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Versions: (draft-thubert-6lo-fragment-recovery)
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6lo P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 4944 (if approved) 11 February 2020
Intended status: Standards Track
Expires: 14 August 2020
6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-12
Abstract
This draft updates RFC 4944 with a simple protocol to recover
individual fragments across a route-over mesh network, with a minimal
flow control to protect the network against bloat.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 14 August 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. References . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . . 6
4. Extending draft-ietf-6lo-minimal-fragment . . . . . . . . . . 6
4.1. Slack in the First Fragment . . . . . . . . . . . . . . . 7
4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 7
4.3. Modifying the First Fragment . . . . . . . . . . . . . . 7
5. New Dispatch types and headers . . . . . . . . . . . . . . . 8
5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12
6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14
6.1.1. Receiving the first fragment . . . . . . . . . . . . 15
6.1.2. Receiving the next fragments . . . . . . . . . . . . 15
6.2. Receiving RFRAG Acknowledgments . . . . . . . . . . . . . 16
6.3. Aborting the Transmission of a Fragmented Packet . . . . 16
6.4. Applying Recoverable Fragmentation along a Diverse
Path . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Management Considerations . . . . . . . . . . . . . . . . . . 18
7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 18
7.2. Observing the network . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
11. Normative References . . . . . . . . . . . . . . . . . . . . 22
12. Informative References . . . . . . . . . . . . . . . . . . . 23
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 26
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 28
Appendix C. Considerations on Flow Control . . . . . . . . . . . 28
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
In most Low Power and Lossy Network (LLN) applications, the bulk of
the traffic consists of small chunks of data (on the order of a few
bytes to a few tens of bytes) at a time. Given that an IEEE Std.
802.15.4 [IEEE.802.15.4] frame can carry a payload of 74 bytes or
more, fragmentation is usually not required. However, and though
this happens only occasionally, a number of mission critical
applications do require the capability to transfer larger chunks of
data, for instance to support the firmware upgrade of the LLN nodes
or the extraction of logs from LLN nodes. In the former case, the
large chunk of data is transferred to the LLN node, whereas in the
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latter, the large chunk flows away from the LLN node. In both cases,
the size can be on the order of 10 kilobytes or more and an end-to-
end reliable transport is required.
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944]
defines the original 6LoWPAN datagram fragmentation mechanism for
LLNs. One critical issue with this original design is that routing
an IPv6 [RFC8200] packet across a route-over mesh requires
reassembling the full packet at each hop, which may cause latency
along a path and an overall buffer bloat in the network. The "6TiSCH
Architecture" [I-D.ietf-6tisch-architecture] recommends using a
fragment forwarding (FF) technique to alleviate those undesirable
effects. "LLN Minimal Fragment Forwarding"
[I-D.ietf-6lo-minimal-fragment] specifies the general behavior that
all FF techniques including this specification follow, and presents
the associated caveats. In particular, the routing information is
fully indicated in the first fragment, which is always forwarded
first. A state is formed and used to forward all the next fragments
along the same path. The datagram_tag is locally significant to the
Layer-2 source of the packet and is swapped at each hop.
"Virtual reassembly buffers in 6LoWPAN"
[I-D.ietf-lwig-6lowpan-virtual-reassembly] (VRB) proposes a FF
technique that is compatible with [RFC4944] without the need to
define a new protocol. However, adding that capability alone to the
local implementation of the original 6LoWPAN fragmentation would not
address the inherent fragility of fragmentation (see
[I-D.ietf-intarea-frag-fragile]) in particular the issues of
resources locked on the receiver and the wasted transmissions due to
the loss of a single fragment in a whole datagram. [Kent] compares
the unreliable delivery of fragments with a mechanism it calls
"selective acknowledgements" that recovers the loss of a fragment
individually. The paper illustrates the benefits that can be derived
from such a method in figures 1, 2 and 3, on pages 6 and 7.
[RFC4944] has no selective recovery and the whole datagram fails when
one fragment is not delivered to the destination 6LoWPAN endpoint.
Constrained memory resources are blocked on the receiver until the
receiver times out, possibly causing the loss of subsequent packets
that cannot be received for the lack of buffers.
That problem is exacerbated when forwarding fragments over multiple
hops since a loss at an intermediate hop will not be discovered by
either the source or the destination, and the source will keep on
sending fragments, wasting even more resources in the network and
possibly contributing to the condition that caused the loss to no
avail since the datagram cannot arrive in its entirety. RFC 4944 is
also missing signaling to abort a multi-fragment transmission at any
time and from either end, and, if the capability to forward fragments
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is implemented, clean up the related state in the network. It is
also lacking flow control capabilities to avoid participating in
congestion that may in turn cause the loss of a fragment and
potentially the retransmission of the full datagram.
This specification provides a method to forward fragments across a
multi-hop route-over mesh, and a selective acknowledgment to recover
individual fragments between 6LoWPAN endpoints. The method is
designed to limit congestion loss in the network and addresses the
requirements that are detailed in Appendix B.
2. Terminology
2.1. BCP 14
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. References
In this document, readers will encounter terms and concepts that are
discussed in "Problem Statement and Requirements for IPv6 over
Low-Power Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]
"LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment]
introduces the generic concept of a Virtual Reassembly Buffer (VRB)
and specifies behaviours and caveats that are common to a large
family of FF techniques including this, which fully inherits from
that specification.
Past experience with fragmentation has shown that misassociated or
lost fragments can lead to poor network behavior and, occasionally,
trouble at the application layer. The reader is encouraged to read
"IPv4 Reassembly Errors at High Data Rates" [RFC4963] and follow the
references for more information.
