LWIG Working Group M. Kovatsch
Internet-Draft ETH Zurich
Intended status: Informational O. Bergmann
Expires: January 02, 2014 Universitaet Bremen TZI
A. Castellani
University of Padova
E. Dijk
Philips Research
C. Bormann, Ed.
Universitaet Bremen TZI
July 01, 2013
CoAP Implementation Guidance
draft-kovatsch-lwig-coap-00
Abstract
The Constrained Application Protocol (CoAP) is designed for resource-
constrained nodes and networks, e.g., sensor nodes in low-power lossy
networks (LLNs). Still, to implement this Internet protocol on Class
1 devices, i.e., ~ 10 KiB of RAM and ~ 100 KiB of ROM, lightweight
implementation techniques are necessary. This document provides
lessons learned from implementing CoAP for tiny, battery-operated
networked embedded systems. The guidelines for transmission state
management and developer APIs can also help with the implementation
of CoAP for less constrained nodes.
Status of This Memo
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This Internet-Draft will expire on January 02, 2014.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Message Processing . . . . . . . . . . . . . . . . . . . . . 3
2.1. Message Buffer Usage . . . . . . . . . . . . . . . . . . 4
2.2. Header Option Parsing and Management . . . . . . . . . . 5
2.2.1. On-the-fly Processing . . . . . . . . . . . . . . . . 5
2.2.2. Internal Data Structure . . . . . . . . . . . . . . . 6
2.3. Retransmissions . . . . . . . . . . . . . . . . . . . . . 6
2.4. Deduplication . . . . . . . . . . . . . . . . . . . . . . 7
2.4.1. Managing Peer MIDs . . . . . . . . . . . . . . . . . 7
2.4.2. Resource-specific Deduplication . . . . . . . . . . . 10
3. Transmission State Management . . . . . . . . . . . . . . . . 10
3.1. Request/Response Layer . . . . . . . . . . . . . . . . . 11
3.2. Message Layer . . . . . . . . . . . . . . . . . . . . . . 12
4. Observing . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Block-wise Transfers . . . . . . . . . . . . . . . . . . . . 13
6. Application Developer API . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
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The Constrained Application Protocol [I-D.ietf-core-coap] has been
designed specifically for machine-to-machine communication in
networks with very constrained nodes. Typical application scenarios
therefore include building automation and the Internet of Things.
The major design objectives have been set on small protocol overhead,
robustness against packet loss, and against high latency induced by
small bandwidth shares or slow request processing in end nodes. To
leverage integration of constrained nodes with the world-wide
Internet, the protocol design was led by the REST architectural style
that accounts for the scalability and robustness of the Hypertext
Transfer Protocol [RFC2616].
Lightweight implementations benefit from this design in many
respects: First, the use of Uniform Resource Identifiers (URIs) for
naming resources and the transparent forwarding of their
representations in a server-stateless request/response protocol make
protocol translation to HTTP a straightforward task. Second, the set
of protocol elements that are unavoidable for the core protocol and
thus must be implemented on every node has been kept very small,
minimizing the unnecessary accumulation of "optional" features.
Options that - when present - are critical for message processing are
explicitly marked as such to force immediate rejection of messages
with unknown critical options. Third, the syntax of protocol data
units is easy to parse and is carefully defined to avoid creation of
state in servers where possible.
Although these features enable lightweight implementations of the
Constrained Application Protocol, there is still a tradeoff between
robustness and latency of constrained nodes on one hand and resource
demands (such as battery consumption, dynamic memory needs, and
static code size) on the other. The present document gives some
guidance on possible strategies to solve this tradeoff for very
constrained nodes (Class 1 in [I-D.ietf-lwig-terminology]). The main
focus is on servers as this is deemed the predominant case where CoAP
applications are faced with tight resource constraints.
Additional considerations for the implementation of CoAP on tiny
sensors are given in [I-D.arkko-core-sleepy-sensors].
2. Message Processing
For constrained nodes of Class 1 or even Class 2, the most limiting
factors for (wireless) network communication usually are RAM size and
battery lifetime. Most applications therefore try to minimize
internal buffer space for both transmit and receive operations, and
to maximize sleeping cycles.
