Multicasting

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Multicasting
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References

• Note Some slides come from the slides associated
  with this book
• Mastering Computer Networks An Internet Lab
  Manual, J. Liebeherr, M. El Zarki,
  Addison-Wesley, 2003.
• The Kurose Ross textbook on computer networking

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Multicasting

• Multicast is an operation that sends a single
  message from one process to each of the members
  of a group of processes.
• Ideally this is done in such a way that the
  membership of the group is transparent to the
  sender.

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Multicasting

• Multicast communications refers to one-to-many or
  many-to-many communications.

Unicast
Broadcast
Multicast
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Applications Using Multicasting

• For replication
• Streaming continuous media
• Example The transfer of the audio, video and
  text of a live lecture to a set of distributed
  lecture participants
• One sender, multiple receivers
• Shared data applications
• Example whiteboard, teleconferencing
• Multiple senders, multiple receivers
• Web cache updates
• Email distribution lists
• Content distribution
• Software distribution
• Interactive gaming

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Approaches to Multicasting

• One-to-all unicast
• Sender uses a separate unicast transport (e.g.,
  TCP or UDP) connection to each of the receivers.
• If using TCP, a connection is made between the
  sender and a receiver. The data is sent and the
  connection is then terminated. This is done for
  the sender and each of the receivers.

routers forward unicast datagrams
multicast receiver (red)
not a multicast receiver (red)
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Approaches to Multicasting

• Application-level multicasting
• This also uses unicast transmission.
• The sender transmits a copy to a small number
  of receivers, which then make copies themselves
  and forward these copies to other receivers which
  may then duplicate and forward copies to yet
  additional receivers.

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Approaches to Multicasting

• Explicit multicast
• Provide explicit multicast support at the network
  layer.
• A single datagram is transmitted from the sending
  host.
• This datagram (or copy) is then replicated at a
  network router whenever it must be forwarded on
  multiple outgoing links in order to reach the
  receivers.
• The data is forwarded in a tree-like fashion.
  The discovery of the tree is dynamic. Lots of
  routing algorithms.
• There is an Internet protocol called IP multicast
  for this.
• An IP address represents a group of processes

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Approaches to Multicast
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IP Multicast Communication

• IP multicast is built on top of IP.
• IP multicast allows the sender to transmit a
  single IP packet to a set of computers that form
  a multicast group.
• Being a member of a multicast group allows a
  computer to receive IP packets sent to the group.
• IP multicast is available only via UDP
• There is no multicast TCP

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IP Multicast Communication

• IP multicast works as follows
• Multicast groups are identified by IP addresses
  in the range 224.0.0.0 - 239.255.255.255 (class D
  address)
• Every host can join and leave a multicast group
  dynamically
• Server does not have to know all the receivers
• Anyone can send to the group
• Routers forward multicast datagrams to hosts that
  have joined the multicast group.
• Anyone can join a group

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IP Multicast Communication

• define MULTICASTADDR "229.8.2.4"
• define PORTNUMBER 5824
• if ((socketDesc socket(AF_INET, SOCK_DGRAM, 0))
  lt 0) perror("open error on socket")
• exit(1)
• myAddr.sin_family AF_INET
• myAddr.sin_addr.s_addr htonl(INADDR_ANY)
• myAddr.sin_port htons(0) / The zero says bind
  to any port /
• if (bind(socketDesc, (struct sockaddr ) myAddr,
  sizeof(myAddr)) lt 0) perror("bind error")
  exit(1)

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IP Multicast Communication

• / And build MULTICAST address /
• destinationAddr.sin_family AF_INET
• destinationAddr.sin_addr.s_addr
  htonl(inet_addr(MULTICASTADDR))
• destinationAddr.sin_port htons(PORTNUMBER)
• / Set the socket to permit multicasts on the
  local LAN /
• if (setsockopt(socketDesc, IPPROTO_IP,
  IP_MULTICAST_TTL, limitLanFlag,
  sizeof(limitLanFlag)) ! 0)
• perror("setsockopt error") exit(1)

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IP Multicast Communication

• / Broadcast message /
• strcpy(outMessageBuf, "Hello world")
• if (sendto(socketDesc, outMessageBuf,
  strlen(outMessageBuf)1, 0, (struct sockaddr
  )destinationAddr, sizeof(destinationAddr)) lt 0)
  perror("socket send error") exit(1)

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IP Multicast Communication

• / And wait for up to 5 seconds for replies to
  come back /
• / Set timeOut value for receive to 5 seconds /
• timeOut.tv_sec 5L
• timeOut.tv_usec 0L
• / Read responses from hosts accepting the
  multicast until a read finally times-out. /
• FD_ZERO(readReadySet)
• FD_SET(socketDesc,readReadySet)
• msgCount 0
• while (select(socketDesc1,readReadySet, NULL,
  NULL, timeOut) gt 0) .

Question What is the networking support needed?
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IP Multicast Communication

• IGMP signaling protocol to establish,
  maintain, remove groups on a subnet.
• Two new operations
• Join-IP-Multicast-Group(group-address, interface)
• Leave-IP-Multicast-Group(group-address,
  interface)

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Joining a Multicast Group

• Host informs local multicast(mcast) router of
  desire to join group
• Local router interacts with other routers to
  receive mcast datagram flow
• many protocols (e.g., DVMRP, MOSPF, PIM)

IGMP
IGMP
wide-area multicast routing
IGMP
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IGMP Internet Group Management Protocol

• The host sends IGMP report when application joins
  mcast group
• The host need not explicitly unjoin group when
  leaving
• The router sends IGMP query at regular intervals
• host belonging to a mcast group must reply to
  query

report
query
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Building Mcast Trees

• Problem Statement Find a tree (or trees)
  connecting routers having local mcast group
  members
• Approaches
• Source-based tree
• One tree per source
• Shortest path trees
• Reverse path forwarding
• Group-shared tree
• Group uses one tree
• Minimal spanning (Steiner) NP Complete Problem
• center-based trees

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Problems with IP Multicast Communication

• Security and privacy issues
• If the reason to use IP multicast is replication
  then you want to make sure that the multicast is
  reliable.
• Not all multicast applications need strong
  reliability of the type provided by TCP. Some can
  tolerate reordering, delay, etc
• Reliability needs application support. This
  suggests that applications may need to send their
  acknowledgemens.
• Ack-implosion if all destinations ack at once
• We will discuss reliability later
• Routers need to maintain state
• A bad link affects a subgroup
• IP multicast is probably sufficient for
  enterprise computing but not Internet computing
• We will discuss more on reliable multicast later
  which does not necessarily depend on using IP
  Multicast communication.

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Summary

• This section briefly summarizes issues in
  multicasting.