< draft-ietf-6lo-minimal-fragment-04.txt   draft-ietf-6lo-minimal-fragment-15.txt >
6lo T. Watteyne, Ed. 6lo T. Watteyne, Ed.
Internet-Draft Analog Devices Internet-Draft Analog Devices
Intended status: Informational C. Bormann Intended status: Standards Track P. Thubert, Ed.
Expires: March 2, 2020 Universitaet Bremen TZI Expires: 24 September 2020 Cisco Systems
P. Thubert C. Bormann
Cisco Universitaet Bremen TZI
August 30, 2019 23 March 2020
6LoWPAN Fragment Forwarding On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network
draft-ietf-6lo-minimal-fragment-04 draft-ietf-6lo-minimal-fragment-15
Abstract Abstract
This document provides a simple method to forwarding 6LoWPAN This document provides generic rules to enable the forwarding of
fragments. When employing adaptation layer fragmentation in 6LoWPAN, 6LoWPAN fragment over a route-over network. Forwarding fragments can
it may be beneficial for a forwarder not to have to reassemble each improve both the end-to-end latency and reliability, and reduce the
packet in its entirety before forwarding it. This has always been buffer requirements in intermediate nodes; it may be implemented
possible with the original fragmentation design of RFC4944. This using RFC 4944 and virtual reassembly buffers.
method reduces the latency and increases end-to-end reliability in
route-over forwarding. It is the companion to the virtual Reassembly
Buffer which is a pure implementation technique.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 2, 2020. This Internet-Draft will expire on 24 September 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Limits of Per-Hop Fragmentation and Reassembly . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Memory Management and Reliability . . . . . . . . . . . . 4 2.2. Referenced Work . . . . . . . . . . . . . . . . . . . . . 3
3. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 5 2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 3. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 4. Limitations of Per-Hop Fragmentation and Reassembly . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Informative References . . . . . . . . . . . . . . . . . . . 7 4.2. Memory Management and Reliability . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 5. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 7
6. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. Normative References . . . . . . . . . . . . . . . . . . . . 11
11. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Overview of 6LoWPAN Fragmentation 1. Introduction
The original 6LoWPAN fragmentation is defined in [RFC4944] and it is The original 6LoWPAN fragmentation is defined in [RFC4944] for use
implicitly defined for use over a single IP hop though possibly over a single Layer 3 hop, though possibly multiple Layer 2 hops in a
multiple Layer-2 hops in a meshed 6LoWPAN Network. Although mesh-under network, and was not modified by the [RFC6282] update.
[RFC6282] updates [RFC4944], it does not redefine 6LoWPAN 6LoWPAN operations including fragmentation depend on a Link-Layer
fragmentation. security that prevents any rogue access to the network.
In a route-over 6LoWPAN network, an IP packet is expected to be
reassembled at each intermediate hop, uncompressed, pushed to Layer 3
to be routed, and then compressed and fragmented again. This draft
introduces an alternate approach called 6LoWPAN Fragment Forwarding
(6FF) whereby an intermediate node forwards a fragment (or the bulk
thereof, MTU permitting) without reassembling if the next hop is a
similar 6LoWPAN link. The routing decision is made on the first
fragment of the datagram, which has the IPv6 routing information.
The first fragment is forwarded immediately and a state is stored to
enable forwarding the next fragments along the same path.
Done right, 6LoWPAN Fragment Forwarding techniques lead to more
streamlined operations, less buffer bloat and lower latency. But it
may be wasteful when fragments are missing, leading to locked
resources and low throughput, and it may be misused to the point that
the end-to-end latency of one packet falls behind that of per-hop
reassembly.
This specification provides a generic overview of 6FF, discusses
advantages and caveats, and introduces a particular 6LoWPAN Fragment
Forwarding technique called Virtual Reassembly Buffer that can be
used while retaining the message formats defined in [RFC4944]. Basic
recommendations such as the insertion of an inter-frame gap between
fragments are provided to avoid the most typical caveats.
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. Referenced Work
Past experience with fragmentation, e.g., as described in "IPv4
Reassembly Errors at High Data Rates" [RFC4963] and references
therein, has shown that mis-associated or lost fragments can lead to
poor network behavior and, occasionally, trouble at the application
layer. That experience led to the definition of the "Path MTU
discovery" [RFC8201] (PMTUD) protocol that limits fragmentation over
the Internet.
"IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security
threats that are linked to using IP fragmentation. The 6LoWPAN
fragmentation takes place underneath the IP Layer, but some issues
described there may still apply to 6LoWPAN fragments (as discussed in
further details in Section 7).
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].
"Multiprotocol Label Switching (MPLS) Architecture" [RFC3031] says
that with MPLS, 'packets are "labeled" before they are forwarded.'
It goes on to say, "At subsequent hops, there is no further analysis
of the packet's network layer header. Rather, the label is used as
an index into a table which specifies the next hop, and a new label".
The MPLS technique is leveraged in the present specification to
forward fragments that actually do not have a network layer header,
since the fragmentation occurs below IP.
2.3. New Terms
This specification uses the following terms:
6LoWPAN Fragment Forwarding endpoints: The 6FF endpoints are the
first and last nodes in an unbroken string of 6LoWPAN Fragment
Forwarding nodes. They are also the only points where the
fragmentation and reassembly operations 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.
Datagram_Tag: An identifier of a datagram that is locally unique to
the Layer 2 sender. Associated with the Link-Layer address of the
sender, this becomes a globally unique identifier for the datagram
within the duration of its transmission.
Fragment_Offset: The offset of a fragment of a datagram in its
Compressed Form.
3. Overview of 6LoWPAN Fragmentation
We use Figure 1 to illustrate 6LoWPAN fragmentation. We assume node We use Figure 1 to illustrate 6LoWPAN fragmentation. We assume node
A forwards a packet to node B, possibly as part of a multi-hop route A forwards a packet to node B, possibly as part of a multi-hop route
between IPv6 source and destination nodes which are neither A nor B. between 6LoWPAN Fragment Forwarding endpoints which may be neither A
nor B, though 6LoWPAN may compress the IP header better when they are
both the 6FF and the 6LoWPAN compression endpoints.
+---+ +---+ +---+ +---+
... ---| A |-------------------->| B |--- ... ... ---| A |-------------------->| B |--- ...
+---+ +---+ +---+ +---+
# (frag. 5) # (frag. 5)
123456789 123456789 123456789 123456789
+---------+ +---------+ +---------+ +---------+
| # ###| |### # | | # ###| |### # |
+---------+ +---------+ +---------+ +---------+
outgoing incoming outgoing incoming
fragmentation reassembly fragmentation reassembly
buffer buffer buffer buffer
Figure 1: Fragmentation at node A, reassembly at node B. Figure 1: Fragmentation at node A, reassembly at node B.
Node A starts by compacting the IPv6 packet using the header Typically, Node A starts with an uncompressed packet and compacts the
compression mechanism defined in [RFC6282]. If the resulting 6LoWPAN IPv6 packet using the header compression mechanism defined in
packet does not fit into a single link-layer frame, node A's 6LoWPAN [RFC6282]. If the resulting 6LoWPAN packet does not fit into a
sublayer cuts it into multiple 6LoWPAN fragments, which it transmits single Link-Layer frame, node A's 6LoWPAN sublayer cuts it into
as separate link-layer frames to node B. Node B's 6LoWPAN sublayer multiple 6LoWPAN fragments, which it transmits as separate Link-Layer
reassembles these fragments, inflates the compressed header fields frames to node B. Node B's 6LoWPAN sublayer reassembles these
back to the original IPv6 header, and hands over the full IPv6 packet fragments, inflates the compressed header fields back to the original
to its IPv6 layer. IPv6 header, and hands over the full IPv6 packet to its IPv6 layer.
In Figure 1, a packet forwarded by node A to node B is cut into nine In Figure 1, a packet forwarded by node A to node B is cut into nine
fragments, numbered 1 to 9. Each fragment is represented by the '#' fragments, numbered 1 to 9 as follows:
symbol. Node A has sent fragments 1, 2, 3, 5, 6 to node B. Node B
has received fragments 1, 2, 3, 6 from node A. Fragment 5 is still * Each fragment is represented by the '#' symbol.
being transmitted at the link layer from node A to node B.
