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IP : Internet ProtocolComputer Network System

Sirak Kaewjamnong

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IP Layer

• Defines a virtual network on top of difference kinds of hardware platform using IP address

• function of IP– route packet– fragmentation– handle type of service– send and receive error and control

message using ICMP

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IP Attributes

• Handle data unit called IP datagrams

• - connectionless protocol doesn’t promise reliable delivery

• best effort delivery• packets maybe lost, out of seque

nce, or duplicated due the variou s reasons

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IP Encapsulation

With Ethernet frame

Ethernet Header

IP Header Data

datagram

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IP Reframe

• IP will reframe the packet when A send data to B

Router

A BIP 172.28.80.96MAC 00:50:ba:49:9d:b9

IP 172.28.85.24MAC 00:02:7a:d2:b3:00

MAC destMAC srctypeIP srcIP dest

MAC destMAC srctypeIP srcIP dest

00:00:1a:2b:3c:4d00:50:ba:49:9d:b90x800172.28.80.96172.28.85.24

00:02:7a:d2:b3:0000:00:2a:3b:4c:5d0x800172.28.80.96172.28.85.24

Packet to router Packet from router

IP 172.28.80.1MAC 00:00:1a:2b:3c:4d

IP 172.28.85.1MAC 00:00:2a:3b:4c:5d

Change MAC address, IP address be the same

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The Internet Protocol (IP)

AppTranspor

tNetwork

Link

TCP / UDP

IP

DataHdr

DataHdr

TCP SegmentIP Datagram

Protocol Stack

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The Internet Protocol (IP)

Characteristics of IP• CONNECTIONLESS: mis-sequencing• UNRELIABLE: may drop packets…• BEST EFFORT: … but only if necessary• DATAGRAM: individually routed

A

R1

R2

R4

R3

BSource

DestinationD H

D H

•Architecture•Links•Topology

Transparent

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The IP Datagram

Offset within original packet

Flags :3

vers : 4

TTL : 8

TOS : 8

Header Checksum : 16

hlen: 4

Total Length :16

Identification : 16 FRAG Offset : 13

Protocol :8

SRC IP Address : 32

DST IP Address : 32

(OPTIONS) (PAD)

<= 64 KBytes

Data..

0 15 16 31

20 Bytes

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IP Header Details• Ver : version = 4• hlen : header length in 32 bit

words, hlen = 5 with no option• TOS : type of service , desired

quality of servicesPrec. D T R 0C

0 1 2 3 4 5 6 7

Bits if 0 if 10-2 Precedence3 Normal delay Low delay4 Normal throughput High throughput5 Normal reliability High reliability6 Normal cost Low cost7 Reserves

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IP Header Details

• Total Length : max size datagrams include header (64KB)

• Identification , Flag, Fragmentation : use to segmentation and reassembly packet

• TTL : Time to Live, maximum number of routers through with the datagrams may pass (hop count)– decrease at each router to prevent forever looping– normally set to 30– if TTL == 0 discard and send ICMP TTL exceeded to

source IP

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IP Header Details

• Protocol : higher level protocol that provi de data

– 1 = datagrams carries an ICMP messages– 6 = datagrams carries an TCP segments– 17 = datagrams carries an UDP datagrams

• Header checksum : 16 bits one’s comple ment, note that there is no data checksu

m

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IP Header Details

• Source address : 32 bits IP source address

• Destination address : 32 bits IP destination address

• Option and Padding : (maximum 40 bytes) addition information to control functions such as routing and security

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Type of Service in some Internet Protocol

Application D T R C

TELNET 1 0 0 0 Minimize delayFTP control data

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01

00

00

Minimize delayMaximize throughput

Meaning

From RFC 1349

TFTP 1 000 Minimize delaySMTP command phase data phase

10

01

00

00

Minimize delayMaximize throughput

DNS UDP query TCP query zone transfer

SNMP Maximize reliability0 0 1 0

100

001

000

000

Minimize delayMaximize throughput

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Routing• Process of choosing a path over which to

send datagrams• IP routes packet by looking at the IP

network number• Routing components

– determine what path are available– selecting the best path for a particular

purpose– using those paths to reach other network

• devices which perform routing are routers(Historically call IP gateways)

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Routing Table

• Every router contains a routing table of the network numbers

• The table record– Which connection can be used to

reach a particular network– plus some indication of the

performance or cost of using connection

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Routing Table Form

Routing table form• network , gateway, other

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Routing Protocol

• Routing protocol manages and updates routing table on each network node

• UNIX often implemented using one of the two daemons– routed : basic routing daemon for interior

routing, normally with RIP– gated : sophisticated daemon for interior

and exterior routing, with additional protocol such as OSPF, BGP

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Fragmentation• Fragmentation : processed used by IP to

reduced size of datagram that are too long for network interface MTUe.g. fragment 2000 bytes to Ethernet MTU 1500 Bytes

• fragmentation should be reassembled at final destination (expensive process)

• how?– each fragment has it own header– each fragment carries the same 16 bits

identification header– each fragment must be aligned with eight-octet

boundary

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Fragmentation Flag• Identification number

– 16 bits integer value used to identify all fragments– This ID is not a sequence number

• Flags : 3 bits control fragmentation

• Fragment offset : indicate the distance of fragment data from the start of original datagram, measure in 8 octets unit

R DF MF

0 = last fragment1 = more fragment

0 = may fragment1 = don’t fragment

Reserve must be 0

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Fragmentation

…….. 232 0 0 0..2000

...2321 0 0 .. 1479

...23201851480 .. 2000

original

Ethernet with MTU 1500

20 bytes 20 bytes

Other headeridentflagsoffset data

Identification numberMore fragment

Post 0

Last fragment

Post 185 * 8 = 1480

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Problem in fragmentation• The end node has know way of knowing how

many fragments there be. The end node has to manage enough buffer space to handle reassembly process.

• If any fragments lost, all datagram must be discarded

• End node starts a timer when received the first fragment, if any fragments fails to alive(usually 30 sec), all datagrams must be discarded

• Since the IP service is connectionless. No attempt is made by IP to recover these situations, though ICMP error massage may be generated

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Avoiding Fragmentation• For datagrams with in the same physical network, the

MTU is known. TCP/UDP then use the MTU to limit the message size pass to IP, message will never be fragmented.

• For datagram pass to difference network, unknown MTU.

• Standard recommend that all networks supporting TCP/IP have an MTU of at least 576 bytes

512 bytes data + 20 bytes TCP header + 20 bytes IP header with options to guarantee that a packet of 576 bytes or less is never fragmented