TCP Segment Size Optimization for Energy Efficiency in LLNs

TCP over low-power and lossy networks: tuning the
segment size to minimize energy consumption

Ahmed Ayadi, Patrick Maill´, David Ros
e

IT/TELECOM Bretagne
Rennes, France

8-9 February 2011

Ahmed Ayadi (IT/TELECOM Bretagne) NTMS Wireless Sensor Networks 2011 Paris, 8-9 February 2011 1 / 22

Motivation
The IETF Working Group 6LoWPAN has recently introduced an adaptation
layer that provides header compression and fragmentation/reassembly
mechanisms to allow sending/receiving IPv6 packets over LLNs (e.g., IEEE
802.15.4),
The 6LoWPANs have given more chance for TCP to be deployed in the
Low-power and Lossy Networks such as Wireless Sensor Networks.


Motivation
The IETF Working Group 6LoWPAN has recently introduced an adaptation
layer that provides header compression and fragmentation/reassembly
mechanisms to allow sending/receiving IPv6 packets over LLNs (e.g., IEEE
802.15.4),
The 6LoWPANs have given more chance for TCP to be deployed in the
Low-power and Lossy Networks such as Wireless Sensor Networks.

However, the IPv6 MTU is 1280 bytes while an 802.15.4 frame can have a
payload limited to 74 bytes,
A TCP segment might end up fragmented into as many as 18 fragments at
the 6LoWPAN layer,
If a single one of those fragments is lost in transmission, all fragments must
be resent,
Sending long TCP segments increases the packet error rate, while sending
short TCP segments increases the overhead.


Outline

1 Introduction

2 TCP energy consumption model
TCP energy consumption model: Notations
Link layer: one-hop model
Multi-hop model
TCP Model

3 Results and discussion
Model assessment
FEC redundancy ratio and energy consumption
Selecting the TCP MSS to minimize energy consumption

4 Conclusion and perspectives


Introduction

Focus on the energy cost when TCP is used in multi-hops LLNs.
Present a simple mathematical model aimed at predicting the energy
consumed by the wireless nodes of an LLN in a bulk-data transfer scenario.
The model estimates TCP energy performance based on the bit error rate,
the maximum number of retransmissions at link layer, the number of hops
between the sender and the receiver, the amount of FEC, and the TCP
maximum segment size.
The proposed model allows us to study the tradeoﬀs involved in sending
short versus long TCP segments.
Applying the model, we study the energy eﬃciency of TCP over an LLN
using 6LoWPAN and IEEE 802.15.4 protocols.


Notations

We assume that the energy consumed by a TCP transmission in a wireless
LLN mainly corresponds to the data emission and reception, and thus
directly depends on the number of bits sent by all nodes.
The following table lists most of the variables used in the model.

Variable Deﬁnition
D Link-layer data frame size
A Link-layer acknowledgement frame size
h Number of hops between source and destination
r Maximum number of link-layer transmission attempts
m Number of fragments corresponding to a single TCP segment (due
to link layer fragmentation)
α FEC redundancy ratio
B Bit error rate


Link Layer Modeling

Automatic Repeat reQuest (ARQ)
ARQ uses the cyclic redundancy check (CRC) error-detecting code that
is added to the data: the receiver uses the error-detecting code number
to check the integrity of the received data
After receiving a correct frame, the receiver replies by an ACK.
If the sender does not receive an ACK before the timeout, it
re-transmits the frame/packet until the sender receives an
acknowledgment or exceeds a predeﬁned number of re-transmissions.


Link Layer Modeling

Automatic Repeat reQuest (ARQ)
ARQ uses the cyclic redundancy check (CRC) error-detecting code that
is added to the data: the receiver uses the error-detecting code number
to check the integrity of the received data
After receiving a correct frame, the receiver replies by an ACK.
If the sender does not receive an ACK before the timeout, it
re-transmits the frame/packet until the sender receives an
acknowledgment or exceeds a predeﬁned number of re-transmissions.
Forward Error Correction (FEC)
The main idea of FEC is to add redundancy to the original frame, to
allow the destination node to detect and correct some bit errors.
The FEC algorithm adds (α × K) redundancy bits to form a frame of
length D.


