< draft-ietf-6lo-fragment-recovery-11.txt   draft-ietf-6lo-fragment-recovery-12.txt >
6lo P. Thubert, Ed. 6lo P. Thubert, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Updates: 4944 (if approved) 10 February 2020 Updates: 4944 (if approved) 11 February 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 13 August 2020 Expires: 14 August 2020
6LoWPAN Selective Fragment Recovery 6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-11 draft-ietf-6lo-fragment-recovery-12
Abstract Abstract
This draft updates RFC 4944 with a simple protocol to recover This draft updates RFC 4944 with a simple protocol to recover
individual fragments across a route-over mesh network, with a minimal individual fragments across a route-over mesh network, with a minimal
flow control to protect the network against bloat. flow control to protect the network against bloat.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 13 August 2020. This Internet-Draft will expire on 14 August 2020.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8 5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11 5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12 6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12
6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14 6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14
6.1.1. Receiving the first fragment . . . . . . . . . . . . 15 6.1.1. Receiving the first fragment . . . . . . . . . . . . 15
6.1.2. Receiving the next fragments . . . . . . . . . . . . 15 6.1.2. Receiving the next fragments . . . . . . . . . . . . 15
6.2. Receiving RFRAG Acknowledgments . . . . . . . . . . . . . 16 6.2. Receiving RFRAG Acknowledgments . . . . . . . . . . . . . 16
6.3. Aborting the Transmission of a Fragmented Packet . . . . 16 6.3. Aborting the Transmission of a Fragmented Packet . . . . 16
6.4. Applying Recoverable Fragmentation along a Diverse 6.4. Applying Recoverable Fragmentation along a Diverse
Path . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Path . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Management Considerations . . . . . . . . . . . . . . . . . . 17 7. Management Considerations . . . . . . . . . . . . . . . . . . 18
7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 17 7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 18
7.2. Observing the network . . . . . . . . . . . . . . . . . . 19 7.2. Observing the network . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
11. Normative References . . . . . . . . . . . . . . . . . . . . 21 11. Normative References . . . . . . . . . . . . . . . . . . . . 22
12. Informative References . . . . . . . . . . . . . . . . . . . 21 12. Informative References . . . . . . . . . . . . . . . . . . . 23
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 24 Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 26
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 25 Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 28
Appendix C. Considerations on Flow Control . . . . . . . . . . . 26 Appendix C. Considerations on Flow Control . . . . . . . . . . . 28
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 27 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction 1. Introduction
In most Low Power and Lossy Network (LLN) applications, the bulk of In most Low Power and Lossy Network (LLN) applications, the bulk of
the traffic consists of small chunks of data (on the order of a few the traffic consists of small chunks of data (on the order of a few
bytes to a few tens of bytes) at a time. Given that an IEEE Std. bytes to a few tens of bytes) at a time. Given that an IEEE Std.
802.15.4 [IEEE.802.15.4] frame can carry a payload of 74 bytes or 802.15.4 [IEEE.802.15.4] frame can carry a payload of 74 bytes or
more, fragmentation is usually not required. However, and though more, fragmentation is usually not required. However, and though
this happens only occasionally, a number of mission critical this happens only occasionally, a number of mission critical
applications do require the capability to transfer larger chunks of applications do require the capability to transfer larger chunks of
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define a new protocol. However, adding that capability alone to the define a new protocol. However, adding that capability alone to the
local implementation of the original 6LoWPAN fragmentation would not local implementation of the original 6LoWPAN fragmentation would not
address the inherent fragility of fragmentation (see address the inherent fragility of fragmentation (see
[I-D.ietf-intarea-frag-fragile]) in particular the issues of [I-D.ietf-intarea-frag-fragile]) in particular the issues of
resources locked on the receiver and the wasted transmissions due to resources locked on the receiver and the wasted transmissions due to
the loss of a single fragment in a whole datagram. [Kent] compares the loss of a single fragment in a whole datagram. [Kent] compares
the unreliable delivery of fragments with a mechanism it calls the unreliable delivery of fragments with a mechanism it calls
"selective acknowledgements" that recovers the loss of a fragment "selective acknowledgements" that recovers the loss of a fragment
individually. The paper illustrates the benefits that can be derived individually. The paper illustrates the benefits that can be derived
from such a method in figures 1, 2 and 3, on pages 6 and 7. from such a method in figures 1, 2 and 3, on pages 6 and 7.
