< draft-ietf-6lo-fragment-recovery-02.txt   draft-ietf-6lo-fragment-recovery-03.txt >
6lo P. Thubert, Ed. 6lo P. Thubert, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Updates: 4944 (if approved) January 23, 2019 Updates: 4944 (if approved) May 20, 2019
Intended status: Standards Track Intended status: Standards Track
Expires: July 27, 2019 Expires: November 21, 2019
6LoWPAN Selective Fragment Recovery 6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-02 draft-ietf-6lo-fragment-recovery-03
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
skipping to change at page 1, line 33 skipping to change at page 1, line 33
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-
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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 July 27, 2019. This Internet-Draft will expire on November 21, 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.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . . . . . . . 5
4. Updating draft-watteyne-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 . . . . . . . . . . . . . . . 6
4.2. Modifying the First Fragment . . . . . . . . . . . . . . 6 4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 6
4.3. Modifying the First Fragment . . . . . . . . . . . . . . 7
5. New Dispatch types and headers . . . . . . . . . . . . . . . 7 5. New Dispatch types and headers . . . . . . . . . . . . . . . 7
5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8 5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 9 5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 10
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 11 6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12
7. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 13 6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14
7.1. Upon the first fragment . . . . . . . . . . . . . . . . . 13 6.1.1. Upon the first fragment . . . . . . . . . . . . . . . 14
7.2. Upon the next fragments . . . . . . . . . . . . . . . . . 13 6.1.2. Upon the next fragments . . . . . . . . . . . . . . . 14
7.3. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 14 6.2. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 6.3. Cancelling a Fragmented Packet . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 7. Management Considerations . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.2. Observing the network . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 15 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 18 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 19 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
Appendix C. Considerations On Flow Control . . . . . . . . . . . 20 11.1. Normative References . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21 11.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 21
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 22
Appendix C. Considerations On Flow Control . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
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 74 bytes or more in all cases,
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
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"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
effects. "LLN Minimal Fragment Forwarding" effects. "LLN Minimal Fragment Forwarding"
[I-D.watteyne-6lo-minimal-fragment] proposes such a technique, in a [I-D.ietf-6lo-minimal-fragment] proposes such a technique, in a
fashion that is compatible with [RFC4944] without the need to define fashion that is compatible with [RFC4944] without the need to define
a new protocol. However, adding that capability alone to the local a new protocol.
implementation of the original 6LoWPAN fragmentation would not
address the bulk of the issues raised against it, and may create new
issues like remnant state in the network.
Another issue against [RFC4944] is that it does not define a However, adding that capability alone to the local implementation of
mechanism to first discover the loss of a fragment along a multi-hop the original 6LoWPAN fragmentation would not address the issues of
path (e.g. having exhausted the link-layer retries at some hop on the resources locked and wasted transmissions due to the loss of a
way), and then to recover that loss. With RFC 4944, the forwarding fragment. [RFC4944] does not define a mechanism to first discover a
of a whole datagram fails when one fragment is not delivered properly fragment loss, and then to recover that loss. With RFC 4944, the
to the destination 6LoWPAN endpoint. End-to-end transport or forwarding of a whole datagram fails when one fragment is not
application-level mechanisms may require a full retransmission of the delivered properly to the destination 6LoWPAN endpoint. Constrained
datagram, wasting resources in an already constrained network. memory resources are blocked on the receiver until the receiver times
out.
In that situation, the source 6LoWPAN endpoint will not be aware that That problem is exacerbated when forwarding fragments over multiple
a loss occurred and will continue sending all fragments for a hops since a loss at an intermediate hop will not be discovered by
datagram that is already doomed. The original support is missing either the source or the destination, and the source will keep on
signaling to abort a multi-fragment transmission at any time and from sending fragments, wasting even more resources in the network and
either end, and, if the capability to forward fragments is possibly contributing to the condition that caused the loss to no
implemented, clean up the related state in the network. It is also avail since the datagram cannot arrive in its entirety. RFC 4944 is
lacking flow control capabilities to avoid participating to a also missing signaling to abort a multi-fragment transmission at any
time and from either end, and, if the capability to forward fragments
is implemented, clean up the related state in the network. It is
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 trigger
the retransmission of the full datagram. 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
skipping to change at page 5, line 18 skipping to change at page 5, line 24
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.watteyne-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 MLPS. 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 The LLN nodes in charge of generating or expanding a 6LoWPAN
header from/to a full IPv6 packet. The 6LoWPAN endpoints are the header from/to a full IPv6 packet. The 6LoWPAN endpoints are the
skipping to change at page 5, line 44 skipping to change at page 5, line 50
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 introduces and new
dispatch types are defined in Section 5. dispatch types are defined in Section 5.