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In the programming styles supported by very simple operating systems,
preemptive multi-threading is not an option. Instead, all operations
are triggered by an event loop system, e.g., in a send-receive-
dispatch cycle. It is also common practice to allocate memory
statically to ensure stable behavior, as no memory management unit
(MMU) or other abstractions are available. For a CoAP node, the two
key parameters for memory usage are the number of (re)transmission
buffers and the maximum message size that must be supported by each
buffer. Often the maximum message size is set far below the
1280-byte MTU of 6LoWPAN to allow more than one open confirmable
transmission at a time (in particular for observe notifications).
Note that implementations on constrained platforms often not even
support the full MTU. Larger messages must then use block-wise
transfers [I-D.ietf-core-block], while a good tradeoff between
6LoWPAN fragmentation and CoAP header overhead must be found.
Usually the amount of available free RAM dominates this decision.
For Class 1 devices, the maximum message size is typically 128 or 256
bytes plus an estimate of the maximum header size with a worst case
option setting.
2.1. Message Buffer Usage
The cooperative multi-threading of an event loop system allows to
optimize memory usage through in-place processing and reuse of
buffers, in particular the IP buffer provided by the OS.
CoAP servers can significantly benefit from in-place processing, as
they can create responses directly in the incoming IP buffer. Note
that an embedded OS usually only has a single buffer for incoming and
outgoing IP packets. Empty ACKs and RST messages can promptly be
assembled and sent using the IP buffer. The first few bytes of the
basic header are usually parsed into an internal data structure and
can be overwritten without harm. Also when a CoAP server only sends
piggy-backed or non-confirmable responses, no additional buffer is
required at the application layer. This, however, requires careful
timing so that no incoming data is overwritten before it was
processed. Because of cooperative multi-threading, this requirement
is relaxed, though.
For clients, this is only an option for non-confirmable requests that
do not need to be kept for retransmission. Using the IP also for
retransmissions would require to forbid any packet reception during
an open request. This is can only be applied in special cases.
Depending on the number of requests that can be handled in parallel,
an implementation might create a stub response filled with any option
that has to be copied from the original request to the separate
response, especially the Token option. The drawback of this
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technique is that the server must be prepared to receive
retransmissions of the previous (confirmable) request to which a new
acknowledgement must be generated. If memory is an issue, a single
buffer can be used for both tasks: Only the message type and code
must be updated, changing the message id is optional. Once the
resource representation is known, it is added as new payload at the
end of the stub response. Acknowledgements still can be sent as
described before as long as no additional options are required to
describe the payload.
2.2. Header Option Parsing and Management
There are two alternatives to handle the header: Either process the
header on the fly when an option is accessed or initially parse all
values into an internal data structure.
2.2.1. On-the-fly Processing
The advantage of on-the-fly processing is that the compact encoding
saves memory and fully reuses the buffer for incoming messages. The
basic message header information should be copied into an internal
data structure, as Message ID and/or Token are required for request/
response matching or generating the response. Once the message is
accepted for further processing, the set of options contained in the
received message must be decoded to check for unknown critical
options. To avoid multiple passes through the option list, the
option parser might maintain a bit-vector where each bit represents
an option number that is present in the received request. With the
wide range of options, the option number itself cannot be used to
indicate the number of left-shift operations to mask the
corresponding bit. Hence, an internal enum should be used to mask
the supported set options of an implementation in the bitmask.
In addition, the byte index of every option is added to a sparse list
(e.g., a one-dimensional array) for fast retrieval. This
particularly enables efficient reduced-function handling of options
that might occur more than once such as Uri-Path. In this
implementation strategy, the delta is zero for any subsequent path
segment, hence the stored byte index for option 9 (Uri-Path) will be
overwritten to hold a pointer to the last occurrence of that option,
i.e., only the last path component actually matters. (Of course,
this requires choosing resource names where the combination of (final
Uri-Path component, final Uri-Query component) is server-wide unique.
Note: Where skipping all but the last path segment is not feasible
for some reason, resource identification could be ensured by some
hash value calculated over the path segments. For each segment
encountered, the stored hash value is updated by the current
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option value. This works if a cheap _perfect hashing_ scheme can
be found for the resource names.
Once the option list has been processed at least up to the highest
option number that is supported by the application, any known
critical option and all elective options can be masked out to
determine if any unknown critical option was present. If this is the
case, this information can be used to create a 4.02 response
accordingly. (Note that the remaining options also must be processed
to add further critical options included in the original request.)
2.2.2. Internal Data Structure
Using an internal data structure for all parsed options has
advantages when processing the values, as they are already in a
variable of corresponding type and integers in host byte order. The
incoming payload and byte strings of the header can be accessed
directly in its IP buffer using pointers. This approach also
benefits from a bitmap. Otherwise special values must be reserved to
encode an unset option, which might require a larger type than
required for the actual value range (e.g., a 32-bit integer instead
of 16-bit).
