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Transport protocols


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Transport protocols

  1. 1. Transport Protocols 1
  2. 2. The Transport Abstraction• Transport provides three key services – Application Multiplexing Several processes can send and receive data between ports using a single host-to-host IP link. – Connection-Orientation data are sent and received as a seamless stream of bytes, rather than fixed-size packets – Error Controlled packets which are lost or received out-of order are transparent to the receiving application• TCP provides all these abstractions• UDP provides only the first one 2
  3. 3. The Transmission Control Protocol (TCP)• End-to-End communication for user processes• Processes can communicate in a simple stream of binary data• Each byte of data will arrive in the order it was sent• If some part of the data cannot be received this will be made explicit by an error being passed to the user code (e.g. an Exception) 3
  4. 4. Fundamentals of TCP• Basically a Sliding Window protocol – Data sent in segments in IP packets – Very large (32-bit) sequence number • represents byte offset of segment 4
  5. 5. TCP Process Communication 5
  6. 6. TCP Process Communication• Two processes communicating via TCP sockets.• Each side of a TCP connection has a socket which can be identified by the pair < IP_address, port_number >• Two processes communicating over TCP form a logical connection that is uniquely identifiable by the two sockets involved, that is by the combination < local_IP_address, local_port, remote_IP_address, remote_port> 6
  7. 7. TCP FacilitiesStream Data Transfer• From the applications viewpoint, TCP transfers a contiguous stream of bytes.• TCP does this by grouping the bytes in TCP segments, which are passed to IP for transmission to the destination.• TCP itself decides how to segment the data and it may forward the data at its own convenience. 7
  8. 8. TCP FacilitiesReliability TCP assigns a sequence number to each byte transmitted, and expects a positive acknowledgment (ACK) from the receiving TCP.• If the ACK is not received within a timeout interval, the data is retransmitted.• The receiving TCP uses the sequence numbers to rearrange the segments when they arrive out of order, and to eliminate duplicate segments. 8
  9. 9. TCP FacilitiesFlow Control• The receiving TCP, when sending an ACK back to the sender, also indicates to the sender the number of bytes it can receive beyond the last received TCP segment, without causing overrun and overflow in its internal buffers.• This is sent in the ACK in the form of the highest sequence number it can receive without problems. 9
  10. 10. TCP FacilitiesMultiplexing• To allow for many processes within a single host to use TCP communication facilities simultaneously, the TCP provides a set of addresses or ports within each host.• Concatenated with the network and host addresses from the internet communication layer, this forms a socket.• A pair of sockets uniquely identifies each connection. 10
  11. 11. TCP FacilitiesLogical Connections• The reliability and flow control mechanisms described above require that TCP initializes and maintains certain status information for each data stream.• The combination of this status, including sockets, sequence numbers and window sizes, is called a logical connection.• Each connection is uniquely identified by the pair of sockets used by the sending and receiving processes. 11
  12. 12. TCP FacilitiesFull Duplex• TCP provides for concurrent data streams in both directions. 12
  13. 13. TCP HEADER• TCP data is encapsulated in an IP datagram.• Its normal size is 20 bytes unless options are present. 13
  14. 14. TCP HEADER 14
  15. 15. TCP HEADER• The SrcPort and DstPort fields identify the source and destination ports,respectively. These two fields plus the source and destination IP addresses, combine to uniquely identify each TCP connection.• The sequence number identifies the byte in the stream of data from the sending TCP to the receiving TCP that the first byte of data in this segment represents. 15
  16. 16. TCP HEADER• The Acknowledgement number field contains the next sequence number that the sender of the acknowledgement expects to receive.• This is therefore the sequence number plus 1 of the last successfully received byte of data.• This field is valid only if the ACK flag is on.• Once a connection is established the Ack flag is always on. 16
  17. 17. TCP HEADER• The Acknowledgement, SequenceNum, and AdvertisedWindow fields are all involved in TCPs sliding window algorithm.• The Acknowledgement and AdvertisedWindow fields carry information about the flow of data going in the other direction.• In TCPs sliding window algorithm the reciever advertises a window size to the sender.• This is done using the AdvertisedWindow field.• The sender is then limited to having no more than a value of AdvertisedWindow bytes of un acknowledged data at any given time.• The receiver sets a suitable value for the AdvertisedWindow based on the amount of memory allocated to the connection for the purpose of buffering data 17
  18. 18. TCP HEADER• The header length gives the length of the header in 32-bit words. This is required because the length of the options field is variable.• The 6-bit Flags field is used to relay control information between TCP peers. The possible flags include SYN, FIN, RESET, PUSH, URG, and ACK. 18
  19. 19. TCP HEADER• The Checksum covers the TCP segment: the TCP header and the TCP data. This is a mandatory field that must be calculated by the sender, and then verified by the receiver.• The Option field is the maximum segment size option, called the MSS. Each end of the connection normally specifies this option on the first segment exchanged. It specifies the maximum sized segment the sender wants to receive.• The data portion of the TCP segment is optional. 19
  20. 20. The User Datagram Protocol (UDP)• UDP is a commonly used transport protocol employed by many types of applications• A connectionless transport – doesn’t guarantee either packet delivery or packets arrive in sequential order• Bytes are grouped together in discrete packets not an ordered sequence of bytes using I/O stream 20
  21. 21. The User Datagram Protocol (UDP)• The packet may travel along different path as selected by the various network routers that distribute traffic flow (network congestion, priority of routes and the cost of transmission)• Possibility of packet missing or discarded exist• No notification is available to update the status of delivery 21
  22. 22. UDP Advantages• UDP communication can be more efficient than guaranteed-delivery data streams. If the amount is small and the data is sent frequently, it may make sense to avoid the overhead of guaranteed delivery• Overhead to establish connection can be reduced as compared to TCP• Real time applications that demand up-to-the-second or better performance maybe the candidates for UDP, since fewer delay due to the error checking and flow control of TCP 22
  23. 23. The User Datagram Protocol (UDP) 23
  24. 24. The User Datagram Protocol (UDP)• UDP Length: length of datagram in bytes, including header and data, max is 65,535 bytes• Checksum: optional -- 16-bit checksum over header and data, or zero 24
  25. 25. TCP vs. UDP• How do we send and reconstruct multiple large objects with each protocol? 25
  26. 26. TCP vs. UDPHow do we send and reconstruct multiple large objects with UDP? – Fragment objects in to byte blocks agree a size and add delimiters between objects which share the same block, number each block and say which object it belongs to – Send blocks one at a time – When a block is received check which object it is for, place it in a buffer for that object – When the end block of an object is found, if that object is complete, return it to the application, if not, request retransmission of missing blocks 26
  27. 27. TCP vs. UDPWhat is UDP useful for?– Fire-and-Forget data • informational (time server updates, keep alives) • wide-area search (service discovery, P2P search)– Real time data • Any application where we don’t care about data from the past, only current data – voice over IP – streaming video/audio – multiplayer games 27