[e2e] TCP in outer space

Cannara cannara at attglobal.net
Wed Apr 25 10:19:35 PDT 2001


My statement about layer boundaries seems to be misunderstood.  I'm in favor
of organized communication across layer interfaces that do more that just pass
PDUs.  My point is that things like "net-layer drops so transport-layer slows"
are not "organized", but kludged -- if only because drops occur for reasons
other than congestion, particularly in the environments suggested by the title
of this email train.  The original, general networking purpose of transport
timeout, retransmission, backoff, etc. was not managing router queues -- at
the very least because it would depend on a specific traffic mix and therefore
be fragile.

We may think the Internet is working well when 30% of packets through a major
peering point disappear during busy hours due to congestion, but when 30%
disappear on a satellite hop because of a solar flare and our transport does
the exactly same thing, we can hardly feel we've addressed the design of
net-to-transport communication in a well-organized, reasonably-complete way.

On the alternative-ideas front, I suggested example ideas that, say, a
master's student could do a little project on.  I wasn't saying: "I know it
needs to be done this way" -- those are your words and the implied words of
folks who seem to feel no improvements but what have been thought of over the
last 20+ years need trying.  In other words, the parochial "TCP-friendly"

You say "the Internet is a work in progress" and we all agree.  The protocols,
however, have not progressed in the way one could reasonably expect, given the
time and subsidies provided to the bodies involved.  I'm obviously not alone
in this opinion.

As to rigidity of admission to the network, we've had such systems for many
years, as you must know.  Internet and corporate links have depended on them.
They've been more effective in throughput delivery than the result of dropping
30% of packets, which is why providers offer services that go around the
Internet that simply drops and hopes.  However, I'm not advocating rigidity,
simply pointing out that the inadequacies of the "depend-on-drops-and-TCP"
trick are simply being worked around by end-system providers as well as
alternate-backbone providers, with the addition of service for fee.  The
result then is not derived as much from things "The IETF technical community
has fairly thoroughly explored..." (whatever "fairly thoroughly" means), but
an internetwork that provides levels of services that serious customers want
and can decide to pay for.  In other words, an internet that minimizes
customer visibility of the Internet.