* Node A has sent fragments 1, 2, 3, 5, 6 to node B.
* Node B has received fragments 1, 2, 3, 6 from node A.
* Fragment 5 is still being transmitted at the link layer from node
A to node B.
The reassembly buffer for 6LoWPAN is indexed in node B by: The reassembly buffer for 6LoWPAN is indexed in node B by:
o a unique Identifier of Node A (e.g., Node A's link-layer address) * a unique Identifier of Node A (e.g., Node A's Link-Layer address)
o the datagram_tag chosen by node A for this fragmented datagram
Because it may be hard for node B to correlate all possible link- * the Datagram_Tag chosen by node A for this fragmented datagram
layer addresses that node A may use (e.g., short vs. long addresses),
node A must use the same link-layer address to send all the fragments
of a same datagram to node B.
Conceptually, the reassembly buffer in node B contains, assuming that Because it may be hard for node B to correlate all possible Link-
node B is neither the source nor the final destination: Layer addresses that node A may use (e.g., short vs. long addresses),
node A must use the same Link-Layer address to send all the fragments
of the same datagram to node B.
o a datagram_tag as received in the incoming fragments, associated Conceptually, the reassembly buffer in node B contains:
to link-layer address of node A for which the received
datagram_tag is unique, * a Datagram_Tag as received in the incoming fragments, associated
o the link-layer address that node B uses to forward the fragments to the interface and the Link-Layer address of node A for which
o the link-layer address of the next hop that is resolved on the the received Datagram_Tag is unique,
first fragment
o a datagram_tag that node B uniquely allocated for this datagram * the actual packet data from the fragments received so far, in a
and that is used when forwarding the fragments of the datagram
o the actual packet data from the fragments received so far, in a
form that makes it possible to detect when the whole packet has form that makes it possible to detect when the whole packet has
been received and can be processed or forwarded, been received and can be processed or forwarded,
o a datagram_size,
o a buffer for the remainder of a previous fragment left to be sent, * a state indicating the fragments already received,
o a timer that allows discarding a partially reassembled packet
* a Datagram_Size,
* a timer that allows discarding a partially reassembled packet
after some timeout. after some timeout.
A fragmentation header is added to each fragment; it indicates what A fragmentation header is added to each fragment; it indicates what
portion of the packet that fragment corresponds to. Section 5.3 of portion of the packet that fragment corresponds to. Section 5.3 of
[RFC4944] defines the format of the header for the first and [RFC4944] defines the format of the header for the first and
subsequent fragments. All fragments are tagged with a 16-bit subsequent fragments. All fragments are tagged with a 16-bit
"datagram_tag", used to identify which packet each fragment belongs "Datagram_Tag", used to identify which packet each fragment belongs
to. Each datagram can be uniquely identified by the sender link- to. Each datagram can be uniquely identified by the sender Link-
layer addresses of the frame that carries it and the datagram_tag Layer addresses of the frame that carries it and the Datagram_Tag
that the sender allocated for this datagram. Each fragment can be that the sender allocated for this datagram. [RFC4944] also mandates
identified within its datagram by the datagram-offset. that the first fragment is sent first and with a particular format
that is different than that of the next fragments. Each fragment but
the first one can be identified within its datagram by the datagram-
offset.
Node B's typical behavior, per [RFC4944], is as follows. Upon Node B's typical behavior, per [RFC4944], is as follows. Upon
receiving a fragment from node A with a datagram_tag previously receiving a fragment from node A with a Datagram_Tag previously
unseen from node A, node B allocates a buffer large enough to hold unseen from node A, node B allocates a buffer large enough to hold
the entire packet. The length of the packet is indicated in each the entire packet. The length of the packet is indicated in each
fragment (the datagram_size field), so node B can allocate the buffer fragment (the Datagram_Size field), so node B can allocate the buffer
even if the first fragment it receives is not fragment 1. As even if the fragment it receives first is not the first fragment. As
fragments come in, node B fills the buffer. When all fragments have fragments come in, node B fills the buffer. When all fragments have
been received, node B inflates the compressed header fields into an been received, node B inflates the compressed header fields into an
IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer. IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer
which performs the route lookup. This behavior typically results in
This behavior typically results in per-hop fragmentation and per-hop fragmentation and reassembly. That is, the packet is fully
reassembly. That is, the packet is fully reassembled, then reassembled, then (re)fragmented, at every hop.
