Transcript Module 11: TCP/IP Transport and Application Layer

Module 11:
TCP/IP Transport and
Application Layer
James Chen
[email protected]
2015/7/21
YuDa college of business
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Outline
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11.1 TCP/IP Transport Layer
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Introduction to transport layer
Flow control
Session establishment, maintenance, and termination overview
Three-way handshake
Windowing
Acknowledgment
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
TCP and UDP port numbers
11.2 The Application Layer
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Introduction to the TCP/IP application layer
DNS
FTP
HTTP
SMTP
SNMP
Telnet
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 11.1 TCP/IP Transport Layer
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Introduction to transport layer
 It establishes a logical connection between the endpoints of the
network.
 It provides transport services from the source host to the destination
host (end-to-end transport services).
 Transport services segment and reassemble several upper-layer
applications onto the same transport layer data stream.
 Reliable transportation
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Ensure that segments delivered will be acknowledged to the sender.
Provide for retransmission of any segments that are not acknowledge.
Put segments back into their correct sequence numbers at the
destination.
Provide congestion avoidance and control.
 Flow control
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Sliding window
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Flow control
 Flow control avoids the problem of a transmitting host overflowing the
buffers in the receiving host.
 TCP provides the mechanism for flow control by allowing the sending
and receiving host to communicate.
 The two hosts then establish a data-transfer rate that is agreeable to
both.
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Session establishment, maintenance,
and termination overview
 Multiple applications can share the same transport connection.
 segment-by-segment basis
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Session establishment, maintenance,
and termination overview (cont.)
 Synchronization handshake
requests
 Synchronize connection
parameters in the opposite
direction.
 An acknowledgment to
inform the destination that
both sides agree that a
connection has been
established.
 After the connection has
been established, data
transfer begins.
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Session establishment, maintenance,
and termination overview (cont.)
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Congestion occurs during data transfering
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When datagrams arrive too quickly for a host or
gateway to process, they are temporarily stored in
memory.
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A high-speed computer might be capable of
generating traffic faster than a network can
transfer it.
If many computers simultaneously need to send
datagrams to a single destination, that destination
can experience congestion (no single source
caused the problem).
If the traffic continues, the host or gateway
eventually exhausts its memory and must discard
additional datagrams that arrive.
To prevent data to be lost
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not ready > stop sign
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ready
> go sign
At the end of data transfer
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The sending host sends a signal that indicates the
end of the transmission.
The receiving host at the end of the data
sequence acknowledges the end of transmission
The connection is terminated.
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Three-way handshake
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For a connection to be established or initialized, the two hosts must synchronize
their Initial Sequence Numbers (ISNs).
Segments that carry the SYN bit are also called “SYNs".
The synchronization requires each side to send its own Initial Sequence Numbers
(ISNs) and to receive a confirmation of exchange in an acknowledgment (ACK)
from the other side.
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Three-way handshake (cont.)
 TCP uses a 3-way handshake to initiate a connection. The handshake
serves two functions:
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It ensures that both sides are ready to transmit data, and that both ends
know that the other end is ready before transmission actually starts.
It allows both sides to pick the initial sequence number to use.
 When opening a new connection, why not simply use an initial sequence
number of 0? Because if connections are of short duration, exchanging
only a small number of segments, we may reuse low sequence numbers
too quickly. Thus, each side that wants to send data must be able to
choose its initial sequence number. The 3-way handshake proceeds as
follows:
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TCP A picks an initial sequence number (A_SEQ) and sends a segment
to B containing: SYN_FLAG=1, ACK_FLAG=0, and SEQ=A_SEQ.
When TCP B receives the SYN, it chooses its initial sequence number
(B_SEQ) and sends a TCP segment to A containing: ACK=(A_SEQ+1),
ACK_BIT=1, SEQ=B_SEQ, SYN_FLAG=1.
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Three-way handshake (cont.)
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When A receives B's response, it acknowledges B's choice
of an initial sequence number by sending a dataless third
segment containing: SYN_FLAG=0, ACK=(B_SEQ+1),
ACK_BIT=1, SEQ=A_SEQ+1(data length = 0).
Data transfer may now begin.
 Note: The sequence number used in SYN segments are actually
part of the sequence number space. That is why the third
segment that A sends contains SEQ=(A_SEQ+1). This is
required so that we don't get confused by old SYNs that we
have already seen. To insure that old segments are ignored,
TCP ignores any segments that refer to a sequence number
outside of its receive window. This includes segments with the
SYN bit set.
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Windowing
 recipient acknowledge :
the receipt of each
packet before the next
packet is sent.
 If the sender must wait
for an acknowledgment
after sending each
packet, throughput
would be low.
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Windowing (cont.)
 Transmit more data
 The number of data packets the sender is allowed to have
outstanding without having received an acknowledgment is
known as the window size, or window.
 TCP uses expectational acknowledgments.
 TCP window sizes are variable during the lifetime of a
connection.
 Each acknowledgement contains a window advertisement that
indicates the number of bytes the receiver can accept.
