1. The Internet Protocol

2. Contents
Understand the role of the Internet Protocol (IP)
Examine IP address classes
Use Address Resolution Protocols (ARPs)
Decode IP packet structure
Examine IP on various physical networks

3. The Internet Protocol

4. Internetworking Concepts
Different networks connected together

5. Physical-Transport Independence

6. IP layer
Provides powerful logical abstraction
Hides Physical Layer dependency
Upper layer processes see a logical IP network

7. Functions of Internet Protocol
Internet Protocol (IP) provides
Datagram service
Phisical network independence for higher layer processing
Logical address for computers on network
Independence from maximum transmission unit size
Fragmentation and reassembly control
These topics are examined in the next several viewgraphs

8. IP Datagram Service Ip makes use of “best efforts” service
Similar to postal services
Advantages
Simplicity and less overhead
Upper layers can build more reliable service
Adequate for many networks
-- LANs, frame relay

9. Need for Uniform Addresses (Logical Addresses)
Phisical networks use different addressing schemes
Ethernet networks use 6-byte addresses
X.25 networks use 14-digit decimal codes
ARCNET networks use 1-byte addresses
How should nodes on a logical network be identified?
Solution: Use a logical address to provide a uniform way of addressing all network nodes rregardless of their physical network connections

10. Message Size Limitations
Physical networks support different maximum frame size
Example: 1518 bytes for Etherne, 512 bytes for ARCNET, etc.
Upper layers (TCP) del with message size of arbitrary length
Problem: How do you send arbitrarily long messages to networks with packet-size constraints?
A solution: datagrams, fragmentation, and reassembly

11. Names and Addresses LAN Addressing
NIC addresses (like National Insurance Number)
Broadcast technology
No geography
Network Addressing
IP addresses (telephone numbers)
Permits subnetting (like county and area codes)
Gives routing capability
Maps to NIC address through ARP
Human Friendly Names
Shows affiliations (like a normal mail address)
Structured independently of IP
Used to identify people, computers, networks, organistations.
Maps to IP through DNS

12. Typical MTU Size Network Typical frame size (bytes) MTU (bytes)
Maximum frame size (bytes)
Ethernet
1024
1500
1518
IEEE 802.5
(4 Mbps)
4464
4508
(16 Mbps)*
1024/4096
17,756
17,800
ARCNET
508
512
X.25
128
4080
4096
*Assuming a token-holding time of 9 ms.
MTU = maximum transmission unit

13. Fragmentation and Reassembly Control
Many applications on hosts tend to use large message size
File transfer, graphic applications
Many wide area networks prefer smaller packet size in comparison with some applications
Better use of buffer memory
Smaller probability of error for each packet
Smaller delay for priority packets
Larger packet size in networks (such as LANs) can result in more efficient data transfer

14. Freagmentation and Reassembly Control (continued)
What would happen to packets going from Host B to Host A?
Note: TCP messages can be fragmented by sender

15. The Internet Protocol

16. Calculating an Address Class (continued)
First Decimal Number in Dotted Decimal Address
Minimum
Maximum A 1
126 B
128
191 C
192
223 D
224
239 E
240
247

17. Range of Assignable Addresses
netid
hostid
Class
Minimum
Maximum A 1
126.
0.0.1 B
128.0
0.1 C .1
254 D
224.
239.
N/A
N/A = not applicable
Why is address 127.x.x.x not assigned?

18. Software Loopback Local machne can be addressed by 127.x.x.x
“x” can be any value
-- Typically, is used for local host
Also referred to software loopback test
-- Packets never transmitted
-- Packets copied from transmit buffer to receive buffer

19. Hostid and Broadcast Addresses
Hostid of 0 is never assigned to an individual host
An internet address with hostid of 0 refers to the network itself
-- Example:
-- Refers to class B network
Directed broadcast addresses
By convention, broadcast addresses have all 1 s in hostid field
-- Example:
Important exception is software derived from BSD 4.2 UNIX
-- Uses all 0s broadcast
Limited broadcast address
Broadcast address of
IP packets with this address usually do not cross router boudary
Not all TCP/IP implementations support it

20. The Internet Protocol

21. The Problem: Need for Address Resolution
How does a host know about another host’s physical address?
Hard code knowledge of physical addresses?
Can you logical (IP address) to determine physical address?
-- Address resolution protocols

22. Dynamic Address Resolution Protocol Mechanism
Host A broadcasts ARP request on network containing B’s IP address
All nodes receive ARP request, but only B responds because its IP
address is included in the ARP request
B replies to A, with B’s physical address
Assumption: requires broadcast capability on network (i.e.,
Ethernet, Token Ring, etc.)

