computer networks

1.2 network hardware

• taxonomy of computer networks
  • transmission technology
    • broadcast links
      • A wireless network
    • broadcasting: every machine
    • multicasting: subset of the machines
    • point-to-point links, sometimes called unicasting
  • scale
    • PANs (Personal Area Networks)
      • Bluetooth
        • The master tells the slaves
          • what addresses to use,
          • when they can broadcast,
          • how long they can transmit,
          • what frequencies they can use,and so on
    • LAN (Local Area Network)
      • a standard for wireless LANs called IEEE 802.11, popularly known as WiFi
      • IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN.
    • MAN (Metropolitan Area Network)
    • WAN (Wide Area Network)
    • Internetworks
      • A collection of interconnected networks is called an internetwork or internet.
      • worldwide Internet (is one specific internet)

1.3.1 Protocol Hierarchies

• each layer passes data and control information to the layer immediately below it, until the lowest layer is reached.
• It is common that different hosts use different implementations of the same protocol
• the lower layers of a protocol hierarchy are frequently implemented in hardware or firmware

1.3.2 Design Issues for the Layers

Reliability

• bits are damaged
  • errors:
    • fluke electrical noise,
    • random wireless signals,
    • hardware flaws,
    • software bugs and so on
  • solution: adding redundant information, used at low layers, to protect packets sent over individual links,and high layers, to check that the right contents were received
    • error detection
      • Information that is incorrectly received can then be retransmitted, until it is received correctly
    • error correction
      • where the correct message is recovered from the possibly incorrect bits that were originally received
• finding a working path through a network
  • may be some links or routers that are broken
  • routing: The network should automatically make this decision

the evolution of the network

• protocol layering
  • internetworking, different network technologies often have different limitations .ex:
    • number messages
      • not all communication channels preserve the order of messages sent on them
    • differences in the maximum size of a message that the networks can transmit
      • disassembling, transmitting, and then reassembling messages
  • traffic jams

resource allocation

• statistical multiplexing
  • share the bandwith dynamically, rather than by giving each host a fixed fraction
  • sharing based on the statistics of demand
• how to keep a fast sender from swamping a slow receiver with data
  • flow control: feedback from the receiver
• congestion: network is oversubscribed, overloading
  • too many computers want to send too much traffic
  • the network cannot deliver it all
• resource
  • bandwidth
  • timeliness
    • live video
    • real-time

security

• eavesdropping[ secretly listening to the private conversation]
  • solution: provide confidentiality
    • authentication: prevent someone from impersonating someone else
    • integrity: prevent surreptitious changes to messages

1.3.3 Connection-Oriented Versus Connectionless Service

Connection-oriented

• Connection-oriented service is modeled after the telephone system.
  • establish
  • use
  • release
• service is modeled after the telephone system.
• A circuit is another name for a connection with associated resources
  • fixed bandwidth
  • in some case negotiation parameters
    • maximum message size
    • quality of service required

    • one side make proposal, the other accept

      reject

      make a counterproposal

  • acts like a tube:
    • sender pushes at one end , receiver takes them at the other end
    • in most case the order is preserved: bits arrive in the order they were sent

Connectionless Service

• connectionless service is modeled after the postal system
  • each msg carries the full destination address
  • a packet is a message at the network layer
  • store-and-forward switching：
    • nodes receive a message in full before sending it to the next
  • cut-through switching:
    • onward transmission of a message at a node starts before it is completely received
  • order is not consistant because of delayed

reliability

both Connection-Oriented or Connectionless Service can further be characterized by its reliability

• implemented by having the receiver acknowledge the receipt of each msg
• Reliable connection-oriented service has two minor variations:
  • message sequences
    • message boundaries are preserved
  • byte streams
    • no message boundaries
• Unreliable (meaning not acknowledged) connectionless service is often called datagram service
  • does not return an acknowledgement to the sender
  • dominant form in most networks
    • reliable communication (in our sense, that is,acknowledged) may not be available in a given layer
      • Ethernet does not provide reliable communication
      • many reliable services are built on top of an unreliable datagram service
      • the delays inherent in providing a reliable service may be unacceptable, especially in real-time applications such as multimedia

