wdiff draft-thubert-6lowpan-simple-fragment-recovery-03.txt draft-thubert-6lowpan-simple-fragment-recovery-04.txt

6LoWPAN P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track March 23, 2009 J. Hui
Expires: September 24, 29, 2009 Arch Rock Corporation
March 28, 2009

LoWPAN simple fragment Recovery
draft-thubert-6lowpan-simple-fragment-recovery-03
draft-thubert-6lowpan-simple-fragment-recovery-04

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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 that might be lost over multiple hops
between 6LoWPAN endpoints.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
3. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 6
5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7
6. New Dispatch types and headers . . . . . . . . . . . . . . . . 7
6.1. Recoverable Fragment Dispatch type and Header . . . . . . 7 8
6.2. Fragment Acknowledgement Dispatch type and Header . . . . 8
7. Outstanding Fragments Control Recovery . . . . . . . . . . . . . . . . 8 . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10 11
11.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 12

1. Introduction

Considering that

In many 6LoWPAN packets can be as large as 2K applications, the majority of traffic is spent
sending small chunks of data (order few bytes and to few tens of bytes)
per packet. Given that
a an 802.15.4 frame with security will can carry in on the order of
80 bytes in the worst case, fragmentation is often not needed for
most application traffic. However, many applications also require
occasional bulk data transfer capabilities to support firmware
upgrades of
effective payload, a packet might be fragmented into about 25
fragments at 6LoWPAN devices or extraction of logs from 6LoWPAN
devices. In the former case, bulk data is transferred to the 6LoWPAN shim layer. This level
device, and in the latter, bulk data flows away from the 6LoWPAN
device. In both cases, the bulk data size is often on the order of fragmentation
10K bytes or more and end-to-end reliable transport is
much higher than that traditionally experienced required.

Mechanisms such as TCP or application-layer segmentation will be used
to support end-to-end reliable transport. One option to support bulk
data transfer over 6LoWPAN links is to set the Internet
with IPv4 fragments. At Maximum Segment Size
to fit within the same time, 802.15.4 MTU. Doing so, however, can add
significant header overhead to each 802.15.4 frame. This causes the
end-to-end transport to be aware of the delivery properties of
6LoWPAN networks, which is a layer violation.

An alternative mechanism combines the use of radios increases 6LoWPAN fragmentation in
addition to transport or application-layer segmentation. Increasing
the Maximum Segment Size reduces header overhead by the probability end-to-end
transport protocol. It also encourages the transport protocol to
reduce the number of transmission loss and Mesh-Under techniques
compound outstanding datagrams, ideally to a single
datagram, thus reducing the need to support out-of-order delivery
common to 6LoWPAN networks.

[RFC4944] defines a datagram fragmentation mechanism for 6LoWPAN
networks. However, because [RFC4944] does not define a mechanism for
recovering fragments that risk over multiple hops. are lost, datagram forwarding fails if even
one fragment is not delivered properly to the next IP hop. End-to-
end transport mechanisms will require retransmission of all
fragments, wasting resources in an already resource-constrained
network.

Past experience with fragmentation has shown that missassociated or
lost fragments can lead to poor network behaviour behavior and, eventually,
trouble at application layer. The reader is encouraged to read
[RFC4963] and follow the references for more information. That
experience led to the definition of the Path MTU discovery [RFC1191]
protocol that limits fragmentation over the Internet.

For one-hop communications, a number of media propose a local
acknowledgement mechanism that is enough to protect the fragments.
In a multihop environment, 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. recovery. 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.

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.

Data logs: 6LoWPAN nodes may generate large logs of sampled data
for later extraction. 6LoWPAN nodes may also generate system
logs to assist in diagnosing problems on the node or network.

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 queuing
and memory congestion may result while waiting for a low power next
hop to emerge from its sleeping state.

To demonstrate the severity of the problem, consider a fairly
reliable 802.15.4 frame delivery rate of 99.9% over a single 802.15.4
hop. The expected delivery rate of a 5-fragment datagram would be
about 99.5% over a single 802.15.4 hop. However, the expected
delivery rate would drop to 95.1% over 10 hops, a reasonable network
diameter for 6LoWPAN applications. The expected delivery rate for a
1280-byte datagram is 98.4% over a single hop and 85.2% over 10 hops.

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.

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.

Minimum acknowledgement overhead

Because the radio is half duplex, and because of silent time spent
in the various medium access mechanisms, an acknowledgement acknowledgment
consumes roughly as many resources as data fragment.

