Subject: Re: Reminder: 2.5g3g wg last call ends Friday

From: Mark Allman <[email-masked]>
Date: Thu, 19 Jul 2001 10:29:50 -0400

I was planning to say something about ...

> These rah-rah-rah capsule descriptions of WAP, i-mode and Ricochet
> are pointless, less than what you'd find in many trade rags, and
> would be out of place in any RFC.

but Lloyd beat me to it.  

An RFC is no place for advertisements, which is precisely what these
sections of the document are.  I raised this issue the last time I
read this document and was told that these were supposed to a sort
of "applicability statement".  I did not respond as I wanted to wait
and see if the document was altered at all.  But, I still feel the
same way.  These sections are essentially marketing.  If you want to
list the technologies that the document is applicable for just put a
very simple list in section 2 and be done with it.  In my opinion
these sections **really** need to be removed.

> Eleven named authors and not one can write clear RFC-quality
> English or run a spelling or grammar checker?

***HUM***

allman

---
Mark Allman -- BBN/NASA GRC -- http://roland.grc.nasa.gov/~mallman/

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From: Joe Touch <[email-masked]>
Date: Thu, 19 Jul 2001 23:03:53 -0700

Mark Allman wrote:
> 
> An RFC is no place for advertisements, 

Beyond that...

I was hoping to see a summary of the link characteristics of 2G/3G,
and a discussion of how the current PILC recommendations need 
specific tuning to this environment. Short of that, the rest of the
discussion seems moot. There is a small amount of this discussion
in the i-mode section, but the others are sorely lacking in this regard.

Other nits:

TCP needs large windows. 3G has large BDP. Let the other
shoe drop - tell us what the BDP _IS_. Also, TCP with support for
large windows is already recommended.

TCP needs large initial windows. But the common implementation
doesn't hold the SYN-ACK (as delayed), and the SYN-ACK opens the window
to 2 anyway. The issue of the two-window start seems specious as a result.

The use of 'split connection' TCP is purported as efficiency-enhancing,
without discussing the E2E semantics implications.

Joe

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From: gabriel montenegro <[email-masked]>
Date: Tue, 31 Jul 2001 19:08:23 +0200

we are currently modifying the draft according to the various comments received during last call.
a significant modification is that we have taken out section 3.2 and replaced it with a much
shorter applicability statement within section 1. we're also incorporating other changes
suggested on the alias. below, please find the suggested new abstract and sections
1 and 2. remaining sections are being modified by others.

tnx,

-gabriel

---------------------------------------
Abstract

   This document describes a profile for optimizing TCP over 2.5G/3G
   wireless networks. We describe the link characteristics of 3G
   wireless by using W-CDMA (Wideband CDMA) as an example. We then
   recommend TCP optimization mechanisms. We also present examples
   of wireless internet services and standardization activities that
   potentially will deploy the profile described in this document.

1. Introduction

   Much development and deployment activity has centered around
   2.5G (GPRS and EDGE) and 3G technologies (Wideband CDMA and
   cdma2000).  A primary motivation for these is data
   communication, and, in particular, Internet access.
   Accordingly, key issues are TCP performance and the several
   techniques which can be applied to optimize it over different
   wireless environments [1].

   This document proposes a profile of such techniques,
   (particularly effective for use with 2.5G and 3G wireless
   networks), derived from previous work at the IETF [34].

   Two example applications of the recommendations in this document
   are:

   The WAP Forum [14] is an industry association that has developed
   standards for wireless information and telephony services on
   digital mobile phones. In order to address WAP functionality for
   high speed networks such as 2.5G and 3G networks, and to aim at
   convergence with Internet standards, the WAP Forum thoroughly
   revised its specifications. The resultant version 2.0 [20]
   adopts TCP as its transport protocol, and recommends TCP
   optimization mechanisms closely aligned with those described in
   this document.

   I-mode [27], with a huge subscriber base, is a wireless Internet
   service deployed on handsets in Japan.  The new version of i-mode that
   operates over W-CDMA is called FOMA [28]. It deploys the profile of
   TCP described in this document.

2. 2.5G and 3G Link Characteristics

   Link layer characteristics of 2.5G/3G networks
   have significant effects on TCP performance.  ARQ and FEC are
   discussed in [1]. Justification for link layer ARQ is discussed in
   [10], [12].

   This section provides further details on a specific 3G technology,
   namely, Wideband CDMA (W-CDMA).

   W-CDMA (Wideband CDMA) uses RLC (Radio Link Control)
   [3], a Selective Repeat and sliding window
   ARQ. RLC uses protocol data units (PDUs) 336 bits long (including a
   16 bit RLC header). This is the unit for link layer retransmission.
   The SDU (IP packet) is fragmented into PDUs for transmission by RLC.

   In W-CDMA, one to twelve PDUs (RLC frames) constitute one FEC frame.
   The actual size of the FEC
   frame depends on the link conditions and bandwidth allocation. The
   FEC frame is the unit of interleaving.

   RLC uses "status report" type acknowledgments: the poll bit in the header
   explicitly solicits the peer for a status report containing the
   sequence number that the peer acknowledged. The use of the poll bit
   is controlled by timers and by the size of available buffer space in
   RLC. Also, when the peer detects a gap between sequence numbers in
   received frames, it can issue a status report to invoke retransmission.
   RLC preserves the order of packet delivery

   The maximum number of retransmissions is a configurable RLC
   parameter, but it cannot exceed a value of 10. Therefore, RLC can be
   described as an ARQ that can be configured for either
   HIGH-PERSISTENCE or LOW-PERSISTENCE, not PERFECT-PERSISTENCE,
   according to the terminology in [10].

   In general, the link layer ARQ and FEC can provide a packet service with a
   negligibly small probability of undetected error (failure of the
   link CRC), and a low level of loss (non-delivery) for the upper
   layer traffic, i.e. IP. The SDU (IP packet) is fragmented into PDUs
   for transmission. Retransmission of these by ARQ introduces latency
   and jitter to the SDU flow. This results in relatively large BDP
   (Bandwidth-Delay Product). Typical values may range around
   50KB, but may exceed 64KB. [These BDP figures to be completed
   according to the DoCoMo measurements.]

   The above explains why the transport layer sees the underlying W-CDMA
   network as a network with a relatively large BDP and
   jitter. The loss rate of IP packets is low due to the ARQ, but the
   recovery in layer two appears as jitter to the higher layers.