[ih] (LONG) - Separating TCP and IP

Ian Peter ian.peter at ianpeter.com
Thu Sep 30 23:36:11 PDT 2004


Some of you will be aware of the Internet history site I am
running at www.nethistory.info. It's a portal, and essentially
aimed at less technical people. But one of the things that comes
up continually is suitable archives and more in depth material,
so it was interesting to see the recent discussion on this
subject.

Meanwhile some people here will be interested in the following
email exchange with Vint Cerf, Bob Frankston and David Reed, on
the subject of early tcp and ip separation. I'm reproducing it
here with the permission of the participants. I'm sure some
people here will want to add comments,and I am sure the record of
conversation will be of interest to researchers.

It's fairly long. I've edited the content a bit to get to the
original discussion sequence. 

Enjoy!


Ian Peter

(Bob Frankston)
While I am biased, I notice that the histories rarely mention
David (Reed of course -- there are too many Davids though David
Reed is not exactly unique either) despite his role in the
end-to-end concept and the related separation TCP from IP. You
can ask the other participants in your list for their comments on
this.

A number of the key concepts could be found in the design of the
Ethernet though they arose more from imitating the AlohaNet than
because of an explicit end-to-end design point. What makes it
interesting is that it the idea of putting a reliable transport
didn't even come up. I remember sitting in class when Bob
(Metcalfe - too many Bobs?) spoke about his project. Of course
the computers, not the network, would take care of assuring that
the message got through. The insight was in making this an
explicit design point of the Internet since the big Arpanet had
separate computers - the IMPs - that could perform all sorts of
magic including assuring a reliable transport.

The defining concept of the Internet is the / between TCP and IP.
Writing it as TCP/IP is unfortunate since they are very far
apart. UDP/IP might be more appropriate though I just talk about
TCP or IP depending on which is appropriate.


(Vint Cerf)
Hi Bob
  
As I recall, David was certainly a proponent of the end/end
philosophy that drove TCP. He also thought that we should use
random sequence number (64 bit?) rather than the Initial Sequence
Number mechanism based on clocks - I was not in favor of the
random method but it had the advantage that you didn't need a
quiet time. I was uncomfortable with the potential for ISN
collision though. I don't recall David's role in arguing for the
split of IP from TCP - what I remember most vividly is Danny
Cohen's argument for the need for a service that was fast if not
reliable or sequenced (to handle real-time traffic such as
voice).
 
David, can you fill in blanks here?
 
vint
 
 
(David Reed) 
I can fill in some blanks about splitting TCP from IP.   There
were a small number of proponents for specifying the Internet
protocols as based on datagrams, with the idea of reliable
in-order delivery being an "application layer" mechanism.   This
all came together at the meeting I attended in Marina del Rey.
John Schoch, Danny Cohen and I each 
presented arguments to that effect, from different points of
view. John argued based on the PUP architecture, which was an
architecture based on datagrams, where streams were one option
among many.

Danny argued for the idea that packet speech did not want
retransmission, but instead just a sequence-numbered stream of
packets, where non-delivery was an option because latency was the
key variable and the application could fill gaps.   I argued that
non-connection based computer-computer protocols, such as those
we were developing for interconnected LANs, could be more
general, and that end-to-end reliability was ultimately up to the
endpoints to assure - for example, there were useful protocols
that involved a packet from A to B, handing off of the request
from B to C, and a response back from C to A were quite useful,
and would be end-to-end reliable at the application level, while
gaining little or no benefit from low level "reliability"
guarantees.   Other protocols, such as multicast, etc. were
essentially 
datagram-oriented.   I remember arguing quite strongly that you
could support streams on top of datagrams, but by requiring a
streams, you'd never get effective or efficient datagram
services.   Danny equally argued that reliable streams would
create latency variability (jitter) where none was necessary.
John Schoch argued that PUP was datagram-based, with streams
built on top, and that architecture was quite effective.

