To: asherman@MIT.EDU
Subject: Re: Internet lecture 
In-reply-to: Your message of "Thu, 16 Jan 1997 11:52:57 EST."
             <9701161652.AA00416@m1-142-9.MIT.EDU> 
Date: Sat, 18 Jan 1997 19:13:38 EST
From: elliot


Hi,

          I liked your internet lecture thanks

Thanks. If you have any other comments, I'd be interested in hearing them.

          1.Could you briefly explain how IP checksum is calculated
          and how does it ensure the authenticity of the IP layer
          header?
          
According to RFC791,

    The checksum algorithm is:

      The checksum field is the 16 bit one's complement of the one's
      complement sum of all 16 bit words in the header.  For purposes of
      computing the checksum, the value of the checksum field is zero.

It doesn't ensure the authenticity - what it does is allow routers and
hosts to notice data corruption in the header. If the checksum taken from
the header doesn't match the given checksum, one of them changed. It's
possible that both could change, or that the header could change in such
a way that the checksum is still correct, but this is unlikely given the
size of the checksum.

          2.I tried using POP and HTTP last night directly from the
          prompt and it worked, I also observed SMTP interaction by
          doing send -watch, but I was wondering if there is a way to
          do a direct SMTP interaction, meaning sendimg mail using
          SMTP directly?

Sure... just telnet to the SMTP port of a mail server. So, for example:
athena% telnet -u pca.mit.edu smtp
Trying 18.69.0.28...
Connected to PACIFIC-CARRIER-ANNEX.MIT.EDU.
Escape character is '^]'.
220 MIT.EDU Sendmail 5.61/1.10 (MIT Mailhub) No collect or third-number calls at Sat, 18 Jan 97 19:12:45 EST
expn postmaster
250-<tom>
250-<tytso@PO9.LOCAL>
250-<chris@PO10.LOCAL>
250-<hoffmann@PO9.LOCAL>
250-<jis@E40-PO.LOCAL>
250-<foley@PO7.LOCAL>
250 <pm-mtg@menelaus.mit.edu>
quit
221 MIT.EDU closing connection
       
The SMTP port is 25, in case you don't happen to have it in your services
file.
   
elliot

          
          