Data and Computer Communications

Internet Protocols

Network Architecture Features Addressing Packet size Access mechanism Timeouts Error recovery Flow control Routing User access control Connection based or connectionless

Connectionless Internetworking Advantages

Flexibility Robust No unnecessary overhead

Unreliable Not guaranteed delivery Not guaranteed order of delivery

Packets can take different routes Reliability is responsibility of next layer up (e.g.

TCP)

IP Operation

Go to Router X

MAC address for Router X

IP PDU

Encapsulated with LAN protocol

Encapsulated with X.25 protocol

Design Issues Routing Datagram lifetime Fragmentation and re-assembly Error control Flow control

Routing End systems and routers maintain routing tables

Indicate next router to which datagram should be sent Static

May contain alternative routes Dynamic

Flexible response to congestion and errors

Source routing Source specifies route as sequential list of routers to be

followed

Datagram Lifetime Datagrams could loop indefinitely

Consumes resources Transport protocol may need upper bound on datagram life

Datagram marked with lifetime Time To Live field in IP Once lifetime expires, datagram discarded (not forwarded) Hop count

Decrement time to live on passing through a each router Time count

Need to know how long since last router

Fragmentation and Re-assembly Different packet sizes When to re-assemble

At destination Results in packets getting smaller as data traverses

internet Intermediate re-assembly

Need large buffers at routers Buffers may fill with fragments All fragments must go through same router

Inhibits dynamic routing

IP Fragmentation (1) IP re-assembles at destination only Uses fields in header

Data Unit Identifier (ID) Identifies end system originated datagram

Source and destination address Protocol layer generating data (e.g. TCP) Identification supplied by that layer

Data length Length of user data in octets

IP Fragmentation (2) Offset

Position of fragment of user data in original datagram In multiples of 64 bits (8 octets)

More flag Indicates that this is not the last fragment

Fragmentation Example

Application data

IP header is the same as origin TCP header is not duplicated –

More is the same as original

Internet Protocol (IP) Part of TCP/IP

Used by the Internet Specifies interface with higher layer

e.g. TCP Specifies protocol format and mechanisms IPv4

addresses are 32 bits wide Its header is 20 bytes at minimum Uses doted-decimal notation (e.g. 43.23.43.56)

IPv6 Provides larges address domain; addresses are 128 bits wide Multiple separate headers are supported Handles audio and video; providing high quality paths Supports unicast, multicast, anycast

Dealing with Failure Re-assembly may fail if some fragments

get lost Requires buffer Need to detect failure – but how?

Re-assembly time out Assigned to first fragment to arrive If timeout expires before all fragments arrive,

discard partial data Use packet lifetime (time to live in IP)

If time to live runs out, kill partial data

Parameters (1) Source address Destination address Protocol

Recipient e.g. TCP Type of Service

Specify treatment of data unit during transmission through networks

Identification Source, destination

address and user protocol Uniquely identifies PDU Needed for re-assembly

and error reporting Send only

+ 0 - 3 4 - 7 8 - 15 16 - 18 19 - 31

0 Version Header length Type of Service Total Length

32 Identification Flags Fragment Offset

64 Time to Live Protocol Header Checksum

96 Source Address

128 Destination Address

160 Options + padding

192  

Data 

IP Packet Format - TTL TTL (time-to-live) refers to the number of router

hops the IP packet is allowed before it must be discarded. Each router that receives a packet subtracts one from

the count in the TTL field. When the count reaches zero, the router detecting it

discards the packet and sends an Internet Control Message Protocol (ICMP) message back to the originating host

IP Header

The IP datagram contains data and IP address

The IP datagram is encapsulated in a frame with physical address

The header changes as the frame goes from one network domain to the next

IP Datagram

Frame

DataIP Address

FrameAddress

Encapsulated IP Packet in Ethernet Frame

MAC and Associated IP address

Ethernet Frame Carrying IP Packet

Protocol Analyzer Display: 0000 00 00 C0 A0 51 24 00 C0 93 21 88 A7 08 00 45 080010 00 5A DC 28 00 00 FF 01 88 08 C0 99 B8 01 C0 990020 B8 03 2a B4 DD …..

