[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03

INTERNET-DRAFT                                                C. Semeria
                                                               T. Maufer
Category: Informational                                 3Com Corporation
                                                            January 1997



               Introduction to IP Multicast Routing

            <draft-ietf-mboned-intro-multicast-00.txt>


Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

Internet Drafts are draft documents valid for a maximum of six months.
Internet Drafts may be updated, replaced, or obsoleted by other
documents at any time.  It is not appropriate to use Internet Drafts as
reference material or to cite them other than as a "working draft" or
"work in progress."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the internet-drafts Shadow
Directories on:

         ftp.is.co.za            (Africa)
         nic.nordu.net           (Europe)
         ds.internic.net  (US East Coast)
         ftp.isi.edu      (US West Coast)
         munnari.oz.au      (Pacific Rim)


FOREWORD

This document is introductory in nature.  We have not attempted to
describe every detail of each protocol, rather to give a concise
overview in all cases, with enough specifics to allow a reader to grasp
the essential details and operation of protocols related to multicast
IP.  Every effort has been made to ensure the accurate representation of
any cited works, especially any works-in-pro- gress.  For the complete
details, we refer you to the relevant specification(s).

If internet-drafts are cited in this document, it is only because they
are the only sources of certain technical information at the time of
this writing.  We expect that many of the internet-drafts which we have
cited will eventually become RFCs.  See the shadow directories on the
previous page for the status of any of these drafts, their follow-on
drafts, or possibly the resulting RFCs.




Semeria & Maufer                                                [Page 1]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


ABSTRACT

The first part of this paper describes the benefits of multicasting,
the MBone, Class D addressing, and the operation of the Internet Group
Management Protocol (IGMP).  The second section explores a number of
different techniques that may potentially be employed by multicast
routing protocols:

    o  Flooding
    o  Spanning Trees
    o  Reverse Path Broadcasting (RPB)
    o  Truncated Reverse Path Broadcasting (TRPB)
    o  Reverse Path Multicasting (RPM)
    o  "Shared-Tree" Techniques

The third part contains the main body of the paper.  It describes how
the previous techniques are implemented in multicast routing protocols
available today (or under development).

    o  Distance Vector Multicast Routing Protocol (DVMRP)
    o  Multicast Extensions to OSPF (MOSPF)
    o  Protocol-Independent Multicast (PIM)
    o  Core-Based Trees (CBT)


                          Table of Contents
Section

1  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  INTRODUCTION
1.1  . . . . . . . . . . . . . . . . . . . . . . . . .  Multicast Groups
1.2  . . . . . . . . . . . . . . . . . . . . . Group Membership Protocol
1.3  . . . . . . . . . . . . . . . . . . . . Multicast Routing Protocols
1.3.1  . . . . . . . . . . .  Multicast Routing vs. Multicast Forwarding
2  . . . . . . . .  MULTICAST SUPPORT FOR EMERGING INTERNET APPLICATIONS
2.1  . . . . . . . . . . . . . . . . . . . . . . . Reducing Network Load
2.2  . . . . . . . . . . . . . . . . . . . . . . . .  Resource Discovery
2.3  . . . . . . . . . . . . . . .  Support for Datacasting Applications
3  . . . . . . . . . . . . . . THE INTERNET'S MULTICAST BACKBONE (MBone)
4  . . . . . . . . . . . . . . . . . . . . .  . . . MULTICAST ADDRESSING
4.1  . . . . . . . . . . . . . . . . . . . . . .  . .  Class D Addresses
4.2  . . . . . . .  Mapping a Class D Address to an IEEE-802 MAC Address
4.3  . . . . . . . . .  Transmission and Delivery of Multicast Datagrams
5  . . . . . . . . . . . . . . INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)
5.1  . . . . . . . . . . . . . . . . . . . . . . . . . .  IGMP Version 1
5.2  . . . . . . . . . . . . . . . . . . . . . . . . . .  IGMP Version 2
5.3  . . . . . . . . . . . . . . . . . . . . . . . . . .  IGMP Version 3
6  . . . . . . . . . . . . . . . . . . . MULTICAST FORWARDING TECHNIQUES
6.1  . . . . . . . . . . . . . . . . . . . . . "Simpleminded" Techniques
6.1.1  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Flooding
6.1.2  . . . . . . . . . . . . . . . . . . . . . . . . . . Spanning Tree
6.2  . . . . . . . . . . . . . . . . . . .  Source-Based Tree Techniques
6.2.1  . . . . . . . . . . . . . . . . . Reverse Path Broadcasting (RPB)

Semeria & Maufer                                                [Page 2]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


6.2.1.1  . . . . . . . . . . . . .  Reverse Path Broadcasting: Operation
6.2.1.2. . . . . . . . . . . . . . . . . . RPB: Benefits and Limitations
6.2.2  . . . . . . . . . . .  Truncated Reverse Path Broadcasting (TRPB)
6.2.3  . . . . . . . . . . . . . . . . . Reverse Path Multicasting (RPM)
6.2.3.1  . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation
6.2.3.2  . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations
6.3  . . . . . . . . . . . . . . . . . . . . . .  Shared Tree Techniques
6.3.1  . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation
6.3.2  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Benefits
6.3.3  . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations
7  . . . . . . . . .  SOURCE-BASED TREE ("DENSE MODE") ROUTING PROTOCOLS
7.1  . . . . . . . .  Distance Vector Multicast Routing Protocol (DVMRP)
7.1.1  . . . . . . . . . . . . . . . . .  Physical and Tunnel Interfaces
7.1.2  . . . . . . . . . . . . . . . . . . . . . . . . . Basic Operation
7.1.3  . . . . . . . . . . . . . . . . . . . . .  DVMRP Router Functions
7.1.4  . . . . . . . . . . . . . . . . . . . . . . . DVMRP Routing Table
7.1.5  . . . . . . . . . . . . . . . . . . . . .  DVMRP Forwarding Table
7.1.6  . . . . . . . . . . . . . . . . . Hierarchical DVMRP (DVMRP v4.0)
7.1.6.1  . . . . . . . . . .  Benefits of Hierarchical Multicast Routing
7.1.6.2  . . . . . . . . . . . . . . . . . . . Hierarchical Architecture
7.2  . . . . . . . . . . . . . . .  Multicast Extensions to OSPF (MOSPF)
7.2.1  . . . . . . . . . . . . . . . . . . Intra-Area Routing with MOSPF
7.2.1.1  . . . . . . . . . . . . . . . . . . . . .  Local Group Database
7.2.1.2  . . . . . . . . . . . . . . . . . Datagram's Shortest Path Tree
7.2.1.3  . . . . . . . . . . . . . . . . . . . . . . .  Forwarding Cache
7.2.2  . . . . . . . . . . . . . . . . . . Mixing MOSPF and OSPF Routers
7.2.3  . . . . . . . . . . . . . . . . . . Inter-Area Routing with MOSPF
7.2.3.1  . . . . . . . . . . . . . . . . Inter-Area Multicast Forwarders
7.2.3.2  . . . . . . . . . . .  Inter-Area Datagram's Shortest Path Tree
7.2.4  . . . . . . . . . Inter-Autonomous System Multicasting with MOSPF
7.3  . . . . . . . . . . . . . . .  Protocol-Independent Multicast (PIM)
7.3.1  . . . . . . . . . . . . . . . . . . . . PIM - Dense Mode (PIM-DM)
8  . . . . . . . . . . . . SHARED TREE ("SPARSE MODE") ROUTING PROTOCOLS
8.1  . . . . . . . Protocol-Independent Multicast - Sparse Mode (PIM-SM)
8.1.1  . . . . . . . . . . . . . .  Directly Attached Host Joins a Group
8.1.2  . . . . . . . . . . . . Directly Attached Source Sends to a Group
8.1.3  . . . . . . .  Shared Tree (RP-Tree) or Shortest Path Tree (SPT)?
8.1.4  . . . . . . . . . . . . . . . . . . . . . . .   Unresolved Issues
8.2  . . . . . . . . . . . . . . . . . . . . . .  Core-Based Trees (CBT)
8.2.1  . . . . . . . . . . . . . . . . . . Joining a Group's Shared Tree
8.2.2  . . . . . . . . . . . . . . . . . . . Primary and Secondary Cores
8.2.3  . . . . . . . . . . . . . . . . . . . . .  Data Packet Forwarding
8.2.4  . . . . . . . . . . . . . . . . . . . . . . .  Non-Member Sending
8.2.5  . . . . . . . . . . . . . . . . . . Emulating Shortest-Path Trees
8.2.6  . . . . . . . . . . . . . . . . .  CBT Multicast Interoperability
9   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCES
9.1 . . . . . . . . . . . . . . . . . . . . Requests for Comments (RFCs)
9.2 . . . . . . . . . . . . . . . . . . . . . . . . . .  Internet Drafts
9.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Textbooks
9.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Other
10  . . . . . . . . . . . . . . . . . . . . . .  SECURITY CONSIDERATIONS
11  . . . . . . . . . . . . . . . . . . . . . . . . . AUTHORS' ADDRESSES

Semeria & Maufer                                                [Page 3]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


1. Introduction

There are three fundamental types of IPv4 addresses:  unicast,
broadcast, and multicast.  A unicast address is used to transmit a
packet to a single destination.  A broadcast address is used to send a
datagram to an entire subnetwork.  A multicast address is designed to
enable the delivery of datagrams to a set of hosts that have been
configured as members of a multicast group across various
subnetworks.

Multicasting is not connection-oriented.  A multicast datagram is
delivered to destination group members with the same "best-effort"
reliability as a standard unicast IP datagram.  This means that
multicast datagrams are not guaranteed to reach all members of a group,
nor to arrive in the same order in which they were transmitted.

The only difference between a multicast IP packet and a unicast IP
packet is the presence of a 'group address' in the Destination Address
field of the IP header.  Instead of a Class A, B, or C IP destination
address, multicasting employs a Class D address format, which ranges
from 224.0.0.0 to 239.255.255.255.

1.1 Multicast Groups

Individual hosts are free to join or leave a multicast group at any
time.  There are no restrictions on the physical location or the number
of members in a multicast group.  A host may be a member of more than
one multicast group at any given time and does not have to belong to a
group to send packets to members of a group.

1.2 Group Membership Protocol

A group membership protocol is employed by routers to learn about the
presence of group members on their directly attached subnetworks.  When
a host joins a multicast group, it transmits a group membership protocol
message for the group(s) that it wishes to receive, and sets its IP
process and network interface card to receive frames addressed to the
multicast group.  This receiver-initiated join process has excellent
scaling properties since, as the multicast group increases in size, it
becomes ever more likely that a new group member will be able to locate
a nearby branch of the multicast delivery tree.





[This space was intentionally left blank.]






Semeria & Maufer                                                [Page 4]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================
                    _    _    _    _
                   |_|  |_|  |_|  |_|
                   '-'  '-'  '-'  '-'
                    |    |    |    |
                  <- - - - - - - - - ->
                           |
                           |
                           v
                        Router
                           ^
                        /     \
 _  ^                 +         +              ^  _
|_|-|               /            \             |-|_|
'_' |             +                +           | '_'
 _  |          v                     v         |  _
|_|-|- - >|Router| <- + - + - + -> |Router|<- -|-|_|
'_' |                                          | '_'
 _  |                                          |  _
|_|-|                                          |-|_|
'_' |                                          | '_'
    v                                          v


LEGEND

<- - - -> Group Membership Protocol
<-+-+-+-> Multicast Routing Protocol

Figure 1: Multicast IP Delivery Service
=======================================================================


1.3 Multicast Routing Protocols

Multicast routers execute a multicast routing protocol to define
delivery paths that enable the forwarding of multicast datagrams
across an internetwork.

1.3.1  Multicast Routing vs. Multicast Forwarding

Multicast routing protocols supply the necessary data to enable the
forwarding of multicast packets.  In the case of unicast routing,
protocols are used to build a forwarding table (commonly called a
routing table).  Unicast destinations are entered in the routing table,
and associated with a metric and a next-hop router toward the
destination.  Multicast routing protocols are usually unicast routing
protocols that facilitate the determination of routes toward a source,
not a destination.  Multicast routing protocols are also used to build
a forwarding table.



Semeria & Maufer                                                [Page 5]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


The key difference between unicast forwarding and multicast forwarding
is that multicast packets must be forwarded away from a source. If a
packet ever goes back toward the source, a forwarding loop could be
formed, possibly leading to a multicast "storm."

A common misconception is that multicast routing protocols pass around
information about groups, represented by class D addresses. In fact, as
long as a router can determine what direction the source is (relative to
itself) and where all the downstream receivers are, then it can build
a forwarding table.  The forwarding table tells the router that for a
certain source sending to a certain group (or in other words, for a
certain (source, group) pair), the packets must all arrive on a certain
interface and be copied to certain "downstream" interface(s).


2. MULTICAST SUPPORT FOR EMERGING INTERNET APPLICATIONS

Today, the majority of Internet applications rely on point-to-point
transmission.  The utilization of point-to-multipoint transmission has
traditionally been limited to local area network applications.  Over the
past few years the Internet has seen a rise in the number of new
applications that rely on multicast transmission.  Multicast IP
conserves bandwidth by forcing the network to do packet replication only
when necessary, and offers an attractive alternative to unicast
transmission for the delivery of network ticker tapes, live stock
quotes, multiparty videoconferencing, and shared whiteboard applications
(among others). It is important to note that the applications for IP
Multicast are not solely limited to the Internet.  Multicast IP can also
play an important role in large commercial internetworks.











[This space was intentionally left blank.]












Semeria & Maufer                                                [Page 6]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


2.1 Reducing Network Load

Assume that a stock ticker application is required to transmit packets
to 100 stations within an organization's network.  Unicast transmission
to this set of stations will require the periodic transmission of 100
packets where many packets may in fact be traversing the same link(s).
Multicast transmission is the ideal solution for this type of
application since it requires only a single packet stream to be
transmitted by the source which is replicated at forks in the multicast
delivery tree.

Broadcast transmission is not an effective solution for this type of
application since it affects the CPU performance of each and every
station that sees the packet.  Besides, it wastes bandwidth.

