4.0 Chapter Introduction
4.0.1 Chapter IntroductionPage 1:
Data networks and the Internet support the human network by supplying seamless, reliable communication between people - both locally and around the globe. On a single device, people can use multiple services such as e-mail, the web, and instant messaging to send messages or retrieve information. Applications such as e-mail clients, web browsers, and instant messaging clients allow people to use computers and networks to send messages and find information.
Data from each of these applications is packaged, transported, and delivered to the appropriate server daemon or application on the destination device. The processes described in the OSI Transport layer accept data from the Application layer and prepare it for addressing at the Network layer. The Transport layer is responsible for the overall end-to-end transfer of application data.
In this chapter, we examine the role of the Transport layer in encapsulating application data for use by the Network layer. The Transport layer also encompasses these functions:
- Enables multiple applications to communicate over the network at the same time on a single device
- Ensures that, if required, all the data is received reliably and in order by the correct application
- Employs error handling mechanisms
Upon completion of this chapter, you will be able to:
- Explain the need for the Transport layer.
- Identify the role of the Transport layer as it provides the end-to-end transfer of data between applications.
- Describe the role of two TCP/IP Transport layer protocols: TCP and UDP.
- Explain the key functions of the Transport layer, including reliability, port addressing, and segmentation.
- Explain how TCP and UDP each handle key functions.
- Identify when it is appropriate to use TCP or UDP and provide examples of applications that use each protocol.
4.0.1 - Chapter Introduction
The diagram depicts the O S I model with the Transport Layer broken out. A group of routers is attached to the Physical Layer, and a PC is at the Application Layer. The Transport Layer is acting as the intermediary between application data and network data. The Transport Layer prepares application data for transport over the network and processes network data for use by applications.
The diagram depicts the O S I model with the Transport Layer broken out. A group of routers is attached to the Physical Layer, and a PC is at the Application Layer. The Transport Layer is acting as the intermediary between application data and network data. The Transport Layer prepares application data for transport over the network and processes network data for use by applications.
4.1 Roles of the Transport Layer
4.1.1 Purpose of the Transport LayerPage 1:
The Transport layer provides for the segmentation of data and the control necessary to reassemble these pieces into the various communication streams. Its primary responsibilities to accomplish this are:
- Tracking the individual communication 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
Any host may have multiple applications that are communicating across the network. Each of these applications will be communicating with one or more applications on remote hosts. It is the responsibility of the Transport layer to maintain the multiple communication streams between these applications.
Segmenting Data
As each application creates a stream data to be sent to a remote application, this data must be prepared to be sent across the media in manageable pieces. The Transport layer protocols describe services that segment this data from the Application layer. This includes the encapsulation required on each piece of data. Each piece of application data requires headers to be added at the Transport layer to indicate to which communication it is associated.
Reassembling Segments
At the receiving host, each piece of data may be directed to the appropriate application. Additionally, these individual pieces of data must also be reconstructed into a complete data stream that is useful to the Application layer. The protocols at the Transport layer describe the how the Transport layer header information is used to reassemble the data pieces into streams to be passed to the Application layer.
Identifying the Applications
In order to pass data streams to the proper applications, the Transport layer must identify the target application. To accomplish this, the Transport layer assigns an application an identifier. The TCP/IP protocols call this identifier a port number. Each software process that needs to access the network is assigned a port number unique in that host. This port number is used in the Transport layer header to indicate to which application that piece of data is associated.
The Transport layer is the link between the Application layer and the lower layer that are responsible for network transmission. This layer accepts data from different conversations and passes it down to the lower layers as manageable pieces that can be eventually multiplexed over the media.
Applications do not need to know the operational details of the network in use. The applications generate data that is sent from one application to another, without regard to the destination host type, the type of media over which the data must travel, the path taken by the data, the congestion on a link, or the size of the network.
Additionally, the lower layers are not aware that there are multiple applications sending data on the network. Their responsibility is to deliver data to the appropriate device. The Transport layer then sorts these pieces before delivering them to the appropriate application.
Data Requirements Vary
Because different applications have different requirements, there are multiple Transport layer protocols. For some applications, segments must arrive in a very specific sequence in order to be processed successfully. In some cases, all of the data must be received for any of it to be of use. In other cases, an application can tolerate some loss of data during transmission over the network.
In today's converged networks, applications with very different transport needs may be communicating on the same network. The different Transport layer protocols have different rules allowing devices to handle these diverse data requirements.
Some protocols provide just the basic functions for efficiently delivering the data pieces between the appropriate applications. These types of protocols are useful for applications whose data is sensitive to delays.
Other Transport layer protocols describe processes that provide additional features, such as ensuring reliable delivery between the applications. While these additional functions provide more robust communication at the Transport layer between applications, they have additional overhead and make larger demands on the network.
4.1.1 - Purpose of the Transport Layer
The diagram depicts the TCP/IP model and focuses on how the Transport Layer enables applications on devices to communicate. Devices on the left include an IP phone, a videoconferencing TV, a PC, and a cell phone. Each device is communicating with a similar device on the right side. The Transport Layer moves data between applications on devices in the network.
The diagram depicts the TCP/IP model and focuses on how the Transport Layer enables applications on devices to communicate. Devices on the left include an IP phone, a videoconferencing TV, a PC, and a cell phone. Each device is communicating with a similar device on the right side. The Transport Layer moves data between applications on devices in the network.
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Separating Multiple Communications
Consider a computer connected to a network that is simultaneously receiving and sending e-mail and instant messages, viewing websites, and conducting a VoIP phone call. Each of these applications is sending and receiving data over the network at the same time. However, data from the phone call is not directed to the web browser, and text from an instant message does not appear in an e-mail.
Further, users require that an e-mail or web page be completely received and presented for the information to be considered useful. Slight delays are considered acceptable to ensure that the complete information is received and presented.
In contrast, occasionally missing small parts of a telephone conversation might be considered acceptable. One can either infer the missing audio from the context of the conversation or ask the other person to repeat what they said. This is considered preferable to the delays that would result from asking the network to manage and resend missing segments. In this example, the user - not the network - manages the resending or replacement of missing information.
4.1.1 - Purpose of the Transport Layer
The diagram depicts how the Transport Layer tracks multiple communications or conversations. A PC is shown with multicolored arrows pointing to multiple applications, including e-mail, instant messaging, a browser with multiple web pages open, a V o IP phone call, streaming video, and a connection to a network cloud. The Transport Layer segments the data and manages the separation of data for different applications. Multiple applications on a device receive the correct data.
The diagram depicts how the Transport Layer tracks multiple communications or conversations. A PC is shown with multicolored arrows pointing to multiple applications, including e-mail, instant messaging, a browser with multiple web pages open, a V o IP phone call, streaming video, and a connection to a network cloud. The Transport Layer segments the data and manages the separation of data for different applications. Multiple applications on a device receive the correct data.
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As explained in a previous chapter, sending some types of data - a video for example - across a network as one complete communication stream could prevent other communications from occurring at the same time. It also makes error recovery and retransmission of damaged data difficult.
Dividing data into small parts, and sending these parts from the source to the destination, enables many different communications to be interleaved (multiplexed) on the same network.
Segmentation of the data, in accordance with Transport layer protocols, provides the means to both send and receive data when running multiple applications concurrently on a computer. Without segmentation, only one application, the streaming video for example, would be able to receive data. You could not receive e-mails, chat on instant messenger, or view web pages while also viewing the video.
At the Transport layer, each particular set of pieces flowing between a source application and a destination application is known as a conversation.
To identify each segment of data, the Transport layer adds to the piece a header containing binary data. This header contains fields of bits. It is the values in these fields that enable different Transport layer protocols to perform different functions.
4.1.1 - Purpose of the Transport Layer
The diagram is similar to the previous diagram. It depicts how the Transport Layer divides the data up into segments that are easier to manage and transport. A series of small color-coded envelopes is moving toward the PC, representing segments from various applications.
The diagram is similar to the previous diagram. It depicts how the Transport Layer divides the data up into segments that are easier to manage and transport. A series of small color-coded envelopes is moving toward the PC, representing segments from various applications.
4.1.2 Controlling the Conversations
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The primary functions specified by all Transport layer protocols include:
Segmentation and Reassembly - Most networks have a limitation on the amount of data that can be included in a single PDU. The Transport layer divides application data into blocks of data that are an appropriate size. At the destination, the Transport layer reassembles the data before sending it to the destination application or service.
Conversation Multiplexing - There may be many applications or services running on each host in the network. Each of these applications or services is assigned an address known as a port so that the Transport layer can determine with which application or service the data is identified.
In addition to using the information contained in the headers, for the basic functions of data segmentation and reassembly, some protocols at the Transport layer provide:
- Connection-oriented conversations
- Reliable delivery
- Ordered data reconstruction
- Flow control
4.1.2 - Controlling the Conversations
The diagram is similar to the previous diagram. It depicts how the segmentation process allows session multiplexing so that multiple applications can use the network link at the same time. Data segmentation facilitates data carriage by the lower network layers. Error checking can be performed on the data in the segment to see whether the segment was changed during transmission.
The diagram is similar to the previous diagram. It depicts how the segmentation process allows session multiplexing so that multiple applications can use the network link at the same time. Data segmentation facilitates data carriage by the lower network layers. Error checking can be performed on the data in the segment to see whether the segment was changed during transmission.
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Establishing a Session
The Transport layer can provide this connection orientation by creating a sessions between the applications. These connections prepare the applications to communicate with each other before any data is transmitted. Within these sessions, the data for a communication between the two applications can be closely managed.
Reliable Delivery
For many reasons, it is possible for a piece of data to become corrupted, or lost completely, as it is transmitted over the network. The Transport layer can ensure that all pieces reach their destination by having the source device to retransmit any data that is lost.
Same Order Delivery
Because networks may provide multiple routes that can have different transmission times, data can arrive in the wrong order. By numbering and sequencing the segments, the Transport layer can ensure that these segments are reassembled into the proper order.
Flow Control
Network hosts have limited resources, such as memory or bandwidth. When Transport layer is aware that these resources are overtaxed, some protocols can request that the sending application reduce the rate of data flow. This is done at the Transport layer by regulating the amount of data the source transmits as a group. Flow control can prevent the loss of segments on the network and avoid the need for retransmission.
As the protocols are discussed in this chapter, these services will be explained in more detail.
4.1.2 - Controlling the Conversations
The diagram depicts Transport Layer services. A PC is shown with multicolored arrows pointing to multiple applications, including e-mail, instant messaging, a browser with multiple web pages open, a V o IP phone call, and streaming video. Transport Layer services include:
Establishing a session, which ensures that the application is ready to receive data.
Reliable delivery means that lost segments are resent so that all the data is received.
Same order delivery ensures that the segments are reassembled into the proper order.
Flow control manages data delivery if there is congestion on the host.
The diagram depicts Transport Layer services. A PC is shown with multicolored arrows pointing to multiple applications, including e-mail, instant messaging, a browser with multiple web pages open, a V o IP phone call, and streaming video. Transport Layer services include:
Establishing a session, which ensures that the application is ready to receive data.
