SAM Research Group M. Waehlisch
Internet-Draft link-lab & FU Berlin
Intended status: Informational T C. Schmidt
Expires: August 1, 2011 HAW Hamburg
S. Venaas
cisco Systems
January 28, 2011
A Common API for Transparent Hybrid Multicast
draft-waehlisch-sam-common-api-05
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 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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 1, 2011.
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Copyright Notice
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Use Cases for the Common API . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Objectives and Reference Scenarios . . . . . . . . . . . . 7
3.2. Group Communication API & Protocol Stack . . . . . . . . . 8
3.3. Naming and Addressing . . . . . . . . . . . . . . . . . . 10
3.4. Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Common Multicast API . . . . . . . . . . . . . . . . . . . . . 12
4.1. Abstract Data Types . . . . . . . . . . . . . . . . . . . 12
4.1.1. Multicast URI . . . . . . . . . . . . . . . . . . . . 12
4.1.2. Interface . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Group Management Calls . . . . . . . . . . . . . . . . . . 13
4.2.1. Create . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.2. Destroy . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.3. Join . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.4. Leave . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.5. Source Register . . . . . . . . . . . . . . . . . . . 14
4.2.6. Source Deregister . . . . . . . . . . . . . . . . . . 15
4.3. Send and Receive Calls . . . . . . . . . . . . . . . . . . 15
4.3.1. Send . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3.2. Receive . . . . . . . . . . . . . . . . . . . . . . . 16
4.4. Socket Options . . . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Get Interfaces . . . . . . . . . . . . . . . . . . . . 16
4.4.2. Add Interface . . . . . . . . . . . . . . . . . . . . 16
4.4.3. Delete Interface . . . . . . . . . . . . . . . . . . . 17
4.4.4. Set TTL . . . . . . . . . . . . . . . . . . . . . . . 17
4.5. Service Calls . . . . . . . . . . . . . . . . . . . . . . 17
4.5.1. Group Set . . . . . . . . . . . . . . . . . . . . . . 17
4.5.2. Neighbor Set . . . . . . . . . . . . . . . . . . . . . 18
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4.5.3. Children Set . . . . . . . . . . . . . . . . . . . . . 18
4.5.4. Parent Set . . . . . . . . . . . . . . . . . . . . . . 19
4.5.5. Designated Host . . . . . . . . . . . . . . . . . . . 19
4.5.6. Update Listener . . . . . . . . . . . . . . . . . . . 20
4.5.7. Update Sender . . . . . . . . . . . . . . . . . . . . 20
5. Functional Details . . . . . . . . . . . . . . . . . . . . . . 20
5.1. Namespaces . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2. Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
9. Informative References . . . . . . . . . . . . . . . . . . . . 22
Appendix A. Practical Example of the API . . . . . . . . . . . . 23
Appendix B. Deployment Use Cases for Hybrid Multicast . . . . . . 24
B.1. DVMRP . . . . . . . . . . . . . . . . . . . . . . . . . . 25
B.2. PIM-SM . . . . . . . . . . . . . . . . . . . . . . . . . . 25
B.3. PIM-SSM . . . . . . . . . . . . . . . . . . . . . . . . . 26
B.4. BIDIR-PIM . . . . . . . . . . . . . . . . . . . . . . . . 26
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
Currently, group application programmers need to make the choice of
the distribution technology that the application will require at
runtime. There is no common communication interface that abstracts
multicast transmission and subscriptions from the deployment state at
runtime. 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:
o Enable any application programmer to implement group-oriented data
communication independent of the underlying delivery mechanisms.
In particular, allow for a late binding of group applications to
multicast technologies that makes applications efficient, but
robust with respect to deployment aspects.
o Allow for a flexible namespace support in group addressing, and
thereby separate naming and addressing/routing schemes from the
application design. This abstraction does not only decouple
programs from specific aspects of underlying protocols, but may
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open application design to extend to specifically flavored group
services.
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.
1.1. Use Cases for the Common API
Four generic use cases can be identified that require an abstract
common API for multicast services:
Application Programming Independent of Technologies Application
programmers are provided with group primitives that remain
independent of multicast technologies and its deployment in target
domains. They are thus enabled to develop programs once that run
in every deployment scenario. The employment 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.