That experience led to the definition of "Path MTU discovery"
[RFC8201] (PMTUD) protocol that limits fragmentation over the
Internet.
Specifically in the case of UDP, valuable additional information can
be found in "UDP Usage Guidelines for Application Designers"
[RFC8085].
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Readers are expected to be familiar with all the terms and concepts
that are discussed in "IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944].
"The Benefits of Using Explicit Congestion Notification (ECN)"
[RFC8087] provides useful information on the potential benefits and
pitfalls of using ECN.
Quoting the "Multiprotocol Label Switching (MPLS) Architecture"
[RFC3031]: with MPLS, 'packets are "labeled" before they are
forwarded' along a Label Switched Path (LSP). At subsequent hops,
there is no further analysis of the packet's network layer header.
Rather, the label is used as an index into a table which specifies
the next hop, and a new label". The MPLS technique is leveraged in
the present specification to forward fragments that actually do not
have a network layer header, since the fragmentation occurs below IP.
2.3. New Terms
This specification uses the following terms:
6LoWPAN endpoints: The LLN nodes in charge of generating or
expanding a 6LoWPAN header from/to a full IPv6 packet. The
6LoWPAN endpoints are the points where fragmentation and
reassembly take place.
Compressed Form: This specification uses the generic term Compressed
Form to refer to the format of a datagram after the action of
[RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts.
datagram_size: The size of the datagram in its Compressed Form
before it is fragmented. The datagram_size is expressed in a unit
that depends on the MAC layer technology, by default a byte.
datagram_tag: An identifier of a datagram that is locally unique to
the Layer-2 sender. Associated with the MAC address of the
sender, this becomes a globally unique identifier for the
datagram.
fragment_offset: The offset of a particular fragment of a datagram
in its Compressed Form. The fragment_offset is expressed in a
unit that depends on the MAC layer technology and is by default a
byte.
RFRAG: Recoverable Fragment
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RFRAG-ACK: Recoverable Fragment Acknowledgement
RFRAG Acknowledgment Request: An RFRAG with the Acknowledgement
Request flag ('X' flag) set.
NULL bitmap: Refers to a bitmap with all bits set to zero.
FULL bitmap: Refers to a bitmap with all bits set to one.
Forward: The direction of a LSP path, followed by the RFRAG.
Reverse: The reverse direction of a LSP path, taken by the RFRAG-
ACK.
3. Updating RFC 4944
This specification updates the fragmentation mechanism that is
specified in "Transmission of IPv6 Packets over IEEE 802.15.4
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 that are lost on the way can be recovered
individually. A new format for fragments is introduced and new
dispatch types are defined in Section 5.
[RFC8138] allows modifying the size of a packet en route by removing
the consumed hops in a compressed Routing Header. This requires that
fragment_offset and datagram_size (see Section 2.3) are also modified
en route, which is difficult to do in the uncompressed form. This
specification expresses those fields in the Compressed Form and
allows modifying them en route (see Section 4.3) easily.
Note that consistent with 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.
4. Extending draft-ietf-6lo-minimal-fragment
This specification implements the generic FF technique specified in
"LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment] in
a fashion that enables end-to-end recovery of fragments and some
degree of flow control.
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4.1. Slack in the First Fragment
[I-D.ietf-6lo-minimal-fragment] 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 next fragment. The reason for
this happening would be the need for space in the outgoing fragment
that was not needed in the incoming fragment, for instance because
the 6LoWPAN Header Compression is not 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 because it matches the
source MAC address, but cannot be on the next hops because the source
MAC address changes.
This specification cannot allow this operation since fragments are
recovered end-to-end based on a sequence 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.
4.2. Gap between frames
This specification introduces a concept of an 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 inter-frame
gap ensures that the next hop has completed processing of 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 inter-frame gap is required to
protect the frame against hidden terminal collisions with the
previous frame of a same flow that is still progressing along a
common path.
The inter-frame gap is useful even for unfragmented datagrams, but it
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
addresses in the IPv6 Header, and of the Routing Header may change en
route in a Route-Over mesh LLN. If the size of the first fragment is
modified, then the intermediate node MUST adapt the datagram_size to
reflect that difference.
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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.
5. New Dispatch types and headers
This specification enables the 6LoWPAN fragmentation sublayer to
provide an MTU up to 2048 bytes to the upper layer, which can be the
6LoWPAN Header Compression sublayer that is defined in the
"Compression Format for IPv6 Datagrams" [RFC6282] specification. In
order to achieve this, this specification enables the fragmentation
and the reliable transmission of fragments over a multihop 6LoWPAN
mesh network.
This specification provides a technique that is derived from MPLS to
forward individual fragments across a 6LoWPAN route-over mesh without
reassembly at each hop. The datagram_tag is used as a label; it is
locally unique to the node that owns the source MAC address of the
fragment, so together the MAC address and the label can identify the
fragment globally. A node may build the datagram_tag in its own
locally-significant way, as long as the chosen datagram_tag stays
unique to the particular datagram 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 each hop as the source MAC address
changes.
This specification extends RFC 4944 [RFC4944] with 2 new Dispatch
types, for Recoverable Fragment (RFRAG) and for the RFRAG
Acknowledgment back. The new 6LoWPAN Dispatch types are taken from
Page 0 [RFC8025] as indicated in Table 1 in Section 9.
In the following sections, a "datagram_tag" extends the semantics
defined in [RFC4944] Section 5.3."Fragmentation Type and Header".
