Energy Efficient Fragment Recovery Techniques for Low-power and Lossy Networks

Energy Eﬃcient Fragment Recovery Techniques for
Low-power and Lossy Networks

Ahmed Ayadi , Pascal Thubert†

IT/TELECOM Bretagne Rennes, France
† Cisco Systems

12 January 2011

Ahmed Ayadi (IT/TELECOM Bretagne) IP and Wireless Sensor Networks’2011 Lyon, 12-13 January 2011 1 / 19

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 IPv6 length is larger than 1280 bytes while an 802.15.4 frame
can have a payload limited to 74 bytes
A IPv6 packet 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.


Outline

1 Link Layer Error Control Mechanisms

2 Simple Fragment Forward and Recovery
Fragment Recovery proposal
Recoverable Fragment: Dispatch type and Header
Fragment Acknowledgement Dispatch type and Header
An SFFR scenario

3 Performance evaluation
Impact of SFFR on the energy consumption of TCP
Impact of SFFR on the energy consumption of UDP
The SFFR rounds improve the energy efciency
When it is better to used SFFR?

4 Conclusion and perspectives


Link Layer Error Control Mechanisms

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.
FEC can adapt to multihop by adopting more redundancy bits, but.


Link Layer Error Control Mechanisms

If the wireless network becomes very lossy, ARQ would increase the
transmission delay between the source and the receiver.
Using ARQ, the source continues to send the remaining fragments,
even if one fragment is already lost.
The reliable transport layer (e.g., TCP) MUST retransmit the
segment and thus all the fragments.
FEC requires more CPU energy and the amount of overhead is diﬃcult
to predict for the rapidly changing conditions of real-world LLNs .


Simple Fragment Forward and Recovery

SFFR is a new end-to-end recovery algorithm recently proposed by
Thubert et Hui for 6LoWPANs.
SFFR allows the sender to recover easily and quickly the lost
fragments.
SFFR uses the datagram ”tag” as a switchable label.
SFFR minimize the acknowledgement overhead by applying a
compressed acknowledgement bitmap
SFFR takes into support the out-of-order fragment delivery.


Fragment Recovery proposal

SFFR uses 32 bits as SACK Bitmap
SFFR deﬁnes 4 new dispatch types:
RFRAG: regular fragments,
RFRAG-AR: the last fragment which request an acknowledgment,
RFRAG-ACK: an new fragment that inform the sender about the
received fragments form the lost one.

Figure: Additional Dispatch Value Bit Patterns


Recoverable Fragment: Dispatch type and Header

Upon the ﬁrst fragment, the routers lay an label along the path that is
followed by that fragment (that is IP routed), and all further fragments are
label switched along that path.

Figure: Recoverable Fragment Dispatch type and Header


Fragment Acknowledgement: Dispatch type and Header

A 32 bits uncompressed bitmap is obtained by prepending zeroes to
the XXX in the pattern below.
else,

Figure: Compressed acknowledgement bitmap encoding


Expanded bitmap examples

(a) Expanding 1 octet encoding

(b) Expanding 3 octets encoding

Figure: Expanded bitmap encoding


An SFFR scenario

Sender Receiver
RFRA
G
RFRA
G
RFRA
G-AR .

RFRAG-ACK
RFRA
G-AR

RFRAG-ACK

Figure: End-to-end simple fragment forwarding and recovery


Parameters

Table: Network parameters.
Parameter Value
Hop number 5
Application data size 1048 kbytes
TCP MSS/ UDP payload size 512/1024 bytes
NHC header 1 bytes
TCPHC header 8 bytes
6LoWPAN header 3 bytes
IEEE 802.15.4 header 23 bytes
IEEE 802.15.4 acknowledgment size 10 bytes
Transmit Energy 0.24 µJ/bit
Receive Energy 0.21 µJ/bit


Impact of SFFR on the energy consumption of TCP (1/2)

103 103
No ARQ, No SFFR No ARQ, No SFFR
No ARQ, SFFR No ARQ, SFFR
ARQ=3, No SFFR ARQ=3, No SFFR
Consumed energy (J)

Consumed energy (J)
ARQ=3, SFFR ARQ=3, SFFR

102 102

10−5 10−4 10−3 10−5 10−4 10−3
BER BER

(a) MSS = 1024 bytes (b) MSS = 512 bytes

Figure: Energy Consumption of an TCP data transfer with vs without SFFR
(number of hops is equal to ﬁve).


Impact of SFFR on the energy consumption of TCP (2/2)

103
1024, No SFFR
1024, SFFR
512, No SFFR

Consumed Energy (J)
512, SFFR

102

2 4 6 8 10
Number of hops

Figure: Energy Consumption of an TCP data transfer with vs without SFFR
SFFR (ARQ=3, B = 5 × 10−4 ).


Impact of SFFR on the energy consumption of UDP (1/2)
control congestion

10−1 10−1
Energy Efficiency

Energy Efficiency
10−2 10−2

No ARQ, No SFFR No ARQ, No SFFR
No ARQ, SFFR No ARQ, SFFR
ARQ=3, No SFFR ARQ=3, No SFFR
ARQ=3, SFFR ARQ=3, SFFR
10−3 −5 10−3 −5
10 10−4 10−3 10 10−4 10−3
BER BER

(a) UDP payload size = 1024 bytes (b) UDP payload size = 512 bytes

Figure: Energy Efficiency of an UDP data transfer with vs without SFFR.


Impact of SFFR on the energy consumption of UDP (2/2)

1024, No SFFR
1024, SFFR
512, No SFFR
512, SFFR
Energy Eﬃciency 10−1

10−2

2 4 6 8 10
Number of hops

Figure: Energy Eﬃciency of an UDP data trasfer with and without SFFR
(ARQ=3, B = 5 × 10−4 ).


The SFFR rounds improve the Energy Efficiency

10−1

Energy Efficiency

10−2 No SFFR
SFFR=1
SFFR=2
SFFR=3

10−3 −5
10 10−4 10−3
BER

Figure: Energy Efficiency of an UDP data transfer with different SFFR rounds
(ARQ=3, 5 hops).


When it is better to used SFFR?
10−3

MSS=256
BER

MSS=512
MSS=768
MSS=1024
MSS=1280

10−4
2 4 6 8 10
Number of Hops (h)

Figure: SFFR in a multi-hop TCP transmission: prefer SFFR above the curves
(ARQ=3).


Conclusion and perspectives

Conclusion
SFFR is a new energy-eﬃcient end-to-end fragment recovery,
Simulations results show that SFFR reduces signiﬁcantly the
consumed energy.

Perspectives
Congestion control due to fragmentation,
Reduces the PER of RFRAG-AR and RFRAG-ACK.


Energy Efficient Fragment Recovery Techniques for Low-power and Lossy Networks

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