draft-ietf-6lo-fragment-recovery-03.txt   draft-ietf-6lo-fragment-recovery-04.txt 
6lo P. Thubert, Ed. 6lo P. Thubert, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Updates: 4944 (if approved) May 20, 2019 Updates: 4944 (if approved) June 11, 2019
Intended status: Standards Track Intended status: Standards Track
Expires: November 21, 2019 Expires: December 13, 2019
6LoWPAN Selective Fragment Recovery 6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-03 draft-ietf-6lo-fragment-recovery-04
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 November 21, 2019. This Internet-Draft will expire on December 13, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 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 Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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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. 6LoWPAN Acronyms . . . . . . . . . . . . . . . . . . . . 4 2.3. 6LoWPAN Acronyms . . . . . . . . . . . . . . . . . . . . 4
2.4. Referenced Work . . . . . . . . . . . . . . . . . . . . . 4 2.4. Referenced Work . . . . . . . . . . . . . . . . . . . . . 4
2.5. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5 2.5. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 5 3. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 6
4. Updating draft-ietf-6lo-minimal-fragment . . . . . . . . . . 6 4. Updating draft-ietf-6lo-minimal-fragment . . . . . . . . . . 6
4.1. Slack in the First Fragment . . . . . . . . . . . . . . . 6 4.1. Slack in the First Fragment . . . . . . . . . . . . . . . 7
4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 6 4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 7
4.3. Modifying the First Fragment . . . . . . . . . . . . . . 7 4.3. Modifying the First Fragment . . . . . . . . . . . . . . 7
5. New Dispatch types and headers . . . . . . . . . . . . . . . 7 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 . . . . . . 10 5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12 6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 13
6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14 6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 15
6.1.1. Upon the first fragment . . . . . . . . . . . . . . . 14 6.1.1. Upon the first fragment . . . . . . . . . . . . . . . 15
6.1.2. Upon the next fragments . . . . . . . . . . . . . . . 14 6.1.2. Upon the next fragments . . . . . . . . . . . . . . . 15
6.2. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 15 6.2. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 16
6.3. Cancelling a Fragmented Packet . . . . . . . . . . . . . 15 6.3. Aborting the Transmission of a Fragmented Packet . . . . 17
7. Management Considerations . . . . . . . . . . . . . . . . . . 16 7. Management Considerations . . . . . . . . . . . . . . . . . . 17
7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 16 7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 17
7.2. Observing the network . . . . . . . . . . . . . . . . . . 17 7.2. Observing the network . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . 18 11.1. Normative References . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . 19 11.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 21 Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 22 Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 24
Appendix C. Considerations On Flow Control . . . . . . . . . . . 23 Appendix C. Considerations On Flow Control . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26
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 (in the order few bytes the traffic consists of small chunks of data (in the order few bytes
to a few tens of bytes) at a time. Given that an IEEE Std. 802.15.4 to a few tens of bytes) at a time. Given that an IEEE Std. 802.15.4
[IEEE.802.15.4] frame can carry 74 bytes or more in all cases, [IEEE.802.15.4] frame can carry a payload of 74 bytes or more,
fragmentation is usually not required. However, and though this fragmentation is usually not required. However, and though this
happens only occasionally, a number of mission critical applications happens only occasionally, a number of mission critical applications
do require the capability to transfer larger chunks of data, for do require the capability to transfer larger chunks of data, for
instance to support a firmware upgrades of the LLN nodes or an instance to support the firmware upgrade of the LLN nodes or the
extraction of logs from LLN nodes. In the former case, the large extraction of logs from LLN nodes. In the former case, the large
chunk of data is transferred to the LLN node, whereas in the latter, chunk of data is transferred to the LLN node, whereas in the latter,
the large chunk flows away from the LLN node. In both cases, the the large chunk flows away from the LLN node. In both cases, the
size can be on the order of 10Kbytes or more and an end-to-end size can be on the order of 10 kilobytes or more and an end-to-end
reliable transport is required. 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 to an IPv6 [RFC8200] packet across a route-over mesh requires to
reassemble the full packet at each hop, which may cause latency along reassemble the full packet at each hop, which may cause latency along
a path and an overall buffer bloat in the network. The "6TiSCH a path and an overall buffer bloat in the network. The "6TiSCH
Architecture" [I-D.ietf-6tisch-architecture] recommends to use a hop- Architecture" [I-D.ietf-6tisch-architecture] recommends to use a hop-
by-hop fragment forwarding technique to alleviate those undesirable by-hop fragment forwarding technique to alleviate those undesirable
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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
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 to a also lacking flow control capabilities to avoid participating to a
congestion that may in turn cause the loss of a fragment and trigger congestion that may in turn cause the loss of a fragment and
the retransmission of the full datagram. potentially the retransmission of the full datagram.
