Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission
draft-ietf-core-new-block-14
CoRE Working Group M. Boucadair
Internet-Draft Orange
Intended status: Standards Track J. Shallow
Expires: November 27, 2021 May 26, 2021
Constrained Application Protocol (CoAP) Block-Wise Transfer Options
Supporting Robust Transmission
draft-ietf-core-new-block-14
Abstract
This document specifies alternative Constrained Application Protocol
(CoAP) Block-Wise transfer options: Q-Block1 and Q-Block2 Options.
These options are similar to, but distinct from, the CoAP Block1 and
Block2 Options defined in RFC 7959. Q-Block1 and Q-Block2 Options
are not intended to replace Block1 and Block2 Options, but rather
have the goal of supporting Non-confirmable messages for large
amounts of data with fewer packet interchanges. Also, the Q-Block1
and Q-Block2 Options support faster recovery should any of the blocks
get lost in transmission.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 27, 2021.
Copyright Notice
Copyright (c) 2021 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Alternative CoAP Block-Wise Transfer Options . . . . . . . . 5
3.1. CoAP Response Code (4.08) Usage . . . . . . . . . . . . . 7
3.2. Applicability Scope . . . . . . . . . . . . . . . . . . . 7
4. The Q-Block1 and Q-Block2 Options . . . . . . . . . . . . . . 8
4.1. Properties of Q-Block1 and Q-Block2 Options . . . . . . . 8
4.2. Structure of Q-Block1 and Q-Block2 Options . . . . . . . 10
4.3. Using the Q-Block1 Option . . . . . . . . . . . . . . . . 11
4.4. Using the Q-Block2 Option . . . . . . . . . . . . . . . . 15
4.5. Using Observe Option . . . . . . . . . . . . . . . . . . 17
4.6. Using Size1 and Size2 Options . . . . . . . . . . . . . . 17
4.7. Using Q-Block1 and Q-Block2 Options Together . . . . . . 18
4.8. Using Q-Block2 Option With Multicast . . . . . . . . . . 18
5. The Use of 4.08 (Request Entity Incomplete) Response Code . . 18
6. The Use of Tokens . . . . . . . . . . . . . . . . . . . . . . 19
7. Congestion Control for Unreliable Transports . . . . . . . . 20
7.1. Confirmable (CON) . . . . . . . . . . . . . . . . . . . . 20
7.2. Non-confirmable (NON) . . . . . . . . . . . . . . . . . . 20
8. Caching Considerations . . . . . . . . . . . . . . . . . . . 25
9. HTTP-Mapping Considerations . . . . . . . . . . . . . . . . . 26
10. Examples with Non-confirmable Messages . . . . . . . . . . . 26
10.1. Q-Block1 Option . . . . . . . . . . . . . . . . . . . . 27
10.1.1. A Simple Example . . . . . . . . . . . . . . . . . . 27
10.1.2. Handling MAX_PAYLOADS Limits . . . . . . . . . . . . 27
10.1.3. Handling MAX_PAYLOADS with Recovery . . . . . . . . 27
10.1.4. Handling Recovery with Failure . . . . . . . . . . . 29
10.2. Q-Block2 Option . . . . . . . . . . . . . . . . . . . . 30
10.2.1. A Simple Example . . . . . . . . . . . . . . . . . . 30
10.2.2. Handling MAX_PAYLOADS Limits . . . . . . . . . . . . 31
10.2.3. Handling MAX_PAYLOADS with Recovery . . . . . . . . 32
10.2.4. Handling Recovery using M-bit Set . . . . . . . . . 33
10.3. Q-Block1 and Q-Block2 Options . . . . . . . . . . . . . 34
10.3.1. A Simple Example . . . . . . . . . . . . . . . . . . 34
10.3.2. Handling MAX_PAYLOADS Limits . . . . . . . . . . . . 35
10.3.3. Handling Recovery . . . . . . . . . . . . . . . . . 36
11. Security Considerations . . . . . . . . . . . . . . . . . . . 38
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
12.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 39
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12.2. Media Type Registration . . . . . . . . . . . . . . . . 39
12.3. CoAP Content-Formats Registry . . . . . . . . . . . . . 40
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
13.1. Normative References . . . . . . . . . . . . . . . . . . 41
13.2. Informative References . . . . . . . . . . . . . . . . . 42
Appendix A. Examples with Confirmable Messages . . . . . . . . . 43
A.1. Q-Block1 Option . . . . . . . . . . . . . . . . . . . . . 43
A.2. Q-Block2 Option . . . . . . . . . . . . . . . . . . . . . 45
Appendix B. Examples with Reliable Transports . . . . . . . . . 47
B.1. Q-Block1 Option . . . . . . . . . . . . . . . . . . . . . 47
B.2. Q-Block2 Option . . . . . . . . . . . . . . . . . . . . . 47
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252], although
inspired by HTTP, was designed to use UDP instead of TCP. The
message layer of CoAP over UDP includes support for reliable
delivery, simple congestion control, and flow control. CoAP supports
two message types (Section 1.2 of [RFC7252]): Confirmable (CON) and
Non-confirmable (NON) messages. Unlike NON messages, every CON
message will elicit an acknowledgement or a reset.
