Internet Engineering Task Force (IETF) P. Thubert, Ed.
Request for Comments: 8931 Cisco Systems
Updates: 4944 November 2020
Category: Standards Track
ISSN: 2070-1721
IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Selective
Fragment Recovery
Abstract
This document updates RFC 4944 with a protocol that forwards
individual fragments across a route-over mesh and recovers them end
to end, with congestion control capabilities to protect the network.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8931.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
2.1. Requirements Language
2.2. Background
2.3. Other Terms
3. Updating RFC 4944
4. Extending RFC 8930
4.1. Slack in the First Fragment
4.2. Gap between Frames
4.3. Congestion Control
4.4. Modifying the First Fragment
5. New Dispatch Types and Headers
5.1. Recoverable Fragment Dispatch Type and Header
5.2. RFRAG Acknowledgment Dispatch Type and Header
6. Fragment Recovery
6.1. Forwarding Fragments
6.1.1. Receiving the First Fragment
6.1.2. Receiving the Next Fragments
6.2. Receiving RFRAG Acknowledgments
6.3. Aborting the Transmission of a Fragmented Packet
6.4. Applying Recoverable Fragmentation along a Diverse Path
7. Management Considerations
7.1. Protocol Parameters
7.2. Observing the Network
8. Security Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Rationale
Appendix B. Requirements
Appendix C. Considerations on Congestion Control
Acknowledgments
Author's Address
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 latter case, the large chunk flows away from the
LLN node. In both cases, the size can be on the order of 10 KB 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 IPv6 over Low-Power Wireless Personal Area
Network (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 the reassembly of
the packet at each hop. "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4" [6TiSCH] indicates that this may cause latency
along a path and impact critical resources such as memory and
battery; to alleviate those undesirable effects, it recommends using
a 6LoWPAN Fragment Forwarding (6LFF) technique.
"On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network"
[RFC8930] specifies the generic behavior that all 6LFF techniques
including this specification follow, and it presents the associated
caveats. In particular, the routing information is fully indicated
in the first fragment, which is always forwarded first. With this
specification, the first fragment is identified by a Sequence of 0 as
opposed to a dispatch type in [RFC4944]. 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; see Section 6. This specification
encodes the Datagram_Tag in 1 byte, which will saturate if more than
256 datagrams transit in fragmented form over a single hop at the
same time. This is not realistic at the time of this writing.
Should this happen in a new 6LoWPAN technology, a node will need to
use several link-layer addresses to increase its indexing capacity.
"Virtual reassembly buffers in 6LoWPAN" [LWIG-FRAG] proposes a 6LFF
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 [RFC8900]), in
particular, the issues of resources locked on the reassembling
endpoint 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
acknowledgments" that recovers the loss of a fragment individually.
The paper illustrates the benefits that can be derived from such a
method; see Figures 1, 2, and 3 in Section 2.3 of [Kent]. [RFC4944]
has no selective recovery, and the whole datagram fails when one
fragment is not delivered to the reassembling endpoint. Constrained
memory resources are blocked on the reassembling endpoint until it
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 fragmenting or the reassembling endpoints. Should this
happen, the source will keep on sending fragments, wasting even more
resources in the network since the datagram cannot arrive in its
entirety, which possibly contributes to the condition that caused the
loss. [RFC4944] is lacking a congestion control to avoid
participating in a saturation that may have caused the loss of the
fragment. It has no signaling to abort a multi-fragment transmission
at any time and from either end, and if the capability to forward
fragments is implemented, clean up the related state in the network.
This specification provides a method to forward fragments over,
typically, a few hops in a route-over 6LoWPAN mesh and a selective
acknowledgment to recover individual fragments between 6LoWPAN
endpoints. The method can help limit the congestion loss in the
network and addresses the requirements in Appendix B. Flow control
is out of scope since the endpoints are expected to be able to store
the full datagram. Deployments are expected to be managed and
homogeneous, and an incremental transition requires a flag day.
2. Terminology
2.1. Requirements Language
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. Background
This document uses 6LoWPAN terms and concepts that are presented in
"IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals" [RFC4919];
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944];
and "Problem Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606].
[RFC8930] discusses the generic concept of a Virtual Reassembly
Buffer (VRB) and specifies behaviors and caveats that are common to a
large family of 6LFF techniques including the mechanism specified by
this document, which is fully inherited from that specification. It
also defines terms used in this document: Compressed Form,
Datagram_Tag, Datagram_Size, Fragment_Offset, and 6LoWPAN Fragment
Forwarding endpoint (commonly abbreviated as only "endpoint").
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 the "Path MTU Discovery for IP version 6" [RFC8201]
protocol that limits fragmentation over the Internet. Specifically,
in the case of UDP, valuable additional information can be found in
"UDP Usage Guidelines" [RFC8085].
"The Benefits of Using Explicit Congestion Notification (ECN)"
[RFC8087] provides useful information on the potential benefits and
pitfalls of using ECN.
