Subject: Re: PILC ASYM: Wording of Motivation (section 3).

From: Gorry Fairhurst <[email-masked]>
Date: Thu, 20 Sep 2001 10:31:08 +0100

I am editing what will hopefully be the last version of the PILC WG
draft on asymmetry prior to the WG last call. 

It has been suggested we restructure section 3 and add text on DOCSIS
and Cable DVB to assist in defining the motivation. 

We have had several contributions of text, thanks to those who
sent text/comments.  

I have tried to these integrate into the ID (see below). 
This is the proposed new text for this section, 
COMMENTS are welcome to me or the list.

Gorry Fairhurst

P.S. I have  tried to reduce discussion of data rates to what 
I hope is a minimum to make it less likely to be outdated by 
advances in modulation, alternative standards variants, etc.

-----

3. Motivation

Asymmetric characteristics are exhibited by several network
technologies, including Hybrid Fiber Coax (HFC) [CR98] and DOCSIS cable
networks [DS00], direct broadcast satellite with an interactive return
channel, Very Small Aperture satellite Terminals (VSAT), Asymmetric
Digital Subscriber Line (ADSL), and several packet radio networks. These
networks are increasingly being deployed as high-speed Internet access
networks, and it is therefore highly desirable to achieve good TCP
performance. However, the asymmetry of the network paths often makes
this challenging. 

Asymmetry may manifest itself as a difference in transmit and receive
capacity, an imbalance in the packet loss rate, or differences between
the transmit and receive paths [RFC3077]. For example, when capacity is
asymmetric, such that there is reduced capacity on reverse path used by
TCP ACKs, slow or infrequent ACK feedback degrades TCP performance in
the forward direction. Even when capacity is symmetric, asymmetry in the
underlying Medium Access Control (MAC) protocol could make it expensive
to transmit ACKs (disproportionately to the size of the ACKs). 

In a wireless packet radio network (or subnetwork), the asymmetry of the
MAC protocol is often a fundamental consequence of the hub-and-spokes
architecture of the network (e.g., a single base station that
communicates with multiple mobile stations) rather than an artifact of
poor engineering choices. In a DOCSIS cable network, upstream capacity
can be limited by the requirement that each node (cable modem) must
first request per-packet bandwidth using a contention MAC protocol
(DOCSIS 1.0 MAC restricts each node to sending at most a single packet
in each upstream time-division interval [DS00]). A satellite network
employing dynamic Bandwidth on Demand (BoD)  is another example of a
system that consumes MAC resources for each packet sent.  In these
schemes the transmission of ACKs may be as costly as transmission of
data packets. These MAC interactions may result in significant
degradation of TCP performance.

Network paths may also be asymmetric because the upstream and downstream
links are implemented using different technologies. An example is an IP
service using a forward satellite link utilising Digital Video Broadcast
(DVB) transmission [EN97] (e.g. 38-45 Mbps), and a slower upstream link
using terrestrial network technology (e.g. dial-up modem, line of sight
microwave, cellular radio) [CLC99]. Another set of examples may be found
in military networks [KSG98] providing Internet access to in-transit or
isolated hosts [Seg00] using a high capacity downstream satellite link
(e.g. 2-3 Mbps) with a narrowband upstream link (e.g. 2.4-9.6kbps) using
UHF/DAMA or Inmarsat satellite links. In DOCSIS/HFC cable networks, the
analogue channels assigned for upstream communication may be narrower
and more noisy than those assigned for the downstream; as a consequence,
the upstream and down stream links differ in their transmission rate and
modulation format. Cable service providers typically therefore offer a
downstream transmission rate (e.g. 27-52 Mbps) which is much higher
(e.g. of 2-200 times) than that used for the upstream direction. The
asymmetry in capacity of these network paths can require mitigations to
provide acceptable overall performance.

The asymmetry in capacity may be substantially increased when TCP ACKs
share the available upstream capacity with higher priority traffic (e.g.
support of Voice over IP using the Unsolicited Grant service in DOSCSIS
[DS00], CBR cells in ATM over ADSL, or IP QoS link scheduling).  When
upstream capacity is shared by a number of end hosts, the asymmetry may
be mitigated in some cases by using multiple upstream links for each
downstream link.

Despite the technological differences between capacity-dependent and
MAC-dependent asymmetries, TCP performance suffers in both these kinds
of network paths for the same fundamental reason: the imperfection and
variability of ACK feedback. This document discusses the problem in
detail and describes several techniques that may reduce or eliminate the constraints.

