TCP Optimization
Feature Description
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Copyright
© Ericsson AB 2016. All rights reserved. No part of this document may be
reproduced in any form without the written permission of the copyright owner.
Disclaimer
The contents of this document are subject to revision without notice due to
continued progress in methodology, design and manufacturing. Ericsson shall
have no liability for any error or damage of any kind resulting from the use of
this document.
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Contents
Contents
1
Introduction
1
1.1
Basic Characteristics
1
1.2
TCP Optimization Summary
1
1.3
Additional Information
2
2
Feature Operation
3
2.1
Network Requirements
3
2.2
Feature Operation Sequence Diagram
3
2.3
Process Steps
4
2.4
Basic Configuration
4
2.5
TCP Congestion Control
5
2.6
PDCP SDU Timer Discard in the Uplink
6
3
Parameters
8
3.1
Feature Configuration Parameters
8
3.2
Affected Parameters
9
4
Network Impact
11
5
Associated Features and Affected Functions
12
5.1
Prerequisite Features
12
5.2
Affected Features
12
5.3
Related Features
12
5.4
Affected System Functions
13
6
Performance
14
6.1
KPIs
14
6.2
Counters
14
6.3
Events
15
7
Activate TCP Optimization
16
8
Deactivate TCP Optimization
17
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TCP Optimization
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Introduction
1
Introduction
This document describes the TCP Optimization feature and its main benefits
and impacts in the LTE RAN. It is assumed that the reader has a deep
understanding of TCP and TCP congestion control.
1.1
Basic Characteristics
This section describes the basic characteristics of the feature.
Feature name:
TCP Optimization
Product identity:
DU Radio Node, see Feature Overview
Baseband Radio Node, see Licensed Feature Overview
Replaces:
N/A
Dependencies
This feature requires the following RAN features to be active:
•
This feature has no prerequisite features.
This feature affects the following RAN features:
1.2
•
Delay-Based Scheduling and Grant Estimation
•
Data Forwarding at Intra-LTE Handover
•
RLC in Unacknowledged Mode
TCP Optimization Summary
This section describes the benefits of this feature.
The TCP Optimization feature is useful to provide a low queuing delay, while
maintaining high link utilization and thereby improving the perceived end-user
performance in terms of system responsiveness without sacrificing throughput.
The feature is also capable of maintaining a more stable cell throughput under
load.
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TCP Optimization
1.3
Additional Information
More information about this feature and related topics can be found in the
following documentation:
2
•
Quality of Service
•
3GPP TS 36.323: Packet Data Convergence Protocol (PDCP)
•
3GPP TS 36.331: Radio Resource Control (RRC); Protocol specification
•
3GPP TS 23.203: Policy and charging control architecture
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Feature Operation
2
Feature Operation
This section describes the TCP Optimization feature in more detail, including
network configuration requirements and operation flows.
2.1
Network Requirements
This is a licensed feature. This means that for the feature to be operational, a
valid license key must be installed and the feature must be explicitly activated
by setting a MOM attribute.
2.2
Feature Operation Sequence Diagram
The TCP Optimization feature uses a delay-based Active Queue Management
(AQM) algorithm discarding packets before the buffer is full, and thus provides
rapid feedback to the traffic sender for Transmission Control Protocol (TCP)
traffic.
Delay-based AQM has two active modes. Mode 1 is for TCP type traffic and
mode 2 is for Guaranteed Bit Rate (GBR) such as Voice over LTE (VoLTE).
See Figure 1.
Packet source
AQM
Packet sink
RBS
UE
Packet sink
PDCP
Packet source
L0000489B
Figure 1 Figure 1 Delay-based AQM Algorithm Implemented in the RBS and
UL PDCP SDU Timer discard in UE.
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TCP Optimization
2.3
Process Steps
This section describes the main process steps for the feature flow.
The delay-based AQM algorithm makes use of the TCP congestion avoidance
Algorithm, see TCP Congestion Control on page 5. TCP assumes that a
packet loss is due to congestion and therefore it will reduce the number of
bytes it has in flight (congestion window). By discarding a packet we can
reduce the TCP congestion window which results in a shorter queue. In LTE
RAN, the AQM algorithm is implemented in RBS for the downlink only.
However, the UE is configured to use PDCP Service Data Unit (SDU) timer
discard in the uplink.
The delay-based AQM algorithm makes use of parameters based on the type
of traffic managed. For non-GBR data traffic, the algorithm discards a packet
only once it reaches a threshold of age in the buffer, at the same time
maintaining a minimum of packets in the buffer, ensuring that the link
utilization is high while keeping the queueing delay low.
