| SAM Research Group | M. Waehlisch |
| Internet-Draft | link-lab & FU Berlin |
| Intended status: Informational | T C. Schmidt |
| Expires: September 08, 2011 | HAW Hamburg |
| S. Venaas | |
| cisco Systems | |
| March 07, 2011 |
A Common API for Transparent Hybrid Multicast
draft-waehlisch-sam-common-api-06
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 using a stable, upper layer protocol controlled 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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 08, 2011.
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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. The standard multicast socket options [RFC3493], [RFC3678] are bound to an IP version and do not distinguish between naming and addressing of multicast identifiers. Group communication, however, is commonly implemented in different flavors such as any source (ASM) vs. source specific mutlicast (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 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:
Multicast technologies may be of various P2P 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 proposes and discusses mapping mechanisms between different namespaces and forwarding technologies. 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.
Four generic use cases can be identified that require an abstract common API for multicast services:
This document uses the terminology as defined for the multicast protocols [RFC2710],[RFC3376],[RFC3810],[RFC4601],[RFC4604]. In addition, the following terms will be used.
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 that
This document refers to a reference scenario that covers the following two hybrid deployment cases displayed in Figure 1:
+-------+ +-------+
| Member| | Member|
| Foo | | G |
+-------+ +-------+
\ /
*** *** *** ***
* ** ** ** *
* *
* MCast Tec A *
* *
* ** ** ** *
*** *** *** ***
+-------+ +-------+ |
| Member| | Member| +-------+
| G | | Foo | | IMG |
+-------+ +-------+ +-------+
| | |
*** *** *** *** *** *** *** ***
* ** ** ** * * ** ** ** *
* * +-------+ * *
* MCast Tec A * --| IMG |-- * MCast Tec B * +-------+
* * +-------+ * * - | Member|
* ** ** ** * * ** ** ** * | G |
*** *** *** *** *** *** *** *** +-------+
It is assumed throughout the document that the domain composition, as well as the node attachment to a specific technology remain unchanged during a multicast session.
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 management:
Multicast applications that use the common API require assistance by a group communication stack. This protocol stack serves two needs:
A general initiation of a multicast communication in this setting proceeds as follows:
The 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 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 communication interfaces. It provides a minimal hybrid function that bridges data between different interfaces and multicast domains. Details of service discovery are out-of-scope of this document.
The extended multicast functions can be implemented by a middleware as conceptually visualized in Figure 2.
*-------* *-------*
| App 1 | | App 2 |
*-------* *-------*
| |
*---------------------* ---|
| Middleware | |
*---------------------* |
| | |
*---------* | |
| Overlay | | \ Group Communication
*---------* | / Stack
| | |
| | |
*---------------------* |
| Underlay | |
*---------------------* ---|
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 namespaces. The namespace of a Group Address reflects a routing technology, while the namespace of a Group Name represents the context in which the application operates.
URIs [RFC3986] are a common way to represent namespace-specific identifiers in applications in the form of an abstract meta-data type. Throughout this document, any kind of Group Name follows a URI notation with the syntax defined in Section 4.2.1. Examples are, ip://224.1.2.3:5000 for a canonical IPv4 ASM group, sip://news@cnn.com for an application-specific naming with service instantiator and default port selection.
An implementation of the group communication middleware can provide convenience functions that detect the namespace of a Group Name and use it to optimize service instantiation. In practice, such a library would provide support for high-level data types to the application, similar to the current socket API (e.g., InetAddress in Java). Using this data type could implicitly determine the namespace. Details of automatic namespace identification is out-of-scope of this document.
All group members subscribe to the same Group Name taken from a common namespace and thereby identify the group in a technology-agnostic way.
Group Names require a mapping to addresses prior to service instantiation at an Interface. Similarly, a mapping is needed at gateways to translate between Group Addresses from different namespaces. Some namespaces facilitate a canonical transformation to default address spaces. For example, ip://224.1.2.3:5000 has an obvious correspondence to 224.1.2.3 in the IPv4 multicast address space. Note that in this example the multicast URI can be completely recovered from any data packet received from this group.
