osi transport layer
Post on 24-Feb-2016
34 Views
Preview:
DESCRIPTION
TRANSCRIPT
© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.0
OSI Transport Layer
Network Fundamentals – Chapter 4
2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Objectives Explain the role of Transport Layer protocols and
services in supporting communications across data networks.
Analyze the application and operation of TCP mechanisms that support reliability.
Analyze the application and operation of TCP mechanisms that support reassembly and manage data loss.
Analyze the operation of UDP to support communicate between two processes on end devices.
3© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services Transport layer is responsible for overall end-to-end
transfer of application data
4© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services Transport layer enables applications on devices to
communicate Fig 4.2
5© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Purpose of Transport Layer
Tracking the individual communications between applications on the source and destination hosts
Segmenting data and managing each piece Reassembling the segments into streams of application
data Identifying the different applications Performing flow control between end users Enabling error recovery Initiating a session
6© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
1. Tracking Individual Conversation Any host can have multiple applications communicating
across the network TL maintain the multiple communication streams
between these applications. Ex. Email, IM, Websites and VoIP simultaneously Fig 4-3
7© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Segmenting Data AL passes large amounts of data to TL TL has to break data into smaller pieces =segments –
suited transmission Without segmentations, only one application would be
able to receive data Fig 4.4
8© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Reassembling Segments
Networks prove multiple routes – diff trans times Data can arrives in the wrong order Numbering and sequencing – TL ensure segments are
reassembled into the proper order Each segment of data reassembled and directed to
appropriate application.
9© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Identifying the Applications To pass data streams to the proper applications TL must identify target application TL assigns an identifier to an application In TCP/IP, the identifier = port number Port number used in TL header to indicate which
application the data is associated Each particular set of pieces flowing between a source
application and destination application is known as conversation
Dividing data into small parts enables many diff comm to be interleaved/multiplexed on the same network
10© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Flow Control
Network hosts have limited resources – memory, bandwidth
When TL aware, some protocol can request the sending application to reduce the rate of data flow
This is done at TL by regulating the amount of data source transmits as a group.
=FLOW CONTROL Prevent the loss of segments on net and avoid
retransmission
11© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Error Recovery Possible Piece of data corrupted or lost while transmitted over
the network TL ensure all pieces reach destination – source device
retransmit any data that is lost
Initiating a Session TL can provide connection orientation by creating a
session between the app. These connections prepare the app to communicate
with each other before any data transmitted Data can be closely managed
12© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services Supporting Reliable Communication
• Diff apps have diff requirements for their data• Diff transport protocols have been developed to meet these
requirements• TCP is a TL protocol – ensure reliable delivery• In networking, reliability means each piece of data the source
sends arrives at the destination• Three basic operation at TL to support reliability
• Tracking transmitted data• Acknowledging received data• Retransmitting any unacknowledged data
• More control data (in Layer 4 header) is exchanged to support acknowledgement, tracking and retransmission
13© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services Supporting Reliable Communication
14© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP and UDP
Two most common TL protocols of TCP/IP are Transmission Control Protocol (TCP) and User
Datagram Protocol (TCP)
15© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Simple and connectionless protocol Low-overhead data delivery 8 bytes of overhead UDP segments = datagrams UDP sends datagrams as ‘best effort’ Applications of UDP
• DNS• Video Streaming• VoIP
16© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services Identify the basic characteristics of the UDP and TCP
protocols
17© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Connection-Oriented protocol Additional overhead to gain functions Additional functions
• Same-order delivery• Reliable delivery• Flow-control
• Each TCP segments has 20 bytes of overhead
• Applications• Web browsers• E-mail• File transfer
18© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services Identify the basic characteristics of the UDP and TCP
protocols
19© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Transport Layer Role and Services
20© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Port Addressing
TCP and UDP based services keep track of various applications.
To differentiate segments and datagrams for each application – TCP and UDP have header fields that uniquely identify these apps.
Header contains source and dest port Server processes have static port number assigned Client dynamically choose a port number for each
conversation.
21© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Port Addressing Identifying Conversations
22© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Port Addressing
The combination of TL port number and NL IP address uniquely identifies a particular process running on specific host device.
