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1 Tutorial on Computer Networks ECE 7750: Distributed Control System Lili Ma Department of Electrical & Computer Engineering Utah State University January.

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Presentation on theme: "1 Tutorial on Computer Networks ECE 7750: Distributed Control System Lili Ma Department of Electrical & Computer Engineering Utah State University January."— Presentation transcript:

1 1 Tutorial on Computer Networks ECE 7750: Distributed Control System Lili Ma Department of Electrical & Computer Engineering Utah State University January 2005

2 2 Contents ►Physical Layer:Physical Layer: Guided Transmission Media Wireless Transmission Media Communication Networks ►Introduction:Introduction: Classification Software ►Data Link Layer:Data Link Layer: Services Framing Error Control Flow Control ►Ethernet:Ethernet: Connecting Devices Evolution of Ethernet Wireless LAN ►Network Layer:Network Layer: Routing Algorithms Congestion Control Quality of Services IP Protocols ►Transport Layer:Transport Layer: UDP and TCPUDPTCP ►Application Layer ProtocolsApplication Layer Protocols Back to TOC

3 3 ►Introduction Back to TOC Distinguish Between Two Important Concepts: Computer Networks: A large number of separate but interconnected computers. Example: LAN, WAN, Internet. Distributed Systems: A software system built on top of a computer network that appears to its users as a single coherent system. Example: WWW (world wide web).

4 4 Usages of Computer Networks Business Applications: – Resource sharing: printer, scanner, custom records, inventories. – Communication medium: email, file update, video conferencing. – Enable companies to do business electronically or online. Home Applications: – Access to remote information. – Person-to-person communication. – Interactive entertainment. Mobile Users: – Mobile & wireless notebook computers. – Portable office, PDS for store inventory. Back to TOC

5 5 Network Classification Transmission Technology: – Broadcasting: Broadcasting or multicasting. – Point-to-point, also called unicasting. Scale: – PAN: processors located in same square meter. – LAN: processors located in same room, building, campus. – WAN: processors located in same city. – MAN: processors located in same country, continent. – Internet: processors located in same planet. Back to TOC

6 6 LAN and MAN Back to TOC Bus Ring LAN: MAN:

7 7 WAN and Internet Back to TOC WAN: – Subnet consists of transmission lines and switching elements (such as routers). Internet: – A collection of interconnected networks.

8 8 Wireless Networks Back to TOC Wireless Networks: Bluetooth Configuration: Master-slave configuration Wireless LAN. Bluetooth ConfigurationWireless LAN

9 9 Software Protocol Hierarchy Back to TOC Information is passed from higher layer to lower layer for transmission. Appropriate headers and/or tailors are added. Pieces of information is delivered from lower layer to higher layer where the data load is extracted. Dashed: Virtual Communication Solid: Actual Communication

10 10 Service vs. Protocol Back to TOC Service: a set of operations that a layer provides to its layer above. Protocol: a set of rules governing the transmitting between peers of the same layer.

11 11 Services Back to TOC Connection-oriented/connectionless: – Whether or not sequence (or order) can be guaranteed. – Connection-oriented: modeled after the telephone system. – Connectionless: modeled after the postal office. Reliable/Unreliable: – Whether or not there are ACKs. Connection-orientedConnectionless Reliable File Transfer We want sequence of data and the correctness of data. Acked Datagram Modeled after the register letter. We don’t care how the letter is routed, but make sure it is arrived by requesting a receipt – ACK Unreliable Voice/Video Transmission We want the sequence but not the reliability since the speed is more important than correctness Called Datagram Service

12 12 OSI vs. TCP/IP Models Back to TOC

13 13 OSI Layers Back to TOC Physical Layer: Transmit raw bits over a communication channel. Data Link Layer: Ensure raw transmission appear to be free of undetected error. Network Layer: Control the operation of subnet. Do the routing. Transport Layer: End-to-end layer. Accept data from above, send to the receiver. Application Layer: A variety of protocols commonly used by users.

14 14 TCP/IP Layers Back to TOC Internet Layer: IP Protocol. Permit packets to be transmitted independently to destination Transport Layer: Allow peers to have conversations. – TCP: Reliable and connection-oriented. – UDP: Unreliable and connectionless. Application Layer: A variety of protocols commonly used by users.

15 15 ►Physical Layer Back to TOC Fact: No transmission facilities can transmit signals without lose of power. Bandwidth: The range of frequencies transmitted without being strongly attenuated. The bandwidth can depend on: – Transmission media. – Transmission distance. – Signal-to-noise ratio. – Thickness, construction, and ….

16 16 Guided Transmission Media Back to TOC Magnetic Media: Write data onto magnetic tape or removal media and physically transport the media to destination. – Advantages: Excellent bandwidth. – Disadvantages: Slow. Unshielded Twisted Pair (UTP): Consists of two twisted copper pair. – Application: Telephone system. – Be able to transmit analog/digital signals. Category 3 UTPCategory 5 UTP

17 17 Guided Transmission Media (cont) Back to TOC Coaxial Cable: – Can span longer compared with twisted pairs. – High bandwidth. – Excellent noise immunity. – Used to be widely used within telephone system, now replaced by fiber optics. Still widely in use in cable TV and MAN.

18 18 Guided Transmission Media (cont) Back to TOC Fiber Optics: – Light Source: pulse (absence) of light indicates “1” (“0”). – Detector (Photodiode): generates electrical pulse in response to light. – Multimode: multiple rays bouncing inside the media – Single-mode: when the fiber’s diameter is reduced to few wavelengths of light, light will propagate in a straight line. (a) Side view of a single fiber (b) End view of a sheath with three fibers. Light Source Multimode Operation

19 19 Wireless Transmission Back to TOC Principle: – When an antenna of appropriate size is attached to an electrical circuit, the electromagnetic waves can be broadcasted and received by a receiver some distance away. – In vacuum, electromagnetic waves travel at the speed of light. – In copper or fiber, this speed slows down. Wireless Transmission Media: – Radio Transmission. – Microwave Transmission. – Infrared and Millimeter Waves. – Lightwave Transmission.

20 20 Electromagnetic Spectrum Back to TOC The electromagnetic spectrum and its uses for communication.

21 21 Wireless Transmission (cont) Back to TOC Radio Transmission: – Easy to generate, subject to interference from electrical devices, frequency dependent. – Low (omnidirectional): pass through objects, power decreases sharply. The transmitter & receiver do not have to be aligned. – High (travel straightly): bounce off objects, absorbed by rain. High FrequencyLow Frequency

22 22 Wireless Transmission (cont) Back to TOC Microwave Transmission: – Travel in nearly straight lines. – Can be narrowly focused. – Transmitter & receiver need to be aligned. – Widely used for long-distance mobile phones. Inferred Transmission: – Used for short-range communication, TV remote control. – Relatively directional, cheap, easy to build. – Drawback: Cannot penetrate objects. – Advantage: Communication in one room will not interfere with neighbors.

