Network Working Group I. Johansson
Internet-Draft Ericsson AB
Intended status: Informational October 14, 2015
Expires: April 16, 2016
Congestion control for 4G and 5G access
draft-johansson-cc-for-4g-5g-00
Abstract
This memo outlines the challenge that 4G and 5G access brings for
transport protocol congestion control and also outlines a few simple
examples that can improve transport protocol congestion control
performance in 4G and 5G access.
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 16, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The 4G protocol stack impact on transport protocols . . . . . 3
2.1. PDCP layer . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. RLC layer . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. MAC layer . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Downlink scheduling . . . . . . . . . . . . . . . . . 6
2.3.2. Uplink scheduling . . . . . . . . . . . . . . . . . . 6
3. 4G and 5G evolution . . . . . . . . . . . . . . . . . . . . . 7
4. Requirements for improved performance . . . . . . . . . . . . 8
5. Congestion control examples . . . . . . . . . . . . . . . . . 9
5.1. HyStartRestart . . . . . . . . . . . . . . . . . . . . . 9
5.2. Hybrid Cubic . . . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Wireless access is becoming more and more widely used, 4G (LTE)
access is one wireless access technology that has built in support
for seamless mobility that gives the end user a feeling of being
always connected. Transport endpoints may even be unaware of the
existence of the 4G access. Everyday use for 4G access includes web,
chat, streaming video and lately also WebRTC. These use cases pose
different requirements on the underlying access. Evolving existing
radio-access technologies like LTE, and new 5G technologies will all
be part of a future flexible and dynamic 5G system. 5G has potential
to offer lower latency and higher peak throughput. The goal of this
document is to provide sufficient input to guide the development of
congestion control that is better suited for 4G and 5G access,
without an explicit need to know about the presence of 4G or 5G
access along the transmission path.
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2. The 4G protocol stack impact on transport protocols
This section will go into the different layers in the 4G protocol
stack. It will not delve in the very details, recommended reading
for more details is found in [LTE]. Rather this section will
illustrate what effect each layer can have on the transport protocol
efficiency. The description is focused mainly on default radio
access bearers, which are commonly used for OTT (Over The Top)
services, these bearers typically use Acknowledged Mode (AM), which
means that packet loss only occurs as the result of packet drops in
AQM (Active Queue manager). Specialized services like VoLTE (Voice
over LTE) use different bearer configurations, this is however
outside the scope of this document. The concept of bearer is
mentioned throughout the document, a bearer is to be seen as a data
channel for a given terminal or UE (User Equipment), there could be
many bearers with different priorities for a given terminal.
------------
| |
| IP |
| |
------------
| PDCP |
------------
| RLC |
------------
| MAC |
------------
| PHY |
------------
LTE protocol stack
2.1. PDCP layer
The PDCP (Packet Data Convergence Protocol Layer), ensures that
intra-RAT (Radio Access Type) handover, i.e. LTE to LTE is reasonably
seamless. The PDCP layer makes sure that packets pending
transmission in one cell to terminal connection, are transmitted in
the new cell to terminal connection. This way all packets will be
ensured to be delivered to the endpoint. PDCP also ensures that all
packets are delivered in order up to higher layers.
Packet retransmission typically means that the amount of data to
transmit increases immediately after the handover, first the
retransmission data needs to be transmitted, then the incoming data
needs to be transmitted. Depending on the available link throughput
after the handover, an increased RTT may be experienced at a handover
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event. In addition, a small temporal delay increase can occur as
packet transmission is inhibited at the handover event. Unnecessary,
retransmission at handover can be minimized by means of PDCP status
reports, but this is not always implemented. Because of the above it
is a good practice to keep the amount of data in flight as small as
possible, without sacrificing throughput. Excessive amounts of data
in flight means potentially more data to retransmit at handover and
thus an even more increased RTT.
Packets are typically not lost at handover in a 4G/LTE system.
Reliable delivery at handover may however be turned off or is simply
not implemented, this means that packets may be lost at handover.
