体验性学习在小学低年级语文教学中 ...pptVIP

15-441 Computer Networking Lecture 18 – More TCP Congestion Control Good Ideas So Far… Flow control Stop wait Parallel stop wait Sliding window (e.g., advertised windows) Loss recovery Timeouts Acknowledgement-driven recovery (selective repeat or cumulative acknowledgement) Congestion control AIMD ? fairness and efficiency How does TCP actually implement these? Outline THE SPOOKY PARTS of TCP If it doesn’t scare you now… it will on the Final! TCP connection setup/data transfer The Candy-exchange Protocol (TCP) TCP reliability How to recover your DEAD packets TCP congestion avoidance Avoiding the death-traps of overloaded routers Sequence Number Space Each byte in byte stream is numbered. 32 bit value Wraps around Initial values selected at start up time TCP breaks up the byte stream into packets. Packet size is limited to the Maximum Segment Size Each packet has a sequence number. Indicates where it fits in the byte stream Establishing Connection:Three-Way handshake Each side notifies other of starting sequence number it will use for sending Why not simply chose 0? Must avoid overlap with earlier incarnation Security issues Each side acknowledges other’s sequence number SYN-ACK: Acknowledge sequence number + 1 Can combine second SYN with first ACK TCP Connection Setup Example Client SYN SeqC: Seq. #4019802004, window 65535, max. seg. 1260 Server SYN-ACK+SYN Receive: #4019802005 (= SeqC+1) SeqS: Seq. #3428951569, window 5840, max. seg. 1460 Client SYN-ACK Receive: #3428951570 (= SeqS+1) TCP State Diagram: Connection Setup Tearing Down Connection Either side can initiate tear down Send FIN signal “I’m not going to send any more data” Other side can continue sending data Half open connection Must continue to acknowledge Acknowledging FIN Acknowledge last sequence number + 1 TCP Connection Teardown Example Session Echo client on 128.2.222.198, server on 128.2.210.194 Client FIN SeqC: 1489294581 Server ACK + FIN Ack: 1489294582 (= SeqC+1) SeqS: 1909787689 Client