Constrained Application Protocol (CoAP) over Bundle Protocol (BP)
draft-gomez-core-coap-bp-02
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draft-gomez-core-coap-bp-02
CoRE Working Group C. Gomez
Internet-Draft A. Calveras
Intended status: Standards Track UPC
Expires: 24 April 2025 October 2024
Constrained Application Protocol (CoAP) over Bundle Protocol (BP)
draft-gomez-core-coap-bp-02
Abstract
The Bundle Protocol (BP) was designed to enable end-to-end
communication in challenged networks. The Constrained Application
Protocol (CoAP), which was designed for constrained-node networks,
may be a suitable application-layer protocol for the scenarios where
BP is used. This document specifies how CoAP is carried over BP.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 4 April 2025.
Copyright Notice
Copyright (c) 2024 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/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements language . . . . . . . . . . . . . . . . . . 3
2.2. Background on previous specifications . . . . . . . . . . 3
2.3. New terms . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Messaging model . . . . . . . . . . . . . . . . . . . . . 4
4.2. Single message format . . . . . . . . . . . . . . . . . . 6
4.3. Payload-length option . . . . . . . . . . . . . . . . . . 6
5. Encapsulating bundle . . . . . . . . . . . . . . . . . . . . 8
6. CoAP parameter settings and related times . . . . . . . . . . 8
7. Observe . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Block-wise transfers . . . . . . . . . . . . . . . . . . . . 11
8.1. Main CoAP block-wise transfer parameters . . . . . . . . 12
9. URI Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Securing CoAP over BP . . . . . . . . . . . . . . . . . . . . 14
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11.1. Creation of two new reserved domains in the .arpa name
space . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1.1. Domain Name Reservation Considerations . . . . . . . 15
11.2. ipn URI Scheme Well-known Service Number for CoAP . . . 16
12. Security Considerations . . . . . . . . . . . . . . . . . . . 16
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
14.1. Normative References . . . . . . . . . . . . . . . . . . 16
14.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Reference CoAP parameter values for interplanetary
communication . . . . . . . . . . . . . . . . . . . . . . 19
Appendix B. Message ID size, EXCHANGE_LIFETIME, and maximum CoAP
message rate . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The Delay-Tolerant Networking (DTN) architecture has been designed to
enable communication in challenged networks, which are characterized
by long delays, intermittent connectivity, and high error rates,
among other constraints [RFC4838][RFC7228]. DTN was mainly intended
for deep space communication (e.g., to enable an Interplanetary
Internet). However, it is also applicable to enable communication on
Earth in environments exhibiting relatively similar features, such as
sensor networks or temporarily disconnected areas.
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The Bundle Protocol (BP) is the fundamental component of DTN. BP is
a message-oriented protocol that operates as a store-carry-forward
overlay atop the transport-layer protocols of a number of constituent
networks [RFC9171]. The protocol data unit of BP is called a bundle.
Application-layer functionality runs atop BP.
The Constrained Application Protocol (CoAP) is an application-layer
protocol that was specifically designed for constrained-node networks
[RFC7252][RFC7228], which are typical in Internet of Things (IoT)
scenarios. Such environments are often characterized by
significantly constrained node and network features, including low
computational capacity, limited energy availability (which often
leads to the use of duty-cycled links), low bandwidth, high latency,
and high loss rates. Accordingly, CoAP offers several features,
which are also suitable for DTN, including lightweight operation,
asynchronous message exchanges, and a significant degree of
flexibility, based on RESTful principles.
The present document specifies how CoAP is carried over BP.
2. Terminology
2.1. Requirements language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP14 [RFC2119], [RFC8174], when, and only when, they appear in all
capitals, as shown here.
2.2. Background on previous specifications
The reader is expected to be familiar with the terms and concepts
defined by the DTN main specifications (e.g., [RFC4838], [RFC9171],
and [RFC9172]), and the CoAP main specifications (e.g., [RFC7252],
[RFC7641], [RFC7959], [RFC8323], and [RFC9177]).
2.3. New terms
Single message: a CoAP message that is carried as the payload of the
underlying layer protocol data unit. In CoAP over BP, a Single
message is carried as the block-type-specific data field of the
Bundle Payload Block of the encapsulating bundle.
Aggregate message: a concatenation of Single messages that carry the
Payload-length option (see Section 4.3). In CoAP over BP, an
Aggregate message is carried as the block-type-specific data field of
the Bundle Payload Block of the encapsulating bundle.
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3. Architecture
Figure 1 illustrates the protocol stack model for CoAP over BP.
(Note: this figure is the same as Figure 1 of RFC 9171, except for
the indication of CoAP's location in the protocol stack model.) In
this model, CoAP entities exchange application-layer messages carried
by BP over an end-to-end path composed of a number of constituent
networks.