23 23 Examples of Communication Networks Back to TOC PSTN (Public Switched Telephone Network): Each local loop is independent. Physically circuit switched. Mobile Phones: The local loop uses wireless transmission media. Frequency reuse. Cable TV: Neighbors share the same physical medium. Competitions exist among neighbors for bandwidth. Security can be an issue.

24 24 Switching Techniques Back to TOC Circuit Switching: – Physical path is established before transmission. – This dedicated path is available till finish. Message Switching: – No physical path is established before transmission. – No limitation on data/block size. – Thus requires router to have disk for storing. – Data is stored at router before being forwarded. Packet Switching: – Strict upper limit about packet size. – Packet can be stored at router’s memory, instead of hard disk. – Be able to handle traffic much better.

25 25 Switching Techniques (cont) Back to TOC (a) Circuit switching(b) Packet switching

26 26 Switching Techniques (cont) Back to TOC Computer network is always packet switching, occasionally circuit-switching, and never message-switching.

27 27 ►Data Link Layer Back to TOC Tasks of Data Link: To achieving reliable and efficient communications between adjacent machines. By adjacent, we mean that two machines are connected by a communication channel, “like” a wire. The “wire-like” channel guarantees the order of bits exactly as they are sent. The data link layer is not an end-to-end layer. Protocols happen among adjacent computers

28 28 Position of Data Link Back to TOC

29 29 Problems Might Encounter Back to TOC Problems that might be encountered: There are errors in the physical communication layer. The physical layer only provides finite data rate. There exists non-zero propagation delay between T-send and T- receive. A simple thinking of data link layer is: “A puts bits on wire and B gets them”.

30 30 Function & Design Issues of Data Link Back to TOC Functions of Data Link: – Provide service interfaces to network layer. – Dealing with transmission errors. – Regulating flow control. Design Issues of Data Link: – Services provided to Network layer. – Framing. – Error control. – Flow control.

31 31 Services Provided by Data Link Back to TOC Un-Acked Connectionless Service: – No logical channel is established. – Independent frames are sent without Acks. – No attempt is made for lost frames (left to higher layer). – Suitable over reliable channels (fiber). Hope for the best! Ack: No. Timer: No. Sequence #: No. Error Control: No.

32 32 Services Provided by Data Link (cont) Back to TOC Acked Connectionless Service: – No logical channel is established. – Frame is individually Acked! – If a frame has not been received for certain amount time, re- send. – Suitable over un-reliable channels (wireless). Ack: Yes. Timer: Yes. Sequence #: No. Error Control: Yes.

33 33 Services Provided by Data Link (cont) Back to TOC Acked Connection-Oriented Service: – Logical channel is established. – Frame is guaranteed to be received! – Each frame is received exactly once! – All frames are received in the right order! – Frame is numbered! – Three phases: Initialization, Send, Release. Ack: Yes. Timer: Yes. Sequence #: Yes. Error Control: Yes.

34 34 Services Provided by Data Link (cont) Back to TOC Acked service is not a necessity in Data Link since higher layer (Network, for example) can also do the job. However, if frames are individually Acked, packets can get through faster. For reliable channels, this might be unnecessary. For un-reliable channels, this is worth doing.

35 35 Framing Back to TOC What is framing? Break raw bits into discrete frames. Why framing? Can add a field for error detection (checksum). Packets from Network Layer  Frames by Data Link

36 36 Framing Methods Back to TOC Character Count: A field in the header tells the # of characters in each frame. Problems of Character Count: – It will not help to re-send, since the sender does not know how many characters to skip over to re-transmit. – Due to the above problem, character count is rarely used anymore alone.

37 37 Framing Methods (cont) Back to TOC Flag Byte: – To get around the problem of re-synchronization. – Each frame starts and ends with special bytes (can be same), called flag byte. – If get lost, the receiver can search for “flag byte” for frame boundary. In actual usages, many protocols use combinations of “character count” plus “flag” for safety purpose.

38 38 Error Control Back to TOC Ultimate Goal: By managing timers and sequence # to ensure that each frame is passed to the layer above (not just received by the receiver) exactly once, no more and no less! Error will be with us for several/many years to come. We have to live with it and deal with it.

39 39 Error Control (cont) Back to TOC The receiver provides Acks to received frames. How about when a whole frame is lost? – Action of receivers: receive nothing, do nothing. – Action of senders: have sent something, get no Ack, wait for certain time (timer is assumed),  re-send. Problem arising from retransmission: – The same frame might be received many times and delivered to “network” layer many times. – To solve this problem, add sequence # in each frame,  receiver will know if the same frame has been received many times.

40 40 Error Control: Piggybacking Back to TOC To improve efficiency. The receiver attaches Acks to outgoing frames to get a “free ride”. The Ack needs to be sent out “soon” (due to sender’s timer). The data link of receiver side has no way to foretell when a new frame will come from Network Layer.  The rule is to wait for certain time, if a new frame comes, then the Ack gets a free ride, else send the Ack independently.

41 41 Flow Control Back to TOC When the receiver is slow and cannot process all the data received, some data might get lost. Methods: – Feedback-based: the receiver updates its status or sends permission for the sender to send. – Rate-based: (usually in higher level).

42 42 Flow Control: Stop-and-Wait Back to TOC Stop-and-Wait: The sender waits for an Ack after sending each frame. Only when an Ack is received, or timer out, next frame goes. – Advantage: simple. – Disadvantage: slow and inefficient since each frame is alone on the wire. Deal with damaged frame? Deal with lost frame? Deal with lost ACK/NACK?

43 43 Flow Control: Sliding Window Back to TOC Sliding Window: The sender can transmit multiple frames at a time, thus increases efficiency. The receiver Acks some of the frames, using a single Ack to confirm the receipt of multiple frames. – Advantage: more efficient. – Disadvantage: complex. Deal with damaged frame? Deal with lost frame? Deal with lost ACK/NACK?

44 44 Sliding Window Protocols Back to TOC Sliding Window Protocols: – One-bit sliding window. – Go back N. – Selective Repeat. The protocols differ in efficiency, complexity, buffer Size. Both the sender and receiver maintain their own windows.

45 45 Sliding Window Protocols (cont) Back to TOC Sending Window: – At any time, the sender maintains a set of Seq.# corresponding to frames it is allowed to send. – Seq.# that falls into the sending window represents frames that have been sent or can be sent but are not Acked. Receiving Window: – Corresponds to frames it may accept. The sending window and receiving window can have different sizes.

46 46 One-Bit Sliding Window Back to TOC Sliding window of size 1: Initially After the first frame is sent After the first frame is received After the first Ack is received

47 47 One-Bit Sliding Window (cont) Back to TOC In one-bit sliding window, though the transmission is bi- directional, the hosts are still blocked by waiting for Acks. When the transmission time is not negligible, it means the available bandwidth is not used properly.  Solution: To allow the sender to send more frames before being blocked. With an appropriate choice of window size, the sender will be able to continuously transmit frames for a time equal to the round-trip transmission time. Bandwidth increases  window size increases. Delay increases  window size increases. The sliding window can help to keep the “line” busy!

