Basic Infrastructure for Electronic Business on the Internet

Basic Infrastructure for Electronic Business on the Internet Authors: Miroslava Mitrovic (mirka@galeb. etf. bg. ac. yu) Veljko Milutinovic (vm@galeb. etf. bg. ac. yu)

Network l l l A network is a group of computers and peripherals connected together by some medium The connection can be direct (through a cable) or indirect (through a modem) The different devices on the network communicate with each other through a predefined set of rules (protocol)

Network Hardware l l Physically connects computers in the network together Main hardware components: Nodes Communication equipment Additional components Topology

Nodes l l Network node (computer, printers, routers) Computers in network can be used as work stations or servers or both PC computers need NIC (Network Interface Card) to be able to be part of LAN (Local Area Network) Different standards, different network cards! (Ethernet, Token Ring, ARC net)

Communication equipment l l l Cables : (coaxial cables, fiber-optic cables, twisted pairs) Wiring centers : (HUB , concentrator, MAU (Multi station Access Units)) Intranetwork links : (connectors, transceivers, repeaters) Internetwork links : (bridges, gateways, routers, switches) Wireless components

Additional Components l l l Peripherals : (printers, fax machines, modems, cd readers) Safety devices : UPS (Uninterruptible Power Supply), SPS (Standby Power Supply) Network tools

Topology l l Defines rules for connecting network nodes Logical topology differs from physical topology! Logical topology defines rules for transmitting information's (bus, ring, star) Physical topology defines rules for electrical connection components in network

Topology

Network Software l l Enables using hardware for communication and exchanging information Protocols (set of rules) give formulas for passing messages, specifying the details of message formats, and for describing how to handle error conditions

Internetting l l Connecting many disparate physical networks and make them function as a coordinate unit Internet technology hides the details of network hardware and permits computers to communicate independent of their physical network connection

Internetting

Internet l l l Internet is network of all networks Interconnected computer networks that communicate, usually via TCP/IP Internet and internet Set of application programs that use the network to carry out useful communication tasks (user’s point of view) No one owns the Internet!

Internet architecture l l How are networks interconnected to form a internet work? Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routers

Internet architecture

Internet architecture l l l Gateways provide all interconnections among physical networks Usually minicomputers Gateways route packets based on destination network, not on destination host!

Internet architecture

Open Systems l l l Architecture is not secret The definition applies equally well to software and hardware UNIX-classic example for an open software platform Open system networking: network based on well-known and understood protocol Three most popular services: File Transfer Electronic mail Remote login

The ISO OSI-RM International Organization for Standardization Open System Interconnection Reference Model l ISO has developed an architecture that defines seven layers of network protocol

Grouping of OSI Layers

Upper Layers Application/Presentation/Session l l l Applications oriented Presents the interface to the user Don’t care how data gets to the application!

Middle Layer Transport l Separate application oriented upper layers from network oriented lower layers

Lower Layers Network/Data Link/Physical l l Oriented toward data transmission Packages, routes, verifies and transmits datagrams Type of data for application doesn't matter Has no application dependencies !

Communication in OSI-RM

Application Layer l l Highest layer in the OSI reference model Concerned with the user’s view of the network Handles network transparency, resource allocation and problem partitioning Sends data and program tasks to the presentation layer, which is responsible for coding data from application layer in the appropriate form

Presentation Layer l l The second highest layer in the OSI reference model Converts network data to application format Performs function as text compression , code or format conversion to try to smooth out differences between hosts Serves application layer, uses session layer

Session Layer l l l l The third highest layer in the OSI reference model Synchronized data exchange between upper and lower layers Manages the current connection, or session, between two computers Handles security and creation of the session Establishing connection Closes connection Uses the transport layer to establish a connection between processes on different hosts Used by the presentation layer

Transport Layer l l l The middle layer in the OSI reference model Uses the network layer to provide a virtual, error-free, point-to-point connection so that host A can send messages to host B and they will arrive uncorrupted and in the correct order Hides details necessary to actually make a connection between two computers Establishes and dissolves connections between hosts Used by the session layer

Network Layer l l l The third lowest layer in the OSI reference model Determines addresses Responsible for routing packets across the network Establishing logical link between two nodes Used by the transport layer, uses the data link layer

Data Link Layer l l l The second lowest layer in the OSI reference model Splits data into frames for sending on the physical layer Receives acknowledgment frames Performs error checking and retransmission Provides an error free virtual channel to the network layer Split into an upper sublayer Logical Link Control (LLC) and lower sublayer Media Access Control (MAC)