The byte strings (e.g., the URI) are usually not required when
generating the response. Thus, this alternative also facilitates the
usage of the IP buffer for message assembly - all important values
are copied from the shared incoming/outgoing buffer.
Setting options for outgoing messages is also easier with an internal
data structure. Application developers can set options independent
from the option number order required for the delta encoding. The
CoAP encoding is then applied in a serialization step before sending.
On-the-fly processing might require extensive memmove operations to
insert new header options or needs to restrict developers to set
options in order.
2.3. Retransmissions
CoAP's reliable transmissions require the before-mentioned
retransmission buffers. Messages, such as the requests of a client,
should be stored in serialized form. For servers, retransmissions
apply for confirmable separate responses and confirmable
notifications [I-D.ietf-core-observe]. As separate responses stem
from long-lasting resource handlers, the response should be stored
for retransmission instead of re-dispatching a stored request (which
would allow for updating the representation). For confirmable
notifications, please see Section 2.6, as simply storing the response
can break the concept of eventual consistency.
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String payloads such as JSON require a buffer to print to. By
splitting the retransmission buffer into header and payload part, it
can be reused. First to generate the payload and then storing the
CoAP message by serializing into the same memory. Thus, providing a
retransmission for any message type can save the need for a separate
application buffer. This, however, requires an estimation about the
maximum expected header size to split the buffer and a memmove to
concatenate the two parts.
For platforms that disable clock tick interrupts in sleep states, the
application must take into consideration the clock deviation that
occurs during sleep (or ensure to remain in idle state until the
message has been acknowledged or the maximum number of
retransmissions is reached). Since CoAP allows up to four
retransmissions with a binary exponential back-off it could take up
to 45 seconds until the send operation is complete. Even in idle
state, this means substantial energy consumption for low-power nodes.
Implementers therefore might choose a two-step strategy: First, do
one or two retransmissions and then, in the later phases of back-off,
go to sleep until the next retransmission is due. In the meantime,
the node could check for new messages including the acknowledgement
for any confirmable message to send.
2.4. Deduplication
If CoAP is used directly on top of UDP (i.e., in NoSec mode), it
needs to cope with the fact that the UDP datagram transport can
reorder and duplicate messages. (In contrast to UDP, DTLS has its
own duplicate detection.) CoAP has been designed with protocol
functionality such that rejection of duplicate messages is always
possible. It is at the discretion of the receiver if it actually
wants to make use of this functionality. Processing of duplicate
messages comes at a cost, but so does the management of the state
associated with duplicate rejection. Hence, a receiver may have good
reasons to decide not to do the duplicate rejection. If duplicate
rejection is indeed necessary, e.g., for non-idempotent requests, it
is important to control the amount of state that needs to be stored.
2.4.1. Managing Peer MIDs
CoAP's duplicate rejection functionality can be straightforwardly
implemented in a CoAP end-point by storing, for each remote CoAP end-
point ("peer") that it communicates with, a list of recently received
CoAP Message IDs (MIDs) along with some timing information. A CoAP
message from a peer with a MID that is in the list for that peer can
simply be discarded.
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The timing information in the list can then be used to time out
entries that are older than the _expected extent of the re-ordering_,
an upper bound for which can be estimated by adding the _potential
retransmission window_ ([I-D.ietf-core-coap] section "Reliable
Messages") and the time packets can stay alive in the network.
Such a straightforward implementation is suitable in case other CoAP
end-points generate random MIDs. However, this storage method may
consume substantial RAM in specific cases, such as:
o many clients are making periodic, non-idempotent requests to a
single CoAP server;
o one client makes periodic requests to a large number of CoAP
servers and/or requests a large number of resources; where servers
happen to mostly generate separate CoAP responses (not piggy-
backed);
For example, consider the first case where the expected extent of re-
ordering is 50 seconds, and N clients are sending periodic POST
requests to a single CoAP server during a period of high system
activity, each on average sending one client request per second. The
server would need 100 * N bytes of RAM to store the MIDs only. This
amount of RAM may be significant on a RAM-constrained platform. On a
number of platforms, it may be easier to allocate some extra program
memory (e.g. Flash or ROM) to the CoAP protocol handler process than
to allocate extra RAM. Therefore, one may try to reduce RAM usage of
a CoAP implementation at the cost of some additional program memory
usage and implementation complexity.
