Internet DRAFT - draft-gomez-6lo-optimized-fragmentation-header
draft-gomez-6lo-optimized-fragmentation-header
6lo Working Group C. Gomez
Internet-Draft J. Paradells
Intended status: Standards Track UPC/i2CAT
Expires: April 26, 2017 J. Crowcroft
University of Cambridge
October 23, 2016
Optimized 6LoWPAN Fragmentation Header
draft-gomez-6lo-optimized-fragmentation-header-00
Abstract
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.
Status of This Memo
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This Internet-Draft will expire on April 26, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. 6LoFH rules and format . . . . . . . . . . . . . . . . . . . 3
3. Changes from RFC 4944 fragmentation header and rationale . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. Annex A. Quantitative performance comparison of RFC 4944
fragmentation header with 6LoFH . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
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 [RFC4944]. One of the 6LoWPAN
protocol suite components is fragmentation, which fulfills the IPv6
MTU requirement of 1280 bytes [RFC2460] over a radio interface with a
layer two (L2) payload size around 100 bytes (in the best case) and
without fragmentation support [RFC4944].
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:
-- 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.
-- 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.
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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.
1.1. Conventions used in this document
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 [RFC2119]
2. 6LoFH rules and format
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 Figure 1.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: First Fragment
The second and subsequent fragments (up to and including the last)
SHALL contain a fragmentation header that conforms to the format
shown in Figure 2.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Subsequent Fragments
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 [RFC4944] of the packet. Note that this
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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).
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.
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.
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.
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.
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).
3. Changes from RFC 4944 fragmentation header and rationale
The main changes introduced in this specification to the
fragmentation header format defined in RFC 4944 are listed below,
together with their rationale:
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-- 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.
-- 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.
-- The datagram offset size is increased from 8 bits to 11 bits.
Rationale: This allows to express the datagram offset in single-octet
increments.
4. IANA Considerations
This document allocates the following sixteen RFC 4944 Dispatch type
values:
11001 000
through
11001 111
and
11010 000
through
11010 111
5. Security Considerations
6LoWPAN fragmentation attacks have been analyzed in the literature.
Countermeasures to these have been proposed as well [HHWH].
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
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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.
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.
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.
6. Acknowledgments
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.
The authors would like to thank Carsten Bormann, Tom Phinney, Ana
Minaburo and Laurent Toutain for valuable comments that helped
improve the document.
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.
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7. Annex A. Quantitative performance comparison of RFC 4944
fragmentation header with 6LoFH
+-------------------------------------------------------+
| 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 |
+------------------+-------------+------+------+------+------+-------------+
Figure 3: Adaptation layer fragmentation overhead (in bytes) required
to transport an IPv6 datagram
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.
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.
8. References
8.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
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[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,
<http://www.rfc-editor.org/info/rfc4944>.
[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,
<http://www.rfc-editor.org/info/rfc6282>.
8.2. Informative References
[HHWH] Hummen et al, R., "6LoWPAN fragmentation attacks and
mitigation mechanisms", 2013.
[I-D.minaburo-lpwan-gap-analysis]
Minaburo, A., Gomez, C., Toutain, L., Paradells, J., and
J. Crowcroft, "LPWAN Survey and GAP Analysis", draft-
minaburo-lpwan-gap-analysis-02 (work in progress), October
2016.
Authors' Addresses
Carles Gomez
UPC/i2CAT
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
Josep Paradells
UPC/i2CAT
C/Jordi Girona, 1-3
Barcelona 08034
Spain
Email: josep.paradells@entel.upc.edu
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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