Network Working Group B. Quinn
Request for Comments: 3170 Celox Networks
Category: Informational K. Almeroth
UC-Santa Barbara
September 2001
IP Multicast Applications:
Challenges and Solutions
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes the challenges involved with designing and
implementing multicast applications. It is an introductory guide for
application developers that highlights the unique considerations of
multicast applications as compared to unicast applications.
To this end, the document presents a taxonomy of multicast
application I/O models and examples of the services they can support.
It then describes the service requirements of these multicast
applications, and the recent and ongoing efforts to build protocol
solutions to support these services.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Focus and Scope. . . . . . . . . . . . . . . . . . . . . . . 3
2. IP Multicast-enabled Network. . . . . . . . . . . . . . . . . . 3
2.1 Essential Protocol Components. . . . . . . . . . . . . . . . 4
2.1.1 EXPedient Joins and Leaves . . . . . . . . . . . . . . . 5
2.1.2 Send without a Join. . . . . . . . . . . . . . . . . . . 5
3. IP Multicast Application Taxonomy . . . . . . . . . . . . . . . 6
3.1 One-to-Many Applications . . . . . . . . . . . . . . . . . . 8
3.2 Many-to-Many Applications. . . . . . . . . . . . . . . . . . 9
3.3 Many-to-One Applications . . . . . . . . . . . . . . . . . .10
4. Common Multicast Service Requirements . . . . . . . . . . . . .13
4.1 Bandwidth Requirements . . . . . . . . . . . . . . . . . . .13
4.2 Delay Requirements . . . . . . . . . . . . . . . . . . . . .13
5. Unique Multicast Service Requirements . . . . . . . . . . . . .14
5.1 Address Management . . . . . . . . . . . . . . . . . . . . .16
5.2 Session Management . . . . . . . . . . . . . . . . . . . . .16
5.3 Heterogeneous Receiver Support . . . . . . . . . . . . . . .18
5.4 Reliable Data Delivery . . . . . . . . . . . . . . . . . . .20
5.5 Security . . . . . . . . . . . . . . . . . . . . . . . . . .21
5.6 Synchronized Play-Out. . . . . . . . . . . . . . . . . . . .23
6. Service APIs. . . . . . . . . . . . . . . . . . . . . . . . . .23
7. Security Considerations . . . . . . . . . . . . . . . . . . . .24
8. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . .24
9. References. . . . . . . . . . . . . . . . . . . . . . . . . . .24
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .27
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . .28
1. Introduction
IP Multicast will play a prominent role on the Internet in the coming
years. It is a requirement, not an option, if the Internet is going
to scale. Multicast allows application developers to add more
functionality without significantly impacting the network.
Developing multicast-enabled applications is ostensibly simple.
Having datagram Access allows any application to send to a multicast
address. A multicast application need only increase the Internet
Protocol (IP) time-to-live (TTL) value to more than 1 (the default
value) to allow outgoing datagrams to traverse routers. To receive a
multicast datagram, applications join the multicast group, which
transparently generates an [IGMPv2, IGMPv3] group membership report.
This apparent simplicity is deceptive, however. Enabling multicast
support in applications and protocols that can scale well on a
heterogeneous network is a significant challenge. Specifically,
sending constant bit rate datastreams, reliable data delivery,
security, and managing many-to-many communications all require
special consideration. Some solutions are available, but many of
these services are still active research areas.
1.1 Motivation
The purpose of this document is to provide a framework for
understanding the challenges of designing and implementing multicast
applications. In order to use multicast communications correctly,
application developers must first understand the various I/O models
and the network services (in addition to basic multicast
communication) that are required. Secondly, application developers
need to be aware of efforts underway to provide these services. Such
efforts range in maturity from deployed commercial products to basic
research efforts to evaluate feasibility.
Multicast-based applications and services will play an important role
in the future of the Internet as continued multicast deployment
encourages their use and development. It is important that
developers be aware of the issues and solutions available--and
especially of their limitations--in order to avoid protocols that
negatively impact networks (thereby counter-acting the benefits of
multicast) or wasting their efforts "re-inventing the wheel".
The hope is that by raising developers' awareness, we can adjust
their expectations of finding solutions and lead them to successful,
scalable, and "network-friendly" development efforts.
1.2 Focus and Scope
Our initial premise is that the multicast infrastructure is
transparent to applications, so it is not directly relevant to this
discussion. Our focus here is on multicast application protocol
services, so this document explicitly avoids any discussion of
multicast routing issues. We identify and describe the multicast-
specific issues involved with developing applications.
We assume the reader has a general understanding of the mechanics of
multicast, and in this respect we intend to compliment other
introductory documents [BeauW, Maufer, Miller]. Since this is an
introductory survey rather than a comprehensive examination, we refer
readers to other multicast application requirements descriptions [RM,
LSMA, Miller] for more detail.
In the remainder of this document we first define the term "IP
multicast enabled network", the multicast infrastructure and
essential multicast services. Next we describe the types of new
functionality that multicast applications can enable and their
requirements. We then examine the services that satisfy these
requirements, the challenges they present, and provide a brief survey
of the solutions available or under development. We wrap up with a
discussion of application programming interfaces (APIs) for multicast
services.
2. IP Multicast Enabled Network
An "IP multicast-enabled network" provides end-to-end services in the
IP network infrastructure to allow any IP host to send datagrams to
an IP multicast address that any number of other IP hosts widely
dispersed can receive.
There are two kinds of multicast-enabled networks available. The
first is based on the original multicast service model as defined in
RFC1112 [Deering]. In this model, a receiver simply joins the group
and does not need to know the identity of the source(s). This model
is known by a number of names including Internet Standard Multicast
(ISM), Internet Traditional Multicast (ITM), Deering multicast, etc.
