Configuring TCP's Initial Window
draft-you-tcpm-configuring-tcp-initial-window-00
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| Authors | Jianjie You , Rachel Huang | ||
| Last updated | 2014-10-19 | ||
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draft-you-tcpm-configuring-tcp-initial-window-00
Tcpm Working Group J. You
Internet-Draft R. Huang
Intended status: Standards Track Huawei
Expires: April 22, 2015 October 19, 2014
Configuring TCP's Initial Window
draft-you-tcpm-configuring-tcp-initial-window-00
Abstract
This document discusses that TCP's initial congestion window is not a
constant in different use cases. It proposes a flexible method to
configure the initial window in order to keep up with the current
network state.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 22, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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This document is subject to BCP 78 and the IETF Trust's Legal
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Current TCP's Initial Window Configuration Methods . . . . . 3
4. Configuring TCP's Initial Window . . . . . . . . . . . . . . 4
4.1. Factors affecting TCP's Initial Window . . . . . . . . . 5
4.1.1. Web Object Size . . . . . . . . . . . . . . . . . . . 5
4.1.2. Bandwidth . . . . . . . . . . . . . . . . . . . . . . 5
4.1.3. Latency . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.4. Packet Loss Rate . . . . . . . . . . . . . . . . . . 6
4.1.5. Concurrent TCP connections . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Example of Configuring TCP's InitCwnd . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Google proposes to increase TCP's initial congestion window
(InitCwnd) to at least ten segments (about 15KB) [RFC6928]. While
for Taobao, the biggest online shopping mall in China, some public
material from Taobao discloses that Taobao is using IW7 in their
network instead of IW10. When the TCP's InitCwnd is set to 7, they
get the best end-user experience in Taobao's experiments. In
Google's experiments, the InitCwnd is configured using the InitCwnd
option in the IP route command. Furthermore, all front-end servers
within a data center are configured with the same InitCwnd. However,
as the network properties at geographically diverse locations differ,
one global InitCwnd for all servers cannot optimize the TCP
performance.
This document discusses that TCP's initial congestion window is not a
constant in different use cases. It proposes a flexible method to
configure the initial window in order to keep up with the current
network state.
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2. Terminology
This section contains definitions of terms used in this document.
TCP: Transmission Control Protocol
RTT: Round-Trip Time
RTO: Retransmission Timeout
3. Current TCP's Initial Window Configuration Methods
The performance of initial TCP connection and congestion control is
often affected by TCP parameters, such as initial congestion window,
slow start threshold etc., which are usually default in systems.
While with the global network access speeds growing, these default
values set by systems may not be suitable for all the usages in
current networks.
For example, Google believes a modest increase of InitCwnd to 10 is
the best solution for the near-term deployment. Google's experiments
consist of enabling a larger initial congestion window on front-end
servers in several data centers at geographically diverse locations.
In their experiments, the front-end servers run Linux with a
reasonably standards compliant TCP implementation (the congestion
control algorithm used is TCP CUBIC), and the initial congestion
window is configured using the initcwnd option in the ip route
command, i.e., InitCwnd=10.
Another example is Taobao, the biggest online shopping mall in China.
Some public material from Taobao discloses that Taobao is using IW7
in their network instead of IW10. In Taobao's experiments, when the
TCP's InitCwnd is set to 7, they get the best end-user experience.
As the application scenarios for Google and Taobao are definitely
different, the InitCwnd values for optimal performance are different
too. So using one fixed InitCwnd value (i.e. 10) for all cases is
not appropriate.
Current TCP's InitCwnd is a global variable on a server or host, for
example, a host is configured with the same initial congestion window
for all the applications. Those parameters can be changed by
modifying the regedit or kernel. However, they can't be modified
dynamically, nor can be based on different granularities, such as per
TCP flow. If they can be adjusted according to peak and off-peak
times of Internet, server capability, network bandwidth, number of
users, etc., maybe the performance of TCP connections could be
effectively improved. For example, when there is no network
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congestion or server overloading, TCP initial window size could be
set bigger. While during the peak time of Internet, e.g. "Double-
11" shopping festival of Taobao, the window size could be set smaller
in order to avoid unnecessary congestion.
4. Configuring TCP's Initial Window
Different applications may require different transport performance to
satisfy various service requirements. This document proposes that
TCP's initial window could be configured via TCP API (Application
Programming Interface) based on different granularities such as TCP
flow, as shown in Figure 1. By using this method, applications could
flexibly customize their own TCP parameters according to their
specific requirements.
+----------------+
| |
| Applications |
| |
+-------+--------+
|
Socket|
APIs|
|
+-------+--------+
|OS |
| |
| +---------+ |
| |TCP Stack| |
| +---------+ |
+----------------+
Figure 1: TCP's Initial Window Configuration
TCP API needs to be extended to allow applications to configure TCP's
initial window per TCP flow granularity or others. A detailed
example can be found in Appendix A.
By using socket APIs to configure TCP's initial window, it is
flexible for applications to customize appropriate TCP parameters
according to service requirements and network availability, to
achieve better user experience. Moreover, TCP's initial window can
be configured based on different granularities, e.g. TCP flow, which
is a finer granularity to meet the service diversity.
