LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDPAcklio2bis rue de la Chataigneraie35510 Cesson-Sevigne CedexFranceana@ackl.ioIMT-Atlantique2 rue de la ChataigneraieCS 1760735576 Cesson-Sevigne CedexFranceLaurent.Toutain@imt-atlantique.frUniversitat Politècnica de CatalunyaC/Esteve Terradas, 708860 CastelldefelsSpaincarlesgo@entel.upc.edulpwan Working GroupThis document describes a header compression scheme and fragmentation functionality
for IPv6/UDP protocols. These techniques are especially tailored for LPWAN (Low Power Wide Area Network) networks
and could be extended to other protocol stacks.The Static Context Header Compression (SCHC) offers a great level of flexibility
when processing the header fields.
Static context means that information stored in the context which, describes field values, does not change during
the packet transmission, avoiding complex resynchronization mechanisms, incompatible
with LPWAN characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small identifier.This document describes the generic compression/decompression process and applies it
to IPv6/UDP headers. Similar mechanisms for other protocols such as CoAP will be described in a
separate document. Moreover, this document specifies fragmentation and reassembly mechanims for SCHC compressed packets exceeding the L2 pdu size and for the case where the SCHC compression is not possible then the IPv6/UDP packet is sent.Header compression is mandatory to efficiently bring Internet connectivity to the node
within a LPWAN network .Some LPWAN networks properties can be exploited for an efficient header
compression:Topology is star oriented, therefore all the packets follow the same path.
For the needs of this draft, the architecture can be summarized to Things or End-Systems
(ES) exchanging information with LPWAN Application Server (LA) through a Network Gateway (NG).Traffic flows are mostly known in advanced, since End-Systems embed built-in
applications. Contrary to computers or smartphones, new applications cannot
be easily installed.The Static Context Header Compression (SCHC) is defined for this environment.
SCHC uses a context where header information is kept in order, this context is
static the values on the header fields do not change during time, avoiding
complex resynchronization mechanisms, incompatible
with LPWAN characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small context identifier.The SCHC header compression is indedependent of the specific LPWAN technology over which it will be used.On the other hand, LPWAN technologies are characterized,
among others, by a very reduced data unit and/or payload size
. However, some of these technologies
do not support layer two fragmentation, therefore the only option for
these to support IPv6 when header compression is not possible (and, in particular, its MTU requirement of
1280 bytes ) is the use of fragmentation mechanism at the
adaptation layer below IPv6. This specification defines fragmentation
functionality to support the IPv6 MTU requirements over LPWAN
technologies.This section defines the terminology and aconyms used in this document.CDF: Compression/Decompression Function. A function that is used for both functionnalities to compress a header field or to recover its original value in the decompression phase.Context: A set of rules used to compress/decompress headersES: End System. Node connected to the LPWAN. An ES may implement SCHC.LA: LPWAN Application. An application sending/consuming IPv6 packets to/from the End System.LC: LPWAN Compressor/Decompressor. A process in the network to achieve compression/decompressing headers. LC uses SCHC rules to perform compression and decompression.MO: Matching Operator. An operator used to compare a value contained in a header field with a value contained in a rule.Rule: A set of header field values.Rule ID: An identifier for a rule, LC and ES share the same rule ID for a specific flow. Rule ID
is sent on the LPWAN.TV: Target value. A value contained in the rule that will be matched with the value of a header field.Static Context Header Compression (SCHC) avoids context synchronization,
which is the most bandwidth-consuming operation in other header compression mechanisms
such as RoHC. Based on the fact
that the nature of data flows is highly predictable in LPWAN networks, a static
context may be stored on the End-System (ES). The context must be stored in both ends. It can
also be learned by using a provisionning protocol that is out of the scope of this draft. based on terminology represents the architecture for
compression/decompression. The Thing or End-System is running applications which produce IPv6 or IPv6/UDP
flows. These flows are compressed by a LPWAN Compressor (LC) to reduce the headers size. Resulting
information is sent on a layer two (L2) frame to the LPWAN Radio Network to a Radio Gateway (RG) which forwards
the frame to a Network Gateway.
