Internet DRAFT - draft-ietf-lwig-coap
draft-ietf-lwig-coap
LWIG Working Group M. Kovatsch
Internet-Draft ETH Zurich
Intended status: Informational O. Bergmann
Expires: January 3, 2019 C. Bormann, Ed.
Universitaet Bremen TZI
July 02, 2018
CoAP Implementation Guidance
draft-ietf-lwig-coap-06
Abstract
The Constrained Application Protocol (CoAP) is designed for resource-
constrained nodes and networks such as sensor nodes in a low-power
lossy network (LLN). Yet to implement this Internet protocol on
Class 1 devices (as per RFC 7228, ~ 10 KiB of RAM and ~ 100 KiB of
ROM) also lightweight implementation techniques are necessary. This
document provides lessons learned from implementing CoAP for tiny,
battery-operated networked embedded systems. In particular, it
provides guidance on correct implementation of the CoAP specification
RFC 7252, memory optimizations, and customized protocol parameters.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 3, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
Kovatsch, et al. Expires January 3, 2019 [Page 1]
Internet-Draft CoAP Implementation Guidance July 2018
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Implementation . . . . . . . . . . . . . . . . . . . 4
2.1. Client/Server Model . . . . . . . . . . . . . . . . . . . 4
2.2. Message Processing . . . . . . . . . . . . . . . . . . . 5
2.2.1. On-the-fly Processing . . . . . . . . . . . . . . . . 5
2.2.2. Internal Data Structure . . . . . . . . . . . . . . . 6
2.3. Message ID Usage . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Duplicate Rejection . . . . . . . . . . . . . . . . . 7
2.3.2. MID Namespaces . . . . . . . . . . . . . . . . . . . 8
2.3.3. Relaxation on the Server . . . . . . . . . . . . . . 8
2.3.4. Relaxation on the Client . . . . . . . . . . . . . . 9
2.4. Token Usage . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.1. Tokens for Observe . . . . . . . . . . . . . . . . . 11
2.4.2. Tokens for Blockwise Transfers . . . . . . . . . . . 12
2.5. Transmission States . . . . . . . . . . . . . . . . . . . 12
2.5.1. Request/Response Layer . . . . . . . . . . . . . . . 12
2.5.2. Message Layer . . . . . . . . . . . . . . . . . . . . 13
2.6. Out-of-band Information . . . . . . . . . . . . . . . . . 14
2.7. Programming Model . . . . . . . . . . . . . . . . . . . . 15
2.7.1. Client . . . . . . . . . . . . . . . . . . . . . . . 16
2.7.2. Server . . . . . . . . . . . . . . . . . . . . . . . 16
3. Optimizations . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Message Buffers . . . . . . . . . . . . . . . . . . . . . 17
3.2. Retransmissions . . . . . . . . . . . . . . . . . . . . . 18
3.3. Observable Resources . . . . . . . . . . . . . . . . . . 18
3.4. Blockwise Transfers . . . . . . . . . . . . . . . . . . . 19
3.4.1. Generic Proxying of Block Messages . . . . . . . . . 19
3.4.2. Atomic Blockwise Operations . . . . . . . . . . . . . 20
3.5. Deduplication with Sequential MIDs . . . . . . . . . . . 20
4. Alternative Configurations . . . . . . . . . . . . . . . . . 23
4.1. Transmission Parameters . . . . . . . . . . . . . . . . . 23
4.2. CoAP over IPv4 . . . . . . . . . . . . . . . . . . . . . 24
5. Binding to specific lower-layer APIs . . . . . . . . . . . . 24
5.1. Berkeley Socket Interface . . . . . . . . . . . . . . . . 24
5.1.1. Responding from the right address . . . . . . . . . . 24
5.1.2. Handling ICMP errors . . . . . . . . . . . . . . . . 25
5.2. Java . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3. Multicast detection . . . . . . . . . . . . . . . . . . . 26
5.4. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Kovatsch, et al. Expires January 3, 2019 [Page 2]
Internet-Draft CoAP Implementation Guidance July 2018
6. CoAP on various transports . . . . . . . . . . . . . . . . . 26
6.1. CoAP over reliable transports . . . . . . . . . . . . . . 27
6.2. Translating between transports . . . . . . . . . . . . . 27
6.2.1. Transport translation by proxies . . . . . . . . . . 27
6.2.2. One-to-one Transport translation . . . . . . . . . . 28
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 28
8. Security considerations . . . . . . . . . . . . . . . . . . . 28
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1. Normative References . . . . . . . . . . . . . . . . . . 28
10.2. Informative References . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
The Constrained Application Protocol [RFC7252] has been designed
specifically for machine-to-machine communication in networks with
very constrained nodes. Typical application scenarios therefore
include building automation, process optimization, 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 [RFC7230].
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 on the other. For constrained nodes of Class 1 or even
Class 2 [RFC7228], the most limiting factors usually are dynamic
memory needs, static code size, and energy. Most implementations
Kovatsch, et al. Expires January 3, 2019 [Page 3]
Internet-Draft CoAP Implementation Guidance July 2018
therefore need to optimize internal buffer usage, omit idle protocol
features, and maximize sleeping cycles.
The present document gives possible strategies to solve this tradeoff
for very constrained nodes (i.e., Class 1). For this, it provides
guidance on correct implementation of the CoAP specification
[RFC7252], memory optimizations, and customized protocol parameters.