The datagram_tag is a locally unique identifier for the datagram from
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
In this specification, if the packet is compressed then the size and
offset of the fragments are expressed with respect to the Compressed
Form of the packet form as opposed to the uncompressed (native)
packet form.
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The format of the fragment header is shown in Figure 1. It is the
same for all fragments. The format has a length and an offset, as
well as a sequence field. This would be redundant if the offset was
computed as the product of the sequence by the length, but this is
not the case. The position of a fragment in the reassembly buffer is
neither correlated with the value of the sequence field nor with the
order in which the fragments are received. This enables out-of-
sequence subfragmenting, e.g., a fragment seq. 5 that is retried end-
to-end as smaller fragments seq. 5, 13 and 14 due to a change of MTU
along the path between the 6LoWPAN endpoints.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 0 1 0 0|E| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| sequence| Fragment_Size | fragment_offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X set == Ack-Request
Figure 1: RFRAG Dispatch type and Header
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
Fragment_Size and the datagram_size of the compressed packet before
it is fragmented, whereas the other fragments carry their
Fragment_Size and fragment_offset. The last fragment for a datagram
is recognized when its fragment_offset and its Fragment_Size add up
to the datagram_size.
Recoverable Fragments are sequenced and a bitmap is used in the RFRAG
Acknowledgment to indicate the received fragments by setting the
individual bits that correspond to their sequence.
X: 1 bit; Ack-Request: when set, the sender requires an RFRAG
Acknowledgment from the receiver.
E: 1 bit; Explicit Congestion Notification; the "E" flag is reset by
the source of the fragment and set by intermediate routers to
signal that this fragment experienced congestion along its path.
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Fragment_Size: 10-bit unsigned integer; the size of this fragment in
a unit that depends on the MAC layer technology. Unless
overridden by a more specific specification, that unit is the
octet, which allows fragments up to 512 bytes.
datagram_tag: 8 bits; an identifier of the datagram that is locally
unique to the sender.
Sequence: 5-bit unsigned integer; the sequence number of the
fragment in the acknowledgement bitmap. Fragments are numbered
[0..N] where N is in [0..31]. A Sequence of 0 indicates the first
fragment in a datagram, but non-zero values are not indicative of
the position in the reassembly buffer.
Fragment_offset: 16-bit unsigned integer.
When the Fragment_offset is set to a non-0 value, its semantics
depend on the value of the Sequence field as follows:
* For a first fragment (i.e., with a Sequence of 0), this field
indicates the datagram_size of the compressed datagram, to help
the receiver allocate an adapted buffer for the reception and
reassembly operations. The fragment may be stored for local
reassembly. Alternatively, it may be routed based on the
destination IPv6 address. In that case, a VRB state must be
installed as described in Section 6.1.1.
* When the Sequence is not 0, this field indicates the offset of
the fragment in the Compressed Form of the datagram. The
fragment may be added to a local reassembly buffer or forwarded
based on an existing VRB as described in Section 6.1.2.
A Fragment_offset that is set to a value of 0 indicates an abort
condition and all state regarding the datagram should be cleaned
up once the processing of the fragment is complete; the processing
of the fragment depends on whether there is a VRB already
established for this datagram, and the next hop is still
reachable:
* if a VRB already exists and is not broken, the fragment is to
be forwarded along the associated Label Switched Path (LSP) as
described in Section 6.1.2, but regardless of the value of the
Sequence field;
* else, if the Sequence is 0, then the fragment is to be routed
as described in Section 6.1.1, but no state is 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 along the path indicated by the IPv6 header (possibly
including a routing header).
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If the fragment cannot be forwarded or routed, then an abort
RFRAG-ACK is sent back to the source as described in
Section 6.1.2.
5.2. RFRAG Acknowledgment Dispatch type and Header
This specification also defines a 4-octet RFRAG Acknowledgment bitmap
that is used by the reassembling endpoint to confirm selectively the
reception of individual fragments. A given offset in the bitmap maps
one-to-one with a given sequence number and indicates which fragment
is acknowledged as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RFRAG Acknowledgment Bitmap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^ ^
| | bitmap indicating whether:
| +----- Fragment with sequence 9 was received
+----------------------- Fragment with sequence 0 was received
Figure 2: RFRAG Acknowledgment Bitmap Encoding
Figure 3 shows an example Acknowledgment bitmap which indicates that
all fragments from sequence 0 to 20 were received, except for
fragments 1, 2 and 16 were lost and must be retried.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 3: Example RFRAG Acknowledgment Bitmap
The RFRAG Acknowledgment Bitmap is included in an RFRAG
Acknowledgment header, as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 0 1 0 1|E| datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RFRAG Acknowledgment Bitmap (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 4: RFRAG Acknowledgment Dispatch type and Header
E: 1 bit; Explicit Congestion Notification Echo
When set, the sender indicates that at least one of the
acknowledged fragments was received with an Explicit Congestion
Notification, indicating that the path followed by the fragments
is subject to congestion. More in Appendix C.
RFRAG Acknowledgment Bitmap: An RFRAG Acknowledgment Bitmap, whereby
setting the bit at offset x indicates that fragment x was
received, as shown in Figure 2. A NULL bitmap that indicates that
the fragmentation process is aborted. A FULL bitmap that
indicates that the fragmentation process is complete, all
fragments were received at the reassembly endpoint.
