A Common API for Transparent Hybrid
Multicastlink-lab & FU BerlinHoenower Str. 35Berlin10318Germanymw@link-lab.nethttp://www.inf.fu-berlin.de/~waehlHAW HamburgBerliner Tor 7Hamburg20099Germanyschmidt@informatik.haw-hamburg.dehttp://inet.cpt.haw-hamburg.de/members/schmidtcisco SystemsTasman DriveSan JoseCA95134USAstig@cisco.comSAM Research GroupGroup 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.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 ,
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: 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.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
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.The following generic use cases can be identified that require an
abstract common API for multicast services: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.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.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.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.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 for mobility-related
aspects).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 : 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 overlay multicast respectively.Using the current socket API, the application programmer needs to
decide in advance on the IP technologies. Additional distribution
techniques, such as overlay multicast, must be integrated specificly
to the application. For each technology the application programmer
needs to create a separate socket and initiates 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). As the souce may distribute content
via a technology that is not supported by the receivers or its
Internet Service Provider (cf., R3) a gateway is required. To build
gateway functions a consistent view on the multicast states at the
gateway is important.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
may then look like:The gateway function for R2 can be implemented by the service calls
similar to:This document uses the terminology as defined for the multicast
protocols ,,,,. In addition,
the following terms will be used.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.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, but represents a logical identifier.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.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.A Multicast Domain hosts nodes and
routers of a common, single multicast forwarding technology and is
bound to a single namespace.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.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), 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 thatprogrammers are enabled to process group names in their
programs without the need to consider technological mappings to
designated deployments in target domains;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 :Multicast Domains running the same multicast technology but
remaining isolated, possibly only connected by network layer
unicast.Multicast Domains running different multicast technologies but
hosting nodes that are members of the same multicast group.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:
provide the minimal API to
instantiate a multicast socket and to manage group membership.provide the minimal API to send
and receive multicast data in a technology-transparent
fashion.provide extension calls for an
explicit configuration of the multicast socket such as setting hop
limits or associated Interfaces.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:It provides system-level support to transfer the abstract
functions of the common API, including namespace support, into
protocol operations at Interfaces.It group communication services across different multicast
technologies at the local host.A general initiation of a multicast communication in this setting
proceeds as follows:An application opens an abstract multicast socket.The application subscribes/leaves/(de)registers to a group
using a Group Name.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.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.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 .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 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 . Examples are, ip://224.1.2.3:5000
for a canonical IPv4 ASM group, sip://news@cnn.com for an
application-specific naming with service instantiator and default port
selection.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.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.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.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.This namespace transparently carries
strings without further syntactical information, meanings or
associated resolution mechanism.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.This namespace covers address strings
immediately valid in a RELOAD overlay network. A
processor handling those strings may pass these values directly
to a corresponding overlay.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.For a given Group Name, identify an Address that is appropriate
for a local distribution instance.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 ). 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.Some Multicast Namespaces defined in 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 received from this
group.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.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 potentially needs to be acquired out of band from a
neighboring node.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
include an attribute which represents the number of elements.The corresponding C signatures are defined in .Multicast Names and Multicast Addresses used in this API follow
an URI scheme that defines a subset of the generic URI specified in
and is compliant with the guidelines
in .The multicast URI is defined as follows:scheme "://" group "@" instantiation ":" port "/"
sec-credentialsThe parts of the URI are defined as follows:refers to the specification of the assigned
identifier which takes the role
of the Multicast Namespace.identifies the group uniquely within the
Namespace given in scheme.identifies the entity that generates
the instance of the group (e.g., a SIP domain or a source in
SSM), 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.identifies a specific application at an
instance of a group. This parameter is optional.used to implement optional
security credentials (e.g., to authorize a multicast group
access). Note that security credentials 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.The Interface denotes the layer and instance on which the
corresponding call will be effective. In agreement with we identify an Interface by an identifier,
which is a positive integer starting at 1.Properties of an Interface are stored in the following
struct:The following function retrieves all available Interfaces from
the system:It extends the functions for Interface Identification defined in
Section 4 of and can be implemented
by: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.An event will be created by the group communication stack and
passed to applications that have registered for events.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 a non-operational
middleware.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.The delete call removes the multicast socket.The s argument identifies the multicast socket for
destruction.On success the out parameter error is 0, otherwise -1.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.The s argument identifies the multicast socket.The groupName argument identifies the group.On success the out parameter error is 0, otherwise -1.The leave call results in an unsubscription for the given Group
Name.The s argument identifies the multicast socket.The groupName identifies the group.On success the out parameter error is 0, otherwise -1.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. It