Global Identification of Groups Groups can be identified
independent of technological instantiations and beyond deployment
domains. Taking advantage of the abstract naming, an application
is thus enabled to match data received from different interface
technologies (e.g., IPv4, IPv6, or overlays) to belong to the same
group. This not only increases flexibility, an application may
for instance combine heterogeneous multipath streams, but
simplifies the design and implementation of gateways and
translators.
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Simplified Service Deployment through Generic Gateways The API
allows for an implementation of abstract gateway functions with
mappings to specific technologies residing at a system level.
Such generic gateways may provide a simple bridging service and
facilitate an inter-domain deployment of multicast.
Mobility-agnostic Group Communication Group naming and management as
foreseen in the 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.
2. Terminology
This document uses the terminology as defined for the multicast
protocols [RFC2710],[RFC3376],[RFC3810],[RFC4601],[RFC4604]. In
addition, the following terms will be used.
Group Address: A Group Address is a routing identifier. It
represents a technological specifier and thus reflects the
distribution technology in use. Multicast packet forwarding is
based on this ID.
Group Name: A Group Name is an application identifier that is used
by applications to manage communication in a multicast group
(e.g., join/leave and send/receive). The Group Name does not
predefine any distribution technologies, even if it syntactically
corresponds to an address, but represents a logical identifier.
Multicast Namespace: A Multicast Namespace is a collection of
designators (i.e., names or addresses) for groups that share a
common syntax. Typical instances of namespaces are IPv4 or IPv6
multicast addresses, overlay group ids, group names defined on the
application layer (e.g., SIP or Email), or some human readable
strings.
Multicast Domain: A Multicast Domain hosts nodes and routers of a
common, single multicast forwarding technology and is bound to a
single namespace.
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.
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Inter-domain Multicast Gateway: 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 IP layer and overlay multicast.
3. Overview
3.1. Objectives and Reference Scenarios
The default use case addressed in this document targets at
applications that participate in a group by using some common
identifier taken from some common namespace. This group name is
typically learned at runtime from user interaction like the selection
of an IPTV channel, from dynamic session negotiations like in the
Session Initiation Protocol (SIP), but may as well have been
predefined for an application as a common group name. Technology-
specific system functions then transparently map the group name to
group addresses such that
o programmers are enabled to process group names in their programs
without the need to consider technological mappings to designated
deployments in target domains;
o applications are enabled to identify packets that belong to a
logically named group, independent of the interface technology
used for sending and receiving packets. The latter shall also
hold for multicast gateways.
This document refers to a reference scenario that covers the
following two hybrid deployment cases displayed in Figure 1:
1. Multicast domains running the same multicast technology but
remaining isolated, possibly only connected by network layer
unicast.
2. Multicast domains running different multicast technologies, but
hosting nodes that are members of the same multicast group.
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+-------+ +-------+
| Member| | Member|
| Foo | | G |
+-------+ +-------+
\ /
*** *** *** ***
* ** ** ** *
* *
* MCast Tec A *
* *
* ** ** ** *
*** *** *** ***
+-------+ +-------+ |
| Member| | Member| +-------+
| G | | Foo | | IMG |
+-------+ +-------+ +-------+
| | |
*** *** *** *** *** *** *** ***
* ** ** ** * * ** ** ** *
* * +-------+ * *
* MCast Tec A * --| IMG |-- * MCast Tec B * +-------+
* * +-------+ * * - | Member|
* ** ** ** * * ** ** ** * | G |
*** *** *** *** *** *** *** *** +-------+
Figure 1: Reference scenarios for hybrid multicast, interconnecting
group members from isolated homogeneous and heterogeneous domains.
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.
3.2. Group Communication API & Protocol Stack
The group communication API consists of four parts. Two parts
combine the essential communication functions, while the remaining
two offer optional extensions for an enhanced management:
Group Management Calls provide the minimal API to instantiate a
multicast socket and manage group membership.
