Transport Working Group R. Penno
Internet Draft Juniper Networks
Intended status: Informational J. Iyengar
Expires: May 2009 Franklin & Marshall College
November 3, 2008
TANA Practices and Recommendations
draft-penno-tana-app-practices-recommendation-01.txt
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Abstract
Applications routinely open multiple TCP connections. For example,
P2P applications maintain connections to a number of different peers
while web browsers perform concurrent download from the same web
server. Application designers pursue different goals when doing so:
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P2P apps need to maintain a well-connected mesh in the swarm while
web browsers mainly use multiple connections to parallelize requests
that involve application latency on the web server side. But this
practice also has impacts to the host and the network as a whole. For
example, an application can obtain a larger fraction of the
bottleneck than if it had used fewer connections. Although capacity
is the most commonly considered bottleneck resource, middlebox state
table entries are also an important resource for an end system
communication.
This documents clarifies the current practices of application design
and reasons behind them, and discusses the tradeoffs surrounding the
use of many concurrent TCP connections to one destination and/or to
different destinations. Other resource types may exist, and the
guidelines are expected to comprehensively discuss them.
Conventions used in this document
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 RFC-2119 Error!
Reference source not found..
Table of Contents
1. Introduction 3
2. Terminology 4
3. Multiple TCP Connections Advantages 4
4. Multiple TCP connections Resources 4
5. Memory Space 4
6. Link Bandwidth 5
7. Middleboxes 5
8. Recommendations 6
8.1. Diffserv 6
8.2. AQM 6
9. Security Considerations 6
10. IANA Considerations 6
11. Conclusions 6
12. Acknowledgments 6
13. References 7
13.1. Normative References 7
13.2. Informative References 7
Author's Addresses 7
Intellectual Property Statement 7
Disclaimer of Validity 8
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1. Introduction
The use of P2P protocols by end users is widespread. These protocols
are meant to exchange, replicate, stream or download files with
little human intervention, trying at the same time to minimize the
download time of the files requested by any single peer. This is done
by opening several connections to multiple peers and downloading one
or more chunks of the file from each one, selecting faster peers,
amongst others.
If we assume that in any file transfer the bottleneck is on the
uploading peer or server side, end users that utilize P2P clients in
general download the file faster and consume more bandwidth within a
specific timeframe than traditional client-server applications. P2P
clients can overcome the server side bottleneck by opening multiple
connections to different peers. Users of P2P applications also
consume bandwidth throughout the whole day since even after a file is
fully downloaded it will continue to be shared with others users
increasing the upstream bandwidth.
We can see then that the advantages of P2P applications come from the
fact that they open multiple TCP connection to different peers in
order to download multiple pieces of a file in parallel, always look
for faster peers.
But the use of multiple TCP connections by an application is not new.
Web Browsers have being it for a decade. But these are usually short-
lived connections as opposed to long-lived connections. A long-lived
connection in this document should be interpreted as strictly
defined, i.e., a TCP connection that is simply in the established
state, but not necessarily continuously transferring data. In the
case of P2P protocols, e.g. Bittorrent, at any point in time a
fraction of these connections is actually sending or receiving data,
the others are idle or exchange occasional control information.
With the popularity of P2P applications, which maintain hundreds of
long-lived TCP connections to multiple hosts, the issue applications
making use of multiple TCP connections has been gaining attention.
This documents clarifies the current practices of application design
and reasons behind them, and discusses the tradeoffs surrounding the
use of many concurrent TCP connections to one destination and/or to
different destinations. Other resource types may exist, and the
guidelines are expected to comprehensively discuss them.
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2. Terminology
Bandwidth: A measure of the amount of data that can be transferred
per second. So, if a 1Gb file were transferred within one second the
bandwidth consumption during the transfer would be 1Gb/s. If it were
transferred within a day, it would be approximately 0.0002Gb/s.
Volume: The total number of bytes transferred during a long time
period. In both examples above the volume within a day would have
been 1Gb.
Capacity: The maximum bandwidth a link can sustain continuously.
