Subject: RE: 2.5g/3g ID revised Bandwidth Oscillation text

From: "Farid Khafizov" <[email-masked]>
Date: Mon, 28 Jan 2002 17:20:58 -0600

Hiroshi et al:

Enclosed please find our revised version of Bandwidth Oscillation text for
2.5g/3g ID.
Mehmet and I feel strongly that the introduction of the draft should have a
statement saying that 
2.5g/3g ID is addressing issues of the networks that have not been widely
deployed. 
Therefore, at the time of writing of the draft we can not know all TCP
issues related to 3G wireless. 
Only time will show what they are and how to address them. Some of the
recommendations
of this text are based on exploratory research.

--Farid

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2.7. Bandwidth Oscillation 

Limited RF spectrum along with high data rate requirement of 2.5G/3G
wireless systems necessitates dynamic resource sharing among concurrent data
users. Various scheduling mechanisms can be deployed in order to maximize
resource utilization. Time division sharing of these resources may result in
TCP throughput degradation. Ideally resources are allocated on per needed
bases (bandwidth on demand) and released when there is no data to send.
However, there could be situations when resources are de-allocated while
significant amount of data is still waiting in the queue to be transmitted.
For example, if multiple users require large data file transfer at the same
time, the system (e.g., the scheduler) may have to repeatedly allocate and
de-allocate resources for each user. In this section we refer to periodic
allocation and de-allocation of high-speed channel as Bandwidth Oscillation.
Bandwidth Oscillation effects such as spurious retransmission were
identified elsewhere (e.g., [17]) as throughput degradation factors. There
are research studies [n3], which show that in some cases Bandwidth
Oscillation can be the single most important factor in reducing throughput. 

One of the ways of detecting congestion in TCP is RTO expiration. RTO
computation algorithm [32] was designed to follow closely round trip time
(RTT), but is known to work poorly when delay variance is high [11]. When a
user has high bandwidth (i.e., low RTT), if resources are allocated for a
sufficiently long time, RTO converges to RTT. When resources are released,
suddenly RTT increases and low RTO expires forcing TCP into the Slow Start
state, while actually none of the TCP segments were lost. 

For fixed TCP parameters the achievable throughput depends on the pattern of
resource allocation. When the frequency of resource allocation and
de-allocation is sufficiently high, there is no throughput degradation.
However, increasing frequency of resource allocation/de-allocation may come
at the expense of increased signaling, and, therefore, may not be desirable
in systems which have interference limited capacity. Standards for 3G
wireless technologies [n1, n2] provide other mechanisms that can be used to
combat adverse effects of Bandwidth Oscillation. It is the consensus of the
PILC WG that the best approach for avoiding adverse effects of Bandwidth
Oscillation is proper wireless sub-network design [11].

In systems that do experience bandwidth oscillation, one can control
throughput degradation by optimizing TCP parameters [n3]. One obvious method
is to adjust computed RTO value (or configure appropriately the minimum RTO
value) at sending TCP. This technique, however, can not be recommended as a
practical solution. Experiments have shown that RTO algorithm implementation
compliant with RFC2988 [32] (e.g., minimum RTO=1 sec and initial RTO=3 sec)
reduce number of spurious re-transmissions. Although RTO timer management
specified in RFC2988 is not mandatory, implementation of retransmission
timer restart when an ACK is received (section 5.3 of RFC2988) will further
reduce (or even eliminate) spurious retransmissions. Secondary effects, such
as TCP segment loss, in combination with Bandwidth Oscillation may not allow
avoiding all spurious re-transmissions.

Analysis of RTO algorithm along with an alternative (Eifel) algorithm are
presented in [17]. Eifel algorithm requires timestamp option and at least
one RTO expiration before TCP "learns" that retransmission was not
necessary. D-SACK option [26] also allows TCP sender to detect spurious RTO
expirations. Enabling timestamp option enables increased RTT sampling which
can reduce spurious re-transmissions due to Bandwidth Oscillation. Other
options that could reduce spurious RTO expirations due to Bandwidth
Oscillation are increase CWND and reduced delay ACK timer at Receiving TCP
to < 100 ms (however, this technique may have side effects in case bandwidth
is limited in the opposite direction).

[n1] 3GPP TS 25.3xx, UMTS MAC and RLC Protocol Specifications, 2001,
ftp://ftp.3gpp.org/specs/latest/R1999/ 
[n2] TIA/EIA/IS-2000.5-A, "Upper Layer (Layer 3) Signaling Standard for
cdma2000 Spread Spectrum Systems", March, 2000 
[n3] F.Khafizov, M.Yavuz, "Running TCP over IS-2000", to appear in Proc. of
IEEE ICC 2002