CS2200 Presentation 26

1
CS2200Presentation 26

• TCP/IP
• see
• Computer Networks and Internets
• Second Edition
• Douglas E. Comer

2
Local Area Networks

• LAN Types
• Ethernet
• Token Ring
• LAN Components
• NIC
• Repeaters
• Bridges
• Switches

3
Wide Area Networks

• Difference between LAN and WAN?
• Scalability
• Packet Switches

Connections to other packet switches
Connections to computers
4
Forming a WAN
Switch Site 1
Switch Site 4
Switch Site 2
Switch Site 3
5
Physical Addressing in a WAN
1,2
Switch Site 1
Switch Site 4
A
B
H
1,5
4,2
Switch Site 2
Switch Site 3
Hierarchical Addressing
C
F
E
D
G
2,2
2,6
3,1
3,4
3,7
6
Next Hop Forwarding
7
Source Independence

• Forwarding is only based on destination
• Example
• Passengers arriving in Atlanta from Boston, Los
  Angeles and Midtown all look in one place to find
  where to board flight to Miami
• Allows compact tables and a single mechanism for
  handling forwarding

8
Routing

• "Next Hop Information" table is commonly called a
  routing table.
• Process of forwarding a packet to its next hop is
  known as routing.
• Hierarchical addressing (i.e. 1,2)
• Computation can be reduced
• Routing table can be made shorter

9
Table Size Reduction
Next Hop Information
Destination
Next Hop
1,anything
Int 2
2,anything
Local
3,anything
Int 4
4,anything
Int 3
10
Routing in a WAN

• As mentioned key element of WAN is scalability
• Capacity of a WAN may be increased by adding
  packet switches (without attached computers) to
  the interior of the network
• Exterior switches have attched computers
• Interior switches do not
• Each switch must have routing table and be able
  to forward packets
• Each routing table must be able to handle all
  possible addresses
• Tables must point to shortest route (Optimal)

11
Routing in a WAN
4
1
3
2
12
Routing Tables
13
Use of Default Routes
Node 1
Node 2
Node 3
Node 4
Dest
Next Hop
Dest
Next Hop
Dest
Next Hop
Dest
Next Hop
1
-
1
(2,1)
3
-
3
(4,3)

(1,2)
2
-
4
(3,4)
4
-
3
(2,3)

(3,2)

(4,2)
4
(2,4)
14
Routing Table Computation

• Routing tables are computed automatically
• Two basic approached are used
• Static routing
• Program runs when packet switch boots
• Advantages Simple with low network overhead
• Disadvantage Inflexible
• Dynamic routing
• Program builds routing table on boot and then as
  conditions change adjusts table
• Advantage Allows network to handle problems
  automatically

15
WAN Technologies

• ARPANET
• One of the first packet switched networks
• X.25
• CCITT X.25
• Popular in Europe
• Originally for ASCII to Host connections
• Frame Relay
• Originally designed to bridge LAN segments
• SMDS (Switched Multi-megabit Data Service)
• ATM (Asynchronous Transfer Mode)
• Intended for voice, video and data over wide
  areas
• Uses fixed size cells
• Can specify quality of service required

16
Internetworking

• Different networking solutions exist
• Why? No single networking technology is best for
  all needs
• Universal service
• System where arbitrary pairs of computers can
  communicate
• Increases productivity
• Networks, by themselves, are incompatible with
  universal service
• Solution Internetworking or an internet

17
Physical Network Connection
Router
Individual Networks
Each cloud represents arbitrary network
technology LAN, WAN, ethernet, token ring, ATM,
etc.
18
Router
A router is a special-purpose computer dedicated
to the task of interconnecting networks. A router
can interconnect networks that use different
technologies, including different media, physical
addressing schemes or frame formats.
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Virtual Network
20
Virtual Network
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TCP/IP

• A number of different protocols have been
  developed to permit internetworking
• TCP/IP (actually a suite of protocols) was the
  first developed.
• Work began in 1970 (same time as LAN's were
  developed)
• Most of the development of TCP/IP was funded by
  the US Government (ARPA)