6. Fragments Recovery
The Recoverable Fragment header RFRAG is used to transport a fragment
and optionally request an RFRAG Acknowledgment that will confirm the
good reception of one or more fragments. An RFRAG Acknowledgment is
carried as a standalone fragment header (i.e., with no 6LoWPAN
payload) in a message that is propagated back to the 6LoWPAN endpoint
that was the originator of the fragments. To achieve this, each hop
that performed an MPLS-like operation on fragments reverses that
operation for the RFRAG_ACK by sending a frame from the next hop to
the previous hop as known by its MAC address in the VRB. The
datagram_tag in the RFRAG_ACK is unique to the receiver and is enough
information for an intermediate hop to locate the VRB that contains
the datagram_tag used by the previous hop and the Layer-2 information
associated to it (interface and MAC address).
The 6LoWPAN endpoint that fragments the packets at the 6LoWPAN level
(the sender) also controls the amount of acknowledgments by setting
the Ack-Request flag in the RFRAG packets. The sender may set the
Ack-Request flag on any fragment to perform congestion control by
limiting the number of outstanding fragments, which are the fragments
that have been sent but for which reception or loss was not
positively confirmed by the reassembling endpoint. The maximum
number of outstanding fragments is controlled by the Window-Size. It
is configurable and may vary in case of ECN notification. When the
6LoWPAN endpoint that reassembles the packets at the 6LoWPAN level
(the receiver) receives a fragment with the Ack-Request flag set, it
MUST send an RFRAG Acknowledgment back to the originator to confirm
reception of all the fragments it has received so far.
The Ack-Request ('X') set in an RFRAG marks the end of a window.
This flag MUST be set on the last fragment if the sender wishes to
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protect the datagram, and it MAY be set in any intermediate fragment
for the purpose of flow control.
This automatic repeat request (ARQ) process MUST be protected by a
Retransmission TimeOut (RTO) timer, and the fragment that carries the
'X' flag MAY be retried upon a time out for a configurable number of
times (see Section 7.1). Upon exhaustion of the retries the sender
may either abort the transmission of the datagram or retry the
datagram from the first fragment with an 'X' flag set in order to
reestablish a path and discover which fragments were received over
the old path in the acknowledgment bitmap. 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 receiver MAY issue unsolicited acknowledgments. An unsolicited
acknowledgment signals to the sender endpoint that it can resume
sending if it had reached its maximum number of outstanding
fragments. Another use is to inform the sender that the reassembling
endpoint aborted the processing of an individual datagram.
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.
In order to protect the datagram, the sender transfers a controlled
number of fragments and flags the last fragment of a window with an
RFRAG Acknowledgment Request. The receiver MUST acknowledge a
fragment with the acknowledgment request bit set. If any fragment
immediately preceding an acknowledgment request is still missing, the
receiver MAY intentionally delay its acknowledgment to allow in-
transit fragments to arrive. Because it might defeat the round-trip
delay computation, delaying the acknowledgment should be configurable
and not enabled by default.
When all the fragments are received, the receiving endpoint
reconstructs the packet, passes it to the upper layer, sends an RFRAG
Acknowledgment on the reverse path with a FULL bitmap, and arms a
short timer, e.g., in the order of an average round-trip delay in the
network. As the timer runs, the receiving endpoint absorbs the
fragments that were still in flight for that datagram without
creating a new state. The receiving endpoint abort the communication
if it keeps going on beyond the duration of the timer.
Note that acknowledgments might consume precious resources so the use
of unsolicited acknowledgments should be configurable and not enabled
by default.
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An observation is that streamlining forwarding of fragments generally
reduces the latency over the LLN mesh, providing room for retries
within existing upper-layer reliability mechanisms. The sender
protects the transmission over the LLN mesh with a retry timer that
is computed according to the method detailed in [RFC6298]. It is
expected that the upper layer retries obey the recommendations in
[RFC8085], in which case a single round of fragment recovery should
fit within the upper layer recovery timers.
Fragments are sent in a round-robin fashion: the sender sends all the
fragments for a first time before it retries any lost fragment; lost
fragments are retried in sequence, oldest first. This mechanism
enables the receiver to acknowledge fragments that were delayed in
the network before they are retried.
When a single frequency is used by contiguous hops, the sender should
insert a delay between fragments of a same datagram that covers
multiple transmissions so as to let a fragment progress a few hops
and avoid hidden terminal issues. This precaution is not required on
channel hopping technologies such as Time Slotted Channel Hopping
(TSCH) [RFC6554], where nodes that communicate at Layer-2 are
scheduled to send and receive respectively, and different hops
operate on different channels.
6.1. Forwarding Fragments
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
specification MUST NOT be used.
This specification extends the Virtual Reassembly Buffer (VRB)
technique to forward fragments with no intermediate reconstruction of
the entire packet. It inherits operations like datagram_tag
switching and using a timer to clean the VRB when the traffic dries
up. The first fragment carries the IP header and it is routed all
the way from the fragmenting endpoint to the reassembling endpoint.
Upon receiving the first fragment, the routers along the path install
a label-switched path (LSP), and the following fragments are label-
switched along that path. As a consequence, the next fragments can
only follow the path that was set up by the first fragment and cannot
follow an alternate route. The datagram_tag is used to carry the
label, which is swapped in each hop. All fragments follow the same
path and fragments are delivered in the order at which they are sent.