Long-lived connection: A TCP connection that is in the established
state but not necessarily continuously transferring data.
3. Multiple TCP Connections Advantages
There are good reasons for an application to use multiple TCP
connections. P2P apps need to maintain a well-connected mesh in the
swarm while web browsers mainly use multiple connections to
parallelize requests that involve application latency on the web
server side
But from a P2P standpoint multiple TCP connections are at the heart
of its functionality. Multiple connections allow for multiple
simultaneous downloads, which improve reliability and speed. Multiple
connections also allow more effective discovery of new peers, and
effective peer-to-peer communication, which allows exchange of
information such as which pieces of a file a client has and is
available.
4. Multiple TCP connections Resources
Every connected application on the Internet competes for resources.
This is not specific to applications that open multiple TCP
connections. The use of multiple TCP connections just amplifies the
issue. In the following sections we discuss these resources and how
they are amplified by an application opening multiple connections.
5. Memory Space
Each TCP connection needs a TCP control block (TCB) or equivalent to
keep state about its connection. In operating systems where the TCP
stack is part of the kernel, this would come from the kernel memory
space, otherwise from userland memory.
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But irrespective from where the memory comes from a TCP control block
requires a significant amount of memory. This is significant issue
for devices that terminate TCP connections from multiple end hosts to
provide functions such as Load-Balancing, Gateway and Tunneling.
Some proposals have been put forward to reduce the amount of memory
occupied by each TCP control block [RFC2140], but the issue remains
significant and is amplified by applications that use multiple TCP
connections.
6. Link Bandwidth
The bottlenecks for these n connections could be shared or separate.
If separate, there's no specific bottleneck where the connections are
hogging bandwidth. But from a network resource point of view, the
application download still gets multiple shares.
If some/all bottlenecks are shared, then two possibilities exist for
shared bottleneck
- bottleneck is a last-hop link (user traffic dominates link), OR
- bottleneck is in-network wide-area link (background traffic
dominates link)
If bottleneck is last-hop, then n transport connections compete with
each other and share link bandwidth.
Although these connections might impact delay-sensitive traffic and
increase delay, in the last hop it only affects end end-user, which
is in control of which applications run on its host. In this case the
user has the option of manually choosing when to run such
application, configuring the end host, amongst others. Alternatively,
or in conjunction with the above, the application can be enhanced to
use Diffserv and new delay sensitive congestion mechanisms.
If shared bottleneck is in-network, then application gets unfair
share of bottleneck bandwidth. This impacts flows belonging to other
users in general, and most importantly delay-sensitive traffic.
7. Middleboxes
Middleboxes re defined as any intermediary box performing functions
apart from normal, standard functions of an IP router on the data
path between a source host and destination host [RFC3234].
Middleboxes can be stand-alone or integrated in another device such
as a router or modem.
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The functions that are relevant to this discussion are those that
require the middlebox to keep per session state, sometimes referred
as transformation services. Some of these functions are, for example,
NAT, Intrusion Detection and Load-Balancing.
It is easy to see that the more sessions a host initiates, the more
state the middlebox will have to keep. The relationship is at least
1:1 but due asymmetric traffic, routing changes and others, this can
be 1:N.
Although application traffic from most broadband subscribers today go
through at least one middlebox (integrated into the broadband modem),
it can traverse other middleboxes that reside within the ISP's
network or close the destination. These middleboxes aggregate traffic
from multiple subscribers and state tables within these devices can
become a premium.
8. Recommendations
8.1. Diffserv
8.2. AQM
9. Security Considerations
None at this time
10. IANA Considerations
None at this time
11. Conclusions
TBD
12. Acknowledgments
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13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13.2. Informative References
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC2140] J. Touch, TCP Control Block Interdependence, RFC 2140
(1997)
Author's Addresses
Reinaldo Penno
Juniper Networks
1194 N Mathilda Aveue
Sunnyvale, CA
Email: rpenno@juniper.net
Jana Iyengar
Franklin & Marshall College
Email: jiyengar@fandm.edu
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