48 48 Frame # Ack # ………… Sliding Window (cont) Back to TOC Illustration to improve efficiency:

49 49 Sliding Window: Go-Back-N Back to TOC Go Back N: – Discard all subsequent frames and send no Acks back to the sender. Send Acks to frames correctly received. – Eventually, timer times out and the sender resends again. – Suitable for reliable channels where error rarely occurs. – Buffer on receiver side: not required (just deliver correct frames to Network Layer above) – Buffer on sender side: required (to remember what to re-send) – The protocol is relatively simple.

50 50 Sliding Window: Selective Repeat Back to TOC Selective Repeat: – Buffer all subsequent frames that are “good” and send NAK (negative Ack) to sender to inform the situation. – The NAK will expedite the re-transmission process – Suitable for un-reliable channels where it wastes a lot of bandwidth on re-transmission – Buffer on receiver side: required (to buffer the subsequent “good” frames) – Buffer on sender side: required (to remember what to re-send) – The protocol is relatively complex

51 51 NAK will help to inform the sender the problem earlier before timer times out. The main difference lies in the buffering & ordering at the receiver side. * Totally lost. * Get damaged. Go-Back-N vs. Selective Repeat Back to TOC

52 52 Error & Flow Control Methods Back to TOC Stop-and-waitGo-Back-NSelective Repeat Damaged Frames Re-send the damaged one (by NAK) Re-send all since the last Acked one (by NAK) Re-send the damaged one (by NAK) Lost Frames Re-send the lost one (by timer) Re-send all since the last Acked one (by out of order) Re-send the lost one (by out of order) Lost Acks Re-send the last one (by timer) Re-send all since the last Acked one (by timer) Re-send all since the last Acked one (by timer)

53 53 ►Ethernet Back to TOC IEEE Standards for LANs: Ethernet: IEEE 802.3 – Widely used Wireless LAN: IEEE 802.11 – Widely used Bluetooth: IEEE 802.15 – Hard to say Wireless WAN: IEEE 802.16 – Hard to say Because Ethernet and IEEE 802.3 has only two differences, people use these two terms interchangeably.

54 54 Ethernet: Classical The Ethernet protocol is the most widely used protocol for LANs. It is first designed in 1973. The classical Ethernet operates at 10 Mbps, uses baseband signaling. – Baseband: The whole capacity of the medium is occupied by the signal and frequency multiplexing is not possible. – Broadband: The capacity of the medium is divided into channels using multiplexing. Back to TOC

55 55 DIX 802.3 Difference:  Length < 1500 < Type.  802.3: Has SOF (Start of Frame).  802.3: The type filed can be specified at the beginning of Data. Ethernet Framing: DIX & IEEE 802.3 Back to TOC

56 56 Preamble Start of frame delimiter Data, the payloadCRC 7 bytes1 byte 46 to 1500 bytes4 bytes Destination addressSource address Type Length 6 bytes 2 bytes IEEE 802 Done in hardwareDone in software Ethernet Framing: IEEE 802.3 Back to TOC

57 57 Preamble: a bit pattern of “1010 1010……” – Preamble is to allow the receiver’s clock to synchronize with the sender’s. SOF (Start of Frame) Delimiter: a byte to demark when preamble has finished. – SOF has a bit pattern of “1010 1011”. – The Ethernet controller chip becomes interested only after the SFD has been received. Back to TOC Preamble and SOF

58 58 Performed by hardware. CRC compares the calculated value with the received one at the time the signal stops. A CRC error is sent to the controller chip and the damaged frame is normally discarded. CRC Checksum Back to TOC

59 59 Global Address: Assigned by the IEEE to ensure that no two stations in the world would have the same addresses. Local Address: Assigned by the network administrator and has no significance outside the local network. Back to TOC Addresses: Global & Local

60 60 Group Address: – Higher order bit is 1. – Allow multiple stations to listen to a single address. – All “1”s in the destination address: broadcasting. Ordinary Address: – Individual address. Addresses: Group & Ordinary Back to TOC

61 61 Minimum length of frame: 64 bytes from destination address to checksum. Reason for this minimum length requirement: – To distinguish between jam frames on the cable all the time. – To prevent a station from completing the transmission of a short frame before detection of possible collision with another frame far away. Minimum Length of Frame Back to TOC

62 62 The station sets the backoff factor to 0. The station monitors the medium, waiting for an ideal channel. If the channel is free, send the frame, otherwise, use 1- persistent strategy to aggressively monitor the channel. The channel is monitored during frame transmission. If a collision is detected, transmission stops immediately, and a special jam signal is generated to inform nearby stations of the collision. The station increases backoff factor and perform binary exponential backoff. After 15 (actually 10) times of retries, report failure. Back to TOC Medium Access: CSMA/CD

63 63 The binary exponential backoff dynamically adapts to the number of stations trying to send. The algorithm ensures a low delay when only a few stations collide; also ensures a reasonable interval when many stations collide. Truncating the backoff interval to be within certain value prevents the upper bound from exploding. The CSMS/CD that we have discussed so far does not provides Acks. For reliable transmission, Acks can be sent to the sender for receipt of good frames. It depends. Back to TOC Medium Access: CSMA/CD (cont)

64 64 Repeater  Bridge  Switch  Router  Gateway Repeater: Operates only on the physical layer. Receive signals before attenuation, amplify/regenerate the original bit pattern, and put back to the link. Back to TOC

65 65 Repeater  Bridge  Switch  Router  Gateway Bridge: Divides a large network into smaller segments and has the logic to keep the traffic for each segment separate. Bridges are repeaters that are smart enough to relay a frame only to the side of the segment containing the intended receipt They filter traffics and are useful for controlling congestion. Bridges don’t modify the structure of the frames and can only be used between segments that use the same protocol. In summary, bridges not only just amplify the input signals, but forward frames to segments to which the destination belongs. Back to TOC

66 66 Repeater  Bridge  Switch  Router  Gateway Switch: Traditionally a device to provide bridging function efficiently. Normally has a buffer for each link it is connected. When it receives a frame, it stores it in its buffer and check its address (sometimes CRC) to find the outgoing link. If the outgoing link is idle, sends the frame to the particular link. Back to TOC

67 67 Repeater  Bridge  Switch  Router  Gateway Router: Repeaters and bridges are simple network devices. Routers are more sophisticated. They have access to network layer addresses and certain software and thus be capable to determine which of several possible paths is the best for the transmission. Back to TOC

68 68 Repeater  Bridge  Switch  Router  Gateway Gateway: A gateway is a protocol converter. A gateway can accept a packet formatted for one protocol and convert it to a packet formatted for another protocol before forwarding it. A gateway is generally software installed within a router. Back to TOC