Quoting "Multiprotocol Label Switching 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".
[RFC8930] leverages MPLS to forward fragments that actually do not
have a network-layer header, since the fragmentation occurs below IP,
and this specification makes it reversible so the reverse path can be
followed as well.
2.3. Other Terms
This specification uses the following terms:
RFRAG: Recoverable Fragment
RFRAG-ACK: Recoverable Fragment Acknowledgment
RFRAG Acknowledgment Request: An RFRAG with the Acknowledgment
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.
Reassembling endpoint: The receiving endpoint.
Fragmenting endpoint: The sending endpoint.
Forward direction: The direction of a path, which is followed by the
RFRAG.
Reverse direction: The reverse direction of a path, which is taken
by the RFRAG-ACK.
3. Updating RFC 4944
This specification updates the fragmentation mechanism that is
specified in [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 (defined in Section 5.1) also be
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 easily (more in Section 4.4).
To be 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 RFC 8930
This specification implements the generic 6LFF technique defined in
[RFC8930] and provides end-to-end fragment recovery and congestion
control mechanisms.
4.1. Slack in the First Fragment
[RFC8930] allows for a refragmentation operation in intermediate
nodes, whereby the trailing bytes from a given fragment may be left
in the VRB to be added as the heading bytes in the next fragment.
This solves the case when the outgoing fragment needs more space than
the incoming fragment; that case may arise when the 6LoWPAN header
compression is not as efficient on the outgoing link or if the Link
MTU is reduced.
This specification cannot allow that refragmentation operation since
the fragments are recovered end to end based on a sequence number.
The Fragment_Size MUST be tailored to fit the minimal MTU along the
path, and the first fragment that contains a 6LoWPAN compressed
header MUST have enough slack to enable a less-efficient compression
in the next hops to still fit within the Link MTU.
For instance, if the fragmenting endpoint is also the 6LoWPAN
compression endpoint, it will elide the Interface ID (IID) of the
source IPv6 address when it matches the link-layer address [RFC6282].
In that case, it MUST leave slack in the first fragment as the if MTU
on the first hop was 8 bytes less, so the next hop can expand the IID
within the same fragment within MTU.
4.2. Gap between Frames
[RFC8930] requires that a configurable interval of time be inserted
between transmissions to the same next hop and, in particular,
between fragments of a same datagram. In the case of half duplex
interfaces, this inter-frame gap ensures that the next hop is done
forwarding 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 the 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. Congestion Control
The inter-frame gap is the only protection that [RFC8930] imposes by
default. This document enables grouping fragments in windows and
requesting intermediate acknowledgments, so the number of in-flight
fragments can be bounded. This document also adds an ECN mechanism
that can be used to protect the network by adapting the size of the
window, the size of the fragments, and/or the inter-frame gap.
This specification enables the fragmenting endpoint to apply a
congestion control mechanism to tune those parameters, but the
mechanism itself is out of scope. In most cases, the expectation is
that most datagrams will require only a few fragments, and that only
the last fragment will be acknowledged. A basic implementation of
the fragmenting endpoint is NOT REQUIRED to vary the size of the
window, the duration of the inter-frame gap, or the size of a
fragment in the middle of the transmission of a datagram, and it MAY
ignore the ECN signal or simply reset the window to 1 (see
Appendix C) until the end of this datagram upon detecting a
congestion.
An intermediate node that experiences a congestion MAY set the ECN
bit in a fragment, and the reassembling endpoint echoes the ECN bit
at most once at the next opportunity to acknowledge back.
The size of the fragments is typically computed from the Link MTU to
maximize the size of the resulting frames. The size of the window
and the duration of the inter-frame gap SHOULD be configurable, to
reduce the chances of congestion and to follow the general
recommendations in [RFC8930], respectively.
4.4. 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, which are
all in the first fragment, 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, encoded in the
Fragment_Size field, 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 Fragment_Offset of
all the subsequent fragments that it forwards for that datagram. In
the case of a Source Routing Header 6LoWPAN Routing Header (SRH-
6LoRH) [RFC8138] being consumed and thus reduced, that difference is
negative, meaning that the Fragment_Offset is decremented by the
number of bytes that were consumed.
5. New Dispatch Types and Headers
This document specifies an alternative to the 6LoWPAN fragmentation
sub-layer [RFC4944] to emulate a Link MTU up to 2048 bytes for the
upper layer, which can be the 6LoWPAN header compression sub-layer
that is defined in "Compression Format for IPv6 Datagrams over IEEE
802.15.4-Based Networks" [RFC6282]. This specification also provides
a reliable transmission of the fragments over a multi-hop 6LoWPAN
route-over mesh network and a minimal congestion control to reduce
the chances of congestion loss.
A 6LoWPAN Fragment Forwarding [RFC8930] technique derived from MPLS
enables the forwarding of 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 link-layer address of the fragment, so together the link-layer
address and the label can identify the fragment globally within the
lifetime of the datagram. 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 its lifetime. The result is
that the label does not need to be globally unique, but it must be
swapped at each hop as the source link-layer address changes.
