CS Computer Networks Dr. Randy L. Ribler 103 Hobbs

Introduction Read Chapter 1 in Kurose and Ross

How do we design networks?  Use layers of “abstractions”  Each layer provides a set of “services”  Upper layers implement the services in terms of lower layer services  Each layer treats the layer above it and below it as an abstraction

Sentence Word Net Talk Sentence Net Talk Send sentence Send Word Route Word Alphanumeric Transmission Word Classroom Demonstration Protocol

Talk Net Word Sen- tence Net Word Sen- tence Host 0Host 1Host 2Host 3Host 4Host 5

ISO OSI Reference Model  International Standards Organization  Open Systems Interconnection (OSI)  The ISO OSI contains 7 layers

OSI Reference Model Application Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Presentation Layer

Application Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Presentation Layer Application Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Presentation Layer Adapted from p34, Tanenbaum Sending ProcessReceiving Process

OSI Reference Model Terminology  Services - What the layer does  like public member functions in OOP  Interface - Parameters and results  like the parameter lists and return values in OOP  Protocols - the communication between peer levels  like the hidden implementation of class in OOP.

Physical Layer  Transmission of “raw” bits  Hardware level concerns  voltages, pins  mechanical, electrical, and procedural interfaces  Communicates between a pair of nodes  Provided by modems and other point-to-point transmission hardware

Data Link Layer  Imposes a structure on the bit stream  data frames (100’s or 1000’s of bytes long)  Acknowledge receipt of frames  Handle damaged, lost, or duplicate frames  Flow regulation  Don’t overflow buffers

The Network Layer  Routing “packets”  Congestion Control  Accounting  Heterogeneous network interconnects

The Transport Layer  Provides multiplexing/demultiplexing between network traffic and individual processes.  Establishes/deletes network connections  End-to-end flow control.  Not needed on routers (unless router is also a host).  Often provides a reliable bit stream.  Transport layer protocols handle “messages.”

The Session Layer  Information about where to obtain service  Access Rights/permissions  establish “sessions” (login or ftp)  Traffic Control ( half/full duplex connections)  The Session Layer is generally under utilized

Presentation Layer General Purpose Application Utilities Utilities that operate on data Data word sizes Encryption Data Compression Code conversions e.g., ASCII EBCDIC

Application Layer  Application specific code  ftp  telnet  http  Anything not implemented elsewhere in the protocol stack.

Internet Protocol (IP)  Internet protocol has been revised  IP version 4 (IPv4) is the most widely used.  IP version 6 (IPv6) will eventually replace it.  IPv4 uses 32-bit host addresses  IPv6 uses 128-bit host addresses

TCP/IP Reference Model Application Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Presentation Layer Application Layer Transport Layer Internet Layer Host-to-network Layer OSI Model TCP/IP Model IP TCP or UDP

Talk Authorize Talk Break Message Apart Route Message Manage Frames Send Bit Stream ASCII-EBCDIC Talk Authorize Talk Reassemble Message Route Message Manage Frames Receive Bit Stream YouYour Friend Application Presentation Session Transport Network Data Link Physical

Internet Protocol (IPv4)  32-bit IP addresses  Store internally as a single word  Usually expressed to user’s using “dotted decimal notation” (e.g., ) where each of the four numbers range from 0 to 255.  Usually aliased to host name of the system (e.g. mail.lynchburg.edu

Domain Name System (DNS)  How can we map host names to IP addresses?  Domain Name Servers provide names in response to client requests.  A hierarchical representation provides name management at each level of domain name.

Client/Server Models  Network interaction is based on an asymmetrical relationship  Server  provides services to authorized clients  typically handles many clients simultaneously  works passively, driven by client requests  Client  requests services  typically initiates

Port Numbers  IP addresses identify host machines only.  Port numbers identify services on machines.  16-bit port numbers provide values ( )  “Well-known ports” (0-1023)  Registered ports ( ) (49152 is 75% of available port numbers)  Dynamic or private ports  Internet Assigned Number Authority (IANA)  Determines (0-1023)  Registers ( )  Does not control ( )

Some Well Known Port Numbers  23 (telnet)  21 (ftp)  13 (daytime)  25 (smtp (mail))  43 (whois)  144 (news)  80 (http web server)

Telnet Client  telnet  provides a general interface to request a service and return a text response.

Ping  See if there is a connection between two machines  See if a particular host is running ping

Trace Route  traceroute (unix)  tracert (dos/windows)

Internetworking lynchburg.edu acavax.lynchburg.edu networkvirginia.net cs.vt.edu cns.vt.edu lasi-main GATEWAYS

Network Protocols Must Adopt One of the Two Byte Orders  Network Byte Order  The protocol byte order  Host Byte Order  The native machine byte order  Conversion Functions  htons : convert 16-bit value from host byte order to network byte order. (ntohs provides the inverse)  htonl: convert 32-bit value from host byte order to network byte order. (ntohl provides the inverse)

Domain Name Management  Each level is responsible for assigning its own names .edu is the top-level domain for educational institutions in the US. We need to get permission from the managers of the.edu domain before we can use lynchburg.edu  lynchburg.edu is the Lynchburg College domain name. We can name our computers anything we want, (e.g., acavax.lynchburg.edu).  If we wanted to we could create a cs.lynchburg.edu subdomain

Lavender Army Magenta Army Distributed processing problem from Data Networks, by Dimitri Bertsekas and Robert Gallager