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CPS110: More Networks Landon Cox March 30, 2009
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Virtual/physical interfaces
Applications Ordered messages Reliable messages Byte streams OS Unordered messages Unreliable messages Distinct messages Hardware
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Ordered messages Networks can re-order IP messages
E.g. Send: A, B. Arrive: B, A How should we fix this? Assign sequence numbers (0, 1, 2, 3, 4, …)
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Ordered messages Do what for a message that arrives out of order?
(0, 1, 3, 2, 4) Save #3 and deliver after #2 is delivered (this is what TCP does) Drop #3, deliver #2, deliver #4 Deliver #3, drop #2, deliver #4 b. and c. are ordered, but not reliable (messages are dropped). Relies on the reliability layer to handle lost messages.
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Ordered messages For a notion of order, first need “connections” Why?
Must know which messages are related to each other Idea in TCP Open a connection Send a sequence of messages Close the connection Opening a connection ties two sockets together Connection is socket-to-socket unique: only these sockets can use it Sequence numbers are connection specific
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Virtual/physical interfaces
Applications Ordered messages Reliable messages Byte streams OS Unordered messages Unreliable messages Distinct messages Hardware
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Reliable messages Usually paired with ordering Hardware interface
TCP provides both ordering and reliability Hardware interface Network drops messages Network duplicates messages Network corrupts messages Application interface Every message is delivered exactly once
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Detecting and fixing drops
How to fix a dropped message? Have sender re-send it How does sender know it’s been dropped? Have receiver tell the sender Receiver may not know it’s been sent Like asking in the car, “If we left you at the theater, speak up.”
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Detecting and fixing drops
Have receiver acknowledge each message Called an “ACK” If sender doesn’t get an ACK Assume message has been dropped Resend original message Is this ok for the sender to assume? No. ACKs can be dropped too (or delayed)
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Detecting and fixing drops
Possible outcomes Message is delayed or dropped ACK is delayed or dropped Strategy Deal with all as though message was dropped Worst case if message wasn’t dropped after all? Need to deal with duplicate messages How to detect and fix duplicate messages? Easy. Just use the sequence number and drop duplicate.
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What about corruption? Messages can also be corrupted
Bits get flipped, etc Especially true over wireless networks How to deal with this? Add a checksum (a little redundancy) Checksum usually = sum of all bits Drop corrupted messages
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What about corruption? Corruption drops Drops duplicates
Dropping corrupted messages is elegant Transforms problem into a dropped message We already know how to deal with drops Common technique Solve one problem by transforming it into another Corruption drops Drops duplicates Drop any duplicate messages (very simple)
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Virtual/physical interfaces
Applications Ordered messages Reliable messages Byte streams OS Unordered messages Unreliable messages Distinct messages Hardware
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Byte streams Hardware interface Application interface
Send information in discrete messages Application interface Send data in a continuous stream Like reading/writing from/to a file
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Byte streams Many apps think about info in distinct messages
What if you want to send more data than fits? UDP max message size is 64 KB What if data never ends? Streamed media TCP provides “byte streams” instead of messages
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Byte streams Sender writes messages of arbitrary size
TCP breaks up the stream into fragments Reassembles the fragments at destination Receiver sees a byte stream Fragments are not visible to either process Programming the receiver Must loop until certain number of bytes arrive Otherwise, might get first fragment and return
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Byte streams UDP makes boundaries visible
TCP makes boundaries invisible (loop until you get everything you need) How to know # of bytes to receive? Size is contained in header Read until you see a pattern (sentinel) Sender closes connection
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Sentinels Idea: message is done when special pattern arrives
Example: C strings How do we know the end of a C string? When you reach the null-termination character (‘\0’) Ok, now say we are sending an arbitrary file Can we use ‘\0’ as a sentinel? No. The data payload may contain ‘\0’ chars What can we do then?
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Course administration
Last day for Project 2 submissions 89, 89, 89, 89, 87, 86, 83, 79, 79, 78, 76, 75, 49, 33, 28 Project 3 will be out next week Hack into a server Socket programming how-to posted Ryan will cover in discussion section this week Will post example client/server code Any questions?
