Transcript Semester 2 Chapter 9

Institute of Technology Sligo - Dept of Computing
Semester 2
Chapter 9
The TCP/IP Protocol Suite
Paul Flynn
Institute of Technology Sligo - Dept of Computing
Lesson Overview
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How TCP/IP operates to ensure communication
across any set of interconnected networks.
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Learn about parts of the TCP/IP protocol stack that
support:
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File transfer (FTP, TFTP),
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e-mail,
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Reliable (TCP) and unreliable (UDP) transport layer
protocols,
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Connectionless datagram (packet) delivery at the
network layer,
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ICMP provides control and message functions at
the network layer,
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ARP and RARP.
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TCP/IP
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Developed by the Defence Advanced Research
Projects Agency (DARPA).
Later included with the Berkeley Software
Distribution of UNIX.
Now the de facto standard for internetwork
communications.
Serves as the transport protocol for the Internet.
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9.1.1 The Internet TCP/IP
protocols and the OSI model
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OSI Layers 7,6 & 5 are rolled into 1 application layer
The Transport layer in both models perform basically the same function
OSI layers 2 & 1 are combined into 1 Network Access layer
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9.1.2 TCP/IP protocol stack and
the application layer
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DNS - Domain Name System
WINS - Windows Internet Naming System
NFS - Network File System (The standard UNIX file
system)
POP3 - Post Office Protocol (An e-mail standard)
SMTP - Simple Mail Transfer Protocol
SNMP - Simple Network Management Protocol
FTP - File Transfer Protocol (Upload/Download files from
the ‘net)
TFTP - Trivial File Transfer Protocol
HTTP - The Internet standard for web browsing
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9.1.2 TCP/IP protocol stack and the application layer
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Other Application Layer protocols used for
troubleshooting a network include:
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Telnet - A standard terminal emulation protocol
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PING - Packet InterNet Groper
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Traceroute - Traces the path of a packet from the source to
the receiver.
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9.1.3 TCP/IP protocol stack and
the transport layer
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The transport layer provides two protocols:
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TCP (Transport Control Protocol)
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Provides reliable, connection-oriented communications
between 2 hosts.
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TCP requires more network overhead because data is
acknowledged as it is received.
UDP (User Datagram Protocol)
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UDP is faster but less reliable, because the recipient does not
acknowledge the data as it is received.
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Communication reliability is left to the Application Layer
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9.1.4 TCP segment format
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source port -- the number of the
calling port
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destination port -- the number of
the called port
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sequence number -- the number
used to ensure correct sequencing
of the arriving data
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acknowledgment number -- the
next expected TCP octet
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HLEN -- the number of 32-bit words
in the header
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reserved -- set to 0
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code bits -- the control functions
(e.g. setup and termination of a
session)
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window -- the number of octets
that the sender is willing to accept
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checksum -- the calculated
checksum of the header and data 8
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Application layer
9.1.4 UDP segment
format
protocols provide for
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reliability.
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UDP uses no windowing
or acknowledgments.
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Designed for applications
that do not need to put
sequences of segments
together.
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Protocols that use UDP
are:
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TFTP
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SNMP
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Network File System
(NFS)
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Domain Name System
(DNS)
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9.1.5 TCP and UDP port
numbers
Both TCP and UDP
use port numbers for communication
between hosts.
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EG When a Host wants to transfer a file using FTP
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Port numbers are similar to phone numbers
Transport layer services can be “called” by their port number.
Uses TCP port 21 to establish and control the connection
Uses TCP port 20 to transfer the data
TCP ports 20 & 21 are called “Well Known Port Numbers” because
applications expect to find FTP services on these ports.
Other “Well Known” Port numbers are:
TCP Port 23
- Telnet
 TCP Port 25
- SMTP (email)
 TCP Port 53
- DNS
 TCP Port 80
- HTTP web services
 UDP Port 53
- DNS
Request For Comment 1700 defines “Well Known
- TFTP
Port Numbers” UDP Port 69
 UDP Port 161 - SNMP
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9.1.6 TCP three-way
handshake/open connection
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For TCP to establish a reliable connection between 2 hosts.
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Uses a “Three Way Handshake”
Transmits 3 packets before the actual data
The 2 Hosts synchronise their “Initial Sequence Number” (ISN)
Ensures that the communications are assembled in the correct order and no
missing packets exist.
