6LoWPAN P. Thubert
Internet-Draft Cisco
Intended status: Standards Track March 18, 2008
Expires: September 19, 2008
LoWPAN simple fragment Recovery
draft-thubert-6lowpan-simple-fragment-recovery-00
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Copyright (C) The IETF Trust (2008).
Abstract
Considering that 6LoWPAN packets can be as large as 2K bytes and that
an 802.15.4 frame with security will carry in the order of 80 bytes
of effective payload, a packet might end up fragmented into as many
as 25 fragments at the 6LoWPAN shim layer. If a single one of those
fragments is lost in transmission, all fragments must be resent,
further contributing to the congestion that might have caused the
initial packet loss. This draft introduces a simple protocol to
recover individual fragments between 6LoWPAN endpoints.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. New Dispatch types and headers . . . . . . . . . . . . . . . . 7
6.1. Recoverable Fragment Dispatch type and Header . . . . . . 7
6.2. Fragment Acknowledgement Dispatch type and Header . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
Considering that 6LoWPAN packets can be as large as 2K bytes and that
a 802.15.4 frame with security will carry in the order of 80 bytes of
effective payload, a packet might be fragmented into about 25
fragments at the 6LoWPAN shim layer. This level of fragmentation is
much higher than that traditionally experienced over the Internet
with IPv4 fragments. At the same time, the use of radios increases
the probability of transmission loss and Mesh-Under techniques
compound that risk over multiple hops.
Past experience with fragmentation has shown that missassociated or
lost fragments can lead to poor network behaviour and, eventually,
trouble at application layer. The reader might start his research
from [I-D.mathis-frag-harmful] and follow the references. That
experience led to the definition of the Path MTU discovery [RFC1191]
protocol that avoids fragmentation over the Internet.
An end-to-end fragment recovery mechanism might be a good complement
to a hop-by-hop MAC level recovery with a limited number of retries.
This draft introduces a simple protocol to recover individual
fragments between 6LoWPAN endpoints. Specifically in the case of
UDP, valuable additional information can be found in UDP Usage
Guidelines for Application Designers [I-D.ietf-tsvwg-udp-guidelines].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Readers are expected to be familiar with all the terms and concepts
that are discussed in "IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944].
ERP
Error Recovery Procedure.
LoWPAN endpoints
The LoWPAN nodes in charge of generating or expanding a 6LoWPAN
header from/to a full IPv6 packet. The LoWPAN endpoints are the
points where fragmentation and reassembly take place.
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3. Rationale
There are a number of usages for large packets in Wireless Sensor
Networks. Such usages may not be the most typical or represent the
largest amount of traffic over the LoWPAN; however, the associated
functionality can be critical enough to justify extra care for
ensuring effective transport of large packets across the LoWPAN.
The list of those usages includes:
Towards the LoWPAN node:
Packages of Commands: A number of commands or a full
configuration can by packaged as a single message to ensure
consistency and enable atomic execution or complete roll back.
Until such commands are fully received and interpreted, the
intended operation will not take effect.
Firmware update: For example, a new version of the LoWPAN node
software is downloaded from a system manager over unicast or
multicast services. Such a reflashing operation typically
involves updating a large number of similar 6LoWPAN nodes over
a relatively short period of time.
From the LoWPAN node:
Waveform captures: A number of consecutive samples are measured
at a high rate for a short time and then transferred from a
sensor to a gateway or an edge server as a single large report.
Large data packets: Rich data types might require more than one
fragment.
Uncontrolled firmware download or waveform upload can easily result
in a massive increase of the traffic and saturate the network.
When a fragment is lost in transmission, all fragments are resent,
further contributing to the congestion that caused the initial loss,
and potentially leading to congestion collapse.
This saturation may lead to excessive radio interference, or random
early discard (leaky bucket) in relaying nodes. Additional queueing
and memory congestion may result while waiting for a low power next
hop to emerge from its sleeping state.
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4. Requirements
This paper proposes a method to recover individual fragments between
LoWPAN endpoints. The method is designed to fit the following
requirements of a LoWPAN (with or without a Mesh-Under routing
protocol):
Number of fragments
The recovery mechanism must support highly fragmented packets,
with a maximum of 32 fragments per packet.
Minimimum acknowledgement overhead
Because the radio is half duplex, and because of silent time spent
in the various medium access mechanisms, an acknowledgement
consumes roughly as many resources as data fragment.
The recovery mechanism should be able to acknowledge multiple
fragments in a single message.
Controlled latency
The recovery mechanism must succeed or give up within the time
boundary imposed by the recovery process of the Upper Layer
Protocols.
Support for out-of-order fragment delivery
A Mesh-Under load balancing mechanism such as the ISA100 Data Link
Layer can introduce out-of-sequence packets. The recovery
mechanism must account for packets that appear lost but are
actually only delayed over a different path.
Optional flow control
The aggregation of multiple concurrent flows may lead to the
saturation of the radio network and congestion collapse.
