TDC561 Network Programming

1. TDC561 Network Programming Week 8: Multicasting; Socket Options; Camelia Zlatea, PhD Email: czlatea@cs.depaul.edu

2. W. Richard Stevens, Network Programming : Networking API: Sockets and XTI, Volume 1, 2nd edition, 1998 (ISBN 0-13-490012-X) Chap. 7, 11, 19, 21, 22 References

3. Addressing in the Internet • Addressing tied to reachability • Every host interface has its own IP address • Router interfaces usually have their own IP addresses • IP is version 4 (IPv4 addresses) • 4 bytes long • two part hierarchy • network number and host number • different types of boundary indicator • class, subnet mask, prefix • Goal of boundaries is address aggregation

4. Address classes • Historical first choice • fixed network-host partition, with 8 bits of network number • Generalization • Class A addresses have 8 bits of network number • Class B addresses have 16 bits of network number • Class C addresses have 24 bits of network number • Distinguished by leading bits of address • leading 0 => class A (first byte < 128) • leading 10 => class B (first byte in the range 128-191) • leading 110 => class C (first byte in the range 192-223) • leading 1110 => class D (multicast) • leading 1111 => Class E (reserved)

5. Address evolution • Class based scheme was too inflexible • Two problems • Too many routes • Too few addresses • Four extensions • Subnetting (flexible boundaries within network) • CIDR (flexible grouping of networks- Classless Inter-domain Routing) • Dynamic host configuration (reuse of addresses) • A bigger address (IPv6) • One issue • Network address translation

6. What is Multicast? • Multicast is a communication paradigm • 1 source, multiple destination • Applications: • bulk-data distribution to subscribers • (e.g., newspaper, software, and video tapes distribution), • connection-time-based charging data distribution • (e.g., financial data, stock market information, and news tickets broadcasting), • streaming (e.g., video/audio real-time distribution), • push applications, web-casting, • distance learning, conferencing, collaborative work, distributed simulation, and interactive games.

7. The Internet group model 140.192.1.8 source source site 2 host_1 host_1 Ethernet host_2 multicast router from logical view... ...to physical view receiver multicast group 225.1.2.3 multicast router site 1 Internet receiver multicast router host_3 host_2 host_3 Ethernet receiver 216.47.143.60 receiver 140.192.1.6 multicast distribution tree • multicast/group communications means... • 1  n as well as n  m • a group is identified by a class D IP address (224.0.0.0 to 239.255.255.255) • abstract notion does not identify any host

8. IP Multicast: Basic Idea • Multicast groups: abstract “rendez-vous” points. • Set up optimal spanning tree spanning participants for each group. • Make it cheap by not providing strong guarantees: send out packets and hope for the best.

9. The Internet group model (cont’) • the group model is an open model • anybody can belong to a multicast group • no authorization is required • a host can belong to many different groups • no restriction • a source can send to a group, no matter whether it belongs to the group or not • membership not required • the group is dynamic, a host can subscribe to or leave at any time • a host (source/receiver) does not know the number/identity of members of the group

10. Mapping IP Multicast onto Ethernet Multicast • IP Multicast (class D IP address): • Class D: 224.x.x.x-239.x.x.x (in HEX: Ex.xx.xx.xx): 28 bits • No further structure (like Class A, B, or C) • Not addresses but identifiers of groups • Some of them are assigned by the IANA to permanent host groups • Mapping a class D IP adr. into an Ethernet multicast adr. • The least 23 bits of the Class D address are inserted into the 23 bits of Ethernet multicast address • Many to one mapping: 5 bits are not used • More filtering has to be done at IP level

11. Ethernet Multicast • Ethernet is a broadcast medium • Every frame can potentially be seen by every host • Ethernet cards have a unique Ethernet address • Broadcast address: • ff:ff:ff:ff:ff:ff • Ethernet Multicast address range for IP: • 01:00:5e:00:00:00 -to- 01:00:5e:7f:ff:ff • Mapping IP Multicast onto Ethernet • Multicast

12. The Internet group model (cont’) • local-area multicast • use the potential diffusion capabilities of the physical layer(e.g. Ethernet) • efficient and straightforward • wide-area multicast • requires to go through multicast routers, use IGMP/multicast routing/... • routing in the same administrative domain is simple and efficient • inter-domain routing is complex, not fully operational