The second kind of multicast modifies the original service model such
that in addition to knowing the group address, a receiver must know
the set of relevant sources. This type of multicast is called Source
Specific Multicast (SSM) [SSM]. It becomes the application's
responsibility to know what kind of multicast capability the network
provides. Currently, the only way for an application to know the
type of multicast is based on the group address. If the group is in
the 232/8 range, the application should assume SSM is the service
model. Otherwise, the application should assume source-generic
multicast is the service model.
At the time of this writing, end-to-end "global" multicast service is
not yet available, but the size of the "multicast-enabled" Internet
is growing. Recent development and deployment of interdomain
multicast routing protocols and multicast-friendly Internet exchanges
have enabled peering between major ISPs. This, along with the
increasing availability of compelling content, is spurring deployment
and availability of the IP Multicast Enabled Network.
In the remainder of this document we assume that the multicast-
enabled network is already ubiquitous. Since such a large and
growing portion of the global Internet is IP multicast-enabled now,
and many enterprise networks (intranets) are also, this perspective
is relevant today.
2.1 Essential Protocol Components
An IP multicast enabled network requires two essential protocol
components:
1) An IP host-based protocol to allow a receiver application to
notify a local router(s) that it has joined the group, and
initiate the data flow from all sender(s) within the scope
2) An IP router-based protocol to allow any routers with multicast
group members (receivers) on their local networks to communicate
with other routers to ensure that all datagrams sent to the
group address are forwarded to all receivers within the intended
scope
Ideally, these protocol components are transparent to multicast
applications. However, there are two ASPects of their functionality
requirements that are worth mentioning specifically, since they
affect application performance and design. These are the multicast
application requirements for:
- Expedient Joins and Leaves
- Sends without a Join
2.1.1 Expedient Joins and Leaves
Some applications require expedient group joins and leaves, as their
usability or functionality are sensitive to the latency involved with
joining and leaving a group.
Join Latency: The time it takes for data to begin flowing after an
application issues a command to join a multicast group
Leave Latency: The time it takes for data to stop flowing after an
application issues a command to leave a multicast group
[IGMPv2,IGMPv3]
For example, using distributed a/v as a multicast-based "television"
must allow users to "channel surf"--changing channels frequently.
Each channel change generates a group leave and group join, so delays
in either will affect usability. In a sense, this is more of a user
requirement than an application requirement.
The functionality of distributed interactive simulations [DIS] is
often sensitive to join/leave latency. This is particularly true
when trying to exchange information to represent fast moving objects
that quickly enter and exit the scope of a simulated environment
(e.g., low-flying, fast-moving aircraft). If the join latency is too
long, the information provided may be old by the time it is received.
A fast leave phase is highly desirable both for effective congestion
control mechanisms, to stop undesirable flows quickly, and for the
network in general, to better filter unwanted traffic [Rizzo].
Applications cannot affect control over either join or leave latency,
but are dependent on the multicast infrastructure to enable expedient
operations. This is a basic multicast service requirement.
2.1.2 Sends without a Join
Applications that send to a multicast address should be able to start
sending immediately, without having to join the destination group
first. This is important for embedded devices and bursty-source
applications with low-delay delivery requirements.
The current IGMP-based multicast host model and all current
implementations allow senders to send to a group without joining it
as a standard feature.
3. IP Multicast Application Taxonomy
With an IP multicast-enabled network available, some unique and
powerful applications and application services are possible.
"Multicast enables coordination - it is well suited to loosely
coupled distributed systems (of people, servers, databases,
processes, devices...)" [Estrin].
A "multicast application" is simply defined as any application that
sends to and/or receives from an IP multicast address. These may or
may not also reference IP unicast addresses, as we describe later.
What differentiates IP multicast applications from one-to-one unicast
applications are the other sender and receiver relationships
multicast enables. There are three general categories of multicast
applications:
One-to-Many (1toM): A single host sending to two or more (n)
receivers
Many-to-Many (MtoM): Any number of hosts sending to the same
multicast group address, as well as receiving from it
Many-to-One (Mto1): Any number of receivers sending data back to a
(source) sender via unicast or multicast
+-----------------------------------+
Host 2->n ("many")
+-------------+---------------------+
One-Way Two-Way
+-------------+---------------------
A B C D E
+-----------+-------------+---------------------+
I/O S(m)/ S(u)/ S(m)/
Operations S(m) R(m) R(m) R(m) R(u)
+-------+---+-----------+-------------+---------------------
1 S(m) 1toM MtoM
Host 2 R(m) Mto1 MtoM
+---+-----------+-------------+
1 3 S(m)/R(m) Mto1 1toM MtoM
4 S(m)/R(u) Mto1
("one") 5 S(u)/R(m) Mto1
+-------+---+-----------+-----------------------------------+
Legend: S: "Send" (u): "unicast"
------ R: "Receive" (m): "multicast"
Table 1: Application types characterized by I/O relationships
and destination address types (multicast or unicast)
Table 1 defines these application types in terms of the I/O
relationships they represent. These categories are defined in terms
of the combination of communication mechanisms they use. At the IP
level, all multicast I/O is only 1toM or MtoM and unicast is always
one-to-one (1to1). The Mto1 category, for example, refers to several
possible combinations of IP-level relationships, including unicast.
We created the Mto1 category to help differentiate between the many
applications and services that use multicast.