69 69 Hub & Switch (1) Hub: All the incoming lines are logically connected, forming a single collision domain. Only one particular station at a time can be transmitting. Hubs only allow half-duplex communication. Switch: Each incoming frame is buffered. Each frame is only forwarded to its specific destination. Switches allow full-duplex communication. Back to TOC

70 70 Hub & Switch (2) Back to TOC

71 71 Ethernet Evolution (1) Bridged Ethernet: The first evolution is the division of a LAN by bridges. Bridges have two functions: – Raising the bandwidth: Bandwidth-wise, each division/segment is independent. – Separating collision domains: The collision domain becomes smaller and the probability of collision is reduced. Back to TOC

72 72 Ethernet Evolution (2) Switched Ethernet: The idea of bridged Ethernet is extended to switched Ethernet. Instead of having multi-port bridge, why not N-port switch, where N is the # of stations in the LAN? In this way, bandwidth is shared only between the switch and each station. The collision domain is further divided. Back to TOC

73 73 Ethernet Evolution (3) Full-Duplex Ethernet: Each station can send and receive at the same time. No need for CSMA/CD: If a switched Ethernet is further full-duplex, there is no need to sense, there is no need for collision detection  the MAC sub- layer can be turned off. Back to TOC

74 74 Ethernet Evolution (4) Fast Ethernet (Operate at 100 Mbps): The idea is to keep the MAC sub-layer untouched. Because advantages of the 10Base-T is so overwhelming, fast Ethernet is based entirely on this design – Star Topology. Gigabit Ethernet (Operate at 1000 Mbps): The whole idea in the evolution of Ethernet was to keep the MAC sub-layer untouched. But, when it comes to the Gigabit rate, the designers feel that changes are needed. Back to TOC

75 75 Fast Ethernet (1) 100Base-T4: Four pairs of UTP in a physical star topology. – One pair is always to the hub/switch. – One pair is always from the hub/switch. – Two are switchable to the current transmission direction. Logical topology: – Using hub: Star for half-duplex mode (CSMA/CD is needed). – Using switch: Bus for full-duplex mode (CSMA/CD can be turned off). – Hub/Switch is allowed. Back to TOC

76 76 Fast Ethernet (2) 100Base-TX: Two pairs of twisted-pair cable (category 5 UPT) in a physical star topology. Logical topology: – Using hub: Star for half-duplex mode (CSMA/CD is needed). – Using switch: Bus for full-duplex mode (CSMA/CD can be turned off). – Hub/Switch is allowed. Back to TOC

77 77 Fast Ethernet (3) 100Base-FX: Two pairs of fiber-optic cable in a physical star topology. Logical topology: – Using switch: Bus for full-duplex mode (CSMA/CD can be turned off). – Only switch is allowed. Back to TOC

78 78 Gigabit Ethernet – The basic idea is similar to fast Ethernet. – Hub or Switch? Though it is foolish to keep a hub thus sacrificing the property of full-duplex, this is added for the purpose of back compatibility. – Fiber or Copper? Gigabit Ethernet implementations support fiber and copper, though copper is unlikely to be used, if not all. Back to TOC

79 79 Software Side So far, for the evolution of Ethernet from Classical  Fast  Gigabit Ethernets, we discussed the implementations in the hardware. Algorithms in the software are also evolving, but will be not discussed in this class. Back to TOC

80 80 Logic Link Control (LLC) We have seen a variety of Ethernet implementations and protocols. The logic link control sub-layer is added to make the data link layer to appear transparent to the layer above, thus different LANs can be connected together. Back to TOC

81 81 So far, we have: – discussed fundamental Medium Access Methods (MA, CSMA, CSMA/CD). – discussed Ethernet and how CSMS/CD is used in Ethernet. Now, let us see: – the MAC methods that are used in Wireless LAN. Wireless LAN Back to TOC

82 82 Wireless LAN Fact: Not all stations are within the range of each other. Problem: Hidden/Exposed State Problem (a) Hidden (b) Exposed Back to TOC

83 83 CSMA/CA: Handshaking (1) Handshaking when A wants to sent to B: (1) A  B: Request to Send (RTS) All stations hearing RTS should be quiet to ensure RTS can be correctly received by B (2) A  B: Clear to Send (CTS) All stations hearing CTS should be quiet to ensure CTS can be correctly received by A (3) A  B: Frame/Data All stations hearing RTS can transmit if they are not in the range of B. All stations hearing CTS should be quite since they are inside range of B. (4) A  B: ACK All stations inside range of A should be quiet to ensure this ACK can be correctly received by A. Back to TOC

84 84 CSMA/CA: Handshaking (2) The handshaking process listed in above seems to be complex. To make things easy, we can have : – All stations hearing either RTS or CTS should be quiet till the final ACK is correctly received by A. – By doing so, we might waste the bandwidth, since when A  B (for frame), stations inside range of A can transmit to another station, such as D, as long as D is not inside range of B. Back to TOC

85 85 Other Stations CSMA/CA: Handshaking (3) NAV: records the duration to wait till start sensing the channel again. The above duration is indicated in RTS and CTS (copied from RTS). Back to TOC

86 86 CSMA/CA: Via Base Station The above handshaking process is completed by the stations themselves. Another method is via base station: – The base station polls the stations to see if they want to send. – Since the transmission order is controlled by the base station, no collision will occur. Back to TOC

87 87 ►Network Layer Back to TOC Data Link: Moving frames from one end of the wire to the other. Network: Getting packets from the source all the way to the destination. – Design Issues. – Routing Algorithms. – Congestion Control. – Quality of Service. – IP Protocol.

88 88 Position of Network Layer Back to TOC

89 89 Functions of Network Layer Back to TOC Packetizing: To encapsulate packets received from the upper layer. Addressing: To uniquely identify a station in the world (IP address). Internetworking: To provide internetworking, logically gluing physical networks together to look like a single network to the upper layer. Routing: To choose one path when multiple choices exist. Fragmenting: Processed by routers when the datagram travels through different networks.