+-----------+ +-----------+
| CoAP | | CoAP |
+---------v-| +->>>>>>>>>>v-+ +->>>>>>>>>>v-+ +-^---------+
| BP v | | ^ BP v | | ^ BP v | | ^ BP |
+---------v-+ +-^---------v-+ +-^---------v-+ +-^---------+
| T1 v | + ^ T1/T2 v | + ^ T2/T3 v | | ^ T3 |
+---------v-+ +-^---------v-+ +-^---------v + +-^---------+
| N1 v | | ^ N1/N2 v | | ^ N2/N3 v | | ^ N3 |
+---------v-+ +-^---------v + +-^---------v-+ +-^---------+
| >>>>>>>>^ >>>>>>>>>>^ >>>>>>>>^ |
+-----------+ +-------------+ +-------------+ +-----------+
| | | |
|<---- A network ---->| |<---- A network ---->|
| | | |
Figure 1: BP and CoAP in the protocol stack model
4. Messages
4.1. Messaging model
The CoAP base specification was produced assuming UDP as the
underlying transport-layer protocol [RFC 7252]. Like UDP, BP is a
message-oriented protocol. Furthermore, BP does not provide bundle
retransmission. Therefore, when CoAP is used over BP, the same
messaging model defined for CoAP in RFC 7252 is used, and the CoAP
signaling messages defined in RFC 8323 (which are intended for use
over reliable transports) MUST NOT be used.
Figure 2 shows the two-sublayer structure of CoAP, when used over BP.
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+----------------------+
| Application |
+----------------------+
+----------------------+ \
| Requests/Responses | |
|----------------------| | CoAP
| Messages | |
+----------------------+ /
+----------------------+
| BP |
+----------------------+
Figure 2: Abstract Layering of CoAP over BP
CoAP follows a client/server model, whereby a client may request an
action on a resource on a server. Upon receipt of a request, the
server sends a response, including a response code, which may also
include a resource representation. (Note that, if a request includes
the "No-Response" option [RFC7967], the server may suppress the
response.) Requests and responses are encapsulated in messages.
CoAP defines four message types: Confirmable (CON), Non-confirmable
(NON), Acknowledgment (ACK), and Reset (RST). CON messages elicit
ACKs, whereas NON messages do not. For CON messages, CoAP uses stop-
and-wait retransmission with exponential back-off. A RST message is
sent by a CoAP endpoint that has received a message but is unable to
process it.
When CoAP is used over BP, a source bundle node MAY set the "request
reporting of bundle delivery" flag in the bundle's status report
request field of a bundle that encapsulates a CoAP CON message. Upon
receipt of a bundle that carries a CoAP CON message with the "request
reporting of bundle delivery" flag set, the receiver MAY opt to only
send the corresponding bundle delivery status report and omit sending
a bundle encapsulating a CoAP ACK message, if and only if the CoAP
ACK message does not carry a payload. In that case, if the CoAP CON
message sender receives the status report sent in response to its
bundle-encapsulated CON message, it MUST assume that the status
report serves as CoAP ACK for the CON message.
(Note: the assumption is that the status report size is shorter than
the size of a bundle encapsulating a CoAP ACK message that does not
carry a payload. To be further confirmed.)
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4.2. Single message format
In CoAP over BP, the format of a Single message (Figure 4) is the
same as the CoAP message format defined in RFC 7252 (Figure 3),
except for the Message ID size, which is increased to 24 bits for
CoAP over BP. The reason for this change is avoiding a severe
limitation on the number of messages a sender can send per time unit,
considering the latency values in the environments where CoAP over BP
may be used, and that, as stated in RFC 7252, "the same Message ID
MUST NOT be reused (in communicating with the same endpoint) within
the EXCHANGE_LIFETIME". See Appendix B for further details.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | TKL | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: CoAP Message Format as defined in RFC 7252
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | TKL | Code | Message ID . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...Message ID | Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Single Message Format over BP
4.3. Payload-length option
CoAP messages destined to the same endpoint MAY be aggregated and
carried as the payload of a single encapsulating bundle.
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An Aggregate message is a concatenation of Single messages that carry
the Payload-length option. The Payload-length option defined in this
subsection (see Figure 5, which extends "Table 4: Options" of
[RFC7252]) indicates the size of the payload of a CoAP message. The
Payload-length option is critical, safe to forward, part of the cache
key, and not repeatable.
+------+---+---+---+---+----------------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+------+---+---+---+---+----------------+--------+--------+---------+
| TBD | x | | | | Payload-length | (*) | 1-5 | (none) |
+------+---+---+---+---+----------------+--------+--------+---------+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable
(*) See below.
Figure 5: The Payload-length option
The format of the Payload-length option value is shown in Figure 6.
The format is inspired by the CoAP frame for reliable transports
[RFC8323].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Len | RSVD | Extended Length (if any, as chosen by Len) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Payload-length option value format
Length (Len): 4-bit unsigned integer. A value between 0 and 12
inclusive indicates the length of the message in bytes, starting with
the first bit of the Options field. Three values are reserved for
special constructs:
13: An 8-bit unsigned integer (Extended Length) follows the initial
byte and indicates the length of options/payload minus 13.
