< draft-ietf-6lo-fragment-recovery-13.txt   draft-ietf-6lo-fragment-recovery-14.txt >
6lo P. Thubert, Ed. 6lo P. Thubert, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Updates: 4944 (if approved) 18 February 2020 Updates: 4944 (if approved) 6 March 2020
Intended status: Standards Track Intended status: Standards Track
Expires: 21 August 2020 Expires: 7 September 2020
6LoWPAN Selective Fragment Recovery 6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-13 draft-ietf-6lo-fragment-recovery-14
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 21 August 2020. This Internet-Draft will expire on 7 September 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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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. References . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. References . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 6 3. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 6
4. Extending draft-ietf-6lo-minimal-fragment . . . . . . . . . . 6 4. Extending draft-ietf-6lo-minimal-fragment . . . . . . . . . . 6
4.1. Slack in the First Fragment . . . . . . . . . . . . . . . 7 4.1. Slack in the First Fragment . . . . . . . . . . . . . . . 6
4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 7 4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 7
4.3. Modifying the First Fragment . . . . . . . . . . . . . . 7 4.3. Flow Control . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Modifying the First Fragment . . . . . . . . . . . . . . 8
5. New Dispatch types and headers . . . . . . . . . . . . . . . 8 5. New Dispatch types and headers . . . . . . . . . . . . . . . 8
5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8 5.1. Recoverable Fragment Dispatch type and Header . . . . . . 9
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11 5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11
6. Fragment Recovery . . . . . . . . . . . . . . . . . . . . . . 12 6. Fragment 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 . . . . . . . . . . . . 16
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 . . . . 17
6.4. Applying Recoverable Fragmentation along a Diverse 6.4. Applying Recoverable Fragmentation along a Diverse
Path . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Path . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Management Considerations . . . . . . . . . . . . . . . . . . 18 7. Management Considerations . . . . . . . . . . . . . . . . . . 18
7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 18 7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 18
7.2. Observing the network . . . . . . . . . . . . . . . . . . 20 7.2. Observing the network . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
11. Normative References . . . . . . . . . . . . . . . . . . . . 22 11. Normative References . . . . . . . . . . . . . . . . . . . . 23
12. Informative References . . . . . . . . . . . . . . . . . . . 23 12. Informative References . . . . . . . . . . . . . . . . . . . 24
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 26 Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 26
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 27 Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 28
Appendix C. Considerations on Flow Control . . . . . . . . . . . 28 Appendix C. Considerations on Flow Control . . . . . . . . . . . 29
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 29 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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end reliable transport is required. end reliable transport is required.
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944] "Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944]
defines the original 6LoWPAN datagram fragmentation mechanism for defines the original 6LoWPAN datagram fragmentation mechanism for
LLNs. One critical issue with this original design is that routing LLNs. One critical issue with this original design is that routing
an IPv6 [RFC8200] packet across a route-over mesh requires an IPv6 [RFC8200] packet across a route-over mesh requires
reassembling the full packet at each hop, which may cause latency reassembling the full packet at each hop, which may cause latency
along a path and an overall buffer bloat in the network. The "6TiSCH along a path and an overall buffer bloat in the network. The "6TiSCH
Architecture" [I-D.ietf-6tisch-architecture] recommends using a Architecture" [I-D.ietf-6tisch-architecture] recommends using a
fragment forwarding (FF) technique to alleviate those undesirable fragment forwarding (FF) technique to alleviate those undesirable
effects. "LLN Minimal Fragment Forwarding" effects.
[I-D.ietf-6lo-minimal-fragment] specifies the general behavior that
all FF techniques including this specification follow, and presents "LLN Minimal Fragment Forwarding" [FRAG-FWD] specifies the general
the associated caveats. In particular, the routing information is behavior that all FF techniques including this specification follow,
fully indicated in the first fragment, which is always forwarded and presents the associated caveats. In particular, the routing
first. A state is formed and used to forward all the next fragments information is fully indicated in the first fragment, which is always
along the same path. The Datagram_Tag is locally significant to the forwarded first. A state is formed and used to forward all the next
Layer-2 source of the packet and is swapped at each hop. fragments along the same path. The Datagram_Tag is locally
significant to the Layer-2 source of the packet and is swapped at
each hop, more in Section 6. With this specification the
Datagram_Tag is encoded in one byte, and will saturate if there are
more than 256 datagram that transit in the fragmented form over a
same hop at the same time. This is not realistic at the time of this
writing. Should this happen in a new 6LoWPAN technology, a node will
need to use several Link-Layer addresses to increase its indexing
capacity.
"Virtual reassembly buffers in 6LoWPAN" "Virtual reassembly buffers in 6LoWPAN"
[I-D.ietf-lwig-6lowpan-virtual-reassembly] (VRB) proposes a FF [I-D.ietf-lwig-6lowpan-virtual-reassembly] (VRB) proposes a FF
technique that is compatible with [RFC4944] without the need to technique that is compatible with [RFC4944] without the need to
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
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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
avail since the datagram cannot arrive in its entirety. RFC 4944 is avail since the datagram cannot arrive in its entirety. RFC 4944 is
also missing signaling to abort a multi-fragment transmission at any also missing signaling to abort a multi-fragment transmission at any
time and from either end, and, if the capability to forward fragments time and from either end, and, if the capability to forward fragments
is implemented, clean up the related state in the network. It is is implemented, clean up the related state in the network. It is
also lacking flow control capabilities to avoid participating in also lacking flow control capabilities to avoid participating in
congestion that may in turn cause the loss of a fragment and congestion that may in turn cause the loss of a fragment and
potentially the retransmission of the full datagram. potentially the retransmission of the full datagram.
This specification provides a method to forward fragments across a This specification provides a method to forward fragments over
multi-hop route-over mesh, and a selective acknowledgment to recover typically a few hops in a route-over 6LoWPAN mesh, and a selective
individual fragments between 6LoWPAN endpoints. The method is acknowledgment to recover individual fragments between 6LoWPAN
designed to limit congestion loss in the network and addresses the endpoints. The method is designed to limit congestion loss in the
requirements that are detailed in Appendix B. network and addresses the requirements that are detailed in
Appendix B.
2. Terminology 2. Terminology
2.1. BCP 14 2.1. BCP 14
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all 14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2.2. References 2.2. References
In this document, readers will encounter terms and concepts that are In this document, readers will encounter terms and concepts that are
discussed in "Problem Statement and Requirements for IPv6 over discussed in "Problem Statement and Requirements for IPv6 over
Low-Power Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606] Low-Power Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]
"LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment] "LLN Minimal Fragment Forwarding" [FRAG-FWD] introduces the generic
introduces the generic concept of a Virtual Reassembly Buffer (VRB) concept of a Virtual Reassembly Buffer (VRB) and specifies behaviours
and specifies behaviours and caveats that are common to a large and caveats that are common to a large family of FF techniques
family of FF techniques including this, which fully inherits from including this, which fully inherits from that specification. It
that specification. also defines terms used in this document: 6LoWPAN endpoints,
Compressed Form, Datagram_Tag, Datagram_Size, and Fragment_Offset.
