INTERNET-DRAFT R.Sallantin
Intended Status: Proposed Standard CNES/TAS/TESA
Expires: July 19, 2014 C.Baudoin
F.Arnal
Thales Alenia Space
E.Dubois
CNES
E.Chaput
A.Beylot
IRIT
January 15, 2014
Safe increase of the TCP's Initial Window
Using Initial Spreading
draft-irtf-iccrg-sallantin-initial-spreading-00.txt
Abstract
This document proposes a new fast start-up mechanism for TCP that can
be used to speed the beginning of an Internet connection and then
improved the short-lived TCP connections performance.
Initial Spreading allows to safely increase the Initial Window size
in any cases, and notably in congested networks.
Merging the increase in the IW with the spacing of the segments
belonging to the Initial Window (IW), Initial Spreading is a very
simple mechanism that improves short-lived TCP flows performance and
do not deteriorate long-lived TCP flows performance.
Status of this Memo
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Initial Spreading mechanism . . . . . . . . . . . . . . . . . . 4
4 Implementation considerations . . . . . . . . . . . . . . . . . 4
4.1 Short Round Trip Time . . . . . . . . . . . . . . . . . . 5
4.2 Delayed Ack . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3 TSO/GSO . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.4 RTT measure . . . . . . . . . . . . . . . . . . . . . . . . 6
5 Open discussions . . . . . . . . . . . . . . . . . . . . . . . 6
5.1 Spacing Interval . . . . . . . . . . . . . . . . . . . . . . 6
5.2 Increasing the upper bound TCP's IW to more than 10
segments . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3 Initial Spreading and LFN . . . . . . . . . . . . . . . . . 7
6 Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1 Normative References . . . . . . . . . . . . . . . . . . . 8
8.2 Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1 Introduction
The long Round Trip Time is probably the most detrimental constraint
in Long Fat Networks (LFN), such as satellite networks, and notably
for short-lived connections when the long delay significantly
downgrades regular slow-start performance [FA11]. Several protocols
and even new network architectures have been proposed to deal with
this issue. The original idea of Initial Spreading [SB13] was to
consider a long RTT as a resource to exploit, rather than as a
constant to bypass. The long RTT can therefore be used as an
opportunity to safely send a large amount of data during the first
RTT after the connection has opened. Spacing the data along the whole
RTT would in fact hopefully guarantee high independent probability
that each segment is successfully received.
This approach resembles a combination of 2 TCP mechanisms: Pacing and
Increase in the Initial Window. Both mechanisms have then been
studied in depth to design Initial Spreading as an efficient fast
start-up TCP mechanism, and notably avoid their respective flaws or
weaknesses.
The original Pacing idea is to space the segments of a same window
along an RTT to prevent generating bursts as far as possible. Hence,
each segment arrives separately at the buffer and the impact on its
queue is minimized. The bit rate can then reach its maximum. However,
[AS00] has pointed out that this lack of bursts is responsible for
poor performance. Pacing has a tendency to overload the network, and
then cause a synchronization of the flows, that seriously damages
both individual and global performance.
RFC 6928 [RFC6928] suggests to enlarge the IW size up to ten
segments. Several articles and studies demonstrated that this would
allow transmission of 90% of the connections in one RTT [DR10]. In
most cases, and when the network is not congested in particular, this
solution is probably the best one for dealing with short-lived TCP
flows. However, in a congested environment, sending a large IW in one
burst is likely to impact the buffers and then deteriorate the
individual connection. Correlation between the segments of a same
burst is responsible for major impairments when regarding the short-
lived connections, and in particular for the connections that can be
sent in one RTT (number of segments to be transmitted inferior to the
upper bound value of the TCP's IW):
o a decrease of the probability to successfully transmit the entire
window.
o an increase of the probability of successive segment losses.
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o a significant reduction of the number of potential Duplicated
Acknowledgements that are necessary to trigger fast loss recovery
mechanisms and avoid to wait for an Retransmission Time Out.
In favor of a conservative approach, [RFC3390] recommended the use of
an IW equal to 3.
Both mechanisms therefore suffer from a burst-related phenomenon, but
in opposite ways.
Initial Spreading has been designed to tackle previous burst issues.
Simulations and experimentations show that Initial Spreading is not
only efficient in case of LFNs but also for other networks with small
RTT.
2 Terminology
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 [RFC2119].
3 Initial Spreading mechanism
Initial Spreading [SB13] mechanism uses the permitted upper bound
value of the TCP's IW (e.g; RFC 6928 [RFC6928] suggests to use 10 for
this value). Initial Spreading spaces out a number of segments
inferior or equal to this value across the first RTT before letting
the TCP algorithm continue conventionally:
(1) The RTT is measured during the SYN-SYN/ACK exchange.
(2) The first RTT is split into n spaces with n the permitted upper
bound value of the TCP's IW. Depending on the number of segments
to be sent, until n segments are sent every RTT/n.
(3) After the transmission of the IW, the regular TCP algorithm is
used.