1.3.4 Service Primitives

A service is formally specified by a set of primitives (operations) available to user processes to access the service

• tell the service to perform some action or report on an action taken by a peer entity
• system calls
  • cause a trap to kernel mode
  • send the necessary packets

primitives for connection-oriented service

steps

six packets are required to complete the protocol

• server executes LISTEN to indicate that it is prepared to accept incoming connections.
  • system call
  • server process blocked until a request for connection appears
• send packets
  • 1.client executes CONNECT to establish a connection with the server
    • sends a packet to the peer asking it to connect
    • parameter of server’s address
    • client suspended until a response
  • 2.server can then establish the connection with the ACCEPT call
    • send a response to accept
    • arrival of response release the client
      • client && server now both running and have a connection established
• server to execute RECEIVE to prepare to accept the first request
  • block the server
  • when
    • normally immediately being released from the LISTEN
    • before the acknowledgement can get back to the client
• send packets
  • 3.client executes SEND to transmit its request
    • client: followed by execution of RECEIVE to get the reply
    • server: arrivial of request unblock the server to handle the request
  • 4.server: use SEND to return the answer
    • client: arrivial of the packet unblock the cient
  • additional requsts can go on(if have)
• send packets
  • 5.client: when done, executes DISCONNECT to terminate the connection
    • an initial DISCONNECT is a blocking call, suspending the client
    • sending a packet to the server saying that the connection is no longer needed
  • 6.server: gets the packet
    • issues a DISCONNECT of its own
    • acknowledging the client and releasing the connection
  • client receive the packet
    • client process is released
    • connection is broken

issues

• timing can be wrong (e.g., the CONNECT is done before the LISTEN),
• packets can get lost
• why not connectionless:
  • in perfect world only two packets would be needed
    • request
    • reply
  • but
    • large messages in either direction (e.g., a megabyte file),
    • transmission errors,
    • lost packets

problems of connectionless

• if packets lost, how would the client know if some pieces were missing?
• How would the client know whether the last packet actually received was really the last packet sent?
• How could it tell packet 1 from the second file from a lost packet 1 from the first file that suddenly found its way to the client?

1.3.5 services vs protocols

image

• service
  • like api
  • is a set of primitives that a layer provides to the layer above it
  • relates to an interface between two layers
    • lower layer -> service provider
    • upper layer -> service user
• protocol
  • like impl
  • relate to the packtets
  • is a set of rules relate to packets
    • governing the format and meaning of
      • the packets,
      • or messages that are exchanged by the peer entities within a layer.
• Many older protocols did not distinguish the service from the protocol
  • a typical layer might have had a service primitive SEND PACKET with the user providing a pointer to a fully assembled packet.
  • all changes to the protocol were immediately visible to the users —– violate the rule/ a serious blunder

1.4 OSI Model && TCP/IP Model

• osi
  • protocols not used any more
  • model itself quite general && valid
  • features at each layer are still very important
• TCP/IP
  • the model itself is not of much use
  • the protocols are widely used.

1.4.1 OSI

• developed by the International Standards Organization (ISO)
• ISO OSI (Open Systems Interconnection) Reference Model
  • means open for communication with other systems
• OSI model itself is not a network architecture
  • it does not specify the exact services and protocols to be used in each layer
    • just tell what each layers should do
  • produced standards for all layers
    • these are not part of refernce model itself

model(in part) widely used, protocols are long forgotten

1.4.1 OSI model layers

image

physical layer

• concerned with transmitting raw bits
• Typical questions
  • what electrical signals should be used to represent a 1 and a 0,
  • how many nanoseconds a bit lasts,
  • whether transmission may proceed simultaneously in both directions,
  • how the initial connection is established,
  • how it is torn down when both sides are finished,
  • how many pins the network connector has, and what each pin is used for.

