< draft-ietf-6lo-minimal-fragment-00.txt   draft-ietf-6lo-minimal-fragment-01.txt >
6lo T. Watteyne, Ed. 6lo T. Watteyne, Ed.
Internet-Draft Analog Devices Internet-Draft Analog Devices
Intended status: Informational C. Bormann Intended status: Informational C. Bormann
Expires: April 21, 2019 Universitaet Bremen TZI Expires: September 12, 2019 Universitaet Bremen TZI
P. Thubert P. Thubert
Cisco Cisco
October 18, 2018 March 11, 2019
LLN Minimal Fragment Forwarding LLN Minimal Fragment Forwarding
draft-ietf-6lo-minimal-fragment-00 draft-ietf-6lo-minimal-fragment-01
Abstract Abstract
This document gives an overview of LLN Minimal Fragment Forwarding. This document gives an overview of LLN Minimal Fragment Forwarding.
When employing adaptation layer fragmentation in 6LoWPAN, it may be When employing adaptation layer fragmentation in 6LoWPAN, it may be
beneficial for a forwarder not to have to reassemble each packet in beneficial for a forwarder not to have to reassemble each packet in
its entirety before forwarding it. This has been always possible its entirety before forwarding it. This has always been possible
with the original fragmentation design of RFC4944. This document is with the original fragmentation design of RFC4944. This document is
a companion document to [I-D.ietf-lwig-6lowpan-virtual-reassembly], a companion document to [I-D.ietf-lwig-6lowpan-virtual-reassembly],
which details the virtual Reassembly Buffer (VRB) implementation which details the virtual Reassembly Buffer (VRB) implementation
technique which reduces the latency and increases end-to-end technique which reduces the latency and increases end-to-end
reliability in route-over forwarding. reliability in route-over forwarding.
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.
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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 April 21, 2019. This Internet-Draft will expire on September 12, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 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 Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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+---------+ +---------+ +---------+ +---------+
| # ###| |### # | | # ###| |### # |
+---------+ +---------+ +---------+ +---------+
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 header compression Node A starts by compacting the IPv6 packet using the header
defined in [RFC6282]. If the resulting 6LoWPAN packet does not fit compression mechanism defined in [RFC6282]. If the resulting 6LoWPAN
into a single link-layer frame, node A's 6LoWPAN sublayer cuts it packet does not fit into a single link-layer frame, node A's 6LoWPAN
into multiple 6LoWPAN fragments, which it transmits as separate link- sublayer cuts it into multiple 6LoWPAN fragments, which it transmits
layer frames to node B. Node B's 6LoWPAN sublayer reassembles these as separate link-layer frames to node B. Node B's 6LoWPAN sublayer
fragments, inflates the compressed header fields back to the original reassembles these fragments, inflates the compressed header fields
IPv6 header, and hands over the full IPv6 packet to its IPv6 layer. back to the original 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. Each fragment is represented by the '#'
symbol. Node A has sent fragments 1, 2, 3, 5, 6 to node B. Node B 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 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. being transmitted at the link layer from node A to node B.
A reassembly buffer for 6LoWPAN contains: Conceptually, a reassembly buffer for 6LoWPAN contains:
o datagram_size, o a datagram_size,
o datagram_tag and link-layer sender and receiver addresses (to o a datagram_tag, associated to the link-layer sender and receiver
which the datagram_tag is local), addresses to which the datagram_tag is local,
o actual packet data from the fragments received so far, in a form o the actual packet data from the fragments received so far, in a
that makes it possible to detect when the whole packet has been form that makes it possible to detect when the whole packet has
received and can be processed or forwarded, been received and can be processed or forwarded,
o a timer that allows discarding the partial packet after a timeout. o a timer that allows discarding a partially reassembled packet
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 fragment can be uniquely identified by the source and to. Each fragment can be uniquely identified by the source and
destination link-layer addresses of the frame that carries it, and destination link-layer addresses of the frame that carries it, and
the datagram_tag. The value of the datagram_tag only needs to be the datagram_tag. The value of the datagram_tag only needs to be
locally unique to nodes A and B. locally unique to nodes A and B.
