Columbia Kermit/kermit.zip/newsgroups/misc.19970104-19970326/[e-mail] _Wed Mar 5 01:25:25 1997.msg

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From: [e-mail] (Joe Doupnik)
Newsgroups: comp.protocols.kermit.misc
Subject: Re: MS-Kermit via INT14h
Message-ID: <[e-mail]>
Date: 4 Mar 97 20:49:43 MDT
References: <urvCz8Q0+/[e-mail]> <0cHGz8Q0+f/[e-mail]>
Organization: Utah State University
Lines: 86
Xref: news.columbia.edu comp.protocols.kermit.misc:6692

In article <0cHGz8Q0+f/[e-mail]>, [e-mail] (Thomas Pothmann) writes:
> [e-mail] (SAM, Chi-Kin) wrote in comp.protocols.kermit.misc:
>
>  > I am playing with MS-Kermit version 3.15 beta 18, I tested it
>  > working with PPPshare fine.
>
> Oops, how that? As far as I know, Kermit can't handle ethernet and
> by reason of this I tried to make it work via INT14h redirection.
>
>  > I do not using INT14h. I just setup up and load PPPshare as usual.
>  > Then I make the following changes in mskermit.ini / mscustom.ini:
>
>  > set port tcp nnn.nnn.nnn.nnn (the host I want to connect) set tcp
>  > addr dhcp (my isp can provide dhcp or bootp) then I can telnet to
>  > any hosts now.
>
> I've tried this way, too. But I always get an error message and Kermit
> complains, that it can not resolve the site called for; thereafter
> the PPP link is broken as I wrote. BTW: My isp doesn't provide neither
> bootp nor dhcp.
----------------
    Let me explain a little, and then I hope the matter will be better
understood.
    Ethernet is a broadcast medium where many stations can share the
same "wire." To talk to a particular station frames on the wire carry
the hardware address of the destination, and of the source for that matter,
which is decoded by all Ethernet boards prior to accepting the frame. To
determine the hardware address of a particular station on the wire TCP/IP
uses an ARP request and expects and ARP reply (Address Resolution Protocol).
ARP replies are sent by the station owning the IP address in the request;
all other stations must not reply.
    The alternative strategy is to emit physical layer broadcasts, which
every station on the wire receives and decodes, and hope the interior of
the frame distinguishes one station from another. NetBIOS works this way.
Naturally enough, folks react badly having stray traffic enter and interrupt
their machine all the time. Thus TCP/IP over Ethernet uses the "know your
friend's hardware address via ARP" approach; it's polite and helps a great
deal.
    An IP gateway is the machine on the local wire which is able to
relay IP datagrams to other IP networks (on other wires). The IP address
of that gateway must be known to the client. Once known the client wishing
to send an IP datagram to a place far away sends the IP datagram to that
place but the Ethernet frame has a physical address of the gateway machine.
Gateways are expected to look at what they receive and relay appropriately.
SLIP is not a broadcast medium so the gateway is automatically the station
at the other end of the wire; no gateway IP address is needed as such.
    If there is a serial port handler which proclaims itself to be
an Ethernet device then it must support the notion of ARP and physical
addresses and broadcast media strategies. If it does not then it is
incompetent.
    But a serial port connection is a point to point connection, not
a broadcast one. SLIP is Serial Line IP, a method of framing IP datagrams
for such a serial point to point link. No ARP and no gateway are used on
SLIP links.
    PPP is a faker, of the kindly type, which can multiplex packets of
different protocols over the same serial (or other) link. There is no
frame kind called PPP to applications programs.
    By now the situation is becoming clearer. A serial handler proclaiming
itself to be of type Ethernet must support intelligent ARP, but very often
it does not. End of that handler; discard it. PPP handlers are many and
all are much different. Some may present an Ethernet interface and do ARP,
others may forget the ARP part, others may provide a SLIP interface, and
still others do none of the above and instead require the applications
program to build in formal details of that handler.
    The MERIT PPP handler provides both Ethernet (minus ARP) and
SLIP interfaces. Use the SLIP interface with MS-DOS Kermit. It works here,
unquote. We provide it in the Crynwr Collection of Packet Drivers area.
    MS-DOS Kermit obeys the many RFCs on TCP/IP, and it deals with SLIP
interfaces in addition to many others. But if the interface is Ethernet, or
Token Ring, or Arcnet or FDDI etc which are broadcast media, then ARP must
be supported by the handler.
    MS-DOS Kermit 3.14 and 3.15 do an ARP for its own IP address, on
suitable wiring, to detect IP address problems before they become a problem.
If an ARP reply is heard that means another station is using Kermit's IP
address, and Kermit declines to transmit another IP datagram. Thus it does
no good for a serial handler to reply itself to an ARP request for Kermit's
IP address. No one should have that IP address except Kermit, and thus no
other station should send an ARP reply to Kermit's IP address. Serial
handlers can make this mistake. It's fatal. End of handler; discard it.
    What is now evident is one must select the serial port handler
with some care. Prefer SLIP interfaces over Ethernet unless you have tested
the handler and find that it does ARP properly. Else keep shopping. PPP
drivers vary considerably, without much standardization, and hence Kermit
can deal with those it knows (Novell's SLIP_PPP and Telebit's PPP) or it
must use another style interface (say SLIP).
    Joe D.