2.2 Resource Discovery

Some applications implement multicast group addresses instead of
broadcasts to transmit packets to group members residing on the same
network. However, there is no reason to limit the extent of a multicast
transmission to a single LAN.  The time-to-live (TTL) field in the IP
header can be used to limit the range (or "scope") of a multicast
transmission.

2.3 Support for Datacasting Applications

Since 1992, the IETF has conducted a series of "audiocast" experiments
in which live audio and video were multicast from the IETF meeting site
to destinations around the world.  In this case, "datacasting" takes
compressed audio and video signals from the source station and transmits
them as a sequence of UDP packets to a group address.  Multicast
delivery today is not limited to audio and video.  Stock quote systems
are one example of a (connectionless) data-oriented multicast
application.  Someday reliable multicast transport protocols may
facilitate efficient inter-computer communication.  Reliable multicast
transport protocols are currently an active area of research and
development.


3. THE INTERNET'S MULTICAST BACKBONE (MBone)

The Internet Multicast Backbone (MBone) is an interconnected set of
subnetworks and routers that support the delivery of IP multicast
traffic.  The goal of the MBone is to construct a semipermanent IP
multicast testbed to enable the deployment of multicast applications
without waiting for the ubiquitous deployment of multicast-capable
routers in the Internet.

The MBone has grown from 40 subnets in four different countries in 1992,
to more than 2800 subnets in over 25 countries by April 1996.  With new
multicast applications and multicast-based services appearing, it seems


Semeria & Maufer                                                [Page 7]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


likely that the use of multicast technology in the Internet will keep
growing at an ever-increasing rate.

The MBone is a virtual network that is layered on top of sections of the
physical Internet.  It is composed of islands of multicast routing
capability connected to other islands by virtual point-to-point links
called "tunnels."  The tunnels allow multicast traffic to pass through
the non-multicast-capable parts of the Internet.  Tunneled IP multicast
packets are encapsulated as IP-over-IP (i.e., the protocol number is set
to 4) so they look like normal unicast packets to intervening routers.
The encapsulation is added on entry to a tunnel and stripped off on exit
from a tunnel.  This set of multicast routers, their directly-connected
subnetworks, and the interconnecting tunnels comprise the MBone.


========================================================================


                      +++++++
                   / |Island | \
                 /T/ |   A   | \T\
                /U/  +++++++++   \U\
              /N/        |         \N\
            /N/          |           \N\
          /E/            |             \E\
        /L/              |               \L\
 ++++++++            +++++++++           ++++++++
| Island |           | Island| ---------| Island |
|    B   |           |   C   |   Tunnel |   D    |
++++++++++           +++++++++ --------- ++++++++
       \ \               |
         \T\             |
           \U\           |
            \N\          |
              \N\    +++++++++
                \E\  |Island |
                  \L\|   E   |
                    \+++++++++


Figure 2: Internet Multicast Backbone (MBone)

========================================================================


Since the MBone and the Internet have different topologies, multicast
routers execute a separate routing protocol to decide how to forward
multicast packets.  The majority of the MBone routers currently use the
Distance Vector Multicast Routing Protocol (DVMRP), although some
portions of the MBone execute either Multicast OSPF (MOSPF) or the
Protocol-Independent Multicast (PIM) routing protocols.  The operation
of each of these protocols is discussed later in this paper.

Semeria & Maufer                                                [Page 8]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


As multicast routing software features become more widely available on
the routers of the Internet, providers may gradually decide to use
"native" multicast as an alternative to using lots of tunnels.

The MBone carries audio and video multicasts of Internet Engineering
Task Force (IETF) meetings, NASA Space Shuttle Missions, US House and
Senate sessions, and live satellite weather photos.  The session
directory (SDR) tool provides users with a listing of the active
multicast sessions on the MBone and allows them to create and/or join
a session.


4. MULTICAST ADDRESSING

A multicast address is assigned to a set of receivers defining a
multicast group.  Senders use the multicast address as the destination
IP address of a packet that is to be transmitted to all group members.

4.1 Class D Addresses

An IP multicast group is identified by a Class D address.  Class D
addresses have their high-order four bits set to "1110" followed by
a 28-bit multicast group ID.  Expressed in standard "dotted-decimal"
notation, multicast group addresses range from 224.0.0.0 to
239.255.255.255 (shorthand:  224.0.0.0/4).

Figure 3 shows the format of a 32-bit Class D address.

========================================================================


      0 1 2 3                                                      31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1|1|1|0|                   Multicast Group ID                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |------------------------28 bits------------------------|


Figure 3: Class D Multicast Address Format
========================================================================

The Internet Assigned Numbers Authority (IANA) maintains a list of
registered IP multicast groups.  The base address 224.0.0.0 is reserved
and cannot be assigned to any group.  The block of multicast addresses
ranging from 224.0.0.1 to 224.0.0.255 is reserved for permanent
assignment to various uses, including routing protocols and other
protocols that require a well-known permanent address.  Multicast
routers should not forward any multicast datagram with destination
addresses in this range, (regardless of the packet's TTL).




Semeria & Maufer                                                [Page 9]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


Some of the well-known groups include:

    "all systems on this subnet"       224.0.0.1
    "all routers on this subnet"       224.0.0.2
    "all DVMRP routers"                224.0.0.4
    "all OSPF routers"                 224.0.0.5
    "all OSPF designated routers"      224.0.0.6
    "all RIP2 routers"                 224.0.0.9
    "all PIM routers"                  224.0.0.13

The remaining groups ranging from 224.0.1.0 to 239.255.255.255 are
assigned to various multicast applications or remain unassigned.  From
this range, the addresses from 239.0.0.0 to 239.255.255.255 are being
reserved for site-local "administratively scoped" applications, not
Internet-wide applications.

The complete list may be found in the Assigned Numbers RFC (RFC 1700 or
its successor) or at the IANA Web Site:

<URL:http://www.isi.edu/div7/iana/assignments.html>

4.2 Mapping a Class D Address to an IEEE-802 MAC Address

The IANA has been allocated a reserved portion of the IEEE-802 MAC-layer
multicast address space.  All of the addresses in IANA's reserved block
begin with 01-00-5E (hex).  A simple procedure was developed to map
Class D addresses to this reserved address block.  This allows IP
multicasting to easily take advantage of the hardware-level multicasting
supported by network interface cards.

For example, the mapping between a Class D IP address and an IEEE-802
(e.g., Ethernet) multicast address is obtained by placing the low-order
23 bits of the Class D address into the low-order 23 bits of IANA's
reserved address block.

Figure 4 illustrates how the multicast group address 224.10.8.5
(E0-0A-08-05) is mapped into an IEEE-802 multicast address.
















Semeria & Maufer                                               [Page 10]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================

   Class D Address: 224.10.8.5 (E0-0A-08-05)

                                |    E      0   |   0
                  Class-D IP    |_______ _______|__ _ _ _
                     Address    |-+-+-+-+-+-+-+-|-+ - - -
                                |1 1 1 0 0 0 0 0|0
                                |-+-+-+-+-+-+-+-|-+ - - -
                                ...................
IEEE-802                           ....not.........
MAC-Layer                            ..............
Multicast                              ....mapped..
Address                                 ...........
|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+ - - -
|0 0 0 0 0 0 0 1|0 0 0 0 0 0 0 0|0 1 0 1 1 1 1 0|0
|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+ - - -
|_______ _______|_______ _______|_______ _______|_______
|   0       1   |   0       0   |   5       E   |   0



    [Address mapping below continued from half above]

         |   0       A   |   0       8   |   0      5    |
         |_______ _______|_______ _______|_______ _______|    Class-D IP
 - - - +-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|    Address
         |  0 0 0 1 0 1 0|0 0 0 0 1 0 0 0|0 0 0 0 0 1 0 1|
 - - - +-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|
            \____________           ____________________/
                         \___   ___/
                             \ /
                              |
                   23 low-order bits mapped
                              |
                              v

 - - - +-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|    IEEE-802
         |  0 0 0 1 0 1 0|0 0 0 0 1 0 0 0|0 0 0 0 0 1 0 1|    MAC-Layer
 - - - +-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|    Multicast
         |_______ _______|_______ _______|_______ _______|    Address
         |   0       A   |   0       8   |   0       5   |


Figure 4: Mapping between Class D and IEEE-802 Multicast Addresses
========================================================================


The mapping in Figure 4 places the low-order 23 bits of the IP multicast
group ID into the low order 23 bits of the IEEE-802 multicast address.
Note that the mapping may place up to 32 different IP groups into the


Semeria & Maufer                                               [Page 11]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


same IEEE-802 address because the upper 5 bits of the IP multicast
group ID are not used. For example, the multicast addresses
224.138.8.5 (E0-8A-08-05) and 225.10.8.5 (E1-0A-08-05) would also be
mapped to the same IEEE-802 multicast address (01-00-5E-0A-08-05) used
in this example.

4.3 Transmission and Delivery of Multicast Datagrams

When the sender and receivers are members of the same (LAN) subnetwork,
the transmission and reception of multicast frames is a straightforward
process.  The source station simply addresses the IP packet to the
multicast group, the network interface card maps the Class D address to
the corresponding IEEE-802 multicast address, and the frame is sent.
Receivers that wish to capture the frame notify their MAC and IP layers
that they want to receive datagrams addressed to the group.

Things become somewhat more complex when the sender is attached to one
subnetwork and receivers reside on different subnetworks. In this case,
the routers must implement a multicast routing protocol that permits the
construction of multicast delivery trees and supports multicast packet
forwarding.  In addition, each router needs to implement a group
membership protocol that allows it to learn about the existence of group
members on its directly attached subnetworks.


5. INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)

The Internet Group Management Protocol (IGMP) runs between hosts and
their immediately-neighboring multicast routers.  The mechanisms of the
protocol allow a host to inform its local router that it wishes to
receive transmissions addressed to a specific multicast group.  Also,
routers periodically query the LAN to determine if known group members
are still active.  If there is more than one router on the LAN
performing IP multicasting, one of the routers is elected "querier" and
assumes the responsibility of querying the LAN for group members.

Based on the group membership information learned from the IGMP, a
router is able to determine which (if any) multicast traffic needs to be
forwarded to each of its "leaf" subnetworks.  Multicast routers use this
information, in conjunction with a multicast routing protocol, to
support IP multicasting across the Internet.

5.1 IGMP Version 1

IGMP Version 1 was specified in RFC-1112.  According to the
specification, multicast routers periodically transmit Host Membership
Query messages to determine which host groups have members on their
directlyattached networks.  Query messages are addressed to the
all-hosts group (224.0.0.1) and have an IP TTL = 1.  This means that
Query messages sourced from a router are transmitted onto the



Semeria & Maufer                                               [Page 12]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


directly-attached subnetwork but are not forwarded by any other
multicast routers.


========================================================================


     Group 1                                   _____________________
       ____            ____                   |  multicast          |
      |    |          |    |                  |            router   |
      |_H2_|          |_H4_|                  |_____________________|
       ----            ----                      +-----+  |
         |               |                 <-----|Query|  |
         |               |                       +-----+  |
         |               |                                |
|---+----+-------+-------+--------+-----------------------+----|
    |            |                |
    |            |                |
  ____         ____              ____
 |    |       |    |            |    |
 |_H1_|       |_H3_|            |_H5_|
  ----         ----              ----
 Group 2      Group 1           Group 1
              Group 2



Figure 5: Internet Group Management Protocol-Query Message
========================================================================


When a host receives an IGMP Query message, it responds with a Host
Membership Report for each group to which it belongs, sent to each group
to which it belongs.  (This is an important point: While IGMP Queries
are sent to the "all hosts on this subnet" class D address (224.0.0.1),
IGMP Reports are sent to the group(s) to which the host(s) belong.
Reports have a TTL of 1, and thus are not forwarded beyond the local
subnetwork.)

In order to avoid a flurry of Reports, each host starts a randomly-
chosen Report delay timer for each of its group memberships.  If, during
the delay period, another Report is heard for the same group, each other
host in that group resets its timer to a new random value. This
procedure spreads Reports out over a period of time and minimizes Report
traffic for each group that has at least one member on a given
subnetwork.

It should be noted that multicast routers do not need to be directly
addressed since their interfaces are required to promiscuously receive
all multicast IP traffic.  Also, a router does not need to maintain a
detailed list of which hosts belong to each multicast group; the router


Semeria & Maufer                                               [Page 13]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


only needs to know that at least one group member is present on a given
network interface.

Multicast routers periodically transmit Queries to update their
knowledge of the group members present on each network interface. If the
router does not receive a Report from any members of a particular group
after a number of Queries, the router assumes that group members are no
longer present on an interface.  Assuming this is a leaf subnet, this
interface is removed from the delivery tree for this (source, group)
pair.  Multicasts will continue to be sent on this interface if the
router can tell (via multicast routing protocols) that there are
additional group members further downstream reachable via this
interface.

When a host first joins a group, it immediately transmits an IGMP Report
for the group rather than waiting for a router's IGMP Query.  This
reduces the "join latency" for the first host to join a given group on
a particular subnetwork.

5.2 IGMP Version 2

IGMP Version 2 was distributed as part of the IP Multicasting (Version
3.3 through Version 3.8) code package.  Initially, there was no detailed
specification for IGMP Version 2 other than this source code.  However,
the complete specification has recently been published in <draft-ietf-
idmr-igmp-v2-05.txt> which will update the informal specification
contained in Appendix I of RFC-1112.  IGMP Version 2 enhances and
extends IGMP Version 1 while maintaining backward compatibility with
Version 1 hosts.

IGMP Version 2 defines a procedure for the election of the multicast
querier for each LAN.  In IGMP Version 2, the router with the lowest IP
address on the LAN is elected the multicast querier.  In IGMP Version 1,
the querier election was determined by the multicast routing protocol.
This could lead to potential problems because each multicast routing
protocol might use unique methods for determining the multicast querier.

IGMP Version 2 defines a new type of Query message:  the Group-Specific
Query.  Group-Specific Query messages allow a router to transmit a Query
to a specific multicast group rather than all groups residing on a
directly attached subnetwork.