Reliable delivery means that lost segments are resent so that all the data is received.
Same order delivery ensures that the segments are reassembled into the proper order.
Flow control manages data delivery if there is congestion on the host.
4.1.3 Supporting Reliable Communication
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Recall that the primary function of the Transport layer is to manage the application data for the conversations between hosts. However, different applications have different requirements for their data, and therefore different Transport protocols have been developed to meet these requirements.
A Transport layer protocol can implement a method to ensure reliable delivery of the data. In networking terms, reliability means ensuring that each piece of data that the source sends arrives at the destination. At the Transport layer the three basic operations of reliability are:
- tracking transmitted data
- acknowledging received data
- retransmitting any unacknowledged data
These reliability processes place additional overhead on the network resources due to the acknowledgement, tracking, and retransmission. To support these reliability operations, more control data is exchanged between the sending and receiving hosts. This control information is contained in the Layer 4 header.
This creates a trade-off between the value of reliability and the burden it places on the network. Application developers must choose which transport protocol type is appropriate based on the requirements of their applications. At the Transport layer, there are protocols that specify methods for either reliable, guaranteed delivery or best-effort delivery. In the context of networking, best-effort delivery is referred to as unreliable, because there is no acknowledgement that the data is received at the destination.
Determining the Need for Reliability
Applications, such as databases, web pages, and e-mail, require that all of the sent data arrive at the destination in its original condition, in order for the data to be useful. Any missing data could cause a corrupt communication that is either incomplete or unreadable. Therefore, these applications are designed to use a Transport layer protocol that implements reliability. The additional network overhead is considered to be required for these applications.
Other applications are more tolerant of the loss of small amounts of data. For example, if one or two segments of a video stream fail to arrive, it would only create a momentary disruption in the stream. This may appear as distortion in the image but may not even be noticeable to the user.
Imposing overhead to ensure reliability for this application could reduce the usefulness of the application. The image in a streaming video would be greatly degraded if the destination device had to account for lost data and delay the stream while waiting for its arrival. It is better to render the best image possible at the time with the segments that arrive and forego reliability. If reliability is required for some reason, these applications can provide error checking and retransmission requests.
4.1.3 - Supporting Reliable Communication
The diagram depicts the required characteristics of Transport Layer protocols for various types of applications. Application developers choose the appropriate Transport Layer protocol based on the nature of the application.
For IP telephony and streaming video applications, the required protocol properties are:
- Fast
- Low overhead
- Does not require acknowledgements
- Does not resend lost data
- Delivers data as it arrives
For SMTP and POP (e-mail) and HTTP applications, the required protocol properties are:
- Reliable
- Acknowledges data
- Resends lost data
- Delivers data in the order sent
The diagram depicts the required characteristics of Transport Layer protocols for various types of applications. Application developers choose the appropriate Transport Layer protocol based on the nature of the application.
For IP telephony and streaming video applications, the required protocol properties are:
- Fast
- Low overhead
- Does not require acknowledgements
- Does not resend lost data
- Delivers data as it arrives
For SMTP and POP (e-mail) and HTTP applications, the required protocol properties are:
- Reliable
- Acknowledges data
- Resends lost data
- Delivers data in the order sent
4.1.4 TCP and UDP
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The two most common Transport layer protocols of TCP/IP protocol suite are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Both protocols manage the communication of multiple applications. The differences between the two are the specific functions that each protocol implements.
User Datagram Protocol (UDP)
UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery. The pieces of communication in UDP are called datagrams. These datagrams are sent as "best effort" by this Transport layer protocol.
Applications that use UDP include:
- Domain Name System (DNS)
- Video Streaming
- Voice over IP (VoIP)
TCP is a connection-oriented protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are the same order delivery, reliable delivery, and flow control. Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data, whereas each UDP segment only has 8 bytes of overhead. See the figure for a comparison.
Applications that use TCP are:
- Web Browsers
- File Transfers
4.1.4 - TCP and UDP
The diagram depicts the structure of a TCP segment and a UDP datagram. Each component is listed with its length in bits.
TCP segment (Header length 20 bytes):
- Source Port (16 bits)
- Destination Port (16 bits)
- Sequence Number (32 bits)
- Acknowledgment Number (32 bits)
- Header Length (4 bits) Reserved (6 bits) Code Bits (6 bits), Window (16 bits)
- Checksum (16 bits), Urgent (16 bits)
- Options (0 or 32 bits, if any)
- Application Layer Data (size varies)
UDP Datagram (Header length 8 bytes):
- Source Port (16 bits)
- Destination Port (16 bits)
- Length (16 bits)
- Checksum (16 bits)
- Application Layer Data (size varies)
The diagram depicts the structure of a TCP segment and a UDP datagram. Each component is listed with its length in bits.
TCP segment (Header length 20 bytes):
- Source Port (16 bits)
- Destination Port (16 bits)
- Sequence Number (32 bits)
- Acknowledgment Number (32 bits)
- Header Length (4 bits) Reserved (6 bits) Code Bits (6 bits), Window (16 bits)
- Checksum (16 bits), Urgent (16 bits)
- Options (0 or 32 bits, if any)
- Application Layer Data (size varies)
UDP Datagram (Header length 8 bytes):
- Source Port (16 bits)
- Destination Port (16 bits)
- Length (16 bits)
- Checksum (16 bits)
- Application Layer Data (size varies)
4.1.5 Port Addressing
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Identifying the Conversations
Consider the earlier example of a computer simultaneously receiving and sending e-mail, instant messages, web pages, and a VoIP phone call.
The TCP and UDP based services keep track of the various applications that are communicating. To differentiate the segments and datagrams for each application, both TCP and UDP have header fields that can uniquely identify these applications. These unique identifiers are the port numbers.
In the header of each segment or datagram, there is a source and destination port. The source port number is the number for this communication associated with the originating application on the local host. The destination port number is the number for this communication associated with the destination application on the remote host.
Port numbers are assigned in various ways, depending on whether the message is a request or a response. While server processes have static port numbers assigned to them, clients dynamically choose a port number for each conversation.
When a client application sends a request to a server application, the destination port contained in the header is the port number that is assigned to the service daemon running on the remote host. The client software must know what port number is associated with the server process on the remote host. This destination port number is configured, either by default or manually. For example, when a web browser application makes a request to a web server, the browser uses TCP and port number 80 unless otherwise specified. This is because TCP port 80 is the default port assigned to web-serving applications. Many common applications have default port assignments.
The source port in a segment or datagram header of a client request is randomly generated from port numbers greater than 1023. As long as it does not conflict with other ports in use on the system, the client can choose any port number from the range of default port numbers used by the operating system. This port number acts like a return address for the requesting application. The Transport layer keeps track of this port and the application that initiated the request so that when a response is returned, it can be forwarded to the correct application. The requesting application port number is used as the destination port number in the response coming back from the server.
The combination of the Transport layer port number and the Network layer IP address assigned to the host uniquely identifies a particular process running on a specific host device. This combination is called a socket. Occasionally, you may find the terms port number and socket used interchangeably. In the context of this course, the term socket refers only to the unique combination of IP address and port number. A socket pair, consisting of the source and destination IP addresses and port numbers, is also unique and identifies the conversation between the two hosts.
For example, an HTTP web page request being sent to a web server (port 80) running on a host with a Layer 3 IPv4 address of 192.168.1.20 would be destined to socket 192.168.1.20:80.
If the web browser requesting the web page is running on host 192.168.100.48 and the Dynamic port number assigned to the web browser is 49152, the socket for the web page would be 192.168.100.48:49152.
4.1.5 - Port Addressing
The diagram depicts using Transport Layer application port addressing to identify conversations. Examples of e-mail, HTML, and Internet chat are shown. Data for different applications is directed to the correct application because each application has a unique port number.
Application: E-mail:
Protocol: POP3
Port number: 110
Application: HTML web page
Protocol: HTTP
Port number: 80
Application: Internet chat
Protocol: IM
Port number: 531
The diagram depicts using Transport Layer application port addressing to identify conversations. Examples of e-mail, HTML, and Internet chat are shown. Data for different applications is directed to the correct application because each application has a unique port number.
Application: E-mail:
Protocol: POP3
Port number: 110
Application: HTML web page
Protocol: HTTP
Port number: 80
Application: Internet chat
Protocol: IM
Port number: 531
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The Internet Assigned Numbers Authority (IANA) assigns port numbers. IANA is a standards body that is responsible for assigning various addressing standards.
There are different types of port numbers:
Well Known Ports (Numbers 0 to 1023) - These numbers are reserved for services and applications. They are commonly used for applications such as HTTP (web server) POP3/SMTP (e-mail server) and Telnet. By defining these well-known ports for server applications, client applications can be programmed to request a connection to that specific port and its associated service.
Registered Ports (Numbers 1024 to 49151) - These port numbers are assigned to user processes or applications. These processes are primarily individual applications that a user has chosen to install rather than common applications that would receive a Well Known Port. When not used for a server resource, these ports may also be used dynamically selected by a client as its source port.
Dynamic or Private Ports (Numbers 49152 to 65535) - Also known as Ephemeral Ports, these are usually assigned dynamically to client applications when initiating a connection. It is not very common for a client to connect to a service using a Dynamic or Private Port (although some peer-to-peer file sharing programs do).
Using both TCP and UDP
Some applications may use both TCP and UDP. For example, the low overhead of UDP enables DNS to serve many client requests very quickly. Sometimes, however, sending the requested information may require the reliability of TCP. In this case, the well known port number of 53 is used by both protocols with this service.
Links
A current list of port numbers can be found at http://www.iana.org/assignments/port-numbers.
4.1.5 - Port Addressing
The diagram depicts TCP and UDP port number ranges and some examples of applications that use them.
TCP/UDP Port Numbers:
Port Number Range: 0 to 1023 - Well Known (commonly used) Ports
Well Known TCP Port examples:
- 21 - FTP
- 23 - Telnet
- 25 - SMTP
- 80 - HTTP
- 110 - POP3
- 194 - Internet Relay Chat (I RC)
- 443 - Secure HTTP (HTTPS)
Well Known UDP Port examples:
- 69 - TFTP
- 520 - RIP
Well Known TCP and UDP Common Port examples:
- 53 - DNS
- 161 - SNMP
- 531 - A O L Instant Messenger, I RC
Port Number Range: 1024 to 49151 - Registered Ports
Registered TCP Port examples:
- 1863 - MSN Messenger
- 2000 - Cisco SCCP (V o IP)
- 8008 - Alternate HTTP
- 8080 - Alternate HTTP
Registered UDP Port examples:
- 1812 - RADIUS Authentication Protocol
- 5004 - RTP (Voice and Video Transport Protocol)
- 5060 - SIP (V o IP)
Registered TCP and UDP Common Port examples:
- 1433 - MS SQL
- 2948 - WAP (MMS)
Port Number Range: 49152 to 65535 - Private and/or Dynamic Ports
The diagram depicts TCP and UDP port number ranges and some examples of applications that use them.