Thus, bursts do not downgrade the transmission of short-lived
connections, but continue to prevent an overload of the network in
the case of long-lived connections.
4 Implementation considerations
In this section, we discuss a number of aspects surrounding the
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Initial Spreading implementations.
4.1 Short Round Trip Time
Whatever the different timer implementations, 2 segments can not be
spaced of less than 1 Kernel timer (e.g: jiffy for Linux kernel).
In case where the time resulting of the division of 1 RTT by the
upper bound value of the TCP's IW is inferior to 1 Kernel timer,
Initial Spreading is not activated and TCP uses a regular slow start
with a large IW.
4.2 Delayed Ack
The use of Delayed Ack (Del Ack) does not downgrade Initial Spreading
efficiency.
Regarding long-lived connections and notably TCP's steady state, the
effects of Del Ack are lessened by new TCP's flavors (such as TCP
Cubic or Compound TCP [HR08][TS06]) which tend to adapt their
congestion algorithm to take into account whether the receiver uses
the Del Ack option or not. In doing so, they can prevent the
connection from being too slow, and still continue to reduce
acknowledgments traffic. In the event of short-lived connections, the
use of Del Ack does not modify the transmission of the IW. There is
then no change in the burst propagation.
4.3 TSO/GSO
TSO/GSO is used to reduce the CPU overhead of TCP/IP on fast
networks. Instead of doing the segmentation in the kernel, large
packets are sent to the Network Interface Card (NIC). The
segmentation is then achieved by the NIC or just before the entry
into the driver's xmit routine.
In its current design, Initial Spreading is not working when TSO or
GSO are activated, but using Initial Spreading with an inactive
TSO/GSO still enables better performance.
Two options can be foreseen for the joint use of Initial Spreading
and TSO/GSO:
(1) disable TSO/GSO for the first RTT, with no impact on performance
since the throughput is limited by the IW.
(2) implement Initial Spreading using the TCP Offload Engine (TOE)
[RFC5522].
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4.4 RTT measure
Initial spreading uses the SYN-SYN/ACK exchange to calculate the
space between two segments. This measurement may not be perfectly
accurate in congested networks when the RTT varies. Two different
scenarios can then occur:
o The measured RTT is superior to the RTT of the first segment of
the IW, and an ACK arrives before that all the segments of the IW
have been sent. Then, Initial Spreading MUST be stopped and only
the segments transmission that is triggered by the received ACK is
done.
o The measured RTT is inferior to the RTT of the first segment of
the IW. Consequences are negligible.
5 Open discussions
In this section, we introduce possible improvements for Initial
Spreading and new perspectives.
5.1 Spacing Interval
It has been observed that most of the savings enabled by the Initial
Spreading in congested environments comes from the independence of
the segments sent during the first RTT. Indeed, experimentations have
shown that preventing the bursts, Initial Spreading enables each
segment of the IW to have an independent loss probability.
Currently, Initial Spreading waits RTT/n seconds before transmitting
two segments of the IW, with n the permitted upper bound value of the
TCP's IW.
This simple mechanism offers very good results but has two minor
drawbacks:
(1) An inaccuracy of the space measure (cf section 4.4).
(2) In uncongested networks, Initial Spreading adds an extra delay
that is equal to (IW-1) * RTT/n, with IW the number of segments
(<= n) that can be sent during the first RTT.
A solution could be to set the space not as a ratio of the measured
RTT but as a minimal space that preserves the independence between
the sent segments. Preliminary results show that smaller spacing
interval may allow to maintain the independence of the segment loss
probability. This may provide the same performances in congested
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networks and improve the average delay in uncongested networks.
5.2 Increasing the upper bound TCP's IW to more than 10 segments
[DR10] have shown that an IW of 10 segments enables to send more than
90% of the web objects in one RTT. So the authors recommend to use
Initial Spreading as a complement to [RFC6928].
If the average size of the web objects continues to evolve, Initial
Spreading can be used to raise the IW size. Simulations and
experiments showed even better results with an IW equal to 12.
Thus, Initial Spreading paves the way for larger IW. Further studies
are needed to assess the impact on the networks, notably in terms of
individual performance, fairness, friendliness and global
performance.
5.3 Initial Spreading and LFN
The space community designed middleboxes to mitigate poor TCP
performance for network with large RTT [FA11]. Proxy Enhancement
Performance (PEP) are generally used in LFN and in particular in
satellite communication systems [RFC3135] and offer very good TCP
performance.
Nevertheless, some recent studies have emphasized major impairments
occasioned by the use of satellite-specific transport solutions, and
notably TCP-PEPs, in a global context. The break of the end-to-end
TCP semantic, which is required to isolate the satellite segment, is
notably responsible for an increased complexity in case of mobility
scenarios or security context. This strongly mitigates PEPs benefits
and reopens the debate on their relevance[DC10].
Many researchers have outlined that new TCP releases perform well for
long-lived TCP connections, even in satellite environment [SC12], but
continue to suffer from very poor performance in case of short-lived
TCP connections.
