interface
The delInterface call removes the Interface from the multicast
socket.
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 is returned.
4.6.4. Set TTL
The setTTL call configures the maximum hop count for the socket that
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 is returned.
4.6.5. Get TTL
The getTTL call returns the maximum hop count that a multicast
message is allowed to traverse for the interface bound to the socket.
getTTL(in Socket s, in Interface if,
out Int h, out Int error);
The s argument identifies a multicast socket.
The if argument identifies an interface that is bound to socket s.
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The h argument holds the maximum number of hops associated with the
interface.
On success, the out parameter error is 0; otherwise, -1 is returned.
4.6.6. 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.
getAtomicMsgSize(in Socket s,
out Int return);
On success, the function returns a positive value of appropriate
message size; otherwise, -1 is returned.
4.7. Service Calls
4.7.1. 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 and 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 is returned.
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4.7.2. 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 information
regarding neighbors is requested.
The neighborsAddresses argument points to a list of neighboring nodes
on a successful return.
On success, the out parameter error is 0; otherwise, -1 is returned.
4.7.3. 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 information
regarding children is requested.
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 is returned.
4.7.4. 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 information
regarding parents is requested.
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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 is returned.
4.7.5. Designated Host
The designatedHost function inquires about whether this host has the
role of a designated forwarder (or querier), or not. Such
information is provided by almost all multicast protocols to prevent
packet duplication, if multiple multicast instances provide service
on the same subnet.
designatedHost(in Interface if, in Uri groupName
out Int return);
The if argument identifies the Interface for which information
regarding designated forwarding is requested.
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. The return value -1 indicates an error. Otherwise, 0 is
returned.
4.7.6. 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 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 4.3.2).
The multicast protocol does not have to support membership tracking
in order to enable this feature. This function can also be
implemented at the middleware layer.
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4.7.7. 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.
4.7.8. 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.
getMaxMsgSize(out Int return);
On success, the function returns a positive value of allowed message
size; otherwise, -1 is returned.
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 user space 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 specifies the "ham" URI scheme that has been registered
by IANA as one of the "Provisional URI Schemes" according to
[RFC4395].
URI scheme name ham
Status provisional
URI scheme syntax See Section 4.2.1.
URI scheme semantics See Section 4.2.2.
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Encoding See Section 4.2.1
considerations
Applications/protocols The scheme is used by multicast applications
that use this URI to access multicast content.
scheme name
Interoperability None
considerations
Security See Section 7.
considerations
Contact Matthias Waehlisch, mw@link-lab.net
Author/Change IRTF
controller
References As specified in this document.
7. Security Considerations
This document does not introduce additional messages or novel
protocol operations.
8. Acknowledgements
We would like to thank the HAMcast team at the HAW Hamburg -- Nora
Berg, Gabriel Hege, Fabian Holler, Alexander Knauf, Sebastian
Meiling, Sebastian Woelke, and Sebastian Zagaria -- for many fruitful
discussions and for their continuous critical feedback while
implementing the common multicast API and hybrid multicast
middleware. Special thanks to Dominik Charousset of the HAMcast team
for in-depth perspectives on the matter of code. 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. We thank Lisandro
Zambenedetti Granville and Tony Li for very careful reviews of the
pre-final versions of this document. Barry Leiba and Graham Klyne
provided very constructive input to find a suitable URI scheme. They
are gratefully acknowledged.
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. References
9.1. Normative References
[RFC1075] Waitzman, D., Partridge, C., and S. Deering, "Distance
Vector Multicast Routing Protocol", RFC 1075,
November 1988.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[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.
Waehlisch, et al. Experimental [Page 32]
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[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, April 2013.
9.2. Informative References
[AMT] Bumgardner, G., "Automatic Multicast Tunneling", Work
in Progress, October 2013.
[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. IEEE GLOBECOM 2010 Workshops, MCS 2010, pp. 913-917,
Piscataway, NJ, USA, IEEE Press, December 2010.
[HAMcast-DEV]
"HAMcast developers",
<http://hamcast.realmv6.org/developers>.
[LCN2012] Meiling, S., Schmidt, T., and M. Waehlisch, "Large-Scale
Measurement and Analysis of One-Way Delay in Hybrid
Multicast Networks", Proc. 37th Annual IEEE Conference on
Local Computer Networks (LCN 2012), Piscataway, NJ, USA,
IEEE Press, October 2012.
[MCAST-v4v6]
Venaas, S., Asaeda, H., SUZUKI, S., and T. Fujisaki, "An
IPv4 - IPv6 multicast translator", Work in Progress,
December 2010.
[MCAST-v4v6-FRAMEWORK]
Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
Multicast Translation", Work in Progress, June 2011.
[RELOAD] Jennings, C., Lowekamp, B., Ed., Rescorla, E., Baset, S.,
and H. Schulzrinne, "REsource LOcation And Discovery
(RELOAD) Base Protocol", Work in Progress, February 2013.
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[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.
[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.
[RFC7019] Buford, J. and M. Kolberg, "Application-Layer Multicast
Extensions to REsource LOcation And Discovery (RELOAD)",
RFC 7019, September 2013.
[SIP-RELOAD]
Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
Schulzrinne, H., and T. Schmidt, Ed., "A SIP Usage for
RELOAD", Work in Progress, July 2013.
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RFC 7046 Common Mcast API December 2013
Appendix A. C Signatures
This section describes the C signatures of the common multicast API
(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,
uint32_t* ifs);
int srcDeregister(int msock,
const char* group_uri,
size_t num_ifs,
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);
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);
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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);
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 enableEvents();
int disableEvents();
int getMaxMsgSize();
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Appendix B. Use Case for the API
For the sake of readability, we demonstrate development of
applications using the API based on a high-level Java-like syntax; we
do not consider error handling.
-- Application above middleware:
//Initialize multicast socket;
//the middleware selects all available Interfaces
MulticastSocket m = new MulticastSocket();
m.join(URI("ham:ip:224.1.2.3:5000"));
m.join(URI("ham:ip:[ff02:0:0:0:0:0:0:3]:6000"));
m.join(URI("ham: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);
//...
}
}
}
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Appendix C. Deployment Use Cases for Hybrid Multicast
This section describes the application of the defined API to
implement an IMG.
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 Distance Vector Multicast Routing Protocol (DVMRP)
[RFC1075] router will not be informed of 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 the IMG 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 noticeable via the Source Register call, as well as by an IMG
that initiates 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 (RPs). 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,
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the IMG joins the group and a shared tree between the RP and the
sources will be established; this shared tree 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 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 be transparently
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 the
underlay and overlay.
A solution can be to intercept the subscription at both source sites
and receiver sites: To monitor multicast receiver subscriptions
("join_event" or "leave_event") in the underlay, the IMG is placed on
the 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 Automatic Multicast
Tunneling [AMT] 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.
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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.
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 case
will be transmitted to the RP 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 of group requests from
its domain, which are then delegated to the overlay.
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RFC 7046 Common Mcast API December 2013
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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