network short notes

7/28/2019 Network Short Notes

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ISO OSI and TCP Model

Layer Description

Physical

This deals with the physical aspect of a network,

i.e. electrical or light signals sent between local

devices. Includes Encoding and Signalling,

physical data transfer, hardware specifications,

topology and design.

Data Link

Deals with low-level data messages between

local devices. Includes logical link control, data

framing, error detection and handling.

Network

Deals with messages between local or remote

devices. Includes addressing, routing, datagram

encapsulation and fragmentation and

reassembly.

Transport

Deals with the communication between software

processes. Includes process-level addressing,

multiplexing / de-multiplexing, segmentation and

re-assembly, ACK and flow control.

Session

Deals with sessions between local and remote

devices. Includes session establishment,

management and termination.

Presentation

Deals with application data representation

including data transmission, compression and

encryption.

ApplicationDeals with application data associated with user

application such as email programs.

Figure 1: TCP vs ISO OSI


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Shannon-Hartley Law

The ShannonHartley theorem tells the maximum rate at which information can be transmitted over a

communications channel of a specified bandwidth in the presence of noise.

= log (1 +)

Automatic Repeat Requests

Stop and Wait

Sender waits for an ACK for each send frame Frames may be damaged or lost, same for ACK No NACKs for lost or error frame Sender uses a timer and resends frame A 1-bit frame number (0 or 1) is needed for data and ACK frames. This value alternates when

data is ACKed (new data sent), but remains the same when data is resent.

Go Back N

Receiver has no buffer It can only accept (and send) a correct frame with the next expected sequence number.

o Sends an ACK for that sequence number, which implies that all previous frames havearrived.

Otherwise it discards the frame and:o Either sends nothing back, oro Sends an ACK for the last accepted frameo Sends a NACK for the next frame

Receiver time to send ACK or NACK in separate frame.Selective Repeat

Receiver buffers frameso Can store out-of-order correct frames

Only 1 frame has to be resent in case of error or loss NACKs are usually used for error or lost frames Sender needs timer of each send frame Receiver timer to send ACK or NACK in separate frame.

Trade-Offs between Go-Back-N and Selective Repeat

Since Go-Back-N only has a buffer on the sender side, if a frame is lost or corrupted, the frameshave to be resent starting from the last ACKed frame.

Since Selective Repeat has a buffer on both the sending and receiving side, only the lost orcorrupted frame needs to be resent.


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Media Access Control (MAC)

CSMA/CD

CSMA/CD is a media access control method. It is a modification of the pure CSMA where a transmission

is terminated if a collision is detected (CD).

Figure 2: CSMA Flow Chart

1. The station that wants to transmit first listens to check if another transmission is in progress.2. If medium is in use, station waits, else it transmits3. Transmitter waits for ACK, if none, retransmits4. If Station transmits and no collisions occur during the time the leading edge of frame takes to

propagate to the farthest station, then no collisions.

5. CD: If collision is detected, terminate transmission.Collisions can still occur when 2 or more stations begin transmitting within a short time of each other.

CSMA/CD is not required with GigabitEthernet since the normal mode of operation for Gigabit Ethernet

is full-duplex, which allows traffic in both directions at the same time. In this configuration, all lines are

buffered, so each computer and switch is free to send frames whenever it wants to. On the computer

and a switch, the computer is the only possible sender on the line to the switch. Since no contention is

possible, the CSMA/CD protocol is not used.

Figure 3 Why collision detection takes 2


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IPV4

Figure 4 IPV4 Header

Field Description

IHL Defines how long the header is, in 32 bit words.

Type of ServiceContains a 3-bit precedence field used for thepriority and 3 flags: delay, throughput and

reliability, to specify what is most important in

the packet

Time to Live (TTL)

The TTL field is a counter to limit packet lifetimes.

It must decrement at each hop and the packet is

discarded when it reaches 0.

Protocol

This field tells the receiving host which transport

process (TCP / UDP / etc) the packet should be

given to.

Header ChecksumThis is used to verify the header only, useful for

detecting errors.

The fields used for fragmentation are the flag and fragmentation offset. Flag is a 3-bit field and is used

to control or identify fragments. The fragment offset is the offset of a particular fragment to the

beginning of the original IP datagram.


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TCP/IP

TCP provides a reliable byte stream over an unreliable internetwork. TCP accepts user data streams

from local processes, breaks them up into pieces not exceeding 64k and sends each piece as a separate

IP datagram. The receiver side gives IP datagrams containing TCP data to its TCP entitiy, which

reconstructs the original byte streams. IP gives no guarantees that datagrams will be delivered properly,so its up to TCP to time out, retransmit, and re-order them in the proper sequence.

Figure 5 TCP/IP Header

Field DescriptionSource Port Identifies the sending port

Destination Port Identifies the receiving port

Sequence Number

If SYN = 1, this is the sequence number of the actual first data byteand the ACK number in the corresponding ACK is this number + 1.

If SYN = 0, this is the accumulated sequence number of the firstdata byte of this segment.

Acknowledgement

Number

If ACK = 1, value of this field is the next sequence number that the receiver

is expecting.