data link layer

• transform a raw transmission facility into a line that appears free of undetected transmission errors
  • by masking the real errors so the network layer does not see them
  • break up the input data into data frames
    • transmit the frames sequentially
    • receiver confirms each frame by sending back an acknowledgement frame.
• issues
  • how to keep a fast transmitter from drowning a slow receiver in data.
  • some traffic regulation mechanism want to konw when the receiver can accept more data
  • Broadcast networks : how to control access to the shared channel.
    • A special sublayer added: medium access control sublayer

the network layer

• controls the operations of the subnet
  • determining how packets are routed from source to destination.
  • handling congestion
  • the quality of service provided (delay, transit time, jitter, etc.
  • allow heterogeneous networks to be interconnected
    • different network may be different
      • in addressing
      • packet might be too large
      • protocol may differ

the transport layer

• basic function
  • accept data from above it,
  • split it up into smaller units if need be,
  • pass these to the network layer,
  • and ensure that the pieces all arrive correctly at the other end.
• determines what type of service to provide to the session layer, and, ultimately, to the users of the network–determined when connection is established
  • most popular: error-free point-to-point channel
    • delivers messages or bytes in the order in which they were sent.
  • others
    • transporting of isolated messages with no guarantee about the order of delivery
    • the broadcasting of messages to multiple destinations.
• true end-to-end layer
  • carries data from source to destination
    • message headers
    • control message
  • lower layers not end-2-end
    • each protocols is between a machine and its immediate neighbors,
    • not between the ultimate source and destination machines,
      • separated by many routers

the session layer

• allows users on different machines to establish sessions between them.
  • dialog control
    • (keeping track of whose turn it is to transmit),
  • token management
    • (preventing two parties from attempting the same critical operation simultaneously)
  • synchronization
    • (checkpointing long transmissions to allow them to pick up from where they left off in the event of a crash and subsequent recovery).

presentation layer

• concerned with the syntax and semantics of the information transmitted.

Application Layer

• ex. http

1.4.2 The TCP/IP Reference Model

image

history

• ARPANET
  • using leased telephone lines
  • connect hundreds of universities and government
  • trouble with interworking with satellite and radio networks
  • a new reference architecture was needed –> tcp/ip
    • goals
      • connect multiple networks in a seamless way
      • be able to survive loss of subnet hardware(Soviet Union attack)
      • others:
        • transferring files
        • real-time speech transmission,

The Link Layer

• describes what links such as serial lines and classic Ethernet must do to meet the needs of this connectionless internet layer.

The Internet Layer

• linchpin that holds the whole architecture together
• deliver IP packets where they are supposed to go
  • IP (Internet Protocol): official packet format and protoco
  • ICMP (Internet Control Message Protocol)
• issues
  • Packet routing
  • congestion

the transport layer

• allow peer entities on the source and destination hosts to carry on a conversation
  • end-to-end transport protocols
    • TCP(Transmission Control Protocol)
      • reliable connection-oriented protocol
      • handles flow control to make sure a fast sender cannot swamp a slow receiver
    • UDP (User Datagram Protocol)
      • unreliable, connectionless protocol
      • for applications that do not want TCP’s sequencing or flow control and wish to provide their own
      • widely used
        • one-shot, client-server-type request-reply queries
        • applications in which prompt delivery is more important than accurate delivery
          • such as transmitting speech or video.

The Application Layer

• virtual terminal (TELNET),
• file transfer (FTP),
• and electronic mail (SMTP)
• Domain Name System (DNS)
• HTTP, the protocol for fetching pages on the World Wide Web,
• RTP, the protocol for delivering real-time media such as voice or movies

1.4.4 OSI vs TCP/IP [compare in reference modules not the corresponding protocol stacks]

• layers:
  • osi: 4 layers
  • tcp/ip: 7 layers
• both: layers up through and including the transport layer provide an end-to-end, network-independent transport service to processes wishing to communicate
• OSI devised before the corresponding protocols
• tcp/ip:protocols come first,
  • model was really just a description of the existing protocols
  • no problems with protocols fitting the model, but model did not fit any other protocol stacks
    • it was not especially useful for describing other, non-TCP/IP networks.

