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LOCAL foilset MultiCast Technologies and Rationale

Given by Marek Podgorny at Large Web Application Class, CPS600/640 Fall 1997 on Fall Semester 97. Foils prepared 17 Feb 1997
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This module covers basics of the multicast technology
It also introduces switching technology for traditional broadcast networks
For multicast, the focus is on the multicast routing protocols
The MBONE is not discussed but will be included later

Table of Contents for full HTML of MultiCast Technologies and Rationale

1 Multicast
2 Abstract of Multicast - Rationale, Technology, Perspectives
3 Multimedia and Multicast
4 Multipoint Transmission Methods
5 Multipoint Transmission Methods (Continued)
6 Multicast support on LANs
7 Multicast Technology Components
8 Multicast Technology Components (Continued)
9 IP Multicast Design
10 IP Multicast Design (Continued)
11 IP Multicast Routing Protocols: DVMRP
12 IP Multicast Routing Protocols: DVMRP (Continued)
13 IP Multicast Routing Protocols: MOSPF
14 IP Multicast Routing Protocols: MOSPF (Continued)
15 IP Multicast Routing: PIM
16 IP Multicast Routing: PIM (Continued)
17 IP Multicast Routing: PIM (2)
18 IP Multicast Routing: PIM (3)
19 IP Multicast Services in the Workgroup
20 Switched Networks
21 Switched Networks
22 Switched Networks (Continued)
23 Switched Networks - Evolution (1)
24 Switched Networks - Evolution (2)
25 Switched Networks
26 Switch Networks (Continued)

This table of Contents Abstract

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Foil 1 Multicast

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Rationale, Technology, Perspectives
Marek Podgorny
NPAC
Syracuse University
111 College Place
Syracuse
New York 13244-4100
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Foil 2 Abstract of Multicast - Rationale, Technology, Perspectives

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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This module covers basics of the multicast technology
It also introduces switching technology for traditional broadcast networks
For multicast, the focus is on the multicast routing protocols
The MBONE is not discussed but will be included later
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Foil 3 Multimedia and Multicast

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Multimedia streams tend to require higher bandwidth compared to text-based applications
Some MM applications require one-to-many connectivity
  • Desktop conferencing (many-to-many), LAN TV, collaborative computing, corporate broadcast)
Unicast method does not scale even on LANs
Broadcast does not scale on complex LANs and on WANs
Multipoint connectivity also benefits traditional applications (example: e-mail, news distribution, electronic journals)
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Foil 4 Multipoint Transmission Methods

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Unicast:
  • Application sends a copy of each packet to each member of the multicast group. Simple to implement, but does not scale for large groups. It also requires extra bandwidth as the same information has to be carried multiple times even on shared links.
Broadcast:
  • Application sends a copy of each packet to a broadcast address. Even simpler than unicast to implement. However, the network must either stop broadcasts at the LAN boundary or send the broadcast everywhere. Sending the broadcast everywhere is a waste of network resources if only a small group actually needs to see the packets.
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Foil 5 Multipoint Transmission Methods (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Multicast:
  • Application sends one copy of each packet and addresses it to the group of computers that want to receive it. Multicast addresses packets to a group of receivers rather than to a single receiver, and it depends on the network to forward the packets to only these networks that registered as receivers.
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Foil 6 Multicast support on LANs

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Most popular LAN technologies support multicast at their data link layer: Ethernet, FDDI, Token Ring. They however use different technologies to achieve the same functionality
  • An individual computer can listen to a unicast address, several multicast addresses, and the broadcast address on Ethernet and FDDI segments.
  • Token Rings have functional addresses that can be used to address groups of receivers.
Many multipoint applications are valuable precisely because they are not limited to a single LAN.
On internets using mixed data link technologies and other networking technologies, multicast must be implemented at the network layer.
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Foil 7 Multicast Technology Components

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Addressing:
  • There must be a network-layer address that is used to communicate with a group of receivers rather than a single receiver. There must be also a mechanism for mapping this address onto data-link layer multicast addresses where they exist.
Dynamic registration:
  • There must be a mechanism for the computer to communicate to the network that it is a member of a particular group. Computers must register to let the global network know which local networks need to receive traffic for each group.
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Foil 8 Multicast Technology Components (Continued)

