Internet DRAFT - draft-gomez-core-coap-space
draft-gomez-core-coap-space
CoRE Working Group C. Gomez
Internet-Draft UPC
Intended status: Informational S. Aguilar
Expires: 21 June 2024 Sateliot
December 2023
CoAP in Space
draft-gomez-core-coap-space-00
Abstract
This document provides guidance on using the Constrained Application
Protocol (CoAP) in deep space environments. The document focuses on
the scenario where an IP protocol stack is used for deep space
communication.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements language . . . . . . . . . . . . . . . . . . 3
3. CoAP transport . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Overview and underlying transport . . . . . . . . . . . . 3
3.2. Main CoAP parameters and times relevant to deep space . . 4
4. Observe . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Block-wise transfers . . . . . . . . . . . . . . . . . . . . 6
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Main related parameters . . . . . . . . . . . . . . . . . 6
6. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Forward Error Correction . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Deep space communication occurs between devices on or orbiting
different celestial bodies (e.g., different planets of the Solar
System). Such environments are characterized by long delays (e.g.,
in the order of minutes or hours), intermittent communication
opportunities, and relatively low bandwidth in some cases. Resources
such as energy may also be particularly limited for remote devices.
The Internet Protocol (IP) stack was considered unsuitable for deep
space communication more than two decades ago, leading to the design
of the Delay-Tolerant Networking (DTN) architecture [RFC4838] and the
Bundle Protocol (BP) [RFC5050] [RFC9171]. However, recent work has
revisited such assessment, and it has discussed solutions to use the
IP protocol stack in deep space communication
[I-D.many-deepspace-ip-assessment][I-D.huitema-quic-in-space].
From the application layer point of view, the analysis in
[I-D.many-deepspace-ip-assessment] focuses on the use of HTTP (over
QUIC [RFC9000]) in deep space scenarios. However, it also explicitly
mentions that the Constrained Application Protocol (CoAP) [RFC7252]
"is worth considering for application transport in deep space".
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CoAP offers several features suitable for its use in deep space
environments, including lightweight operation, asynchronous message
exchanges, and a significant degree of flexibility. This document
provides guidance on the use of CoAP for deep space communication.
Use of CoAP over BP [RFC9171] is outside the scope of this document.
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.
3. CoAP transport
3.1. Overview and underlying transport
CoAP was originally designed to use UDP as its underlying transport
protocol [RFC7252]. The message layer of CoAP over UDP supports
optional message reliability, simple congestion control, and flow
control. A CoAP message that requires reliable delivery is marked as
a Confirmable (CON) message. The recipient needs to send an
Acknowledgment (ACK) message to confirm successful reception of a CON
message. A sender uses a retransmission mechanism with a default
timeout and an exponential back-off between retransmissions. A CoAP
message that does not require reliability is marked as a Non-
confirmable (NON) message. NON messages are not acknowledged.
Subsequently, CoAP was adapted to be carried also over other
transports, such as TCP, Transport Layer Security (TLS), and
WebSockets [RFC8323]. However, due to the long delays in deep space
environments, initial handshake exchanges (e.g., the three-way
handshake of TCP) penalize communication performance significantly.
In addition, when TCP is used as the underlying transport-layer
protocol, the ability of optionally requesting reliable delivery for
a given message (as offered by CoAP over UDP) is lost. Two further
advantages of UDP-based CoAP transport are a shorter header size and
support for multicast. Therefore, this document will focus on CoAP
as used over UDP as the underlying transport [RFC7252].
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3.2. Main CoAP parameters and times relevant to deep space
This section discusses the main parameters and times that are
relevant in a deep space context. (Note that the complete set of
parameters, assumptions, default values, and related times in CoAP
can be found in Section 4.8 of RFC7252.)
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.
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 deep space scenarios, ACK_TIMEOUT should be set to a value of at
least the expected RTT in such scenarios, which may be of an order of
magnitude 2-3 times greater than the default one.
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 deep space 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 deep space, it may be suitable to set
this parameter to a value lower than the default one.
