Transcript P - School of Computer Science

IP Multicast
G53ACC
Chris Greenhalgh
School of Computer Science
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Contents

What is Multicasting
Applications
Application Semantics
Implementation
Multicast addresses and scopes
Research Issues
Example: DIVE
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Book: COMER, ch 9.6, 9.7, 18.5, 27.15
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What is Multicasting?

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Defines “multicast groups”…
– like a host (destination) address
– but identifies a logical destination or group
Any number of hosts can
– sends packets to a multicast group
– join a multicast group


=> receive packets sent to that group
Plus:
– each packet crosses any network at most once
– packets are filtered in the network and host
NICs for interest (joined)
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Multicasting example
Host
Host
Host
P
P
Host
P
P
joined G,
sees P
Router
LAN
P
P
Host
P
P
LAN
Forwards
P if required
P
P
Host
Sends P to
multicast group G
Host
Host
Joined G,
sees P
Network interface
ignores P
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Multicast Applications

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One-to-many
– A/V distribution, push media, file distribution,
cacheing, announcements, monitoring (e.g.
stocks).
Many-to-many
– A/V conferencing, synchronized resources (e.g.
distributed database), concurrent processing,
collaboration, distance learning, chat groups,
Distributed Interaction Simulation (DIS), multiplayed games, jam sessions.
One-to-any / many-to-one (“any-cast”)
– resource discovery, data collection, auctions 5
Application Semantics
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For UDP only (unreliable datagrams).
Uses socket interface: java.net.MulticastSocket
– See over
Send multicast packets from a normal (unicast) UDP
socket, just give a class D destination IP address
– or use a multicast socket, if TTL must be specified.
Receive multicast packets only on a MulticastSocket.
– joinGroup(addr) performs a JOIN operation,
– leaveGroup(addr) or destroying a socket performs
a LEAVE operation.
– (More complex options with multi-homed
machines, using java.net.NetworkInterface)
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Java API: java.net.MulticastSocket

public class MulticastSocket
extends java.net.DatagramSocket
{
public MulticastSocket()
throws IOException;
public MulticastSocket(int port)
throws IOException;
public void joinGroup(InetAddress mcastaddr)
throws IOException;
public void leaveGroup(InetAddress mcastaddr)
throws IOException;
public void setTimeToLive(int ttl)
throws IOException;
}
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ReverseServerMulticast.java excerpts

...
int port = Integer.parseInt(args[1]);
InetAddress group =
InetAddress.getByName(args[0]);
MulticastSocket socket =
new MulticastSocket(port);
socket.joinGroup(group);
…
socket.receive(request);
…
// as per unicast server
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Defining Packets/Protocols


E.g. Real-Time Protocol (RTP) (RFC-1889, etc.)
Or make them up…
– Remember UDP semantics
Unreliable
 Packet-preserving
 Unordered

– Also consider
Time to join a group (start-up delay)
 Time to leave a group (extra traffic)
 Locally available number of groups (router state)

– Tens rather than thousands!
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Scope (see later slides...)
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Implementation (1)
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Multicast group =
Class D destination IP address
– starts 11102
= 224.0.0.0/4
 = 224.0.0.0 with netmask 240.0.0.0
 = 111000000000000000000000000000002 mask
111100000000000000000000000000002
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Mapped to underlying multicast
– not ARP!
– (LAN) Ethernet multicast addresses
– Takes bits from IP address, use in Ether. addr.
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But not all 28 variable bits! (24 bits only)
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Implementation (2)
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Hosts use Internet Group Management Protocol
(IGMP) (v.2 RFC-2236, v.3 RFC-3376)
– tells network router(s) about groups joined/left
Routers use multicast routing protocol(s) (e.g.
DVMRP, MOSPF)
– forward a single packet on multiple interfaces
– Many routers do NOT do multicast routing and
will drop packets
– Routers may “tunnel” multicast packets in
unicast packets across non-multicast regions

E.g. MBone = Multicast BackBone (historical)
– = Multicast “overlay” network, using unicast wide-area
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Choosing Multicast Addresses (1):
Statically allocated Global addresses
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RFC-1700 / IANA (www.iana.org)
– e.g. 224.2.0.0-224.2.127.253 Multimedia
Conference Calls
– Apply to IANA for an allocation
Allocate locally within Autonomous Unit
(RFC-3180, “GLOP addressing in 233/8”)
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Choosing Multicast Addresses (2):
Administrative Scoped Multicast
Addresses (RFC-2365)
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Configured in network
Link-local scope (i.e. LAN)
– 224.0.0.0/24
 i.e. first 24 bits count, like a netmask
255.255.255.0
Local scope (e.g. department)
– 239.255.0.0/16
Organisation-local scope (e.g. UoN)
– 239.192.0.0/14
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Choosing Multicast Addresses (3):
choosing an address in a range


Make one up and hope (check w. RFCs & scopes)
See also
http://www.29west.com/docs/THPM/multicast-addressassignment.html
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Hash to an address range and hope
Optionally augment with a monitoring process
which detects duplicate use and negotiates a
change to a new address
– As in the SDR MBone tool
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Choosing Multicast Addresses (4)
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Source-specific multicast address (if supported)
– RFC-4607 [1-to-many applications only]
Dynamically allocated using Multicast Address
Allocation Service
– E.g. SDR MBone tool for MM conferences or
IETF malloc working group outputs???
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TTL Scope (How Far do Packets Travel?)
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Use IP packet TTL (Time To Live) to determine
scope of sending:
– TTL 0 = on the same machine

iff “loopback” is on for sending socket and (on
Windows) an Ethernet interface is “up”.
– TTL 1 = on the same LAN (esp. Ethernet)
– TTL >1 = internetwork
on a multicast-capable WAN;
 on the MBone (Multicast Backbone).

– E.g.
31 = campus? 63 = country?
 127 = continent? 255 = whole world?
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Multicast Research & Deployment Issues
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Reliable delivery
– N.B. only a subset of receivers may have
missed a packet.
Flow and congestion control
– N.B. only a subset (one?) of receivers may be
experiencing congestion or buffer overflow.
=> lowest common denominator??
Deployment
– MBone, PIM, native (ISPs??)
– Application-specific bridges and reflectors
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Example: DIVE System Outline
(Swedish Institute of Computer Science,
www.sics.se/dive)
Process (Agent)
World X
Process (Agent)
Join: TCP state transfer World X
World Y
World multicast group:
updates, audio, video.
Data uses Scalable Reliable Multicast (shared NAcks).
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Example: DIVE System Bootstrapping
DIVE mgmt.
multicast group
First Agent
Diveserver
World 
multicast group
(1)
HTTP Server
(3)
(5)
World X
multicast group
(2) Diveserver finds
World X’s
multicast group.
(3) Agent pings
multicast group.
(2)
World X...
(1) Agent locates
diveserver (m/c).
World X definition
(files)
(4) Agent downloads
world definition.
(5) Agent creates
world locally.
(4) HTTP transfer
(c.f. state transfer)
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