1. Module 11: TCP/IP Transport and Application Layer
James Chen
2018/11/12
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Outline
11.1 TCP/IP Transport Layer
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
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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4. 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
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
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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6. Session establishment, maintenance, and termination overview
Multiple applications can share the same transport connection.
segment-by-segment basis
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7. 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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8. Session establishment, maintenance, and termination overview (cont.)
Congestion occurs during data transfering
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).
When datagrams arrive too quickly for a host or gateway to process, they are temporarily stored in memory.
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
not ready > stop sign
ready > go sign
At the end of data transfer
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
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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10. Three-way handshake (cont.)
TCP uses a 3-way handshake to initiate a connection. The handshake serves two functions:
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:
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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11. Three-way handshake (cont.)
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 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
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
It requires a recipient to communicate with the source.
It send back an acknowledgment message when the data is received.
Retransmission
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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16. Transmission Control Protocol (TCP)
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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17. User Datagram Protocol (UDP)
Error processing and retransmission must be handled by higher 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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18. TCP and UDP port numbers
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).
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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20. 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.
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).
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 messages in ASCII format using TCP.
Collecting mail > POP3 and IMAP4
Sending mail > SMTP
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SNMP
Simple Network Management Protocol (SNMP)
Application layer protocol
UDP
Exchange management information between network devices.
Network management system (NMS)
It executes applications that monitor and control managed devices.
Managed device
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
software modules
It translates information into a form compatible with SNMP.
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Telnet
Terminal emulation
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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