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Transport Control Protocol

Transport Control Protocol. TCP. Connection-Oriented Service. Connection-oriented service has a handshake period During this time, a logical connection is made with the destination node The connection is ‘logical’, since all packets are forwarded individually, just like with UDP

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Transport Control Protocol

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  1. Transport Control Protocol TCP

  2. Connection-Oriented Service • Connection-oriented service has a handshake period • During this time, a logical connection is made with the destination node • The connection is ‘logical’, since all packets are forwarded individually, just like with UDP • Typically, connection-oriented service provides reliability, meaning: • Acknowledgements are used to ensure packets arrive • Checksums/CRCs are used to ensure data integrity

  3. Transport Control Protocol (TCP) • Like X.25, TCP provides connection-oriented delivery at a high level layer • X.25 provides it at the Transport OSI layer • TCP provides it at the Transport IP layer • Providing connection-oriented delivery at a high level allows TCP to be applied to any network • Thus the ability to use TCP/IP over Ethernet, Token Ring, etc. • However, providing connection-orientation at a high level means that the network is not necessarily optimized for connection-oriented delivery • For example, Ethernet is optimized for connectionless delivery

  4. TCP • The essence of TCP is to provide an apparently continuous stream of data • Thus, above the transport layer: • Data is not fragmented (into packets) • Data is in order • Lost packets do not occur • Thus, the transport layer (and layers below it) must handle: • Segmentation and reassembly (SAR) • Acknowledgements

  5. Segmentation vs. Fragmentation • Segmentation is basically the same as fragmentation, with a few differences: • Fragmentation (IP layer): • …only occurs when transmitting a packet whose size is larger than the MTU of the destination network • Any router (connecting two different network types) could theoretically fragment packets • Fragmentation can almost be considered an emergency practice (what to do when something goes wrong) • Segmentation (TCP layer): • Occurs for all data streams, to divide the data into packets (above TCP layer data is continuous) • Only the source host will segment packets • Segmentation is a normal part of TCP’s job

  6. TCP • TCP is a reliable protocol • All data sent through TCP is automatically divided into packets • Each of these packets is ensured to be sent by requiring the destination acknowledge the packets when they are received • The destination, knowing it will eventually receive all messages, only has to reorder those messages into an apparently continuous stream of data flow

  7. TCP: Stream Delivery Protocol • TCP abstracts data communication to appear as an apparent stream of flowing data: • The source sends data as a stream into the network • The destination node receives data from the network in an identical form • The data arrives in the same order as it was sent • All data sent, arrives (in its proper position) • This is known as ‘stream orientation’, a format where the data is oriented in such a way as to appear as a direct stream from source to destination • In reality, however, the data is sent as packets (using IP datagrams, for example)

  8. TCP: Stream Delivery Protocol • TCP is normally achieved by using buffering • Data is collected from the stream (and stored into memory), until a certain amount has been obtained • This data is packaged into one or more network packets (e.g. IP datagrams) and sent to the destination using connectionless delivery • The destination should send an acknowledgement back to the source • If this acknowledgement fails to arrive after a specified length of time, the source will retransmit the packet • The destination node buffers the incoming packets into memory, where they can be read (byte by byte)

  9. TCP: Stream Delivery Protocol • The TCP/IP service layers do not contain a Presentation layer • For this reason, both UDP and TCP require that the applications recognize their own data formats • For example, using TCP to connect and send an E-Mail message can be achieved using the existing data format (or language) known as SMTP (Simple Mail Transfer Protocol) • Using TCP to connect and request WWW pages can be achieved using HTTP (HyperText Transfer Protocol) • These protocols are implemented (generated and recognized) inside the applications themselves

  10. TCP: Stream Delivery Protocol • Since TCP is actually implemented using packets (e.g. IP datagrams), it was possible for TCP to ensure bi-directional communication across its connections • Transfer across TCP streams is full duplex

  11. Connection Establishment • TCP uses a three-way handshake to establish a connection • This means 3 messages are exchanged before a connection exists • The first message (SYN), sent by the machine issuing the ‘active open’ request (A), is a request for connection to the destination (B) • The second message (SYN/ACK), both an acknowledgement of the first message as well as a request for connection to A, is sent by B • The third message (ACK) is an acknowledgement to B (from A) for the second message

  12. TCP Handshake SYN Connection Establishment (Handshake) SYN/ACK ACK Transmission of data

  13. TCP Reliability • TCP provides reliability by requiring recipient nodes to send acknowledgments • Acknowledgements are sometimes called ACKs • When a packet is received by the destination, an ACK is sent back to the source • When the source receives the ACK, it sends the next packet • And so on, and so on, …