In the following sections, a Datagram_Tag extends the semantics
defined in "Fragmentation Type and Header" (see Section 5.3 of
[RFC4944]). 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 [RFC8930].
This specification extends [RFC4944] with two new dispatch types for
RFRAG and the RFRAG-ACK that is received back. The new 6LoWPAN
dispatch types are taken from [RFC8025], as indicated in Table 1 of
Section 9.
5.1. Recoverable Fragment Dispatch Type and Header
In this specification, if the packet is compressed, the size and
offset of the fragments are expressed with respect to the compressed
form of the packet, as opposed to the uncompressed (native) form.
The format of the fragment header is shown in Figure 1. It is the
same for all fragments even though the Fragment_Offset is overloaded.
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 correlated with neither the
value of the Sequence field nor the order in which the fragments are
received. This enables splitting fragments to cope with an MTU
deduction; see the example of fragment Sequence 5 that is retried end
to end as smaller fragment Sequences 13 and 14 in Section 6.2.
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 stored Datagram_Size of the packet identified by the sender
link-layer address and the Datagram_Tag.
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
X: 1 bit; Ack-Request. When set, the fragmenting endpoint requires
an RFRAG Acknowledgment from the reassembling endpoint.
E: 1 bit; Explicit Congestion Notification. The "E" flag is cleared
by the source of the fragment and set by intermediate routers to
signal that this fragment experienced congestion along its path.
Fragment_Size: 10-bit unsigned integer. The size of this fragment
in a unit that depends on link-layer technology. Unless
overridden by a more specific specification, that unit is the
byte, which allows fragments up to 1023 bytes.
Datagram_Tag: 8 bits. An identifier of the datagram that is locally
unique to the link-layer sender.
Sequence: 5-bit unsigned integer. The sequence number of the
fragment in the acknowledgment bitmap. Fragments are numbered as
[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-zero 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 reassembling endpoint 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 states 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 if the next hop is still
reachable:
* if a VRB already exists and the next hop is still reachable,
the fragment is to be forwarded along the associated LSP as
described in Section 6.1.2, without checking 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).
* else (the Sequence is non-zero and either no VRB exists or the
next hop is unavailable), the fragment cannot be forwarded or
routed; the fragment is discarded and an abort RFRAG-ACK is
sent back to the source as described in Section 6.1.2.
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.
There is no requirement on the reassembling endpoint to check that
the received fragments are consecutive and non-overlapping. This may
be useful, in particular, in the case where the MTU changes and a
fragment Sequence is retried with a smaller Fragment_Size, with the
remainder of the original fragment being retried with new Sequence
values. The fragmenting endpoint knows that the datagram is fully
received when the acknowledged fragments cover the whole datagram,
which is implied by a FULL bitmap.
5.2. RFRAG Acknowledgment Dispatch Type and Header
This specification also defines a 4-byte RFRAG Acknowledgment Bitmap
that is used by the reassembling endpoint to selectively confirm 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 RFRAG Acknowledgment Bitmap that indicates
that all fragments from Sequence 0 to 20 were received, except for
fragments 1, 2, and 16, which 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: RFRAG Acknowledgment Dispatch Type and Header
E: 1 bit; Explicit Congestion Notification Echo.
When set, the fragmenting endpoint 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. See more details in
Appendix C.
Datagram_Tag: 8 bits; an identifier of the datagram that is locally
unique to the link-layer recipient.
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 indicates that the
fragmentation process is aborted. A FULL bitmap indicates that
the fragmentation process is complete; all fragments were received
at the reassembly endpoint.
6. Fragment Recovery
The RFRAG header is used to transport a fragment and optionally
request an RFRAG-ACK that confirms the reception of one or more
fragments. An RFRAG-ACK is carried as a standalone fragment header
(i.e., with no 6LoWPAN payload) in a message that is propagated back
to the fragmenting endpoint. 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 link-layer address in the VRB. The
Datagram_Tag in the RFRAG-ACK is unique to the reassembling endpoint
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 with it (interface and link-layer address).
The fragmenting endpoint (i.e., the node that fragments the packets
at the 6LoWPAN level) also controls the number of acknowledgments by
setting the Ack-Request flag in the RFRAG packets.
The fragmenting endpoint 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 endpoint that reassembles the packets at the
6LoWPAN level receives a fragment with the Ack-Request flag set, it
MUST send an RFRAG-ACK 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 fragmenting
endpoint wishes to perform an automatic repeat request (ARQ) process
for the datagram, and it MAY be set in any intermediate fragment for
the purpose of congestion control.