23. ARP Request/Response Packet Structure
Numbers in ( ) represent bits

24. ARP Refinements: Caching
In previous example
Host A uses ARP reply to build a local cache
-- Cache contains <IP addr., Physical addr.> pair
Host B is likely to reply to A
Use ARP request to store A’s <IP addr., Psysical addr.> in cache
Other host extract A’s <IP addr., Physical addr> from ARP request
Machines booting on netork announce their <IP addr., Physical addr.>
Other machines cache this information
Also used for duplicate IP address detection

25. IP Address for Disless Nodes
Workstations store their IP addresses in local storage media
How do diskless workstations store their IP addresses?
A solution: use Reverse ARP (RARP)
-- Keep <IP addr., Physical addr.? Bindings on RARP server
-- Potential for simplifying IP adress administration?

26. RARP Operation RARP uses same packet structure as ARP
Sender address – PA
Destination address = broadcast
Ethertype = 8035 hex
RARP uses same packet structure as ARP

27. RARP Operation (continued)
Send broadcasts RARP request
SENDER HA ← Sender’s physical address
TARGET HA ← Sender’s physical address
RARP servers respond with
OPERATION TYPE ← reply
TARGET IP ← Answer (requester’s IP address)
DATA LINK DA ← Requester’s physical address
Lssues
RARP request storms
Primary and backup RARP servers

28. BOOTP
BOOTP makes use of UDP/IP to obtain IP addresses and other information
BOOTP does not provide clients with bootstrap image
It provides the name of the boot image
Boot image is transferred using Trivial File Transfer Protocol (TFTP)
To forward BOOTP requests across routers, routers must be configured with rekay agents to foeward BOOTP packets

29. Troubleshooting Duplicate IP Addresses and ARP Tables
Nodes on an IP network must have unique IP addresses
Otherwise, ARP tables are initialized with incorrect <IP addr., Physical addr.> mappings
Symptoms of bad ARP tables are
-- Users unable to access TCP/IP hosts
-- Workstations and servers crashing
-- Intermittent problems with applications not working
Common results of duplicate IP addresses are
ARP table corruption at workstations
ARP table corruption at servers

30. Duplicate IP Addresses at Workstation
Step 1
Workstation initiates FTP session to server

31. Duplicate IP Addresses at Workstation (continued)
Step 2
Second workstation with duplicate IP address initiates FTP session to server

32. Duplicate IP Addresses at Workstation (continued)
If the server receives a TCP/IP connection request from a second workstation with a duplicate IP address, the TCP/IP software may
Ignore the second request
Overwrite the server ARP cache entry with hardware address from second workstation
Get confused and crash
In either of the above choices, one or both of the workstations with the duplicate IP address will have connection problems

33. Duplicate IP Addresses at the Server
Workstation tries to connect to VAX at IP address
If the SUN server at duplicate IP address returns an ARP reply faster than the VAX, the workstation connects to the SUN server instead of the VAX
What happens if the SUN server and VAX server also act as routers?

34. ARP Display Utilities
Resolving duplicate IP address problems can be a challenging task on large networks
Keeping good records of IP address assignments and hardware addresses of devices can help
Use utilities to display and fix ARP cache entries
-- Most UNIX systems have the arp utility
arp -a
arp -d hostname
arp -s hostname hardware_addr
Display all ARP entries in table
Delete an entry from ARP table
Add a new entry in ARP table. Entry is not
timed out!