[RFC4944] as no selective recovery and the whole datagram fails when [RFC4944] has no selective recovery and the whole datagram fails when
one fragment is not delivered to the destination 6LoWPAN endpoint. one fragment is not delivered to the destination 6LoWPAN endpoint.
Constrained memory resources are blocked on the receiver until the Constrained memory resources are blocked on the receiver until the
receiver times out, possibly causing the loss of subsequent packets receiver times out, possibly causing the loss of subsequent packets
that cannot be received for the lack of buffers. that cannot be received for the lack of buffers.
That problem is exacerbated when forwarding fragments over multiple That problem is exacerbated when forwarding fragments over multiple
hops since a loss at an intermediate hop will not be discovered by hops since a loss at an intermediate hop will not be discovered by
either the source or the destination, and the source will keep on either the source or the destination, and the source will keep on
sending fragments, wasting even more resources in the network and sending fragments, wasting even more resources in the network and
possibly contributing to the condition that caused the loss to no possibly contributing to the condition that caused the loss to no
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elided by the originator on the first hop because it matches the elided by the originator on the first hop because it matches the
source MAC address, but cannot be on the next hops because the source source MAC address, but cannot be on the next hops because the source
MAC address changes. MAC address changes.
This specification cannot allow this operation since fragments are This specification cannot allow this operation since fragments are
recovered end-to-end based on a sequence number. This means that the recovered end-to-end based on a sequence number. This means that the
fragments that contain a 6LoWPAN-compressed header MUST have enough fragments that contain a 6LoWPAN-compressed header MUST have enough
slack to enable a less efficient compression in the next hops that slack to enable a less efficient compression in the next hops that
still fits in one MAC frame. For instance, if the IID of the source still fits in one MAC frame. For instance, if the IID of the source
IPv6 address is elided by the originator, then it MUST compute the IPv6 address is elided by the originator, then it MUST compute the
fragment_size as if the MTU was 8 bytes less. This way, the next hop Fragment_Size as if the MTU was 8 bytes less. This way, the next hop
can restore the source IID to the first fragment without impacting can restore the source IID to the first fragment without impacting
the second fragment. the second fragment.
4.2. Gap between frames 4.2. Gap between frames
This specification introduces a concept of an inter-frame gap, which This specification introduces a concept of an inter-frame gap, which
is a configurable interval of time between transmissions to a same is a configurable interval of time between transmissions to a same
next hop. In the case of half duplex interfaces, this inter-frame next hop. In the case of half duplex interfaces, this inter-frame
gap ensures that the next hop has completed processing of the gap ensures that the next hop has completed processing of the
previous frame and is capable of receiving the next one. previous frame and is capable of receiving the next one.
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order in which the fragments are received. This enables out-of- order in which the fragments are received. This enables out-of-
sequence subfragmenting, e.g., a fragment seq. 5 that is retried end- sequence subfragmenting, e.g., a fragment seq. 5 that is retried end-
to-end as smaller fragments seq. 5, 13 and 14 due to a change of MTU to-end as smaller fragments seq. 5, 13 and 14 due to a change of MTU
along the path between the 6LoWPAN endpoints. along the path between the 6LoWPAN endpoints.
1 2 3 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 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|E| datagram_tag | |1 1 1 0 1 0 0|E| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| sequence| fragment_size | fragment_offset | |X| sequence| Fragment_Size | fragment_offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X set == Ack-Request X set == Ack-Request
Figure 1: RFRAG Dispatch type and Header Figure 1: RFRAG Dispatch type and Header
There is no requirement on the receiver to check for contiguity of There is no requirement on the receiver to check for contiguity of
the received fragments, and the sender MUST ensure that when all the received fragments, and the sender MUST ensure that when all
fragments are acknowledged, then the datagram is fully received. fragments are acknowledged, then the datagram is fully received.