Physical Layer l l l The lowest layer in OSI model Transmits data How bits are passed over the network medium, what control signals are used, and the mechanical properties of the network itself (cable size, connector, and so on) Only layer in which actual communication occur Used by the data link layer

Data Transfer over the Internet

OSI vs. TCP/IP

The TCP/IP protocol family l l l l l TCP UDP IP ICMP RIP OSPF ARP DNS RARP BOOTP l l l FTP Telnet EGP GGP IGP BGP NFS NIS RPC SMTP SNMP

Transport l l Transmission Control Protocol (TCP): connection-based services User Datagram Protocol (UDP): connectionless services

Routing l l Internet Protocol (IP): handles transmission of information Internet Control Message Protocol (ICMP): handles status messages for IP Routing Information Protocol (RIP): determines routing Open Shortest Path First (OSPF): alternate protocol for determining routing

Network Addresses l l l Address Resolution Protocol (ARP): determines addresses Domain Name System (DNS): determines addresses from machine names Reverse Address Resolution Protocol (RARP): determines addresses

User Services l l l Boot Protocol (BOOTP): starts up a network machine File Transfer Protocol (FTP): transfers files Telnet: allows remote logins

Gateway Protocols l l Exterior Gateway Protocol (EGP): transfers routing information for external networks Gateway-to-Gateway Protocol (GGP): transfers routing information between gateways Interior Gateway Protocol (IGP): transfers routing information for internal networks Border Gateway Protocol (BGP) newer than EGP, used within an autonomous network

Others l l l Network File System (NFS): enables directories on one machine to be mounted on another Network Information Service (NIS): maintains user accounts across networks Remote Procedure Call (RPC): enables remote applications to communicate Simple Mail Transfer Protocol (SMTP): transfers electronic mail Simple Network Management Protocol (SNMP): sends status messages about the network

Datagram l l l Basic unit of data send thought an internet Moving packets between routers and/or hosts Consists of a header followed by the data Header: information necessary to move the packet across the Internet Connectionless transport mechanism

Datagram

Datagram Field ver Function The version of the IP protocol that created the datagram. IHL The length of the IP header in 32 -bit words (the minimum value is 5). Type of The quality of service (precedence, delay, service throughput, and reliability) desired for the datagram. Total The length of the datagram (both header length and user data), in octets. Identific A 16 -bit value assigned by the originator of ation the datagram, used during reassembly.

Datagram Flags Control bits indicating whether the datagram may be fragmented, and if so, whether other later fragments exist. Fragment The offset in the original datagram of the offset data being carried in this datagram, for fragmented datagrams. Time to The time in seconds the datagram is live allowed to remain in the internet system. Protocol The high level protocol used to create the message (analogous to the type field in an Ethernet packet). Header A checksum of the header. checksum

Datagram Source IP 32 -bit IP address of the sender. address Destination 32 -bit IP address of the recipient. IP address Options An optional field primarily used for network testing or debugging. Padding All bits set to zero - used to pad the datagram header to a length that is a multiple of 32 bits. User data The actual data being sent.

Internet Addresses l l l Globally accepted method of identifying computers Each host on a TCP/IP internet is assigned a unique 32 -bit address that is used in all communication with that host Must not be confused with physical addresses! Each address is a pair (netid, hostid), where netid identifies a network, and hostid identifies a host on that network In practice, each IP address must have one of the first three forms shown in the picture that follows

Internet Addresses Class Maximum number of possible networks hosts per network A B C D E 127 16, 777, 216 16, 384 65, 536 2, 097, 152 255 Reserved Class

Internet Addresses

Internet Addresses l The IP address is expressed in dotted decimal notation (DDN) by taking the 32 binary bits and forming 4 groups of 8 bits, each separated by a dot - 10. 4. 8. 2 is a class A address - 10 is the DDN assigned network number -. 4. 8 are (possibly) user assigned subnet numbers -. 2 is the user assigned host number - 172. 16. 9. 190 is a class B address - 172. 16 is the DDN assigned network number -. 9 is the user assigned subnet number -. 190 is the user assigned host number

Internet Addresses l l l Host: any attached device on a subnet, including PC’s, mainframes and routers Most hosts are connected to only one network, and they have a single IP address Routers are connected to more than one network and they can have multiple IP addresses Problem: IP type of addressing defines connection, not host! Problem: what when an organization which has a class C address finds that they (. org) need to upgrade to class B