This specification proposes a method to forward fragments across a This specification proposes a method to forward fragments across a
multi-hop route-over mesh, and to recover individual fragments multi-hop route-over mesh, and to recover individual fragments
between LLN endpoints. The method is designed to limit congestion between LLN endpoints. The method is designed to limit congestion
loss in the network and addresses the requirements that are detailed loss in the network and addresses the requirements that are detailed
in Appendix B. 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 the following documents: discussed in "Problem Statement and Requirements for IPv6 over Low-
Power Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]
o "Problem Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]
2.3. 6LoWPAN Acronyms 2.3. 6LoWPAN Acronyms
This document uses the following acronyms: This document uses the following acronyms:
6BBR: 6LoWPAN Backbone Router 6BBR: 6LoWPAN Backbone Router
6LBR: 6LoWPAN Border Router 6LBR: 6LoWPAN Border Router
6LN: 6LoWPAN Node 6LN: 6LoWPAN Node
6LR: 6LoWPAN Router 6LR: 6LoWPAN Router
LLN: Low-Power and Lossy Network LLN: Low-Power and Lossy Network
2.4. Referenced Work 2.4. Referenced Work
Past experience with fragmentation has shown that miss-associated 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 application layer. The reader is encouraged to read "IPv4 trouble at application layer. The reader is encouraged to read "IPv4
Reassembly Errors at High Data Rates" [RFC4963] and follow the 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.
Specifically in the case of UDP, valuable additional information can Specifically in the case of UDP, valuable additional information can
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that are discussed in "IPv6 over Low-Power Wireless Personal Area that are discussed in "IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4 Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944]. Networks" [RFC4944].
"The Benefits of Using Explicit Congestion Notification (ECN)" "The Benefits of Using Explicit Congestion Notification (ECN)"
[RFC8087] provides useful information on the potential benefits and [RFC8087] provides useful information on the potential benefits and
pitfalls of using ECN. pitfalls of using ECN.
Quoting the "Multiprotocol Label Switching (MPLS) Architecture" Quoting the "Multiprotocol Label Switching (MPLS) Architecture"
[RFC3031]: with MPLS, "packets are "labeled" before they are [RFC3031]: with MPLS, 'packets are "labeled" before they are
forwarded. At subsequent hops, there is no further analysis of the forwarded'. At subsequent hops, there is no further analysis of the
packet's network layer header. Rather, the label is used as an index packet's network layer header. Rather, the label is used as an index
into a table which specifies the next hop, and a new label". The into a table which specifies the next hop, and a new label". The
MPLS technique is leveraged in the present specification to forward MPLS technique is leveraged in the present specification to forward
fragments that actually do not have a network layer header, since the fragments that actually do not have a network layer header, since the
fragmentation occurs below IP. fragmentation occurs below IP.
"LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment] "LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment]
introduces the concept of a Virtual Reassembly Buffer (VRB) and an introduces the concept of a Virtual Reassembly Buffer (VRB) and an
associated technique to forward fragments as they come, using the associated technique to forward fragments as they come, using the
Datagram_tag as a label in a fashion similar to MLPS. This datagram_tag as a label in a fashion similar to MPLS. This
specification reuses that technique with slightly modified controls. specification reuses that technique with slightly modified controls.
2.5. New Terms 2.5. New Terms
This specification uses the following terms: This specification uses the following terms:
6LoWPAN endpoints 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.
The LLN nodes in charge of generating or expanding a 6LoWPAN Compressed Form This specification uses the generic term Compressed
header from/to a full IPv6 packet. The 6LoWPAN endpoints are the Form to refer to the format of a datagram after the action of
points where fragmentation and reassembly take place. [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.
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.
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.
RFRAG: Recoverable Fragment
RFRAG-ACK: Recoverable Fragment Acknowledgement
RFRAG Acknowledgment Request: An RFRAG with the Acknowledgement
Request flag ('X' flag) set.
All 0's: Refers to a bitmap with all bits set to zero.
All 1's: Refers to a bitmap with all bits set to one.
3. Updating RFC 4944 3. Updating RFC 4944
This specification updates the fragmentation mechanism that is This specification updates the fragmentation mechanism that is
specified in "Transmission of IPv6 Packets over IEEE 802.15.4 specified in "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944] for use in route-over LLNs by providing a model Networks" [RFC4944] for use in route-over LLNs by providing a model
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 fragment is introduces and new individually. A new format for fragment is introduced and new
dispatch types are defined in Section 5. dispatch types are defined in Section 5.
[RFC8138] allows to modify the size of a packet en-route by removing [RFC8138] allows to modify the size of a packet en-route by removing
the consumed hops in a compressed Routing Header. It results that the consumed hops in a compressed Routing Header. It results that
the fragment_offset and datagram_size cannot be signaled in the fragment_offset and datagram_size (see Section 2.5) must also be
uncompressed form. This specification expresses those fields in the modified en-route, whcih is difficult to do in the uncompressed form.
compressed form and allows to modify them en-route (see Section 4.3. This specification expresses those fields in the Compressed Form and
allows to modify them en-route (see Section 4.3) easily.