TCP/UDP Port Numbers:
Port Number Range: 0 to 1023 - Well Known (commonly used) Ports
Well Known TCP Port examples:
- 21 - FTP
- 23 - Telnet
- 25 - SMTP
- 80 - HTTP
- 110 - POP3
- 194 - Internet Relay Chat (I RC)
- 443 - Secure HTTP (HTTPS)
Well Known UDP Port examples:
- 69 - TFTP
- 520 - RIP
Well Known TCP and UDP Common Port examples:
- 53 - DNS
- 161 - SNMP
- 531 - A O L Instant Messenger, I RC
Port Number Range: 1024 to 49151 - Registered Ports
Registered TCP Port examples:
- 1863 - MSN Messenger
- 2000 - Cisco SCCP (V o IP)
- 8008 - Alternate HTTP
- 8080 - Alternate HTTP
Registered UDP Port examples:
- 1812 - RADIUS Authentication Protocol
- 5004 - RTP (Voice and Video Transport Protocol)
- 5060 - SIP (V o IP)
Registered TCP and UDP Common Port examples:
- 1433 - MS SQL
- 2948 - WAP (MMS)
Port Number Range: 49152 to 65535 - Private and/or Dynamic Ports
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Sometimes it is necessary to know which active TCP connections are open and running on a networked host. Netstat is an important network utility that can be used to verify those connections. Netstat lists the protocol in use, the local address and port number, the foreign address and port number, and the state of the connection.
Unexplained TCP connections can pose a major security threat. This is because they can indicate that something or someone is connected to the local host. Additionally, unnecessary TCP connections can consume valuable system resources thus slowing down the host's performance. Netstat should be used to examine the open connections on a host when performance appears to be compromised.
Many useful options are available for the netstat command.
4.1.5 - Port Addressing
The diagram depicts using the Netstat utility from a PC command line. For network connections, Netstat lists the protocol in use, the local address and port number, the foreign address and port number, and the state of the connection. The following is an example of an active connection from the output shown.
C:\>netstat
Proto Local Address Foreign Address State:
TCP ken pc:3126 www.cisco.com:http ESTABLISHED
The following elements in the above output are highlighted:
Protocol used : TCP
Source port: 3126
Address or name of remote host: www.cisco.com
Destination port: http
Connection state: Established
The diagram depicts using the Netstat utility from a PC command line. For network connections, Netstat lists the protocol in use, the local address and port number, the foreign address and port number, and the state of the connection. The following is an example of an active connection from the output shown.
C:\>netstat
Proto Local Address Foreign Address State:
TCP ken pc:3126 www.cisco.com:http ESTABLISHED
The following elements in the above output are highlighted:
Protocol used : TCP
Source port: 3126
Address or name of remote host: www.cisco.com
Destination port: http
Connection state: Established
4.1.6 Segmentation and Reassembly - Divide and Conquer
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A previous chapter explained how PDUs are built by passing data from an application down through the various protocols to create a PDU that is then transmitted on the medium. At the destination host, this process is reversed until the data can be passed up to the application.
Some applications transmit large amounts of data - in some cases, many gigabytes. It would be impractical to send all of this data in one large piece. No other network traffic could be transmitted while this data was being sent. A large piece of data could take minutes or even hours to send. In addition, if there were any error, the entire data file would have to be lost or resent. Network devices would not have memory buffers large enough to store this much data while it is transmitted or received. The limit varies depending on the networking technology and specific physical medium being in use.
Dividing application data into pieces both ensures that data is transmitted within the limits of the media and that data from different applications can be multiplexed on to the media.
TCP and UDP Handle Segmentation Differently.
In TCP, each segment header contains a sequence number. This sequence number allows the Transport layer functions on the destination host to reassemble segments in the order in which they were transmitted. This ensures that the destination application has the data in the exact form the sender intended.
Although services using UDP also track the conversations between applications, they are not concerned with the order in which the information was transmitted, or in maintaining a connection. There is no sequence number in the UDP header. UDP is a simpler design and generates less overhead than TCP, resulting in a faster transfer of data.
Information may arrive in a different order than it was transmitted because different packets may take different paths through the network. An application that uses UDP must tolerate the fact that data may not arrive in the order in which it was sent.
4.1.6 - Segmentation and Reassembly - Divide and Conquer
The diagram depicts the Transport Layer dividing Application Layer data into pieces and adding either a TCP header or UDP header for delivery over the network. Application Layer data is divided into three UDP datagrams, each with a header, and also into three TCP segments, each with a header.
UDP header provides:
- Source and destination ports
TCP header provides:
- Source and destination ports
- Sequencing for same order delivery
- Acknowledgement of received segments
- Flow control and congestion management
The diagram depicts the Transport Layer dividing Application Layer data into pieces and adding either a TCP header or UDP header for delivery over the network. Application Layer data is divided into three UDP datagrams, each with a header, and also into three TCP segments, each with a header.
UDP header provides:
- Source and destination ports
TCP header provides:
- Source and destination ports
- Sequencing for same order delivery
- Acknowledgement of received segments
- Flow control and congestion management
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In this activity, you will "look inside" packets to see how DNS and HTTP use port numbers.
Click the Packet Tracer icon to launch the Packet Tracer activity.
4.1.6 - Segmentation and Reassembly - Divide and Conquer
Link to Packet Tracer Exploration: UDP and TCP Port Numbers
In this activity, you look inside packets to see how DNS and HTTP use port numbers.
Link to Packet Tracer Exploration: UDP and TCP Port Numbers
In this activity, you look inside packets to see how DNS and HTTP use port numbers.
4.2 The TCP Protocol - Communicating with Reliability
4.2.1 TCP - Making Conversations ReliablePage 1:
The key distinction between TCP and UDP is reliability. The reliability of TCP communication is performed using connection-oriented sessions. Before a host using TCP sends data to another host, the Transport layer initiates a process to create a connection with the destination. This connection enables the tracking of a session, or communication stream between the hosts. This process ensures that each host is aware of and prepared for the communication. A complete TCP conversation requires the establishment of a session between the hosts in both directions.
After a session has been established, the destination sends acknowledgements to the source for the segments that it receives. These acknowledgements form the basis of reliability within the TCP session. As the source receives an acknowledgement, it knows that the data has been successfully delivered and can quit tracking that data. If the source does not receive an acknowledgement within a predetermined amount of time, it retransmits that data to the destination.
Part of the additional overhead of using TCP is the network traffic generated by acknowledgements and retransmissions. The establishment of the sessions creates overhead in the form of additional segments being exchanged. There is also additional overhead on the individual hosts created by the necessity to keep track of which segments are awaiting acknowledgement and by the retransmission process.
This reliability is achieved by having fields in the TCP segment, each with a specific function, as shown in the figure. These fields will be discussed later in this section.
4.2.1 - TCP - Making Conversations Reliable
The diagram depicts the structure of a TCP segment with each component listed. The fields of the TCP header enable TCP to provide connection-oriented, reliable communications.
TCP segment Header Fields:
- Source Port Number: TCP session on the device that opened a connection. Normally a random value above 1023.
- Destination Port Number: Identifies the upper layer protocol or application on the remote site.
- Sequence Number: Specifies the number of the last octet (byte) in a segment.
- Acknowledgment Number: Specifies the next octet expected by the receiver.
- Header Length: Specifies the length of the segment header in bytes.
- Reserved: Set to zero.
- Control Flags: Used in session management and in the treatment of segments.
- Window size: Value of the dynamic window - how many octets can be sent before waiting for an acknowledgement.
- TCP Checksum: Used for error-checking the header and data.
- Urgent Pointer: Only used with an URG (Urgent) flag.
- Options (if any): Optional information.
- Data (size varies): Application data.
The diagram depicts the structure of a TCP segment with each component listed. The fields of the TCP header enable TCP to provide connection-oriented, reliable communications.
TCP segment Header Fields:
- Source Port Number: TCP session on the device that opened a connection. Normally a random value above 1023.
- Destination Port Number: Identifies the upper layer protocol or application on the remote site.
- Sequence Number: Specifies the number of the last octet (byte) in a segment.
- Acknowledgment Number: Specifies the next octet expected by the receiver.
- Header Length: Specifies the length of the segment header in bytes.
- Reserved: Set to zero.
- Control Flags: Used in session management and in the treatment of segments.
- Window size: Value of the dynamic window - how many octets can be sent before waiting for an acknowledgement.
- TCP Checksum: Used for error-checking the header and data.
- Urgent Pointer: Only used with an URG (Urgent) flag.
- Options (if any): Optional information.
- Data (size varies): Application data.
4.2.2 TCP Server Processes
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As discussed in the previous chapter, application processes run on servers. These processes wait until a client initiates communication with a request for information or other services.
Each application process running on the server is configured to use a port number, either by default or manually by a system administrator. An individual server cannot have two services assigned to the same port number within the same Transport layer services. A host running a web server application and a file transfer application cannot have both configured to use the same port (for example, TCP port 8080). When an active server application is assigned to a specific port, that port is considered to be "open" on the server. This means that the Transport layer accepts and processes segments addressed to that port. Any incoming client request addressed to the correct socket is accepted and the data is passed to the server application. There can be many simultaneous ports open on a server, one for each active server application. It is common for a server to provide more than one service, such as a web server and an FTP server, at the same time.
One way to improve security on a server is to restrict server access to only those ports associated with the services and applications that should be accessible to authorized requestors.
The figure shows the typical allocation of source and destination ports in TCP client/server operations.
4.2.2 - TCP Server Processes
The diagram depicts two clients, Client 1 and Client 2, sending TCP requests to a server. The destination ports on the server are well-known port numbers.
Client 1 HTTP request
Source Port: 49152
Destination Port: 80
Server HTTP response to Client 1 request
Source Port: 80
Destination Port: 49152
Client 2 SMTP request
Source Port: 51152
Destination Port: 25
Server SMTP response to Client 2 request
Source Port: 25
Destination Port: 51152
The diagram depicts two clients, Client 1 and Client 2, sending TCP requests to a server. The destination ports on the server are well-known port numbers.
Client 1 HTTP request
Source Port: 49152
Destination Port: 80
Server HTTP response to Client 1 request
Source Port: 80
Destination Port: 49152
Client 2 SMTP request
Source Port: 51152
Destination Port: 25
Server SMTP response to Client 2 request
Source Port: 25
Destination Port: 51152
4.2.3 TCP Connection Establishment and Termination
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When two hosts communicate using TCP, a connection is established before data can be exchanged. After the communication is completed, the sessions are closed and the connection is terminated. The connection and session mechanisms enable TCP's reliability function.
See the figure for the steps to establish and terminate a TCP connection.
The host tracks each data segment within a session and exchanges information about what data is received by each host using the information in the TCP header.
Each connection involves one-way communication streams, or sessions to establish and terminate the TCP process between end devices. To establish the connection, the hosts perform a three-way handshake. Control bits in the TCP header indicate the progress and status of the connection. The three-way handshake:
- Establishes that the destination device is present on the network
- Verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use for the session
- Informs the destination device that the source client intends to establish a communication session on that port number
1. The initiating client sends a segment containing an initial sequence value, which serves as a request to the server to begin a communications session.