90 90 Services: Opinions of Two Camps One Camp, the Internet Camp: - Routers’ job is moving packets around and nothing else. - Thus, the Network layer services – Connectionless. - Example: Internet. - Internet begins to provide connection-oriented services. - Need routing for each packet. The Other Camp, the telephone companies: - Need to provide a reliable, connection-oriented service. - Example: ATM. - Need routing at the set-up stage (session routing). Back to TOC

91 91 Connectionless Service - Packets are injected into the subnet individually and routed independently. - No advance setup is needed. - Frames are called datagrams. The subnet is called datagram subnet. - Internal tables of the router can be dynamic. Back to TOC

92 92 Connection-oriented Service - Avoid choosing a new route for every packet by setting up a route in advance. - The connection is called virtual circuit (VC). The subnet is called VC subnet. - Each packet carries an identifier telling which virtual circuit it belongs to. - Routers have the ability to replace identifiers to avoid confliction. Back to TOC

93 93 Routing vs. Forwarding Routing: Be responsible for filling in and updating the routing table. Here, this is what we call the routing. Forwarding: Look up the routing table when a new packet arrives and forward it out. Back to TOC

94 94 Adaptive/Non-adaptive Routing Non-adaptive Routing (Static): – The routing table is computed in advance, off-line, and downloaded to the routers when the network is booted. – The routing table is not updated frequently. Adaptive Routing (Dynamic): – Routing decisions are changeable to reflect changes in topology and traffic. – Differs in where to get input information, routers in adjacent or all – local or global. Back to TOC

95 95 Routing Principle Routing Principle: If B is on the optimal path from router A to router C, then the optimal path from B to C also falls in the same path. How about an inverse question: Given A  C optimal, is C  A optimal? A BC Back to TOC

96 96 Sink Tree Sink Tree: From the routing principle, there is a tree illustrating the optimal path from all sources to the destination, rooted at the destination. (a) A subnet (b) sink tree rooted at B Back to TOC

97 97 Unicast Routing Algorithms 1.Shortest path routing 2.Flooding 3.Distance Vector Routing 4.Link State Routing 5.Hierarchical Routing Back to TOC

98 98 Shortest Path Routing Criteria: Hops, delay, physical distance, … Back to TOC

99 99 Flooding Flooding: – Every packet is sent out on every outgoing line except the one it comes in. – The disadvantages are so obvious ……, but still suitable to applications where …… Selective Flooding: The packet is sent out to outgoing lines that are approximately/roughly in the right direction. Back to TOC

100 100 Distance Vector Routing (1) Operation: Each router maintains a table containing the best known distance to each destination and the line to get there. Other Names: Bellman-Ford, Ford-Fulkerson, Routing Information Protocol (RIP). Dynamic, since: Every T seconds, each router sends to its neighbor its estimated cost to each destination. Shortcoming: – Slow in convergence. – Stability? Back to TOC

101 101 Distance Vector Routing (2) Operation in details: Each router periodically shares its knowledge about the entire internet with its neighbors. 1. Sharing knowledge about the entire network with neighbors: It sends out whatever it knows. 2. Sharing only with neighbors. 3. Sharing at regular intervals. 4. Old routing table is not used in the calculation. Thus, 1. Scope of knowledge to share: entire. 2. To whom to share the knowledge: only its neighbors. 3. When to share the knowledge: periodically. Back to TOC

102 102 Distance Vector Routing (3) Back to TOC

103 103 The count-to-infinity problem. Distance Vector Routing (4) Good news is distributed/propagated at a reasonable speed. Bad news propagates slowly. Cost of each link: Hop. Dot: denotes infinity. Info is distributed at the same time. Only consider router’s info about A. Back to TOC

104 104 Distance Vector Routing (5) Some remedies for the in-stability/slow convergence: Triggered Update: If there are no changes in the network, updates are sent regularly about 30 s. If there are changes in the network, updates are sent out immediately. Split Horizon: Be selective in sending out updates. If a router has received updates from an interface, then this updated info should not be sent back to this interface. Poison Reverse: It is a variation of the split horizon. Updated info is sent to all interfaces. But, the entry that comes from one interface is set to be 16 (denote infinity). Back to TOC

105 105 Link State Routing (1) RIP “is replaced” by Link State Routing because RIP: Does not take bandwidth into consideration. The algorithm takes too long to converge to the right solutions. Unstable? When this happens, a packet could go from one router to another in a dead loop. Back to TOC

106 106 Link State Routing (1) Main difference between RIP and Link State Routing Link State Routing (2) Link State Routing does the following: Scope of knowledge to share: – “Entire” knowledge about the net. – Whatever the router knows. To whom to share the knowledge: sharing with neighbors, then neighbors, …, and finally the whole nodes in the net. When to share the knowledge: when there is a “change” in the net. Back to TOC

107 107 Link State Routing (3) Five Steps: – Discover its neighbors, learn their network address. – Measure the delay or cost to each of its neighbors. – Construct a packet telling all it has just learned. – Send this packet to all other routers. – Compute the shortest path to every other router using the shortest-path algorithm discussed before. Back to TOC

108 108 Hierarchical Routing (1) As the network grows in size, as the # of routers increases, it is reasonable to do a hierarchical routing to reduce the entries inside a router’s table. When doing hierarchical routing, the best route is with respect to regions, instead of individual station. Back to TOC

109 109 Hierarchical Routing (2) Back to TOC

110 110 What is Congestion? Congestion in a network may occur if the load on the network is greater than the capacity of the network. Load – the number of packets sent to the network. Capacity – the number of packets a network can handle. Congestion happens in any system that involves waiting. Congestion happens on a freeway because any abnormality in the flow, such as an accident during the rush hour, creates blockage. Back to TOC

111 111 Reason of Congestion Congestion in a network or internetwork occurs because routers and switches have queues – buffers that hold the packets before and after processing. Back to TOC

112 112 Factors that May Cause Congestion Suddenly appeared packets requiring the same line: – Due to the finite buffer size of router, packets will get lost. – Increase the buffer size can only help to a “point”. – Due to Nagle’s study. Slow receiver will cause queue up. Low bandwidth. Back to TOC

113 113 Network Performance Congestion control involves two factors that measure the performance of a network: delay and throughput. Back to TOC

114 114 Congestion Control Congestion control refers to techniques and mechanisms that can either: - Prevention: prevent congestion before it happens. - Removal: remove congestion after it happened. Congestion Control Open-Loop Closed-Loop Back to TOC

115 115 To overcome congestion Reduce the load Increase the resources Congestion in the Network Layer (IP) Congestion in the Transport Layer (TCP) Congestion Control Back to TOC

116 116 Congestion Control: Open-Loop (1) Retransmission Policy: – A good retransmission policy can prevent congestion. – The retransmission policy and the retransmission timers should be designed to optimize efficiency and at the same time prevent congestion. Window Policy: – The type of window at the sender can also affect congestion. – The Selective Repeat is better than Go-Back-N for congestion control. Back to TOC

117 117 Congestion Control: Open-Loop (2) ACK Policy: – The ACK policy at the receivers may also affect congestion. – If the receiver does not ACK every packet it receivers, it may help prevent congestion (piggybacking). Discarding Policy: – Good discarding policy by routers may prevent congestion. – In audio transmission, if the policy is to discard less sensitive packets when congestion is likely to occur, the quality of sound is still preserved and congestion is prevented. Admission Policy: – A quality of service (QoS) mechanism. – Routers or switches in a flow first check the resource requirement before admitting it to the network. Back to TOC

118 118 Congestion Control: Closed-Loop (1) Chock Packet: A chock packet is the packet sent by routers to the sender to inform the congestion. Hop-by-Hop Chock Packet: - If a router is congested, it informs the previous upstream router to reduce the rate of outgoing packets. - Recursive all the way to the sender. Back to TOC

