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Versions: (draft-thubert-6lo-fragment-recovery)
00 01 02 03 04 05 06 07
6lo P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 4944 (if approved) July 22, 2019
Intended status: Standards Track
Expires: January 23, 2020
6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-05
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 January 23, 2020.
Copyright Notice
Copyright (c) 2019 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. 6LoWPAN Acronyms . . . . . . . . . . . . . . . . . . . . 4
2.4. Referenced Work . . . . . . . . . . . . . . . . . . . . . 4
2.5. 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 . . . . . . . . . . . . . . 8
5. New Dispatch types and headers . . . . . . . . . . . . . . . 8
5.1. Recoverable Fragment Dispatch type and Header . . . . . . 9
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 11
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12
6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14
6.1.1. Upon the first fragment . . . . . . . . . . . . . . . 15
6.1.2. Upon the next fragments . . . . . . . . . . . . . . . 15
6.2. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 16
6.3. Aborting the Transmission of a Fragmented Packet . . . . 16
7. Management Considerations . . . . . . . . . . . . . . . . . . 17
7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 17
7.2. Observing the network . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 23
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 24
Appendix C. Considerations On Flow Control . . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
In most Low Power and Lossy Network (LLN) applications, the bulk of
the traffic consists of small chunks of data (in the order few bytes
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
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chunk of data is transferred to the LLN node, whereas in the latter,
the large chunk flows away from the LLN node. In both cases, the
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 to
reassemble 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 to use a hop-
by-hop fragment forwarding technique to alleviate those undesirable
effects. "LLN Minimal Fragment Forwarding"
[I-D.ietf-6lo-minimal-fragment] proposes such a technique, in a
fashion 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 ina 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, pages 6
and 7. [RFC4944] as 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 can not 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
is implemented, clean up the related state in the network. It is
also lacking flow control capabilities to avoid participating to a
congestion that may in turn cause the loss of a fragment and
potentially the retransmission of the full datagram.
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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]
2.3. 6LoWPAN Acronyms
This document uses the following acronyms:
6BBR: 6LoWPAN Backbone Router
6LBR: 6LoWPAN Border Router
6LN: 6LoWPAN Node
6LR: 6LoWPAN Router
LLN: Low-Power and Lossy Network
2.4. Referenced Work
Past experience with fragmentation has shown that misassociated or
lost fragments can lead to poor network behavior and, occasionally,
trouble at 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.
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Specifically in the case of UDP, valuable additional information can
be found in "UDP Usage Guidelines for Application Designers"
[RFC8085].
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'. 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.
"LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment]
introduces the concept of a Virtual Reassembly Buffer (VRB) and an
associated technique to forward fragments as they come, using the
datagram_tag as a label in a fashion similar to MPLS. This
specification reuses that technique with slightly modified controls.
2.5. 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.
fragment_offset: The offset of a particular fragment of a datagram
in its Compressed Form. The fragment_offset is expressed in a
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unit that depends on the MAC layer technology and is by default a
byte.
datagram_tag: An identifier of a datagram that is locally unique to
the Layer-2 sender. Associated with the MAC address of the
sender, this becomes a globally unique identifier for the
datagram.
RFRAG: Recoverable Fragment
RFRAG-ACK: Recoverable Fragment Acknowledgement
RFRAG Acknowledgment Request: An RFRAG with the Acknowledgement
Request flag ('X' flag) set.
All 0's: Refers to a bitmap with all bits set to zero.
All 1's: Refers to a bitmap with all bits set to one.
3. Updating RFC 4944
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 fragment is introduced and new
dispatch types are defined in Section 5.
[RFC8138] allows to modify the size of a packet en-route by removing
the consumed hops in a compressed Routing Header. It results that
fragment_offset and datagram_size (see Section 2.5) must also be
modified en-route, whcih is difficult to do in the uncompressed form.
This specification expresses those fields in the Compressed Form and
allows to modify them en-route (see Section 4.3) easily.