The amount of lost packets is then proportional to the number of
packets in flight, it is therefore instrumental that the amount of
packets in flight is as small as possible, with the objective to
reach full link utilization, nothing more. Bufferbloat [REF] can for
instance lead to that 1000s of packets are lost at handover, which is
of course undesired as it can affect media quality quite
considerably.
2.2. RLC layer
The role of the RLC (Radio Link Control) layer is to ensure that
packets are delivered in order up to the higher layers, in addition
the RLC layer corrects errors that can occur on the MAC layer. The
in order delivery constraint means that additional HoL (Head of Line)
blocking delay can occur due to errors on the MAC layer.
The throughput on lower layers can vary quite considerably, this
manifests itself as a varying size of the available transport. The
transport block size depends on how much of the available resources
is allocated to a given bearer at a given time instant and also on
the channel quality. Because of this the size of the transport
blocks can vary from tens of bytes, up to more than 10000 bytes. The
rate of change in transport block size also varies with terminal
mobility as higher terminal mobility means faster changing channel
fading and thus a faster changing channel quality.
For optimal spectrum efficiency, it is important that a sufficient
amount of data is available to fill the transport blocks, this data
needs to be available already when a bearer is scheduled, in practice
within a fraction of a millisecond. To satisfy this requirement,
packets need to be queued up and ready for immediate transmission,
either on the RLC or the PDCP layer. The transport protocol server
(TCP, QUIC) is typically too far away to satisfy this need.
The need to instantly provide sufficient data for optimal spectrum
efficiency, given the variability in transport block sizes and
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scheduling opportunities for a given bearer, quite naturally leads to
a variation in queuing delay and this variation can be larger than
what is to be expected from e.g. fixed line access.
The requirement above to have data available can be seen something
that contradicts the strive for low latency, and there is indeed a
balance to be struck here. What to aim for, maximum throughput or
low latency, is something that depends on the requirements from the
application. A desire for very low latency comes generally at the
cost of reduced peak throughput, this applies to default radio access
bearers. QoS classed bearers can have different characteristics and
may well be able to deliver both very low latency and high
throughput.
2.3. MAC layer
The MAC (Media Access Control) layer handles transmission of
transport blocks, the outcome of a transmission attempt can be either
success or failure. The signaling of the success is handled with a
single ACK/NAK bit. Upon indicated failure, the transport block is
retransmitted (with a different channel coding), soft decoding is
utilized and the softbits of the first transmission and the
retransmission are combined, hopefully with a successful result, if
not the case a 3rd retransmission can occur and so on. This is
referred to as HARQ (Hybrid Automatic Repeat Request). The maximum
number of retransmissions is configurable, if the maximum number of
retransmission is reached without successful transmission then the
RLC layer will have to handle the retransmission instead.
Errors may also happen in the transmission of the ACK/NAK bit. The
event that ACK is decoded as NAK will only lead to that an extra
superfluous retransmission occur. The event that a NAK is decoded as
an ACK will be handled by the RLC layer as the result of a detected
RLC checksum error.
MAC layer retransmissions naturally lead to out of order delivery up
to higher layers as some transport blocks are transmitted error free
while others need retransmission. The role of the RLC layer is to
ensure in order delivery, the effect of this is that HARQ
retransmissions and HARQ failures lead to additional delays.
Scheduling of many bearers has the effect that available resources
have to be shared between two or more bearers. When new bearers with
data to transmit are added in a cell (either handover, or new
traffic), it means that the amount of resources need to be shared
with an additional party. This can give a large drop in available
throughput for already existing users, with the effect of an
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increased queuing delay that decreased only when the transmission
rate over the bearer is reduced.
The downlink and uplink scheduling differs in some details which are
described in the following sub-sections.
2.3.1. Downlink scheduling
Downlink scheduling, or scheduling of packets to terminals, is
controlled by the base station. For each TTI (1ms interval) a
decision is made on which bearer is to become scheduled, i.e.
packets are forwarded from the queue to the terminal in question.