Note that consistently with in Section 2 of [RFC6282] for the Note that consistently 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. Updating draft-ietf-6lo-minimal-fragment 4. Updating draft-ietf-6lo-minimal-fragment
This specification updates the fragment forwarding mechanism This specification updates the fragment forwarding mechanism
specified in "LLN Minimal Fragment Forwarding" specified in "LLN Minimal Fragment Forwarding"
[I-D.ietf-6lo-minimal-fragment] by providing additional operations to [I-D.ietf-6lo-minimal-fragment] by providing additional operations to
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a given fragment may be left in the VRB to be added to the next a given fragment may be left in the VRB to be added to the next
fragment. The reason for this to happen would be the need for space fragment. The reason for this to happen would be the need for space
in the outgoing fragment that was not needed in the incoming in the outgoing fragment that was not needed in the incoming
fragment, for instance because the 6LoWPAN Header Compression is not fragment, for instance because the 6LoWPAN Header Compression is not
as efficient on the outgoing link, e.g., if the Interface ID (IID) of as efficient on the outgoing link, e.g., if the Interface ID (IID) of
the source IPv6 address is elided by the originator on the first hop the source IPv6 address is elided by the originator on the first hop
because it matches the source MAC address, but cannot be on the next because it matches the source MAC address, but cannot be on the next
hops because the source MAC address changes. hops because the source 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 the fragment number. This means that recovered end-to-end based on a sequence number. This means that the
the fragments that contain a 6LoWPAN-compressed header MUST have fragments that contain a 6LoWPAN-compressed header MUST have enough
enough slack to enable a less efficient compression in the next hops slack to enable a less efficient compression in the next hops that
that still fits in one MAC frame. For instance, if the IID of the still fits in one MAC frame. For instance, if the IID of the source
source IPv6 address is elided by the originator, then it MUST compute IPv6 address is elided by the originator, then it MUST compute the
the fragment_size as if the MTU was 8 bytes less. This way, the next fragment_size as if the MTU was 8 bytes less. This way, the next hop
hop can restore the source IID to the first fragment without can restore the source IID to the first fragment without impacting
impacting the second fragment. the second fragment.
4.2. Gap between frames 4.2. Gap between frames
This specification introduces a concept of Inter-Frame Gap, which is This specification introduces a concept of Inter-Frame Gap, which is
a configurable interval of time between transmissions to a same next a configurable interval of time between transmissions to a same next
hop. In the case of half duplex interfaces, this InterFrameGap hop. In the case of half duplex interfaces, this InterFrameGap
ensures that the next hop has progressed the previous frame and is ensures that the next hop has progressed the previous frame and is
capable of receiving the next one. 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 InterFrameGap is required protect omnidirectional antennas, a larger InterFrameGap is required to
the frame against hidden terminal collisions with the previous frame protect the frame against hidden terminal collisions with the
of a same flow that is still progressing alon a common path. previous frame of a same flow that is still progressing along a
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. 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, and of the Routing Header may change en-route in a Route- addresses in the IPv6 Header, and of the Routing Header, may change
Over mesh LLN. If the size of the first fragment is modified, then en-route in a Route-Over mesh LLN. If the size of the first fragment
the intermediate node MUST adapt the datagram_size to reflect that is modified, then the intermediate node MUST adapt the datagram_size
difference. to 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 specification enables the 6LoWPAN fragmentation sublayer to
provide an MTU up to 2048 bytes to the upper layer, which can be the provide an MTU up to 2048 bytes to the upper layer, which can be the
6LoWPAN Header Compression sublayer that is defined in the 6LoWPAN Header Compression sublayer that is defined in the
"Compression Format for IPv6 Datagrams" [RFC6282] specification. In "Compression Format for IPv6 Datagrams" [RFC6282] specification. In
order to achieve this, this specification enables the fragmentation order to achieve this, this specification enables the fragmentation
and the reliable transmission of fragments over a multihop 6LoWPAN and the reliable transmission of fragments over a multihop 6LoWPAN
mesh network. mesh network.
This specification provides a technique that is derived from MPLS in This specification provides a technique that is derived from MPLS to
order to forward individual fragments across a 6LoWPAN route-over forward individual fragments across a 6LoWPAN route-over mesh without
mesh. The Datagram_tag is used as a label; it is locally unique to reassembly at each hop. The datagram_tag is used as a label; it is
the node that is the source MAC address of the fragment, so together locally unique to the node that owns the source MAC address of the
the MAC address and the label can identify the fragment globally. A fragment, so together the MAC address and the label can identify the
node may build the Datagram_tag in its own locally-significant way, fragment globally. A node may build the datagram_tag in its own
as long as the selected tag stays unique to the particular datagram locally-significant way, as long as the chosen datagram_tag stays
for the lifetime of that datagram. It results that the label does unique to the particular datagram for the lifetime of that datagram.
not need to be globally unique but also that it must be swapped at It results that the label does not need to be globally unique but
each hop as the source MAC address changes. also that it must be swapped at each hop as the source MAC address
changes.
This specification extends RFC 4944 [RFC4944] with 4 new Dispatch This specification extends RFC 4944 [RFC4944] with 2 new Dispatch
types, for Recoverable Fragment (RFRAG) headers with or without types, for Recoverable Fragment (RFRAG) and for the RFRAG
Acknowledgment Request (RFRAG vs. RFRAG-ARQ), and for the RFRAG Acknowledgment back.
Acknowledgment back, with or without ECN Echo (RFRAG-ACK vs. RFRAG-
ECHO).
(to be confirmed by IANA) The new 6LoWPAN Dispatch types use the (to be confirmed by IANA) The new 6LoWPAN Dispatch types use the
Value Bit Pattern of 11 1010xx from page 0 [RFC8025], as follows: Value Bit Pattern of 11 1010xx from Page 0 [RFC8025], as follows:
Pattern Header Type Pattern Header Type
+------------+------------------------------------------+ +------------+------------------------------------------+
| 11 10100x | RFRAG - Recoverable Fragment | | 11 10100x | RFRAG - Recoverable Fragment |
| 11 10101x | RFRAG-ACK - RFRAG Acknowledgment | | 11 10101x | RFRAG-ACK - RFRAG Acknowledgment |
+------------+------------------------------------------+ +------------+------------------------------------------+
Figure 1: Additional Dispatch Value Bit Patterns Figure 1: Additional Dispatch Value Bit Patterns
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]. [I-D.ietf-6lo-minimal-fragment].