2. Protocol Implementation
In the programming styles supported by very simple operating systems
as found on constrained nodes, 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 parallel
observe notifications [RFC7641]). Note that implementations on
constrained platforms often not even support the full MTU. Larger
messages must then use blockwise transfers [RFC7959], 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 (blockwise) payload plus an estimate of the maximum
header size for the worst case option setting.
2.1. Client/Server Model
In general, CoAP servers can be implemented more efficiently than
clients. REST allows them to keep the communication stateless and
piggy-backed responses are not stored for retransmission, saving
buffer space. The use of idempotent requests also allows to relax
deduplication, which further decreases memory usage. It is also easy
to estimate the required maximum size of message buffers, since URI
paths, supported options, and maximum payload sizes of the
application are known at compile time. Hence, when the application
is distributed over constrained and unconstrained nodes, the
constrained ones should preferably have the server role.
HTTP-based applications have established an inverse model because of
the need for simple push notifications: A constrained client uses
POST requests to update resources on an unconstrained server whenever
an event (e.g., a new sensor reading) is triggered. This requirement
is solved by the Observe option [RFC7641] of CoAP. It allows servers
Kovatsch, et al. Expires January 3, 2019 [Page 4]
Internet-Draft CoAP Implementation Guidance July 2018
to initiate communication and send push notifications to interested
client nodes. This allows a more efficient and also more natural
model for CoAP-based applications, where the information source is an
origin server, which can also benefit from caching. Publish-
subscribe brokers [I-D.ietf-core-coap-pubsub] may be deployed to act
in the server role on behalf of constrained clients.
2.2. Message Processing
Apart from the required buffers, message processing is symmetric for
clients and servers. First the base header has to be parsed and
thereby checked if it is a valid CoAP message. For UDP datagrams,
the version identifier or a size smaller than four bytes identify
non-CoAP data. These datagrams need to be silently ignored. Other
message format errors, such as an incomplete datagram or the usage of
reserved values, may need to be rejected with a Reset (RST) message
(see Section 4.2 and 4.3 of [RFC7252] for details).
As CoAP over TCP has a different base header, the Length field must
be parsed to determine the message size. As this field may have up
to five bytes, it may be extend over TCP segment boundaries. For
CoAP over WebSockets the actual message length is given by the
WebSocket frame hence the Length field is always zero.
Next, the token length is read based on the TKL field which is for
all transports contained in the four least significant bits of the
first byte. The (possibly empty) Token itself is located immediately
after the four-byte base header for UDP, while for TCP and
WebSockets, it follows the variable Length field and Code byte.
For the options following the Token, there are two alternatives:
either process them 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 no additional memory
needs to be allocated to store the option values, which are stored
efficiently inline in the buffer for incoming messages. 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 and sparse range of option numbers, the
number itself cannot be used to indicate the number of left-shift
operations to mask the corresponding bit. Hence, an implementation-
specific enum of supported options should be used to mask the present
Kovatsch, et al. Expires January 3, 2019 [Page 5]
Internet-Draft CoAP Implementation Guidance July 2018
options of a message in the bitmap. In addition, the byte index of
every option (a direct pointer) can be added to a sparse list (e.g.,
a one-dimensional array) for fast retrieval.
This particularly enables efficient 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 this option (e.g., 11 for Uri-Path) would
be overwritten to hold a pointer to only the last occurrence of that
option. The Uri-Path can be resolved on the fly, though, and a
pointer to the targeted resource stored directly in the sparse list.
Once the option list has been processed, all known critical option
and all elective options can be masked out in the bit-vector 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 full processing must only be done up to the
highest supported option number. Beyond that, only the least
significant bit (Critical or Elective) needs to be checked.
Otherwise, if all critical options are supported, the sparse list of
option pointers is used for further handling of the message.
2.2.2. Internal Data Structure
Using an internal data structure for all parsed options has an
advantage when working on the option values, as they are already in a
variable of corresponding type (e.g., an integer in host byte order).
The incoming payload and byte strings of the header can be accessed
directly in the buffer for incoming messages using pointers (similar
to on-the-fly processing). 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).
Many of the byte strings (e.g., the URI) are usually not required
when generating the response. When all important values are copied
(e.g., the Token, which needs to be mirrored), the internal data
structure facilitates using the buffer for incoming messages also for
the assembly of outgoing messages - which can be the shared IP buffer
provided by the operating system.
Setting options for outgoing messages is also easier with an internal
data structure. Application developers can set options independent
from the option number and do not need to care about the order for
the delta encoding. The CoAP encoding is applied in a serialization
step before sending. In contrast, assembling outgoing messages with
on-the-fly processing requires either extensive memmove operations to
Kovatsch, et al. Expires January 3, 2019 [Page 6]
Internet-Draft CoAP Implementation Guidance July 2018
insert new options, or restrictions for developers to set options in
their correct order.
2.3. Message ID Usage
Many applications of CoAP use unreliable transports, in particular
UDP, which can lose, reorder, and duplicate messages. Although
DTLS's replay protection deals with duplication by the network,
losses are addressed with DTLS retransmissions only for the handshake
protocol and not for the application data protocol. Furthermore,
CoAP implementations usually send CON retransmissions in new DTLS
records, which are not considered duplicates at the DTLS layer.