The Network Gateway sends the data to a LC for decompression which shares the same rules with the ES. The LC can be
located on the Network Gateway or in another places if a tunnel is established between the NG and the LC.
This architecture forms a star topology. After decompression, the packet can be sent on the Internet to one
or several LPWAN Application Servers (LA).The principle is exactly the same in the other direction.The context contains a list of rules (cf. ). Each rule contains
itself a list of fields descriptions composed of a field identifier (FID), a target
value (TV), a matching operator (MO) and a Compression/Decompression Function
(CDF).The rule does not describe the original packet format which
must be known from the compressor/decompressor. The rule just describes the
compression/decompression behavior for the header fields. In the rule, it is recommended
to describe the header field in the same order they appear in the packet.The main idea of the compression scheme is to send the rule id to the other end instead
of known field values. When a value is known by both
ends, it is not necessary to send it on the LPWAN network.The field description is composed of different entries:A Field ID (FID) is a unique value to define the field.A Target Value (TV) is the value used to make the comparison with
the packet header field. The Target Value can be of any type (integer, strings,…).
It can be a single value or a more complex structure (array, list,…). It can
be considered as a CBOR structure.A Matching Operator (MO) is the operator used to make the comparison between
the field value and the Target Value. The Matching Operator may require some
parameters, which can be considered as a CBOR structure. MO is only used during
the compression phase.A Compression Decompression Function (CDF) is used to describe the compression
and the decompression process. The CDF may require some
parameters, which can be considered as a CBOR structure.Rule IDs are sent between both compression/decompression elements. The size
of the rule ID is not specified in this document and can vary regarding the
LPWAN technology, the number of flows,…Some values in the rule ID space may be reserved for goals other than header
compression, for example fragmentation.Rule IDs are specific to an ES. Two ESs may use the same rule ID for different
header compression. The LC needs to combine the rule ID with the ES L2 address
to find the appropriate rule.The compression/decompression process follows several steps:compression rule selection: the goal is to identify which rule(s) will be used
to compress the headers. Each field is associated to a matching operator for
compression. Each header field’s value is compared to the corresponding target
value stored in the rule for that field using the matching operator. If all
the fields in the packet’s header satisfied all the matching operators of
a rule, the packet is processed using Compression Decompression Function associated
with the fields. Otherwise the next rule
is tested. If no eligible rule is found, then the packet is sent without compression,
which may require using the fragmentation procedure.sending: The rule ID is sent to the other end followed by information resulting
from the compression of header fields. This information is sent in the order expressed in the rule for the matching
fields. The way the rule ID is sent depends on the
layer two technology and will be specified in a specific document. For example,
it can either be included in a Layer 2 header or sent in the first byte of
the L2 payload.decompression: The receiver identifies the sender through its device-id
(e.g. MAC address) and selects the appropriate rule through the rule ID. This
rule gives the compressed header format and associates these values to header fields.
It applies the CDF function to reconstruct the original
header fields. CDF of Compute-* must be applied after the other CDFs.This document describes basic matching operators (MO)s which must be known by both LC, endpoints involved in the header compression/decompression. They are
not typed and can be applied indifferently to integer, string or any other type. The MOs and their definition are provided next:equal: a field value in a packet matches with a field value in a rule if
they are equal.ignore: no check is done between a field value in a packet and a field value
in the rule. The result of the matching is always true.MSB(length): a field value of a size equal to “length” bits in a packet matches with a field value
in a rule if the most significant “length” bits are equal.match-mapping: The goal of mapping-sent is to reduce the size of a field by allocating
a shorter value. The Target Value contains a list of pairs. Each pair is composed of
a value and a short ID. This operator matches if a field value is equal to one of the pairs’
values.Matching Operators may need a list of parameters to proceed to the matching. For instance MSB requires an
integer indicating the number of bits to test.The Compression Decompression Functions (CDF) describes the action taken during
the compression of headers fields, and inversely, the action taken by the decompressor to restore
the original value. sumarizes the functions defined to compress and decompress
a field. The first column gives the function’s name. The second and third
columns outlines the compression/decompression behavior.Compression is done in the rule order and compressed values are sent in that order in the compressed
message. The receiver must be able to find the size of each compressed field
which can be given by the rule or may be sent with the compressed header.Not-sent function is generally used when the field value is specified in the rule and
therefore known by the both Compressor and Decompressor. This function is generally used with the
“equal” MO. If MO is “ignore”, there is a risk to have a decompressed field
value different from the compressed field.The compressor does not send any value on the compressed header for that field on which compression is applied.The decompressor
restores the field value with the target value stored in the matched rule.The value-sent function is generally used when the field value is not known by both Compressor and Decompressor.