5.1. Recoverable Fragment Dispatch type and Header 5.1. Recoverable Fragment Dispatch type and Header
In this specification, the size and offset of the fragments are In this specification, if the packet is compressed then the size and
expressed on the compressed packet form as opposed to the offset of the fragments are expressed on the Compressed Form of the
uncompressed - native - packet form. packet form as opposed to the uncompressed - native - packet form.
The format of the fragment header is the same for all fragments. The The format of the fragment header is shown in Figure 2. It is the
format indicates both a length and an offset, which seem be redundant same for all fragments. The format has a length and an offset, as
with the sequence field, but is not. The position of a fragment in well as a sequence field. This would be redundant if the offset was
the recomposition buffer is neither correlated with the value of the computed as the product of the sequence by the length, but this is
sequence field nor with the order in which the fragments are not the case. The position of a fragment in the reassembly buffer is
received. This enables out-of-sequence and overlapping fragments, neither correlated with the value of the sequence field nor with the
e.g., a fragment 5 that is retried as smaller fragments 5, 13 and 14 order in which the fragments are received. This enables out-of-
due to a change of MTU. sequence and overlapping fragments, e.g., a fragment 5 that is
retried as smaller fragments 5, 13 and 14 due to a change of MTU.
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, whereas fragment_size and the datagram_size of the compressed packet before
the other fragments carry their fragment_size and fragment_offset. it is fragmented, whereas the other fragments carry their
fragment_size and fragment_offset. The last fragment for a datagram
The last fragment for a datagram is recognized when its is recognized when its fragment_offset and its fragment_size add up
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.
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 2: RFRAG Dispatch type and Header Figure 2: RFRAG Dispatch type and Header
X: 1 bit; Ack-Request: when set, the sender requires an RFRAG
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. By default, that a unit that depends on the MAC layer technology. By default, that
unit is the octet which allows fragments up to 512 bytes. For unit is the octet which allows fragments up to 512 bytes. For
IEEE Std. 802.15.4, the unit is octet, and the maximum fragment IEEE Std. 802.15.4, the unit is octet, and the maximum fragment
size, when it is constrained by the maximum frame size of 128 size, when it is constrained by the maximum frame size of 128
octet minus the overheads of the MAC and Fragment Headers, is not octet minus the overheads of the MAC and Fragment Headers, is not
limited by this encoding. limited by this encoding.
X: 1 bit; Ack-Request: when set, the sender requires an RFRAG datagram_tag: 16 bits; an identifier of the datagram that is locally
Acknowledgment from the receiver. 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 recomposition buffer. the position in the reassembly buffer.
Fragment_offset: 16 bit unsigned integer; Fragment_offset: 16 bit unsigned integer;
* When the Fragment_offset is set to a non-0 value, its semantics * When the Fragment_offset is set to a non-0 value, its semantics
depend on the value of the Sequence field. depend on the value of the Sequence field.
+ For a first fragment (i.e. with a Sequence of 0), this field + For a first fragment (i.e. with a Sequence of 0), this field
indicates the datagram_size of the compressed datagram, to indicates the datagram_size of the compressed datagram, to
help the receiver allocate an adapted buffer for the help the receiver allocate an adapted buffer for the
reception and reassembly operations. The fragment may reception and reassembly operations. The fragment may be
stored for local recomposition, or it may be routed based on stored for local reassembly. Alternatively, it may be
the destination IPv6 address, in which case a VRB state must routed based on the destination IPv6 address. In that case,
be installed as described in Section 6.1.1. a VRB state must be installed as described in Section 6.1.1.
+ When the Sequence is not 0, this field indicates the offset + When the Sequence is not 0, this field indicates the offset
of the fragment in the compressed form. The fragment may be of the fragment in the Compressed Form of the datagram. The
added to a local recomposition buffer or forwarded based on fragment may be added to a local reassembly buffer or
an existing VRB as described in Section 6.1.2. forwarded based on an existing VRB as described in
Section 6.1.2.
* A Fragment_offset that is set to a value of 0 indicates an * A Fragment_offset that is set to a value of 0 indicates an
abort condition and all state regarding the datagram should be abort condition and all state regarding the datagram should be
cleaned up once the processing of the fragment is complete; the cleaned up once the processing of the fragment is complete; the
processing of the fragment depends on whether there is a VRB processing of the fragment depends on whether there is a VRB
already established for this datagram, and the next hop is already established for this datagram, and the next hop is
still reachable: still reachable:
+ if a VRB already exists and is not broken, the fragment is + if a VRB already exists and is not broken, the fragment is
to be forwarded along the associated Label Switched Path to be forwarded along the associated Label Switched Path
skipping to change at page 10, line 35 skipping to change at page 11, line 32
+ else, if the Sequence is 0, then the fragment is to be + else, if the Sequence is 0, then the fragment is to be
routed as described in Section 6.1.1 but no state is routed as described in Section 6.1.1 but no state is
conserved afterwards. In that case, the session if it conserved afterwards. In that case, the session if it
exists is aborted and the packet is also forwarded in an exists is aborted and the packet is also forwarded in an
attempt to clean up the next hops as along the path attempt to clean up the next hops as along the path
indicated by the IPv6 header (possibly including a routing indicated by the IPv6 header (possibly including a routing
header). 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. RFRAG-ACK is sent back to the source as described in
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-octet RFRAG Acknowledgment bitmap
that is used by the reassembling end point to confirm selectively the that is used by the reassembling end point 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. one to one with a given sequence number.