(re)fragmented, at every hop.
2. Limits of Per-Hop Fragmentation and Reassembly 4. Limitations of Per-Hop Fragmentation and Reassembly
There are at least 2 limits to doing per-hop fragmentation and There are at least 2 limitations to doing per-hop fragmentation and
reassembly. See [ARTICLE] for detailed simulation results on both reassembly. See [ARTICLE] for detailed simulation results on both
limits. limitations.
2.1. Latency 4.1. Latency
When reassembling, a node needs to wait for all the fragments to be When reassembling, a node needs to wait for all the fragments to be
received before being able to generate the IPv6 packet, and possibly received before being able to reform the IPv6 packet, and possibly
forward it to the next hop. This repeats at every hop. forward it to the next hop. This repeats at every hop.
This may result in increased end-to-end latency compared to a case This may result in increased end-to-end latency compared to a case
where each fragment is forwarded without per-hop reassembly. where each fragment is forwarded without per-hop reassembly.
2.2. Memory Management and Reliability 4.2. Memory Management and Reliability
Constrained nodes have limited memory. Assuming 1 kB reassembly Constrained nodes have limited memory. Assuming a reassembly buffer
buffer, typical nodes only have enough memory for 1-3 reassembly for a 6LoWPAN MTU of 1280 bytes as defined in section 4 of [RFC4944],
buffers. typical nodes only have enough memory for 1-3 reassembly buffers.
To illustrate this we use the topology from Figure 2, where nodes A, To illustrate this we use the topology from Figure 2, where nodes A,
B, C and D all send packets through node E. We further assume that B, C and D all send packets through node E. We further assume that
node E's memory can only hold 3 reassembly buffers. node E's memory can only hold 3 reassembly buffers.
+---+ +---+ +---+ +---+
... --->| A |------>| B | ... --->| A |------>| B |
+---+ +---+\ +---+ +---+\
\ \
+---+ +---+ +---+ +---+
skipping to change at page 5, line 25 skipping to change at page 7, line 29
... --->| C |------>| D | ... --->| C |------>| D |
+---+ +---+ +---+ +---+
Figure 2: Illustrating the Memory Management Issue. Figure 2: Illustrating the Memory Management Issue.
When nodes A, B and C concurrently send fragmented packets, all 3 When nodes A, B and C concurrently send fragmented packets, all 3
reassembly buffers in node E are occupied. If, at that moment, node reassembly buffers in node E are occupied. If, at that moment, node
D also sends a fragmented packet, node E has no option but to drop D also sends a fragmented packet, node E has no option but to drop
one of the packets, lowering end-to-end reliability. one of the packets, lowering end-to-end reliability.
3. Virtual Reassembly Buffer (VRB) Implementation 5. Forwarding Fragments
Virtual Reassembly Buffer (VRB) is the implementation technique A 6LoWPAN Fragment Forwarding technique makes the routing decision on
described in [I-D.ietf-lwig-6lowpan-virtual-reassembly] in which a the first fragment, which is always the one with the IPv6 address of
forwarder does not reassemble each packet in its entirety before the destination. Upon receiving a first fragment, a forwarding node
forwarding it. (e.g. node B in a A->B->C sequence) that does fragment forwarding
MUST attempt to create a state and forward the fragment. This is an
atomic operation, and if the first fragment cannot be forwarded then
the state MUST be removed.
VRB overcomes the limits listed in Section 2. Nodes do not wait for Since the Datagram_Tag is uniquely associated to the source Link-
the last fragment before forwarding, reducing end-to-end latency. Layer address of the fragment, the forwarding node MUST assign a new
Datagram_Tag from its own namespace for the next hop and rewrite the
fragment header of each fragment with that Datagram_Tag.
When a forwarding node receives a fragment other than a first
fragment, it MUST look up state based on the source Link-Layer
address and the Datagram_Tag in the received fragment. If no such
state is found, the fragment MUST be dropped; otherwise the fragment
MUST be forwarded using the information in the state found.