22
Layered Model
Application
5
Transport
4
Internet
3
Network Interface
2
Physical
1
23
Layer upon layer upon layer...

• Layer 1 Physical
• Basic network hardware (same as ISO model Layer
  1)
• Layer 2 Network Interface
• How to organize data into frames and how to
  transmit over network (similar to ISO model Layer
  2)
• Layer 3 Internet
• Specify format of packets sent across the
  internet as well as forwarding mechanisms ised by
  routers
• Layer 4 Transport
• Like ISO Layer 4 specifies how to ensure reliable
  transfer
• Layer 5 Application
• Corresponds to ISO Layers 6 and 7. Each Layer 5
  protocol specifies how one application uses an
  internet

24
Host Computers, Routers and Protocol Layers

• Host computer (or sometimes "Host")
• Any computer system that connects to an internet
  and runs applications
• Use all layers of TCP/IP model
• Router
• Connect networks to form internet
• Do not use protocols from all layers of TCP/IP
• In particular, does not use layer 5

25
IP Internet Protocol Addresses

• Recall The various networking schemes (LAN's and
  WAN's) we discussed used physical addresses
• To achieve a seamless network with universal
  connectivity we need addresses for the virtual
  internet
• The internet is an abstraction created in
  software which can use addresses, packet format
  and delivery techniques independent of the
  physical hardware

26
IP Addressing

• Each host in the internet must have a unique
  address
• Users, application programs and software
  operating in the higher layers of the protocol
  stack use these addresses
• In the IP protocol each host is assigned a unique
  32 bit address. Any packet destined for a host on
  the internet will contain the destination IP
  address.

27
IP Address Hierarchy

• Addresses are broken into a prefix and a suffix
  for routing efficiency
• The Prefix is uniquely assigned to an individual
  network.
• The Suffix is uniquely assigned to a host within
  a given network

1
1
Network 1
2
Network 2
3
3
5
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Guarantee

• Each computer has a unique address
• The full address contains both a prefix and a
  suffix assigned to guarantee uniqueness.
• Although network numbers must be assigned
  globally, suffixes can be assigned locally
  without global coordination

29
How many bits?

• How should the 32 bit address be divided?
• In other words how many bits for prefix, how many
  for suffix?
• Example 1
• 16 bits for each
• 65536 max networks, 65536 max hosts/network
• Example 2
• 24 bits for prefix, 8 bits for suffix
• 8,388,608 max networks, 256 max hosts/network
• Other possibilities?

30
More Flexible System

• Create system with different classes of address.
  Each class has different size for the prefix and
  the suffix
• (Up to) the first 4 bits determine the class
• Five classes are defined

31
Five Classes of IP Address
32
Five Classes of IP Address
Primary Classes
33
Computingthe Class
34
Dotted Decimal Notation

• Conventionally 32 bit IP addresses are expressed
  in dotted decimal notation
• Each byte is expressed as a decimal number
  (0-255). The bytes are separated by decimal
  points
• Addresses range from 0.0.0.0 to 255.255.255.255

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Classes and Dotted Decimal

• Class
• A
• B
• C
• D
• E

• Range of Values
• 0 through 127
• 128 through 191
• 192 through 223
• 224 through 239
• 240 through 255

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Division of the Address Space
Address Class
Bits in Prefix
Maximum Number of Networks
Bits in Suffix
Maximum Number of Hosts per Network
A B C
7 14 21
128 16384 2097152
24 16 8
16777216 65536 256
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Addressing Example
128.10
128.211
128.10.0.1
128.10.0.2
128.211.28.4
128.211.6.115
10
192.5.48
10.0.0.37
10.0.0.49
192.5.48.3
192.5.48.85
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Special IP Addresses

• Network Address
• Directed Broadcast Address
• Limited Broadcast Address
• This Computer Address
• Loopback Address
• Berkley Broadcast Address Form