 TCP also maintains a congestion-control window.
 This window is normally the same size as the window of the
receiver.
 This window is cut in half when a packet is lost, perhaps as a
result of network congestion.
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Windowing (cont.)
 The sender sends three packets
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before expecting an ACK.
If the receiver can handle a
window size of only two packets,
the window drops packet three,
specifies three as the next
packet, and specifies a new
window size of two.
The sender sends the next two
packets, but still specifies a
window size of three.
This means that the sender will
still expect a three packet
acknowledgement from the
receiver.
The receiver replies by
requesting packet five and
again specifying a window size
of two.
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Acknowledgment
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Positive acknowledgment with
retransmission guarantees that a
stream of data sent from one device
is delivered through a data link to
another device without duplication
or data loss.
Positive acknowledgment
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It requires a recipient to
communicate with the source.
It send back an acknowledgment
message when the data is
received.
Retransmission
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The sender keeps a record of
each data packet (TCP segment),
that it sends and expects an
acknowledgment.
The sender also starts a timer
when it sends a segment and will
retransmit a segment if the timer
expires before an
acknowledgment arrives.
all segments / part of segments
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Transmission Control Protocol (TCP)
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Source port – Number of the calling port
Destination port – Number of the called port
Sequence number – Number used to ensure correct sequencing of the arriving data
Acknowledgment number – Next expected TCP octet
HLEN – Number of 32-bit words in the header
Reserved – Set to zero
Code bits – Control functions, such as setup and termination of a session
Window – Number of octets that the sender is willing to accept
Checksum – Calculated checksum of the header and data fields
Urgent pointer – Indicates the end of the urgent data
Option – One option currently defined, maximum TCP segment size
Data – Upper-layer protocol data
FTP / HTTP / SMTP / telnet
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User Datagram Protocol (UDP)
 Error processing and retransmission must be handled by higher
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layer protocols.
Source port – Number of the calling port
Destination port – Number of the called port
Length – Number of bytes including header and data
Checksum – Calculated checksum of the header and data fields
Data – Upper-layer protocol data
TFTP / SNMP / DHCP / DNS
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TCP and UDP port numbers
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Both TCP and UDP use port numbers to pass information to the upper layers.
socket = IP + port number
Port numbers are used to keep track of different conversations crossing the
network at the same time.
Internet Assigned Numbers Authority (IANA).
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Numbers below 1024 are considered well-known ports numbers.
Numbers above 1024 are dynamically assigned ports numbers.
Registered port numbers are those registered for vendor-specific applications.
Most of these are above 1024.
End systems use port numbers to select the proper application.
The source host dynamically assigns originating source port numbers.
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 11.2 The Application Layer
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Introduction to the TCP/IP application
layer
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DNS
 The Domain Name System (DNS) is a system used on the
Internet for translating names of domains and their publicly
advertised network nodes into IP addresses.
 A domain name is a string of characters, number, or both.
 There are more than 200 top-level domains on the Internet.
 .us – United States
 .uk – United Kingdom
 .edu – educational sites
 .com – commercial sites
 .gov – government sites
 .org – non-profit sites
 .net – network service
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FTP
 FTP is a reliable, connection-oriented service that uses TCP to transfer
files between systems.
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FTP first establishes a control connection between the client and the
server.
Then a second connection is established, which is a link between the
computers through which the data is transferred.
Data transfer can occur in ASCII mode or in binary mode.
 TFTP is a connectionless service that uses User Datagram Protocol
(UDP).
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TFTP is designed to be small and easy to implement.
It lacks most of the features of FTP.
It cannot list directories and currently has no provisions for user
authentication.
TFTP is used on the router to transfer configuration files and Cisco IOS
images and to transfer files between systems.
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HTTP
 Hypertext Transfer Protocol (HTTP)
 client-server application
 A Web browser presents data in multimedia formats on Web
pages that use text, graphics, sound, and video.
 Hyperlinks make the World Wide Web easy to navigate.
 Uniform Resource Locator (URL)
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SMTP
 Simple Mail Transfer Protocol (SMTP)
 The SMTP protocol transports email messages in ASCII format
using TCP.
 Collecting mail > POP3 and IMAP4
 Sending mail > SMTP
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SNMP
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Simple Network Management Protocol
(SNMP)
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Network management system (NMS)
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It executes applications that monitor
and control managed devices.
Managed device
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Application layer protocol
UDP
Exchange management information
between network devices.
network node
An SNMP agent reside in it.
It collects and stores management
information for NMS.
routers, access servers, switches,
bridges, hubs, computer hosts,
printers.
Agents
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software modules
It translates information into a form
compatible with SNMP.
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Telnet
 Terminal emulation
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Telnet operation uses none of the processing
power from the transmitting computer.
It transmits the keystrokes to the remote host
and sends the resulting screen output back to
the local monitor.
 A Telnet client is referred to as a local host.
 A Telnet server uses special software called a
daemon, is referred to as a remote host.
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 END
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