Chapter 9: Transport Layer

to terminate a single conversation supported by TCP

-4 exchanges are needed to end both sessions note: termination process can be initiated by any 2 hosts that have an open session

flags

-6 bits n Control Bits field of TCP segment header -a bit that is either set to on or off

TCP Feature: Same-Order Delivery

-Because networks may provide multiple routes that can have different transmission rates, data can arrive in wrong order -By numbering and sequencing segments, TCP can ensure that these segments are reassembled into proper order -ensures that segments are reassembled into proper order

Transport Layer Responsibility: Segmenting Data and Reassembling Segments

-Data must be prepared to be sent across media in manageable pieces -Most networks have a limitation on amount of data that can be included in a single packet -Transport layer protocols have services that segment application data into blocks that are an appropriate size -This service includes encapsulation required on each piece of data -At the destination, transport layer must be able to reconstruct pieces of data into a complete data stream that is useful to application layer -The protocols at transport layer describe how the transport layer header information is used to reassemble data pieces into streams to be passed to application layer

three-way handshake

-Establishes that destination device is present on network -Verifies that destination device has an active service and is accepting requests on destination port number that the initiating client intends to use -Informs destination device that source client intends to establish a communication session on that port number Note: to establish connection, hosts perform a 3-way handshake

TCP provides other services

-Establishing a session -Reliable Delivery Same-Order Delivery -Flow Control

to close a connection

-Finish (FIN) control flag must be set in segment header

Multiplexing

-Multiple applications can use network at same time

TCP Feature: Flow Control

-Network hosts have limited resources, like memory and processing power. -When TCP is aware that these resources are overtaxed, it can request that sending application reduce rate of data flow. -This is done by TCP regulating amount of data source transmits. -Flow control can prevent need for retransmission of data when receiving host's resources are overwhelmed. -ensures that receiver is able to process data received

TCP, 3 basic operations of reliability

-Numbering and tracking data segments transmitted to a specific host from a specific application -Acknowledging received data -Retransmitting any unacknowledged data after a certain period of time

Transport Layer services

-Segmentation allows conversation multiplexing -segmenting the data into smaller chunks enables many different communications, from many different users, to be interleaved (multiplexed) on the same network

conversation multiplexing

-Sending some types of data (like streaming video) across a network, as one complete communication stream, can consume all of available bandwidth -This will then prevent other communications from occurring at the same time -It would also make error recovery and retransmission of damaged data difficult

each TCP segment has 20 bytes of overhead in the header encapsulating the application layer data:

-Source Port (16 bits) and Destination Port (16 bits): Used to identify the application -Sequence number (32 bits): Used for data reassembly purposes -Acknowledgment number (32 bits): Indicates data has been received and next byte expected from source -Header length (4 bits): Known as ʺdata offsetʺ. Indicates length of TCP segment header. -Reserved (6 bits): This field is reserved for the future. -Control bits (6 bits): Includes bit codes, or flags, which indicate the purpose and function of the TCP segment -Window size (16 bits): Indicates number of bytes that can be accepted at one time -Checksum (16 bits): Used for error checking of segment header and data -Urgent (16 bits): Indicates if data is urgent

TCP connection is established in 3 steps

-Step 1: Initiating client requests a client-to-server communication session with the server -Step 2: Server acknowledges client-to-server communication session and requests a server-to-client communication session -Step 3: initiating client acknowledges server-to-client communication session

TCP Connection Termination

-Step 1: When the client has no more data to send in stream, it sends a segment with FIN flag set -Step 2: server sends an ACK to acknowledge receipt of FIN to terminate session from client to server -Step 3: server sends a FIN to client to terminate server-to-client session -Step 4: client responds with an ACK to acknowledge FIN from the server Note: when all segments have been acknowledged, session is closed

TCP Header

-TCP is a stateful protocol -To track state of a session, TCP records which information it has sent and which information has been acknowledged -The stateful session begins with session establishment and ends when closed with session termination.