14: A 16-bit unsigned integer (Extended Length) in network byte order
follows the initial byte and indicates the length of options/payload
minus 269.
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15: A 32-bit unsigned integer (Extended Length) in network byte order
follows the initial byte and indicates the length of options/payload
minus 65805.
RSVD: this is a 4-bit field reserved for future use.
5. Encapsulating bundle
In order to transmit a CoAP message (either a Single message or an
Aggregate message) over BP, the CoAP message MUST be carried as the
block-type-specific data field of the Bundle Payload Block (block
type 1) of an encapsulating bundle.
The lifetime field of the bundle encapsulating a CON Single message
MUST be set to EXCHANGE_LIFETIME (see Section 6). The lifetime field
of the bundle encapsulating a NON Single message MUST be set to
NON_LIFETIME (see Section 6). If an Aggregate message comprises at
least one CON message and one NON message, the lifetime field of the
encapsulating bundle is set to the maximum of EXCHANGE_LIFETIME and
NON_LIFETIME.
In some cases, upon receipt of a CoAP message, the receiving endpoint
needs to transmit a CoAP message in response to the sender. The
Destination EID and Source Node ID fields of the primary bundle block
of the bundle encapsulating such a CoAP message sent as a response
SHALL be set as follows:
Destination EID: The Destination EID SHALL be identical to the Source
Node ID of the bundle encapsulating the received CoAP message that
produces the response.
Source Node ID: The Source Node ID SHALL be identical to the
Destination EID of the bundle encapsulating the received CoAP message
that produces the response.
6. CoAP parameter settings and related times
This section discusses the main CoAP parameters and times that are
relevant in the environments where BP may be used. (Note that the
complete set of parameters, assumptions, default values, and related
times in CoAP can be found in Section 4.8 of RFC 7252.)
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Most of these CoAP parameters and times are relevant for CON
messages. Note that, in some scenarios, the protocols operating
below BP may support reliability and congestion control. In that
case, using NON messages might suffice to achieve a reasonable degree
of reliability. The congestion control considerations for NON
message transmission would still apply, though (see Sections 4.7 and
4.8 of RFC 7252).
As a congestion control measure, the maximum number of outstanding
interactions between a client and a given server is limited to
NSTART, which is set to a default value of 1. A greater value for
NSTART can be used only when mechanisms that ensure congestion
control safety are used [RFC 7252].
The main parameters related with CON messages are indicated next.
ACK_TIMEOUT and ACK_RANDOM_FACTOR. These two parameters determine
the duration of the initial retransmission timeout, which is set to a
randomly chosen value between ACK_TIMEOUT and ACK_TIMEOUT *
ACK_RANDOM_FACTOR. The default values for ACK_TIMEOUT and
ACK_RANDOM_FACTOR are 2 s and 1.5, respectively. Therefore, the
default initial retransmission timeout in CoAP is between 2 and 3 s.
For CoAP over BP, ACK_TIMEOUT should be set to a value of at least
the expected RTT, which may be of an order of magnitude several times
greater than the default one (see Appendix A).
ACK_RANDOM_FACTOR needs to be at least equal to or greater than 1.0.
The default value of 1.5 is intended to avoid synchronization effects
among different senders when RTTs are in the order of seconds.
However, the greater latency in delay-tolerant environments may
reduce the risk of synchronization effects therein. In such case, a
lower ACK_RANDOM_FACTOR may help reduce total message delivery
latency when retries are performed.
MAX_RETRANSMIT. This parameter defines the maximum number of retries
for a given CON message. The default value for this parameter is 4.
Since there is an exponential back-off between retransmissions, and
considering the delay values in environments where BP is used, it may
be suitable to set this parameter to a value lower than the default
one (see Appendix A).
The following assumptions on the characteristics of the network and
the nodes need to be considered:
MAX_LATENCY is the maximum time a datagram is expected to take from
the start of its transmission to the completion of its reception. In
RFC 7252, this value is arbitrarily set to 100 s, which is close to
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the historic Maximum Segment Lifetime (MSL) of 120 s defined in the
TCP specification [RFC9293]. However, such value assumes
communication in non-challenged environments. Therefore, in
environments where BP is used, MAX_LATENCY may need to be increased
by at least 2-3 orders of magnitude.
PROCESSING_DELAY is the time since a node receives a CON message
until it transmits an ACK in response. In RFC 7252, this value is
assumed to be of at most the default ACK_TIMEOUT value of 2 s. For
the sake of limiting latency, it is assumed that the same value can
be used also in environments where BP is used.
