Distributed systems		29.4.2004

Exercise 10

No X-exercises this week. Last lecture at Wed 28.4.2004

2nd intermediate exam at Tue 4th May 08.00 -- 10.00 at T/B106.

Please fill in the course evaluation form
(http://cs.joensuu.fi/~arvio/english.html)

46) Design a binary protocol for dinner time agreement (basic version &
    algorithm of exercise 3). Assume that we use persistent TCP
    connections, i.e., the pair of hosts keep the connection if they
    expect to transfer more data later on. Specify each byte and bit of
    the message. Make sure that the receiving end can always reliably
    interpret the contents of the message. Assume first that we are
    agreeing the time only for one day, and that only full hours are
    used.

47) Refine the previous protocol to support a variable sized list of
    times with more accuracy and longer time span. (At least minutes
    accuracy, at least several months time span).

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.

    CPU speed (Moper/s)			2000
    Average reply length (B)		20000
    Server number of CPUs		1
    Session decrypting speed (oper/B)	1000
    Client network bandwidth (kb/s)	28
    Network delay (ms)			50
    Server network bandwidth (Mb/s)	20
    SSL handshake total length (B)	1000
    Average request length (B)		200
    # of SSL handshake messages		6
    Session encryption speed (oper/B)	2000
    SSL handshake oper/participant	5*10^8

48) 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) 28 kbit/s
    (modem), b) 512 kbit/s (ADSL). 1) with SSL handshake, 2) without SSL
    handshake. You need to compute also the time used by the server.

49) How many new requests/second the server can handle a) with one
    processor, b) with 10 processors, c) with 1000 processors if the
    server Internet connection speed is 20 Mbit/s. 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.

50) Recompute the performance of the previous exercise (a, b, c, 1, 2)
    if server Internet connection speed is 625 Mbit/s.