COP 5611 Operating Systems Spring 2010 Dan C. Marinescu Office: HEC 439 B Office hours: M-Wd 2:00-3:00 PM

Lecture 10 Reading Assignment: Chapter 7 from the online textbook A progress report for the project is due on every Monday. Last time:  Network properties Today:  Layering  Link layer  Network layer Next time  Routing

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4 Recovery of lost packets

5 Duplicate requests

6 Delays and recovery lead to duplicate response

7 Layering Simplify the design Example- RPC

8 Client-server communication based on RPC

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Multiple protocols may be supported at each layer 11

12 Multiplexing and Demultiplexing

Network layers for store and forward networks Every module has three interfaces:  To the layers above  To the layers below  To the peer layer on another system Each layer hides the operation of the layer below from the layer above and provides its own interpretation of the important features of the layers below. A minimum of three layers are necessary:  Link layer  move data from one point to another. Includes  Network layer  forward data through intermediate node towards the destination  End-to-end-layer  provides all the functions required by the application including Encapsulation: each layer treats information received from the layers above as raw data and adds control information (headers and trailers) understood only by its peer. 13

14 Link layer

15 Network layer

End-to-end layer 16

17 Layering in different network architectures OSI –has 7 layers  The upper four layers are: Application Presentation  translate data formats and emulate the semnatics of RPC Transport  deal with lost, duplicate, out of order, segments Session  deals with connectivity Internet is based on a model including  Application  Transport – transports segments  Network – transports packets  Data Link – transports frames Applications are very diverse and it makes no sense for a lower layer to implement functions required by higher layers. The end-to-end argument  application knows best

Example of an end-to end argument Should we have a checksum implemented at the link layer or should leave it for the application protocol e.g., a file transfer protocol? If we implement it at the link layer   Advantage: detect earlier an error  Disadvantage: the data may still be affected by errors when buffered at the upper layers. 18

Application, Transport, Network, and Data Link Layer Protocols

20 Hourglass communication model

21 Network composition Network composition:  Mapped composition  A network layer is built directly on another network layer by mapping directly higer-layer network addresses to lower-layer network addresses. Example: Internetworking  interconnect several networks together, e.g., the Internet  Recursive composition  a network layer rests on a link layer that it is itself a complete network. Examples: Overlay networks. E.g., Gnutella

22 Network composition. The overlay network Gnutella uses for its link layer an end-to-end transport protocol of the Internet. In turn, the Internet uses for one of its links an end-to-end transport protocol of a dial-up phone system

23 More about the link layer Function: push bits from one place to another In this architecture the link layer combines:  MAC (medium access control layer)  The physical layer Analog worlds Capacity of a communication channel Capacity of a noisy communication channel C= B x log (1+ signal/noise) B is the bandwidth in Hz signal/noise – ratio of signal power to noise power Signals attenuation Signals are distorted over long distances

24 Serial transmission

25 How to push bits from A to B which do not share the same clock? First raise the READY line

26 Signal attenuation and shape distortion

Error detection and error correction Encode data with an error detection code. Use error correction if necessary. 27

28 Framing A pattern of bits serve as a frame delimiter – e.g., seven 1’s Bit stuffing:  The sender: add a 0 whenever it encounters a pattern of six 1’s in data  The receiver: remove a 0 following a pattern of six 1’s in data Add a frame header Add a frame trailer

29 Sender bit stuffing procedure

30 Receiver bit stuffing procedure

31 A network protocol may use multiple data link protocols

32 Multiple transport and data link protocols

33 Sending a frame

34 Receiving a frame

Link properties We distinguish several types of links:  Point-to-point  Broadcast links Each link imposes a maximum transmission unit (MTU)  the largest frame size which affects  Timing, un upper bound on link commitment time  Error, the longer the frame the larger the probability of error for a given error rate on the link  Buffer space at each end 35

The network layer A network can be viewed as consisting of  Network core – carries out segments from one communicating entity to another  Network periphery – hosting the communicating entities at attachment points Addressing – each communicating entity must have a unique address. 36

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38 Internet Core and Edge

39 Router

40 Router supporting QoS (Quality of Service)

Network model The network layer transports packets The network core consists of routers which forward packets using routing tables The input for network layer on a host at the periphery of the network is a segment in a segment buffer at the transport layer. The output is a link buffer 41

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All hosts must have logical addresses All devices have a physical address of the interface Must obtain a logical address 45

Dynamic IP address assignment -DHCP