39
Network Address

• Useful to have an address which represents a
  network
• Formed by adding a 0 suffix
• Example
• 128.10 ? 128.10.0.0
• 10 ? 10.0.0.0
• 192.5.48 ? 192.5.48.0
• A network address should never appear as a
  destination in a packet

40
Directed Broadcast Address

• Often convenient to send a message to all hosts
  on a single network
• Directed broadcast address formed by adding a
  suffix containing all 1 bits
• Once the direct broadcast message arrives in the
  destination network it is sent to all host on the
  network via
• The local networks hardware broadcast facility or
  if none present
• Individual messages sent to each host

41
Limited Broadcast Address

• Typically used on startup by a computer that
  doesn't yet know the network number
• Message must contain all 1 bits
• Message remains on local net

42
This Computer Address

• A computer needs to know its IP address to send
  or receive internet packets
• TCP/IP contains protocols which allow a computer
  to obtain its IP address automatically when it
  boots
• These startup protocols use IP to communicate
• Sending an IP packet requires a source address
• Address 0.0.0.0 means "this computer"

43
Loopback Address

• During testing it is often convenient to have two
  applications which will eventually communicate
  run on the same computer.
• A message can travel down the stack from one
  application and back up the stack to the other
  application
• IP reserves class A network prefix 127 for this
  purpose (the suffix doesn't matter)
• By convention 127.0.0.1 is most often used

44
Berkley Broadcast Address Form

• UC Berkley developed and distributed an early
  version of TCP/IP as part of BSD UNIX
• Instead of a directed broadcast address suffix of
  all 1 bits they used a suffix of all 0 bits. This
  is known as a Berkley Broadcast
• Many early computer manufacturers derived their
  software from the Berkley Implementation
• Some can accept either, some both

45
Special IP Address Summary
Prefix
Suffix
Type of Address
Purpose
All-0's
All-0's
This computer
Used during bootstarp
Network
All-0's
Network
Identifies a network
Network
all-1's
Directed broadcast
Broadcast on specified net
All-1's
All-1's
Limited broadcast
Broadcast on local net
127
Any
Loopback
Testing
Network
All-0's
Directed broadcast
Berkley broadcast
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Routers and IP Addressing

• Each host has an address
• Each router has two (or more) addresses!
• Why?
• A router has connections to multiple physical
  networks
• Each IP address contains a prefix that specifies
  a physical network
• An IP address does not really identify a specific
  computer but rather a connection between a
  computer and a network.
• A computer with multiple network connections
  (e.g. a router) must be assigned an IP address
  for each connection

47
Example
Ethernet 131.108.0.0
Token Ring 223.240.129.0
131.108.99.5
223.240.129.2
223.240.129.17
78.0.0.17
WAN 78.0.0.0
Note!
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Multi-homed Hosts

• Can a host have multiple network connections?
• Yes! Why?
• Increase reliability
• Increase performance
• Like router, need one address for each connection

49
Address Resolution Protocol

• IP addresses are virtual
• LAN/WAN hardware doesn't understand IP addresses
• Frame transmitted across a network must have
  hardware address of destination (in that network)
• Three basic mechanisms for resolving addresses

50
Resolving Addresses

• 1. Address translation table
• Used primarily in WAN's
• 2. Translation by mathematical function
• 3. Distributed computation across network
• Protocol addresses are abstractions
• Physical hardware does not know how to locate a
  computer from its protocol addess
• Protocol address of next hop must be must be
  translated to hardware address

