One document matched: draft-gomez-6lo-optimized-fragmentation-header-00.xml
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<rfc category="std" docName="draft-gomez-6lo-optimized-fragmentation-header-00"
ipr="trust200902">
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<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
full title is longer than 39 characters -->
<title abbrev="6lo fragmentation header">
Optimized 6LoWPAN Fragmentation Header</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="Carles Gomez" initials="C.G" surname="Gomez">
<organization>UPC/i2CAT</organization>
<address>
<postal>
<street>C/Esteve Terradas, 7</street>
<city>Castelldefels</city>
<region/>
<code>08860</code>
<country>Spain</country>
</postal>
<phone/>
<facsimile/>
<email>carlesgo@entel.upc.edu</email>
<uri/>
</address>
</author>
<author fullname="Josep Paradells" initials="J.P" surname="Paradells">
<organization>UPC/i2CAT</organization>
<address>
<postal>
<street>C/Jordi Girona, 1-3</street>
<city>Barcelona</city>
<region/>
<code>08034</code>
<country>Spain</country>
</postal>
<phone/>
<facsimile/>
<email>josep.paradells@entel.upc.edu</email>
<uri/>
</address>
</author>
<author fullname="Jon Crowcroft" initials="J.C" surname="Crowcroft">
<organization>University of Cambridge</organization>
<address>
<postal>
<street>JJ Thomson Avenue</street>
<city>Cambridge</city>
<region>CB3 0FD</region>
<code/>
<country>United Kingdom</country>
</postal>
<phone/>
<facsimile/>
<email>jon.crowcroft@cl.cam.ac.uk</email>
<uri/>
</address>
</author>
<!-- uri and facsimile elements may also be added -->
<date month="October" year="2016"/>
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<area>INT</area>
<workgroup>6lo Working Group</workgroup>
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<!---->
<abstract>
<t> RFC 4944 specifies 6LoWPAN fragmentation, in order to support the IPv6 MTU
requirement over IEEE 802.15.4-2003 networks. The 6LoWPAN fragmentation
header format comprises a 4-byte format for the first fragment, and a 5-byte
format for subsequent fragments. This specification defines a more efficient
3-byte, optimized 6LoWPAN fragmentation header for all fragments.
</t>
</abstract>
</front>
<middle>
<section title="Introduction ">
<t>IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) was originally
designed as an adaptation layer intended to enable IPv6 over IEEE 802.15.4-
2003 networks <xref
target="RFC4944"/>. One of the 6LoWPAN protocol suite components is
fragmentation, which fulfills the IPv6 MTU requirement of 1280 bytes
<xref
target="RFC2460"/> over a radio interface with a layer two (L2) payload size around
100 bytes (in the best case) and without fragmentation support <xref
target="RFC4944"/>. </t>
<t>RFC 4944 defines the 6LoWPAN fragmentation header format, which comprises a
4-byte format for the first fragment, and a 5-byte format for subsequent
fragments. This specification defines a more efficient 3-byte, optimized
6LoWPAN Fragmentation Header (6LoFH). The benefits of using 6LoFH
are the following:</t>
<t> -- Reduced overhead for transporting an IPv6 packet that requires
fragmentation (see Annex A). This decreases consumption of energy and
bandwidth, which are typically limited resources in the scenarios where
6LoWPAN fragmentation is used.</t>
<t> -- Because the datagram offset can be expressed in increments of a
single octet, 6LoFH enables the transport of IPv6 packets over L2
data units with a maximum payload size as small as only 4 bytes in
the most extreme case. Note that RFC 4944 fragmentation can only be used over
L2 technologies with a maximum L2 payload size of at least 13 bytes.</t>
<t> In comparison with the 6LoWPAN fragmentation header, parsing of the 6loFH format is also simplified, as the format has a constant size, and a 'symmetric' shape for both the first fragment and subsequent fragments. However, receiver buffer management will involve greater complexity as explained in Section 3. </t>
<section title="Conventions used in this document">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"/></t>
</section>
</section>
<section title="6LoFH rules and format">
<t>If an entire payload (e.g., IPv6) datagram fits within a single
L2 data unit, it is unfragmented and a fragmentation header is not needed. If the datagram does not fit within a single L2 data unit, it SHALL be broken into fragments. The first fragment SHALL contain the first fragment header as defined in <xref
target="fig.firstfragment"/>.</t>
<figure anchor="fig.firstfragment"
title="First Fragment">
<artwork><![CDATA[
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 0 0 1| datagram_size | datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>
The second and subsequent fragments (up to and including the
last) SHALL contain a fragmentation header that conforms to the
format shown in <xref target="fig.nextfragments"/>. </t>
<figure anchor="fig.nextfragments"
title="Subsequent Fragments">
<artwork><![CDATA[
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 0 1 0| datagram_offset | datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>
datagram_size: This 11-bit field encodes the size of the entire IP
packet before link-layer fragmentation (but after IP layer
fragmentation). For IPv6, the datagram size SHALL be
40 octets (the size of the uncompressed IPv6 header) more than the
value of Payload Length in the IPv6 header <xref
target="RFC4944"/> of the
packet. Note that this packet may already be fragmented by hosts
involved in the communication, i.e., this field needs to encode a
maximum length of 1280 octets (the required by IPv6).</t>
<t>
datagram_tag: The value of datagram_tag (datagram tag) SHALL be the
same for all fragments of a payload (e.g., IPv6) datagram.
