Multicasting CSE 6590 Winter December 2015

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. 3

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) 2 ways of doing IP multicast at the link layer: – Link-layer (Ethernet) broadcast – Link-layer (Ethernet) multicast Both cases need filtering at IP layer. 4

IPv4 Address Formats 5

Multicast Addressing (2) 1. Link-layer (Ethernet) broadcast IP multicast packet is encapsulated in an Ethernet broadcast frame and transmitted on the bus. Every host picks up the Ethernet frame and does filtering at the IP layer to decide whether to keep or discard the frame. Redundant reception by many hosts. 2. Link-layer (Ethernet) multicast Requires a mapping of IP multicast address to an Ethernet multicast address (see next slide). There are up to 32 IP class-D addresses mapped to the same Ethernet multicast address. The IP module still has to filter out packets for non-member hosts. 6

Mapping from Class D IP adress to Ethernet multicast adress 7

Multicast Protocols Transport layer UDP Real-time Transport Protocol (RTP): for multimedia content ReSerVation Protocol (RSVP): for bandwidth reservation in a multicast distribution 8

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) 9

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) 10 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 (2) 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 13 query report

IGMP (3) 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 14

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 Message Formats Membership Query sent by multicast router three subtypes: general query, group-specific query, group- and-source specific query

Membership Query Fields type max response time checksum group address S flag QRV (querier's robustness variable) QQIC (querier's querier interval code) number of sources source addresses

IGMP Message Formats Membership Report

IGMP Message Formats Group Record

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 – all sources except those listed –I–INCLUDE mode – only from sources listed to join send IGMP membership report message address field multicast address of group sent in IP datagram current group members receive & learn 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 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 own if timer expires, host sends report only one member of each group reports to router

IGMP Operation - Leaving host leaves group by sending a leave group message to the all-routers static multicast address – sends a membership report message with EXCLUDE option and null list of source addresses router determines if have any remaining group members using group-specific query message

Group Membership with IPv6 IGMP defined for IPv4 – uses 32-bit addresses IPv6 internets need the same functionality. IGMP functions included in Internet Control Message Protocol v6 (ICMPv6). – ICMPv6 has functionality of ICMPv4 & IGMP. ICMPv6 includes group-membership query and group-membership report messages as IGMP.

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. – Note: feedback implosion  uses a random timer. 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 25

Multicast Routing

Shortest Path Tree Multicast cast forwarding tree: tree of shortest path routes from source to all receivers. – Dijkstra’s algorithm. 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 27

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). 28

Reverse Path Forwarding Building a loop-free broadcast tree No knowledge of group membership 29

Reverse Path Forwarding (2) if (multicast datagram received on incoming link on shortest path back to sender) then flood datagram onto all outgoing links else ignore datagram  rely on router’s knowledge of unicast shortest path from it to sender  each router has simple forwarding behavior: 30

Reverse Path Forwarding: Example 31 A B G D E c F

Spanning-Tree Broadcast 32 A B G D E c F A B G D E c F (a) Broadcast initiated at A (b) Broadcast initiated at D

Internet Multicasting Routing: DVMRP DVMRP: distance vector multicast routing protocol, RFC1075. Flood and prune: reverse path forwarding, source- based tree. – initial datagram to multicast group is flooded everywhere via RPF – routers not wanting the multicast data: send prune messages to upstream neighbors 33

DVMRP Example 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 34

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

DVMRP Details Soft state: DVMRP router periodically (1 min.) “forgets” that branches are pruned: – multicast data again flows down unpruned branches. – downstream routers: reprune or else continue to receive data. Routers can quickly re-graft to tree following an IGMP join at a leaf router by sending a “graft” message upstream. Deployment: – commonly implemented in commercial routers. – Mbone routing done using DVMRP. Works well in small autonomous domains. 36

DVMRP Prune Sent from routers receiving multicast traffic for which they have no downstream group members “Prunes” the tree created by DVMRP Stops needless data from being sent

DVMRP Graft Used after a branch has been pruned back Sent upstream by a router that has a host that joins a multicast group Goes from router to router until a router active on the multicast group is reached Sent for the following cases – A new host member joins a group – A new dependent router joins a pruned branch – A dependent router restarts on a pruned branch – If a Graft Ack is not received before the timeout

DVMRP Graft ACK Used to acknowledge receipt of Graft message Allows sending downstream router to know upstream router has received and processed its request If not received within 5 sec. Graft message sent again

DVMRP Issues Hop count used as metric Scalability issues – Performs periodic flooding – Maintains its own routing table Older versions lack pruning Even though it is a main part of MBone, MBone is being phased out

DVMRP: Summary Distance Vector Multicast Routing Protocol Leaf router sends a prune message to neighbouring routers when there is no group member on the subnet. Intermediate routers perform pruning whenever possible. Flooding and pruning are repeated periodically, when the current state times out. Between flooding rounds, a leaf router can re-join a group by sending a graft message upstream. Intermediate routers propagates the graft message upstream until the path is re-connected. 41

MBone Multicast backbone of the Internet. Not all routers support multicast routing protocols and IGMP. Connecting multicast-capable routers using (virtual) IP tunnels. Was a long-running experimental approach to enabling multicast between sites through the use of tunnels. No longer operational. 42

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. 43