1toM: MtoM: Mto1:
R1 S1/R1 S1
/ / \ S-R2 S2/R2-+-S3/R3 S2-R
\... \ / .../
Rn Sn/Rn Sn
Legend: S: "Sender"
------ R: "Receiver"
Figure 1: Generalization of the three application categories
Figure 1 illustrates the general model for each of the three
multicast application categories. Again it is worth emphasizing that
Mto1 is an artificial category defined by the application-level
relationship between sender(s) and receiver. At the IP-level,
multicast does not provide an Mto1 I/O mechanism, since it does not
allow senders to limit receivers, nor even know who they are.
Receiver information and limitations are enabled at the application
level, as required by the multicast application.
We describe each of these general application types in more detail
and provide application examples of each in the sub-sections below.
The list of examples is not comprehensive, but attempts to define the
prominent multicast application and service types in each of the
three general categories. We reference the items in these lists in
the remainder of this document as we describe their specific service
requirements, define the challenges they present, and reference
solutions available or under development.
3.1 One-to-Many Applications
One-to-Many (1toM) applications have a single sender, and multiple
simultaneous receivers. Entry B1 in Table 1 represents the classic
1toM relationship. Entry B3 differs only slightly, as the sender
also acts as receiver (i.e., it has loopback enabled).
When people think of multicast, they most often think of broadcast-
based multimedia applications: television (video) and radio (audio).
This is a reasonable analogy and indeed these are significant
multicast applications, but these are far from the extent of
applications that multicast can enable. Audio/Video distribution
represents a fraction of the multicast application possibilities, and
most do not have analogs in today's consumer broadcast industry.
a) Scheduled audio/video (a/v) distribution: Lectures,
presentations, meetings, or any other type of scheduled event
whose multimedia coverage could benefit an audience (i.e.
television and radio "broadcasts"). One or more constant-bit-
rate (CBR) datastreams and relatively high-bandwidth demands
characterize these applications. When more than one datastream
is present--as with an audio/video combination--the two are
synchronized and one typically has a higher priority than the
other(s). For example, in an a/v combination it is more
important to ensure an intelligible audio stream, than perfect
video.
b) Push media: News headlines, weather updates, sports scores, or
other types of non-essential dynamic information. "Drip-feed",
relatively low-bandwidth data characterize these applications.
c) File Distribution and Caching: Web site content, executable
binaries, and other file-based updates sent to distributed
end-user or replication/caching sites
d) Announcements: Network time, multicast session schedules,
random numbers, keys, configuration updates, (scoped) network
locality beacons, or other types of information that are
commonly useful. Their bandwidth demands can vary, but
generally they are very low bandwidth.
e) Monitoring: Stock prices, Sensor equipment (seismic activity,
telemetry, meteorological or oceanic readings), security
systems, manufacturing or other types of real-time information.
Bandwidth demands vary with sample frequency and resolution,
and may be either constant-bit-rate or bursty (if event-
driven).
3.2 Many-to-Many Applications
In many-to-Many (MtoM) applications two or more of the receivers also
act as senders. In other Words, MtoM applications are characterized
by two-way multicast communications.
The many-to-many capabilities of IP multicast enable the most unique
and powerful applications. Each host running an MtoM application may
receive data from multiple senders while it also sends data to all of
them. As a result, many-to-many applications often present complex
coordination and management challenges.
f) Multimedia Conferencing: Audio/Video and whiteboard comprise
the classic conference application. Having multiple
datastreams with different priorities characterizes this type
of application. Co-ordination issues--such as determining who
gets to talk when--complicate their development and usability.
There are common heuristics and "rules of play", but no
standards exist for managing conference group dynamics.
g) Synchronized Resources: Shared distributed databases of any
type (schedules, Directories, as well as traditional
Information System databases).
h) Concurrent Processing: Distributed parallel processing.
i) Collaboration: Shared document editing.
j) Distance Learning: This is a one-to-many a/v distribution
application with "upstream" capability that allows receivers to
question the speaker(s).
k) Chat Groups: These are like text-based conferences, but may
also provide simulated representations ("avatars") for each
"speaker" in simulated environments.
l) Distributed Interactive Simulations [DIS]: Each object in a
simulation multicasts descriptive information (e.g., telemetry)
so all other objects can render the object, and interact as
necessary. The bandwidth demands for these can be tremendous,
as the number of objects and the resolution of descriptive
information increases.
m) Multi-player Games: Many multi-player games are simply
distributed interactive simulations, and may include chat group
capabilities. Bandwidth usage can vary widely, although
today's first-generation multi-player games attempt to minimize
bandwidth usage to increase the target audience (many of whom
still use dial-up modems).
n) Jam Sessions: Shared encoded audio (e.g., music). The
bandwidth demands vary based on the encoding technique, sample
rate, sample resolution, number of channels, etc.
3.3 Many-to-One Applications
Unlike the one-to-many and many-to-many application categories, the
many-to-one (Mto1) category does not represent a communications
mechanism at the IP layer. Mto1 applications have multiple senders
and one (or a few) receiver(s), as defined by the application layer.
Table 1 shows that Mto1 applications can be one-way or use a two-way
request/response type protocol, where either senders or receiver(s)
may generate the request. Figure 2 characterizes the I/O
relationship possibilities in Mto1 applications:
Mto1 applications have many scaling issues. Too many simultaneous
senders can potentially overwhelm receiver(s), a condition
characterized as an "implosion problem". Another considerable
scaling problem is the large amount of state in the network that
having many multicast senders generates. This is largely transparent
to applications and the effect may be diminished in the future with
the use of bi-directional trees in multicast routing protocols, but
nonetheless it should be considered by application designers.
