1. Introduction to Communication Networks
Lecture 7
Internetworking
Introduction to Communication Networks 2/2005

2. Motivation LANs WANs Low cost Limited distance High cost
Unlimited distance.
Introduction to Communication Networks 2/2005

3. Goal: Universal Service
Arbitrary pair of computers can communicate
Pioneered by telephone system
Fundamental concept in networking
Incompatibilities among networks
Electrical properties
Signaling and data encoding
Packet formats
Addresses
Introduction to Communication Networks 2/2005

4. An Internetwork or internet
Introduction to Communication Networks 2/2005

5. Internet Architecture
Multiple
Networks
Routers interconnecting networks
Host computer connects to a network
Single router has insufficient
CPU power and memory
I/O capability
Introduction to Communication Networks 2/2005

6. Virtual Network Seamless Uniform General-purpose Universal
Hides heterogeneity from user
Introduction to Communication Networks 2/2005

7. Hide Heterogeneity Create ‘‘virtual’’ network
Addressing scheme
Naming scheme
Implement with: Protocol software
Note: protocol software needed on both hosts and routers
Introduction to Communication Networks 2/2005

8. The Internet Network layer
Host, router network layer functions:
routing
table
Routing protocols
path selection
RIP, OSPF, BGP
IP protocol
addressing conventions
datagram format
packet handling conventions
ICMP protocol
error reporting
router “signaling”
Transport layer: TCP, UDP
Link layer
physical layer
Network
layer
Introduction to Communication Networks 2/2005

9. Internet Reference Model
Layer 1: Physical. Basic network hardware
Layer 2: Network Interface
MAC frame format
MAC addressing
Interface between computer and network (NIC)
Layer 3: Internet. Facilities to send packets across internet composed of multiple routers
Layer 4: Transport: Transport from an application on one computer to application on another
Layer 5: Application. Everything else
Introduction to Communication Networks 2/2005

10. Internet Protocol (IP)
Only protocol at Layer 3
Defines
Internet addressing
Internet packet format
Internet routing
We have talked until now about general routing and route establishment and maintenance protocols
Introduction to Communication Networks 2/2005

11. IP Addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host, router and physical link
router’s typically have multiple interfaces
host may have multiple interfaces
IP addresses associated with interface, not host, or router =
223 1 1 1
Introduction to Communication Networks 2/2005

12. IP Addressing IP address:
network part
high order bits
host part
low order bits
What’s a network ? (from IP address perspective)
device interfaces with same network part of IP address
can physically reach each other without intervening router
LAN
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
Introduction to Communication Networks 2/2005

13. IP Addressing How to find the networks?
Detach each interface from router, host
create “islands of isolated networks
Interconnected
system consisting
of six networks
Introduction to Communication Networks 2/2005

14. IP Addresses given notion of “network”, let’s re-examine IP addresses:
“class-full” addressing:
class A to
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits
Introduction to Communication Networks 2/2005

15. IP addressing: CIDR Classfull addressing:
inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network
CIDR: Classless Inter Domain Routing
network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in network portion of address
network
part
host
/23
Introduction to Communication Networks 2/2005

16. IP Addressing - Problem
Address classes were too “rigid”. For most organizations, Class C were too small and Class B too big. Led to inefficient use of address space, and a shortage of addresses.
Organizations with internal routers needed to have a separate (Class C) network ID for each link.
And then every other router in the Internet had to know about every network ID in every organization, which led to large address tables.
Small organizations wanted Class B in case they grew to more than 255 hosts. But there were only about 16,000 Class B network IDs.
Introduction to Communication Networks 2/2005

17. IP Addressing cont. Two solutions were introduced:
Subnetting within an organization to subdivide the organization’s network ID.
Classless Interdomain Routing (CIDR) in the Internet backbone was introduced in 1993 to provide more efficient and flexible use of IP address space.
CIDR is also known as “supernetting” because subnetting and CIDR are basically the same idea.
Introduction to Communication Networks 2/2005