  14. TCP Reliability S Network D M A M M A M M M A A M M M A M A M M A M A M A

  15. TCP Reliability • If a packet is sent, and no ACK is received within a certain time, the message will be retransmitted • This time is called the ‘timeout’ • It is possible that the original packet was received, but the ACK was somehow lost • TCP networks treat both situations identically • The destination will receive the packet again, ignore it (it already has the data), and acknowledge it again • Hopefully this time, the acknowledgement will be received

  16. M M M M 4 3 2 1 TCP Reliability S Network D M M M M M M M 10 6 5 9 8 7

  17. TCP Reliability • If each node waited for acknowledgements without transmitting data, it would involve wasteful delays between packets in a series • TCP uses a scheme called the ‘sliding window technique’ to solve this problem

  18. Sliding Window Technique • The sliding window technique allows a transmitting node to transmit more than one packet without waiting for an ACK • Nodes cannot transmit more than S packets beyond the first unacknowledged packet • S is known as the window size • Thus, transmitting nodes have a ‘window’ of up to S packets, all of which have already been sent • Some of these packets may be acknowledged • At least the first packet is unacknowledged (but sent) • When an ACK is received for the first packet in the window, another packet can be sent • The window index can be increased by one

  19. Sliding Window Technique • Let’s see an example without a sliding window: • Packet size: 4, Data: abcd efgh ijkl mnop SYN SYN/ACK ACK 1: abcd Connection established Passage of Time ACK: 1 2: efgh ACK: 2 3: ijkl ACK: 3 …etc… 4: mnop

  20. Sliding Window Technique • Let’s see the same example with a sliding window: • Packet size: 4, Window size: 3, Data: abcd efgh ijkl mnop SYN SYN/ACK ACK 1: abcd Connection established 2: efgh 3: ijkl Passage of Time ACK: 1 ACK: 2 ACK: 3 4: mnop ACK: 4

  21. Piggybacked ACKs • Often two node communicate back and forth • When an acknowledgement is to be sent from A to B, as well as a data packet, the ACK can be added to the packet and sent to B as one packet • Essentially, only the sequencing number is required to indicate that a message has been received • Sequencing numbers are discussed later • Rather than send a small packet (ACK) followed by a larger packet (data), the node sends a single larger packet (data with piggybacked acknowledgement)

  22. TCP Layers Application Transport Control Protocol (TCP) Internet Protocol (IP) Network Interface Hardware

  23. TCP/UDP Layers Application TCP UDP Internet Protocol (IP) Network Interface Hardware

  24. TCP Header Information • As stated previously, TCP is built on top of IP datagrams • These datagrams must arrive correctly • Therefore, TCP streams are often created using the same information as would be used in the IP datagram header: • Address (network and machine portion) • Header checksum • etc,

  25. Ports • Multiple TCP streams can be active on any machine • Therefore, ports should be used to represent which stream is which • These ports are the same ones used for UDP • This makes sense, considering both use IP datagrams for their implementation

  26. Sockets • Sockets, to programmers, represent connections to the network • In some sense, a socket are associated with a network port on the machine • A machine (and even a single program) may have several open sockets at any time

  27. Sockets • In UDP, sockets can be shared: • Datagrams from different destinations can be received on the same socket • Datagrams can be sent to multiple destinations through the same socket • In TCP, sockets can not be shared: • TCP sockets (or stream sockets) represent an active connection with the other side • Both source and destination must have an active socket open for communication to occur

  28. Stream Sockets • With stream sockets, one side must initiate the connection • The side that will accept a connection requests a ‘passive open’ with its operating system • This indicates that the OS should accept incoming connection requests • A port is associated with the passive open, and can be used by the initiating node when requesting the connection • The side that initiates a connection requests an ‘active open’ • The initiating node requests a connection with a given machine (specified by its address) at a particular port • If the machine has a passive open registered at that port, the connection will be accepted, otherwise it will not

  29. Stream Sockets • The socket that represents a ‘passive open’: • Is called a server socket • Represents the willingness to accept connections • The socket that represents an ‘active open’: • Is called a client socket • Represents the act of actually connecting to a server socket

  30. Data Corruption • TCP uses a 16 octet checksum to ensure that data has not been corrupted • If data is changed in any way, the checksum computed using the data at the destination will be different than the checksum computed on the source side (and transmitted along with the data) • If checksums do not match: • Data is corrupt • The checksum is corrupt • Both situations are treated identically in TCP, data is retransmitted

  31. Retransmission • Similar to re-collision avoidance backoff, unacknowledged packets are sent after increasing timeouts • This prevents packets from being indefinitely lost because the timeout value is too short for extremely high network usage situations • Unless a message is undeliverable, in any amount of time, the message will eventually reach its destination and be acknowledged

  32. Connection Use • Once a connection has been made, sequence numbers are used to represent packets that make up the data stream • Sequence numbers indicate the position of the data in the packet in the data stream

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