6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8931.
|
|
|---|---|---|---|
| Author | Pascal Thubert | ||
| Last updated | 2019-05-20 | ||
| Replaces | draft-thubert-6lo-fragment-recovery | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-6lo-fragment-recovery-03
6lo P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 4944 (if approved) May 20, 2019
Intended status: Standards Track
Expires: November 21, 2019
6LoWPAN Selective Fragment Recovery
draft-ietf-6lo-fragment-recovery-03
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 November 21, 2019.
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 . . . . . . . . . . . . . . . . . . . . . . 5
4. Updating draft-ietf-6lo-minimal-fragment . . . . . . . . . . 6
4.1. Slack in the First Fragment . . . . . . . . . . . . . . . 6
4.2. Gap between frames . . . . . . . . . . . . . . . . . . . 6
4.3. Modifying the First Fragment . . . . . . . . . . . . . . 7
5. New Dispatch types and headers . . . . . . . . . . . . . . . 7
5.1. Recoverable Fragment Dispatch type and Header . . . . . . 8
5.2. RFRAG Acknowledgment Dispatch type and Header . . . . . . 10
6. Fragments Recovery . . . . . . . . . . . . . . . . . . . . . 12
6.1. Forwarding Fragments . . . . . . . . . . . . . . . . . . 14
6.1.1. Upon the first fragment . . . . . . . . . . . . . . . 14
6.1.2. Upon the next fragments . . . . . . . . . . . . . . . 14
6.2. Upon the RFRAG Acknowledgments . . . . . . . . . . . . . 15
6.3. Cancelling a Fragmented Packet . . . . . . . . . . . . . 15
7. Management Considerations . . . . . . . . . . . . . . . . . . 16
7.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 16
7.2. Observing the network . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 21
Appendix B. Requirements . . . . . . . . . . . . . . . . . . . . 22
Appendix C. Considerations On Flow Control . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
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 74 bytes or more in all cases,
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 a firmware upgrades of the LLN nodes or an
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 10Kbytes 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 issues of
resources locked and wasted transmissions due to the loss of a
fragment. [RFC4944] does not define a mechanism to first discover a
fragment loss, and then to recover that loss. With RFC 4944, the
forwarding of a whole datagram fails when one fragment is not
delivered properly to the destination 6LoWPAN endpoint. Constrained
memory resources are blocked on the receiver until the receiver times
out.
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 trigger
the retransmission of the full datagram.
This specification proposes a method to forward fragments across a
multi-hop route-over mesh, and to recover individual fragments
between LLN endpoints. The method is designed to limit congestion
loss in the network and addresses the requirements that are detailed
in Appendix B.
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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 the following documents:
o "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 miss-associated 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.
Specifically in the case of UDP, valuable additional information can
be found in "UDP Usage Guidelines for Application Designers"
[RFC8085].
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Readers are expected to be familiar with all the terms and concepts
that are discussed in "IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944].
"The Benefits of Using Explicit Congestion Notification (ECN)"
[RFC8087] provides useful information on the potential benefits and
pitfalls of using ECN.
Quoting the "Multiprotocol Label Switching (MPLS) Architecture"
[RFC3031]: with MPLS, "packets are "labeled" before they are
forwarded. 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 MLPS. 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.
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 introduces 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
the fragment_offset and datagram_size cannot be signaled in the
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uncompressed form. This specification expresses those fields in the
compressed form and allows to modify them en-route (see Section 4.3.
Note that consistently with in Section 2 of [RFC6282] for the
fragmentation mechanism described in Section 5.3 of [RFC4944], any
header that cannot fit within the first fragment MUST NOT be
compressed when using the fragmentation mechanism described in this
specification.
4. Updating draft-ietf-6lo-minimal-fragment
This specification updates the fragment forwarding mechanism
specified in "LLN Minimal Fragment Forwarding"
[I-D.ietf-6lo-minimal-fragment] by providing additional operations to
improve the management of the Virtual Reassembly Buffer (VRB).
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 the fragment number. This means that
the fragments that contain a 6LoWPAN-compressed header MUST have
enough slack to enable a less efficient compression in the next hops
that still fits in one MAC frame. For instance, if the IID of the
source IPv6 address is elided by the originator, then it MUST compute
the fragment_size as if the MTU was 8 bytes less. This way, the next
hop can restore the source IID to the first fragment without
impacting the second fragment.
