Low-Latency
Adaptive Streaming
Over TCP
Author
Ashvin Goal
(University of Toronto)
Charles Krasic
(University of British Columbia)
Jonathan Walpole (Portland State University)
Presented By
Kulkarni Ameya.s
JongHwa Song
Concept of Paper
Benefits of TCP for Media streaming –
Congestion control delivery
Flow control
Reliable delivery -Packet loss recovery
 Problem:- TCP introduces latency at application level
 Solution proposed:- adaptive buffer-size tuning technique
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Agenda
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First 2 sections talk about the ‘Challenge’
Next section analyses ways in which TCP introduces
latency
Adaptive send-buffer technique for reducing TCP latency
Effect on throughput and Tradeoff – Network throughput
and latency
Implementation
Real streaming application example
Justification of Benefits
The Challenge
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Application must adapt media quality in response to TCP’s
estimate of current bandwidth availability
Adaptive streaming applications - prioritized data
dropping and dynamic rate shaping
Application must make adaptation decisions far in
advance of data transmission
Result –adaptation is unresponsive and performs poorly
as the available bandwidth varies over time
TCP induced latency
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End to end latency consists of
1. Application level latency
2. Protocol latency
TCP introduces protocol latency in 3 ways:1. Packet retransmission
2. Congestion control
3. Sender side buffering
TCP congestion window
Window size (CWND)= max number of
unacknowledged and distinct packets in flight
 Send buffer keeps copies of packets in flight
 Throughput of TCP stream = CWND/RTT
 TCP induces latency in following ways:1. Packet retransmission – at least 1 RTT delay
2. Congestion control- at least 1.5 RTT delay
3. Sender side buffering
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Adaptive send buffer tuning
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Latency is introduced into the network because of
blocked packets in the send buffer
Reduce the send buffer size to CWND
Send Buffer size should never be less than CWND
Tune the send buffer size to follow the CWND
Since the CWND changes dynamically over time this
technique is called Adaptive send buffer tuning and TCP
connection is MIN_BUF TCP flow
MIN_BUF TCP
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Blocks an application from writing data to a socket when
there are CWND packets in send buffer
Application can write packet to socket only when an ack
arrives and the CWND is opened up
MIN_BUF TCP moves the latency due to blocked packets
to the application level
Application has much greater control in sending timecritical data
Evaluation
Forward path congestion topology
Reverse path congestion topology
Comparison of Latencies
Other factors affecting latency
Experiments performed with smaller bandwidth 10 Mb/s
at the router
 Experiments performed with smaller round trip time (25
ms and 50 ms)
 Observation –
1. Latency increases with lesser available BW
2. Latency decreases with lower RTT
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Experiment with ECN
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ECN – External Congestion Notification
ECN MIN_BUF TCP still performs better than TCP flows
TCP with ECN but without MIN_BUF drops few packets
but still suffers large delays because of blocked packets
Thus, MIN_BUF TCP gives more control and flexibility to
the application in terms of what data to be sent and when
it should be sent
Eg – high priority data
Next – we will delve on TCP throughput
EFFECT ON THROUGHPUT
Consideration
: Latency + Throughput
Packet for the ACK arrival
Standard TCP -> Send Immediately
MIN_BUF TCP-> Wait until writing the next
packet
Solution : Adjusting the buffer size
(slightly larger than CWND)
Considering Event
A. ACK Arrival
Standard TCP
When an ACK arrives for the first packet in TCP window,
the window admits a new packet in the window.
MIN_BUF TCP
Buffer one additional packet to send immediately
B. Delayed ACK
Purpose : To save bandwidth in the reverse direction
Function : One ACK for every two data packets received.
Each ACK arrival opens TCP’s window by two packets
MIN_BUF TCP
Buffer two instead of one additional packets
5. EFFECT ON THROUGHPUT
Considering Event
C. CWND Increase
TCP increments CWND by 1 on every round-trip time
ACK arrival -> releasing two packets
Delayed ACKs -> Three additional packets.
