draft-ietf-6lo-minimal-fragment-11.txt   draft-ietf-6lo-minimal-fragment-12.txt 
6lo T. Watteyne, Ed. 6lo T. Watteyne, Ed.
Internet-Draft Analog Devices Internet-Draft Analog Devices
Intended status: Standards Track P. Thubert, Ed. Intended status: Standards Track P. Thubert, Ed.
Expires: 10 August 2020 Cisco Systems Expires: 15 August 2020 Cisco Systems
C. Bormann C. Bormann
Universitaet Bremen TZI Universitaet Bremen TZI
7 February 2020 12 February 2020
On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network
draft-ietf-6lo-minimal-fragment-11 draft-ietf-6lo-minimal-fragment-12
Abstract Abstract
This document introduces the capability to forward 6LoWPAN fragments. This document introduces the capability to forward 6LoWPAN fragments.
This method reduces the latency and increases end-to-end reliability This method reduces the latency and increases end-to-end reliability
in route-over forwarding. It is the companion to using virtual in route-over forwarding. It is the companion to using virtual
reassembly buffers which is a pure implementation technique. reassembly buffers which is a pure implementation technique.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 10 August 2020. This Internet-Draft will expire on 15 August 2020.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Simplified BSD License. provided without warranty as described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Referenced Work . . . . . . . . . . . . . . . . . . . . . 3 2.2. Referenced Work . . . . . . . . . . . . . . . . . . . . . 3
2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 4 3. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 4
4. Limits of Per-Hop Fragmentation and Reassembly . . . . . . . 6 4. Limitations of Per-Hop Fragmentation and Reassembly . . . . . 6
4.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Memory Management and Reliability . . . . . . . . . . . . 6 4.2. Memory Management and Reliability . . . . . . . . . . . . 6
5. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 7 5. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 7
6. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 9 6. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. Normative References . . . . . . . . . . . . . . . . . . . . 11 10. Normative References . . . . . . . . . . . . . . . . . . . . 11
11. Informative References . . . . . . . . . . . . . . . . . . . 11 11. Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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pushed to Layer-3 to be routed, and then fragmented again if the next pushed to Layer-3 to be routed, and then fragmented again if the next
hop is another similar 6LoWPAN link. This draft introduces an hop is another similar 6LoWPAN link. This draft introduces an
alternate approach called 6LoWPAN Fragment Forwarding (FF) whereby an alternate approach called 6LoWPAN Fragment Forwarding (FF) whereby an
intermediate node forwards a fragment as soon as it is received if intermediate node forwards a fragment as soon as it is received if
the next hop is a similar 6LoWPAN link. The routing decision is made the next hop is a similar 6LoWPAN link. The routing decision is made
on the first fragment, which has all the IPv6 routing information. on the first fragment, which has all the IPv6 routing information.
The first fragment is forwarded immediately and a state is stored to The first fragment is forwarded immediately and a state is stored to
enable forwarding the next fragments along the same path. enable forwarding the next fragments along the same path.
Done right, 6LoWPAN Fragment Forwarding techniques lead to more Done right, 6LoWPAN Fragment Forwarding techniques lead to more
streamlined operations, less buffer bloat and lower latency. It may streamlined operations, less buffer bloat and lower latency. But it
be wasteful if some fragments are missing after the first one since may be wasteful when fragments are missing, leading to locked
the first fragment will still continue until the 6LoWPAN endpoint resources and low throughput, and it may be misused to the point that
that will attempt to perform the reassembly, and may be misused to the end-to-end latency of one packet falls behind that of per-hop
the point that the end-to-end latency falls behind that of per-hop
recomposition. recomposition.
This specification provides a generic overview of FF, discusses This specification provides a generic overview of FF, discusses
advantages and caveats, and introduces a particular 6LoWPAN Fragment advantages and caveats, and introduces a particular 6LoWPAN Fragment
Forwarding technique called Virtual Reassembly Buffer that can be Forwarding technique called Virtual Reassembly Buffer that can be
used while conserving the message formats defined in [RFC4944]. used while conserving 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. Terminology
2.1. BCP 14 2.1. BCP 14
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all 14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
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that with MPLS, 'packets are "labeled" before they are forwarded.' that with MPLS, 'packets are "labeled" before they are forwarded.'
It goes on to say, "At subsequent hops, there is no further analysis 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 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". an index into a table which specifies the next hop, and a new label".
The MPLS technique is leveraged in the present specification to The MPLS technique is leveraged in the present specification to
forward fragments that actually do not have a network layer header, forward fragments that actually do not have a network layer header,
since the fragmentation occurs below IP. since the fragmentation occurs below IP.
2.3. New Terms 2.3. New Terms
This specification defines the following terms: This specification uses the following terms:
6LoWPAN endpoints: The 6LoWPAN endpoints are the first and last 6LoWPAN endpoints: The 6LoWPAN endpoints are the first and last
nodes in an unbroken string of 6LoWPAN nodes. They are in charge nodes in an unbroken string of 6LoWPAN nodes. They are in charge
of generating or expanding a 6LoWPAN header from/to a full IPv6 of generating or expanding a 6LoWPAN header from/to a full IPv6
packet. They are also the points where the fragmentation and packet. They are also the points where the fragmentation and
reassembly operations take place. reassembly operations take place.
Compressed Form: This specification uses the generic term Compressed Compressed Form: This specification uses the generic term Compressed
Form to refer to the format of a datagram after the action of Form to refer to the format of a datagram after the action of
[RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts. [RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts.
