Lecture Class 3 Advanced Socket Programming Broadcast Multicast

Lecture Class 3 Advanced Socket Programming: Broadcast & Multicast Concepts

Broadcast and Multicast Concepts See syllabus for supporting links

Unicast Review Our discussion so far has been about unicast sockets, where one server talks directly (and individually) to one or more clients TCP is unicast biased—it can only talk to unicast addresses Unicast works across both TCP and UDP connections

Unicast Datagrams

Broadcast Concepts Multicasting is supported in IPv 4 but is mandatory in IPv 6 Broadcasting has been dropped in IPv 6 (IPv 4 broadcasting must be recoded to multicasting in IPv 6) Broadcasting and multicasting only work across UDP—not TCP

Broadcast Concepts Key difference between unicast and broadcast: Whereas unicast sends a message from one NIC* to another NIC*, broadcast sends the same message to all the NICs across a sub-network Thus broadcasting implements a one-to-many model In IPv 4, broadcasting is used in ARP, BOOTP, NTP, and routed

Broadcast Concepts In IPv 4, broadcasting is used in Address Resolution Protocol (ARP) IP address of X. X please ID yourself with hardware address Bootstrap Protocol (BOOTP predecessor to DHCP) From 0. 0 to 255 broadcast Network Time Protocol (NTP) time broadcast to all clients on a LAN routed (broadcasts its routing table on the LAN

Broadcast Concepts A broadcast goes out to every host on the subnet, which includes the sender itself (if the sender has bound to the target port) So, every IPv 4 host on the subnet receives the broadcast and must completely process the UDP datagram all the way up the protocol stack, up to and including the Transport Layer What if only 10% of the computers on the LAN care about the broadcast? What if a high data rate is being sent, as in audio or video?

Broadcast Datagrams

Sending Broadcasts int enabled = 1; setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &enabled, sizeof(enabled)); You must set the originating socket option to SO_BROADCAST in order to avoid error EACCES (except Solaris 2. 5) Then call sendto(X. X. X. 255) as usual (remember: you are using SOCK_DGRAM and UDP) Note that each individual datagrams sent out to X. X. X. 255 is a unicast datagram

Multicast Concepts Whereas broadcast is a shotgun approach sending the same datagram to every node on the network, multicast is a full barrel shotgun, sending a "tight group" to a pre-defined subset of nodes on the network A multicast datagram is only received by those hosts interested in such receipt Multicast thus sits "in between" unicast and broadcast Note: We will skip PIM (Protocol Independent Multicast and IGMP (Internet Group Management Protocol)

Multicast Distribution Models One-to-Many (1 -M) Many-to-Many (M-M) Many-to-One (M-1) Audio/Video (lectures, presentations, concerts) Conferencing (A/V conferencing, whiteboards) Resource Discovery (location services, device discovery) Push Games (multi-player, Media/Announcements simulators) (news, weather, time) Data Collection (data monitoring applications, video surveillance) Distribution (binary executables) Resource Sharing Miscellaneous (distributed databases) (auctions, polling) Monitoring (stock prices) Distributed OS (concurrent collaboration, distance learning)

Unicast vs. Multicast ©Cisco, see also Williamson, op. cit.

Bandwidth Efficiency Williamson, Developing IP Multicast Networks, v 1.

Scalability Unicast Multicast Makofske & Almeroth, Multicast Sockets: Practical Guide for Programmers

IPv 4 Multicast Concepts Whereas broadcasting is normally limited to a LAN subnet, multicasting can be used on a LAN or a WAN (e. g. MBone) Class D addresses in the range of 224. 0. 0. 0 through 239. 255 is the IPv 4 multicast address range 224. 0. 0. 1 is the all-hosts group 224. 0. 0. 2 is the all-routers group

IPv 4 Multicast Datagrams

IPv 4 Multicast Address Ranges While technically any Class D addresses in the range of 224. 0. 0. 0 through 239. 255 can be used. . . The Address range 224. 0. 0. 0 through 224. 0. 0. 255 (aka 224. 0. 0. 0/24) is termed the link local range These packets are never forwarded by a multicast router, and are reserved for low-level topology discovery or maintenance protocols Gist is you should stay away from 224. 0. 0. X

Bits and Bytes The high-order 24 bits of an Ethernet (not token ring) IPv 4 multicast address is always: 01: 00: 5 E (always 01: 00: 5 E: 00 effectively) The high-order byte of an IPv 6 multicast address is always FF. The FF is followed by: 4 -bit flags (well-known = 0; transient = 1) 4 -bit scope (how "far" the packet travels)

IPv 4 Scoping IPv 4 does not define scope, but uses a Time To Live (TTL) value in the IP header A TTL value of 0 means node-local A TTL value of 1 means link-local (subnet only) A TTL value of 2 -31 means site-local (subnet only) A TTL value of 32 -63 means region-local A TTL value of 64 -127 means "continent" A TTL value of 128 -255 means global scope

IPv 6 Scoping node-local link-local site-local organization-local global

Administrative Scope Administrative scope is 239. 0. 0. 0 to 239. 255 Organizations must configure routers never to forward multicast packets destined to these addresses In summary:

The Reality of Global Scope So it sounds like Multicast is the solution for any wide area distribution, right? Not so fast. . . Firewalls almost never accept incoming Multicast messages Most organizations configure routers to never let Multicast messages out ISP routers never forward Multicast messages And this includes commercial applications