2.3.1. Duplicate Rejection
CoAP's messaging sub-layer has been designed with protocol
functionality such that rejection of duplicate messages is always
possible. It is realized through the Message IDs (MIDs) and their
lifetimes with regard to the message type.
Duplicate detection is under the discretion of the recipient (see
Section 4.5 of [RFC7252], Section 2.3.3, Section 2.3.4). Where it is
desired, the receiver needs to keep track of MIDs to filter the
duplicates for at least NON_LIFETIME (145 s). This time also holds
for CON messages, since it equals the possible reception window of
MAX_TRANSMIT_SPAN + MAX_LATENCY.
On the sender side, MIDs of CON messages must not be re-used within
the EXCHANGE_LIFETIME; MIDs of NONs respectively within the
NON_LIFETIME. In typical scenarios, however, senders will re-use
MIDs with intervals far larger than these lifetimes: with sequential
assignment of MIDs, coming close to them would require 250 messages
per second, much more than the bandwidth of constrained networks
would usually allow for.
In cases where senders might come closer to the maximum message rate,
it is recommended to use more conservative timings for the re-use of
MIDs. Otherwise, opposite inaccuracies in the clocks of sender and
recipient may lead to obscure message loss. If needed, higher rates
can be achieved by using multiple endpoints for sending requests and
managing the local MID per remote endpoint instead of a single
counter per system (essentially extending the 16-bit message ID by a
16-bit port number and/or an 128-bit IP address). In controlled
scenarios, such as real-time applications over industrial Ethernet,
the protocol parameters can also be tweaked to achieve higher message
rates (Section 4.1).
Kovatsch, et al. Expires January 3, 2019 [Page 7]
Internet-Draft CoAP Implementation Guidance July 2018
2.3.2. MID Namespaces
MIDs are assigned under the control of the originator of CON and NON
messages, and they do not mix with the MIDs assigned by the peer for
CON and NON in the opposite direction. Hence, CoAP implementors need
to make sure to manage different namespaces for the MIDs used for
deduplication. MIDs of outgoing CONs and NONs belong to the local
endpoint; so do the MIDs of incoming ACKs and RSTs. Accordingly,
MIDs of incoming CONs and NONs and outgoing ACKs and RSTs belong to
the corresponding remote endpoint. Figure 1 depicts a scenario where
mixing the namespaces would cause erroneous filtering.
Client Server
| |
| CON [0x1234] |
+----------------->|
| |
| ACK [0x1234] |
|<-----------------+
| |
| CON [0x4711] |
|<-----------------+ Separate response
| |
| ACK [0x4711] |
+----------------->|
| |
A request follows that uses the same MID as the last separate response
| |
| CON [0x4711] |
+----------------->|
Response is filtered | |
because MID 0x4711 | ACK [0x4711] |
is still in the X<-----------------+ Piggy-backed response
deduplication list | |
Figure 1: Deduplication must manage the MIDs in different namespace
corresponding to their origin endpoints.
2.3.3. Relaxation on the Server
Using the de-duplication functionality is at the discretion of the
receiver: Processing of duplicate messages comes at a cost, but so
does the management of the state associated with duplicate rejection.
The number of remote endpoints that need to be managed might be vast.
This can be costly in particular for less constrained nodes that have
throughput in the order of hundreds of thousands requests per second
(which needs about 16 GiB of RAM just for duplicate rejection).
Deduplication is also heavy for servers on Class 1 devices, as also
Kovatsch, et al. Expires January 3, 2019 [Page 8]
Internet-Draft CoAP Implementation Guidance July 2018
piggy-backed responses need to be stored for the case that the ACK
message is lost. Hence, a receiver may have good reasons to decide
not to perform deduplication. This behavior is possible when the
application is designed with idempotent operations only and makes
good use of the If-Match/If-None-Match options.
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. It can be reduced, for instance, by deduplication at
resource level: Knowledge of the application and supported
representations can minimize the amount of state that needs to be
kept.
2.3.4. Relaxation on the Client
Duplicate rejection on the client side can be simplified by choosing
clever Tokens that are virtually not re-used (e.g., through an
obfuscated sequence number in the Token value) and only filter based
on the list of open Tokens. If a client wants to re-use Tokens
(e.g., the empty Token for optimizations), it requires strict
duplicate rejection based on MIDs to avoid the scenario outlined in
Figure 2.
Kovatsch, et al. Expires January 3, 2019 [Page 9]
Internet-Draft CoAP Implementation Guidance July 2018
Client Server
| |
| CON [0x7a10] |
| GET /temp |
| (Token 0x23) |
+----------------->|
| |
| ACK [0x7a10] |
|<-----------------+
| |
... Time Passes ...
| |
| CON [0x23bb] |
| 4.04 Not Found |
| (Token 0x23) |
|<-----------------+
| |
| ACK [0x23bb] |
+--------X |
| |
| CON [0x7a11] |
| GET /resource |
| (Token 0x23) |
+----------------->|
| |
| CON [0x23bb] |
Causing an implicit | 4.04 Not Found |
acknowledgement if | (Token 0x23) |
not filtered through X<-----------------+ Retransmission
duplicate rejection | |
Figure 2: Re-using Tokens requires strict duplicate rejection.
2.4. Token Usage
Tokens are chosen by the client and help to identify request/response
pairs that span several message exchanges (e.g., a separate response,
which has a new MID). Servers do not generate Tokens and only mirror
what they receive from the clients. Tokens must be unique within the
namespace of a client throughout their lifetime. This begins when
being assigned to a request and ends when the open request is closed
by receiving and matching the final response. Neither empty ACKs nor
notifications (i.e., responses carrying the Observe option) terminate
the lifetime of a Token.
