Distributed Real-Time Control Systems

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1 Distributed Real-Time Control Systems Lecture 18 More on Boost ASIO Ethernet Communications Sockets in C++ 1

2 Boost Asio Communication Functions Boost Asio abstracts communication channels as streams. Streams can be sockets (UDP, TCP, ICMP), serial I/O, files. It provides the following functions to read or write synchronously or asynchronously to streams: async_read(stream, stream_buffer, [completion_test,] completion_handler); read(stream, stream_buffer, [completion_test]); async_write(stream, stream_buffer, [completion_test,] completion_handler); write(stream, stream_buffer, [completion_test]); async_read_until(stream, stream_buffer, delim, completion_handler); read_until(stream, stream_buffer, delim); async_read_at(stream, offset, buffer [, completion_test], completion_handler); read_at(stream, offset, buffer [, completion_test]); async_write_at(stream, offset, buffer [, completion_test], completion_handler) write_at(stream, offset, buffer [, completion_test]) 2

3 Boost Asio Completion Test The completion function is called after each successful read, and tells Boost.Asio if the read/write operation is complete (if not, it will continue to read/write); Function prototype is: size_t completion(const boost::system::error_code& err, size_t bytes_transfered) ; Boost.Asio comes with a few helper completion_test functions: transfer_at_least(n) transfer_exactly(n) transfer_all() 3

4 Boost Asio Completion Handler Each read or write operation will end when one of these conditions occur: The supplied buffer is full (for read) or all the data in the buffer has been written (for write). The completion function returns 0 (if you supplied one such function). A delimiter is provided in the (async_)read_until functions (a char, string or regex). An error occurs When the operation ends, the data will be available at the provided buffer (if read), and the user provided completion_handler is invoked: void handler(const boost::system::error_ code & err, size_t bytes) 4

5 Boost Asio Buffers boost::asio::buffer( ) is a function that creates and object that manages raw memory. If can manage POD types (plain old data arrays), vectors of POD and strings. char *d0[1024]; auto b0 boost::asio::buffer(d0, 128)); char d1[128]; auto b1 = boost::asio::buffer(d1); std::vector<char> d2(128); auto b2 = boost::asio::buffer(d2); std::array<char, 128> d3; auto b3 = boost::asio::buffer(d3); 5

6 Boost Asio Timers Boost asio provides the following timer classes: deadline_timer high_resolution_timer steady_timer system_timer All classes provide function async_wait which can be used to manage pauses and check for deadlines asynchronously. io_service service; bool read = false; char data[512]; ip::tcp::socket sock(service); deadline_timer t(service, boost::posix_time::milliseconds(100)); void deadline_handler(const boost::system::error_code &) { std::cout << (read? "read successfully" : "read failed") << std::endl; } void read_handler(const boost::system::error_code &) { read = true; } sock.async_read_some(buffer(data, 512), read_handler); t.async_wait(&deadline_handler); service.run(); 6

7 The Ethernet Frame-based Technology Ethernet stations communicate by sending each other data packets. Each Ethernet station is given a single 48-bit MAC address, which is used both to specify the destination and the source of each data packet. 7

8 The OSI stack model APDU Application Application Protocol Application 7 PPDU Presentation Presentation Protocol Presentation 6 SPDU Session Session Protocol Session 5 TPDU Transport Trasport Protocol Host-Router communication boundary Transport 4 packet Network Network Network Network 3 Frame Data Link Data Link Data Link Data Link 2 Bit Physical Physical Physical Physical 1 Host-Router internal protocol 8

9 The First Ethernet Designed by Bob Metcalf at PARC in 1972/3 9

10 Ethernet Evolution Ethernet today is not the same as it was when it was invented 30 years ago. Improvements have consisted of : Higher bandwidth Dual duplex Switching Prioritisation VLANs 10

11 Original Ethernet Original Ethernet composed of one collision domain. Only one device at a time can talk. 11

12 Switching Switched Ethernet has broken up the collision domains Now lots of little collision domains. 12

13 Full Duplex Switched Ethernet with full duplex communication Now there are no collision domains and hence no collisions! 13

14 Ethernet Performance : Case Study Application with one Master, 31 Slaves and 4 RS2-Switches 14

15 Ethernet Performance: Timings 100Mbps = 10 nsec / bit Payload, for example 54 byte Ethernet frame: 14 bytes (header) 54 bytes (payload) 4 bytes (CRC) Total: 72 byte = 576 bit 576 bit * 10 nsec / bit = 5.76 us Latency in switches: 4us (Typ). 15

16 Multicast Performance SW1 to SW2 16

17 Multicast Performance 17

18 Scan Times Scan time of 100Mbps network 52 μs + 240μs = 292 μs Scan time of a 10Mbps network 312 μs μs = 2505 μs Scan time of 10Mbps network with 100Mbps links between Switches 156 μs μs = 924 μs 18

