Assignment 5Reliable networking with minisocketsAri RabkinA filesystem postmortemIt’s hard, we know. We aren’t out to get you; grading won’t be too brutal.Late submission until 3am tonight without penalty.Also, we’ll drop lowest project scoreRemember; if you need extra time, or clarification, ask early.Project overviewDatagram networking is ugly: Packets get lostForces programmer to think about packets (too low level)We’re going to build reliable data streams, which have neither drawback.Closely mimics TCPSocketsA socket is an endpoint of a reliable communication streamProgram can read or write any quantity of data; stream interface divides into packets.Yours are called minisockets -- see minisocket.h and .cServer and Client SocketsSockets come in two kinds: server (listening) and clientServer socket created by minisocket_server_create(); call blocks until client connects.Clients create sockets with minisocket_client_create(), which connects.Creating a socketSYNSYNACKACKClient ServerThe three-way handshakeTwo-way-nessNote that after connection setup, the stream is two-way. The server and client sockets become interchangeable after the create succeeds.Closing sockets gracefullyEither side can send a close message. Other side responds with ACKAfter receiving ACK or timeout, close-initiator can treat socket as closed. Other side waits 15 seconds, then treats it as closed. (why?)Networks lose packets for lots of reasons.Easy fix: retransmit if needed.Catch: what if destination is down?Packet lossUsing ACKsDetecting failure perfectly is impossible. In practice, we do well enough.Send a packet with sequence #, wait for ACK to that packet.Resend if no ACK received by timeout.RetransmittingWait 100 ms for ACK, then retransmit Retransmit seven times, doubling wait each time after each failure.Use alarms and semaphores: note that if a packet arrives, should wake up right away.So sleep-with-timeout is no good here.Special casesIn the three-way handshake, the SYN and the SYNACK are retransmitted reliably.So is the CLOSE on socket teardown.Packet FormatsRemember that ‘type’ field in packets from Project 3? Here’s where it comes in handy.Need to separate minisocket and minimsg packets.Probably also wise to divide socket control and data packets.Minisocket PacketsYou need SYN, SYNACK, DATA, ACK, CLOSE.StreamsKey idea of streams is that the application doesn’t see packet boundaries.All app sees is a stream of data, some of which is available.Stream must be reliable, since app can’t easily implement reliability again on top.Read and writeWe’re building streams on packets: minisocket_send() with a big buffer should split it into packets and send them reliably.Likewise, minisocket_receive() should return whatever is available, or a full buffer.If more data than a buffer is available, can’t lose it. (Save it in queue somehow)The State MachineUse the state machine abstraction in designing, and also in implementing. Track a state for each socket, and when a packet comes in, switch on socket state and packet type.See the TCP state machine (online) for further insight.NotesThread safety is important here.Don’t sleep when you have work to do: need to wake listening thread as soon as packet
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