Basic Networking Concepts - GMU

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© 2002 D. Menascé. All Rights Reserved.

Basic Networking Concepts

Dr. Daniel A. MenascéDept. of Computer ScienceGeorge Mason University

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Copyright Notice

These slides are copyrighted by Dr. Daniel A. Menascé and cannot be copied, in part or in their entirety, into any medium without the written authorization of their author.

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Learning Objectives

• Packet Switched Networks.• Protocols.• The ISO Open System Architecture• TCP/IP Main Features.

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A

B

host computer

router

computer network

high-speed link

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3

2

5

4

6

A packet switched network.

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P1 P4

M E S S A G E

four packets

P2 P3

A message broken down into four packets. Packets have a maximum size.

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P1 P4

M E S S A G E

four packets

P2 P3AB

AB

AB

AB

packet header

A message broken down into four packets. Each packet has a header, which includes source (A) and destination (B).

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Host A Host Brouter 1 router 2 router 4 router 6

234

1

234

1

234

1

234

1

234

1

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Hello, Alice speaking.

Hi Alice, this is Bob.

Ring Ring Ring

Bye Alice. Nice talking to you.

Bye Bob. Nice talking to you too.

Bob Alice

Connectiontermination

phase

Hi Bob, bla bla bla … [PAUSE]

Bla bla bla …?

Protocol Used in a Phone Conversation.

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Payload

Fields of theheader.

Protocol Data Unit (PDU)

Trailer

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At the sending computer: data is generated at the application layer, which adds the application layer

header (H) before passing the PDU down.

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Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application Layer

1 0 1 0

H FrameTrailer01100011100101101100011 Frame

Packet Data Packet

Transport Layer Data

H

H

H Session Layer Data

H Presentation Layer Data

H Application Data

ISO Open System Interconnection Reference Model

Segment

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At the receiving computer: PDUs are processedand their payload is passed one level up until

the application layer is reached.

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Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application Layer

1 0 1 0

H FrameTrailer01100011100101101100011 Frame

Packet Data Packet

Transport Layer Data

H

H

H Session Layer Data

H Presentation Layer Data

H Application Data

ISO Open System Interconnection Reference Model

Segment

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ISO OSI Model• Physical layer: electrical signal levels.• Data link layer: point to point (e.g., between

two routers, between host and router). Error detection, error recovery, and flow control at the data link layer. Example: HDLC.

• Network level: host-to-host and includes routers. Main functions: routing and addressing. No end-to-end error recovery: best-effort. Example: IP.

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ISO OSI Model (cont’d)• Transport level: End-to-end process to process.

– Connection-oriented: End to end error detection and recovery, flow control, and congestion control. Example: Transport Control Protocol (TCP).

– Connectionless: no end to end functionality except process addressing. Best effort. Example: Universal Datagram Protocol (UDP).

• Session level: End-to-end session establishment. Example: Secure Sockets Layer (SSL). Note: the encryption and compression part of SSL fits best into the Presentation level.

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ISO OSI Model (cont’d)• Presentation level: in charge of data

transformations (e.g., encryption, compression, code conversion).

• Application level: what is left.

The levels above the transport level are not necessarily organized according to the OSI reference model.

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Window mechanism

• Many protocols use a sliding window mechanism as part of flow and error recovery control.

• Examples:– Transmission Control Protocol (TCP)– High-level Data Link Protocols (HDLC)

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7 8 9 10 11 12 13 14 15 17 18 19 20 216 16

sender receiver

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Window

PDU 7 sentbut notACKed

Window mechanism

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7 8 9 10 11 12 13 14 15 17 18 19 20 216 16

sender receiver

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Window

PDUs 7 & 8 sent but not

ACKed

Window mechanism

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7 8 9 10 11 12 13 14 15 17 18 19 20 216 16

sender receiver

789

Window

PDUs 7, 8 & 9 sent but not

ACKed

Window mechanism

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7 8 9 10 11 12 13 14 15 17 18 19 20 216 16

sender receiver

78910

Window

PDUs 7-10 sent but not

ACKed

Window mechanism

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7 8 9 10 11 12 13 14 15 17 18 19 20 216 16

sender receiver

78910

ACK 7,8

Window

PDUs 7 & 8 are ACKed. The window

slides.

Window mechanism

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TCP Basics

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TCP/IP

IPIP

TCP

HTTP

IP

TCP

HTTP

network 1 network 2

Host A

Router

Host BGET http://www.server.com/doc.html

file doc.html

SYN, FIN, DATA, ACK

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TCP Connection Establishment

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

application data

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Application datais segmented

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

threepackets

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP HeaderTCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

application data

TCP Segmentation

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

application data

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Application datais segmented

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

threepackets

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

Middle packetwas dropped

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Middle packetwas dropped:TCP requestsretransmission

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP Header

Packet Header

TCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

TCP HeaderTCP Retransmission

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IP

TCP

APP

IP

IP

IP

TCP

APP

Network

Host A Host B

Router Router

application data

TCP Retransmission

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TCP: Window Mechanism

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TCP: Window size vs time (in RTTs)

WindowSize(segments)

Time (in RTTs)

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TCP Throughput and Window Size

• TCP’s slow start mechanism: since TCP does not know the RTT nor the effective bandwidth of a TCP connection, it starts with a small window size (low throughput) and adjusts the window size according to the rateACKs are received.

• Short-lived TCP connections tend to operate well below the maximum throughput!

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TPC Window Size Variation

• During slow start: window size doubles every RTT.

• During congestion avoidance: window size increases by one each RTT.

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TCP Throughput

• Depends on:– Packet Loss Ratio– Round Trip Time– Wm: Maximum Receiver Window Size (advertised by

the receiver at connection establishment time)– TCP timeout– Network Bandwith– Maximum Segment Size

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TCP Throughput

RTT=0.04 sec; Bandwidth = 12,500 KB/secTimeout = 2 sec;Maximum Segment Size = 1,460 bytes

Basic Networking Concepts - GMU

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