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Broadcasting & Multicasting with UDP sockets

Broadcasting & Multicasting with UDP sockets. Chap 20, 21. Broadcasting. Introduction. Use of broadcasting to know server address on the local subnet: resource discovery to minimize network traffic on a LAN Internet applications ARP(Address Resolution Protocol) BOOTP(Bootstrap Protocol)

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Broadcasting & Multicasting with UDP sockets

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  1. Broadcasting & Multicastingwith UDP sockets Chap 20, 21

  2. Broadcasting

  3. Introduction • Use of broadcasting • to know server address on the local subnet: resource discovery • to minimize network traffic on a LAN • Internet applications • ARP(Address Resolution Protocol) • BOOTP(Bootstrap Protocol) • NTP(Network Time Protocol) • Routing daemons: broadcast routing table

  4. Broadcast Addresses • Subnet-directed: [subnetid, -1] • most commonly used today • e.g.) ping 203.253.70.255 • Limited broadcast: 255.255.255.255 • All hosts on the same LAN • must not forwarded by a router • Some systems do not understand a subnet-directed broadcast address and only interpret 255.255.255.255 as a broadcast • TFTP and BOOTP use this limited broadcast address to know IP address of its diskless workstation on bootstrapping procedure

  5. Unicast versus Broadcast

  6. Before broadcasting, set SO_BROADCAST socket option 문제점 만일 timeout이 짧다면 ?? SIGALRM signal이 blocked system call (recvfrom) 이외에서 deliver되면 loop을 빠져 나오지 못함 해결방안 1: recvfrom 외에는 ALRM signal이 뜨지 않게 만들자. 즉, signal을 block시킴. (Fig. 20.6, bcast/dgclibcast3.c) 그래도 race condition이 발생할 수 있음 Race Condition: shared data를 여러 process들이 동시에 access할 때 time-dependent error 발생 Case 1: shared data (global variable) among threads Case 2: dealing with signals  signal handler도 일종의 thread로 봐야 dg_cli Function thatbroadcasts bcast/dgclibcast1.c

  7. Correct Solutions for Avoiding Race Condition bcast/dgclibcast5.c

  8. Multicasting

  9. Multicast Address • Multicast(Group) address: IPv4 Class D Addresses • IPv4 • 224.0.0.0 - 239.255.255.255(first 4bit: 1110) • Special IPv4 multicast addresses • 224.0.0.1 all-hosts group (on a subnet, i.e. link-local) • 224.0.0.2 all-routers group (on a subnet, i.e. link-local) • IPv6 Multicast Addresses • ff01::1, ff02::1 all-nodes group (interface-local, link-local) • ff01::2, ff02::2, ff05::2 all-routers group(…, site-local) Scope of multicast addresses

  10. Multicasting in broadcast-capable LANs • Just Map IP address to Ethernet Address • Then, LAN will delivers.

  11. IP Multicasting 특정 Group에 Join해야 수신할 수 있음 Sender는 단지 Group 주소(Class D address)로 하나의 packet을 보냄 Network(즉, 라우터)가 Group에속한 member에게 전달할 책임 있음 (copy는 router 책임) Join/leave a group: IGMP( Internet Group Membership Protocol) Application은 setsockopt() 으로 호출 Multicast Routing Protocol Find multicast tree E.g) PIM, M-OSPF LAN 내부에서의 multicast에는 필요 없음 Multicast Types Any-source Multicast (ASM) (*, G) Source-Specific Multicast (SSM) Similar to TV channels Member may join a channel (S, G) Multicast Protocols

  12. Multicast versus Broadcast

  13. Multicast Session • A multicast session • Defined by (IP multicast addr, UDP port) • Always use different ports • Sometimes use different groups • Audio/video conference may require two multicast sessions • A session for audio • A session for video

  14. Senders: Sending Multicast Datagrams • Specify interface, TTL, and enable/disable loop back • Default, • Interface for outgoing datagram will be chosen by kernel • TTL or hop limit == 1 • Enable loop back: sender도 보낸 패킷을 받음 • e.g. Disable loop back setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_LOOP, &flag, sizeof(flag)); • Send datagrams to all the members of group G (ASM인 경우) • Sender app: set destination socket addr to (G, port); sendto(); • Sender의 send socket에 꼭 bind할 필요는 없음. Why? • Internet은 Multicast routing protocol에 의해 receiver host들을 담당하는 local router까지의 Multicast tree를 구성함 • When? Dynamic(PIM-SM, PIM-DM, DVMRP, OSPF) or static (shared tree: e,g CBT) • Multicast tree를 따라 중간 라우터는 copy하여 forwarding하고, 결국, local router는 다시 receiver의 host로 forward함

  15. Receivers: Receiving Multicast Datagrams • Join the multicast group • Ask the kernel for joining the group struct ip_mreq mreq; struct in_addr group_addr; memcpy(&mreq.imr_multiaddr, group_addr, sizeof(group_addr)); setsockopt(sockfd, IPPROTO_IP, IP_ADDMEMBERSHIP, &mreq, sizeof(mreq)); • Kernel joins the Group using IGMP • 이 group G로 보내는 IP multicast packet들이 receiver의 host computer (i.e IP protocol)에게도 보내달라고 Internet (router)에게 요청함 • Bind [the group address and] port for the multicast session • Multicast session (G, port)으로 전달된 packet을 Kernel이 Receiver application의 해당 socket으로 전달하도록 요청 • Receive datagrams: recvfrom()

  16. Multicast Socket Options IPv4, ASM IPv4, SSM IPv6, ASM IPv4/v6, ASM IPv4/v6, SSM

  17. UNP Library For Supporting Protocol-independent Multicast Application

  18. Joining Multicast Group lib/mcast_join.c

  19. Sending and Receiving Multicast Packets mcast/main.c mcast/send.c mcast/recv.c

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