Internet Engineering Task Force G. Chen
Internet-Draft H. Deng
Intended status: Informational China Mobile
Expires: September 8, 2011 March 7, 2011
Problem Statement of Long-lived TCP Connection
draft-chen-long-lived-connection-ps-00
Abstract
This memo describes issues encountered by a long-lived TCP
connection. Long-lived TCP connections are served for several
applications, which are quite popularly used in our daily life, such
as chat and messaging(MSN, Skype). Others are computer-to-computer
communications, which are also demanding at long-lived TCP
connection. Issues such as network resources consumption happen when
there are frequent keepalive message has been transmitted
periodically. Moreover, other issues are occurring during long TCP
connection, such as TCP congestion, TCP connection recovery.
Depending on raised issues, protocol optimization could be considered
to optimize network quality.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 8, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Traffic Features of Long-lived TCP Connection . . . . . . . . . 4
3. Problems caused by Long-Lived TCP Connection . . . . . . . . . 5
3.1. Keepalive issues . . . . . . . . . . . . . . . . . . . . . 5
3.2. TCP Congestion Issues . . . . . . . . . . . . . . . . . . . 5
3.3. TCP connection Recovery Issues . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Traffic in the Internet is a complex mix of effects from protocols,
different application characteristics, and user behaviors.
Understanding traffic is essential for network protocol optimization
and quality improvement. This memo describes issues encountered by a
long-lived TCP connection. Long-lived TCP connection is served for
several applications, which are quite popularly used in our daily
life, such as chat and messaging(MSN, Skype). Others are computer-
to-computer communications. Such traffic is growing due to automated
control and sensing, online backup, and distributed processing in the
cloud and across distributed data centers. This kind applications
are demanding at long-lived TCP connection.
The long-lived TCP connections have impacts on normal network
behavior.Issues such as network resources consumption happen when
there are frequent keepalive message has been transmitted
periodically. Moreover, long TCP connection will lead to others
issues, such as TCP congestion, TCP connection recovery. Depending
on raised issues, protocol optimization could be considered to
optimize the network quality.
2. Traffic Features of Long-lived TCP Connection
The traffic features of long-lived TCP depends on specific
applications usages characteristics.
In some cases, TCP is not able to fully utilize the transport or
network layer resources because the application does not produce data
fast enough. The application is producing small amounts of data at a
relatively constant rate for the TCP layer. This results in small
bursts of packets, in the extreme case a single packet of size less
than the maximum segment size of the connection. Typical examples
are live streaming applications such as Skype that transfer data over
TCP at a constant rate of 32 Kbit/s. Also, applications that use
permanent TCP connections and send keep-alive packets during inactive
periods, fall in this category (BitTorrent exhibits this behavior
during choke periods).
In some scenarios, application is producing data in bursts separated
from each other by idle periods. An example of such behavior is web
browsing with persistent HTTP connections. The user clicks on a link
to load a web page, causing a transfer period, reads the page,
causing an idle period, and clicks on another link, causing another
transfer period.
From duration point of view, hour time-scales are dominated by web
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(HTTP, HTTPS, ports 80 and 443) destinations.Day-long flows contain
background traffic, with computer-driven but human initiated
protocols like chat and messaging. Week-long flows are almost all
computer-to-computer protocols that run without human involvement,
such as time synchronization (ntp) and multicast control (sd, pim,
sapv1).
3. Problems caused by Long-Lived TCP Connection
This section describes possible problems caused by long-lived TCP
connection.
3.1. Keepalive issues
Keepalive is to prevent inactivity from disconnecting the channel.
It's a very common phenomenons, when you are behind a NAT proxy or a
firewall, you could be disconnected without a reason. This behavior
is caused by the connection tracking procedures implemented in
NAT/FW, which keep track of all connections that pass through them.
Because of the physical limits of these machines, they can only keep
a finite number of connections in their memory. The most common and
logical policy is to keep newest connections and to discard old and
inactive connections first.
Frequent TCP keepalive message will help to constantly maintain
mapping records in NAT/FW, and eliminate interruptions of TCP
connections. However, it will cause significant network resource
consumption, especially, in wireless environment. The dedicated air
channel resource need to be assigned to each keepalive signaling.
The numerous keepalive messages might cause air resource depletion,
and degrade other application performance. Currently, PCP [PCP] is a
way to optimize this situation.
3.2. TCP Congestion Issues
When long-lived TCP connections produce data in bursts, it mostly
operates in congestion avoidance phase probing for bandwidth with
larger windows, they produce larger packet bursts. TCP connections
with large window sizes are more tolerant of packet loss than those
with small windows. While long-lived TCP flows may recover from
multiple packet losses in one round-trip time (RTT), short-lived TCP
flows may have to wait for a timeout period to recover from a single
packet loss. It is observed that long-lived TCP flows may completely
shut off short-lived TCP flows. This causes performance problems for
short-lived TCP flows, which generally carry interactive/delay
sensitive data.
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3.3. TCP connection Recovery Issues
TCP was not initially designed to cope with some of the extreme
conditions which cause frequent disconnections. In this sceanrio,
TCP connection will try to re-transmit to re-establish TCP
connection. It will cause bad user experiences and network
consumptions.
4. IANA Considerations
This memo includes no request to IANA.
5. Security Considerations
TBD
6. Normative References
[PCP] Wing, D., "Pinhole Control Protocol (PCP)",
draft-ietf-pcp-base-06.txt (work in progress), February 2011.
Authors' Addresses
Gang Chen
China Mobile
53A,Xibianmennei Ave.,
Xuanwu District,
Beijing 100053
China
Email: chengang@chinamobile.com
Hui Deng
China Mobile
53A,Xibianmennei Ave.
Beijing 100053
P.R.China
Phone: +86-13910750201
Email: denghui02@gmail.com
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