Compared to Section 3, the conceptual reassembly buffer in node B now
contains, assuming that node B is neither the source nor the final
destination:
* a Datagram_Tag as received in the incoming fragments, associated
to the interface and the Link-Layer address of node A for which
the received Datagram_Tag is unique
* the Link-Layer address that node B uses as source to forward the
fragments
* the interface and the Link-Layer address of the next hop C that is
resolved on the first fragment
* a Datagram_Tag that node B uniquely allocated for this datagram
and that is used when forwarding the fragments of the datagram
* a buffer for the remainder of a previous fragment left to be sent,
* a timer that allows discarding the stale FF state after some
timeout. The duration of the timer should be longer than that
which covers the reassembly at the receiving end point.
A node that has not received the first fragment cannot forward the
next fragments. This means that if node B receives a fragment, node
A was in possession of the first fragment at some point. To keep the
operation simple and consistent with [RFC4944], the first fragment
MUST always be sent first. When that is done, if node B receives a
fragment that is not the first and for which it has no state, then
node B treats it as an error and refrains from creating a state or
attempting to forward. This also means that node A should perform
all its possible retries on the first fragment before it attempts to
send the next fragments, and that it should abort the datagram and
release its state if it fails to send the first fragment.
Fragment forwarding obviates some of the benefits of the 6LoWPAN
header compression [RFC6282] in intermediate hops. In return, the
memory used to store the packet is distributed along the path, which
limits the buffer bloat effect. Multiple fragments may progress
simultaneously along the network as long as they do not interfere.
An associated caveat is that on a half duplex radio, if node A sends
the next fragment at the same time as node B forwards the previous
fragment to a node C down the path then node B will miss it. If node
C forwards the previous fragment to a node D at the same time and on
the same frequency as node A sends the next fragment to node B, this
may result in a hidden terminal problem. In that case, the
transmission from C interferes at node B with that from A unbeknownst
of node A. Consecutive fragments of a same datagram MUST be
separated with an inter-frame gap that allows one fragment to
progress beyond the next hop and beyond the interference domain
before the next shows up. This can be achieved by interleaving
packets or fragments sent via different next-hop routers.
6. Virtual Reassembly Buffer (VRB) Implementation
The Virtual Reassembly Buffer (VRB) [LWIG-VRB] is a particular
incarnation of a 6LoWPAN Fragment Forwarding that can be implemented
without a change to [RFC4944].
VRB overcomes the limitations listed in Section 4. Nodes do not wait
for the last fragment before forwarding, reducing end-to-end latency.
Similarly, the memory footprint of VRB is just the VRB table, Similarly, the memory footprint of VRB is just the VRB table,
reducing the packet drop probability significantly. reducing the packet drop probability significantly.
There are, however, limits: There are other caveats, however:
Non-zero Packet Drop Probability: The abstract data in a VRB table Non-zero Packet Drop Probability: The abstract data in a VRB table
entry contains at a minimum the MAC address of the predecessor entry contains at a minimum the Link-Layer address of the
and that of the successor, the datagram_tag used by the predecessor and that of the successor, the Datagram_Tag used by
predecessor and the local datagram_tag that this node will swap the predecessor and the local Datagram_Tag that this node will
with it. The VRB may need to store a few octets from the last swap with it. The VRB may need to store a few octets from the
fragment that may not have fit within MTU and that will be last fragment that may not have fit within MTU and that will be
prepended to the next fragment. This yields a small footprint prepended to the next fragment. This yields a small footprint
that is 2 orders of magnitude smaller compared to needing a that is 2 orders of magnitude smaller compared to needing a
1280-byte reassembly buffer for each packet. Yet, the size of 1280-byte reassembly buffer for each packet. Yet, the size of the
the VRB table necessarily remains finite. In the extreme case VRB table necessarily remains finite. In the extreme case where a
where a node is required to concurrently forward more packets node is required to concurrently forward more packets that it has
that it has entries in its VRB table, packets are dropped. entries in its VRB table, packets are dropped.
No Fragment Recovery: There is no mechanism in VRB for the node that No Fragment Recovery: There is no mechanism in VRB for the node that
reassembles a packet to request a single missing fragment. reassembles a packet to request a single missing fragment.