Finally, IGMP Version 2 defines a Leave Group message to lower IGMP's
"leave latency."  When the last host to respond to a Query with a Report
wishes to leave that specific group, the host transmits a Leave Group
message to the all-routers group (224.0.0.2) with the group field set to
the group to be left.  In response to a Leave Group message, the router
begins the transmission of Group-Specific Query messages on the inter-
face that received the Leave Group message.  If there are no Reports in
response to the Group-Specific Query messages, then if this is a leaf



Semeria & Maufer                                               [Page 14]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


subnet, this interface is removed from the delivery tree for this
(source, group) pair (as was the case of IGMP version 1).  Again,
multicasts will continue to be sent on this interface if the router can
tell (via multicast routing protocols) that there are additional group
members further downstream reachable via this interface.

5.3 IGMP Version 3

IGMP Version 3 is a preliminary draft specification published in
<draft-cain-igmp-00.txt>.  IGMP Version 3 introduces support for Group-
Source Report messages so that a host can elect to receive traffic from
specific sources of a multicast group.  An Inclusion Group-Source Report
message allows a host to specify the IP addresses of the specific
sources it wants to receive.  An Exclusion Group-Source Report message
allows a host to explicitly identify the sources that it does not want
to receive.  With IGMP Version 1 and Version 2, if a host wants to
receive any traffic for a group, the traffic from all sources for the
group must be forwarded onto the host's subnetwork.

IGMP Version 3 will help conserve bandwidth by allowing a host to select
the specific sources from which it wants to receive traffic.  Also,
multicast routing protocols will be able to make use this information to
conserve bandwidth when constructing the branches of their multicast
delivery trees.

Finally, support for Leave Group messages first introduced in IGMP
Version 2 has been enhanced to support Group-Source Leave messages.
This feature allows a host to leave an entire group or to specify the
specific IP address(es) of the (source, group) pair(s) that it wishes to
leave.


6. MULTICAST FORWARDING TECHNIQUES

IGMP provides the final step in a multicast packet delivery service
since it is only concerned with the forwarding of multicast traffic from
a router to group members on its directly-attached subnetworks.  IGMP is
not concerned with the delivery of multicast packets between neighboring
routers or across an internetwork.

To provide an internetwork delivery service, it is necessary to define
multicast routing protocols.  A multicast routing protocol is
responsible for the construction of multicast delivery trees and
enabling multicast packet forwarding.  This section explores a number of









Semeria & Maufer                                               [Page 15]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


different techniques that may potentially be employed by multicast
routing protocols:

    o "Simpleminded" Techniques
       - Flooding
       - Spanning Trees

    o  Source-Based Tree (SBT) Techniques
       - Reverse Path Broadcasting (RPB)
       - Truncated Reverse Path Broadcasting (TRPB)
       - Reverse Path Multicasting (RPM)

    o "Shared-Tree" Techniques

Later sections will describe how these algorithms are implemented in the
most prevalent multicast routing protocols in the Internet today  (e.g.,
Distance Vector Multicast Routing Protocol (DVMRP), Multicast extensions
to OSPF (MOSPF), Protocol-Independent Multicast (PIM), and Core-Based
Trees (CBT).

6.1 "Simpleminded" Techniques

Flooding and Spanning Trees are two algorithms that can be used to build
primitive multicast routing protocols.  The techniques are primitive due
to the fact that they tend to waste bandwidth or require a large amount
of computational resources within the multicast routers involved.  Also,
protocols built on these techniques may work for small networks with few
senders, groups, and routers, but do not scale well to larger numbers of
senders, groups, or routers.  Also, the ability to handle arbitrary
topologies may not be present or may only be present in limited ways.

6.1.1 Flooding

The simplest technique for delivering multicast datagrams to all routers
in an internetwork is to implement a flooding algorithm. The flooding
procedure begins when a router receives a packet that is addressed to a
multicast group.  The router employs a protocol mechanism to determine
whether or not it has seen this particular packet before.  If it is the
first reception of the packet, the packet is forwarded on all
interfaces--except the one on which it arrived--guaranteeing that the
multicast packet reaches all routers in the internetwork.  If the router
has seen the packet before, then the packet is discarded.

A flooding algorithm is very simple to implement since a router does not
have to maintain a routing table and only needs to keep track of the
most recently seen packets.  However, flooding does not scale for
Internet-wide applications since it generates a large number of
duplicate packets and uses all available paths across the internetwork
instead of just a limited number.  Also, the flooding algorithm makes
inefficient use of router memory resources since each router is required
to maintain a distinct table entry for each recently seen packet.


Semeria & Maufer                                               [Page 16]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


6.1.2 Spanning Tree

A more effective solution than flooding would be to select a subset of
the internetwork topology which forms a spanning tree.  The spanning
tree defines a structure in which only one active path connects any two
routers of the internetwork.  Figure 6 shows an internetwork and a
spanning tree rooted at router RR.

Once the spanning tree has been built, a multicast router simply
forwards each multicast packet to all interfaces that are part of the
spanning tree except the one on which the packet originally arrived.
Forwarding along the branches of a spanning tree guarantees that the
multicast packet will not loop and that it will eventually reach all
routers in the internetwork.

A spanning tree solution is powerful and would be relatively easy to
implement since there is a great deal of experience with spanning tree
protocols in the Internet community.  However, a spanning tree solution
can centralize traffic on a small number of links, and may not provide
the most efficient path between the source subnetwork and group members.
Also, it is computationally difficult to compute a spanning tree in
large, complex topologies.


6.2 Source-Based Tree Techniques

The following techniques all generate a source-based tree by various
means.  The techniques differ in the efficiency of the tree building
process, and the bandwidth and router resources (i.e., state tables)
used to build a source-based tree.

6.2.1 Reverse Path Broadcasting (RPB)

A more efficient solution than building a single spanning tree for the
entire internetwork would be to build a group-specific spanning tree for
each potential source [subnetwork].  These spanning trees would result
in source-based delivery trees emanating from the subnetwork directly
connected to the source station.  Since there are many potential sources
for a group, a different delivery tree is constructed emanating from
each active source.

6.2.1.1 Reverse Path Broadcasting: Operation

The fundamental algorithm to construct these source-based trees is
referred to as Reverse Path Broadcasting (RPB).  The RPB algorithm is
actually quite simple.  For each (source, group) pair, if a packet
arrives on a link that the local router believes to be on the shortest
path back toward the packet's source, then the router forwards the
packet on all interfaces except the incoming interface.  If the packet
does not arrive on the interface that is on the shortest path back



Semeria & Maufer                                               [Page 17]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================

A Sample Internetwork

                     #----------------#
                   / |\              / \
                  |  | \           /    \
                  |  |   \       /       \
                  |  |    \    /          \
                  |  |      \ /            \
                  |  |       #------#       \
                  |  |      /       | \      \
                  |  |     /        |  \      \
                  |   \   /         |   \-------#
                  |    \ /          |     -----/|
                  |     #-----------#----/      |
                  |    /|\---    --/|    \      |
                  |   / |    \  /    \    \     |
                  |  /   \    /\     |     \   /
                  | /      \ /   \   |      \ /
                  #---------#--   \  |   ----#
                               \   \ |  /
                                \--- #-/

A Spanning Tree for this Sample Internetwork

                     #                #
                      \              /
                       \           /
                         \       /
                          \    /
                            \ /
                             #------RR
                                    | \
                                    |  \
                                    |   \-------#
                                    |
                        #-----------#----
                       /|           |    \
                      / |            \    \
                     /   \           |     \
                    /      \         |      \
                   #        #        |       #
                                     |
                                     #
LEGEND

#   Router
RR  Root Router

Figure 6: Spanning Tree
========================================================================

Semeria & Maufer                                               [Page 18]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


toward the source, then the packet is discarded.  The interface over
which the router expects to receive multicast packets from a particular
source is referred to as the "parent" link.  The outbound links over
which the router forwards the multicast packet are called "child" links
for this group.

This basic algorithm can be enhanced to reduce unnecessary packet
duplication.  If the local router making the forwarding decision can
determine whether a neighboring router on a child link is "downstream,"
then the packet is multicast toward the neighbor. (A "downstream"
neighbor is a neighboring router which considers the local router to be
on the shortest path back toward a given source.) Otherwise, the packet
is not forwarded on the potential child link since the local router
knows that the neighboring router will just discard the packet (since it
will arrive on a non-parent link for the (source, group) pair, relative
to that downstream router).


========================================================================


                 Source
                    .   ^
                    .   |    shortest path back to the
                    .   |     source for THIS router
                    .   |
               "parent link"
                    _
            ______|!2|_____
           |               |
--"child -|!1|           |!3| - "child --
    link"  |    ROUTER     |      link"
           |_______________|


Figure 7: Reverse Path Broadcasting - Forwarding Algorithm
========================================================================


The information to make this "downstream" decision is relatively easy to
derive from a link-state routing protocol since each router maintains a
topological database for the entire routing domain.  If a distance-
vector routing protocol is employed, a neighbor can either advertise its
previous hop for the (source, group) pair as part of its routing update
messages or "poison reverse" the route toward a source if it is not on
the distribution tree for that source.  Either of these techniques
allows an upstream router to determine if a downstream neighboring
router is on an active branch of the delivery tree for a certain source
sending to a certain group.




Semeria & Maufer                                               [Page 19]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


Please refer to Figure 8 for a discussion describing the basic operation
of the enhanced RPB algorithm.


======================================================================

 Source Station------>O
                    A #
                     +|+
                    + | +
                   +  O  +
                  +       +
                 1         2
                +           +
               +             +
              +               +
          B  +                 +  C
          O-#- - - - -3- - - - -#-O
           +|+                 -|+
          + | +               - | +
         +  O  +             -  O  +
        +       +           -       +
       +         +         -         +
      4           5       6           7
     +             +     -             +
    +               + E -               +
   +                 + -                 +
D #- - - - -8- - - - -#- - - - -9- - - - -# F
  |                   |                   |
  O                   O                   O


LEGEND

O   Leaf
+ + Shortest-path
- - Branch
#   Router


Figure 8: Reverse Path Broadcasting - Example
=======================================================================


Note that the source station (S) is attached to a leaf subnetwork
directly connected to Router A.  For this example, we will look at the
RPB algorithm from Router B's perspective. Router B receives the
multicast packet from Router A on link 1.  Since Router B considers link
1 to be the parent link for the (source, group) pair, it forwards the
packet on link 4, link 5, and the local leaf subnetworks if they contain
group members.  Router B does not forward the packet on link 3 because


Semeria & Maufer                                               [Page 20]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


it knows from routing protocol exchanges that Router C considers link 2
as its parent link for the (source, group) pair.  Router B knows that if
it were to forward the packet on link 3, it would be discarded by Router
C since the packet would not be arriving on Router C's parent link for
this (source, group) pair.

6.2.1.2 RPB: Benefits and Limitations

The key benefit to reverse path broadcasting is that it is reasonably
efficient and easy to implement.  It does not require that the router
know about the entire spanning tree, nor does it require a special
mechanism to stop the forwarding process (as flooding does).  In
addition, it guarantees efficient delivery since multicast packets
always follow the "shortest" path from the source station to the
destination group.  Finally, the packets are distributed over multiple
links, resulting in better network utilization since a different tree is
computed for each (source, group) pair.

One of the major limitations of the RPB algorithm is that it does not
take into account multicast group membership when building the delivery
tree for a (source, group) pair.  As a result, datagrams may be
unnecessarily forwarded to subnetworks that have no members in the
destination group.

6.2.2 Truncated Reverse Path Broadcasting (TRPB)

Truncated Reverse Path Broadcasting (TRPB) was developed to overcome the
limitations of Reverse Path Broadcasting.  With the help of IGMP,
multicast routers determine the group memberships on each leaf
subnetwork and avoid forwarding datagrams onto a leaf subnetwork if it
does not contain at least one member of the destination group.  Thus,
the delivery tree is "truncated" by the router if a leaf subnetwork has
no group members.

Figure 9 illustrates the operation of TRPB algorithm.  In this example
the router receives a multicast packet on its parent link for the
(Source, G1) pair.  The router forwards the datagram on interface 1
since that interface has at least one member of G1. The router does not
forward the datagram to interface 3 since this interface has no members
in the destination group.  The datagram is forwarded on interface 4 if
and only if a downstream router considers this subnetwork to be part of
its "parent link" for the (Source, G1) pair.

TRPB removes some limitations of RPB but it solves only part of the
problem.  It eliminates unnecessary traffic on leaf subnetworks but it
does not consider group memberships when building the branches of the
delivery tree.






Semeria & Maufer                                               [Page 21]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


======================================================================

                      Source
                          .   |
                          .   |
                          .   |     (Source, G1)
                          .   v
                          |
                     "parent link"
                          |
        "child link"     ___
    G1            _______|2|_____
     \           |               |
   G3\\ _____   ___    ROUTER   ___      ______ / G2
      \| hub |--|1|             |3|-----|switch|/
      /|_____|  ^--     ___     -- ^    |______|\
      /        ^ |______|4|_____|   ^          \
    G1       ^         ^---            ^         G3
            ^        ^   |              ^
        Forward->->-^ "child link"  Truncate
                         |

Figure 9: Truncated Reverse Path Broadcasting - (TRPB)
======================================================================



6.2.3 Reverse Path Multicasting (RPM)

Reverse Path Multicasting (RPM) is an enhancement to Reverse Path
Broadcasting and Truncated Reverse Path Broadcasting.

RPM creates a delivery tree that spans only:

    o  Subnetworks with group members, and

    o  Routers and subnetworks along the shortest
       path to subnetworks with group members.

RPM allows the source-based "shortest-path" tree to be pruned so that
datagrams are only forwarded along branches that lead to active members
of the destination group.