This ARQ process MUST be protected by a Retransmission Timeout (RTO)
timer, and the fragment that carries the "X" flag MAY be retried upon
a timeout for a configurable number of times (see Section 7.1) with
an exponential backoff. Upon exhaustion of the retries, the
fragmenting endpoint may either abort the transmission of the
datagram or resend the first fragment with an "X" flag set in order
to establish a new path for the datagram and obtain the list of
fragments that were received over the old path in the acknowledgment
bitmap. When the fragmenting endpoint 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 reassembling endpoint MAY issue unsolicited acknowledgments. An
unsolicited acknowledgment signals to the fragmenting endpoint that
it can resume sending in case it has reached its maximum number of
outstanding fragments. Another use is to inform the fragmenting
endpoint that the reassembling endpoint aborted the processing of an
individual datagram.
The RFRAG Acknowledgment carries an ECN indication for congestion
control (see Appendix C). The reassembling endpoint of a fragment
with the "E" (ECN) flag set MUST echo that information at most once
by setting the "E" (ECN) flag in the next RFRAG-ACK.
In order to protect the datagram, the fragmenting endpoint transfers
a controlled number of fragments and flags to the last fragment of a
window with an RFRAG Acknowledgment Request. The reassembling
endpoint MUST acknowledge a fragment with the acknowledgment request
bit set. If any fragment immediately preceding an acknowledgment
request is still missing, the reassembling endpoint MAY intentionally
delay its acknowledgment to allow in-transit fragments to arrive.
Because it might defeat the round-trip time computation, delaying the
acknowledgment should be configurable and not enabled by default.
When enough fragments are received to cover the whole datagram, the
reassembling endpoint reconstructs the packet, passes it to the upper
layer, sends an RFRAG-ACK on the reverse path with a FULL bitmap, and
arms a short timer, e.g., on the order of an average round-trip time
in the network. The FULL bitmap is used as opposed to a bitmap that
acknowledges only the received fragments to let the intermediate
nodes know that the datagram is fully received. As the timer runs,
the reassembling endpoint absorbs the fragments that were still in
flight for that datagram without creating a new state, acknowledging
the ones that bear an Ack-Request with an FRAG Acknowledgment and the
FULL bitmap. The reassembling endpoint aborts the communication if
fragments with a matching source and Datagram-Tag continue to be
received after the timer expires.
Note that acknowledgments might consume precious resources, so the
use of unsolicited acknowledgments SHOULD be configurable and not
enabled by default.
An observation is that streamlining the forwarding of fragments
generally reduces the latency over the LLN mesh, providing room for
retries within existing upper-layer reliability mechanisms. The
fragmenting endpoint protects the transmission over the LLN mesh with
a retry timer that is configured for a use case and may be adapted
dynamically, e.g., according to the method detailed in [RFC6298]. It
is expected that the upper-layer retry mechanism obeys the
recommendations in [RFC8085], in which case a single round of
fragment recovery should fit within the upper-layer recovery timers.
Fragments MUST be sent in a round-robin fashion: the sender MUST send
all the fragments for a first time before it retries any lost
fragment; lost fragments MUST be 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 radio frequency is used by contiguous hops, the
fragmenting endpoint SHOULD insert a delay between the frames (e.g.,
carrying fragments) that are sent to the same next hop. The delay
SHOULD cover multiple transmissions so as to let a frame 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
This specification inherits from [RFC8930] and proposes a Virtual
Reassembly Buffer technique to forward fragments with no intermediate
reconstruction of the entire datagram.
The IPv6 header MUST be placed in the first fragment in full to
enable the routing decision. The first fragment is routed and
creates an LSP from the fragmenting endpoint to the reassembling
endpoint. The next fragments are label switched along that LSP. As
a consequence, the next fragments can only follow the path that was
set up by the first fragment; they cannot follow an alternate route.
The Datagram_Tag is used to carry the label, which is swapped in each
hop.
If the first fragment is too large for the path MTU, it will
repeatedly fail and never establish an LSP. In that case, the
fragmenting endpoint MAY retry the same datagram with a smaller
Fragment_Size, in which case it MUST abort the original attempt and
use a new Datagram_Tag for the new attempt.
6.1.1. Receiving the First Fragment
In route-over mode, the source and destination link-layer addresses
in a frame change at each hop. The label that is formed and placed
in the Datagram_Tag by the sender is associated with the source link-
layer address and only valid (and temporarily unique) for that source
link-layer address.
Upon receiving the first fragment (i.e., with a Sequence of 0), an
intermediate router creates a VRB and the associated LSP state
indexed by the incoming interface, the previous-hop link-layer
address, and the Datagram_Tag and forwards the fragment along the
IPv6 route that matches the destination IPv6 address in the IPv6
header until it reaches the reassembling endpoint, as prescribed by
[RFC8930]. The LSP state enables matching the next incoming
fragments of a datagram to the abstract forwarding information of the
next interface, source and next-hop link-layer addresses, and the
swapped Datagram_Tag.
In addition, the router also forms a reverse LSP state indexed by the
interface to the next hop, the link-layer address the router uses as
source for that datagram, and the swapped Datagram_Tag. This reverse
LSP state enables matching the tuple (interface, destination link-
layer address, Datagram_Tag) found in an RFRAG-ACK to the abstract
forwarding information (previous interface, previous link-layer
address, Datagram_Tag) used to forward the RFRAG-ACK back to the
fragmenting endpoint.