Three concepts are central to the OSI model:

• 1. Services.[methods , operations]
     • tells what the layer does,
     • not how entities above it access it or how the layer works.
     • It defines the layer’s semantics.
• 1. Interfaces.[methods’ parameter && result]
     • tells the processes above it how to access it
     • specific parameters
     • not or how the layer works.
• 1. Protocols.[code internal]
     • the layer’s own business.
     • can use any protocols to finish its work

The TCP/IP model did not originally clearly distinguish between services, interfaces, and protocols

• ex. the only real services offered by the internet layer are SEND IP PACKET and RECEIVE IP PACKET
• the protocols in the OSI model compare that in the TCP/IP model
  • better hidden
  • replaced relatively easily as the technology changes

connectionless vs connection-oriented

• osi
  • support both in network layer
  • only connection-oriented in transport layer
• tcp/ip
  • connectionless in network layer
  • both in transport layer
    • giving the user a choice–important for simple request-response protocols

1.4.5 critique of osi

bad timing

• TCP/IP protocols were already in widespread use by research universities by the time the OSI protocols appeared
• When OSI came around, they did not want to support a second protocol stack until they were forced to

bad technology

• seven layers was more political than technical, and two of the layers
  • (session and presentation) are nearly empty,
  • (data link and network) are overfull.
• extraordinarily complex
  • difficult to implement and inefficient in operation
• functions, such as addressing, flow control, and error control, reappear again and again in each layer.
  • unnecessary and inefficient.

bad implementations

• implementations were huge, unwieldy, and slow.
• In contrast, one of the first implementations of TCP/IP was part of Berkeley UNIX and was quite good && free

bad politics

• TCP/IP was thought as part of UNIX
• osi supported by the government but not work

1.4.6 critique of tcp/ip

• not clearly distinguish the concepts of services, interfaces, and protocols.
  • not much of a guide for designing new networks using new technologies
• the TCP/IP model is not at all general and is poorly suited to describing any protocol stack other than TCP/IP.
  • ex: completely impossible to use the TCP/IP model to describe Bluetooth
• the link layer is not a layer
  • it is an interface between the network and data link layer
• does not distinguish between the physical and data link layers.
  • physical layer:
    • copper wire, fiber optics, and wireless communication
  • data link layer:
    • delimit the start and end of frames and get them from one side to the other with the desired degree of reliability.
• some protocols are ad hoc,
  • ex. TELNET–The virtual terminal protocol
    • was designed for a ten-character-per-second mechanical Teletype terminal.
    • knows nothing of graphical user interfaces and mice
    • but still in used some 30 years later

Ad hoc是拉丁文常用短语中的一个短语。这个短语的意思是“特设的、特定目的的（地）、即席的、临时的、将就的、专案的”。这个短语通常用来形容一些特殊的、不能用于其它方面的的，为一个特定的问题、任务而专门设定的解决方案

1.5.1 the internet

not really a network at all

• a vast collection of different networks
  • use certain common protocols
  • provide certain common services

The ARPANET

• 1950 DoD wants a command-and-control network that could survive a nuclear war.
  • telephone network
• 1960s, Baran proposed using digital packet-switching technology. AT&T dismissed
• 1957, Soviet Union launch of the first artificial satellite, Sputnik–beat US
  • President Eisenhower–ARPA, the Advanced Research Projects Agency.
    • 1967, ARPA – ARPANET (minicomputers + IMPs[Interface Message Processors])
      • first electronic storeand-forward packet-switching network
    • NPL system – National Physical Laboratory in England
      • demonstrated that packet switching could be made to work
• an experimental network went online
  • 1969 with four nodes: at UCLA, UCSB, SRI, and the University of Utah.
    • all had a large number of ARPA contracts
    • all had different and completely incompatible host computers (just to make it more fun).
  • ARPA also funded research on the use of satellite networks and mobile packet radio networks
  • demonstrated that the existing ARPANET protocols were not suitable for running over different networks
    • 1974–TCP/IP was specifically designed to handle communication over internetworks
• ARPA awarded several contracts to implement TCP/IP on different computer platforms,
  • IBM, DEC, and HP systems, as well as for Berkeley UNIX.
  • Berkeley UNIX–rewrote TCP/IP with a new programming interface called sockets for the upcoming 4.2BSD release of Berkeley UNIX
• 1980s, additional networks, especially LANs, connected to the ARPANET
  • DNS(Domain Name System) was created