13. Multicast and the TCP/IP layered model reliability mgmt congestion control other building blocks Application higher-level services user space kernel space Socket layer UDP TCP multicast routing ICMP IP / IP multicast IGMP device drivers

14. What is Multicast? • Several applications need efficient means to transmit data to multiple destinations with: • less bandwidth • higher throughput • lower delay • higher reliability • Classification • Data dissemination • Transactions • Large Scale Virtual Environments • Build on top of the existing Internet and take into account group communication constraints • Manage groups • Create and maintain multicast routes • Efficient end-to-end delay (reliability, flow control, time constraints)

15. Ideal Multicast • Senders (S) and Receivers (R) not aware of each other’s position in the network. • Scalable. • Low latency (join, data propagation). • Low bandwidth and processing overhead. • “Reliable”, if this is cheap (“end-to-end”?) • Easy to join/leave.

16. Why IP multicast? client access line content server use unicast? ISP and Internet client client access line content server ISP and Internet client • scalability... • scales to an unlimited number of users • reduced costs... • cheaper equipment and access line • increased speed... • increases the delivery speed ...or multicast?

17. Multicast Features: Multicast Scope Control • Who gets which packets? • Send everything to everybody .. • TTL scope • To keep multicast traffic within an administrative domain by setting ttl thresholds on interfaces on the border router • Administratively scoped addresses • A multicast boundary can be setup on the borders for addresses in range of 239.0.0.0–239.255.255.255 • Better than ttl scope

18. Multicasting: Receiving multicast message • For a process to receive multicast messages it needs to perform the following steps: 1. Create a UDP socket msd msd = socket(AF_INET,SOCK_DGRAM, 0); 2. Bind it to a UDPport, e.g., 1234. All processes must bind to the same port in order to receive the multicast messages. struct sockaddr_in groupHost; groupHost.sin_family = AF_INET; groupHost.sin_port = htons(UDPport); groupHost.sin_addr.s_addr = htonl(INADDR_ANY); bind(msd, (struct sockaddr *) &groupHost, sizeof(groupHost))

19. Multicasting: Receiving multicast message • (cont’) 3. Join a multicast group address GroupIPaddress , e.g., 224.111.112.113 joinGroup (msd, GroupIPaddress); 4. Use recv or recvfrom to read the messages, e.g., nbytes = recv(msd, recvBuf, BufLen,0);

20. Multicast Groups and Addresses • Every IP multicast group has a group address. • IP multicast provides only open groups • it is not necessary to be a member of a group in order to send datagrams to the group. • Multicast address are like IP addresses used for single hosts, and is written in the same way: A.B.C.D. • Multicast addresses will never clash with host addresses because a portion of the IP address space is specifically reserved for multicast. 224.0.0.0 to 239.255.255.255. • Multicast addresses from 224.0.0.0 to 224.0.0.255 are reserved for multicast routing information; • Application programs should use multicast addresses outside this range.

21. Multicasting: Receiving multicast message /* This function sets the socket option to make the local host join the multicast group */ void joinGroup(int s, char *group) { struct sockaddr_in groupStruct; struct ip_mreq mreq; /* multicast group info structure */ if((groupStruct.sin_addr.s_addr = inet_addr(group))== -1) printf("error in inet_addr\n"); /* check if group address is indeed a Class D address */ mreq.imr_multiaddr = groupStruct.sin_addr; mreq.imr_interface.s_addr = INADDR_ANY; if ( setsockopt(s,IPPROTO_IP,IP_ADD_MEMBERSHIP,(char *) &mreq, sizeof(mreq)) == -1 ) { printf("error in joining group \n"); exit(-1); } }

22. Receiving Multicast Datagrams • Join a particular multicast group. This is done using another call to setsockopt: struct ip_mreq mreq; setsockopt(sock,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq)); • The definition of struct ip_mreq is as follows: struct ip_mreq { struct in_addr imr_multiaddr; /* multicast group to join */ struct in_addr imr_interface; /* interface to join on */ }