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Multicast routing:
  • The network must be able to build packet distribution trees that allow sources to send packets to all receivers. Packet distribution trees need to ensure that each packet exists only one time on any given network - if there are multiple receivers on a given branch, there should only be one copy of the packets on that branch.
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Foil 9 IP Multicast Design

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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Addressing:
  • The IP address space is divided into four pieces: Class A, Class B, Class C, and Class D. Class D is reserved for multicast traffic. Class D addresses are allocated dynamically.
Dynamic Registration:
  • RFC 1112 defines the Internet Group Membership Protocol (IGMP). IGMP specifies how the host should inform the network that it is a member of a particular multicast group.
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Foil 10 IP Multicast Design (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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Multicast Routing:
  • RFC 1075 defines the Distance Vector Multicast Routing Protocol (DVMRP).
  • RFC 1584 defines the Multicast Open Shortest Path First (MOSPF) protocol -- OSFP extension to OSPF supporting IP Multicast.
  • PIM is a multicast protocol that can be used in conjunction with all unicast IP routing protocols. Relevant IETF drafts:
    • Protocol-Independent Multicast (PIM): Motivation and Architecture
    • Protocol-Independent Multicast (PIM): Protocol Specification.
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Foil 11 IP Multicast Routing Protocols: DVMRP

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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DVMRP uses Reverse Path Forwarding. When a router receives a packet, it floods the packet out of all paths except the one that leads back to the packet's source. Data stream reaches all LANs (possibly multiple times). If a router is attached to a set of LANs that do not want to receive a particular multicast group, the router can send a "prune" message back up the distribution tree to stop subsequent packets from traveling where there are no members.
Since new hosts may want to join the multicast group at any time, DVMRP must periodically re-flood. This creates a scaling problem, especially if pruning not effective or not implemented.
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Foil 12 IP Multicast Routing Protocols: DVMRP (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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DVMRP implements its own unicast routing protocol (similar to RIP) to determine which interface leads back to the source of the data stream. The path that the multicast traffic follows may not be the same as the path that the unicast traffic follows.
DVMRP has been used to build the MBONE by building tunnels between DVMRP-capable machines.
DVMRP is state of the art today.
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Foil 13 IP Multicast Routing Protocols: MOSPF

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Multicast OSPF (MOSPF) was defined as an extension to the OSPF unicast routing protocol. OSPF works by having each router in a network understand all of the available links in the network. Each OSPF router calculates routes from itself to all possible destinations.
MOSPF works by including multicast information in OSPF link state advertisements. An MOSPF router learns which multicast groups are active on which LANs.
MOSPF builds a distribution tree for each source/group pair and computes a tree for active sources sending to the group. The tree state is cached, and trees must be recomputed when a link state change occurs or when the cache times out.
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Foil 14 IP Multicast Routing Protocols: MOSPF (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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MOSPF works only in internetworks that are using OSPF.
MOSPF is best suited for environments that have relatively few source/group pairs active at any given time. It will work less well in environments that have many active sources or environments that have unstable links.
OSPF is not widely used ....
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Foil 15 IP Multicast Routing: PIM