Past experience with fragmentation has shown that misassociated or Past experience with fragmentation has shown that misassociated or
lost fragments can lead to poor network behavior and, occasionally, lost fragments can lead to poor network behavior and, occasionally,
trouble at the application layer. The reader is encouraged to read trouble at the application layer. The reader is encouraged to read
"IPv4 Reassembly Errors at High Data Rates" [RFC4963] and follow the "IPv4 Reassembly Errors at High Data Rates" [RFC4963] and follow the
references for more information. references for more information.
That experience led to the definition of "Path MTU discovery" That experience led to the definition of "Path MTU discovery"
[RFC8201] (PMTUD) protocol that limits fragmentation over the [RFC8201] (PMTUD) protocol that limits fragmentation over the
Internet. Internet.
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there is no further analysis of the packet's network layer header. there is no further analysis of the packet's network layer header.
Rather, the label is used as an index into a table which specifies Rather, the label is used as an index into a table which specifies
the next hop, and a new label". The MPLS technique is leveraged in the next hop, and a new label". The MPLS technique is leveraged in
the present specification to forward fragments that actually do not the present specification to forward fragments that actually do not
have a network layer header, since the fragmentation occurs below IP. have a network layer header, since the fragmentation occurs below IP.
2.3. New Terms 2.3. New Terms
This specification uses the following terms: This specification uses the following terms:
6LoWPAN endpoints: The LLN nodes in charge of generating or
expanding a 6LoWPAN header from/to a full IPv6 packet. The
6LoWPAN endpoints are the points where fragmentation and
reassembly take place.
Compressed Form: This specification uses the generic term Compressed
Form to refer to the format of a datagram after the action of
[RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts.
Datagram_Size: The size of the datagram in its Compressed Form
before it is fragmented. The Datagram_Size is expressed in a unit
that depends on the MAC layer technology, by default a byte.
Datagram_Tag: An identifier of a datagram that is locally unique to
the Layer-2 sender. Associated with the MAC address of the
sender, this becomes a globally unique identifier for the
datagram.
Fragment_Offset: The offset of a particular fragment of a datagram
in its Compressed Form. The Fragment_Offset is expressed in a
unit that depends on the MAC layer technology and is by default a
byte.
RFRAG: Recoverable Fragment RFRAG: Recoverable Fragment
RFRAG-ACK: Recoverable Fragment Acknowledgement RFRAG-ACK: Recoverable Fragment Acknowledgement
RFRAG Acknowledgment Request: An RFRAG with the Acknowledgement RFRAG Acknowledgment Request: An RFRAG with the Acknowledgement
Request flag ('X' flag) set. Request flag ('X' flag) set.
NULL bitmap: Refers to a bitmap with all bits set to zero. NULL bitmap: Refers to a bitmap with all bits set to zero.
FULL bitmap: Refers to a bitmap with all bits set to one. FULL bitmap: Refers to a bitmap with all bits set to one.
Forward: The direction of a LSP path, followed by the RFRAG. Forward: The direction of a LSP path, followed by the RFRAG.
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where fragments can be forwarded end-to-end across a 6LoWPAN LLN, and where fragments can be forwarded end-to-end across a 6LoWPAN LLN, and
where fragments that are lost on the way can be recovered where fragments that are lost on the way can be recovered
individually. A new format for fragments is introduced and new individually. A new format for fragments is introduced and new
dispatch types are defined in Section 5. dispatch types are defined in Section 5.
[RFC8138] allows modifying the size of a packet en route by removing [RFC8138] allows modifying the size of a packet en route by removing
the consumed hops in a compressed Routing Header. This requires that the consumed hops in a compressed Routing Header. This requires that
Fragment_Offset and Datagram_Size (see Section 2.3) are also modified Fragment_Offset and Datagram_Size (see Section 2.3) are also modified
en route, which is difficult to do in the uncompressed form. This en route, which is difficult to do in the uncompressed form. This
specification expresses those fields in the Compressed Form and specification expresses those fields in the Compressed Form and
allows modifying them en route (see Section 4.3) easily. allows modifying them en route (see Section 4.4) easily.
Note that consistent with Section 2 of [RFC6282], for the Note that consistent with Section 2 of [RFC6282], for the
fragmentation mechanism described in Section 5.3 of [RFC4944], any fragmentation mechanism described in Section 5.3 of [RFC4944], any
header that cannot fit within the first fragment MUST NOT be header that cannot fit within the first fragment MUST NOT be
compressed when using the fragmentation mechanism described in this compressed when using the fragmentation mechanism described in this
specification. specification.
4. Extending draft-ietf-6lo-minimal-fragment 4. Extending draft-ietf-6lo-minimal-fragment
This specification implements the generic FF technique specified in This specification implements the generic FF technique defined in
"LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment] in "LLN Minimal Fragment Forwarding" [FRAG-FWD], provides end-to-end
a fashion that enables end-to-end recovery of fragments and some fragment recovery and mechanisms that can be used for flow control.
degree of flow control.
4.1. Slack in the First Fragment 4.1. Slack in the First Fragment
[I-D.ietf-6lo-minimal-fragment] allows for refragmenting in [FRAG-FWD] allows for refragmenting in intermediate nodes, meaning
intermediate nodes, meaning that some bytes from a given fragment may that some bytes from a given fragment may be left in the VRB to be
be left in the VRB to be added to the next fragment. The reason for added to the next fragment. The reason for this happening would be
this happening would be the need for space in the outgoing fragment the need for space in the outgoing fragment that was not needed in
that was not needed in the incoming fragment, for instance because the incoming fragment, for instance because the 6LoWPAN Header
the 6LoWPAN Header Compression is not as efficient on the outgoing Compression is not as efficient on the outgoing link, e.g., if the
link, e.g., if the Interface ID (IID) of the source IPv6 address is Interface ID (IID) of the source IPv6 address is elided by the
elided by the originator on the first hop because it matches the originator on the first hop because it matches the source Link-Layer
source MAC address, but cannot be on the next hops because the source address, but cannot be on the next hops because the source Link-Layer
MAC address changes. 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 [FRAG-FWD] requires that a configurable interval of time is inserted
is a configurable interval of time between transmissions to the same between transmissions to the same next hop and in particular between
next hop. In the case of half duplex interfaces, this inter-frame fragments of a same datagram. In the case of half duplex interfaces,
gap ensures that the next hop has completed processing of the this inter-frame gap ensures that the next hop is done forwarding the
previous frame and is capable of receiving the next one. previous frame and is capable of receiving the next one.
In the case of a mesh operating at a single frequency with In the case of a mesh operating at a single frequency with
omnidirectional antennas, a larger inter-frame gap is required to omnidirectional antennas, a larger inter-frame gap is required to
protect the frame against hidden terminal collisions with the protect the frame against hidden terminal collisions with the
previous frame of the same flow that is still progressing along a previous frame of the same flow that is still progressing along a
common path. common path.
The inter-frame gap is useful even for unfragmented datagrams, but it The inter-frame gap is useful even for unfragmented datagrams, but it
becomes a necessity for fragments that are typically generated in a becomes a necessity for fragments that are typically generated in a
fast sequence and are all sent over the exact same path. fast sequence and are all sent over the exact same path.