23. Multicasting: Receiving multicast message /* This function removes the process from the group */ void leaveGroup(int recvSock,char *group) { struct sockaddr_in groupStruct; struct ip_mreq dreq; /* multicast group info structure */ if((groupStruct.sin_addr.s_addr = inet_addr(group))== -1) printf("error in inet_addr\n"); dreq.imr_multiaddr = groupStruct.sin_addr; dreq.imr_interface.s_addr = INADDR_ANY; if( setsockopt(recvSock,IPPROTO_IP,IP_DROP_MEMBERSHIP, (char *) &dreq,sizeof(dreq)) == -1 ) { printf("error in leaving group \n"); exit(-1); } printf("process quitting multicast group %s \n",group); }

24. Multicasting: Sending multicast message For a process to send multicast messages it needs to perform the following: 1. use the UDP socket msd for sending multicast messages struct sockaddr_in dest; dest.sin_family = AF_INET; dest.sin_port = UDPport; dest.sin_addr.s_addr = inet_addr(GroupIPaddress); sendto (msd, sendBuf, BufLen,0, (struct sockaddr *) &dest, sizeof(dest)) ;

25. Multicasting: Sending multicast message • (cont’) 2. Join a multicast group address GroupIPaddress , e.g., 224.111.112.113 joinGroup (msd, GroupIPaddress); 3. Use recv or recvfrom to read the messages, e.g., nbytes = recv(msd, recvBuf, BufLen,0);

26. Multicasting: Sending multicast message /* This function sets the socket option to make the local host join the multicast group */ void joinGroup(int s, char *group) { struct sockaddr_in groupStruct; struct ip_mreq mreq; /* multicast group info structure */ if((groupStruct.sin_addr.s_addr = inet_addr(group))== -1) printf("error in inet_addr\n"); /* check if group address is indeed a Class D address */ mreq.imr_multiaddr = groupStruct.sin_addr; mreq.imr_interface.s_addr = INADDR_ANY; if ( setsockopt(s,IPPROTO_IP,IP_ADD_MEMBERSHIP,(char *) &mreq, sizeof(mreq)) == -1 ) { printf("error in joining group \n"); exit(-1); } }

27. Multicasting • Time-to-live • control how far the messages can go, e.g., 2 means at most 2 routers away. (default is 1- which will result in multicast packets going only to other hosts on the local network. ) u_char TimeToLive; TimeToLive = 2; setTTLvalue (s, &TimeToLive); /* This function sets the Time-To-Live value */ void setTTLvalue(int s,u_char *ttl_value) { if( setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, (char *) ttl_value, sizeof(u_char)) == -1 ) { printf("error in setting loopback value\n"); } }

28. Multicasting • Time-to-live • To provide meaningful scope control, multicast routers enforce the following "thresholds" on forwarding based on the TTL field: 0 restricted to the same host 1 restricted to the same subnet 32 restricted to the same site 64 restricted to the same region 128 restricted to the same continent 255 unrestricted

29. Multicasting • Loop-back • allow the process to get a copy of its own transmission we use: u_char loop; loop = 1; setLoopback (s, &loop); void setLoopback(int s,u_char loop) { if( setsockopt(s,IPPROTO_IP,IP_MULTICAST_LOOP,(char *) &loop, sizeof(u_char)) == -1 ) { printf("error in disabling loopback\n"); } } By default, messages sent to the multicast group are looped back to the local host. this function disables that. loop = 1 /* means enable loopback (default) loop = 0 /* means disable loopback

30. Multicasting • Reuse-port • allow multiple multicast processes to to run on the same host: reusePort (s); /* This function sets a socket option that allows multiple processes to bind to the same port */ void reusePort(int s) { int one=1; if ( setsockopt(s,SOL_SOCKET,SO_REUSEADDR,(char *) &one,sizeof(one)) == -1 ) { printf("error in setsockopt,SO_REUSEPORT \n"); exit(-1); } }

31. Multicasting - Example • http://condor.depaul.edu/~czlatea/TDC561/LectureNotes/TDC561_week8/ • multicast.h • multicastUtilities.c • multicastChat.c

32. Reliable One-One Communication • Use reliable transport protocols (TCP) or handle at the application layer • Client/Server semantics in the presence of failures • Possibilities • Client unable to locate server • Lost request messages • Server crashes after receiving request • Lost reply messages • Client crashes after sending request

33. Reliable One-Many Communication • Reliable multicast • Lost messages => need to retransmit • Possibilities • ACK-based schemes • Sender can become bottleneck • NACK-based schemes

34. Atomic Multicast • Reliable Group Communication • Processes can fail • Atomicity of Multicast is required • Atomicity? • Group Membership • Multicast and a corresponding group of recipients • Failures of processes can be viewed as changes to group membership. • System Model • Separating receiving a message and delivering it to a application • Group View: a list of processes associated with a message • View Change • A special multicast message • Race between m and vc • Condition • Either m is delivered to all processes before a process is delivered a new vc • Or, m is not delivered at all.

