Transport Working Group R. Penno
Internet Draft S. Raghunath
Intended status: Informational Juniper Networks
Expires: August 2010 R. Woundy
Comcast
V. Gurbani
Bell Labs, Alcatel-Lucent
J. Touch
USC/ISI
February 26, 2010
LEDBAT Practices and Recommendations for Managing Multiple
Concurrent TCP Connections
draft-ietf-ledbat-practices-recommendations-00.txt
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Abstract
Applications routinely open multiple TCP connections. For example,
P2P applications maintain connections to a number of different peers
and web browsers perform concurrent download from the same web
server. Application designers pursue different goals when doing so:
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. However
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 document clarifies the current practices of application design
involving concurrent TCP connections and reasons behind them, and
discusses the tradeoffs surrounding their use, whether to one
destination 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 RFC2119.
Table of Contents
1. Introduction 3
2. Terminology 4
3. Multiple control versus data connections 5
4. Multiple TCP Connections Advantages 6
4.1. Avoiding head-of-line blocking 6
4.2. Logical partitioning at application level 7
4.3. Multiple streams with different properties 7
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4.4. Signaling application layer request completion 7
4.5. High bandwidth-delay links 7
4.6. Error resiliency and reliability 8
4.7. Leveraging multiple processors in a system 8
4.8. Overcoming TCP Limitations 8
5. Multiple TCP connections Disadvantages 8
5.1. Additional connection setup overhead 8
5.2. Memory Space 9
5.3. Link Bandwidth 9
5.4. Middleboxes 10
6. Conclusion and Recommendations 10
6.1. Diffserv 10
6.2. Window scale negotiation 10
6.3. Number of Connections 11
6.3.1. HTTP 11
6.4. Bi-Directional HTTP 12
7. Security Considerations 12
8. IANA Considerations 12
9. Acknowledgments 12
10. References 13
10.1. Normative References 13
10.2. Informative References 13
Author's Addresses 15
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
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order to download multiple pieces of a file in parallel, and that
they always look for faster peers.
But the use of multiple TCP connections by an application is not new.
Web Browsers have been doing 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 only a
fraction of these connections is actually sending or receiving data,
while 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 of
applications making use of multiple TCP connections has been gaining
attention.
This document 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.
2. Terminology
Bandwidth: A measure of the amount of data that can be transferred
within a time period, often expressed in bits per second. Bit rate
prefixes are expressed in decimal, so 1 kilobit per second is 1,000
bits per second, and 1 megabit per second is 1,000,000 bits per
second. So, if one million bits are transferred within one second,
the average bandwidth consumption during the transfer would be 1
megabit per second (1 Mbps). If the same amount of data were
transferred within a day, the bandwidth would be approximately 11.574
bits per second.
Volume: The total number of bytes (or octets) transferred during a
time period. Byte prefixes are expressed in binary, so 1 kilobyte is
1,024 bytes, and 1 megabyte is 1,024 * 1,024 = 1048576 bytes. In both
examples above the volume within a day would have been 125,000 bytes
or about 122.07 kilobytes (122.07 KB).
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.
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3. Multiple control versus data connections
The traditional model of applications interacting with each other
using TCP started off as a single socket opened between a client and
a server for data communications. Control signaling was usually
passed on the same channel as well. Telnet [RFC854] and rlogin
protocols [RFC1282] are good examples of this approach. File
Transfer Protocol [RFc959] was one of the first known protocols that
used more than one connection between a client and a server. In FTP,
the client in the normal client-server fashion opens the control
connection. This connection is used for commands from the client to
the server and replies from the server to the client. Distinguishing
FTP from other protocols was its use of a second data connection.
The client initiates this data connection passively, and the port
number is sent to the server. The server subsequently establishes an
active connection to this port. A data connection is created each
time a file is transferred between the client and the server.
However, unlike the control connection, it does not persist for the
duration of the FTP session.
These early protocols limited TCP connections between a pair of
machines. This changed with the advent of the Hypertext Transfer
Protocol [RFC2616]. In HTTP, a client (browser) downloads a document
from a server and analyses it to render the document on a display
device of some sort. As part of the analysis, the browser may open
one or more connections to either the same host from which the
original document was downloaded, or to different hosts that serve
other content referenced in the document. However, these connections
were usually short lived (the current phenomenon of "long polling"
notwithstanding). Here, the client (browser) opens up multiple TCP
connections to possibly multiple servers simultaneously. The Session
Initiation Protocol [RFC3261] can use TCP connection in the same vein
as HTTP did, namely to contact multiple servers simultaneously.