119 119 Congestion Control: Closed-Loop (2) Explicit Signaling: - The routers that experience congestion can send an explicit signal, such as by setting one bit in a packet, to inform the sender or the receiver of the congestion. - Forward direction: to warn to receivers.. - Backward direction: to warn the sources. Implicit Signaling: - The sender can detect an implicit signal of congestion and slow down its sending rate. - Ex: the mere delay in receiving an ACK can be a signal that the network might be congested. Back to TOC

120 120 Note: TCP assumes that the cause of a lost segment is due to congestion in the network. Back to TOC

121 121 Note: If the cause of the lost segment is congestion, retransmission of the segment does not remove the cause—it aggravates it. Back to TOC

122 122 What is Quality of Service? QoS is an internetworking issue that has been discussed more than defined. We can informally define quality of service (QoS) as something a flow seeks to attain. Back to TOC

123 123 Flow Characteristics (1) Four types of characteristics are attributed to a flow: Reliability, Delay, Jitter, and Bandwidth. Back to TOC

124 124 Flow Characteristics (2) Reliability: Lack of reliability means losing a packet or ACK, which will cause retransmission. The sensitivity of applications to reliability is not the same: – More important for Emails. – Less important for video/audio transmissions. Delay: Applications can tolerate delays in different degrees: – File transfer/Emails: delays are less important. – Video conferencing/remote log-in: delays are important. Back to TOC

125 125 Flow Characteristics (3) Jitter: Jitter is the variation in delay for packets belonging to same flow. Applications can tolerate jitters in different degrees: – File transfer/Emails: jitters are less important. – Video conferencing: jitters are important. Bandwidth: Applications have different requirements on bandwidth. – Video conferencing: need to send millions of bits per second. – Emails: total number of bits might not exceed a million. Back to TOC

126 126 Resource Reservation Resource Reservation: - A flow of data needs resources such as a buffer, bandwidth, CPU time, and … - The quality of service can be improved if these resources can be reserved beforehand. - If we have a specific route for a flow, the above reservation can be possible. Back to TOC

127 127 Admission Control Admission Control: – Ad mission control refers to a mechanism used by a router, or a switch, to accept or reject a flow based on predefined parameters, called flow specifications. – Before a router accepts a flow for processing, it checks its specifications to see if the router’s capacity (buffer, CPU time) and its previous commitment to other flows can handle this new flow. Back to TOC

128 128 Scheduling Scheduling: To treat packets from different interfaces in a fair and appropriate manner. – FIFO Queuing. – Priority Queuing. – Weighted Fair Queuing. Back to TOC

129 129 Scheduling (1) FIFO Queuing: - First-in first-out. - If the average arrival rate is higher than the processing rate, the queue will fill up and new packets will be discarded. Back to TOC

130 130 Scheduling (2) Priority Queuing: - Packets are assigned to a priority class. - Each priority class has its own queue. - Packets in higher-priority queue are processed first. - Packets in lower-priority queue are processed last. - The system does not stop serving a queue until it is empty. Back to TOC

131 131 Scheduling (3) Weighted Fair Queuing: - Packets are still assigned to priority classes. - Packets of different priority are assigned to different queues. - Packets in different queues are processed based on corresponding weights. Back to TOC

132 132 Traffic Shaping Traffic Shaping: Traffic shaping is a mechanism to control the amount and the rate of the traffic sent to the network. – Leaky Bucket. – Token Bucket. Back to TOC

133 133 Traffic Shaping: Leaky Bucket Leaky Bucket: – If a bucket has a hole, the water leaks from the bucket at a constant rate independent on the rate at which the water is input to the bucket as long as the bucket is not empty. – Idea: the input rate can vary, the output rate remains constant. – Thus, leaky bucket can smooth traffics. Back to TOC

134 134 Traffic Shaping: Leaky Bucket The leaky bucket shapes burst traffic into fixed-rate traffic by averaging the data rate. Leaky Bucket’s Restriction: If a host is not sending for a while, its bucket becomes empty. Now, if the host has burst data, the leaky bucket algorithm allows only an average rate. The time that the host is idle is not taken into account. Back to TOC

135 135 Traffic Shaping: Token Bucket Token Bucket: Allows idle hosts to accumulate credits for a future use in the form of tokens. – Tokens are assigned to the bucket at a constant rate. – Packets need tokens to be processed/transmitted. – The host can consume all the tokens at once or slowly. – The host can send burst data as long as the bucket is not empty. Differences from Leaky Bucket: – Allowing varying output rate. Token bucket allows burst outgoing traffic. Back to TOC

136 136 Traffic Shaping: Token Bucket Token Bucket Illustration: Back to TOC

137 137 IP Protocol The IP Protocol is designed with the internetworking in mind. The glue of the whole Internet is the Network Layer Protocol: IP Protocol. Back to TOC

138 138 Top Principles in Designs Keep it simple: When in doubt, use the simplest solution. That is, fight features: if a feature is not absolutely necessary, leave it out. Make clear choices: If there are several ways to do one job, use one. Look for good design: Though it might not handle some weird situations, leave that burden to those users. Be strict when sending but tolerant when receiving… Back to TOC

139 139 The IPv4 (Internet Protocol) header. IP Header (v4) Back to TOC

140 140 IP Header (v4) Version: To keep track of which version the datagram belongs to. IHL: To tell how long the header is. Type of Service: Originally designed to distinguish between different classes of services. But usually ignored. Total Length: Include everything in the datagram, header + data. Identification: To let the receiver know which segment a newly arrived datagram belongs to. DF (Don’t Fragment): In case the destination can not reassembly pieces back together. MF (More Fragments): All fragments except the last one have this bit set. Fragment Offset: To indicate the current position of the datagram in the segment. Time to Live: A counter to limit packet lifetimes  avoid dead loop. Protocol: To indicate the Transport Protocol, such as TCP, UDP. Header Checksum: Checksum of the header. Back to TOC

141 141 IP Addresses Function: To uniquely identify each device on the Internet to allow global communication between all devices. Analog to our daily live: The telephone system, where each telephone subscriber has a unique telephone number, given that the country code and the area code are part of the identification scheme. What is an IP Address? An IP address, in the current version of protocol, is a 32-bit binary address that uniquely and universally defines the connection of a host or a router to the Internet. An IP address is a 32-bit binary address. Back to TOC

142 142 IP Addresses Uniquely: IP addresses are unique in the sense that each address defines one, and only one, connection to the Internet. Two devices on the Internet can never has the same address at the same time. If a device has two connections to the Internet via two networks, it has two IP addresses. Universally: IP addresses are universal in the sense that the addressing system must be accepted by any host wants to be connected to the Internet. Back to TOC