[RFC8138] allows to modifies the size of a packet en-route by [RFC8138] allows to modify the size of a packet en-route by removing
removing the consumed hops in a compressed Routing Header. It the consumed hops in a compressed Routing Header. It results that
results that the fragment_offset and datagram_size cannot be signaled the fragment_offset and datagram_size cannot be signaled in the
in the uncompressed form. This specification expresses those fields uncompressed form. This specification expresses those fields in the
in the compressed form and allows to modify them en-route (see compressed form and allows to modify them en-route (see Section 4.3.
Section 4.2.
Note that consistantly with in Section 2 of [RFC6282] for the Note that consistently with in 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-watteyne-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.watteyne-6lo-minimal-fragment] by providing additional [I-D.ietf-6lo-minimal-fragment] by providing additional operations to
operations to improve the management of the Virtual Reassembly Buffer improve the management of the Virtual Reassembly Buffer (VRB).
(VRB).
4.1. Slack in the First Fragment 4.1. Slack in the First Fragment
At the time of this writing, [I-D.watteyne-6lo-minimal-fragment] At the time of this writing, [I-D.ietf-6lo-minimal-fragment] allows
allows for refragmenting in intermediate nodes, meaning that some for refragmenting in intermediate nodes, meaning that some bytes from
bytes from a given fragment may be left in the VRB to be added to the a given fragment may be left in the VRB to be added to the next
next fragment. The reason for this to happen would be the need for fragment. The reason for this to happen would be the need for space
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 the fragment number. This means that
the fragments that contain a 6LoWPAN-compressed header MUST have the fragments that contain a 6LoWPAN-compressed header MUST have
enough slack to enable a less efficient compression in the next hops enough 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 that 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 source 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 the 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 hop can restore the source IID to the first fragment without
impacting the second fragment. impacting the second fragment.
4.2. Modifying the First Fragment 4.2. Gap between frames
This specification introduces a concept of Inter-Frame Gap, which is
a configurable interval of time between transmissions to a same next
hop. In the case of half duplex interfaces, this InterFrameGap
ensures that the next hop has progressed the previous frame and is
capable of receiving the next one.
In the case of a mesh operating at a single frequency with
omnidirectional antennas, a larger InterFrameGap is required protect
the frame against hidden terminal collisions with the previous frame
of a same flow that is still progressing alon a common path.
The Inter-Frame Gap is useful even for unfragmented datagrams, but it
becomes a necessity for fragments that are typically generated in a
fast sequence and are all sent over the exact same path.