The idea of the large, randomly chosen connection
identifier/sequence number was a part of the technique used in
the LAN-centric internetworking protocol "DSP" (Datagram/Stream
Protocol, or Dave's Simple Protocol) I had developed for MIT to
allow for both datagram-like and stream-like behavior to coexist,
because the connection identifier would minimize the problem of
collisions in sequence space so that one need not do a 3-way
handshake to authenticate an incoming message.   But this was a
part of a more general argument I made that related to how one
could achieve idempotence in protocols - the 3-way connection
setup handshake in TCP was a way to prevent delayed duplicate SYN
requests from having a non-idempotent effect, i.e. doing the same
command twice by accident.  In many cases, I argued, idempotence
is best done by the application layer, since the command itself
is idempotent (i.e. the "compare and swap" instruction in a
processor is inherently idempotent, because a delayed duplicate
command will not compare correctly).   This was one of the first
end-to-end arguments, i.e. an argument that the function
(idempotence) should not be done in the network layer but at a
higher layer, and began my conversation with Jerry Saltzer, since
he was my adviser at the time.

As I recall, we 3 people, plus Steve Crocker, conspired to argue
that we needed a datagram service so we could continue to do
research on more general protocols, and in the heat of the
argument proposed why not split out the addressing layer from the
stream layer, just as we had split the TCP from the functions of
the Telnet layer.   (you may remember that I also was involved in
changing the interaction of the TCP/Telnet layers to use a
byte-oriented "urgent pointer" that was a pointer into the stream
of bytes, rather than a general "process interrupt" mechanism
that was being proposed, which was problematic because it
embedded operating system process semantics into the network
layer).   In the heat of the Marina del Rey meeting, we 3
(sitting next to each other) agreed to push for splitting the TCP
packet into two 
layers, the routing header and the end-to-end stream payload.
This resulted in a sidebar meeting in the hall during the next
break, where I remember it was Jon, you, Danny, me, Steve, and
John Schoch, and you agreed that we should try defining how we'd
split the layers, and see if the overhead would be significant.
This resulted in 3 protocols, IP, 
TCP, and UDP (for us datagram nuts).   Danny went off being happy
that he could define a packet speech protocol layered on UDP, I
went off happy that I didn't need to pursue DSP anymore, but
could focus on how to use UDP for protocols like TFTP, which we
built at MIT shortly thereafter.


Vinton G. Cerf wrote:

>Dave,
>
>Thanks a million for this rendering - I had not recalled your 
>involvement in the MDR event so this fills in an unintentional
blank in my own recollection of this passage in Internet history.
>  
>

(David Reed)
Glad to be helpful.   As a young graduate student, this
particular meeting was at the core of my interest, so the details
are vivid in my memory.  It was one of the best groups that I
have ever worked with with intense argument, but a focus on
constructive results.   This particular interaction taught me a
lot about groups, about protocol design, etc.

Too bad there are so few opportunities out there for young
students to participate in such processes.   The "open source"
community is the one area where that is still happening, and it's
sad that DARPA, the current IETF, and industry don't get how to
harness such things.


(Reed) 

Pouzin was really arguing for packet networking, not free
datagrams, sans connections.   X.25, which only had streams, but
was a packet network, was a descendant of Pouzin's work.
I think he invented the term "virtual circuit", which illustrates
where his head was at.  He couldn't break free from the "circuit"
idea, though he liberalized it a lot.   This is where Gallegher
and the folks at 
today's LIDS got it wrong, too.  They defined a network as a way
to multiplex a bunch of streams/circuits onto a more efficient
infrastructure.   They were trapped by their mindset and their
model of what a network was.

The real issues that bogged people down in those days were "flow
control" and "reliability", whose very definitions (at least the
definitions use by the networking community) were defined in
terms of connections or stream or circuits.  In fact, the term
"congestion control" appears in no literature before the Internet
design time-frame, because it was subsumed into "flow control".

Though it wasn't the best idea, the idea of the "source quench"
message was invented to provide a crude mechanism of congestion
control that could deal with datagram-only connections, and the
current TCP windowing system was initially understood as only
serving an end-to-end flow control function.   So inventing
"source quench" was a way of breaking a 
controversy that surrounded the idea that gateways between
networks would have no way to signal buffer overflow back to the
sources.   It was only later that the community (Van Jacobsen
played a role, I think) invented the idea that packet drops could
serve as the sensor that closed the control loop about congestion
with the source.

One MUST remember that we were designing the Internet protocols
as*overlays* to interconnect existing networks with heterogeneous
designs, but which had internal congestion control and
reliability.   This 
allowed the Internet protocols to focus on the big picture - the
"end-to-end" functions.