1) S1 2) S1 3) S1 4) S1
\ \ \ S2-R S2-R S2-R S2-R
.../ .../ .../ .../
Sn Sn Sn Sn
Data(m) Request(m) Request(m) Request(u)
------> ----------> <---------- ---------->
Response(u) Response(u) Response(m)
<----------- -----------> <----------
Figure 2: Characterization of Mto1 I/O possibilities
No standards yet exist for alternate and equivalent Mto1 application
designs, but there are a number of possibilities to consider [HNRS].
Since the main advantage of using multicast in a Mto1 application is
that senders need not know the receiver(s) unicast address(es), one
alternative is for each receiver to advertise its unicast address via
multicast. However, since this strategy still leaves the potential
for implosion on each receiver, additional strategies may be needed
to distribute the load. For example, it may be possible to increase
the number of receivers (in a "flat" receiver topology) or establish
localized receivers (in a "hierarchical" topology) as used in "local
recovery" (section 5.3). Such methods have coordination issues, and
since standard solutions have not yet been identified, Mto1
application developers should consider their alternatives carefully.
o) Resource Discovery: Service Location, for example, leverages IP
Multicast to enable something like a "host anycasting service"
capability [AnyCast]: A multicast receiver to send a query to a
group address, to elicit responses from the closest host so
they can satisfy the request. The responses might also contain
information that allows the receiver to determine the most
appropriate (e.g., closest) service provider to use.
In Table 1, this application is entry D4. It is also
illustrated in Figure 2 by possibility number 3.
Alternately, the response could be to a multicast rather
than unicast address, although technically that would make
it an MtoM application type (this is how the Service
Location Protocol [SLP] operates, when a user agent attempts
to locate a directory agent).
p) Data Collection: This is the converse of a one-to-many
"monitoring" application described earlier. In this case there
may be any number of distributed "sensors" that send data to a
data collection host. The sensors might send updates in
response to a request from the data collector, or send
continuously at regular intervals, or send spontaneously when a
pre-defined event occurs. Bandwidth demands can vary based on
sample frequency and resolution.
This is illustrated in Table 1 by entries A1 and A3, the only
difference being that A3 has a loopback interface. In Figure
2, this is possibility number 1. Since the number of receivers
can easily be more than one, this is really an MtoM
application.
q) Auctions: The "auctioneer" starts the bidding by describing
whatever it is for sale (product or service or whatever), and
receivers send their bids privately or publicly (i.e., to a
unicast or multicast address).
This is possibility number 2 in Figure 2, and D5 in Table 1.
The response could be sent to a multicast address, although
technically that would make it an MtoM application.
r) Polling: The "pollster" sends out a question, and the "pollees"
respond with answers. This is possibility number 2 in Figure
2, and could also be characterized as an MtoM application if
the response is to a multicast address.
s) Jukebox: Allows near-on-demand a/v playback. Receivers use an
"out-of-band" protocol mechanism (via web, email, unicast or
multicast requests, etc.) to send their playback request into a
scheduling queue [IMJ].
This is characterized by possibility number 4 in Figure 2, and
entry D4 in Table 1. The initial unicast request is the only
difference between this type of application and a typical 1toM.
If that initial request were sent to a multicast address, this
would effectively be an MtoM application.
t) Accounting: This is basically data collection but is worth
separating since it is such an important application. In some
multicast applications it is imperative to know information
about each receiver, possibly in real-time. But such
information can be overwhelming [MRM]. Current mechanisms,
like RTCP (which is actually MtoM since it is multicast but
could be made Mto1), use scaling techniques but trade-off
information granularity. As a group grows the total amount of
feedback is constant but each receiver sends less.
4. Common Multicast Service Requirements
Some multicast application service requirements are common to unicast
network applications as well. We characterize two of them here--
bandwidth and delay requirements.
4.1 Bandwidth Requirements
Figure 3 shows multicast applications approximate bandwidth
requirements.
Unicast and multicast applications both need to design applications
to adapt to the variability of network conditions. But as we
describe in section 5.3, it is the need to accommodate multiple
heterogeneous multicast receivers--with their diversity of bandwidth
capacity and delivery delays--that presents the unique challenge for
multicast applications to satisfy these requirements.
1toM b, d c, e a
MtoM k g, i f, h, j, l, m, n
Mto1 o, q, r p, t s
+-----------------------------------------------
Low Bandwidth High Bandwidth
Figure 3: Bandwidth Requirements of applications
4.2 Delay Requirements
Aside from those with time-sensitive data (e.g., stock prices, and
real-time monitoring information), most one-to-many applications have
a high tolerance for delay and delay variance (jitter). Constant
bit-rate (CBR) data--such as streaming media (audio/video)--are
sensitive to jitter, but applications commonly counteract the effects
by buffering data and delaying playback.
Most many-to-one and many-to-many multicast applications are
intolerant of delays because they are bidirectional, interactive and
request/response dependent. As a result, delays should be minimized,
since they can adversely affect the application's usability.
This need to minimize delays is most evident in (two-way) conference
applications, where users cannot converse effectively if the audio or
video is delayed more than 500 milliseconds. For this and other
examples see Figure 4, which plots multicast applications on a
(coarse) scale of sensitivity to delivery delays.
1toM b, c a, d e
MtoM g, i, j, k f, h, l, m, n
Mto1 r o, p, s, t q
+-----------------------------------------------
Delay Tolerant Delay Intolerant
Figure 4: Delay tolerance of application types
For delay-intolerant multicast (or unicast) applications, quality of
service (QoS) is the only option. IP networks currently provide only
"best effort" delivery, so data are subject to variable router
queuing delays and loss due to network congestion (router queue
overflows). IP QoS standards do exist now [RSVP] and efforts to
enable end-to-end QoS support in the Internet are underway [E2EQOS].
However, QoS support is an IP network infrastructure consideration.