The recovery mechanism should be able to acknowledge multiple
fragments in a single message. message and not require an acknowledgement
at all if fragments are already protected at a lower layer.

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 congestion 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.

5. Overview

Considering that a multi-hop LoWPAN can be a very sensitive
environment due to the limited queueing queuing capabilities of a large
population of its nodes, this draft recommends a simple and
conservative approach to congestion control, based on TCP congestion
avoidance.

Congestion on the forward path is assumed in case of packet loss, and
packet loss is assumed upon time out.

Congestion on the forward path can also be indicated by an Explicit
Congestion Notification (ECN) mechanism. Though whether and how ECN
[RFC3168] is carried out over the LoWPAN is out of scope, this draft
provides a way for the destination endpoint to echo an ECN indication
back to the source endpoint in an acknowledgement message as
represented in Figure 3 in Section 6.2.

From the standpoint of a source LoWPAN endpoint, an outstanding
fragment is a fragment that was sent but for which no explicit
acknowledgement
acknowledgment was received yet. This means that the fragment might
be on the way, received but not yet acknowledged, or the
acknowledgement
acknowledgment might be on the way back. It is also possible that
either the fragment or the acknowledgement acknowledgment was lost on the way.

Because a meshed LoWPAN might deliver frames out of order, it is
virtually impossible to differentiate these situations. In other
words, from the sender standpoint, all outstanding fragments might
still be in the network and contribute to its congestion. There is
an assumption, though, that after a certain amount of time, a frame
is either received or lost, so it is not causing congestion anymore.
This amount of time can be estimated based on the round trip delay
between the LoWPAN endpoints. The method detailed in [RFC2988] is
recommended for that computation.

The reader is encouraged to read through "Congestion Control
Principles" [RFC2914]. Additionally [RFC2309] and [RFC2581] provide
deeper information on why this mechanism is needed and how TCP
handles Congestion Control. Basically, the goal here is to manage
the amount of fragments present in the network; this is achieved by
to reducing the number of outstanding fragments over a congested path
by throttling the sources.

Section 7 describes how the sender decides how many fragments are
(re)sent before an acknowledgement is required, and how the sender
adapts that number to the network conditions.

6. New Dispatch types and headers

This specification extends "Transmission of IPv6 Packets over IEEE
802.15.4 Networks" [RFC4944] with 4 new dispatch types, for
Recoverable Fragments (RFRAG) headers with or without Acknowledgement Acknowledgment
Request, and for the Acknowledgement back, with or without ECN Echo. Acknowledgment back.

Pattern Header Type
+------------+-----------------------------------------------+
| 11 101000 | RFRAG - Recoverable Fragment |
| 11 101001 | RFRAG-AR - RFRAG with Ack Request |
| 11 101010 10101x | RFRAG-ACK - RFRAG Acknowledgement |
| 11 101011 | RFRAG-AEC - RFRAG Ack with ECN Echo Acknowledgment |
+------------+-----------------------------------------------+

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

Figure 2: Recoverable Fragment Dispatch type and Header

X bit

When set, the sender requires an Acknowledgement Acknowledgment from the receiver

Sequence

The sequence number of the fragment. Fragments are numbered
[0..N] where N is in [0..31].

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 Y| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgement Acknowledgment Bitmap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^ ^
| | Y set == ECN echo is reserved
| |
| | bitmap indicating whether
| +-----Fragment with sequence 10 was received
+-------------------------Fragment with sequence 00 was received

Figure 3: Fragment Acknowledgement Dispatch type and Header

Y bit

When set, the sender indicates that at least one of the
acknowledged fragments was received with an Explicit Congestion
Notification, indicating that the path followed by the fragments
is subject to congestion.

Reserved.

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

A NULL bitmap (All zeroes) means that the fragment was dropped because it
corresponds .

7. Fragments Recovery

The node that fragments the packets at 6LoWPAN level (the sender)
controls the Fragment Acknowledgements. If may do that at any
fragment to implement its own policy or perform congestion control
which is out of scope for this document. When the sender of the
fragment knows that an obsolete datagram_tag. This happens if underlying mechanism protects the
packet was Fragments
already reassembled and passed to the network upper
layer, or it MAY refrain from using the packet expired Acknowledgement mechanism, and was dropped.

7. Outstanding Fragments Control

A mechanism based on TCP congestion avoidance dictates
never set the maximum
number of outstanding fragments. Ack Requested bit. The maximum number of outstanding fragments for a given packet toward
a given LoWPAN endpoint is initially set to a configured value,
unless recent history indicates otherwise.

