Internet Engineering Task Force C. Raiciu
Internet-Draft University Politehnica of
Intended status: Experimental Bucharest
Expires: January 30, 2012 M. Handley
D. Wischik
University College London
July 29, 2011
Coupled Congestion Control for Multipath Transport Protocols
draft-ietf-mptcp-congestion-07
Abstract
Often endpoints are connected by multiple paths, but communications
are usually restricted to a single path per connection. Resource
usage within the network would be more efficient were it possible for
these multiple paths to be used concurrently. Multipath TCP is a
proposal to achieve multipath transport in TCP.
New congestion control algorithms are needed for multipath transport
protocols such as Multipath TCP, as single path algorithms have a
series of issues in the multipath context. One of the prominent
problems is that running existing algorithms such as standard TCP
independently on each path would give the multipath flow more than
its fair share at a bottleneck link traversed by more than one of its
subflows. Further, it is desirable that a source with multiple paths
available will transfer more traffic using the least congested of the
paths, achieving a property called resource pooling where a bundle of
links effectively behaves like one shared link with bigger-capacity.
This would increase the overall efficiency of the network and also
its robustness to failure.
This document presents a congestion control algorithm which couples
the congestion control algorithms running on different subflows by
linking their increase functions, and dynamically controls the
overall aggressiveness of the multipath flow. The result is a
practical algorithm that is fair to TCP at bottlenecks while moving
traffic away from congested links.
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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Drafts is at http://datatracker.ietf.org/drafts/current/.
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Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Coupled Congestion Control Algorithm . . . . . . . . . . . . . 6
4. Implementation Considerations . . . . . . . . . . . . . . . . 7
4.1. Computing alpha in Practice . . . . . . . . . . . . . . . 8
4.2. Implementation Considerations when CWND is Expressed
in Packets . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. 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 RFC 2119 [RFC2119] .
2. Introduction
Multipath TCP (MPTCP, [I-D.ietf-mptcp-multiaddressed]) is a set of
extensions to regular TCP [RFC0793] that allows one TCP connection to
be spread across multiple paths. MPTCP distributes load through the
creation of separate "subflows" across potentially disjoint paths.
How should congestion control be performed for multipath TCP? First,
each subflow must have its own congestion control state (i.e. cwnd)
so that capacity on that path is matched by offered load. The
simplest way to achieve this goal is to simply run standard TCP
congestion control on each subflow. However this solution is
unsatisfactory as it gives the multipath flow an unfair share when
the paths taken by its different subflows share a common bottleneck.
Bottleneck fairness is just one requirement multipath congestion
control should meet. The following three goals capture the desirable
properties of a practical multipath congestion control algorithm:
o Goal 1 (Improve Throughput) A multipath flow should perform at
least as well as a single path flow would on the best of the paths
available to it.
o Goal 2 (Do no harm) A multipath flow should not take up more
capacity from any of the resources shared by its different paths,
than if it was a single flow using only one of these paths. This
guarantees it will not unduly harm other flows.
o Goal 3 (Balance congestion) A multipath flow should move as much
traffic as possible off its most congested paths, subject to
meeting the first two goals.
Goals 1 and 2 together ensure fairness at the bottleneck. Goal 3
captures the concept of resource pooling [WISCHIK]: if each multipath
flow sends more data through its least congested path, the traffic in
the network will move away from congested areas. This improves
robustness and overall throughput, among other things. The way to
achieve resource pooling is to effectively "couple" the congestion
control loops for the different subflows.
We propose an algorithm that couples the additive increase function
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of the subflows, and uses unmodified TCP behavior in case of a drop.
The algorithm relies on the traditional TCP mechanisms to detect
drops, to retransmit data, etc.
Detecting shared bottlenecks reliably is quite difficult, but is just
one part of a bigger question. This bigger question is how much
bandwidth a multipath user should use in total, even if there is no
shared bottleneck.
The congestion controller aims to set the multipath flow's aggregate
bandwidth to be the same as a regular TCP flow would get on the best
path available to the multipath flow. To estimate the bandwidth of a
regular TCP flow, the multipath flow estimates loss rates and round
trip times and computes the target rate. Then it adjusts the overall
aggresiveness (parameter alpha) to achieve the desired rate.
While the mechanism above applies always, its effect depends on
whether the multipath TCP flow influences or does not influence the
link loss rates (low vs. high statistical multiplexing). If MPTCP
does not influence link loss rates, MPTCP will get the same
throughput as TCP on the best path. In cases with low statistical
multiplexing, where the multipath flow influences the loss rates on
the path, the multipath throughput will be strictly higher than a
single TCP would get on any of the paths. In particular, if using
two idle paths, multipath throughput will be sum of the two paths'
throughput.