This may be useful in particular in the case where the MTU changes This may be useful in particular in the case where the MTU changes
and a fragment sequence is retried with a smaller fragment_size, the and a fragment sequence is retried with a smaller Fragment_Size, the
remainder of the original fragment being retried with new sequence remainder of the original fragment being retried with new sequence
values. values.
The first fragment is recognized by a sequence of 0; it carries its The first fragment is recognized by a sequence of 0; it carries its
fragment_size and the datagram_size of the compressed packet before Fragment_Size and the datagram_size of the compressed packet before
it is fragmented, whereas the other fragments carry their it is fragmented, whereas the other fragments carry their
fragment_size and fragment_offset. The last fragment for a datagram Fragment_Size and fragment_offset. The last fragment for a datagram
is recognized when its fragment_offset and its fragment_size add up is recognized when its fragment_offset and its Fragment_Size add up
to the datagram_size. to the datagram_size.
Recoverable Fragments are sequenced and a bitmap is used in the RFRAG Recoverable Fragments are sequenced and a bitmap is used in the RFRAG
Acknowledgment to indicate the received fragments by setting the Acknowledgment to indicate the received fragments by setting the
individual bits that correspond to their sequence. individual bits that correspond to their sequence.
X: 1 bit; Ack-Request: when set, the sender requires an RFRAG X: 1 bit; Ack-Request: when set, the sender requires an RFRAG
Acknowledgment from the receiver. Acknowledgment from the receiver.
E: 1 bit; Explicit Congestion Notification; the "E" flag is reset by E: 1 bit; Explicit Congestion Notification; the "E" flag is reset by
the source of the fragment and set by intermediate routers to the source of the fragment and set by intermediate routers to
signal that this fragment experienced congestion along its path. signal that this fragment experienced congestion along its path.
Fragment_size: 10-bit unsigned integer; the size of this fragment in Fragment_Size: 10-bit unsigned integer; the size of this fragment in
a unit that depends on the MAC layer technology. Unless a unit that depends on the MAC layer technology. Unless
overridden by a more specific specification, that unit is the overridden by a more specific specification, that unit is the
octet, which allows fragments up to 512 bytes. octet, which allows fragments up to 512 bytes.
datagram_tag: 8 bits; an identifier of the datagram that is locally datagram_tag: 8 bits; an identifier of the datagram that is locally
unique to the sender. unique to the sender.
Sequence: 5-bit unsigned integer; the sequence number of the Sequence: 5-bit unsigned integer; the sequence number of the
fragment in the acknowledgement bitmap. Fragments are numbered fragment in the acknowledgement bitmap. Fragments are numbered
[0..N] where N is in [0..31]. A Sequence of 0 indicates the first [0..N] where N is in [0..31]. A Sequence of 0 indicates the first
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the datagram_tag used by the previous hop and the Layer-2 information the datagram_tag used by the previous hop and the Layer-2 information
associated to it (interface and MAC address). associated to it (interface and MAC address).