For GBR traffic, the timer threshold is given by the Packet Delay Budget (PDB)
value, as defined in 3GPP TS 23.203.
The queues apply "drop from tail", which means that the incoming packets are
dropped. This delays the congestion signal to the TCP sender which can lead
to more packet loss.
There are two mechanisms for packet discard, regardless whether TCP
Optimization is activated:
Packet discard due to full buffer
This could either be due to the maximum number of
PDCP SDUs per bearer or per eNodeB. The maximum
number of PDCP SDUs per radio bearer is dependent
of the hardware and the UE category. The maximum
number of PDCP SDUs per eNodeB is dependant on
the memory. Incoming packets are discarded.
Stale packets discard
Packets older than the maximum age threshold are
discarded, see Table 3. The threshold is 1 second in
AQM Mode 0. The oldest, i.e. the first, packets are
discarded.
2.4
Basic Configuration
The TCP Optimization feature introduces a new aqmMode parameter under
the qciProfilePredefined MO in the QCI table (see Table 3).
The internal configuration of the AQM algorithm depends on the AQM mode
for the Quality of Service Class Indicator (QCI) of the bearer in question.
Configuration is done slightly different depending on the AQM mode. There
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Feature Operation
are two active AQM modes; the first mode is optimized for GBR services and
the second is optimized for non-GBR services assumed to use TCP. In
general, these map to the resource type with some exceptions for important
traffic, see Table 3.
The AQM Mode can be one of the following:
2.5
AQM mode 0
No AQM is used. Packets are discarded if the
maximum number of packets for each radio bearer or
per eNodeB has been reached. Packets may also be
discarded if they become older than 1 second. The UE
is configured to not use PDCP SDU timer discard.
AQM mode 1
Primarily for non-GBR bearers or bearers with TCP
type traffic. The UE is configured to use PDCP SDU
timer discard. A packet is discarded if it becomes older
than the minimum age threshold. All packets older than
the maximum age threshold are discarded. If there is
packet in queue, there is a time between discards. Let
minimumInterDropTime = 2*minimumAgeThreshold.
Keep lower drop threshold = 5 packets in queue
AQM mode 2
Used for GBR bearers. Packets older than the PDB
may be discarded. Packets older than twice the PDB
are discarded. The UE is configured to use PDCP SDU
timer discard. Keep lower drop threshold = 5 packets in
queue.
TCP Congestion Control
The congestion control in TCP comprises four intertwined algorithms: the
slow-start, the congestion avoidance, the fast-retransmit and fast-recovery
algorithms. This document focuses on the congestion avoidance phase, even
though these principles are equally applicable during the whole lifetime of a
connection.
An end-point in the network cannot know the true Pipe Capacity (PC) for the
connection; instead it has to probe for the PC and therefore the bottleneck
rate. TCP uses three types of signals; if an ACK is received it is a signal that
more bandwidth is available, if a packet is dropped it is a signal of light
congestion, and if there are many packet drops, or, a time-out, it is a signal of
serious congestion. TCP acts on these signals by changing its congestion
window or by starting all over with the initial settings.
Packets are lost for two reasons – they are lost in transit or the network is
congested. The TCP congestion control assumes that a packet was lost due
to congestion.
This assumption is used by this feature. Since the congestion window is
halved at a congestion event (or put in another way, the TCP sender stops
sending for one RTT), the minimum age threshold is optimally set to one PC,
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TCP Optimization
assuming a standard compliant TCP sender and TCP New Reno. This
ensures that the queue is not emptied after a packet discard.
Measured in time, the optimal threshold becomes one RTT. The estimated
RTT is by default 100 ms in order to prioritize throughput when PC changes
suddenly.
Since TCP increases the congestion window slowly, and due to the high
likelihood of improved radio conditions in the target cell after handover, the
minimum age threshold is set to twice the estimated RTT. This has the benefit
that the link utilization will be high also after handover. The drawback is that
the delay in normal operation is slightly higher.
The TCP congestion control, with an IETF Standard compliant TCP sender
and optimal configuration of the eNodeB queue size is shown in Figure 2.
Outstanding
bytes
Discard
2 PC
Packets in
the eNB queue
1 PC
Packets in
the pipe
Non-empty queue
0
t
L0000618A
Figure 2 TCP congestion control, with an IETF Standard compliant TCP
sender and optimal configuration of the eNodeB queue size. The aim is to
have a non-empty queue after a packet discard.
2.6
PDCP SDU Timer Discard in the Uplink
If the AQM Mode is set to 1 or 2, the UE will be configured to use PDCP SDU
timer discard through RRC signalling. If AQM Mode is set to 0 (effectively
disabling the feature) the PDCP SDU timer discard will be configured with a
value of infinity.