Each time node that maximum number of recomposes the
packets at 6LoWPAN level (the receiver) MUST acknowledge the
fragments is fully acknowledged,
that number can be incremented by 1. ECN echo it has received when asked to, and packet loss cause
the number to be divided by 2. MAY slightly defer that
acknowledgement.

The sender transfers a controlled number of fragments and flags MAY flag
the last fragment of a series with an acknowledgement acknowledgment request. The sender arms
received MUST acknowledge a timer to cover the fragment that carries the
Acknowledgement request. Upon time out, with the sender assumes that all acknowledgment request
bit set. If any fragment immediately preceding an acknowledgment
request is still missing, the receiver MAY intentionally delay its
acknowledgment to allow in-transit fragments on to arrive. delaying the way are received or lost. It divides
acknowledgement might defeat the
maximum number of outstanding fragments by 2 round trip delay computation so it
should be configurable and resets the number of
outstanding fragments to 0.

Upon receipt of an Acknowledgement request, the not enabled by default.

The receiver responds interacts with the sender using an Acknowledgement containing Acknowledgment
message with a bitmap that indicates which fragments were actually
received. The bitmap is a 32bit DWORD, SWORD, 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
with sequence n was received, otherwise the bit is set to 0. All
zeroes is a NULL bitmap that indicates that the fragmentation process
was cancelled by the receiver for that datagram.

The receiver MAY issue unsolicited acknowledgements. acknowledgments. An unsolicited
acknowledgement
acknowledgment enables the sender endpoint to resume sending if it
had reached its maximum number of outstanding fragments. fragments or indicate
that the receiver has cancelled the process of an individual
datagram. Note that
acknowledgements acknowledgments might consume precious resources
so the use of unsolicited acknowledgements acknowledgments should be configurable and
not enabled by default.

The received MUST acknowledge sender arms a fragment with retry timer to cover the acknowledgement
request bit set. If any fragment immediately preceding that carries the
Acknowledgment request. Upon time out, the sender assumes that all
the fragments on the way are received or lost. The process must
completed within an
acknowledgement request acceptable time that is still missing, within the receiver MAY
intentionally delay its acknowledgement to allow in-transit fragments
to arrive. This mechanism might defeat boundaries of
upper layer retries. The method detailed in [RFC2988] is recommended
for the round trip delay computation so it of the retry timer. It is expected that the
upper layer retries obey the same or friendly rules in which case a
single round of fragment recovery should be configurable and not enabled by default. fit within the upper layer
recovery timers.

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.

The process must complete within an acceptable time that is within
the boundaries of upper layer retries. Additional work is required
to define how this is achieved.

When the source endpoint sender decides that a packet should be dropped and the
fragmentation process
cancelled, canceled, it sends a pseudo fragment with the
datagram_offset, sequence and datagram_size all set to zero, and no
data. Upon reception of this message, the receiver should clean up
all resources for the packet associated to the datagram_tag. If an
acknowledment is requested, the receiver responds with a NULL bitmap.

The receiver might need to cancel the process of a fragmented packet
for internal reasons, for instance if it is out of recomposition
buffers, or considers that this packet is already fully recomposed
and passed to the upper layer. In that case, the receiver SHOULD
indicate so to the sender with a NULL bitmap. Upon an
acknowledgement with a NULL bitmap, the sender MUST drop the
datagram.

8. Security Considerations

The process of recovering fragments does not appear to create any
opening for new threat. threat compared to "Transmission of IPv6 Packets over
IEEE 802.15.4 Networks" [RFC4944].

9. IANA Considerations

Need extensions for formats defined in "Transmission of IPv6 Packets
over IEEE 802.15.4 Networks" [RFC4944].

10. Acknowledgments

The author wishes to thank Jay Werb, Christos Polyzois, Soumitri
Kolavennu and Harry Courtice for their contribution and review.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.

[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.

11.2. Informative References

[I-D.ietf-tsvwg-udp-guidelines]
Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers",
draft-ietf-tsvwg-udp-guidelines-11 (work in progress),
October 2008.

[I-D.mathis-frag-harmful]
Mathis, M., "Fragmentation Considered Very Harmful",
draft-mathis-frag-harmful-00 (work in progress),
July 2004.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.

[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998.

[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.

[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.

[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.

[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.

Author's Address

Authors' Addresses

Pascal Thubert (editor)
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

Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, California 94107
USA

Phone: +415 692 0828
Email: jhui@archrock.com