4.2. 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.
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In the case of a mesh operating at a single frequency with
omnidirectional antennas, a larger InterFrameGap is required protect
the frame against hidden terminal collisions with the previous frame
of a same flow that is still progressing alon a common path.
The Inter-Frame Gap is useful even for unfragmented datagrams, but it
becomes a necessity for fragments that are typically generated in a
fast sequence and are all sent over the exact same path.
4.3. Modifying the First Fragment
The compression of the Hop Limit, of the source and destination
addresses, and of the Routing Header may change en-route in a Route-
Over mesh LLN. If the size of the first fragment is modified, then
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 in
order to forward individual fragments across a 6LoWPAN route-over
mesh. The Datagram_tag is used as a label; it is locally unique to
the node that is the source MAC address of the fragment, so together
the 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 selected 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 4 new Dispatch
types, for Recoverable Fragment (RFRAG) headers with or without
Acknowledgment Request (RFRAG vs. RFRAG-ARQ), and for the RFRAG
Acknowledgment back, with or without ECN Echo (RFRAG-ACK vs. RFRAG-
ECHO).
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(to be confirmed by IANA) The new 6LoWPAN Dispatch types use the
Value Bit Pattern of 11 1010xx from page 0 [RFC8025], as follows:
Pattern Header Type
+------------+------------------------------------------+
| 11 10100x | RFRAG - Recoverable Fragment |
| 11 10101x | RFRAG-ACK - RFRAG Acknowledgment |
+------------+------------------------------------------+
Figure 1: Additional Dispatch Value Bit Patterns
In the following sections, a "Datagram_tag" extends the semantics
defined in [RFC4944] Section 5.3."Fragmentation Type and Header".
The Datagram_tag is a locally unique identifier for the datagram from
the perspective of the sender. This means that the datagram-tag
identifies a datagram uniquely in the network when associated with
the source of the datagram. As the datagram gets forwarded, the
source changes and the Datagram_tag must be swapped as detailed in
[I-D.ietf-6lo-minimal-fragment].
5.1. Recoverable Fragment Dispatch type and Header
In this specification, the size and offset of the fragments are
expressed on the compressed packet form as opposed to the
uncompressed - native - packet form.
The format of the fragment header is the same for all fragments. The
format indicates both a length and an offset, which seem be redundant
with the sequence field, but is not. The position of a fragment in
the recomposition buffer is neither correlated with the value of the
sequence field nor with the order in which the fragments are
received. This enables out-of-sequence and overlapping fragments,
e.g., a fragment 5 that is retried as smaller fragments 5, 13 and 14
due to a change of MTU.
There is no requirement on the receiver to check for contiguity of
the received fragments, and the sender MUST ensure that when all
fragments are acknowledged, then the datagram is fully received.
This may be useful in particular in the case where the MTU changes
and a fragment sequence is retried with a smaller fragment_size, the
remainder of the original fragment being retried with new sequence
values.
The first fragment is recognized by a sequence of 0; it carries its
fragment_size and the datagram_size of the compressed packet, whereas
the other fragments carry their fragment_size and fragment_offset.
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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.
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 2: RFRAG Dispatch type and Header
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. By default, that
unit is the octet which allows fragments up to 512 bytes. For
IEEE Std. 802.15.4, the unit is octet, and the maximum fragment
size, when it is constrained by the maximum frame size of 128
octet minus the overheads of the MAC and Fragment Headers, is not
limited by this encoding.
X: 1 bit; Ack-Request: when set, the sender requires an RFRAG
Acknowledgment from the receiver.
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 recomposition 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
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reception and reassembly operations. The fragment may
stored for local recomposition, or it may be routed based on
the destination IPv6 address, in which case a VRB state must
be installed as described in Section 6.1.1.
+ When the Sequence is not 0, this field indicates the offset
of the fragment in the compressed form. The fragment may be
added to a local recomposition buffer or forwarded based on
an existing VRB as described in Section 6.1.2.