Some CoAP clients generate MID values by a using a Message ID
variable [I-D.ietf-core-coap] that is incremented by one each time a
new MID needs to be generated. (After the maximum value 65535 it
wraps back to 0.) We call this behavior "sequential" MIDs. One
approach to reduce RAM use exploits the redundancy in sequential MIDs
for a more efficient MID storage in CoAP servers.
Naturally such an approach requires, in order to actually reduce RAM
usage in an implementation, that a large part of the peers follow the
sequential MID behavior. To realize this optimization, the authors
therefore RECOMMEND that CoAP end-point implementers employ the
"sequential MID" scheme if there are no reasons to prefer another
scheme, such as randomly generated MID values.
Security considerations might call for a choice for
(pseudo)randomized MIDs. Note however that with truly randomly
generated MIDs the probability of MID collision is rather high in use
cases as mentioned before, following from the Birthday Paradox. For
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example, in a sequence of 52 randomly drawn 16-bit values the
probability of finding at least two identical values is about 2
percent.
From here on we consider efficient storage implementations for MIDs
in CoAP end-points, that are optimized to store "sequential" MIDs.
Because CoAP messages may be lost or arrive out-of-order, a solution
has to take into account that received MIDs of CoAP messages are not
actually arriving in a sequential fashion, due to lost or reordered
messages. Also a peer might reset and lose its MID counter(s) state.
In addition, a peer may have a single Message ID variable used in
messages to many CoAP end-points it communicates with, which partly
breaks sequentiality from the receiving CoAP end-point's perspective.
Finally, some peers might use a randomly generated MID values
approach. Due to these specific conditions, existing sliding window
bitfield implementations for storing received sequence numbers are
typically not directly suitable for efficiently storing MIDs.
Table 1 shows one example for a per-peer MID storage design: a table
with a bitfield of a defined length _K_ per entry to store received
MIDs (one per bit) that have a value in the range [MID_i + 1 , MID_i
+ K].
+----------+----------------+-----------------+
| MID base | K-bit bitfield | base time value |
+----------+----------------+-----------------+
| MID_0 | 010010101001 | t_0 |
| | | |
| MID_1 | 111101110111 | t_1 |
| | | |
| ... etc. | | |
+----------+----------------+-----------------+
Table 1: A per-peer table for storing MIDs based on MID_i
The presence of a table row with base MID_i (regardless of the
bitfield values) indicates that a value MID_i has been received at a
time t_i. Subsequently, each bitfield bit k (0...K-1) in a row i
corresponds to a received MID value of MID_i + k + 1. If a bit k is
0, it means a message with corresponding MID has not yet been
received. A bit 1 indicates such a message has been received already
at approximately time t_i. This storage structure allows e.g. with
k=64 to store in best case up to 130 MID values using 20 bytes, as
opposed to 260 bytes that would be needed for a non-sequential
storage scheme.
The time values t_i are used for removing rows from the table after a
preset timeout period, to keep the MID store small in size and enable
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these MIDs to be safely re-used in future communications. (Note that
the table only stores one time value per row, which therefore needs
to be updated on receipt of another MID that is stored as a single
bit in this row. As a consequence of only storing one time value per
row, older MID entries typically time out later than with a simple
per-MID time value storage scheme. The end-point therefore needs to
ensure that this additional delay before MID entries are removed from
the table is much smaller than the time period after which a peer
starts to re-use MID values due to wrap-around of a peer's MID
variable. One solution is to check that a value t_i in a table row
is still recent enough, before using the row and updating the value
t_i to current time. If not recent enough, e.g. older than N
seconds, a new row with an empty bitfield is created.) [Clearly,
these optimizations would benefit if the peer were much more
conservative about re-using MIDs than currently required in the
protocol specification.]
The optimization described is less efficient for storing randomized
MIDs that a CoAP end-point may encounter from certain peers. To
solve this, a storage algorithm may start in a simple MID storage
mode, first assuming that the peer produces non-sequential MIDs.
While storing MIDs, a heuristic is then applied based on monitoring
some "hit rate", for example, the number of MIDs received that have a
Most Significant Byte equal to that of the previous MID divided by
the total number of MIDs received. If the hit rate tends towards 1
over a period of time, the MID store may decide that this particular
CoAP end-point uses sequential MIDs and in response improve
efficiency by switching its mode to the bitfield based storage.
