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Telebit Corporation           Revision 1.02                01 APRIL 1990
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       TCP/IP, DIAL-UP IP (SLIP), AND THE INTERNET PRIMER

  By Michael Ballard, Director, Network Systems, Telebit Corp.
  Edited by C. E. Castillo, UNIX Communications Specialist, Telebit Corp. 


WHAT IS TCP/IP?

TCP/IP (Transmission Control Protocol/Internet Protocol) is a pair of protocols
developed to allow cooperating computers to share resources across a network.
It was developed by a community of researchers centered around the ARPAnet.
Certainly the ARPAnet is the best known TCP/IP network.  However, as of
June 87, at least 150 different vendors had products that support TCP/IP,
and tens of thousands of networks of all kinds use it.

WHAT IS THE INTERNET?

The INTERNET is a collection of networks, including the ARPAnet, NFSnet,
USEnet, CSnet, regional networks such as NYSERnet, local networks at a
number of universities and research institutions and a number of military
networks.  The term "INTERNET" applies to this entire set of networks. 
The subset of those managed by the Department of Defense is referred to as
the "DDN", (Defense Data Network).  All of these networks are connected to
each other.  Users can send messages from any of them to any other, except
where there are security or other policy restrictions on access.  

Officially speaking, the Internet Protocol documents, known as RFC (Request
for Comments) are simply standards adopted by the INTERNET community for its
own use.  More recently, the Department of Defense issued a MILSPEC definition
of TCP/IP.  This was intended to be a more formal definition, appropriate for
use in purchasing specifications.  However, most of the TCP/IP community
continues to use the INTERNET standards.  The MILSPEC version is intended 
to be consistent with it.

MULTI-LEVEL PROTOCOLS OFFER DELEGATED RESPONSIBILITY

TCP by itself is that portion of the protocol that keeps track of packets by
validation of CRC (Cyclical Redundancy Check), handles re-transmission, and
actually does the "thinking".

IP is that portion of the protocol that moves these packets, however built
and tracked, across the physical medium, usually ethernet.  IP has no error
checking or correcting.  It merely sends or receives packets handing them to
the "higher" protocol for it to decipher.  That is why other protocols can be
implemented "on top" of IP.  In fact, NFS (Sun's Network File System), is a
third layer of protocol that sits on top of UDP (Uni- Directional Protocol,
used in place of TCP), that in turn sits on top of IP.  Each layer has a
descending set of responsibility.

It is key to note that the design of TCP/IP was based directly on connected
hard-wired networks.  Nowhere in the specification is the notion of come and
go, dial-up connections considered.  Ethernet is the common carrier.

SLIP

Serial Line Internet Protocol (SLIP) was developed and distributed for Berkeley
4.3 BSD (Berkeley Standard Distribution) systems.  This program allowed the
network routing code to send any IP packet through a serial port as opposed to
an ethernet port.  The profound difference was performance.  Ethernets run
typically at 10 megabits per second as opposed to typical top speeds of
19.2K bps (bits per second) on asynchronous serial ports.  Nevertheless, it
works.

At 1200 bps, previous modem speeds, performance was so dismal as to discourage
the use of SLIP over dial-up lines entirely.  With the advent of high-speed
modems, dial-up takes on a new life.

TCP/IP - A MESSAGE CARRIER FOR APPLICATIONS

TCP and IP are two cooperating methods that work together to encapsulate data
for passage across a network.  Just like placing a letter in an envelope, the
receiver may discard the envelope after safely receiving the contents.  The
envelope served only to transport the contained information.

Likewise, applications in a network environment become the senders and 
receivers of electronic letters packaged in TCP/IP envelopes.  These 
applications, FTP (File Transfer Protocol), SMTP (Simple Mail Transfer 
Protocol), TELNET (virtual terminal emulation access across the network), 
PING (a "hello" message to any host on the net to validate its healthy 
presence), RLOGIN (similar to TELNET providing remote login to other host 
computers on the net), RCP (similar to FTP providing file transfer to other 
host computers on the net) and many others, all envelope their cross-net 
traffic inside TCP/IP envelopes.

TCP/IP IMPOSES HIGH OVERHEAD

TCP's contribution to the data envelope is 20 bytes of overhead.  Likewise IP 
contributes its own 20 bytes.  SLIP prepends and appends another 2 bytes for 
a grand total of 42 bytes of overhead.  FTP file transfers, today, run well 
with Telebit modems achieving 12,000- 14,000 bps throughput.  This is true for 
all bulk data applications.

