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An Analysis of TCP Startup over
an Experimental DVB-RCS Platform
Alberto Gotta, Francesco Potortì, Raffaello Secchi
Italian National Research Council (CNR)
Istituto di Scienza e Tecnologie dell’Informazione “A. Faedo” (ISTI)
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Outline
• The Skyplex satellite platform
• TCP measurement over Skyplex
• Focusing on the TCP startup phase
– The congestion window model
– Transfer time during startup
• Consideration on parameters choice
• Conclusions
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Assumptions
• High performance in satellite mesh
networks
– Powerful error-correcting scheme and
adaptive modulation and coding (DVB-S2)
– High link availability: transmission virtually
immune from packet losses for most of time
• Share of satellite bandwidth (DVB-RCS)
– Demand Assigned Multiple Access (DAMA)
schemes used to share uplink channel (CRA,
RBDC, VBDC, FCA)
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Research topics (Motivations)
• TCP issues
– TCP universally used for reliable data delivery across the
Internet
– TCP startup phase is critical
• The short flows (web, interactive applications, etc.) are concluded within the SlowStart phase
• Current DAMA Implementations
– Satellite bandwidth allocation schemes do not consider the
efficiency of short connection transfers
– The long loop delay of satellite links slows down the Slow-Start
phase
• Bandwidth assigned on the basis of stations’ requests, which in turn depend on the
current transmission rate (transmission backlog respectively)
• Demand-assignment delay is at least 500 ms on satellite links
• Since the throughput of TCP increases at each RTT, the allocated bandwidth is
always less than the offered traffic
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
SkyplexTM platform overview
•
Skyplex Data from Eutelsat
– Enables low-cost meshed networks
– Support a range of data, television and audio applications
•
Eutelsat will offer its Skyplex Data service
– Ka band transponders on its Hot Bird 6 satellite located at 13° E.
– Wide bandwidth
• the downlink stream is up to 36 Mb/s from 6 carriers
•
User Terminal
– User terminals can employ small antennas 90 cm
– Low-power consumption (2 W SSPAs)
– The terminal is a compact unit (30 cm wide by 4.5 cm high)
– Low-cost equipment
•
Skyplex Data bandwidth access
– Permanent bandwidth assignment
– Multiple access mode (Demand Assignment Multiple Access)
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Test-bed platform
DVB-RCS
DVB-S
LAN
Ethernet
LAN
Ethernet
testbed specifications
channel capacity
active stations
available bandwidth
2.1 Mb/s
Medium Access Control
TDMA/TDM
•
The bandwidth allocation uses a
DAMA scheme
•
It implements a variant of Rate
Based Dynamic Capacity (RBDC)
18
~1.2 Mb/s
slots per frame
48
frame duration
273 ms
bandwidth granularity
44 kb/s
guaranteed bandwidth
44 kb/s
mean RTT
840 ms
Madrid –September 15th 2006
•
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Experiment Setup
• Linux 2.6 TCP/IP stack
– Windows scale option enable
net.ipv4.tcp_window_scaling
– Sack option was enabled/disabled
net.ipv4.tcp_sack
– Transmitting and receiving buffer are sized big
enough not to limit TCP throughput
• FreeBSD 5.4 TCP/IP stack
– Uses standard settings
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Linux 2.6 without SACK
•
The bottleneck buffer fills up at the end
of the Slow Start phase
– many packet losses when the buffer
overflows
Madrid –September 15th 2006
•
The subsequent Fast Recovery phase
is not fast enough to avoid a timeout
•
TCP retransmits a high number of
already transmitted packets, whose
duplicate ACKs do not contribute to
increasing the congestion window
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
TCP Newreno on DAMA
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Linux 2.6 with SACK
•
Linux with SACK behaves much
better
– The loss recovery is fast thanks
to SACKs
Madrid –September 15th 2006
•
The resulting throughput is, on
average, as fast as the channel
permits
•
The flow of packets is very
irregular
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
FreeBSD 5.4 stack
•
FreeBSD estimates available
bandwidth using ACK interarrival
times
– the estimate is used to cap the
rate of packet transmission
Madrid –September 15th 2006
•
A single connection occupies
almost all the channel capacity
•
The packet rate is extremely
regular without even a loss
•
The RTT is only slightly greater
than the minimum.
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Focusing on TCP Startup
• Relevant parameters observed during
TCP startup
– Initial Window (IW)
• Linux 2.6 sets the Initial Window to three packets according to
the specification of RFC 3390
• FreeBSD 5.4 conservatively sets the IW to one packet
– Round Trip Time (RTT)
– Allocation Delay
• the delay between making a request and receiving the
respective allocation
– Delayed ACK
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
TCP Startup Comparison
FreeBSD TCP
Linux TCP
bandwidth
limited
increasing
phase
•
We analyze the first 20 sec of transmission
–
•
about 1.5MB are transmitted during this phase, which is enough
for most interactive applications
The only significant difference between TCP
flavors during startup is the Initial Window
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Basic DAMA scheme
• Each station periodically submits to a master station an
allocation request based on the incoming traffic rate or
traffic backlog at the terminal
– The time interval between subsequent allocation requests is
called request period (Tr)
• The master station collects the request of all the active
stations, computes a burst time plan and broadcasts it
back to the terminals
– The allocation period (Ta) is the time interval between
subsequent allocation assignments
• When the stations receive the burst time plan, they get to
know their allocation
– The allocation delay is the overall time between making a
request and receiving the corresponding allocation
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Timeline for DAMA scheme
mean request
period (Tr/2) half RTT (t/2)
mean allocation
period (Ta/2)
half RTT (t/2)
master
the terminal
issues the request
the request is
received
the request is
delivered
the allocation t is
delivered
the allocation is
received
terminal
Average allocation delay
Ta
Tr
Da  t 