35. The Internet Protocol

36. Hands-On Exercise 3.1: Address Resolution
Your instructor will guide you to Hands-On Exercise 3.1 in the Exercise Manual

37. The Internet Protocol

38. Hands-On Exercise 3.2: Observing Effects of Duplicate IP Addresses
Your instructor will guide you to Hands-On Exercise 3.2 in the Exercise Manual

39. The Internet Protocol

40. IP Packet Structure Background:
This exercise is a guided tour on the structure of IP packets. It will be done concurrently with the lecture, which will explain the IP structure. You will use the packet trace that you saved in an earlier exercise for understanding the IP packet structure.
Objectives:
Examine the IP packet fields
Understand the functionality of the IP protocol

41. IP Packet Structure (continued)
Run LANWatch at your workstation.
If you forgot how to run LANWatch, see page 16 in the Exercise Manual.
Load the filt TELNET. TR1 that contains the TELNET packet trace youstored in an earlier exercise.
Highlight one of the red packets that contain IP protocol information and display it in the detailed format.
Follow the instructions given to you by the instructor.

42. IP Field: Version Version field Indicates format of IP header
Declares version of protocol to which datagram belongs
Allows development of new protocols while network is operational
What is the version of the IP packet on your screen?

43. IP Field: Internet Header Length
Measured in 32-bit words
Required because IP header contains variable length options field
What is the internet header length of the IP packet on your screen?___________________________
Does the IP packet have an options field?
Yes No

44. IP Field: Type of Service (TOS)
Informs networks on Quality Of Service (QOS) desired

45. IP Field: Type of Service (TOS)
What is the bit pattern for TOS of IP packet on your screen? __________________________
What is the TOS value? _________________

46. IP Field: Total Length Total length
Length of datagram (octets), including IP header and data portion
Maximum datagram size is 65,535 octets
All hosts must be prepared to receive datagrams of 576 octets
512 octets of data and 64 octets of protocol overhead
What is the total length for the IP packet on your screen? _________________ octets?

47. IP Field: Identification
Set uniquely for each datagram
Used as an aid in assembling fragments of a datagram
What is the identification value for the IP packet on your screen? ___________

48. IP Field: Identification (continued)
Use cursor keys (↑, ↓) to examine identification field values of IP packets before and after this IP packet
What is the identification value of the previous IP packet?______
What is the identification value of the next IP packet?________

49. IP Field: Flags
What are the flag settings for the IP packet on your screen?
DF flag =_______ MF flag = _______

50. IP Field: Fragment Offset
Position of fragment's data relative to the beginning of data carried in original datagram
Maximum of 8192 fragments per datagram
Identification field is same for all fragments

51. IP Field: Fragment Offset
What is the fragment offset for the IP packet on your screen?_________________________

52. IP Field: Time to Live Time to live
Maximum time IP datagram can remain on internet
When TTL = 0, IP datagram is destroyed (dropped)
Decreased by time for IP header processing, but must be decreased by at least 1

53. What is the TTL field value for the IP datagram on your screen

54. IP Field: Protocol Protocol field
Indicates which Upper Layer Protocol (ULP) is to receive data portion of IP datagram
What is the protocol field value for the IP packet on your screen?

55. IP Field: Protocol (continued)
Protocol field value
Keyboard
Description
Reserved 1
ICMP
Internet Control Message Protocol 6
TCP
Transmission Control Protocol 8
EGP
Exterior Gateway Protocol 9
IGP
Any private Interior Gateway Protocol 11
NVP
Network Voice Protocol 17
UDP
User Datagram Protocol 22
XNS IDP
Xerox Network System’s Internet Datagram Protocol 29
ISO TP4
ISO Transport Protocol class 4 89
OSPF
Open shortest path first

56. IP Field: Header Checksum
Covers only the IP header
Add up 1's complement of each data item (16-bit) and then the 1 's complement of the sum
Recomputed at every route because TTL field changes
What is the header checksum field value of the IP packet?_______________________

57. IP Field: Source Address, Destination Address
Source and destination addresses are divided in netid and hostid fields
What are the source-address and destination-address fields of the IP packet on your screen?
Source address: _______________
Destination address: ____________

58. IP Field: Options Options Officially defined options are
-- Security, loose source routing
-- Strict source routing, record route
-- Stream ID, Internet timestamp
Options are of two types

59. Are there any options defined in the IP packet on your screen

60. Bonus
Pick an IP packet (that appeals to you!) within LANWatch and analyze it on your own. Try to identify the blank fields in the IP datagram above. Label these fields and enter the value in the fields for the IP packet you are analyzing.