NSFNET

• 1970s, NSF (the U.S. National Science Foundation) saw enormous impact of ARPANET
  • fund the Computer Science Network (CSNET) in 1981,
    • have to have a research contract with the DoD to get on ARPANET, many have no contract
    • connect to ARPANET via dial-up and leased lines
    • backbone network to connect supercomputer centers
      • Each supercomputer given a microcomputer called a fuzzball
        • fuzzball connected with 56kb lines to form subnet
          • same hardware technology the ARPANET used
        • fuzzball use TCP/IP right from the start—first TCP/IP WAN
  • NSFNET: the complete network
    • funded some regional networks connected to the backbone
  • NSF encouraged MERIT, MCI, and IBM to form a nonprofit corporation, ANS (Advanced Networks and Services)
    • ANS took over NSFNET and upgraded to form ANSNET
    • 5 years later sold to America Online
  • NSF awarded contracts to four different network operators to establish a NAP (Network Access Point)
    • To ease the transition && make sure regional networks could communicate with each other
• 1990s, many other countries and regions also built national research networks,
  • often patterned on the ARPANET and NSFNET

1.5.1 Architecture of the Internet

• ISP: Internet Service Provider
• DSL: Digital Subscriber Line
  • DSL modem
    • modem is short for ‘modulator demodulator’ and refers to any device that converts between digital bits and analog signals.
  • DSLAM (Digital Subscriber Line Access Multiplexer)
• DSL + telephone line : dial-up
  • limited to 56 kbps
• DSL + cable : much greater
• FTTH (Fiber to the Home)
  • optical fiber, 光纤
• POP (Point of Presence)
  • the ISP’s POP: locaiton at which customer packets enter the ISP network for service
• TV system
  • cable modem
  • CMTS (Cable Modem Termination System)
• ISP’s POP (Point of Presence).
• backbone
  • long-distance transmission lines that interconnect routers at POPs in the different cities that the ISPs serve
• IXPs (Internet eXchange Points)
  • ISPs connect their networks to exchange traffic at IXPs
  • Basically, an IXP is a room full of routers,
    • at least one per ISP
    • A LAN in the room connects all the routers
  • a small ISP might pay a larger ISP for Internet connectivity
  • two large ISPs might decide to exchange traffic
• packet’s path
  • depends on the peering choices of the ISPs
    • with destination ISP :directly to its peer
    • or else route the packet to the nearest paid transit provider
• tier 1 ISPs
  • form the backbone of the Internet
  • thousands of routers connected by high-bandwidth fiber optic links
  • AT&T, Sprint…
• data centers
  • Google, Yahoo…
  • server farm: rack upon rack of machines
  • Colocation/hosting (机器托管)
    • short, fast connections can be made between the servers and the ISP backbones

1.5.2 Mobile Phone Networks

generation

AMPS : Advanced Mobile Phone System , 1G

1
2
- transmitted voice calls as continuously varying(Analog) signals
- each voice call on a specific frequency band

GSM : Global System for Mobile communications

1
2
3
4
5
- transmitting voice calls in digital form #### UMTS (Universal Mobile Telecommunications System) || WCDMA (Wideband Code Division Multiple Access)
- offer both digital voice and broadband digital data services
- each cell to use all frequencies,
- a tolerable level of interference to the neighboring cells.
- based on Code Division Multiple Access (CDMA)

radio access network

1
2
3
- cellular base station
- controller node or RNC (Radio Network Controller)
    - controls how the spectrum is used.

core network: carries the traffic for the radio access network

• UMTS core network evolved from 2G GSM system’s core network
  • connectionless subnets–packet networks
    • if too many packets, router will choke and lose packets
      • sender will resend but service will be jerky
  • connection-oriented subnets–circuit networks
    • set up establishes a route
      • all words or packets follow the same route
      • if a line or switch on the path goes down, call is aborted
    • support quality of service more easily
      • subnet can reserve resources(link bandwidth, switch buffer space, CPU)
      • if resource is insufficient, call will be rejected
      • once a connection is connected, connection would get good service
• old mobile phone networks–connectionless
  • MSC (Mobile Switching Center), GMSC (Gateway Mobile Switching Center)
  • MGW (Media Gateway) elements that set up connections over a circuit-switched core network such as the PSTN (Public Switched Telephone Network).
• GPRS(General Packet Radio Service)
  • ran at tens of kbps
• UMTS core network nodes connect directly to a packet-switched network
  • SGSN (Serving GPRS Support Node) and the GGSN (Gateway GPRS Support Node)
  • voice calls over a packet data network

