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

From: "Lai, Wai S (Waisum), ALSVC" <[email-masked]>
Date: Thu, 20 Sep 2001 08:45:05 -0400

Gorry,

>>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
>>S00], CBR cells in ATM over ADSL, or IP QoS link scheduling). 

I think the above capacity reduction may or not affect TCP performance,
depending on the relative proportion of voice and data users.  Therefore,
careful traffic engineering of upstream capacity for different services
should be a prerequisite for the techniques described in this draft.  I
propose adding the following sentence after the above sentence.

"In this case, careful traffic engineering is required to ensure that
upstream capacity is adequate for the lower-priority data traffic
[ITU01]."

Reference [ITU01] ITU-T Recommendation E.681, "Traffic Engineering Methods
For IP Access Networks Based on Hybrid Fiber/Coax System," September
2001.

(I would be happy to send you separately a copy of this document.)

Thanks, Waisum.

-----Original Message-----
From: Gorry Fairhurst [mailto:[email-masked]]
Sent: Thursday, September 20, 2001 5:31 AM
To: [email-masked]
Subject: Re: PILC ASYM: Wording of Motivation (section 3).

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I hope is a minimum to make it less likely to be outdated by 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: Gorry Fairhurst <[email-masked]>
Date: Tue, 25 Sep 2001 14:15:51 +0100

"TRAFFIC ENGINEERING METHODS FOR IP ACCESS NETWORKS
BASED ON HYBRID FIBER/COAX SYSTEM" 
ITU Draft Recommendation E.681.

I've cited the above ITU document in the references, and added a
reference in section 3.

The document seems to focus mainly on how to provision networks
(DOCSIS/HFC) to offer IP services. In some ways this is the 
opposite of what ASYM is trying to say - i.e. the implications
if you can't dimension the network, and we could argue that
guaranteeing sufficient capacity for TCP services is particularly
difficult, given that we don't know how much each session will
send or how bursty transmission will be... Never the less, 
I found the document very relevant to Voice over IP, and
interesting background for TCP over cable networks.

People (who able to get a copy) may also find it useful to look at and
comment on this.

Gorry