So How is this Done? Multicast messages need to be encapsulated in some fashion in order to "pass" muster Protocol Independent Multicast is sometimes used to wrap Multicast messages and forward those messages over a VPN (PIM=>GRE=>IPsec) You can always write your own Multicast router

Joining In In order for a host to receive multicast messages, that host must join the multicast group and bind() a UDP socket to a port for receiving multicast messages Joining is done via a call to setsockopt(): struct ip_mreq mcastreq; setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mcastreq, sizeof(mcastreq)); The IP layer then tells the datalink layer to receive Ethernet packets destined for this node (in our example, to 01: 00: 5 E: 00: 01)

IPv 4 Multicast Datagrams

Multicast Request Structure struct ip_mreq { struct in_addr imr_multiaddr; //IPv 4 mcast address struct in_addr imr_interface; //IPv 4 local interface }; struct ipv 6_mreq { struct in 6_addr imv 6 mr_multiaddr; //IPv 6 mcast addr unsigned int imv 6 mr_interface; //interface index or 0 }; If the local interface is INADDR_ANY (IPv 4) or an index of 0 (IPv 6), the kernel will choose the local interface for the app

Joining the Multicast Party Nothing is required for the server to actually send a multicast message. . . the server does not even have to join the multicast group itself As multicast support in IPv 4 is optional for any host, any host that has specifically not joined the group will ignore multicast messages at the datalink layer

Joining the Multicast Party More than one join is allowed on a given socket as long as each join is for: a different multicast address; or for the same multicast address but different interface There may exist a limit (IP_MAX_MEMBERSHIPS in socket. h or in. h) to the number of joins on a single socket (BSD/OSX/Linux defines as 20)

Joining the Multicast Party If a host has joined one or more multicast groups, the datalink layer passes the message to the IP layer The IP layer checks to see if this message was sent to a multicast group this node has joined If so, the IP layer hands the packet to the UDP layer for direct processing by the specified server port

Any vs. Specific Sourcing Any-Source Multicast (ASM): By default, any source is accepted. Individual sources may be turned off and back on as needed over time. This is also known as "exclude" mode, since the source filter contains a list of excluded sources. Source-Specific Multicast (SSM): Only sources in a given list are allowed. The list may change over time. This is also known as "include" mode, since the source filter contains a list of included sources. SSM assumes a one-to-many model

Join Socket Options Six socket functions may be set to manage group membership Socket Option* IP_ADD_MEMBERSHIP IPV 6_JOIN_GROUP MCAST_JOIN_GROUP (generic) Meaning Join a particular any-source multicast group on a specified local interface. IP_DROP_MEMBERSHIP Leave a particular any-source multicast group on a IPV 6_LEAVE_GROUP specified local interface (auto-leave on socket close) MCAST_LEAVE_GROUP (generic) IP_BLOCK_SOURCE MCAST_BLOCK_SOURCE Block receipt of messages on this socket from a given group (may be useful in denying rogues and DOS attacks) IP_UNBLOCK_SOURCE MCAST_UNBLOCK_SOURCE Un. Block (open) receipt of messages on this socket from a previously blocked group IP_ADD_SOURCE_MEMBERSHIP MCAST_JOIN_SOURCE_GROUP Join a source-specific group on a specific local interface IP_DROP_SOURCE_MEMBERSH Leave a source-specific group on a specific local IP interface MCAST_LEAVE_SOURCE_GROU See Stevens Unix Network Programming, v 1, 3 rd ed. for structures *

Outbound Socket Options There are three socket functions that may be set when using multicast sockets to send messages All three options are set with setsockopt() and are optional: Socket Option Meaning IP_MULTICAST_IF IPV 6_MULTICAST_IF Specify the default interface for outgoing multicast datagrams sent on this socket IP_MULTICAST_TTL IPV 6_MULTICAST_HOPS Set the TTL (IPv 4) or hop limit (IPv 6) for outgoing multicast datagrams (both default to 1 which restricts to local subnet only) IP_MULTICAST_LOOP IPV 6_MULTICAST_LOOP Enable or disable loopback of multicast datagrams (default is enabled). Enables sending host receipt of datagrams sent, but only if host has joined the multicast group (cf. Broadcast)

Joining Review So, for a node to receive multicast datagrams, the client must: call setsockopt() with socket option IP_ADD_MEMBERSHIP (IPv 4) to join the multicast group and call bind() to bind to a specific port on which to receive the multicast messages For more information on multicast, see: http: //tools. ietf. org/html/rfc 3678

Multicast Sender (IPv 4) Create a socket: socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); //or 0 Call setsockopt for SO_REUSEADDR (optional) Call setsockopt to set the TTL: unsigned char mc_ttl = 1; //keep to localnet setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, (void*) &mc_ttl, sizeof(mc_ttl))); Send data as usual close() the socket when done

Multicast Receiver (IPv 4) Create a socket: socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); //or 0 Call setsockopt for SO_REUSEADDR Call bind() as normal Call setsockopt to join the multicast group: struct ip_mreq mc_req; //multicast request structure setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void*) &mc_req, sizeof(mc_req)); Send and receive data as usual Call setsockopt to leave the multicast group when done: setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, (void*) &mc_req, sizeof(mc_req)); close() the socket when done

The Downside IP Multicast over UDP suffers from the standard limitations of the IP protocol itself with UDP: Unreliable packet delivery Duplicate packet delivery Network congestion (as there is no built-in congestion avoidance mechanism as in TCP)