Download TCP/IP: An overview

TCP/IP: An overview
Yatindra Nath Singh
EE/ACES, IIT Kanpur-208016
Email: [email protected]
web: http://home.iitk.ac.in/~ynsingh
Copyright 2001: YNS
TCP/IP: Overview
1
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.
Copyright 2001: YNS
TCP/IP: Overview
2
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
Copyright 2001: YNS
TCP/IP: Overview
3
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
Copyright 2001: YNS
TCP/IP: Overview
4
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.
TCPTransmission control protocol.
IPInternet protocol
• Can be used within the network also.
Internet – Network of networks
Copyright 2001: YNS
TCP/IP: Overview
5
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.
Copyright 2001: YNS
TCP/IP: Overview
6
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.
Copyright 2001: YNS
TCP/IP: Overview
7
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
Net work
Copyright 2001: YNS
Host
TCP/IP: Overview
8
A Typical IP Header ( IPv4)
32 Bits
Vers
IHL Type of Serv
Total length
D M
F F
Identification
Time to live Protocol
Fragment Offset
Header Check Sum
Source address
Destination Address
Option (0 or more words)
Copyright 2001: YNS
TCP/IP: Overview
9
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.
Copyright 2001: YNS
TCP/IP: Overview
10
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.
Copyright 2001: YNS
TCP/IP: Overview
11
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.
Network
Fragments
reassembled
Fragmentation of
IP packet payload.
Max packet size is limited.
(Happens when links are
low speed links)
Copyright 2001: YNS
TCP/IP: Overview
Destination
TCP
12
All these fragments belonging to same IP packet – identified
by a Identification number
One unused bits, DF, MF
Dont fragment
When fragmentation
along the way is not
desired, set to one
More fragment
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.
Copyright 2001: YNS
TCP/IP: Overview
13
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.
Copyright 2001: YNS
TCP/IP: Overview
14
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.
Copyright 2001: YNS
TCP/IP: Overview
15
Source and Destination address - 32 bits
Contain Network number, Host-number
0
31
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.
Copyright 2001: YNS
TCP/IP: Overview
16
IPAdderess
0
32 bits
Network
10 Network
Host
110 Network
1110
11110
Copyright 2001: YNS
Class A
Host
Host
Class B
Class C
Multicast Address
Reserved for future use
TCP/IP: Overview
17
Used only at
boot time
All 0’s
All 0’s
This host
Host
Broadcast on
local network
All 1’s
Network
127
Copyright 2001: YNS
TCP/IP: Overview
A host on this
network
All 1’s
Broadcast on
distant N/W
(Anything)
Loop back
18
CIDR (Classless Inter Domain Routing)
• All class ‘C’ IP addresses can be allocated in variable
sizes.
• Netmask  11111…….11
0000……00000
When netmask ANDED with IP address Network address
is found
• Each router maintains network address and netmask for
each entry.
Copyright 2001: YNS
TCP/IP: Overview
19
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
Copyright 2001: YNS
TCP/IP: Overview
20
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.
Copyright 2001: YNS
TCP/IP: Overview
21
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.
Copyright 2001: YNS
TCP/IP: Overview
22
TCP Does.
• Flow control
• Retramission (IP packet can be lost)
• Reassembly of received datagrams.
Transport layer
Processes
Transport layer
IP
IP Layer
Multiplexing and demultiplexing
Copyright 2001: YNS
TCP/IP: Overview
23
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.
Copyright 2001: YNS
TCP/IP: Overview
24
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
Copyright 2001: YNS
TCP/IP: Overview
25
Flow Control
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
Copyright 2001: YNS
TCP/IP: Overview
26
32 Bits
Source port
destination port
Sequence no
Acknowledgement no
6 bits
TCP
4Bits
Header
Length
U A
R C
G R
P
S
H
Unused
Check Sum
R
S
T
S
Y
N
F
I
N
Window
Urgent pointer
Option (0 or more 32 bits words)
Data (Optional)
Copyright 2001: YNS
TCP/IP: Overview
27
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 .
Copyright 2001: YNS
TCP/IP: Overview
28
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
Copyright 2001: YNS
TCP/IP: Overview
29
for checksum pseudo header is included with segment header
31
0000000000
0
source
Destination
Protocol TCP Segment
(6)
length
Byte count for TCP segment
including header
Option 
• Used for deciding segment sizes between two hosts
• Extended window size
Copyright 2001: YNS
TCP/IP: Overview
30
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
Copyright 2001: YNS
TCP/IP: Overview
31