Although there are multicast-specific challenges for implementing QoS
in the network for multicast flows, they are beyond the control of
applications, so further discussion of the QoS protocols and services
is beyond the scope of this document.
5. Unique Multicast Service Requirements
The three application categories described earlier are very general
in nature. Within each category and even among each of the
application types, specific application instances have a variety of
application requirements. One-to-many application types are
relatively simple to develop, but as we pointed out there are
challenges involved with developing many-to-one and many-to-many
applications. Some of these have requirements bandwidth and delay
requirements, similar to unicast applications.
Multicast applications can be further differentiated from unicast
applications and from each other by the services they require. In
this section we provide a survey of the various services that have
unique requirements for multicast applications.
+--------------------------------------------------------------+
Multicast Application
+--------------------------------------+ +-------------------+
+-------------------------------------+ +--------++--------+
Multicast Security
+----------------------+ +----------+ System
+----------++---------+ +---------+ Time Codecs
Reliable Address Session
Delivery Mgt Mgt
+----------++---------++---++---------++---++--------++--------+
+----------------------------------------++--------------------+
Basic IP Multicast Service IP Unicast
(e.g., UDP and IGMPv2/v3) Service
+----------------------------------------++--------------------+
Figure 5: Multicast service requirements summary
Here's the list of multicast application service requirements:
Address Management - Selection and coordinated of address
allocation. The need is to provide assurances against "address
collision" and to provide address ownership.
Session Management - Perform application-layer services on top of
multicast transport. These services depend heavily on the
application but include functions like session advertisement,
billing, group member monitoring, key distribution, etc.
Heterogeneous Receiver Support - Sending to receivers with a wide
variety of bandwidth capacities, latency characteristics, and
network congestion requires feedback to monitor receiver
performance.
Reliable Data Delivery - Ensuring that all data sent is received
by all receivers.
Security - Ensuring content privacy among dynamic multicast group
memberships, and limiting senders.
Synchronized Play-Out - Allow multiple receivers to "replay" data
received in synchronized fashion.
In the remainder of this section, we describe each of these
application services in more detail, the challenges they present, and
the status of standardized solutions.
5.1 Address Management
One of the first questions facing a multicast application developer
is what multicast address to use. Multicast addresses are not
assigned to individual hosts, assignments can change dynamically, and
addresses sometimes have semantics of their own (e.g., Admin
Scoping). Multicast applications require an address management
service that provides address allocation or assignment queries.
There are a number of ways for applications to learn about multicast
addresses:
Hard-Coded: Software configuration, encoded in a binary
executable, or burned into ROM in embedded devices. These
applications typically reference IANA statically allocated
multicast addresses (including relative addresses).
Advertised: Session announcements (as described in the next
section), or via another "out-of-band" query or discovery protocol
mechanism.
Algorithmically Derived: Using a programmatic algorithm to
allocate a statistically random (unused) address.
1toM c, e a, b d
MtoM f, j, k, n g, h, i, l, m
Mto1 r o, p, s q, t
+-----------------------------------------------
Hard-Coded Advertised Algorithmic
Figure 6: Multicast address usage for application types
In almost all cases, application designers should assume that
multicast addresses are to be dynamic. Very little of the multicast
address space is available for static assignment by IANA [MADDR].
Also, given the host-specific addressing available with SSM,
Internet-wide, static address assignment is expected to be very rare.
5.2 Session Management
Session management is one of the most misunderstood services with
respect to multicast. Most application developers assume that
multicast will provide services like security, encryption,
reliability, session advertisement, monitoring, billing, etc. In
fact, multicast is simply a transport mechanism that provides end-
to-end delivery. All of the other services are application-layer
services that must be provided by each particular application.
Furthermore, in most cases there are not defined standards for how
these functions should be provided. The particular functions are
dependent on the particular needs of the application, and no single
method (or standard) can be made to be sufficient for all cases.
While there are no generic solutions which provide all session
management functions, there are some protocols and common techniques
that provide support for some of the functions. Techniques for
congestion control and heterogeneous receiver support are discussed
in Section 5.3. Protocols for reliability are discussed in Section
5.4. Security considerations are discussed in Section 5.5.
With respect to session advertisement, there are a number of
mechanisms for advertising sessions. One commonly used technique is
to advertise sessions via the WWW. Users can join a group by
clicking on URLs, and then having a response returned to the user
that includes the group address and maybe information about group
source(s). Another mechanism is the session description protocol
[SDP]. It provides a format for representing information about
sessions, but it does not provide the transport for dissemination of
these session descriptions, nor does it provide address allocation
and management. SDP only provides the syntax for describing session
attributes.
SDP session descriptions may be conveyed publicly or privately by
means of any number of transports including web (HTTP) and MIME
encoded email. The session announcement protocol [SAP] is the de
facto standard transport and many multicast-enabled applications
currently use it. SAP limits distribution via multicast scoping, but
the current protocol definition has scaling issues that need to be
addressed. Specifically, the initialization latency for a session
directory can be quite long, and it increases in proportion to the
number of session announcements. This is to an extent a multicast
infrastructure issue, however, as this level of protocol detail
should be transparent to applications.
The session management service needs to:
- Advertise scheduled sessions
- Provide a query mechanism for retrieving
information about session schedules
5.3 Heterogeneous Receiver Support
The Internet is a network of networks. IP's strength is its ability
to enable seamless interoperability between hosts on disparate
network media, the heterogeneous network.
When two hosts communicate via unicast--one-to-one--across an IP
network, it is relatively easy for senders to adapt to varying
network conditions. The Transmission Control Protocol (TCP) provides
reliable data transport, and is the model of "network friendly"
adaptability.
