==============
Administrivia:
==============
- Course evaluations are this week.  Be sure to show up for them if
  you want to provide official feedback for the course.
- Lab L4 is happening.  It is due on Friday at midnight.

================
Text Formatting:
================
- printf() can be very powerful.  Take advantage of it.
  - printf("0x%02X", some_uint & 0xff) will print the low order byte with a
  leading zero.
  - printf("%c", isprint(some_char) ? some_char : '.') will print a
  character if it is printable, and a period otherwise.

=================
Berkeley Sockets:
=================
- Allows network connections to behave like files.
- Based on connections to ports.
- Processes create sockets with the socket() syscall call.
- Connect your socket to another socket with connect().
- Give the socket a name with bind().
- Tell the socket to allow connections with listen().
- Accept a connection request with accept().

======================
Servers and Protocols:
======================

SMTP summary from RFC 821
-------------------------
- protocol for transferring mail
- any reliable transport will do
- communication starts from machine on which mail is sent
- message transferred via SMTP to next machine (final or intermediate)
- next machine can reject the mail
- to transfer, the source machine connects to a server on the next machine
  (assigned number: port 25)

- the protocol is a series of commands issued by the sender,
- responses from the receiver with error codes

- mention full BNF, error codes to be found in rfc821
- protocol state diagrams also in RFC

  sender commands: (followed by <CRLF>)
   "HELO <domain>" (identify source machine, start of connection)
   "MAIL FROM:<reverse-path>" (id of sender, optional reverseroute for errors)
   "RCPT TO:<forward-path>" (id of recipient, optional forward source route)
   "DATA" (then enter the text, INCLUDING HEADERS, <CRLF>.<CRLF> to end)
   "QUIT" (close connection)
  
   "NOOP" (nothing)
   "RSET" (reset buffers - forward route, reverse route, msg buffer)
   "HELP" (handy-dandy protocol command help)
  
  receiver replies:
   first digit: good, bad, incomplete response
   2nd digit:   category (syntax, information, etc)
   3rd digit:   diversity within category
  
    220 (Service ready)
    250 (OK)
    421 (Connection terminated)
    500 (Syntax Error)

- Sample session: [Make a slide of this]
    > telnet coltrane.stampede.cs.cmu.edu smtp
    Trying 128.2.181.76...
    Connected to coltrane.stampede.cs.cmu.edu.
    Escape character is '^]'.
    220 coltrane.stampede.cs.cmu.edu SMTP, Problems to: Help@FAC.CS.CMU.EDU
    helo gs138.sp.cs.cmu.edu
    250 coltrane.stampede.cs.cmu.edu
    mail from:<colohan@cs.cmu.edu>
    250 OK
    rcpt to:<kwalker@cs.cmu.edu>
    250 Rcpt OK
    data
    354 Enter Mail, end with a '.'
    This is a sample message to show the class how a session works.
    .
    250 Sub & q (msg.aa02024)
    quit
    221 Goodbye
    Connection closed by foreign host.
- If a period starts a line, a period is prefixed to the line.  The
  receiver strips out leading periods.
- Protocol has state:  the commands must occur in the order "helo",
  "mail", "rcpt"*n, "data".
- Defined in RFC821
    /afs/andrew/common/rfc/rfcXXXX.txt
    /afs/andrew/common/internet-drafts/...

HTTP 1.0 summary from RFC 1945
------------------------------

- stateless application-level protocol
- can be used for name-servers and distributed object management systems
- includes notion of data-types --> MIME (rfc1521)
- da Web

- basic mechanism:
  naming - URI, transport

- lots of nasty terminology (entity, resource, user agent, proxy, etc)
- lots of nasty BNF
- deals with variety of date formats, character sets, data types, encoding
  schemes, all of which makes it hairy to learn

- request/response mechanism with NO STATE (unlike SMTP)
  state must accompany the request in the form of headers

- basic request types: "GET", "HEAD", "POST"
  (assigned number: port 80)

  telnet www.theonion.com http
  -->"GET / HTTP/1.0<CRLF><CRLF>"
  <--"HTTP/1.1 200 OK", headers, HTML of the root page!
  
  telnet www.theonion.com http
  -->"GET http://www.cnn.com/ HTTP/1.0<CRLF><CRLF>"
  <--"HTTP/1.0 403 Forbidden", headers, HTML of the error page
  
  telnet www.cs.cmu.edu http
  -->"HEAD /~kwalker/index.html HTTP/1.0<CRLF><CRLF>"
  <--"HTTP/1.1 200 OK", headers, including data (Fri, 28 Aug 1998 20:00:46 GMT)
  
  telnet www.cs.cmu.edu http
  -->"GET /~kwalker/index.html HTTP/1.0<CRLF>
      If-Modified-Since: <date><CRLF><CRLF>"
  <--"HTTP/1.0 304 Not Modified"

- response codes:
  2xx good news
  3xx redirection (and not modified)
  4xx client error
  5xx server error

==========
fork()ing:
==========
- Fork is what is used in Unix to start all processes
- Splits a process into two (almost) identical processes
- Will return 0 to the child process, and the child's PID to the
  parent
- Correct usage:
  if(fork() == 0) {
      /* I am the child.  Do stuff. */
  } else {
      /* I am the parent.  Do stuff. */
      
      /* Wait for the child to finish: */
      wait();
  }
- Incorrect usage:  What does this do?
  while(1) {
      fork();
  }
  [Fork bomb -- infinitely forks until the machine can't handle all of
   the processes.  This is *bad*.]
- How about this?
  while(fork() == 0) {}
  [Morphing program.  Constantly changes PID.  Very hard to kill.]
- When debugging a program that forks, check to see if you have left
  behind extra processes.
       System V:  ps -eu colohan
	      e = info about every process
	      u = restrict to user "colohan"
       BSD: ps x 
	      x = include processes not bound to a terminal
- To kill an errant process:
  kill <PID>
- To toast it with predjudice:
  kill -9 <PID>
- To kill every process you have authority to kill (DO NOT RUN AS ROOT):
  kill -9 -1