operating systems and computer networks...faculty of engineering institute of computer engineering...

Faculty of Engineering

Institute of Computer Engineering

Prof. Dr.-Ing. Axel Hunger

Operating Systems and

Computer Networks Exercises

Pascal A. Klein & Alexander Maxeinerti.uni-due.de


Computer NetworksTCP/IP







Linux System Structure

Corbet et al, Linux Device Drivers, O‘Reilly , 2005







Process Control

– load, execute, end, abort, create process (fork),

terminate process,…

File management

– create file, delete file, open, close, read, write,...

Device Management

– request device, release device, read, write,…

Information Maintenance

– get/set time or date, get/set system data,…

Communication

– create, delete communication connection, send,

receive messages, transfer status information,…

Categories of System Calls







Network Fundamentals







Types of transmission techniques

Broadcast LinksMulticastingWLANBus-/Ring-System

Point to Point LinksUnicastingLink between Pairs of Computers







Connection oriented vs. connectionless oriented

Connection orientedTelephoneByte stream

Connectionless orientedPostal systemDatagram







Unacknowledged vs. Acknowledged Connection

Unacknowledged connectionless service.

– No attempt to recover lost frame

Acknowledged connectionless service.

– Sender receives an acknowledgement for each and every successfully received frame

Acknowledged connection-oriented service.

– A connection is established prior to sending frames

– Sender receives an acknowledgement for each and every successfully received frame

– Sequenced delivery of frames







Example

Service Primitives:-Listen-Connect-Receive-Send-Disconnect

Connection oriented Service







Data Transmission in OSI

ApplicationLayer

PresentationLayer

Session Layer

Transport Layer

Network Layer

Data Link Layer

PhysicalLayer

ApplicationLayer

PresentationLayer

Session Layer

Transport Layer

Network Layer

Data Link Layer

PhysicalLayer

BitsDH DT

DataNH

TH DataNetwork

protocol

DataSHTransport

protocol

PH DataSession

protocol

AH DataPresentation

protocol

DataApplication

protocol







TCP/IP Model vs OSI Model

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer1

2

3

4

5

6

7 Application Layer

Transport Layer

Internet Layer

Network Layer

OSI TCP/IP







The End-to-End Principle

Ford, A., Raiciu, C., Handley, M. & Bonaventure, O., 2013. RFC 6824: TCP Extensions for Multipath Operation with Multiple

Addresses, Internet Engineering Task Force.







Leon-Garcia, A. & Widjaja, I., 2004. Communiation Networks – Fundamental Concepts and Key Architectures, 2. Ausg., McGraw-Hill.







Transport Control Protocol - TCP







Tasks and functions of TCP/IP

Logical addressing: IPv4- (32 Bit) / IPv6-Adressing (128 Bit)

Routing: Next lecture

Error and Flow control: ACK-/NAK-Transmissions

Application support: Software abstraction

Name Resolution: DNS (Domain Name System), works as a translator for Internet Domains to IP-addresses.

TCP IP Overview







Webserver Listening

Client Server







Client sends Request

SYN

Client Server







Server sends Acknowledgment

SYN/ACK

Client Server







Client sends Acknowledgment

ACK

Client Server







Connection established

Client Server







Server reads Request

Client Server

Data 01








Client Server

Data 02 Data 01








Data 03

Client Server

Data 02 Data 01







Server send Acknowledgments

Client Server

Data 03 Data 02

ACK 01








Client Server

Data 03

ACK 02ACK 01








Client ServerACK 03ACK 02ACK 01







Lost Packet

Data 03

Client Server

Data 01Data 02








Client ServerACK 03ACK 01







TCP State Machine: Open and Closing







Most important Transport Layer Protocol on theInternet (developed 1974 by Cerf&Kahn)

Reliable and Connection oriented

Connection between two endpoints

Connection endpoint 5-Tupel<local IP address, local port number, protocol no, remote IP address, remote port number>

Byte stream bidirectional data communication

Connection in TCP







Sending a file

ApplicationLayer

File

Divide in Segements

Socket

Transport Layer

Send tonext Layer

Network Layer







In-order delivery via sequence numbers

Automatic Repeat Request (ARQ) via Acknowledgements of each received packet (ACK)

Checksums for reliable data delivery(Sum of 16-bit words in Ones-complement)

Flow Control to not flood receiver

Congestion Control to not flood the network

Reliability of TCP







Synchronization of Data between Sender & Receiver

Payload (Byte) increasingly numbered (use first Databyte)

e.g.: DS0: 0, DS1: 1059, DS2: 2118, ......