TCP Header Length Specifies the size of the Header in 32 bit words.

Flags (SYN, FIN, ACK, etc)Number of 1 bit flags used for dealing with acknowledgements,

terminations, etcWindow Size

The size of the receive window which specifies the number of window size

units that the sender of the segment is currently willing to receive.

Checksum Used for error checking of both header and Data

Urgent PointerIf flag URG = 1, the value is an offset from the sequence number indicating

the last urgent data byte.

OptionsUsed to add padding so that the TCP header ends (and the data begins) on

a 32 bit boundary.


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TCP Connection Protocol Full-Duplex

Figure 6 TCP half duplex 3-way handshake

1. The initiator (A) sends a SYN to the listener (B), and sets the sequence number to a randomvalue .

2. In response, B replies with a SYN-ACK. The acknowledgement number is set to + 1, and thesequence number is another random value .

3. A sends an ACK back to B, the sequence number is set to the received acknowledgment number( + 1) and the acknowledgement number is set to + 1.

Figure 7Special Case TCP connection

This diagram shows what happens when two devices try to open a connection to each other at the same

time. In this case instead of a three-way handshake, each sends a SYN and receives an ACK. They each

follow the same sequence of states, which differs from both sequences in the normal three-wayhandshake.


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TCP Connection Termination

Figure 8 TCP Connection Termination

TCP Congestion Control

Slow Start:

Slow start is part of the congestion control strategy used by TCP. It is used to avoid sending more data

than the network is capable of transmitting, that is, to avoid network congestion.

The congestion window is doubled on each packet successfully sent (an ACK received before timeout).

This exponential increase continues until the threshold (initially 32k) is reached, after which the increase

is linear. When a timeout occurs, the threshold is set for half the current congestion window, and the

slow start is repeated.

Congestion Avoidance:

Upon receiving an ACK the congestion window (cwind) is increased by

. This is an additiveincrease, as opposed the multiplicative increase of the basic slow start.

Accurate retransmission timeout estimation:

If the retransmission timer is too short, unnecessary retransmissions will occur. On the other hand, if

too long, a long transmission delay will occur.

A number of algorithms are available to replace the basic RTT timer value, including Jacobson and Karns

algorithms.

Error Detection / Correction

On channels that are highly reliable, such as fiber, it is cheaper to use an error detecting code. On channels such as wireless links, that have many errors, it is better to add enough redundancy

to each block for the receiver to be able to figure out what the original block was.


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Hamming Code Example

Bit stream: 10101111

20 2 2 231 2 3 4 5 6 7 8 9 10 11 12

P1 P2 1 P4 0 1 0 P8 1 1 1 1

Bit Index Made up of:

Bit 3 P1 + P2

Bit 5 P1 + P4

Bit 6 P2 + P4

Bit 7 P1 + P2 + P4

Bit 9 P1 + P8

Bit 10 P2 + P8

Bit 11 P1 + P2 + P8

Bit 12 P4 + P8

Odd Parity

P1 = 3, 5, 7, 9, 11

(Bit Value) 1 0 0 1 1 0

P2 = 3, 6, 7, 10, 11

(Bit Value) 1 1 0 1 1 1

P3 = 5, 6, 7, 12

(Bit Value) 0 1 0 1 1

P4 = 9, 10, 11, 12

(Bit Value) 1 1 1 1 1

Final Bit Stream:

01110101111

Cyclic Redundancy Check (CRC)

The CRC bit is typically put in the trailer so that the calculations, for both the sender and receiver, can be

both made without the need of memory. From the sender side, as the data is being transmitted, the

CRC value can be calculated and after all the data is finished being transmitted (and the CRC being

updated each step), the CRC value is then added to the trailer. On the receiving side, as the data is

coming in, the CRC may be calculated (as the data is updated) and then checked with the CRC value inthe trailer. If the CRC were to be in the header, the sender would have to calculate the CRC based on

the data, send it in the header, and then send the data. The receiver would receive the CRC first, store

it, receive the data, calculate the CRC and compare it to the saved data. Thus putting CRC in the trailer

allows the process to be real-time (on the go).


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User Datagram protocol (UDP)

Figure 9

UPD Header

No Connection establishment No reliability No ordering No Flow or congestion Control

UDP is basically IP with a shorter header added. The port numbers indicate the sending and receiving

transport endpoints. When a UDP packet arrives its payload is sent to the process attached to the

destination port.

The checksum is optional and stored as 0 if not computed, a calculated 0 checksum is stored as all 1s.

UDP does not do flow control, error control or retransmission upon receipt of bad datagrams.

Domain Name System (DNS)

The DNS maps host names to IP addresses and Vice versa.

Application calls resolver with name Resolver contacts local DNS server (using UDP) passing the name Server returns with corresponding IP address

By using a distributed hierarchy of servers the system has better scalability and does not present a single

point of failure. If DNS were to crash, one could only use IP addresses and not host names when

accessing servers on the internet.

Real Time Transport Protocol (RTP) is intended for real time multimedia applications, it multiplexes

several real-time data streams into a single stream of UDP packets.