The 6LoWPAN endpoint that fragments the packets at the 6LoWPAN level The 6LoWPAN endpoint that fragments the packets at the 6LoWPAN level
(the sender) also controls the amount of acknowledgments by setting (the sender) also controls the amount of acknowledgments by setting
the Ack-Request flag in the RFRAG packets. The sender may set the the Ack-Request flag in the RFRAG packets. The sender may set the
Ack-Request flag on any fragment to perform congestion control by Ack-Request flag on any fragment to perform congestion control by
limiting the number of outstanding fragments, which are the fragments limiting the number of outstanding fragments, which are the fragments
that have been sent but for which reception or loss was not that have been sent but for which reception or loss was not
positively confirmed by the reassembling endpoint. The maximum positively confirmed by the reassembling endpoint. The maximum
number of outstanding fragments is the Window-Size. It is number of outstanding fragments is controlled by the Window-Size. It
configurable and may vary in case of ECN notification. When the is configurable and may vary in case of ECN notification. When the
6LoWPAN endpoint that reassembles the packets at the 6LoWPAN level 6LoWPAN endpoint that reassembles the packets at the 6LoWPAN level
(the receiver) receives a fragment with the Ack-Request flag set, it (the receiver) receives a fragment with the Ack-Request flag set, it
MUST send an RFRAG Acknowledgment back to the originator to confirm MUST send an RFRAG Acknowledgment back to the originator to confirm
reception of all the fragments it has received so far. reception of all the fragments it has received so far.
The Ack-Request ('X') set in an RFRAG marks the end of a window. The Ack-Request ('X') set in an RFRAG marks the end of a window.
This flag MUST be set on the last fragment if the sender wishes to This flag MUST be set on the last fragment if the sender wishes to
protect the datagram, and it MAY be set in any intermediate fragment protect the datagram, and it MAY be set in any intermediate fragment
for the purpose of flow control. for the purpose of flow control.
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Note that acknowledgments might consume precious resources so the use Note that acknowledgments might consume precious resources so the use
of unsolicited acknowledgments should be configurable and not enabled of unsolicited acknowledgments should be configurable and not enabled
by default. by default.
An observation is that streamlining forwarding of fragments generally An observation is that streamlining forwarding of fragments generally
reduces the latency over the LLN mesh, providing room for retries reduces the latency over the LLN mesh, providing room for retries
within existing upper-layer reliability mechanisms. The sender within existing upper-layer reliability mechanisms. The sender
protects the transmission over the LLN mesh with a retry timer that protects the transmission over the LLN mesh with a retry timer that
is computed according to the method detailed in [RFC6298]. It is is computed according to the method detailed in [RFC6298]. It is
expected that the upper layer retries obey the recommendations in expected that the upper layer retries obey the recommendations in
"UDP Usage Guidelines" [RFC8085], in which case a single round of [RFC8085], in which case a single round of fragment recovery should
fragment recovery should fit within the upper layer recovery timers. fit within the upper layer recovery timers.
Fragments are sent in a round-robin fashion: the sender sends all the 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 for a first time before it retries any lost fragment; lost
fragments are retried in sequence, oldest first. This mechanism fragments are retried in sequence, oldest first. This mechanism
enables the receiver to acknowledge fragments that were delayed in enables the receiver to acknowledge fragments that were delayed in
the network before they are retried. the network before they are retried.
When a single frequency is used by contiguous hops, the sender should When a single frequency is used by contiguous hops, the sender should
insert a delay between fragments of a same datagram that covers insert a delay between fragments of a same datagram that covers
multiple transmissions so as to let a fragment progress a few hops multiple transmissions so as to let a fragment progress a few hops
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of hops or the minimal value of MTU along those hops. But should the of hops or the minimal value of MTU along those hops. But should the
minimal MTU decrease, it is possible to retry a long fragment (say minimal MTU decrease, it is possible to retry a long fragment (say
sequence of 5) with first a shorter fragment of the same sequence (5 sequence of 5) with first a shorter fragment of the same sequence (5
again) and then one or more other fragments with a sequence that was again) and then one or more other fragments with a sequence that was
not used before (e.g., 13 and 14). Note that Path MTU Discovery is not used before (e.g., 13 and 14). Note that Path MTU Discovery is
out of scope for this document. out of scope for this document.
6.3. Aborting the Transmission of a Fragmented Packet 6.3. Aborting the Transmission of a Fragmented Packet
A reset is signaled on the forward path with a pseudo fragment that A reset is signaled on the forward path with a pseudo fragment that
has the fragment_offset, sequence, and fragment_size all set to 0, has the fragment_offset, sequence, and Fragment_Size all set to 0,
and no data. and no data.
