Transcript Chp. 4, Part I - comp

Chapter 4: Internetworking
(Introduction)
Dr. Rocky K. C. Chang
16 March 2004
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1. The internetworking problem
• Problem: How to interconnect heterogeneous
networks effectively?
• Three problems with interconnection at the
data-link layer:
– Do not scale to the number of data-link
technologies.
– Do not scale to the number of hosts (or networks).
– Do not have a common addressing space.
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1. The internetworking problem
Network 1 (Ethernet)
H7
H2
H1
S3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
S1
S2
H4
Network 3 (FDDI)
H5
H6
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1.1 Scaling to data-link technologies
• Conversion between frame structures.
• Scalability problem as the number of data-link
technologies supported increases, e.g.,
Ethernet
PPP
Token ring
FDDI
Frame conversion
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1.2 Scaling to the network size
• A switched LAN is a “flat” network---A single
broadcast frame reaches every LAN.
– VLAN can relieve this problem at the expense of
managing VLAN membership.
• Spanning tree protocol does not scale well to
the network size.
– Take a longer time for the protocol to converge.
– Take a longer time to respond to network state
changes.
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1.3 Uncommon MAC address space
• The number of bits used in a MAC address may
differ.
– 48-bit IEEE MAC addresses
– IBM recommends another locally administered
MAC addresses (overriding the burned-in MAC
addresses).
• Each address in a data-link technology must be
universally unique, but its uniqueness is not
guaranteed when several networks are bridged.
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2. A layer-three internetworking solution
• Use IP, XNS, IPX, etc on top of the networks.
• Replace LAN switches with layer-three
switches, more commonly known as routers.
• Add IP software to each end host (with the
whole protocol suite software).
• Assign an IP address to each network interface.
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2. A layer-three internetworking solution
Network 1 (Ethernet)
H7
H2
H1
R3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
H4
Network 3 (FDDI)
H5
H6
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2.1.1 IP: Scaling to data-link technologies
Ethernet
Token ring
IP
PPP
FDDI
Encapsulation and demultiplexing
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2.1.2 IP: Scaling to the network size
• IP network uses hierarchy to achieve scalability.
• There are at least three levels:
– A single IP host (csultra6.comp.polyu.edu.hk)
– A IP subnet (four subnets in comp.polyu.edu.hk)
– An autonomous system (polyu.edu.hk)
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2.1.3 IP: Uncommon MAC Address space
• Create a logical (unicast) address space to identify
network interfaces.
• Classes A-C for unicast and a class D for
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multicast:
(a)
(b)
(c)
0
1
1
Network
0
1
Host
14
16
Network
Host
0
21
8
Network
Host
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(d)
1
1
1
0
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2.2 IP software at end hosts
• The IP software mainly consists of modules for
–
–
–
–
Application layer, such as DNS
Transport layer: TCP, UDP
Routing layer: IP, ICMP, and others.
Data-link layer: MAC-IP-addresses binding
Host names
DNS
ARP
IP addresses
MAC addresess
RARP
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2.2.1 An example
www.sun.com
140.20.1.1
m1.sun.com
140.20.1.2
• A HTTP client is running in m1.sun.com to
connect to a HTTP server at www.sun.com.
• The DNS client at the m1.sun.com first obtains
the IP address of www.sun.com.
• The application data (HTTP+TCP) will then be
encapsulated by an IP datagram with
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2.2.1 An example
– IP source address = 140.20.1.2
– IP destination address = 140.20.1.1
• Now m1.sun.com needs to run ARP to obtain
the MAC address of www.sun.com’s network
interface to the LAN.
• The IP datagram is then encapsulated in an
Ethernet frame with
– MAC source address = that of m1.sun.com
– MAC destination address = that of www.sun.com
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2.2.2 IP software at routers
• The software at routers is mainly used for
routing and datagram forwarding.
• Each router is running at least a “routing
protocol” to construct a routing (or forwarding)
table.
– Each entry in a routing table consists of IP
destination address and the next-hop’s IP address.
• Upon receiving a datagram, a router forwards it
based on a set of forwarding rules and the
routing table.
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2.3 Encapsulation and address binding
• To transmit IP datagrams over any data-link
network, two requirements are needed:
– A standard way to encapsulate IP datagrams
– Address resolution between IP addresses and MAC
addresses
• Standard RFCs for specifying datagram encapsulations and possibly address resolutions, e.g.,
Ethernet (RFC 894), IEEE 802 (RFC 1042), etc.
• A shared medium uses an Address Resolution
Protocol (ARP) for address binding.
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2.3.1 Data encapsulation
• You have seen
–
–
–
–
IP over DIX Ethernet (slide 19 in Chapter 2, part I)
IP over IEEE 802.3 (slide 22 in Chapter 2, part II)
IP over PPP (slide 22 in Chapter 2, part I)
IP over ATM via AAL 5 (slide 15 in Chapter 3, part
III)
Send out to the network interface
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2.3.2 Address resolution protocol
• An ARP request message is data-link
broadcasted on the LAN with the target IP
address.
• Every IP host picks up a copy of the message
and examine the target IP address.
– If matching its IP address, send an ARP reply
message back to the sender with its MAC address.
– Else, drop the message.
• To reduce broadcast traffic, each host uses an
ARP cache to remember the recent binding.
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2.3.2 Address resolution protocol
0
8
16
Hardware type = 1
HLen = 48
PLen = 32
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ProtocolType = 0x0800
Operation
SourceHardwareAddr (bytes 0 – 3)
SourceHardwareAddr (bytes 4 – 5) SourceProtocolAddr (bytes 0 – 1)
SourceProtocolAddr (bytes 2 – 3) TargetHardwareAddr (bytes 0 – 1)
TargetHardwareAddr (bytes 2 – 5)
TargetProtocolAddr (bytes 0 – 3)
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2.4 An internetworking example
• On each “hop or link,” both data encapsulation
and address resolution occur.
H1
H8
TCP
R1
IP
IP
ETH
R2
ETH
R3
IP
FDDI
FDDI
IP
PPP
PPP
TCP
IP
ETH
ETH
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