The scheduling decision is based on channel quality, and possibly
historic bitrate for the given bearers, or it may be just a simple
round robin scheduler. The very details of the scheduling algorithms
are vendor specific.
DRX (Discontinuous Reception) is a feature implemented to save
battery power, in which the terminal sleeps and only checks for the
presence on downlink data only at regular intervals.
Given the facts above, downlink data cannot always be transmitted
immediately, this has the effect that additional jitter may be added
(in the order of 10-20ms). Congestion control algorithms that are
tuned with a high sensitivity to delay can by mistake treat this
jitter as congestion.
2.3.2. Uplink scheduling
As is the case with downlink scheduling, uplink scheduling is
controlled by the base station. A terminal that has data to transmit
will first transmit a scheduling request to the base station. The
scheduling request does not indicate how many bytes that are in the
uplink queue. The base station transmits a scheduling grant back,
with a delay that depends on the overall load level. The scheduling
grant indicates how many bytes that can be transmitted by the
terminal. After this the terminal can transmit the allowed number of
bytes, if there is still data in the queue, then a BSR (Buffer Status
Report) is attached to the uplink transmission which will in turn
trigger an additional scheduling grant from the base station to the
terminal and so on until all the data in the queue is transmitted.
The uplink scheduling regime outlined above can break up packet
trains, for instance a set of 10 ACKs in the uplink may be broken up
to an initial transmission of 2 ACKs followed by the transmission of
the remaining 8 ACKs, the HARQ RTT is typically 8ms, which means that
the remaining 8 ACKs are delivered upstream 8ms later. This can
cause problems for congestion control algorithms that depend on e.g.
packet train based estimation of throughput. Also, algorithms that
depend on precise RTT estimates may by mistake treat the occurrence
above as emerging congestion in the downlink.
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This behavior can also trigger coalescing issues similar to those
experienced when ACK compression occur.
Worth notice is also that the above effects can occur at low as well
as high network load levels.
ACK traffic in uplink can also be delayed due to for instance lack of
signaling channel resources for instance if many terminals generate
ACK traffic that is so sparse that scheduling requests need to be
generated with high frequency. A transport protocol design that
tries to limit the amount of ACK traffic can have a performance
benefit under such circumstances as the limitation is then more
controlled and the protocol can be optimized for this. Reduced ACKs
can unfortunately cause coalescing, something that may be mitigated
to some extent by means of packet pacing.
3. 4G and 5G evolution
It is currently unclear in what aspect a 5G protocol stack will
affect transport protocol performance. Listed below are however some
features of evolved 4G and 5G that have relevance in this context:
o Shorter TTIs (Transmission Time Interval) has the potential to
reduce the latency. Given that shorter TTIs have impact on the
scheduling and also the retransmissions, the impact of shorter
TTIs is that errors on the MAC layer will cause less jitter.
o Larger throughput variations can occur as a result of techniques
like carrier aggregation and dual connectivity. Carrier
aggregation means that additional carriers (possibly in very high
frequency bands) are added. Dual connectivity can combine two
similar or different radio access technologies on lower layers
(below IP). Both technologies mentioned above can lead to large
variations in available throughput.
o ECN is specified in 3GPP 36.300 [TS_36300]. ECN can provide with
prompt indication of congestion without the need for packet drop
caused by normal AQM operation, this can provide with a benefit
for e.g. latency sensitive traffic. ECN also gives a explicit
indication of congestion, opposed to the implicit congestion
indications that loss and delay gives.