18. Subnetting CLASS “B” e.g. Company 10 Net ID Host-ID Subnet ID (20)14 16
0000
Subnet ID (20)
Subnet
Host ID (12)
1111
000000
Subnet ID (22)
Host ID (10)
Subnet ID (26)
Host ID (6)
e.g. Site
e.g. Dept
Introduction to Communication Networks 2/2005

19. Subnetting Subnetting is a form of hierarchical routing.
Subnets are usually represented via an address plus a subnet mask or “netmask”.
Netmask ffffff00: the first 24 bits are the subnet ID, and the last 8 bits are the host ID.
Can also be represented by a “prefix + length”, e.g /24, or just /24.
Introduction to Communication Networks 2/2005

20. Classless Inter-domain Routing (CIDR) Addressing
The IP address space is broken into line segments.
Each line segment is described by a prefix.
A prefix is of the form x/y where x indicates the prefix of all addresses in the line segment, and y indicates the length of the segment.
e.g. The prefix 128.9/16 represents the line segment containing addresses in the range: …
232-1
128.9/16
216
142.12/19
65/8
Introduction to Communication Networks 2/2005

21. Classless Interdomain Routing (CIDR) Addressing
/24
/24
/20
/20
Most specific route = “longest matching prefix”
128.9/16
232-1
Introduction to Communication Networks 2/2005

22. Classless Interdomain Routing (CIDR) Addressing
Prefix aggregation:
If a service provider serves two organizations with prefixes, it can (sometimes) aggregate them to form a shorter prefix.
Other routers can refer to this shorter prefix, and so reduce the size of their address table.
E.g. ISP serves /24 and /24, it can tell other routers to send it all packets belonging to the prefix /23.
ISP Choice:
In principle, an organization can keep its prefix if it changes service providers.
Introduction to Communication Networks 2/2005

23. Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us
Introduction to Communication Networks 2/2005

24. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
Introduction to Communication Networks 2/2005

25. IP addresses: how to get one?
Network (network portion):
get allocated portion of ISP’s address space:
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Introduction to Communication Networks 2/2005

26. IP addresses: how to get one?
Hosts (host portion):
hard-coded by system admin in a file
DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play”
host broadcasts “DHCP discover” msg
DHCP server responds with “DHCP offer” msg
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
The common practice in LAN and home access (why?)
Introduction to Communication Networks 2/2005

27. IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes
Introduction to Communication Networks 2/2005

28. Getting a datagram from source to dest.
routing table in A
Dest. Net. next router Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A B E
datagram remains unchanged, as it travels source to destination
addr fields of interest here
mainly dest. IP addr
Introduction to Communication Networks 2/2005

29. Getting a datagram from source to dest.
misc
fields
data
Dest. Net. next router Nhops
Starting at A, given IP datagram addressed to B:
look up net. address of B
find B is on same net. as A
link layer will send datagram directly to B inside link-layer frame
B and A are directly connected A B E
Introduction to Communication Networks 2/2005

30. Getting a datagram from source to dest.
misc
fields
data
Dest. Net. next router Nhops
Starting at A, dest. E:
look up network address of E
E on different network
A, E not directly attached
routing table: next hop router to E is
link layer sends datagram to router inside link-layer frame
datagram arrives at
continued….. A B E
Introduction to Communication Networks 2/2005

31. Getting a datagram from source to dest.
network router Nhops interface
Dest next
misc
fields
data
Arriving at , destined for
look up network address of E
E on same network as router’s interface
router, E directly attached
link layer sends datagram to inside link-layer frame via interface
datagram arrives at !!! (hooray!) A B E
Introduction to Communication Networks 2/2005

32. Special Addresses Network address not used in packets
Loopback never leaves local computer
Introduction to Communication Networks 2/2005

33. Mapping Computer Names to IP addresses The Domain Naming System (DNS)
Names are hierarchical and belong to a domain:
e.g. cse.bgu.ac.il
Common domain names: .com, .gov, .org, .net, .il (or other country-specific domain).
Top-level names are assigned by the Internet Corporation for Assigned Names and Numbers (ICANN).
A unique name is assigned to each organization.
DNS Client-Server Model
DNS maintains a hierarchical, distributed database of names.
Servers are arranged in a hierarchy.
Each domain has a “root” server.
An application needing an IP address is a DNS client.
Introduction to Communication Networks 2/2005