143 143 IP Address Notations IP Address Notations: There are two common notations to represent an IP address: Binary notation: 01110101 10010101 00011101 11101010 Dotted-decimal notation: Back to TOC

144 144 We are running out of IP addresses! Short-Term Solutions: 1. Classless Addressing  CIDR (Classless InterDomain Routing): – To allocate the remaining IP addresses in variable-sized blocks, without regard to the classes. – Ex: if someone needs 2000 addresses, it will be given 2048. – Size of routing table is more likely to be increased (thus need hierarchy routing). 2. NAT (Network Address Translation): Each stations inside a domain has unique address for internal traffic/communication. Outgoing packets experience an address translation – to use single or several assigned global IP address. Long-Term Solution: Use IPv6. Back to TOC

145 145 TCP/IP Protocol Suite Back to TOC

146 146 ICMP: Motivation Motivation: The IP protocol is a best-effort service that delivers a datagram from its source to destination. However, it has two deficiencies: – Lack of Error Control. – Lack of Assistant Mechanism. Solution: – ICMP (Internet Control Message Protocol). – ICMP is a companion to IP Protocol. Back to TOC

147 147 ICMP: Message Type Error-Reporting: To report problems that a router or a host (destination) may encounter when it processes an IP packet. Query: – Occur in pairs. – To help a host or a network manager to get a specific information from a router or another host. Back to TOC

148 148 ARP: Background Info Delivery of packets need two levels of addressing: logical and physical. We need to be able to map a logical address to its corresponding physical address, and vice visa. This can be done statically or dynamically. ARP: Address Resolution Protocol. RARP: Reverse Address Resolution Protocol. Back to TOC

149 149 Static & Dynamic Static: A table is created and maintained that contains logical addresses and their corresponding physical ones. – This table is stored in each station in the network. – Disadvantages: Physical address may change. Dynamic: The machine can find out the required physical/logical address using a protocol. Back to TOC

150 150 ARP Idea: The sender asks the receiver to announce its physical address, if needed. Operation: (1) Each time a host wants to find the physical address of a receiver, it sends an ARP query packet. (2) The ARP query packet contains the source physical + IP addresses, and the destination IP address. (3) The ARP query packet is sent via broadcast. (4) Only the receiver replies its physical address via unicast. Back to TOC

151 151 ARP: Query and Reply Back to TOC

152 152 Proxy ARP Proxy ARP: – A proxy ARP is an ARP that acts on behalf of a set of hosts. – Whenever a router running a proxy APR receives an ARP request regarding one of these hosts, it will send an ARP reply announcing its own physical address (the router’s). Back to TOC

153 153 RARP RARP: RARP finds the logical address for a machine only knowing its physical address. Situations: Diskless machines. Machines booted the first time. Back to TOC

154 154 BOOT & DHCP Each host attached to a TCP/IP internet must know: (1) Its IP address. (2) Its subnet mask. (3) The IP address of a router. (4) The IP address of a name server (DNS: to discuss later). The above info is usually stored in a configuration file and accessed by the computer during the boot-up process. BOOT and DHCP are designed to provide the listed information. DHCP is more popularly implemented. Back to TOC

155 155 DHCP DHCP stands for Dynamic Host Configuration Protocol. DHCP is an extension to BOOTP. It enhances BOOTP and is backward compatible with BOOTP. DHCP has two databases: – The 1 st database statically binds physical with logical addresses – same as the database of a BOOTP server. – The 2 nd database has a pool of available IP addresses – makes DHCP dynamic! Back to TOC

156 156 DHCP Components Two Components: – A protocol for delivering host-specific configuration parameters from a server to a host – A mechanism for allocation of network addresses to hosts. DHCP supports three mechanisms for IP address allocation. – Automatic allocation -- in which a permanent IP address is assigned to the client. – Dynamic allocation -- in which the address is assigned for a limited period of time (a "lease"). – Manual allocation -- in which the address is assigned manually by the network administrator. Back to TOC

157 157 DHCP Terminologies DHCP Server: Host that provides and manages the configuration parameters for many “clients” hosts using UDP Transport (port 67). DHCP Client: Host that requests configuration parameters from a DHCP Server, also known as a DHCP Daemon (DHCPD). It also uses the UDP transport (port 68). BOOTP Relay Agent: A host or router that passes DHCP messages between DHCP clients and DHCP servers. Binding: A binding is a collection of configuration parameters, including at least an IP address, associated with or "bound to" a DHCP client. Back to TOC

158 158 DHCP State Diagram Back to TOC

159 159 ►Transportation Layer Back to TOC Node-to-Note Delivery: The data link layer is responsible for delivery of frames between two nodes over a link. Host-to-Host Delivery: The network layer is responsible for delivery of packets between two hosts (might not be on the same network). Process-to-Process Delivery: The transport layer is responsible for delivery of datagrams/segments, parts of messages, between two processes (application programs) on two hosts.

160 160 Node, Host, and Process Deliveries Node-to-Note Delivery: The data link layer is responsible for delivery of frames between two nodes over a link. Host-to-Host Delivery: The network layer is responsible for delivery of packets between two hosts (might not be on the same network). Process-to-Process Delivery: The transport layer is responsible for delivery of datagrams/segments, parts of messages, between two processes (application programs) on two hosts. Back to TOC

161 161 Process-to-Process Delivery Process-to-Process Delivery Illustration: Back to TOC

162 162 Client-Server Paradigm (1) Although there are several ways to achieve process to process communication, the most common one is through client- server paradigm. (2) A process on the local host, called client, needs service from a process on a remote machine, called server. (3) Both processes have the same name: HTTP client, HTTP server. Back to TOC

163 163 Addressing Physical Layer Addressing (node-to-note): Physical address  MAC address in Ethernet (48-bit). Network Layer Addressing (host-to-host): Logical address  IP address (32-bit in IPv4). Transport Layer Addressing (process-to-process): logical address  Port number (16-bit). (1) Client’s port number can be randomly chosen by its transport protocol. (2) Server’s port number is decided to be universal. Back to TOC

164 164 IP + Port Number IP address selects the specific host. Port number selects the specific process running on the host. (1) Client’s port number can be randomly chosen by its transport protocol. (2) Server’s port number is decided to be universal for easy communication: well-known port numbers. Back to TOC

165 165 Socket Address Fact: Process-to-process communication needs two identifiers: IP address + Port Number. Socket Address: The combination of IP address and Port Number is called socket address. The client socket address defines the client process uniquely. The server socket address defines the server process uniquely. IP header provides IP address. TCP/UDP header provides port number. Back to TOC

166 166 Connection-Oriented/Connectionless Service Connectionless Service: (1) The packet is sent from one party to another with no need for connection establishment or connection release. (2) The packets are not numbered, may be delayed, lost, or arrive out of order. There is no ACK.  UDP (unreliable and connectionless) Connection-Oriented Service: (1) Connection is first established. (2) Data is transmitted. (3) Connection is released.  TCP (reliable and connection-oriented) Back to TOC