The sender SHALL increment datagram_tag for successive, fragmented
datagrams. The incremented value of datagram_tag SHALL wrap from
255 back to zero. This field is 8 bits long, and its initial
value is not defined.</t>
<t>
datagram_offset: This field is present only in the second and
subsequent fragments and SHALL specify the offset, in
increments of 1 octet, of the fragment from the beginning of the
payload datagram. The first octet of the datagram (e.g., the
start of the IPv6 header) has an offset of zero; the implicit
value of datagram_offset in the first fragment is zero. This
field is 11 bits long. </t>
<t>
The recipient of link fragments SHALL use (1) the sender's L2
source address, (2) the destination's L2 address, (3)
datagram_size, and (4) datagram_tag to identify all the
fragments that belong to a given datagram. </t>
<t>
Upon receipt of a link fragment, the recipient starts constructing
the original unfragmented packet whose size is datagram_size. It
uses the datagram_offset field to determine the location of the
individual fragments within the original unfragmented packet. For
example, it may place the data payload (except the encapsulation
header) within a payload datagram reassembly buffer at the location
specified by datagram_offset. The size of the reassembly buffer
SHALL be determined from datagram_size. </t>
<t>
If a fragment recipient disassociates from its L2 network, the recipient
MUST discard all link fragments of all partially
reassembled payload datagrams, and fragment senders MUST discard all
not yet transmitted link fragments of all partially transmitted
payload (e.g., IPv6) datagrams. Similarly, when a node first
receives a fragment with a given datagram_tag, it starts a reassembly
timer. When this time expires, if the entire packet has not been
reassembled, the existing fragments MUST be discarded and the
reassembly state MUST be flushed. The reassembly timeout MUST be set
to a maximum of TBD seconds). </t>
</section>
<section title="Changes from RFC 4944 fragmentation header and rationale">
<t>The main changes introduced in this specification to the fragmentation header format defined in RFC 4944 are listed below, together with their rationale:
</t>
<t>-- The datagram size field is only included in the first fragment. Rationale: In the RFC 4944 fragmentation header, the datagram size was included in all fragments to ease the task of reassembly at the receiver, since in an IEEE 802.15.4 mesh network, the fragment that arrives earliest to a destination is not necessarily the first fragment transmitted by the source. Nevertheless, the
fragmentation format defined in this document supports reordering, at the
expense of additional complexity in this regard. </t>
<t>-- The datagram tag size is reduced from 2 bytes to 1 byte. Rationale: Given the low bit rate, as well as the relatively low message rate
in IEEE 802.15.4 scenarios, ambiguities due to datagram tag wrapping events
are unlikely despite the reduced tag space. </t>
<t>-- The datagram offset size is increased from 8 bits to 11 bits. Rationale: This allows to express the datagram offset in single-octet increments.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This document allocates the following sixteen RFC 4944 Dispatch type values:</t>
<t> 11001 000</t>
<t>through</t>
<t> 11001 111</t>
<t> and</t>
<t> 11010 000</t>
<t>through</t>
<t> 11010 111</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>6LoWPAN fragmentation attacks have been analyzed in the literature. Countermeasures to these have been proposed as well <xref
target="HHWH"/>.</t>
<t>A node can perform a buffer reservation attack
by sending a first fragment to a target. Then, the receiver will reserve
buffer space for the whole packet on the basis of the datagram size
announced in that first fragment. Other incoming fragmented packets will be
dropped while the reassembly buffer is occupied during the reassembly
timeout. Once that timeout expires, the attacker can repeat the same
procedure, and iterate, thus creating a denial of service attack. The (low)
cost to mount this attack is linear with the number of buffers at the
target node. However, the cost for an attacker can be increased if
individual fragments of multiple packets can be stored in the reassembly
buffer. To further increase the attack cost, the reassembly buffer
