CS 2200 TCPIP

1
CS 2200 TCP/IP

• (Lectures based on the work of Jay Brockman,
  Sharon Hu, Randy Katz, Peter Kogge, Bill Leahy,
  Ken MacKenzie, Richard Murphy, and Michael
  Niemier)

2
The Internet

• IP
• One protocol to rule them all
• Address assignment
• IP Routing (layer-3 switching)
• What exactly do we mean by this???
• Name Resolution
• ARP IP -gt ethernet MAC (media access control)
• DNS name -gt IP
• TCP
• Reliable in-order streams
• Built atop IP
• What exactly do we mean by this???

(i.e. how do we find the address of computer we
want to go to? 129.10.10.128 is more like a VA)
3
Forming a WAN
Switch Site 1
Switch Site 4
Switch Site 2
Switch Site 3
4
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
5
Next Hop Forwarding
This table is for switch 2.
6
Layer-2 Switches

• Each switch learns MAC addresses on its ports.
• Maintains a table of all MAC addresses seen
• For each packet, search the table(probably a
  hash table)

48-bit MAC port 00423765fe01 1 0
0423765fe02 2 0197fbcd0137 2
7
Scalability?

• Tables get large
• Must store all the MAC addresses in the universe

8
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

9
Internetworking

• Different networking solutions exist
• Why? No single networking technology is best for
  all needs
• Universal service
• System where any pair of computers can
  communicate
• Increases productivity
• Networks, by themselves, dont have universal
  service
• Solution Internetworking or an internet

Literally Communicating between networks of the
same and/or different types
10
Step oneDefine universal packet format
11
Step twoEncapsulate the universal packetsin
(any) local network frame format
Used to send msg. from 1 network to another (or
wi/the same)but we want a uniform standard.
Frame Header
Frame Data
Used to communicate within 1 network
12
Physical Network Connection
Router
Router facilitates communication
between networks
Individual Networks
Each cloud represents arbitrary network
technology LAN, WAN, ethernet, token ring, ATM,
etc.
13
Routers

• A router is
• A computer dedicated to the taskof
  interconnecting networks.
• A router can connect nets with different
  technologies
• different media, addressing schemes, or frame
  formats

Router
14
Router operation

• Unpack IP packet from frame format of source net
• Perform routing decision
• Pack IP packet into frame format of destination
  net

15
Virtual Network
16
Virtual Network
17
MTU and Fragmentation

• For any given network there is aMaximum
  Transmission Unit (MTU)
• If a datagram (packet) 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
• (Again, see the board 32 bytes?16 bytes?32
  bytes)

18
Reassembly

• Fragments are not re-assembled until final
  destination
• Why?
• Routers dont have to deal with
  re-assembly.Packet arrives, router simply
  forwards it
• Allows routes to change dynamically.Cant
  re-assemble if fragments take different paths.

19
Example
Source Host
Net 1
header 1
Router 1
Net 2
header 2
Router 2
Net 3
header 3
Destination Host
20
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)

21
Layered Model
TCP/IP Model
Application
5
Transport
4
Internet
3
Network Interface
2
Physical
1
22
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 used 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

23
IP Addresses
24
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
• (sound familiar???)

25
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.

26
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
2
Network 1
Network 2
3
3
5
27
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

28
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?

29
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

30
Five Classes of IP Address
31
Five Classes of IP Address
Primary Classes
32
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

28
28
28
28
33
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
(on the board)
34
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
192.5.48.3
192.5.48.85
10.0.0.37
10.0.0.49
(on the board)
35
Special IP Addresses

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

36
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

(quiz question)
37
Directed Broadcast Address

• Often convenient to send a messageto all hosts
  on a single network
• Directed broadcast address formedby adding a
  suffix containing all 1 bits
• When direct broadcast message arrives to the
  destination network, it is sent to all hosts
  there via
• Local networks hardware broadcast facility, or
• Individual messages sent to each host

(quiz question)
38
Limited Broadcast Address

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

(quiz question)
39
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"

40
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

(goto print menu)
41
Berkeley Broadcast Address Form

• UC Berkeley 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 Berkeley Implementation
• Some can accept either, some both

42
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
43
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

44
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!
(on the board)
45
Multi-homed Hosts

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

46
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

47
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

48
Address Resolution
A
C
E
R2
R1
B
D
F
49
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
50
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
51
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
52
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
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 R2 determines that
message is intended for host on local net.
Address of F is resolved by R2 and message is
sent to F
54
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

(more detail about items 1-3 on earlier slide)
55
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
56
Table Lookup IIDirect Lookup

• IP Address
• 197.15.3.4

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

Must also do array bounds checking
57
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

58
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
59
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

60
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

61
ARP Message Delivery
62
ARP Message Delivery
63
ARP Message Delivery
64
ARP Message Delivery
65
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).

66
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
(see board)
67
Best-Effort Delivery

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

68
IP Routing
69
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
70
Next Hop Forwarding
71
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

72
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

73
Table Size Reduction
Next Hop Information
Destination
Next Hop
1,anything
Int 2
2,anything
Local
3,anything
Int 4
4,anything
Int 3
74
Testing Reachability

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

75
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

76
Some ICMP Messages

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

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

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
77
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

78
ICMP Message Transport
79
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!!!

80
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

81
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)

82
TCP
83
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

84
Services Provided by TCP

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

85
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).

86
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

87
Packet Loss and Retransmission
88
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

89
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

90
Aside Breaking TCP/IP

• Dont cooperate with congestion control
• ?