167 167 UDP: Datagram Format UDP (User Datagram Protocol): UDP is a connectionless, unreliable protocol that has no error control and no flow control (simple and faster). UDP packets are called user datagrams. User Datagram Format: The calculation of checksum and its inclusion in the user datagram is optional. Back to TOC

168 168 UDP: Applications UDP is a convenient transport-layer protocol for applications that provide flow and error control. It is also used by multimedia applications. Back to TOC

169 169 TCP TCP (Transition Control Protocol): TCP is a connection-oriented, reliable transport protocol (complex and slower). Transmission is full duplex. Transmission unit is called segment. Connection is established and released for communication Has error control and flow control. Has timer and Acks. Numbering is regarding “byte”. …… Back to TOC

170 170 PortProtocolDescription 7EchoEchoes a received datagram back to the sender 9DiscardDiscards any datagram that is received 13DaytimeReturns the date and the time 20FTP, DataFile Transfer Protocol (data connection) 21FTP, ControlFile Transfer Protocol (control connection) 23TELNETTerminal Network 25SMTPSimple Mail Transfer Protocol 53DNSDomain Name Server 67BOOTPBootstrap Protocol 80HTTPHypertext Transfer Protocol TCP: Port Numbers Back to TOC

171 171 Basic Idea of Flow Control: Similar to the mechanisms in Data Link Layer One Serious Problem – Silly Window Syndrome: In sliding window operation, when either the sending process creates data slowly, or the receiving process consumes data slowly, we are not using the capacity of the network efficiently. This problem is called silly window syndrome. Proposed Solutions: From the side of the sending process: From the side of the receiving process: TCP: Flow Control Back to TOC

172 172 Syndrome Created by the Sender: The sending TCP may create a silly window syndrome if it is serving an application program that creates data slowly, for example, byte by byte. The solution is to prevent the sending TCP from sending the data byte by byte. Q: How long should the sending TCP wait? If wait long, it may delay the process. If wait shortly, it may end up with small segments. A: Nagle’s Algorithm. TCP: Silly Window Syndrome Back to TOC

173 173 Nagle’s Algorithm: for the sending TCP. 1. The sending TCP sends the first piece of data it receives from the sending application even if it is 1 byte. 2. After sending the first segment, the sending TCP accumulates data until either it receives an ACK or enough data has been accumulated to fill a max-size segment. At this time, the sending TCP can send. 3. The above Step-2 is repeated… TCP: Nagle’s Algorithm Back to TOC

174 174 TCP: Nagle’s Algorithm The Elegance of Nagle’s Algorithm: Lies in its simplicity and the fact that it takes into account the speed of the application program that creates data and the speed of the network. If the application is faster than the network, segments will be transmitted with max-size. If the application is slower than the network, smaller segments (not max-size) will be transmitted. Back to TOC

175 175 Syndrome Created by the Receiver: The receiving TCP may create a silly window syndrome if it is serving an application program that consumes data slowly, for example, byte by byte. One solution is to delay Acks: Acks are not sent immediately. Delayed Acks: Can reduce traffic. However, it is possible that it may force the sender to retransmit the un-Acked segments. The protocol balances the advantage and disadvantage and specifies that the delayed Acks should not exceed 500ms. TCP: Silly Window Syndrome Back to TOC

176 176 TCP: Congestion Control TCP assumes that the cause of a lost segment is due to the congestion in the network. If the cause of the lost segment is congestion, retransmission of the segment does not remove the cause—it aggravates it. Back to TOC

177 177 TCP: Congestion Control In TCP, the sending window is not only determined by the receiver, but also by the congestion in the network. Three Phases of Congestion Control in TCP: Slow Start, Additive Increase, and Multiplicative Decrease. Back to TOC

178 178 TCP: Congestion Control (1) Slow Start ( Van Jacobson’s Algorithm): At the beginning of a connection, TCP sets the congestion window size to be the max segment size. For each segment Acked, TCP increases the congestion window size by one max segment size, until it reaches a threshold of one-half of the allowable window size. The above is called slow start, though the actual operation is now slow, but exponential. Back to TOC

179 179 TCP: Congestion Control (2) Additive Increase: To avoid congestion before it happens, we must slow down this exponential process. After the congestion window size reaches the threshold, the size is increased one max segment for each Ack even if an Ack is for several segments. The additive-increase strategy continues until as long as Acks arrives before timer times out, and the congestion window size does not reach the receiver window value. Back to TOC

180 180 TCP: Congestion Control (3) Multiplicative Decrease: If congestion occurs, the congestion window size must be decreased. If the sending TCP does receive an ACK before times out, it assumes that there is a congestion. Then, the threshold must be set to one-half of the last congestion window size, and the congestion window size should start from 1 again. In other words, the sender returns to the “slow start” phase. Back to TOC

181 181 ►Application Layer Protocols Back to TOC DNS (Domain Name System): To identify an entity, TCP/IP protocols use the IP address. However, people prefer to use names instead of IP address. DNS provides a mapping between an IP address and a name. DNS can use the services of UDP or TCP, using the well- known port 53. DNS uses UDP as a transport protocol, for the most part for normal resource records lookup. The exceptions are zone transfers and very large responses, which use TCP.

182 182 Protocols Back to TOC Telnet/Rlogin (Terminal Network/Remote Login): A TELNET connection is a Transmission Control Protocol (TCP) connection used to transmit data. It uses the well-known port 23. FTP (File Transfer Protocol): A standard mechanism provided by TCP/IP for copying a file from one host to another. FTP uses two well-known TCP ports: Port 21 is used for the control connection, and port 20 is used for the data connection.

183 183 Protocols TFTP (Trivial FTP): TFTP uses the services of UDP on the well-known port 69. SMTP (Simple Mail Transfer Protocol): The TCP/IP protocol that support electronic mail on the Internet. Uses the TCP well-known port 25. SNMP (Simple Network Management Protocol): A framework for managing devices in an internet. An application-level protocol in which a few manager stations control a set of agents. Uses the services of UDP on two well-known ports: 161 and 162. Back to TOC

184 184 Protocols HTTP (Hypertext Transfer Protocol): A protocol used mainly to access data on the World Wide Web. Uses the services of TCP on well-known port 80. WWW (World Wide Web): A repository of information spread all over the world and linked together. When accessing the WWW, DNS is first queried for the IP address (UDP). Then, data is transferred using TCP. Back to TOC

185 185 Protocols RTP (Real-time Transport Protocol): A protocol to handle real-time traffic on the Internet. RTP stands between UDP and the application program. RTP does not have a delivery mechanism and it must be used with UDP. RTP is treated as an application-level program and is encapsulated into a UDP datagram. RTP uses a temporary UDP port. RTCP (Real-time Transport Control Protocol): Work with RTP for real-time traffic transmission. Like RTP, uses a temporary UDP port. Back to TOC

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