Internet DRAFT - draft-inamura-docomo
draft-inamura-docomo
Internet Engineering Task Force H. Inamura
INTERNET DRAFT T. Ishikawa
NTT DoCoMo, Inc
14 July 2000
A TCP profile for W-CDMA: 3G wireless packet service.
draft-inamura-docomo-00.txt
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Abstract
The third generation, 3G, wireless networks that enable high-speed
data transfer are now being introduced. Internet access is one of
the most important applications of such networks. TCP is one of the
key technologies that enable Internet applications for such networks.
A number of TCP optimization techniques have been studied to enhance
the performance of TCP transmission for various wireless
environments.
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This document proposes a TCP profile that is a set of TCP features,
derived from the previous studies in IETF, and is specifically
effective for the use in W-CDMA (Wide-band CDMA) 3G wireless packet
services.
We conducted simulations to verify the performances of the profile.
The features are discussed with the result of our simulation.
We publish this memo to IETF for informational purpose, since there
is no major modification to the IETF standard protocols.
1 Introduction
IETF has been discussing performance enhancements to TCP for various
wireless environments. The efforts are summarized in RFC2757 [LTN]
that serves as a good survey for various options available for TCP.
In order to define the most suitable TCP enhancements for the W-CDMA
network, we have evaluated the options listed in the RFC based on our
performance simulation, and compiled a profile of features for TCP
that is a set of the options from the RFC and is effective in
enhancing the performance of TCP over the W-CDMA network.
In this document we propose the TCP profile for implementation
guidelines and parameters for 3G W-CDMA wireless packet services for
the Internet community.
The list of features for the profile is small and conservative.
Through the simulation, we confirmed that they work well under the 3G
wireless packet service network environment. One of the important
criteria for selecting the options is to ensure the interoperability
with and minimal impact on existing TCP implementations.
The profile consists of the following TCP options:
* Choice of MTU size in link layer
* Appropriate receive window size
* Increased initial congestion window
* Use of Selective ACK
In the following sections, we introduce some of key aspects in W-CDMA
infrastructure, and describe each of TCP features.
2 Architecture of W-CDMA packet service
The architecture of W-CDMA is an implementation of 3GPP stan-
dard[3GPP]. The W-CDMA network can accommodate high speed traffic up
to 2Mbps (384kbps for best effort in the first deployment).
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In the following section, we introduce an Internet-enabled mobile
phone architecture and, then, we focus on the layer two ARQ and its
impact on the TCP performance.
2.1 The architecture for Internet-enabled mobile phone on W-CDMA
L7 |HTTP | |HTTP |HTTP | |HTTP |
L4 | TCP | | TCP | TCP | | TCP |
L3 | IP | | IP | IP | | IP | IP | | IP |
L2 | RLC | | RLC |fiber| |fiber| any | | any |
L1 |WCDMA| |WCDMA| any | | any | any | | any |
Hand set BTS/RNC Gateway Web
server
Fig 1: The architecture of Internet-enabled mobile phone on W-CDMA
As depicted in Fig. 1, The hand set is the browser equipped phone
that talks TCP/IP. In BTS/RNC, IP traffic carried by the layer two
ARQ over the air goes through the wire-line to the gateway. The
gateway serves for the central point for call control to enable push
functionality, charging and fire-walling, etc.
2.2 Layer two ARQ
The key component in the W-CDMA network in terms of performance is
the behavior of the layer two ARQ (L2 ARQ) protocol. The protocol,
RLC (Radio Link Control) [RLC], is a kind of Selective Repeat ARQ.
RLC defines PDU as its frame length to 336b (including 16b RLC
header) that is the unit for retransmission. The SDU (in this compo-
sition, IP packet) is fragmented into PDUs at RLC.
The FEC (forward error correction) and interleaving is provided in
the layer 1 ("WCDMA" in the Fig. 1). Two to twelve PDU (RLC frames)
constitutes one FEC frame. The actual number of aggregation varies
depending on the air condition and the allocation of bandwidth. The
FEC frame is the unit of interleaving. The block error rate (BLER)
is defined by the PDU (the unit of RLC frame) loss rate. The PDU
loss becomes visible after interleave and the FEC are processed by
the layer 1.
RLC uses "status report" as an acknowledgment. It does not use sim-
ple ack-clocking that is used by TCP, but it uses the poll bit in the
header to explicitly solicit the peer for status report containing
the sequence number that the peer acknowledged. The use of poll bit
is controlled by timers and the size of available buffer space in
RLC. Also, the peer can issue status report when it detects a gap
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between sequence numbers in received frames, and it invokes retrans-
mission.
In summary, the RLC provides an almost error-free packet service to
the upper layer traffic, i.e. IP. The SDU (IP packet) is fragmented
into PDUs for RLC retransmission. The retransmission by RLC intro-
duces latency and jitter to the SDU flow.