Note that consistently with 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 extends the fragment forwarding mechanism
specified in "LLN Minimal Fragment Forwarding"
[I-D.ietf-6lo-minimal-fragment] by providing additional operations to
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improve the management of the Virtual Reassembly Buffer (VRB) in the
context of recoverable fragments.
4.1. Slack in the First Fragment
At the time of this writing, [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 to happen 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 Inter-Frame Gap, which is
a configurable interval of time between transmissions to a same next
hop. In the case of half duplex interfaces, this InterFrameGap
ensures that the next hop has progressed the previous frame and is
capable of receiving the next one.
In the case of a mesh operating at a single frequency with
omnidirectional antennas, a larger InterFrameGap is required 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.
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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.
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].
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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 on the Compressed Form of the
packet form as opposed to the uncompressed - native - packet form.
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.
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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.
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: 16 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.
+ 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:
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+ 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 as along the path
indicated by the IPv6 header (possibly including a routing
header).
If the fragment cannot be forwarded or routed, then an abort
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 end point 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 that 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
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The RFRAG Acknowledgment Bitmap is included in a 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 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.
All 0's is a NULL bitmap that indicates that the fragmentation
process is aborted. All 1's is a FULL bitmap that indicates that
the fragmentation process is complete, all fragments were received
at the reassembly end point.
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).
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The 6LoWPAN endpoint that fragments the packets at 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 the Window-Size. It is
configurable and may vary in case of ECN notification. When the
6LoWPAN endpoint that reassembles the packets at 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 SHOULD be set on the last fragment to protect the datagram,
and it MAY be set in any intermediate fragment for the purpose of
flow control. This ARQ process MUST be protected by a timer, and the
fragment that carries the 'X' flag MAY be retried upon time out a
configurable amount 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 RFRAG Acknowledgment can optionally carry an ECN indication for
flow control (see Appendix C). The receiver of a fragment with the
'E' (ECN) flag set MUST echo that information by setting the 'E'
(ECN) flag in the next RFRAG Acknowledgment.
The sender transfers a controlled number of fragments and MAY flag
the 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.
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 that the reassembling endpoint
aborted the process of an individual datagram.
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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 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
"UDP Usage Guidelines" [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
wait a reasonable amount of time between fragments 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]
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. In more details, the first fragment carries the IP header and it
is routed all the way from the fragmenting end point to the
reassembling end point. Upon 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, that is swapped at 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. Upon the first fragment
In Route-Over mode, the source and destination MAC addressed in a
frame change at each hop. The label that is formed and placed in the
datagram_tag is associated to the source MAC and only valid (and
unique) for that source MAC. Upon a first fragment (i.e. with a
sequence of zero), a VRB and the associated LSP state are created 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]. 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 to match 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.
6.1.2. Upon the next fragments
Upon a next fragment (i.e. with a non-zero sequence), the router
looks up a LSP indexed by the tuple (MAC address, datagram_tag) found
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:
o The source and destination MAC addresses are swapped from those
found in the fragment
o The datagram_tag set to the datagram_tag found in the fragment
o A NULL bitmap is used to signal the abort condition
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.
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6.2. Upon the RFRAG Acknowledgments
Upon 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.
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 to the datagram_tag. If an
acknowledgment is requested, the receiver responds with a NULL
bitmap.
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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, or considers that this packet is already fully
reassembled and passed to the upper layer. In that case, the
receiver SHOULD indicate so to the sender with a NULL bitmap in a
RFRAG Acknowledgment. Upon an acknowledgment with a NULL bitmap, the
sender endpoint MUST abort the transmission of the fragmented
datagram.
7. Management Considerations
7.1. Protocol Parameters
There is no particular configuration on the receiver, as echoing ECN
is always on. The configuration only applies to the sender, which is
in control of the transmission. The management system SHOULD be
capable of providing the parameters below:
MinFragmentSize: The MinFragmentSize is the minimum value for the
Fragment_Size.
OptFragmentSize: The MinFragmentSize is the value for the
Fragment_Size that the sender should use to start with.