The value is sent in the compressed message header. Both Compressor and Decompressor must know the
size of the field, either implicitely (the size is known by both sides)
or explicitely in the compressed header
field by indicating the length. This function is generally used with the “ignore” MO.The compressor sends the Target Value stored on the rule in the compressed
header message. The decompressor restores the field value with the one received
from the LPWANLSB function is used to send a fixed part of the packet field header to the other end.
This function is used together with the “MSB” MOThe compressor sends the “length” Least Significant Bits. The decompressor
combines with an OR operator the value received with the Target Value.These functions are used to process respectively the End System and the LA
Device Identifier (DID).The IID value is computed from the device ID present in the Layer 2 header. The
computation depends on the technology and the device ID size.mapping-sent is used to send a smaller index associated to the field value
in the Target Value. This function is used together with the “match-mapping” MO.The compressor looks in the TV to find the field value and send the corresponding index.
The decompressor uses this index to restore the field value.These functions are used by the decompressor to compute the compressed field value based on received information.
Compressed fields are elided during the compression and reconstructed during the decompression.compute-length: compute the length assigned to this field. For instance, regarding
the field ID, this CDF may be used to compute IPv6 length or UDP length.compute-checksum: compute a checksum from the information already received by the LC.
This field may be used to compute UDP checksum.This section lists the different IPv6 and UDP header fields and how they can be compressed.This field always holds the same value, therefore the TV is 6, the MO is “equal”
and the CDF “not-sent”.If the DiffServ field identified by the rest of the rule do not vary and is known
by both sides, the TV should contain this wellknown value, the MO should be “equal”
and the CDF must be “not-sent.If the DiffServ field identified by the rest of the rule varies over time or is not
known by both sides, then there are two possibilities depending on the variability of the value,
the first one there is without compression and the original value is sent, or
the sencond where the values can be computed by sending only the LSB bits:TV is not set, MO is set to “ignore” and CDF is set to “value-sent”TV contains a stable value, MO is MSB(X) and CDF is set to LSB(8-X)If the Flow Label field identified by the rest of the rule does not vary and is known
by both sides, the TV should contain this well-known value, the MO should be “equal”
and the CDF should be “not-sent”.If the Flow Label field identified by the rest of the rule varies during time or is not
known by both sides, there are two possibilities dpending on the variability of the value,
the first one is without compression and then the value is sent
and the second where only part of the value is sent and the decompressor needs to compute the original value:TV is not set, MO is set to “ignore” and CDF is set to “value-sent”TV contains a stable value, MO is MSB(X) and CDF is set to LSB(20-X)If the LPWAN technology does not add padding, this field can be elided for the
transmission on the LPWAN network. The LC recompute the original payload length
value. The TV is not set, the MO is set to “ignore” and the CDF is “compute-IPv6-length”.If the payload is small, the TV can be set to 0x0000, the MO set to “MSB (16-s)” and the
CDF to “LSB (s)”. The ‘s’ parameter depends on the maximum packet length.On other cases, the payload length field must be sent and the CDF is replaced by “value-sent”.If the Next Header field identified by the rest of the rule does not vary and is known
by both sides, the TV should contain this Next Header value, the MO should be “equal”
and the CDF should be “not-sent”.If the Next header field identified by the rest of the rule varies during time or is not
known by both sides, then TV is not set, MO is set to “ignore” and CDF is set to
“value-sent”.The End System is generally a host and does not forward packets, therefore the