issues

• handover
  • soft handover
    • connect to the new base station before disconnecting from the old base station
  • hard handover
• how to find a mobile in the first place when there is an incoming call
  • Each mobile phone network has a HSS (Home Subscriber Server)
    • the location of each subscriber,
    • profile information that is used for authentication and authorization
• security
  • SIM card, short for Subscriber Identity Module
    • authenticate subscribers
    • cryptographic keys on the SIM card are used to encrypt transmissions

1.5.3 wifi, 802.11

• operate in unlicensed bands such as the ISM (Industrial, Scientific, and Medical) bands
  • defined by ITU-R (e.g., 902-928 MHz, 2.4-2.5 GHz, 5.725-5.825 GHz).
• two style
  • Wired network with APs(access points)
  • ad hoc network: clients talk directly
• multipath fading
  • radio signals reflected off solid objects
    • multiple echoes of a transmission
      • cancel
      • reinforce
  • solutions:
    • path diersity: sending along multiple, independent paths
    • using different frequencies
    • repeating bits over different periods of time
• multiple transmissions collide
  • CSMA (Carrier Sense Multiple Access)
    • Computers wait for a short random interval before transmitting, and defer their transmissions if they hear that someone else is already transmitting
• security
  • WEP (Wired Equivalent Privacy)
  • WiFi Protected Access, initially called WPA but now replaced by WPA2

1.5.4 RFID and Sensor Networks

Radio Frequency IDentification (RFID)

• consists of a small microchip with a unique identifier and an antenna that receives radio transmissions.

battery

• passive RFID
  • all of the energy needed to operate them is supplied in the form of radio waves by RFID readers.
• active RFID
  • there is a power source on the tag

distance

• UHF RFID (Ultra-High Frequency RFID).
  • shipping pallets and some drivers licenses
  • backscatter：Tags communicate at distances of several meters by changing the way they reflect the reader signals
• HF RFID (High Frequency RFID)
  • passport, credit cards, books, and noncontact payment systems
  • based on induction
• LFRFID (Low Frequency RFID)
  • for animal tracking

problem

• multiple tags
  • tags wait for a short random interval before responding with their identification
• security
  • weak measures: like passwords

sensor network

• Sensor nodes are small computers
• self-organize to relay messages for each other

1.7 METRIC UNITS

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3.1 data link layer

uses the services of the physical layer to send and receive bits over communication channels

• Providing a well-defined service interface to the network layer.
• Dealing with transmission errors.
• Regulating the flow of data

3.1.1 services provided to the natwork layer

Unacknowledged connectionless service.

having the source machine send independent frames to the destination machine without having the destination machine acknowledge them

• ex. Ethernet,
• no logical connection
• if a frame is lost, no attempt to detect the loss or recover
• appropriate when the error rate is very low, appropriate for real-time traffic, such as voice

Acknowledged connectionless service.

• no logical connections
• each frame sent is individually acknowledged
• is useful over unreliable channels, such as wireless systems. ex. 802.11 (WiFi)

Acknowledged connection-oriented service

providing acknowledgements in the data link layer is just an optimization, never a requirement

• can send packets and wait for ack, resend when expired
  • inefficient,
• network layer does not know hardware parameters, and send a big packet(ex.10 frames, 2 lost):
  • links have a strict maximum frame length imposed by hardware
  • known propagation delays.
• On reliable channels, such as fiber, the overhead of a heavyweight data link protocol may be unnecessary,
• but on (inherently unreliable) wireless channels it is well worth the cost

connection-oriented

• Each frame sent over the connection is numbered,
• guarantees that each frame sent is indeed received.
• guarantees that each frame is received exactly once and that all frames are received in the right order.
• appropriate over long, unreliable links such as a satellite channel or a long-distance telephone circuit.
  • if connectionless, cause a frame to be sent and received several times, wasting bandwidth.