4.3. Modifying the First Fragment 4.3. Flow Control
The inter-frame gap is the only protection that [FRAG-FWD] imposes by
default. This document enables to group fragments in windows and
request intermediate acknowledgements so the number of in-flight
fragments can be bounded. This document also adds an ECN mechanism
that can be used to adapt the size of the window, the size of the
fragments, and/or the inter-frame gap to protect the network.
This specification enables the source endpoint to apply a flow
control mechanism to tune those parameters, but the mechanism itself
is out of scope. In most cases, the expectation is that most
datagrams will represent only a few fragments, and that only the last
fragment will be acknowledged. A basic implementation of the source
endpoint is NOT REQUIRED to variate the size of the window, the
duration of the inter-frame gap or the size of a fragment in the
middle of the transmission of a datagram, and it MAY ignore the ECN
signal or simply reset the window to 1 (see Appendix C for more) till
the end of this datagram upon detecting a congestion.
The size of the fragments is typically computed from the Link MTU to
maximize the size of the resulting frames. The size of the window
and the duration of the inter-frame gap SHOULD be configurable, to
roughly adapt the size of the window to the number of hops in an
average path, and to follow the general recommendations in
[FRAG-FWD], respectively.
4.4. Modifying the First Fragment
The compression of the Hop Limit, of the source and destination The compression of the Hop Limit, of the source and destination
addresses in the IPv6 Header, and of the Routing Header may change en addresses in the IPv6 Header, and of the Routing Header may change en
route in a Route-Over mesh LLN. If the size of the first fragment is route in a Route-Over mesh LLN. If the size of the first fragment is
modified, then the intermediate node MUST adapt the Datagram_Size to modified, then the intermediate node MUST adapt the Datagram_Size to
reflect that difference. reflect that difference.
The intermediate node MUST also save the difference of Datagram_Size The intermediate node MUST also save the difference of Datagram_Size
of the first fragment in the VRB and add it to the Datagram_Size and of the first fragment in the VRB and add it to the Datagram_Size and
to the Fragment_Offset of all the subsequent fragments for that to the Fragment_Offset of all the subsequent fragments for that
datagram. datagram.
5. New Dispatch types and headers 5. New Dispatch types and headers
This specification enables the 6LoWPAN fragmentation sublayer to This document specifies an alternate to the 6LoWPAN fragmentation
provide an MTU up to 2048 bytes to the upper layer, which can be the sublayer [RFC4944] to emulate an Link MTU up to 2048 bytes for the
6LoWPAN Header Compression sublayer that is defined in the upper layer, which can be the 6LoWPAN Header Compression sublayer
"Compression Format for IPv6 Datagrams" [RFC6282] specification. In that is defined in the "Compression Format for IPv6 Datagrams"
order to achieve this, this specification enables the fragmentation [RFC6282] specification. This specification also provides a reliable
and the reliable transmission of fragments over a multihop 6LoWPAN transmission of the fragments over a multihop 6LoWPAN route-over mesh
mesh network. network and a minimal flow control to reduce the chances of
congestion loss.
This specification provides a technique that is derived from MPLS to A LoWPAN Fragment Forwarding [FRAG-FWD] technique derived from MPLS
forward individual fragments across a 6LoWPAN route-over mesh without enables the forwarding of individual fragments across a 6LoWPAN
reassembly at each hop. The Datagram_Tag is used as a label; it is route-over mesh without reassembly at each hop. The Datagram_Tag is
locally unique to the node that owns the source MAC address of the used as a label; it is locally unique to the node that owns the
fragment, so together the MAC address and the label can identify the source Link-Layer address of the fragment, so together the Link-Layer
fragment globally. A node may build the Datagram_Tag in its own address and the label can identify the fragment globally. A node may
locally-significant way, as long as the chosen Datagram_Tag stays build the Datagram_Tag in its own locally-significant way, as long as
unique to the particular datagram for the lifetime of that datagram. the chosen Datagram_Tag stays unique to the particular datagram for
The result is that the label does not need to be globally unique but the lifetime of that datagram. The result is that the label does not
also that it must be swapped at each hop as the source MAC address need to be globally unique but also that it must be swapped at each
changes. hop as the source Link-Layer address changes.
This specification extends RFC 4944 [RFC4944] with 2 new Dispatch This specification extends RFC 4944 [RFC4944] with 2 new Dispatch
types, for Recoverable Fragment (RFRAG) and for the RFRAG types, for Recoverable Fragment (RFRAG) and for the RFRAG
Acknowledgment back. The new 6LoWPAN Dispatch types are taken from Acknowledgment back. The new 6LoWPAN Dispatch types are taken from
Page 0 [RFC8025] as indicated in Table 1 in Section 9. Page 0 [RFC8025] as indicated in Table 1 in Section 9.
In the following sections, a "Datagram_Tag" extends the semantics In the following sections, a "Datagram_Tag" extends the semantics
defined in [RFC4944] Section 5.3."Fragmentation Type and Header". defined in [RFC4944] Section 5.3."Fragmentation Type and Header".
The Datagram_Tag is a locally unique identifier for the datagram from The Datagram_Tag is a locally unique identifier for the datagram from
the perspective of the sender. This means that the Datagram_Tag the perspective of the sender. This means that the Datagram_Tag
identifies a datagram uniquely in the network when associated with identifies a datagram uniquely in the network when associated with
the source of the datagram. As the datagram gets forwarded, the the source of the datagram. As the datagram gets forwarded, the
source changes and the Datagram_Tag must be swapped as detailed in source changes and the Datagram_Tag must be swapped as detailed in
[I-D.ietf-6lo-minimal-fragment]. [FRAG-FWD].
5.1. Recoverable Fragment Dispatch type and Header 5.1. Recoverable Fragment Dispatch type and Header
In this specification, if the packet is compressed then the size and In this specification, if the packet is compressed then the size and
offset of the fragments are expressed with respect to the Compressed offset of the fragments are expressed with respect to the Compressed
Form of the packet form as opposed to the uncompressed (native) Form of the packet form as opposed to the uncompressed (native)
packet form. packet form.
The format of the fragment header is shown in Figure 1. It is the The format of the fragment header is shown in Figure 1. It is the
same for all fragments. The format has a length and an offset, as same for all fragments. The format has a length and an offset, as
well as a Sequence field. This would be redundant if the offset was well as a Sequence field. This would be redundant if the offset was
computed as the product of the Sequence by the length, but this is computed as the product of the Sequence by the length, but this is
not the case. The position of a fragment in the reassembly buffer is not the case. The position of a fragment in the reassembly buffer is
neither correlated with the value of the Sequence field nor with the neither correlated with the value of the Sequence field nor with the
order in which the fragments are received. This enables out-of- order in which the fragments are received. This enables
sequence subfragmenting, e.g., a fragment seq. 5 that is retried end- refragmenting to cope with an MTU deduction, see the example of the
to-end as smaller fragments seq. 5, 13 and 14 due to a change of MTU fragment seq. 5 that is retried end-to-end as smaller fragments seq.
along the path between the 6LoWPAN endpoints. 13 and 14 in Section 6.2.