This algorithm ensures bottleneck fairness and fairness in the
broader, network sense. We acknowledge that current TCP fairness
criteria are far from ideal, but a multipath TCP needs to be
deployable in the current Internet. If needed, new fairness criteria
can be implemented by the same algorithm we propose by appropriately
scaling the overall aggressiveness.
It is intended that the algorithm presented here can be applied to
other multipath transport protocols, such as alternative multipath
extensions to TCP, or indeed any other congestion-aware transport
protocols. However, for the purposes of example this document will,
where appropriate, refer to the MPTCP protocol.
The design decisions and evaluation of the congestion control
algorithm are published in [NSDI].
The algorithm presented here only extends standard TCP congestion
control for multipath operation. It is foreseeable that other
congestion controllers will be implemented for multipath transport to
achieve the bandwidth-scaling properties of the newer congestion
control algorithms for regular TCP (such as Compound TCP and Cubic).
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3. Coupled Congestion Control Algorithm
The algorithm we present only applies to the increase phase of the
congestion avoidance state specifying how the window inflates upon
receiving an ack. The slow start, fast retransmit, and fast recovery
algorithms, as well as the multiplicative decrease of the congestion
avoidance state are the same as in standard TCP[RFC5681].
Let cwnd_i be the congestion window on the subflow i. Let tot_cwnd
be the sum of the congestion windows of all subflows in the
connection. Let p_i, rtt_i and mss_i be the loss rate, round trip
time (i.e. smoothed round trip time estimate used by TCP) and maximum
segment size on subflow i.
We assume throughout this document that the congestion window is
maintained in bytes, unless otherwise specified. We briefly describe
the algorithm for packet-based implementations of cwnd in section
Section 4.2.
Our proposed "Linked Increases" algorithm MUST:
o For each ack received on subflow i, increase cwnd_i by
alpha * bytes_acked * mss_i bytes_acked * mss_i
min ( --------------------------- , ------------------- ) (1)
tot_cwnd cwnd_i
The increase formula (1) takes the minimum between the computed
increase for the multipath subflow (first argument to min), and the
increase TCP would get in the same scenario (the second argument).
In this way, we ensure that any multipath subflow cannot be more
aggressive than a TCP flow in the same circumstances, hence achieving
goal 2 (do no harm).
"alpha" is a parameter of the algorithm that describes the
aggresiveness of the multipath flow. To meet Goal 1 (improve
throughput), the value of alpha is chosen such that the aggregate
throughput of the multipath flow is equal to the throughput a TCP
flow would get if it ran on the best path.
To get an intuition of what the algorithm is trying to do, let's take
the case where all the subflows have the same round trip time and
MSS. In this case the algorithm will grow the total window by
approximately alpha*MSS per RTT. This increase is distributed to the
individual flows according to their instantaneous window size.
Subflow i will increase by alpha*cwnd_i/tot_cwnd segments per RTT.
Note that, as in standard TCP, when tot_cwnd is large the increase
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may be 0. In this case the increase MUST be set to 1. We discuss
how to implement this formula in practice in the next section.
We assume implementations use an approach similar to appropriate byte
counting (ABC, [RFC3465]), where the bytes_acked variable records the
number of bytes newly acknowledged. If this is not the case,
bytes_acked SHOULD be set to mss_i.
To compute tot_cwnd, it is an easy mistake to sum up cwnd_i across
all subflows: when a flow is in fast retransmit, its cwnd is
typically inflated and no longer represents the real congestion
window. The correct behavior is to use the ssthresh value for flows
in fast retransmit when computing tot_cwnd. To cater for connections
that are app limited, the computation should consider the minimum
between flight_size_i and cwnd_i, and flight_size_i and ssthresh_i
where appropriate.
The total throughput of a multipath flow depends on the value of
alpha and the loss rates, maximum segment sizes and round trip times
of its paths. Since we require that the total throughput is no worse
than the throughput a single TCP would get on the best path, it is
impossible to choose a-priori a single value of alpha that achieves
the desired throughput in every occasion. Hence, alpha must be
computed based on the observed properties of the paths.
The formula to compute alpha is:
cwnd_i
max --------
i 2
rtt_i
alpha = tot_cwnd * ---------------- (2)
/ cwnd_i \ 2
| sum ---------|
\ i rtt_i /
The formula (2) is derived by equalizing the rate of the multipath
flow with the rate of a TCP running on the best path, and solving for
alpha.
4. Implementation Considerations
Equation (2) implies that alpha is a floating point value. This
would require performing costly floating point operations whenever an
ACK is received, Further, in many kernels floating point operations
are disabled. There is an easy way to approximate the above
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calculations using integer arithmetic.
4.1. Computing alpha in Practice
Let alpha_scale be an integer. When computing alpha, use alpha_scale
* tot_cwnd instead of tot_cwnd, and do all the operations in integer
arithmetic.