Dropping a fragment requires the whole packet to be resent. This
causes unnecessary traffic, as fragments are forwarded even when
the destination node can never construct the original IPv6 packet.
Dropping a fragment requires the whole packet to be resent. This
causes unnecessary traffic, as fragments are forwarded even when
the destination node can never construct the original IPv6
packet.
No Per-Fragment Routing: All subsequent fragments follow the same No Per-Fragment Routing: All subsequent fragments follow the same
sequence of hops from the source to the destination node as the sequence of hops from the source to the destination node as the
first fragment, because the IP header is required to route the first fragment, because the IP header is required in order to
fragment and is only present in the first fragment. A side route the fragment and is only present in the first fragment. A
effect is that the first fragment must always be forwarded first. side effect is that the first fragment must always be forwarded
first.
The severity and occurrence of these limits depends on the link-layer The severity and occurrence of these caveats depends on the Link-
used. Whether these limits are acceptable depends entirely on the Layer used. Whether they are acceptable depends entirely on the
requirements the application places on the network. requirements the application places on the network.
If the limits are present and not acceptable for the application, If the caveats are present and not acceptable for the application,
future specifications may define new protocols to overcome these alternative specifications may define new protocols to overcome them.
limits. One example is [I-D.ietf-6lo-fragment-recovery] which One example is [FRAG-RECOV] which specifies a 6LoWPAN Fragment
defines a protocol which allows fragment recovery. Forwarding technique that allows the end-to-end fragment recovery
between the 6LoWPAN FF endpoints.
4. Security Considerations 7. Security Considerations
An attacker can perform a Denial-of-Service (DoS) attack on a node An attacker can perform a Denial-of-Service (DoS) attack on a node
implementing VRB by generating a large number of bogus "fragment 1" implementing VRB by generating a large number of bogus "fragment 1"
fragments without sending subsequent fragments. This causes the VRB fragments without sending subsequent fragments. This causes the VRB
table to fill up. Note that the VRB does not need to remember the table to fill up. Note that the VRB does not need to remember the
full datagram as received so far but only possibly a few octets from full datagram as received so far but only possibly a few octets from
the last fragment that could not fit in it. It is expected that an the last fragment that could not fit in it. It is expected that an
implementation protects itself to keep the number of VRBs within implementation protects itself to keep the number of VRBs within
capacity, and that old VRBs are protected by a timer of a reasonable capacity, and that old VRBs are protected by a timer of a reasonable
duration for the technology and destroyed upon timeout. duration for the technology and destroyed upon timeout.
Secure joining and the link-layer security that it sets up protects Secure joining and the Link-Layer security that it sets up protects
against those attacks from network outsiders. against those attacks from network outsiders.
5. IANA Considerations "IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security
threats and other caveats that are linked to using IP fragmentation.
The 6LoWPAN fragmentation takes place underneath the IP Layer, but
some issues described there may still apply to 6LoWPAN fragments.
No requests to IANA are made by this document. * Overlapping fragment attacks are possible with 6LoWPAN fragments
but there is no known firewall operation that would work on
6LoWPAN fragments at the time of this writing, so the exposure is
limited. An implementation of a firewall SHOULD NOT forward
fragments but instead should recompose the IP packet, check it in
the u ncompressed form, and then forward it again as fragments if
necessary. Overlapping fragments are acceptable as long as they
contain the same payload. The firewall MUST drop the whole packet
if overlapping fragments are encountered that result in different
data at the same offset.
6. Acknowledgments * Resource exhaustion attacks are certainly possible and a sensitive
issue in a constrained network. An attacker can perform a Denial-
of-Service (DoS) attack on a node implementing VRB by generating a
large number of bogus first fragments without sending subsequent
fragments. This causes the VRB table to fill up. When hop-by-hop
reassembly is used, the same attack can be more damaging if the
node allocates a full Datagram_Size for each bogus first fragment.
With the VRB, the attack can be performed remotely on all nodes
along a path, but each node suffers a lesser hit. This is because
the VRB does not need to remember the full datagram as received so
far but only possibly a few octets from the last fragment that
could not fit in it. An implementation MUST protect itself to
keep the number of VRBs within capacity, and ensure that old VRBs
are protected by a timer of a reasonable duration for the
technology and destroyed upon timeout.
