networks2.pdf September 8,

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1 Contents SCI Computer networks 2 Layer model Problems and solutions Lennart Herlaar September 8, 2003 SCI X SCI X OSI Reference Model OSI = Open Systems Interconnection, from the ISO (International Standards Organisation) 7 pplication 6 Presentation 5 Session 4 Transport 3 Network 2 Datalink 1 Physical subnet 3 Network 2 Datalink 1 Physical 2. Datalink Layer Purpose: reliable communication between two points The bits that should be transported are divided into frames Each frame gets some additional information such that beginning and end can be recognized Each frame gets additional information to detect errors (checksum) Frames that get lost or garbled must be retransmitted (protocol) SCI X SCI X Checksum The checksum is a function f(frame), such that a small change in frame will cause a big change in f(frame). Usually f(frame) will give 16 or 32 bits that will be appended to the frame (the checksum) The receiver will check that checksum = f(frame) f must be such that the probability of a garbled frame giving the same checksum is extremely small and can be ignored Checksum algorithms have been designed specifically to catch the most common errors It is also possible to use codes for repairing errors but these are much more complicated and use many more bits. Protocol To correct lost or garbled frames a protocol is used protocol is a collection of rules and agreements about how to send messages back and forth It regulates: The form and contents of the messages The meaning of the messages The order in which messages must be sent How to handle errors In a simple protocol the receiver sends an acknowledge (CK) if a message has been received correctly. It is also possible to send a negatieve ack (NK) if a message arrives in error SCI X SCI X networks2.pdf September 8,

2 Protocol Problems 1 Protocol Problems 2 message message message disappears cannot do anything because it doesn t know that sent something. Therefore has to find out on its own that something is wrong CK disappears Now must also find out on its own that something is wrong SCI X SCI X Datalink Protocol Sliding Window Protocol uses a timeout: if an CK doesn t arrive within a certain time, assumes that the message was lost and retransmits it sliding window protocol is a protocol where messages are sent without waiting In case 2 this is a problem, as will get the message twice (e.g. transfer 100 euros from my bank account to another account) Solution: the messages (frames) are numbered and the CK contains the number of the last (properly) received message So every frame has: beginning and end indications, sequence number, checksum and the kind of frame (CK, NK, Data) How many sequence numbers do we need? For communication with a large delay (e.g. satellite) this stop-and-wait protocol would be too slow (calculate) Only if something goes wrong an action is taken m1 m2 m3 m4 CK disappears SCI X SCI X Sliding Window Protocol 3. Network Layer The sender must keep the messages until an CK is recevied The message sequence numbers that the sender keeps are called the sender window. The most important problem the network layer has to solve is routing It is limited by the amount of memory that the sender reserves When an CK is received the sender advances its window (sliding) The receiver must also have a window because it has to wait until missing messages have been retransmitted If the receiver window is smaller than the sender window the receiver may have to drop correctly received messages How do I get a message from to? The nodes in the network have to exchange information about the topology of the network What is the shortest/cheapest/fastest route? SCI X SCI X networks2.pdf September 8,

3 roadcast Network Problem: medium When two or more computers transmit a message at approximately the same time they garble each others messages For this kind of networks there is a separate sublayer (Medium ccess Control Layer) that regulates the access to the medium MC loha Original system: University of Hawai Radio transmitters (stations) on various islands Central computer to access When more than 1 station transmits the message arrives with a checksum error (collision) The datalink protocol causes a retransmission The more messages are transmitted, the more collisions, the less successful transmissions Maximal efficiency = 18% SCI X SCI X MC CSM/CD Improvements used in modern broadcast systems (Ethernet) Carrier Sense: efore transmission a stations listens if another station is sending and waits until the channel is free Collision Detection: During transmission the station detects if a collission occurs and aborts transmission if so Requirements ll stations must be able to receive each others transmissions transmission must be long enough so that a collision is detected before the transmission is finished I.e. minimum frame size, maximum distance Wireless Networks In a large wireless network repeaters may be necessary to cover the whole area Each station must be able to communicate with at least one repeater Different repeaters or different stations may not be able to see each other I.e. CSM/CD may not work Special protocols are used where a sender first announces its intention to transmit and the size of the frame The intended receiver acknowledges with the same information Neighbouring stations will keep quiet during the time necessary to send the frame The announcement messages may still cause collissions SCI X SCI X Routing Network Design 1 In every node in the network for every other node there should be routing info to 1 2 a e c d C e d The second choice can be used if the first choice fails or is too busy C e a Node d b c queues outgoing messages ecause the network layer is inherently unreliable, it is in fact useless to acknowledge packets The data networks of the telephone companies still do it (gives an impression of reliability!!) The layer above must add reliability anyway Most networks have a limited packet size messages must be split into packets and be assembled at the destination When the packets pass a network with even smaller packet size they must be split again! packet-switching SCI X SCI X networks2.pdf September 8,

4 Network Design 2 How should the routes be changed if a node or connection fails? How should we route in order to minimize congestion? node can overflow packets disappear node can stop idem Packets can travel in circles if the routing is not perfect Deadlock: two or more nodes wait for each other indefinitely = The communication from to is imperfect in spite of the datalink layer Connecting Networks nother network layer task Connecting networks, e.g. the university networks in the Internet In such a network routing tables will become too large Routing is done in the first place per network Every node has an address network number node number (within the network) The routing tables contain info about routes to other networks (on network number) routes to other nodes within its own network (on node number) Hierarchical routing SCI X SCI X Internet 4. Transport Layer The transport layer must solve the unreliability of the subnet process virtual channel router etween the networks there are special nodes whose purpose it is to route the packets (routers or gateways) In large networks a similar system is used to divide the network in subnets. E.g. a subnet per faculty or building lso for security SCI X process SCI X Transport Layer Problems We want to have a reliable connection between two processes in different computers. It must behave like a pipeline Solutions are comparable to datalink layer dditional problems because packets can roam for a long time in the network Or they can overtake each other Reliability of Transport Layer The transport layer guarantees that messages arrive Or it guarantees that the sender knows that it cannot arrive This is not always enough TM messages request euro 100 for bank account xxx request accepted Question: when will the money be dispensed and when will it be booked? SCI X SCI X networks2.pdf September 8,

5 Scenario 1 Scenario 2 TM TM req. 100 euro req. 100 euro book 100 euro check 100 euro pay 100 euro pay 100 euro dispensed dispensed TM breaks down before the money is dispensed ad for customer? book 100 euro ank computer stops after dispensing ad for bank SCI X SCI X Solution with CKs? Send CK first, then do the action Fails if machine breaks down just after the CK First do action, then send CK Fails if machine breaks down just before the CK Problem can only be solved in higher layers Example: sequence numbers that survive machine failure Storage on permanent storage medium necessary 5. Session Layer For structuring two-way communication Sessions (e.g. login-session) Whose turn is it? Synchronization and restart e.g. printer paper fails: go back to previous page SCI X SCI X 6. Presentation Layer (Re)presentation of data Meaning of bits Representation of text, images, numbers, sound etc. Different computers can have different representations: transformation and uniformization Data compression Encryption 7. pplication Layer Here are all things for the user Databases Remote login File transfer Electronic mail Electronic news WWW No limits... SCI X SCI X networks2.pdf September 8,