6.1.2. Receiving the Next Fragments
Upon receiving the next fragment (i.e., with a non-zero Sequence), an
intermediate router looks up an LSP indexed by the tuple (incoming
interface, previous-hop link-layer address, Datagram_Tag) found in
the fragment. If it is found, the router forwards the fragment using
the associated VRB as prescribed by [RFC8930].
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 link-layer addresses are swapped from
those found in the fragment, and the same interface is used
* The Datagram_Tag is set to the Datagram_Tag found in the fragment
* 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
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.
[RFC8930] indicates that the reassembling 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 is implementation
specific, but it 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 interface and destination link-layer address of the
received frame and the received Datagram_Tag in the RFRAG-ACK. If it
is found, the router forwards the fragment using the associated VRB
as prescribed by [RFC8930], but it uses the reverse LSP so that the
RFRAG-ACK flows back to the fragmenting 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 states are destroyed. Until the timer
elapses, fragments of that datagram may still be received, e.g., if
the RFRAG-ACK was lost on the path back, and the source retried the
last fragment. In that case, the router generates an RFRAG-ACK with
a FULL bitmap back to the fragmenting endpoint if an acknowledgment
was requested; else, it silently drops the fragment.
This specification does not provide a method to discover the number
of hops or the minimal value of MTU along those hops. In a typical
case, the MTU is constant and is the same across the network. But
should the minimal MTU along the path decrease, it is possible to
retry a long fragment (say a Sequence of 5) with several shorter
fragments with a Sequence that was not used before (e.g., 13 and 14).
Fragment 5 is marked as abandoned and will not be retried anymore.
Note that when this mechanism is in place, it is hard to predict the
total number of fragments that will be needed or the final shape of
the bitmap that would cover the whole packet. This is why the FULL
bitmap is used when the reassembling endpoint gets the whole datagram
regardless of which fragments were actually used to do so.
Intermediate nodes will know unambiguously that the process is
complete. Note that Path MTU Discovery is out of scope for this
document.
6.3. Aborting the Transmission of a Fragmented Packet
A reset is signaled on the forward path with a pseudo fragment that
has the Fragment_Offset set to 0. The sender of a reset SHOULD also
set the Sequence and Fragment_Size field to 0.
When the fragmenting endpoint or a router on the path 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 along the path forwards the pseudo fragment in turn based
on the VRB state. If an acknowledgment is not requested, the VRB and
all associated states are destroyed.
Upon reception of the pseudo fragment, the reassembling endpoint
cleans up all resources for the packet associated with the
Datagram_Tag. If an acknowledgment is requested, the reassembling
endpoint responds with a NULL bitmap.
On the other hand, the reassembling endpoint might need to abort the
processing 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 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
reassembling endpoint SHOULD indicate so to the fragmenting endpoint
with a NULL bitmap in an RFRAG-ACK.
The RFRAG-ACK is forwarded all the way back to the source of the
packet and cleans up all resources on the path. Upon an
acknowledgment with a NULL bitmap, the fragmenting 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. The IPv6 over the TSCH mode of
IEEE 802.15.4e (6TiSCH) architecture (see Section 4.5.3 of [6TiSCH])
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 reaches
back to the source.
7. Management Considerations
This specification extends [RFC8930] and requires the same parameters
in the reassembling endpoint and on intermediate nodes. There is no
new parameter as echoing ECN is always on. These parameters
typically include the reassembly timeout at the reassembling
endpoint, an inactivity cleanup 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 fragmenting endpoint, 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 10 Kbps to several Mbps at the
Physical Layer (PHY) PHY
layer), in traffic density, and in optimizations that are desired
(e.g., the selection of a Routing Protocol for LLNs (RPL) [RFC6550]
Objective Function [RFC6552] impacts the shape of the routing graph).
For that reason, only very generic guidance can be given on the
settings of the fragmenting endpoint and on whether complex
algorithms are needed to perform congestion control or to estimate
the round-trip time. To cover the most complex use cases, this
specification enables the fragmenting endpoint to vary the fragment
size, the window size, and the inter-frame gap based on the number 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, and an implementation MUST abide by those
parameters and, in particular, never exceed the minimum and maximum
configured boundaries.
An implementation should consider the generic recommendations from
the IETF in the matter of congestion control and rate management for
IP datagrams in [RFC8085]. An implementation may perform congestion
control by using a dynamic value of the window size (Window_Size),
adapting the fragment size (Fragment_Size), and potentially reducing
the load by inserting an inter-frame gap that is longer than
necessary. In a large network where nodes contend for the bandwidth,
a larger Fragment_Size consumes less bandwidth but also reduces
fluidity and incurs higher chances of loss in transmission.