The configured value depends on the downlink value. There are only 8
allowed values for discardTimer in the IE PDCP-Config defined in 3GPP
TS 36.331: Radio Resource Control (RRC); Protocol specification, so the
nearest value greater than the minimum age threshold is chosen (see Table
1).
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Feature Operation
Table 1
PDCP SDU timer discard values (discardTimer) in the uplink
Enum
Name
Description
0
ms50
Packet discarded after 50 ms
1
ms100
Packet discarded after 100 ms
2
ms150
Packet discarded after 150 ms
3
ms300
Packet discarded after 300 ms
4
ms500
Packet discarded after 500 ms
5
ms750
Packet discarded after 750 ms
6
ms1500
Packet discarded after 1500 ms
7
infinity
Packet never discarded
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TCP Optimization
3
Parameters
This section describes configuration parameters for the TCP Optimization
feature and parameters affected by activating the feature.
3.1
Feature Configuration Parameters
The structure of the TCP Optimization MOM is shown in Figure 3.
<<MO Class>>
ManagedElement
1
1
1
1
<<MO Class>>
ENodeBFunction
<<MO Class>>
SystemFunctions
1
1
1
<<MO Class>>
EUtranCellFDD
1
<<MO Class>>
QciTable
1
<<MO Class>>
Licensing
estimatedE2ERTT
1
1
1
10
<<MO Class>>
QciProfilePredefined
<<MO Class>>
OptionalFeatures
aqmMode
pdb
1
1
<<MO Class>>
TCPOptimization
L0000617D
Figure 3
TCP Optimization MOM structure
Attributes to configure AQM are described in Table 2.
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Parameters
Table 2
AQM
Attributes of MO QciProfilePredefined and QciProfileOperatorDefined to Configure
Parameter
Description
MO QciProfilePredefined: aqmMode
MO QciProfileOperatorDefined: aqmMode
Indicates the operating modes of the AQM algorithm for specific QCIprofile
QCI characteristics are shown in Table 3.
Table 3
QCI Characteristics
Resource Type
QCI
AQM
Mode
PDB
Minimum Age
Threshold
Maximum Age
Threshold
Example services
GBR
1
2
100
ms
PDB
2• PDB
Conversational voice
2
2
150
ms
PDB
2• PDB
Conversational video (live
streaming)
3
2
50
ms
PDB
2• PDB
Real time gaming
4
2
300
ms
PDB
2• PDB
Non-conversational video
(buffered streaming)
5
0
100
ms
N/A
1s
IMS Signalling
6
1
300
ms
200
ms
1s
Video (Buffered
Streaming) TCP-based (for
example, www, email,
chat, ftp, p2p file sharing,
progressive video, and so
on)
7
1
100
ms
200
ms
1s
Voice, Video (Live
Streaming) Interactive
Gaming
8
1
200
ms
200
ms
1s
Video (Buffered
Streaming),TCP-based (for
example, www, email,
chat, ftp, p2p file sharing,
progressive video, and so
on)
9
1
200
ms
200
ms
1s
Video (Buffered
Streaming), TCP-based
(for example, www, email,
chat, ftp, p2p file sharing,
progressive video, and so
on)
NonGBR
3.2
Affected Parameters
The Packet Delay Budget from the pdb parameter in MO
QciProfilePredefined and QciProfileOperatorDefined is used in
AQM Mode 2 as the minimum age threshold. The maximum age threshold is
twice the PDB.
The pdb parameter is described in Table 4.
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TCP Optimization
Table 4
Affected parameters in MO QciProfilePredefined and QciProfileOperatorDefined
Parameter
Description
MO QciProfilePredefined: pdb
MO QciProfileOperatorDefined: pdb
The contribution from eNodeB to the Packet Delay Budget
(PDB) for a QCI. Packet delays outside eNodeB, for example in
the transport network, are excluded. For more information
about PDB, see 3GPP TS 23.203: Policy and charging control
architecture.
10
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Network Impact
4
Network Impact
This section describes how the TCP Optimization feature impacts the network
functions and capabilities.
This is a feature that impacts the end-to-end behavior, especially TCP
behavior.
The TCP Optimization feature has an impact on ULPacketLossRate. This
happens because UE discards the PDCP packet, resulting in a discontinuous
sequence number in eNodeB. Also, retainability is improved.
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TCP Optimization
5
Associated Features and Affected
Functions
This section describes how the TCP Optimization feature affects other
features and functions.
5.1
Prerequisite Features
This feature has no prerequisite features.
5.2
Affected Features
This section provides information on features that are affected by TCP
Optimization.