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Address Resolution
A
C
E
R2
R1
B
D
F
52
Address Resolution
A
C
E
R2
R1
B
D
F
Application sends message from A to B using B's
IP address Protocol software on A resolves IP
address of B to physical hardware address and
sends frame directly using hardware address
53
Address Resolution
A
C
E
R2
R1
B
D
F
Application sends message from A to F using F's
IP address Software on A does NOT resolve F's
address
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Address Resolution
A
C
E
R2
R1
B
D
F
Application sends message from A to F using F's
IP address Software on A first determines that
message must pass through router R1. Address of
R1 is resolved and message is sent to R1
55
Address Resolution
A
C
E
R2
R1
B
D
F
Application sends message from A to F using F's
IP address Software on R1 determines that
message must pass through router R2. Address of
R2 is resolved and message is sent to R2
56
Address Resolution
A
C
E
R2
R1
B
D
F
Application sends message from A to F using F's
IP address Software on R2 determines that
message is intended for host on local net.
Address of F is resolved by R2 and message is
sent to F
57
How to Resolve Addresses

• Table Lookup
• Store bindings/mapping in table which software
  can search
• Closed-form Computation
• Protocol addresses are chosen to allow
  computation of hardware address from protocol
  address using basic boolean and arithmetic
  operations
• Message Exchange
• Computers exchange messages across a network to
  resolve addresses. One computer sends a message
  requesting a translation and another computer
  replies

58
Table Lookup I

• IP Address
• 197.15.3.2
• 197.15.3.3
• 197.15.3.4
• 197.15.3.5
• 197.15.3.6
• 197.15.3.7

• Hardware Address
• 0A074B128236
• 0A9CBC71328D
• 0A119A680199
• 0A078290CC1F
• 0A7776EF0328
• 0A828F5ABEFA

For large tables may use hashing or direct lookup
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Table Lookup IIDirect Lookup

• IP Address
• 197.15.3.4

• Hardware Address
• 0A074B128236
• 0A9CBC71328D
• 0A119A680199
• 0A078290CC1F
• 0A7776EF0328
• 0A828F5ABEFA

Must also do array bounds checking
60
Address Resolution with Closed-form Computation

• Some networks have configurable hardware
  addresses
• NIC can be assigned any physical address
• By judiciously selecting hardware and IP
  addresses, efficient computation of a hardware
  address can be made from an IP address

61
Example

• IP Address
• network 220.123.5.0
• 220.123.5.1
• 220.123.5.2
• 220.123.5.3
• 220.123.5.4
• 220.123.5.5

• Assigned Hardware Address
• 1
• 2
• 3
• 4
• 5

hardwareAddress IPAddress 0xFF
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Message Exchange

• Computer needing address resolved sends message
  across network
• The message carries an address that needs to be
  resolved
• The reply contains the appropriate hardware
  address
• Two possible designs
• Special address resolution servers are
  established known to computers on net
• Request is broadcast to net and each computer is
  responsible for replying when request is for its
  address

63
Address Resolution Protocol

• TCP/IP can use any of the three methods
• Table lookup usually used in a WAN
• Closed-form computation is used with configurable
  networks
• Message exchanged used in LAN's with static
  addressing
• To insure that all computers agree TCP/IP
  includes an Address Resolution Protocol
• Two types of messages are supported
• Request a hardware address given a protocol
  address
• Reply containing IP Address and hardware request

64
ARP Message Delivery
65
ARP Message Delivery
66
ARP Message Delivery
67
ARP Message Delivery
68
Caching ARP Responses

• When a ARP response is received the result is
  cached (new responses replace old ones)
• Cache size is limited
• Entries are removed after some amount of time if
  unused (e.g. 20 minutes)
• When an ARP response is sent, the sender puts the
  address binding in its cache
• Communication is typically two-way
• Space is limited (i.e. computers do not attempt
  to listen to the net and store all addressing
  information that passes).

