TCP

TCP (Transmission Control Protocol)

A [Transport Layer](Transport_Layer#Transport Protocols) protocol.

• Reliable byte stream protocol over IP
  • Adds reliability
  • Adds congestion control
• Doesn’t provide framing
  • Delivers an ordered byte stream, the application must impose structure
• Adds a 16 bit port number as a service identifier
  • Port numbers in the range [0, 65535]

TCP Connection Setup

The 3-way Handshake using TCP sockets

happens during the connect()/accept() calls Call send() to transmit data

• Will block until the data can be written, and returns amount of data sent
• Might not be able to send all the data, if the connection is congested
• Returns actual number of bytes sent
• Also see sendall() Call recv(X) to read up to X bytes of data from a connection
• Will block until some data is available or the connection is closed
• Returns a bytes object with the data that was received from the socket
• Received data is not null terminated - potential security risk? All errors handled through exceptions

Communication over TCP

• The send() call enqueues data for transmission
• This data is split into segments
• Each segment is placed in a TCP packet
• That packet is sent when allowed by the congestion control algorithm • If the data in a send() call is too large to fit into one segment, the TCP implementation will split it into several segments
  • Similarly, several send() requests might be aggregated into a single TCP segment
  • Both are done transparently by the TCP implementation and are invisible to the application
• Implication: the data returned by recv() doesn’t necessarily correspond to a single send() call
• The recv() call can return data in unpredictably sized chunks – applications must be written to cope with this

Congestion Control

TCP uses a window-based congestion control algorithm Maintains a sliding window onto the available data that determines how much can be sent according to the AMID algorithm TCP congestion control highly effective at keeping bottleneck link fully utilised Provided sufficient buffering in the network: buffer size = bandwidth × delay Packets queued in buffer → delay TCP trades some extra delay to ensure high throughput TCP assumes loss is due to congestion Too much traffic queued at an intermediate link → some packets dropped