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
skipping to change at page 10, line 8 skipping to change at page 10, line 19
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 1024 bytes. byte, which allows fragments up to 1024 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
fragment in a datagram, but non-zero values are not indicative of fragment in a datagram, but non-zero values are not indicative of
the position in the reassembly buffer. the position in the reassembly buffer.
skipping to change at page 11, line 11 skipping to change at page 11, line 22
and the packet is also forwarded in an attempt to clean up the and the packet is also forwarded in an attempt to clean up the
next hops along the path indicated by the IPv6 header (possibly next hops along the path indicated by the IPv6 header (possibly
including a routing header). including a routing header).
If the fragment cannot be forwarded or routed, then an abort If the fragment cannot be forwarded or routed, then an abort
RFRAG-ACK is sent back to the source as described in RFRAG-ACK is sent back to the source as described in
Section 6.1.2. Section 6.1.2.
5.2. RFRAG Acknowledgment Dispatch type and Header 5.2. RFRAG Acknowledgment Dispatch type and Header
This specification also defines a 4-octet RFRAG Acknowledgment bitmap This specification also defines a 4-byte RFRAG Acknowledgment bitmap
that is used by the reassembling endpoint to confirm selectively the that is used by the reassembling endpoint to confirm selectively the
reception of individual fragments. A given offset in the bitmap maps reception of individual fragments. A given offset in the bitmap maps
one-to-one with a given sequence number and indicates which fragment one-to-one with a given sequence number and indicates which fragment
is acknowledged as follows: is acknowledged as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RFRAG Acknowledgment Bitmap | | RFRAG Acknowledgment Bitmap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 12, line 30 skipping to change at page 12, line 42
6. Fragment Recovery 6. Fragment Recovery
The Recoverable Fragment header RFRAG is used to transport a fragment The Recoverable Fragment header RFRAG is used to transport a fragment
and optionally request an RFRAG Acknowledgment that will confirm the and optionally request an RFRAG Acknowledgment that will confirm the
good reception of one or more fragments. An RFRAG Acknowledgment is good reception of one or more fragments. An RFRAG Acknowledgment is
carried as a standalone fragment header (i.e., with no 6LoWPAN carried as a standalone fragment header (i.e., with no 6LoWPAN
payload) in a message that is propagated back to the 6LoWPAN endpoint payload) in a message that is propagated back to the 6LoWPAN endpoint
that was the originator of the fragments. To achieve this, each hop that was the originator of the fragments. To achieve this, each hop
that performed an MPLS-like operation on fragments reverses that that performed an MPLS-like operation on fragments reverses that
operation for the RFRAG_ACK by sending a frame from the next hop to operation for the RFRAG_ACK by sending a frame from the next hop to
the previous hop as known by its MAC address in the VRB. The the previous hop as known by its Link-Layer address in the VRB. The
Datagram_Tag in the RFRAG_ACK is unique to the receiver and is enough Datagram_Tag in the RFRAG_ACK is unique to the receiver and is enough
information for an intermediate hop to locate the VRB that contains information for an intermediate hop to locate the VRB that contains
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 with it (interface and MAC address). associated with it (interface and Link-Layer 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 number of acknowledgments by setting (the sender) also controls the number 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 controlled by the Window-Size. It number of outstanding fragments is controlled by the Window-Size. It
is configurable and may vary in case of ECN notification. When the is configurable and may vary in case of ECN notification. When the
skipping to change at page 13, line 10 skipping to change at page 13, line 22
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.
This automatic repeat request (ARQ) process MUST be protected by a This automatic repeat request (ARQ) process MUST be protected by a
Retransmission Time Out (RTO) timer, and the fragment that carries Retransmission Time Out (RTO) timer, and the fragment that carries
the 'X' flag MAY be retried upon a time out for a configurable number the 'X' flag MAY be retried upon a time out for a configurable number
of times (see Section 7.1). Upon exhaustion of the retries the of times (see Section 7.1) with an exponential backoff. Upon
sender may either abort the transmission of the datagram or retry the exhaustion of the retries the sender may either abort the
datagram from the first fragment with an 'X' flag set in order to transmission of the datagram or resend the first fragment with an 'X'
reestablish a path and discover which fragments were received over flag set in order to establish a new path for the datagram and obtain
the old path in the acknowledgment bitmap. When the sender of the the list of fragments that were received over the old path in the
fragment knows that an underlying link-layer mechanism protects the acknowledgment bitmap. When the sender of the fragment knows that an
fragments, it may refrain from using the RFRAG Acknowledgment underlying link-layer mechanism protects the fragments, it may
mechanism, and never set the Ack-Request bit. refrain from using the RFRAG Acknowledgment mechanism, and never set
the Ack-Request bit.
The receiver MAY issue unsolicited acknowledgments. An unsolicited The receiver MAY issue unsolicited acknowledgments. An unsolicited
acknowledgment signals to the sender endpoint that it can resume acknowledgment signals to the sender endpoint that it can resume
sending if it had reached its maximum number of outstanding sending if it had reached its maximum number of outstanding
fragments. Another use is to inform the sender that the reassembling fragments. Another use is to inform the sender that the reassembling
endpoint aborted the processing of an individual datagram. endpoint aborted the processing of an individual datagram.
The RFRAG Acknowledgment can optionally carry an ECN indication for The RFRAG Acknowledgment has an ECN indication for flow control (see
flow control (see Appendix C). The receiver of a fragment with the Appendix C). The receiver of a fragment with the 'E' (ECN) flag set
'E' (ECN) flag set MUST echo that information by setting the 'E' MUST echo that information by setting the 'E' (ECN) flag in the next
(ECN) flag in the next RFRAG Acknowledgment. RFRAG Acknowledgment.
In order to protect the datagram, the sender transfers a controlled In order to protect the datagram, the sender transfers a controlled
number of fragments and flags the last fragment of a window with an number of fragments and flags the last fragment of a window with an
RFRAG Acknowledgment Request. The receiver MUST acknowledge a RFRAG Acknowledgment Request. The receiver MUST acknowledge a
fragment with the acknowledgment request bit set. If any fragment fragment with the acknowledgment request bit set. If any fragment
immediately preceding an acknowledgment request is still missing, the immediately preceding an acknowledgment request is still missing, the
receiver MAY intentionally delay its acknowledgment to allow in- receiver MAY intentionally delay its acknowledgment to allow in-
transit fragments to arrive. Because it might defeat the round-trip transit fragments to arrive. Because it might defeat the round-trip
delay computation, delaying the acknowledgment should be configurable delay computation, delaying the acknowledgment should be configurable
and not enabled by default. and not enabled by default.
When enough fragments are received to cover the whole datagram, the When enough fragments are received to cover the whole datagram, the
receiving endpoint reconstructs the packet, passes it to the upper receiving endpoint reconstructs the packet, passes it to the upper
layer, sends an RFRAG Acknowledgment on the reverse path with a FULL layer, sends an RFRAG Acknowledgment on the reverse path with a FULL
bitmap, and arms a short timer, e.g., on the order of an average bitmap, and arms a short timer, e.g., on the order of an average
round-trip delay in the network. As the timer runs, the receiving round-trip delay in the network. The FULL bitmap is used as opposed
endpoint absorbs the fragments that were still in flight for that to a bitmap that acknowledges only the received fragments to let the
datagram without creating a new state. The receiving endpoint aborts intermediate nodes know that the datagram is fully received. As the
the communication if it keeps going on beyond the duration of the timer runs, the receiving endpoint absorbs the fragments that were
timer. still in flight for that datagram without creating a new state. The
receiving endpoint aborts the communication if it keeps going on
beyond the duration of the timer.