However, a problem arises when interactive terminal emulation  applications 
like RLOGIN or TELNET operate.  When running a TELNET or RLOGIN session, that 
is simply typing on a screen to a host somewhere on the network, each character
typed is enveloped in 42 bytes and mailed to the remote machine.  When running 
across a 10 megabit per second ethernet (the medium this whole process was 
designed for) enduring that much overhead is not elegant, however, it is 
functional.  At 9600 bps on a high-speed modem, the overhead is impeding.

TELEBIT MODEM BEHAVIOR - NOT THE BEST FOR TCP/SLIP

Micro-short packets potentially hold about 10 data bytes.  Once the first ten 
bytes of a TCP/SLIP packet fill the first micro-short modem packet, the modem 
builds a 256 byte long packet to house the following plus or minus 32 bytes.  
This inefficiently stuffed packet travels the link consuming unnecessary 
transmission time.  The action of sending packets back and forth causes the 
modem to thrash between micro-short and long packets and the resulting 
interactive TCP/SLIP performance is poor.

These interactive applications currently run with about a 1.2 to 2.0 second 
round trip character echo delay.  This interactive performance is unacceptable.
Nevertheless, many Telebit modem users are today running SLIP.  The resources 
available through TCP/SLIP are forcing them to accept the unacceptable.

VAN JACOBSON'S WORK

Van Jacobson is a research scientist currently at Lawrence Berkeley Labs.  He 
is, without a doubt, one of the world's premier TCP/IP network researchers.  
His recent work with TCP has found its way into the TCP releases from such 
companies as Sun Microsystems, Digital Equipment Corporation, The Wollongong 
Group, Xerox, Cisco Systems, Convex, IBM and SRI International.  TCP has 
continued to evolve as an open systems networking protocol capable of 
efficiently moving data between dissimilar computers.  Much of the work that 
has led to the increased efficiency of TCP/IP belongs to Van Jacobson.

Most recently Van's work has focused on what are known as the header 
compression and prediction algorithms.  These are a set of rules that allow 
TCP and IP to pass single byte tokens between themselves to represent their 
larger volume 20 byte headers.  These rules provide for compressing repetitive 
data and prediction of that portion of the header that changes from packet to 
packet.  The result is a header that leaves the host computer entering the 
modem without the offending overhead.  This packet can fit into a micro-short 
packet and eliminate the thrashing effect of swapping between micro-short and 
long packets in Telebit modems.

Van has been given several Telebit high-speed modems and is currently 
implementing a program that employs the header compression and prediction 
algorithms.  This program will operate on Berkeley BSD UNIX systems like that 
of Sun Microsystems.  Performance with this version of dial-up IP should prove 
to eliminate objections to the interactive use of dial-up IP across Telebit 
high-speed modems.  It is designed to operate asynchronously and will, for all 
intents and purposes, meet the needs of all Telebit users requiring 
"asynchronous" dial-up TCP/IP connectivity.

As of the creation of this document, Van Jacobson's compress TCP/SLIP (TCP/CSLIP)
is in distribution to the Internet on systems such as DECWRL.COM. Announcements
made at the Winter USENIX 1990, indicate that Van Jacobson's TCP/CSLIP has been
included in the BSD 4.4 release.  Official release date(s) were not announced for
BSD 4.4.  The Internet RFC is still in the drafting stages.  Updates may be obtained
from the network newsgroup comp.protocols.tcp-ip.

The introduction of the Point-to-Point Protocol (PPP [RFC-1134]) promises to 
contain TCP/CSLIP in its initial source distribution.  The PPP sources will 
be posted at various archive locations throughout the INTERNET including those 
of UUNET Communications Services (UUNET.UU.NET).  Again, more information may
be obtained from the newsgroup comp.protocols.tcp-ip.

This covers most of the commonly asked questions about TCP/IP, TCP/SLIP, and 
the INTERNET.   If any of the above information is unclear, or you have 
questions regarding other aspects of internetworking and modem technology or 
performance, send mail to:


	Michael Ballard/Cerafin E. Castillo
	Telebit Corporation
	1315 Chesapeake Terrace
	Sunnyvale, CA  94089
        1-800-TELEBIT

            UUCP:     {ames, uunet, sun, pyramid, decwrl}!telebit!modems

        INTERNET:     modems@telebit.com
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