6.2.3.1 Operation

When a multicast router receives a packet for a (source, group) pair,
the first packet is forwarded following the TRPB algorithm across all
routers in the internetwork.  Routers on the edge of the network (which
have only leaf subnetworks) are called leaf routers.  The TRPB algorithm
guarantees that each leaf router will receive at least the first
multicast packet.  If there is a group member on one of its leaf


Semeria & Maufer                                               [Page 22]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


subnetworks, a leaf router forwards the packet based on this IGMP Report
(or a statically-defined local group on an interface).


========================================================================

                   Source
                      . |
                      . | (Source, G)
                      . |
                      | v
                      |
                    o-#-G
                      |**********
                    ^ |         *
                    , |         *
                    ^ |         *  o
                    , |         * /
                    o-#-o       #***********
                    ^ |\      ^ |\         *
                    ^ | o     ^ | G        *
                    , |       , |          *
                    ^ |       ^ |          *
                    , |       , |          *
                      #         #          #
                     /|\       /|\        /|\
                    o o o     o o o      G o G
LEGEND

 #    Router
 o    Leaf without group member
 G    Leaf with group member
***   Active Branch
---   Pruned Branch
,>,   Prune Message (direction of flow -->

Figure 10: Reverse Path Multicasting  (RPM)
========================================================================


If none of the subnetworks connected to the leaf router contain group
members, the leaf router may transmit a "prune" message on its parent
link, informing the upstream router that it should not forward packets
for this particular (source, group) pair on the child interface on which
it received the prune message.  Prune messages are sent just one hop
back toward the source.

An upstream router receiving a prune message is required to store the
prune information in memory.  If the upstream router has no recipients
on local leaf subnetworks and has received prune messages on each of the
child interfaces for this (source, group) pair, then the upstream router


Semeria & Maufer                                               [Page 23]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


does not need to receive additional packets for the (source, group)
pair.  This implies that the upstream router can also generate a prune
message of its own, one hop further back toward the source.  This
cascade of prune messages results in an active multicast delivery tree,
consisting exclusively of "live" branches (i.e., branches that lead to
active receivers).

Since both the group membership and internetwork topology can change
dynamically , the pruned state of the multicast delivery tree must be
refreshed periodically.  At regular intervals, the prune information
expires from the memory of all routers and the next packet for the
(source, group) pair is forwarded toward all downstream routers.  This
results in a new burst of prune messages allowing the multicast
forwarding tree to adapt to the ever-changing multicast delivery
requirements of the internetwork.

6.2.3.2 Limitations

Despite the improvements offered by the RPM algorithm, there are still
several scaling issues that need to be addressed when attempting to
develop an Internet-wide delivery service.  The first limitation is that
multicast packets must be periodically flooded across every router in
the internetwork, onto every leaf subnetwork.  This flooding is wasteful
of bandwidth (until the updated prune state is constructed).

This "flood and prune" paradigm is very powerful, but it wastes
bandwidth and does not scale well, especially if there are receivers at
the edge of the delivery tree which are connected via low-speed
technologies (e.g., ISDN or modem).  Also, note that every router
participating in the RPM algorithm must either have a forwarding table
entry for a (source, group) pair, or have prune state information for
that (source, group) pair.

It is clearly wasteful (especially as the number of active sources and
groups increase) to place such a burden on routers that are not on every
(or perhaps any) active delivery tree.  Shared tree techniques are an
attempt to address these scaling issues, which become quite acute when
most groups' senders and receivers are sparsely distributed across the
internetwork.

6.3 Shared Tree Techniques

The most recent additions to the set of multicast forwarding techniques
are based on a shared delivery tree.  Unlike shortest-path tree
algorithms which build a source-based tree for each (source, group)
pair, shared tree algorithms construct a single delivery tree that is
shared by all members of a group.  The shared tree approach is quite
similar to the spanning tree algorithm except it allows the definition
of a different shared tree for each group.  Stations that wish to
receive traffic for a multicast group are required to explicitly join



Semeria & Maufer                                               [Page 24]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


the shared delivery tree.  Multicast traffic for each group is sent and
received over the same delivery tree, regardless of the source.

6.3.1 Operation

A shared tree may involve a single router, or set of routers, which
comprise the "core" of a multicast delivery tree.  Figure 11 illustrates
how a single multicast delivery tree is shared by all sources and
receivers for a multicast group.


========================================================================


               Source        Source        Source
                  |             |             |
                  |             |             |
                  v             v             v

                 [#] * * * * * [#] * * * * * [#]
                                *
                  ^             *             ^
                  |             *             |
             join |             *             | join
                  |            [#]            |
                 [x]                         [x]
                  :                           :
                member                      member
                 host                        host


LEGEND

[#]  Shared Tree Core Routers
* *  Shared Tree Backbone
[x]  Member-hosts' directly-attached routers

Figure 11: Shared Multicast Delivery Tree

========================================================================


The directly attached router for each station wishing to belong to a
particular multicast group is required to send a "join" message toward
the shared tree of the particular multicast group.  The directly
attached router only needs to know the address of one of the group's
core routers in order to transmit a join request (via unicast).  The
join request is processed by all intermediate routers, each of which
identifies the interface on which the join was received as belonging to
the group's delivery tree.  The intermediate routers continue to forward
the join message toward the core, marking local downstream interfaces


Semeria & Maufer                                               [Page 25]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


until the request reaches a core router (or a router that is already on
the active delivery tree).  This procedure allows each member-host's
directly-attached router to define a branch providing the shortest path
between itself and a core router which is part of the group's shared
delivery tree.

Similar to other multicast forwarding algorithms, shared tree algorithms
do not require that the source of a multicast packet be a member of a
destination group.  Packets sourced by a non-group member are simply
unicast toward the core until they reach the first router that is a
member of the group's delivery tree.  When the unicast packet reaches a
member of the delivery tree, the packet is multicast to all outgoing
interfaces that are part of the tree except the incoming link.  This
guarantees that traffic follows the shortest path from source station to
the shared tree. It also ensures that multicast packets are forwarded to
all routers on the core tree which in turn forward the traffic to all
receivers that have joined the shared tree.

6.3.2 Benefits

In terms of scalability, shared tree techniques have several advantages
over source-based trees.  Shared tree algorithms make efficient use of
router resources since they only require a router to maintain state
information for each group, not for each (source, group) pair. (Remember
that source-based tree techniques required all routers in an
internetwork to either be a) on the delivery tree for a given (source,
group) pair, or b) have prune state for that (source, group) pair:  So
the entire internetwork must participate in the source-based tree
protocol.)  This improves the scalability of applications with many
active senders since the number of source stations is no longer a
scaling issue.  Also, shared tree algorithms conserve network bandwidth
since they do not require that multicast packets be periodically flooded
across all multicast routers in the internetwork onto every leaf
subnetwork.  This can offer significant bandwidth savings, especially
across low-bandwidth WAN links, and when receivers sparsely populate the
domain of operation.  Finally, since receivers are required to
explicitly join the shared delivery tree, data only ever flows over
those links that lead to active receivers.

6.3.3 Limitations

Despite these benefits, there are still several limitations to protocols
that are based on a shared tree algorithm.  Shared trees may result in
traffic concentration and bottlenecks near core routers since traffic
from all sources traverses the same set of links as it approaches the
core.  In addition, a single shared delivery tree may create suboptimal
routes (a shortest path between the source and the shared tree, a
suboptimal path across the shared tree, a shortest path between the
egress core router and the receiver's directly attached router)
resulting in increased delay which may be a critical issue for some
multimedia applications.  (Simulations indicate that latency over a


Semeria & Maufer                                               [Page 26]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


shared tree may be approximately 10% larger than source-based trees in
many cases, but by the same token, this may be negligible for many
applications.)  Finally, new algorithms need to be developed to support
all aspects of core management which include core router selection and
(potentially) dynamic placement strategies.


7. SOURCE-BASED TREE ("DENSE MODE") ROUTING PROTOCOLS

An established set of multicast routing protocols define a source-based
delivery tree which provides the shortest path between the source and
each receiver.

These routing protocols include:

o  Distance Vector Multicast Routing Protocol (DVMRP),

o  Multicast Extensions to Open Shortest Path First (MOSPF),

o  Protocol Independent Multicast - Dense Mode (PIM-DM).

Each of these routing protocols is designed to operate in an environment
where group members are relatively densely populated and internetwork
bandwidth is plentiful.  Their underlying designs assume that the amount
of protocol overhead (in terms of the amount of state that must be
maintained by each router, the number of router CPU cycles required, and
the amount of bandwidth consumed by protocol operation) is appropriate
since receivers densely populate the area of operation.

7.1. Distance Vector Multicast Routing Protocol (DVMRP)

The Distance Vector Multicast Routing Protocol (DVMRP) is a
distance-vector routing protocol designed to support the forwarding of
multicast datagrams through an internetwork.  DVMRP constructs
source-based multicast delivery trees using variants of the Reverse Path
Broadcasting (RPB) algorithm.  Originally, the entire MBone ran DVMRP.
Today, over half of the MBone routers still run some version of DVMRP.

DVMRP was first defined in RFC-1075.  The original specification was
derived from the Routing Information Protocol (RIP) and employed the
Truncated Reverse Path Broadcasting (TRPB) technique. The major
difference between RIP and DVMRP is that RIP was concerned with
calculating the next-hop to a destination, while DVMRP is concerned with
computing the previous-hop back to a source.  It is important to note
that the latest mrouted version 3.8 and vendor implementations have
extended DVMRP to employ the Reverse Path Multicasting (RPM) algorithm.
This means that the latest implementations of DVMRP are quite different
from the original RFC specification in many regards.  There is an active
effort within the IETF Inter-Domain Multicast Routing (IDMR) working
group to specify DVMRP version 3 in a standard form (as opposed to the
current spec, which is written in C).


Semeria & Maufer                                               [Page 27]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


The current DVMRP v3 Internet-Draft is:

    <draft-ietf-idmr-dvmrp-v3-03.txt>, or
    <draft-ietf-idmr-dvmrp-v3-03.ps>

7.1.1 Physical and Tunnel Interfaces

The ports of a DVMRP router may be either a physical interface to a
directly-attached subnetwork or a tunnel interface to another multicast
island.  All interfaces are configured with a metric that specifies the
cost for the given port and a TTL threshold that limits the scope of a
multicast transmission.  In addition, each tunnel interface must be
explicitly configured with two additional parameters:  the IP address of
the local router's interface and the IP address of the remote router's
interface.


========================================================================

   TTL                                Scope
Threshold
________________________________________________________________________
    0                                Restricted to the same host
    1                                Restricted to the same subnetwork
   15                                Restricted to the same site
   63                                Restricted to the same region
  127                                Worldwide
  191                                Worldwide; limited bandwidth
  255                                Unrestricted in scope


Table 1:   TTL Scope Control Values
========================================================================


A multicast router will only forward a multicast datagram across an
interface if the TTL field in the IP header is greater than the TTL
threshold assigned to the interface.  Table 1 lists the conven- tional
TTL values that are used to restrict the scope of an IP multicast.  For
example, a multicast datagram with a TTL of less than 16 is restricted
to the same site and should not be forwarded across an interface to
other sites in the same region.

TTL-based scoping is not always useful, so the IETF MBoneD working group
is considering the definition and usage of a range of multi- cast
addresses for "administrative" scoping.  In other words, such addresses
would be usable within a certain administrative scope, a corporate
network, for instance, but would not be forwarded across the global
MBone.  At the moment, the range from 239.0.0.0 through 239.255.255.255
is being reserved for administratively scoped applications, but the
structure and usage of this block has yet to be completely formalized.


Semeria & Maufer                                               [Page 28]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


7.1.2 Basic Operation

DVMRP implements the Reverse Path Multicasting (RPM) algorithm.
According to RPM, the first datagram for any (source, group) pair is
forwarded across the entire internetwork (providing the packet's TTL and
router interface thresholds permit this).  The initial datagram is
delivered to all leaf routers which transmit prune messages back toward
the source if there are no group members on their directly attached leaf
subnetworks.  The prune messages result in the removal of branches from
the tree that do not lead to group members, thus creating a source-based
shortest path tree with all leaves having group members. After a period
of time, the pruned branches grow back and the next datagram for the
(source, group) pair is forwarded across the entire internetwork
resulting in a new set of prune messages.

DVMRP also implements a mechanism to quickly "graft" back a previously
pruned branch of a group's delivery tree.  If a router that previously
sent a prune message for a (source, group) pair discovers new group
members on a leaf network, it sends a graft message to the group's
previous-hop router.  When an upstream router receives a graft message,
it cancels out the previously-received prune message.  Graft messages
will cascade back toward the source (until reaching the nearest "live"
branch point on the delivery tree), thus allowing previously pruned
branches to be quickly restored as part of the active delivery tree.

7.1.3 DVMRP Router Functions

When there is more than one DVMRP router on a subnetwork, the Dominant
Router is responsible for the periodic transmission of IGMP Host
Membership Query messages.  Upon initialization, a DVMRP router
considers itself to be the Dominant Router for the subnetwork until it
receives a Host Membership Query message from a neighbor router with a
lower IP address.  Figure 12 illustrates how the router with the lowest
IP address functions as the Dominant Router for the subnetwork.

In order to avoid duplicate multicast datagrams when there is more than
one DVMRP router on a subnetwork, one router is elected the Dominant
Router for the particular source subnetwork (see fig. 12).  In  Figure
13, Router C is downstream and may potentially receive datagrams from
the source subnetwork from Router A or Router B.  If Router A's metric
to the source subnetwork is less than Router B's metric, then Router A
is dominant over Router B for this source.

This means that Router A will forward traffic from the source sub-
network and Router B will discard traffic from that source subnet- work.
 However, if Router A's metric is equal to Router B's metric, then
router with the lower IP address on its downstream interface (child
link) becomes the Dominant Router for this source.  Note that on a
subnetwork with multiple routers forwarding to groups with multiple
sources, different routers may be dominant for each source.