Link Layer Modeling

ACKs are sent without FEC.

Sender Receiver Sender Receiver Sender Receiver
Data frame Data Data
. frame frame

Ack frame e
. Ack fram

(a) Failure. (b) Partial failure. (c) Success.
c
D
Pfail = 1 − B i (1 − B)D−i ,
i
i=0

Ppartial = (1 − Pfail )(1 − (1 − B)A )
Psucc = (1 − Pfail )(1 − B)A


Link Layer Modeling

F i : Probability that a destination node does not receive a link layer data frame
after r attempts (i th hop)
r
F = Pfail .


Link Layer Modeling

r
F = Pfail .
Hf : Expected number of bits sent after r attempts knowing that the (one-hop)
transmission has failed
Hf = r × D.


Link Layer Modeling

r
F = Pfail .
Hf : Expected number of bits sent after r attempts knowing that the (one-hop)
transmission has failed
Hf = r × D.
Hs : Expected number of bits sent within r attempts knowing that the
(one-hop) transmission has succeeded
r
1 r i r −i
Hs = ( Ppartial Pfail (r D + iA)
1 − F i=1 i
r k−1
k −1 i k−1−i
+ Psucc Ppartial Pfail (kD + (i + 1)A))
i
k=1 i=0


Multi-hop model

Sender Receiver Sender Receiver

. .
.
.
(d) End-to-end failure scenario: the (e) End-to-end success scenario:
frame cannot be forwarded after r the frame arrives at the destination.
unsuccessful retransmissions. This scenario may also include par-
tial failures over one or more hops
(not depicted).

Figure: Failure and success scenarios in a multi-hop transmission.


Multi-hop model

Q s : Probability of an end-to-end packet transmission success
h
Qs = (1 − F i )
i=1


Multi-hop model

h
Qs = (1 − F i )
i=1

Es : Expected number of bits sent for a successful end-to-end packet transmission
knowing that it has succeeded
h
Es = Hsi
i=1


Multi-hop model

h
Qs = (1 − F i )
i=1

Es : Expected number of bits sent for a successful end-to-end packet transmission
knowing that it has succeeded
h
Es = Hsi
i=1

Ef : Expected number of bits sent for an end-to-end packet transmission knowing
that it has failed
h k−1 k−1
k=1 ( i=1 Hsi + Hfk ) j=1 (1 − F j )F k
Ef = h
1− i=1 (1 − Fi)


TCP Model

Ps : The success probability of a TCP segment transmission attempt is simply the
probability that all m data fragments be correctly sent to the destination, and the
TCP ACK be successfully sent back to the source:

Ps = Q m × Q s,ack ,
s


TCP Model

Ps : The success probability of a TCP segment transmission attempt is simply the
probability that all m data fragments be correctly sent to the destination, and the
TCP ACK be successfully sent back to the source:

Ps = Q m × Q s,ack ,
s

Knowing that a transmission is successful at the TCP level (i.e., the TCP ACK is
correctly received by the TCP source, which implies that all m fragments correctly
reached the destination), the expected total number of bits sent by all nodes
equals:
Ss = Es × m + Es,ack


TCP Model

Knowing that a TCP transmission attempt has failed, the expected number of
bits sent end-to-end by all nodes is:
1
Sf = If (1 − Q m )
s + (Es × m + Ef ,ack )Q m (1 − Q s,ack )
s ,
1 − Ps
end-to-end transmission failure end-to-end transmission failure of the TCP ACK
of one or more of the m fragments

m
m
If = kEf +(m−k)Es (1−Q s )k (Q s )m−k = m(1−Q s )Ef +mEs Q s (1−Q m ).
s
k
k=1

This therefore corresponds to a total number of bits sent (per segment) of

S = Sf (1/Ps − 1) + Ss .