3 TCP profile for W-CDMA
We have selected four features for TCP on W-CDMA.
* Choice of MTU size in link layer
* Appropriate receive window size
* Increased initial congestion window
* Use of Selective ACK
To confirm the effects of these TCP features, we modeled the W-CDMA
infrastructure into a simulation test bed running on OpNet [MIL3].
TCP/IP ----------queue---------------queue------------------TCP/IP
RLC -[BLER]---RLC----[no loss]----Dedicated-[no loss]----Dedicated
442kbps(down) 384kbps line 1.5Mbps line
64kbps(up)
Handset BTS/RNC Gate Switch Gateway/
Web server
Fig 2: Simulation composition
The attributes for each link is shown in Fig. 2. The two nodes, the
BTS/RNC and the Gate Switch, each have a queues for speed adjustment.
Without the queuing delay, we assumed 600ms latency from the Handset
to the Gateway/Web server in roud trip, that is a realistic load sit-
uation. The queuing delay and jitter are introduced by the simulated
behavior of RLC under specific block error rate and pattern and TCP
traffic.
3.1 Choice of MTU size in link layer
One of the important link layer parameters is the MTU (Maximum Trans-
fer Unit). In TCP, the slow start mechanism tries to find an ade-
quate rate for the link layer. The larger MTU allows TCP to grow the
congestion window faster, because the window is counted in unit of
segments, especially when the link condition is good. In contrast,
under a high BER situation, a smaller MTU is better in terms of the
increased chance of successful transmission. With the L2 ARQ, the
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upper layer can enjoy a larger MTU even in a relatively high BER con-
dition. This is due to the result of trading a high BER with an
increased latency, by the L2 ARQ. Due to the effect of the L2 ARQ, a
large MTU and mss, such as 1500B, is recommended for the use with W-
CDMA. In the following sections, we use 1500B MTU for the simulation
parameter to discuss the performance.
3.2 Appropriate receive window size
To utilize link capacity in full, a TCP sender has to send segments
"on the fly". To achieve the maximum link efficiency for TCP, the
advertised receive window size needs to be greater than the product
of delay and bandwidth of the link. The link capacity varies by
physical layers, even by specific wireless technologies. The
receiver must advertise the appropriate receive window size to the
link connected to.
BLER\Rwin 32KB 64KB 128KB
--------------------------------------------------------
1e-2 340 kbps 369 kbps 369 kbps
5e-2 275 kbps 340 kbps 340 kbps
1e-1 230 kbps 320 kbps 320 kbps
Table 1: TCP throughput with given BLER and receive window sizes.
Table 1 shows the result of our simulation. It shows the TCP
throughput for specific BLERs (block error rate) and receive window
sizes. It models the transfer of 5MB data from a server to a W-CDMA
handset. From the result, a receive window size of 64KB is recom-
mended for the scenario of the simulation.
3.3 Increased initial congestion window
TCP controls its transmit rate using the congestion window mechanism.
In tradition, the initial value of the window is one segment.
Because the delayed Ack mechanism is widely deployed, a TCP sender
should have an increased initial congestion window of two segments
[TCC]. This effectively cancels the delayed Ack by sending two seg-
ments at once in the very first slow start turn. This contributes to
avoiding the overhead in connection creation.
Furthermore, the increased initial window option [IW] is also effec-
tive especially for small data to be transmitted, which is commonly
seen in such an application as the Internet-enabled mobile phone ser-
vice. For large data transfer, on the other hand, the effect of this
option is negligible. [All97] describes evaluations of this option
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by simulation.
According to our simulation, the increased initial window improves
time to transfer 100KB by 2 second, reducing the time from 9 second
to 7second. With RFC2414, contents around this size (more precisely,
up to 4380B) can be transfered in one RTT just after the three-way
handshake.
Although RFC2414 is experimental status, there is no impact of using
RFC2414 on the majority of the Internet as long as the gateway archi-
tecture is used to limit the use of RFC2414 only between the handset
and the gateway.
Due to the fact that the delayed Ack mechanism is the standard and
that the increased initial window option is especially effective for
small data transfer that is common for such applications the i-mode
service, this option is recommended.
3.4 Use of Selective ACK
The Selective ACKnowledgement option (SACK) [SACK] is effective when
multiple TCP segments are lost in a single TCP window. In particu-
lar, if the link has a large delay-bandwidth product and a high
packet loss rate, the ratio of multiple segment losses grows high.
In such cases, SACK performs better than traditional and Reno TCP
[FF96]. This is the case of 3G and SACK is recommended.
As we see, the L2 ARQ handles the most of retransmission caused by
wireless conditions. TCP retransmission occurs in window learning
phase and due to the packets loss caused by bottle-neck routers.
[FF96] shows the superiority of SACK and our simulation backs up this
result. By setting the queue resource at the bottleneck router small
(20 packets), that is simulating congestion situation, we see 20 sec-
onds difference in time to transfer 1MB data. 40 seconds observed
SACK is used for the transfer, while 60 seconds needed if Reno is
used.
