draft-ietf-6lo-fragment-recovery-00.txt   draft-ietf-6lo-fragment-recovery-01.txt 
6lo P. Thubert, Ed. 6lo P. Thubert, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Updates: 4944 (if approved) September 20, 2018 Updates: 4944 (if approved) January 22, 2019
Intended status: Standards Track Intended status: Standards Track
Expires: March 29, 2019 Expires: July 26, 2019
6LoWPAN Selective Fragment Recovery 6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-00 draft-ietf-6lo-fragment-recovery-01
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-
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 December 29, 2018. This Internet-Draft will expire on July 26, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 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. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 3 2. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 3
3. Updating draft-watteyne-6lo-minimal-fragment . . . . . . . . 4 2.1. Updating draft-watteyne-6lo-minimal-fragment . . . . . . 4
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Slack in the First Fragment . . . . . . . . . . . . . . . 4
4.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Modifying the First Fragment . . . . . . . . . . . . . . 5
4.2. References . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. 6LoWPAN Acronyms . . . . . . . . . . . . . . . . . . . . 4 3.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Referenced Work . . . . . . . . . . . . . . . . . . . . . 5 3.2. References . . . . . . . . . . . . . . . . . . . . . . . 5
4.5. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3. 6LoWPAN Acronyms . . . . . . . . . . . . . . . . . . . . 5
5. New Dispatch types and headers . . . . . . . . . . . . . . . 6 3.4. Referenced Work . . . . . . . . . . . . . . . . . . . . . 6
5.1. Recoverable Fragment Dispatch type and Header . . . . . . 7 3.5. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 9 4. New Dispatch types and headers . . . . . . . . . . . . . . . 7
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 10 4.1. Recoverable Fragment Dispatch type and Header . . . . . . 8
7. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 12 4.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 9
7.1. Upon the first fragment . . . . . . . . . . . . . . . . . 12 5. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 11
7.2. Upon the next fragments . . . . . . . . . . . . . . . . . 13 6. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 13
7.3. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 13 6.1. Upon the first fragment . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 6.2. Upon the next fragments . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 6.3. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 14
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 14 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . 15 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . 15
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 18 10.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix C. Considerations On Flow Control . . . . . . . . . . . 19 Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 19
Appendix C. Considerations On Flow Control . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
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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in Appendix B. in Appendix B.
2. Updating RFC 4944 2. 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 4.
3. Updating draft-watteyne-6lo-minimal-fragment [RFC8138] allows to modifies the size of a packet en-route by
removing the consumed hops in a compressed Routing Header. It
results that the fragment_offset and datagram_size cannot be signaled
in the uncompressed form. This specification expresses those fields
in the compressed form and allows to modify them en-route (see
Section 2.3.
Note that consistantly with in Section 2 of [RFC6282] for the
fragmentation mechanism described in Section 5.3 of [RFC4944], any
header that cannot fit within the first fragment MUST NOT be
compressed when using the fragmentation mechanism described in this
specification.
2.1. Updating draft-watteyne-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 events to [I-D.watteyne-6lo-minimal-fragment] by providing additional
iprove the management of the Virtual Reassembly Buffer (VRB). operations to improve the management of the Virtual Reassembly Buffer
(VRB).
2.2. 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.watteyne-6lo-minimal-fragment]
allows for refragmenting in intermediate nodes, meaning that some allows for refragmenting in intermediate nodes, meaning that some
bytes from a given fragment may be left in the VRB to be added to the bytes from 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 next fragment. The reason for this to happen would be the need for
space in the outgoing fragment that was not needed in the incoming space 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 IID of the source as efficient on the outgoing link, e.g., if the Interface ID (IID) of
IPv6 address is elided on the first hop because it matches the MAC the source IPv6 address is elided by the originator on the first hop
address, but cannot be on the next hops. This specification does not because it matches the source MAC address, but cannot be on the next
allow this since fragments are recovered end-to-end. This means that hops because the source MAC address changes.
the fragments that contain 6LoWPAN-compressed data must have enough
slack in them to enable a lesser efficient compression in the next
hops to still fit in one frame.
4. Terminology This specification cannot allow this operation since fragments are
recovered end-to-end based on the fragment number. This means that
the fragments that contain a 6LoWPAN-compressed header MUST have
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
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
hop can restore the source IID to the first fragment without
impacting the second fragment.