Byte-counting Algorithm
Mitigating the impact of delayed ACKs on the growth of CWND
D. ACK Compression
Function : At routers, ACK can arrive at the sender in a busty
Worst Case : The ACKs for all the CWND packets arrive together
Solution of MIN_BUF TCP
2* CWND packets(default send buffer size is large)
5. EFFECT ON THROUGHPUT
Considering Event
E. Dropped ACK
When the reverse path is congested, ACK packets drops
Later ACK -> acknowledge more than two packets
Act similar to ACK Compression
5. EFFECT ON THROUGHPUT
MIN_BUF TCP Streams
A*CWND + B (A>0, B≥0)
(A) Handling any bandwidth reduction caused by ACK compression
(B) Taking ACK arrivals, delayed ACK arrivals, delayed ACKs and
CWND increase into account
(A ≥ 2, B≥1, TCP send & acknowledged packets is unaffected so
Throughput between MIN_BUF TCP and TCP is comparable)
Tradeoff of A and B => latency and throughput
(every additional block packet -> increase latency)
MIN_BUF(A,B) ( default size : MIN_BUF(1,0) )
5. EFFECT ON THROUGHPUT
Evaluation
MIN_BUF(1,0) -> original stream
MIN_BUF(1,3) -> take ACK arrivals, delayed ACKs
CWND increase into account
MIN_BUF(2,0) -> take ACK compression and dropped ACKs
5. EFFECT ON THROUGHPUT
Evaluation
X- axis : protocol latency in milliseconds
Y- axis : percentage of packets that arrive at the receiver
within a delay threshold
5. EFFECT ON THROUGHPUT
Evaluation
160 500
160 ms threshold : Requirement of interactive application
such as video conferencing
500 ms threshold : Requirement of media control operations
Each experiment performs 8 times & latencies accumulated
over all the runs
5. EFFECT ON THROUGHPUT
Forward path topology
160 ms threshold
MIN-BUF(1,0)
MIN-BUF(1,3)
:less than 2%
MIN-BUF(2,0)
:10%
TCP
:30%
5. EFFECT ON THROUGHPUT
Reverse path topology
Acknowledgement
->Drops
Slightly
increase delay
160 ms threshold
MIN-BUF(1,0)
MIN-BUF(1,3)
:less than 10%
TCP
:40%
5. EFFECT ON THROUGHPUT
Normalized Throughput
MIN-BUF(2,0) : Close to Std TCP
MIN-BUF(1,0) : Least Throughput
TCP has no new packets in the send buffer after each ACK is received
MIN-BUF(1,3) : 95% -> Achieving low latency and good throughput
5. EFFECT ON THROUGHPUT
System Overhead
Write Data to the Kernel
: High system overhead because more system calls are invoked to
transfer the same amount of data.
Write (MIN_BUF(1,0) slightly over)
MIN_BUF TCP one packet at a time
TCP several packets at a time
TCP amortized context switching overhead
5. EFFECT ON THROUGHPUT
System Overhead
Poll calls (MIN_BUF(1,0) significantly more overhead)
Standard TCP poll calls after every 14 writes
MIN_BUF(1,0) called everytime
Ratio between two is 12.66
5. EFFECT ON THROUGHPUT
System Overhead
Total CPU time (MIN_BUF(1,0) three times)
As a result of fine-grained write
To redece overhead -> Larger values of the MIN_BUF parameters
5. EFFECT ON THROUGHPUT
IMPLEMENTATION
MIN BUF TCP approach with a small modification
in the Linux 2.4 kernal :
Using a new SO_TCP_MIN BUF option
Limit send buffer A∗CWND+MIN(B, CWND)segments
(segments are packets of maximum segment size or MSS)
The send buffer size is at least CWND
because A must be an integer greater than zero and
B is zero or larger
Default A is one and B is zero
IMPLEMENTATION
A. Sack Correction
Term “sacked out” to A ∗ CWND + MIN(B, CWND).
The sacked out term is maintained by a TCP SACK sender and is
the number of selectively acknowledged packets.
To ensure that the send buffer limit includes this window and is
thus at least CWND+sacked out.
Without this correction,TCP SACK is unable to send new packets
for a MIN_BUF flow and assumes that the flow is application
limited
6. IMPLEMENTATION
IMPLEMENTATION
B. Alternate Application-Level Implementation
The application would stop writing data when the socket buffer
has a fill level of packets.
The problem with this approach is that the application
has to poll the socket fill level.
Polling is potentially both expensive in terms of CPU consumption
and inaccurate since the application is not informed immediately
when the socket-fill level goes below the threshold
6. IMPLEMENTATION
IMPLEMENTATION
C. Application Model
MIN BUF TCP should explicitly align their data
Two benefits:
(1) it minimizes any latency due to coalescing or fragmenting of packets
below the application layer.
(2) it ensures that low-latency applications are aware of the latency cost
and throughput overhead of coalescing or fragmenting application data
into network packets.
For alignment, an application should write maximum segment size
(MSS) packets on each write
TCP CORK socket option in Linux
improves throughput &not affect protocol latency
6. IMPLEMENTATION
Application-LEVEL Evaluation
Evaluate timing behavior of a real live streaming
MIN_BUF TCP helps improving end to end latency
Qstream
(open-source adaptive streaming application)
- Adaptive media format
- Adaptation mechanism
Application-LEVEL Evaluation
Adaptive media format :
SPEQ(scalable MPEQ)
Variant of MPEG-1 that supports layered encoding of video
data that allows dynamic data dropping
Adaptation mechanism :
PSS(priority-progress streaming)
The key idea is an adaptation period, which determines
how often the sender drops data. Within each adaptation
period, the sender sends data packets in priority order,
from the highest priority to the lowest priority
7. Application level evaluation
Evaluation Methodology
7. Application level evaluation
Result
7. Application level evaluation
Result
7. Application level evaluation
CONCLUSIONS
Tuning TCP’s send buffer
: low latency streaming over TCP
: show the significant effect on TCP of latency
at the application level
Extra Packet(Blocked Packet)
: Help to recover throughput without increasing
protocol latency
Used layered media encoding for evaluation
: reducing end-to-end latency and variation in media
quality
Questions