Even if a system supports setting the initial window via the new API
proposed, not all applications will take advantage of this, and so a
system-wide default initial window still will apply to these
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applications. For those applications don't knowing how to pick an
optimal value for the initial window, it is not recommended to used
this function.
4.1. Factors affecting TCP's Initial Window
This section discusses some factors that need to be considered when
determining an appropriate TCP initial congestion window. Regarding
how to find the proper InitCwnd, it is TBD.
4.1.1. Web Object Size
[RFC3390] stated that the main motivation for increasing the initial
window to 4 KB was to speed up connections that only transmit a small
amount of data, e.g., email and web. The majority of transfers back
then were less than 4 KB and could be completed in a single RTT
(Round-Trip Time).
However, nowadays, the size of the average web page of the top 1000
websites passed 1600K for the first time in July 2014. At the same
time the number of objects in the average web page increased to 112
objects. Google proposed to increase TCP's initial congestion window
to at least ten segments (about 15KB) [RFC6928], while Taobao thinks
7 segments is optimal.
4.1.2. Bandwidth
For low bandwidth networks, such as GSM (Global System for Mobile
Communication) and GPRS (General Packet Radio Service), injecting
high-speed data into low-speed links leads to congestion or even
collapse. In such case, small initial congestion window for TCP
connections is relatively safe, e.g. 1 to 4, to prevent network
congestion caused by a sudden influx of data into network. However,
for networks with sufficient bandwidth capacity, the value of TCP
initial congestion window could be set bigger, but we also need to
consider other factors such as latency, packet loss, etc.
4.1.3. Latency
In high latency networks, the duration of slow start stage has a big
impact on the whole TCP performance. A small initial congestion
window usually leads to a long slow start stage, which may seriously
decrease the TCP performance. Especially for short-lived services,
e.g., HTTP Web transaction, most data would be transmitted at the low
speed rate if the slow start stage is too long. For such kind of
application, increasing InitCwnd enables transmission to be finished
in fewer RTTs. This would shorten the duration of slow start stage
and avoid RTO (Retransmission Timeout).
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Our experiments show the relations between the initial congestion
window (horizontal axis) and transmission time when sending 50k data
(vertical axis) in a lab simulation environment. We compare the
results using different initial congestion windows (from 1 to 10)
under different latency (50ms, 100ms, 200ms, 300ms) when sending 50k
data. As we can see, when the initial congestion window is bigger,
the duration is smaller.
4.1.4. Packet Loss Rate
Packet loss is usually caused by network congestion due to
insufficient bandwidth discussed in Section 4.1.3, or network device
problems, e.g. not enough storage in routers. Increasing InitCwnd
would lead to data stream burst into networks then would aggravate
the packet loss. So in high congestion environment, TCP initial
congestion window should be set relatively small, e.g. 2 or 4.
Our experiments show the relations between the initial congestion
window (horizontal axis) and transmission time when sending 50k data
with different packet loss rate (vertical axis) in a lab simulation
environment. We compare the results using different initial
congestion windows (from 1 to 10) under packet loss rate (0.10%,
0.20%, 0.40%, 0.60%, 0.80%) when sending 50k data with fixed
latency=50ms. As we can see, when the initial congestion window is
bigger, the duration is smaller.
4.1.5. Concurrent TCP connections
For applications with too many concurrent TCP connections, it is not
suggested to set too large initial congestion window since too many
network resources would be occupied in a very short time. It's
against the TCP fairness and also easily results in network
congestion.
5. IANA Considerations
This document does not introduce any new IANA considerations.
6. Security considerations
This document introduces a new method to configure the initial
congestion window for TCP connections. This method facilitates
application developers to tune TCP for their benefits. But it also
has the possibility that packet loss may caused by inappropriate
setting. However, as RFC6928 says, it is unlikely to lead to a
persistent state of network congestion or collapse. So it does not
introduce any new security issues.
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7. Acknowledgement
The authors would like to thank Yoshifumi Nishida and Wesley Eddy for
their detailed review and comments.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928, April 2013.
Appendix A. Example of Configuring TCP's InitCwnd
This example illustrates how to set TCP initial congestion window
parameter.
/* Server establishes listen socket */
int = listen_fd = socket( AF_INET, SOCK_STREAM, 0 );
/* Server creates accept socket to accept the data received from the
listen socket from client */
int conn_fd = accept( listen_fd, ..., ... );
/* Receiving data and analyzing it (take HTTP as an example, the data
may be the GET message from the client to request some resources */
read( conn_fd, buf, size );
/* e.g. Server calculates the initial cwnd based on the size of the
resources */
int cwnd = (send_size+(mss-1))/mss;
/* MAX_INITCWND could be bigger than IW10 */
if (cwnd > MAX_INITCWND) cwnd = MAX_INITCWND;
setsockopt(conn_fd, IPPROTO_TCP, TCP_INITCWND, (void*)&cwnd,
sizeof(cwnd));
In this example, TCP initial cwnd is a new extended parameter for
socket API.
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Authors' Addresses
Jianjie You
Huawei
101 Software Avenue, Yuhuatai District
Nanjing, 210012
China
Email: youjianjie@huawei.com
Rachel Huang
Huawei
101 Software Avenue, Yuhuatai District
Nanjing, 210012
China
Email: rachel.huang@huawei.com
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