"J. Noel Chiappa" wrote:
>     > From: Cannara <cannara at attglobal.net>
>     > then we hear principles like: "thou shalt not cross layer bounds". This
>     > despite whole trains of design and discussion history on using router
>     > code, that assumes something about a particular transport, to delete
>     > frames and thus trick far end transports into backing off, just so the
>     > near router code can hope to reduce its filling queues -- how many
>     > layers are involved here?
> First, you're reading the "layer boundaries" rule far too simplistically.
> See, for example, RFC-817, "Modularity and Efficiency in Protocol
> Implementation".
> Second, in this particular instance layers are being perfectly respected. The
> current service model (i.e. the abstract model presented to higher-level
> clients) of the Internet layer is quite simple in how it deals with
> congestion, and how it signals congestion to its clients: it silently drops
> packets. That's the only signal the higher layers currently get.
> Higher layers which are potentially sending a lot of traffic MUST (I don't
> have the appropriate RFC number at hand, but it is some sort of standard) be
> able to detect packet losses and deal with the case in which those losses are
> due to congestion, and back off their sending rate. But that all happens at
> the higher layer.
> You may not *like* that architecture, but it's one we've evolved to, and one
> that works pretty well (but by no means Really Well) *in practise*, as part of
> *an overall system architecture*.
>     > As for comments on "conservative design", I'm always in favor of that,
>     > yet we open a vast network to partial control, which actually generates
>     > control inputs of the wrong polarity on occasion, and we assume it's ok
>     > ... I'd say luck, rather than engineering conservatism, is in force
>     > here
> First, the world is full of large networks with the characteristic that they
> have less-than-optimal control systems - and far more severe penalties when
> those control systems malfunction. Try looking at the road network in most
> major industrialized countries. Somehow we survive.
> The Internet's congestion control mechanisms actually work pretty well, thank
> you - in part because of clever design which informs the characteristic that
> if people try and get more than their "fair share", they almost always
> actually get less.
> Which is not to say they couldn't be improved further - I think most people
> can see potential further improvements - but the situation is no where near
> as dire as your comments above would indicate.
> Further, if I'm hearing you correctly, you seem to feel that authoritarian
> rate limiting is the only answer - and therefore by definition anything else
> (such as our current discard-based mechanism) is wrong. Please don't be
> surprised if many (most?) of the rest of us don't agree with you.
> Second, the Internet is a work in progress, and there are large parts of it
> which bear a strong resemblence to Swiss cheese. Congestion control is, to my
> mind, rather far along, compared to, say, routing and addressing.
> (Those who've been at meetings with me will no doubt have amusing
> recollections of the clouds of steam that come out of my ears when people
> doing long-range planning speak of tweaking the current system, as opposed to
> "throwing it with great force" [as Dorothy Parker said] in the trash; lock,
> stock and barrel. :-)
> So, again, your complaints about the awful state of congestion control are
> likely to fall on fairly deaf ears.
>     > The fact is that network processors are now available that have (per
>     > chip) >500,000,000 RISC cycles available per second .. (corresponding
>     > to about 2M packets/sec).
> Still nowhere near fast enough to deploy in the core of the network -
> although we can do some traffic shaping at the edges. But this is something
> that's already being worked on in the IETF.
>     > Since basic RFC1812 routing takes <200 such cycles per packet
> No high-performance router that I know of uses software forwarding on a
> general purpose processor. Everyone's gone to specialized ASIC's.
>     > router/switch code can do far more these days than ever imagined when
>     > the decision to offload performance and capacity decisions from
>     > 'gateways' (routers) was made years ago.
> I'm a bit confused by this version of history. It doesn't at all correspond
> to the one I recall living through.
> The decisions on where to put various functions was *not* primarily driven by
> efficiency/switch-processing-capacity. Those decisions were principally
> driven by things like i) the end-end principle (the version in "End-End
> Considerations in System Design", not the *other* end-end principle, which
> seems to be about global names, not tweaking packets, etc), ii) robustness,
> iii) the ability to work over a fairly wide range of technologies, with
> varying underlying capabilities, etc, etc.
> For example, if you have a network technology where *you can't tell if the
> network is congested*, and *the hardware silently drops packets when
> congetsion occurs*, so that when a shared link congests *the switches cannot
> detect it* (and yes, we had some of those), then building a congestion
> control system that *depends* on the switches noticing, and signalling,
> congestion is clearly not going to work.
> (Not that we really had a lot of problems with congestion in the early days,
> otherwise the poor TCP congestion control would have bit us sooner than it
> did - but I digress.)
> Your guess about switch processing power is exactly back to front, as a
> matter of fact. When the architecture was being set, in the late 70's, it was
> often taking about 10K instructions to switch a packet, mostly because of OS
> overhead. When I got it down to around 1K packets (and in a higher-level
> language to boot), that was a big step. The most common network technology,
> ARPANet, had all sorts of wierd back-pressure stuff which meant you had to
> keep *all sorts* of fancy meters, queues, etc. Putting in the better output
> queue management needed for switch-based congesion control would have been a
> cost so incremental it would have been in the noise.
> The reason we didn't do it was i) because we didn't have anything like the
> understanding of, or focus on, congestion control that we do now, and ii) to
> the extent we *did* focus on it, we preferred (for architectural reasons) to
> do in the hosts. We did throw in a signal (ICMP SQ) for the routers to notify
> the hosts of congestion. However, at that point we still didn't know whether
> closed-loop congestion control was even feasible! (Some thought traffic
> levels would vary much faster than the RTT, making any closed-loop congestion
> control infeasible.)
>     > So, for example, rather than simply using the hardware RED capability
>     > now available to drop packets, use it to generate a more intelligent
>     > control statement to the sender.
> The IETF technical community has fairly thoroughly explored the topic of
> congestion control and how to do it over the years. Some things have been
> tried and discarded, others have been tried and adopted, and still others
> are being experimented with now.
> Flat assertions that "I know it needs to be done this way" are therefore
> likely to have very little impact - and, indeed, likely to merely annoy
> people.
> Particularly when it's clear, as above, that the person making the statements
> has a very poor understanding of what has been done, and why.
>         Noel

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