As already mentioned, a clever assignment of Tokens can help to
simplify duplicate rejection. Yet this is also important for coping
with client crashes. When a client restarts during an open request
Kovatsch, et al. Expires January 3, 2019 [Page 10]
Internet-Draft CoAP Implementation Guidance July 2018
and (unknowingly) re-uses the same Token, it might match the response
from the previous request to the current one. Hence, when only the
Token is used for matching, which is always the case for separate
responses, randomized Tokens with enough entropy should be used. The
8-byte range for Tokens can even allow for one-time usage throughout
the lifetime of a client node. When DTLS is used, client crashes/
restarts will lead to a new security handshake, thereby solving the
problem of mismatching responses and/or notifications.
2.4.1. Tokens for Observe
In the case of Observe [RFC7641], a request will be answered with
multiple notifications and it is important to continue keeping track
of the Token that was used for the request - its lifetime will end
much later. Upon establishing an Observe relationship, the Token is
registered at the server. Hence, the client's use of that specific
Token is now limited to controlling the Observation relationship. A
client can use it to cancel the relationship, which frees the Token
upon success (i.e., the message with an Observe Option with the value
set to 'deregister' (1) is confirmed with a response; see [RFC7641]
section 3.6). However, the client might never receive the response
due to a temporary network outage or worse, a server crash. Although
a network outage will also affect notifications so that the Observe
garbage collection could apply, the server might simply happen not to
send CON notifications during that time. Alternative Observe
lifetime models such as Stubbornness(tm) might also keep
relationships alive for longer periods.
Thus, it is best to carefully choose the Token value used with
Observe requests. (The empty value will rarely be applicable.) One
option is to assign and re-use dedicated Tokens for each Observe
relationship the client will establish. The choice of Token values
also is critical in NoSec mode, to limit the effectiveness of
spoofing attacks. Here, the recommendation is to use randomized
Tokens with a length of at least four bytes (see Section 5.3.1 of
[RFC7252]). Thus, dedicated ranges within the 8-byte Token space
should be used when in NoSec mode. This also solves the problem of
mismatching notifications after a client crash/restart.
When the client wishes to reinforce its interest in a resource, maybe
not really being sure whether the server has forgotten it or not, the
Token value allocated to the Observe relationship is used to re-
register that observation (see Section 3.3.1 of [RFC7641] for
details): If the server is still aware of the relationship (an entry
with a matching endpoint and token is already present in its list of
observers for the resource), it will not add a new relationship but
will replace or update the existing one (Section 4.1 of [RFC7641]).
Kovatsch, et al. Expires January 3, 2019 [Page 11]
Internet-Draft CoAP Implementation Guidance July 2018
If not, it will simply establish a new registration which of course
also uses the Token value.
If the client sends an Observe request for the same resource with a
new Token, this is not a protocol violation, because the
specification allows the client to observe the same resource in a
different Observe relationship if the cache-key is different (e.g.,
requesting a different Content-Format). If the cache-key is not
different, though, an additional Observe relationship just wastes the
server's resources, and is therefore not allowed; the server might
rely on this for its housekeeping.
2.4.2. Tokens for Blockwise Transfers
In general, blockwise transfers are independent from the Token and
are correlated through client endpoint address and server address and
resource path (destination URI). Thus, each block may be transferred
using a different Token. Still it can be beneficial to use the same
Token (it is freed upon reception of a response block) for all
blocks, e.g., to easily route received blocks to the same response
handler.
When Block2 is combined with Observe, notifications only carry the
first block and it is up to the client to retrieve the remaining
ones. These GET requests do not carry the Observe option and need to
use a different Token, since the Token from the notification is still
in use.
2.5. Transmission States
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 RR_EVT() action triggers a
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).
2.5.1. Request/Response Layer
Figure 3 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
Kovatsch, et al. Expires January 3, 2019 [Page 12]
Internet-Draft CoAP Implementation Guidance July 2018
cancels the request and triggers cancellation of a CON transmission
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)--------------------> | |
|WAITING| | IDLE |
+-------+ -------RR_EVT(rx)[is Response]---------> | |
^ / M_CMD(accept) +------+
| |
+--------------------REQUEST----------------------+
/ M_CMD((un)reliable_send)
Figure 3: 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 4, 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) |
^ |
+------------------------RR_EVT(rx)[is CON]--------+
Figure 4: CoAP Server Request/Response Layer FSM
2.5.2. Message Layer
Figure 5 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
Kovatsch, et al. Expires January 3, 2019 [Page 13]
Internet-Draft CoAP Implementation Guidance July 2018
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 5: CoAP Message Layer FSM
T.B.D.: (i) Rejecting messages (can be triggered at message and
request/response layer). (ii) ACKs can also be triggered at both
layers.
2.6. Out-of-band Information
The CoAP implementation can also leverage out-of-band information,
that might also trigger some of the transitions shown in Section 2.5.
In particular ICMP messages can inform about unreachable remote
endpoints or whole network outages. This information can be used to
Kovatsch, et al. Expires January 3, 2019 [Page 14]
Internet-Draft CoAP Implementation Guidance July 2018
pause or cancel ongoing transmission to conserve energy. Providing
ICMP information to the CoAP implementation is easier in constrained
environments, where developers usually can adapt the underlying OS
(or firmware). This is not the case on general purpose platforms
that have full-fledged OSes and make use of high-level programming
frameworks.