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6.1.1. Receiving the first fragment
In Route-Over mode, the source and destination MAC addresses in a
frame change at each hop. The label that is formed and placed in the
datagram_tag is associated with the source MAC address and only valid
(and unique) for that source MAC address. Upon a first fragment
(i.e., with a sequence of zero), an intermediate router creates a VRB
and the associated LSP state for the tuple (source MAC address,
datagram_tag) and the fragment is forwarded along the IPv6 route that
matches the destination IPv6 address in the IPv6 header as prescribed
by [I-D.ietf-6lo-minimal-fragment], where the receiving endpoint
allocates a reassembly buffer.
The LSP state enables to match the (previous MAC address,
datagram_tag) in an incoming fragment to the tuple (next MAC address,
swapped datagram_tag) used in the 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 hop and the swapped
datagram_tag. This reverse LSP state also points at the VRB and
enables matching the (next MAC address, swapped_datagram_tag) found
in an RFRAG Acknowledgment to the tuple (previous MAC address,
datagram_tag) used when forwarding a Fragment Acknowledgment (RFRAG-
ACK) back to the sender endpoint.
The first fragment may be received a second time, indicating that it
did not reach the destination and was retried. In that case, it
SHOULD follow the same path as the first occurrence. It is up to
sending endpoint to determine whether to abort a transmission and
then retry it from scratch, which may build an entirely new path.
6.1.2. Receiving the next fragments
Upon receiving a next fragment (i.e., with a non-zero sequence), an
intermediate router 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 using the associated VRB as prescribed by
[I-D.ietf-6lo-minimal-fragment].
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
the following information:
* The source and destination MAC addresses are swapped from those
found in the fragment
* The datagram_tag is set to the datagram_tag found in the fragment
* A NULL bitmap is used to signal the abort condition
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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
all the way to the source using the reverse LSP state in the VRBs in
the intermediate routers as described in the next section.
[I-D.ietf-6lo-minimal-fragment] indicates that the receiving endpoint
stores "the actual packet data from the fragments received so far, in
a form that makes it possible to detect when the whole packet has
been received and can be processed or forwarded". How this is
computed in implementation specific but relies on receiving all the
bytes up to the datagram_size indicated in the first fragment. An
implementation may receive overlapping fragments as the result of
retries after an MTU change.
6.2. Receiving RFRAG Acknowledgments
Upon receipt of an RFRAG-ACK, the router looks up a reverse LSP
indexed by the tuple (MAC address, datagram_tag), which are
respectively the source MAC address of the received frame and the
received datagram_tag. If it is found, the router forwards the
fragment using the associated VRB as prescribed by
[I-D.ietf-6lo-minimal-fragment], but using the reverse LSP so that
the RFRAG-ACK flows back to the sender endpoint.
If the reverse LSP is not found, the router MUST silently drop the
RFRAG-ACK message.
Either way, if the RFRAG-ACK indicates that the fragment was entirely
received (FULL bitmap), it arms a short timer, and upon timeout, the
VRB and all the associated state are destroyed. Until the timer
elapses, fragments of that datagram may still be received, e.g. if
the RFRAG-ACK was lost on the way back and the source retried the
last fragment. In that case, the router forwards the fragment
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). Note that Path MTU Discovery is
out of scope for this document.
6.3. Aborting the Transmission of 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.
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When the sender or a router on the way decides that a packet should
be dropped and the fragmentation process aborted, 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 with the datagram_tag. If an
acknowledgment is requested, the receiver responds with a NULL
bitmap.
The other way around, the receiver might need to abort the process of
a fragmented packet for internal reasons, for instance if it is out
of reassembly buffers, already uses all 256 possible values of the
datagram_tag, or if it keeps receiving fragments beyond a reasonable
time while it considers that this packet is already fully reassembled
and was passed to the upper layer. In that case, the receiver SHOULD
indicate so to the sender with a NULL bitmap in an RFRAG
Acknowledgment. The RFRAG Acknowledgment is forwarded all the way
back to the source of the packet and cleans up all resources on the
way. Upon an acknowledgment with a NULL bitmap, the sender endpoint
MUST abort the transmission of the fragmented datagram with one
exception: In the particular case of the first fragment, it MAY
decide to retry via an alternate next hop instead.
6.4. Applying Recoverable Fragmentation along a Diverse Path
The text above can be read with the assumption of a serial path
between a source and a destination. Section 4.5.3 of the "6TiSCH
Architecture" [I-D.ietf-6tisch-architecture] defines the concept of a
Track that can be a complex path between a source and a destination
with Packet ARQ, Replication, Elimination and Overhearing (PAREO)
along the Track. This specification can be used along any subset of
the complex Track where the first fragment is flooded. The last
RFRAG Acknowledgment is flooded on that same subset in the reverse
direction. Intermediate RFRAG Acknowledgments can be flooded on any
sub-subset of that reverse subset that reach back to the source.
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7. Management Considerations
This specification extends "On Forwarding 6LoWPAN Fragments over a
Multihop IPv6 Network" [I-D.ietf-6lo-minimal-fragment] and requires
the same parameters in the receiver and on intermediate nodes. There
is no new parameter as echoing ECN is always on. This parameters
typically include the reassembly time-out at the receiver and an
inactivity clean-up timer on the intermediate nodes, and the number
of messages that can be processed in parallel in all nodes.
The configuration settings introduced by this specification only
apply to the sender, which is in full control of the transmission.
LLNs vary a lot in size (there can be thousands of nodes in a mesh),
in speed (from 10Kbps to several Mbps at the PHY layer), in traffic
density, and in optimizations that are desired (e.g., the selection
of a RPL [RFC6550] Objective Function [RFC6552] impacts the shape of
the routing graph).
