CS 716 Advanced Computer Networks By Dr. Amir Qayyum 1
Lecture No. 3
Protocol Machinery Multiplexing and Demultiplexing (demux key) Encapsulation (header/body) in peer-to-peer interfaces indirect communication (except at hardware level) each protocol adds a header part of header includes demultiplexing field (e. g. , pass up to request/reply or to message stream? )
Encapsulation Host 1 Host 2 Application program Data RRP RRP Data HHP HHP RRP Data 4
Message Transmission Using Layers 5
Standard Architectures • Open System Interconnect (OSI) Architecture – International Standards Organization (ISO) – International Telecommunications Union (ITU), formerly CCITT – “X dot” series: X. 25, X. 400, X. 500 – Primarily a reference model 6
OSI Architecture End host Application User level OS kernel Application Presentation Data formatting Session Connection management Transport Process-to-process communication channel Network Data link Physical Host-to-host packet delivery Framing of data bits Transmission of raw bits One or more nodes within the network 7
Internet Architecture • TCP/IP Architecture – Developed with ARPANET and NSFNET – Internet Engineering Task Force (IETF) • Culture: implement, then standardize • OSI culture: standardize, then implement – Became popular with release of Berkeley Software Distribution (BSD) Unix; i. e. free software – Standard suggestions traditionally debated publically through “Request For Comments” (RFC’s) 8
Internet Architecture • Implementation and design done together • Hourglass Design (bottleneck is IP) • Application vs Application Protocol (FTP, HTTP) FTP HTTP NV TFTP UDP TCP IP NET 1 NET 2 … NETn 9
Internet Architecture • Layering is not very strict Application TCP UDP IP Network 10
Network Models 11
How Layers Fit Together in Practice 12
Networking in the Internet Age 13
Protocol Acronyms • • • (T)FTP – (Trivial) File Transfer Protocol HTTP – Hyper Text Transport Protocol NV – Network Video SMTP – Simple Mail Transfer Protocol NTP – Network Time Protocol TCP – Transmission Control Protocol UDP – User Datagram Protocol IP – Internet Protocol FDDI – Fiber Distributed Data Interface ATM – Asynchronous Transfer Mode 14
Elements of a Protocol Implementation • Outline – – Service Interface Process Model Common Subroutines Example Protocol 15
Network Software • Major factors for runaway success of the Internet: – most functionalities provided by software running on general-purpose computers • new services can be added readily with just a small matter of programming • Understanding how to implement network software is essential to understand computer networks 16
Network Application Programming Interface (API) • Interface that the OS provides to its networking subsystem – – most network protocols are implemented in software all systems implement network protocols as part of the OS each OS is free to define its own network API applications can be ported from one OS to another if APIs are similar • *IF* application program does not interact with other parts of the OS other than the network (file system, fork processes, display …) 17
Protocols and API • Protocols provide a certain set of services • API provides a syntax by which those services can be invoked • Implementation is responsible for mapping API syntax onto protocol services 18
Socket API • Use sockets as “abstract endpoints” of communication • Issues – Creating & identifying sockets – Sending & receiving data • Mechanisms – UNIX system calls and library routines socket process 19
Socket API • Creating a socket int socket(int domain, int type, int protocol) • domain (family) = AF_INET, PF_UNIX, AF_OSI • type = SOCK_STREAM, SOCK_DGRAM • protocol = TCP, UDP, UNSPEC • return value is a handle for the newly created socket 20
Sockets (cont) • Passive Open (on server) int bind(int socket, struct sockaddr *addr, int addr_len) int listen(int socket, int backlog) int accept(int socket, struct sockaddr *addr, int addr_len) • Active Open (on client) int connect(int socket, struct sockaddr *addr, int addr_len) 21
Sockets (cont) • Sending Messages int send(int socket, char *msg, int mlen, int flags) • Receiving Messages int recv(int socket, char *buf, int blen, int flags) 22
Protocol-to-Protocol Interface • A protocol interacts with a lower level protocol like an application interacts with underlying network • Why not using available network APIs for PPI ? – Inefficiencies built into the socket interface • application programmer tolerate them to simplify their task – inefficiency at one level • protocol implementers do not tolerate them 23
Protocol-to-Protocol Interface Issues • Configure multiple layers – static versus extensible • Process Model – avoid context switches • Buffer Model – avoid data copies 24
Process Model procedure call inter-process communication (a) Process-per-Protocol (b) Process-per-Message 25
Process Model (cont. ) int send(Protocol llp, Msg *message) int deliver(Protocol hlp, Msg *message) 26
Buffer Model buffer copy 27
Message Library • Add header m • Strip header m abcdefg bcopy (“ xyz” , hdr , 3); msg. Add. Hdr (m, hdr, 3); hdr =msg. Strip. Hdr (m, 3); m m defg + hdr = “ abc” xyzabcdefg 28
Message Library (cont) • Fragment message m m 1 abcdefg m + m 2 abcd msg. Fragment (m, new, 3); new defg • Reassemble messages abc efg msg. Reassemble(new, m 1, m 2) new abcdefg 29
Network Programming • Things to learn – Internet protocols (IP, TCP, UDP, …) – Sockets API (Application Programming Interface) • Why IP and sockets • allows a common name space across most of Internet – IP (Internet Protocol) is standard • reduces number of translations, which incur overhead – Sockets: reasonably simple and elegant Unix interface (most servers run Unix) 30
OSI Model application layers Internet Protocols FTP network physical NV TFTP UDP TCP transport data link HTTP IP FDDI Ethernet … ATM 31
Programming transport network • learn to use Internet for communication (with focus on implementation of networking concepts) data link physical Principles and concepts • learn to build network from ground up 32
Socket Programming • Reading: Stevens 2 nd edition, Chapter 1 -6 • Sockets API: a transport layer service interface – – introduced in 1981 by BSD 4. 1 implemented as library and/or system calls similar interfaces to TCP and UDP can also serve as interface to IP (for super-user); known as “raw sockets” – Linux also provides interface to MAC layer (for superuser); known as “data-link sockets” 33
Outline • • • Client-server model TCP connections UDP services Addresses and data Sockets API Example of usage 34
Client-Server Model • Asymmetric relationship • Server/daemon – well-known name – waits for contact – process requests, sends replies • Client server client – initiates contact – waits for response 35
Client-Server Model • Bidirectional communication channel • Service models – sequential: server processes only one client’s requests at a time – concurrent: server processes multiple clients’ requests simultaneously – hybrid: server maintains multiple connections, but processes requests sequentially • Server and client categories not disjoint – server can be client of another server – server as client of its own client (peer-to-peer architecture) 36
Review Lecture 3 • • Protocol Machinery - Encapsulation Standard architectures: OSI, Internet Network models and layering Elements of protocol implementation Network software Network API – Socket API – PPI Process and buffer model Programming – Client server model 37
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