When the sender or a router on the way decides that a packet should When the sender or a router on the way decides that a packet should
be dropped and the fragmentation process aborted, it generates a be dropped and the fragmentation process aborted, it generates a
reset pseudo fragment and forwards it down the fragment path. reset pseudo fragment and forwards it down the fragment path.
Each router next along the path the way forwards the pseudo fragment Each router next along the path the way forwards the pseudo fragment
based on the VRB state. If an acknowledgment is not requested, the based on the VRB state. If an acknowledgment is not requested, the
VRB and all associated state are destroyed. VRB and all associated state are destroyed.
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Track that can be a complex path between a source and a destination Track that can be a complex path between a source and a destination
with Packet ARQ, Replication, Elimination and Overhearing (PAREO) with Packet ARQ, Replication, Elimination and Overhearing (PAREO)
along the Track. This specification can be used along any subset of along the Track. This specification can be used along any subset of
the complex Track where the first fragment is flooded. The last the complex Track where the first fragment is flooded. The last
RFRAG Acknowledgment is flooded on that same subset in the reverse RFRAG Acknowledgment is flooded on that same subset in the reverse
direction. Intermediate RFRAG Acknowledgments can be flooded on any direction. Intermediate RFRAG Acknowledgments can be flooded on any
sub-subset of that reverse subset that reach back to the source. sub-subset of that reverse subset that reach back to the source.
7. Management Considerations 7. Management Considerations
This specification extends "On Forwarding 6LoWPAN Fragments over a
Multihop IPv6 Network" [I-D.ietf-6lo-minimal-fragment] and requires
the same parameters in the receiver and on intermediate nodes. There
is no new parameter as echoing ECN is always on. This parameters
typically include the reassembly time-out at the receiver and an
inactivity clean-up timer on the intermediate nodes, and the number
of messages that can be processed in parallel in all nodes.
The configuration settings introduced by this specification only
apply to the sender, which is in full control of the transmission.
LLNs vary a lot in size (there can be thousands of nodes in a mesh),
in speed (from 10Kbps to several Mbps at the PHY layer), in traffic
density, and in optimizations that are desired (e.g., the selection
of a RPL [RFC6550] Objective Function [RFC6552] impacts the shape of
the routing graph).
For that reason, only a very generic guidance can be given on the
settings of the sender and on whether complex algorithms are needed
to perform flow control or estimate the round-trip time. To cover
the most complex use cases, this specification enables the sender to
vary the fragment size, the window size and the inter-frame gap,
based on the amount of losses, the observed variations of the round-
trip time and the setting of the ECN bit.
7.1. Protocol Parameters 7.1. Protocol Parameters
There is no particular configuration on the receiver, as echoing ECN The management system SHOULD be capable of providing the parameters
is always on. The configuration only applies to the sender, which is listed in this section.
in control of the transmission. The management system SHOULD be
capable of providing the parameters below: An implementation must control the rate at which it sends packets
over a same path to allow the next hop to forward a packet before it
gets the next. In a wireless network that uses a same frequency
along a path, more time must be inserted to avoid hidden terminal
issues between fragments. This is controlled by the following
parameter:
inter-frame gap: Indicates a minimum amount of time between
transmissions. All packets to a same destination, and in
particular fragments, may be subject to receive while transmitting
and hidden terminal collisions with the next or the previous
transmission as the fragments progress along a same path. The
inter-frame gap protects the propagation of one transmission
before the next one is triggered and creates a duty cycle that
controls the ratio of air time and memory in intermediate nodes
that a particular datagram will use.
An implementation should consider the generic recommendations from
the IETF in the matter of flow control and rate management in
[RFC5033]. To control the flow, an implementation may use a dynamic
value of the window size (Window_Size), adapt the fragment size
(Fragment_Size) and insert an inter-frame gap that is longer than
necessary. In a large network where node contend for the bandwidth,
a larger Fragment_Size consumes less bandwidth but also reduces the
fluidity and incurs higher chances of loss in transmission. This is
controlled by the following parameters:
MinFragmentSize: The MinFragmentSize is the minimum value for the MinFragmentSize: The MinFragmentSize is the minimum value for the
Fragment_Size. Fragment_Size.