4.1. BCP 14 2.3. Modifying the First Fragment
The compression of the Hop Limit, of the source and destination
addresses, and of the Routing Header may change en route in a Route-
Over mesh network. If the size of the first fragment is modified,
then the intermediate node MUST adapt te datagram_size to reflect
that difference.
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
to the fragment_offset of all the subsequent fragments for that
datagram.
3. Terminology
3.1. 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.
4.2. References 3.2. 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 the following documents:
o "Problem Statement and Requirements for IPv6 over Low-Power o "Problem Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606] Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]
4.3. 6LoWPAN Acronyms 3.3. 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
4.4. Referenced Work 3.4. Referenced Work
Past experience with fragmentation has shown that miss-associated or Past experience with fragmentation has shown that miss-associated 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.
skipping to change at page 6, line 5 skipping to change at page 6, line 46
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.watteyne-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.
4.5. New Terms 3.5. 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
points where fragmentation and reassembly take place. points where fragmentation and reassembly take place.
5. New Dispatch types and headers 4. 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
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| 11 101011 | RFRAG-ECHO - RFRAG Ack with ECN Echo | | 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, the semantics of "datagram_tag" are
unchanged from [RFC4944] Section 5.3. "Fragmentation Type and unchanged from [RFC4944] Section 5.3. "Fragmentation Type and
Header." and is compatible with the fragment forwarding operation Header." and is compatible with the fragment forwarding operation
described in [I-D.watteyne-6lo-minimal-fragment]. described in [I-D.watteyne-6lo-minimal-fragment].
5.1. Recoverable Fragment Dispatch type and Header 4.1. 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 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 X|E|fragment_size| datagram_tag | |1 1 1 0 1 0 0|E| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|sequence | fragment_offset | |X| sequence| fragment_size | fragment_offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X set == Ack Requested
Figure 2: RFRAG Dispatch type and Header X set == Ack Requested
X: 1 bit; Ack Requested: when set, the sender requires an RFRAG Figure 2: RFRAG Dispatch type and Header
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: 7 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. For IEEE Std.
802.15.4, the unit is octet, and the maximum fragment size, which 802.15.4, the unit is octet, and the maximum fragment size, which
is constrained by the maximum frame size of 128 octet minus the is constrained by the maximum frame size of 128 octet minus the
overheads of the MAC and Fragment Headers, is not limited by this overheads of the MAC and Fragment Headers, is not limited by this
encoding. encoding.
X: 1 bit; Ack Requested: when set, the sender requires an RFRAG
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. Fragments are sequence numbered [0..N] where N is in
[0..31]. A sequence of 0 indicates the first fragment in a [0..31]. A sequence of 0 indicates the first fragment in a
datagram. For IEEE Std. 802.15.4, as long as the overheads enable datagram. For IEEE Std. 802.15.4, as long as the overheads enable
a fragment size of 64 octets or more, this enables to fragment a a fragment size of 64 octets or more, this enables to fragment a
packet of 2047 octets. packet of 2047 octets.
Fragment_offset: 11 bit unsigned integer; Fragment_offset: 16 bit unsigned integer;
* When set to a non-0 value, the semantics of the Fragment_offset * When set to a non-0 value, the semantics of the Fragment_offset
depends on the value of the Sequence. depends on the value of the Sequence.
+ 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 should of the fragment in the compressed form. The fragment should
be forwarded based on an existing VRB as described in be forwarded based on an existing VRB as described in
Section 7.2, or silently dropped if none is found. Section 6.2, or silently dropped if none is found.
+ 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
is overloaded to indicate the total_size of the compressed is overloaded to indicate the total_size of the compressed
packet, to help the receiver allocate an adapted buffer for packet, to help the receiver allocate an adapted buffer for
the reception and reassembly operations. This format limits the reception and reassembly operations. This format limits
the maximum MTU on a 6LoWPAN link to 2047 bytes, but 1280 the maximum MTU on a 6LoWPAN link to 2047 bytes, but 1280
bytes is the recommended value to avoid issues with IPV6 bytes is the recommended value to avoid issues with IPV6
Path MTU Discovery [RFC8201]. The fragment should be routed Path MTU Discovery [RFC8201]. The fragment should be routed
based on the destination IPv6 address, and an VRB state based on the destination IPv6 address, and an VRB state
should be installed as described in Section 7.1. should be installed as described in Section 6.1.