19 Ethernet for Real-Time Control? Can Ethernet cope with the demands of a real-time control network? Scan times are as quick if not quicker than contemporary bus systems. Deterministic in a controlled traffic network. Worldwide, open standards means all vendors support Ethernet. Evolutionary history means it can only get better! Specialized implementations of Real-Time Ethernet (RTE): PROFINET, ETHERNET Powerlink, EtherCAT 19

20 Address Resolution The Address Resolution Protocol (ARP) is a method for finding a host s hardware address when only its network layer address is known. E.g. IP addresses to MAC addresses. ARP is used in four cases of two hosts communicating: 1. When two hosts are on the same network and one desires to send a packet to the other 2. When two hosts are on different networks and must use a gateway/router to reach the other host 3. When a router needs to forward a packet for one host through another router 4. When a router needs to forward a packet from one host to the destination host on the same network 20

21 The Internet Protocol (IP) Packetization Data is encapsulated in one or more Packets/Datagrams Best-Effort Delivery - unspecified variable bit rate and delivery time, depending on the current traffic load Reliability Only ensures the IP packet header is error-free through the use of a checksum. Addressing - how end hosts become assigned IP addresses and how subnetworks of IP host addresses are divided and grouped together Evolving Protocols - IPv4, IPv6 21

22 IP Addressing There are two versions of an IP address : IPv4 and IPv6. Because of its prevalence, "IP address" typically refers to those defined by IPv4. Using reserved values for the first octet, network addresses are broken into classes: Class A very large networks (up to 2 24 hosts) Class B large networks (up to 2 16 hosts) Class C small networks (up to 255 hosts) Class D multi-cast messages to multiple hosts Class E addresses not allocated and reserved. 22

23 IP Classes 7 24 Class A: 0 Network ID Host ID Class B: 1 0 Network ID Host ID 21 8 Class C: Network ID Host ID 28 Class D (multicast): Multicast address Class E (reserv ed): unused Class Range within 1st octet Network ID Host ID 27 Possible number of networks Possible number of hosts A a b.c.d ,777,214 B a.b c.d 16,384 65,534 C a.b.c d 2,097,

24 Special Addresses Adapter Loopback Private addresses (will not be routed)

Subnet Masking Subnetwork or Subnet is a range of logical addresses within the address space that is assigned to an organization. Subnetworking simplifies routing.

25 Subnet Masking Subnetwork or Subnet is a range of logical addresses within the address space that is assigned to an organization. Subnetworking simplifies routing. Dot-decimal Address Full Network Address Subnet Mask Network Portion Host Portion Binary

26 Packet Forwarding Forwarding is the relaying of packets from one network segment to another by nodes in a computer network. Usually done by network switches or routers. unicasting Unicasting packet relay from link to link. Broadcasting packets are duplicated and copies sent to all links. multicasting Multicasting packets are selectively duplicated and sent to a set of links. broadcasting 26

27 Routing Techniques What is the best path between nodes J and E? Fastest Shortest I 6 G 4 D 1 B Connection cost Host J 1 7 H 3 2 C F A 6 K 5 2 E Local minimization of a cost function whose parameters are obtained by local information exchange between neighbour routers ( IGCP protocol). Typical cost functions: Shortest path. Fastest path. Least switches path. Flooding technique: Test all possible paths. 27

28 Traffic Control Traffic control tries to optimize the existing resourses in the transmission of packets in a network. This is a dificult and still open problem. Modeling, control strategies, and stability analysis are under active research. Optimal strategy Desireable strategy Número de pacotes entregues Maximum network capacity Congestion # packets 28

29 Delays and Losses in IP Delays and message losses are important factors in real time communications. The Internet Protocol (IP) offers few guarantees of real-time. Delays in the packet switching network may occur at any point due to: 1. Processing delays. 2. Waiting queue delays. 3. Transmission delays. 4. Propagation Delays. Packets get lost when the waiting queues are full. Lost packets can be retransmitted either by the transport layer (TCP) or by the application layer if using connectionless transports (UDP). 29

30 Network Programming with Sockets Sockets are probably the most widely used objects in programming networked communications. What is a socket? To the kernel, a socket is an endpoint of communication: IP address, port number, and protocol (TCP, UDP, etc). To an application, a socket is a file descriptor that lets the application read/write from/to the network Clients and servers communicate with each by reading from and writing to socket. 30

Remote address: IP address and port number.

