TCP, UDP, IP

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Chapter 4
TCP, UDP, IP
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TCP/IP Protocol Suite

• De-facto standard for open system
  interconnection.
• Provide highest degree of interoperability.
• Usually resides in O/S, shared by all application
  programs running on the machine.

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An Internet According to TCP/IP
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TCP/IP and the OSI Model
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RFC (Request for Comments)

• Documentation for TCP/IP Protocols
• Available on-line from
• - Network Information Center, www.nic.mil
• - other mirror sites
• Examples
• - IP, RFC 760
• - TCP, RFC 761
• - FTP, RFC 959
• - HTTP, RFC 2068

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IP (Internet Protocol)

• Described in RFC 760.
• Responsible for communication at the computer
  level (i.e. host-to-host communication).
• Best-try datagram (or connectionless) service.
• Data transferred across the Internet (datagram)
  is self-contained, individually-addressed.
• IP is independent of underlying network
  technologies.
• IP address is used to determine the source and
  destination of a datagram.
• Operate in layer 3 (network layer) of the OSI
  model.

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IP Datagram
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• VER
• Defines version of the IP.
• HLEN (Header length)
• Defines the length of the datagram header in
  4-btye words.
• TOS (Type of Service)
• For specifying special handling of the packet
  Replaced by Differentiated Services to handle the
  shortcomings of Integrated Services.
• Total Length
• Defines the total length (header plus data) of
  the IP datagram in bytes.
• Identification, Flags, Fragmentation offset
• These fields are related to fragmentation and
  reassembly of an IP diagram.

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• Time To Live (TTL)
• Defines the maximum time for which a datagram is
  allowed to be in transit across the Internet.
  But, in practice, it counts hops.
• Protocol
• Defines the higher-level protocols that uses the
  services of the IP layer
• e.g. TCP (6), UDP(17), OSPF(89).
• Checksum
• It covers only the header, not the data.
• Source IP address
• Defines the IP address of the source.
• Destination IP address
• Defines the IP address of the destination.
• Option
• Used for network testing and debugging.

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Example of checksum calculation
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Maximum Transmission Unit ( MTU)
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Fragmentation example
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Internet Address
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Internet Classes
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IP Address Range
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Private Address

• Address ranges defined for use within private
  intranets
• Can be used without registration
• Class A
• 10.0.0.0 to 10.255.255.255
• Class B
• 172.16.0.0 to 172.31.255.255
• Class C
• 192.168.0.0 to 192.168.255.255

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Network and Host Addresses
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A Network with Two Levels of Hierarchy
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A Network with Three Levels of Hierarchy
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Subnetwork Address

• Allow standard class address to be further
  subdivided for routing within an organization.
• Subnet mask is used to divide a network into
  subnets.
• Cannot define all 0s or all 1s as a host
  number
• - all 0s is the subnetid
• - all 1s is the subnet broadcast address

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Example 1
 

• Determine the netid, subnetid, hostid, subnet
  broadcast address of the following IP address
• 10.84.60.5 / 255.240.0.0
• Answer
• netid 10
• subnetid 10.80.0.0
• hostid 4.60.5
• subnet broadcast 10.95.255.255

 
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Example 2

• With the following subnet assignment, list all
  available subnet addresses
• 192.60.20.0 / 255.255.255.224
• Answer
• 192.60.20.32, 192.60.20.64
• 192.60.20.96, 192.60.20.128
• 192.60.20.160, 192.60.20.192

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Transport Layer Process-to-Process Delivery
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Port numbers
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IP addresses versus port numbers
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Socket address
IANA ranges
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Well-known ports used by TCP
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Well-known ports used by UDP
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TCP (Transmission Control Protocol)

• Described in RFC 761.
• Responsible for delivery of the message to the
  appropriate program (process-to-process
  communication).
• Provide a reliable, end-to-end, byte stream (data
  pipe) over an unreliable internetwork.
• Operate in layer 4 (transport layer) of the OSI
  model.
• Default segment size 556 bytes (20 536).
• Error-detection by CHECKSUM field.
• Flow-control by Sequence No., Acknowledgement No.
  and Window Size.

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TCP Segment Format
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• Source port address
• Defines the port number of the application
  program in the sending host.
• Destination port address
• Defines the port number of the application
  program in the receiving host.
• Sequence number
• Defines the number assigned to the first byte of
  data contained in the segment.
• Acknowledgement number
• Defines the byte number that the sender of the
  segment is expecting to receive from the other
  party.
• Header length
• Indicates the number of 4-byte words in the TCP
  header.

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• Reserved
• Reserved for future use.
• Control bits
• One or more of these bits can be set at a time.
  These bits enable flow control, connection
  establishment and termination, and the mode of
  data transfer in TCP.
• Windows size
• Defines the size of window in bytes.
• Checksum
• Used to detect errors over the entire segment
  (header plus data).
• Urgent pointer
• It is used when the segment contains urgent data.
  The number is added to the sequence number to
  obtain the number of the last urgent byte in the
  data section of the segment.
• Options
• Up to 40 bytes of option information.

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Description of flags in the control field
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Example 1
Imagine a TCP connection is transferring a file
of 6000 bytes. The first byte is numbered 10010.
What are the sequence numbers for each segment if
data are sent in five segments with the first
four segments carrying 1000 bytes and the last
segment carrying 2000 bytes?
Solution
The following shows the sequence number for each
segment Segment 1 gt sequence number
10,010 (range 10,010 to 11,009) Segment 2
gt sequence number 11,010 (range 11,010 to
12,009) Segment 3 gt sequence number
12,010 (range 12,010 to 13,009) Segment
4 gt sequence number 13,010 (range 13,010 to
14,009) Segment 5 gt sequence number
14,010 (range 14,010 to 16,009)
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Three-step connection establishment
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Four-step connection termination
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Sliding Window

• A sliding window is used to make transmission
  more efficient as well as to control the flow of
  data so that the destination does not become
  overwhelmed with data. TCPs sliding windows are
  byte-oriented.
• In TCP, the sender window size is totally
  controlled by the receiver window value (the
  number of empty locations in the receiver
  buffer). However, the actual window size can be
  smaller if there is congestion in the network.

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Lost segment
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Lost acknowledgment
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UDP (User Datagram Protocol)

• Described in RFC 768.
• UDP is a connectionless, unreliable protocol that
  has no flow and error control. It uses port
  numbers to multiplex data from the application
  layer.
• The calculation of checksum and its inclusion in
  the user datagram are optional.
• UDP is a convenient transport-layer protocol for
  applications that provide flow and error control.
  It is also used by multimedia applications.

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UDP Datagram Format
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Why UDP?

• Simple
• Less overhead, 8 bytes header in UDP vs 20 bytes
  header in TCP
• No acknowledgement, therefore much less delay in
  communication.
• Less computer resources requirement, e.g.
  buffers.

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Common UDP Applications

• Characteristics of UDP applications
• single packet based transaction (request, reply
  or update)
• distributed servers
• no pre-determined server
• loss of packets does not affect service
• Examples SNMP, BOOTP, DNS, DHCP, RIP