Then, scale down the increase per ack by alpha_scale. The resulting
algorithm is a simple change from Equation (1):
o For each ack received on subflow i, increase cwnd_i by
alpha * bytes_acked * mss_i bytes_acked * mss_i
min ( --------------------------- , ------------------- ) (3)
alpha_scale * tot_cwnd cwnd_i
The alpha_scale parameter denotes the precision we want for computing
alpha. Observe that the errors in computing the numerator or the
denominator in the formula for alpha are quite small, as the cwnd in
bytes is typically much larger than the RTT (measured in ms).
With these changes, all the operations can be done using integer
arithmetic. We propose alpha_scale be a small power of two, to allow
using faster shift operations instead of multiplication and division.
Our experiments show that using alpha_scale=512 works well in a wide
range of scenarios. Increasing alpha_scale increases precision, but
also increases the risk of overflow when computing alpha. Using
64bit operations would solve this issue. Another option is to
dynamically adjust alpha_scale when computing alpha; in this way we
avoid overflow and obtain maximum precision.
It is possible to implement the algorithm by calculating tot_cwnd on
each ack, however this would be costly especially when the number of
subflows is large. To avoid this overhead the implementation MAY
choose to maintain a new per connection state variable called
tot_cwnd. If it does so, the implementation will update tot_cwnd
value whenever the individual subflows' windows are updated.
Updating only requires one more addition or subtraction operation
compared to the regular, per subflow congestion control code, so its
performance impact should be minimal.
Computing alpha per ack is also costly. We propose alpha to be a per
connection variable, computed whenever there is a drop and once per
RTT otherwise. More specifically, let cwnd_new be the new value of
the congestion window after it is inflated or after a drop. Update
alpha only if the quotient of cwnd_i/mss_i differs from the quotient
of cwnd_new_i/mss_i.
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In certain cases with small RTTs, computing alpha can still be
expensive. We observe that if RTTs were constant, it is sufficient
to compute alpha once per drop, as alpha does not change between
drops (the insight here is that cwnd_i/cwnd_j = constant as long as
both windows increase). Experimental results show that even if round
trip times are not constant, using average round trip time per
sawtooth instead of instantaneous round trip time (i.e. TCP's
smoothed RTT estimator) gives good precision for computing alpha.
Hence, it is possible to compute alpha only once per drop using a
modified version of equation (2) where rtt_i is replaced with
rtt_avg_i.
If using average round trip time, rtt_avg_i will be computed by
sampling the rtt_i whenever the window can accommodate one more
packet, i.e. when cwnd / mss < (cwnd+increase)/mss. The samples are
averaged once per sawtooth into rtt_avg_i. This sampling ensures
that there is no sampling bias for larger windows.
Given tot_cwnd and alpha, the congestion control algorithm is run for
each subflow independently, with similar complexity to the standard
TCP increase code [RFC5681].
4.2. Implementation Considerations when CWND is Expressed in Packets
When the congestion control algorithm maintains cwnd in packets
rather than bytes, the algorithms above must change to take into
account path mss.
To compute the increase when an ack is received, the implementation
for multipath congestion control is a simple extension of the
standard TCP code. In standard TCP cwnd_cnt is an additional state
variable that tracks the number of segments acked since the last cwnd
increment; cwnd is incremented only when cwnd_cnt > cwnd; then
cwnd_cnt is set to 0.
In the multipath case, cwnd_cnt_i is maintained for each subflow as
above, and cwnd_i is increased by 1 when cwnd_cnt_i > max(alpha_scale
* tot_cwnd / alpha, cwnd_i).
When computing alpha for packet-based stacks, the errors in computing
the terms in the denominator are larger (this is because cwnd is much
smaller and rtt may be comparatively large). Let max be the index of
the subflow used in the numerator. To reduce errors, it is easiest
to move rtt_max (once calculated) from the numerator to the
denominator, changing equation (2) to obtain the equivalent formula
below.
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cwnd_max
alpha = alpha_scale * tot_cwnd * ----------------------- (4)
/ rtt_max * cwnd_i \ 2
| sum -----------------|
\ i rtt_i /
Note that the calculation of alpha does not take into account path
mss, and is the same for stacks that keep cwnd in bytes or packets.
With this formula, the algorithm for computing alpha will match the
rate of TCP on the best path in B/s for byte-oriented stacks, and in
packets/s in packet-based stacks. In practice, mss rarely changes
between paths so this shouldn't be a problem.
However, it is simple to derive formulae allowing packet-based stacks
to achieve byte rate fairness (and viceversa) if needed. In
particular, for packet-based stacks wanting byte-rate fairness,
equation (4) above changes as follows: cwnd_max is replaced by
cwnd_max * mss_max * mss_max, while cwnd_i is replaced with cwnd_i *
mss_i.
