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000215_news@newsmaster….columbia.edu _Wed Jun 17 03:55:26 1998.msg
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From: jaltman@watsun.cc.columbia.edu (Jeffrey Altman)
Newsgroups: comp.protocols.misc,comp.bbs,alt.comp,alt.computer,alt.computer.security,aus.computers.sun,comp.bbs.misc,comp.dcom.modem,comp.protocols.tcp-ip,comp.protocols.tcpip,comp.protocols.ppp,comp.protocols.kermit.misc,comp.unix.sun,comp.unix.solaris,comp.unix
Subject: Re: Z-Modem vs FTP: Is the following an accurate comparison?
Date: 17 Jun 1998 07:44:39 GMT
Organization: Columbia University
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In article <35867DA0.A4FB2608@interlog.com>,
James Coe <jcoe@interlog.com> wrote:
:
:
: Z-Modem vs FTP
:
: Speed
: Z-Modem is usually much faster than FTP. This is because most
: implementations of Z-Modem use internal data compression routines and do
: not perform Cyclic Redundancy Checks (CRC).
: FTP is slower than Z-Modem since no data compression is used and CRC is
: performed.
Have you done any real performance testing?
If so, with what products?
FTP relies on TCP/IP to perform all error correction and data verification.
It does not perform any additional data checks. It has no need to.
When Zmodem is used over a Telnet or Rlogin connection not only does
the data transfer occur on the same TCP/IP link as is used by FTP,
the data is also being passed through a psuedo-terminal and the
telnetd or rlogind which must perform per-character checking.
I would have an extremely hard time believing that Zmodem would be
significantly faster unless the data that is being transfered is
highly compressible with Run-Length Encoding. The RLE compression
is only available in Zmodem-90 implementations and not in the Zmodem
implementations available in most non-Omen Technologies implementations.
: Error Checking/Reliability
: Since Z-Modem does not use CRC routines, transmission errors are not
: easily detected or corrected.
: FTP has full error detection capabilities as well as very good error
: recovery procedures.
Since both FTP and ZModem are being used over TCP/IP, neither one needs
to worry about data corruption (except that Zmodem needs to know how
to properly process data over Telnet protocol or a 7-bit NVT connection
if Bidirectional Binary mode is not negotiated.) The only item that
FTP and Zmodem have to worry about is the connection being dropped
prematurely.
Many implementations of Zmodem protocol support a RESUME option
to restart pre-maturely terminated connections. Some FTP implementations
have similar support but most do not.
Zmodem also has a potential problem with TEXT file transfers that
require RESUME because of the way it handles end of line translation.
: Additional Purchasing
: Z-Modem is not part of the standard Sun Solaris Operating System. Thus,
: it must be purchased from some third party vendor.
: FTP is part of the standard Sun Solaris configuration.
Public domain implemenations of Zmodem are available from the
Free Software Foundation although the versions from Omen Technology
are more feature complete.
: Customizing for GPS Needs
: For the GPS system, Z-modem must be customized so that there is some
: indicator that the file upload is complete and processing can proceed.
: In order to do this, source code must be available.
: FTP must also be customized in this way. Oasis customized the FTP
: daemon so that the file will be transferred to a processing directory
: when the file upload is complete.
Zmodem source code may be licensed from Omen Technologies.
: Connection
: In order to use Z-Modem, the user must dial into the UNIX box and be
: connected as a �real� UNIX user. The user can then initiate a Z-Modem
: protocol upload. Some sort of server software which allows terminal
: access is necessary on the UNIX box.
: In order to use FTP, the user connects to the PPP server and then
: launches an FTP application. This application is connected as a FTP
: user.
Zmodem can be used over a PPP link via Telnet or Rlogin just as is done
with FTP. If you are attempting to compare Dial-Up connections to
PPP links then that is the most likely cause for the discrepencies
of your comparisions. PPP adds a great deal of overhead. On a dial-up
connection there is no additional overhead. So Zmodem will be faster.
However, if you are going over a dial-up connection then everything
I said earlier about error correction goes out the window. On a dial-up
connection Zmodem will have to use CRC checks to detect errors. If
it does not, and an error occurs, Zmodem has to wait until the entire
file is transfered before it can detect the error and then rollback.
This means that even one error will erase all the performance benefits
you may have gained.
The "user" is the same regardless of whether a dial-up or FTP
connection is made. One is given shell access the other is not.
That is the only difference.
: Security
: Since the user is connected as a �real� UNIX user, there are fewer
: restrictions that can be placed on them.
The user is given a shell. The Shell can be replaced with anything
that you wish.
: Since the user is connected as a �ftp� UNIX user, there are many
: restrictions that can be placed on the account. For example, the ftp
: account can be restricted such that they can only put the file on the
: system. They will not be able to �get� files, change from their
: directory, list files, etc.
: Also, this method allows the use of 2-levels of authentication. That
: is, the user must sign into their PPP account and then into their ftp
: account. Both of these accounts could have different passwords (or even
: different User ID�s) and the PPP account could use CHAP for password
: authentication (password not sent �in the clear�).
Since the PPP connection can be made from anywhere you can't trust
any aspect of the PPP authentication when the FTP authentication
takes place.
: UNIX Support
: Since �real� UNIX accounts need to be created, a UNIX system
: administrator is necessary to create and manage/maintain the UNIX user
: accounts.
: �FTP� UNIX accounts can be generated automatically.
Since the accounts are one and the same there is no difference in
administrative overhead.
: PC Support
: Z-Modem is much easier to support as it is just a terminal connection.
: That is, no TCP/IP set-up is required.
: FTP is more difficult to install and support since it requires Win95
: dial-up networking to be configured.
I would agree that Dial-Up Networking is difficult to automate
on Win95. There is no scriptable way to connect and disconnect
a PPP connection.
The ease of use of the terminal connection is really determined
by the application that you are using.
: General
: Considering the speed and error issues, Z-Modem should only be used when
: the file is large and can be easily resent. In our case, the maximum
: size of a single transaction is only 150 kb and we expect a frequency
: of � transaction per merchant per day.
: FTP is better when file stability is necessary and speed is not an issue
: as is true in our case. FTP is also preferable in terms of security,
: availability and support on the UNIX side. The biggest drawback to FTP
: appears to be the PC support issues.
These conclusions are unwarranted. The comparisons are not equivalent.
---
The decision you have to make is whether or not you want to use
Dial-Up Networking to make a PPP connection. If the answer to that
question is 'yes' then you need to compare FTP, Zmodem, and Kermit
over the PPP connection for performance, reliability, and automation.
If the answer to the question is 'no', then you have to compare
various dial-up packages which implement Zmodem and Kermit for
performance, reliability, and automation.
I believe that regardless of which way you decide that C-Kermit for
Solaris and Kermit 95 for Win95 will provide you with the best
solution for your project.
See http://www.kermit-project.org/ for details.
Jeffrey Altman * Sr.Software Designer * Kermit-95 for Win32 and OS/2
The Kermit Project * Columbia University
612 West 115th St #716 * New York, NY * 10025
http://www.kermit-project.org/k95.html * kermit-support@kermit-project.org