The most important ICMP messages are host, network, port, or protocol
unreachable errors. After appropriate vetting (cf. [RFC5927]), they
should cause the cancellation of ongoing CON transmissions and
clearing (or deferral) of Observe relationships. Requests to this
destination should be paused for a sensible interval. In addition,
the device could indicate of this error through a notification to a
management endpoint or external status indicator, since the cause
could be a misconfiguration or general unavailability of the required
service. Problems reported through the Parameter Problem message are
usually caused through a similar fundamental problem.
The CoAP specification recommends to ignore Source Quench and Time
Exceeded ICMP messages, though. Source Quench messages were
originally intended to inform the sender to reduce the rate of
packets. However, this mechanism is deprecated through [RFC6633].
CoAP also comes with its own congestion control mechanism, which is
already designed conservatively. One advanced mechanism that can be
employed for better network utilization is CoCoA,
[I-D.ietf-core-cocoa]. Time Exceeded messages often occur during
transient routing loops (unless they are caused by a too small
initial Hop Limit value).
2.7. Programming Model
The event-driven approach, which is common in event-loop-based
firmware, has also proven very efficient for embedded operating
systems [TinyOS], [Contiki]. Note that an OS is not necessarily
required and a traditional firmware approach can suffice for Class 1
devices. Event-driven systems use split-phase operations (i.e.,
there are no blocking functions, but functions return and an event
handler is called once a long-lasting operation completes) to enable
cooperative multi-threading with a single stack.
Bringing a Web transfer protocol to constrained environments does not
only change the networking of the corresponding systems, but also the
programming model. The complexity of event-driven systems can be
hidden through APIs that resemble classic RESTful Web service
implementations.
Kovatsch, et al. Expires January 3, 2019 [Page 15]
Internet-Draft CoAP Implementation Guidance July 2018
2.7.1. Client
An API for asynchronous requests with response handler functions goes
hand-in-hand with the event-driven approach. Synchronous requests
with a blocking send function can facilitate applications that
require strictly ordered, sequential request execution (e.g., to
control a physical process) or other checkpointing (e.g., starting
operation only after registration with the resource directory was
successful). However, this can also be solved by triggering the next
operation in the response handlers. Furthermore, as mentioned in
Section 2.1, it is more like that complex control flow is done by
more powerful devices and Class 1 devices predominantly run a CoAP
server (which might include a minimal client to communicate with a
resource directory).
2.7.2. Server
On CoAP servers, the event-driven nature can be hidden through
resource handler abstractions as known from traditional REST
frameworks. The following types of RESTful resources have proven
useful to provide an intuitive API on constrained event-driven
systems:
NORMAL A normal resource defined by a static Uri-Path and an
associated 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 under a given
base path by programmatically evaluating the Uri-Path. Defining a
URI template (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-
specific interval. It can be used to sample a sensor or perform
similar periodic updates of its state. Usually, a periodic
resource is observable and sends the notifications by triggering
its normal resource handler from the periodic handler. 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.
Kovatsch, et al. Expires January 3, 2019 [Page 16]
Internet-Draft CoAP Implementation Guidance July 2018
SEPARATE Separate responses are usually used when handling a request
takes more time, e.g., due to a slow sensor or UART-based
subsystems. To not fully block the system during this time, the
handler should also employ split-phase execution: The resource
handler returns as soon as possible and an event handler resumes
responding when the result is ready. The separate resource type
can abstract from the split-phase operation and take care of
temporarily storing the request information that is required later
in the result handler to send the response (e.g., source address
and Token).
3. Optimizations
3.1. Message Buffers
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 or firmware.
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. The first few bytes of the basic header are
usually parsed into an internal data structure and can be overwritten
without harm. Thus, empty ACKs and RST messages can promptly be
assembled and sent using the IP buffer. 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. Once the message is sent, the IP buffer can
accept new messages again. This does not work for Confirmable
messages, however. They need to be stored for retransmission and
would block any further IP communication.
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
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.
Kovatsch, et al. Expires January 3, 2019 [Page 17]
Internet-Draft CoAP Implementation Guidance July 2018
3.2. 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 [RFC7641]. 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.
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.
3.3. Observable Resources
For each observer, the server needs to store at least address, port,
token, and the last outgoing message ID. The latter is needed to
match incoming RST messages and cancel the observe relationship.
It is favorable to have one retransmission buffer per observable
resource that is shared among all observers. 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
Kovatsch, et al. Expires January 3, 2019 [Page 18]
Internet-Draft CoAP Implementation Guidance July 2018
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.
3.4. Blockwise Transfers
Blockwise 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 [RFC7959] also
anticipates atomic handling of blocks, i.e., only fully received CoAP
messages, this is not possible on Class 1 devices.
When receiving a blockwise transfer, each block 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 [RFC7959], it is beneficial for Class 1
devices to only allow ordered transmission of blocks. Otherwise on-
the-fly processing would not be possible.
When sending a blockwise transfer out of dynamically generated
information, 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. For instance, if the CoRE Link Format at /.well-known/
core is dynamically generated, 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.