For that reason, only a very generic guidance can be given on the
settings of the sender and on whether complex algorithms are needed
to perform flow control or estimate the round-trip time. To cover
the most complex use cases, this specification enables the sender to
vary the fragment size, the window size and the inter-frame gap,
based on the amount of losses, the observed variations of the round-
trip time and the setting of the ECN bit.
7.1. Protocol Parameters
The management system SHOULD be capable of providing the parameters
listed in this section.
An implementation must control the rate at which it sends packets
over a same path to allow the next hop to forward a packet before it
gets the next. In a wireless network that uses a same frequency
along a path, more time must be inserted to avoid hidden terminal
issues between fragments. This is controlled by the following
parameter:
inter-frame gap: Indicates a minimum amount of time between
transmissions. All packets to a same destination, and in
particular fragments, may be subject to receive while transmitting
and hidden terminal collisions with the next or the previous
transmission as the fragments progress along a same path. The
inter-frame gap protects the propagation of one transmission
before the next one is triggered and creates a duty cycle that
controls the ratio of air time and memory in intermediate nodes
that a particular datagram will use.
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An implementation should consider the generic recommendations from
the IETF in the matter of flow control and rate management in
[RFC5033]. To control the flow, an implementation may use a dynamic
value of the window size (Window_Size), adapt the fragment size
(Fragment_Size) and insert an inter-frame gap that is longer than
necessary. In a large network where node contend for the bandwidth,
a larger Fragment_Size consumes less bandwidth but also reduces the
fluidity and incurs higher chances of loss in transmission. This is
controlled by the following parameters:
MinFragmentSize: The MinFragmentSize is the minimum value for the
Fragment_Size.
OptFragmentSize: The OptFragmentSize is the value for the
Fragment_Size that the sender should use to start with. It is
more than or equal to MinFragmentSize. It is less than or equal
to MaxFragmentSize. On the first fragment, it must enable the
expansion of the IPv6 addresses and of the Hop Limit field within
MTU. On all fragments, it is a balance between the expected
fluidity and the overhead of MAC and 6LoWPAN headers. For a small
MTU, the idea is to keep it close to the maximum, whereas for
larger MTUs, it might makes sense to keep it short enough, so that
the duty cycle of the transmitter is bounded, e.g., to transmit at
least 10 frames per second.
MaxFragmentSize: The MaxFragmentSize is the maximum value for the
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.
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. It is more than or
equal to MinWindowSize. It is less than or equal to
MaxWindowSize. The Window_Size should be maintained below the
number of hops in the path of the fragment to avoid stacking
fragments at the bottleneck on the path. If an inter-frame gap is
used to avoid interference between fragments then the Window_Size
should be at most in the order of the estimation of the trip time
divided by the inter-frame gap.
MaxWindowSize: The maximum value of Window_Size that the sender can
use. The value MUST be less than 32.
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An implementation may perform its estimate of the RTO or use a
configured one. The ARQ process is controlled by the following
parameters:
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 RTO, that is
amount of time that a sender should wait for an RFRAG
Acknowledgment before it takes a next action. It is more than or
equal to MinARQTimeOut. It is less than or equal to
MaxARQTimeOut.
MaxARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action. It must cover
the longest expected round-trip time, and be several times less
than the time-out that covers the recomposition buffer at the
receiver, which is typically in the order of the minute. See
Appendix C for recommendations on computing the round-trip time.
MaxFragRetries: The maximum number of retries for a particular
fragment.
MaxDatagramRetries: The maximum number of retries from scratch for a
particular datagram.
An implementation may be capable to perform flow control based on
ECN, more in Appendix C. This is controlled by the following
parameter:
UseECN: Indicates whether the sender should react to ECN. The
sender may react to ECN by varying the Window_Size between
MinWindowSize and MaxWindowSize, varying the Fragment_Size between
MinFragmentSize and MaxFragmentSize and/or by increasing the
inter-frame gap.
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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 both the sender
and the receiver, and may tune the optimum size of Fragment_Size and
of the Window_Size, OptDatagramSize 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 inter-frame gap is another tool that can be
used to increase the spacing between fragments of the same datagram
and reduce the ratio of time when a particular intermediate node
holds a fragment of that datagram.
8. Security Considerations
This document specifies an instantiation of a 6LoWPAN Fragment
Forwarding technique. [I-D.ietf-6lo-minimal-fragment] provides the
generic description of Fragment Forwarding and this specification
inherits from it. The generic considerations in the Security
sections of [I-D.ietf-6lo-minimal-fragment] apply equally to this
document.
This specification does not recommend a particular algorithm for the
estimation of the duration of the RTO that covers the detection of
the loss of a fragment with the 'X' flag set; regardless, an attacker
on the path may slow down or discard packets, which in turn can
affect the throughput of fragmented packets.
Compared to "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944], this specification reduces the datagram_tag to 8
bits and the tag wraps faster than with [RFC4944]. But for a
constrained network where a node is expected to be able to hold only
one or a few large packets in memory, 256 is still a large number.
Also, the acknowledgement mechanism allows cleaning up the state
rapidly once the packet is fully transmitted or aborted.
The abstract Virtual Recovery Buffer inherited from
[I-D.ietf-6lo-minimal-fragment] may be used to perform a Denial-of-
Service (DoS) attack against the intermediate Routers since the
routers need to maintain a state per flow. The particular VRB
implementation technique described in
[I-D.ietf-lwig-6lowpan-virtual-reassembly] allows realigning which
data goes in which fragment, which causes the intermediate node to
store a portion of the data, which adds an attack vector that is not
present with this specification. With this specification, the data
that is transported in each fragment is conserved and the state to
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keep does not include any data that would not fit in the previous
fragment.