4. Summary and Conclusion
In order to enable efficient TCP transport for 3G wireless packet
service using W-CDMA, we have proposed a profile of TCP features,
that is a set of TCP options derived from the previous works in IETF
to enhance the performance of TCP in wireless environments. In
selecting the features a special attention has been paid to ensure
the interoperability with and minimal impact on existing TCP imple-
mentations. We recognize that the proposed profile is small and con-
servative.
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Based on our simulation of the W-CDMA network, we have demonstrated
and discussed the effectiveness of the profile.
The features for the proposed TCP profile for W-CDMA are summarized
in Table 2.
Feature Parameter/Recommendation RFC/Status
-------------------------------------------------------------------
MTU size 1500B N/A
Window size 64KB RFC 793 Standard
Initial window 2 mss RFC 2581 Proposed Standard
Initial window up to 4380B RFC 2414 Experimental
Use of SACK Recommend RFC 2018 Proposed Standard
Table 2: Summary of implementation guidelines and parameters
References
[LTN] G. Montenegro, S. Dawkins, M. Kojo, V. Magret, N. Vaidya,
"Long Thin Networks" RFC 2757, January 2000
[3GPP] http://www.3gpp.org
[RLC] 3G TS 25.322: "RLC Protocol Specification"
[TLS] T. Dierks, C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999
[MIL3] http://www.mil3.com
[TCC] M. Allman, V. Paxson, W. Stevens, "TCP Congestion Control",
RFC 2581, April 1999
[IW] M. Allman, S. Floyd, C. Partridge, "Increased TCP's Inital
Window" RFC2414, September 1998
[All97] M.Allman, "An Evaluation of TCP with Larger Initial Windows",
40th IETF Meeting -- TCP Implementations WG. December 1997.
[SACK] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, "TCP Selective
Acknowledgment Options" RFC 2018, October 1996
[FF96] K.Fall, S.Floyd, "Simulation-based Comparisons of Tahoe,
Reno, and SACK TCP," Computer Communication Review, 26(3),
July 1996.
Authors' Address
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Hiroshi Inamura
NTT DoCoMo, Inc.
Multimedia Laboratories.
3-5 Hikarinooka Yokosuka-shi
Kanagawa-ken 239-8536 Japan
Phone: +81-468-40-3329
Fax: +81-468-40-3788
EMail: inamura@mml.yrp.nttdocomo.co.jp
Taro Ishikawa
NTT DoCoMo, Inc.
Multimedia Laboratories.
3-5 Hikarinooka Yokosuka-shi
Kanagawa-ken 239-8536 Japan
Phone: +81-468-40-3033
Fax: +81-468-40-3788
EMail: taro@mml.yrp.nttdocomo.co.jp
Appendix A DoCoMo's W-CDMA and background
W-CDMA is one of the 3G wireless standards. NTT DoCoMo plans to use
TCP as one of the core technologies to enable the wireless packet
services. In order to achieve the high performance, we recognized
that it is necessary to enhance the TCP performance for the W-CDMA
environments.
The W-CDMA network can accommodate high speed traffic up to 384kbps
for best effort in the first deployment. The tariff is charged per
packet basis.
NTT DoCoMo plans to provide IP wireless packet services on W-CDMA
(Wide-band CDMA) in Spring 2001, the next generation i-mode service
over W-CDMA.
Appendix B i-mode
Since February 1999, NTT DoCoMo has been providing the "i-mode" ser-
vice, that is a comprehensive mobile Internet service using Internet-
enabled mobile phones, each equipped with a web browser in addition
to the e-mail service and the digital mobile phone service. The num-
ber of subscribers has exceeded 8 million in Japan, as of the date of
submission, and is growing rapidly.
The first generation of i-mode is implemented on top of the PDC (Per-
sonal Digital Cellular) packet service using gateway architecture
that acts as an HTTP proxy. NTT DoCoMo has evolved the PDC voice
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cellular network into packet capable network by adding packet
switches and routers.
For the next generation i-mode service using the W-CDMA technology,
we re-designed the network and made it into a much more IP-centric
network, using the IPv4 addressing and routing technology.
The traffic pattern specific to the NTT DoCoMo's i-mode service, size
of an html page is limited to 5KB in the original phase. Although
this limitation will be relaxed in the W-CDMA, we foresee such num-
bers are still be the majority.
When a user wants an end-to-end security solution, which is espe-
cially needed for services such as on-line banking, the handset and
the Web sever will communicate on TLS [TLS]. In that case, the gate-
way only relays the TCP traffic.
The profiled TCP that is proposed in this document will be imple-
mented in the next generation of the i-mode service that will be a
higher-transfer-rate wireless packet service enabled by the W-CDMA
technology.
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