may remain without effect in some multicast technologies.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.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.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.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.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.The receive call passes multicast data and the corresponding
Group Name to the application.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.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.The msgBuf argument points to the payload of the received
multicast data.On success the out parameter error is 0, otherwise -1.The following calls configure an existing multicast socket.The getInterface call returns an array of all available multicast
communication Interfaces associated with the multicast socket.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.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.The s and if arguments identify a multicast socket and Interface,
respectively.On success the value 0 is returned, otherwise -1.The delInterface call removes the Interface if from the multicast
socket.The s and if arguments identify a multicast socket and Interface,
respectively.On success the out parameter error is 0, otherwise -1.The setTTL call configures the maximum hop count for the socket a
multicast message is allowed to traverse.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.The getTTL call returns the maximum hop count a multicast message
is allowed to traverse for the socket.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.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.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.The neighborSet function returns the set of neighboring nodes for
a given Interface as seen by the multicast routing protocol.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.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.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.The parentSet function returns the set of neighbors from which
the current node receives multicast data at a given Interface for
the specified group.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.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.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.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.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., ).The multicast protocol has not to support membership tracking to
enable this feature. This function can also be implemented at the
middelware layer.The disableEvents function deactivates the information about
group state changes.On success the stack will not pass membership events to the
application.A reference implementation of the Common API for Transparent Hybrid
Multicast is available with the HAMcast stack, . 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.This document makes no request of IANA.This draft does neither introduce additional messages nor novel
protocol operations.We would like to thank the HAMcast-team, Dominik Charousset, Gabriel
Hege, Fabian Holler, Alexander Knauf, Sebastian Meiling, and Sebastian
Woelke, 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 and
Mario Kolberg for 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, which is part of
G-Lab.HAMcast developersSystem-assisted Service Evolution for a Future Internet - The
HAMcast Approach to Pervasive MulticastThis section describes the C signatures of the common multicast API,
which are defined in .This section describes the application of the defined API to
implement an IMG.The following procedure describes a transparent mapping of a
DVMRP-based any source multicast service to another many-to-many
multicast technology.An arbitrary DVMRP 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. 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 for DVMRP routers on the path from the
new receiver subnet to the source, but in general not for an IMG. To
overcome this problem, data of multicast senders will be flooded in
the overlay as well as in the underlay. Hence, an IMG has to initiate
an all-group join to the overlay using the namespace extension of the
API. Each IMG is initially required to forward the received overlay
data to the underlay, independent of native multicast receivers.
Subsequent prunes may limit unwanted data distribution thereafter.The following procedure describes a transparent mapping of a
PIM-SM-based any source multicast service to another many-to-many
multicast technology.The Protocol Independent Multicast Sparse Mode (PIM-SM) establishes rendezvous points (RP). These
entities receive listener and source subscriptions of a domain. To be
continuously updated, an IMG has to be co-located with a 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 a sufficient number of data has been delivered. 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
transparently be forwarded in the underlay network and distributed
through the RP instance.The following procedure describes a transparent mapping of a
PIM-SSM-based source specific multicast service to another one-to-many
multicast technology.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 . 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 are 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 the sense of PIM-SSM, the signaling
should not be flooded in underlay and overlay.One solution could 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 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.The following procedure describes a transparent mapping of a
BIDIR-PIM-based any source multicast service to another many-to-many
multicast technology.Bidirectional PIM 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). However, 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 address,
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.The following changes have been made from
draft-irtf-samrg-common-api-03Added section "Illustrative Example"Added section "Implementation"Minor clarificationsThe following changes have been made from
draft-irtf-samrg-common-api-02Added use case of multicast flavor supportRestructured Section 3Major update on namespaces and on mappingC signatures completedMany clarifications and editorial improvements The following changes have been made from
draft-irtf-samrg-common-api-01Pseudo syntax for lists objects changedEditorial improvementsThe following changes have been made from
draft-irtf-samrg-common-api-00Incorrect pseudo code syntax fixedMinor editorial improvementsThe following changes have been made from
draft-waehlisch-sam-common-api-06no 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-05Description of the Common API using pseudo syntax addedC signatures of the Comon API moved to appendixupdateSender() and updateListener() calls replaced by eventsFunction destroyMSocket renamed as deleteMSocket.The following changes have been made from
draft-waehlisch-sam-common-api-04updateSender() added.The following changes have been made from
draft-waehlisch-sam-common-api-03Use cases added for illustration.Service calls added for inquiring on the multicast distribution
system.Namespace examples added.Clarifications and editorial improvements.The following changes have been made from
draft-waehlisch-sam-common-api-02Rename init() in createMSocket().Added calls srcRegister()/srcDeregister().Rephrased API calls in C-style.Cleanup code in "Practical Example of the API".Partial reorganization of the document.Many editorial improvements.The following changes have been made from
draft-waehlisch-sam-common-api-01Document restructured to clarify the realm of document overview
and specific contributions s.a. naming and addressing.A clear separation of naming and addressing was drawn. Multicast
URIs have been introduced.Clarified and adapted the API calls.Introduced Socket Option calls.Deployment use cases moved to an appendix.Simple programming example added.Many editorial improvements.