Encapsulated IP Packet in Ethernet Frame

Ethernet Frame Carrying IP Packet

IP starting with 45 Hex indicates IPv4 with standard length of 20 bytes

IP starting with 4F Hex indicates IPv4 with standard length of 60 bytes

Remember: 24=16; 45= 0100 0101= One Byte

An Ethernet frame containing IP information has 08 00 in its type field

99 is one byte

1001 1001

Example:

IP Addressing Two address types

Physical address (the frame has the physical address) Embedded in the hardware (NIC, e.g., 00 00 11 00 11 33) Also called the Media Address Control (MAC) address

Logical IP datagram contains the logical IP address

To transport IP packets both physical and IP addresses must be known Static address resolution Dynamic address resolution

How to map physical and IP addresses Local tables of IP addresses Centralized directory Address resolution mechanism

Address resolution mechanisms ARP (address resolution protocol) – IP48 bit Ethernet address RARP (reverse) address resolution protocol are used to convert

MAC to IP address and vice versa

IP Addressing – IPv4 A network address is divided into Netid and Hostid IP Address classification

(number of hosts per network)

Class Leading bits NetworkAddress (Netid)

Host Address (Hostid)

Class A     0     7 bit (125)*     24 bit (16,777,151,750)

Class B     10     14 bit (16,368)     16 bit (65,534)

Class C     110     21 bit (2,096,896)     8 bit (254)

Class D (multicast)     1110

Class E (reserved)     1111

* Some values are reserved!

IP Addressing Classification

Network Address Host Address

Network Address

Host Address

Network Address

Host Address

Reserved for Internet research

Multicast Address

Example of IP Addressing

Q1: Determine the network address for the following IP addresses:

1- 84.42.58.11 (84 = 54 Hex = 0101 0100) Netid=84.0.0.0 Class A Hostid=0.42.58.11

2- 144.54.67.5 (144 = 90 Hex = 1001 0000) Netid=144.62.0.0 Class B Hostid=0.0.67.5

Q2: What type of IP address classificationwill a large organization with 1000individual users in 150 dispersed buildings use? Class B

IP Routing Protocols

Routing packets requires having knowledge about the network Partial (know your own neighbors) Full (know the entire network elements)

Retrieving network information (network discovery protocols) RIP (routing information protocol): routing based on the least

number of hops OSPF (open shortest path first): Routing based on number of hops,

link speed, congestion

TCP/IP Stack Protocol

Bridge IS used to connect two LANs

using similar LAN protocols Address filter passing on packets

to the required network only OSI layer 2 (Data Link)

Router Connects two (possibly

dissimilar) networks Uses internet protocol present in

each router and end system OSI Layer 3 (Network)

Subnets and Subnet Masks Each LAN assigned subnet number Site looks to rest of internet like single network Host portion of address partitioned into subnet

number and host number Local routers route within subnetted network Subnet mask indicates which bits are subnet

number and which are host number Allow arbitrary complexity of internetworked LANs

within organization Insulate overall internet from growth of network

numbers and routing complexity

00 10 00 00

Routing Using Subnets

Masking

IP Address of B: 192.228.17.57 00 1|1 10 01IP Address of A: 192.228.17.33 00 1|0 00 01IP Address of X: 192.228.17.32 00 1|0 00 00

Subnet mask: 255.255.255.224 11 10 00 00

Note: if we XOR IP Address of B & Subnet MaskWe will have: 00 11 10 01 XOR 00 10 00 00 = 00 01 10 01Host number is 25 00 01 10 01;

192 1100 0000, hence, Class C network (8-bit host/subnet) !

5-bit host

3-bit subnet

NetId SubNetId Host

Why Change IP? Address space exhaustion

Two level addressing (network and host) wastes space

Network addresses used even if not connected to Internet

Growth of networks and the Internet Extended use of TCP/IP Single address per host

Requirements for new types of service

IP v6 Header vs. IPV4

128 bits

128 bits

Features: Extended address spaceImproved option mechanismDynamic address assignmentMulticasting and anycastingFlow routing

Note: IPv5 used for Stream Protocol- IP-layer protocol that provides end-to-end guaranteed service across a network.

Internet Addressing Over half million networks are connected to the

Internet Network numbers are managed by ICNN (Internet

Corporation for Assigned Names and Numbers) - http://www.icann.org/ Delegates part of address assignments to regional

authorities IP addresses are given to ISPs and companies

IP addresses are based on dotted decimal notation: 192.41.7.32 IP address 0.0.0.0 refer to machine’s own network when

it is being booted (This host) 255.255.255.255 broadcast on the LAN 127.x.y.z reserved for loopback testing

More about subnets…. Routers can be connected to multiple LANs LANS are divided into subnets each identified by a subnet

mask: 255.255.252.0 (… 1111 1100 0000 0000) 32-10=22 to identify the subnet! Mask: netID + SunnetID or /22 (subnet mask is 22 bit long) Subnets are not visible outside the network

Example: Assume subnet mask is 255.255.252.0/22 Subnet 1: 130.50.4.1 Subnet 2: 130.50.8.1 …000010|00 00000001 Subnet 3: 130.50.12.1 …000011|00

00000001 Assume a packet’s destination is 130.50.15.6 which subnet

does it belong to? Mask: … 1111 11|00 0000 0000 Adrs: … 0000 11|11 0000 0110 Hence: the packet must go to Subnet 3

Classes and subnets… Classful routing is not very efficient Having IP address classes creates issues

Addresses can be under utilized (Class A) Addresses can be over utilized (Class C) Management of addresses may be difficult Organizations can grow!