However, mapping in general can be more complex and need not be invertible. 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. For example, it is not obvious how to map a large identifier space (e.g., IPv6) to a smaller, collision-prone set like IPv4.
Two (or more) Multicast Addresses from different namespaces may belong to
This decision can be based on invertible mappings. However, the application of such functions depends on the cardinality of the namespaces and thus does not hold in general. It is not obvious how to map a large identifier space (e.g., IPv6) to a smaller set (e.g., IPv4).
A mapping can be realized by embedding smaller in larger namespaces or selecting an arbitrary, unused ID in the target space. The relation between logical and technical ID is maintained by mapping functions which can be stateless or stateful. The middleware thus queries the mapping service first, and creates a new technical group ID only if there is no identifier available for the namespace in use. The Group Name is associated with one or more Group Addresses, which belong to different namespaces. Depending on the scope of the mapping service, it ensures a consistent use of the technical ID in a local or global domain.
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 corresponding C signatures are defined in Appendix Appendix A.
Multicast Names and Multicast Addresses used in this API follow an URI scheme that defines a subset of the generic URI specified in [RFC3986] and is compliant with the guidelines in [RFC4395].
The multicast URI is defined as follows:
The parts of the URI are defined as follows:
The interface denotes the layer and instance on which the corresponding call will be effective. In agreement with [RFC3493] we identify an interface by an identifier, which is a positive integer starting at 1.
Properties of an interface are stored in the following struct:
struct if_prop {
unsigned int if_index; /* 1, 2, ... */
char *if_name; /* "eth0", "eth1:1", "lo", ... */
char *if_addr; /* "1.2.3.4", "abc123", ... */
char *if_tech; /* "ip", "overlay", ... */
};
The following function retrieves all available interfaces from the system:
getInterfaces(out Int num_ifs, out Interface <if>);
It extends the functions for Interface Identification in [RFC3493] (cf., Section 4) and can be implemented by:
struct if_prop *if_prop(void);
A membership event is triggered by a mutlicast state change, which is observed by the current node. It is related to a specific Group Name and may be receiver or source oriented.
event_type {
join_event,
leave_event,
new_source_event
};
event {
event_type event,
Uri group_name
};
An event will be created by the middleware and passed to applcations that are 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 may return an error code in the case of failures, e.g., due to a non-operational middleware.
createMSocket(in Interface <if>,
out Socket s, out Int error);
The if 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.
deleteMSocket(in Socket s, out Int error);
The s argument identifies the multicast socket for destruction.
On success the value 0 is returned, otherwise -1.
The join call initiates a subscription for the given group. Depending on the interfaces that are associated with the socket, this may result in an IGMP/MLD report or overlay subscription, for example.
join(in Socket s, in Uri group_name, out Int error);
The s argument identifies the multicast socket.
The group_name argument identifies the group.
On success the value 0 is returned, otherwise -1.
The leave call results in an unsubscription for the given Group Name.
leave(in Socket s, in Uri group_name, out Int error);
The s argument identifies the multicast socket.
The group_name identifies the group.
On success the value 0 is returned, 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, the creation of sub-overlays, for example. Not all multicast technologies require his call.
srcRegister(in Socket s, in Uri group_name,
in Int num_ifs, in Interface <ifs>,
out Int error);
The s argument identifies the multicast socket.
The group_name argument identifies the multicast group to which a source intends to send data.
The num_ifs argument holds the number of elements in the ifs array. This parameter is optional.
The ifs argument points to the list of interface indexes for which the source registration failed. If num_ifs was 0 on output, a NULL pointer is returned. This parameter is optional.
If source registration succeeded for all interfaces associated with the socket, the value 0 is returned, 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.
srcDeregister(in Socket s, in Uri group_name,
in Int num_ifs, in Interface <ifs>,
out Int error);
The s argument identifies the multicast socket.
The group_name argument identifies the multicast group to which a source has stopped to send multicast data.
The num_ifs argument holds the number of elements in the ifs array.
The ifs argument points to the list of interfaces for which the source deregistration failed. If num_ifs was 0 on output, a NULL pointer is returned.