Send/Receive Calls provide the minimal API to send and receive
multicast data in a technology-transparent fashion.
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Socket Options provide extension calls for an explicit configuration
of the multicast socket like 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.
Multicast applications that use the common API require assistance by
a group communication stack. This protocol stack serves two needs:
o It provides system-level support to transfer the abstract
functions of the common API, including namespace support, into
protocol operations at interfaces.
o It bridges data distribution between different multicast
technologies.
A general initiation of a multicast communication in this setting
proceeds as follows:
1. An application opens an abstract multicast socket.
2. The application subscribes/leaves/(de)registers to a group using
a logical group identifier.
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 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
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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 | |
*---------------------* ---|
Figure 2: A middleware for offering uniform access to multicast in
underlay and overlay
3.3. Naming and Addressing
Applications use Group Names to identify groups. Names can uniquely
determine a group in a global communication context and hide
technological deployment for data distribution from the application.
In contrast, multicast forwarding operates on Group Addresses. Even
though both identifiers may be identical in symbols, they carry
different meanings. They may also belong to different 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 4.1.1. Examples are,
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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.
3.4. Mapping
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
a. the same logical group (i.e., same Multicast Name)
b. different multicast channels (i.e., different technical IDs).
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).
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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.
4. Common Multicast API
4.1. Abstract Data Types
4.1.1. Multicast URI
Multicast Names and Multicast Addresses used in this API follow an
URI scheme that defines a subset of the generic URI specified in
[RFC3986] and is compliant with the guidelines in [RFC4395].
The multicast URI is defined as follows:
scheme "://" group "@" instantiation ":" port "/" sec-credentials
The parts of the URI are defined as follows:
scheme refers to the specification of the assigned identifier
[RFC3986] which takes the role of the namespace.
group identifies the group uniquely within the namespace given in
scheme.
instantiation identifies the entity that generates the instance of
the group (e.g., a SIP domain or a source in SSM) using the
namespace given in scheme.
port identifies a specific application at an instance of a group.
sec-credentials used to implement security credentials (e.g., to
authorize a multicast group access).
4.1.2. Interface
The interface denotes the layer and instance on which the
corresponding call will be effective. In agreement with [RFC3493] we
identify an interface by an identifier, which is a positive integer
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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:
struct if_prop *if_prop(void);
It extends the functions for Interface Identification in [RFC3493]
(cf., Section 4).
4.2. Group Management Calls
4.2.1. 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 may return an
error code in the case of failures, e.g., due to a non-operational
middleware.
int createMSocket(uint32_t *if);
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.
4.2.2. Destroy
The destroy call removes the multicast socket.
int destroyMSocket(int s);
The s argument identifies the multicast socket for destruction.
On success the value 0 is returned, otherwise -1.
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4.2.3. Join
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.
int join(int s, const uri group_name);
The s argument identifies the multicast socket.
The group_name argument identifies the group.
On success the value 0 is returned, otherwise -1.
4.2.4. Leave
The leave call results in an unsubscription for the given Group Name.
int leave(int s, const uri group_name);
The s argument identifies the multicast socket.
The group_name identifies the group.
On success the value 0 is returned, otherwise -1.
4.2.5. 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, the creation of sub-overlays, for example. Not
all multicast technologies require his call.
int srcRegister(int s, const uri group_name,
uint_t num_ifs, uint_t *ifs);
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.
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If source registration succeeded for all interfaces associated with
the socket, the value 0 is returned, otherwise -1.
4.2.6. Source Deregister
The srcDeregister indicates that a source does no longer intend to
send data to the multicast group. This call may remain without
effect in some multicast technologies.
int srcDeregister(int s, const uri group_name,
uint_t num_ifs, uint_t *ifs);
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.
4.3. Send and Receive Calls
4.3.1. Send
The send call passes multicast data for a Multicast Name from the
application to the multicast socket.
int send(int s, const uri group_name,
size_t msg_len, const void *buf);
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 buf argument passes the multicast data to the multicast socket.
On success the value 0 is returned, otherwise -1.
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4.3.2. Receive
The receive call passes multicast data and the corresponding Group
Name to the application.
int receive(int s, const uri group_name,
size_t msg_len, msg *msg_buf);
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.