2. The server responds with a segment containing an acknowledgement value equal to the received sequence value plus 1, plus its own synchronizing sequence value. The value is one greater than the sequence number because the ACK is always the next expected Byte or Octet. This acknowledgement value enables the client to tie the response back to the original segment that it sent to the server.
3. Initiating client responds with an acknowledgement value equal to the sequence value it received plus one. This completes the process of establishing the connection.
Within the TCP segment header, there are six 1-bit fields that contain control information used to manage the TCP processes. Those fields are:
URG - Urgent pointer field significant
ACK - Acknowledgement field significant
PSH - Push function
RST - Reset the connection
SYN - Synchronize sequence numbers
FIN - No more data from sender
These fields are referred to as flags, because the value of one of these fields is only 1 bit and, therefore, has only two values: 1 or 0. When a bit value is set to 1, it indicates what control information is contained in the segment.
Using a four-step process, flags are exchanged to terminate a TCP connection.
4.2.3 - TCP Connection Establishment and Termination
The diagram depicts TCP connection establishment (SYN ACK) and termination (FIN ACK) between two hosts, A and B.
Connection Establishment (SYN ACK) Steps:
Step 1. Host A sends a SYN request (SEQ=100, CTL=SYN) to host B. CTL = Which controls bits in the TCP header are set to 1.
Step 2. Host B sends an ACK response and SYN request (SEQ=300, ACK=101, CTL=SYN, ACK) to host A.
Step 3. Host A sends an ACK response (SEQ=101, ACK=301, CTL=ACK) to host B.
Connection Termination (FIN ACK) Steps:
Step 1. Host A sends a FIN to host B.
Step 2. Host B sends an ACK response to host A.
Step 3. Host B sends a FIN to host A.
Step 4. Host A sends an ACK response Host B, and the session is terminated.
The diagram depicts TCP connection establishment (SYN ACK) and termination (FIN ACK) between two hosts, A and B.
Connection Establishment (SYN ACK) Steps:
Step 1. Host A sends a SYN request (SEQ=100, CTL=SYN) to host B. CTL = Which controls bits in the TCP header are set to 1.
Step 2. Host B sends an ACK response and SYN request (SEQ=300, ACK=101, CTL=SYN, ACK) to host A.
Step 3. Host A sends an ACK response (SEQ=101, ACK=301, CTL=ACK) to host B.
Connection Termination (FIN ACK) Steps:
Step 1. Host A sends a FIN to host B.
Step 2. Host B sends an ACK response to host A.
Step 3. Host B sends a FIN to host A.
Step 4. Host A sends an ACK response Host B, and the session is terminated.
4.2.4 TCP Three-Way Handshake
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Using the Wireshark outputs, you can examine the operation of the TCP 3-way handshake:
Step 1
A TCP client begins the three-way handshake by sending a segment with the SYN (Synchronize Sequence Number) control flag set, indicating an initial value in the sequence number field in the header. This initial value for the sequence number, known as the Initial Sequence Number (ISN), is randomly chosen and is used to begin tracking the flow of data from the client to the server for this session. The ISN in the header of each segment is increased by one for each byte of data sent from the client to the server as the data conversation continues.
As shown in the figure, output from a protocol analyzer shows the SYN control flag and the relative sequence number.
The SYN control flag is set and the relative sequence number is at 0. Although the protocol analyzer in the graphic indicates the relative values for the sequence and acknowledgement numbers, the true values are 32 bit binary numbers. We can determine the actual numbers sent in the segment headers by examining the Packet Bytes pane. Here you can see the four bytes represented in hexadecimal.
4.2.4 - TCP Three-Way Handshake
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the initial client request (SYN) in a TCP three-way handshake. In the screenshot, various information lines are highlighted.
Frame 14: Source IP 10.1.1.1, Destination IP 192.168.254.254.
Frame 14 detail:
Transmission Control Protocol, Source Port: 1069 (1069), Destination Port: http (80), Seq: 0, Len: 0.
Flags: 0x02 (SYN)
00000010=Syn: Set
The initial client request in frame 14 shows the following TCP segment information:
- SYN flag set to validate an initial sequence number.
- Randomized sequence number valid (relative value is 0).
- Random source port 1069.
- Well-known destination port 80 (HTTP port) indicating web server (httpd).
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the initial client request (SYN) in a TCP three-way handshake. In the screenshot, various information lines are highlighted.
Frame 14: Source IP 10.1.1.1, Destination IP 192.168.254.254.
Frame 14 detail:
Transmission Control Protocol, Source Port: 1069 (1069), Destination Port: http (80), Seq: 0, Len: 0.
Flags: 0x02 (SYN)
00000010=Syn: Set
The initial client request in frame 14 shows the following TCP segment information:
- SYN flag set to validate an initial sequence number.
- Randomized sequence number valid (relative value is 0).
- Random source port 1069.
- Well-known destination port 80 (HTTP port) indicating web server (httpd).
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Step 2
The TCP server needs to acknowledge the receipt of the SYN segment from the client to establish the session from the client to the server. To do so, the server sends a segment back to the client with the ACK flag set indicating that the Acknowledgment number is significant. With this flag set in the segment, the client recognizes this as an acknowledgement that the server received the SYN from the TCP client.
The value of the acknowledgment number field is equal to the client initial sequence number plus 1. This establishes a session from the client to the server. The ACK flag will remain set for the balance of the session. Recall that the conversation between the client and the server is actually two one-way sessions: one from the client to the server, and the other from the server to the client. In this second step of the three-way handshake, the server must initiate the response from the server to the client. To start this session, the server uses the SYN flag in the same way that the client did. It sets the SYN control flag in the header to establish a session from the server to the client. The SYN flag indicates that the initial value of the sequence number field is in the header. This value will be used to track the flow of data in this session from the server back to the client.
As shown in the figure, the protocol analyzer output shows that the ACK and SYN control flags are set and the relative sequence and acknowledgement numbers are shown.
4.2.4 - TCP Three-Way Handshake
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the server response (SYN, ACK) in a TCP three-way handshake. In the screenshot, various information lines are highlighted.
Frame 15: Source IP 192.168.254.254, Destination IP 10.1.1.1.
Frame 15 detail:
Transmission Control Protocol, Source Port: http (80), Destination Port: 1069 (1069), Seq: 0, Ack: 1, Len: 0.
Flags: 0x12 (SYN, ACK)
00010000=Acknowledgment: Set
00000010=Syn: Set
[SEQ/ACK analysis]
[This is an ACK to the segment in frame: 14]
The server response in frame 15 shows:
- ACK flag set to indicate a valid acknowledgement number.
- Acknowledgement number response to the initial sequence number as a relative value of 1.
- SYN flag set to indicate the initial sequence number for the server-to-client session.
- Destination port number 1069 corresponds to the clients source port.
- Source port number 80 (HTTP) indicates the web server service (httpd).
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the server response (SYN, ACK) in a TCP three-way handshake. In the screenshot, various information lines are highlighted.
Frame 15: Source IP 192.168.254.254, Destination IP 10.1.1.1.
Frame 15 detail:
Transmission Control Protocol, Source Port: http (80), Destination Port: 1069 (1069), Seq: 0, Ack: 1, Len: 0.
Flags: 0x12 (SYN, ACK)
00010000=Acknowledgment: Set
00000010=Syn: Set
[SEQ/ACK analysis]
[This is an ACK to the segment in frame: 14]
The server response in frame 15 shows:
- ACK flag set to indicate a valid acknowledgement number.
- Acknowledgement number response to the initial sequence number as a relative value of 1.
- SYN flag set to indicate the initial sequence number for the server-to-client session.
- Destination port number 1069 corresponds to the clients source port.
- Source port number 80 (HTTP) indicates the web server service (httpd).
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Step 3
Finally, the TCP client responds with a segment containing an ACK that is the response to the TCP SYN sent by the server. There is no user data in this segment. The value in the acknowledgment number field contains one more than the initial sequence number received from the server. Once both sessions are established between client and server, all additional segments exchanged in this communication will have the ACK flag set.
As shown in the figure, the protocol analyzer output shows the ACK control flag set and the relative sequence and acknowledgement numbers are shown.
Security can be added to the data network by:
- Denying the establishment of TCP sessions
- Only allowing sessions to be established for specific services
- Only allowing traffic as a part of already established sessions
4.2.4 - TCP Three-Way Handshake
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the client response to the server (ACK) in a TCP three-way handshake. In the screenshot, various information lines are highlighted.
Frame 16: Source IP 10.1.1.1, Destination IP 192.168.254.254.
Frame 16 detail:
Transmission Control Protocol, Source Port: 1069 (1069), Destination Port: http (80) Seq: 1, Ack: 1, Len: 0.
Flags: 0x10 (ACK)
00010000=Acknowledgment: Set
[SEQ/ACK analysis]
[This is an ACK to the segment in frame: 15]
The TCP segment in this frame shows:
- ACK flag set to indicate a valid acknowledgement number.
- Acknowledgement number response to the initial sequence number as a relative value of 1.
- Source port number 1069 to corresponds to the clients source port.
- Source port number 80 (HTTP) indicates the web server service (httpd).
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the client response to the server (ACK) in a TCP three-way handshake. In the screenshot, various information lines are highlighted.
Frame 16: Source IP 10.1.1.1, Destination IP 192.168.254.254.
Frame 16 detail:
Transmission Control Protocol, Source Port: 1069 (1069), Destination Port: http (80) Seq: 1, Ack: 1, Len: 0.
Flags: 0x10 (ACK)
00010000=Acknowledgment: Set
[SEQ/ACK analysis]
[This is an ACK to the segment in frame: 15]
The TCP segment in this frame shows:
- ACK flag set to indicate a valid acknowledgement number.
- Acknowledgement number response to the initial sequence number as a relative value of 1.
- Source port number 1069 to corresponds to the clients source port.
- Source port number 80 (HTTP) indicates the web server service (httpd).
4.2.5 TCP Session Termination
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To close a connection, the FIN (Finish) control flag in the segment header must be set. To end each one-way TCP session, a two-way handshake is used, consisting of a FIN segment and an ACK segment. Therefore, to terminate a single conversation supported by TCP, four exchanges are needed to end both sessions. Note: In this explanation, the terms client and server are used in this description as a reference for simplicity, but the termination process can be initiated by any two hosts that complete the session:
1. When the client has no more data to send in the stream, it sends a segment with the FIN flag set.
2. The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server.
3. The server sends a FIN to the client, to terminate the server to client session.
4. The client responds with an ACK to acknowledge the FIN from the server.
When the client end of the session has no more data to transfer, it sets the FIN flag in the header of a segment. Next, the server end of the connection will send a normal segment containing data with the ACK flag set using the acknowledgment number, confirming that all the bytes of data have been received. When all segments have been acknowledged, the session is closed.