5. Discussion
The algorithm we've presented fully achieves Goals 1 and 2, but does
not achieve full resource pooling (Goal 3). Resource pooling
requires that no traffic should be transferred on links with higher
loss rates. To achieve perfect resource pooling, one must couple
both increase and decrease of congestion windows across subflows, as
in [KELLY].
There are a few problems with such a fully-coupled controller.
First, it will probe insufficiently paths with high loss rates, and
will fail to detect free capacity when it becomes available. Second,
such controllers tend to exhibit "flappiness": when the paths have
similar levels of congestion, the congestion controller will tend to
allocate all the window to one random subflow, and allocate zero
window to the other subflows. The controller will perform random
flips between these stable points. This doesn't seem desirable in
general, and is particularly bad when the achieved rates depend on
the RTT (as in the current Internet): in such a case, the resulting
rate with fluctuate unpredictably depending on which state the
controller is in, hence violating Goal 1.
By only coupling increases our proposal probes high-loss paths,
detecting free capacity quicker. Our proposal does not suffer from
flappiness but also achieves less resource pooling. The algorithm
will allocate window to the subflows such that p_i * cwnd_i =
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constant, for all i. Thus, when the loss rates of the subflows are
equal, each subflow will get an equal window, removing flappiness.
When the loss rates differ, progressively more window will be
allocated to the flow with the lower loss rate. In contrast, perfect
resource pooling requires that all the window should be allocated on
the path with the lowest loss rate. Further details can be found in
[NSDI].
6. Security Considerations
One security concern relates to what we call the traffic-shifting
attack: on-path attackers can drop packets belonging to a multipath
subflow, which in turn makes the path seem congested and will force
the sender's congestion controller to avoid that path and push more
data over alternate subflows.
The attacker's goal is to create congestion on the corresponding
alternative paths. This behaviour is entirely feasible, but will
only have minor effects: by design, the coupled congestion controller
is less (or similarly) aggressive on any of its paths than a single
TCP flow. Thus, the biggest effect this attack can have is to make a
multipath subflow be as aggressive as a single TCP flow.
Another effect of the traffic-shifting attack is that the new path
can monitor all the traffic, whereas before it could only see a
subset of traffic. We believe that if privacy is needed, splitting
traffic across multiple paths with MPTCP is not the right solution in
the first place; end-to-end encryption should be used instead.
Besides the traffic-shifting attack mentioned above, the coupled
congestion control algorithm defined in this draft adds no other
security considerations to those found in
[I-D.ietf-mptcp-multiaddressed] and [RFC6181]. Detailed security
analysis for the Multipath TCP protocol itself is included in
[I-D.ietf-mptcp-multiaddressed] and [RFC6181].
7. Acknowledgements
We thank Christoph Paasch for his suggestions for computing alpha in
packet-based stacks. The authors are supported by Trilogy
(http://www.trilogy-project.org), a research project (ICT-216372)
partially funded by the European Community under its Seventh
Framework Program. The views expressed here are those of the
author(s) only. The European Commission is not liable for any use
that may be made of the information in this document.
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8. IANA Considerations
This document does not require any action from IANA.
9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
9.2. Informative References
[I-D.ietf-mptcp-multiaddressed]
Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", draft-ietf-mptcp-multiaddressed-04 (work in
progress), July 2011.
[KELLY] Kelly, F. and T. Voice, "Stability of end-to-end
algorithms for joint routing and rate control", ACM
SIGCOMM CCR vol. 35 num. 2, pp. 5-12, 2005,
<http://portal.acm.org/citation.cfm?id=1064415>.
[NSDI] Wischik, D., Raiciu, C., Greenhalgh, A., and M. Handley,
"Design, Implementation and Evaluation of Congestion
Control for Multipath TCP", Usenix NSDI , March 2011, <htt
p://www.cs.ucl.ac.uk/staff/c.raiciu/files/mptcp-nsdi.pdf>.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, February 2003.
[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for
Multipath Operation with Multiple Addresses", RFC 6181,
March 2011.
[WISCHIK] Wischik, D., Handley, M., and M. Bagnulo Braun, "The
Resource Pooling Principle", ACM SIGCOMM CCR vol. 38 num.
5, pp. 47-52, October 2008,
<http://ccr.sigcomm.org/online/files/p47-handleyA4.pdf>.
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Authors' Addresses
Costin Raiciu
University Politehnica of Bucharest
Splaiul Independentei 313
Bucharest
Romania
Email: costin.raiciu@cs.pub.ro
Mark Handley
University College London
Gower Street
London WC1E 6BT
UK
Email: m.handley@cs.ucl.ac.uk
Damon Wischik
University College London
Gower Street
London WC1E 6BT
UK
Email: d.wischik@cs.ucl.ac.uk
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