Generally -- although there are exceptions -- in SIP just like HTTP,
these connections are typically short-lived.
More recent protocols like Skype (http://www.skype.com) and
BitTorrent (http://www.bittorrent.com) have a much different view on
the number of TCP connections they are willing to open and manage.
While earlier protocols were parsimonious with connections, the
modern peer-to-peer protocols do not appear to be wary of this to the
same degree. Part of the reason why this is the case is the
assumption that the older protocols (Telnet, rlogin, HTTP) were
operating under was that relatively few bytes will be transferred
from the client to the server while many more bytes will be
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transferred in the opposite direction. With current peer-to-peer
protocols, where the resource to be accessed is distributed among the
peers, a requesting peer has to open multiple TCP connections to more
than one peer in order to efficiently download the data represented
by that resource.
In summary, trying to establish a boundary between data connections
and control connections is something of a fool's errand. Protocols
evolve to match the capabilities and characteristics of the network.
While early protocols may have opened up a pair of connections to
communicate, more recent protocols are not inhibited in the same
manner.
Similarly, while earlier protocols may have established different
control channel from a data channel, this was not a design rule that
was carried forward faithfully. While SIP falls in the former camp
of a control channel that is distinct from a data channel, HTTP falls
in the latter camp (i.e., same TCP connection serves to send control
messages and the data itself.) BitTorrent and Skype perform control
and data communications over the very same TCP connection as well.
BitTorrent, in particular, attempts to open multiple connections to
many peers, even though only a small subset of these connections are
involved in the actual data transfer. In Skype, a peer does not open
multiple connections to access a resource; rather multiple
connections are opened and maintained to a discrete set of neighbors
to help in routing of subsequent messages [Skype-analysis].
4. 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.
4.1. Avoiding head-of-line blocking
Web browsers started using multiple TCP connections partly because of
this reason [STEVENS]. This is especially true when the multiple TCP
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connections are between a pair of hosts. Originally, individual HTTP
transactions each used their own TCP connection, because HTTP lacked
a response length marker. The client sent a request to the server,
and the server's response to the client was completed when the TCP
connection was closed, i.e., CLOSE was interpreted as ''end of
transaction''. This caused numerous problems, notably the buildup of
connections in the TIME-WAIT state at the server [Fab1999]. HTTP
added persistent connections to v1.0 (they were later included in the
1.1 spec), and they became the default. In persistent connections,
transactions complete but the connection remains open for subsequent
responses. Responses are pipelined, not interleaved, however,
resulting in Head-of-Line (HOL) blocking.
HTTP clients currently open 4-8 connections to each endpoint. This
partly avoids HOL blocking, but also allows increased performance.
Separate connections open independently, increasing bandwidth, and
also can use separate endpoint processes, increasing computational
parallelism that maps more effectively to multiprocessors and multi-
core systems.
4.2. Logical partitioning at application level
Some applications such as FTP use a separate connection for control
and data transfers. The advantage is that this allows a model where
the data transfer is actually happening between hosts that are not
local (see [RFC959], sections 2.3 & 5.2).
4.3. Multiple streams with different properties
The application may need different properties on multiple streams of
data (e.g., Nagle's algorithm, socket buffer sizes etc).
4.4. Signaling application layer request completion
If the application assumes that connection close indicates the
completion of a request, it becomes necessary to have new connections
for multiple requests. This was a reason for multiple connections in
HTTP 1.0.
4.5. High bandwidth-delay links
In the presence of a large bandwidth-delay product, the 16-bit window
size parameter in TCP header does not allow the application to fully
utilize the link. In such situations, the current practice is to
negotiate the Window Scale Option [RFC1323]. In addition multiple TCP
connections can allow the application to achieve an effectively
larger window size so that it can better utilize a link with high
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bandwidth-delay product (e.g. iSCSI [SCSIREF]), although this can
result in mutual escalation, where TCP fairness is ensured only for
endpoints opening multiple connections.
4.6. Error resiliency and reliability
When multiple connections are used to download a single file or
webpage, for instance, there is lesser chance of a single failure on
one connection having a negative impact on the whole download.
Especially with P2P applications, this makes the network robust to
failures and churn in participants.
4.7. Leveraging multiple processors in a system
With multiple processor systems, there can be higher performance with
parallelism and multiple connections spread over different
processors.
This presumes that the kernel is parallelized; the potential for TCP
parallelism is limited (http://www.isi.edu/touch/pubs/pfhsn94.html)
4.8. Overcoming TCP Limitations
The performance of a single TCP connection given a certain link is
well understood today [PARATCPSCK], [FF99], [PFTK98] and the general
rule of thumb is that a TCP connection can utilize 75% of its
capacity. This means more than one connection to one or more servers
would be needed to saturate the link.