The session in the other direction is closed using the same process. The receiver indicates that there is no more data to send by setting the FIN flag in the header of a segment sent to the source. A return acknowledgement confirms that all bytes of data have been received and that session is, in turn, closed.
As shown in the figure, the FIN and ACK control flags are set in the segment header, thereby closing a HTTP session.
It is also possible to terminate the connection by a three-way handshake. When the client has no more data to send, it sends a FIN to the server. If the server also has no more data to send, it can reply with both the FIN and ACK flags set, combining two steps into one. The client replies with an ACK.
4.2.5 - TCP Session Termination
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the TCP session termination request (FIN) and the resulting ACK to acknowledge the receipt of the FIN to terminate the session from the client to the server. In the screenshot, various information lines are highlighted:
FIN Frame 20:
The protocol analyzer shows details of frame 20, the TCP FIN request, including the destination and source ports and the header field contents and values.
ACK Frame 20: Source IP 192.168.254.254, Destination IP 10.1.1.1.
FIN Frame 20 detail:
Transmission Control Protocol, Source Port: http (80), Destination Port: 1069 (1069) Seq: 440, Ack: 414, Len: 0.
Flags: 0x11 (FIN, ACK).
00010000=Acknowledgment: Set.
00000001=Fin: Set.
ACK Frame 21:
The protocol analyzer shows details of frame 21, the TCP ACK response, including the destination and source ports and the header field contents and values.
ACK Frame 21: Source IP 10.1.1.1, Destination IP 192.168.254.254.
ACK Frame 21 detail:
Transmission Control Protocol, Source Port: 1069 (1069), Destination Port: http (80) Seq: 414, Ack: 440, Len: 0.
Flags: 0x10 (ACK)
00010000=Acknowledgment: Set
[SEQ/ACK analysis]
[This is an ACK to the segment in frame: 20]
The diagram depicts a Wireshark screenshot of an Ethernet frame showing the TCP session termination request (FIN) and the resulting ACK to acknowledge the receipt of the FIN to terminate the session from the client to the server. In the screenshot, various information lines are highlighted:
FIN Frame 20:
The protocol analyzer shows details of frame 20, the TCP FIN request, including the destination and source ports and the header field contents and values.
ACK Frame 20: Source IP 192.168.254.254, Destination IP 10.1.1.1.
FIN Frame 20 detail:
Transmission Control Protocol, Source Port: http (80), Destination Port: 1069 (1069) Seq: 440, Ack: 414, Len: 0.
Flags: 0x11 (FIN, ACK).
00010000=Acknowledgment: Set.
00000001=Fin: Set.
ACK Frame 21:
The protocol analyzer shows details of frame 21, the TCP ACK response, including the destination and source ports and the header field contents and values.
ACK Frame 21: Source IP 10.1.1.1, Destination IP 192.168.254.254.
ACK Frame 21 detail:
Transmission Control Protocol, Source Port: 1069 (1069), Destination Port: http (80) Seq: 414, Ack: 440, Len: 0.
Flags: 0x10 (ACK)
00010000=Acknowledgment: Set
[SEQ/ACK analysis]
[This is an ACK to the segment in frame: 20]
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In this activity, you will study the TCP 3-way handshake for session establishment and the TCP process for session termination. Many application protocols use TCP, and visualizing the session establishment and termination processes with Packet Tracer will deepen your understanding.
Click the Packet Tracer icon to launch the Packet Tracer activity.
4.2.5 - TCP Session Termination
Link to Packet Tracer Exploration: TCP Session Establishment and Termination.
In this activity, you study the TCP three-way handshake for session establishment and the TCP process for session termination. Many application protocols use TCP. Visualizing the session establishment and termination processes with Packet Tracer will deepen your understanding.
Link to Packet Tracer Exploration: TCP Session Establishment and Termination.
In this activity, you study the TCP three-way handshake for session establishment and the TCP process for session termination. Many application protocols use TCP. Visualizing the session establishment and termination processes with Packet Tracer will deepen your understanding.
4.3 Managing TCP Sessions
4.3.1 TCP Segment ReassemblyPage 1:
Resequencing Segments to Order Transmitted
When services send data using TCP, segments may arrive at their destination out of order. For the original message to be understood by the recipient, the data in these segments is reassembled into the original order. Sequence numbers are assigned in the header of each packet to achieve this goal.
During session setup, an initial sequence number (ISN) is set. This initial sequence number represents the starting value for the bytes for this session that will be transmitted to the receiving application. As data is transmitted during the session, the sequence number is incremented by the number of bytes that have been transmitted. This tracking of data byte enables each segment to be uniquely identified and acknowledged. Missing segments can be identified.
Segment sequence numbers enable reliability by indicating how to reassemble and reorder received segments, as shown in the figure.
The receiving TCP process places the data from a segment into a receiving buffer. Segments are placed in the proper sequence number order and passed to the Application layer when reassembled. Any segments that arrive with noncontiguous sequence numbers are held for later processing. Then, when the segments with the missing bytes arrive, these segments are processed.
4.3.1 - TCP Segment Reassembly
The diagram depicts how different segments can take different routes through a network and arrive at the destination out of order. When this occurs, they are resequenced by TCP in the original order transmitted at the destination.
Network Topology:
PC1 is connected to one of a group of routers with multiple paths between them. PC2 is connected to one of the other routers on the opposite side of the network. Arrows from PC1 through various routers to PC2 indicate that packets with segments from the same data stream pass through different routers on their way from one PC to the other. TCP compensates by sequencing the segments.
The diagram depicts how different segments can take different routes through a network and arrive at the destination out of order. When this occurs, they are resequenced by TCP in the original order transmitted at the destination.
Network Topology:
PC1 is connected to one of a group of routers with multiple paths between them. PC2 is connected to one of the other routers on the opposite side of the network. Arrows from PC1 through various routers to PC2 indicate that packets with segments from the same data stream pass through different routers on their way from one PC to the other. TCP compensates by sequencing the segments.
4.3.2 TCP Acknowledgement with Windowing
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Confirming Receipt of Segments
One of TCP's functions is making sure that each segment reaches its destination. The TCP services on the destination host acknowledge the data that it has received to the source application.
The segment header sequence number and acknowledgement number are used together to confirm receipt of the bytes of data contained in the segments. The sequence number is the relative number of bytes that have been transmitted in this session plus 1 (which is the number of the first data byte in the current segment). TCP uses the acknowledgement number in segments sent back to the source to indicate the next byte in this session that the receiver expects to receive. This is called expectational acknowledgement.
The source is informed that the destination has received all bytes in this data stream up to, but not including, the byte indicated by the acknowledgement number. The sending host is expected to send a segment that uses a sequence number that is equal to the acknowledgement number.
Remember, each connection is actually two one-way sessions. Sequence numbers and acknowledgement numbers are being exchanged in both directions.
In the example in the figure, the host on the left is sending data to the host on the right. It sends a segment containing 10 bytes of data for this session and a sequence number equal to 1 in the header.
The receiving host on the right receives the segment at Layer 4 and determines that the sequence number is 1 and that it has 10 bytes of data. The host then sends a segment back to the host on the left to acknowledge the receipt of this data. In this segment, the host sets the acknowledgement number to 11 to indicate that the next byte of data it expects to receive in this session is byte number 11. Note, the Ack. value in the source host stays 1 to indicate that the segment is part of an ongoing conversation and the number in the Acknowledgment Number field is valid.
When the sending host on the left receives this acknowledgement, it can now send the next segment containing data for this session starting with byte number 11.
Looking at this example, if the sending host had to wait for acknowledgement of the receipt of each 10 bytes, the network would have a lot of overhead. To reduce the overhead of these acknowledgements, multiple segments of data can be sent before and acknowledged with a single TCP message in the opposite direction. This acknowledgement contains an acknowledgement number based on the total number of bytes received in the session.
For example, starting with a sequence number of 2000, if 10 segments of 1000 bytes each were received, an acknowledgement number of 12000 would be returned to the source.
The amount of data that a source can transmit before an acknowledgement must be received is called the window size. Window Size is a field in the TCP header that enables the management of lost data and flow control.
4.3.2 - TCP Acknowledgement and Windowing
The diagram depicts how TCP helps to ensure reliable delivery through data segment acknowledgment. The TCP frame header contains the source and destination ports and sequence and acknowledgement numbers to accomplish this.
Network Topology:
PC1 and PC2 are connected to each other through WAN links to a network cloud. The PC1 speech bubble says: "Start with byte number 1. I am sending 10 bytes. The PC2 speech bubble says: "I received 10 bytes starting with byte number 1. I expect byte number 11 next."
PC1 sends 10 bytes to PC2 with the following information in the header:
Source Port: 1028
Destination Port: 23
Sequence Number: 1
Acknowledgement number: 1
PC2 sends an acknowledgement of the 10 bytes to PC1, with the following information in the header:
Source Port: 23
Destination Port: 1028
Sequence Number: 1
Acknowledgement number: 11
PC1 sends another 10 bytes to PC2 with the following information in the header:
Source Port: 1028
Destination Port: 23
Sequence Number: 11
Acknowledgement number: 1
The diagram depicts how TCP helps to ensure reliable delivery through data segment acknowledgment. The TCP frame header contains the source and destination ports and sequence and acknowledgement numbers to accomplish this.
Network Topology:
PC1 and PC2 are connected to each other through WAN links to a network cloud. The PC1 speech bubble says: "Start with byte number 1. I am sending 10 bytes. The PC2 speech bubble says: "I received 10 bytes starting with byte number 1. I expect byte number 11 next."
PC1 sends 10 bytes to PC2 with the following information in the header:
Source Port: 1028
Destination Port: 23
Sequence Number: 1
Acknowledgement number: 1
PC2 sends an acknowledgement of the 10 bytes to PC1, with the following information in the header:
Source Port: 23
Destination Port: 1028
Sequence Number: 1
Acknowledgement number: 11
PC1 sends another 10 bytes to PC2 with the following information in the header:
Source Port: 1028
Destination Port: 23
Sequence Number: 11
Acknowledgement number: 1
4.3.3 TCP Retransmission
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Handling Segment Loss
No matter how well designed a network is, data loss will occasionally occur. Therefore, TCP provides methods of managing these segment losses. Among these is a mechanism to retransmit segments with unacknowledged data.
A destination host service using TCP usually only acknowledges data for contiguous sequence bytes. If one or more segments are missing, only the data in the segments that complete the stream are acknowledged.
For example, if segments with sequence numbers 1500 to 3000 and 3400 to 3500 were received, the acknowledgement number would be 3001. This is because there are segments with the sequence numbers 3001 to 3399 that have not been received.
When TCP at the source host has not received an acknowledgement after a predetermined amount of time, it will go back to the last acknowledgement number that it received and retransmit data from that point forward.
The retransmission process is not specified by the RFC, but is left up to the particular implementation of TCP.
For a typical TCP implementation, a host may transmit a segment, put a copy of the segment in a retransmission queue, and start a timer. When the data acknowledgment is received, the segment is deleted from the queue. If the acknowledgment is not received before the timer expires, the segment is retransmitted.