69
IP Datagrams and Datagram Forwarding

• At the IP layer the service provided is
  connectionless
• Messages are sent and forwarded across network
• Service is not reliable!
• The packet used by IP to send information is
  called an IP Datagram
• The IP Datagram will be placed into physical
  frames

70
IP Addresses and Routing Table Entries
71
IP Addresses and Routing Table Entries
R1
R2
R3
Assume message with IP address
192.4.10.3 arrives at router R2
for each entry in table if(Mask Addr)
Dest forward to NextHop
72
Best-Effort Delivery

• IP does not handle the problems of
• Datagram duplication
• Delayed or out-of-order delivery
• Corruption of data
• Datagram loss

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IP Encapsulation
Frame Header
Frame Data
74
Transmission Across an Internet
Source Host
Net 1
header 1
Router 1
Net 2
header 2
Router 2
Net 3
header 3
Destination Host
75
MTU and Fragmentation

• For any given network there is a Maximum
  Transmission Unit or MTU
• If a datagram arrives at a network an exceeds the
  MTU the protocol software will break the Datagram
  up into smaller pieces called fragments
• The format of a fragment is the same except for
  bits which are set to indicate a fragment

76
Reassembly

• Fragments are never reassembled until the final
  destination
• Why?
• Reduce amount of state information in routers.
  When packets arrive at a router they can simply
  be forwarded
• Allows routes to change dynamically. Intermediate
  reassembly would be problematic if all fragments
  didn't arrive.

77
Error Reporting (ICMP)

• TCP/IP includes a protocol used by IP to send
  messages when problems are detected Internet
  Control Message Protocol
• IP uses ICMP to signal problems
• ICMP uses IP to send messages
• When IP detects an error (e.g. corrupt packet) it
  sends an ICMP packet
• Exception Problems with ICMP messages are not
  reported? Why?

78
Some ICMP Messages

• 0
• 3
• 4
• 5
• 6
• 8
• 9
• 10
• 11
• 12

• Echo Reply
• Destination unreachable
• Source quench
• Redirect
• Alternate host address
• Echo
• Router advertisement
• Router selection
• Time exceeded
• Parameter problem

13 14 15 16 17 18 30 31 Note Max message 255
Timestamp Timestamp reply Info request Info
reply Addr mask req Addr mask reply Traceroute Dat
agram conv error
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Sample Messages

• Source Quench - Sent by router when out of buffer
  space (and discards a datagram). Sent to the
  originator of the datagram. Sender must reduce
  transmission rate.
• Time Exceeded - Sent by router when discarding a
  datagram whose Time to Live field has reached 0.
  Also, sent if reassembly timer expires before all
  fragments have arrived.
• Destination Unreachable - Router that determines
  a message cannot be delivered to its final
  destination sends to originator
• An entire network is disconnected from internet
• A given host is offline
• Note Some ICMP messages are not error messages

80
ICMP Message Transport
81
ICMP Message Transport

• Where should ICMP message be sent?
• ICMP messages are always created in response to a
  Datagram.
• Router sends ICMP message to source of datagram
• What happens if Datagram containing ICMP message
  encounters an error
• Nothing!!!

82
Testing Reachability

• Ping
• Sends an ICMP echo request message
• Starts a timer
• If no answer...retransmits, etc.

83
Using ICMP to Trace a Route

• Datagram has "TIME TO LIVE" field.
• Upon reaching a router the "TIME TO LIVE" field
  is decremented
• If the field reaches 0, Datagram is discarded and
  ICMP message is sent to originator
• We can use this operation to trace a route

84
Tracing a Route

• Send a Datagram to the destination with the "TIME
  TO LIVE" field set to 1
• At the first router "TIME TO LIVE" will be set to
  0 and an ICMP message will be returned
• Send a Datagram to the destination with the "TIME
  TO LIVE" field set to 2
• etc.
• (Some details omitted)

85
TCP Reliable Transport Service

• TCP must use an inherently unreliable service,
  IP, to provide reliable service
• TCP must supply a service that guarantees
• Prompt, reliable communication
• Data delivery in the same order sent
• No loss
• No duplication

86
Services Provided by TCP

• Connection Orientation
• Point-To-Point Communication
• Complete Reliability
• Full Duplex Communication
• Stream Interface
• Reliable Connection Startup
• Graceful Connection Shutdown

87
End to End Services

• TCP provides a connection from one application on
  a computer to an application on a remote computer
• Connection is virtual - provided by software
  passing messages
• TCP messages are encapsulated in IP Datagrams
• Upon arrival IP passes the TCP message on to the
  TCP layer.
• TCP exists at both end of the connection but not
  at intermediate points (routers).

