Data and Computer Communications

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Data and Computer Communications

• Internet Protocols

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Network Architecture Features

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

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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)

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IP Operation
Go to Router X
MAC address for Router X
IP PDU
Encapsulated with LAN protocol
Encapsulated with X.25 protocol
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Design Issues

• Routing
• Datagram lifetime
• Fragmentation and re-assembly
• Error control
• Flow control

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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

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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

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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

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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

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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

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Fragmentation Example
Application data
IP header is the same as origin TCP header is
not duplicated
More is the same as original
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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

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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

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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 Total Length
32 Identification Identification Identification Flags Fragment Offset
64 Time to Live Time to Live Protocol Header Checksum Header Checksum
96 Source Address Source Address Source Address Source Address Source Address
128 Destination Address Destination Address Destination Address Destination Address Destination Address
160 Options padding Options padding Options padding Options padding Options padding
192  Data   Data   Data   Data   Data 
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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

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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

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Encapsulated IP Packet in Ethernet Frame
Ethernet Frame Carrying IP Packet
MAC and Associated IP address
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Encapsulated IP Packet in Ethernet Frame
Ethernet Frame Carrying IP Packet
An Ethernet frame containing IP information has
08 00 in its type field
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 2416 45
0100 0101 One Byte
Example
99 is one byte 1001 1001
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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) IP?48 bit
  Ethernet address
• RARP (reverse) address resolution protocol are
  used to convert MAC to IP address and vice versa

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IP Addressing IPv4

• A network address is divided into Netid and
  Hostid
• IP Address classification
• (number of hosts per network)

Class Leading bits Network Address (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!
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IP Addressing Classification
Network Address
Host Address
Multicast Address
Network Address
Host Address
Reserved for Internet research
Network Address
Host Address
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Example of IP Addressing
Q1 Determine the network address for the
following IP addresses 1- 84.42.58.11 (84 54
Hex 0101 0100) ? Netid84.0.0.0 ? Class A ?
Hostid0.42.58.11 2- 144.54.67.5 (144 90 Hex
1001 0000) ? Netid144.62.0.0 ? Class B ?
Hostid0.0.67.5 Q2 What type of IP address
classification will a large organization with
1000 individual users in 150 dispersed buildings
use? ? Class B
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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

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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)

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Subnets and Subnet Masks
00 10 00 00

• 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

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Routing Using Subnets
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Masking
3-bit subnet
IP Address of B 192.228.17.57 00 11 10 01 IP
Address of A 192.228.17.33 00 10 00 01 IP
Address of X 192.228.17.32 00 10 00 00 Subnet
mask 255.255.255.224 11 10 00 00
5-bit host
Note if we XOR IP Address of B Subnet Mask We
will have 00 11 10 01 XOR 00 10 00 00 00 01
10 01 Host number is 25 ? 00 01 10 01
192 ? 1100 0000, hence, Class C network (8-bit
host/subnet) !
NetId
SubNetId
Host
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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

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IP v6 Header vs. IPV4
Note IPv5 used for Stream Protocol- IP-layer
protocol that provides end-to-end guaranteed
service across a network.
Features Extended address space Improved option
mechanism Dynamic address assignment Multicasting
and anycasting Flow routing
128 bits
128 bits
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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 machines 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

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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) ? 32-1022 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? 00001000 00000001
• Subnet 3 130.50.12.1? 00001100 00000001
• Assume a packets destination is 130.50.15.6 ?
  which subnet does it belong to?
• Mask 1111 1100 0000 0000
• Adrs 0000 1111 0000 0110
• Hence the packet must go to Subnet 3

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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

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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
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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?

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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

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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)

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Internet Control Protocols

• IP protocol only deal with the data transfer
• We need control protocol to check the network
• ICMP, ARP, RARP, BOOTP, HDCP

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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
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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

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ARP Example

• Assume 1 is sending a message to 2
  (rose_at_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

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ARP Example

• Assume 1 is sending a message to 4
  (rose_at_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)?F1?F3?4
  Each router looks are the IP address and passes
  it to the next node using the routing table

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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!

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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

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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

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References

• Tanenbaum
• Tomasi Text Book
• Comer Text book