HTTP

HTTP is the transfer protocol used throughout the world wide web. It specifies what messages clients

may send to servers and what responses to get in return. All clients and servers must obey this protocol.

Each HTTP request consists of one of more lines of ASCII text, with the first word on the first line being

the name of the method requested. There are a number of methods, such as: GET, HEAD, PUT, etc

Client Side

Clicking in a browser on http://www.cs.ru.nl/~ths/index.html.

The steps that occur then are:

1. The browser determines the URL (by seeing what was selected)2. The browser asks DNS for the IP address of www.cs.ru.nl3. DNS answers with the IP number1544. The browser makes a TCP connection to that number on port 80


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5. It then sends a GET /~ths/index.html command6. The www.cs.ru.nl server sends the file index.html7. The TCP connection is released8. The browser displays all the text in index.html9. The browser fetches all images indicated in index.html, by establishing a TCP connection for

each of them, and displays them.

Server Side

This performs the following steps in its main loop:

1. Accept a TCP connection from a client.2. Resolve the name of the page requested.3. Authenticate the client if needed.4. Perform access control on the client, can the requested page be sentgiven the client's identity

and location.

5. Perform access control on the web page, some pages may only been sent to clients on particulardomains, e.g. inside the company.

6. Check the cache if the page is there, otherwise get it from disk.7. Determine the MIME type and include it in the header of the reply.8. Other possible tasks, like building a user profile, gathering statistics or making an entry in a

logfile.

9. Return a reply, either the requested file or error information10.Release the TCP connection

Types of Networks

Circuit switching

For each connection, physical switches are set in the network to create a physical circuit. Switches are

set up at the beginning of the connection and maintained throughout the connection. Network

resources are reserved and dedicated.

Example: Telephone Network

Packet Switching

In this switching method, instead of establishing a dedicated line between the sender and receiver, the

message is sent to the nearest, directly connected switching node. This node stores the message,

checks for errors, selects the best available route and forwards the message to the next intermediate

node. To reduce the required storage size messages are divided into subsets of equal length called

packets. Each packet is composed of the payload (data), and a header, which contains information

useful for network layer functions such as the source and destination addresses.

Example: The internet (TCP/UDP)

Virtual Circuit Switching

At connection establishment time, the path from source to destination is selected and used throughout

the connection lifetime. Thus, packets passing though the route can have a short header, containing

only a virtual circuit identifier (VCI) and not their destination.


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Example: Frame Relay

Figure 10

Virtual Circuit Vs. Datagram Subnet

Repeaters, Hubs, Bridges, Switches, Routers and Gateways

A bridge connects two or more LANs A switch is more often used to connect individual computers A router gets the packet out of a frame and uses the information in the packet header, for

example the IP addresses.

A transport gateway receives e.g. a TCP packet and uses the header information to decide whatto do with the packet.

An application gateway understands the format and content of the data. It can translatemessages from on format to another. Might also be used for security, for example, blocking

messages.

Figure 11 Comparison to Layers


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Ethernet

Figure 12 Ethernet Frame Format

Field Description

Preamble 7 bytes used to synchronize clocks

Start of Frame (SOF) Contains 10101011

Addresses

Unique values in the world of 6 bytes each. Address containing all 1s is reserved for broadcast, a

message destined to all receivers.

LengthVariable uses, most common is type field that indicates that the data

is a higher level protocol packet.

Data Data

Pad Used so that the frame has always the same lengthChecksum Used for error checking

Dynamic Routing

Distance Vector routing

A routing table in each router contains, for each router, the preferred outgoing line for the router and

the estimate cost to that destination. The cost metric might be number of hops, queue length, time

delay, etc

Once every T msec each router sends its neighbours a list of estimated costs to each destination.

Link State Routing

In Link State routing each router sends the costs to the immediate neighbours to all the routers in the

network.

Each router must:

1. Discover its neighbours, learn their network addresses2. Measure the delay or cost to each of its neighbours3. Construct a packet telling all it has just learned4. Send this packet to neighbouring routers5. Compute the shortest path to every other router

Summary:

In Distance Vector, each node talks only to its directly connected neighbours, but it tells themeverything it has learned the distance to all nodes.

In Link State, each node talks to all other nodes, but it tells them only the state of its directlyconnected links.


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Fragmentation

Figure 13 Transparent (a) and Non-transparent (b) fragmentation

Fragmentation happens when a message is larger than the maximum allowable network packet size.

Transparent:

Small packets are transparent to other networks Packet is reassembled at exit gateway

Other networks are not aware fragmentation occurred Exit Gateway must know that all pieces were received (fragment counter or end of

packet bit)

May have to repeatedly fragment and reassemble to travel through a series ofnetworks.

Non-Transparent

Reassembly only occurs at destination host Each Fragment becomes a separate packet and may be routed independently. Every packet (fragment) must carry header until it reaches destination host.

Internet Control Message Protocol (ICMP)

When something unexpected occurs in a router or host, this event is reported by ICMP. It is also used

by routers to test the internet or to obtain information to be used in routing decisions (ECHO).

Messages include: Time Exceeded, Echo, and Echo reply.

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