DNS – The Domain Name System l l l Converts symbolic node name to IP address Uses distributed database Network Information Center (NIC) maintains names and network gateway addresses Name Resolver on each network tries to complete address from own database Examines Name Server for full NIC database if unsuccessful resolution

ARP - Address Resolution Protocol l l Allows host to find the physical address of a target host on the same physical network, given only the target’s IP address It does this by sending out an ARP broadcast packet containing both the source and destination IP address The broadcast is media-dependant For Ethernet LANs the broadcast address is a packet whose destination address bits are all `1'

ARP - Address Resolution Protocol l l All stations on the LAN receive this address, but only one host is able to recognize its own IP address It replies, thereby giving the original host its physical address.

ARP - Address Resolution Protocol

RARP - Reverse Address Resolution Protocol l l How does the diskless machine, one without access to secondary storage, determines its IP address at system startup? A diskless machine uses RARP to obtain its IP address from a server

RARP - Reverse Address Resolution Protocol

IP - Internet Protocol l l Defines unreliable, connectionless delivery mechanism IP provides three important definitions: - Defines the basic unit of data transfer used throughout a TCP/IP internet - Performs routing function, choosing a path over which data will be sent - Includes a set of rules that embody the idea of unreliable packet delivery

ICMP - Internet Control Message Protocol l l Integral part of IP Allows gateways to send error or control messages to other gateways or hosts Provides communication between the Internet Protocol software on one machine and the Internet Protocol Software on another Only reports error conditions to the original source! ICMP message is encapsuled in an IP datagram

ICMP-The Internet Control Message Protocol

Type field Value Description 0 Echo Reply 3 Destination Not Reachable 4 Source Quench 5 Redirection Required 8 Echo Request 11 Time to Live Exceeded 12 Parameter Problem 13 Timestamp Request 14 Timestamp Reply 15 Information Request (now obsolete) 16 Information Reply (now obsolete) 17 Address Mask Request 18 Address Mask Reply

UDP - User Datagram Protocol l l l Connectionless protocol Provides the primary mechanism that application programs use to send datagrams to other application programs Provides protocol ports used to distinguish among multiple programs executing on a single machine Uses IP to transport a message from one machine to another Does not use acknowledgements to make sure messages arrive UDP message is called a user datagram

UDP - User Datagram Protocol

UDP - User Datagram Protocol l l Source port: An optional field with the port number If a port number is not specified, the field is set to 0 Destination port: The port on the destination machine Length: The length of the datagram, including header and data Checksum: A 16 -bit one's complement of the one's complement sum of the datagram, including a pseudo header similar to that of TCP

UDP - User Datagram Protocol Conceptual Layering

TCP - Transmission Control Protocol l l l TCP is a communication protocol, not a piece of software! Provides reliable stream service Positive acknowledgement with retransmission Specifies the format of the data and acknowledgements that two computers exchange to achieve a reliable transfer Specifies the procedures the computers use to ensure that the data arrives correctly Specifies how TCP software distinguishes among multiple destinations on given machine Specifies how communicating machines recover from errors like lost or duplicated packets

TCP - Transmission Control Protocol l l l Specifies how two computers initiate a TCP stream transfer and how they agree when it is complete Does not dictate the details of the interface between an application program and TCP Does not specify the exact application programs to access these operations Reason: flexibility TCP uses the connection, not the protocol port, as its fundamental abstraction Connections are identified by a pair of endpoints

TCP - Transmission Control Protocol l TCP port number can be shared by a multiple connection on the same machine The basic unit of transfer used by TCP is a segment Segments are used to pass control information or data

TCP - Transmission Control Protocol Server Site Network Messages Receiver Site Send Packet 1 Receive ACK 1 Send Packet 2 Receive Packet 2 Send ACK 2 Receive ACK 2 Sender awaits an acknowledgement for each packet sent

TCP - Transmission Control Protocol Server Site Send Packet 1 Start Timer Network Messages. Receiver Site Packet lost Packet should arrive ACK should be sent ACK would normally Arrive at this time Timer Expires Retransmit Packet 1 Start Timer Receive Packet 1 Receive ACK 1 Cancel Timer Send ACK 1 Timeout and retransmission that occurs when a packet is lost