The key goal was to make the gateways simple and stateless and
stupid (that was the "best efforts" insight that Vint and Bob
Kahn brought to the world, which was their unique contribution.
Bob often said to me that I was one of the few who really got
what he meant by "best efforts").    Many people want to rewrite
the history as if the Internet 
Protocols were designed to be implemented directly on the wire,
handling in a single layer all of the problems of wireless,
wired, multicast, ... transmission.   In fact, the engineering
approach was to accomodate MANY 
DIFFERENT solutions at the lower layers, and to encourage
heterogeneity.   Which is why I and John Schoch shared a very
different viewpoint from the BBN guys, which was different from
Danny Cohen.   
John and I were "LAN guys" who were working on networks that were
inherently message-oriented, computer-computer, multicast
technologies.   We wanted those advantages to shine through to
applications, not turned into virtual dialup telephone calls at
teletype rates.   Danny was a media carriage guy, who was
interested in conversational packet voice over unreliable
channels.

To design a protocol with that diversity of undercarriage
required us to resist all sorts of attempts by the "Router Guys"
(like Tomlinson and McQuillen) to "optimize" for the same old
ARPANET constraints (50Kb/s private lines connected by IMPs).
Cisco's heritage is as Router Guys, and too many of the IETF guys
these days are Router Guys.  They are the new "bellheads".

(Cerf)
David, I think there is something incorrect about your rendition
regarding Louis Pouzin.

Louis was the datagram guru. The other French guy was Remi
Despres and he was the one who did the Reseau Communication Par
Packet - a predecessor to X.25.
The latter was developed jointly by Remi, Larry Roberts/Barry
Wessler/Dave Horton/and John Wedlake. When Larry Roberts was
building Telenet he asked what protocols to use and I suggested
TCP/IP but he rejected that claiming he could not sell datagrams
and that people would only buy "virtual circuits" (sigh). 

Virtual Circuits were never in Louis' network world - he was all
datagrams until you got to the end/end transport layer and there
he introduced the voie virtuelle (virtual circuit) - not unlike
TCP over IP. When another of Louis' team, Hubert Zimmerman, wrote
the first OSI architecture spec, I think he had X.25 in mind as
the network layer with virtual circuits built in. When I "called
him" on it, he said he could not sell datagrams to the rest of
the OSI community - but thought AFTER he got the X.25-based OSI
specs agreed he might be able to get people to accept a
connectionless addition. Eventually there was a CLNP
(connectionless Network Protocol) but it was never widely
implemented - nor was most of OSI except for X.400 I suppose.

Remi's work had the VCs built into the network while we and Louis
preferred a datagram interface and service. Oddly enough, the
ARPANET had a datagram interface but an internal virtual circuit!

Your comments about best efforts and the use of gateways to link
networks with very different characteristics are spot on - we
lost that in many respects when Cisco started routing IP directly
and not encapsulating in anything other than PPP or the moral
equivalent or over ethernet. I really liked the indirectness of
encapsulating IP in a network layer.




(Reed)

Vint - you are closer to Pouzin than I ever was, so I could very
well be wrong.  I don't remember him ever advocating pure
best-efforts datagrams without circuits at the top layer, all I
remember was that he pointed 
out that circuit functions were better done on top of datagrams.
But his normal audience was the people who used circuit
abstractions (comms people), so that might have shaped the
context so he would never have talked about pure datagrams as a
useful capability to have.   I should probably go back and read
some of his stuff again, because I haven't read that literature
for 25 years, and it has a way of blurring together into the OSI
world, etc.

Larry Roberts has always resisted the idea of moving functions
out of the network, since I've known him.   I remember arguing
with the Telenet people repeatedly that they should think more
about internetworking and less about being a homogeneous and
optimized network for terminal traffic.   They just never got the
"Internet" idea - they were just 
doing the ARPANET again.   They were competing with Tymnet and
SNA, and seemed to think that they should move towards them.



(Cerf)

David,

You may be correct that Louis always included VC on top of DG -
he kept the VC at the edge however and not in the net. I am not
sure he had applications that were purely datagram in nature -I
have some old slides of his in my files from the1970s and if I
ever get to it I will unearth them (I am in Hong Kong at the
moment and in the middle of three weeks of travel).

This is a great interchange so I'm archiving it - hope that's ok
with all of you

Vint

(permission to post this discussion obtained from the
participants)



Ian Peter
Senior Partner
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