The offset of the bit in the bitmap indicates which fragment is The offset of the bit in the bitmap indicates which fragment is
acknowledged as follows: acknowledged as follows:
skipping to change at page 11, line 35 skipping to change at page 12, line 35
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Example RFRAG Acknowledgment Bitmap Figure 4: Example RFRAG Acknowledgment Bitmap
The RFRAG Acknowledgment Bitmap is included in a RFRAG Acknowledgment The RFRAG Acknowledgment Bitmap is included in a RFRAG Acknowledgment
header, as follows: header, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 0 1 0 1|E| Datagram_tag | |1 1 1 0 1 0 1|E| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RFRAG Acknowledgment Bitmap (32 bits) | | RFRAG Acknowledgment Bitmap (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: RFRAG Acknowledgment Dispatch type and Header Figure 5: RFRAG Acknowledgment Dispatch type and Header
E: 1 bit; Explicit Congestion Notification Echo E: 1 bit; Explicit Congestion Notification Echo
When set, the sender indicates that at least one of the When set, the sender indicates that at least one of the
acknowledged fragments was received with an Explicit Congestion acknowledged fragments was received with an Explicit Congestion
skipping to change at page 12, line 13 skipping to change at page 13, line 13
RFRAG Acknowledgment Bitmap RFRAG Acknowledgment Bitmap
An RFRAG Acknowledgment Bitmap, whereby setting the bit at offset An RFRAG Acknowledgment Bitmap, whereby setting the bit at offset
x indicates that fragment x was received, as shown in Figure 3. x indicates that fragment x was received, as shown in Figure 3.
All 0's is a NULL bitmap that indicates that the fragmentation All 0's is a NULL bitmap that indicates that the fragmentation
process is aborted. All 1's is a FULL bitmap that indicates that process is aborted. All 1's is a FULL bitmap that indicates that
the fragmentation process is complete, all fragments were received the fragmentation process is complete, all fragments were received
at the reassembly end point. at the reassembly end point.
6. Fragments Recovery 6. Fragments Recovery
The Recoverable Fragment headers RFRAG and RFRAG-ARQ are used to The Recoverable Fragment header RFRAG is used to transport a fragment
transport a fragment and optionally request an RFRAG Acknowledgment and optionally request an RFRAG Acknowledgment that will confirm the
that will confirm the good reception of a one or more fragments. An good reception of one or more fragments. An RFRAG Acknowledgment is
RFRAG Acknowledgment is carried as a standalone header in a message carried as a standalone fragment header (i.e. with no 6LoWPAN
that is sent back to the 6LoWPAN endpoint that was the source of the payload) in a message that is propagated back to the 6LoWPAN endpoint
fragments, as known by its MAC address. The process ensures that at that was the originator of the fragments. To achieve this, each hop
every hop, the source MAC address and the Datagram_tag in the that performed an MPLS-like operation on fragments reverses that
received fragment are enough information to send the RFRAG operation for the RFRAG_ACK by sending a frame from the next hop to
Acknowledgment back towards the source 6LoWPAN endpoint by reversing the previous hop as known by its MAC address in the VRB. The
the MPLS operation. 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
the datagram_tag used by the previous hop and the Layer-2 information
associated to it (interface and MAC address).
The 6LoWPAN endpoint that fragments the packets at 6LoWPAN level (the The 6LoWPAN endpoint that fragments the packets at 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 Ack- Ack-Request flag in the RFRAG packets. The sender may set the Ack-
Request flag on any fragment to perform congestion control by 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. Te maximum number positively confirmed by the reassembling endpoint. The maximum
of outstanding fragments is the Window-Size. It is configurable and number of outstanding fragments is the Window-Size. It is
may vary in case of ECN notification. When it receives a fragment configurable and may vary in case of ECN notification. When the
with the Ack-Request flag set, the 6LoWPAN endpoint that reassembles 6LoWPAN endpoint that reassembles the packets at 6LoWPAN level (the
the packets at 6LoWPAN level (the receiver) MUST send back an RFRAG receiver) receives a fragment with the Ack-Request flag set, it MUST
Acknowledgment to confirm reception of all the fragments it has send an RFRAG Acknowledgment back to the originator to confirm
received so far. reception of all the fragments it has received so far.
The Ack-Request bit marks the end of a window. It SHOULD be set on The Ack-Request ('X') set in an RFRAG marks the end of a window.
the last fragment to protect the datagram, and MAY be used in This flag SHOULD be set on the last fragment to protect the datagram,
intermediate fragments for the purpose of flow control. This ARQ and it MAY be set in any intermediate fragment for the purpose of
process MUST be protected by a ARQ timer, and the fragment that flow control. This ARQ process MUST be protected by a timer, and the
carries the Ack-Request flag MAY be retried upon time out a fragment that carries the 'X' flag MAY be retried upon time out a
configurable amount of times. Upon exhaustion of the retries the configurable amount of times (see Section 7.1). Upon exhaustion of
sender may either abort the transmission of the datagram or retry the the retries the sender may either abort the transmission of the
datagram from the first fragment with an Ack-Request in order to datagram or retry the datagram from the first fragment with an 'X'
reestablish a path and discover which fragments were received over flag set in order to reestablish a path and discover which fragments
the old path. When the sender of the fragment knows that an were received over the old path in the acknowledgment bitmap. When
underlying link-layer mechanism protects the fragments, it may the sender of the fragment knows that an underlying link-layer
refrain from using the RFRAG Acknowledgment mechanism, and never set mechanism protects the fragments, it may refrain from using the RFRAG
the Ack-Request bit. Acknowledgment mechanism, and never set the Ack-Request bit.
