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Recitation 12: ProxyLab Part 1
Instructor: TA(s)
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

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⬛ Feedback
⬛ Networking
⬛ PXYDRIVE Demo
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

⬛ Please sign up for code reviews for Shell Lab by Thursday
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

⬛ Checkpoint is worth 2%, due Tuesday, November 23rd
⬛ Final is worth 6%, due Thursday, December 2nd
⬛ Current situation w/ grace / late days (subject to change):
⬛ 1 grace / late day allowed for both checkpoint and final
⬛ You are submitting an entire project ▪ Modify the makefile
▪ Split source file into separate pieces
⬛ Submit regularly to verify proxy builds on Autolab
⬛ Your proxy is a server, it should not crash!
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Why Proxies?
Proxies are both clients and servers
Can perform useful functions as requests and responses pass by
▪ Examples: Caching, logging, anonymization, filtering, transcoding Request foo.html
Proxy cache
Request foo.html foo.html
Origin Server
Request foo.html Client
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

2. Start client Client
1. Start server Server
Echo Server + Client
Await connection request from client
3. Exchange data
Connection request
Client / Server Session
4. Disconnect client
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third
5. Drop client
open_client
rio_writen
open_listen
rio_readlineb
rio_readline
rio_writen
rio_readline

Transferring HTTP Data
If something requests a file from a web server, how does it know that the transfer is complete?
A) It reads a NULL byte.
B) The connection closes.
C) It reads a blank line.
D) The HTTP header specifies the number of bytes to receive. E) The reading function receives EOF.
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Introducing PXYDRIVE1
⬛ A REPL for testing your proxy implementation ⬛ We also grade using this
⬛ Typical pre-f18 proxy debugging experience:
⬛ Open up three terminals:
for Tiny server, gdb proxy and curl
⬛ Can make multiple requests, but need more terminals
for multiple instances of the Tiny server
⬛ If the data is corrupted, need to manually inspect lines
of gibberish binary data to check error
⬛ Not anymore with PXYDRIVE!
1 Not typing PXYDRIVE in small-caps is a style violation.
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Introducing PXYDRIVE
⬛ General workflow
⬛ Generate text and
binary data to test
your proxy with
⬛ Create (multiple) server
⬛ Make transactions
⬛ Trace transactions to
inspect headers and response data
⬛ Transaction
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Some practice
⬛ Get the tarball
⬛ $ wget https://www.cs.cmu.edu/~213/activities/proxy-r ecitation13.tar
⬛ $ tar –xvf proxy-recitation13.tar
⬛ $ cd pxydrive-tutorial
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Trying out PXYDRIVE
⬛ It’s a REPL: the user can run commands
⬛ $ ./pxy/pxydrive.py
⬛ Just starts PXYDRIVE
⬛ Try entering commands:
⬛ >help help help help help help… ⬛ >quit
⬛ $ ./pxy/pxydrive.py –p ./proxy-ref
⬛ Starts PXYDRIVE and specifies a proxy to run
⬛ Proxy set up at :30104
⬛ Picks the right port and starts the proxy
⬛ ./proxy-ref is the reference proxy
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 1
⬛ Introducing basic procedures:
generate data, create server, fetch / request file from server, trace transaction
⬛ Open s01-basic-fetch.cmd
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 1
⬛ >generate data1.txt 1K
⬛ Generates a 1K text file called data1.txt
⬛ >serve s1
⬛ Launches a server called s1
⬛ >fetch f1 data1.txt s1
⬛ Fetches data1.txt from server s1, in a transaction called f1
⬛ Waits for all transactions to finish
⬛ Needed in the trace, not in the command-line
⬛ >trace f1
⬛ Traces the transaction f1
⬛ >check f1
⬛ Checks the transaction f1
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 1
⬛ Run trace with –f option:
⬛ $ ./pxy/pxydrive.py –f s01-basic-fetch.cmd –p
./proxy-ref
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Look at the trace of the transaction!
⬛ Identify:
⬛ GET command
⬛ Host header
⬛ Other headers
⬛ Request from client to proxy
⬛ Request from proxy to server
⬛ Response by server to proxy
⬛ Response by proxy to client
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 1
⬛ Run a different trace
⬛ $ ./pxy/pxydrive.py –f s02-basic-request.cmd –p
./proxy-ref
⬛ You should get a different output from the first trace
⬛ Why? Let’s look at this trace…
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 1
⬛ >generate data1.txt 1K
⬛ >serve s1
⬛ >request r1 data1.txt s1
⬛ Requests data1.txt from server s1, in a transaction called r1
⬛ >trace r1
⬛ >respond r1
⬛ Allow server to respond to the transaction r1
⬛ >trace r1
⬛ >check r1
⬛ Checks the transaction r1 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 1
⬛ The fetch command makes the server immediately respond to a request.
⬛ All steps of a transaction are complete after a fetch. fetch
Client Proxy Server
⬛ The request command does not complete a transaction.
⬛ A request needs a respond to complete its transaction.
Client Proxy Server
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 2
⬛ Debugging a proxy that clobbers responses
⬛ Run the same trace but with a faulty proxy
⬛ $ ./pxy/pxydrive.py –f s01-basic-fetch.cmd –p ./proxy-corrupt
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

