Multicasting EECS June 2016

Multicast One-to-many, many-to-many communications Applications: – Teleconferencing – Database – Distributed computing – Real-time work groups – Stock quote updates – Software and software update distribution 2

Internet Multicast Service Model Multicast group concept: use of indirection a host “sends” IP datagrams to multicast group. routers forward multicast datagrams to hosts that have “joined” that multicast group multicast group

Multicast Groups  Class D Internet addresses reserved for multicast:  Host group semantics: oanyone can “join” (receive from) multicast group. oanyone can send to multicast group. ono network-layer identification to hosts of members.  Needed: infrastructure to deliver multicast-addressed datagrams to all hosts that have joined that multicast group. 4

Multicast Addressing Class D address (see next slide) Source: unicast IP address S Receivers: multicast group ID G, a class-D address Each group is identified by (S, G) Ethernet broadcast address (all 1’s) 5

IPv4 Address Formats 6

Multicast Protocols Transport layer UDP Real-time Transport Protocol (RTP): for multimedia content ReSerVation Protocol (RSVP): for bandwidth reservation in a multicast distribution 7

Multicast Protocols (2) Routing, delivery On a local network (join/leave): – Internet Group Management Protocol (IGMP) – Multicast Listener Discovery (MLD): similar to IGMP but for IPv6 Intra-domain (routing): – MOSPF, PIM, DVMRP Inter-domain (routing): – Multicast Border Gateway Protocol (MBGP) 8

Joining a multicast group: 2-step process Local: host informs local multicast router of desire to join group: IGMP (Internet Group Management Protocol) Wide area: local router interacts with other routers to receive multicast datagram flow – many protocols (e.g., DVMRP, MOSPF, PIM) 9 IGMP wide-area multicast routing

IGMP

Internet Group Management Protocol (IGMP) RFC 3376 used to exchange multicast group information between hosts & routers on a LAN hosts send messages to routers to subscribe to (join) and unsubscribe from (leave) multicast group routers check which multicast groups are of interest to which hosts IGMP currently at version 3

IGMP: Main Ideas Router: sends IGMP query at regular intervals – hosts belonging to a multicast group must reply to query if wishing to join or stay in the group. Host: sends IGMP report (reply) when application wishes to join a multicast group. – IP_ADD_MEMBERSHIP socket option – hosts need not explicitly “unsubscribe” when leaving 12 query report

IGMP Router: broadcasts Host Membership Query message on LAN. Host: replies with Host Membership Report message to indicate group membership – randomized delay before responding – may send multiple times – implicit leave via no reply to Query Group-specific Query Leave Group message – Last host replying to Query can send explicit Leave Group message – Router performs group- specific query to see if any hosts left in group – Introduced in RFC 2236 IGMP v3: current version 13

Operation of IGMP v1 and v2 IGMPv1 – hosts could join group – routers used timer to unsubscribe members IGMPv2 enabled hosts to unsubscribe operational model: – receivers have to subscribe to groups – sources do not have to subscribe to groups – any host can send traffic to any multicast group Problems: spamming of multicast groups establishment of distribution trees is problematic (source location unknown) finding globally unique multicast addresses difficult (duplicate addresses)

IGMP v3 addresses weaknesses by: – allowing hosts to specify list from which they want to receive traffic – blocking traffic from other hosts at routers – allowing hosts to block packets from sources that send unwanted traffic

IGMP Operation - Joining IGMP host wants to make itself known as group member to other hosts and routers on LAN IGMPv3 allows hosts to specify wanted or unwanted sources (filtering capabilities) –E–EXCLUDE mode – to receive from all sources except those listed –I–INCLUDE mode – to receive only from sources listed To join a group, a host sends IGMP membership report message address field is the multicast address of the group sent in IP datagram current group members receive and learn about the new member routers listen to all IP multicast addresses to hear all reports

IGMP Operation – Keeping Lists Valid routers periodically issue IGMP general query message (group address is null) in datagram with all-hosts multicast address hosts must read such datagrams hosts respond with report message router doesn’t know every host in a group needs to know at least one group member still active each host in group sets timer with random delay host hearing another report cancels its own report if timer expires, host sends report only one member of each group reports to router

IGMP Operation - Leaving A host leaves a group G by sending a “leave group” message to the all-routers static multicast address. – “leave” = a membership report message with multicast address of group G, INCLUDE option and null list of source addresses. – Note: a membership report message with multicast address of group G, EXCLUDE option and null list of source addresses means to receive from all sources in group G. Router determines if there are any remaining members in that group using a group-specific query message (a membership query with the multicast address of G).

Membership Query Message

Membership Report Message

Group Record

IGMP: Summary For membership management. Between a host on a subnet (Ethernet) and the router for the subnet. The router periodically broadcast an IGMP host-membership query message on its subnet. A host subscribes to a group replies by multicasting a host- membership report message. – Feedback implosion  uses a random timer to suppress duplicates. The report is sent 3 times (for reliability). IGMP-1: hosts send no report  leaving the group IGMP-2: hosts send explicit host-membership leave messages to reduce leave latency. IGMP-3: filtering and blocking 22

Multicast Routing

Shortest Path Tree Multicast cast forwarding tree: tree of shortest path routes from source to all receivers. – Dijkstra’s algorithm. – MOSPF routing protocol R1 R2 R3 R4 R5 R6 R i router with attached group member router with no attached group member link used for forwarding, i indicates order link added by algorithm LEGEND S: source 24

How MOSPF Fits into Multicasting IGMP - used for discovery of hosts in multicast groups, messages exchanged between neighbors MOSPF - routes multicast datagrams within an AS MBGP (Multicast BGP) - routes multicast datagrams between ASs

MOSPF Extends OSPF for multicasting. Every router has the complete topology of its own network. A receiver joins a multicast group G by exchanging IGMP messages with its end-router R. The end-router R broadcasts its group membership to the whole network in the form (G, R). Every router in the network maintains a group membership table with each entry being a tuple [S, G, ]. A sender simply sends data packets as they are available. Each router uses the network topology, the group membership table, and the multicast group ID in the data packets to compute the route(s) to the destination(s). 26

Reading Section 19.1, Stallings More references: Multicasting on the Internet and Its Applications, Sanjoy Paul, Kluwer Academic Publishers, 1998, chapters 2, 4, 5. Computer Networking: A Top-Down Approach, 5 th edition, Kurose and Ross. 27