The RFRAG Acknowledgment can optionally carry an ECN indication for The RFRAG Acknowledgment can optionally carry an ECN indication for
flow control (see Appendix C). The receiver of a fragment with the flow control (see Appendix C). The receiver of a fragment with the
'E' (ECN) flag set MUST echo that information by setting the 'E' 'E' (ECN) flag set MUST echo that information by setting the 'E'
(ECN) flag in the next RFRAG Acknowledgment. (ECN) flag in the next RFRAG Acknowledgment.
The sender transfers a controlled number of fragments and MAY flag The sender transfers a controlled number of fragments and MAY flag
the last fragment of a series with an RFRAG Acknowledgment Request. the last fragment of a window with an RFRAG Acknowledgment Request.
The receiver MUST acknowledge a fragment with the acknowledgment The receiver MUST acknowledge a fragment with the acknowledgment
request bit set. If any fragment immediately preceding an request bit set. If any fragment immediately preceding an
acknowledgment request is still missing, the receiver MAY acknowledgment request is still missing, the receiver MAY
intentionally delay its acknowledgment to allow in-transit fragments intentionally delay its acknowledgment to allow in-transit fragments
to arrive. Delaying the acknowledgment might defeat the round trip to arrive. Because it might defeat the round trip delay computation,
delay computation so it should be configurable and not enabled by delaying the acknowledgment should be configurable and not enabled by
default. default.
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 that the reassembling endpoint fragments. Another use is to inform that the reassembling endpoint
has canceled the process of an individual datagram. Note that aborted the process of an individual datagram. Note that
acknowledgments might consume precious resources so the use of acknowledgments might consume precious resources so the use of
unsolicited acknowledgments should be configurable and not enabled by unsolicited acknowledgments should be configurable and not enabled by
default. 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
skipping to change at page 14, line 13 skipping to change at page 15, line 13
Time Slotted Channel Hopping (TSCH) [RFC6554] Time Slotted Channel Hopping (TSCH) [RFC6554]
6.1. Forwarding Fragments 6.1. Forwarding Fragments
It is assumed that the first Fragment is large enough to carry the It is assumed that the first Fragment is large enough to carry the
IPv6 header and make routing decisions. If that is not so, then this IPv6 header and make routing decisions. If that is not so, then this
specification MUST NOT be used. specification MUST NOT be used.
This specification extends the Virtual Reassembly Buffer (VRB) This specification extends the Virtual Reassembly Buffer (VRB)
technique to forward fragments with no intermediate reconstruction of technique to forward fragments with no intermediate reconstruction of
the entire packet. It inherits operations like Datagram_tag the entire packet. It inherits operations like datagram_tag
Switching and using a timer to clean the VRB when the traffic dries Switching and using a timer to clean the VRB when the traffic dries
up. In more details, the first fragment carries the IP header and it up. In more details, the first fragment carries the IP header and it
is routed all the way from the fragmenting end point to the is routed all the way from the fragmenting end point to the
reassembling end point. Upon the first fragment, the routers along reassembling end point. Upon the first fragment, the routers along
the path install a label-switched path (LSP), and the following the path install a label-switched path (LSP), and the following
fragments are label-switched along that path. As a consequence, fragments are label-switched along that path. As a consequence, the
alternate routes not possible for individual fragments. The next fragments can only follow the path that was set up by the first
Datagram_tag is used to carry the label, that is swapped at each hop. fragment and cannot follow an alternate route. The datagram_tag is
All fragments follow the same path and fragments are delivered in the used to carry the label, that is swapped at each hop. All fragments
order at which they are sent. follow the same path and fragments are delivered in the order at
which they are sent.
6.1.1. Upon the first fragment 6.1.1. Upon the first fragment
In Route-Over mode, the source and destination MAC addressed in a In Route-Over mode, the source and destination MAC addressed in a
frame change at each hop. The label that is formed and placed in the frame change at each hop. The label that is formed and placed in the
Datagram_tag is associated to the source MAC and only valid (and datagram_tag is associated to the source MAC and only valid (and
unique) for that source MAC. Upon a first fragment (i.e. with a unique) for that source MAC. Upon a first fragment (i.e. with a
sequence of zero), a VRB and the associated LSP state are created for sequence of zero), a VRB and the associated LSP state are created for
the tuple (source MAC address, Datagram_tag) and the fragment is the tuple (source MAC address, datagram_tag) and the fragment is
forwarded along the IPv6 route that matches the destination IPv6 forwarded along the IPv6 route that matches the destination IPv6
address in the IPv6 header as prescribed by address in the IPv6 header as prescribed by
[I-D.ietf-6lo-minimal-fragment]. The LSP state enables to match the [I-D.ietf-6lo-minimal-fragment]. The LSP state enables to match the
(previous MAC address, Datagram_tag) in an incoming fragment to the (previous MAC address, datagram_tag) in an incoming fragment to the
tuple (next MAC address, swapped Datagram_tag) used in the forwarded tuple (next MAC address, swapped datagram_tag) used in the forwarded
fragment and points at the VRB. In addition, the router also forms a fragment and points at the VRB. In addition, the router also forms a
Reverse LSP state indexed by the MAC address of the next hop and the Reverse LSP state indexed by the MAC address of the next hop and the
swapped Datagram_tag. This reverse LSP state also points at the VRB swapped datagram_tag. This reverse LSP state also points at the VRB
and enables to match the (next MAC address, swapped_Datagram_tag) and enables to match the (next MAC address, swapped_datagram_tag)
found in an RFRAG Acknowledgment to the tuple (previous MAC address, found in an RFRAG Acknowledgment to the tuple (previous MAC address,
Datagram_tag) used when forwarding a Fragment Acknowledgment (RFRAG- datagram_tag) used when forwarding a Fragment Acknowledgment (RFRAG-
ACK) back to the sender endpoint. ACK) back to the sender endpoint.