61. The Internet Protocol

62. Duplicate IP Address Problem
IP addresses must be unique
Most network software assumes trusted hosts
Duplicate IP addresses result in
Network software becoming confused, malfunctioning
Routing problems
-- Because routing information is encoded in IP address netid and hostid

63. Buffer Reassembly Problem
Not all IP implementations are equally robust
Some IP implementations may not reassemble datagram fragments correctly
Solution: Configure IP software for DF = 1
-- Problem:
DF flag may not be configurable by network manager
Fragmentation may be required if IP datagram traverses networks with small MTU

64. IP Trailers: 4BSD UNIX
Software derived from BSD 4.2 UNIX may use alternate IP encapsulation
Done for efficient memory management
-- To place data information on page boundary
Berkeley-style trailer encapsulation
Will not interoperate with normal IP encapsulation (example: IP routers)

65. Avoiding IP Trailers
On many UNIX systems, IP trailer encapsulation can be controlled by the if conf ig utility
Example:
ifconfig ethO -trailers
ifconfig ethO
ethO: flags=23<UP,BROADCAST,NOTRAILERS>
inet netmask ffffOOOO broadcast

66. All O's Broadcast
IP software implemented on earlier BSD 4.2 UNIX may use all O's broadcast
Can cause confusion with most systems that use all 1's broadcast
Use if conf ig utility to enable all 1's broadcast
May not work if broadcast mechanism has been hard-coded
Example:
ifconfig ethO broadcast
ifconfig ethO
ethO: flags=23<UP,BROADCAST,NOTRAILERS>
inet netmask ffffOOOO broadcast

67. The Internet Protocol

68. Using Unique Internet Addresses
If building your own private internet
Decide on an IP address class
-- Popular choices are class B, class C
If you decide not to connect to the Internet
You can select your own IP network number
If you decide to connect to the Internet
You should apply to Network information Center (NIC) for unique internetwork number
Alternatively, use IP address translation devices such as application-level gateways
-- Application-level gateways also can be used to implement
-- firewalls for enhancing security
-- Can be used to avoid duplicate IP address conflicts

69. Application-Level Security Gateway (Firewall)
Application-level firewall provides
Isolation between duplicate IP addresses
Security by restricting access between internal and external networks at the Application Layer
Alternatively, use a private address

70. Obtaining Unique IP Network Number
To obtain Internet number to connect to the Internet, apply to
Network Solutions InterNIC Registration Services 505 Huntmar Park Drive Herndon, VA USA
Users wanting to connect to the MILNET must still apply to
DDN Network Information Center Park Meadow Drive, Suite 200 Chantilly, VA USA
See Appendix D for application form for Obtaining IP Network Number

71. IP Address Database Local management of IP addresse RARP servers can
Configuration database kept on local machine
-- Often simple text files, such as /etc/hosts (UNIX) or net . cfg and config . tel, etc. (MS-DOS)
RARP servers
BOOTP server
DHCP server (covered in Course 154)
Name servers
RARP servers can
Simplify IP address maintenance
Problems:
-- Updates when data-link address changes
-- RARP storms
-- Single point of failure

72. The Internet Protocol

73. IP on IEEE 802 LANs Initial IP implementation on LANs was on Ethernet
Ethertype field in Ethernet header is used to indicate IP packet
-- Ethertype = 800 hex for IP packets
There is no Ethertype field in IEEE LANs
How do you indicate Ethertype information?
-- Use lEEE802.2LLC

74. IEEE 802.2 Logical Link Control
IEEE LANs use a sublayer called LLC to indicate protocol (software) addresses

75. The SNAP Protocol
A special DSAP or SSAP value in IEEE field indicates that Ethertype field is in the data field
This mechanism is called SubNet Access Erotocol (SNAP)

76. IP on IEEE 802.3, 802.5, and FDDI

77. IP on X.25
First octet in X.25 call request data field indicates IP protocol
X.25 virtual circuit is used to transmit datagrams
-- Closed after a period of inactivity
-- Treated as a point-to-point circuit

78. IP on ATM
ATM transmits data in fixed 53-byte cells (5 bytes header and 48 bytes data)
ATM cells available on demand with low latency
-- Real-time audio/video
-- Multimedia applications
ATM provides virtual channels with quality of service (QoS) parameters

79. Chapter Summary You have learned about
The role of the Internet Protocol (IP)
IP address classes
Address Resolution Protocols
IP packet structure
IP on various physical networks