If source deregistration succeeded for all interfaces associated with the socket, the value 0 is returned, otherwise -1.
The send call passes multicast data for a Multicast Name from the application to the multicast socket.
send(in Socket s, in Uri group_name,
in Size msg_len, in Masg msg_buf,
in Int error);
The s argument identifies the multicast socket.
The group_name argument identifies the group to which data will be sent.
The msg_len argument holds the length of the message to be sent.
The msg_buf argument passes the multicast data to the multicast socket.
On success the value 0 is returned, otherwise -1.
The receive call passes multicast data and the corresponding Group Name to the application.
receive(in Socket s, in Uri group_name,
in Size msg_len, in Msg msg_buf,
in Int error);
The s argument identifies the multicast socket.
The group_name argument identifies the multicast group for which data was received.
The msg_len argument holds the length of the received message.
The msg_buf argument points to the payload of the received multicast data.
On success the value 0 is returned, 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.
getInterfaces(in Socket s, out Int num_ifs,
out Interface <ifs>, out Int error);
The s argument identifies the multicast socket.
The num_ifs argument holds the number of interfaces in the ifs list.
The ifs argument points to an array of interface index identifiers.
On success the value 0 or lager is returned, 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.
addInterface(in Socket s, in Interface if,
out Int error);
The s and if arguments identify a multicast socket and interface, respectively.
On success the value 0 is returned, otherwise -1.
The delnterface call removes the interface if 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 value 0 is returned, otherwise -1.
The setTTL call configures the maximum hop count for the socket a multicast message is allowed to traverse.
setTTL(in Socket s, in Int h,
in Int num_ifs, in Interface <ifs>,
out Int error);
The s and h arguments identify a multicast socket and the maximum hop count, respectively.
The num_ifs argument holds the number of interfaces in the ifs list. This parameter is optional.
The ifs argument points to an array of interface index identifiers. This parameter is optional.
On success the value 0 is returned, otherwise -1.
The getTTL call returns the maximum hop count a multicast message is allowed to traverse for the socket.
getTTL(in Socket s,
out Int h, out Int error);
The s argument identifies a multicast socket.
The h argument holds the maximum number of hops associated with socket s.
On success the value 0 is returned, 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.
int groupSet(in Interface if, out Int num_groups,
out GroupSet <groupSet>, out Int error);
struct GroupSet {
uri group_name; /* registered multicast group */
int type; /* 0 = listener state, 1 = sender state,
2 = sender & listener state */
}
The if argument identifies the interface for which states are maintained.
The num_groups argument holds the number of groups in the groupSet array.
The groupSet argument points to a lilst of group states.
On success the value 0 is returned, otherwise -1.
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 Int num_neighbors,
out Uri <neighbor_address>, out Int error);
The if argument identifies the interface for which neighbors are inquired.
The num_neighbors argument holds the number of addresses in the neighbor_address array.
The neighbor_address argument points to a list of neighboring nodes on a successful return.
On success the value 0 is returned, 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.
childrenSet(in Interface if, in Uri group_name,
out Int num_children, out Uri <child_address>,
out Int error);
The if argument identifies the interface for which children are inquired.
The group_name argument defines the multicast group for which distribution is considered.
The num_children argument holds the number of addresses in the child_address array.
The child_address argument points to a list of neighboring nodes on a successful return.
On success the value 0 is returned, 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.
parentSet(in Interface if, in Uri group_name,
out Int num_parents, out Uri parent_address,
out Int error);
The if argument identifies the interface for which parents are inquired.
The group_name argument defines the multicast group for which distribution is considered.
The num_parents argument holds the number of addresses in the parent_address array.
The parent_address argument points to a list of neighboring nodes on a successful return.
On success the value 0 is returned, otherwise -1.
The designatedHost function inquires whether the 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.
designatedHost(in Interface if, in Uri group_name
out Int return);
The if argument identifies the interface for which designated forwarding is inquired.
The group_name 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 middleware to inform the application about a group change. This is the result of receiver new subscriptions or leaves as well as the observation of source changes. The group service may call other service calls to get additional information.
void enableEvents();
Calling this function, the middleware starts to pass membership events to the application. Each event includes an event type identifier and a Group Name (cf., Section 4.2.3).