TCP receivers send acknowledgements back to the sender for data
delivered. A TCP sender detects data loss from the data sent that is
not acknowledged. When it detects data loss, TCP infers that there
is network congestion or a low-bandwidth link, and adapts by
throttling down its send rate [SlowStart].
User Datagram Protocol (UDP) does not enable a receiver feedback loop
the way TCP does, since UDP does not provide reliable data delivery
service. As a result, it also does not have a loss detection and
adaptive congestion control mechanism as TCP does. However, it is
possible for a unicast UDP application to enable similar adaptive
algorithms to achieve the same result, or even improve on it.
A unicast UDP application that uses a feedback mechanism to detect
data loss and adapt the send rate, can do so better than TCP. TCP
automatically reduces the "congestion window" when data loss is
detected, although the updated send rate may be slower than a CBR
audio/video stream requires. When a UDP application detects loss, it
can adapt the data itself to accommodate the lower send rate. For
example, a UDP application can:
- Reduce the data resolution (e.g., send lower fidelity
audio/video by reducing sample frequency or frame rate) to
reduce data rate.
- Modify the data encoding to add redundant data (e.g., forward
error correction) offset in time to avoid fate sharing. This
could also be "layered", so a percentage of data loss will
simply reduce fidelity rather than corrupt the data.
- Reduce the send rate of one datastream in order to favor another
of higher priority (e.g., sacrifice video in order to ensure
audio delivery).
- Send data at a lower rate (i.e., with a different encoding) on a
separate multicast address and/or port number for high-loss
receivers.
However, with multicast applications--one-to-many or many-to-many--
which have multiple receivers, the feedback loop design needs
modification. If all receivers return data loss reports
simultaneously, the sender is easily overwhelmed in the storm of
replies. This is known as the "implosion problem".
Another problem is that heterogeneous receiver capabilities can vary
widely due to the wide range of (static) network media bandwidth
capabilities and dynamically due to transient traffic conditions. If
a sender adapts its send rate and data resolution based on the loss
rate of its worst receiver(s), then it can only service the lowest
common denominator. Hence, a single "crying baby" can spoil it for
all other receivers.
Strategies exist for dealing with these heterogeneous receiver
problems. Here are two examples:
Shared Learning - When loss is detected (i.e., a sequenced packet
isn't received), a receiver starts a random timer. If it
receives a data loss report sent by another receiver as it waits
for the timer to expire, it stops the timer and does not send a
report. Otherwise, it sends a report when the timer expires.
The Real-Time Protocol and its feedback-loop counterpart Real-
Time Control Protocol [RTP/RTCP] employ a strategy similar to
this to keep feedback traffic to 5 percent or less than the
overall session traffic. This technique was originally utilized
in IGMP.
Local Recovery - Some receivers may be designated as local
distribution points or "transcoders" that either re-send data
locally (possibly via unicast) when loss is reported or they re-
encode the data for lower bandwidth receivers before re-sending.
No standards exist for these strategies, although "local
recovery" is used by several reliable multicast protocols.
Adaptive multicast application design for heterogeneous receivers is
still an active area of research. The fundamental requirements are
to maximize application usability, while accommodating network
conditions in a "network friendly" manner. In other words,
congestion detection and avoidance are (at least) as important in
protocol design as the user experience. The adaptive mechanisms must
also be stable, so they do not adapt too quickly--changing encoding
and rates based on too little information about what may be a
transient condition--to avoid oscillation.
This "feedback loop" service necessary for support of heterogeneous
receivers is not illustrated in the services summary in Figure 4,
although it could be added alongside "Reliable Transport" and the
others. This service could be implemented within an application or
accessed externally, as provided by the operating system or a third
party. See [HNRS] for a taxonomy of strategies for providing
feedback for multicast, with the ultimate goal of developing a common
multicast feedback protocol.
5.4 Reliable Data Delivery
Many of the multicast application examples in our list--like
audio/video distribution--have loss-tolerant data content. In other
words, the data content itself can remain useful even if some of it
is lost. For example, audio might have a short gap or lower fidelity
but will remain intelligible despite some data loss.
Other application examples--like caching and synchronized resources-
-require reliable data delivery. They deliver content that must be
complete, unchanged, in sequence, and without duplicates. The "Loss
Intolerant" column in Figure 7 shows a list of applications with this
requirement, while the others can tolerate varying levels of data
loss. The tolerance levels are typically determined by the nature of
the data and the encoding in use.
1toM b a, d c, e
MtoM f, j, k, l, m, n g, h, i
Mto1 o, p, r, s, t q
+------------------------------------------------
Loss Tolerant Loss Intolerant
Figure 7: Reliability Requirements of Application types
Some of the challenges involved with enabling reliable multicast
transport are the same as those of sending to heterogeneous
receivers, and some solutions are similar also. For example, many
reliable multicast transport protocols avoid the implosion problem by
using negative acknowledgements (NAKs) from receivers to indicate
what was lost. They also use "shared learning" whereby receivers
listen to others' NAKs and then listen for the resulting
retransmission of data, rather than requesting retransmission by
sending a NAK themselves.
Although reliable delivery cannot change the data sent--except,
perhaps, to use a loss-less data compression algorithm--they can use
other adaptive techniques like sending redundant data, or adjusting
the send rate.
Although many reliable multicast protocol implementations exist
[Obraczka], and a few are already available in commercial products,
none of them are standardized. Work is ongoing in the "Reliable
Multicast" research group of the Internet Research Task Force [IRTF]
to provide a better definition of the problem, the multicast
transport requirements, and protocol mechanisms.
Scalability is the paramount concern, and it implies the general need
for "network friendly" protocols that detect and avoid congestion as
they provide reliable delivery. Other considerations are protocol
robustness, support for "late joins", group management and security
(which we discuss next).