What went wrong?
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 3
⬛ Debugging a proxy that clobbers headers
⬛ Run the same trace but with another faulty proxy
⬛ $ ./pxy/pxydrive.py –f s01-basic-fetch.cmd –p ./proxy-strip –S 3
⬛ -S specifies strictness level
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

What went wrong?
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 4
⬛ Debugging a proxy that crashes
⬛ Run the same trace but with yet another faulty proxy
⬛ $ ./pxy/pxydrive.py –f s03-overrun.cmd –p ./proxy-overrun
⬛ Is the error message helpful?
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

PXYDRIVE Tutorial 4
⬛ We resort to multi-window debugging
⬛ Set up another window and run GDB in one:
⬛ $ gdb ./proxy-overrun
⬛ (gdb) run ⬛ In the other window, run PXYDRIVE:
./port-for-user.pl
Run this to get your unique port!
⬛ $ ./pxy/pxydrive.py –P localhost: –f s03-overrun.cmd
⬛ -P specifies the host and port the proxy is running on
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

⬛ Read the writeup
⬛ One grace / late day for both checkpoint and final
⬛ So you really have to start early
▪ Come to office hours this week, before it gets crowded!
⬛ Work incrementally and take breaks
⬛ Simpler tests should be completed in the first week!
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

So you wanna TA for 213?
⬛ What qualifications are we looking for?
○ Decent class performance, but also critical thinking skills
○ Like computer systems + want to help others like systems!
○ Have a reasonable ability to gauge your schedule + responsibilities
○ Leadership potential! Take initiative, we love to see it 😌
○ Ability to tell students:
■ “Did you write your heap checker”
■ “Run backtrace for me”
■ rinse and repeat, it’s mouthwash baby
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Appendix on echoserver / client
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

⬛ See the instructions written in the telnet results to set up the echo server. Get someone nearby to connect using the echo client.
⬛ What does echoserver output? (Sample output:)
$ ./echoserver 10101
Accepted connection from hammerheadshark.ics.cs.cmu.edu :46422 hammerheadshark.ics.cs.cmu.edu:46422 sent 6 bytes Disconnected from hammerheadshark.ics.cs.cmu.edu:46422
Server listening port
Client host
Client port
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

⬛ Look at echoclient.c
− Opens a connection to the server
− Reads/writes from the server
⬛ Look at echoserver output
− Why is the printed client port different from the
server’s listening port?
− Server opens one “listening” port
● Incoming clients connect to this port
− Once server accepts a connection, it talks to client on a
different “ephemeral” port Listening port
Ephemeral port
Client connects to server
GET /~213/recitations/rec12.html HTTP/1.0
HTTP/1.1 200 OK Content-Type: text/html…
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

⬛ Try to connect two clients to the same server.
⬛ What happens?
− Second client has to wait for first client to finish!
− Server doesn’t even accept second client’s connection
− Where/why are we getting stuck?
⬛ Because we’re stuck in echo() talking to the first client, echoserver can’t handle any more clients
⬛ Solution: multi-threading
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Echo Server Multithreaded
⬛ How might we make this server multithreaded? (Don’t look at echoserver_t.c)
while (1) {
// Allocate space on the stack for client info
client_info client_data;
client_info *client = &client_data;
// Initialize the length of the address
client->addrlen = sizeof(client->addr);
// Accept() will block until a client connects to the port
client->connfd = Accept(listenfd,
(SA *) &client->addr, &client->addrlen);
// Connection is established; echo to client
echo(client);
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

Echo Server Multithreaded
⬛ echoserver_t.c isn’t too different from echoserver.c
− To see the changes: `diff echoserver.c echoserver_t.c`
⬛ Making your proxy multithreaded will be very similar
⬛ However, don’t underestimate the difficulty of addressing
race conditions between threads!
− Definitely the hardest part of proxylab
− More on this next time…
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third

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