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 configured for a use case and may be adapted dynamically, e.g.,
expected that the upper layer retries obey the recommendations in according to the method detailed in [RFC6298]. It is expected that
[RFC8085], in which case a single round of fragment recovery should the upper layer retries obey the recommendations in [RFC8085], in
fit within the upper layer recovery timers. which case a single round of fragment recovery should 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 the frames (e.g., carrying fragments) that are insert a delay between the frames (e.g., carrying fragments) that are
sent to the same next hop. The delay should cover multiple sent to the same next hop. The delay should cover multiple
skipping to change at page 15, line 7 skipping to change at page 15, line 21
receiving the first fragment, the routers along the path install a receiving the first fragment, the routers along the path install a
label-switched path (LSP), and the following fragments are label- label-switched path (LSP), and the following fragments are label-
switched along that path. As a consequence, the next fragments can switched along that path. As a consequence, the next fragments can
only follow the path that was set up by the first fragment and cannot only follow the path that was set up by the first fragment and cannot
follow an alternate route. The Datagram_Tag is used to carry the follow an alternate route. The Datagram_Tag is used to carry the
label, which is swapped in each hop. All fragments follow the same label, which is swapped in each hop. All fragments follow the same
path and fragments are delivered in the order in which they are sent. path and fragments are delivered in the order in which they are sent.
6.1.1. Receiving the first fragment 6.1.1. Receiving the first fragment
In Route-Over mode, the source and destination MAC addresses in a In Route-Over mode, the source and destination Link-Layer addresses
frame change at each hop. The label that is formed and placed in the in a frame change at each hop. The label that is formed and placed
Datagram_Tag is associated with the source MAC address and only valid in the Datagram_Tag is associated with the source Link-Layer address
(and unique) for that source MAC address. Upon receiving the first and only valid (and unique) for that source Link-Layer address. Upon
fragment (i.e., with a Sequence of zero), an intermediate router receiving the first fragment (i.e., with a Sequence of zero), an
creates a VRB and the associated LSP state for the tuple (source MAC intermediate router creates a VRB and the associated LSP state for
address, Datagram_Tag) and the fragment is forwarded along the IPv6 the tuple (source Link-Layer address, Datagram_Tag) and the fragment
route that matches the destination IPv6 address in the IPv6 header as is forwarded along the IPv6 route that matches the destination IPv6
prescribed by [I-D.ietf-6lo-minimal-fragment], where the receiving address in the IPv6 header as prescribed by [FRAG-FWD], where the
endpoint allocates a reassembly buffer. receiving endpoint allocates a reassembly buffer.
The LSP state enables to match the (previous MAC address, The LSP state enables to match the (previous Link-Layer address,
Datagram_Tag) in an incoming fragment to the tuple (next MAC address, Datagram_Tag) in an incoming fragment to the tuple (next Link-Layer
swapped Datagram_Tag) used in the forwarded fragment and points at address, swapped Datagram_Tag) used in the forwarded fragment and
the VRB. In addition, the router also forms a reverse LSP state points at the VRB. In addition, the router also forms a reverse LSP
indexed by the MAC address of the next hop and the swapped state indexed by the MAC address of the next hop and the swapped
Datagram_Tag. This reverse LSP state also points at the VRB and Datagram_Tag. This reverse LSP state also points at the VRB and
enables matching the (next MAC address, swapped_Datagram_Tag) found enables matching the (next Link-Layer address, swapped_Datagram_Tag)
in an RFRAG Acknowledgment to the tuple (previous MAC address, found in an RFRAG Acknowledgment to the tuple (previous Link-Layer
Datagram_Tag) used when forwarding a Fragment Acknowledgment (RFRAG- address, Datagram_Tag) used when forwarding a Fragment Acknowledgment
ACK) back to the sender endpoint. (RFRAG-ACK) back to the sender endpoint.
The first fragment may be received a second time, indicating that it The first fragment may be received a second time, indicating that it
did not reach the destination and was retried. In that case, it did not reach the destination and was retried. In that case, it
SHOULD follow the same path as the first occurrence. It is up to SHOULD follow the same path as the first occurrence. It is up to
sending endpoint to determine whether to abort a transmission and sending endpoint to determine whether to abort a transmission and
then retry it from scratch, which may build an entirely new path. then retry it from scratch, which may build an entirely new path.
6.1.2. Receiving the next fragments 6.1.2. Receiving the next fragments
Upon receiving the next fragment (i.e., with a non-zero Sequence), an Upon receiving the next fragment (i.e., with a non-zero Sequence), an
intermediate router looks up a LSP indexed by the tuple (MAC address, intermediate router looks up a LSP indexed by the tuple (Link-Layer
Datagram_Tag) found in the fragment. If it is found, the router address, Datagram_Tag) found in the fragment. If it is found, the
forwards the fragment using the associated VRB as prescribed by router forwards the fragment using the associated VRB as prescribed
[I-D.ietf-6lo-minimal-fragment]. by [FRAG-FWD].
If the VRB for the tuple is not found, the router builds an RFRAG-ACK If the VRB for the tuple is not found, the router builds an RFRAG-ACK
to abort the transmission of the packet. The resulting message has to abort the transmission of the packet. The resulting message has
the following information: the following information:
* The source and destination MAC addresses are swapped from those * The source and destination Link-Layer addresses are swapped from
found in the fragment those found in the fragment
* The Datagram_Tag is set to the Datagram_Tag found in the fragment * The Datagram_Tag is set to the Datagram_Tag found in the fragment
* A NULL bitmap is used to signal the abort condition * A NULL bitmap is used to signal the abort condition
At this point the router is all set and can send the RFRAG-ACK back At this point the router is all set and can send the RFRAG-ACK back
to the previous router. The RFRAG-ACK should normally be forwarded to the previous router. The RFRAG-ACK should normally be forwarded
all the way to the source using the reverse LSP state in the VRBs in all the way to the source using the reverse LSP state in the VRBs in
the intermediate routers as described in the next section. the intermediate routers as described in the next section.
[I-D.ietf-6lo-minimal-fragment] indicates that the receiving endpoint [FRAG-FWD] indicates that the receiving endpoint stores "the actual
stores "the actual packet data from the fragments received so far, in packet data from the fragments received so far, in a form that makes
a form that makes it possible to detect when the whole packet has it possible to detect when the whole packet has been received and can
been received and can be processed or forwarded". How this is be processed or forwarded". How this is computed is implementation
computed is implementation specific but relies on receiving all the specific but relies on receiving all the bytes up to the
bytes up to the Datagram_Size indicated in the first fragment. An Datagram_Size indicated in the first fragment. An implementation may
implementation may receive overlapping fragments as the result of receive overlapping fragments as the result of retries after an MTU
retries after an MTU change. change.