34. Mapping Computer Names to IP addresses The Domain Naming System (DNS)
A DNS Query
Client asks local server.
If local server does not have address, it asks the root server of the requested domain.
Addresses are cached in case they are requested again.
huji.ac.il
bgu.ac.il
cse.bgu.ac.il
.ac
“What is the IP address of
e.g. gethostbyname()
Client
application
Introduction to Communication Networks 2/2005

35. Summary Internetworking Internet concept
Solves problem of heterogeneity
Includes LANs and WANs
Internet concept
Virtual network
Seamless
Universal
Introduction to Communication Networks 2/2005

36. Summary: Architecture
Internet architecture
Multiple networks
Interconnected by routers
Router
Special-purpose computer system
Interconnects two or more networks
Uses table to forward datagrams
Introduction to Communication Networks 2/2005

37. Summary: Internet Protocol (IP)
Fundamental piece of TCP/ IP
Defines
Internet addressing
Delivery semantics
Internet packet format (IP datagram)
Introduction to Communication Networks 2/2005

38. Internet Transmission Paradigm
Source host
Forms datagram
Includes destination address
Sends to nearest router
Intermediate routers
Forward datagram to next router
Final router
Delivers to destination host
Introduction to Communication Networks 2/2005

39. Datagram Transmission
Datagram sent across conventional network
From source host and router
Between intermediate routers
From final router to destination host
Network hardware does not recognize
Datagram format
IP addresses
Encapsulation needed
Introduction to Communication Networks 2/2005

40. Created and understood only by software
Internet Packets
Created and understood only by software
Contains sender and destination addresses
Size depends on data being carried
Called IP datagram
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41. IP Datagram
Introduction to Communication Networks 2/2005

42. Datagram Header Source IP address Destination IP address Type ver TTL
Flags
ver
TTL
TOS
checksum
H-Len
Total Length ID
FRAG Offset
Protocol
SRC IP Address
DST IP Address
(OPTIONS)
(PAD)
<=64 KBytes
Offset within original packet
Hop count
Beginning of data
Source IP address
Destination IP address
Type
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43. IP Semantics IP is connectionless Routes can change at any time
Datagram contains identity of destination
Each datagram sent / handled independently
Routes can change at any time
Introduction to Communication Networks 2/2005

44. Frame and Datagram Destination Addresses
Frame address
Hardware (MAC) address
Next hop
Datagram address
IP address
Ultimate destination
Introduction to Communication Networks 2/2005

45. Resolving Addresses Hardware only recognizes MAC addresses
IP only uses IP addresses
Consequence: software needed to perform translation
Part of network interface
Known as address resolution
Introduction to Communication Networks 2/2005

46. Address Resolution Layer 2 protocol Given Find Technique
A locally-connected network, N
IP address C of computer on N
Find
Hardware address for C
Technique
Address Resolution Protocol
Introduction to Communication Networks 2/2005

47. Address Resolution Protocol (ARP)
Keep bindings in table
Table entry contains pair of addresses for one computer
IP address
Hardware address
Build table automatically as needed
Introduction to Communication Networks 2/2005

48. ARP Table Only contains entries for computers on local network
IP network prefix in all entries identical
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49. ARP Lookup Algorithm Look for target IP address, T, in ARP table
If not found
Send ARP request message to T
Receive reply with T’s hardware address
Add entry to table
Return hardware address from table
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50. ARP Exchange
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51. ARP Message Format (Ethernet)
Length of hardware address fields depend on network type
Ethernet uses 48-bit addresses
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52. ARP Message Transmission
ARP message sent in payload area of frame
Called encapsulation
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53. Addressing Example H1 H2 H3 R1 R2 H7 H8 H4 H6 H5 Network 1 (Ethernet)
Network 3 (FDDI)
Network 2 (Ethernet)
Network 4 (PTP)
Introduction to Communication Networks 2/2005

54. Frames and Datagrams
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55. Maximum Frame Size Each network technology imposes maximum frame size
Called Maximum Transmission Unit (MTU)
MTUs differ in different technologies
Internet
Can contain heterogeneous technologies
Must accommodate multiple MTUs
Introduction to Communication Networks 2/2005