Hop Limit value is constant. So the TV is set with a default value, the MO
is set to “equal” and the CDF is set to “not-sent”.Otherwise the value is sent on the LPWAN: TV is not set, MO is set to ignore and
CDF is set to “value-sent”.As in 6LoWPAN , IPv6 addresses are split into two 64-bit long fields;
one for the prefix and one for the Interface Identifier (IID). These fields should
be compressed. To allow a single rule, these values are identified by their role
(ES or LA) and not by their position in the frame (source or destination). The LC
must be aware of the traffic direction (upstream, downstream) to select the appropriate
field.Both ends must be synchronized with the appropriate prefixes. For a specific flow,
the source and destination prefix can be unique and stored in the context. It can
be either a link-local prefix or a global prefix. In that case, the TV for the
source and destination prefixes contains the values, the MO is set to “equal” and
the CDF is set to “not-sent”.In case the rule allows several prefixes, static mapping must be used. The
different prefixes are listed in the TV associated with a short ID. The MO is set
to “match-mapping” and the CDF is set to “mapping-sent”.Otherwise the TV contains the prefix, the MO is set to “equal” and the CDF is set to
value-sent.If the ES or LA IID are based on an LPWAN address, then the IID can be reconstructed
with information coming from the LPWAN header. In that case, the TV is not set, the MO
is set to “ignore” and the CDF is set to “ESiid-DID” or “LAiid-DID”. Note that the
LPWAN technology is generally carrying a single device identifier corresponding
to the ES. The LC may also not be aware of these values.For privacy reasons or if the ES address is changing over time, it maybe better to
use a static value. In that case, the TV contains the value, the MO operator is set to
“equal” and the CDF is set to “not-sent”.If several IIDs are possible, then the TV contains the list of possible IID, the MO is
set to “match-mapping” and the CDF is set to “mapping-sent”.Otherwise the value variation of the IID may be reduced to few bytes. In that case, the TV is
set to the stable part of the IID, the MO is set to MSB and the CDF is set to LSB.Finally, the IID can be sent on the LPWAN. In that case, the TV is not set, the MO is set
to “ignore” and the CDF is set to “value-sent”.No extension rules are currently defined. They can be based on the MOs and
CDFs described above.To allow a single rule, the UDP port values are identified by their role
(ES or LA) and not by their position in the frame (source or destination). The LC
must be aware of the traffic direction (upstream, downstream) to select the appropriate
field. The following rules apply for ES and LA port numbers.If both ends knows the port number, it can be elided. The TV contains the port number,
the MO is set to “equal” and the CDF is set to “not-sent”.If the port variation is on few bits, the TV contains the stable part of the port number,
the MO is set to “MSB” and the CDF is set to “LSB”.If some well-known values are used, the TV can contain the list of this values, the
MO is set to “match-mapping” and the CDF is set to “mapping-sent”.Otherwise the port numbers are sent on the LPWAN. The TV is not set, the MO is
set to “ignore” and the CDF is set to “value-sent”.If the LPWAN technology does not introduce padding, the UDP length can be computed
from the received data. In that case the TV is not set, the MO is set to “ignore” and
the CDF is set to “compute-UDP-length”.If the payload is small, the TV can be set to 0x0000, the MO set to “MSB” and the
CDF to “LSB”.On other cases, the length must be sent and the CDF is replaced by “value-sent”.IPv6 mandates a checksum in the protocol above IP. Nevertheless, if a more efficient
mechanism such as L2 CRC or MIC is carried by or over the L2 (such as in the
LPWAN fragmentation process (see XXXX)), the UDP checksum transmission can be avoided.
In that case, the TV is not set, the MO is set to “ignore” and the CDF is set to
“compute-UDP-checksum”.In other cases the checksum must be explicitly sent. The TV is not set, the MO is set to
“ignore” and the CDF is set to “value-sent”.This section gives some scenarios of the compression mechanism for IPv6/UDP.