A relevant CON message derived time is EXCHANGE_LIFETIME. This time
indicates the maximum possible time since a CON message is sent for
the first time, until ACK reception (which may potentially occur
after several retries). EXCHANGE_LIFETIME includes the following
components: the total time since the first transmission attempt of a
CON message until the last one (called MAX_TRANSMIT_SPAN in RFC
7252), a MAX_LATENCY for the CON, PROCESSING_DELAY, and a MAX_LATENCY
for the ACK. The default value for EXCHANGE_LIFETIME is 247 s.
However, in challenged environments (e.g., deep space), and
considering the increased values for protocol parameters and network
characteristics described above, EXCHANGE_LIFETIME will be at least 2
(and perhaps a greater number of) orders of magnitude greater than
the default one (see Appendix A).
The main time related with NON messages is NON_LIFETIME. This is the
time since a NON message is transmitted until its Message ID can be
safely reused. This time is actually equal to MAX_LATENCY, therefore
its default value is 100 s. However, as described earlier, in
challenged environments (e.g, deep space) it may need to be increased
by 2-3 orders of magnitude.
Note that CoAP implementations may also need to be adapted if they
have been designed to use 8-bit timers to handle CON or NON message
lifetimes (e.g., to retire Message IDs) in seconds.
7. Observe
The CoAP Observe Option allows a server to send notifications
carrying a representation of the current state of a resource to
interested clients called observers [RFC7641]. The latter need to
initially register at a specific server that they are interested in
being notified whenever the resource state changes.
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Observe generally provides significant performance benefits, since,
after the registration, the client does not have to send a request to
receive a notification. This feature is particularly beneficial in
environments where end-to-end latency is high, and energy and
bandwidth resources may be constrained.
As per the Observe specification, when the time between the two last
notifications received by a CoAP client is greater than 128 seconds,
it can be concluded that the last one received is also the latest
sent by the server. The duration of 128 seconds was chosen as a
number greater than the default MAX_LATENCY value of the base CoAP
specification. When CoAP is used over BP, determining whether a
notification was sent by the server later than another notification
MUST be performed based on the creation timestamps of the
corresponding bundles encapsulating the two notifications. The
duration of 128 seconds may be insufficient in many scenarios. In
such cases, the duration needs to be chosen as a value greater than
the MAX_LATENCY of the scenario (see Appendix A).
8. Block-wise transfers
CoAP supports functionality that allows carrying large payloads by
means of block-wise transfers [RFC7959], [RFC9177]. BP also supports
fragmentation and reassembly functionality. RFC 7959 states, in the
context of fragmentation and reassembly functionality being available
at several protocol stack layers, that "the fragmentation/reassembly
process burdens the lower layers with conversation state that is
better managed in the application layer". However, an implicit
assumption in RFC 7959 is that details on the data unit sizes that
can be carried over the different links of an end-to-end path are
known in advance by the sender.
When CoAP is used over BP, CoAP block-wise transfers MAY be used if
the source knows in advance the duration and type of expected
contacts (e.g., scheduled or predicted) between the BP nodes that
will forward the bundles from the source bundle node to the
destination bundle node. This does not preclude the use of BP
fragmentation and reassembly when deemed necessary.
There exist two CoAP specifications that allow to perform block-wise
transfers: [RFC7959] and [RFC9177].
As per RFC 7959, a CoAP endpoint can only ask for (or send) the next
block after the previous block has been transferred. Furthermore,
RFC 7959 recommends the use of CON messages. Therefore,
communication follows a stop-and-wait pattern, which is not suitable
for environments with long delays.
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RFC 9177 is particularly suitable for DTN environments, as it enables
block-wise transfers using NON messages. Thus, blocks can be
transmitted serially without having to wait for a response or next
request from the remote CoAP peer. Recovery of multiple missing
blocks (which can be reported at once in a single CoAP message) is
also supported.
8.1. Main CoAP block-wise transfer parameters
The following new parameters are defined by RFC 9177, for use with
NON messages and the Q-Block1 and Q-Block2 options: MAX_PAYLOADS,
NON_TIMEOUT, NON_TIMEOUT_RANDOM, NON_RECEIVE_TIMEOUT,
NON_MAX_RETRANSMIT, NON_PROBING_WAIT, and NON_PARTIAL_TIMEOUT.
MAX_PAYLOADS indicates the number of consecutive blocks an endpoint
can transmit without eliciting a message from the other endpoint.
The default value defined for this parameter is 10, which is in line
with the initial window size currently defined for TCP [RFC6928].
TO-DO: MAX_PAYLOADS for deep space?
NON_TIMEOUT is the minimum time between sending two consecutive sets
of MAX_PAYLOADS blocks that correspond to the same body. The actual
time between sending two consecutive sets of MAX_PAYLOADS blocks is
called NON_TIMEOUT_RANDOM, which is calculated as NON_TIMEOUT *
ACK_RANDOM_FACTOR. In RFC 9177, NON_TIMEOUT is defined as having the
same value as ACK_TIMEOUT. ACK_RANDOM_FACTOR is set to 1.5,
following RFC 7252. As a result, by default, NON_TIMEOUT_RANDOM is
equal to a randomly chosen value between 2 and 3 s.