5.2.1
Delay-Based Scheduling and Grant Estimation
The feature Delay-Based Scheduling and Grant Estimation delays packets so
that TCP Optimization discards are triggered for GBR bearers if AQM Mode 2
is selected and SchedulingAlgorithm MO in QciProfilePredefined or
QciProfileOperatorDefined is set to DELAY_BASED.
In case AQM Mode 0 is selected there is no dependency for the QCI in
question.
5.2.2
Data Forwarding at Intra-LTE Handover
Packets that are forwarded over S1 or X2 are rejuvenated when they arrive in
the target eNodeB, since their age is unknown to the receiving side. Packets
are timestamped when they are arriving in PDCP, and TCP Optimization acts
on the age only when these packets have been sent to RLC.
Due to the improved radio conditions in the target cell, a higher PC is needed
to have high link utilization. The minimum age threshold is therefore doubled
at the expense of higher delay in normal operation. The gain is higher
throughput after handover.
5.3
Related Features
This feature is not related to any other feature.
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Associated Features and Affected Functions
5.4
Affected System Functions
This feature affects no system functions.
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TCP Optimization
6
Performance
This section describes performance indicators, counters, and events
associated with the TCP Optimization feature.
6.1
KPIs
This feature has no associated Key Performance Indicators (KPIs).
6.2
Counters
Table 5 lists the counters associated with the TCP Optimization feature.
Note:
Table 5
The counters exist in the following MO classes:
•
EUtranCellFDD
•
EUtranCellTDD
Counters
Counter
Description
pmPdcpPktDiscDlAqm
Number of discarded PDCP (Packet Data Convergence Protocol) packets in
downlink due to the AQM algorithm
This is a PEG counter that is incremented by one for each discarded packet.
pmPdcpPktDiscDlAqmQci
The number of discarded PDCP packets in downlink due to AQM algorithm for
each specific QCI
This is a Probability Density Function (PDF) counter where each QCI interval is
incremented by one for each discarded packet for that QCI.
More information about counters can be found in Managed Object Model
(MOM).
Impacted counters are listed in Table 6.
Note:
14
The counters exist in the following MO classes:
•
EUtranCellFDD
•
EUtranCellTDD
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Performance
Table 6
Impacted counters
Counter
Description
pmRlcDelayTimeDl
Aggregated time for the downlink RLC delay measure. The
time for each sample is the time difference between reception
of a PDCP SDU until it is scheduled and sent to the MAC layer
for transmission over the air. Only applicable to DRB packets.
pmRlcDelayPktTransDl
Number of samples for RLC delay measurements.
pmRlcDelayTimeDlQci
Aggregated DL RLC delay for a measurement period per QCI.
The time for each sample is the time difference between
reception of a PDCP SDU until it is scheduled and sent to the
MAC layer for transmission over the air. Only applicable to DRB
packets.
pmRlcDelayPktTransDlQci
Number of samples for DL RLC delay measurements during a
measurement period per QCI.
pmPdcpPktLostUI
Total number of DRB packets (PDCP SDUs) lost in the uplink.
6.3
Events
Table 7 lists the events associated with the TCP Optimization feature.
Table 7
Events
Event
Event Parameter
Description
INTERNAL_PER_UE_RB_TRAFFI
C_REP
EVENT_PARAM_PER_DRB_PACKET_DISC_A
QM_DL
Total number of packets (PDCP SDUs) for
which no part has been transmitted over the
air in the downlink direction that are
discarded due to AQM algorithm
For a full list with detailed information about PM events, see the list files in the
List Files library folder.
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TCP Optimization
7
Activate TCP Optimization
Prerequisites
•
The license key is installed in the node.
•
Continuous Cell Trace Recording (CCTR) is activated since at least one
week. This ensures there is troubleshooting data available if something
goes wrong.
Steps
1. Set the attribute featureState to ACTIVATED in the applicable MO
instance, depending on node type:
Node Type
License Control MO
DU Radio Node
OptionalFeatureLicense=TcpOptimizat
ion
Baseband-based Node FeatureState=CXC4011050
After This Task
Let the CCTR be active for one week, for continued collection of
troubleshooting data.
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Deactivate TCP Optimization
8
Deactivate TCP Optimization
Prerequisites
Continuous Cell Trace Recording (CCTR) is activated since at least one week.
This ensures there is troubleshooting data available if something goes wrong.
Steps
1. Set the attribute featureState to DEACTIVATED in the applicable MO
instance, depending on node type:
Node Type
License Control MO
DU Radio Node
OptionalFeatureLicense=TcpOptimizat
ion
Baseband-based
FeatureState=CXC4011050
After This Task
Let the CCTR be active for one week, for continued collection of
troubleshooting data.
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