This is controlled by the following parameters:
inter-frame gap: The inter-frame gap indicates the minimum amount of
time between transmissions. The inter-frame gap controls the rate
at which fragments are sent, the ratio of air time, and the amount
of memory in intermediate nodes that a particular datagram will
use. It can be used as a flow control, a congestion control, and/
or a collision control measure. It MUST be set at a minimum to a
value that protects the propagation of one transmission against
collision with next [RFC8930]. In a wireless network that uses
the same frequency along a path, this may represent the time for a
frame to progress over multiple hops (see more in Section 4.2).
It SHOULD be augmented beyond this as necessary to protect the
network against congestion.
MinFragmentSize: The MinFragmentSize is the minimum value for the
Fragment_Size. It MUST be lower than the minimum value of
smallest 1-hop MTU that can be encountered along the path.
OptFragmentSize: The OptFragmentSize is the value for the
Fragment_Size that the fragmenting endpoint should use to start
with. It is greater than or equal to MinFragmentSize. It is less
than or equal to MaxFragmentSize. For the first fragment, it must
account for the expansion of the IPv6 addresses and of the Hop
Limit field within MTU. For all fragments, it is a balance
between the expected fluidity and the overhead of link-layer and
6LoWPAN headers. For a small MTU, the idea is to keep it close to
the maximum, whereas for larger MTUs, it might make 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 maximum value of the
smallest 1-hop MTU that can be encountered 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 byte.
Window_Size: The Window_Size MUST be at least 1 and less than 33.
* If the round-trip time is known, the Window_Size SHOULD be set
to the round-trip time divided by the time per fragment; that
is, the time to transmit a fragment plus the inter-frame gap.
Otherwise:
* A window_size of 32 indicates that only the last fragment is to
be acknowledged in each round. This is the RECOMMENDED value
in a half-duplex LLN where the fragment acknowledgment consumes
roughly the same bandwidth on the same links as the fragments
themselves.
* If it is set to a smaller value, more acks are generated. In a
full-duplex network, the load on the forward path will be
lower, and a small value of 3 SHOULD be configured.
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 minimum amount of time a node should wait for an
RFRAG Acknowledgment before it takes the next action. It MUST be
more than the maximum expected round-trip time in the respective
network.
OptARQTimeOut: The initial value of the RTO, which is the amount of
time that a fragmenting endpoint should wait for an RFRAG
Acknowledgment before it takes the next action. It is greater
than or equal to MinARQTimeOut. It is less than or equal to
MaxARQTimeOut. See Appendix C for recommendations on computing
the round-trip time. By default, a value of 3 times the maximum
expected round-trip time in the respective network is RECOMMENDED.
MaxARQTimeOut: The maximum amount of time a node should wait for the
RFRAG Acknowledgment before it takes the next action. It must
cover the longest expected round-trip time and be several times
less than the timeout that covers the recomposition buffer at the
reassembling endpoint, which is typically on the order of the
minute. An upper bound can be estimated to ensure that the
datagram is either fully transmitted or dropped before an upper
layer decides to retry it.
MaxFragRetries: The maximum number of retries for a particular
fragment. A default value of 3 is RECOMMENDED. An upper bound
can be estimated to ensure that the datagram is either fully
transmitted or dropped before an upper layer decides to retry it.
MaxDatagramRetries: The maximum number of retries from scratch for a
particular datagram. A default value of 1 is RECOMMENDED. An
upper bound can be estimated to ensure that the datagram is either
fully transmitted or dropped before an upper layer decides to
retry it.
An implementation may be capable of performing congestion control
based on ECN; see Appendix C. This is controlled by the following
parameter:
UseECN: Indicates whether the fragmenting endpoint should react to
ECN. The fragmenting endpoint may react to ECN by varying the
Window_Size between MinWindowSize and MaxWindowSize, varying the
Fragment_Size between MinFragmentSize and MaxFragmentSize, and/or
increasing or reducing the inter-frame gap. With this
specification, if UseECN is set and a fragmenting endpoint detects
a congestion, it may apply a congestion control method until the
end of the datagram, whereas if UseECN is reset, the endpoint does
not react to congestion. Future specifications may provide
additional parameters and capabilities.
7.2. Observing the Network
The management system should monitor the number of retries and ECN
settings that can be observed from the perspective of the fragmenting
endpoint with respect to the reassembling endpoint and reciprocally.
It may then tune the optimum size of Fragment_Size and of
Window_Size, OptFragmentSize, and OptWindowSize, respectively, at the
fragmenting endpoint towards a particular reassembling endpoint,
which is applicable to the next datagrams. It will preferably tune
the inter-frame gap to increase the spacing between fragments of the
same datagram and reduce the buffer bloat in the intermediate node
that holds one or more fragments of that datagram.
8. Security Considerations
This document specifies an instantiation of a 6LFF technique and
inherits from the generic description in [RFC8930]. The
considerations in the Security Considerations section of [RFC8930]
equally apply to this document.