6.1.2. Upon the next fragments 6.1.2. Upon the next fragments
Upon a next fragment (i.e. with a non-zero sequence), the router Upon a next fragment (i.e. with a non-zero sequence), the router
looks up a LSP indexed by the tuple (MAC address, Datagram_tag) found looks up a LSP indexed by the tuple (MAC address, datagram_tag) found
in the fragment. If it is found, the router forwards the fragment in the fragment. If it is found, the router forwards the fragment
using the associated VRB as prescribed by using the associated VRB as prescribed by
[I-D.ietf-6lo-minimal-fragment]. [I-D.ietf-6lo-minimal-fragment].
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:
o The source and destination MAC addresses are swapped from those o The source and destination MAC addresses are swapped from those
found in the fragment found in the fragment
o The Datagram_tag set to the Datagram_tag found in the fragment o The datagram_tag set to the datagram_tag found in the fragment
o A NULL bitmap is used to signal the abort condition o 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.
6.2. Upon the RFRAG Acknowledgments 6.2. Upon the RFRAG Acknowledgments
Upon an RFRAG-ACK, the router looks up a Reverse LSP indexed by the Upon an RFRAG-ACK, the router looks up a Reverse LSP indexed by the
tuple (MAC address, Datagram_tag), which are respectively the source tuple (MAC address, datagram_tag), which are respectively the source
MAC address of the received frame and the received Datagram_tag. If MAC address of the received frame and the received datagram_tag. If
it is found, the router forwards the fragment using the associated it is found, the router forwards the fragment using the associated
VRB as prescribed by [I-D.ietf-6lo-minimal-fragment], but using the VRB as prescribed by [I-D.ietf-6lo-minimal-fragment], but using the
Reverse LSP so that the RFRAG-ACK flows back to the sender endpoint. Reverse LSP so that the RFRAG-ACK flows back to the sender 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 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). not used before (e.g., 13 and 14). Note that Path MTU Discovery is
out of scope for this document.
6.3. Cancelling 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, and has the fragment_offset, sequence and fragment_size all set to 0, and
no data. 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 canceled, 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.
Upon reception of the pseudo fragment, the receiver cleans up all Upon reception of the pseudo fragment, the receiver cleans up all
resources for the packet associated to the Datagram_tag. If an resources for the packet associated to the datagram_tag. If an
acknowledgment is requested, the receiver responds with a NULL acknowledgment is requested, the receiver responds with a NULL
bitmap. bitmap.
The other way around, the receiver might need to cancel the process The other way around, the receiver might need to abort the process of
of a fragmented packet for internal reasons, for instance if it is a fragmented packet for internal reasons, for instance if it is out
out of reassembly buffers, or considers that this packet is already of reassembly buffers, or considers that this packet is already fully
fully reassembled and passed to the upper layer. In that case, the reassembled and passed to the upper layer. In that case, the
receiver SHOULD indicate so to the sender with a NULL bitmap in a receiver SHOULD indicate so to the sender with a NULL bitmap in a
RFRAG Acknowledgment. Upon an acknowledgment with a NULL bitmap, the RFRAG Acknowledgment. Upon an acknowledgment with a NULL bitmap, the
sender endpoint MUST abort the transmission of the fragmented sender endpoint MUST abort the transmission of the fragmented
datagram. datagram.
7. Management Considerations 7. Management Considerations
7.1. Protocol Parameters 7.1. Protocol Parameters
There is no particular configuration on the receiver, as echoing ECN There is no particular configuration on the receiver, as echoing ECN
should always be on. The configuration only applies to the sender is always on. The configuration only applies to the sender, which is
that is in control of the transmission. The management system SHOULD in control of the transmission. The management system SHOULD be
be capable of providing the parameters below: capable of providing the parameters below:
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 MinFragmentSize 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.
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
skipping to change at page 17, line 14 skipping to change at page 18, line 18
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.
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.
UseECN: Indicates whether the sender should react to ECN. When the
sender reacts to ECN the sender SHOULD adapt the Window_Size
between MinWindowSize and MaxWindowSize and it MAY adapt the
Fragment_Size if that is supported.