2
2
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Interaction of DAMA with TCP
•
During the Slow Start phase the growth of the segment transmit rate
depends on the ACK reception rate
•
The ACK reception rate depends on the rate the other TCP end
receives the segments, that is, on the bandwidth allocation
•
When using DAMA, the bandwidth allocation received by the earth
station has been computed based on the segment transmit rate that
the TCP connection had an allocation delay before (at least 500 ms)
•
Thus, the larger the allocation delay, the slower the growth of the
congestion window
–
DAMA delays the time the bandwidth is available to TCP connections,
which in turns delay congestion window growth
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Modeling TCP startup on DAMA
• We model the performance of TCP during startup on a
rate-based DAMA
• We derive the expression of the congestion window
evolution as a function of the initial window, RTT and
allocation delay
• We show that the congestion window evolution is similar
to the one without DAMA, but with an increased RTT
• Using the expression for the congestion window, we
estimate the connection completion time for short flows
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Model Assumptions
• Single greedy connection
• No losses due to corruption
– ideal channel conditions (e.g. clear sky)
• No throughput limitations
– receiving and transmitting buffer sufficiently large
• Satellite link idle before connection starts
• Slow Start threshold big enough that the Slow Start phase
does not end before the channel is saturated
• No delayed ACK
– Modern TCP implementations disable DelACK during startup
– Even if DelACK is enabled, the ACK inter-arrival time is larger
that DelACK retransmission timeout
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Congestion Window Equation
• During Slow-Start phase the congestion window growth is equal to
the ACK reception rate
dw(t )
da (t )

dt
dt
w(t) congestion window in segments
a(t) number of ACK in [0:t]
• If enough space is available on the channel, the resource scheduler
assign the requested bandwidth
dw(t )
da(t )
w(t  T )

 k
dt
dt
t
ACK rate
Madrid –September 15th 2006
T  t  Da
Requested/Assigned
bandwidth
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Model Analysis
•
We solve this equation using a second order approximation of its Laplace
transform and we obtain a closed form representation of congestion window
w(t ) 
 2 s T
2
W0 
 s2T  s1T




s 2T
e
0  t  ts
SN
•
•
The expression is valid for 0 < t < ts, where ts is
the time when the TCP rate equals the
available capacity m
For t > ts the ACK rate is capped by m and the
growth of the congestion window depends on
the exact implementation
time
ts
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Equivalent RTT for rate-based DAMA
• This expression of w(t) just derived is equivalent to having
a fixed allocation policy but with increased RTT:
Equivalent
RTT
t eq    t 
1   / 2  1  3    / 2 
2
3/ 2 
17 / 4

T
t
– The equivalent RTT is an increasing function of ρ, that
is, of allocation delay
– Rate-based DAMA slows down the Slow Start phase
of TCP connections
– This effect can be partially compensated by
requesting more bandwidth for starting connections
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Transfer Time During Startup
• From the expression for w(t), we can estimate the amount
of data N(t) transferred in the time interval [0:t]
• By integrating the TCP throughput (w(t)/t, we obtain the
time D required to transmit N packets as a function of the
initial window W0, the round trip time t and the value of T
 1  N  W0 s2T s2T  s1T  


 ln 
W0
 2  s2T  
s
D 2 
 1 N  N (t )   t
s
s
 m
Madrid –September 15th 2006
N  N (t s )
N  N (t s )
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Transfer Time Comparison
actual
values
max DAMA
performance
constant
allocation
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Consideration on Parameter Choice
• The harmful effect of DAMA on TCP startup could be compensated
by assigning more bandwidth to a starting TCP connection
– Ideally, a DAMA scheduler should provide enough bandwidth to double
the congestion window every RTT
• The terminal requests k times the measured incoming traffic rate
when detects a new connections is initiated from that station
• Using our model, we determine the condition over k allowing to
exploit the assigned bandwidth share without buffering and without
wasting channel resources
– In the case of Skyplex, we estimate k=4.8
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Connection Duration
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
•
The completion time
improves with
increasing k (down to
a value of k near to
4.8)
•
Improvements are
most significant for
bigger data volume
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Assigned Slot Utilization
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
•
The channel utilization
worsens with
increasing k
•
Worsening is most
significant for smaller
volume of data
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”
Conclusions
• Experimental and simulative studies on Skyplex Data highlights how
rate-based DAMA can significantly slow down the transmission of
small volume of data using TCP
• We presented an analytical modeling of this phenomenon
– we show it is similar to what would happen on a traditional fixed-allocation
satellite system if the RTT was increased
• A small change in allocation policy can bring significant benefits to
short TCP connections in terms of channel utilization
• Further studies required in order to modify the allocation algorithm to
make it more responsive to starting TCP connections
Madrid –September 15th 2006
Raffaello Secchi – IWSSC06
Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”