5. Multiple TCP connections Disadvantages
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.
5.1. Additional connection setup overhead
The TCP's mechanisms for starting up the connection and then probing
the available bandwidth have to be repeated for each new connection.
So there may be lesser leverage of network information. There is also
the overhead of additional control traffic that may have been
avoided.
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5.2. 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.
But irrespective of 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.
5.3. Link Bandwidth
The bottlenecks for these N multiple 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:
o the bottleneck is a last-hop link (user traffic dominates link),
OR
o the bottleneck is an in-network wide-area link (background traffic
dominates link)
If bottleneck is the 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 they only affect the 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 each
application, configuring the end host, amongst other choices.
Alternatively, or in conjunction with the above, the application can
be enhanced to use Diffserv and new delay sensitive congestion
mechanisms.
If the shared bottleneck is in-network, then the application gets an
unfair share of bottleneck bandwidth. This impacts flows belonging to
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other users in general, and most importantly may impact delay-
sensitive traffic.
5.4. Middleboxes
Middleboxes are 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.
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 other
considerations, this can be 1:N.
Although application traffic from most broadband subscribers today go
through at least one middlebox (as a stand-alone device in the home
network, or integrated into the broadband modem), it can traverse
other middleboxes that reside within the ISP's network or close to
the destination. These middleboxes aggregate traffic from multiple
subscribers, and state tables within these devices can become a
premium.
6. Conclusion and Recommendations
6.1. Diffserv
REC-1: Applications involved in bulk data transfer with low priority
in time could mark their packets according with the guidelines of RFC
3662 [RFC3662].
6.2. Window scale negotiation
REC-2: Where appropriate, sender & receiver window should be scaled
using RFC1323 based negotiation in order to make the best use of
network resources. Recommendations to adjust window size are not new
and have been recommended in networks where the BDP (Bandwidth Delay
Product) is large [RFC3481].
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6.3. Number of Connections
Multiple connections to the same or different servers provide a
significant speedup as compared to a single connection. The
motivation to use multiple connections is to achieve throughput
efficiency and the cause for such deficiency could be head of line
blocking, slow servers, server availability or simply overcoming TCP
throughput limitations. [DYNPARACON],[PARATCPSCK].
In the case of multiple parallel connection homogeneous connection
sharing a link of capacity c, it was found that 6 connections are
sufficient to reach 95% download utilization [PARATCPSCK].
Interestingly this is comparable to the number of configured active
transfers (5) of the most used BitTorrent client [UTORRENT]. It is
worth noticing that during a large file transfer BitTorrent clients
will prefer peers that provide the largest upload rate, thus
theoretically saturating its download link. In reality, packet drops,
upload caps and others transient effects would require clients to
have more than 5 connections in order to saturate the link, but the
overall effect of these issues in terms of bandwidth decrease is
something already measured by BitTorrent clients and used in the
(optimistic) unchoke/choke algorithm. Therefore the number of active
connections should not be much higher than 5 since the idea is to
saturate the link by choosing the best connections and not
necessarily more connections.
REC-3: Applications should only open more than 6 connections to
download the same object if the first hop link is not saturated.
6.3.1. HTTP
The case of web browsing (HTTP) is quite different from P2P. One
could argue that the number of active connections used by HTTP is
much higher than that used by BitTorrent, but the scenarios are quite
different. In the case of dynamic pages, different objects are
downloaded from (and exclusively available from) certain locations.
Moreover, time is of the essence since there is an expectation that a
page is downloaded and rendered within a few seconds. Finally,
objects in a webpage are quite small, with the majority (75%) below
6KB [HTTPDATA]; therefore many connections are needed to saturate the
link since TCP congestion avoidance never has time to ramp up to its
maximum bandwidth. If multiple small HTTP objects can be retrieved
from the same server, the use of HTTP/1.1 Pipelining is recommended
since it can dramatically reduce the number of packets and connection
overhead between client and server [HTTPPERF].
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REC-4: HTTP based applications should use HTTP/1.1 pipelining when
transferring multiple small objects from the same server.
6.4. Bi-Directional HTTP
Recent frameworks like Ajax allow application developers to write
applications that allow a delay between when the HTTP server receives
a request and sends the corresponding response a response. This
technique, called "long polling", works by having the HTTP server
delay sending the response to a request back until it has some
additional data to sent to the client. HTTP streaming is a technique
where the server keeps the connection open indefinitely by using
chunked Transfer-Encoding mechanism to send incremental responses
spread over time.