Semeria & Maufer                                               [Page 29]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================


        _____________                               _____________
       | Router A    |                             | Router B    |
       |             |                             |      DR     |
        -------------                               -------------
   128.2.3.4  |                                  <-Query  |  128.2.1.1
              |                                           |
 ---------------------------------------------------------------------
                                       |
                          128.2.3.1    |
                                 _____________
                                | Router C    |
                                |             |
                                 -------------


Figure 12. DVMRP Dominant Router Election
========================================================================

========================================================================


                                   To
              .-<-<-<-<-<-<-Source Subnetwork->->->->->->->->--.
              v                                                v
              |                                                |
          parent link                                      parent link
              |                                                |
        _____________                                    _____________
       | Router A    |                                  | Router B    |
       |             |                                  |             |
        -------------                                    -------------
              |                                                |
         child link                                       child link
              |                                                |
 ---------------------------------------------------------------------
                                       |
                                  parent link
                                       |
                                 _____________
                                | Router C    |
                                |             |
                                 -------------
                                       |
                                  child link
                                       |

Figure 13. DVMRP Dominant Router in a Redundant Topology
========================================================================


Semeria & Maufer                                               [Page 30]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


7.1.4 DVMRP Routing Table

The DVMRP process periodically exchanges routing table updates with its
DVMRP neighbors.  These updates are logically independent of those
generated by any unicast Interior Gateway Protocol.

Since the DVMRP was developed to route multicast and not unicast
traffic, a router will probably run multiple routing processes in
practice:  One to support the forwarding of unicast traffic and another
to support the forwarding of multicast traffic. (This can be convenient:
A router can be configured to only route multicast IP, with no unicast
IP routing.  This may be a useful capability in firewalled
environments.)

Consider Figure 13:  There are two types of routers in this figure:
dominant and subordinate; assume in this example that Router B is
dominant, Router A is subordinate, and Router C is part of the
downstream distribution tree.  In general, which routers are dominant
or subordinate may be different for each source!  A subordinate router
is one that is NOT on the shortest path tree back toward a source.  The
dominant router can tell this because the subordinate router will
'poison-reverse' the route for this source in its routing updates which
are sent on the common LAN (i.e., Router A sets the metric for this
source to 'infinity').  The dominant router keeps track of subordinate
routers on a per-source basis...it never needs or expects to receive a
prune message from a subordinate router.  Only routers that are truly on
the downstream distribution tree will ever need to send prunes to the
dominant router.  If a dominant router on a LAN has received either a
poison-reversed route for a source, or prunes for all groups emanating
from that source subnetwork, then it may itself send a prune upstream
toward the source (assuming also that IGMP has told it that there are no
local receivers for any group from this source).

A sample routing table for a DVMRP router is shown in Figure 14. Unlike


========================================================================

Source      Subnet     From           Metric   Status   TTL
 Prefix      Mask       Gateway


128.1.0.0  255.255.0.0  128.7.5.2       3        Up     200
128.2.0.0  255.255.0.0  128.7.5.2       5        Up     150
128.3.0.0  255.255.0.0  128.6.3.1       2        Up     150
128.3.0.0  255.255.0.0  128.6.3.1       4        Up     200


Figure 14: DVMRP Routing Table
========================================================================



Semeria & Maufer                                               [Page 31]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


the table that would be created by a unicast routing protocol such as
the RIP, OSPF, or the BGP, the DVMRP routing table contains Source
Prefixes and From-Gateways instead of Destination Prefixes and Next-Hop
Gateways.

The routing table represents the shortest path (source-based) spanning
tree to every possible source prefix in the internetwork--the Reverse
Path Broadcasting (RPB) tree.  The DVMRP routing table does not
represent group membership or received prune messages.

The key elements in DVMRP routing table include the following items:

Source Prefix          A subnetwork which is a potential or actual
                       source of multicast datagrams.

Subnet Mask            The subnet mask associated with the Source
                       Prefix.  Note that the DVMRP provides the subnet
                       mask for each source subnetwork (in other words,
                       the DVMRP is classless).

From-Gateway           The previous-hop router leading back toward a
                       particular Source Prefix.

TTL                    The time-to-live is used for table management
                       and indicates the number of seconds before an
                       entry is removed from the routing table.  This
                       TTL has nothing at all to do with the TTL used
                       in TTL-based scoping.

7.1.5 DVMRP Forwarding Table

Since the DVMRP routing table is not aware of group membership, the
DVMRP process builds a forwarding table based on a combination of the
information contained in the multicast routing table, known groups, and
received prune messages.  The forwarding table represents the local
router's understanding of the shortest path source-based delivery tree
for each (source, group) pair--the Reverse Path Multicasting (RPM) tree.


========================================================================

Source      Multicast     TTL   InPort   OutPorts
 Prefix      Group

 128.1.0.0  224.1.1.1     200    1 Pr      2p3p
            224.2.2.2     100    1         2p3
            224.3.3.3     250    1         2
 128.2.0.0  224.1.1.1     150    2         2p3

Figure 15: DVMRP Forwarding Table
========================================================================


Semeria & Maufer                                               [Page 32]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


The forwarding table for a sample DVMRP router is shown in Figure 15.
The elements in this display include the following items:

Source Prefix           The subnetwork sending multicast datagrams
                        to the specified groups (one group per row).

Multicast Group         The Class D IP address to which multicast
                        datagrams are addressed.  Note that a given
                        Source Prefix may contain sources for several
                        Multicast Groups.

InPort                  The parent port for the (source, group) pair.
                        A 'Pr' in this column indicates that a prune
                        message has been sent to the upstream router
                        (the From-Gateway for this Source Prefix in
                        the DVMRP routing table).

OutPorts                The child ports over which multicast datagrams
                        for this (source, group) pair are forwarded.
                        A 'p' in this column indicates that the router
                        has received a prune message(s) from a (all)
                        downstream router(s) on this port.

7.1.6 Hierarchical DVMRP (DVMRP v4.0)

The rapid growth of the MBone is placing ever-increasing demands on its
routers.  Essentially, today's MBone is deployed as a single, "flat"
routing domain where each router is required to maintain detailed
routing information to every possible subnetwork on the MBone.  As the
number of subnetworks continues to increase, the size of the routing
tables and of the periodic update messages will continue to grow.  If
nothing is done about these issues, the processing and memory
capabilities of the MBone routers will eventually be depleted and
routing on the MBone will be degraded, or fail.

To overcome these potential scaling issues, a hierarchical version of
the DVMRP is under development.  In hierarchical routing, the MBone
would be divided into a number of individual routing domains.  Each
routing domain executes its own instance of an "intra-domain" multicast
routing protocol.  Another protocol, or another instance of the same
protocol, would be used for routing between the individual domains.

7.1.6.1 Benefits of Hierarchical Multicast Routing

Hierarchical routing reduces the demand for router resources because
each router only needs to know the explicit details about routing
packets to destinations within its own domain, but needs to know little
or nothing about the detailed topological structure of any of the other
domains.  The protocol running between the domains is envisioned to
maintain information about the interconnection of the domains, but not
about the internal topology of each domain.


Semeria & Maufer                                               [Page 33]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================
                                                      _________
                      ________       _________       /         \
                     /        \     /         \     | Region D  |
     ___________     |Region B|-L2-|           |-L2-\___________/
    /           \-L2-\________/    |           |      ___________
   |             |     |    |      |           |     /           \
   | Region A    |    L2   L2      | Region C  |-L2-| Region E    |
   |             |     |    |      |           |    |             |
    \___________/     ________     |           |    \_____________/
                     /        \-L2-|           |
                     |Region F|    \___________/
                     \________/


Figure 16. Hierarchical DVMRP
========================================================================


In addition to reducing the amount of routing information, there are
several other benefits to be gained from the development and deployment
of a hierarchical version of the DVMRP:

    o  Different multicast routing protocols may be deployed
       in each region of the MBone.  This permits the testing
       and deployment of new protocols on a domain-by-domain
       basis.

    o  The effects of an individual link or router failures
       are limited to only those routers operating within a
       single domain. Likewise, the effects of any change to
       the topological interconnection of regions is limited
       to only inter-domain routers.  These enhancements are
       especially important when deploying a distance-vector
       routing protocol which can result in relatively long
       convergence times.

    o  The count-to-infinity problem associated with distance-
       vector routing protocols places limitations on the
       maximum diameter of the MBone topology.  Hierarchical
       routing limits these diameter constraints to a single
       domain, instead of to the entire MBone.

7.1.6.2 Hierarchical Architecture

Hierarchical DVMRP proposes the creation of non-intersecting regions
where each region has a unique Region-ID.  The routers internal to a
region execute any multicast routing protocols such as DVMRP, MOSPF,
PIM, or CBT as a "Level 1" (L1) protocol.  Each region is required to
have at least one "boundary router" which is responsible for providing



Semeria & Maufer                                               [Page 34]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


inter-regional connectivity.  The boundary routers execute DVMRP as a
"Level 2" (L2) protocol to forward traffic between regions.

The L2 routers exchange routing information in the form of Region-IDs
instead of the individual subnetwork prefixes contained within each
region.  With DVMRP as the L2 protocol, the inter-regional multicast
delivery tree is constructed based on the (region_ID, group) pair rather
than the usual (source, group) pair.

When a multicast packet originates within a region, it is forwarded
according to the L1 protocol to all subnetworks containing group
members.  In addition, the datagram is forwarded to each of the boundary
routers (L2) configured for the source region.  The L2 routers tag the
packet with the Region-Id and place it inside an encapsulation header
for delivery to other regions.  When the packet arrives at a remote
region, the encapsulation header is removed before delivery to group
members by the L1 routers.


7.2. Multicast Extensions to OSPF (MOSPF)

Version 2 of the Open Shortest Path First (OSPF) routing protocol is
defined in RFC-1583.  It is an Interior Gateway Protocol (IGP)
specifically designed to distribute unicast topology information among
routers belonging to a single Autonomous System.  OSPF is based on
link-state algorithms which permit rapid route calculation with a
minimum of routing protocol traffic.  In addition to efficient route
calculation, OSPF is an open standard that supports hierarchical
routing, load balancing, and the import of external routing information.

The Multicast Extensions to OSPF (MOSPF) are defined in RFC-1584.  MOSPF
routers maintain a current image of the network topology through the
unicast OSPF link-state routing protocol.  MOSPF enhances the OSPF
protocol by providing the ability to route multicast IP traffic.  The
multicast extensions to OSPF are built on top of OSPF Version 2 so that
a multicast routing capability can be incrementally introduced into an
OSPF Version 2 routing domain.  The enhancements that have been added
are backwards-compatible so that routers running MOSPF will interoperate
with non-multicast OSPF routers when forwarding unicast IP data traffic.
Note that MOSPF, unlike DVMRP, does not provide support for tunnels.

7.2.1 Intra-Area Routing with MOSPF

Intra-Area Routing describes the basic routing algorithm employed by
MOSPF.  This elementary algorithm runs inside a single OSPF area and
supports multicast forwarding when a source and all destination group
members reside in the same OSPF area, or when the entire OSPF Autonomous
System is a single area.  The following discussion assumes that the
reader is familiar with the basic operation of the OSPF routing
protocol.



Semeria & Maufer                                               [Page 35]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


7.2.1.1 Local Group Database

Similar to the DVMRP, MOSPF routers use the Internet Group Management
Protocol (IGMP) to monitor multicast group membership on directly-
attached subnetworks.  MOSPF routers are required to implement a "local
group database" which maintains a list of directly attached groups and
determines the local router's responsibility for delivering multicast
datagrams to these groups.

On any given subnetwork, the transmission of IGMP Host Membership
Queries is performed solely by the Designated Router (DR).  Also, the
responsibility of listening to IGMP Host Membership Reports is performed
only by the Designated Router (DR) and the Backup Designated Router
(BDR).  This means that in a mixed environment containing both MOSPF and
OSPF routers, an MOSPF router must be elected the DR for the subnetwork
if IGMP Queries are to be generated.  This can be achieved by simply
assigning all non-MOSPF routers a RouterPriority of 0 to prevent them
from becoming the DR or BDR, thus allowing an MOSPF router to become the
DR for the subnetwork.

The DR is responsible for communicating group membership information to
all other routers in the OSPF area by flooding Group-Membership LSAs.
The DR originates a separate Group-Membership LSA for each multicast
group having one or more entries in the DR's local group database.
Similar to Router-LSAs and Network-LSAs, Group-Membership LSAs are
flooded throughout a single area only.  This ensures that all remotely-
originated multicast datagrams are forwarded to the specified subnetwork
for distribution to local group members.

7.2.1.2 Datagram's Shortest Path Tree

The datagram's shortest path tree describes the path taken by a
multicast datagram as it travels through the internetwork from the
source subnetwork to each of the individual group members.  The shortest
path tree for each (source, group) pair is built "on demand" when a
router receives the first multicast datagram for a particular (source,
group) pair.

When the initial datagram arrives, the source subnetwork is located in
the MOSPF link state database.  The MOSPF link state database is simply
the standard OSPF link state database with the addition of Group-
Membership LSAs.  Based on the Router- and Network-LSAs in the
MOSPF link state database, a source-based shortest-path tree is
constructed using Dijkstra's algorithm.  After the tree is built, Group-
Membership LSAs are used to prune those branches that do not lead to
subnetworks containing members of this group.  The output of these
algorithms is a pruned source-based tree rooted at the datagram's
source.

To forward multicast datagrams to downstream members of a group, each
router must determine its position in the datagram's shortest path


Semeria & Maufer                                               [Page 36]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


tree.  Assume that Figure 17 illustrates the shortest path tree
for a particular (source, group) pair.  Router E's upstream node is


========================================================================


                      S
                      |
                      |
                   A  #
                     / \
                    /   \
                   1     2
                  /       \
               B #         # C
                / \         \
               /   \         \
              3     4         5
             /       \         \
          D #         # E       # F
                     / \         \
                    /   \         \
                   6     7         8
                  /       \         \
               G #         # H       # I


LEGEND

 #   Router


Figure 17. Shortest Path Tree for a (S, G) pair
========================================================================


Router B and there are two downstream interfaces:  one connecting to
Subnetwork 6 and another connecting to Subnetwork 7.