OptFragmentSize: The MinFragmentSize is the value for the OptFragmentSize: The OptFragmentSize is the value for the
Fragment_Size that the sender should use to start with. Fragment_Size that the sender should use to start with. It is
more than or equal to MinFragmentSize. It is less than or equal
to MaxFragmentSize. On the first fragment, it must enable the
expansion of the IPv6 addresses and of the Hop Limit field within
MTU. On all fragments, it is a balance between the expected
fluidity and the overhead of MAC and 6LoWPAN headers. For a small
MTU, the idea is to keep it close to the maximum, whereas for
larger MTUs, it might makes sense to keep it short enough, so that
the duty cycle of the transmitter is bounded, e.g., to transmit at
least 10 frames per second.
MaxFragmentSize: The MaxFragmentSize is the maximum value for the MaxFragmentSize: The MaxFragmentSize is the maximum value for the
Fragment_Size. It MUST be lower than the minimum MTU along the Fragment_Size. It MUST be lower than the minimum MTU along the
path. A large value augments the chances of buffer bloat and path. A large value augments the chances of buffer bloat and
transmission loss. The value MUST be less than 512 if the unit transmission loss. The value MUST be less than 512 if the unit
that is defined for the PHY layer is the octet. that is defined for the PHY layer is the octet.
UseECN: Indicates whether the sender should react to ECN. When the
sender reacts to ECN the Window_Size will vary between
MinWindowSize and MaxWindowSize.
MinWindowSize: The minimum value of Window_Size that the sender can MinWindowSize: The minimum value of Window_Size that the sender can
use. use.
OptWindowSize: The OptWindowSize is the value for the Window_Size OptWindowSize: The OptWindowSize is the value for the Window_Size
that the sender should use to start with. that the sender should use to start with. It is more than or
equal to MinWindowSize. It is less than or equal to
MaxWindowSize. The Window_Size should be maintained below the
number of hops in the path of the fragment to avoid stacking
fragments at the bottleneck on the path. If an inter-frame gap is
used to avoid interference between fragments then the Window_Size
should be at most in the order of the estimation of the trip time
divided by the inter-frame gap.
MaxWindowSize: The maximum value of Window_Size that the sender can MaxWindowSize: The maximum value of Window_Size that the sender can
use. The value MUSt be less than 32. use. The value MUST be less than 32.
inter-frame gap: Indicates a minimum amount of time between An implementation may perform its estimate of the RTO or use a
transmissions. All packets to a same destination, and in configured one. The ARQ process is controlled by the following
particular fragments, may be subject to receive while transmitting parameters:
and hidden terminal collisions with the next or the previous
transmission as the fragments progress along a same path. The
inter-frame gap protects the propagation of one transmission
before the next one is triggered and creates a duty cycle that
controls the ratio of air time and memory in intermediate nodes
that a particular datagram will use.
MinARQTimeOut: The maximum amount of time a node should wait for an MinARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action. RFRAG Acknowledgment before it takes a next action.
OptARQTimeOut: The starting point of the value of the amount of time OptARQTimeOut: The starting point of the value of the RTO, that is
that a sender should wait for an RFRAG Acknowledgment before it amount of time that a sender should wait for an RFRAG
takes a next action. Acknowledgment before it takes a next action. It is more than or
equal to MinARQTimeOut. It is less than or equal to
MaxARQTimeOut.
MaxARQTimeOut: The maximum amount of time a node should wait for an MaxARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action. RFRAG Acknowledgment before it takes a next action. It must cover
the longest expected round-trip time, and be several times less
than the time-out that covers the recomposition buffer at the
receiver, which is typically in the order of the minute. See
Appendix C for recommendations on computing the round-trip time.
MaxFragRetries: The maximum number of retries for a particular MaxFragRetries: The maximum number of retries for a particular
fragment. fragment.