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, and of the Routing Header may change en-route in a Route-
Over mesh LLN. If the size of the first fragment is modified, then Over mesh LLN. If the size of the first fragment is modified, then
the intermediate node MUST adapt the datagram_size to reflect that the intermediate node MUST adapt the datagram_size to reflect that
difference. 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 in
order to forward individual fragments across a 6LoWPAN route-over order to forward individual fragments across a 6LoWPAN route-over
mesh. The datagram_tag is used as a label; it is locally unique to mesh. The Datagram_tag is used as a label; it is locally unique to
the node that is the source MAC address of the fragment, so together the node that is the source MAC address of the fragment, so together
the MAC address and the label can identify the fragment globally. A the MAC address and the label can identify the fragment globally. A
node may build the datagram_tag in its own locally-significant way, node may build the Datagram_tag in its own locally-significant way,
as long as the selected tag stays unique to the particular datagram as long as the selected tag stays unique to the particular datagram
for the lifetime of that datagram. It results that the label does for the lifetime of that datagram. It results that the label does
not need to be globally unique but also that it must be swapped at not need to be globally unique but also that it must be swapped at
each hop as the source MAC address changes. 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 4 new Dispatch
types, for Recoverable Fragment (RFRAG) headers with or without types, for Recoverable Fragment (RFRAG) headers with or without
Acknowledgment Request (RFRAG vs. RFRAG-ARQ), and for the RFRAG Acknowledgment Request (RFRAG vs. RFRAG-ARQ), and for the RFRAG
Acknowledgment back, with or without ECN Echo (RFRAG-ACK vs. RFRAG- Acknowledgment back, with or without ECN Echo (RFRAG-ACK vs. RFRAG-
ECHO). 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 101000 | RFRAG - Recoverable Fragment | | 11 10100x | RFRAG - Recoverable Fragment |
| 11 101001 | RFRAG-ARQ - RFRAG with Ack Request | | 11 10101x | RFRAG-ACK - RFRAG Acknowledgment |
| 11 101010 | RFRAG-ACK - RFRAG Acknowledgment | +------------+------------------------------------------+
| 11 101011 | RFRAG-ECHO - RFRAG Ack with ECN Echo |
+------------+------------------------------------------+
Figure 1: Additional Dispatch Value Bit Patterns Figure 1: Additional Dispatch Value Bit Patterns
In the following sections, the semantics of "datagram_tag" are In the following sections, a "Datagram_tag" extends the semantics
unchanged from [RFC4944] Section 5.3. "Fragmentation Type and defined in [RFC4944] Section 5.3."Fragmentation Type and Header".
Header." and is compatible with the fragment forwarding operation The Datagram_tag is a locally unique identifier for the datagram from
described in [I-D.watteyne-6lo-minimal-fragment]. the perspective of the sender. This means that the datagram-tag
identifies a datagram uniquely in the network when associated with
the source of the datagram. As the datagram gets forwarded, the
source changes and the Datagram_tag must be swapped as detailed in
[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, the size and offset of the fragments are
expressed on the compressed packet form as opposed to the expressed on the compressed packet form as opposed to the
uncompressed - native - packet form. uncompressed - native - packet form.
The format of the fragment header is the same for all fragments. The
format indicates both a length and an offset, which seem be redundant
with the sequence field, but is not. The position of a fragment in
the recomposition buffer is neither correlated with the value of the
sequence field nor with the order in which the fragments are
received. This enables out-of-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
the received fragments, and the sender MUST ensure that when all
fragments are acknowledged, then the datagram is fully received.
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
remainder of the original fragment being retried with new sequence
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, whereas
the other fragments carry their fragment_size and fragment_offset. the other fragments carry their fragment_size and fragment_offset.
The last fragment for a datagram is recognized when its The last fragment for a datagram is recognized when its
fragment_offset and its fragment_size add up to the datagram_size. fragment_offset and its fragment_size add up 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 Requested X set == Ack-Request
Figure 2: RFRAG Dispatch type and Header Figure 2: RFRAG Dispatch type and Header
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. For IEEE Std. a unit that depends on the MAC layer technology. By default, that
802.15.4, the unit is octet, and the maximum fragment size, which unit is the octet which allows fragments up to 512 bytes. For
is constrained by the maximum frame size of 128 octet minus the IEEE Std. 802.15.4, the unit is octet, and the maximum fragment
overheads of the MAC and Fragment Headers, is not limited by this size, when it is constrained by the maximum frame size of 128
encoding. octet minus the overheads of the MAC and Fragment Headers, is not
limited by this encoding.
X: 1 bit; Ack Requested: 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.
Sequence: 5 bit unsigned integer; the sequence number of the Sequence: 5 bit unsigned integer; the sequence number of the
fragment. Fragments are sequence numbered [0..N] where N is in fragment in the acknowledgement bitmap. Fragments are numbered
[0..31]. A sequence of 0 indicates the first fragment in a [0..N] where N is in [0..31]. A Sequence of 0 indicates the first
datagram. For IEEE Std. 802.15.4, as long as the overheads enable fragment in a datagram, but non-zero values are not indicative of
a fragment size of 64 octets or more, this enables to fragment a the position in the recomposition buffer.
packet of 2047 octets.
