Chapter 4 IP Multicast

1
Chapter 4IP Multicast

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

2
Announcements

• Netstat portion of Homework 3 on Web, due March
  12 (two weeks)
• Programming Assignment 2 coming Friday
• Midterm March 14
• Next, IP multicast

3
Network Topology for Programming Assignment 2
1. Distance Vector Routing 2. Link State Routing
4
Recap of Previous Lecture

• More on Hierarchy in the Internet
• BGP, IBGP
• OSPF
• Subnets and CIDR prefixes
• DHCP
• Dynamic IP address
• Local client broadcasts to DHCP Relay, which
  unicasts to a DHCP Server
• ICMP
• Reports IP packet delivery errors back to source
• Ping ICMP echo and echo reply, smurf attack
• Traceroute ICMP time expired
• Router advertisement and solicitation

5
IP Multicast

• Unicast one source to one destination
• Broadcast one source to every destination
• Multicast one source to N destinations
• Variant N sources to N destinations
• Application Scenarios
• Video conference
• Audio concert
• Interactive games
• Example
• C wants to multicast
• to D and F

Multicast Receiver
Multicast Receiver
Multicast Sender
6
IP Multicast (2)

• How do we multicast 1-gtN efficiently?
• N unicast connections can waste bandwidth over
  shared links
• not scalable sender would have to send 100,000
  copies of the same packet to 100,000 subscribers
• Hint use the shortest path spanning tree to
  efficiently route multicast packets

7
Link-State IP Multicast

• Dijkstra gives us the shortest path spanning tree
• Root B of multicast tree sends packets only to
  nearest neighbors on tree, rather than to 100,000
  members
• Neighbors forward to their neighbors along
  shortest path tree, and so on
• Also called MOSPF

8
Link-State IP Multicast (2)

• How does a router know an IP packet is a
  multicast packet, and not a unicast packet?
• IP packets have a special multicast address
• Class D IP addresses top 4 bits are 1110
  followed by 28 bits to identify the multicast
  group address G
• Not all nodes want to be part of the multicast
  tree
• Participating nodes announce that they want to be
  part of the tree
• A Designated Router on each LAN hears this and
  floods Link State Packets that this LAN has a
  multicast group member
• All routers learn multicast membership of entire
  network

9
Link-State IP Multicast (3)

• Forwarding of packets
• When a router R receives a packet from S with
  destination multicast address G, it
• Performs Dijkstra calculation with S as root (not
  R) if not already in cache
• Finds itself (R ) in tree
• For each subtree from R with a group member,
• Cache info for this subtree
• Forward packet to
• subtree
• Example SB,
• RE

Member
Sender
Member
10
Distance Vector Multicast (DVMRP)

• How do we use distance vectors to achieve
  multicast?
• Dijkstras shortest path tree was more intuitive
• Distance vectors tell us next-hop path for
  shortest route to destination
• Basic method Flood, then Prune
• Flood Reverse-Path Broadcast
• When receiving a multicast packet to group G from
  source S
• Does packet arrive from the shortest-hop link to
  S?
• If yes, then multicast on all other outgoing
  links
• This efficiently floods the network
• Any source can send to any node along a shortest
  hop tree

11
DVMRP (2)

• Flooding Example Reverse-Path Broadcast
• When E receives a multicast packet to group G
  from source SC,
• Does packet arrive from the shortest-hop link to
  B?
• Yes, it arrived on B-E link which is next hop
  using the spanning tree centered on E
• Since yes, then multicast
• on all other outgoing
• links
• To A, F, and D
• This floods the network
• Compare to LSPs
• reliable flooding

12
DVMRP (3)

• Problems with Reverse-Path Broadcast
• Since were flooding on all outgoing links, then
  multiple routers on same Ethernet send copies of
  same packet
• Solution assign a parent router on each link.
  For each source S, the parent router is the one
  closest to S.

Parent
B-C link is Ethernet LAN
13
DVMRP (4)

• Problems with Reverse-Path Broadcast
• Flooding the network reaches nodes who dont want
  to be part of the multicast group
• Solution Prune using Reverse-Path Multicast
• Same parent router knows its a leaf when it is
  the only router in a network
• Each host that is a member of G periodically
  broadcasts its membership
• Parent router hears this, and will forward
  multicast packets to this LAN
• If parent hears no members, then sends no
  members up the shortest path tree

14
DVMRP (5)

• Pruning Example
• B sends to multicast group G consisting of F and
  D
• This message floods
• E sends a request to its leaf nodes is anyone
  part of group G?
• F and D respond, A does not
• E prunes A (remembers not to forward to A when it
  receives a multicast dest addr G)
• C is also pruned from B

Member
Sender
Member
15
Protocol Independent Multicast (PIM)

• Problems with MOSPF and DVMRP
• Each router has to have state
• Flooding is costly in DVMRP
• Routers not part of multicast group are flooded,
  then prune via No members messages
• When only a few sparse nodes in multicast tree,
  then flooding and state storage become excessive

16
PIM (2)

• PIM sparse mode
• Create a Rendezvous Point (RP) for each multicast
  group
• From a multicast tree rooted from the well-known
  RP
• Reverse path tree formed from unicast Joins by
  leaf nodes
• Senders unicast to RP, which then multicasts
  along tree

Rendezvous Point
17
PIM (3)

• PIM sparse mode
• If a particular sender transmits frequently then
• Receiver sees many packets with
• ltsource IP addr, multicast group addrgt
• Then, receiver sends a Join to the source
• Reverse path multicast tree is formed to the
  source, rather than to RP

18
Internet Group Management Protocol (IGMP)

• Used to join and leave multicast groups
• A multicast-enabled router on a LAN sends
  membership_query IGMP message
• Is anyone part of multicast group G?
• A multicast-enabled host replies with an IGMP
  membership_report
• Yes, Im part of multicast group G
• Host can send unsolicited membership_report to
  explicitly join a multicast group
• Router only needs to know that one host on LAN is
  a member of group G, not which host nor how many
  hosts
• Feedback suppression

19
Other Multicasting Issues

• Source doesnt have to be a member of a multicast
  tree!
• Any malicious user can overwhelm a multicast tree
• Source doesnt have to know in advance who is in
  multicast tree
• Clean decoupling of senders and receivers
• Receiver-driven multicast model Deering, 1990
• Any host can send an IGMP Join message to its
  attached IP multicast-enabled router
• No control over who joins an IP multicast group
• How do you prevent someone from subscribing at
  the IP router level?
• Once in a multicast group, can eavesdrop

20
Other Multicasting Issues (2)

• Multicast is unreliable
• Uses UDP datagrams over IP multicast datagrams
• How do we design a reliable multicast protocol?
• Suppose B reliably multicasts, and F and D are
  both members of multicast tree
• F and D both send ACKs upstream
• Source can get swamped with ACKs
• Solution put ACK filters in
• intermediate routers
• Would have to
• understand TCP
• semantics
• Controversial