Hybrid of client-server and P2P. Pure P2P Architecture. App-layer Protocols. Communicating Processes. Transport Service Requirements

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1 Announcements CS 5565 Network Architecture and Protocols Lecture 5 Godmar Back Problem Set 1 due Feb 17 Project 1 handed out shortly 2 Layer The Layer Let s look at some s (in keeping with top-down) architectures: Client/Server, P2P, Hybrid Client/Server Architecture vs Client Process/Server Process Transport Layer Requirements 4 Creating a app Write programs that run on different end systems and communicate over a. No software typically written for devices in core Network core devices do not function at app layer This design allows for rapid app development Exception: extensible routers Research: active s Client-Server Architecture server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 5 6 1

2 Pure P2P Architecture no always on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses example: Gnutella Highly scalable But difficult to manage 7 Hybrid of client-server and P2P (Original) BitTorrent File transfer P2P; Tracker centralized Instant messaging Presence detection/location centralized: User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies Chatting between two users can be P2P File transfer between users typically P2P 8 App-layer Protocols Transport Service Requirements Defines: Types of messages exchanged, e.g., request & response messages Syntax of message types: what fields in messages & how fields are delineated Semantics of the fields, ie, meaning of information in fields Rules for when and how es send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary protocols: e.g., Skype file transfer web documents real-time audio/video stored audio/video interactive games instant messaging Data loss l t Bandwidth audio: 5kbps-1Mbps video:10kbps-5mbps same as above few kbps up Time Sensitive no no no (?) yes, 100 s msec yes, few secs yes, 100 s msec yes and no 9 10 Internet apps:, protocols remote terminal access Web file transfer streaming multimedia Internet telephony layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Skype) Underlying protocol or UDP typically UDP 11 Communicating Processes Process: program running within a host. Access services through s Client : that initiates communication Server : that waits to be contacted Note 1: client/server is really a property of state Note 2: can be both server & client for different communications when multiple s are used 12 2

3 Addressing Processes For a to receive messages, it must have an identifier A host has a unique IP address (or more than one, details later) Not sufficient because multiple es can be on same host Add 16-bit port number Identifier includes both the IP address and port number associated with the on the host. Example port numbers: HTTP server: 80 Mail server: 25 Well-known port numbers /etc/services UDP & Socket Programming 13 UDP Service provided Demultiplexing Payload checksum Passes segments straight to IP No congestion control So simple it fits on one slide Length, in bytes of UDP segment, including header 32 bits source port # dest port # length data (message) checksum UDP segment format Socket Programming Goal: learn how to build client/server that communicate using s Socket API a host-local, introduced in BSD 4.1 -created, UNIX, 1981 explicitly created, used, released by apps used for both local and remote communication OS-controlled interface (a door ) into which can both send and receive messages to/from another Network Socket Programming Socket: (narrow definition:) a door between and end-end- protocol (UDP or ) developer operating system Stack w/ buffers, variables host or server internet Stack w/ buffers, variables host or server developer operating system 17 Addressing For UDP/IP or /IP communication, generally need 4 parameters: Source Identifier (32-bit IPv4 or 128-bit IPv6 Address) Source Port (16-bit) Destination Identifier (32-bit IP or 128-bit IPv6 Address) Destination Port (16-bit) Notice that the relationship of local and remote (also called peer ) to source/destination depends on direction of communication Note: UDP uses only Destination (IP+Port) for demultiplexing uses Source + Destination (quadruple: Src IP, Src Port, Dst IP, Dest Port) 18 3

4 BSD Socket API UDP Sockets: Overview API Programming Interface Provides access to services Specified in C language Implemented on many platforms in one way or the other (Windows: WinSock2, CSocket MFC classes for BSD-like look) Sockets (in Unix) are file descriptors General idea: writing to the is sending data to, reading from is receiving data Good because read(2), write(2), close(2) and others (select(2), poll(2), ioctl(2), SIGIO, fcntl(2)) can be reused Bad because? Study C API first and observe how other languages embedded s connect() (optional) send() recv() acts like acts like Client sendto() recvfrom() bind() recvfrom() sendto() Server (2) int (int domain, int type, int protocol) domain: PF_INET, PF_UNIX, PF_INET6,. type: SOCK_DGRAM, SOCK_STREAM, protocol: 0 for Unspecified (or IPPROTO_UDP) returns integer file descriptor entirely between and OS no actions involved whatsoever man pages: ip(7), udp(7), tcp(7), (2), (7), unix(7) type man 2, man 7 Note: passing constants to is deprecated; use getaddrinfo() instead! 21 bind(2) int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen) sockfd: return by my_addr: address addrlen length of address (address is variable-sized data structure) binds to (local) address specified This affects protocol namespace, but no information is transmitted over Typically: one per port, exception: multicast Note: passing set-up addresses to bind() is deprecated; use getaddrinfo() instead! 22 How are addresses represented? struct sockaddr { /* GENERIC TYPE, should be abstract */ sa_family_t sa_family; /* address family */ char sa_data[14]; /* address data */ /* This is the concrete subtype you ll use in IPv4 */ struct sockaddr_in { sa_family_t sin_family; /* address family: AF_INET */ u_int16_t sin_port; /* port in byte order */ struct in_addr sin_addr; /* internet address */ /* Internet address. */ struct in_addr { u_int32_t s_addr; /* address in byte order */ More on bind(2) Address specified is the local address Which is destination for receive and source for sends. sin_addr.s_addr useful for multi-homed hosts, otherwise use INADDR_ANY sin_port may be zero if any port would do Use getsockname() to retrieve assigned port s_addr and sin_port are specified in byte order This convention holds throughout API

5 Common Pitfalls (1) Network vs. Host Byte order Network order is big endian, most-significant byte first Host order may be either big or little: little endian on x86 Use ntohs(), ntohl(), htons(), htonl() to convert 16-bit ( short ) and 32-bit ( long ) values portably Never convert addresses/ports in sockaddr_in structures in place Not as much an issue in languages that hide data representation (e.g., Java) 25 Common Pitfalls (2) Pay attention to length parameters Example int getsockname(int s, struct sockaddr *name, socklen_ t *namelen); namelen here is not OUT, its INOUT aka value-result You know what it is, the OS doesn t! Always check return codes! 26 sendto(2), recvfrom(2) UDP & connect(2) Then what does connect() do??? ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen); ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen); s, buf, len as in read/write flags: MSG_OOB, MSG_PEEK mostly 0 to/from are of type struct sockaddr_in These are remote/peer addresses: where did the packet come from, where should it be sent to NB: fromlen is value-result! No notion of connections in UDP connect(2) can be used to tell OS default destination for future sends Then can use send(2) and write(2) instead of sendto(2), or can omit the destination address in sendto(2) What does ECONNREFUSED mean for UDP? Courtesy extended by other nodes which send ICMP packet saying no one s listening for UDP packets at the port number you sent it to OS relayed this information to UDP UDP Demultiplexing S: :3045 D: :80 Payload sendto( :80) bind(*, 80) S: :80 S: : :??? from:10002:3045 from: : from: :512 S: :53 D: :80 Payload bind(*, 53) from: : D: :53 Payload bind( ,512) 29 5

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