In addition to the threats detailed therein, an attacker that is on
path can prematurely end the transmission of a datagram by sending a
RFRAG Acknowledgment to the fragmenting endpoint. It can also cause
extra transmissions of fragments by resetting bits in the RFRAG
Acknowledgment Bitmap and of RFRAG Acknowledgments by forcing the
Ack-Request bit in fragments that it forwards.
As indicated in [RFC8930], secure joining and link-layer security are
REQUIRED to protect against those attacks, as the fragmentation
protocol does not include any native security mechanisms.
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 [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 acknowledgment mechanism allows cleaning up the state
rapidly once the packet is fully transmitted or aborted.
The abstract Virtual Recovery Buffer from [RFC8930] 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 [LWIG-FRAG]
allows realigning which data goes in which fragment; this 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 keep does not include any data that would
not fit in the previous fragment.
9. IANA Considerations
This document allocates two patterns for a total of four dispatch
values for Recoverable Fragments from the "Dispatch Type Field"
registry that was created by [RFC4944] and reformatted by "IPv6 over
Low-Power Wireless Personal Area Network (6LoWPAN) Paging Dispatch"
[RFC8025].
+-------------+------+----------------------------------+-----------+
| Bit Pattern | Page | Header Type | Reference |
+-------------+------+----------------------------------+-----------+
| 11 10100x | 0 | RFRAG - Recoverable Fragment | RFC 8931 |
+-------------+------+----------------------------------+-----------+
| 11 10100x | 1-14 | Unassigned | |
+-------------+------+----------------------------------+-----------+
| 11 10100x | 15 | Reserved for Experimental Use | RFC 8025 |
+-------------+------+----------------------------------+-----------+
| 11 10101x | 0 | RFRAG-ACK - RFRAG | RFC 8931 |
| | | Acknowledgment | |
+-------------+------+----------------------------------+-----------+
| 11 10101x | 1-14 | Unassigned | |
+-------------+------+----------------------------------+-----------+
| 11 10101x | 15 | Reserved for Experimental Use | RFC 8025 |
+-------------+------+----------------------------------+-----------+
Table 1: Additional Dispatch Value Bit Patterns
10. References
10.1. Normative References
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[RFC8930] Watteyne, T., Ed., Thubert, P., Ed., and C. Bormann, "On
Forwarding 6LoWPAN (IPv6 over Low-Power Wireless Personal
Area Network) Fragments over a Multi-Hop IPv6 Network",
RFC 8930, DOI 10.17487/RFC8930, November 2020,
<https://www.rfc-editor.org/info/rfc8930>.
10.2. Informative References
[6TiSCH] Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", Work in Progress, Internet-Draft,
draft-ietf-6tisch-architecture-29, 27 August 2020,
<https://tools.ietf.org/html/draft-ietf-6tisch-
architecture-29>.
[IEEE.802.15.4]
IEEE, "IEEE Standard for Low-Rate Wireless Networks",
IEEE Standard 802.15.4-2015,
DOI 10.1109/IEEESTD.2016.7460875, April 2016,
<http://ieeexplore.ieee.org/document/7460875/>.
[Kent] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
SIGCOMM '87: Proceedings of the ACM workshop on Frontiers
in computer communications technology, pp. 390-401,
DOI 10.1145/55483.55524, August 1987,
<http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-
87-3.pdf>.
[LWIG-FRAG]
Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
lwig-6lowpan-virtual-reassembly-02, 9 March 2020,
<https://tools.ietf.org/html/draft-ietf-lwig-6lowpan-
virtual-reassembly-02>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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)",
RFC 6552, DOI 10.17487/RFC6552, March 2012,
<https://www.rfc-editor.org/info/rfc6552>.
[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>.
[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>.
[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>.
[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>.
[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>.
[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.
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 rollback.
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 are 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 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 sleep state.
Considering that RFC 4944 defines an MTU as 1280 bytes, and that in
most incarnations (except 802.15.4g) an IEEE Std 802.15.4 frame can
limit the link-layer 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.
However, doing so can add significant header overhead to each
802.15.4 frame and cause extraneous acknowledgments across the LLN
compared to the method in this specification.
Appendix B. Requirements
For one-hop communications, a number of LLN link layers propose a
local acknowledgment mechanism that is enough to detect and recover
the loss of fragments. In a multi-hop environment, an end-to-end
fragment recovery mechanism might be a good complement to a hop-by-
hop Medium Access Control (MAC) recovery. This document introduces a
simple protocol to recover individual fragments between 6LFF
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 Congestion 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 document 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 timeout. This document allows
controlling the number of outstanding fragments that have been
transmitted, but for which an acknowledgment was not yet received,
and that are still covered by the ARQ timer.
Congestion on the forward path can also be indicated by an ECN
mechanism. Though whether and how ECN [RFC3168] is carried out over
the LoWPAN is out of scope, this document provides a way for the
destination endpoint to echo an ECN indication back to the
fragmenting endpoint in an acknowledgment message as represented in
Figure 4 in Section 5.2.