Scenario & Parameters

TCP Sender n-1 n n+1 TCP Receiver

Figure: Chain Topology


Scenario & Parameters

TCP Sender n-1 n n+1 TCP Receiver

Figure: Chain Topology

Parameter Value
h 5
r 3
α 0
BER B 3 × 10−4
Link-layer Ack frame size 40 bits
Link-layer data frame header 120 bits
IP header 160 bits
TCP header 160 bits


Model assessment (1/2)

103
MSS = 512 (model)
MSS = 512 (simulation)
MSS = 64 (model)
Consumed energy (J) MSS = 64 (simulation)
102

101

10−6 10−5 10−4 10−3
BER

Figure: Energy consumption with long or short TCP segments, as a function of
the BER B.


Model assessment (2/2)
102
MSS = 512 (model)
Consumed energy (J)
MSS = 64 (model)

101

2 3 4 5 6 7
Maximum link layer attempts

Figure: Energy consumption with short or long TCP segments, as a function of
the number of link layer attempts r (with B = 5 × 10−4 ).


FEC redundancy ratio and energy consumption (1/2)
103
MSS = 512, r =1
MSS = 512, r =3
MSS = 64, r =1
Consumed energy (J)
MSS = 64, r =3
102

101

100 −3
10 10−2 10−1 100
Redundancy ratio (α)

Figure: Consumed energy using short or long TCP segment, as a function of the
redundancy ratio α (B = 3 × 10−4 , h = 5).


FEC redundancy ratio and energy consumption (2/2)
103
MSS = 512, B = 10−3
MSS = 512, B = 10−4
MSS = 64, B = 10−3
Consumed energy (J) 102 MSS = 64, B = 10−4

101

100
10−2 10−1 100
Redundancy ratio (α)

Figure: Consumed energy using short or long TCP segment, as a function of the
redundancy ratio α (r = 1, h = 5).


102
MSS = 512, B = 4 × 10−4
MSS = 64, B = 4 × 10−4

Consumed energy (J)

101

100
2 4 6 8 10
Number of Hops (h)

Figure: Energy consumption for short and long TCP segment sizes, as a function
of the network size (r = 3).


Depending on the transmission distance and the BER. We remark that for a given
BER, short MSSs tend to outperform long MSSs when the distance grows: it is
more and more interesting to use short MSS values instead of long ones.

10−1
MSS=64
MSS=512
−2
10
BER

10−3

10−4

10−5
2 4 6 8
Number of Hops (h)

Figure: Long (MSS=512 bytes) versus short (MSS=64 bytes) in a multi-hop
transmission (r = 3).


The impact of ARQ max attempts

10−3 r =7
r =5
r =4
r =3
BER

r =2
10−4

r =1

10−5
2 4 6 8
Number of Hops (h)

Figure: Long (MSS=512 bytes) versus short (MSS=64 bytes) in a multi-hop TCP
transmission: prefer the short MSS above the curves, the long one below.


The eﬀect of FEC mechanisms
Not surprisingly (the FEC reducing the eﬀect of transmission errors), redundancy
makes large MSSs outperform small MSSs due to the overhead reduction they
allow.
10−1
α = 10−1

10−2

α = 10−2
BER

−3
10

α = 10−3
10−4

10−5
2 4 6 8
Number of Hops (h)

Figure: Long (MSS=512 bytes) versus short (MSS=64 bytes) in a multi-hop TCP
transmission: prefer the short MSS above the curves, the long one below.


Conclusion and perspectives
Conclusion
We have proposed an analytical model to estimate the number of bits
sent by all wireless nodes in a TCP session in a Low power and Lossy
Network, in order to evaluate the overall energy consumption,
We have shown that using a large TCP segment size is less
energy-consuming in small, low-error networks, while it becomes
interesting to reduce the MSS when the network is large or very lossy,

Perspectives
A ﬁrst interesting direction would be to model the collision process,
that has been observed in our simulations for large MSS values,
We would also like to consider the case when duplicate frames are not
detected at the link layer; likewise, the transport layer modeling could
be extended to encompass TCP’s delayed acknowledgement
mechanism, and also larger TCP windows,
We are currently investigating an adaptation algorithm which
dynamically adjusts the TCP segment size to the optimal MSS value.


TCP Segment Size Optimization for Energy Efficiency in LLNs

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