The NON_TIMEOUT_RANDOM inactivity interval described above is
introduced to avoid causing congestion due to the transmission of
MAX_PAYLOADS itself. As discussed previously, in challenged
networks, ACK_TIMEOUT should be set to a value greater than default.
When CoAP is used in deep space, NON_TIMEOUT, and thus
NON_TIMEOUT_RANDOM, need to be adjusted considering the
characteristics of the end-to-end path, independent of ACK_TIMEOUT.
NON_RECEIVE_TIMEOUT is the initial time that a receiver will wait for
a missing block within MAX_PAYLOADS before requesting retransmission
for the first time. Every time the missing payload is re-requested,
the time to wait value doubles. NON_RECEIVE_TIMEOUT has a default
value of 2*NON_TIMEOUT. As described earlier, in challenged
networks, NON_TIMEOUT needs to be adjusted considering the
characteristics of the end-to-end path.
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NON_MAX_RETRANSMIT is the maximum number of times a request for the
retransmission of missing payloads can occur without a response from
the remote peer. By default, NON_MAX_RETRANSMIT has the same value
as MAX_RETRANSMIT (Section 4.8 of [RFC7252]). Accordingly, when CoAP
is used in deep space, the same considerations regarding
MAX_RETRANSMIT in Section 5 apply to NON_MAX_RETRANSMIT as well.
That is, when CoAP is used in space, while the default value for this
parameter is 4, it may be suitable to set this parameter to a value
lower than the default one.
9. URI Scheme
The URI scheme for CoAP over BP is "coap" as per the recommendation
of Section 6 of [draft-ietf-core-transport-indication]. The "coap"
scheme is defined in Section 6 of [RFC7252].
When the endpoint ID of the target resource is based on the "dtn"
scheme, the authority component of the URI is formed as the reg-name
of the endpoint ID, followed by .dtn.arpa.
When the endpoint ID of the target resource is based on the "ipn"
scheme, the authority component of the URI is formed as the service-
nbr, followed by the nbr-delim (".") and the node-nbr of the endpoint
ID, followed by .ipn.arpa.
User information and port are always absent with the URI scheme used
in CoAP over BP.
For example, under the rules of Section 6 of [RFC7252], the URI of a
request for the discovery resource of a CoAP over BP entity with
endpoint ID dtn://JupiterSensor would be:
coap://JupiterSensor.dtn.arpa/.well-known/core
Similarly, the URI of a request for the discovery resource of a CoAP
over BP entity with endpoint ID ipn:81.2 would be:
coap://2.81.ipn.arpa/.well-known/core
TO-DO: request a Well-known Service Number for CoAP in the ipn URI
Scheme Well-known Service Numbers for BPv7 registry [draft-ietf-dtn-
ipn-update].
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10. Securing CoAP over BP
The base CoAP specification defines a binding to Datagram Transport
Layer Security (DTLS) [RFC7252][RFC9147]. There are four possible
DTLS security modes: NoSec, PreSharedKey, RawPublicKey, and
Certificate. The NoSec and RawPublicKey modes are mandatory to
implement.
Subsequently, Object Security for Constrained RESTful Environments
(OSCORE) was specified [RFC8613]. OSCORE is a security protocol for
CoAP that allows to protect an application-layer data payload end-to-
end, even in the presence of untrusted proxies in the path between
two endpoints. The Group OSCORE protocol [draft-ietf-core-oscore-
groupcomm] is used to secure CoAP group communication [draft- ietf-
core-groupcomm-bis].
In OSCORE, the communicating endpoints require a shared security
context. An interesting aspect of OSCORE for the environments where
BP is used is that, if the materials used to establish such context
are pre-shared, there is no initial handshake prior to actual
communication, thus avoiding a significant latency penalty. In
contrast, DTLS does require an initial handshake. For this reason,
the use of DTLS to secure CoAP over BP is generally NOT RECOMMENDED,
possible exceptions being environments where the latency penalty is
considered acceptable.
On the other hand, Bundle Protocol Security (BPSec) [RFC 9172]
provides security services for BP bundles, allowing to protect (with
integrity and/or confidentiality) one or more blocks of a bundle.
When CoAP is carried over BP, the CoAP message will be carried as the
block-type-specific data field of the Bundle Payload Block (block
type 1) of an encapsulating bundle. If OSCORE is used to protect
CoAP, only the CoAP message payload, one CoAP message header field,
and some of the CoAP options are protected. Currently, all CoAP
message fields that are protected are provided with confidentiality
and integrity protection.
In order to offer protection against replay attacks, OSCORE uses by
default an anti-replay sliding window, with a window size of 32 [RFC
8613]. If a greater window size is deemed necessary (e.g., due to
high latency in an intended scenario), that window size needs to be
known by both sender and receiver at the moment of security context
establishment. Note that BP provides additional protection against
replay attacks, since a bundle includes a creation timestamp and a
lifetime field. If the bundle is replayed outside of its lifetime,
the bundle will be discarded and the replay attack will fail (see
Section 8.2.4 of RFC 9172).