The following assumptions on the characteristics of the network and
the nodes need to be considered:
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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
the historic Maximum Segment Lifetime (MSL) of 120 s defined in the
TCP specification [RFC9293]. However, such value assumes
communication between devices on Earth. Therefore, in deep space,
MAX_LATENCY may need to be increased by 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 deep space environments.
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 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.
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 deep
space environments it may need to be increased by 2-3 orders of
magnitude.
Note that 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.
4. Observe
The 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
deep space environments, where end-to-end latency is high, and energy
and bandwidth resources may be constrained.
5. Block-wise transfers
5.1. Overview
There exist two CoAP specifications that define functionality that
allows to carry large CoAP payloads (i.e., payloads that do not fit a
single packet) by means of block-wise transfers: [RFC7959] and
[RFC9177].
RFC 7959 defines the Block1 and Block2 options, whereby, in a block-
wise transfer, 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.
RFC 9177, which defines the Q-Block1 and Q-Block2 options, is
particularly suitable for deep space 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.
The Q-Block1 option is defined for payload-bearing (e.g., POST, PUT,
FETCH, PATCH, and iPATCH) requests and their responses. The Q-Block2
option is useful for requests (e.g., GET, POST, PUT, FETCH, PATCH,
and iPATCH) and their payload-bearing responses.
5.2. Main related 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?
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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 in Section 3.2, in deep space,
ACK_TIMEOUT should be set to a value greater than default. However,
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, when CoAP is used in
deep space, NON_TIMEOUT needs to be adjusted considering the
characteristics of the end-to-end path.
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 2.2 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.
6. Security
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.
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Subsequently, Object Security for Constrained RESTful Environments
(OSCORE) was specified [RFC8613]. OSCORE is a CoAP option 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. OSCORE is used to secure CoAP group communication (which
uses UDP/IP multicast as underlying transport) [I-D.ietf-core-
groupcomm-bis].
In OSCORE, the communicating endpoints require a shared security
context. An interesting aspect of OSCORE in deep space 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.
7. Forward Error Correction
As of the writing, no proposal has been made to add support of
Forward Error Correction (FEC) to CoAP. However, considering the
significant latency penalty of deep space environments, FEC might
allow to reduce the probability of incurring additional latency (due
to retries) in order to sucessfully deliver a message to its intended
destination.
8. IANA Considerations
This document has no IANA considerations
9. Security Considerations
TO-DO
10. Acknowledgments
Marisa Catalan and Julia Igual from i2cat contributed to this
document.
Carles Gomez has been funded in part by the Spanish Government
through project PID2019-106808RA-I00, and by Secretaria
d'Universitats i Recerca del Departament d'Empresa i Coneixement de
la Generalitat de Catalunya 2017 through grant SGR 376 and 2021
throught grant SGR 00330.
11. References
11.1. Normative References
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[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>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.
[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>.
[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>.
[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>.
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[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>.
11.2. Informative References
[I-D.huitema-quic-in-space]
Huitema, C. and M. Blanchet, "QUIC in Space", Work in
Progress, Internet-Draft, draft-huitema-quic-in-space-00,
24 September 2023, <https://datatracker.ietf.org/doc/html/
draft-huitema-quic-in-space-00>.
[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-
10, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
groupcomm-bis-10>.
[I-D.many-deepspace-ip-assessment]
Blanchet, M., Huitema, C., and D. Bogdanović, "Revisiting
the Use of the IP Protocol Stack in Deep Space: Assessment
and Possible Solutions", Work in Progress, Internet-Draft,
draft-many-deepspace-ip-assessment-00, 8 September 2023,
<https://datatracker.ietf.org/doc/html/draft-many-
deepspace-ip-assessment-00>.
[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>.
[RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, DOI 10.17487/RFC5050, November
2007, <https://www.rfc-editor.org/info/rfc5050>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[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>.
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[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
Authors' Addresses
Carles Gomez
UPC
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carles.gomez@upc.edu
Sergio Aguilar
Sateliot
C/Berlin 61, Esc A Entresuelo
08029 Barcelona
Spain
Email: sergio.aguilar@sateliot.com
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