9. IANA Considerations
This document allocates 2 patterns for a total of 4 dispatch values
in Page 0 for recoverable fragments from the "Dispatch Type Field"
registry that was created by "Transmission of IPv6 Packets over IEEE
802.15.4 Networks" [RFC4944] and reformatted by "6LoWPAN Paging
Dispatch" [RFC8025].
The suggested patterns (to be confirmed by IANA) are indicated in
Table 1.
+-------------+------+----------------------------------+-----------+
| Bit Pattern | Page | Header Type | Reference |
+=============+======+==================================+===========+
| 11 10100x | 0 | RFRAG - Recoverable Fragment | THIS RFC |
+-------------+------+----------------------------------+-----------+
| 11 10100x | 1-14 | Unassigned | |
+-------------+------+----------------------------------+-----------+
| 11 10100x | 15 | Reserved for Experimental Use | RFC 8025 |
+-------------+------+----------------------------------+-----------+
| 11 10101x | 0 | RFRAG-ACK - RFRAG | THIS RFC |
| | | Acknowledgment | |
+-------------+------+----------------------------------+-----------+
| 11 10101x | 1-14 | Unassigned | |
+-------------+------+----------------------------------+-----------+
| 11 10101x | 15 | Reserved for Experimental Use | RFC 8025 |
+-------------+------+----------------------------------+-----------+
Table 1: Additional Dispatch Value Bit Patterns
10. Acknowledgments
The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent
Toutain, Carles Gomez Montenegro, Thomas Watteyne, and Michael
Richardson for in-depth reviews and comments. Also many thanks to
Peter Yee, Colin Perkins, Tirumaleswar Reddy Konda and Erik Nordmark
for their careful reviews and for helping through the IETF Last Call
and IESG review process, and to Jonathan Hui, Jay Werb, Christos
Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret Wasserman, Richard
Kelsey, Carsten Bormann and Harry Courtice for their various
contributions in the long process that lead ot this document.
11. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.
[RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
RFC 8025, DOI 10.17487/RFC8025, November 2016,
<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
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[I-D.ietf-6lo-minimal-fragment]
Watteyne, T., Thubert, P., and C. Bormann, "On Forwarding
6LoWPAN Fragments over a Multihop IPv6 Network", Work in
Progress, Internet-Draft, draft-ietf-6lo-minimal-fragment-
10, 1 February 2020, <https://tools.ietf.org/html/draft-
ietf-6lo-minimal-fragment-10>.
12. Informative References
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
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[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, DOI 10.17487/RFC4919, August 2007,
<https://www.rfc-editor.org/info/rfc4919>.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
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RFC 6552, DOI 10.17487/RFC6552, March 2012,
<https://www.rfc-editor.org/info/rfc6552>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/info/rfc5033>.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing",
RFC 6606, DOI 10.17487/RFC6606, May 2012,
<https://www.rfc-editor.org/info/rfc6606>.
[I-D.ietf-lwig-6lowpan-virtual-reassembly]
Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
lwig-6lowpan-virtual-reassembly-01, 11 March 2019,
<https://tools.ietf.org/html/draft-ietf-lwig-6lowpan-
virtual-reassembly-01>.
[I-D.ietf-intarea-frag-fragile]
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", Work
in Progress, Internet-Draft, draft-ietf-intarea-frag-
fragile-17, 30 September 2019,
<https://tools.ietf.org/html/draft-ietf-intarea-frag-
fragile-17>.
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[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", Work in Progress, Internet-Draft,
draft-ietf-6tisch-architecture-28, 29 October 2019,
<https://tools.ietf.org/html/draft-ietf-6tisch-
architecture-28>.
[IEEE.802.15.4]
IEEE, "IEEE Standard for Low-Rate Wireless Networks",
IEEE Standard 802.15.4, DOI 10.1109/IEEE
P802.15.4-REVd/D01,
<http://ieeexplore.ieee.org/document/7460875/>.
[Kent] Kent, C. and J. Mogul, ""Fragmentation Considered
Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in
Computer Communications Technology",
DOI 10.1145/55483.55524, August 1987,
<http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-
87-3.pdf>.
Appendix A. Rationale
There are a number of uses for large packets in Wireless Sensor
Networks. Such usages may not be the most typical or represent the
largest amount of traffic over the LLN; however, the associated
functionality can be critical enough to justify extra care for
ensuring effective transport of large packets across the LLN.
The list of those usages includes:
Towards the LLN node: Firmware update: For example, a new version
of the LLN node software is downloaded from a system manager
over unicast or multicast services. Such a reflashing
operation typically involves updating a large number of similar
LLN nodes over a relatively short period of time.
Packages of Commands: A number of commands or
a full configuration can be packaged as a single message to
ensure consistency and enable atomic execution or complete roll
back. Until such commands are fully received and interpreted,
the intended operation will not take effect.
From the LLN node: Waveform captures: A number of consecutive
samples are measured at a high rate for a short time and then
transferred from a sensor to a gateway or an edge server as a
single large report.
Data logs: LLN nodes may generate large logs of
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sampled data for later extraction. LLN nodes may also generate
system logs to assist in diagnosing problems on the node or
network.
Large data packets: Rich data types might
require more than one fragment.
Uncontrolled firmware download or waveform upload can easily result
in a massive increase of the traffic and saturate the network.