Note the following properties of the basic MOSPF routing algorithm:

    o  For a given multicast datagram, all routers within an OSPF
       area calculate the same source-based shortest path delivery
       tree.  Tie-breakers have been defined to guarantee that if
       several equal-cost paths exist, all routers agree on a single
       path through the area.  Unlike unicast OSPF, MOSPF does not
       support the concept of equal-cost multipath routing.

    o  Synchronized link state databases containing Group-Membership
       LSAs allow an MOSPF router to effectively perform the Reverse


Semeria & Maufer                                               [Page 37]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


       Path Multicasting (RPM) computation "in memory."   Unlike the
       DVMRP, this means that the first datagram of a new transmis-
       sion does not have to be flooded to all routers in an area.

    o  The "on demand" construction of the source-based delivery tree
       has the benefit of spreading calculations over time, resulting
       in a lesser impact for participating routers.  Of course, this
       may strain the CPU(s) in a router if many (source, group) pairs
       appear at about the same time, or if there are a lot of events
       which force the router to flush and rebuild its forwarding cache.
       In a stable topology with long-lived multicast sessions, these
       effects should be minimal.

7.2.1.3 Forwarding Cache

Each MOSPF router makes its forwarding decision based on the contents of
its forwarding cache.  The forwarding cache is built from the source-
based shortest-path tree for each (source, group) pair and the router's
local group database.  After the router discovers its position in the
shortest path tree, a forwarding cache entry is created containing the
(source, group) pair, the upstream interface, and the downstream
interface(s). At this point, all resources associated with the creation
of the tree are deleted.  From this point on, the forwarding cache entry
is used to quickly forward all subsequent datagrams from this source to
this group.

Figure 18 displays the forwarding cache for an example MOSPF router.
The elements in the display include the following items:

Dest. Group            The destination group address to which matching
                       datagrams are forwarded.

Source                 The datagram's source host address.  Each (Dest.
                       Group, Source) pair uniquely identifies a
                       separate forwarding cache entry.


========================================================================


Dest. Group     Source       Upstream     Downstream   TTL

224.1.1.1       128.1.0.2      11          12   13      5
224.1.1.1       128.4.1.2      11          12   13      2
224.1.1.1       128.5.2.2      11          12   13      3
224.2.2.2       128.2.0.3      12          11           7


Figure 18: MOSPF Forwarding Cache
========================================================================



Semeria & Maufer                                               [Page 38]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


Upstream               The interface from which a matching datagram
                       must be received

Downstream             The interface(s) over which a matching datagram
                       will be forwarded to reach known Destination
                       group members

TTL                    The minimum number of hops a datagram will
                       travel to reach the multicast group members.
                       This allows the router to discard datagrams
                       that do not have a high enough TTL to reach a
                       certain group member.

The information in the forwarding cache is not aged or periodically
refreshed.  It is maintained as long as there are system resources
available (e.g., memory) or until the next topology change.  In general,
the contents of the forwarding cache will change when:

    o  The topology of the OSPF internetwork changes, forcing all of
       the shortest path trees to be recalculated.  (Once the cache
       has been flushed, entries are not rebuilt until another packet
       for one of the previous (Dest. Group, Source) pairs is
       received.)

    o  There is a change in the Group-Membership LSAs indicating that
       the distribution of individual group members has changed.

7.2.2 Mixing MOSPF and OSPF Routers

MOSPF routers can be combined with non-multicast OSPF routers. This
permits the gradual deployment of MOSPF and allows experimentation with
multicast routing on a limited scale.  When MOSPF and non-MOSPF routers
are mixed within an Autonomous System, all routers will interoperate in
the forwarding of unicast datagrams.

It is important to note that an MOSPF router is required to eliminate
all non-multicast OSPF routers when it builds its source-based shortest-
path delivery tree.  An MOSPF router can easily determine the multicast
capability of any other router based on the setting of the multicast-
capable bit (MC-bit) in the Options field of each router's link state
advertisements.  The omission of non-multicast routers can create a
number of potential problems when forwarding multicast traffic:

    o  The Designated Router for a multi-access network must be an
       MOSPF router.  If a non-multicast OSPF router is elected the
       DR, the subnetwork will not be selected to forward multicast
       datagrams since a non-multicast DR cannot generate Group-
       Membership LSAs for its subnetwork (because it is not running
       IGMP, so it won't hear IGMP Host Membership Reports).  To use
       MOSPF, it is a good idea to ensure that at least two of the
       MOSPF routers on each LAN have higher router_priority values


Semeria & Maufer                                               [Page 39]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


       than any non-MOSPF routers.  A possible strategy would be to
       configure any non-MOSPF routers with a router_priority of
       zero, so that they cannot become (B)DR.

    o  Multicast datagrams may be forwarded along suboptimal routes
       since the shortest path between two points may require traversal
       of a non-multicast OSPF router.

    o  Even though there is unicast connectivity to a destination,
       there may not be multicast connectivity.  For example, the
       network may partition with respect to multicast connectivity
       since the only path between two points could require traversal
       of a non-multicast-capable OSPF router.

    o  The forwarding of multicast and unicast datagrams between
       two points may follow entirely different paths through the
       internetwork.  This may make some routing problems a bit more
       challenging to debug.

7.2.3 Inter-Area Routing with MOSPF

Inter-area routing involves the case where a datagram's source and some
of its destination group members reside in different OSPF areas.  It
should be noted that the forwarding of multicast datagrams continues to
be determined by the contents of the forwarding cache which is still
built from the local group database and the datagram source-based trees.
The major differences are related to the way that group membership
information is propagated and the way that the inter-area source-based
tree is constructed.

7.2.3.1 Inter-Area Multicast Forwarders

In MOSPF, a subset of an area's Area Border Routers (ABRs) function as
"inter-area multicast forwarders."  An inter-area multicast forwarder is
responsible for the forwarding of group membership information and
multicast datagrams between areas. Configuration parameters determine
whether or not a particular ABR also functions as an inter-area
multicast forwarder.

Inter-area multicast forwarders summarize their attached areas' group
membership information to the backbone by originating new Group-
Membership LSAs into the backbone area.  It is important to note that
the summarization of group membership in MOSPF is asymmetric.  This
means that group membership information from non-backbone areas is
flooded into the backbone.  However, group membership from the backbone
or from other non-backbone areas is not flooded into any non-backbone
area(s).

To permit the forwarding of multicast traffic between areas, MOSPF
introduces the concept of a "wild-card multicast receiver." A wild-card
multicast receiver is a router that receives all multicast traffic


Semeria & Maufer                                               [Page 40]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


generated in an area, regardless of the multicast group membership.  In
non-backbone areas, all inter-area multicast forwarders operate as
wild-card multicast receivers.  This guarantees that all multicast
traffic originating in a non-backbone area is delivered to its inter-
area multicast forwarder, and then if necessary into the backbone area.


========================================================================


                 -------------------------
                /      Backbone Area      \
                |                         |
                |      ^           ^      |
                |   ___|___     ___|___   |
                \__|       |___|       |__/
                   |---*---|   |---*---|
                       |           |
                    _______     _______
                   /       \   /       \
                   | Area  |   | Area  |
                   |   1   |   |   2   |
                   |-------|   |-------|


LEGEND

   ^
   |    Group Membership LSAs
 _____
|_____| Area Border Router and
        Inter-Area Multicast Forwarder

*       Wild-Card Multicast
        Receiver Interface


Figure 19. Inter-Area Routing Architecture
========================================================================


Since the backbone has group membership knowledge for all areas, the
datagram can then be forwarded to group members residing in the
backbone and other non-backbone areas.  The backbone area does not
require wild-card multicast receivers because the routers in the
backbone area have complete knowledge of group membership information
for the entire OSPF system.






Semeria & Maufer                                               [Page 41]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


7.2.3.2 Inter-Area Datagram Shortest-Path Tree

In the case of inter-area multicast routing, it is often impossible to
build a complete datagram shortest-path delivery tree.  Incomplete trees
are created because detailed topological and group membership
information for each OSPF area is not distributed between OSPF areas.
To overcome these limitations, topological estimates are made through
the use of wild-card receivers and OSPF Summary-Links LSAs.

There are two cases that need to be considered when constructing an
inter-area shortest-path delivery tree.  The first involves the
condition when the source subnetwork is located in the same area as the
router performing the calculation.  The second situation occurs when the


========================================================================


                 ----------------------------------
                |              S                   |
                |              |     Area 1        |
                |              |                   |
                |              #                   |
                |             / \                  |
                |            /   \                 |
                |           /     \                |
                |          /       \               |
                |       O-#         #-O            |
                |        / \         \             |
                |       /   \         \            |
                |      /     \         \           |
                |     /       \         \          |
                |  O-#         #         #-O       |
                |             / \         \        |
                |            /   \         \       |
                |           /     \         \      |
                |          /       \         \     |
                |       O-#         #-O       ---  |
                 ----------------------------| ? |-
                                              ---
                                               To
                                            Backbone
LEGEND

S   Source Subnetwork
O   Subnet Containing Group Members
#   Intra-Area MOSPF Router
?   WildCard Multicast Receiver

Figure 20. Datagram Shortest Path Tree (Source in Same Area)
========================================================================


Semeria & Maufer                                               [Page 42]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


source subnetwork is located in a different area than the router
performing the calculation.

If the source of a multicast datagram resides in the same area as the
router performing the calculation, the pruning process must be careful
to ensure that branches leading to other areas are not removed from the
tree.  Only those branches having no group members nor wild-card
multicast receivers are pruned.  Branches containing wild-card multicast
receivers must be retained since the local routers do not know if there
are group members residing in other areas.


========================================================================

                               S
                               |
                               #
                               |
                       Summary-Links LSA
                               |
                              ---
                 ------------| ? |-----------------
                |             ---    Area 1        |
                |              |                   |
                |              #                   |
                |             / \                  |
                |            /   \                 |
                |           /     \                |
                |          /       \               |
                |       O-#         #-O            |
                |        / \         \             |
                |       /   \         \            |
                |      /     \         \           |
                |     /       \         \          |
                |  O-#         #         #-O       |
                |             / \         \        |
                |            /   \         \       |
                |           /     \         \      |
                |          /       \         \     |
                |       O-#         #-O       #-O  |
                 ----------------------------------

LEGEND

S   Source Subnetwork
O   Subnet Containing Group Members
#   Inter-Area MOSPF Router
?   Intra-Area Multicast Forwarder

Figure 21. Shortest Path Tree (Source in Different Area)
========================================================================


Semeria & Maufer                                               [Page 43]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


If the source of a multicast datagram resides in a different area than
the router performing the calculation, the details describing the local
topology surrounding the source station are not known.  However, this
information can be estimated using information provided by Summary-Links
LSAs for the source subnetwork.  In this case, the base of the tree
begins with branches directly connecting the source subnetwork to each
of the local area's inter-area multicast forwarders.  The inter-area
multicast forwarders must be included in the tree since any multicast
datagrams originating outside the local area will enter the area via an
inter-area multicast forwarder.

Since each inter-area multicast forwarder is also an ABR, it must
maintain a separate link state database for each attached area. This
means that each inter-area multicast forwarder is required to calculate
a separate forwarding tree for each of its attached areas.  After the
individual trees are calculated, they are merged into a single
forwarding cache entry for the (source, group) pair and then the
individual trees are discarded.

7.2.4 Inter-Autonomous System Multicasting with MOSPF

Inter-Autonomous System multicasting involves the situation where a
datagram's source and at least some of its destination group members
reside in different OSPF Autonomous Systems.  It should be emphasized
that in OSPF terminology "inter-AS" communication also refers to
connectivity between an OSPF domain and another routing domain which
could be within the same Autonomous System from the perspective of an
Exterior Gateway Protocol.

To facilitate inter-AS multicast routing, selected Autonomous System
Boundary Routers (ASBRs) are configured as "inter-AS multicast
forwarders."  MOSPF makes the assumption that each inter-AS multicast
forwarder executes an inter-AS multicast routing protocol (e.g., DVMRP)
which forwards multicast datagrams in a reverse path forwarding (RPF)
manner.  Each inter-AS multicast forwarder functions as a wild-card
multicast receiver in each of its attached areas.  This guarantees that
each inter-AS multicast forwarder remains on all pruned shortest-path
trees and receives all multicast datagrams, regardless of the multicast
group membership.

Three cases need to be considered when describing the construction of an
inter-AS shortest-path delivery tree.  The first occurs when the source
subnetwork is located in the same area as the router performing the
calculation.  For the second case, the source subnetwork resides in a
different area than the router performing the calculation.  The final
case occurs when the source subnetwork is located in a different AS
than the router performing the calculation.

The first two cases are similar to the inter-area examples described in
the previous section.  The only enhancement is that inter-AS multicast
forwarders must also be included on the pruned shortest path delivery


Semeria & Maufer                                               [Page 44]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


tree.  Branches containing inter-AS multicast forwarders must be
retained since the local routers do not know if there are group members
residing in other Autonomous Systems.  When a multicast datagram arrives
at an inter-AS multicast forwarder, it is the responsibility of the ASBR
to determine whether the datagram should be forwarded outside of the
local Autonomous System.

Figure 22 illustrates a sample inter-AS shortest path delivery tree when
the source subnetwork resides in the same area as the router performing
the calculation.


========================================================================


                 -----------------------------------
                |              S     Area 1         |
                |              |                    |
                |              #                    |
                |             / \                   |
                |            /   \                  |
                |           /     \                 |
                |          /       \                |
                |       O-#         #-O             |
                |        / \         \              |
                |       /   \         \             |
                |      /     \         \            |
                |     /       \         \           |
                |  O-#         #         #-O        |
                |             / \         \         |
                |            /   \         \        |
                |           /     \         \       |
                |          /       \         \      |
                |         /         #-O       \     |
                |       ---                    ---  |
                 ------| & |------------------| ? |-
                        ---                    ---
                 To other Autonomous      To Backbone
                     Systems

LEGEND

S   Source Subnetwork
O   Subnet Containing Group Members
#   Intra-Area MOSPF Router
?   Inter-Area Multicast Forwarder
&   Inter-AS Multicast Forwarder


Figure 22. Inter-AS Datagram Shortest Path Tree (Source in Same Area)
========================================================================


Semeria & Maufer                                               [Page 45]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


If the source of a multicast datagram resides in a different Autonomous
System than the router performing the calculation, the details
describing the local topology surrounding the source station are not
known.   However, this information can be estimated using the multicast-
capable AS-External Links describing the source subnetwork.  In this
case, the base of the tree begins with branches directly connecting the
source subnetwork to each of the local area's inter-AS multicast
forwarders.