Transport Layer Responsibility: Identifying the Applications

-To pass data streams to proper applications, the transport layer must identify the target application -To accomplish this, the transport layer assigns each application an identifier called a port number -Each software process that needs to access network is assigned a port number unique to that host -transport layer ensures that even with multiple applications running on a device, all applications receive correct data

2 transport layer protocols TCP/IP provides

-Transmission Control Protocol (TCP) -User Datagram Protocol

Port Number groups

-Well-known Ports (Numbers 0 to 1023) -Registered Ports (Numbers 1024 to 49151) -Dynamic or Private Ports (Numbers 49125 to 65535) Note: Some client operating systems may use registered port numbers instead of dynamic port numbers for assigning source ports

Example of TCP Server Processes

-a host running a web server application and a file transfer application cannot have both configured to use same port (for example, TCP port 80) -An active server application assigned to a specific port is considered to be open, which means that transport layer accepts and processes segments addressed to that port -Any incoming client request addressed to correct socket is accepted, and data is passed to server application -There can be many ports open simultaneously on a server, one for each active server application

stateful protocol

-a protocol that keeps track of state of communication session

to end each one-way TCP session

-a two-way handshake, consisting of a FIN segment and an Acknowledgement (ACK) segment, is used

Dynamic or Private Ports (Numbers 49152 o 65535)

-also known as ephemeral ports -assigned dynamically by client's OS hen a connection to a service is initiated -dynamic port is then used to identify client application during communication

TCP transport

-analogous to sending packages -tracked from source to destination -more robust communication between applications -additional overhead could reduce usefulness of application and can even be detrimental -possible delays in transmission

UDP is used as the transport protocol in these situations

-application can tolerate some data loss during transmission over network, but delays in transmission are unacceptable note: -UDP is the better choice for these applications because less network overhead is required -UDP is preferable for applications like streaming live audio, live video, and Voice over IP (VoIP) -Acknowledgments and retransmission would slow down delivery

Example of when applications are designed to use TCP

-applications like databases, web browsers, and email clients, require that all data that is sent arrives at destination in its original condition -Any missing data could cause a corrupt communication that is either incomplete or unreadable

Registered Ports (Numbers 1024 to 49151)

-are assigned by IANA to a requesting entity to use with specific 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 number -EX: Cisco has registered port 1985 for its Hot Standby Routing Protocol (HSRP) process

source port numbers acts

-as a return address for requesting application -transport layer keeps track of this port and application that initiated request so that when a response is returned, it can be forwarded to correct pplication

Unexplained TCP connections

-can pose a major security threat -indicate that something r someone is connected to local host -sometimes it is necessary to know which active TCP connections are open and running on a networked host

Destination Port

-client places a destination port number in the segment to tell destination server what service is being requested -EX: when a client specifies port 80 in the destination port, server that receives message knows that web services are being requested. A server can offer more than one service simultaneously like web services on port 80 at same time that it offers File Transfer Protocol (FTP) connection establishment on port 21.

Socket

-combination of source IP address and source port number -or, destination IP address and destination port number -used to identify server and service being requested by client

UDP Features

-considered a best-effort transport protocol -lightweight transport protocol that offers same data segmentation and reassembly as TCP, but without TCP reliability nd flow control -simple protocol that it is usually described in terms f what it does not do compared to TCP

most important requirements for delivering live video and voice over network

-data continues to flow quickly -Live video and voice applications can tolerate some data loss with minimal or no noticeable effect, and are perfectly suited to UDP

TCP Feature: Establishing a Session

-devices negotiate amount f traffic that can be forwarded at a given time -communication data between 2 can be closely managed -ensures application is ready to receive data

Source Port

-dynamically generated by sending device to identify a conversation between 2 devices -process allows multiple conversations to occur simultaneously -common for a device to send multiple HTTP service request to a web server at same time -each separate HTTP conversation is tracked based on source ports

TCP Server Processes

-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 same port number within same transport layer services

IP uses TCP and UDP protocols to

-enable hosts to communicate and transfer data

reliability (in networking terms)

-ensuring that each segment that source sends arrives at destination

Segmentation (9.1.1.3 Conversation Multiplexing)

-facilitates data transport by lower network layers

TCP is used as the transport protocol in these situations

-for some application, segments must arrive in a very specific sequence to be processed successfully -other applications, all data must be fully received before any is considered useful

TCP connections

-host clients establishes connection with the server Connections on the network is similar like 2 people greeting each other by shaking each other's hand

Transport Layer Responsibility: Tracking Individual Conversations

-host may have multiple applications that are communicating across network simultaneously -each applications communicates with 1 or more applications on 1 or more remote hosts -responsibility of transport layer to maintain and track these multiple conversations -tracks each individual conversation flowing between a source application and a destination application separately