3.4.1. Generic Proxying of Block Messages
Proxies cannot ignore the Block options by specification, because the
options Block1 and Block2 are not safe-to-forward. The rationale
behind this design decision is that servers might not be able to
distinguish blocks originating from different senders once they have
been forwarded by a CoAP proxy. For atomic operations where all
blocks are assembled before actually executing the desired operation,
this could lead to inconsistent state on the server side.
To ensure that this does not happen, a proxy can add the Request-Tag
option (see [I-D.ietf-core-echo-request-tag]) containing data that
uniquely identifies the originating endpoint in the proxy namespace.
Kovatsch, et al. Expires January 3, 2019 [Page 19]
Internet-Draft CoAP Implementation Guidance July 2018
3.4.2. Atomic Blockwise Operations
When an implementation needs to assemble blocks from block-wise
transfers, applications need to create an identifier to group
messages that belong together. This "Block Key" at least contains:
o The source endpoint (e.g., IP address and port in the UDP case),
o the destination endpoint,
o the Cache-Key (as updated in [RFC7252]), and
o all options that are proxy unsafe and not explicitly described as
safe for block-wise assembly.
The only known options safe for block-wise assembly are the options
Block1 and Block2 [RFC7959].
For the Block1 phase, the request payload is excluded from the
identifier generation as it is just being assembled.
If a message is received that is not the start of a block-wise
operation has a Block Key that is not known, and the implementation
needs to act atomically on a request body, it must answer 4.08
(Request Entity Incomplete).
Conversely, clients should be aware that requests whose Block Key
matches can be interpreted by the server atomically. This especially
affects proxies (see Section 3.4.1).
3.5. Deduplication with Sequential MIDs
CoAP's duplicate rejection functionality can be straightforwardly
implemented in a CoAP endpoint by storing, for each remote CoAP
endpoint ("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.
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_ ([RFC7252] section "Reliable Messages") and
the time packets can stay alive in the network.
Such a straightforward implementation is suitable in case other CoAP
endpoints generate random MIDs. However, this storage method may
consume substantial RAM in specific cases, such as:
Kovatsch, et al. Expires January 3, 2019 [Page 20]
Internet-Draft CoAP Implementation Guidance July 2018
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 [RFC7252] 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 endpoint 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
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 endpoints, 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.
Kovatsch, et al. Expires January 3, 2019 [Page 21]
Internet-Draft CoAP Implementation Guidance July 2018
In addition, a peer may have a single Message ID variable used in
messages to many CoAP endpoints it communicates with, which partly
breaks sequentiality from the receiving CoAP endpoint'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
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 endpoint 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
Kovatsch, et al. Expires January 3, 2019 [Page 22]
Internet-Draft CoAP Implementation Guidance July 2018
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 endpoint 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
endpoint uses sequential MIDs and in response improve efficiency by
switching its mode to the bitfield based storage.
4. Alternative Configurations
4.1. Transmission Parameters
When a constrained network of CoAP nodes is not communicating over
the Internet, for instance because it is shielded by a proxy or a
closed deployment, alternative transmission parameters can be used.
Consequently, the derived time values provided in [RFC7252] section
4.8.2 will also need to be adjusted, since most implementations will
encode their absolute values.
Static adjustments require a fixed deployment with a constant number
or upper bound for the number of nodes, number of hops, and expected
concurrent transmissions. Furthermore, the stability of the wireless
links should be evaluated. ACK_TIMEOUT should be chosen above the
xx% percentile of the round-trip time distribution.
ACK_RANDOM_FACTOR depends on the number of nodes on the network.
MAX_RETRANSMIT should be chosen suitable for the targeted
application. A lower bound for LEISURE can be calculated as
lb_Leisure = S * G / R
where S is the estimated response size, G the group size, and R the
target data transfer rate (see [RFC7252] section 8.2). NSTART and
PROBING_RATE depend on estimated network utilization. If the main
cause for loss are weak links, higher values can be chosen.
Dynamic adjustments will be performed by advanced congestion control
mechanisms such as [I-D.ietf-core-cocoa]. They are required if the
Kovatsch, et al. Expires January 3, 2019 [Page 23]
Internet-Draft CoAP Implementation Guidance July 2018
main cause for message loss is network or endpoint congestion. Semi-
dynamic adjustments could be implemented by disseminating new static
transmission parameters to all nodes when the network configuration
changes (e.g., new nodes are added or long-lasting interference is
detected).
4.2. CoAP over IPv4
CoAP was designed for the properties of IPv6, which is dominating in
constrained environments because of the 6LoWPAN adaption layer
[RFC6282]. In particular, the size limitations of CoAP are tailored
to the minimal MTU of 1280 bytes. Until the transition towards IPv6
converges, CoAP nodes might also communicate over IPv4, though.
Sections 4.2 and 4.6 of the base specification [RFC7252] already
provide guidance and implementation notes to handle the smaller
minimal MTUs of IPv4.
Another deployment issue in legacy IPv4 deployments is caused by
Network Address Translators (NATs). The session timeouts are
unpredictable and NATs may close UDP sessions with timeout as short
as 60 seconds. This makes CoAP endpoints behind NATs practically
unreachable, even when they contact the remote endpoint with a public
IP address first. Incorrect behavior may also arise when the NAT
session heuristic changes the external port between two successive
CoAP messages. For the remote endpoint, this will look like two
different CoAP endpoints on the same IP address. Such behavior can
be fatal for the resource directory registration interface.