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The process occurs by
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Host 1 sends a SYN packet to Host 2 (1st data packet)
 Host 2 ACK the packet from Host 1 (2nd data packet)
 Host 2 includes its’ own SYN data for Host 1 (2nd data packet)
 Host 1 ACK packet from Host 2 (3rd data packet)
It is call the 3 way handshake because only 3 packets are exchanged
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The SYN packet from Host 2 also serves as the acknowledgment to the SYN packet
from Host 1
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9.1.7 TCP simple
acknowledgment and
windowing
TCP “Sliding Windows” control the flow
and efficiency of communication
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Quite simply, once the window fills with
data, the destination host sends an ACK for
packets received in that window
The window slides over in order to select
new packets.
TCP window sizes can change during the
life of the connection so that the window
can be
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9.2.1 TCP/IP and the Internet
The Internet layer of
the TCP/IP stack
Layer
corresponds to the network layer of the OSI
model.
Several protocols operate at the TCP/IP Internet
layer that corresponds to the OSI network
layer:
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IP -- provides connectionless, best-effort delivery
routing of datagrams; is not concerned with the
content of the datagrams; looks for a way to move
the datagrams to their destination
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ICMP -- provides control and messaging capabilities
ARP -- determines the data link layer address for
known IP addresses
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9.2.2 The IP datagram
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An IP datagram contains an IP header and data,
and is surrounded by the Media Access Control
(MAC) layer header and MAC layer trailer. One
message may be transmitted
as
a series
of
 VERS
-- version
number
 HLEN -- header
length, the
in 32-bit words
datagrams that are reassembled
into
 type of service -- how the datagram should be handled
message at the receiving location.
 total length -- total length (header + data)
 identification, flags, flag offset -- provides
fragmentation of datagrams to allow differing MTUs in
the internetwork
 TTL -- Time-To-Live
 protocol -- the upper-layer (Layer 4) protocol sending
the datagram
 header checksum -- an integrity check on the header
 source IP address and destination IP address -- 32-bit
IP addresses
 IP options -- network testing, debugging, security, and
other options
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9.2.2 The IP datagram
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The protocol field
determines the
Layer 4 protocol
being carried within
an IP datagram.
Although most IP
traffic uses TCP,
other protocols can
also use IP.
Each IP header must
identify the
destination Layer 4
protocol for the
datagram.
Transport layer
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9.2.3 Internet Control
Message
Protocol
(ICMP)
Provides message control and error reporting
Institute of Technology Sligo - Dept of Computing
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services between 2 TCP/IP hosts or between
a host server and a gateway to the Internet.
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The PING utility uses ICMP messages to check
connections between 2 points.
EG: A router receives a packet that it is
unable to deliver to its final destination.
The router sends an ICMP unreachable
message to the source.
The message might be undeliverable
because there is no known route to the
destination.
An echo reply is a successful reply to a
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ping command.
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9.2.4 How ARP works
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ARP maps IP addresses to MAC addresses.
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The source of the network packet sends the
information to the destination.
Unique MAC addresses reference the end points in
the exchange.
Networked devices maintain details of MAC and IP
addresses of other devices on the network.
This is called an ARP table or cache
Maintained in RAM
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9.2.4 How ARP works
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A computer transmits a packet on the network.
It checks its’ ARP cache for an IP to MAC address
mapping
If source finds an appropriate IP to MAC mapping, the
source uses the IP and MAC address to encapsulate the
data that it is ready to transmit.
Source computer sends the packet directly to the
destination.
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Institute of Technology Sligo - Dept of Computing
9.2.4 How ARP works
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The IP address is listed first, the MAC address
next, followed by information about whether
the entry is static or dynamic.
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9.2.4 How ARP works
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If a source computer cannot
locate an IP to MAC address
mapping in its’ ARP table, it
must obtain the correct
mapping.
Source initiates an ARP request
to all hosts to gain the
destinations’ MAC address.
The one host that sees that the
ARP packet is meant for its’ IP
address responds to the ARP
request.
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How RARP works
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RARP is similar to ARP in that computers use
it to bind MAC addresses to IP addresses.
RARP is used by Diskless Workstations which
have a MAC address burned into their
network cards but no IP address.
A clients IP configuration is stored on a RARP
server
RARP servers maintain a table of MAC and IP
address mapping for RARP clients.
During the boot process, a RARP client will
call the RARP server to obtain their IP
configuration.
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The preceding presentation
was put together using the
Cisco curriculum and the
CCNA Guide to Cisco
Networking Fundamentals by
Kurt Hudson and Kelly
Cannon.
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