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TCP Details

TCP Details. Introduction to Networking Yan Gao TA Jan 27, 2005 Recital 3. The TCP Project Important Handouts:. Minet The Minet Technical Report The Minet Socket Interface TCP Guides RFC 793   (in convenient HTML format), RFC 1122

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TCP Details

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  1. TCP Details Introduction to Networking Yan Gao TA Jan 27, 2005 Recital 3 Netprog: TCP Details

  2. The TCP ProjectImportant Handouts: • Minet • The Minet Technical Report • The Minet Socket Interface • TCP Guides • RFC 793  (in convenient HTML format), RFC 1122 • A very useful summary and picture of the TCP State diagram • TCP, UDP and IP pocket guide includes header details • Brief overview of TCP contains a nice summary of the essentials • Here is a page with nice TCP animations. They explain TCP connection startup, termination, data flow and flow control and cumulative ack concepts. Please see animations 20_1 to 20_5. Netprog: TCP Details

  3. TCP Lingo • When a client requests a connection, it sends a “SYN” segment (a special TCP segment) to the server port. • SYN stands for synchronize. The SYN message includes the client’s ISN. • ISN is Initial Sequence Number. Netprog: TCP Details

  4. More... • Every TCP segment includes a Sequence Number that refers to the first byte of data included in the segment. • Every TCP segment includes a Request Number (Acknowledgement Number) that indicates the byte number of the next data that is expected to be received. Netprog: TCP Details

  5. And more... • There are a bunch of control flags: • URG: urgent data included. • ACK: this segment is (among other things) an acknowledgement. • RST: error - abort the session. • SYN: synchronize Sequence Numbers (setup) • FIN: polite connection termination. Netprog: TCP Details

  6. And more... • MSS: Maximum segment size (A TCP option) • Window: Every ACK includes a Window field that tells the sender how many bytes it can send before the receiver will have to toss it away (due to fixed buffer size). Netprog: TCP Details

  7. TCP Connection Creation • A server accepts a connection. • Must be looking for new connections! • A client requests a connection. • Must know where the server is! Netprog: TCP Details

  8. Client Starts • A client starts by sending a SYN segment with the following information: • Client’s ISN (generated pseudo-randomly) • Maximum Receive Window for client. • Optionally (but usually) MSS (largest datagram accepted). • No payload! (Only TCP headers) Netprog: TCP Details

  9. Sever Response • When a waiting server sees a new connection request, the server sends back a SYN segment with: • Server’s ISN (generated pseudo-randomly) • Request Number is Client ISN+1 • Maximum Receive Window for server. • Optionally (but usually) MSS • No payload! (Only TCP headers) Netprog: TCP Details

  10. Finally • When the Server’s SYN is received, the client sends back an ACK with: • Request Number is Server’s ISN+1 Netprog: TCP Details

  11. SYN ISN=X 1 SYN ISN=Y ACK=X+1 2 ACK=Y+1 3 Server Client time Netprog: TCP Details

  12. Why 3-Way? • Why is the third message necessary? • HINTS: • TCP is a reliable service. • IP delivers each TCP segment. • IP is not reliable. Netprog: TCP Details

  13. TCP Data and ACK • Once the connection is established, data can be sent. • Each data segment includes a sequence number identifying the first byte in the segment. • Each segment (data or empty) includes a request number indicating what data has been received. Netprog: TCP Details

  14. Buffering • Keep in mind that TCP is (usually) part of the Operating System. It takes care of all these details asynchronously. • The TCP layer doesn’t know when the application will ask for any received data. • TCP buffers incoming data so it’s ready when we ask for it. Netprog: TCP Details

  15. TCP Buffers • Both the client and server allocate buffers to hold incoming and outgoing data • The TCP layer does this. • Both the client and server announce with every ACK how much buffer space remains (the Window field in a TCP segment). Netprog: TCP Details

  16. Send Buffers • The application gives the TCP layer some data to send. • The data is put in a send buffer, where it stays until the data is ACK’d. • it has to stay, as it might need to be sent again! • The TCP layer won’t accept data from the application unless (or until) there is buffer space. Netprog: TCP Details

  17. ACKs • A receiver doesn’t have to ACK every segment (it can ACK many segments with a single ACK segment). • Each ACK can also contain outgoing data (piggybacking). • If a sender doesn’t get an ACK after some time limit (MSL) it resends the data. Netprog: TCP Details

  18. TCP Segment Order • Most TCP implementations will accept out-of-order segments (if there is room in the buffer). • Once the missing segments arrive, a single ACK can be sent for the whole thing. • Remember: IP delivers TCP segments, and IP in not reliable - IP datagram can be lost or arrive out of order. Netprog: TCP Details

  19. Termination • The TCP layer can send a RST segment that terminates a connection if something is wrong. • Usually the application tells TCP to terminate the connection politely with a FIN segment. Netprog: TCP Details

  20. FIN • Either end of the connection can initiate termination. • A FIN is sent, which means the application is done sending data. • The FIN is ACK’d. • The other end must now send a FIN. • That FIN must be ACK’d. Netprog: TCP Details

  21. FIN SN=X 1 ACK=X+1 2 FIN SN=Y 3 ACK=Y+1 4 App2 App1 ... Netprog: TCP Details

  22. Netprog: TCP Details

  23. Implementing TCP Interactive Data Flow • DF_listen • DF_SynRcvd • DF_SynSent • DF_Established • DF_SendData • DF_CloseWait • DF_FinWait1 • DF_Closing • DF_LastAck • DF_FinWait2 • DF_TimeWait Netprog: TCP Details

  24. Implementing TCP-Socket Layer interface • SOCK_connect • Create an active connection • Add new connection to list • Return a STATUS with the same connection, no data, no byte count and the error code • SOCK_accept • Perform the passive open • Return a STATUS with only the error code set • SOCK_write • Push data into the connection’s output queue • SOCK_forward • Forward matching packets • Message ignored, but Send back result code • SOCK_close • Received a CLOSE request from Sock Layer • Remove the connection, and create a FIN packet • SOCK_status • Status update • Byte count actually reflects the number of bytes read from the WRITE • Attempt to send the remaining bytes Netprog: TCP Details

  25. Thank you ! Netprog: TCP Details

  26. Implementing TCP Interactive Data Flow • DF_Listen{ if(IS_SYN(flags)) //Store remote side sequence number and connection information with a new connection, send SYN/ACK with my sequence number } • DF_SynRcvd{ if(IS_ACK(flags)) //Send nothing and progress to ESTABLISHED state if valid ACK else if(IS_SYN(flags)) //Store remote side sequence number and connection information //with a new connection, send SYN/ACK with my sequence number. //If we get here, we recvd another syn before timing out. else if(IS_RST(flags)) //close the connection, return to listen state } Close Netprog: TCP Details

  27. Samples //Send back result code SockRequestResponse repl; repl.type=STATUS; repl.connection=req.connection; repl.bytes=0; repl.error=EOK; MinetSend(sock,repl); Close Netprog: TCP Details

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