31 Stream Sockets Connection oriented, require handshake Use TCP protocol Local address: IP address and port number. Remote address: IP address and port number. Datagram Sockets Connectionless, low overhead Use UDP protocol Types of Sockets 31

32 Connection-Oriented Model The connection-oriented model is implemented with Stream Sockets using the TCP protocol. TCP provides a variety or error correction and performance features for transmission of byte streams. Implements a client-server architecture. 32

33 Connection Oriented Model Server Listening socket Welcomes some initial contact from a client Connection socket It is created at initial contact of client New socket that is dedicated to the particular client Client Client socket Specify the address of the server process, namely, the IP address of the server and the port number of the process 33

34 Connection-Oriented Model Server Process socket() Client Process socket() bind() listen() accept() get a blocked client read() write() read() 3-way handshake client request server response close notification connect() write() read() close() close() 34

35 Socket Primitives socket() creates a new socket of a certain socket type (tcp,udp), identified by an integer number, and allocates system resources to it. bind() is typically used on the server side, and associates a socket with a socket address structure, i.e. a specified local port number and IP address. listen() is used on the server side, and causes a bound TCP socket to enter listening state. connect() is used on the client side, and assigns a free local port number to a socket. In case of a TCP socket, it causes an attempt to establish a new TCP connection. accept() is used on the server side. It accepts a received incoming attempt to create a new TCP connection from the remote client, and creates a new socket associated with the socket address pair of this connection. send() and recv(), or write() and read(), or sendto() and recvfrom(), are used for sending and receiving data to/from a remote socket. 35

36 Boost Stream Sockets Boost Stream Sockets are part of the ASIO library: #include <boost/asio.hpp> Important classes ip::tcp::acceptor ip::tcp::endpoint ip::tcp::iostream ip::tcp::resolver ip::tcp::socket Important functions socket::socket() acceptor::acceptor() acceptor::accept() ip::tcp::connect() ip::tcp::read() ip::tcp::write() 36

37 //TCPSERVER.CPP #include <boost/asio.hpp> using namespace boost::asio; using ip::tcp; Boost Synchronous TCP Example Compile with g++ V4.7.0 or bigger: g++ -std=c++11 tcpserver.cpp lpthread lboost_system-mt int main() { //initialize services io_service io; //create a listening socket // bind and start listening at port tcp::acceptor acceptor(io, tcp::endpoint(tcp::v4(), 10000)); for (;;) { //create service socket tcp::socket socket(io); //wait client to connect tcp::acceptor.accept(socket); //client isaccessingservice write(socket, buffer( Hello World\n )); } } //TCPCLIENT.CPP #include <iostream> #include <boost/array.hpp> #include <boost/asio.hpp> using namespace boost::asio; using ip::tcp; int main() { io_service io; boost::system::error_code err; tcp::resolver resolver(io); tcp::resolver::query query( , ); tcp::resolver::iterator endpoint = resolver.resolve(query); tcp::socket socket(io); socket.connect(*endpoint,err); //connect and wait for (;;) { boost::array<char, 128> buf; size_t len = socket.read_some(buffer(buf), err); if (err == error::eof) break; // Closed cleanly by peer. else if (err) std::cout << Unknown Error ; std::cout.write(buf.data(), len); } } 37

38 Connectionless-Oriented Model The connectionless model uses UDP protocol with no guarantee of delivery. UDP packets may arrive out of order, multiple times, or not at all. UDP has considerably less overhead than TCP Implements a peer-to-peer architecture. 38

39 Connectionless-Oriented Model Server Process socket() bind() Client Process socket() bind() read_from() write_to() request response write_to() read_from() close() close() 39

40 Boost Datagram Sockets Important classes ip::udp::endpoint ip::udp::resolver ip::udp::socket Important functions socket::receive_from() socket::send_to() 40

41 Boost Synchronous UDP Example //UDP SERVER #iinclude <iostream> #include <boost/asio.hpp> using namespace boost::asio; using ip::udp; int main() { io_service io; udp::socket socket(io,udp::endpoint(udp::v4(),10000)); for (;;) { boost::array<char, 1> recv; udp::endpoint client; boost::system::error_code err; //client endpoint retrieved on receive_from socket.receive_from(buffer(recv),client, 0, err); if (err && err!= error::message_size) { std::cout << Error << std::endl; return -1; } boost::system::error_code ignored; socket.send_to(buffer( Hello\n ), client, 0, ignored); } } // UDP CLIENT #iinclude <iostream> #include <boost/asio.hpp> using namespace boost::asio; using ip::udp; int main() { io_service io; udp::resolver resolver(io); udp::resolver::query query(udp::v4(), " ", "10000"); udp::endpoint receiver = *resolver.resolve(query); udp::socket socket(io); socket.open(udp::v4()); boost::array<char, 1> send_buf = {{ 0 }}; //send dummy data to initiate communication socket.send_to(buffer(send_buf), receiver); boost::array<char, 128> recv_buf; udp::endpoint sender; size_t len = socket.receive_from( buffer(recv_buf), sender); //write received data to console std::cout.write(recv_buf.data(), len); } 41

Distributed Real-Time Control Systems. Module 26 Sockets

Distributed Real-Time Control Systems Module 26 Sockets 1 Network Programming with Sockets Sockets are probably the most widely used objects in programming networked communications. What is a socket? To