Concurrent Server Using Selective Repeat Data Link Layer

Transcription

1 S4514 HELP Session 3 oncurrent Server Using Selective Repeat Data Link Layer Mingzhe Li lmz@cs.wpi.edu 12/05/2005

2 Description Objective: To implement a simple concurrent server and clients having four emulated network protocol stacks. pplication layer: ustomized pplications Network layer: Message Packet (send&recv) Datalink layer: Packet Frame and Selective Repeat sliding window protocol Physical layer: TP connection. To get full credit, you need a sending and receiving windows size >=4. Your programs should compile and work on any host of ccc.wpi.edu. 2

3 System Overview (1) Server lient1 (3) (2) fork() (4) fork() hild Process1 lient2 You can either use multiprocesses (fork()) or multithreading (pthread) hild Process2 You need to implement concurrent access to the database. 3

4 System Framework lient PP Layer NW Layer DLL PHL Server PP Layer NW Layer DLL PHL TP onnection Four Layer stacks 4

5 5 Selective Repeat B 0 time error N B

6 How the System Works: Layer by Layer pplication Layer Messages lienti Server hild Process i Messages PP PP lient Requests: pplication depended nwl_send ( msg ) nwl_recv ( msg ) Server Responses: pplication depended t least 5 operations, there is at least one long operation in each direction that will easily test your sliding window 6

7 How the System Works: Layer by Layer Network Layer lienti Server Message Message NWL NWL n_packets n_packets End of Message an be an special packet, OR a special position in each packet, eg. 1 st byte dll_send ( pkt ) dll_recv ( pkt ) Tasks for NWL Disassemble and assemble packets om Msg. No in this layer Two Bytes sequence no. for packets Note: The max_size of a packet is 256 bytes, The network layer will send packets until blocked by the Data Link Layer. But HOW? 7

8 How the System Works: Layer by Layer DataLink Layer or N? lienti Server Piggyback? packet packet n_ames 1 Byte End of Packet 2 Bytes Error Detection 2 Bytes SEQ# DLL Selective Repeat phl_send ( m ) phl_recv ( m ) DLL n_ames Note: The max_size of a ame payload is 100 bytes Sliding window size >=4 8

9 How the System Works: Layer by Layer lienti Server ame ame PHL PHL read ( data ) write ( data ) TP onnection Identify client when start Force Single Bit Errors - Data: 8 th ame -ck: 6 th ame - Server and clients 9

10 How the Functions Work: Layer by Layer client PP User input server child process PP fork() Pick a command nwl_recv( ) q cmd? No Yes q cmd? No Yes Build Msg Build Msg nwl_send( msg ) nwl_send( msg ) nwl_recv( ack ) 10

11 How the Functions Work: Layer by Layer nwl_send ( msg ) nwl_recv ( msg ) Split msg into pkts Pick a pkt dll_recv ( pkt ) Last pkt? No Last pkt? Yes hang EOM No Yes Reassemble pkts into msg Return msg to PP dll_send( pkt ) Note: you need have a mechanism to decide the last packet in a message (EOM). 11

12 How the Functions Work: Layer by Layer dll_send ( pkt ) Split a packet into payloads Sliding window size =1 reate a new ame Start a Timer Send a ame to PHL Wait for receiving a ame phl_send ( ) phl_recv ( ) Retransmit ames if timeout or error ame! Receive a ame correctly, then continue... 12

13 How the Functions Work: Layer by Layer dll_recv ( pkt ) Receive a ame om PHL phl_recv ( ) ompute ED byte and check error Drop if error detected Drop if duplicate, else send phl_send ( ) Reassemble the packet If EOP, forward the packet to NWL Question: When is the correct time to send N or? Not after ED drop, but on receiving next ame or dup ame. 13

14 Project Tips-1 Sliding Window Protocol: Selective repeat (N>=4) Try to implement windows size 1 first Then implement N (multiple timers) Follow the example in the book (protocol 6) How to terminate client process: When the client gets the response to the quit message clean way to terminate the server child process? Use wait()! 14

15 Project Tips-2 Simulate multiple timer in software pproach I Using link list or array pp.223 on textbook() Need signal() pproach II Using link list or array Update the struct timeval for next select() call 15

16 Project Tip3 How could the NWL eep sending packets until blocked by the Data Link Layer? Our suggestion is that you could use pipe to implement it: NWL keeps writing packets to the pipe until the pipe is full. simple code of pipe could be found at amples/examples.html Pipe is more like a socket between local processes. 16

17 oncurrent TP Server Example (fork) pid_t pid; int listenfd, connfd; /* 1. create a socket socket() */ if ((listenfd = socket(f_inet, SO_STREM, 0)) < 0 ) err_quit("build server socket error\n", -1); /* 2. fill in sockaddr_in{ } with server's well-known port */ /* 3. bind socket to a sockaddr_in structure bind() */ bind (listenfd,...); /* 4. specify the backlog of incoming connection requests listen() */ listen (listenfd, LISTENQ); while(1){ } connfd = accept(listenfd,... ); /* probably blocks */ if(( pid = fork()) == 0){ } close(listenfd); /* child closes listening socket */ doit(connfd); /* process the request */ close(connfd); /* done with this client */ exit(0); close(connfd); /* parent closes connected socket */ 17

18 Questions? 18