Example of when applications are designed to use UDP

-if one or two segments of a live video stream fail to arrive, it creates a momentary disruption in the stream -This may appear as distortion in the image or sound, but may not be noticeable to the user -If destination device had to account for lost data, stream could be delayed while waiting for retransmissions, therefore causing the image or sound to be greatly degraded -In this case, it is better to render best media possible with segments received, and forego reliability

Netstat

-important network utility that can be used to verify those connections

TCP header

-increases size of packet -increases delay

Control bits in the TCP header

-indicate progress -status of connection

netstat command

-lists protocols in use -lists local address and port numbers -lists foreign address and port numbers -lists connection state -will attempt to resolve IP addresses to domain names and port numbers to well-known applications

Transport layer is also responsible

-managing reliability requirements of a conversation -different applications have different transport reliability requirements

Socket enables

-multiple processes -running on a client -to distinguish themselves from each other -multiple connections to a server process to be distinguished from each other

Multiple separate conversations on transport layer

-must be able to separate and manage multiple communications with different transport requirement needs -Users expect to be able to simultaneously receive and send email and instant messages, view websites, and conduct a VoIP phone call -Each of these applications is sending and receiving data over network at same time, despite different reliability requirements. -Additionally, data from phone call is not directed to the web browser, and text from an instant message does not appear in an email.

stateless protocol

-neither client, nor server, is obligated to keep track of state of communication session

9.2.1.5: VD- TCP 3-Way Handshake

-no notes

Error Checking

-performed on data in segment to check if segment was changed during transmission

datagrams

-pieces of communication in UDP -are sent as best-effort by transport layer protocol -UDP has a low overhead of 8 bytes

TCP Feature: Reliable Delivery

-possible for a segment to become corrupted or lost completely, as it is transmitted over network -ensures that receiver is able to process data received

UDP transport/best-effort delivery protocol

-provides basic function for delivering data segments between appropriate applications -very little overhead -data checking -no transport layer processes that inform sender of a successful delivery -EX: similar to placing a regular, non-registered, letter in the mail

TCP

-reliable -full-featured transport layer protocol, which ensures that all of data arrives at destination -requires additional fields in TCP header

Well-known Ports (Numbers 0 to 1023)

-reserved for services and applications -commonly used for applications like web browsers, email clients, and remote access clients -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

Transport layer

-responsible for establishing a temporary communication session between 2 applications and delivering data between them -application generates data that is sent from an application on a source host to an application on a destination host -without regard to destination host type. type of media over which data must travel, path taken by data, congestion on a link, or size of network -link between application layer and lower layers that are responsible for network transmission

source and destination ports are placed with the segment

-segments are then encapsulated within an IP packet. -IP packet contains the IP address of source and destination

After the communication is completed

-sessions are closed, and connection is terminated -connection and session mechanisms enable TCP's reliability function

Conversation

-set of data flowing between a source application and a destination application

UDP

-simpler transport layer protocol -does not provide for reliability -fewer fields -faster than TCP

Port numbers

-source port number is associated with originating application on local host -destination port number is associated with destination application on remote host

Transport protocols

-specify how to transfer messages between hosts

Internet Assigned Numbers Authority (IANA)

-standards body responsible for assigning various addressing standards -including port numbers

IP concerned only

-structure -addressing -routing -does not specify how delivery or transportation of packets takes place

identify each segment of data

-transport layer adds a header containing binary data organized into several fields -It is values in these fields that enable various transport layer protocols to perform different functions in managing data communication

port number

-transport layer assigns each application an identifier

Segmentation

-transport layer divides data into segments that are easier to manage and transport

context of networking, best-effort delivery

-unreliable because there is no acknowledgment that data is received at destination

Header

-used for reassembly -is added to each block of data -used to track data stream

-n option

-used to display IP addresses and port numbers in their numerical form

RST flag

-used to reset a connection when an error or timeout occurs

TCP and UDP manages these multiple simultaneous conversations by

-using header fields that can uniquely identify these applications -these unique identifiers are port numbers

connection-oriented protocol

-what TCP is -one that negotiates and establishes a permanent connection (or session) between source and destination devices prior to forwarding any traffic

client socket might look like

-with 1099 representing source port number: 192.168.1.5:1099 -socket on web server might be: 192.168.1.7:80 -together, these 2 sockets combine to form a socket pair: 192.168.1.5:1099, 192.168.1.7:80

9.1.2.6: Port Numbers

Note: Data for different applications is directed to correct application because each application has a unique port number