Tutorial – Summer 2007 1 317 Summer 2007 - Tutorial.

317 Summer 2007 - Tutorial 1

CS 317: INTERNET COMPUTING

Tutorial – Summer 2007


The Tutorial Session

Total 6 weeks during the whole term. Two separate sessions

1 hour duration each Each session twice a week. T1A - Tue, Thu – 12:30 t0 13:30 T1B – Tue, Thu – 13:30 to 14:30 T1C – (has been blocked)


Your TAs

Anirban Sinha (you can call me Ani) Gitika Aggarwal Kan Cai

We are all affiliated to the Distributed Systems Research group under the supervision of separate profs.


Tutorial Organization In strict correspondence with the

assignments. Three assignments, each of 2 weeks

duration. Similarly, each of us will lead 2 weeks of

the tutorial . First two weeks, I will concentrate on the first

assignment. Next two weeks, Gitika will discuss the

second assignment. Last two weeks, Kan will help you out with

the last and final assignment.


What else to discuss?

Questions related to the weekly quiz. Midterm questions and their

solutions . Grading (only general ones)

be careful to ask individual grading questions offline.

Debugging/testing/good practices.


What else to discuss?

General queries/doubts/comments. We will try our best to answer the

questions. If unable, we will answer it in the WebCT. Please try to attend the tutorial sessions.

It will help you a lot in solving the

assignments and the quizzes. 1% bonus for being there in the tutorial !


What not to discuss

Direct solutions to assignments We have not solved the problems ourselves. Violation of UBC policies.

What are the likely questions for midterm/final/quizzes We are not mind readers – we do not know

what Dr. Vuong would like to give in the exams.

Anything that is not related to 317- Canucks game?! I would like to discuss that offline though


What if I get stuck?!

Use WEBCT discussion group! It is more likely that one of your peers

has faced the same problem and solved it!

Group discussion helps to come up with a solution together – much better than getting off-the-shelf answer from the TAs.

If totally stuck with no one knowing what to do, Kan and possibly me will be there to help.

Do *not* email problems to TAs individually *unless* you really think its necessary.


Useful suggestions, tips … Read carefully the text and the lecture

slides. Understand the basic concepts. Ask

questions if in doubt. Write small simple code to test out your

concepts – an excellent way to learn new things.

Know what is exactly being asked for. Don’t procastinate.

Distribute the work over a longer period – helps your mind to think and come up with ideas.


Useful suggestions, tips … If stuck, go back to the text and the

lecture slides – do not cry out for help at the first sign of trouble.

Talk to peers, help is not far away! Be patient and take a deep breath

when things go wrong.


End of Introduction

Any questions, concerns, worries?


What you need for the assignments? Single most important resource:

Beej's Guide to Network Programming, Using Internet Sockets.http://beej.us/guide/bgnet/

All you will ever need is there in this book.

Study the examples and the network APIs carefully.


Assignment 1: Proxy server Work in groups of three

Smaller groups preferred but no bigger. 15 hours total working time per

team. (excluding review time). If you have not read the assignment,

here it is in nutshell: Design a *caching* proxy server. Design a mechanism for cleaning

expired items from cache.


Assignment 1: Proxy server A proxy server works in between

clients and the web server. Helps several clients to share a

single Internet connection – cost effective.

Also caches the requested pages so that pages that are revisited can be delivered immediately.

Firewalls and IP filters are generally set up along with the proxy servers.


Basic Socket Programming

Client Side Programming: Call socket(): get a new client socket

socket(PF_INET, SOCK_STREAM, 0): creates a new TCP/IP socket.

Fill in a structure with the server address & port:

struct sockaddr_in their_addrOften we use gethostbyname() to obtain the remote server IP from its host name.



Client Side Programming: Connect() to the server.

connect(sockfd, (struct sockaddr *)&their_addr, sizeof(struct sockaddr)) : Connect to the remote client using the client side socket, sockfd obtained through call to socket() earlier.

Start Receiving data: Call Recv(): recv(sockfd, buf, MAXDATASIZE-1, 0), where buf is the buffer where the data is received!



Server Side Programming: Generally servers “bind” to a specific

well-known port. For example, all web-servers “bind” & then “listen” on port 80.

All clients (browsers) connect to this port by default.

Thus, server side programming involves a little more work.



Server Side Programming: Call Socket() to get a server side socket. Call

is identical as before. Fill in a structure as before.

struct sockaddr_in my_addrmy_addr.sin_family = AF_INET; my_addr.sin_port = htons(MYPORT); my_addr.sin_addr.s_addr = INADDR_ANY; memset(my_addr.sin_zero, '\0', sizeof my_addr.sin_zero);

Fill in the structure with local port (MYPORT) and local server address (INADDR_ANY).