32-Bit-Sequence Number

Receiver is able to :

reassemble Data in right order

identify gaps or duplicates

TCP Sequencing

Data 03 Data 02 Data 01







Three Way Handshake

:Host A :Host B







Sequencing

:Host A :Host BSEQ=0

SEQ=1059

SEQ=2118







Duplicate ACK & Resending

:Host A :Host B

Tim

er

SEQ=1059

SEQ=0

SEQ=1059

SEQ=2118







Sending of a Datasegment Start Timer

Timeout of Timer: Datasegment loss Resending of Datasegment

Timeout Calculation:

Timeout = SRTT + 4 RTTVAR

where SRTT = α SRTT + (1 - α) RTT

RTTVAR = β RTTVAR + (1 – β) |SRTT - RTT|

α = 7/8, β = 3/4

Timeout








Client ServerACK 03ACK 01

Timer runs at Client site if no ACK – resending data

Client must buffer packets until ACKed

If packet is lost (or coming late) Server must buffer all incoming packets until problem is solved







Flow Control

– Sender only sends as much data the receiver can take

– Packet size depends on receiver’s buffer/window(MSS – Maximum Segment Size)

MSS calculated with Maximum Transmission Unit (MTU)

– Value between 64 .. 1460 Byte

Calculation of Receiver Window:

Receiv_wnd = 2 BDP

where BDP = Bandwidth One-Way-Delay

Current Window Size sent by each ACK-Segment

Sliding Window







TCP Frame Structure







Data Transmission – if possible: Maximum Utilization

Prevention of Network Congestion by

– Limiting data flow in the network

– Adjusting the sending buffer/window

Option: Measuring of Segment losses

The more segments error free, the less congestion

Size of Congestion Window correspond to number of possibleBytes the Network can admit

Congestion Control







Four Algorithms:

1. Slow Start

2. Congestion Avoidance (Additive Increase)

3. Fast Retransmit

4. Fast Recovery

Slow Start-Algorithm: Datasegments sent slowly

Finding out available capacity of network

Not to flood network

Slow Start Threshold = MAX (Flight Size/2 , 2 MSS)

where Flight Size = Size of all unacknowledged Segments onthe Network

Congestion Control







Congestion Window ≥ Threshold Congestion AvoidanceAlgorithmus

Congestion Avoidance Algorithm: Congestion Windowincreased linearly with one Segment size per RTT

if Packet loss: Slow Start starts again

Congestion Control







Algorithms Fast Retransmit & Fast Recovery:

decrease Error Detection time

Sender assumes at a timeout that subsequent

packages are lost as well and retransmits them.

Duplicate ACK

ACK with same Sequence Number

Sequence number indicates the package the

receiver is waiting for

Duplicate ACK indicates loss of frames during

transmissions

Congestion Control I







If 3 Dup-ACKs then:

Multiple frames lost during transmission

Slow Start Threshold = Congestion Window / 2

Congestion Window = Slow Start Threshold + 3 MSS

Congestion Avoidance Algorithmus continues...

Congestion Control II







Tanenbaum, A.S. & Wetherall, D.J., 2012. Computernetzwerke, 5. Ausg., München, BRD, Pearson Deutschland GmbH.







Sameer Seth, M. Ajaykumar Venkatesulu, TCP/IP Architecture, Design andImplementation in Linux, IEEE Computer Society, 2008

Andrew S. Tanenbaum, Computernetworks, 5th Edition, Pearson, 2012

Cerf, V., & Kahn, R. (5 1974). A Protocol for Packet Network Intercommunication. IEEE Transactions on Communications, 22(5), S. 637-648.

Postel, J. (09/1981). RFC 793, Transmission Control Protocol. Internet Engineering Task Force.

Jacobson, V. (1988). Congestion Avoidance and Control. SIGCOMM '88 Symposium proceedings on Communications architectures and protocols Aug 16-18, 18(4), S. 314–329.

Fall, K., & Stevens, W. (2012). TCP/IP Illustrated, Volume 1, The Protocols (2 Ausg.). Michigan, USA: Pearson Education Inc.

Wireshark, Protocol Analyzer, [Online], 2017, https://www.wireshark.org/.

References

operating systems and computer networks...faculty of engineering institute of computer engineering...

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