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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.
This behavior typically results in per-hop fragmentation and This behavior typically results in per-hop fragmentation and
reassembly. That is, the packet is fully reassembled, then reassembly. That is, the packet is fully reassembled, then
(re)fragmented, at every hop. (re)fragmented, at every hop.
2. Limits of Per-Hop Fragmentation and Reassembly 2. Limits of Per-Hop Fragmentation and Reassembly
There are at least 2 limits to doing per-hop fragmentation and There are at least 2 limits to doing per-hop fragmentation and
reassembly: reassembly. See [ARTICLE] for detailed simulation results on both
limits.
2.1. Latency 2.1. 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 generate 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 the case This may result in increased end-to-end latency compared to a case
where each fragment would be forwarded without per-hop reassembly. where each fragment is forwarded without per-hop reassembly.
2.2. Memory Management and Reliability 2.2. Memory Management and Reliability
Constrained nodes have limited memory. Assuming 1 kB reassembly Constrained nodes have limited memory. Assuming 1 kB reassembly
buffers, typical nodes only have enough memory for 1-3 reassembly buffers, typical nodes only have enough memory for 1-3 reassembly
buffers. buffers.
Assuming the topology from Figure 2, where nodes A, B, C and D all Assuming the topology from Figure 2, where nodes A, B, C and D all
send packets through node E. We further assume that node E's memory send packets through node E. We further assume that node E's memory
can only hold 3 reassembly buffers. can only hold 3 reassembly buffers.
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the destination node can never construct the original IPv6 the destination node can never construct the original IPv6
packet. 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 sequence of hops from the source to the destination node as
fragment 1. fragment 1.
The severity and occurrence of these limits depends on the link-layer The severity and occurrence of these limits depends on the link-layer
used. Whether these limits are acceptable depends entirely on the used. Whether these limits are acceptable depends entirely on the
requirements the application places on the network. requirements the application places on the network.
If the limits are both present and not accepted by the application, If the limits are present and not acceptable for the application,
future specifications may define new protocols to overcome these future specifications may define new protocols to overcome these
limits. One example is [I-D.thubert-6lo-fragment-recovery] which limits. One example is [I-D.thubert-6lo-fragment-recovery] which
defines a protocol which allows fragment recovery. defines a protocol which allows fragment recovery.
4. Security Considerations 4. Security Considerations
An attacker can perform a DoS attack on a node implementing VRB by An attacker can perform a DoS attack on a node implementing VRB by
generating a large number of bogus "fragment 1" fragments without generating a large number of bogus "fragment 1" fragments without
sending subsequent fragments. This causes the VRB table to fill up. sending subsequent fragments. This causes the VRB table to fill up.
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No requests to IANA are made by this document. No requests to IANA are made by this document.
6. Acknowledgments 6. Acknowledgments
The authors would like to thank Yasuyuki Tanaka for his in-depth The authors would like to thank Yasuyuki Tanaka for his in-depth
review of this document. review of this document.
7. Informative References 7. Informative References
[ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment
Forwarding", IEEE Communications Standards Magazine ,
2009.
[BOOK] Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons, [BOOK] Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons,
Ltd monograph, DOI 10.1002/9780470686218, November 2009. Ltd monograph, DOI 10.1002/9780470686218, November 2009.
[I-D.ietf-lwig-6lowpan-virtual-reassembly] [I-D.ietf-lwig-6lowpan-virtual-reassembly]
Bormann, C. and T. Watteyne, "Virtual reassembly buffers Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00 in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00
(work in progress), July 2018. (work in progress), July 2018.
[I-D.thubert-6lo-fragment-recovery] [I-D.thubert-6lo-fragment-recovery]
Thubert, P., "6LoWPAN Selective Fragment Recovery", draft- Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
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