6.2. Receiving RFRAG Acknowledgments 6.2. Receiving RFRAG Acknowledgments
Upon receipt of an RFRAG-ACK, the router looks up a reverse LSP Upon receipt of an RFRAG-ACK, the router looks up a reverse LSP
indexed by the tuple (MAC address, Datagram_Tag), which are indexed by the tuple (Link-Layer address, Datagram_Tag), which are
respectively the source MAC address of the received frame and the respectively the source Link-Layer address of the received frame and
received Datagram_Tag. If it is found, the router forwards the the received Datagram_Tag. If it is found, the router forwards the
fragment using the associated VRB as prescribed by fragment using the associated VRB as prescribed by [FRAG-FWD], but
[I-D.ietf-6lo-minimal-fragment], but using the reverse LSP so that using the reverse LSP so that the RFRAG-ACK flows back to the sender
the RFRAG-ACK flows back to the sender endpoint. endpoint.
If the reverse LSP is not found, the router MUST silently drop the If the reverse LSP is not found, the router MUST silently drop the
RFRAG-ACK message. RFRAG-ACK message.
Either way, if the RFRAG-ACK indicates that the fragment was entirely Either way, if the RFRAG-ACK indicates that the fragment was entirely
received (FULL bitmap), it arms a short timer, and upon timeout, the received (FULL bitmap), it arms a short timer, and upon timeout, the
VRB and all the associated state are destroyed. Until the timer VRB and all the associated state are destroyed. Until the timer
elapses, fragments of that datagram may still be received, e.g. if elapses, fragments of that datagram may still be received, e.g. if
the RFRAG-ACK was lost on the way back and the source retried the the RFRAG-ACK was lost on the way back and the source retried the
last fragment. In that case, the router forwards the fragment last fragment. In that case, the router forwards the fragment
according to the state in the VRB. according to the state in the VRB.
This specification does not provide a method to discover the number This specification does not provide a method to discover the number
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 several shorter fragments with a Sequence that
again) and then one or more other fragments with a Sequence that was was not used before (e.g., 13 and 14). Fragment 5 is marked as
not used before (e.g., 13 and 14). Note that Path MTU Discovery is abandoned and will not be retried anymore. Note that when thi
out of scope for this document. smechanism is in place, it is hard to predict the total number of
fragments that will be needed or the final shape of the bitmap that
would cover the whole packet. This is why the FULL bitmap is used
when the receiving endpoint gets the whole datagram regardless of
which fragments were actually used to do so. Intermediate nodes will
unabiguously knw that the process is complete. Note that Path MTU
Discovery is 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.
skipping to change at page 18, line 8 skipping to change at page 18, line 23
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 This specification extends "On Forwarding 6LoWPAN Fragments over a
Multihop IPv6 Network" [I-D.ietf-6lo-minimal-fragment] and requires Multihop IPv6 Network" [FRAG-FWD] and requires the same parameters in
the same parameters in the receiver and on intermediate nodes. There the receiver and on intermediate nodes. There is no new parameter as
is no new parameter as echoing ECN is always on. These parameters echoing ECN is always on. These parameters typically include the
typically include the reassembly time-out at the receiver and an reassembly time-out at the receiver and an inactivity clean-up timer
inactivity clean-up timer on the intermediate nodes, and the number on the intermediate nodes, and the number of messages that can be
of messages that can be processed in parallel in all nodes. processed in parallel in all nodes.
The configuration settings introduced by this specification only The configuration settings introduced by this specification only
apply to the sender, which is in full control of the transmission. 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), LLNs vary a lot in size (there can be thousands of nodes in a mesh),
in speed (from 10 Kbps to several Mbps at the PHY layer), in traffic in speed (from 10 Kbps to several Mbps at the PHY layer), in traffic
density, and in optimizations that are desired (e.g., the selection density, and in optimizations that are desired (e.g., the selection
of a RPL [RFC6550] Objective Function [RFC6552] impacts the shape of of a RPL [RFC6550] Objective Function [RFC6552] impacts the shape of
the routing graph). the routing graph).
For that reason, only a very generic guidance can be given on the For that reason, only a very generic guidance can be given on the
settings of the sender and on whether complex algorithms are needed settings of the sender and on whether complex algorithms are needed
to perform flow control or estimate the round-trip time. To cover to perform flow control or estimate the round-trip time. To cover
the most complex use cases, this specification enables the sender to the most complex use cases, this specification enables the sender to
vary the fragment size, the window size, and the inter-frame gap, vary the fragment size, the window size, and the inter-frame gap,
based on the number of losses, the observed variations of the round- based on the number of losses, the observed variations of the round-
trip time and the setting of the ECN bit. trip time and the setting of the ECN bit.
7.1. Protocol Parameters 7.1. Protocol Parameters
The management system SHOULD be capable of providing the parameters The management system SHOULD be capable of providing the parameters
listed in this section. listed in this section and an implementation MUST abide by those
parameters and in particular never exceed the minimum and maximum
configured boundaries.
An implementation must control the rate at which it sends packets An implementation must control the rate at which it sends packets
over the same path to allow the next hop to forward a packet before over the same path to allow the next hop to forward a packet before
it gets the next. In a wireless network that uses the same frequency it gets the next. In a wireless network that uses the same frequency
along a path, more time must be inserted to avoid hidden terminal along a path, more time must be inserted to avoid hidden terminal
issues between fragments (more in Section 4.2). issues between fragments (more in Section 4.2).
This is controlled by the following parameter: This is controlled by the following parameter:
inter-frame gap: Indicates the minimum amount of time between inter-frame gap: Indicates the minimum amount of time between
skipping to change at page 19, line 30 skipping to change at page 19, line 48
expected fluidity and the overhead of MAC and 6LoWPAN headers. expected fluidity and the overhead of MAC and 6LoWPAN headers.
For a small MTU, the idea is to keep it close to the maximum, 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 whereas for larger MTUs, it might makes sense to keep it short
enough, so that the duty cycle of the transmitter is bounded, enough, so that the duty cycle of the transmitter is bounded,
e.g., to transmit at least 10 frames per second. 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 byte.
MinWindowSize: The minimum value of Window_Size that the sender can MinWindowSize: The minimum value of Window_Size that the sender can
use. use. A value of 1 is RECOMMENDED.
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. It is greater than or that the sender should use to start with. It is greater than or
equal to MinWindowSize. It is less than or equal to equal to MinWindowSize. It is less than or equal to
MaxWindowSize. The Window_Size should be maintained below the MaxWindowSize. A rule of a thumb for OptWindowSize could be an
number of hops in the path of the fragment to avoid stacking estimation of the one-way trip time divided by the inter-frame
fragments at the bottleneck on the path. If an inter-frame gap is gap. If the acknowledgement back is too costly, it is possible to
used to avoid interference between fragments then the Window_Size set this to 32, meaning that only the last Fragment is
should be at most on the order of the estimation of the trip time acknowledged in the first round.
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 strictly less than 33.
An implementation may perform its estimate of the RTO or use a An implementation may perform its estimate of the RTO or use a
configured one. The ARQ process is controlled by the following configured one. The ARQ process is controlled by the following
parameters: parameters:
MinARQTimeOut: The maximum amount of time a node should wait for an MinARQTimeOut: The minimum amount of time a node should wait for an
RFRAG Acknowledgment before it takes the next action. RFRAG Acknowledgment before it takes the next action. It MUST be
more than the maximum expected round-trip time in the respective
network.