InterFrameGap: Indicates a minimum amount of time between InterFrameGap: Indicates a minimum amount of time between
transmissions. All packets to a same destination, and in transmissions. All packets to a same destination, and in
particular fragments, may be subject to receive while particular fragments, may be subject to receive while
transmitting and hidden terminal collisions with the next or transmitting and hidden terminal collisions with the next or
the previous transmission as the fragments progress along a the previous transmission as the fragments progress along a
same path. The InterFrameGap protects the propagation of one same path. The InterFrameGap protects the propagation of one
transmission before the next one is triggered and creates a transmission before the next one is triggered and creates a
duty cycle that controls the ratio of air time and memory in duty cycle that controls the ratio of air time and memory in
intermediate nodes that a particular datagram will use. intermediate nodes that a particular datagram will use.
skipping to change at page 18, line 12 skipping to change at page 19, line 11
respectively, at the sender. The values should be bounded by the respectively, at the sender. The values should be bounded by the
expected number of hops and reduced beyond that when the number of expected number of hops and reduced beyond that when the number of
datagrams that can traverse an intermediate point may exceed its datagrams that can traverse an intermediate point may exceed its
capacity and cause a congestion loss. The InterFrameGap is another capacity and cause a congestion loss. The InterFrameGap is another
tool that can be used to increase the spacing between fragments of a tool that can be used to increase the spacing between fragments of a
same datagram and reduce the ratio of time when a particular same datagram and reduce the ratio of time when a particular
intermediate node holds a fragment of that datagram. intermediate node holds a fragment of that datagram.
8. Security Considerations 8. Security Considerations
The considerations in the Security section of [I-D.ietf-core-cocoa]
apply equally to this specification.
The process of recovering fragments does not appear to create any The process of recovering fragments does not appear to create any
opening for new threat compared to "Transmission of IPv6 Packets over opening for new threat compared to "Transmission of IPv6 Packets over
IEEE 802.15.4 Networks" [RFC4944]. IEEE 802.15.4 Networks" [RFC4944].
9. IANA Considerations 9. IANA Considerations
Need extensions for formats defined in "Transmission of IPv6 Packets Need extensions for formats defined in "Transmission of IPv6 Packets
over IEEE 802.15.4 Networks" [RFC4944]. over IEEE 802.15.4 Networks" [RFC4944].
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, Thomas Watteyne and Michael Richardson for in-depth reviews Toutain, Carles Gomez Montenegro, Thomas Watteyne and Michael
and comments. Also many thanks to Jonathan Hui, Jay Werb, Christos Richardson for in-depth reviews and comments. Also many thanks to
Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret Wasserman, Richard Jonathan Hui, Jay Werb, Christos Polyzois, Soumitri Kolavennu, Pat
Kelsey, Carsten Bormann and Harry Courtice for their various Kinney, Margaret Wasserman, Richard Kelsey, Carsten Bormann and Harry
contributions. Courtice for their various contributions.
11. References 11. References
11.1. Normative References 11.1. Normative References
[I-D.ietf-6lo-minimal-fragment] [I-D.ietf-6lo-minimal-fragment]
Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
Fragment Forwarding", draft-ietf-6lo-minimal-fragment-01 Fragment Forwarding", draft-ietf-6lo-minimal-fragment-01
(work in progress), March 2019. (work in progress), March 2019.
skipping to change at page 19, line 37 skipping to change at page 20, line 37
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>.
11.2. Informative References 11.2. Informative References
[I-D.ietf-6tisch-architecture] [I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work
in progress), March 2019. in progress), March 2019.
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-03 (work in progress), February 2018.
[IEEE.802.15.4] [IEEE.802.15.4]
IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE, "IEEE Standard for Low-Rate Wireless Networks",
IEEE Standard 802.15.4, DOI 10.1109/IEEE IEEE Standard 802.15.4, DOI 10.1109/IEEE
P802.15.4-REVd/D01, P802.15.4-REVd/D01,
<http://ieeexplore.ieee.org/document/7460875/>. <http://ieeexplore.ieee.org/document/7460875/>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000, RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>. <https://www.rfc-editor.org/info/rfc2914>.
skipping to change at page 20, line 30 skipping to change at page 21, line 35
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>. <https://www.rfc-editor.org/info/rfc5681>.
[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>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[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>.
[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,
skipping to change at page 22, line 13 skipping to change at page 23, line 23
assist in diagnosing problems on the node or network. assist in diagnosing problems on the node or network.
Large data packets: Rich data types might require more than one Large data packets: Rich data types might require more than one
fragment. fragment.
Uncontrolled firmware download or waveform upload can easily result Uncontrolled firmware download or waveform upload can easily result
in a massive increase of the traffic and saturate the network. in a massive increase of the traffic and saturate the network.
When a fragment is lost in transmission, the lack of recovery in the When a fragment is lost in transmission, the lack of recovery in the
original fragmentation system of RFC 4944 implies that all fragments original fragmentation system of RFC 4944 implies that all fragments
are resent, further contributing to the congestion that caused the would need to be resent, further contributing to the congestion that
initial loss, and potentially leading to congestion collapse. caused the initial loss, and potentially leading to congestion
collapse.
This saturation may lead to excessive radio interference, or random This saturation may lead to excessive radio interference, or random
early discard (leaky bucket) in relaying nodes. Additional queuing early discard (leaky bucket) in relaying nodes. Additional queuing
and memory congestion may result while waiting for a low power next and memory congestion may result while waiting for a low power next
hop to emerge from its sleeping state. hop to emerge from its sleeping state.
Considering that RFC 4944 defines an MTU is 1280 bytes and that in Considering that RFC 4944 defines an MTU is 1280 bytes and that in
most incarnations (but 802.15.4g) a IEEE Std. 802.15.4 frame can most incarnations (but 802.15.4g) a IEEE Std. 802.15.4 frame can
limit the MAC payload to as few as 74 bytes, a packet might be limit the MAC payload to as few as 74 bytes, a packet might be
fragmented into at least 18 fragments at the 6LoWPAN shim layer. fragmented into at least 18 fragments at the 6LoWPAN shim layer.
 End of changes. 69 change blocks. 
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