Fragment_offset: 16 bit unsigned integer; Fragment_offset: 16 bit unsigned integer;
* When set to a non-0 value, the semantics of the Fragment_offset * When the Fragment_offset is set to a non-0 value, its semantics
depends on the value of the Sequence. depend on the value of the Sequence field.
+ When the Sequence is not 0, this field indicates the offset + For a first fragment (i.e. with a Sequence of 0), this field
of the fragment in the compressed form. The fragment should indicates the datagram_size of the compressed datagram, to
be forwarded based on an existing VRB as described in help the receiver allocate an adapted buffer for the
Section 7.2, or silently dropped if none is found. reception and reassembly operations. The fragment may
stored for local recomposition, or it may be routed based on
the destination IPv6 address, in which case a VRB state must
be installed as described in Section 6.1.1.
+ For a first fragment (i.e. with a sequence of 0), this field + When the Sequence is not 0, this field indicates the offset
is overloaded to indicate the total_size of the compressed of the fragment in the compressed form. The fragment may be
packet, to help the receiver allocate an adapted buffer for added to a local recomposition buffer or forwarded based on
the reception and reassembly operations. This format limits an existing VRB as described in Section 6.1.2.
the maximum MTU on a 6LoWPAN link to 2047 bytes, but 1280
bytes is the recommended value to avoid issues with IPV6
Path MTU Discovery [RFC8201]. The fragment should be routed
based on the destination IPv6 address, and an VRB state
should be installed as described in Section 7.1.
* When set to 0, this field indicates an abort condition and all * A Fragment_offset that is set to a value of 0 indicates an
state regarding the datagram should be cleaned up once the abort condition and all state regarding the datagram should be
processing of the fragment is complete; the processing of the cleaned up once the processing of the fragment is complete; the
fragment depends on whether there is a VRB already established processing of the fragment depends on whether there is a VRB
for this datagram, and the next hop is still reachable: already established for this datagram, and the next hop is
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
(LSP) as described in Section 7.2, but regardless of the (LSP) as described in Section 6.1.2, but regardless of the
value of the Sequence field; value of the Sequence field;
+ 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 7.1 but no state is conserved routed as described in Section 6.1.1 but no state is
afterwards. conserved afterwards. In that case, the session if it
exists is aborted and the packet is also forwarded in an
attempt to clean up the next hops as along the path
indicated by the IPv6 header (possibly 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. RFRAG-ACK is sent back to the source.
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:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^ ^ ^ ^
| | bitmap indicating whether: | | bitmap indicating whether:
| +--- Fragment with sequence 10 was received | +----- Fragment with sequence 9 was received
+----------------------- Fragment with sequence 00 was received +----------------------- Fragment with sequence 0 was received
Figure 3: RFRAG Acknowledgment bitmap encoding Figure 3: RFRAG Acknowledgment bitmap encoding
Figure 4 shows an example Acknowledgment bitmap which indicates that Figure 4 shows an example Acknowledgment bitmap which indicates that
all fragments from sequence 0 to 20 were received, except for all fragments from sequence 0 to 20 were received, except for
fragments 1, 2 and 16 that were either lost or are still in the fragments 1, 2 and 16 that were lost and must be retried.
network over a slower path.
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|0|0|1|1|1|1|1|1|1|1|1|1|1|1|1|0|1|1|1|1|0|0|0|0|0|0|0|0|0|0|0| |1|0|0|1|1|1|1|1|1|1|1|1|1|1|1|1|0|1|1|1|1|0|0|0|0|0|0|0|0|0|0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Expanding 3 octets encoding 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 Y| 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
Y: 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
Notification, indicating that the path followed by the fragments Notification, indicating that the path followed by the fragments
is subject to congestion. is subject to congestion. More in Appendix C.