OptARQTimeOut: The initial value of the RTO, which is the amount of OptARQTimeOut: The initial value of the RTO, which is the amount of
time that a sender should wait for an RFRAG Acknowledgment before time that a sender should wait for an RFRAG Acknowledgment before
it takes the next action. It is greater than or equal to it takes the next action. It is greater than or equal to
MinARQTimeOut. It is less than or equal to MaxARQTimeOut. See MinARQTimeOut. It is less than or equal to MaxARQTimeOut. See
Appendix C for recommendations on computing the round-trip time. Appendix C for recommendations on computing the round-trip time.
By default a value of 3 times the maximum expected round-trip time
in the respective network is RECOMMENDED.
MaxARQTimeOut: The maximum amount of time a node should wait for the MaxARQTimeOut: The maximum amount of time a node should wait for the
RFRAG Acknowledgment before it takes the next action. It must RFRAG Acknowledgment before it takes the next action. It must
cover the longest expected round-trip time, and be several times cover the longest expected round-trip time, and be several times
less than the time-out that covers the recomposition buffer at the less than the time-out that covers the recomposition buffer at the
receiver, which is typically on the order of the minute. receiver, which is typically on the order of the minute. An upper
bound can be estimated to ensure that the datagram is either fully
transmitted or dropped before an upper layer decides to retry it.
MaxFragRetries: The maximum number of retries for a particular MaxFragRetries: The maximum number of retries for a particular
fragment. fragment. A default value of 3 is RECOMMENDED. An upper bound
can be estimated to ensure that the datagram is either fully
transmitted or dropped before an upper layer decides to retry it.
MaxDatagramRetries: The maximum number of retries from scratch for a MaxDatagramRetries: The maximum number of retries from scratch for a
particular datagram. particular datagram. A default value of 1 is RECOMMENDED. An
upper bound can be estimated to ensure that the datagram is either
fully transmitted or dropped before an upper layer decides to
retry it.
An implementation may be capable of performing flow control based on An implementation may be capable of performing flow control based on
ECN; see in Appendix C. This is controlled by the following ECN; see in Appendix C. This is controlled by the following
parameter: parameter:
UseECN: Indicates whether the sender should react to ECN. The UseECN: Indicates whether the sender should react to ECN. The
sender may react to ECN by varying the Window_Size between sender may react to ECN by varying the Window_Size between
MinWindowSize and MaxWindowSize, varying the Fragment_Size between MinWindowSize and MaxWindowSize, varying the Fragment_Size between
MinFragmentSize and MaxFragmentSize, and/or by increasing the MinFragmentSize and MaxFragmentSize, and/or by increasing or
inter-frame gap. reducing the inter-frame gap.
7.2. Observing the network 7.2. Observing the network
The management system should monitor the number of retries and of ECN The management system should monitor the number 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 with regards to the other endpoint. It may then
of Window_Size, OptFragmentSize, and OptWindowSize, respectively, at tune the optimum size of Fragment_Size and of Window_Size,
the sender. The values should be bounded by the expected number of OptFragmentSize, and OptWindowSize, respectively, at the sender
hops and reduced beyond that when the number of datagrams that can towards a particular receiver, applicable to the next datagrams. The
traverse an intermediate point may exceed its capacity and cause a values should be bounded by the expected number of hops and reduced
congestion loss. The inter-frame gap is another tool that can be beyond that when the number of datagrams that can traverse an
used to increase the spacing between fragments of the same datagram intermediate point may exceed its capacity and cause a congestion
and reduce the ratio of time when a particular intermediate node loss. The inter-frame gap is another tool that can be used to
holds a fragment of that datagram. increase the spacing between fragments of the same datagram and
reduce the ratio of time when a particular intermediate node 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. [I-D.ietf-6lo-minimal-fragment] provides the Forwarding technique. [FRAG-FWD] provides the generic description of
generic description of Fragment Forwarding and this specification Fragment Forwarding and this specification inherits from it. The
inherits from it. The generic considerations in the Security generic considerations in the Security sections of [FRAG-FWD] apply
sections of [I-D.ietf-6lo-minimal-fragment] apply equally to this 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.
Compared to "Transmission of IPv6 Packets over IEEE 802.15.4 Compared to "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944], this specification reduces the Datagram_Tag to 8 Networks" [RFC4944], this specification reduces the Datagram_Tag to 8
bits and the tag wraps faster than with [RFC4944]. But for a bits and the tag wraps faster than with [RFC4944]. But for a
constrained network where a node is expected to be able to hold only constrained network where a node is expected to be able to hold only
one or a few large packets in memory, 256 is still a large number. one or a few large packets in memory, 256 is still a large number.
Also, the acknowledgement mechanism allows cleaning up the state Also, the acknowledgement mechanism allows cleaning up the state
rapidly once the packet is fully transmitted or aborted. rapidly once the packet is fully transmitted or aborted.
The abstract Virtual Recovery Buffer inherited from The abstract Virtual Recovery Buffer inherited from [FRAG-FWD] may be
[I-D.ietf-6lo-minimal-fragment] may be used to perform a Denial-of- used to perform a Denial-of-Service (DoS) attack against the
Service (DoS) attack against the intermediate Routers since the intermediate Routers since the routers need to maintain a state per
routers need to maintain a state per flow. The particular VRB flow. The particular VRB implementation technique described in
implementation technique described in
[I-D.ietf-lwig-6lowpan-virtual-reassembly] allows realigning which [I-D.ietf-lwig-6lowpan-virtual-reassembly] allows realigning which
data goes in which fragment, which causes the intermediate node to data goes in which fragment, which causes the intermediate node to
store a portion of the data, which adds an attack vector that is not store a portion of the data, which adds an attack vector that is not
present with this specification. With this specification, the data present with this specification. With this specification, the data
that is transported in each fragment is conserved and the state to that is transported in each fragment is conserved and the state to
keep does not include any data that would not fit in the previous keep does not include any data that would not fit in the previous
fragment. fragment.
9. IANA Considerations 9. IANA Considerations
skipping to change at page 22, line 30 skipping to change at page 23, line 11
+-------------+------+----------------------------------+-----------+ +-------------+------+----------------------------------+-----------+
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
Roman Danyliw, Peter Yee, Colin Perkins, Tirumaleswar Reddy Konda, Roman Danyliw, Peter Yee, Colin Perkins, Tirumaleswar Reddy Konda,
and Erik Nordmark for their careful reviews and for helping through Eric Vyncke, Benjamin Kaduk, Warren Kumari, Magnus Westerlund, Mirja
the IETF Last Call and IESG review process, and to Jonathan Hui, Jay Kuhlewind, and Erik Nordmark for their careful reviews and for
Werb, Christos Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret helping through the IETF Last Call and IESG review process, and to
Wasserman, Richard Kelsey, Carsten Bormann, and Harry Courtice for Jonathan Hui, Jay Werb, Christos Polyzois, Soumitri Kolavennu, Pat
their various contributions in the long process that lead to this Kinney, Margaret Wasserman, Richard Kelsey, Carsten Bormann, and
document. Harry Courtice for their various contributions in the long process
that lead to this document.
11. Normative References 11. Normative References
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent, [RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, "Computing TCP's Retransmission Timer", RFC 6298,
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>.