5. Binding to specific lower-layer APIs
Implementing CoAP on specific lower-layer APIs appears to
consistently bring up certain less-known aspects of these APIs. This
section is intended to alert implementers to such aspects.
5.1. Berkeley Socket Interface
5.1.1. Responding from the right address
In order for a client to recognize a reply (response or
acknowledgement) as coming from the endpoint to which the initiating
packet was addressed, the source IPv6 address of the reply needs to
match the destination address of the initiating packet.
Implementers that have previously written TCP-based applications are
used to binding their server sockets to INADDR_ANY. Any TCP
connection received over such a socket is then more specifically
bound to the source address from which the TCP connection setup was
received; no programmer action is needed for this.
Kovatsch, et al. Expires January 3, 2019 [Page 24]
Internet-Draft CoAP Implementation Guidance July 2018
For stateless UDP sockets, more manual work is required. Simply
receiving a packet from a UDP socket bound to INADDR_ANY loses the
information about the destination address; replying to it through the
same socket will use the default address established by the kernel.
Two strategies are available:
o Only use sockets bound to a specific address (not INADDR_ANY). A
system with multiple interfaces (or addresses) will thus need to
bind multiple sockets and send replies back on the same socket the
initiating packet was received on.
o Use IPV6_RECVPKTINFO [RFC3542] to configure the socket, and mirror
back the IPV6_PKTINFO information for the reply (see also
Section 5.1.1.1).
5.1.1.1. Managing interfaces
For some applications, it may further be relevant what interface is
chosen to send to an endpoint, beyond the kernel choosing one that
has a routing table entry for the destination address. E.g., it may
be natural to send out a response or acknowledgment on the same
interface that the packet prompting it was received. The end of the
introduction to section 6 of [RFC3542] describes a simple technique
for this, where that RFC's API (IPV6_PKTINFO) is available. The same
data structure can be used for indicating an interface to send a
packet that is initiating an exchange. (Choosing that interface is
too application-specific to be in scope for the present document.)
5.1.2. Handling ICMP errors
Sockets that use the connect and send functions usually receive ICMP
errors in the form of error codes, sockets that use sendto or sendmsg
do not receive them at all.
Neither is sufficient to implement the guidance in Section 2.6, as
the vetting of the message requires access to the CoAP headers in the
ICMP error. The necessary information can be obtained by using the
IPV6_RECVERR option.
5.2. Java
Java provides a wildcard address (0.0.0.0) to bind a socket to all
network interface. This is useful when a server is supposed to
listen on any available interface including the lookback address.
For UDP, and hence CoAP this poses a problem, however, because the
DatagramPacket class does not provide the information to which
address it was sent. When replying through the wildcard socket, the
JVM will pick the default address, which can break the correlation of
Kovatsch, et al. Expires January 3, 2019 [Page 25]
Internet-Draft CoAP Implementation Guidance July 2018
messages when the remote endpoint did not send the message to the
default address. This is in particular precarious for IPv6 where it
is common to have multiple IP addresses per network interface. Thus,
it is recommended to bind to all adresses explicitly and manage the
destination address of incoming messages within the CoAP
implementation.
5.3. Multicast detection
Similar to the considerations above, Section 8 of [RFC7252] requires
a node to detect whether a packet that it is going to reply to was
sent to a unicast or to a multicast address. On most platforms,
binding a UDP socket to a unicast address ensures that it only
receives packets addressed to that address. Programmers relying on
this property should ensure that it indeed applies to the platform
they are using. If it does not, IPV6_PKTINFO may, again, help for
Berkeley Socket Interfaces. For Java, explicit management of
different sockets (in this case a MulticastSocket) is required.
5.4. DTLS
CoAPS implementations require access to the authenticated user/device
prinicipal to realize access control for resources. How this
information can be accessed heavily depends on the DTLS
implementation used. Generic and portable CoAP implementations might
want to provide an abstraction layer that can be used by application
developers that implement resource handlers. It is recommended to
keep the API of such an application layer close to popular HTTPS
solutions that are available for the targeted platform, for instance,
mod_ssl or the Java Servlet API.
6. CoAP on various transports
As specified in [RFC7252], CoAP is defined for two underlying
transports: UDP and DTLS. These transports are relatively similar in
terms of the properties they expose to their users. (The main
difference, apart from the increased security, is that DTLS provides
an abstraction of a connection, into which the endpoint abstraction
is placed; in contrast, the UDP endpoint abstraction is based on
four-tuples of IP addresses and ports.)
Recently, the need to carry CoAP over other transports
[I-D.silverajan-core-coap-alternative-transports] has led to
specifications such as CoAP over TLS or TCP or WebSockets[RFC8323],
or even over non-IP transports such as SMS
[I-D.becker-core-coap-sms-gprs]. This section discusses
considerations that arise when handling these different transports in
an implementation.
Kovatsch, et al. Expires January 3, 2019 [Page 26]
Internet-Draft CoAP Implementation Guidance July 2018
6.1. CoAP over reliable transports
To cope with transports without reliable delivery (such as UDP and
DTLS), CoAP defines its own message layer, with acknowledgments,
timers, and retransmission. When CoAP is run over a transport that
provides its own reliability (such as TCP or TLS), running this
machinery would be redundant. Worse, keeping the machinery in place
is likely to lead to interoperability problems as it is unlikely to
be tested as well as on unreliable transports. Therefore,
[I-D.silverajan-core-coap-alternative-transports] was defined by
removing the message layer from CoAP and just running the request/
response layer directly on top of the reliable transport. This also
leads to a reduced (from the UDP/DTLS 4-byte header) header format.