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Distributed systems You use distributed systems everyday.
Both on your PC and behind the scenes.
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Motivation for distributed apps
Performance
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Motivation for distributed apps
Performance Co-location Can locate computer near local resources Examples of local resources People, sensors, sensitive database
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Motivation for distributed apps
Performance Co-location Reliability Not all computers will go down at once Due to floods, fires, earthquakes, etc Better chance of continuous service
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Motivation for distributed apps
Performance Co-location Reliability Already have multiple machines Can’t put everyone on one machine Try to stitch existing machines together
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Building up to distributed apps
Needed two things in multi-threaded programs Atomic primitives to control thread interleavings (interrupt disable/enable, atomic test&set) Way to share information between threads (shared address space) These won’t work for distributed applications No interrupt disable/enable or atomic test&set No shared memory
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Building up to distributed apps
Can only communicate through messages Send messages Receive messages If no shared data, are there race conditions? Sure, message might reflect an inconsistent state Races can cause other problems too Incorrect event ordering (fire missile after command) Mutual exclusion (two people adding last spot in class)
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Send/receive primitives
So we need sharing and synchronization Obvious we can use send/receive to share Each message communicates information Messages instead of load/store to shared memory Can we use send/receive for synchronization? Depends on the atomicity of our hardware primitives “Try to build large atomic regions from small ones.”
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Send/receive primitives
Atomic operation provided by hardware Can send a single Ethernet frame atomically Ethernet detects simultaneous sends Called a collision Ethernet either allows one or none A bigger problem in wireless networks (why?) Idea: build up from atomic send X
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Send/receive primitives
Interleaved incoming packets OS separates the packets Forms whole messages from fragments Passes messages up to various apps How does OS know which packet goes to which app? Port number in L4 (TCP, UDP, etc) header
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Send/receive primitives
Interleaved outgoing packets OS ensures packets are whole One app’s packets won’t change another’s How does OS control access to the NIC? Device driver “serializes” send requests Use locks inside driver code
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Client-server Many different distributed architectures
Client-server is the most common Also called request-response Basic interaction “GET /images/fish.gif HTTP/1.1”
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Client-server Clients are machines you sit in front of
Send request to server Wait for a response Clients generally initiate communication Writes are similar POST /cgi-bin/uploadfile.pl HTTP/1.1 Content-Type: application/x-www-form-urlencoded Content-Length: 49 filename=foo.txt&file+content=content+of+foo.txt
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Producer-consumer Soda drinker (consumer) Delivery person (producer)
Vending machine (buffer)
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Producer-consumer Have a server manage the coke machine
Clients can call two functions client_produce () client_consume () Both send a request to the server Both return when the request is done
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Need to wait for cokes/space.
Producer-consumer client_produce () { send message to add coke wait for response } client_consume () { send message to get coke wait for response } server () { receive request (from any producer or consumer client) if (request is from a producer) { wait for empty spot in machine put coke in machine } else { wait for coke in machine take coke out of machine } send response Anything missing? Need to wait for cokes/space.
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Producer-consumer Does this work? No. Now we’ll deadlock.
client_produce () { send message to add coke wait for response } client_consume () { send message to get coke wait for response } server () { receive request (from any producer or consumer client) if (request is from a producer) { wait for empty spot in machine put coke in machine } else { wait for coke in machine take coke out of machine } send response Does this work? No. Now we’ll deadlock.
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Use threads at the server.
Producer-consumer client_produce () { send message to add coke wait for response } client_consume () { send message to get coke wait for response } server () { receive request (from any producer or consumer client) if (request is from a producer) { wait for empty spot in machine put coke in machine } else { wait for coke in machine take coke out of machine } send response How do we solve this? Use threads at the server.
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Producer-consumer server () { while (1) {
receive request (from any producer or consumer client) if (request is from a producer) { create a thread to handle the produce reqeust } else { create a thread to handle the consumer request } server_produce () { lock while (machine is full) wait put coke in machine send response to client unlock } server_consume () { lock while (machine is empty) wait take coke from machine send response to client unlock }
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Producer-consumer Note how we used send/receive
Used to share information (request/response messages) Provides before/after constraint (client waits for server) Receive is like wait/down, send is like signal/up Solution creates a thread per request Can we lower the per-request overhead?