When a fragment is lost in transmission, the lack of recovery in the
original fragmentation system of RFC 4944 implies that all fragments
would need to be resent, further contributing to the congestion that
caused the initial loss, and potentially leading to congestion
collapse.
This saturation may lead to excessive radio interference, or random
early discard (leaky bucket) in relaying nodes. Additional queuing
and memory congestion may result while waiting for a low power next
hop to emerge from its sleeping state.
Considering that RFC 4944 defines an MTU is 1280 bytes and that in
most incarnations (but 802.15.4g) a IEEE Std. 802.15.4 frame can
limit the MAC payload to as few as 74 bytes, a packet might be
fragmented into at least 18 fragments at the 6LoWPAN shim layer.
Taking into account the worst-case header overhead for 6LoWPAN
Fragmentation and Mesh Addressing headers will increase the number of
required fragments to around 32. This level of fragmentation is much
higher than that traditionally experienced over the Internet with
IPv4 fragments. At the same time, the use of radios increases the
probability of transmission loss and Mesh-Under techniques compound
that risk over multiple hops.
Mechanisms such as TCP or application-layer segmentation could be
used to support end-to-end reliable transport. One option to support
bulk data transfer over a frame-size-constrained LLN is to set the
Maximum Segment Size to fit within the link maximum frame size.
Doing so, however, can add significant header overhead to each
802.15.4 frame. In addition, deploying such a mechanism requires
that the end-to-end transport is aware of the delivery properties of
the underlying LLN, which is a layer violation, and difficult to
achieve from the far end of the IPv6 network.
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Appendix B. Requirements
For one-hop communications, a number of Low Power and Lossy Network
(LLN) link-layers propose a local acknowledgment mechanism that is
enough to detect and recover the loss of fragments. In a multihop
environment, an end-to-end fragment recovery mechanism might be a
good complement to a hop-by-hop MAC level recovery. This draft
introduces a simple protocol to recover individual fragments between
6LoWPAN endpoints that may be multiple hops away. The method
addresses the following requirements of an LLN:
Number of fragments: The recovery mechanism must support highly
fragmented packets, with a maximum of 32 fragments per packet.
Minimum acknowledgment overhead: Because the radio is half duplex,
and because of silent time spent in the various medium access
mechanisms, an acknowledgment consumes roughly as many resources
as a data fragment.
The new end-to-end fragment recovery mechanism should be able to
acknowledge multiple fragments in a single message and not require
an acknowledgment at all if fragments are already protected at a
lower layer.
Controlled latency: The recovery mechanism must succeed or give up
within the time boundary imposed by the recovery process of the
Upper Layer Protocols.
Optional congestion control: The aggregation of multiple concurrent
flows may lead to the saturation of the radio network and
congestion collapse.
The recovery mechanism should provide means for controlling the
number of fragments in transit over the LLN.
Appendix C. Considerations on Flow Control
Considering that a multi-hop LLN can be a very sensitive environment
due to the limited queuing capabilities of a large population of its
nodes, this draft recommends a simple and conservative approach to
Congestion Control, based on TCP congestion avoidance.
Congestion on the forward path is assumed in case of packet loss, and
packet loss is assumed upon time out. The draft allows controlling
the number of outstanding fragments that have been transmitted but
for which an acknowledgment was not received yet. It must be noted
that the number of outstanding fragments should not exceed the number
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of hops in the network, but the way to figure the number of hops is
out of scope for this document.
Congestion on the forward path can also be indicated by an Explicit
Congestion Notification (ECN) mechanism. Though whether and how ECN
[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
back to the source endpoint in an acknowledgment message as
represented in Figure 4 in Section 5.2.
It must be noted that congestion and collision are different topics.
In particular, when a mesh operates on a same channel over multiple
hops, then the forwarding of a fragment over a certain hop may
collide with the forwarding of a next fragment that is following over
a previous hop but in a same interference domain. This draft enables
end-to-end flow control, but leaves it to the sender stack to pace
individual fragments within a transmit window, so that a given
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
6TiSCH [I-D.ietf-6tisch-architecture], which operates over the
TimeSlotted Channel Hopping [RFC7554] (TSCH) mode of operation of
IEEE802.14.5, a fragment is forwarded over a different channel at a
different time and it makes full sense to transmit the next fragment
as soon as the previous fragment has had its chance to be forwarded
at the next hop.
From the standpoint of a source 6LoWPAN endpoint, an outstanding
fragment is a fragment that was sent but for which no explicit
acknowledgment was received yet. This means that the fragment might
be on the way, received but not yet acknowledged, or the
acknowledgment might be on the way back. It is also possible that
either the fragment or the acknowledgment was lost on the way.
From the sender standpoint, all outstanding fragments might still be
in the network and contribute to its congestion. There is an
assumption, though, that after a certain amount of time, a frame is
either received or lost, so it is not causing congestion anymore.
This amount of time can be estimated based on the round-trip delay
between the 6LoWPAN endpoints. The method detailed in "Computing
TCP's Retransmission Timer" [RFC6298] is recommended for that
computation.
The reader is encouraged to read through "Congestion Control
Principles" [RFC2914]. Additionally [RFC7567] and [RFC5681] provide
deeper information on why this mechanism is needed and how TCP
handles Congestion Control. Basically, the goal here is to manage
the amount of fragments present in the network; this is achieved by
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to reducing the number of outstanding fragments over a congested path
by throttling the sources.
Section 6 describes how the sender decides how many fragments are
(re)sent before an acknowledgment is required, and how the sender
adapts that number to the network conditions.
Author's Address
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 MOUGINS - Sophia Antipolis
France
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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