[ih] Exterior Gateway Protocol

Wed Sep 2 14:10:39 PDT 2020

Craig et al.,
   1987 was an interesting year.  The NSFnet-related regional networks 
were connecting new sites and had to solve interesting routing problems.
   EGP would tell you that a given network was reachable via a given 
gateway router / AS.  But that network might be reachable via two 
different gateways.
   During that era, our "Sesquinet" regional network in Texas was 
connected to two different valuable "backbone" type networks: the 
Fuzzball-based proto-NSFnet via Boulder and the ARPAnet via Austin. 
Both were useful.  Both had 56kb/s circuits.  Both were congested.
   At that time, there were a few hundred networks, and I recall going 
through the whole list, one by one, figuring out whether, if they were 
reachable via both the ARPAnet and the proto-NSFnet, which should be 
preferred.  I considered it worth doing, but it was of course Quixotic, 
given the rapidly growing number of connected networks.

   Phill Gross asked me to lead an "Interconnectivity Working Group" 
within the IETF and work to solve some of these problems.  It was a 
great group, with members from the ARPAnet, NSFnet, NASA, ESnet, and 
other communities.  Understanding the various "backbones", their 
differing technical and policy natures, and the limitations of EGP all 
made for fascinating discussions, which did allow us to make progress.
   One of our explicit marching orders was *not* to invent a new protocol.
   But we did discuss the problem of how to choose which of two exterior 
routes to use when both advertised a given network.
   One line of thought was to consider that the Internet, while not 
having a "tree topology", did have a notion of levels of hierarchy, 
e.g., campus, then regional, then national backbone, then international 
links.  I am grateful that we fairly quickly realized that relying on 
that notion of hierarchy would be building on sand.
   But what kind of "metric" could be used to help make routing decisions?
   One idea, based on Cicso's IGRP protocol was to characterize each 
transit network with a bandwidth and a delay metric.  Then minimum of 
bandwidth along a path and sum of delay along a path could be used. 
That did not get traction.
   I'd been teaching a computer scientist where an idea called the 
"zero, one, infinity rule" was discussed.  As applied here, if a single 
scalar number would not suffice as a metric, then allow a metric of 
variable length.  For example, a whole AS-path could be an interesting 
kind of metric could be used.  But, particularly during that era, 
variable-length fields in protocols were not in favor, and we did not 
pursue this idea.
   Except that, at our next working group meeting (at the January 1989 
IETF meeting at UT-Austin), Kirk Lougheed (Cisco) and Yakov Rekhter 
(IBM) invented BGP in one of the Internet's most famous napkins.  The 
complete AS-path, variable though it was, was a key idea.
   BGP solved several problems with EGP.
   I should emphasize that the AS-path was never exactly regarded as a 
"metric", but it provided valuable information.  I'll avoid going 
further with the evolution of BGP, but it was so much better than its 
predecessor and came along at a very fortunate time.

	-- Guy

On 9/2/20 3:57 PM, Craig Partridge via Internet-history wrote:
> There was a SIGCOMM '87 paper by Mills and Hans-Werner Braun that discussed
> what happened when you tried to break the tree topology.
> 
> Craig
> 
> On Wed, Sep 2, 2020 at 1:52 PM Scott O. Bradner via Internet-history <
> internet-history at elists.isoc.org> wrote:
> 
>> I found a printout of my late 1990s notes which said that EGP was "limited
>> to a tree structured Internet"
>> although I recall that people were hacking it to expand its usefulness but
>> the result was not pretty
>>
>> Scott
>>
>>
>>> On Sep 2, 2020, at 3:45 PM, Barbara Denny <b_a_denny at yahoo.com> wrote:
>>>
>>> That is my recollection too. EGP had topology constraints.
>>>
>>> barbara
>>>
>>> On Wednesday, September 2, 2020, 08:07:37 AM PDT, Scott O. Bradner via
>> Internet-history <internet-history at elists.isoc.org> wrote:
>>>
>>>
>>> my recollection was that EGP could not be twisted enough to be able to
>> deal with the actual
>>> topology that was evolving on the Internet
>>>
>>> Scott
>>>
>>> ps - I tried to open an old powerpoint presentation (from the late
>> 1990s) where I discussed EGP & BGP
>>> but it seem like the oldest version of PowerPoint I have had evolved
>> enough that it will no longer open
>>> that version  - I mention that because there is yet again a discussion
>> on the IETF list about RFC formats
>>> and some people have argued, as people have argued for at least 20
>> years, that moving to MS Word
>>> would be a good thing
>>>
>>>
>>>
>>>> On Sep 2, 2020, at 10:39 AM, Grant Taylor via Internet-history <
>> internet-history at elists.isoc.org> wrote:
>>>>
>>>> On 9/2/20 7:55 AM, Dan York via Internet-history wrote:
>>>>> Grant,
>>>>
>>>> Hi,
>>>>
>>>>> It also needs more explanation that EGP was replaced by BGP. (The
>> current sentence there says “essentially replaced” and is a bit vague with
>> no references.)
>>>>
>>>> Hum.
>>>>
>>>>> If any of you all here know of any RFCs that explicitly indicate EGP
>> was replaced/obsoleted, or if you know of any journal articles, academic
>> papers, historical documents, etc., that could be useful, I would be glad
>> to update the article a bit. Or if you can point me to any info about the
>> creation of EGP (there’s a line that needs a source). Or any other info you
>> think would be useful in this Wikipedia article, that would be great.
>>>>
>>>> I found a some information about EGP in and around the gated routing
>> daemon.  I wonder if there might be some more information that could help
>> you.
>>>>
>>>>> (Note that for info to appear in the English version of Wikipedia, it
>> needs to be backed up by a “reliable source” -
>> https://urldefense.com/v3/__https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources__;!!KwNVnqRv!VGbu8UR1ds9tLcdTmqj_BlSaJwdAR7VMX3YliVLEE7V0gQyXuH0Pp6ld3QEI5A$  - which includes
>> journal articles, academic papers, news articles, RFCs, etc.)
>>>>
>>>> I wonder if you can find any graphs on the number of Internet
>> connections using BGP.  If similar ever existed for EGP, you can probably
>> compare / contrast the two.
>>>>
>>>> I also think that the lack of contemporary EGP implementations is
>> evidence of BGP's replacement of EGP.  As is the fact that BGP supports
>> things that EGP does not.  Things which are used all over the Internet,
>> e.g. multipath.
>>>>
>>>>
>>>>
>>>> --
>>>> Grant. . . .
>>>> unix || die
>>>>
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> 
> 