The shorter TTI feature is part of the 5G standardization, it should
however be stated that the
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4. Requirements for improved performance
The above considerations lead to a few things to consider when
congestion control is designed for optimal performance in 4G and 5G
networks:
o Avoid dependencies on precise RTT estimates: A typical real life
case is the Hybrid Slowstart algorithm bundled with the Cubic
congestion control. Uplink scheduling can break up transmission
of ACKs which will in turn lead to increased RTTs that can falsely
be interpreted as congestion.
o Use timestamps: Related to RTT estimate issue above. For instance
a modified Hybrid Slowstart algorithm can take timestamp values
into account and thus limit the effect of uplink scheduling
effects that can distort the transmission of ACKs.
o Minimize latency under load: The quickly changing throughput in
4G/5G calls for a sensible balance between latency and throughput.
Some amount of bufferbloat needs to be accepted in order to have
enough data to utilize the radio resources optimally and get a
high spectrum efficiency, this can however make the reaction to
reduced throughput more sluggish. Hybrid loss/delay based
congestion control can here be used to find a good balance between
latency and throughput.
o ECN support: The transport protocols should support negotiation
and use of ECN.
o Faster congestion window increase: Traditional AIMD (Additive
Increase Multiplicative Decrease) based congestion avoidance
algorithms are too slow to gain the benefits of e.g added
carriers, therefore, more high speed alternatives should be
considered, that are still reactive to congestion.
o Packet pacing: ACK compression effects can easily occur in 4G/5G
networks, packet pacing should be encouraged to mitigate the
coalescing effects caused by ACK compression, and will at the same
time make ECN detection algorithms more robust.
o ACK reduction: Consider if it is possible to reduce the intensity
of acknowledgements, especially in the uplink. Packet pacing may
here be beneficial as it can mitigate the coalescing effects that
can occur due to reduced ACK intensity.
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5. Congestion control examples
This section lists a few examples of algorithm that can be useful
o Default slowstart algorithms generally only operates at flow
start-up and after a retransmission timeout. The drawback with
this approach is that the congestion control cannot quickly grab
new available capacity due to e.g the addition of an extra
carrier. HyStartRestart is a simple add-on to the Hybrid
Slowstart algorithm that resumes slowstart if it is detected that
the bottleneck is underutilized for a while.
o Hybrid Cubic borrows the OWD (One Way "extra" Delay) estimation
from LEDBAT [RFC6817]. With this addition it is possible to set a
target queuing delay that adds a limit to the congestion window
based on network queuing delay in addition to the already existing
loss based control of the congestion window.
o Various High Speed congestion control algorithms such as SIAD
(Scalable Increase Adaptive Decrease) [TCP_SIAD] can provide with
improved performance in the presence of large changes in available
throughput resulting from e.g added carriers.
The HyStartRestart and Hybrid Cubic algorithms are described in more
detail below. The code samples are shown with the Linux kernel 4.4
version of tcp_cubic.c as basis.
5.1. HyStartRestart
The idea behind HyStartRestart is simply to increase the ssthresh
(slow start threshold) if the RTT has been only marginally higher
than the min RTT for a number of round trips. The HyStart delay
algorithm used for this purpose. Code for this is shown below with
the code from Linux tcp_cubic.c as basis
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Function bictcp_acked(..) is modified according
to the code snippet below
New state variables are added
u32 last_rtt_high
u32 last_hyrestart
New constants
#define N_RTT_LOW 5
#define N_RTT_HYRESTART 10
/** Old code **/
/* first time call or link delay decreases */
if (ca->delay_min == 0 || ca->delay_min > delay)
ca->delay_min = delay;
/** New code **/
/*
* Function bictcp_acked is modified to increase snd_ssthresh when
* RTT is lower than a given value for a given number of RTTs
*/
if (ca->curr_rtt > ca->delay_min +
HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
ca->last_rtt_high = bictcp_clock();
} else {
u32 now = bictcp_clock();
if (now - ca->last_rtt_high > N_RTT_LOW*ca->delay_min &&
now - ca->last_hyrestart > N_RTT_HYRESTART*ca->delay_min) {
/* Double ssthresh */
tp->snd_ssthresh = tp->snd_ssthresh << 1;
}
}
/** End of new code **/
/** Old code **/
/* hystart triggers when cwnd is larger than some threshold */
if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
tp->snd_cwnd >= hystart_low_window)
hystart_update(sk, delay);
HyStartRestart code
The code above needs to be complemented with a limitation to
ssthresh. Furthermore ca->last_hyrestart should be updated to
current time whenever a loss or ECN event is detected.