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Achieving Reliability

• Causes of problems
• Failure of the IP system to deliver information
  reliably
• Messages may be duplicated, lost, delayed or
  delivered out of order
• Reboot of a host computer
• Two programs make a connection
• One computer reboots
• New connection is formed
• Messages from first session now arrive

90
Packet Loss and Retransmission

• Host 1
• Send message 1
• Start timer
• Receive ack 1
• Send message 2
• Start timer
• Timer expires
• Retransmit message 2
• Start timer
• Receive ack 2

• Host 2
• Receive message 1
• Send ack 1
• Receive message 2
• Send ack 2

Packet Loss
How long to set timer for?
91
Adaptive Retransmission

• Whenever TCP sends a message it records the time
  and then the time when a response is received
• A statistical function is used to maintain a
  current estimate of expected delay
• Timer can be set to a value depending on
• Stable conditions
• Increasing delay
• Decreasing delay

92
Buffers and Windows

• Receiving host can have a buffer
• Acknowledgements can contain amount of free
  buffer space available (Window)
• Sender will not send more data than buffer will
  hold
• As buffer space increases (i.e. application
  consumes data from buffer) additional acks can be
  sent updating buffer space available

93
Congestion Control

• Upon sensing congestion (lost message)
• Send a single message
• If received okay
• Send twice as much data
• Keep increasing until amount of data is 50 of
  receivers advertised window size

94
Client-Server Interaction

• An internet system provides a basic communication
  service
• The protocol software cannot initiate contact
  with or accept contact from another computer
• Instead, applications programs must participate
  in any communications
• One application initiates communication
• Client
• One application accepts communication
• Server

95
Sockets

• There must be an API between the application
  software and the communication protocol software
• A common (de facto standard) is the socket
  interface
• BSD
• Solaris
• Windows
• Open-Read-Write-Close Paradigm

96
Procedures

• descriptor socket(protofamily, type, protocol)
• protofamily TCP/IP, DECNET
• type Connection oriented stream
• Connectionless oriented message
• protocol Particular transport model (e.g. TCP)
• close(socket)
• bind(socket, localaddr, addrlen)
• listen(socket, queuesize)
• newsock accept(socket, caddress, caddresslen)
• connect(socket, saddress, saddresslen)
• send(socket, data, length, flags)
• sendto(socket, data, length, flags, destaddress,
  adresslen)
• recv(socket, buffer, length, flags)
• recvfrom(socket, buffer, length, flags, sndraddr,
  saddrlen)

97
Proxy Server

• A server that sits between a client application,
  such as a Web browser, and a real server. It
  intercepts all requests to the real server to see
  if it can fulfill the requests itself. If not, it
  forwards the request to the real server.

98
Purpose of Proxy Server

• Improve Performance
• A proxy server can dramatically improve
  performance for groups of users.
• It saves the results of all requests for a
  certain amount of time.
• Example
• Both user X and user Y access the World Wide Web
  through a proxy server.
• User X requests a certain Web page, which we'll
  call Page 1.
• Sometime later, user Y requests the same page.
• Instead of forwarding the request to the Web
  server where Page 1 resides, which can be a
  time-consuming operation, the proxy server simply
  returns the Page 1 that it already fetched for
  user X.
• Since the proxy server is often on the same
  network as the user, this is a much faster
  operation.
• Real proxy servers support hundreds or thousands
  of users.
• The major online services such as Compuserve and
  America Online, for example, employ an array of
  proxy servers.

99
Purpose of Proxy Server

• Filter Requests
• Proxy servers can also be used to filter
  requests.
• Example A company might use a proxy server to
  prevent its employees from accessing a specific
  set of Web sites.

100
Questions?
101
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