35. Atomic Multicast • Atomic multicast: a guarantee that all process received the message or none at all • Replicated database example • Problem: how to handle process crashes? • Solution: group view • Each message is uniquely associated with a group of processes • View of the process group when message was sent • All processes in the group should have the same view (and agree on it) Virtually Synchronous Multicast

36. Reliable Mcast Transport Protocol Smart “session manager” elects DR’s and sets parameters. How? Just like that... • S, R use windows • Designated Receivers eliminate ACK implosion • ACK’s sent to DR’s • DR’s and S cache data and retransmit it when needed.

37. RMTP(2) • After set up S starts sending data. Receivers send periodic ACK’s after first packet received. • If no ACK’s for a long time, connection terminates. • DR’s or S retransmit info using unicast or multicast, depending on number of errors. • Immediate TX request sent to DR’s, for receivers that join the session. • Sender window advance determined by slowest receiver. • ACK’s must not be repeated too often. Measure RTT to AP. • S adjusts (decreases) send window to 1 if many errors; then increases linearly. • DR’s are fixed, but each R chooses its DR. (DR sends SND_ACK_TOME with TTL fixed to a known value).

38. Socket Options • Various attributes that are used to determine the behavior of sockets. • Setting options tells the OS/Protocol Stack the behavior we want. • Support for generic options (apply to all sockets) and protocol specific options.

39. Option types • Many socket options are Boolean flags indicating whether some feature is enabled (1) or disabled (0). • Other options are associated with more complex types including int, timeval, in_addr, sockaddr, etc. • Read-Only Socket Options • Some options are readable only (we can’t set the value).

40. Setting and Getting option values getsockopt() gets the current value of a socket option. setsockopt() is used to set the value of a socket option. #include <sys/socket.h>

41. getsockopt() int getsockopt( int sockfd, int level, int optname, void *opval, socklen_t *optlen); level specifies whether the option is a general option or a protocol specific option (what level of code should interpret the option).

42. setsockopt() int setsockopt( int sockfd, int level, int optname, const void *opval, socklen_t optlen);

43. General Options • Protocol independent options. • Handled by the generic socket system code. • Some general options are supported only by specific types of sockets (SOCK_DGRAM, SOCK_STREAM).

44. Some Generic Options SO_BROADCAST SO_DONTROUTE SO_ERROR SO_KEEPALIVE SO_LINGER SO_RCVBUF,SO_SNDBUF SO_REUSEADDR

45. SO_BROADCAST • Boolean option: enables/disables sending of broadcast messages. • Underlying DL layer must support broadcasting! • Applies only to SOCK_DGRAM sockets. • Prevents applications from inadvertently sending broadcasts (OS looks for this flag when broadcast address is specified).

46. SO_DONTROUTE • Boolean option: enables bypassing of normal routing. • Used by routing daemons.

47. SO_ERROR • Integer value option. • The value is an error indicator value (similar to errno). • Readable only • Reading (by calling getsockopt()) clears any pending error.

48. SO_KEEPALIVE • Boolean option: enabled means that STREAM sockets should send a probe to peer if no data flow for a “long time”. • Used by TCP - allows a process to determine whether peer process/host has crashed. • Consider what would happen to an open telnet connection without keepalive.

49. SO_LINGER Value is of type: struct linger { int l_onoff; /* 0 = off */ int l_linger; /* time in seconds */ }; • Used to control whether and how long a call to close will wait for pending ACKS. • connection-oriented sockets only.

50. SO_LINGER usage • By default, calling close() on a TCP socket will return immediately. • The closing process has no way of knowing whether or not the peer received all data. • Setting SO_LINGER means the closing process can determine that the peer machine has received the data (but not that the data has been read() !).