The recovery mechanism should provide means for controlling the
number of fragments in transit over the LoWPAN.
Backward compatibility
A node that implements this draft should be able to communicate
with a node that implements [RFC4944]. This draft assumes that
compatibility information about the remote LoWPAN endpoint is
obtained by external means.
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5. Overview
The fragmentation/reassembly of a packet must complete within an
acceptable overall latency, otherwise the packet expires and must be
dropped. This latency must be smaller than Upper Layer Protocol
retry values, and smaller than expiration period of the information
transported.
The sender transfers a controlled number of fragments (possibly all
of them) and flags the last fragment of a series with an
Acknowledgement request.
The sender sets a retry timer for the fragment that carries the
Acknowledgement request. That fragment is retransmitted individually
upon time out. This is repeated until an Acknowledgement comes back
or the packet expires.
Upon receipt of an Acknowledgement request, the receiver responds
with an Acknowledgement containing a bitmap that indicates which
fragments were actually received. The bitmap is a 32bit DWORD, which
accommodates up to 32 fragments and is sufficient for the 6LoWPAN
MTU. For all n in [0..31], bit n is set to 1 in the bitmap to
indicate that fragment n was received, otherwise the bit is set to 0.
If any fragment immmediately preceding the acknowledgement request is
missing, the receiver MAY intentionally delay its response to allow
in-transit fragments to arrive.
The sender has either one or no Acknowledgement pending. An
Acknowledgement that is not expected or does not acknowledge the
pending sequence in the bitmap is a duplicate and is ignored.
When a valid Acknowledgement is received, the sender resumes sending
fragments and the process is repeated until all fragments are
acknowledged or the packet expires.
Fragments are sent in a round robin fashion: the sender sends all the
fragments for a first time before it retries any lost fragment; lost
fragments are retried in sequence, oldest first. This mechanism
enables the receiver to acknowledge fragments that were delayed in
the network before they are actually retried.
It is up to the sender to decide how many fragments are (re)sent
before an acknowledgement is received, and the sender can adapt that
number to the network conditions. This way, the number of
outstanding fragments can be used as a flow control mechanism to
protect the network.
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6. New Dispatch types and headers
This specification extends "Transmission of IPv6 Packets over IEEE
802.15.4 Networks" [RFC4944] with 3 new dispatch types, for
Recoverable Fragments (RFRAG) headers with or without Acknowledgement
Request, and for the Acknowledgement back.
Pattern Header Type
+------------+-----------------------------------------------+
| 11 101000 | RFRAG - Recoverable Fragment |
| 11 101001 | RFRAG-AR - RFRAG with Acknowledgement Req |
| 11 101010 | RFRAG-ACK - RFRAG Acknowledgement |
+------------+-----------------------------------------------+
Figure 1: Additional Dispatch Value Bit Patterns
In the following sections, the semantics of "datagram_tag,"
"datagram_offset" and "datagram_size" and the reassembly process are
unchanged from [RFC4944] Section 5.3. "Fragmentation Type and
Header."
6.1. Recoverable Fragment Dispatch type and Header
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 0 1 0 0 X|datagram_offset| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sequence | datagram_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X set == Ack Requested
Figure 2: Recoverable Fragment Dispatch type and Header
X bit
When set, the sender requires an Acknowledgement from the receiver
Sequence
The sequence number of the fragment. Fragments are numbered
[0..N] where N is in [0..31].
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6.2. Fragment Acknowledgement Dispatch type and Header
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 0 1 0 1 0| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgement Bitmap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+
^ ^
| | bitmap indicating whether
| +-----Fragment with sequence 10 was received
+-------------------------Fragment with sequence 00 was received
Figure 3: Fragment Acknowledgement Dispatch type and Header
Acknowledgement Bitmap
Each bit in the Bitmap refers to a particular fragment: bit n set
indicates that fragment with sequence n was received, for n in
[0..31].
All zeroes means that the fragment was dropped because it
corresponds to an obsolete datagram_tag. This happens if the
packet was already reassembled and passed to the network upper
layer, or the packet expired and was dropped.
7. Security Considerations
The process of recovering fragments does not appear to create any
opening for new threat.
8. IANA Considerations
Need extensions for formats defined in "Transmission of IPv6 Packets
over IEEE 802.15.4 Networks" [RFC4944].
9. Acknowledgments
The author wishes to thank Jay Werb, Christos Polyzois, Soumitri
Kolavennu and Harry Courtice for their contribution and review.
10. References
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10.1. Normative References
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
10.2. Informative References
[I-D.ietf-tsvwg-udp-guidelines]
Eggert, L. and G. Fairhurst, "UDP Usage Guidelines for
Application Designers", draft-ietf-tsvwg-udp-guidelines-05
(work in progress), February 2008.
[I-D.mathis-frag-harmful]
Mathis, M., "Fragmentation Considered Very Harmful",
draft-mathis-frag-harmful-00 (work in progress),
July 2004.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, August 2007.
Author's Address
Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
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