Distributed and Concurrent Systems			18.4.2008
Exercise 6

No X-exercise this week. Remaining exercises (with X) at 26.4. and
6.5 (10.30 - 12).

Course exam 6.5. 08.00-10.00.

In the following exercises we estimate the performance of a situation
where a single server and multiple clients are connected by the
Internet. In all cases, compute the performance 1) with SSL handshake,
2) without SSL handshake. In addition to the throughput computation,
state also the possible bottleneck of the transaction.

A SSL handshake includes 6 messages of total length 1000 bytes. The key
generation and checking of the SSL handshake takes 500 million
operations for both participants (client and server). After SSL
handshake, the communication proceeds using symmetric encryption (which
also takes time). If there is not SSL handshake (e.g., continuing an
earlier session), the participants directly start with encrypted request
and reply.

The following table presents these and other values needed to calculate
the performances. Those values that are in italics have several values
in the following exercises. B is byte, b is bit. The values marked by *,
there are options at text.


    CPU speed (Moper/s)					3000
    Average reply length (B)				20000
    Server number of CPUs.				1*
    Symmetric (session) decrypting speed (oper/B)	100
    Client network bandwidth (kb/s)			100*
    Network delay (ms)					20
    Server network bandwidth (Mb/s)			625
    SSL handshake total length (B)			1000
    Average request length (B)				100
    # of SSL handshake messages				6
    Symmetric (session) encryption speed (oper/B)	200
    SSL handshake oper/participant			5*10^8

31) How long (real, wall-clock time) it takes for a client to make and
    complete a request (from start to finishing the decrypting of the
    reply) if the client network connection speed is a) 100 kbit/s
    (EDGE), b) 2 Mbit/s (ADSL). 1) with SSL handshake, 2) without SSL
    handshake. You need to compute also the time used by the server.

32) How many new requests/second the server can handle a) with one
    processor, b) with 1000 processors. 1) with SSL handshake, 2)
    without SSL handshake. The "new" stands for the fact that even if
    each request takes more than 1 second, it does not require 1 second
    of server resources, and thus the server can time-share its
    resources for several concurrent clients. In other words, compute
    the throughput.

33) Familiarize yourself with Java API class URL
    (http://java.sun.com/j2se/1.5.0/docs/api/java/net/ URL.html,
    http://java.sun.com/docs/books/tutorial/networking/urls/readingURL.html)
    and modify exercise 29 (week 5) to compute the hash from the
    document retrieved from the URL given at command line.

34) Modify the program MD5Client (at www-page) to support computing
    of several hashes during the same TCP connection. Before sending the
    actual file, the client sends the length of the file in bytes (an
    integer of 4 bytes, network byte order). Helper methods at www-page.

35) Correspondingly, modify the MD5Server (at www-page) to support
    computing several hashes during the same TCP connection. Before each
    document, the server receives the length of the document (an integer
    of 4 bytes, network byte order), receives the document, computes the
    hash, sends the answer, and waits for new documents until the client
    closes the connection.

36) Modify the previous MD5Server to multi-threaded one so that a new
    thread is started for each new connection. Thus, the server can
    serve several clients concurrently.