Server Side Programming: Call bind() : It will bind the server socket

to the specific port. This is done only on the server side, in

order to listen on a specific port. bind(sockfd, (struct

sockaddr*)&my_addr, sizeof(struct sockaddr))Here, sockfd is the socket file descriptor obtained from call to socket() and my_addr is the same as discussed before.


Basic Socket Programming Server Side Programming:

Now “listen” for connections from clients: listen(sockfd, BACKLOG), where backlog is the

number of outstanding connection requests to queue. Generally, its something around 10 to 20 depending on the server load expected.

Now “accept” new connections from clients. Accept() blocks (in general, unless you set the socket

into non-blocking mode) until a client connect()s to a server.

It returns a new socked file descriptor for the corresponding connection.

Also fills a struct sockaddr_in with information about the remote client address.



Server Side Programming: There are two ways to deal with multiple

incoming client requests: Set the server side socket into non-

blocking mode, poll the socket at certain intervals for any outstanding requests from clients and dispatch an event handler to service the request – event driven programming.

Fork a new thread to service the request and the main thread keeps accepting new connections in a tight loop: Multithreaded programming.



Server Side programming: Unless you really know event driven style, I

would suggest you to use multithreaded style Simpler to understand and code You are familiar with threaded programming

already from 213. The thread processing client requests can

now send() or recv() data from clients. If you really want to serve only one client, no

need of threads. Just one main thread can do all the work.



General APIs Close() and shutdown() : Used to de-allocate

the socket resources and shut down the connection.

Gethostname() : get the local host name of the machine.

fcntl(sockfd, F_SETFL, O_NONBLOCK): - set a socket in non-blocking mode. More details on blocking issues in the guide.

setsockopt(listener,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) : to get rid of “address already in use message”


Basic Socket Programming – Tips and Suggestions

Read the APIs carefully. Use man pages when help is needed with

APIs. Always check for return values and take

appropriate actions on errors. Work in steps – do not try to do everything

all at once. Use tools like telnet, ftp etc to test run your

program before proceeding to next steps.


Assignment 1: Proxy serverBasic Idea Create a server socket and listen for incoming

client request. When a new client connect()s, fork a new

thread to process the client request. Parse for remote server address from the client

request. Create a client socket and connect to the

remote server. Forward your client’s request to the remote

server. Send back response (separate threads?). Keep doing it in a loop or terminate after one iteration (?)


Assignment 1: Proxy serverCaching Cache requested pages. No need for

elaborate caching scheme. If requested page is already in cache,

do not go to the server. Terminate when either remote server

or your client disconnects.


Assignment 1: Proxy serverCache Invalidation & Cleanup Objects in cache expire with time.

Use a suitable mechanism to find expired items.

Fork a new thread that cleans up expired items from cache.

The expired time limit is a command line parameter that is provided by the user.


Discussion … (5/10/07)

Concurrency Issues how many threads are needed?

Caching Issues How to detect expired items?

Persistent HTTP connection issues. How to detect a change in requested

server address etc … How robust should be your code?!



Tutorials T1A and T1B will probably be combined into a single one … Any problems with anyone here?

Any other administrative issues?



How’s things are with WebCT? Questions related to text (eg. prop &

trans delay) They are in the text itself! Besides TAs, you can ask Son the next

day in class. Assignment 1 is due this Thursday

Issues, problems, questions? ... Browser configuration (demo)



Tutorials T1A and T1B has been merged! Any conflicts?

Do not forget to check the attendance sheet in my homepage later and report any inconsistencies immediately.

Final set of updated slides will be posted today.

Gitika takes over from next week Will discuss assignment 2.



Assignment 1 is due next Tuesday, 22nd of May.

No more discussion on Assignment 1 after today!

I will try to grade Assignment 1 as soon as I can, according to the guidelines provided by Son. Make sure you put the names of *all* group

members in the source code itself. Use electronic handin – no written material. All members of a group will be treated uniformly. Be patient for the grades and comments. I will try to be lenient. yay!!!!


Discussion … (05/17/07) Grading (Assignment 1) …

If you have questions on grading, you can send me email directly ([email protected]) BUT: Send all your questions in a single mail all at once. Only one mail per group will be answered. Write names, stud ID and CWL of all group

members in email. No later than 5 days after the grades/comments

has been sent out. Check instructor’s policy in the course webpage.

Any response will follow the policy guidelines in the course webpage.



Quiz 1 will be graded by all TAs together. Have patience. Any question on Quiz1 – ask any TA

during the tutorials. Grades will be posted in the WebCT as

soon as possible. That’s all from me!

Any questions, comments??

Tutorial – Summer 2007 1 317 Summer 2007 - Tutorial.

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