can be split into fragment-sized buffer slots. Once a packet is complete,
it is processed normally. If buffer overload occurs, a receiver can discard
packets based on the sender behavior, which may help identify which
fragments have been sent by an attacker.</t>
<t>In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a fragment, it can
send a spoofed duplicate (e.g. with random payload) to the destination. A
receiver cannot distinguish legitimate from spoofed fragments. Therefore,
the original IPv6 packet will be considered corrupt and will be dropped. To
protect resource-constrained nodes from this attack, it has been proposed
to establish a binding among the fragments to be transmitted by a node, by
applying content-chaining to the different fragments, based on
cryptographic hash functionality. The aim of this technique is to allow a
receiver to identify illegitimate fragments.</t>
<t>Further attacks may involve sending overlapped fragments (i.e. comprising
some overlapping parts of the original datagram) or announcing a datagram
size in the first fragment that does not reflect the actual amount of data
carried by the fragments. Implementers should make sure that correct
operation is not affected by such events.</t>
</section>
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<section anchor="ACKs" title="Acknowledgments">
<t>In section 2, the authors have reused extensive parts of text available in section 5.3 of RFC 4944, and would like to thank the authors of RFC 4944.</t>
<t>The authors would like to thank Carsten Bormann, Tom Phinney, Ana Minaburo and Laurent Toutain for valuable comments that helped improve the document.</t>
<t>Carles Gomez has been funded in part by the Spanish Government (Ministerio de Educacion, Cultura y Deporte) through the Jose Castillejo grant CAS15/00336. Part of his contribution to this work has been carried out during his stay as a visiting scholar at the Computer Laboratory of the University of Cambridge.</t>
</section>
<section title="Annex A. Quantitative performance comparison of RFC 4944 fragmentation header with 6LoFH">
<figure anchor="AdaptOverhead"
title="Adaptation layer fragmentation overhead (in bytes) required to transport an IPv6 datagram">
<artwork><![CDATA[
+-------------------------------------------------------+
| IPv6 datagram size (bytes) |
+-------------+-------------+-------------+-------------+
| 40 | 100 | 640 | 1280 |
+------------------+-------------+-------------+-------------+-------------+
|L2 payload (bytes)| 4944 |6LoFH | 4944 |6LoFH | 4944 |6LoFH | 4944 |6LoFH |
+------------------+-------------+-------------+-------------+-------------+
| 10 | ---- | 18 | ---- | 45 | ---- | 276 | ---- | 549 |
+------------------+-------------+------+------+------+------+-------------+
| 20 | 19 | 9 | 59 | 18 | 394 | 114 | 794 | 228 |
+------------------+-------------+------+------+------+------+-------------+
| 40 | 0 | 0 | 19 | 9 | 99 | 54 | 199 | 105 |
+------------------+-------------+------+------+------+------+-------------+
| 60 | 0 | 0 | 9 | 6 | 69 | 36 | 134 | 69 |
+------------------+-------------+------+------+------+------+-------------+
| 80 | 0 | 0 | 9 | 6 | 44 | 27 | 89 | 51 |
+------------------+-------------+------+------+------+------+-------------+
| 100 | 0 | 0 | 0 | 0 | 39 | 21 | 74 | 42 |
+------------------+-------------+------+------+------+------+-------------+
]]></artwork>
</figure>
<t>Note 1: while IEEE 802.15.4-2003 allows a maximum L2 payload size between 81
and 102 bytes, a range of L2 payload size between 10 and 100 bytes is
considered in the study to illustrate the performance of 6LoFH also for other
potential L2 technologies with short payload size and without fragmentation
support.</t>
<t>Note 2: with the RFC 4944 fragmentation header it is not possible to transport IPv6 datagrams of the considered sizes over a 10-byte payload L2 technology.</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
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<?rfc include='reference.I-D.minaburo-lpwan-gap-analysis'?>
<reference anchor="HHWH">
<front>
<title>6LoWPAN fragmentation attacks and mitigation mechanisms</title>
<author fullname="Rene" initials="R." surname=" Hummen et al">
</author>
<date year="2013"/>
</front>
</reference>
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