MaxDatagramRetries: The maximum number of retries from scratch for a MaxDatagramRetries: The maximum number of retries from scratch for a
particular datagram. particular datagram.
An implementation may be capable to perform flow control based on
ECN, more in Appendix C. This is controlled by the following
parameter:
UseECN: Indicates whether the sender should react to ECN. The
sender may react to ECN by varying the Window_Size between
MinWindowSize and MaxWindowSize, varying the Fragment_Size between
MinFragmentSize and MaxFragmentSize and/or by increasing the
inter-frame gap.
7.2. Observing the network 7.2. Observing the network
The management system should monitor the amount of retries and of ECN The management system should monitor the amount of retries and of ECN
settings that can be observed from the perspective of both the sender settings that can be observed from the perspective of both the sender
and the receiver, and may tune the optimum size of Fragment_Size and and the receiver, and may tune the optimum size of Fragment_Size and
of the Window_Size, OptDatagramSize and OptWindowSize respectively, of the Window_Size, OptDatagramSize and OptWindowSize respectively,
at the sender. The values should be bounded by the expected number at the sender. The values should be bounded by the expected number
of hops and reduced beyond that when the number of datagrams that can of hops and reduced beyond that when the number of datagrams that can
traverse an intermediate point may exceed its capacity and cause a traverse an intermediate point may exceed its capacity and cause a
congestion loss. The inter-frame gap is another tool that can be congestion loss. The inter-frame gap is another tool that can be
used to increase the spacing between fragments of the same datagram used to increase the spacing between fragments of the same datagram
and reduce the ratio of time when a particular intermediate node and reduce the ratio of time when a particular intermediate node
holds a fragment of that datagram. holds a fragment of that datagram.
8. Security Considerations 8. Security Considerations
This document specifies an instantiation of a 6LoWPAN Fragment This document specifies an instantiation of a 6LoWPAN Fragment
Forwarding technique. "On Forwarding 6LoWPAN Fragments over a Forwarding technique. [I-D.ietf-6lo-minimal-fragment] provides the
Multihop IPv6 Network" [I-D.ietf-6lo-minimal-fragment] provides the
generic description of Fragment Forwarding and this specification generic description of Fragment Forwarding and this specification
inherits from it. The generic considerations in the Security inherits from it. The generic considerations in the Security
sections of [I-D.ietf-6lo-minimal-fragment] apply equally to this sections of [I-D.ietf-6lo-minimal-fragment] apply equally to this
document. document.
This specification does not recommend a particular algorithm for the This specification does not recommend a particular algorithm for the
estimation of the duration of the RTO that covers the detection of estimation of the duration of the RTO that covers the detection of
the loss of a fragment with the 'X' flag set; regardless, an attacker the loss of a fragment with the 'X' flag set; regardless, an attacker
on the path may slow down or discard packets, which in turn can on the path may slow down or discard packets, which in turn can
affect the throughput of fragmented packets. affect the throughput of fragmented packets.
skipping to change at page 20, line 46 skipping to change at page 22, line 42
| 11 10101x | 15 | Reserved for Experimental Use | RFC 8025 | | 11 10101x | 15 | Reserved for Experimental Use | RFC 8025 |
+-------------+------+----------------------------------+-----------+ +-------------+------+----------------------------------+-----------+
Table 1: Additional Dispatch Value Bit Patterns Table 1: Additional Dispatch Value Bit Patterns
10. Acknowledgments 10. Acknowledgments
The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent
Toutain, Carles Gomez Montenegro, Thomas Watteyne, and Michael Toutain, Carles Gomez Montenegro, Thomas Watteyne, and Michael
Richardson for in-depth reviews and comments. Also many thanks to Richardson for in-depth reviews and comments. Also many thanks to
Peter Yee, Tirumaleswar Reddy Konda and Erik Nordmark for their Peter Yee, Colin Perkins, Tirumaleswar Reddy Konda and Erik Nordmark
careful reviews and for helping through the IETF Last Call and IESG for their careful reviews and for helping through the IETF Last Call
review process, and to Jonathan Hui, Jay Werb, Christos Polyzois, and IESG review process, and to Jonathan Hui, Jay Werb, Christos
Soumitri Kolavennu, Pat Kinney, Margaret Wasserman, Richard Kelsey, Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret Wasserman, Richard
Carsten Bormann and Harry Courtice for their various contributions in Kelsey, Carsten Bormann and Harry Courtice for their various
the long process that lead ot this document. contributions in the long process that lead ot this document.