2.4.2. Resource-specific Deduplication
Deduplication is heavy for Class 1 devices, as the number of peer
addresses can be vast. Servers should be kept stateless, i.e., the
REST API should be designed idempotent whenever possible. When this
is not the case, the resource handler could perform an optimized
deduplication by exploiting knowledge about the application.
Another, server-wide strategy is to only keep track of non-idempotent
requests.
3. Transmission State Management
CoAP endpoints must keep transmission state to manage open requests,
to handle the different response modes, and to implement reliable
delivery at the message layer. The following finite state machines
(FSMs) model the transmissions of a CoAP exchange at the request/
response layer and the message layer. These layers are linked
through actions. The M_CMD() action triggers a corresponding
transition at the message layer and the FF_EVT() action triggers a
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transition at the request/response layer. The FSMs also use guard
conditions to distinguish between information that is only available
through the other layer (e.g., whether a request was sent using a CON
or NON message).
3.1. Request/Response Layer
Figure 1 depicts the two states at the request/response layer of a
CoAP client. When a request is issued, a "reliable_send" or
"unreliable_send" is triggered at the message layer. The WAITING
state can be left through three transitions: Either the client
cancels the request and triggers cancellation of a CON transission at
the message layer, the client receives a failure event from the
message layer, or a receive event containing a response.
+------------CANCEL-------------------------------+
| / M_CMD(cancel) V
| +------+
| | |
+-------+ -------RR_EVT(fail)--------------------> | IDLE |
|WAITING| | |
+-------+ -------RR_EVT(rx)[is Response]---------> +------+
^ / M_CMD(accept) |
| |
+--------------------REQUEST----------------------+
/ M_CMD((un)reliable_send)
Figure 1: CoAP Client Request/Response Layer FSM
A server resource can decide at the request/response layer whether to
respond with a piggy-backed or a separate response. Thus, there are
two busy states in Figure 2, SERVING and SEPARATE. An incoming
receive event with a NON request directly triggers the transition to
the SEPARATE state.
+--------+ <----------RR_EVT(rx)[is NON]---------- +------+
|SEPARATE| | |
+--------+ ----------------RESPONSE--------------> | IDLE |
^ / M_CMD((un)reliable_send) | |
| +---> +------+
|EMPTY_ACK | |
|/M_CMD(accept) | |
| | |
| | |
+--------+ | |
|SERVING | --------------RESPONSE------------+ |
+--------+ / M_CMD(accept) |
^ |
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+------------------------RR_EVT(rx)[is CON]--------+
Figure 2: CoAP Server Request/Response Layer FSM
3.2. Message Layer
Figure 3 shows the different states of a CoAP endpoint per message
exchange. Besides the linking action RR_EVT(), the message layer has
a TX action to send a message. For sending and receiving NONs, the
endpoint remains in its CLOSED state. When sending a CON, the
endpoint remains in RELIABLE_TX and keeps retransmitting until the
transmission times out, it receives a matching RST, the request/
response layer cancels the transmission, or the endpoint receives an
implicit acknowledgement through a matching NON or CON. Whenever the
endpoint receives a CON, it transitions into the ACK_PENDING state,
which can be left by sending the corresponding ACK.
+-----------+ <-------M_CMD(reliable_send)-----+
| | / TX(con) \
| | +--------------+
| | ---TIMEOUT(RETX_WINDOW)------> | |
|RELIABLE_TX| / RR_EVT(fail) | |
| | ---------------------RX_RST--> | | <----+
| | / RR_EVT(fail) | | |
+-----------+ ----M_CMD(cancel)------------> | CLOSED | |
^ | | \ \ | | --+ |
| | | \ +-------------------RX_ACK---> | | | |
+*1+ | \ / RR_EVT(rx) | | | |
| +----RX_NON-------------------> +--------------+ | |
| / RR_EVT(rx) ^ ^ ^ ^ | | | | | |
| | | | | | | | | | |
| | | | +*2+ | | | | |
| | | +--*3--+ | | | |
| | +----*4----+ | | |
| +------*5------+ | |
| +---------------+ | |
| | ACK_PENDING | <--RX_CON-------------+ |
+----RX_CON----> | | / RR_EVT(rx) |
/ RR_EVT(rx) +---------------+ ---------M_CMD(accept)---+
/ TX(ack)
*1: TIMEOUT(RETX_TIMEOUT) / TX(con)
*2: M_CMD(unreliable_send) / TX(non)
*3: RX_NON / RR_EVT(rx)
*4: RX_RST / REMOVE_OBSERVER
*5: RX_ACK
Figure 3: CoAP Message Layer FSM
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T.B.D.: (i) Rejecting messages (can be triggered at message and
request/response layer). (ii) ACKs can also be triggered at both
layers.