The goal is to illustrate the SCHC behavior.The most common case using the mechanisms defined in this document will be a
LPWAN end-system that embeds some applications running over
CoAP. In this example, three flows are considered. The first flow is for the device management based
on CoAP using
Link Local IPv6 addresses and UDP ports 123 and 124 for ES and LA, respectively.
The second flow will be a CoAP server for measurements done by the end-system
(using ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to beta::1/64.
The last flow is for legacy applications using different ports numbers, the
destination IPv6 address prefix is gamma::1/64. presents the protocol stack for this End-System. IPv6 and UDP are represented
with dotted lines since these protocols are compressed on the radio link.Note that in some LPWAN technologies, only the End Systems have a device ID.
Therefore, when such technologie are used, it is necessary to define statically an IID for the Link
Local address for the LPWAN compressor.All the fields described in the three rules are present
in the IPv6 and UDP headers. The ESDevice-ID value is found in the L2
header.The second and third rules use global addresses. The way the ES learns the
prefix is not in the scope of the document.The third rule compresses port numbers to 4 bits.Fragmentation support in LPWAN is mandatory and it is used if, after SCHC header compression,
the size of the resulting packet is larger than the L2 data unit maximum payload. Fragmentation is also used if SCHC header compression has not been able to compress a packet that is larger than the L2 data unit maximum payload. In LPWAN technologies the L2 data unit size typically varies from tens to hundreds of bytes.
If the entire IPv6 datagram fits within a single L2
data unit, the fragmentation mechanism is not used and the packet is sent unfragmented.
If the datagram does not fit within a single L2 data unit,
it SHALL be broken into fragments.Moreover, LPWAN technologies impose some strict limitations on traffic;
therefore it is desirable to enable optional fragment retransmission, while
a single fragment loss should not lead to retransmitting the full datagram.
To preserve energy, Things (End Systems) are sleeping most of the time
and may receive data during a short period of time after transmission.In order to adapt to the capabilities of various LPWAN technologies,
this specification allows for a gradation of fragment delivery reliability.
There are three main options: Unreliable (UnR) mode, Reliable per-Packet (RpP) mode and Reliable per-Window (RpW) mode.
Additionally, the specification provides the option to withhold acknowledgments (ACK) in case of success, making effectively the ACK a Negative ACK (NACK). It is up to the underlying LPWAN technology to decide which setting to use and whether the same setting applies to all IPv6 packets. Note that the fragment delivery reliability
option to be used is not necessarily tied to the particular characteristics of the
underlying L2 LPWAN technology (e.g. UnR may be used on top of an L2
LPWAN technology with symmetric characteristics for uplink and downlink).The same reliability option MUST be used for all fragments of a packet.In UnR mode, the receiver MUST NOT issue acknowledgments. In RpP mode, the receiver may transmit one acknowledgment (ACK) after all fragments carrying an IPv6 packet have been transmitted. The ACK informs the sender about received
and missing fragments from the IPv6 packet. In RpW mode, an ACK may be transmitted by
the fragment receiver after a window of fragments have been sent. A window of fragments is
a subset of the full set of fragments needed to carry an IPv6 packet. In this mode, the ACK informs the
sender about received and missing fragments from the window of fragments. In either mode,
upon receipt of an ACK that informs about any lost fragments, the sender may retransmit the
lost fragments. The maximum number of ACK and retransmission rounds is TBD.Some LPWAN deployments may benefit from conditioning the creation and transmission of an ACK
to the detection of at least one fragment loss (per-packet or per-window), thus leading to
NACK-oriented behavior, while not having such condition may be preferred for other scenarios.This document does not make any decision as to whether UnR, RpP or RpW modes are used, or
or whether the transmission of ACKs is conditioned to the detection of fragment losses or not.
A complete specification of the receiver and sender behaviors that correspond to each
acknowledgment policy is also out of scope. Nevertheless, this document does provide
examples of the different reliability options described.A fragment comprises a fragmentation header and a fragment payload, and conforms
to the format shown in . The fragment payload carries a subset of either the
IPv6 packet after header compression or an IPv6 packet which could not be compressed.