This combination = socket Ex : Web server on host 192.168.1.20, web browser
(dynamically assigned port 49152) on host 192.168.100.48
Destined socket = 192.168.1.10:80 Socket for the web page = 192.168.100.48:49152
23© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Port Addressing
24© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Port Addressing
Different types of port numbers • Well-known ports (0-1023)
• Reserved for services and apps• FTP 20, SMTP 25, HTTP 80
• Registered Ports (1024-49151)• Assigned to user processes or application
• Dynamic or private ports (49152-65535)
• Netstat command
25© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Connection Establishment and Termination TCP Three-Way Handshake Three steps in TCP connection establishment:
1. Initiating client sends a segment containing an initial sequence value.2. Server responds with segment containing an acknowledgement value of received sequence+1, plus its own synchronizing sequence value3. Initiating client responds with an acknowledgement received sequence+1
Fig 4.11
26© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Connection Establishment and Termination
27© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Session Termination
Client has no more data to send, it sends a segment with FIN flag set
The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server
The server sends a FIN to the client to terminate server to client session
Client responds with ACK to acknowledge the FIN from server
Fig 4-12
28© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Session Termination
29© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Managing TCP Sessions Describe how TCP sequence numbers are used to
reconstruct the data stream with segments placed in the correct order
30© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Managing TCP Sessions TCP Acknowledgement with Windowing
• The sequence number indicates the relative number of bytes that have been transmitted in this session, including the bytes in the current segment.
• TCP use acknowledgement number in segment sent back to indicate the next byte expects to receive.
• Fig 4-13• But if A had to wait for ack of the receipt of each 10
bytes – lot of overhead
31© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Managing TCP Sessions TCP Acknowledgement with Windowing
32© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Managing TCP Sessions TCP Acknowledgement with Windowing
• Multiple segments of data can be sent and ack with a single TCP message in opposite direction.
• Ex start with SEQ = 2000, if 10 segments of 1000bytes were received, an ACK = 12001 would be returned to the source.
• Amount of data can be transmitted before ACK = window size
• Window size is a field in TCP header used for management of lost data and flow control
33© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Retransmission Data loss will occasionally occur Dest host using TCP only ack data for contiguous
sequence bytes If one or more segments are missing, only segments
that complete the stream is ack Ex. Segments with SEQ=1500 to 3000 and SEQ=3400
to 3500 were received, the ACK will be =3001
34© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Retransmission
35© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Congestion Control
Flow Control & Dynamic Window Sizes
36© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Congestion Control
Flow Control• Adjusting the effective rate of data flow.• Window size field in TCP header specifies amount of data can
be transmitted before an ACK• Initial window size determined through 3-way handshake• TCP feedback mechanism adjusts the effective rate to the
maximum flow network and destination can support without loss• Fig 4-14• During the delay in receiving the ACK, sender will not sent
additional segments for the session
37© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Congestion Control
Dynamic Window Size• When network resources are constrained, TCP can reduce the
window size• After no data losses or constrained resources, receiver will
begin to increase the window size field• This dynamic increasing and decreasing of window size is a
continuous process in TCP
38© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
TCP Congestion Control
39© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Protocol Communicating with low overhead
40© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Protocol Simple protocol Basic TL functions Not connection oriented Does not provide sophisticated retransmission,
sequencing and flow control mechanism UDP does not provide reliability..so be careful.. but, are not always unreliable just the reliability is not
provided
41© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Datagram Reassembly Session are not established When app has data to send, it simply sends the data When larger amounts of data – split into multiple
segments =datagrams Multiple datagrams – may take different paths and
arrives in the wrong order UDP has no way to reorder the datagrams into their
transmission order. UDP simply reassembles the data in order it was
received and forward to app If seq is important for app, the app will have to identify
the proper sequence and how it should be processed
42© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Datagram Reassembly
43© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Protocol Servers use port numbers to identify a specified
application layer process and direct segments to the proper service or application
44© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
UDP Protocol UDP protocol and port numbers are utilized in client-
server communication
45© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
46© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
top related