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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 first fragment it receives is not fragment 1. 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 which performs the route lookup. This behavior typically results in
per-hop fragmentation and reassembly. That is, the packet is fully per-hop fragmentation and reassembly. That is, the packet is fully
reassembled, then (re)fragmented, at every hop. reassembled, then (re)fragmented, at every hop.
4. Limits of Per-Hop Fragmentation and Reassembly 4. Limitations of Per-Hop Fragmentation and Reassembly
There are at least 2 limitations 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
limitations. limitations.
4.1. 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.
4.2. Memory Management and Reliability 4.2. Memory Management and Reliability
Constrained nodes have limited memory. Assuming a reassembly buffer Constrained nodes have limited memory. Assuming a reassembly buffer
for a 6LoWPAN MTU of 1280 bytes as defined in section 4 of [RFC4944], for a 6LoWPAN MTU of 1280 bytes as defined in section 4 of [RFC4944],
typical nodes only have enough memory for 1-3 reassembly buffers. typical nodes only have enough memory for 1-3 reassembly buffers.
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* a buffer for the remainder of a previous fragment left to be sent, * a buffer for the remainder of a previous fragment left to be sent,
* a timer that allows discarding the stale FF state after some * a timer that allows discarding the stale FF state after some
timeout. The duration of the timer should be longer than that timeout. The duration of the timer should be longer than that
which covers the reassembly at the receiving end point. which covers the reassembly at the receiving end point.
A node that has not received the first fragment cannot forward the A node that has not received the first fragment cannot forward the
next fragments. This means that if node B receives a fragment, node 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 A was in possession of the first fragment at some point. To keep the
operation simple, it makes sense to be consistent with [RFC4944] and operation simple and consistent with [RFC4944], the first fragment
enforce that the first fragment is always sent first. When that is must always be sent first. When that is done, if node B receives a
done, if node B receives a fragment that is not the first and for fragment that is not the first and for which it has no state, then
which it has no state, then node B treats this as an error and node B treats it as an error and refrains from creating a state or
refrains from creating a state or attempting to forward. This also attempting to forward. This also means that node A should perform
means that node A should perform all its possible retries on the all its possible retries on the first fragment before it attempts to
first fragment before it attempts to send the next fragments, and send the next fragments, and that it should abort the datagram and
that it should abort the datagram and release its state if it fails release its state if it fails to send the first fragment.
to send the first fragment.
One benefit of Fragment Forwarding is that the memory that is used to One benefit of Fragment Forwarding is that the memory that is used to
store the packet is now distributed along the path, which limits the store the packet is now distributed along the path, which limits the
buffer bloat effect. Multiple fragments may progress in parallel buffer bloat effect. Multiple fragments may progress in parallel
along the network as long as they do not interfere. An associated 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 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 fragment at the same time as node B forwards the previous fragment to
a node C down the path then node B will miss the next fragment from a node C down the path then node B will miss the next fragment from
node A. If node C forwards the previous fragment to a node D at the node A. 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 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 at B whereby to node B, this may result in a hidden terminal problem. In that
the transmission from C interferes with that from A unbeknownst of case, the transmission from C interferes at node B with that from A
node A. It results that consecutive fragments must be reasonably unbeknownst of node A.
spaced to avoid the 2 forms of collision described above. A node
that has multiple packets or fragments to send via different next-hop Consecutive fragments of a same datagram must be separated with an
routers may interleave the messages in order to alleviate those inter-frame gap that allows one fragment to progress before the next
effects. shows up. This can be achieved by interleaving packets or fragments
sent via different next-hop routers.
6. Virtual Reassembly Buffer (VRB) Implementation 6. Virtual Reassembly Buffer (VRB) Implementation
Virtual Reassembly Buffer (VRB) is the implementation technique Virtual Reassembly Buffer (VRB) is the implementation technique
described in [LWIG-VRB] in which a forwarder does not reassemble each described in [LWIG-VRB] in which a forwarder does not reassemble each
packet in its entirety before forwarding it. packet in its entirety before forwarding it.
VRB overcomes the limitations listed in Section 4. Nodes do not wait VRB overcomes the limitations listed in Section 4. Nodes do not wait
for the last fragment before forwarding, reducing end-to-end latency. 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,
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first fragment, because the IP header is required in order to first fragment, because the IP header is required in order to
route the fragment and is only present in the first fragment. A route the fragment and is only present in the first fragment. A
side effect is that the first fragment must always be forwarded side effect is that the first fragment must always be forwarded
first. first.
The severity and occurrence of these caveats depends on the Link- The severity and occurrence of these caveats depends on the Link-
Layer used. Whether they 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 caveats 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 them. One alternative specifications may define new protocols to overcome them.
example is [FRAG-RECOV] which defines a protocol which allows One example is [FRAG-RECOV] which specifies a 6LoWPAN Fragment
fragment recovery. Forwarding technique that allows the end-to-end fragment recovery
between the 6LoWPAN endpoints.
7. Security Considerations 7. Security Considerations
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.
"IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security "IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security
threats that are linked to using IP fragmentation. The 6LoWPAN threats that are linked to using IP fragmentation. The 6LoWPAN
fragmentation takes place underneath, but some issues described there fragmentation takes place underneath, but some issues described there
may still apply to 6LoWPAN fragments. may still apply to 6LoWPAN fragments.
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