Conversely, where reliable transports provide a byte stream
abstraction, some form of message delimiting had to be added, which
now needs to be handled in the CoAP implementation. The use of
reliable transports may reduce the disincentive for using messages
larger than optimal link layer packet sizes. Where different message
sizes are chosen by an application for reliable and for unreliable
transports, this can pose additional challenges for translators
(Section 6.2).
Where existing CoAP APIs expose details of the the message layer
(e.g., CON vs. NON, or assigning application layer semantics to
ACKs), using a reliable transport may require additional adjustments.
6.2. Translating between transports
One obvious way to convey CoAP exchanges between different transports
is to run a CoAP proxy that supports both transports. The usual
considerations for proxies apply. Section 6.2.1 discusses some
additional considerations.
Where not much of the functionality of CoAP proxies (such as caching)
is required, a simpler 1:1 translation may be possible, as discussed
in Section 6.2.2.
6.2.1. Transport translation by proxies
(TBD. In particular, point out the obvious: fan-in/fan-out means
that separate message ID and token spaces need to be maintained at
the ends of the proxy.)
One more CoAP specific function of a transport translator proxy may
be to convert between different block sizes, e.g. between a TCP
connection that can tolerate large blocks and UDP over a constrained
node network.
Kovatsch, et al. Expires January 3, 2019 [Page 27]
Internet-Draft CoAP Implementation Guidance July 2018
6.2.2. One-to-one Transport translation
A translator with reduced requirements for state maintenance can be
constructed when no fan-in or fan-out is required, and when the
namespace lifetimes of the two sides can be made to coincide. For
this one-to-one translation, there is no need to manage message-ID
and Token value spaces for both sides separately. So, a simple UDP-
to-UDP one-to-one translator could simply copy the messages (among
other applications, this might be useful for translation between IPv4
and IPv6 spaces). Similarly, a DTLS-to-TCP translator could be built
that executes the message layer (deduplication, retransmission) on
the DTLS side, and repackages the CoAP header (add/remove the length
information, and remove/add the message ID and message type) between
the DTLS and the TCP side.
By definition, such a simple one-to-one translator needs to shut down
the connection on one side when the connection on the other side
terminates. However, a UDP-to-TCP one-to-one translator cannot
simply shut down the UDP endpoint when the TCP endpoint vanishes
because the TCP connection closes, so some additional management of
state will be necessary.
7. IANA considerations
This document has no actions for IANA.
8. Security considerations
TBD
9. Acknowledgements
Esko Dijk contributed the sequential MID optimization. Xuan He
provided help creating and improved the state machine charts.
Christian Amsuess provided input on forwarding block messages by
proxies and usage of the Request-Tag option.
10. References
10.1. Normative References
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-03 (work in progress), February 2018.
Kovatsch, et al. Expires January 3, 2019 [Page 28]
Internet-Draft CoAP Implementation Guidance July 2018
[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,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[RFC6633] Gont, F., "Deprecation of ICMP Source Quench Messages",
RFC 6633, DOI 10.17487/RFC6633, May 2012,
<https://www.rfc-editor.org/info/rfc6633>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
10.2. Informative References
[Contiki] Dunkels, A., Groenvall, B., and T. Voigt, "Contiki - a
Lightweight and Flexible Operating System for Tiny
Networked Sensors", Proceedings of the First IEEE
Workshop on Embedded Networked Sensors, November 2004.
[I-D.becker-core-coap-sms-gprs]
Kuladinithi, K., Becker, M., Li, K., and T. Poetsch,
"Transport of CoAP over SMS", draft-becker-core-coap-sms-
gprs-06 (work in progress), February 2017.
Kovatsch, et al. Expires January 3, 2019 [Page 29]
Internet-Draft CoAP Implementation Guidance July 2018
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-04 (work in
progress), March 2018.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Echo and
Request-Tag", draft-ietf-core-echo-request-tag-02 (work in
progress), June 2018.
[I-D.silverajan-core-coap-alternative-transports]
Silverajan, B. and T. Savolainen, "CoAP Communication with
Alternative Transports", draft-silverajan-core-coap-
alternative-transports-11 (work in progress), March 2018.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003,
<https://www.rfc-editor.org/info/rfc3542>.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<https://www.rfc-editor.org/info/rfc5927>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[TinyOS] Levis, P., Madden, S., Polastre, J., Szewczyk, R.,
Whitehouse, K., Woo, A., Gay, D., Woo, A., Hill, J.,
Welsh, M., Brewer, E., and D. Culler, "TinyOS: An
Operating System for Sensor Networks", Ambient
intelligence, Springer (Berlin Heidelberg),
ISBN 978-3-540-27139-0, 2005.
Authors' Addresses
Kovatsch, et al. Expires January 3, 2019 [Page 30]
Internet-Draft CoAP Implementation Guidance July 2018
Matthias Kovatsch
ETH Zurich
Universitaetstrasse 6
CH-8092 Zurich
Switzerland
Email: kovatsch@inf.ethz.ch
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Email: bergmann@tzi.org
Carsten Bormann (editor)
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Kovatsch, et al. Expires January 3, 2019 [Page 31]