========================================================================

                               S
                               |
                               :
                               |
                       AS-External links
                               |
                              ---
                 ------------| & |-----------------
                |             ---                  |
                |             / \                  |
                |            /   \     Area 1      |
                |           /     \                |
                |          /       \               |
                |       O-#         #-O            |
                |        / \         \             |
                |       /   \         \            |
                |      /     \         \           |
                |     /       \         \          |
                |  O-#         #         #-O       |
                |             / \         \        |
                |            /   \         \       |
                |           /     \         \      |
                |          /       \         \     |
                |         /         #-O       #-O  |
                |       ---                        |
                 ------| ? |-----------------------
                        ---
                        To
                     Backbone
LEGEND

S   Source Subnetwork
O   Subnet Containing Group Members
#   Intra-Area MOSPF Router
?   Inter-Area Multicast Forwarder
&   Inter-AS Multicast Forwarder

Figure 23. Inter-AS Datagram Shortest Path Tree (Source in Different AS)
========================================================================


Semeria & Maufer                                               [Page 46]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


Figure 23 shows a sample inter-AS shortest-path delivery tree when the
inter-AS multicast forwarder resides in the same area as the router
performing the calculation.  If the inter-AS multicast forwarder is
located in a different area than the router performing the calculation,
the topology surrounding the source is approximated by combining the
Summary-ASBR Link with the multicast capable AS-External Link.

As a final point, it is important to note that AS External Links are not
imported into Stub areas.  If the source is located outside of the stub
area, the topology surrounding the source is estimated by the Default
Summary Links originated by the stub area's intra-area multicast
forwarder rather than the AS-External Links.


7.3 Protocol-Independent Multicast (PIM)

The Protocol Independent Multicast (PIM) routing protocol is currently
under development by the Inter-Domain Multicast Routing (IDMR) working
group of the IETF.  The objective of the IDMR working group is to
develop one--or possibly more than one--standards-track multicast
routing protocol(s) that can provide scaleable inter-domain multicast
routing across the Internet.

PIM receives its name because it is not dependent on the mechanisms
provided by any particular unicast routing protocol.  However, any
implementation supporting PIM requires the presence of a unicast routing
protocol to provide routing table information and to adapt to topology
changes.

PIM makes a clear distinction between a multicast routing protocol that
is designed for dense environments and one that is designed for sparse
environments.  Dense-mode refers to a protocol that is designed to
operate in an environment where group members are relatively densely
packed and bandwidth is plentiful.  Sparse-mode refers to a protocol
that is optimized for environments where group members are distributed
across many regions of the Internet and bandwidth is not necessarily
widely available.  It is important to note that sparse-mode does not
imply that the group has a few members, just that they are widely
dispersed across the Internet.

The designers of PIM argue that DVMRP and MOSPF were developed for
environments where group members are densely distributed. They emphasize
that when group members and senders are sparsely distributed across a
wide area, DVMRP and MOSPF do not provide the most efficient multicast
delivery service.  DVMRP periodically sends multicast packets over
many links that do not lead to group members, while MOSPF can send group
membership information over links that do not lead to senders or
receivers.





Semeria & Maufer                                               [Page 47]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


7.3.1 PIM-Dense Mode (PIM-DM)

While the PIM architecture was driven by the need to provide scaleable
sparse-mode delivery trees, PIM also defines a new dense-mode protocol
instead of relying on existing dense-mode protocols such as DVMRP and
MOSPF.  It is envisioned that PIM-DM would be deployed in resource rich
environments, such as a campus LAN where group membership is relatively
dense and bandwidth is likely to be readily available.  PIM-DM's control
messages are similar to PIM-SM's by design.

PIM - Dense Mode (PIM-DM) is similar to DVMRP in that it employs the
Reverse Path Multicasting (RPM) algorithm.  However, there are several
important differences between PIM-DM and DVMRP:

    o  To find routes back to sources, PIM-DM relies on the presence
       of an existing unicast routing table.  PIM-DM is independent of
       the mechanisms of any specific unicast routing protocol.  In
       contrast, DVMRP contains an integrated routing protocol that
       makes use of its own RIP-like exchanges to build its own unicast
       routing table (so a router may orient itself with respect to
       active source(s).  MOSPF augments the information in the OSPF
       link state database, thus MOSPF must run in conjunction with
       OSPF.

    o  Unlike the DVMRP which calculates a set of child interfaces for
       each (source, group) pair, PIM-DM simply forwards multicast
       traffic on all downstream interfaces until explicit prune
       messages are received.  PIM-DM is willing to accept packet
       duplication to eliminate routing protocol dependencies and
       to avoid the overhead inherent in determining the parent/child
       relationships.

For those cases where group members suddenly appear on a pruned branch
of the delivery tree, PIM-DM, like DVMRP, employs graft messages to
re-attach the previously pruned branch to the delivery tree.


8.  SHARED TREE ("SPARSE MODE") ROUTING PROTOCOLS

The most recent additions to the set of multicast routing proto- cols
are based on a shared delivery tree.

These emerging routing protocols include:

    o  Protocol Independent Multicast - Sparse Mode (PIM-SM), and

    o  Core-Based Trees (CBT).

Each of these routing protocols is designed to operate efficiently
over a wide area network where bandwidth is scarce and group members may



Semeria & Maufer                                               [Page 48]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


be sparsely distributed.  Their ultimate goal is to provide scaleable
interdomain multicast routing across the Internet.


8.1  Protocol-Independent Multicast - Sparse Mode (PIM-SM)

As described previously, PIM also defines a "dense-mode" or source-based
tree variant.  The two protocols are quite unique, and other than
control messages, they have very little else in common.  Because PIM
integrates control message processing and data packet forwarding among
PIM-Sparse and -Dense Modes, a single PIM router can run different modes
for different groups, as desired.

PIM-Sparse Mode (PIM-SM) is being developed to provide a multicast
routing protocol that provides efficient communication between members
of sparsely distributed groups--the type of groups that are likely to
be common in wide-area internetworks. PIM's designers observe that
several hosts wishing to participate in a multicast conference do not
justify flooding the entire internetwork periodically with the group's
multicast traffic.

Noting today's existing MBone scaling problems, and extrapolating to a
future of ubiquitous multicast (overlaid with perhaps thousands of
small, widely dispersed groups), it is not hard to imagine that existing
multicast routing protocols will experience scaling problems.  To
eliminate these potential scaling issues, PIM-SM is designed to limit
multicast traffic so that only those routers interested in receiving
traffic for a particular group "see" it.

PIM-SM differs from existing dense-mode protocols in two key ways:

    o  Routers with adjacent or downstream members are required to
       explicitly join a sparse mode delivery tree by transmitting
       join messages.  If a router does not join the pre-defined
       delivery tree, it will not receive multicast traffic addressed
       to the group.

       In contrast, dense-mode protocols assume downstream group
       membership and forward multicast traffic on downstream links
       until explicit prune messages are received.  Thus, the default
       forwarding action of dense-mode routing protocols is to forward
       all traffic, while the default action of a sparse-mode protocol
       is to block traffic unless it has been explicitly requested.

    o  PIM-SM evolved from the Core-Based Trees (CBT) approach in that
       it employs the concept of a "core" (or rendezvous point (RP) in
       PIM-SM terminology) where receivers "meet" sources.  The creator
       of each multicast group selects a primary RP and a small set of
       alternative RPs, known as the RP-set.  For each group, there is
       only a single active RP (which is uniquely determined by a hash
       function).


Semeria & Maufer                                               [Page 49]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================

                    S1                                      S2
                   ___|___                                 ___|___
                        |                                   |
                        |                                   |
                        #                                   #
                         \                                 /
                          \            Primary            /
                           \_____________RP______________/
                                        /|\
                      ________________// | \\_______________
                     /         _______/  |  \______         \
                     #         #         #         #         #
                  ___|___   ___|___   ___|___   ___|___   ___|___
                        |     |   |        |     |            |
                        R     R   R        R     R            R
LEGEND

   #   PIM Router
   R   Multicast Receiver


Figure 24 Primary Rendezvous Point
========================================================================


When joining a group, each receiver uses IGMP to notify its directly-
attached router, which in turn joins the multicast delivery tree by
sending an explicit PIM-Join message hop-by-hop toward the group's
primary RP.  A source uses the RP to announce its presence, and act as
a conduit to members that have joined the group.  This model requires
sparse-mode routers to maintain a bit of state (i.e., the RP-set for
each defined sparse-mode group) prior to the arrival of data. In
contrast, dense mode protocols are data-driven, since they do not store
any state for a group until the arrival of the first data packet.

8.1.1 Directly Attached Host Joins a Group

When there is more than one PIM router connected to a multi-access LAN,
the router with the highest IP address is selected to function as the
Designated Router (DR) for the LAN.  The DR may or may not be
responsible for the transmission of IGMP Host Membership Query messages,
but does send Join/Prune messages toward the RP, and maintains the
status of the active RP for local senders to multicast groups.

When the DR receives an IGMP Report message for a new group, the DR
determines if the group is RP-based or not by examining the group
address.  If the address indicates a SM group (by virtue of the group-
specific state that even inactive groups have stored in all PIM
routers), the DR performs a deterministic hash function over the


Semeria & Maufer                                               [Page 50]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


========================================================================


                                Source (S)
                                _|____
                                   |
                                   |
                                   #
                                  / \
                                 /   \
                                /     \
                               #       #
                              /         \
             Designated      /           \
 Host      | Router         /             \  Rendezvous Point
      -----|- # - - - - - -#- - - - - - - -RP   for group G
(receiver) |  ----Join-->  ----Join-->
           |

LEGEND

   #   PIM Router                 RP  Rendezvous Point

Figure 25: Host Joins a Multicast Group
========================================================================


group's RP-set to uniquely determine the primary RP for the group.
(Otherwise, this is a dense-mode group and dense-mode forwarding rules
apply.)

After performing the lookup, the DR creates a multicast forwarding cache
entry for the (*, group) pair and transmits a unicast PIM-Join message
toward the primary RP for this specific group.  The (*, group) notation
indicates an (any source, group) pair.  The intermediate routers forward
the unicast PIM-Join message, creating a forwarding cache entry for the
(*, group) pair only if such a forwarding entry does not yet exist.
Intermediate routers must create a forwarding cache entry so that they
will be able to forward future traffic downstream toward the DR which
originated the PIM-Join message.

8.1.2 Directly Attached Source Sends to a Group

When a source first transmits a multicast packet to a group, its DR
forwards the datagram to the primary RP for subsequent distribution
along the group's delivery tree.  The DR encapsulates the initial
multicast packets in a PIM-SM-Register packet and unicasts them toward
the primary RP for the group.  The PIM-SM-Register packet informs the
RP of a new source which causes the active RP to transmit PIM-Join
messages back toward the source's DR.  The routers between the RP and
the source's DR use the re- ceived PIM-Join messages (from the RP) to


Semeria & Maufer                                               [Page 51]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


create forwarding state for the new (source, group) pair.  Now all
routers from the active RP for this sparse-mode group to the source's DR
will be able to forward future unencapsulated multicast packets from
this source subnetwork to the RP.  Until the (source, group) state has
been created in all the routers between the RP and source's DR, the DR
must continue to send the source's multicast IP packets to the RP as
unicast packets encapsulated within unicast PIM-Register packets.  The
DR may stop forwarding multicast packets encapsulated in this manner
once it has received a PIM-Register-Stop message from the active RP for
this group.  The RP may send PIM-Register-Stop messages if there are no
downstream receivers for a group, or if the RP has successfully joined
the (source, group) tree (which originates at the source's DR).


========================================================================

                                Source (S)
                                _|____
                                   |
                                   |
                                   #
                                  / \
                                 /  ^\
                                /    .\
                               #      ^#
                              /        .\
             Designated      /          ^\
 Host      | Router         /            .\ v             |       Host
      -----|-#- - - - - - -#- - - - - - - -RP- - - # - - -|-----
(receiver) |  <~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~>   | (receiver)

LEGEND

   #   PIM Router
   RP  Rendezvous Point
       PIM-Register
< . <  PIM-Join
~ ~ ~  Resend to group members

Figure 26: Source sends to a Multicast Group
========================================================================


8.1.3 Shared Tree (RP-Tree) or Shortest Path Tree (SPT)?

The RP-tree provides connectivity for group members but does not
optimize the delivery path through the internetwork.  PIM-SM allows
receivers to either continue to receive multicast traffic over the
shared RP-tree or over a source-based shortest-path tree that a receiver
subsequently creates.  The shortest-path tree allows a group member to
reduce the delay between itself and a particular source.


Semeria & Maufer                                               [Page 52]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


A PIM router with local receivers has the option of switching to the
source's shortest-path tree (i.e., source-based tree) once it starts
receiving data packets from the source.  The change- over may be
triggered if the data rate from the source exceeds a predefined
threshold.  The local receiver's DR does this by sending a Join
message toward the active source.  After the source-based SPT is
active, protocol mechanisms allow a Prune message for the same source
to be transmitted to the active RP, thus removing this router from the
shared RP-tree.  Alternatively, the DR may be configured to continue
using the shared RP-tree and never switch over to the source-based SPT,
or a router could perhaps use a different administrative metric to
decide if and when to switch to a source-based tree.