* When set to 0, this field indicates an abort condition and all * When set to 0, this field indicates an abort condition and all
state regarding the datagram should be cleaned up once the state regarding the datagram should be cleaned up once the
processing of the fragment is complete; the processing of the processing of the fragment is complete; the processing of the
fragment depends on whether there is a VRB already established fragment depends on whether there is a VRB already established
for this datagram, and the next hop is still reachable: 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 6.2, 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 but no state is conserved
afterwards. afterwards.
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 4.2. 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
skipping to change at page 10, line 5 skipping to change at page 10, line 36
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: Expanding 3 octets encoding
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 Y| 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 Y: 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.
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 5. 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 can optionally carry an ECN indication; it is
carried as a standalone header in a message that is sent back to the carried as a standalone header in a message that is sent back to the
6LoWPAN endpoint that was the source of the fragments, as known by 6LoWPAN endpoint that was the source of the fragments, as known by
its MAC address. The process ensures that at every hop, the source its MAC address. The process ensures that at every hop, the source
MAC address and the datagram_tag in the received fragment are enough MAC address and the datagram_tag in the received fragment are enough
information to send the RFRAG Acknowledgment back towards the source information to send the RFRAG Acknowledgment back towards the source
skipping to change at page 12, line 19 skipping to change at page 13, line 7
The receiver might need to cancel the process of a fragmented packet The receiver might need to cancel the process of a fragmented packet
for internal reasons, for instance if it is out of reassembly for internal reasons, for instance if it is out of reassembly
buffers, or considers that this packet is already fully reassembled buffers, or considers that this packet is already fully reassembled
and passed to the upper layer. In that case, the receiver SHOULD and passed to the upper layer. In that case, the receiver SHOULD
indicate so to the sender with a NULL bitmap in a RFRAG indicate so to the sender with a NULL bitmap in a RFRAG
Acknowledgment. Upon an acknowledgment with a NULL bitmap, the 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. Forwarding Fragments 6. 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. 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. 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.watteyne-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 the tuple (next MAC address, swapped datagram_tag) used in the
forwarded fragment and points at the VRB. In addition, the router forwarded fragment and points at the VRB. In addition, the router
also forms a Reverse LSP state indexed by the MAC address of the next also forms a Reverse LSP state indexed by the MAC address of the next
hop and the swapped datagram_tag. This reverse LSP state also points hop and the swapped datagram_tag. This reverse LSP state also points
at the VRB and enables to match the (next MAC address, at the VRB and enables to match the (next MAC address,
swapped_datagram_tag) found in an RFRAG Acknowledgment to the tuple swapped_datagram_tag) found in an RFRAG Acknowledgment to the tuple
(previous MAC address, datagram_tag) used when forwarding a Fragment (previous MAC address, datagram_tag) used when forwarding a Fragment
Acknowledgment (RFRAG-ACK) back to the sender endpoint. Acknowledgment (RFRAG-ACK) back to the sender endpoint.
7.2. Upon the next fragments 6.2. 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.watteyne-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:
skipping to change at page 13, line 34 skipping to change at page 14, line 23
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 6.3. 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.watteyne-6lo-minimal-fragment], but using
the Reverse LSP so that the RFRAG-ACK flows back to the sender the 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 either an error (NULL bitmap)
or that the fragment was entirely received (FULL bitmap), arms a or that the fragment was entirely received (FULL bitmap), arms a
short timer, and upon timeout, the VRB and all associate state are short timer, and upon timeout, the VRB and all associate state are
destroyed. During that time, fragments of that datagram may still be destroyed. During that time, fragments of that datagram may still be
received, e.g. if the RFRAG-ACK was lost on the way back and the received, e.g. if the RFRAG-ACK was lost on the way back and the
source retried the last fragment. In that case, the router sends an source retried the last fragment. In that case, the router sends an
abort RFRAG-ACK along the Reverse LSP to complete the clean up. abort RFRAG-ACK along the Reverse LSP to complete the clean up.