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 headers RFRAG and RFRAG-ARQ are used to
transport a fragment and optionally request an RFRAG Acknowledgment transport a fragment and optionally request an RFRAG Acknowledgment
that will confirm the good reception of a one or more fragments. An that will confirm the good reception of a one or more fragments. An
RFRAG Acknowledgment can optionally carry an ECN indication; it is RFRAG Acknowledgment is carried as a standalone header in a message
carried as a standalone header in a message that is sent back to the that is sent back to the 6LoWPAN endpoint that was the source of the
6LoWPAN endpoint that was the source of the fragments, as known by fragments, as known by its MAC address. The process ensures that at
its MAC address. The process ensures that at every hop, the source every hop, the source MAC address and the Datagram_tag in the
MAC address and the datagram_tag in the received fragment are enough received fragment are enough information to send the RFRAG
information to send the RFRAG Acknowledgment back towards the source Acknowledgment back towards the source 6LoWPAN endpoint by reversing
6LoWPAN endpoint by reversing the MPLS operation. the MPLS operation.
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 when the reassembling end point sends the RFRAG sender) also controls the amount of acknowledgments by setting the
Acknowledgments by setting the Ack Requested flag in the RFRAG Ack-Request flag in the RFRAG packets. The sender may set the Ack-
packets. It may set the Ack Requested flag on any fragment to Request flag on any fragment to perform congestion control by
perform congestion control by limiting the number of outstanding limiting the number of outstanding fragments, which are the fragments
fragments, which are the fragments that have been sent but for which that have been sent but for which reception or loss was not
reception or loss was not positively confirmed by the reassembling positively confirmed by the reassembling endpoint. Te maximum number
endpoint. When the sender of the fragment knows that an underlying of outstanding fragments is the Window-Size. It is configurable and
link-layer mechanism protects the Fragments, it may refrain from may vary in case of ECN notification. When it receives a fragment
using the RFRAG Acknowledgment mechanism, and never set the Ack with the Ack-Request flag set, the 6LoWPAN endpoint that reassembles
Requested bit. When it receives a fragment with the ACK Request flag the packets at 6LoWPAN level (the receiver) MUST send back an RFRAG
set, the 6LoWPAN endpoint that reassembles the packets at 6LoWPAN Acknowledgment to confirm reception of all the fragments it has
level (the receiver) sends back an RFRAG Acknowledgment 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 last fragment to protect the datagram, and MAY be used in
intermediate fragments for the purpose of flow control. This ARQ
process MUST be protected by a ARQ timer, and the fragment that
carries the Ack-Request flag MAY be retried upon time out a
configurable amount of times. Upon exhaustion of the retries the
sender may either abort the transmission of the datagram or retry the
datagram from the first fragment with an Ack-Request in order to
reestablish a path and discover which fragments were received over
the old path. When the sender of the fragment knows that an
underlying link-layer mechanism protects the fragments, it may
refrain from using the RFRAG Acknowledgment mechanism, and never set
the Ack-Request bit.
The RFRAG Acknowledgment can optionally carry an ECN indication for
flow control (see Appendix C). The receiver of a fragment with the
'E' (ECN) flag set MUST echo that information by setting the 'E'
(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 series with an RFRAG Acknowledgment Request.
The received 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. Delaying the acknowledgment might defeat the round trip
delay computation so it should be configurable and not enabled by delay computation so it 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
skipping to change at page 12, line 29 skipping to change at page 13, line 42
protects the transmission over the LLN mesh with a retry timer that protects the transmission over the LLN mesh with a retry timer that
is computed according to the method detailed in [RFC6298]. It is is computed according to the method detailed in [RFC6298]. It is
expected that the upper layer retries obey the recommendations in expected that the upper layer retries obey the recommendations in
"UDP Usage Guidelines" [RFC8085], in which case a single round of "UDP Usage Guidelines" [RFC8085], in which case a single round of
fragment recovery should fit within the upper layer recovery timers. 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 actually 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
wait a reasonable amount of time between fragments so as to let a wait a reasonable amount of time between fragments so as to let a
fragment progress a few hops and avoid hidden terminal issues. This fragment progress a few hops and avoid hidden terminal issues. This
precaution is not required on channel hopping technologies such as precaution is not required on channel hopping technologies such as
Time Slotted CHannel Hopping (TSCH) [RFC6554] Time Slotted Channel Hopping (TSCH) [RFC6554]
When the sender decides that a packet should be dropped and the
fragmentation process canceled, it sends a pseudo fragment with the
fragment_offset, sequence and fragment_size all set to 0, and no
data. Upon reception of this message, the receiver should clean up
all resources for the packet associated to the datagram_tag. If an
acknowledgment is requested, the receiver responds with a NULL
bitmap.