* A Fragment_offset that is set to a value of 0 indicates an
abort condition and all state regarding the datagram should be
cleaned up once the processing of the fragment is complete; the
processing of the fragment depends on whether there is a VRB
already established for this datagram, and the next hop is
still reachable:
+ if a VRB already exists and is not broken, the fragment is
to be forwarded along the associated Label Switched Path
(LSP) as described in Section 6.1.2, but regardless of the
value of the Sequence field;
+ else, if the Sequence is 0, then the fragment is to be
routed as described in Section 6.1.1 but no state is
conserved afterwards. In that case, the session if it
exists is aborted and the packet is also forwarded in an
attempt to clean up the next hops 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.
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.
The offset of the bit in the bitmap indicates which fragment is
acknowledged as follows:
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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 3: RFRAG Acknowledgment bitmap encoding
Figure 4 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 4: Example RFRAG Acknowledgment Bitmap
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 5: 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
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An RFRAG Acknowledgment Bitmap, whereby setting the bit at offset
x indicates that fragment x was received, as shown in Figure 3.
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 headers RFRAG and RFRAG-ARQ are used to
transport a fragment and optionally request an RFRAG Acknowledgment
that will confirm the good reception of a one or more fragments. An
RFRAG Acknowledgment is carried as a standalone header in a message
that is sent back to the 6LoWPAN endpoint that was the source of the
fragments, as known by its MAC address. The process ensures that at
every hop, the source MAC address and the Datagram_tag in the
received fragment are enough information to send the RFRAG
Acknowledgment back towards the source 6LoWPAN endpoint by reversing
the MPLS operation.
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. Te maximum number
of outstanding fragments is the Window-Size. It is configurable and
may vary in case of ECN notification. When it receives a fragment
with the Ack-Request flag set, the 6LoWPAN endpoint that reassembles
the packets at 6LoWPAN level (the receiver) MUST send back an RFRAG
Acknowledgment to confirm reception of all the fragments it has
received so far.
The Ack-Request bit marks the end of a window. It SHOULD be set on
the last fragment to protect the datagram, and MAY be used in
intermediate fragments for the purpose of flow control. This ARQ
process MUST be protected by a ARQ timer, and the fragment that
carries the Ack-Request flag MAY be retried upon time out a
configurable amount of times. Upon exhaustion of the retries the
sender may either abort the transmission of the datagram or retry the
datagram from the first fragment with an Ack-Request in order to
reestablish a path and discover which fragments were received over
the old path. When the sender of the fragment knows that an
underlying link-layer mechanism protects the fragments, it may
refrain from using the RFRAG Acknowledgment mechanism, and never set
the Ack-Request bit.
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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 series 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. Delaying the acknowledgment might defeat the round trip
delay computation so it 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
has canceled the process of an individual datagram. 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]
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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,
alternate routes not possible for individual fragments. 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.
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].
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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.
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).
6.3. Cancelling a Fragmented Packet
A reset is signaled on the forward path with a pseudo fragment that
has the fragment_offset, sequence and fragment_size all set to 0, and
no data.
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When the sender or a router on the way decides that a packet should
be dropped and the fragmentation process canceled, it generates a
reset pseudo fragment and forwards it down the fragment path.
Each router next along the path the way forwards the pseudo fragment
based on the VRB state. If an acknowledgment is not requested, the
VRB and all associated state are destroyed.
Upon reception of the pseudo fragment, the receiver cleans up all
resources for the packet associated to the Datagram_tag. If an
acknowledgment is requested, the receiver responds with a NULL
bitmap.
The other way around, the receiver might need to cancel the process
of a fragmented packet for internal reasons, for instance if it is
out of reassembly buffers, or considers that this packet is already
fully reassembled and passed to the upper layer. In that case, the
receiver SHOULD indicate so to the sender with a NULL bitmap in a
RFRAG Acknowledgment. Upon an acknowledgment with a NULL bitmap, the
sender endpoint MUST abort the transmission of the fragmented
datagram.
7. Management Considerations
7.1. Protocol Parameters
There is no particular configuration on the receiver, as echoing ECN
should always be on. The configuration only applies to the sender
that is in control of the transmission. The management system SHOULD
be capable of providing the parameters below:
MinFragmentSize: The MinFragmentSize is the minimum value for the
Fragment_Size.