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Producer-consumer Keep a pool of worker threads
When main loop gets a request Pass request to an existing worker thread When worker thread is done Wait for next request from dispatcher Similar to disk scheduler in P1
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Other approaches Don’t have to use worker threads
Just want slow operations to run in parallel What are the slow operations? Producing/consuming a coke Receiving a network request
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Other approaches Want server to work while waiting for requests
Could poll (using the select() system call) Instead of blocking in recv() Poll to see if there is a new message, call recv() if there is Also must avoid calling wait() if there is no coke/space Could use signals (SIGIO) Incoming network messages generate a signal The signal interrupts the thread blocked in wait() Threads are a much easier solution!
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Network abstractions We’ve been using send/receive
Client sends a request to the server Server receives request Server sends response to client What else in CS is this interaction like? Calling a function
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Remote procedure call (RPC)
RPC makes request/response look local Provide a function call abstraction RPC isn’t a really a function call In a normal call, the PC jumps to the function Function then jumps back to caller This is similar to request/response though Stream of control goes from client to server And then returns back to the client
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The RPC illusion How to make send/recv look like a function call?
Client wants Send to server to look like calling a function Reply from server to look like function returning Server wants Receive from client to look like a function being called Wants to send response like returning from function
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RPC stub functions Key to making RPC work Stub functions
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RPC stub functions Client stub Server stub call send recv return send
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RPC stub functions Client stub Server stub
1) Builds request message with server function name and parameters 2) Sends request message to server stub (transfer control to server stub) 8) Receives response message from server stub 9) Returns response value to client Server stub 3) Receives request message 4) Calls the right server function with the specified parameters 5) Waits for the server function to return 6) Builds a response message with the return value 7) Sends response message to client stub
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RPC notes Client makes a normal function call
Can’t tell that it’s to a remote server (sort of) Server function is called like a normal function Can’t tell that it’s returning to a remote client Control returns to client as from a function Common technique to add functionality Leaving existing component interfaces in-place Implement both sides between existing layers
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RPC example Client calls produce(5) Client stub: Server stub:
// must be named produce! int produce (int n) { int status; send (sock, &n, sizeof(n)); recv (sock, &status, sizeof(status)); return status; } // could be named anything void produce_stub () { int n; int status; recv (sock, &n, sizeof(n)); // produce func on server! status = produce (n); send (sock, &status, sizeof(status)); } Server stub code can be generated automatically (C/C++: rpcgen, Java: rmic) What info do you need to generate the stubs? Input parameter types and the return value type.
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Java RMI example Remote calculator program
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Problems with RPC How is RPC different from local function calls?
Hard to pass pointers (and global variables) What happens if server dereferences a passed-in pointer? Pointer will access server’s memory (not client’s) How do we solve this? Send all data reachable from pointer to the server Change the pointers on the server to point to the copy Copy data back when server function returns
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Example RPC with pointers
On client: int a[100]; Want to send “a” (a pointer to an array) Copy entire array to server Have server’s pointer point to copy of a Copy array back to client on return What if a is more complicated? A linked list? Have to marshal the transitive closure of pointer
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Problems with RPC Data may be represented differently
Machines have different “endianness” Byte 0 may be least or most significant Must agree on standard network representation RPC has different failure modes Server or client can fail during a call In local case, client and server fail simultaneously
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Finishing RPC Where have you used RPC in CPS 110?
Project 2 infrastructure libvm_app.a and libvm_pager.a hide send/recv C library interface to system calls is similar Makes something look like a function call
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Structuring a concurrent system
Talked about two ways to build a system
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Alternative structure
Can also give cooperating threads an address space Each thread is basically a separate process Use messages instead of shared data to communicate Why would you want to do this? Protection Each module runs in its own address space Reasoning behind micro-kernels Each service runs as a separate process Mach from CMU (influenced parts Mac OS X) Vista’s handling of device drivers
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Rest of the semester On Wednesday we’ll begin security
Project 3 will be a new security project After security, we’ll cover file systems Probably a guest lecture along the way Finish up with Google
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