Copyright 2001: YNSTCP/IP: Overview1 TCP/IP: An overview Yatindra Nath Singh EE/ACES, IIT...

Copyright 2001: YNS TCP/IP: Overview 1

TCP/IP: An overview

Yatindra Nath Singh

EE/ACES, IIT Kanpur-208016

Email: [email protected]

web: http://home.iitk.ac.in/~ynsingh


Abstraction levels (layers)

• Computer Network - Interconnection of machines.• Studied and designed at various level of abstractions –

ease of study and design.• Five levels of abstraction (Layers)

– Physical layer– Data link layer– Network layer– Transport layer– Application layer

• Each layer uses functionality of lower layer; provides functionality to higher layer.


Physical Layer (PL)

• Modulation formats

• Bit rates

• Phsical interface design

• Examples - SDH, E1, E3, ISDN, ADSL

Data Link Control (DLC)

• HDLC

• Reliable transport of frames in sequence

• Sublayer – MAC for physical broadcast network


Network layer and Transport layer.

• Problems faced in the network layer– Identification of destination.– Reaching destination– Lost packets– Maximum packet size in various networks

• Problems faced by transport layer– Data multiplexing and demultiplexing– Segmentation and reassembly– End to end flow control– Congestion Control


Internetworking

• Interworking between networks (de-facto standard)

• All networks need to used TCP/IP suite when communicating with each other.

• Within them- network can use any protocol of thier choice. They need to convert between TCP/IP and protocol being used.

TCPTransmission control protocol.

IPInternet protocol

• Can be used within the network also.

Internet – Network of networks


IP Address and functionality

In Internet - Each machine is identified by unique ID.– Known as IP address.

In each machine, Network Layer (IP)

• When packet received for itself, forward the payload to layer above it (IP passes payload to layer above).

• When multiple protocols can sit above network layer, payload passed to the one indicated in packet.

• When packet received destined for someone else.

– Routing table is looked into.

– Packet forwarded to appropriate node.


IP layer functionality (contd.)

– Packet is forward to appropriate node.

– Usual routing table entry

destination network address, netmask, interface ID,

next hop gateway. Protocol running in IP Layer • Protocols to generated mapping between IP address and

MAC address.arp, rarp, BOOTP

• Protocols for topology discovery and tracking any change in it

RIP-2,OSPF-2,BGP-4• ICMP (Internet Control Message Protocol) for monitoring

network.


IPv4 – Older version of Internet protocol and mostly used one

Future- IPv6 (IP for next generation)

IPv4 Addressing (32 bit or four Octets)

Two parts

HostNet work


Vers IHL Type of Serv Total length

Identification Fragment Offset

Time to live Protocol Header Check Sum

Source address

Destination Address

Option (0 or more words)

32 Bits

A Typical IP Header ( IPv4)

DF

MF


Version (4 Bits) Keep track, of protocol version

e.g. Helps router in identifying IPv4 and IPv6 packets

IHL (4Bits) Due to options, header length variable

– tells the length header (in 32 bits words - max header length 15 i.e., 60 bytes)

– Minimum, value – five (5) header at least contain uptill destination address.


Type of service (8Bits)Used by routers to provide quality of service (QoS) based routing

• First 3 bits- precedence (max value 7, min 0)

• Next 3 bits- (D) delay, (T) throughput, (R)

Reliability

• Used for choosing between multiple routes available

for destination based on low delay, high throughput,

high reliability bit values.

• Remaining 2 bits are unused.


Total length –Length of IP packet (including header and data)

Max value – 65535 byte.– In high speed networks (Gbps or more) large packet

size is desirable.

TCP

Network

Max packet size is limited. (Happens when links are low speed links)

Fragmentation of IP packet payload.

Fragmentsreassembled Destination


All these fragments belonging to same IP packet – identified by a Identification number

One unused bits, DF, MF

Dont fragment More fragment

When fragmentation along the way is not desired, set to one

Tells whether this is last fragment of the IP packet. Set to 0 for last fragment.

All routers, machine should accepts fragment of 576 bytes or less.


Fragment offset (13 bits)• Tells the position of the fragment in the original IP packet.• All fragments except last one-should be multiple of 8

bytes.13 bits => 8192 fragments=> max data gram length 8192*8= 65536.

Time–to-Live(TTL) field. (8bits) • Every time packet is forwarded, count is decreamented by

‘one’.• When TTL is ‘zero’, packet is discarded and warning

ICMP sent back to source.