The current consensus is that due to the wide variety of multicast
application requirements--some of which are at odds--no single
multicast transport will likely be appropriate for all applications.
As a result, most believe that we will eventually standardize a
number of reliable multicast protocols, rather than a single one
[BULK, RMT].
5.5 Security
For any IP network application--unicast or multicast--security is
necessary because networks comprise users with different levels of
trust.
Network application security is challenging, even for unicast. And
as the need for security increases--gauged by the risks of being
without it--the challenges increase also. Security system complexity
and overhead is commensurate with the protection it provides. "No
one can guarantee 100% security. But we can work toward 100% risk
acceptance...Strong cryptography can withstand targeted attacks up to
a point--the point at which it becomes easier to get the information
some other way...A good design starts with a threat model: what the
system is designed to protect, from whom, and for how long."
[Schneier]
Multicast applications are no different than unicast applications
with respect to their need for security, and they require the same
basic security services: user authentication, data integrity, data
privacy and user privacy (anonymity). However, enabling security for
multicast applications is even more of a challenge than for unicast.
Having multiple receivers makes a difference, as does their
heterogeneity and the dynamic nature of multicast group memberships.
Multicast security requirements can include any combination of the
following services:
Limiting Senders - Controlling who can send to group addresses
Limiting Receivers - Controlling who can receive
Limiting Access - Controlling who can "read" multicast content
either by encrypting content or limiting receivers (which isn't
possible yet)
Verifying Content - Ensuring that data originated from an
authenticated sender and was not altered en route
Protecting Receiver Privacy - Controlling whether sender(s) or
other receivers know receiver identity
Firewall Traversal - Proxying outgoing "join" requests through
firewalls, allowing incoming or outgoing traffic through, and
(possibly) authenticating receivers for filtering purposes and
security [Finlayson].
This list is not comprehensive, but includes the most commonly needed
security services. Different multicast applications and different
application contexts can have very different needs with respect to
these services, and others. Two main issues emerge, where the
performance of current solutions leaves much to be desired [MSec].
Individual authentication - how is sender identity verified for
each multicast datagram received?
Membership revocation - how is further group access disabled for
group members that leave the group (e.g., encryption keys in their
possession disabled)?
Performance is largely a factor when a user joins or leaves a group.
For example, methods used to authenticate potential group members
during joins or re-keying current members after a member leaves can
involve significant processing and protocol overhead and result in
significant delays that affect usability.
Like reliable multicast, secure multicast is also under investigation
in the Internet Research Task Force [IRTF]. Protocol mechanisms for
many of the most important of these services--such as limiting
senders--have not yet been defined, let alone developed and deployed.
As is true for reliable multicast, the current consensus is that no
single security protocol will satisfy the wide diversity of
sometimes-contradictory requirements among multicast applications.
Hence, multicast security will also likely require a number of
different protocols.
5.6 Synchronized Play-Out
This refers to having all receivers simultaneously play-out the
multicast data they received. This may be necessary for fairness--
playing-out prices for auctions, or stock-prices--or to ensure
synchronization with other receivers, such as when playing music.
Here is an analogy to illustrate: Imagine a multi-speaker stereo
system that is wired throughout a home (via analog). With the stereo
playing on all speaker sets, you will hear continuous music as you
walk from room-to-room.
Now imagine a house full of multi-media and network enabled computer
systems. Although they will all receive the same music datastream
simultaneously via multicast, they will provide discontinuous music
playback as you walk room-to-room.
To provide synchronized playback that would enable continuous music
from room-to-room would require three things:
1) system clocks on all systems should be synchronized
2) datastreams must be framed with timestamps
3) you must know the playback latency of the multimedia hardware
The third of these is the most difficult to achieve at this time.
Hardware and drivers don't provide any mechanism for retrieving this
information, although different audio and video devices have a wide-
range of performance.
6. Service APIs
In some cases, the protocol services mentioned in this document can
be enabled transparently by passive configuration mechanisms and
"middleware". For example, it is conceivable that a UDP
implementation could implicitly enable a reliable multicast protocol
without the explicit interaction of the application.
Sometimes, however, applications need explicit access to these
services for flexibility and control. For example, an adaptive
application sending to a heterogeneous group of receivers using RTP
may need to process RTCP reports from receivers in order to adapt
accordingly (by throttling send rate or changing data encoders, for
example) [RTP API]. Hence, there is often a need for service APIs
that allow an application to qualify and initiate service requests,
and receive event notifications. In Figure 5, the top edge of the
box for each service effectively represents its API.
Network APIs generally reflect the protocols they support. Their
functionality and argument values are a (varying) subset of protocol
message types, header fields and values. Although some protocol
details and actions may not be exposed in APIs--since many protocol
mechanics need not be exposed--others are crucial to efficient and
flexible application operation.
A more complete examination of the application services described in
this document might also identify the protocol features that could be
mapped to define a (generic) API definition for that service. APIs
are often controversial, however. Not only are there many language
differences, but it is also possible to create different APIs by
exposing different levels of detail in trade-offs between flexibility
and simplicity.
7. Security Considerations
See section 5.4
8. Acknowledgements
The authors would like to acknowledge and thank the following
individuals for their helpful feedback: Ran Canetti, Brian Haberman,
Eric A. Hall, Kenneth C. Miller, and Dave Thaler.
9. References
[AnyCast] Partridge, C., Mendez, T. and W. Milliken, "Host
Anycasting Service", RFC1546, November 1993.
[BeauW] B. Williamson, "Developing IP Multicast Networks, Volume
I", (c) 2000 Cisco Press, Indianapolis IN, ISBN 1-57870-
077-9.