3.1.2 framing

• channel is noisy
• Breaking up the bit stream into frames
  • Byte count.
    • 无法解决乱序问题
  • Flag bytes with byte stuffing.
    • two consecutive flag bytes indicate the end of one frame and the start of the next
    • insert special escape byte(ESC) to avoid interfering
    • flaw: tied to the use of 8-bit bytes
  • Flag bits with bit stuffing.
    • framing done at the bit level
    • Each frame begins and ends with a special bit pattern, 01111110 or 0x7E in hexadecimal.
      • to avoid conflict in the body
        • sender: stuffs 0, when encounters five consecutive 1s
        • receiver: automatically destuffs the 0 bits when sees
  • Physical layer coding violations.

A common pattern used for Ethernet and 802.11 is to have a frame begin with a well-defined pattern called a preamble.

3.1.3 Error Control

ensure that each frame is ultimately passed to the network layer at the destination exactly once, no more and no less

• provide the sender with some feedback
  • positive ack: arrived
  • negative ack: something go wrong
• if a frame vanish?
  • introducing timers
    • expire after an interval
• if either the frame or the ack is lost?
  • just transmit the frame again
• if sender transmit the same frame multiple times, receiver accept the same frame two or more time
  • assign sequence numbers to outgoing frames

3.1.4 flow control

a sender that systematically wants to transmit frames faster than the receiver can accept them.

issues

• sender: a fast, powerful computer and the receiver: a slow, low-end machine.
  • phone request a web page from powerful server

commonly used approaches

• feedback-based flow control
  • receivers send back information to the sender to send more data
  • or at least telling the sender how the receiver is doing
• rate-based flow control
  • the protocol has a built-in mechanism that limits the rate at which senders may transmit data,
  • without using feedback from the receiver.

feedback vs rate

• Feedback-based schemes are seen at both the link layer and higher layers
  • more common:
    • the link layer hardware is designed to run fast enough that it does not cause loss.
    • hardware implementations of the link layer as NICs (Network Interface Cards) can handle frames as fast as they can arrive on the link
      • so handled by higher layers
  • various feedback-based flow control schemes: the same basic principle
    • well-defined rules about when a sender may transmit the next frame.
    • These rules often prohibit frames from being sent until the receiver has granted permission,
• rate-based schemes are only seen as part of the transport layer

3.2 ERROR DETECTION AND CORRECTION

strategies dealing with errors

• error-correcting codes

  FEC (Forward Error Correction).

  • include enough redundant information to enable the receiver to deduce what the transmitted data must have been.
  • used On channels that are highly reliable, such as fiber
    • it is cheaper to use an error-detecting code and just retransmit the occasional block found to be faulty
  • seen in the physical layer, particularly for noisy channels
  • in higher layers, particularly for real-time media and content distribution
• error-detecting codes
  • include only enough redundancy to allow the receiver to deduce that an error has occurred (but not which error)
    • have it request a retransmission.
  • FEC is used on noisy channels because retransmissions are just as likely to be in error as the first transmission
    • wireless links: make many errors
  • commonly used in link, network, and transport layers

Neither error-correcting codes nor error-detecting codes can handle all possible errors

3.2.1 Error-correcting codes–todo

1. Hamming codes.
2. Binary convolutional codes.
3. Reed-Solomon codes.
4. Low-Density Parity Check codes.

3.2.2 Error-Detecting Codes–todo

1. Parity.
2. Checksums.
3. Cyclic Redundancy Checks (CRCs).

3.4 sliding window

The network layer, in contrast, is always fed data in the proper order, regardless of the data link layer’s window size

• each outbound frame contains a sequence number,ranging from 0 up to some maximum.
• sender maintains a set of sequence numbers corresponding to frames it is permitted to send.
  • These frames are said to fall within the sending window
• receiver also maintains a receiving window corresponding to the set of frames it is permitted to accept.

sequence number in sender

• The sequence numbers within the sender’s window represent
  • frames that have been sent
  • or can be sent but are as yet not acknowledged.
• maintains a list of unacknowledged frames
  • when a new packet arrives
    • it is given the next highest sequence number,
    • the upper edge of the window is advanced by one

sequence number in receiver

• frame receive,
  • if (frame.sq == window.lowerEdge)
    • it is passed to network layer
    • window is rotated by one
  • any frame falling outside the window is discarded
• ack
  • a window size of 1 means that the data link layer only accepts frames in order,
    • but for larger windows this is not so