OptFragmentSize: The MinFragmentSize is the value for the
Fragment_Size that the sender should use to start with.
MaxFragmentSize: The MaxFragmentSize is the maximum value for the
Fragment_Size. It MUST be lower than the minimum MTU along the
path. A large value augments the chances of buffer bloat and
transmission loss. The value MUST be less than 512 if the unit
that is defined for the PHY layer is the octet.
UseECN: Indicates whether the sender should react to ECN. When the
sender reacts to ECN the Window_Size will vary between
MinWindowSize and MaxWindowSize.
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MinWindowSize: The minimum value of Window_Size that the sender can
use.
OptWindowSize: The OptWindowSize is the value for the Window_Size
that the sender should use to start with.
MaxWindowSize: The maximum value of Window_Size that the sender can
use. The value MUSt be less than 32.
UseECN: Indicates whether the sender should react to ECN. When the
sender reacts to ECN the sender SHOULD adapt the Window_Size
between MinWindowSize and MaxWindowSize and it MAY adapt the
Fragment_Size if that is supported.
InterFrameGap: Indicates a minimum amount of time between
transmissions. All packets to a same destination, and in
particular fragments, may be subject to receive while
transmitting and hidden terminal collisions with the next or
the previous transmission as the fragments progress along a
same path. The InterFrameGap protects the propagation of one
transmission before the next one is triggered and creates a
duty cycle that controls the ratio of air time and memory in
intermediate nodes that a particular datagram will use.
MinARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action.
OptARQTimeOut: The starting point of the value of the amount that a
sender should wait for an RFRAG Acknowledgment before it takes
a next action.
MaxARQTimeOut: The maximum amount of time a node should wait for an
RFRAG Acknowledgment before it takes a next action.
MaxFragRetries: The maximum number of retries for a particular
Fragment.
MaxDatagramRetries: The maximum number of retries from scratch for a
particular Datagram.
7.2. Observing the network
The management system should monitor the amount of retries and of ECN
settings that can be observed from the perspective of the both the
sender and the receiver, and may tune the optimum size of
Fragment_Size and of the Window_Size, OptWindowSize and OptWindowSize
respectively, at the sender. The values should be bounded by the
expected number of hops and reduced beyond that when the number of
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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 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].
9. IANA Considerations
Need extensions for formats defined in "Transmission of IPv6 Packets
over IEEE 802.15.4 Networks" [RFC4944].
10. Acknowledgments
The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent
Toutain, 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-01
(work in progress), March 2019.
[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>.
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[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-20 (work
in progress), March 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/>.
[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>.
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[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>.
[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>.
[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>.
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[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>.
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.
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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
are resent, further contributing to the congestion that caused the
initial loss, and potentially leading to congestion collapse.
This saturation may lead to excessive radio interference, or random
early discard (leaky bucket) in relaying nodes. Additional queuing
and memory congestion may result while waiting for a low power next
hop to emerge from its sleeping state.
Considering that RFC 4944 defines an MTU is 1280 bytes and that in
most incarnations (but 802.15.4g) a IEEE Std. 802.15.4 frame can
limit the MAC payload to as few as 74 bytes, a packet might be
fragmented into at least 18 fragments at the 6LoWPAN shim layer.
Taking into account the worst-case header overhead for 6LoWPAN
Fragmentation and Mesh Addressing headers will increase the number of
required fragments to around 32. This level of fragmentation is much
higher than that traditionally experienced over the Internet with
IPv4 fragments. At the same time, the use of radios increases the
probability of transmission loss and Mesh-Under techniques compound
that risk over multiple hops.
Mechanisms such as TCP or application-layer segmentation could be
used to support end-to-end reliable transport. One option to support
bulk data transfer over a frame-size-constrained LLN is to set the
Maximum Segment Size to fit within the link maximum frame size.
Doing so, however, can add significant header overhead to each
802.15.4 frame. In addition, deploying such a mechanism requires
that the end-to-end transport is aware of the delivery properties of
the underlying LLN, which is a layer violation, and difficult to
achieve from the far end of the IPv6 network.
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:
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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.
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
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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 5 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
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.
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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
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