MaxFragmentSize: The MaxFragmentSize is the maximum value for the
Fragment_Size. It MUST be lower than the minimum MTU along the
path. A large value augments the chances of buffer bloat and
transmission loss. The value MUST be less than 512 if the unit
that is defined for the PHY layer is the octet.
UseECN: Indicates whether the sender should react to ECN. When the
sender reacts to ECN the Window_Size will vary between
MinWindowSize and MaxWindowSize.
MinWindowSize: The minimum value of Window_Size that the sender can
use.
OptWindowSize: The OptWindowSize is the value for the Window_Size
that the sender should use to start with.
MaxWindowSize: The maximum value of Window_Size that the sender can
use. The value MUSt be less than 32.
InterFrameGap: Indicates a minimum amount of time between
transmissions. All packets to a same destination, and in
particular fragments, may be subject to receive while
transmitting and hidden terminal collisions with the next or
the previous transmission as the fragments progress along a
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same path. The InterFrameGap protects the propagation of one
transmission before the next one is triggered and creates a
duty cycle that controls the ratio of air time and memory in
intermediate nodes that a particular datagram will use.
MinARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action.
OptARQTimeOut: The starting point of the value of the amount that a
sender should wait for an RFRAG Acknowledgment before it takes
a next action.
MaxARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action.
MaxFragRetries: The maximum number of retries for a particular
Fragment.
MaxDatagramRetries: The maximum number of retries from scratch for a
particular Datagram.
7.2. Observing the network
The management system should monitor the amount of retries and of ECN
settings that can be observed from the perspective of the both the
sender and the receiver, and may tune the optimum size of
Fragment_Size and of the Window_Size, OptWindowSize and OptWindowSize
respectively, at the sender. The values should be bounded by the
expected number of hops and reduced beyond that when the number of
datagrams that can traverse an intermediate point may exceed its
capacity and cause a congestion loss. The InterFrameGap is another
tool that can be used to increase the spacing between fragments of a
same datagram and reduce the ratio of time when a particular
intermediate node holds a fragment of that datagram.
8. Security Considerations
The considerations in the Security section of [I-D.ietf-core-cocoa]
apply equally to this specification.
The process of recovering fragments does not appear to create any
opening for new threat compared to "Transmission of IPv6 Packets over
IEEE 802.15.4 Networks" [RFC4944].
The technique of Virtual Recovery Buffers 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. Note that as opposed to
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the VRB described in [I-D.ietf-lwig-6lowpan-virtual-reassembly] 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 4 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 values (to be confirmed by IANA) are indicated in
Table 1.
+-------------+------+----------------------------------+-----------+
| Bit Pattern | Page | Header Type | Reference |
+-------------+------+----------------------------------+-----------+
| 11 10100x | 0 | RFRAG - Recoverable Fragment | RFC THIS |
| 11 10101x | 0 | RFRAG-ACK - RFRAG Acknowledgment | RFC THIS |
+-------------+------+----------------------------------+-----------+
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
Jonathan Hui, Jay Werb, Christos Polyzois, Soumitri Kolavennu, Pat
Kinney, Margaret Wasserman, Richard Kelsey, Carsten Bormann and Harry
Courtice for their various contributions.
11. References
11.1. Normative References
[I-D.ietf-6lo-minimal-fragment]
Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
Fragment Forwarding", draft-ietf-6lo-minimal-fragment-02
(work in progress), June 2019.
[I-D.ietf-lwig-6lowpan-virtual-reassembly]
Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-01
(work in progress), March 2019.
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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>.
11.2. Informative References
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-24 (work
in progress), July 2019.
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-03 (work in progress), February 2018.
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[I-D.ietf-intarea-frag-fragile]
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", draft-
ietf-intarea-frag-fragile-15 (work in progress), July
2019.
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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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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 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
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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.
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 a 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 data fragment.
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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 to control
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
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
an end-to-end flow control, but leaves it to the sender stack to pace
individual fragments within a transmit window, so that a given
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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 [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
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
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Thubert Expires January 23, 2020 [Page 26]
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