3.4.1 Sliding Window Protocol

go-back-n

is for the receiver simply to discard all subsequent frames, sending no acknowledgements for the discarded frames.

selective repeat

a bad frame that is received is discarded, but any good frames received after it are accepted and buffered.

flow control

error control

4 the medium access control sublayer

5.1.2 Services Provided to the Transport Layer

provides services to the transport layer

rules:

• The services should be independent of the router technology
• transport layer should be shielded from the number, type, and topology of the routers present
• network addresses should use a uniform numbering plan, even across LANs and WANs

two camps

• connectionoriented service
  • routers’job is moving packets around and nothing else
  • host: error control(error detection and correction) && flow control
• connectionless service
  • represented by telephone companies
  • ATM vs IP
  • connection-oriented technologies
    • MPLS (MultiProtocol Label Switching)
    • VLANs

5.2.1 the optimality principle

6.1.1 Services Provided to the Upper Layers

different kinds similar with network service

• connection-oriented
• connectionless,

why need transport layer

make the transport service to be more reliable

• The transport code runs entirely on the users’ machines, but the network layer mostly runs on the routers
  • routers operated by the carrier
  • users have no real control over network layer
    • cannot solve the problem of poor service by using better routers
    • cannot putting more error handling in the data link layer
  • solution: put on top of the network layer another layer–transport layer
    • packets are lost or mangled in connectionless
      • transport layer detect the problem and compensate for it by retransmit
    • in connnection-oriented network, connection is abrutly terminated
      • set up new network connection

hiding the network service behind a set of transport service primitives

• transport primitives can be implemented as call to library procedures
  • independent of the network primimitives
• application programmers can write code according to a standard set of primitives
  • without having to worry about different network interfaces and reliability

transport layer–key position

• the bottom four layers–transport service provider
• the upper layers –transport service user

6.1.2

6.1.3 Berkeley Sockets

what

• A common transport layer interface
• another set of transport primitives,
• Sockets the de facto standard for abstracting transport services to applications.
  • often used with the TCP protocol to provide a connection-oriented service called a reliable byte stream
    • other protocols work as well
  • used with a connectionless transport service

parameters of the call specify

• the addressing format to be used,
• the type of service desired (e.g., reliable byte stream),
• and the protocol

The socket primitives for TCP

• SOCKET Create a new communication endpoint
• BIND Associate a local address with a socket
• LISTEN Announce willingness to accept connections; give queue size
• ACCEPT Passively establish an incoming connection
• CONNECT Actively attempt to establish a connection
• SEND Send some data over the connection
• RECEIVE Receive some data from the connection
• CLOSE Release the connection

6.2 transport protocol vs data link protocol

major difference: operating environment

data link layer – communicate directly via a physical channel(wired

wireless)

transport layer – communicate via the entire network

addressing

• data link layer:
  • not necessary for a router to specify which router it wants to talk to
• transport layer:
  • addressing of destinations is required

establishing a connection

• data link layer : simple
  • the other end is always there
• transport layer: complicated

the potential existence of storage capacity in the network

• data link layer:
  • a packet may arrive or be lost
• transport layer:
  • delay and duplicate packets
    • take scenic route
    • arrive late
    • out of expected order

degree: buffering and flow control

• data link layer:
  • a large and varying number of connections with bandwith that fluctuates
  • not buffers to each line
• transport layer:
  • buffers to each line

6.2.2 connection establishment

network can lose, delay, corrupt, duplicate packets

issues e.g.

• delayed in the network and pop out much later, when the sender thought that they had been lost

prevent packets from being duplicated and delayed

• …
  • throwaway transport addresses
    • more difficult to connect with a process in the first place
  • give each connection a unique identifier
    • requires each transport entity to maitain a certain amount of history infomation indefinitely
• kill off aged packets that hobbling about
  • restricted network design – prevents packets from looping
  • hop counter
  • timestamping

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