4. Observing
At the server, the list of observers should be stored per resource to
only have a handle per observable resource in a superordinate list
instead of one resource handle per observer entry. Then for each
observer, at least address, port, token, and the last outgoing
message ID has to be stored. The latter is needed to match incoming
RST messages and cancel the observe relationship.
Besides the list of observers, it is best to have one retransmission
buffer per observable resource. Each notification is serialized once
into this buffer and only address, port, and token are changed when
iterating over the observer list (note that different token lengths
might require realignment). The advantage becomes clear for
confirmable notifications: Instead of one retransmission buffer per
observer, only one buffer and only individual retransmission counters
and timers in the list entry need to be stored. When the
notifications can be sent fast enough, even a single timer would
suffice. Furthermore, per-resource buffers simplify the update with
a new resource state during open deliveries.
5. Block-wise Transfers
Block-wise transfers have the main purpose of providing fragmentation
at the application layer, where partial information can be processed.
This is not possible at lower layers such as 6LoWPAN, as only
assembled packets can be passed up the stack. While
[I-D.ietf-core-block] also anticipates atomic handling of blocks,
i.e., only fully received CoAP messages, this is not possible on
Class 1 devices.
When receiving a block-wise transfer, each blocks is usually passed
to a handler function that for instance performs stream processing or
writes the blocks to external memory such as flash. Although there
are no restrictions in [I-D.ietf-core-block], it is beneficial for
Class 1 devices to only allow ordered transmission of blocks.
Otherwise on-the-fly processing would not be possible.
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When sending a block-wise transfer, Class 1 devices usually do not
have sufficient memory to print the full message into a buffer, and
slice and send it in a second step. When transferring the CoRE Link
Format from /.well-known/core for instance, a generator function is
required that generates slices of a large string with a specific
offset length (a 'sonprintf()'). This functionality is required
recurrently and should be included in a library.
6. Application Developer API
Bringing a Web transfer protocol to constrained environments does not
only change the networking of the corresponding systems, but also the
way they should be programmed. A CoAP implementation should provide
a developer API similar to REST frameworks in traditional computing.
A server should not be created around an event loop with several
function calls, but rather by implementing handlers following the
resource abstraction.
So far, the following types of RESTful resources were identified:
NORMAL A normal resource defined by a static Uri-Path that is
associated with a resource handler function. Allowed methods
could already be filtered by the implementation based on flags.
This is the basis for all other resource types.
PARENT A parent resource manages several sub-resources by
programmatically evaluating the Uri-Path, which may be longer than
that of the parent resource. Defining a URI templates (see
[RFC6570]) would be a convenient way to pre-parse arguments given
in the Uri-Path.
PERIODIC A resource that has an additional handler function that is
triggered periodically by the CoAP implementation with a resource-
defined interval. It can be used to sample a sensor or perform
similar periodic updates. Usually, a periodic resource is
observable and sends the notifications in the periodic handler
function. These periodic tasks are quite common for sensor nodes,
thus it makes sense to provide this functionality in the CoAP
implementation and avoid redundant code in every resource.
EVENT An event resource is similar to an periodic resource, only
that the second handler is called by an irregular event such as a
button.
7. References
7.1. Normative References
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[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, March 2012.
7.2. Informative References
[I-D.arkko-core-sleepy-sensors]
Arkko, J., Rissanen, H., Loreto, S., Turanyi, Z., and O.
Novo, "Implementing Tiny COAP Sensors", draft-arkko-core-
sleepy-sensors-01 (work in progress), July 2011.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-12 (work in progress), June 2013.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-08 (work in progress), February 2013.
[I-D.ietf-lwig-terminology]
Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained Node Networks", draft-ietf-lwig-terminology-04
(work in progress), April 2013.
Authors' Addresses
Matthias Kovatsch
ETH Zurich
Universitaetstrasse 6
CH-8092 Zurich
Switzerland
Email: kovatsch@inf.ethz.ch
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Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Email: bergmann@tzi.org
Angelo Castellani
University of Padova
Via Gradenigo 6/B
I-35131 Padova
Italy
Email: castellani@dei.unipd.it
Esko Dijk
Philips Research
Email: esko.dijk@philips.com
Carsten Bormann (editor)
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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