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5.2. Hybrid Cubic
The Hybrid Cubic algorithm adds delay sensitivity to the Cubic
congestion avoidance algorithm. It is, in the description below
assumed that the timestamp option is enabled and that OWD samples are
computed, according to the description in LEDBAT [RFC6817].
Furthermore it is assumed than the timestamp clock frequency in
sender and receiver are identical or that the sender can infer the
timestamp clock frequency of the receiver and recompute timestamp
values based on this information.
The function bictcp_update is updated according to the
code snippet below.
New state variables
float owd
u32 last_hycubic_cwnd_reduced
New constants
#define OWD_TARGET 0.1
#define OWD_GAIN_UP 100.0
#define OWD_GAIN_DOWN 0.1
/** New code, inserted before tcp_friendlness: **/
/*
* The cnt variable is modified depending on the
* relation between the OWD and the OWD target
*/
if (ca->owd < OWD_TARGET) {
float tmp = ca->owd/OWD_TARGET;
int cnt_d = (int) (tmp*OWD_GAIN_UP);
/*
* OWD is less than OWD target
* Increase cnt as OWD is approaching target
* This will slow down congestion window growth
* when owd increases
*/
ca->cnt += cnt_d;
} else {
/*
* Set cnt to a low value, this will result in an
* immediate reduction of CWND
*/
ca->cnt = 1;
}
/** End of new code **/
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/* TCP Friendly */
if (tcp_friendliness) {
u32 scale = beta_scale;
delta = (cwnd * scale) >> 3;
/** New code **/
if (ca->owd < OWD_TARGET) {
/** End of new code **/
while (ca->ack_cnt > delta) { /* update tcp cwnd */
ca->ack_cnt -= delta;
ca->tcp_cwnd++;
}
/** New code **/
} else {
u32 now = bictcp_clock();
if (now-ca->last_hycubic_cwnd_reduced > delay) {
/* At most one reduction per RTT
float overshoot = (owd-OWD_TARGET)/delay;
float alpha = MIN(0.5,overshoot*OWD_GAIN_DOWN);
ca->tcp_cwnd = (int)(ca->tcp_cwnd*(1.0-alpha));
ca->ssthresh = ca->tcp_cwnd;
ca->epoch_start = 0;
ca->last_hycubic_cwnd_reduced = now;
}
}
/** End of new code **/
if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */
delta = ca->tcp_cwnd - cwnd;
max_cnt = cwnd / delta;
if (ca->cnt > max_cnt)
ca->cnt = max_cnt;
}
}
Hybrid Cubic
Note that the code is not fully functional, for instance the floating
point arithmetic need to be converted to fixed point ditto.
6. IANA Considerations
This document makes no request of IANA.
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7. Security Considerations
The possible outcome of this work has the same possible security
considerations as other work around congestion control.
8. Acknowledgements
The following persons have contributed with comments and suggestions
for improvements: Kristofer Sandlund, Mats Nordberg, Hans Hannu,
Torsten Dudda and Szilveszter Nadas.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[LTE] "4G: LTE/LTE-Advanced for Mobile Broadband, Second
Edition", 2013.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<http://www.rfc-editor.org/info/rfc6817>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<http://www.rfc-editor.org/info/rfc7567>.
[TCP_SIAD]
"TCP SIAD: Congestion Control
https://www.ietf.org/proceedings/91/slides/slides-91-
iccrg-5.pdf", 2015.
[TS_36300]
"3GPP TS 36.300", 2015.
Author's Address
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Ingemar Johansson
Ericsson AB
Laboratoriegraend 11
Luleaa 977 53
Sweden
Phone: +46 730783289
Email: ingemar.s.johansson@ericsson.com
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