A fragment is the payload in the L2 protocol data unit (PDU).Fragments except the last one SHALL
contain the fragmentation header as defined in . The total size of this fragmentation header is R bits.The last fragment SHALL contain a fragmentation header that conforms to
the format shown in . The total size of this fragmentation
header is R+M bits.Rule ID: this field has a size of R – N bits in all
fragments. Rule ID may be used to signal whether UnR, RpP or RpW mode is in use,
and within the latter, whether window mode or packet mode are used.CFN: CFN stands for Compressed Fragment Number. The size of the CFN field is N bits.
In UnR mode, N=1. For RpP or RpW modes, N equal to or greater than 3 is recommended. This field
is an unsigned integer that carries a non-absolute fragment number. The CFN MUST be set
sequentially decreasing from 2^N - 2 for the first fragment, and MUST wrap from 0 back to
2^N - 2 (e.g. for N=3, the first fragment has CFN=6, subsequent CFNs are set sequentially
and in decreasing order, and CFN will wrap from 0 back to 6). The CFN for the last fragment
has all bits set to 1. Note that, by this definition, the CFN value of 2^N - 1 is only used
to identify a fragment as the last fragment carrying a subset of the IPv6 packet being transported,
and thus the CFN does not strictly correspond to the N least significant bits of the actual
absolute fragment number. It is also important to note that, for N=1, the last fragment
of the packet will carry a CFN equal to 1, while all previous fragments will carry a CFN of 0.MIC: MIC stands for Message Integrity Check. This field has a size of M
bits. It is computed by the sender over the complete IPv6 packet before fragmentation by
using the TBD algorithm. The MIC allows to check for errors in the reassembled IPv6 packet,
while it also enables compressing the UDP checksum by use of SCHC.The values for R, N and M are not specified in this document, and have to be determined by the underlying
LPWAN technology.The format of an ACK is shown in :Rule ID: In all ACKs, Rule ID has a size of R bits and SHALL be set to
TBD_ACK to signal that the message is an ACK.bitmap: size of the bitmap field of an ACK can be equal to 0 or
Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments denotes
the number of fragments of a window (in RpW mode) or the number of fragments
that carry the IPv6 packet (in RpP mode). The bitmap is a sequence of bits,
where the n-th bit signals whether the n-th fragment transmitted has been
correctly received (n-th bit set to 1) or not (n-th bit set to 0). Remaining bits
with bit order greater than the number of fragments sent (as determined by
the receiver) are set to 0, except for the last bit in the bitmap, which is set to 1
if the last fragment (carrying the MIC) has been correctly received, and 0 otherwise.
Absence of the bitmap in an ACK confirms correct reception of all fragments to be
acknowledged by means of the ACK. shows an example of an ACK in packet mode, where the bitmap
indicates that the second and the ninth fragments have not been correctly
received. In this example, the IPv6 packet is carried by eleven fragments
in total, therefore the bitmap has a size of two bytes. shows an example of an ACK in RpW (N=3), where the bitmap
indicates that the second and the fifth fragments have not been correctly received. illustrates an ACK without bitmap.The receiver of link fragments SHALL use (1) the sender’s L2 source address (if present),
(2) the destination’s L2 address (if present), and (3) Rule ID to identify all the fragments
that belong to a given datagram. The fragment receiver SHALL determine the fragment delivery
reliability option in use for the fragment based on the Rule ID field in that fragment.Upon receipt of a link fragment, the receiver starts constructing the original
unfragmented packet. It uses the CFN and the order of arrival of each fragment to
determine the location of the individual fragments within the original unfragmented packet.