The animation demonstrates the retransmission of lost segments.
Hosts today may also employ an optional feature called Selective Acknowledgements. If both hosts support Selective Acknowledgements, it is possible for the destination to acknowledge bytes in discontinuous segments and the host would only need to retransmit the missing data.
4.3.3 - TCP Retransmission
The animation depicts how TCP handles segment loss using retransmission.
Network Topology:
The animation begins with a user at a PC connected to a router at ISP1. The ISP1 router is connected through a network cloud to a router at ISP2. The ISP2 router is connected to a switch, which is connected to a server farm. The server farm contains an FTP server.
As the animation progresses, TCP packet segments are passed between the user PC and the FTP server at ISP2. The following speech bubbles are displayed:
PC1: I am sending this field with FTP using TCP to make sure you receive it!
FTP Server: I received the first three. I will send an acknowledgement.
PC1: I got an acknowledgment. I will send the next group.
FTP Server: I missed the second group. I will send no acknowledgment.
PC1: I received no acknowledgement. I will resend the last group.
FTP Server: I received the next group. I will send an acknowledgement.
The FTP server then sends an acknowledgement back to PC1.
The animation depicts how TCP handles segment loss using retransmission.
Network Topology:
The animation begins with a user at a PC connected to a router at ISP1. The ISP1 router is connected through a network cloud to a router at ISP2. The ISP2 router is connected to a switch, which is connected to a server farm. The server farm contains an FTP server.
As the animation progresses, TCP packet segments are passed between the user PC and the FTP server at ISP2. The following speech bubbles are displayed:
PC1: I am sending this field with FTP using TCP to make sure you receive it!
FTP Server: I received the first three. I will send an acknowledgement.
PC1: I got an acknowledgment. I will send the next group.
FTP Server: I missed the second group. I will send no acknowledgment.
PC1: I received no acknowledgement. I will resend the last group.
FTP Server: I received the next group. I will send an acknowledgement.
The FTP server then sends an acknowledgement back to PC1.
4.3.4 TCP Congestion Control - Minimizing Segment Loss
Page 1:
Flow Control
TCP also provides mechanisms for flow control. Flow control assists the reliability of TCP transmission by adjusting the effective rate of data flow between the two services in the session. When the source is informed that the specified amount of data in the segments is received, it can continue sending more data for this session.
This Window Size field in the TCP header specifies the amount of data that can be transmitted before an acknowledgement must be received. The initial window size is determined during the session startup via the three-way handshake.
TCP feedback mechanism adjusts the effective rate of data transmission to the maximum flow that the network and destination device can support without loss. TCP attempts to manage the rate of transmission so that all data will be received and retransmissions will be minimized.
See the figure for a simplified representation of window size and acknowledgements. In this example, the initial window size for a TCP session represented is set to 3000 bytes. When the sender has transmitted 3000 bytes, it waits for an acknowledgement of these bytes before transmitting more segments in this session.
Once the sender has received this acknowledgement from the receiver, the sender can transmit an additional 3000 bytes.
During the delay in receiving the acknowledgement, the sender will not be sending any additional segments for this session. In periods when the network is congested or the resources of the receiving host are strained, the delay may increase. As this delay grows longer, the effective transmission rate of the data for this session decreases. The slowdown in data rate helps reduce the resource contention.
4.3.4 - TCP Congestion Control - Minimizing Segment Loss
The diagram depicts TCP segment acknowledgement and window sizing between a TCP client and server. A 3000 bytes window size is set up between the sender and receiver.
The window size determines the number of bytes sent before an acknowledgement is expected. The acknowledgement number is the number of the next expected byte.
- Send sequence number 1, 1500 bytes, receive 1 through 1500
- Send sequence number 1501, 1500 bytes, receive 1501 through 3000
- Receive acknowledgement, acknowledgement number 3001
- Send sequence number 3001, 1500 bytes, receive 3001 t through 4500
- Send sequence number 4501, 1500 bytes, receive 4501 through 6000
- Receive acknowledgement, acknowledgement number 6001
The diagram depicts TCP segment acknowledgement and window sizing between a TCP client and server. A 3000 bytes window size is set up between the sender and receiver.
The window size determines the number of bytes sent before an acknowledgement is expected. The acknowledgement number is the number of the next expected byte.
- Send sequence number 1, 1500 bytes, receive 1 through 1500
- Send sequence number 1501, 1500 bytes, receive 1501 through 3000
- Receive acknowledgement, acknowledgement number 3001
- Send sequence number 3001, 1500 bytes, receive 3001 t through 4500
- Send sequence number 4501, 1500 bytes, receive 4501 through 6000
- Receive acknowledgement, acknowledgement number 6001
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Reducing Window Size
Another way to control the data flow is to use dynamic window sizes. When network resources are constrained, TCP can reduce the window size to require that received segments be acknowledged more frequently. This effectively slows down the rate of transmission because the source waits for data to be acknowledged more frequently.
The TCP receiving host sends the window size value to the sending TCP to indicate the number of bytes that it is prepared to receive as a part of this session. If the destination needs to slow down the rate of communication because of limited buffer memory, it can send a smaller window size value to the source as part of an acknowledgement.
As shown in the figure, if a receiving host has congestion, it may respond to the sending host with a segment with a reduced window size. In this graphic, there was a loss of one of the segments. The receiver changed the window field in the TCP header of the returning segments in this conversation from 3000 down to 1500. This caused the sender to reduce the window size to 1500.
After periods of transmission with no data losses or constrained resources, the receiver will begin to increase the window field. This reduces the overhead on the network because fewer acknowledgments need to be sent. Window size will continue to increase until there is data loss, which will cause the window size to be decreased.
This dynamic increasing and decreasing of window size is a continuous process in TCP, which determines the optimum window size for each TCP session. In highly efficient networks, window sizes may become very large because data is not being lost. In networks where the underlying infrastructure is being stressed, the window size will likely remain small.
Links
Details of TCP's various congestion management features can be found in RFC 2581.
http://www.ietf.org/rfc/rfc2581.txt
4.3.4 - TCP Congestion Control - Minimizing Segment Loss
The diagram depicts how TCP segment acknowledgement and the window size provide flow control between a TCP client and server. A window size of 3000 bytes is set up between the sender and receiver.
- Send sequence number 1, 1500 bytes, receive 1 through 1500
- Send sequence number 1501, 1500 bytes, receive 1501 through 3000
- Receive acknowledgement, acknowledgement number 3001
- Send sequence number 3001, 1500 bytes, segment 3 is lost because of congestion at the receiver.
- Send sequence number 4501, 1500 bytes, receive 4501 through 6000
- Receive acknowledgement, acknowledgement number 3001, window size = 1500
If segments are lost because of congestion, the receiver acknowledges the last received sequential segment and replies with a reduced window size.
The diagram depicts how TCP segment acknowledgement and the window size provide flow control between a TCP client and server. A window size of 3000 bytes is set up between the sender and receiver.
- Send sequence number 1, 1500 bytes, receive 1 through 1500
- Send sequence number 1501, 1500 bytes, receive 1501 through 3000
- Receive acknowledgement, acknowledgement number 3001
- Send sequence number 3001, 1500 bytes, segment 3 is lost because of congestion at the receiver.
- Send sequence number 4501, 1500 bytes, receive 4501 through 6000
- Receive acknowledgement, acknowledgement number 3001, window size = 1500
If segments are lost because of congestion, the receiver acknowledges the last received sequential segment and replies with a reduced window size.
4.4 The UDP Protocol - Communicating with Low Overhead
4.4.1 UDP - Low Overhead vs. ReliabilityPage 1:
UDP is a simple protocol that provides the basic Transport layer functions. It has a much lower overhead than TCP, since it is not connection-oriented and does not provide the sophisticated retransmission, sequencing, and flow control mechanisms.
This does not mean that applications that use UDP are always unreliable. It simply means that these functions are not provided by the Transport layer protocol and must be implemented elsewhere if required.
Although the total amount of UDP traffic found on a typical network is often relatively low, key Application layer protocols that use UDP include:
- Domain Name System (DNS)
- Simple Network Management Protocol (SNMP)
- Dynamic Host Configuration Protocol (DHCP)
- Routing Information Protocol (RIP)
- Trivial File Transfer Protocol (TFTP)
- Online games
The low overhead of UDP makes it very desirable for such applications.
4.4.1 - UDP - Low Overhead versus Reliability
The diagram depicts IP phones and streaming video applications communicating using UDP. UDP provides low-overhead data transport because it does not establish a connection before sending data. UDP has a small datagram header with no network management traffic between sender and receiver.
The diagram depicts IP phones and streaming video applications communicating using UDP. UDP provides low-overhead data transport because it does not establish a connection before sending data. UDP has a small datagram header with no network management traffic between sender and receiver.
4.4.2 UDP Datagram Reassembly
Page 1:
Because UDP is connectionless, sessions are not established before communication takes place as they are with TCP. UDP is said to be transaction-based. In other words, when an application has data to send, it simply sends the data.
Many applications that use UDP send small amounts of data that can fit in one segment. However, some applications will send larger amounts of data that must be split into multiple segments. The UDP PDU is referred to as a datagram, although the terms segment and datagram are sometimes used interchangeably to describe a Transport layer PDU.
When multiple datagrams are sent to a destination, they may take different paths and arrive in the wrong order. UDP does not keep track of sequence numbers the way TCP does. UDP has no way to reorder the datagrams into their transmission order. See the figure.
Therefore, UDP simply reassembles the data in the order that it was received and forwards it to the application. If the sequence of the data is important to the application, the application will have to identify the proper sequence of the data and determine how the data should be processed.
4.4.2 - UDP Datagram Reassembly
The diagram depicts how different segments can take different routes through a network and arrive at the destination out of order. When this occurs with UDP, they are not resequenced in the order transmitted at the destination.
Network Topology:
PC1 is connected to one of the routers in a group that has multiple paths between the routers. PC2 is connected to one of the other routers on the opposite side of the network. Data from PC1 is divided into datagrams. Arrows from PC1 through various routers to PC2 indicate that datagrams from the same data stream pass through different routers on their way from one PC to the other. UDP does not compensate for this by resequencing the datagrams, and lost datagrams are not resent.
The diagram depicts how different segments can take different routes through a network and arrive at the destination out of order. When this occurs with UDP, they are not resequenced in the order transmitted at the destination.
Network Topology:
PC1 is connected to one of the routers in a group that has multiple paths between the routers. PC2 is connected to one of the other routers on the opposite side of the network. Data from PC1 is divided into datagrams. Arrows from PC1 through various routers to PC2 indicate that datagrams from the same data stream pass through different routers on their way from one PC to the other. UDP does not compensate for this by resequencing the datagrams, and lost datagrams are not resent.
4.4.3 UDP Server Processes and Requests
Page 1:
Like TCP-based applications, UDP-based server applications are assigned Well Known or Registered port numbers. When these applications or processes are running, they will accept the data matched with the assigned port number. When UDP receives a datagram destined for one of these ports, it forwards the application data to the appropriate application based on its port number.