Upon reception of all fragments, Network layer need to find “what to do with payload?”

Protocol field – Identifies transport or other process to which payload to be given.TCI, UDP, OSPF, ICMP are examples.

Header Checksum - To Detect Errors• All 16 bits half words added using 1’s complement

arithmetic's.• 1’s complement of result gives checksum• Changes with every hop.


Source and Destination address - 32 bits

Contain Network number, Host-number

31 0

Network address Host address

Option - All optional information can be transported using these.

For IPv4 – Five option

Security, Strict source routing, Loose source routing, Record route, Time stamp.


IPAdderess

0 Network Host

10 Network Host

110 Network Host

1110 Multicast Address

11110 Reserved for future use

32 bits

Class A

Class B

Class C


All 0’s

All 0’s Host

All 1’s

Network All 1’s

127 (Anything)

Used only at boot time

This host

A host on this network

Broadcast on local network

Broadcast on distant N/W

Loop back


CIDR (Classless Inter Domain Routing)

• All class ‘C’ IP addresses can be allocated in variable

sizes.

• Netmask

When netmask ANDED with IP address Network address

is found

• Each router maintains network address and netmask for

each entry.

11111…….11 0000……00000


Router handling of CIDR

• For each passing IP packet, destination IP address

ANDED with netmask.• output compared to network addresses in routing table

entries.

When more than one entry matches the output• Routing table with longest string of 1’s is netmask

chosen.• Packet is forwarded to corresponding output port


IP Version 6

• Due to shortage of address and to improve the performance of networks.

– has 16 bytes (128bits) source and destination address

– Header is simple, packet processing in routers faster.


TCP (Transport Control Protocol)

- Forwards end to end reliable byte stream.

- Accepts byte stream from processes – breaks them into 64KB or less size chunks

- Each chunks mapped to separate datagram (which is further mapped to IP payload)

- On reception of IP packet, the payload is given to TCP entity.

- Creates the byte stream back.


TCP Does.

• Flow control

• Retramission (IP packet can be lost)

• Reassembly of received datagrams.

Transport layer

IP

Transport layer

IP Layer

Multiplexing and demultiplexing

Processes


TSAP- Identification by 16 bit number called port no.

Port number below 256- well known ports reserved for standard Services (RFC 1700)

FTP-21, Telnet-23

All TCP connections-Full duplex.

Point to point (exactly two end point)

Multicasting / broadcasting not supported

Every byte transmitted by TCP

Has 32 bit sequence no in header

Used for acknowledgement and window flow Control.


TCP transmits bytes in segments

Each segment has 20 byte header followed by zero or more data bytes

Size of payload decided by (minimum of the two)

Each segment including TCP header should fit in 65,535 byte IP packet

For each network - defined MTU (Maximum Transfer Unit)

A segment can be broken into smaller segments at intermediate router

Each smaller segment provided with separate TCP and IP header. This is different than IP fragmentation


TCP uses Sliding Window Protocol

• Sends the segments

• starts the timer

• Receiving entity sends ack (sequence no of next expected byte)

• If ack received before timer expiry transmission window moves ahead

Else

• retransmission of segments in transmission window is done

Flow Control


Source port destination port

Sequence no

Acknowledgement no

Check Sum Urgent pointer

Option (0 or more 32 bits words)

Data (Optional)

4Bits6 bits

Unused

ACR

PSH

RST

SYN

FIN Window

32 Bits

TCP

Header

Length

URG


URG set to 1 if urgent pointer is used

Byte offset from current sequence number to find urgent

data

(Used for sending interrupts)

Acknowledgement 1 then acknowledgement no field is

valid else ignore it .


RST - used to reset the connection

SYN- used to setup the connection

connection request SYN=1 ACK=0

connection response SYN=1, ACK=1

FIN - used to release the connection

sender has no more data to send

Window -Tells the transmitter

How many bytes can be sent starting at acknowledge number

Window=0 is legal

Check sum for extreme reliability


for checksum pseudo header is included with segment header

31 0

source

Destination

0000000000Protocol (6)

TCP Segment length

Byte count for TCP segment including header

Option

• Used for deciding segment sizes between two hosts

• Extended window size


Summary

IP and TCP protocols : Overview

Lot more is there to IP and TCP - This is just an introduction

Similar concepts are used in other network layers and transport layers implementations

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Copyright 2001: YNSTCP/IP: Overview1 TCP/IP: An overview Yatindra Nath Singh EE/ACES, IIT...

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