4.4. Socket Options
The following calls configure an existing multicast socket.
4.4.1. Get Interfaces
The getInterface call returns an array of all available multicast
communication interfaces associated with the multicast socket.
int getInterfaces(int s, uint_t num_ifs, uint_t *ifs);
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.
4.4.2. Add Interface
The addInterface call adds a distribution channel to the socket.
This may be an overlay or underlay interface, e.g., IPv6 or DHT.
Multiple interfaces of the same technology may be associated with the
socket.
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int addInterface(int s, uint32_t if);
The s and if arguments identify a multicast socket and interface,
respectively.
On success the value 0 is returned, otherwise -1.
4.4.3. Delete Interface
The delnterface call removes the interface if from the multicast
socket.
int delInterface(int s, uint32_t if);
The s and if arguments identify a multicast socket and interface,
respectively.
On success the value 0 is returned, otherwise -1.
4.4.4. Set TTL
The setTTL call configures the maximum hop count for the socket a
multicast message is allowed to traverse.
int setTTL(int s, int h, uint_t num_ifs, uint_t *ifs);
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.
4.5. Service Calls
4.5.1. 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.
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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 */
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 an array of group states.
On success the value 0 is returned, otherwise -1.
4.5.2. Neighbor Set
The neighborSet function returns the set of neighboring nodes for a
given interface as seen by the multicast routing protocol.
int neighborSet(uint32_t if, uint_t *num_neighbors,
const uri *neighbor_address);
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.
4.5.3. 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.
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int childrenSet(uint32_t if, const uri group_name,
uint_t *num_children, const uri *child_address);
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.
4.5.4. 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.
int parentSet(uint32_t if, const uri group_name, uint_t *num_parents,
const uri *parent_address);
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.
4.5.5. Designated Host
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.
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int designatedHost(uint32_t if, const uri *group_name);
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.
4.5.6. Update Listener
The updateListener function is invoked to inform a group service
about a change of listener states for a group. This is the result of
receiver new subscriptions or leaves. The group service may call
groupSet to get updated information.
const uri *updateListener();
On success the updateListener function points to the Group Name that
experienced a state change, otherwise NULL is returned.
4.5.7. Update Sender
The updateSender function is invoked to inform a group service about
a change of sender states for a group. The group service may call
groupSet to get updated information.
const uri *updateSender();
On success the updateListener function points to the Group Name that
experienced a state change, otherwise NULL is returned.
5. Functional Details
In this section, we describe specific functions of the API and the
associated system middleware in detail.
5.1. Namespaces
Namespace identifiers in URIs are placed in the scheme element and
characterize syntax and semantic of the group identifier. They
enable the use of convenience functions and high-level data types
while processing URIs. When used in names, they may facilitate a
default mapping and a recovery of names from addresses. They
characterize its type, when used in addresses.
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Compliant to the URI concept, namespace-schemes can be added.
Examples of schemes and functions currently foreseen include
IP This namespace is comprised of regular IP node naming, i.e., DNS
names and addresses taken from any version of the Internet
Protocol. A processor dealing with the IP namespace is required
to determine the syntax (DNS name, IP address version) of the
group expression.
OLM This namespace covers address strings immediately valid in an
overlay network. A processor handling those strings need not be
aware of the address generation mechanism, but may pass these
values directly to a corresponding overlay.
SIP The SIP namespace is an example of an application-layer scheme
that bears inherent group functions (conferencing). SIP
conference URIs may be directly exchanged and interpreted at the
application, and mapped to group addresses on the system level to
generate a corresponding multicast group.
Opaque This namespace transparently carries strings without further
syntactical information, meanings or associated resolution
mechanism.
5.2. Mapping
Group Name to Group Address, SSM/ASM TODO
6. IANA Considerations
This document makes no request of IANA.
7. Security Considerations
This draft does neither introduce additional messages nor novel
protocol operations. TODO
8. Acknowledgements
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.
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This work is partially supported by the German Federal Ministry of
Education and Research within the HAMcast project, which is part of
G-Lab.