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TO-DO: security requirements of CoAP requests and responses over BP.
11. IANA Considerations
11.1. Creation of two new reserved domains in the .arpa name space
IANA is asked to create two new reserved domain names in the .arpa
name space as described in [RFC6761]: the suffixes .dtn.arpa and
.ipn.arpa.
The expectation for application software is that no DNS resolution is
attempted; instead, the prefix is processed into an endpoint ID, and
any operation on that endpoint ID is pointed to the BP node(s)
registered in that endpoint ID.
11.1.1. Domain Name Reservation Considerations
The Domain Reservation Considerations from Section 5 of [RFC6761] for
both domain names (.dtn.arpa and .ipn.arpa) are:
* Users: users are not expected to recognize those names as special.
* Application Software: application software is expected to pass
those names on to their CoAP over BP implementation. CoAP over BP
implementations are expected to recognize those names as BP endpoint
IDs and MUST NOT pass them on to DNS-based resolvers (unless the name
resolution API happens to explicitly support resolution into endpoint
ID, see below).
* Name resolution APIs and libraries: name resolution APIs and
libraries MAY indicate that .dtn.arpa and .ipn.arpa names resolve to
the endpoint ID encoded inside them (but no details for this are
specified in known resolution APIs or libraries). Otherwise, they
SHOULD report them as NXDOMAIN.
* Caching DNS Servers: caching DNS servers MAY recognize the special
domains and report them as NXDOMAIN. Otherwise, they will cache the
.arpa DNS servers' responses.
* Authoritative DNS Servers: authoritative DNS servers MAY recognize
the special domains and report them as NXDOMAIN.
* DNS Server Operators: No impact on DNS server operators is
expected.
* DNS Registries/Registrars: Any changes to .dtn.arpa or .ipn.arpa
require updates to this document and the corresponding process
through IANA.
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11.2. ipn URI Scheme Well-known Service Number for CoAP
IANA is requested to assign a Well-known Service Number for CoAP in
the ipn URI Scheme Well-known Service Numbers for BPv7 registry
[draft-ietf-dtn-ipn-update].
12. Security Considerations
TO-DO
13. Acknowledgments
The authors would like to thank (in alphabetical order) Christian
Amsuess, Edward J. Birrane, Marc Blanchet, Carsten Bormann, Scott
Burleigh, Joshua Deaton, Jaime Jimenez, Achim Kraus, Brian Sipos,
Rick Taylor, Marco Tiloca, Laurent Toutain, Rodney Van Meter, and
Magnus Westerlund for useful design considerations, reviews and
comments.
Carles Gomez and Anna Calveras have been funded in part by
MCIU/AEI/10.13039/501100011033/FEDER/UE through projects PID2019-
106808RA-I00, PID2023-146378NB-I00, and by Secretaria d'Universitats
i Recerca del Departament d'Empresa i Coneixement de la Generalitat
de Catalunya with the grant number 2021 SGR 00330.
14. References
14.1. Normative References
[I-D.ietf-core-transport-indication]
Amsüss, C. and M. S. Lenders, "CoAP Transport Indication",
Work in Progress, Internet-Draft, draft-ietf-core-
transport-indication-06, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
transport-indication-06>.
[I-D.ietf-dtn-ipn-update]
Taylor, R. and E. J. Birrane, "Update to the ipn URI
scheme", Work in Progress, Internet-Draft, draft-ietf-dtn-
ipn-update-14, 27 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dtn-ipn-
update-14>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
April 2007, <https://www.rfc-editor.org/info/rfc4838>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[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>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<https://www.rfc-editor.org/info/rfc7595>.
[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>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
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[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9171] Burleigh, S., Fall, K., and E. Birrane, III, "Bundle
Protocol Version 7", RFC 9171, DOI 10.17487/RFC9171,
January 2022, <https://www.rfc-editor.org/info/rfc9171>.
[RFC9172] Birrane, III, E. and K. McKeever, "Bundle Protocol
Security (BPSec)", RFC 9172, DOI 10.17487/RFC9172, January
2022, <https://www.rfc-editor.org/info/rfc9172>.
[RFC9177] Boucadair, M. and J. Shallow, "Constrained Application
Protocol (CoAP) Block-Wise Transfer Options Supporting
Robust Transmission", RFC 9177, DOI 10.17487/RFC9177,
March 2022, <https://www.rfc-editor.org/info/rfc9177>.
14.2. Informative References
[Conf] S.M. Davidovich, J. Whittington, "Concept for continuous
inter-planetary communications", May 1999.
[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
11, 24 April 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-groupcomm-bis-11>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and R. Höglund, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-23, 26
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-23>.
[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>.