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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PIM (Protocol-Independent Multicast):
  • Works with all existing unicast routing protocols.
  • Supports two different types of multipoint traffic distribution patterns:
    • Dense-mode PIM: uses Reverse Path Forwarding and looks a lot like DVMRP. However, dense-mode PIM is that PIM works with whatever unicast protocol is being used
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Foil 16 IP Multicast Routing: PIM (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Sparse-mode PIM: optimized for environments where there are many multipoint data streams with each data stream goes to a relatively small number of the LANs in the internetwork.
  • Reverse Path Forwarding wastes bandwidth for such traffic pattern.
  • Sparse-mode PIM works by defining a Rendezvous Point. Senders send data to and the receivers receive data from the Rendezvous Point. .
  • For each data stream the routers in the path will optimize the path automatically to remove any unnecessary hops.
  • Sparse-mode PIM assumes that no hosts want the multicast traffic unless they specifically ask for it.
PIM can concurrently support both modes
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Foil 17 IP Multicast Routing: PIM (2)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Dense Mode PIM routing is useful when:
  • Senders and receivers are in close proximity to one another.
  • There are few senders and many receivers.
  • The volume of multicast traffic is high.
  • The stream of multicast traffic is cons
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Foil 18 IP Multicast Routing: PIM (3)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Sparse Mode PIM routing is useful when:
  • There are few receivers in a group.
  • Senders and receivers are separated by WAN links.
  • The type of traffic is intermittent.
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Foil 19 IP Multicast Services in the Workgroup

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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LAN switches are a natural bandwidth enhancement tool able to improve the performance of time-critical or bandwidth-intensive multimedia applications.
In order for multicast applications to work in the LAN, both switching and routing systems must support IP multicast capabilities.
As bandwidth enhancement and multicast are both critical, LAN switches must have sufficient internetworking intelligence to forward multicast traffic only to those workgroup segments that will use this traffic. Otherwise, multicast traffic will be indiscriminately broadcast to all workgroup segments, needlessly robbing network bandwidth from other users' applications.
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Foil 20 Switched Networks

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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Transition from routed to switched networks
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Foil 21 Switched Networks

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Why the transition?
  • Switched internetworks integrate switching devices into existing shared-media networks to optimize the benefits of both routing and switching.
  • Switched networks provide users with new services and capabilities, such as virtual LANs (VLANs), multimedia support, and more efficient tools for network management.
  • Higher bandwidth to the desktop (microsegmentation) is one of the main driving forces, directly related to multimedia applications.
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Foil 22 Switched Networks (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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Switches (unlike hubs and routers) can (a) forward traffic quickly and directly to its destination; (b) provide non-blocking services and concurrent connections
In addition, switches allow to de-couple the physical and logical network layouts: they enable virtual LANs
Switches, in general, have better scalability than bridges and routers.
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Foil 23 Switched Networks - Evolution (1)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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Phase one: the microsegmentation
phase. Companies retain hubs and
routers but insert a LAN switch to
enhance performance.
Phase two: ATM technology and routing
between switches. LAN switches perform
switch processing and provide dedicated
bandwidth to the desktop and to shared-
media hubs. Backbone routers are
clustered by ATM switches to increase
backbone bandwidth, matching the
increased bandwidth in the wiring
closet.
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Foil 24 Switched Networks - Evolution (2)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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Phase three connects the ATM core
switch directly to LAN switches in the
wiring closet and to centralized or
distributed ATM routers. The network
backbone is now ATM-centric, with all
other devices at the periphery. Multilayer
switches have the intelligence to forward
packets between the different VLANs
locally, or layer 2 switches, or a
combination of both.
Phase four is the end-to-end switching
with integral VLAN and multilayer
switching capability. Route and Switch
Processors are distributed over the
ATM fabric.
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Foil 25 Switched Networks

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * See also color IMAGE
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What are the challenges?
  • Interoperability: legacy (hub/router) networks have to coexist with switched networks
  • Switching capabilities must be implemented at both layer 2 and layer 3 of the OSI model (multilayer switching)
  • LAN emulation must become commonplace - it isn't now
  • Or, network layer protocols must be modified to operate directly over ATM in native (AAL) mode (RFC 1577). This, BTW, would make ATM native QoS guarantees available for higher level protocols
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Foil 26 Switch Networks (Continued)

From MultiCast Technologies and Rationale Large Web Application Class, CPS600/640 Fall 1997 -- Fall Semester 97. * Critical Information in IMAGE
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At present, neither LANE nor RFC 1577 exploit ATM QoS capabilities
RSVP must be implemented on both routers and switches
Multicast routing protocols must be extended to switching platforms
New network management methodologies and tools are required for switched networks to exploit their flexibility
Northeast Parallel Architectures Center, Syracuse University, npac@npac.syr.edu

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