========================================================================

                                Source (S)
                                _|____
                                   |
                                  %|
                                 % #
                                % / \*
                               % /   \*
                              % /     \*
          Designated         % #       #*
           Router           % /         \*
                           % /           \*
 Host      |  <-% % % % % % /             \v
      -----|-#- - - - - - -#- - - - - - - -RP
(receiver) |  <* * * * * * * * * * * * * * *
           |


LEGEND

  #   PIM Router
  RP  Rendezvous Point
* *   RP Tree
% %   SPT Tree

Figure 27: Shared RP-Tree and Shortest Path Tree (SPT)
========================================================================


8.1.4 Unresolved Issues

It is important to note that PIM is an Internet draft.  This means that
it is still early in its development cycle and clearly a "work in





Semeria & Maufer                                               [Page 53]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


progress."  There are several important issues that require further
research, engineering, and/or experimentation:

    o  PIM-SM requires routers to maintain a non-trivial
       amount of state information to describe sources
       and groups.

    o  Some multicast routers will be required to have
       both PIM interfaces and non-PIM interfaces.  The
       interaction and sharing of multicast routing
       information between PIM and other multicast
       routing protocols is still being defined.

Due to these reasons, especially the need to get operational experience
with the protocol, when PIM is finally published as an RFC, it will not
immediately be placed on the standards-track; rather it will be
classified as experimental.  After sufficient operational experience
has been obtained, presumably a slightly altered specification will be
defined that incorporates lessons learned during the experimentation
phase, and that new specification will then be placed on the standards
track.


8.2 Core-Based Trees (CBT)

Core Based Trees is another multicast architecture that is based on a
shared delivery tree.  It is specifically intended to address the
important issue of scalability when supporting multicast applications
across the public Internet.  CBT is also designed to enable
interoperability between distinct "clouds" on the Internet, each
executing a different multicast routing protocol.

Similar to PIM, CBT is protocol-independent.  CBT employs the
information contained in the unicast routing table to build its shared
delivery tree.  It does not care how the unicast routing table is
derived, only that a unicast routing table is present. This feature
allows CBT to be deployed without requiring the presence of any specific
unicast routing protocol.

8.2.1 Joining a Group's Shared Tree

When a multi-access network has more than one CBT router, one of the
routers is elected the designated router (DR) for the subnetwork.  The
DR is responsible for transmitting IGMP Queries and for initiating the
construction of a branch that links directly-attached group members to
the shared distribution tree for the group. The router on the subnetwork
with the lowest IP address is elected the IGMP Querier and also serves
as the CBT DR.

When the DR receives an IGMP Host Membership Report for a new group, it
transmits a CBT Join-Request to the next-hop router on the unicast path


Semeria & Maufer                                               [Page 54]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


to the "target core" for the multicast group. The identification of the
"target core" is based on static configuration.

The Join-Request is processed by all intermediate CBT routers, each of
which identifies the interface on which the Join-Request was received as
part of this group's delivery tree.  The intermediate routers continue
to forward the Join-Request toward the target core and to mark local
interfaces until the request reaches either 1) a core router, or 2) a
router that is already on the distribution tree for this group.

In either case, this router stops forwarding the Join-Request and
responds with a Join-Ack which follows the path back to the DR which
initiated the Join-Request.  The Join-Ack fixes the state in each of the
intermediate routers causing the interfaces to become part of the
distribution tree for the multicast group.  The newly constructed branch
is made up of non-core (i.e., "on-tree") routers providing the shortest
path between a member's directly attached DR and a core.

Once a branch is created, each child router monitors the status of its
parent router with a keepalive mechanism.  A child router periodically
unicasts a CBT-Echo-Request to its parent router which is then required
to respond with a unicast CBT-Echo-Reply message.


========================================================================


                                            #- - - -#- - - - -#
                                                    |          \
                                                    |           #
                                                    |
                                                    # - - - - #
      member  |                                     |
       host --|                                     |
              |     --Join-->  --Join-->  --Join--> |
              |- [DR] - - - [:] - - - -[:] - - - - [@]
              |     <--ACK--   <--ACK--   <--ACK--
              |


     LEGEND

        [DR]  CBT Designated Router
         [:]  CBT Router
         [@]  Target Core Router
          #   CBT Router that is already on the shared tree


Figure 28: CBT Tree Joining Process
========================================================================



Semeria & Maufer                                               [Page 55]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


It is only necessary to implement a single "keepalive" mechanism on each
link regardless of the number of multicast groups that are sharing the
link.  If for any reason the link between the child and parent should
fail, the child is responsible for re-attaching itself and its
downstream children to the shared delivery tree.

8.2.2 Primary and Secondary Cores

Instead of a single active "core" or "rendezvous point," CBT may have
multiple active cores to increase robustness.  The initiator of a
multicast group elects one of these routers as the Primary Core, while
all other cores are classified as Secondary Cores. The Primary Core must
be uniquely identified for the entire multi- cast group.

Whenever a group member joins to a secondary core, the secondary core
router ACKs the Join-Request and then joins toward the Primary Core.
Since each Join-Request contains the identity of the Primary Core for
the group, the secondary core can easily determine the identity of the
Primary Core for the group.  This simple process allows the CBT tree
to become fully connected as individual members join the multicast
group.


========================================================================


                  +----> [PC] <-----------+
                  |       ^               |
             Join |       | Join          | Join
                  |       |               |
                  |       |               |
         [SC]    [SC]    [SC]    [SC]    [SC] <-----+
                  ^       ^               ^         |
                  |       |               |         |
             Join |       | Join     Join |    Join |
                  |       |               |         |
                  |       |               |         |
                 [x]     [x]             [x]       [x]
                  :       :               :         :
                member  member          member    member
                 host    host            host      host


     LEGEND

     [PC]  Primary Core Router
     [SC]  Secondary Core Router
      [x]  Member-hosts' directly-attached routers

Figure 29: Primary and Secondary Core Routers
========================================================================


Semeria & Maufer                                               [Page 56]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


8.2.3 Data Packet Forwarding

After a Join-Ack is received by an intermediate router, it creates a CBT
forwarding information base (FIB) entry listing all interfaces that
are part of the specified group's delivery tree.  When a CBT router
receives a packet addressed to the multicast group, it simply forwards
the packet over all outgoing interfaces as specified by the FIB entry
for the group.

A CBT router may forward a multicast data packet in either "CBT Mode" or
"Native Mode."

    o  CBT Mode is designed for operation in heterogeneous
       environments that may include non-multicast capable
       routers or mrouters that do not implement (or are not
       configured for) CBT.  Under these conditions, CBT Mode
       is used to encapsulate the data packet in a CBT header
       and "tunnel" it between CBT-capable routers (or islands).

    o  Native Mode is designed for operation in a homogeneous
       environment where all routers implement the CBT routing
       protocol and no specialized encapsulation is required.

8.2.4 Non-Member Sending

Similar to other multicast routing protocols, CBT does not require that
the source of a multicast packet be a member of the multicast group.
However, for a multicast data packet to reach the core tree for the
group, at least one CBT-capable router must be present on the non-member
source station's subnetwork.  The local CBT-capable router employs CBT
Mode encapsulation and unicasts the data packet toward a core for the
multicast group.  When the encapsulated packet encounters an on-tree
router (or the target core), the packet is forwarded as required by the
CBT specification.

8.2.5 Emulating Shortest-Path Trees

The most common criticism of shared tree protocols is that they offer
sub-optimal routes and that they create high levels of traffic
concentration at the core routers.  One recent proposal in CBT
technology is a mechanism to dynamically reconfigure the core-based tree
so that it becomes rooted at the source station's local CBT router. In
effect, the CBT becomes a source-based tree but still remains a CBT (one
with a core that now happens to be adjacent to the source).  If
successfully tested and demonstrated, this technique could allow CBT to
emulate a shortest-path tree, providing more-optimal routes and reducing
traffic concentration among the cores.  These new mechanisms are being
designed with an eye toward preserving CBT's simplicity and scalability,
while addressing key perceived weaknesses of the CBT protocol.  Note
that PIM-SM also has a similar technique whereby a source-based delivery
tree can be selected by certain receivers.


Semeria & Maufer                                               [Page 57]

INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


For this mechanism, every CBT router is responsible for monitoring the
transmission rate and duration of each source station on a directly
attached subnetwork.  If a pre-defined threshold is exceeded, the local
CBT router may initiate steps to transition the CBT tree so that the
group's receivers become joined to a "core" that is local to the source
station's subnetwork.  This is accomplished by having the local router
encapsulate traffic in CBT Mode and place its own IP address in the
"first-hop router" field.  All routers on the CBT tree examine the
"first-hop router" field in every CBT Mode data packet.  If this field
contains a non-NULL value, each router transmits a Join-Request toward
the address specified in the "first-hop router" field.  It is important
to note that on the publication date of this "Introduction to IP
Multicast Routing" RFC, these proposed mechanisms to support dynamic
source-migration of cores have not yet been tested, simulated, or
demonstrated.

8.2.6 CBT Multicast Interoperability

Multicast interoperability is being defined in several stages. Stage 1
is concerned with the attachment of non-DVMRP stub domains to a DVMRP
backbone (e.g., the MBone).  Work is currently underway in the IDMR
working group to describe the attachment of stub-CBT and stub-PIM
domains to a DVMRP backbone.  The next stage will focus on developing
methods of connecting non-DVMRP transit domains to a DVMRP backbone.


========================================================================


         /---------------\        /---------------\
         |               |        |                |
         |               |        |                |
         |    DVMRP      |--[BR]--|  CBT Domain    |
         |   Backbone    |        |                |
         |               |        |                |
         \---------------/        \---------------/


     Figure 30: Domain Border Routers (BRs)
========================================================================


CBT interoperability will be achieved through the deployment of domain
border routers (BRs) which enable the forwarding of multicast traffic
between the CBT and DVMRP domains.  The BR implements DVMRP and CBT on
different interfaces and is responsible for forwarding data across the
domain boundary.

The BR is also responsible for exporting selected routes out of the CBT
domain into the DVMRP domain.  While the CBT domain never needs to
import routes, the DVMRP backbone needs to import routes to sources of


Semeria & Maufer                                               [Page 58]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


traffic from within the CBT domain.  The routes must be imported so that
DVMRP can perform the RPF check (which is required for construction of
its forwarding table).


9. REFERENCES

9.1 Requests for Comments (RFCs)

    1075   "Distance Vector Multicast Routing Protocol," D. Waitzman,
            C. Partridge, and S. Deering, November 1988.

    1112   "Host Extensions for IP Multicasting," Steve Deering,
            August 1989.

    1583   "OSPF Version 2," John Moy, March 1994.

    1584   "Multicast Extensions to OSPF," John Moy, March 1994.

    1585   "MOSPF: Analysis and Experience," John Moy, March 1994.

    1700    "Assigned Numbers," J. Reynolds and J. Postel, October
             1994. (STD 2)

    1800    "Internet Official Protocol Standards," Jon Postel,
             Editor, July 1995.

    1812    "Requirements for IP version 4 Routers," Fred Baker,
             Editor, June 1995

9.2 Internet Drafts

   "Core Based Trees (CBT) Multicast: Architectural Overview,"
    <draft-ietf-idmr-cbt-arch-03.txt>, A. J. Ballardie, September 19,
    1996.

   "Core Based Trees (CBT) Multicast: Protocol Specification," <draft-
    ietf-idmr-cbt-spec-06.txt>, A. J. Ballardie, November 21, 1995.

   "Hierarchical Distance Vector Multicast Routing for the MBone,"
    Ajit Thyagarajan and Steve Deering, July 1995.

   "Internet Group Management Protocol, Version 2," <draft-ietf-
    idmr-igmp-v2-05.txt>, William Fenner, October 25, 1996.

   "Internet Group Management Protocol, Version 3," <draft-cain-
    igmp-00.txt>, Brad Cain, Ajit Thyagarajan, and Steve Deering,
    Expires March 8, 1996.





Semeria & Maufer                                               [Page 59]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


   "Protocol Independent Multicast (PIM): Motivation and Architecture,"
    <draft-ietf-idmr-pim-arch-04.ps>, S. Deering, D. Estrin,
    D. Farinacci, V. Jacobson, C. Liu, and L. Wei, September 11, 1996.

   "Protocol Independent Multicast (PIM), Dense Mode Protocol
    Specification," <draft-ietf-idmr-pim-dm-spec-04.ps>, D. Estrin,
    D. Farinacci, V. Jacobson, C. Liu, L. Wei, P. Sharma, and
    A. Helmy, September 16, 1996.

   "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
    Specification," <draft-ietf-idmr-pim-sm-spec-09.ps>, S. Deering,
    D. Estrin, D. Farinacci, V. Jacobson, C. Liu, L. Wei, P. Sharma,
    and A Helmy, September 19, 1996.

9.3 Textbooks

    Comer, Douglas E. Internetworking with TCP/IP Volume 1 Principles,
    Protocols, and Architecture Second Edition, Prentice Hall, Inc.
    Englewood Cliffs, New Jersey, 1991

    Huitema, Christian. Routing in the Internet, Prentice Hall, Inc.
    Englewood Cliffs, New Jersey, 1995

    Stevens, W. Richard. TCP/IP Illustrated: Volume 1 The Protocols,
    Addison Wesley Publishing Company, Reading MA, 1994

    Wright, Gary and W. Richard Stevens. TCP/IP Illustrated: Volume 2
    The Implementation, Addison Wesley Publishing Company, Reading MA,
    1995

9.4 Other

    Deering, Steven E. "Multicast Routing in a Datagram
    Internetwork," Ph.D. Thesis, Stanford University, December 1991.

    Ballardie, Anthony J. "A New Approach to Multicast Communication
    in a Datagram Internetwork," Ph.D. Thesis, University of London,
    May 1995.


10. SECURITY CONSIDERATIONS

Security issues are not discussed in this memo.










Semeria & Maufer                                               [Page 60]


INTERNET-DRAFT    Introduction to IP Multicast Routing      January 1997


11. AUTHORS' ADDRESSES

    Chuck Semeria
      3Com Corporation
      5400 Bayfront Plaza
      P.O. Box 58145
      Santa Clara, CA 95052-8145

      Phone:  +1 408 764-7201
      Email:  <Chuck_Semeria@3Com.com>


    Tom Maufer
      3Com Corporation
      5400 Bayfront Plaza
      P.O. Box 58145
      Santa Clara, CA 95052-8145

      Phone:  +1 408 764-8814
      Email:  <maufer@3Com.com>

































Semeria & Maufer                                               [Page 61]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/