-----

Please tsend comments to:
[email-masked]
[email-masked]

---

From: Dan Grossman <[email-masked]>
Date: Thu, 20 Sep 2001 14:35:10 -0400

> and Cable DVB to assist in defining the motivation. 
I've also suggested adding some text on ADSL.

> advances in modulation, alternative standards variants, etc.

I can see why you'd want to do that, but I'd advise against it.  The issues 
are rate related.  The assymetry ratios may or may not change in the future 
along with the rates.  At least in the case of consumer Internet access 
technologies (cable and DSL), the size of the installed base will keep the 
current technologies in the field for many years, even despite advances in 
modulation.  My suggestion is to use weasel words like "at the present time" 
or "at the time of the writing of this memo", or to specifically reference 
current technology as examples.
> technologies, including Hybrid Fiber Coax (HFC) [CR98] and DOCSIS cable

Probably some misunderstandings here (I don't have the [CR98] reference).  
First, there are two standards (plus a significant deployed base of 
non-standard stuff) for data over CATV.  The dominant one is DOCSIS, which 
also has a European variant for use over plants with 8MHz downstream channels 
and other differences.  There is a smaller base which uses the DAVIC 1.2/DVB 
standard.  I believe the issues apply to both the standards, and at least to 
the proprietary systems that I'm familiar with.  Second, both DOCSIS and DAVIC 
1.2 can run over purely coax systems, as long as the reverse path is enabled;  
in practice, transmission performance over long cascades is usually poor, but 
it is occasionally done (leading to worst case assymetries).  More proper to 
say CATV rather than HFC. 
> networks [DS00], direct broadcast satellite with an interactive return
<<snip>>
> poor engineering choices.

First,  "poor engineering choices" is extremely prejudicial.  In many of these 
cases, asymmetry is due to dealing with existing constraints in the best way 
possible.  Second, the common thread is that in these centralized 
architectures, burst mode operation at the physical layer and contention 
access at the MAC layer consume a significant portion of the total capacity of 
the upstream channel.  Contention access also has significant latency.  
> In a DOCSIS cable network, upstream capacity
    ^
I don't understand this sentence.  The MAC and below don't care whether a PDU 
is TCP data or TCP ACK.  Are you really trying to say that upstream 
transmission is more costly than downstream transmission?
> analogue channels assigned for upstream communication may be narrower
                                                         ^
                                                         are
> and more noisy than those assigned for the downstream; as a consequence,
      ^
      usually more

> the upstream and down stream links differ in their transmission rate and
                                                       ^
                                                     possible symbol rate (or 
you could say "transmission rate" and omit modulation format).
> (e.g. of 2-200 times) than that used for the upstream direction. The
Well, no, not quite.  They offer the best rates they can get out of their 
plants, divided by the number of active subscriber.  Try:
In DOCSIS 1.0 and 1.1,  rates of upstream data bursts
(including Physical Layer overhead)  can range from 166.4 kbit/s to 9.022 
Mbit/s, while downstream rates are either 26.97 Mbit/s or 38.81 Mbit/s.  The 
rates available in each direction depend upon analog parameters in the cable 
plant.  Various complexities related to burst operation and the MAC further 
add to upstream to downstream assymetry.  

> be mitigated in some cases by using multiple upstream links for each
A few corrections (a bit more than nits):  the problem occurs with TCP ACKs 
over a best effort service with no reserved capacity;  if there's reserved 
upstream capacity, the problem is mitigated or goes away.  Second, the problem 
is really about symmetrical rate services  (like VoIP over DOCSIS unsolicited 
grant service or VoATM over ADSL using CBR VCCs) reserving symmetrical 
capacity on asymmetrical rate transmission systems.  Meaning that if it were 
asymmetrical with the same or lower upstream/downstream assymetry, there 
wouldn't be a problem.  Third, it's incorrect to talk about "CBR cells";  "CBR 
virtual connections" is correct.  Last, I think you meant "When downstream 
capacity is shared..."  And this should be in a separate paragraph. 
> detail and describes several techniques that may reduce or eliminate the constraints.

Here's some ADSL related text:

In ADSL, the available rates depend upon channel quality and system
configuration.  The most widely deployed transmission technology 
[T1.413][G.992.1] provides for point-to-point transport of ATM virtual 
channels at rates which are multiples of 32 kbit/s, up to 6.144 Mbit/s 
downstream and 640 kbit/s upstream.  In order to provide consistent service 
offerings over inconsistent outside plant, most operators offer significantly 
lower rates.  For example,  residential service is typically offered at 1.536 
Mbit/s or 768 kbit/s  downstream, 256 kbit/s or 128 kbit/supstream.  Business 
service offerings are often more generous.