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Appendix A. Reference CoAP parameter values for interplanetary
communication
Figure 7 shows the Round-Trip Time (RTT) between two endpoints on (or
close to) different celestial bodies of the Solar System, for the
maximum distances between such endpoints [Conf], and in an idealized
scenario where communication latency only comprises a propagation
delay component. (Note that message storing until the next
connectivity opportunity may significantly increase total
communication latency.) The RTT also provides a lower bound on (and
an approximation of) the ACK_TIMEOUT values required to avoid
spurious retransmission timer expiration.
Figure 8 provides approximate EXCHANGE_LIFETIME values that would
stem from the use of ACK_TIMEOUT values such as those shown in
Figure 5, for MAX_RETRANSMIT=1. (Note that the values provided in
Figure 5 are also approximately equal to EXCHANGE_LIFETIME, for
MAX_RETRANSMIT=0, under the conditions considered.)
For the sake of comparison, Figure 9 also provides the hypothetical,
approximate EXCHANGE_LIFETIME values that would correspond to
MAX_RETRANSMIT= 1, but with a retransmission scheme using a constant
RTO value for message retries.
Finally, Figure 10 provides the one-way delay for communication
between endpoints on (or close to) different celestial bodies of the
Solar System, for the maximum distances between such endpoints, and
assuming an idealized scenario where communication latency only
comprises a propagation delay component. The values in this figure
correspond approximately to MAX_LATENCY in the described scenarios.
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------------------------------------------------------------------------
------------------------------------------------------------------------
| RTT, ACK_TIMEOUT (or EXCHANGE_LIFETIME, for MAX_RETRANSMIT=0) |
------------------------------------------------------------------------
| |Sun|Mercury|Venus|Earth| Mars|Jupiter|Saturn|Uranus|Neptune|
------------------------------------------------------------------------
| Sun| - | 466| 727|1,014|1,661| 5,444|10,007|20,214| 30,288|
------------------------------------------------------------------------
|Mercury| - | - |1,181|1,448|1,968| 5,751|10,340|20,548| 30,554|
------------------------------------------------------------------------
| Venus| - | - | - |1,735|2,382| 6,158|10,741|20,948| 30,955|
------------------------------------------------------------------------
| Earth| - | - | - | - |2,642| 6,424|11,008|21,215| 31,222|
------------------------------------------------------------------------
| Mars| - | - | - | - | - | 6,805|11,408|21,615| 31,622|
------------------------------------------------------------------------
|Jupiter| - | - | - | - | - | - |14,944|25,151| 35,425|
------------------------------------------------------------------------
| Saturn| - | - | - | - | - | - | - |29,220| 39,961|
------------------------------------------------------------------------
| Uranus| - | - | - | - | - | - | - | - | 50,168|
------------------------------------------------------------------------
|Neptune| - | - | - | - | - | - | - | - | - |
------------------------------------------------------------------------
Figure 7: ACK_TIMEOUT or EXCHANGE_LIFETIME (for
MAX_RETRANSMIT=0), expressed in seconds.
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------------------------------------------------------------------------
------------------------------------------------------------------------
| EXCHANGE_LIFETIME (for MAX_RETRANSMIT=1) |
------------------------------------------------------------------------
| |Sun|Mercury|Venus|Earth| Mars|Jupiter|Saturn|Uranus|Neptune|
------------------------------------------------------------------------
| Sun| - | 1,397|2,182|3,042|4,983| 16,331|30,021|60,642| 90,863|
------------------------------------------------------------------------
|Mercury| - | - |3,542|4,343|5,904| 17,252|31,021|61,643| 91,663|
------------------------------------------------------------------------
| Venus| - | - | - |5,204|7,145| 18,473|32,222|62,843| 92,864|
------------------------------------------------------------------------
| Earth| - | - | - | - |7,925| 19,273|33,023|63,644| 93,665|
------------------------------------------------------------------------
| Mars| - | - | - | - | - | 20,414|34,224|64,845| 94,866|
------------------------------------------------------------------------
|Jupiter| - | - | - | - | - | - |44,831|75,452|106,274|
------------------------------------------------------------------------
| Saturn| - | - | - | - | - | - | - |87,661|119,883|
------------------------------------------------------------------------
| Uranus| - | - | - | - | - | - | - | - |150,504|
------------------------------------------------------------------------
|Neptune| - | - | - | - | - | - | - | - | - |
------------------------------------------------------------------------
Figure 8: EXCHANGE_LIFETIME (for MAX_RETRANSMIT=1), expressed in
seconds.
