Knowledge

Note on 23 Jul 2002 Why is Security? Authentication (may include authorisation) Integrity Privacy --> can the password easily break Non-repudiation --> dishonest (e.g. buy things from net, endup said never bought) Threats Eavesdropping Masquerading Message tampering Interuption Replaying Encryption Most wellknown aspect of security Obsfucate the message via some means of reversibly changing the characters in that message Simple Cipher Often known as Ceasar ciphers Displace each character by a given amount Very simple - trivial to break Block ciphers Simple ciphers are to easy Even if the cipher uses a random permutation of key, regularities in English mean that it is easy to break. Better solution is to use a block substuition (e.g. DES) Even better is to do substitution multiple times (e.g. 3DES) Stream Cipher Problem occurs when data is coming in byte/bit at a time (e.g. streaming data)

Note on 22 Jul 2002 Services provided Goal of the transport layer is to provide efficient reliable & cost effective service to higher levels (normally applications) Normally provides connection oriented & connectionless services Functions in a similar manner to network layer QoS Another way of looking at transport layer is that it's primary function is to enhance the "Quality of Service" of the network layer What exactly good is, is best left up to the user Connection establishment delay Connection establishment failure probability Throughput Transit delay Residual error ratio Protection Priority Resilience Transport Service primitives Many programs & programmers use transport layer so primitives must be easy to use and understand Primitive PDPU sent Meaning LISTEN none Block until some process tries to connect CONNECT CONNECTION REQ. Ac

Note on 22 Jul 2002 Properties and Services The network layer normally offers one of the following services to the transport layer Connection-oriented Connectionless Network layers are designed with the following goals in mind To provide services independent of subnet technology To shield the transport layer from the number, type & topology of the subnets present To use a uniform addressing scheme across both LAN & WAN Virtual Circuits Used in connection oriented services Whenever two machines wish to communicate - a route from source to receiver is setup and remembered Advantages easy congestion control quick to parse subsequent packets Disadvantages setup time vulnerable to crashes Datagrams Used in connectionless oriented services Everytime a packet is sent out, a new route maybe chosen Advantages robust no initia

Note on 18 Jul 2002 MAC sub layer Medium Access Control sub layer Sub layer of Data Link layer Job is to control traffic on a multi-access channel Channel Allocation Problem Static channel allocation Simple - each user allocated a fixed amount of bandwidth Provides good performance for few heavily loaded users Doesn't work well with bursty traffic Wastes bandwidth under normal use Dynamic channel allocation Tries to reduce bandwidth wasteage by allocating bandwidth as it is needed Complex Dynamic channel assumptions Station model - The model consist of N stations. The probability of frame transmission is λΔt in a given time Δt. Stations block until frame has been successfully transmitted. Single channel assumption - there is only one channel for all communication Collision assumption - if two frames are transmitted simultaniously, the collide . All stations can detect collision Co

Note on 17 Jul 2002  Data Layer Responsibility of the data layer Framing Error detection (sometimes) Error Correction (sometimes) Flow Control Broadcast channels (MAS sub-layer) Services provided Unacknowledged connectionless service Acknowledged connectionless service Acknowledged connnection-oriented service Framing Used to help data link layer detect and correct errors Data stream is aplit up into regular chunks (frames) Also need to consider timing issues Synchronous - common clock or encoded timing information Asynchronous - no common clock Asynchronous timing Primarily used on slow serial links (75-115200b/s) No common clock Individual characters are enclosed by known start and stop bits Signal is then split up into chunks and centre point of chunks is analysed Synchronous Faster devices (9600+b/s) Can use common network clock (known as heartbeat or clock encoding Sets (frames) of ch

Note on 17 Jul 2002 Sockets Sockets are the method by which & network application sends and receives data. Socket applications use ports to determine what application handles a given message Information can be sent to and fro by writing to and reading from the socket.  Ports Data Port# > TCP/UDP > # > Application TCP & UDP TCP (Transport Control Protocol) Commonest internet protocol Provides guaranteed connection-oriented communication Normally 2 types, server & client UDP (Universal Datagram Protocol) Provides connectionless, unguaranteed communication Faster than TCP TCP - client side Open a socket Open input and output streams to the socket Do any necessary reads and writes to the sockets Close the streams Close the socket TCP - server side Open server socket Listen for communication When communication occurs, create new socket to deal with it Comm

Note on 16 Jul 2002 Topologies - Bus & Ring Bus is a network topology in which all nodes connect to the network via a central cable called the bus. The bus acts as the shared communication medium that the devices are attached to. Any device that wants to communicate with other device on the network will send its data over the bus which will be send to all attached devices but the intended recipient will only process that packet. Thus bus topology is good and easy to setup for only a small number of devices, as devices and network utilization increases the performance issues and problems arise. If the bus is damaged then the whole network fails making bus topology a less preferred option. In a Ring topology every device/node is connected to exactly two other nodes one on either side of it in closed loop fashion. All messages travel though the ring either in a clockwise direction or anti-clockwise direction. Ring topology is very rarely used today because they are expensive

Note on 15 Jul 2002  Why Networks? Communication Sharing of Data Providing remotes services Sharing of Resources Failsafe Social Reason A brief history of networks - the birth of the internet 1980's ATM matures 1982 - TCP/IP developed - the internet as we know it is born 1984 - DNS introduced - users no longer have to remember obscure IP addresses 1984 - Number of internet hosts reaches 1000 1984 - JANet introduced in the UK 1986 - NFSNET created - massive explosion of networks within US universities Glossary Guilded Transmission - signal is sent down physical medium Unguilded Transmission - no physical medium POTS - Plain Old/Ordinary Telephone System Bandwidth - The amount of information that can be in transit at any one time Latency - The amount of time needed to send/receive information A brief history of networks - prehistory Earliest established networks - runners A medical refinement -