4.4.3 - UDP Server Processes and Requests
The diagram depicts a UDP server listening for requests from two client PC's that are connected.
Server application examples:
- Client DNS requests are received on port 53.
- Client RADIUS requests are received on port 1812.
Client requests to servers have well-known port numbers as the destination port.
The diagram depicts a UDP server listening for requests from two client PC's that are connected.
Server application examples:
- Client DNS requests are received on port 53.
- Client RADIUS requests are received on port 1812.
Client requests to servers have well-known port numbers as the destination port.
4.4.4 UDP Client Processes
Page 1:
As with TCP, client/server communication is initiated by a client application that is requesting data from a server process. The UDP client process randomly selects a port number from the dynamic range of port numbers and uses this as the source port for the conversation. The destination port will usually be the Well Known or Registered port number assigned to the server process.
Randomized source port numbers also help with security. If there is a predictable pattern for destination port selection, an intruder can more easily simulate access to a client by attempting to connect to the port number most likely to be open.
Because there is no session to be created with UDP, as soon as the data is ready to be sent and the ports identified, UDP can form the datagram and pass it to the Network layer to be addressed and sent on the network.
Remember, once a client has chosen the source and destination ports, the same pair of ports is used in the header of all datagrams used in the transaction. For the data returning to the client from the server, the source and destination port numbers in the datagram header are reversed.
4.4.4 - UDP Client Processes
The diagram depicts two clients, Client 1 and Client 2, sending UDP requests to a server. The clients send requests to the server using well-known port numbers as the destination ports. The server responds to both clients using well-known port numbers as the source ports.
Client 1 DNS request
Source Port: 49152
Destination Port: 53
Server DNS response to Client 1 request
Source Port: 53
Destination Port: 49152
Client 2 RADIUS request
Source Port: 51152
Destination Port: 1812
Server RADIUS response to Client 2 request
Source Port: 1812
Destination Port: 51152
The diagram depicts two clients, Client 1 and Client 2, sending UDP requests to a server. The clients send requests to the server using well-known port numbers as the destination ports. The server responds to both clients using well-known port numbers as the source ports.
Client 1 DNS request
Source Port: 49152
Destination Port: 53
Server DNS response to Client 1 request
Source Port: 53
Destination Port: 49152
Client 2 RADIUS request
Source Port: 51152
Destination Port: 1812
Server RADIUS response to Client 2 request
Source Port: 1812
Destination Port: 51152
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In this activity, how DNS uses UDP is examined.
Click the Packet Tracer icon to launch the Packet Tracer activity.
4.4.4 - UDP Client Processes
Link to Packet Tracer Exploration: UDP Operation
In this activity, you examine how DNS uses UDP.
Link to Packet Tracer Exploration: UDP Operation
In this activity, you examine how DNS uses UDP.
4.5 Lab Activities
4.5.1 Observing TCP and UDP using NetstatPage 1:
In this lab, you will examine the netstat (network statistics utility) command on a host computer, and adjust netstat output options to analyze and understand TCP/IP Transport layer protocol status.
Click the Lab icon to see more details.
4.5.1 - Observing TCP and UDP Using Netstat
Link to Hands-on Lab: Observing TCP and UDP Using Netstat
In this lab, you examine the netstat command, the network statistics utility, on a host computer, and adjust netstat output options to analyze and understand TCP/IP Transport Layer protocol status.
Link to Hands-on Lab: Observing TCP and UDP Using Netstat
In this lab, you examine the netstat command, the network statistics utility, on a host computer, and adjust netstat output options to analyze and understand TCP/IP Transport Layer protocol status.
4.5.2 TCP/IP Transport Layer Protocols, TCP and UDP
Page 1:
In this lab, you will use Wireshark to capture and identify TCP header fields and operation during an FTP session and UDP header fields and operation during a TFTP session.
Click the Lab icon to see more details.
4.5.2 - TCP/IP Transport Layer Protocols: TCP and UDP
Link to Hands-on Lab: TCP/IP Transport Layer Protocols, TCP and UDP
In this lab, you use Wireshark to capture and identify TCP header fields and operation during an FTP session, and UDP header fields and operation during a TFTP session.
Link to Hands-on Lab: TCP/IP Transport Layer Protocols, TCP and UDP
In this lab, you use Wireshark to capture and identify TCP header fields and operation during an FTP session, and UDP header fields and operation during a TFTP session.
4.5.3 Application and Transport Layer Protocols
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In this lab, you will use Wireshark to monitor and analyze client application (FTP and HTTP) communications between a server and clients.
Click the Lab icon to see more details.
4.5.3 - Application and Transport Layer Protocols
Link to Hands-on Lab: Application and Transport Layer Protocols Examination
In this lab, you use Wireshark to monitor and analyze client application (FTP and HTTP) communications between a server and clients.
Link to Hands-on Lab: Application and Transport Layer Protocols Examination
In this lab, you use Wireshark to monitor and analyze client application (FTP and HTTP) communications between a server and clients.
Page 2:
In this activity, you will use Packet Tracer's Simulation mode to capture and analyze packets a web request using a URL.
Click the Packet Tracer icon to launch the Packet Tracer activity.
4.5.3 - Application and Transport Layer Protocols
Link to Packet Tracer Exploration: Application and Transport Layer Protocols Examination
In this activity, you use Packet Tracer's Simulation mode to capture and analyze packets from a web request using a URL.
Link to Packet Tracer Exploration: Application and Transport Layer Protocols Examination
In this activity, you use Packet Tracer's Simulation mode to capture and analyze packets from a web request using a URL.
4.6 Chapter Summary
4.6.1 Summary and ReviewPage 1:
The Transport layer provides for data network needs by:
- Dividing data received from an application into segments
- Adding a header to identify and manage each segment
- Using the header information to reassemble the segments back into application data
- Passing the assembled data to the correct application
UDP datagrams and TCP segments have headers prefixed to the data that include a source port number and destination port number. These port numbers enable data to be directed to the correct application running on the destination computer.
TCP does not pass any data to the network until it knows that the destination is ready to receive it. TCP then manages the flow of the data and resends any data segments that are not acknowledged as being received at the destination. TCP uses mechanisms of handshaking, timers and acknowledgements, and dynamic windowing to achieve these reliable features. This reliability does, however, impose overhead on the network in terms of much larger segment headers and more network traffic between the source and destination managing the data transport.
If the application data needs to be delivered across the network quickly, or if network bandwidth cannot support the overhead of control messages being exchanged between the source and the destination systems, UDP would be the developer's preferred Transport layer protocol. Because UDP does not track or acknowledge the receipt of datagrams at the destination - it just passes received datagrams to the Application layer as they arrive - and does not resend lost datagrams. However, this does not necessarily mean that the communication itself is unreliable; there may be mechanisms in the Application layer protocols and services that process lost or delayed datagrams if the application has these requirements.
The choice of Transport layer protocol is made by the developer of the application to best meet the user requirements. The developer bears in mind, though, that the other layers all play a part in data network communications and will influence its performance.
4.6.1 - Summary and Review
In this chapter, you learned to:
- Explain the need for the Transport Layer.
- Identify the role of the Transport Layer as it provides the end-to-end transfer of data between applications.
- Describe the role of two TCP/IP Transport Layer protocols: TCP and UDP.
- Explain the key functions of the Transport Layer, including reliability, port addressing, and segmentation.
- Explain how TCP and UDP each handle these key functions.
- Identify when it is appropriate to use TCP or UDP, and provide examples of applications that use each protocol.
In this chapter, you learned to:
- Explain the need for the Transport Layer.
- Identify the role of the Transport Layer as it provides the end-to-end transfer of data between applications.
- Describe the role of two TCP/IP Transport Layer protocols: TCP and UDP.
- Explain the key functions of the Transport Layer, including reliability, port addressing, and segmentation.
- Explain how TCP and UDP each handle these key functions.
- Identify when it is appropriate to use TCP or UDP, and provide examples of applications that use each protocol.
Page 2:
4.6.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Where do Transport Layer processes occur?
Answer: Transport Layer processes occur between the Application Layer and Internet Layer of the TCP/IP model, and between the Session Layer and Network Layer of the O S I model.
Question 2. What are the responsibilities of the Transport Layer?
Answer: The Transport Layer is responsible for:
- Keeping track of the individual conversations taking place between applications on the source and destination hosts.
- Segmenting data and adding a header to identify and manage each segment.
- Using the header information to reassemble the segments back into application data.
- Passing the assembled data to the correct application.
Question 3. What does segmentation provide to communications?
Answer: Segmentation of the data, in accordance with Transport Layer protocols, provides the means to both send and receive data when running multiple applications concurrently on a computer.
Question 4. What are the primary functions specified by all Transport Layer protocols?
Answer: The primary functions specified by all Transport Layer protocols include:
- Conversation Multiplexing: There are many applications or services running on each host in the network. Each of these applications or services is assigned an address known as a port so that the Transport Layer can determine with which application or service the data is identified.
- Segmentation and Reassembly: Most networks have a limitation on the amount of data that can be included in a single PDU. The Transport Layer divides application data into blocks of data that are an appropriate size. At the destination, the Transport Layer reassembles the data before sending it to the destination application or service.
- Error Checking: Basic error checking can be performed on the data in the segment to determine if the data was changed during transmission.
Question 5. In networking terms, what is reliability?
Answer: In networking terms, reliability means ensuring that each segment that the source sends arrives at the destination.
Question 6. List three applications that use TCP.
Answer: Applications that use TCP are:
- Web browsing
- E-mail
- File transfers
Question 7. List three applications that use UDP.
Answer: Applications that use UDP include:
- Domain Name Resolution
- Video streaming
- Voice over IP (V o IP)
Question 8. What are the different types of port numbers?
Answer:
The different types of port numbers are:
Well Known Ports (Numbers 0 to 1023): These numbers are reserved for services and applications. They are commonly used for applications such as HTTP (web server) POP3 and SMTP (e-mail server), and Telnet. By defining these well-known ports for server applications, client applications can be programmed to request a connection to that specific port and its associated service.
Registered Ports (Numbers 1024 to 49151): These port numbers are assigned to user processes or applications. They are primarily individual applications that a user has chosen to install rather than common, universal applications that would receive a well-known port.
Dynamic or Private Ports (Numbers 49152 to 65535): Also known as ephemeral ports, these ports are usually assigned dynamically to client applications when initiating a connection. It is not very common for a client to connect to a service using a dynamic or private port, although some peer-to-peer file-sharing programs do.
Question 9. What is contained in the header of each segment or datagram?
Answer: The source and destination port numbers.
Question 10. What is the purpose of a sequence number?
Answer: A sequence number allows the Transport Layer functions on the destination host to reassemble segments in the order in which they were transmitted.
Question 11. What is one way to improve security on a server?
Answer: One way to improve security on a server is to restrict server access to only those ports associated with the services and applications that should be accessible to authorized requestors.
Question 12. Describe the TCP three-way handshake:
Answer: The three-way handshake does the following:
- Establishes that the destination device is present on the network.
- Verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use for the session.