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------------------------------------------------------------------------
------------------------------------------------------------------------
| EXCHANGE_LIFETIME (for MAX_RETRANSMIT=1 and no exponential backoff) |
------------------------------------------------------------------------
| |Sun|Mercury|Venus|Earth| Mars|Jupiter|Saturn|Uranus|Neptune|
------------------------------------------------------------------------
| Sun| - | 931|1,454|2,028|3,322| 10,888|20,014|40,428| 60,575|
------------------------------------------------------------------------
|Mercury| - | - |2,362|2,895|3,936| 11,501|20,681|41,095| 61,109|
------------------------------------------------------------------------
| Venus| - | - | - |3,469|4,763| 12,315|21,482|41,896| 61,909|
------------------------------------------------------------------------
| Earth| - | - | - | - |5,284| 12,849|22,015|42,429| 62,443|
------------------------------------------------------------------------
| Mars| - | - | - | - | - | 13,609|22,816|43,230| 63,244|
------------------------------------------------------------------------
|Jupiter| - | - | - | - | - | - |29,887|50,301| 70,849|
------------------------------------------------------------------------
| Saturn| - | - | - | - | - | - | - |58,440| 79,922|
------------------------------------------------------------------------
| Uranus| - | - | - | - | - | - | - | - |100,336|
------------------------------------------------------------------------
|Neptune| - | - | - | - | - | - | - | - | - |
------------------------------------------------------------------------
Figure 9: Hypothetical EXCHANGE_LIFETIME (for MAX_RETRANSMIT=1),
assuming CoAP message retransmission without exponential backoff,
expressed in seconds.
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------------------------------------------------------------------------
------------------------------------------------------------------------
| MAX_LATENCY |
------------------------------------------------------------------------
| |Sun|Mercury|Venus|Earth| Mars|Jupiter|Saturn|Uranus|Neptune|
------------------------------------------------------------------------
| Sun| - | 233| 364| 507| 831| 2,722| 5,003|10,107| 15,144|
------------------------------------------------------------------------
|Mercury| - | - | 590| 724| 984| 2,875| 5,170|10,274| 15,277|
------------------------------------------------------------------------
| Venus| - | - | - | 867|1,191| 3,079| 5,370|10,474| 15,477|
------------------------------------------------------------------------
| Earth| - | - | - | - |1,321| 3,212| 5,504|10,607| 15,611|
------------------------------------------------------------------------
| Mars| - | - | - | - | - | 3,402| 5,704|10,807| 15,811|
------------------------------------------------------------------------
|Jupiter| - | - | - | - | - | - | 7,472|12,575| 17,712|
------------------------------------------------------------------------
| Saturn| - | - | - | - | - | - | - |14,610| 19,980|
------------------------------------------------------------------------
| Uranus| - | - | - | - | - | - | - | - | 25,084|
------------------------------------------------------------------------
|Neptune| - | - | - | - | - | - | - | - | - |
------------------------------------------------------------------------
Figure 10: Approximate MAX_LATENCY, expressed in seconds.
Appendix B. Message ID size, EXCHANGE_LIFETIME, and maximum CoAP
message rate
With default settings [RFC 7252], and a 16-bit message ID size, CoAP
supports the transmission of up to 265 messages/s between a sender
and its destination endpoint. If CoAP is used in scenarios involving
much greater latencies (e.g., deep space), the greater
EXCHANGE_LIFETIME would significantly limit the CoAP message rate.
Figure 9 provides the maximum possible message rates for message ID
sizes of 16 and 24 bits, and a range of EXCHANGE_LIFETIME values.
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------------------------------------------------------------------------
------------------------------------------------------------------------
|Message ID 16 bits 24 bits |
------------------------------------------------------------------------
#Messages per EXCHANGE_LIFETIME 65,536 16,777,216
------------------------------------------------------------------------
------------------------------------------------------------------------
------------------------------------------------------------------------
|Message rate (messages/second) |
------------------------------------------------------------------------
EXCHANGE_LIFETIME (s) Message ID_16 bits Message_ID 24 bits
247 (default) 265.3 (default) 67,924
500 131.1 33,554
1,000 65.5 16,777
1,500 43.7 11,184
2,000 32.8 8,388
2,500 26.2 6,710
3,000 21.8 5,592
3,500 18.7 4,793
4,000 16.4 4,194
4,500 14.6 3,728
5,000 13.1 3,355
5,500 11.9 3,050
6,000 10.9 2,796
6,500 10.1 2,581
7,000 9.4 2,396
7,500 8.7 2,237
10,000 6.6 1,677
20,000 3.3 838
30,000 2.2 559
40,000 1.6 419
50,000 1.3 335
60,000 1.1 279
70,000 0.9 239
80,000 0.8 209
90,000 0.7 186
100,000 0.7 167
110,000 0.6 152
120,000 0.5 139
130,000 0.5 129
140,000 0.5 119
150,000 0.4 111
------------------------------------------------------------------------------------------------------------------------------------------------
Figure 11: Maximum CoAP message rate imposed by the Message ID
size and EXCHANGE_LIFETIME, expressed in messages/s.
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Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carles.gomez@upc.edu
Anna Calveras
UPC
C/Jordi Girona, 1-3
08034 Barcelona
Spain
Email: anna.calveras@upc.edu
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