While the support of echoing the ECN at the reassembling endpoint is
mandatory, this specification only provides a minimalistic behavior
on the fragmenting endpoint. If an "E" flag is received, the window
SHOULD be reduced at least by 1 and at max to 1. Halving the window
for each "E" flag received could be a good compromise, but it needs
further experimentation. A very simple implementation may just reset
the window to 1, so the fragments are sent and acknowledged one by
one.
Note that any action that has been performed upon detection of
congestion only applies for the transmission of one datagram, and the
next datagram starts with the configured Window_Size again.
The exact use of the Acknowledgment Request flag and of the window
are left to implementation. An optimistic implementation could send
all the fragments up to Window_Size, setting the Acknowledgment
Request "X" flag only on the last fragment; wait for the bitmap,
which means a gap of half a round-trip time; and resend the losses.
A pessimistic implementation could set the "X" flag on the first
fragment to check that the path works and open the window only upon
receiving the RFRAG-ACK. It could then set an "X" flag again on the
second fragment and use the window as a credit to send up to
Window_Size before it is blocked. In that case, if the RFRAG-ACK
comes back before the window starves, the gating factor is the inter-
frame gap. If the RFRAG-ACK does not arrive in time, the Window_Size
is the gating factor, and the transmission of the datagram is
delayed.
It must be noted that even though the inter-frame gap can be used as
a flow control or a congestion control measure, it also plays a
critical role in wireless collision avoidance. In particular, when a
mesh operates on the same channel over multiple hops, the forwarding
of a fragment over a certain hop may collide with the forwarding of
the next fragment that is following over a previous hop but that is
in the same interference domain. To prevent this, the fragmenting
endpoint is required to pace individual fragments within a transmit
window with an inter-frame gap. This is needed to ensure 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 [6TiSCH], which operates over the Time-Slotted
Channel Hopping (TSCH) mode of operation of IEEE 802.15.4 [RFC7554],
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.
Depending on the setting of the Window_Size and the inter-frame gap,
how the window is used, and the number of hops, the Window_Size may
or may not become the gating factor that blocks the transmission. If
the sender uses the Window_Size as a credit:
* a conservative Window_Size of, say, 3 will be the gating factor
that limits the transmission rate of the sender -- and causes
transmission gaps longer than the inter-frame gap -- as soon as
the number of hops exceeds 3 in a TSCH network and 5-9 in a single
frequency mesh. The more hops the more the starving window will
add to latency of the transmission.
* The recommendation to align the Window-Size to the round-trip time
divided by the time per fragment aligns the Window-Size to the
time it takes to get the RFAG_ACK before the window starves. A
Window-Size that is higher than that increases the chances of a
congestion but does not improve the forward throughput.
Considering that the RFRAG-ACK takes the same path as the fragment
with the assumption that it travels at roughly the same speed, an
inter-frame gap that separates fragments by 2 hops leads to a
Window_Size that is roughly the number of hops.
* Setting the Window-Size to 32 minimizes the cost of the
acknowledgment in a constrained network and frees bandwidth for
the fragments in a half-duplex network. Using it increases the
risk of congestion if a bottleneck forms, but it optimizes the use
of resources under normal conditions. When it is used, the only
protection for the network is the inter-frame gap, which must be
chosen wisely to prevent the formation of a bottleneck.
From the standpoint of a source 6LoWPAN endpoint, an outstanding
fragment is a fragment that was sent but for which no explicit
acknowledgment was yet received. This means that the fragment might
be on the path or received but not yet acknowledged, or the
acknowledgment might be on the path back. It is also possible that
either the fragment or the acknowledgment was lost on the way.
From the fragmenting endpoint 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 time between the 6LoWPAN endpoints. For the lack of a more
adapted technique, the method detailed in "Computing TCP's
Retransmission Timer" [RFC6298] may be used for that computation.
This specification provides the necessary tools for the fragmenting
endpoint to take congestion control actions and protect the network,
but it leaves the implementation free to select the action to be
taken. The intention is to use it to build experience and specify
more precisely the congestion control actions in one or more future
specifications. "Congestion Control Principles" [RFC2914] and
"Specifying New Congestion Control Algorithms" [RFC5033] provide
indications and wisdom that should help through this process.
[RFC7567] and [RFC5681] provide deeper information on why congestion
control is needed and how TCP handles it. Basically, the goal here
is to manage the number of fragments present in the network; this is
achieved by reducing the number of outstanding fragments over a
congested path by throttling the sources.
Acknowledgments
The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent
Toutain, Carles Gomez Montenegro, Thomas Watteyne, and Michael
Richardson for their in-depth reviews and comments. Also, many
thanks to Roman Danyliw, Peter Yee, Colin Perkins, Tirumaleswar
Reddy.K, Eric Éric Vyncke, Warren Kumari, Magnus Westerlund, Erik
Nordmark, and especially Benjamin Kaduk and Mirja Kuehlewind Kühlewind for their
careful reviews and help during the IETF Last Call and IESG review
process. Thanks 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 to this document.
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