Both these techniques originated as a counter mechanism to the normal
manner of polling for events in HTTP: sending multiple requests where
the inter-request frequency is fairly small. Such a polling mechanism
tends to overwhelm the server if the polling frequency is set too
low.
Both long polling and HTTP streaming affect the number of TCP
connections open over a period of time and the network in the
following way:
o Reducing the overhead of opening/closing connections
o Increasing memory consumption in both clients and servers
7. Security Considerations
None at this time
8. IANA Considerations
None at this time
9. Acknowledgments
J. Iyengar was one of the presenters on the first BOF and worked on
the original version of this document.
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10. References
10.1. Normative References
[RFC959] J. Postel and J. Reynolds, "File Transfer Protocol (FTP)",
RFC 959 (1985).
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
10.2. Informative References
[RFC1323] V. Jacobson, B. Braden, D. Borman, TCP Extensions for High
Performance, RFC 1323, May 1992
[RFC2140] J. Touch, TCP Control Block Interdependence, RFC 2140
(1997)
[RFC2616] R. Fielding et al, Hypertext Transfer Protocol HTTP/1.1,
RFC 2616 (1999)
[RFC3481] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A., and F.
Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
Wireless Networks", BCP 71, RFC 3481, February 2003.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services", RFC
3662, December 2003.
[SCSIREF] K.Z. Meth, J. Satran, Design of the iSCSI protocol, Storage
Conference (2003)
[STEVENS] W. Richard Stevens et al, ''Unix Network Programming, The
Sockets Networking API'', Volume 1, Third Edition (2003),
section 10.5, page 293.
[HOLBLCK] Head-of-line Blocking in TCP and SCTP: Analysis and
Measurements
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[EXPPARA] S. Philopoulos and M. Maheswaran. Experimental Study of
Parallel Downloading Schemes for Internet Mirror Sites. In
Thirteenth IASTED International Conference on Parallel and
Distributed Computing Systems (PDCS '01), Aug. 2001.
[HTTPPERF] Network Performance Effects of HTTP/1.1, CSS1, and PNG.
http://www.w3.org/Protocols/HTTP/Performance/Pipeline
[DYNPARACON] P. Rodriguez and E. W. Biersack, ''Dynamic Parallel-
Access to Replicated Content in the Internet'', IEEE/ACM
Transactions on Networking, August 2002
[UTORRENT] http://www.utorrent.com
[PARATCPSCK] Altman, E., Barman, D., Tu.n, B., Vojnovic, M.Parallel
TCP Sockets: Simple Model, Throughput and Validation. In:
IEEE INFOCOM 2006, Barcelona, Spain (2006)
[HTTPDATA] Y. C. Chehadeh, A. Z. Hatahet, A. E. Agamy, M. A.
Bamakhrama, and S. A. Banawan, "Investigating distribution
of data of http traffic: An empirical study," in
Innovations in Information Technology, 2006.
[PFTK98] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose. ''Modeling
TCP Throughput: A Simple Model and its Empirical
Validation''. SIGCOMM Symposium on Communications
Architectures and Protocols, Aug. 1998.
[FF99] S. Floyd and K. Fall. ''Promoting the Use of End-to-End
Congestion Control in the Internet''. IEEE/ACM Transactions
on Networking, Aug. 1999.
[Fab1999] Faber, T., Touch, J. and W. Yue, "The TIME-WAIT state in
TCP and Its Effect on Busy Servers", Proc. Infocom 1999 pp.
1573-1583.
[Skype-analysis] S. Baset and H. Schulzrinne, "An Analysis of the
Skype Peer-to-Peer Internet Telephony Protocol". IEEE
Infocom", Apr. 2006
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Author's Addresses
Reinaldo Penno
Juniper Networks
1194 N Mathilda Aveue
Sunnyvale, CA
Email: rpenno@juniper.net
Satish Raghunath
Juniper Networks
1194 N Mathilda Aveue
Sunnyvale, CA
Email: satishr@juniper.net
Vijay K. Gurbani
Bell Labs, Alcatel-Lucent
1960 Lucent Lane
Room 9C-533
Naperville, IL
60566
USA
Email: vkg@bell-labs.com
Richard Woundy
Comcast Cable Communications
27 Industrial Avenue
Chelmsford, MA 01824
US
Email: richard_woundy@cable.comcast.com
URI: http://www.comcast.com
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Email: touch@isi.edu
URL: http://www.isi.edu/touch
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