The authors would like to thank Yasuyuki Tanaka, for his in-depth * Attacks based on predictable fragment identification values are
review of this document. Also many thanks to Georgies Papadopoulos also possible but can be avoided. The Datagram_Tag SHOULD be
and Dominique Barthel for their own reviews. assigned pseudo-randomly in order to defeat such attacks. A
larger size of the Datagram_Tag makes the guessing more difficult
and reduces the chances of an accidental reuse while the original
packet is still in flight, at the expense of more space in each
frame. Attacks based on predictable fragment identification
values are also possible but can be avoided. The Datagram_Tag
SHOULD be assigned pseudo-randomly in order to reduce the risk of
such attacks. Nonetheless, some level of risk remains that an
attacker able to authenticate to and send traffic on the network
can guess a valid Datagram_Tag value, since there are only a
limited number of possible values.
7. Informative References * Evasion of Network Intrusion Detection Systems (NIDS) leverages
ambiguity in the reassembly of the fragment. This attack makes
little sense in the context of this specification since the
fragmentation happens within the LLN, meaning that the intruder
should already be inside to perform the attack. NDIS systems
would probably not be installed within the LLN either, but rather
at a boittleneck at the exterior edge of the network.
[ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment 8. IANA Considerations
Forwarding", IEEE Communications Standards Magazine ,
2019.
[I-D.ietf-6lo-fragment-recovery] No requests to IANA are made by this document.
Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
ietf-6lo-fragment-recovery-05 (work in progress), July
2019.
[I-D.ietf-lwig-6lowpan-virtual-reassembly] 9. Acknowledgments
Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-01 The authors would like to thank Carles Gomez Montenegro, Yasuyuki
(work in progress), March 2019. Tanaka, Ines Robles and Dave Thaler for their in-depth review of this
document and improvement suggestions. Also many thanks to Georgios
Papadopoulos and Dominique Barthel for their own reviews, and to
Roman Danyliw, Barry Leiba, Murray Kucherawy, Derrell Piper, Sarah
Banks, Joerg Ott, Francesca Palombini, Mirja Kuhlewind, Eric Vyncke,
and especially Benjamin Kaduk for their constructive reviews through
the IETF last call and IESG process.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>. <https://www.rfc-editor.org/info/rfc4944>.
[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>.
11. Informative References
[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>.
[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>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011, DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>. <https://www.rfc-editor.org/info/rfc6282>.
[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>.
[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>.
[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>.
[FRAG-ILE] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", Work
in Progress, Internet-Draft, draft-ietf-intarea-frag-
fragile-17, 30 September 2019,
<https://tools.ietf.org/html/draft-ietf-intarea-frag-
fragile-17>.
[LWIG-VRB] Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
lwig-6lowpan-virtual-reassembly-02, 9 March 2020,
<https://tools.ietf.org/html/draft-ietf-lwig-6lowpan-
virtual-reassembly-02>.
[FRAG-RECOV]
Thubert, P., "6LoWPAN Selective Fragment Recovery", Work
in Progress, Internet-Draft, draft-ietf-6lo-fragment-
recovery-20, 20 March 2020, <https://tools.ietf.org/html/
draft-ietf-6lo-fragment-recovery-20>.
[ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment
Forwarding", IEEE Communications Standards Magazine ,
2019.
Authors' Addresses Authors' Addresses
Thomas Watteyne (editor) Thomas Watteyne (editor)
Analog Devices Analog Devices
32990 Alvarado-Niles Road, Suite 910 32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587 Union City, CA 94587
USA United States of America
Email: thomas.watteyne@analog.com Email: thomas.watteyne@analog.com
Carsten Bormann Pascal Thubert (editor)
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Email: cabo@tzi.org
Pascal Thubert
Cisco Systems, Inc Cisco Systems, Inc
Building D Building D
45 Allee des Ormes - BP1200 45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254 06254 Mougins - Sophia Antipolis
France France
Phone: +33 497 23 26 34
Email: pthubert@cisco.com Email: pthubert@cisco.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Email: cabo@tzi.org
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