Initial Spreading enables to reduce the RTT consequences for short-
lived TCP connections and could be an end-to-end alternative to PEP.
6 Security Considerations
The security considerations found in [RFC5681] apply to this
document. No additional security problems have been identified with
Initial Spreading at this time.
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7 IANA Considerations
This document contains no IANA considerations.
8 References
8.1 Normative References
[RFC3390] A. Allman and S. Floyd, "Increasing tcp's initial window,"
RFC 3390, IETF, Proposed Standard, 2002.
[RFC5532] T. Talpey, C. Juszczak, "Network File System (NFS) Remote
Direct Memory Access (RDMA) Problem Statement," RFC 5532,
IETF, Informational, May 2009.
[RFC6928] J. Chu, N. Dukkipati, Y. Cheng, and M. Mathis, "Increasing
tcp's initial window," RFC 6928, IETF, Experimental, Jan.
2013.
[AH98] A. Allman, C. Hayes, and S. Ostermann, "An evaluation of TCP
with Larger Initial Windows," ACM Computer Communication
Review, 1998.
[AS00] A. Aggarwal, S. Savage, and T. Anderson, "Understanding the
performance of TCP pacing," in INFOCOM, vol. 3, mar 2000,
pp. 1157-1165.
[DR10] N. Dukkipati, T. Refice, Y. Cheng, J. Chu, T. Herbert, A.
Agarwal, A. Jain, and N. Sutin, "An Argument for
Increasing TCP's Initial Congestion Window," SIGCOMM
Comput. Commun. Rev., vol. 40, no. 3, pp. 26-33, Jun.
2010.
[SB13] R. Sallantin, C. Baudoin, E. Chaput, E. Dubois, F. Arnal, and
A. Beylot, "Initial spreading: a fast start-up tcp
mechanism," proceedings of LCN, 2013.
8.2 Informative References
[RFC3135] J. Border, M. Kojo, J. Griner, G. Montenegro, Z. Shelby,
"Performance Enhancing Proxies Intended to Mitigate Link-
Related Degradations," RFC 3135, IETF, Informational, June
2001.
[DF10] E. Dubois, J. Fasson, C. Donny, and E. Chaput, "Enhancing tcp
based communications in mobile satellite scenarios: Tcp
peps issues and solutions," in Proc. 5th Advanced
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satellite multimedia systems conference (asma) and the
11th signal processing for space communications workshop
(spsc), pages 476-483, 2010.
[FA11] A. Fairhurst, G. Arjuna, H. Cruickshank, and C. Baudoin,
"Transport challenges facing a next generation hybrid
satellite internet," in International Journal of Satellite
Communications and networking, 2011.
[HR08] S. Ha, I. Rhee, and L. Xu, "CUBIC: A New TCP-Friendly High-
Speed TCP Variant," SIGOPS Oper. Syst. Rev., vol. 42, no.
5, pp. 64-74, Jul. 2008.
[LC09] R. Lacamera, D. Caini, C. Firrincieli, "Comparative
performance evaluation of tcp variants on satellite
environments," in ICC'09 Proceedings of the 2009 IEEE
international conference on Communications, pages Pages
5161-5165, 2009.
[SC12] R. Sallantin, E. Chaput, E. P. Dubois, C. Baudoin, F. Arnal,
and A.-L.Beylot, "On the sustainability of PEPs for
satellite Internet access," in ICSSC. AIAA, 2012.
[TS06] K. Tan, J. Song, Q. Zhang, and M. Sridharan, "Compound TCP: A
Scalable and TCP-friendly Congestion Control for High-
speed Networks," in 4th International workshop on
Protocols for Fast Long-Distance Networks (PFLDNet), 2006.
Authors' Addresses
Comments are solicited and should be addressed to the working group's
mailing list at iccrg@irtf.org and/or the authors:
Renaud Sallantin
CNES/TAS/TESA
IRIT/ENSEEIHT 2, rue Charles Camichel BP 7122
31071 Toulouse Cedex 7
France
Phone: +33 6 48 07 86 44
Email: renaud.sallantin@gmail.com
Cedric Baudoin
Thales Alenia Space (TAS)
26 Avenue Jean Francois Champollion,
31100 Toulouse
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France
Email: cedric.baudoin@thalesaleniaspace.com
Fabrice Arnal
Thales Alenia Space
Email: fabrice.arnal@thalesaleniaspace.com
Emmanuel Dubois
Centre National des Etudes Spatiales (CNES)
18 Avenue Edouard Belin
31400 Toulouse
France
Email: emmanuel.Dubois@cnes.Fr
Emmanuel Chaput
IRIT
IRIT / ENSEEIHT 2, rue Charles Camichel BP 7122
31071 Toulouse Cedex 7
France
Email: emmanuel.chaput@enseeiht.fr
Andre-Luc Beylot
IRIT
Email: andre-Luc.Beylot@enseeiht.fr
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