The receiver might need to cancel the process of a fragmented packet
for internal reasons, for instance if it is out of reassembly
buffers, or considers that this packet is already fully reassembled
and passed to the upper layer. In that case, the receiver SHOULD
indicate so to the sender with a NULL bitmap in a RFRAG
Acknowledgment. Upon an acknowledgment with a NULL bitmap, the
sender endpoint MUST abort the transmission of the fragmented
datagram.
7. 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. The first fragment carries the IP header and it the entire packet. It inherits operations like Datagram_tag
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
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,
alternate routes not possible for individual fragments. The alternate routes not possible for individual fragments. The
datagram_tag is used to carry the label, that is swapped at each hop. Datagram_tag is used to carry the label, that is swapped at each hop.
All fragments follow the same path and fragments are delivered in the All fragments follow the same path and fragments are delivered in the
order at which they are sent. order at which they are sent.
7.1. 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.watteyne-6lo-minimal-fragment]. The LSP state enables to match [I-D.ietf-6lo-minimal-fragment]. The LSP state enables to match the
the (previous MAC address, datagram_tag) in an incoming fragment to (previous MAC address, Datagram_tag) in an incoming fragment to the
the tuple (next MAC address, swapped datagram_tag) used in the tuple (next MAC address, swapped Datagram_tag) used in the forwarded
forwarded fragment and points at the VRB. In addition, the router fragment and points at the VRB. In addition, the router also forms a
also forms a Reverse LSP state indexed by the MAC address of the next Reverse LSP state indexed by the MAC address of the next hop and the
hop and the swapped datagram_tag. This reverse LSP state also points swapped Datagram_tag. This reverse LSP state also points at the VRB
at the VRB and enables to match the (next MAC address, and enables to match the (next MAC address, swapped_Datagram_tag)
swapped_datagram_tag) found in an RFRAG Acknowledgment to the tuple found in an RFRAG Acknowledgment to the tuple (previous MAC address,
(previous MAC address, datagram_tag) used when forwarding a Fragment Datagram_tag) used when forwarding a Fragment Acknowledgment (RFRAG-
Acknowledgment (RFRAG-ACK) back to the sender endpoint. ACK) back to the sender endpoint.
7.2. 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.watteyne-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.
7.3. 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.watteyne-6lo-minimal-fragment], but using VRB as prescribed by [I-D.ietf-6lo-minimal-fragment], but using the
the Reverse LSP so that the RFRAG-ACK flows back to the sender Reverse LSP so that 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 either an error (NULL bitmap) Either way, if the RFRAG-ACK indicates that the fragment was entirely
or that the fragment was entirely received (FULL bitmap), arms a received (FULL bitmap), it arms a short timer, and upon timeout, the
short timer, and upon timeout, the VRB and all associate state are VRB and all the associated state are destroyed. until the timer
destroyed. During that time, fragments of that datagram may still be elapses, fragments of that datagram may still be received, e.g. if
received, e.g. if the RFRAG-ACK was lost on the way back and the the RFRAG-ACK was lost on the way back and the source retried the
source retried the last fragment. In that case, the router sends an last fragment. In that case, the router forwards the fragment
abort RFRAG-ACK along the Reverse LSP to complete the clean up. according to the state in the VRB.
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
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
again) and then one or more other fragments with a sequence that was
not used before (e.g., 13 and 14).
6.3. Cancelling a Fragmented Packet
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
no data.
When the sender or a router on the way decides that a packet should
be dropped and the fragmentation process canceled, it generates a
reset pseudo fragment and forwards it down the fragment path.
Each router next along the path the way forwards the pseudo fragment
based on the VRB state. If an acknowledgment is not requested, the
VRB and all associated state are destroyed.
Upon reception of the pseudo fragment, the receiver cleans up all
resources for the packet associated to the Datagram_tag. If an
acknowledgment is requested, the receiver responds with a NULL
bitmap.