11. Normative References 11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
skipping to change at page 23, line 5 skipping to change at page 24, line 51
DOI 10.17487/RFC6298, June 2011, DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>. <https://www.rfc-editor.org/info/rfc6298>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550, Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012, DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>. <https://www.rfc-editor.org/info/rfc6550>.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, DOI 10.17487/RFC6552, March 2012,
<https://www.rfc-editor.org/info/rfc6552>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using [RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554, Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015, DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>. <https://www.rfc-editor.org/info/rfc7554>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200, (IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>. <https://www.rfc-editor.org/info/rfc8200>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>. March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using [RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087, Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017, DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>. <https://www.rfc-editor.org/info/rfc8087>.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/info/rfc5033>.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing", Wireless Personal Area Network (6LoWPAN) Routing",
RFC 6606, DOI 10.17487/RFC6606, May 2012, RFC 6606, DOI 10.17487/RFC6606, May 2012,
<https://www.rfc-editor.org/info/rfc6606>. <https://www.rfc-editor.org/info/rfc6606>.
[I-D.ietf-lwig-6lowpan-virtual-reassembly] [I-D.ietf-lwig-6lowpan-virtual-reassembly]
Bormann, C. and T. Watteyne, "Virtual reassembly buffers Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf- in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
lwig-6lowpan-virtual-reassembly-01, 11 March 2019, lwig-6lowpan-virtual-reassembly-01, 11 March 2019,
skipping to change at page 27, line 33 skipping to change at page 29, line 42
acknowledgment was received yet. This means that the fragment might acknowledgment was received yet. This means that the fragment might
be on the way, received but not yet acknowledged, or the be on the way, received but not yet acknowledged, or the
acknowledgment might be on the way back. It is also possible that acknowledgment might be on the way back. It is also possible that
either the fragment or the acknowledgment was lost on the way. either the fragment or the acknowledgment was lost on the way.
From the sender standpoint, all outstanding fragments might still be From the sender standpoint, all outstanding fragments might still be
in the network and contribute to its congestion. There is an in the network and contribute to its congestion. There is an
assumption, though, that after a certain amount of time, a frame is assumption, though, that after a certain amount of time, a frame is
either received or lost, so it is not causing congestion anymore. either received or lost, so it is not causing congestion anymore.
This amount of time can be estimated based on the round-trip delay This amount of time can be estimated based on the round-trip delay
between the 6LoWPAN endpoints. The method detailed in [RFC6298] is between the 6LoWPAN endpoints. The method detailed in "Computing
recommended for that computation. TCP's Retransmission Timer" [RFC6298] is recommended for that
computation.
The reader is encouraged to read through "Congestion Control The reader is encouraged to read through "Congestion Control
Principles" [RFC2914]. Additionally [RFC7567] and [RFC5681] provide Principles" [RFC2914]. Additionally [RFC7567] and [RFC5681] provide
deeper information on why this mechanism is needed and how TCP deeper information on why this mechanism is needed and how TCP
handles Congestion Control. Basically, the goal here is to manage handles Congestion Control. Basically, the goal here is to manage
the amount of fragments present in the network; this is achieved by the amount of fragments present in the network; this is achieved by
to reducing the number of outstanding fragments over a congested path to reducing the number of outstanding fragments over a congested path
by throttling the sources. by throttling the sources.
Section 6 describes how the sender decides how many fragments are Section 6 describes how the sender decides how many fragments are
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