8. Security Considerations 7. 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 8. 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 9. Acknowledgments
The author wishes to thank Thomas Watteyne and Michael Richardson for The author wishes to thank Thomas Watteyne and Michael Richardson for
in-depth reviews and comments. Also many thanks to Jonathan Hui, Jay in-depth reviews and comments. Also many thanks to Jonathan Hui, Jay
Werb, Christos Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret Werb, Christos Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret
Wasserman, Richard Kelsey, Carsten Bormann and Harry Courtice for Wasserman, Richard Kelsey, Carsten Bormann and Harry Courtice for
their various contributions. their various contributions.
11. References 10. References
11.1. Normative References 10.1. Normative References
[I-D.watteyne-6lo-minimal-fragment] [I-D.watteyne-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-watteyne-6lo-minimal-
fragment-01 (work in progress), March 2018. fragment-02 (work in progress), July 2018.
[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 15, line 16 skipping to change at page 16, line 5
Routing Header for Source Routes with the Routing Protocol Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012, DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>. <https://www.rfc-editor.org/info/rfc6554>.
[RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power [RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch", Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
RFC 8025, DOI 10.17487/RFC8025, November 2016, RFC 8025, DOI 10.17487/RFC8025, November 2016,
<https://www.rfc-editor.org/info/rfc8025>. <https://www.rfc-editor.org/info/rfc8025>.
[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/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 10.2. 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-14 (work of IEEE 802.15.4", draft-ietf-6tisch-architecture-19 (work
in progress), April 2018. in progress), December 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 19, line 47 skipping to change at page 20, line 44
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
Congestion Notification (ECN) mechanism. Though whether and how ECN Congestion Notification (ECN) mechanism. Though whether and how ECN
[RFC3168] is carried out over the LoWPAN is out of scope, this draft [RFC3168] is carried out over the LoWPAN is out of scope, this draft
provides a way for the destination endpoint to echo an ECN indication provides a way for the destination endpoint to echo an ECN indication
back to the source endpoint in an acknowledgment message as back to the source endpoint in an acknowledgment message as
represented in Figure 5 in Section 5.2. represented in Figure 5 in Section 4.2.
It must be noted that congestion and collision are different topics. It must be noted that congestion and collision are different topics.
In particular, when a mesh operates on a same channel over multiple In particular, when a mesh operates on a same channel over multiple
hops, then the forwarding of a fragment over a certain hop may hops, then the forwarding of a fragment over a certain hop may
collide with the forwarding of a next fragment that is following over collide with the forwarding of a next fragment that is following over
a previous hop but in a same interference domain. This draft enables a previous hop but in a same interference domain. This draft enables
an end-to-end flow control, but leaves it to the sender stack to pace an end-to-end flow control, but leaves it to the sender stack to pace
individual fragments within a transmit window, so that a given individual fragments within a transmit window, so that a given
fragment is sent only when the previous fragment has had a chance to fragment is sent only when the previous fragment has had a chance to
progress beyond the interference domain of this hop. In the case of progress beyond the interference domain of this hop. In the case of
skipping to change at page 20, line 40 skipping to change at page 21, line 36
recommended for that computation. recommended for that computation.
The reader is encouraged to read through "Congestion Control The reader is encouraged to read through "Congestion Control
Principles" [RFC2914]. Additionally [RFC7567] and [RFC5681] provide Principles" [RFC2914]. Additionally [RFC7567] and [RFC5681] provide
deeper information on why this mechanism is needed and how TCP deeper information on why this mechanism is needed and how TCP
handles Congestion Control. Basically, the goal here is to manage handles Congestion Control. Basically, the goal here is to manage
the amount of fragments present in the network; this is achieved by the amount of fragments present in the network; this is achieved by
to reducing the number of outstanding fragments over a congested path to reducing the number of outstanding fragments over a congested path
by throttling the sources. by throttling the sources.
Section 6 describes how the sender decides how many fragments are Section 5 describes how the sender decides how many fragments are
(re)sent before an acknowledgment is required, and how the sender (re)sent before an acknowledgment is required, and how the sender
adapts that number to the network conditions. adapts that number to the network conditions.
Author's Address Author's Address
Pascal Thubert (editor) Pascal Thubert (editor)
Cisco Systems, Inc Cisco Systems, Inc
Building D Building D
45 Allee des Ormes - BP1200 45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254 MOUGINS - Sophia Antipolis 06254
FRANCE FRANCE
Phone: +33 497 23 26 34 Phone: +33 497 23 26 34
Email: pthubert@cisco.com Email: pthubert@cisco.com
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