The disableEvents function deactivates the information about group state changes.
void disableEvents();,
On success the middleware will not pass membership events to the application.
In this section, we describe specific functions of the API and the associated system middleware in detail.
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 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 and functions currently foreseen include
Group Name to Group Address, SSM/ASM TODO
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 API and a hybrid multicast middleware.
This work is partially supported by the German Federal Ministry of Education and Research within the HAMcast project, which is part of G-Lab.
This section describes the C signatures of the common multicast API, which is defined in Section 4.
int createMSocket(uint32_t *if);
int deleteMSocket(int s);
int join(int s, const uri group_name);
int leave(int s, const uri group_name);
int srcRegister(int s, const uri group_name,
uint_t num_ifs, uint_t *ifs);
int srcDeregister(int s, const uri group_name,
uint_t num_ifs, uint_t *ifs);
int send(int s, const uri group_name,
size_t msg_len, const void *buf);
int receive(int s, const uri group_name,
size_t msg_len, msg *msg_buf);
int getInterfaces(int s, uint_t num_ifs, uint_t *ifs);
int addInterface(int s, uint32_t if);
int delInterface(int s, uint32_t if);
int setTTL(int s, int h, uint_t num_ifs, uint_t *ifs);
int getTTL(int s, int h);
int groupSet(uint32_t if, uint_t *num_groups,
struct groupSet *groupSet);
struct groupSet {
uri group_name; /* registered multicast group */
int type; /* 0 = listener state, 1 = sender state,
2 = sender & listener state */
int neighborSet(uint32_t if, uint_t *num_neighbors,
const uri *neighbor_address);
int childrenSet(uint32_t if, const uri group_name,
uint_t *num_children, const uri *child_address);
int parentSet(uint32_t if, const uri group_name, uint_t *num_parents,
const uri *parent_address);
int designatedHost(uint32_t if, const uri *group_name);
-- Application above middleware:
//Initialize multicast socket;
//the middleware selects all available interfaces
MulticastSocket m = new MulticastSocket();
m.join(URI("ip://224.1.2.3:5000"));
m.join(URI("ip://[FF02:0:0:0:0:0:0:3]:6000"));
m.join(URI("sip://news@cnn.com"));
-- Middleware:
join(URI mcAddress) {
//Select interfaces in use
for all this.interfaces {
switch (interface.type) {
case "ipv6":
//... map logical ID to routing address
Inet6Address rtAddressIPv6 = new Inet6Address();
mapNametoAddress(mcAddress,rtAddressIPv6);
interface.join(rtAddressIPv6);
case "ipv4":
//... map logical ID to routing address
Inet4Address rtAddressIPv4 = new Inet4Address();
mapNametoAddress(mcAddress,rtAddressIPv4);
interface.join(rtAddressIPv4);
case "sip-session":
//... map logical ID to routing address
SIPAddress rtAddressSIP = new SIPAddress();
mapNametoAddress(mcAddress,rtAddressSIP);
interface.join(rtAddressSIP);
case "dht":
//... map logical ID to routing address
DHTAddress rtAddressDHT = new DHTAddress();
mapNametoAddress(mcAddress,rtAddressDHT);
interface.join(rtAddressDHT);
//...
}
}
}
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 [RFC1075] router will not be informed about new receivers, but will learn about new sources immediately. The concept of DVMRP does not provide any central multicast instance. Thus, the IMG can be placed anywhere inside the multicast region, but requires a DVMRP neighbor connectivity. 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) [RFC4601] 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 [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 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 [I-D.ietf-mboned-auto-multicast] by anycast. Consequently, the source address S of the group (S,G) should be built based on an anycast prefix. The corresponding IMG anycast address for a source domain is then derived from the prefix of S.
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 [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). 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-waehlisch-sam-common-api-05
The following changes have been made from draft-waehlisch-sam-common-api-04
The following changes have been made from draft-waehlisch-sam-common-api-03
The following changes have been made from draft-waehlisch-sam-common-api-02
The following changes have been made from draft-waehlisch-sam-common-api-01