- Informs the destination device that the source client intends to establish a communication session on that port number.
Question 13. What are TCP sequence numbers used for?
Answer: For the original message to be understood by the recipient, the data in these segments is reassembled in the original order.
Question 14. Explain an expectational acknowledgement.
Answer: TCP uses the acknowledgement number in segments sent back to the source to indicate the next byte in this session that the receiver expects to receive.
Question 15. After a predetermined amount of time, what does TCP do when it has not received an acknowledgement?
Answer: When TCP at the source host has not received an acknowledgement after a predetermined amount of time, it goes back to the last acknowledgement number that it received and retransmits data from that point forward.
Question 16. What term is used to refer to the amount of data that can be transmitted before a TCP acknowledgement must be received?
Answer: Window size.
Question 17. List key Application Layer protocols that use UDP.
Answer: Application Layer protocols that use UDP include:
- Domain Name System (DNS)
- Simple Network Management Protocol (SNMP)
- Dynamic Host Configuration Protocol (DHCP)
- Routing Information Protocol (RIP)
- Trivial File Transfer Protocol (TFTP)
- Online games
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Where do Transport Layer processes occur?
Answer: Transport Layer processes occur between the Application Layer and Internet Layer of the TCP/IP model, and between the Session Layer and Network Layer of the O S I model.
Question 2. What are the responsibilities of the Transport Layer?
Answer: The Transport Layer is responsible for:
- Keeping track of the individual conversations taking place between applications on the source and destination hosts.
- Segmenting data and adding a header to identify and manage each segment.
- Using the header information to reassemble the segments back into application data.
- Passing the assembled data to the correct application.
Question 3. What does segmentation provide to communications?
Answer: Segmentation of the data, in accordance with Transport Layer protocols, provides the means to both send and receive data when running multiple applications concurrently on a computer.
Question 4. What are the primary functions specified by all Transport Layer protocols?
Answer: The primary functions specified by all Transport Layer protocols include:
- Conversation Multiplexing: There are many applications or services running on each host in the network. Each of these applications or services is assigned an address known as a port so that the Transport Layer can determine with which application or service the data is identified.
- Segmentation and Reassembly: Most networks have a limitation on the amount of data that can be included in a single PDU. The Transport Layer divides application data into blocks of data that are an appropriate size. At the destination, the Transport Layer reassembles the data before sending it to the destination application or service.
- Error Checking: Basic error checking can be performed on the data in the segment to determine if the data was changed during transmission.
Question 5. In networking terms, what is reliability?
Answer: In networking terms, reliability means ensuring that each segment that the source sends arrives at the destination.
Question 6. List three applications that use TCP.
Answer: Applications that use TCP are:
- Web browsing
- File transfers
Question 7. List three applications that use UDP.
Answer: Applications that use UDP include:
- Domain Name Resolution
- Video streaming
- Voice over IP (V o IP)
Question 8. What are the different types of port numbers?
Answer:
The different types of port numbers are:
Well Known Ports (Numbers 0 to 1023): These numbers are reserved for services and applications. They are commonly used for applications such as HTTP (web server) POP3 and SMTP (e-mail server), and Telnet. By defining these well-known ports for server applications, client applications can be programmed to request a connection to that specific port and its associated service.
Registered Ports (Numbers 1024 to 49151): These port numbers are assigned to user processes or applications. They are primarily individual applications that a user has chosen to install rather than common, universal applications that would receive a well-known port.
Dynamic or Private Ports (Numbers 49152 to 65535): Also known as ephemeral ports, these ports are usually assigned dynamically to client applications when initiating a connection. It is not very common for a client to connect to a service using a dynamic or private port, although some peer-to-peer file-sharing programs do.
Question 9. What is contained in the header of each segment or datagram?
Answer: The source and destination port numbers.
Question 10. What is the purpose of a sequence number?
Answer: A sequence number allows the Transport Layer functions on the destination host to reassemble segments in the order in which they were transmitted.
Question 11. What is one way to improve security on a server?
Answer: One way to improve security on a server is to restrict server access to only those ports associated with the services and applications that should be accessible to authorized requestors.
Question 12. Describe the TCP three-way handshake:
Answer: The three-way handshake does the following:
- Establishes that the destination device is present on the network.
- Verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use for the session.
- Informs the destination device that the source client intends to establish a communication session on that port number.
Question 13. What are TCP sequence numbers used for?
Answer: For the original message to be understood by the recipient, the data in these segments is reassembled in the original order.
Question 14. Explain an expectational acknowledgement.
Answer: TCP uses the acknowledgement number in segments sent back to the source to indicate the next byte in this session that the receiver expects to receive.
Question 15. After a predetermined amount of time, what does TCP do when it has not received an acknowledgement?
Answer: When TCP at the source host has not received an acknowledgement after a predetermined amount of time, it goes back to the last acknowledgement number that it received and retransmits data from that point forward.
Question 16. What term is used to refer to the amount of data that can be transmitted before a TCP acknowledgement must be received?
Answer: Window size.
Question 17. List key Application Layer protocols that use UDP.
Answer: Application Layer protocols that use UDP include:
- Domain Name System (DNS)
- Simple Network Management Protocol (SNMP)
- Dynamic Host Configuration Protocol (DHCP)
- Routing Information Protocol (RIP)
- Trivial File Transfer Protocol (TFTP)
- Online games
Page 3:
In this activity, a process which occurs every time you request a web page on Internet - the interaction of DNS, HTTP, UDP, and TCP - is examined in depth.
Packet Tracer Skills Integration Instructions (PDF)
Click the Packet Tracer icon to launch the Packet Tracer activity.
4.6.1 - Summary and Review
Link to Packet Tracer Exploration: Skills Integration Challenge: Analyzing the Application and Transport Layers
In this activity, the interaction of DNS, HTTP, UDP, and TCP, the process that occurs every time you request a web page on the Internet, is examined in depth.
Link to Packet Tracer Exploration: Skills Integration Challenge: Analyzing the Application and Transport Layers
In this activity, the interaction of DNS, HTTP, UDP, and TCP, the process that occurs every time you request a web page on the Internet, is examined in depth.
Page 4:
To Learn More
Reflection Questions
Discuss the requirements of an Application Layer application that would determine whether the developer selected UDP or TCP as the Transport Layer protocol to be used.
If a network application required its data to be delivered reliably, discuss how UDP could be used as the Transport Layer protocol and under what circumstances this would be used.
Links
Introduction to internetworking
http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html
4.6.1 - Summary and Review
The diagram depicts a collage of people using computers and networks.
The diagram depicts a collage of people using computers and networks.
4.7 Chapter Quiz
4.7.1 Chapter QuizPage 1:
4.7.1 - Chapter Quiz
1.Match the TCP port numbers with the correct protocol. (Not all options are used.)
Port Numbers:
20
23
25
27
63
80
110
Protocols:
HTTP
Telnet
FTP
SMTP
2.Categorize the options based on whether they describe TCP or UDP.
Options:
A.Reliable
B.No flow control
C.Reassembles messages at destination host
D.Resends anything not received
E.Does not reassemble incoming messages
F.Unreliable
G.Connectionless
H.Connection-oriented
Categories:
TCP - Transmission Control Protocol
UDP - User Datagram Protocol
3.At the Transport Layer, which of the following controls is used to avoid a transmitting host overflowing the buffers of a receiving host?
A.Best effort
B.Encryption
C.Flow control
D.Compression
E.Congestion avoidance
4.End systems use port numbers to select the proper application. What is the lowest port number that can be dynamically assigned by a host system?
A.1
B.64
C.128
D.256
E.512
F.1024
5.During data transfer, what are the main responsibilities of the receiving host? (Choose two.)
A.Throughput
B.Encapsulation
C.Acknowledgement
D.Bandwidth
E.Segmentation
F.Reassembly
6.At which layer of the TCP/IP model does TCP operate?
A.Session
B.Transport
C.Network
D.Data Link
7.What determines how much data a sending station running TCP/IP can transmit before it must receive an acknowledgement?
A.Segment size
B.Transmission rate
C.Bandwidth
D.Window size
E.Sequence number
8.What is the purpose of the sequence number in the TCP header?
A.Reassembles the segments into data.
B.Identifies the Application Layer protocol.
C.Indicates the number of the next expected byte.
D.Shows the maximum number of bytes allowed during a session.
9.Refer to the diagram description to answer the question.
Diagram description:
The sender is sending TCP segments to the receiver, three at a time, and has sent the sixth segment. The receiver is sending an ACK back to the sender. Which acknowledgement number is sent by the receiver?
A.3
B.4
C.6
D.7
E.9
F.12
10. What is the purpose of TCP/UDP port numbers?
A.Indicates the beginning of a three-way handshake.
B.Reassembles the segments into the correct order.
C.Identifies the number of data packets that may be sent without acknowledgement.
D.Tracks different conversations crossing the network at the same time.
1.Match the TCP port numbers with the correct protocol. (Not all options are used.)
Port Numbers:
20
23
25
27
63
80
110
Protocols:
HTTP
Telnet
FTP
SMTP
2.Categorize the options based on whether they describe TCP or UDP.
Options:
A.Reliable
B.No flow control
C.Reassembles messages at destination host
D.Resends anything not received
E.Does not reassemble incoming messages
F.Unreliable
G.Connectionless
H.Connection-oriented
Categories:
TCP - Transmission Control Protocol
UDP - User Datagram Protocol
3.At the Transport Layer, which of the following controls is used to avoid a transmitting host overflowing the buffers of a receiving host?
A.Best effort
B.Encryption
C.Flow control
D.Compression
E.Congestion avoidance
4.End systems use port numbers to select the proper application. What is the lowest port number that can be dynamically assigned by a host system?
A.1
B.64
C.128
D.256
E.512
F.1024
5.During data transfer, what are the main responsibilities of the receiving host? (Choose two.)
A.Throughput
B.Encapsulation
C.Acknowledgement
D.Bandwidth
E.Segmentation
F.Reassembly
6.At which layer of the TCP/IP model does TCP operate?
A.Session
B.Transport
C.Network
D.Data Link
7.What determines how much data a sending station running TCP/IP can transmit before it must receive an acknowledgement?
A.Segment size
B.Transmission rate
C.Bandwidth
D.Window size
E.Sequence number
8.What is the purpose of the sequence number in the TCP header?
A.Reassembles the segments into data.
B.Identifies the Application Layer protocol.
C.Indicates the number of the next expected byte.
D.Shows the maximum number of bytes allowed during a session.
9.Refer to the diagram description to answer the question.
Diagram description:
The sender is sending TCP segments to the receiver, three at a time, and has sent the sixth segment. The receiver is sending an ACK back to the sender. Which acknowledgement number is sent by the receiver?
A.3
B.4
C.6
D.7
E.9
F.12
10. What is the purpose of TCP/UDP port numbers?
A.Indicates the beginning of a three-way handshake.
B.Reassembles the segments into the correct order.
C.Identifies the number of data packets that may be sent without acknowledgement.
D.Tracks different conversations crossing the network at the same time.
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