For example, it may place the data payload of the fragments within a payload datagram
reassembly buffer at the location determined from the CFN and order of arrival of the
fragments, and the fragment payload sizes. Note that the size of the original, unfragmented
IPv6 packet cannot be determined from fragmentation headers.In RpW mode, when a fragment with all CFN bits set to 0 is received, the recipient MAY transmit
an ACK for the last window of fragments sent. Note that the first fragment of the window is
the one sent with CFN=2^N-2. In RpW mode, the fragment with CFN=0 is considered the
last fragment of its window, except for the last fragment of the whole packet (with all CFN bits set to 1), which is also the last fragment of the last window.Once the recipient has received the last fragment, it checks for the integrity of the
reassembled IPv6 datagram, based on the MIC received. In UnR mode, if the integrity
check indicates that the reassembled IPv6 datagram does not match the original IPv6 datagram
(prior to fragmentation), the reassembled IPv6 datagram MUST be discarded. In RpP or in RpW mode, upon
receipt of the last fragment (i.e. with all CFN bits set to 1), the recipient MAY transmit an ACK for the whole set of fragments sent that carry the complete IPv6 packet.In RpP mode or in RpW mode, the sender retransmits any lost fragments reported in the ACK. A maximum
of TBD iterations of ACK and fragment retransmission rounds are allowed per-window or per-IPv6-packet
in RpP mode or in RpW mode, respectively. A complete specification of the mechanisms needed to
enable the above described fragment delivery reliability options is out of the scope of this document.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 of a packet, 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).TBDThis subsection describes potential attacks to LPWAN fragmentation
and proposes countermeasures, based on existing analysis of attacks
to 6LoWPAN fragmentation {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
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 IPv6 datagram).
Implementers should make sure that correct operation is not affected
by such event.Thanks to Dominique Barthel, Carsten Bormann, Arunprabhu Kandasamy, Antony Markovski, Alexander
Pelov, Pascal Thubert, Juan Carlos Zuniga for useful design consideration.Transmission of IPv6 Packets over IEEE 802.15.4 NetworksThis document describes the frame format for transmission of IPv6 packets and the method of forming IPv6 link-local addresses and statelessly autoconfigured addresses on IEEE 802.15.4 networks. Additional specifications include a simple header compression scheme using shared context and provisions for packet delivery in IEEE 802.15.4 meshes. [STANDARDS-TRACK]Internet Protocol, Version 6 (IPv6) SpecificationThis document specifies version 6 of the Internet Protocol (IPv6), also sometimes referred to as IP Next Generation or IPng. [STANDARDS-TRACK]LP-WAN GAP AnalysisLow Power Wide Area Networks (LP-WAN) are different technologies covering different applications based on long range, low bandwidth and low power operation. The use of IETF protocols in the LP-WAN technologies should contribute to the deployment of a wide number of applications in an open and standard environment where actual technologies will be able to communicate. This document makes a survey of the principal characteristics of these technologies and covers a cross layer analysis on how to adapt and use the actual IETF protocols, but also the gaps for the integration of the IETF protocol stack in the LP-WAN technologies.LPWAN OverviewLow Power Wide Area Networks (LPWAN) are wireless technologies with characteristics such as large coverage areas, low bandwidth, possibly very small packet and application layer data sizes and long battery life operation. This memo is an informational overview of the set of LPWAN technologies being considered in the IETF and of the gaps that exist between the needs of those technologies and the goal of running IP in LPWANs.This section provides examples of different fragment delivery reliability options possible
on the basis of this specification. illustrates the transmission of an IPv6 packet that needs 11 fragments
in UnR mode. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpP mode, for N=3, NACK-oriented, without losses. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpP mode, for N=3, NACK-oriented, with three losses. illustrates the transmission of an IPv6 packet that needs 11 fragments in RpW mode, for N=3, without losses. Receiver feedback is NACK-oriented. Note: in RpW mode, an additional bit will be needed to number windows. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpW mode, for N=3, with three losses. Receiver feedback is NACK-oriented. Note: in RpW mode,
an additional bit will be needed to number windows. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpP mode, for N=3, without losses. Receiver feedback is positive-ACK-oriented. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpP mode, for N=3, with three losses. Receiver feedback is positive-ACK-oriented. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpW mode, for N=3, without losses. Receiver feedback is positive-ACK-oriented. Note: in RpW mode,
an additional bit will be needed to number windows. illustrates the transmission of an IPv6 packet that needs 11 fragments
in RpW mode, for N=3, with three losses. Receiver feedback is positive-ACK-oriented. Note: in RpW mode,
an additional bit will be needed to number windows.Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish Government
through project TEC2016-79988-P. 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.