Classless InterDomain Routing (CIDR) Allocate remaining IP addresses in variable-sized block;

no regard to class! Use 32-bit mask! Uses a single routing table

Classless Routing - Example

What happens if a packet has an address of 194.24.17.4? Where does it go?

O: ….1111 0000 0000 0000 AND 0001 0001 0000 0100 …. 0001 0000 0000 0000 (194.24.16.0)

How do you represent class B using CIDR?16-bit NetID + 16-bit HostID /16

Network Address Translation (NAT) Addresses are growing! What is the

solution? Use IPV6 Use NAT

NAT: Allows using one IP address per company Internally new nodes can be added!

How?

NAT Operation IP reserved addresses

10.x.y.z 172.16.x.y 192.168.x.y

Receiving a packet from the Internet

Sender Add IP address TCP will have the destination port (0-

1023) The port determines which server on

the remote (destination) side to process the packet

NAT box: Using the PORT address in TCP,

change the IP address to a designated address (10.0.01)

Sending a packet into the Internet NAT box:

Changes 10.0.0.1 to 198.60.42.12

198.60.42.12

NAT Issues… Addresses are not unique: many 10.0.0.1! NAT controls are the incoming and

outgoing packets – reliability! NAT accesses TCP and IP layers – layers

should work independent of one another NAT only allows TCP/IP or UDP/IP NAT does not support applications which

insert the IP address in the body (FTP or H.323)

Internet Control Protocols IP protocol only deal with the data transfer We need control protocol to check the

network ICMP, ARP, RARP, BOOTP, HDCP

Internet Control Message Protocol (ICMP) ICMP error messages are used by routers and hosts to tell a

device that sent a datagram about problems encountered in delivering it Used to test the network Messages are encapsulated in the IP packet Has many message types

Code field is used for subtypes

ARP (address resolution protocol) Exploits broadcast property of a LAN Each host on LAN maintains a a table of IP

subnetwork addresses If the address can not be found ARP

broadcasts a request Shouting: Who know about this IP address?

Other hosts listen and reply The reply includes IP address and MAC Any interested host can learn about the new

information

ARP Example Assume 1 is sending a message to 2 (rose@sonoma.edu)

Sonoma.edu is the host Host 1 sends a message to Domain Name System (DNS): what is the IP address

for Sonoma.edu? 192.31.65.5 What is the MAC address for 192.31.65.5? Use ARP broadcast!

Host 2 respond: it is E2 Host 1 maps IP and MAC; encapsulate the IP message in the Ethernet frame and

send it Cashing can enhance ARP operation

ARP Example Assume 1 is sending a message to 4 (rose@sonoma.edu)

Sonoma.edu is the host Host 1 sends a message to Domain Name System (DNS): what is the IP address

for Sonoma.edu? 192.31.65.5 What is the MAC address for 192.31.65.5? ARP cannot pass through the router!

Two choices: Reconfigure routers to response to ARP (Proxy ARP) Send the message to the LAN router (E3)F1F34 – Each router looks are the IP

address and passes it to the next node using the routing table

RARP and BOOTP Reverse ARP translates the Ethernet address to IP

address A diskless machine when it is booking can ask: My MAC

is 12.03.23.43.23.23; what is my IP? RARP broadcasts the question (destination

address is all one) Not passed through the router!

Each LAN needs a RARP server! Bootstrap protocol uses UDP and forwards over

routers Mapping must be done manually in each router! Uses one server but harder to implement!

Dynamic Host Configuration Protocol DHCP allows many and automatic configuration Replaces BOOTP and RARP DHCP sends a DISCOVER Packet

What is the IP address for this MAC? Even when an IP address is assigned, how long is it good

for? Before the IP address is removed find another IP

address….called Leasing

Remember… This is My MAC; what is my IP address?

RARP / DHCP This is the destination host name, what it

is IP address? DNS Server This is the IP address, what is your` MAC

address? ARP

References Tanenbaum Tomasi Text Book Comer Text book