TCP - Transmission Control Protocol l l A simple positive acknowledgement protocol wastes a substantial amount of network bandwidth Delays sending a new packet until it receives an acknowledgement for the previous packet Sliding window protocol Keeps the network completely saturated with packets Higher throughput than a simple positive acknowledgment protocol

TCP - Transmission Control Protocol Sender Site Network Messages Receiver Site Send Packet 1 Send Packet 2 Send Packet 3 Receive ACK 1 Receive ACK 2 Receive Packet 1 Send ACK 1 Receive Packet 2 Send ACK 2 Receive Packet 3 Send ACK 3 Receive ACK 3 Three packet transmitted using a sliding window protocol

TCP - Transmission Control Protocol Conceptual Layering

Telnet – remote login protocol Allows user at one site to establish a TCP connection to a login server at another site, and then it passes keystrokes from the user’s terminal directly to the remote machine as if they had been typed at a terminal on the remote machine l Carries output from the remote machine back to the user’s terminal l

Telnet – remote login protocol l l l The service is called transparent Telnet client software User specifies a remote machine either by giving its domain name or IP address Telnet offers three basic services Defines a network virtual terminal (NVT) Includes a mechanism that allows the client and server to negotiate options Treats both ends of the connection symmetrically

Telnet – remote login protocol client reads from terminal server sends to pseudo terminal client sends to server receives from terminal

FTP – File Transfer Protocol l l l Manages files across machines without remote Telnet session Transfers files, manages directories, accesses e-mail Does not enable access to remote machine for program execution Uses two channels for simultaneous transfer of commands and data Conducts all transfers in foreground Does not use queues or spoolers so you are watching the transfer process in real time

FTP – File Transfer Protocol l Relies on TCP to provide connection management Protocol Interpreter PI transfers instructions using TCP port 21 Data Transfer Process DTP transfers files using TCP port 20

FTP – File Transfer Protocol FTP channel connections

FTP – File Transfer Protocol l Login Verifies user ID and password Anonymous FTP allows access without login account Access and file transfer requires proper permissions on the remote side Define directory Identifies starting directory Local and remote file system Define File Transfer Mode Text uses ASCII characters separated by CR and NL Binary transfers without conversion or formatting

FTP – File Transfer Protocol l l Start Data Transfer Enables user commands Stop Data Transfer Closes connection

TFTP – Trivial File Transfer Protocol l l Does not log on to remote machine Uses UDP connectionless transport TFTP transfer identifiers (TID’s) set ends of connection Support basic error messages Any kind of error can result in transfer failure Uses to boot diskless terminals and load applications Small executable size and memory requirements Cannot execute FTP until loaded with OS Lax access regulations pose security risks

Electronic Mail l l l The most widely used application service Differs fundamentally from other uses of network Usually timeout and retransmission if no acknowledgement Sender don’t want to wait for the remote machine to become available before continuing work! User don't want to have transfer abort merely because communication with the remote machine becomes temporary unavailable! Spooling

Conceptual components of an electronic mail system

SMTP - Simple Mail Transfer Protocol l l Standard for the exchange of mail between machines Focuses specifically on how the underlying mail delivery system passes messages across a link from one machine to another Messages transferred as 7 -bit ASCII Uses spools or queues Does not specify how the mail system accepts mail from a user or how the user interface presents the user with incoming mail

SMTP - Simple Mail Transfer Protocol l Does not specify how mail is stored or how frequently the mail system attempts to send messages Mail command identifies sender and provides message information Response is acknowledgment with identification of recipient

Example of SMTP Transfer S: 220 Beta. gov Simple Mail Transfer Service Ready C: HELO Alpha. edu S: 250 Beta. gov C: MAIL FROM: <Smith@Alpha. edu> S: 250 OK C: RCPT TO: <Green@Beta. gov> S: 550 No such user here

Example of SMTP Transfer C: RCPT TO: <Brown@Beta. gov> S: 250 OK C: DATA S: 354 Start mail input; end with <CR><LF> C: …sends body of mail message… C: …continues for as many lines as message contains C: <CR><LF> S: 250 OK C: QUIT S: 221 Beta. gov Service closing transmission channel

References l l l The Internet Unleashed, SAMS Publishing Internetworking With TCP/IP, Douglas Comer Teach yourself TCP/IP in 14 days, Tim Parker Novell’s Complete Encyclopedia of Networking, Werner Feibel http: //www. ieee-occs. org

Conclusion