[BULK] Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
Floyd, S. and M. Luby, "Reliable Multicast Transport
Building Blocks for One-to-Many Bulk-Data Transfer", RFC
3048, January 2001.
[Deering] Deering, S., "Host Extensions for IP Multicasting", STD
5, RFC1112, August 1989.
[DIS] Pullen, J., Mytak, M. and C. Bouwens, "Limitations of
Internet Protocol Suite for Distributed Simulation in the
Large Multicast Environment", RFC2502, February 1999.
[E2EQOS] Bernet, Y., Yavatkar, R., Ford, P., Baker, F., Zhang, L.,
Speer, M., Braden, R. and B. Davie, "Integrated Services
Operation over Diffserv Networks", RFC2998, November
2000.
[Estrin] D. Estrin, "Multicast: Enabler and Challenge", Caltech
Earthlink Seminar Series, April 22, 1998.
[Finlayson] Finlayson, R., "IP Multicast and Firewalls", RFC2588,
May 1999.
[HNRS] Hofman, Nonnenmacher, Rosenberg, Schulzrinne, "A Taxonomy
of Feedback for Multicast", June 1999, Work in Progress.
[IGMPv2] Fenner, B., "Internet Group Management Protocol, Version
2", RFC2236, November 1997.
[IGMPv3] Cain, B., Deering, S., Kouvelas, I. and A. Thyagarajan,
"Internet Group Management Protocol, Version 3", Work in
Progress.
[IMJ] K. Almeroth and M. Ammar, "The Interactive Multimedia
Jukebox (IMJ): A New Paradigm for the On-Demand Delivery
of Audio/Video", Proceedings of the Seventh International
World Wide Web Conference, Brisbane, AUSTRALIA, April
1998.
[IRTF] Weinrib, A. and J. Postel, "The IRTF Guidelines and
Procedures", BCP 8, RFC2014, January 1996.
[Kermode] Kermode, R., "MADCAP Multicast Scope Nesting State
Option", RFC2907, September 2000.
[LSMA] Bagnall, P., Briscoe, R. and A. Poppitt, "Taxonomy of
Communication Requirements for Large-scale Multicast
Applications", RFC2729, December 1999.
[MADDR] Albanna, Z., Almeroth, K., Meyer, D. and M. Schipper,
"IANA Guidelines for IPv4 Multicast Address Assignments",
BCP 51, RFC3171, August 2001.
[MASC] Estrin, D., Govindan, R., Handley, M., Kumar, S.,
Radoslavov, P. and D. Thaler, "The Multicast Address-Set
Claim (MASC) Protocol", RFC2909, September 2000.
[Maufer] T. Maufer, "Deploying IP Multicast in the Enterprise",
(c) 1998 Prentice Hall, Upper Saddle River NJ ISBN 0-13-
897687-2.
[Miller] C. K. Miller, "Multicast Networking and Applications",
(c) 1999 Addison Wesley Longman, Reading MA ISBN 0-201-
30979-3.
[MADCAP] Hanna, S., Patel, B. and M. Shah, "Multicast Address
Dynamic Client Allocation Protocol (MADCAP)", RFC2730,
December 1999.
[MRM] K. Sarac, K. Almeroth, "Supporting Multicast Deployment
Efforts: A Survey of Tools for Multicast Monitoring",
Journal of High Speed Networking--Special Issue on
Management of Multimedia Networking, March 2001
[MSec] Multicast Security (msec) IETF Working Group charter
[MZAP] Handley, M., Thaler, D. and R. Kermode, "Multicast-Scope
Zone Announcement Protocol (MZAP)", RFC2776, February
2000.
[Obraczka] K. Obraczka "Multicast Transport Mechanisms: A Survey and
Taxonomy", IEEE Communications Magazine, Vol. 36 No. 1,
January 1998.
[Rizzo] L. Rizzo, "Fast Group management in IGMP", HIPPARC 98
workshop, June 1998, UCL London
http://www.iet.unipi.it/~luigi/hipparc98.ps.gz
[RM] Mankin, A., Romanow, A., Bradner, S. and V. Paxson,
"IETF Criteria for Evaluating Reliable Multicast
Transport and Application Protocols", RFC2357, June
1998.
[RSVP] Wroclawski, J., "The Use of RSVP with IETF Integrated
Services", RFC2210, September 1997.
[RTP API] H. Schulzrinne, et al, "RTP Library API Specification,"
http://www.cs.columbia.edu/IRT/software/rtplib/rtplib-
1.0a1/rtp_api.Html
[RTP/RTCP] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC1889, January 1996.
[SAP] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC2974, October 2000.
[SDP] Handley, M., and V. Jacobson, "SDP: Session Description
Protocol", RFC2327, April 1998.
[Schneier] B. Schneier, "Why Cryptography Is Harder Than It Looks",
December 1996, http://www.counterpane.com/whycrypto.html
[SlowStart] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC2001,
January 1997.
[SLP] Veizades, J., Guttman, E., Perkins, C. and S. Kaplan,
"Service Location Protocol", RFC2165, June 1997.
[SSM] Holbrook, H. and B. Cain, "Specific Multicast for IP",
Work in Progress.
10. Authors' Addresses
Bob Quinn
Celox Networks
2 Park Central Drive
Southborough, MA 01772
Phone: +1 508 305 7000
EMail: bquinn@celoxnetworks.com
Kevin Almeroth
Department of Computer Science
University of California
Santa Barbara, CA 93106-5110
Phone: +1 805 893 2777
EMail: almeroth@cs.ucsb.edu
11. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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Funding for the RFCEditor function is currently provided by the
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