[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,
skipping to change at page 23, line 30 skipping to change at page 24, line 14
[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie, [RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network "IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>. April 2017, <https://www.rfc-editor.org/info/rfc8138>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[I-D.ietf-6lo-minimal-fragment] [FRAG-FWD] Watteyne, T., Thubert, P., and C. Bormann, "On Forwarding
Watteyne, T., Thubert, P., and C. Bormann, "On Forwarding
6LoWPAN Fragments over a Multihop IPv6 Network", Work in 6LoWPAN Fragments over a Multihop IPv6 Network", Work in
Progress, Internet-Draft, draft-ietf-6lo-minimal-fragment- Progress, Internet-Draft, draft-ietf-6lo-minimal-fragment-
10, 1 February 2020, <https://tools.ietf.org/html/draft- 13, 5 March 2020, <https://tools.ietf.org/html/draft-ietf-
ietf-6lo-minimal-fragment-10>. 6lo-minimal-fragment-13>.
12. Informative References 12. Informative References
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201, "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017, DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>. <https://www.rfc-editor.org/info/rfc8201>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management", Recommendations Regarding Active Queue Management",
skipping to change at page 27, line 29 skipping to change at page 28, line 12
higher than that traditionally experienced over the Internet with higher than that traditionally experienced over the Internet with
IPv4 fragments. At the same time, the use of radios increases the IPv4 fragments. At the same time, the use of radios increases the
probability of transmission loss and Mesh-Under techniques compound probability of transmission loss and Mesh-Under techniques compound
that risk over multiple hops. that risk over multiple hops.
Mechanisms such as TCP or application-layer segmentation could be Mechanisms such as TCP or application-layer segmentation could be
used to support end-to-end reliable transport. One option to support used to support end-to-end reliable transport. One option to support
bulk data transfer over a frame-size-constrained LLN is to set the bulk data transfer over a frame-size-constrained LLN is to set the
Maximum Segment Size to fit within the link maximum frame size. Maximum Segment Size to fit within the link maximum frame size.
Doing so, however, can add significant header overhead to each Doing so, however, can add significant header overhead to each
802.15.4 frame. In addition, deploying such a mechanism requires 802.15.4 frame and cause extraneous acknowledgements across the LLN
that the end-to-end transport is aware of the delivery properties of compared to the method in this specification.
the underlying LLN, which is a layer violation, and difficult to
achieve from the far end of the IPv6 network.
Appendix B. Requirements Appendix B. Requirements
For one-hop communications, a number of Low Power and Lossy Network For one-hop communications, a number of Low Power and Lossy Network
(LLN) link-layers propose a local acknowledgment mechanism that is (LLN) link-layers propose a local acknowledgment mechanism that is
enough to detect and recover the loss of fragments. In a multihop enough to detect and recover the loss of fragments. In a multihop
environment, an end-to-end fragment recovery mechanism might be a environment, an end-to-end fragment recovery mechanism might be a
good complement to a hop-by-hop MAC level recovery. This draft good complement to a hop-by-hop MAC level recovery. This draft
introduces a simple protocol to recover individual fragments between introduces a simple protocol to recover individual fragments between
6LoWPAN endpoints that may be multiple hops away. The method 6LoWPAN endpoints that may be multiple hops away.
addresses the following requirements of an LLN:
The method addresses the following requirements of an LLN:
Number of fragments: The recovery mechanism must support highly Number of fragments: The recovery mechanism must support highly
fragmented packets, with a maximum of 32 fragments per packet. fragmented packets, with a maximum of 32 fragments per packet.
Minimum acknowledgment overhead: Because the radio is half duplex, Minimum acknowledgment overhead: Because the radio is half duplex,
and because of silent time spent in the various medium access and because of silent time spent in the various medium access
mechanisms, an acknowledgment consumes roughly as many resources mechanisms, an acknowledgment consumes roughly as many resources
as a data fragment. as a data fragment.
The new end-to-end fragment recovery mechanism should be able to The new end-to-end fragment recovery mechanism should be able to
skipping to change at page 28, line 31 skipping to change at page 29, line 15
Appendix C. Considerations on Flow Control Appendix C. Considerations on Flow Control
Considering that a multi-hop LLN can be a very sensitive environment Considering that a multi-hop LLN can be a very sensitive environment
due to the limited queuing capabilities of a large population of its due to the limited queuing capabilities of a large population of its
nodes, this draft recommends a simple and conservative approach to nodes, this draft recommends a simple and conservative approach to
Congestion Control, based on TCP congestion avoidance. Congestion Control, based on TCP congestion avoidance.
Congestion on the forward path is assumed in case of packet loss, and Congestion on the forward path is assumed in case of packet loss, and
packet loss is assumed upon time out. The draft allows controlling packet loss is assumed upon time out. The draft allows controlling
the number of outstanding fragments that have been transmitted but the number of outstanding fragments that have been transmitted but
for which an acknowledgment was not received yet. It must be noted for which an acknowledgment was not received yet.
that the number of outstanding fragments should not exceed the number
of hops in the network, but the way to figure the number of hops is
out of scope for this document.
Congestion on the forward path can also be indicated by an Explicit Congestion on the forward path can also be indicated by an Explicit
Congestion Notification (ECN) mechanism. Though whether and how ECN Congestion Notification (ECN) mechanism. Though whether and how ECN
[RFC3168] is carried out over the LoWPAN is out of scope, this draft [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 provides a way for the destination endpoint to echo an ECN indication
back to the source endpoint in an acknowledgment message as back to the source endpoint in an acknowledgment message as
represented in Figure 4 in Section 5.2. represented in Figure 4 in Section 5.2. While the support of echoing
the ECN at the receiver in mandatory, this specification does not
provide the flow control mechanism that react to the congestion at
teh sender endpoint. A minimalistic behaviour could be to reset the
window to 1 so the fragments are sent and acknowledged one by one
till the end of the datagram.
It must be noted that congestion and collision are different topics. It must be noted that congestion and collision are different topics.
In particular, when a mesh operates on the same channel over multiple In particular, when a mesh operates on the same channel over multiple
hops, then the forwarding of a fragment over a certain hop may hops, then the forwarding of a fragment over a certain hop may
collide with the forwarding of the next fragment that is following collide with the forwarding of the next fragment that is following
over a previous hop but in the same interference domain. This draft over a previous hop but in the same interference domain. This draft
enables end-to-end flow control, but leaves it to the sender stack to enables end-to-end flow control, but leaves it to the sender stack to
pace individual fragments within a transmit window, so that a given pace individual fragments within a transmit window, so that a given
fragment is sent only when the previous fragment has had a chance to fragment is sent only when the previous fragment has had a chance to
progress beyond the interference domain of this hop. In the case of progress beyond the interference domain of this hop. In the case of
skipping to change at page 29, line 22 skipping to change at page 30, line 9
fragment is a fragment that was sent but for which no explicit fragment is a fragment that was sent but for which no explicit
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 time
between the 6LoWPAN endpoints. The method detailed in "Computing between the 6LoWPAN endpoints. For the lack of a more adapted
TCP's Retransmission Timer" [RFC6298] is recommended for that technique, the method detailed in "Computing TCP's Retransmission
computation. Timer" [RFC6298] may be used 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 number of fragments present in the network; this is achieved by the number 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
 End of changes. 67 change blocks. 
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