The other way around, the receiver might need to cancel the process
of a fragmented packet for internal reasons, for instance if it is
out of reassembly buffers, or considers that this packet is already
fully reassembled and passed to the upper layer. In that case, the
receiver SHOULD indicate so to the sender with a NULL bitmap in a
RFRAG Acknowledgment. Upon an acknowledgment with a NULL bitmap, the
sender endpoint MUST abort the transmission of the fragmented
datagram.
7. Management Considerations
7.1. Protocol Parameters
There is no particular configuration on the receiver, as echoing ECN
should always be on. The configuration only applies to the sender
that is in control of the transmission. The management system SHOULD
be capable of providing the parameters below:
MinFragmentSize: The MinFragmentSize is the minimum value for the
Fragment_Size.
OptFragmentSize: The MinFragmentSize is the value for the
Fragment_Size that the sender should use to start with.
MaxFragmentSize: The MaxFragmentSize is the maximum value for the
Fragment_Size. It MUST be lower than the minimum MTU along the
path. A large value augments the chances of buffer bloat and
transmission loss. The value MUST be less than 512 if the unit
that is defined for the PHY layer is the octet.
UseECN: Indicates whether the sender should react to ECN. When the
sender reacts to ECN the Window_Size will vary between
MinWindowSize and MaxWindowSize.
MinWindowSize: The minimum value of Window_Size that the sender can
use.
OptWindowSize: The OptWindowSize is the value for the Window_Size
that the sender should use to start with.
MaxWindowSize: The maximum value of Window_Size that the sender can
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
transmissions. All packets to a same destination, and in
particular fragments, may be subject to receive while
transmitting and hidden terminal collisions with the next or
the previous transmission as the fragments progress along a
same path. The InterFrameGap protects the propagation of one
transmission before the next one is triggered and creates a
duty cycle that controls the ratio of air time and memory in
intermediate nodes that a particular datagram will use.
MinARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action.
OptARQTimeOut: The starting point of the value of the amount that a
sender should wait for an RFRAG Acknowledgment before it takes
a next action.
MaxARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action.
MaxFragRetries: The maximum number of retries for a particular
Fragment.
MaxDatagramRetries: The maximum number of retries from scratch for a
particular Datagram.
7.2. Observing the network
The management system should monitor the amount of retries and of ECN
settings that can be observed from the perspective of the both the
sender and the receiver, and may tune the optimum size of
Fragment_Size and of the Window_Size, OptWindowSize and OptWindowSize
respectively, at the sender. The values should be bounded by the
expected number of hops and reduced beyond that when the number of
datagrams that can traverse an intermediate point may exceed its
capacity and cause a congestion loss. The InterFrameGap is another
tool that can be used to increase the spacing between fragments of a
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
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 Thomas Watteyne and Michael Richardson for The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent
in-depth reviews and comments. Also many thanks to Jonathan Hui, Jay Toutain, Thomas Watteyne and Michael Richardson for in-depth reviews
Werb, Christos Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret and comments. Also many thanks to Jonathan Hui, Jay Werb, Christos
Wasserman, Richard Kelsey, Carsten Bormann and Harry Courtice for Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret Wasserman, Richard
their various contributions. Kelsey, Carsten Bormann and Harry Courtice for their various
contributions.
11. References 11. References
11.1. Normative References 11.1. Normative References
[I-D.watteyne-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-watteyne-6lo-minimal- Fragment Forwarding", draft-ietf-6lo-minimal-fragment-01
fragment-02 (work in progress), July 2018. (work in progress), March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>. <https://www.rfc-editor.org/info/rfc4944>.
skipping to change at page 16, line 18 skipping to change at page 19, line 34
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>.
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-19 (work of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work
in progress), December 2018. in progress), March 2019.
[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 29 skipping to change at page 23, line 40
saturation of the radio network and congestion collapse. saturation of the radio network and congestion collapse.
The recovery mechanism should provide means for controlling the The recovery mechanism should provide means for controlling the
number of fragments in transit over the LLN. number of fragments in transit over the LLN.
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 to control packet loss is assumed upon time out. The draft allows to control
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. It must be noted
that the number of outstanding fragments should not exceed the number 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 of hops in the network, but the way to figure the number of hops is
out of scope for this document. 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
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