9. Informative References
[I-D.ietf-mboned-auto-multicast]
Thaler, D., Talwar, M., Aggarwal, A., Vicisano, L., and T.
Pusateri, "Automatic IP Multicast Without Explicit Tunnels
(AMT)", draft-ietf-mboned-auto-multicast-10 (work in
progress), March 2010.
[RFC1075] Waitzman, D., Partridge, C., and S. Deering, "Distance
Vector Multicast Routing Protocol", RFC 1075,
November 1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[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,
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"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, August 2006.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, October 2007.
Appendix A. Practical Example of the API
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-- Application above middleware:
//Initialize multicast socket;
//the middleware selects all available interfaces
MulticastSocket m = new MulticastSocket();
m.join(URI("ip://224.1.2.3:5000"));
m.join(URI("ip://[FF02:0:0:0:0:0:0:3]:6000"));
m.join(URI("sip://news@cnn.com"));
-- Middleware:
join(URI mcAddress) {
//Select interfaces in use
for all this.interfaces {
switch (interface.type) {
case "ipv6":
//... map logical ID to routing address
Inet6Address rtAddressIPv6 = new Inet6Address();
mapNametoAddress(mcAddress,rtAddressIPv6);
interface.join(rtAddressIPv6);
case "ipv4":
//... map logical ID to routing address
Inet4Address rtAddressIPv4 = new Inet4Address();
mapNametoAddress(mcAddress,rtAddressIPv4);
interface.join(rtAddressIPv4);
case "sip-session":
//... map logical ID to routing address
SIPAddress rtAddressSIP = new SIPAddress();
mapNametoAddress(mcAddress,rtAddressSIP);
interface.join(rtAddressSIP);
case "dht":
//... map logical ID to routing address
DHTAddress rtAddressDHT = new DHTAddress();
mapNametoAddress(mcAddress,rtAddressDHT);
interface.join(rtAddressDHT);
//...
}
}
}
Appendix B. Deployment Use Cases for Hybrid Multicast
This section describes the application of the defined API to
implement an IMG.
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B.1. DVMRP
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 updateSender() call is
triggered. 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.
B.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.
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 updateSender(). 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
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messages invokes the updateListener() call 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.
B.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.
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
(updateListener()) 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.
B.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.
Bidirectional PIM [RFC5015] is a variant of PIM-SM. In contrast to
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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 updateSender(). 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 updateListener() invocation, an IMG will thereby be
informed about group requests from its domain, which are then
delegated to the overlay.
Appendix C. Change Log
The following changes have been made from
draft-waehlisch-sam-common-api-03
1. Use cases added for illustration.
2. Service calls added for inquiring on the multicast distribution
system.
3. Namespace examples added.
4. Clarifications and editorial improvements.
The following changes have been made from
draft-waehlisch-sam-common-api-02
1. Rename init() in createMSocket().
2. Added calls srcRegister()/srcDeregister().
3. Rephrased API calls in C-style.
4. Cleanup code in "Practical Example of the API".
5. Partial reorganization of the document.
6. Many editorial improvements.
The following changes have been made from
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draft-waehlisch-sam-common-api-01
1. Document restructured to clarify the realm of document overview
and specific contributions s.a. naming and addressing.
2. A clear separation of naming and addressing was drawn. Multicast
URIs have been introduced.
3. Clarified and adapted the API calls.
4. Introduced Socket Option calls.
5. Deployment use cases moved to an appendix.
6. Simple programming example added.
7. Many editorial improvements.
Authors' Addresses
Matthias Waehlisch
link-lab & FU Berlin
Hoenower Str. 35
Berlin 10318
Germany
Email: mw@link-lab.net
URI: http://www.inf.fu-berlin.de/~waehl
Thomas C. Schmidt
HAW Hamburg
Berliner Tor 7
Hamburg 20099
Germany
Email: schmidt@informatik.haw-hamburg.de
URI: http://inet.cpt.haw-hamburg.de/members/schmidt
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Stig Venaas
cisco Systems
Tasman Drive
San Jose, CA 95134
USA
Email: stig@cisco.com
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