56. Datagram Fragmentation
Performed by routers
Needed when datagram larger than MTU of network
Divides datagram into pieces called fragments
Each fragment has datagram header
Fragments sent separately
Ultimate destination reassembles fragments
Introduction to Communication Networks 2/2005

57. Illustration of Datagram Fragmentation
Problem: A router may receive a packet larger than the maximum transmission unit (MTU) of the outgoing link.
Solution: R1 fragments the IP datagram into multiple, self-contained datagrams. A
Ethernet
MTU=1500 bytes B
Source
Destination
MTU<1500 bytes R1 R2
Data
HDR (ID=x)
Offset>0
More Frag=0
Offset=0
More Frag=1
Introduction to Communication Networks 2/2005

58. Datagram Fragmentation
Each fragment has IP datagram header
Header fields
Identify original datagram
Indicate where fragment fits
Fragments are re-assembled by the destination host; not by intermediate routers.
To avoid fragmentation, hosts commonly use path MTU discovery to find the smallest MTU along the path.
Most links use MTU>=1500bytes today.
Introduction to Communication Networks 2/2005

59. Path MTU Discovery
IP datagram header contains a bit to specify no fragmentation allowed
ICMP sends an error message when fragmentation required but not permitted
Path MTU discovery involves sending various size datagrams until they do not require fragmentation along the path.
Note: MTU not guaranteed if routes change
Introduction to Communication Networks 2/2005

60. Fragment Loss Receiver
Collects incoming fragments
Reassembles when all fragments arrive
Does not know identity of router that did fragmentation
Cannot request missing pieces
Consequence: Loss of one fragment means entire datagram lost
Introduction to Communication Networks 2/2005

61. IP Semantics IP is best-effort Datagrams can be Lost Delayed
Duplicated
Delivered out of order
Corrupted
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62. Error Detection IP does not Errors detected Introduce errors
Ignore all errors
Errors detected
Corrupted bits
Illegal addresses
Routing loops
Fragment loss
Introduction to Communication Networks 2/2005

63. Problems and Solutions
Corrupted header bits
Header checksum
Illegal destination address
Routing tables
Routing loop
Time-To-Live (TTL) field
Fragment loss
Timeout
Introduction to Communication Networks 2/2005

64. Internet Control Message Protocol (ICMP)
Separate protocol for
Errors
Information
Required part of IP
Sends error messages to original source
Introduction to Communication Networks 2/2005

65. ICMP Messages: Source Quench & Time Exceeded
Source Quench (overfull)
Sent by router
Triggered by datagram overrun
Requests sending host(s) to slow down
Time Exceeded
TTL on datagram reached zero
Not a request for retransmission
Sent by host
Reassembly timeout (some fragments lost)
Introduction to Communication Networks 2/2005

66. ICMP Messages: Destination unreachable & Redirect
Specifies whether
Destination network unreachable
Destination host unreachable
Protocol port on destination unreachable
Redirect
Sent by router
Goes to host on local network
Host used incorrect initial router
Requests host to change routes
Introduction to Communication Networks 2/2005

67. ICMP Message Transport
Error messages go back to original source (may cross internet)
Messages carried in IP
Two levels of encapsulation
IP type field specifies ICMP
Introduction to Communication Networks 2/2005

68. Summary: Address Resolution
Needed to map IP address to equivalent hardware address
Part of network interface
Uses table
Automatically updates table entries
Broadcasts requests
Introduction to Communication Networks 2/2005

69. Summary: Maximum Payload UNIT
Network hardware has maximum payload size
Called MTU
Datagram must be smaller than hardware MTU
Datagram fragmentation
Accommodates multiple MTUs
Performed by router
Divides datagram into pieces
Ultimate destination reassembles
Introduction to Communication Networks 2/2005

70. Summary: Internet Control Message Protocol
Mechanism to detect errors
Header checksum
Time-to-live field
Internet Control Message Protocol
Has both error and informational messages
Closely integrated with IP
ICMP messages
Encapsulated in IP
Sent back to original source
Introduction to Communication Networks 2/2005