The CoAP specification recommends that a CoAP message should fit
within a single IP packet (i.e., avoid IP fragmentation). To handle
data records that cannot fit in a single IP packet, [RFC7959]
introduced the concept of block-wise transfer and the companion CoAP
Block1 and Block2 Options. However, this concept is designed to work
exclusively with Confirmable messages (Section 1 of [RFC7959]). Note
that the block-wise transfer was further updated by [RFC8323] for use
over TCP, TLS, and WebSockets.
The CoAP Block1 and Block2 Options work well in environments where
there are no, or minimal, packet losses. These options operate
synchronously, i.e., each individual block has to be requested. A
CoAP endpoint can only ask for (or send) the next block when the
transfer of the previous block has completed. Packet transmission
rate, and hence block transmission rate, is controlled by Round Trip
Times (RTTs).
There is a requirement for blocks of data larger than a single IP
datagram to be transmitted under network conditions where there may
be asymmetrical transient packet loss (e.g., acknowledgment responses
may get dropped). An example is when a network is subject to a
Distributed Denial of Service (DDoS) attack and there is a need for
DDoS mitigation agents relying upon CoAP to communicate with each
other (e.g., [RFC8782][I-D.ietf-dots-telemetry]). As a reminder,
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[RFC7959] recommends the use of CON responses to handle potential
packet loss. However, such a recommendation does not work with a
flooded pipe DDoS situation (e.g., [RFC8782]).
This document introduces the CoAP Q-Block1 and Q-Block2 Options which
allow block-wise transfer to work with series of Non-confirmable
messages, instead of lock-stepping using Confirmable messages
(Section 3). In other words, this document provides a missing piece
of [RFC7959], namely the support of block-wise transfer using Non-
confirmable where an entire body of data can be transmitted without
the requirement that intermediate acknowledgments be received from
the peer (but recovery is available should it be needed).
Similar to [RFC7959], this specification does not remove any of the
constraints posed by the base CoAP specification [RFC7252] it is
strictly layered on top of.
2. Terminology
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 BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers should be familiar with the terms and concepts defined in
[RFC7252], [RFC7959], and [RFC8132]. Particularly, the document uses
the following key concepts:
Token: is used to match responses to requests independently from the
underlying messages (Section 5.3.1 of [RFC7252]).
ETag: is used as a resource-local identifier for differentiating
between representations of the same resource that vary over time
(Section 5.10.6 of [RFC7252]).
The terms "payload" and "body" are defined in [RFC7959]. The term
"payload" is thus used for the content of a single CoAP message
(i.e., a single block being transferred), while the term "body" is
used for the entire resource representation that is being transferred
in a block-wise fashion.
Request-Tag refers to an option that allows a CoAP server to match
message fragments belonging to the same request
[I-D.ietf-core-echo-request-tag].
MAX_PAYLOADS is the maximum number of payloads that can be
transmitted at any one time.
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MAX_PAYLOADS_SET is the set of blocks identified by block numbers
that, when divided by MAX_PAYLOADS, have the same numeric result.
For example, if MAX_PAYLOADS is set to '10', a MAX_PAYLOADS_SET could
be blocks #0 to #9, #10 to #19, etc. Depending on the overall data
size, there could be fewer than MAX_PAYLOADS blocks in the final
MAX_PAYLOADS_SET.
3. Alternative CoAP Block-Wise Transfer Options
This document introduces the CoAP Q-Block1 and Q-Block2 Options.
These options are designed to work in particular with NON requests
and responses.
Using NON messages, faster transmissions can occur as all the blocks
can be transmitted serially (akin to fragmented IP packets) without
having to wait for a response or next request from the remote CoAP
peer. Recovery of missing blocks is faster in that multiple missing
blocks can be requested in a single CoAP message. Even if there is
asymmetrical packet loss, a body can still be sent and received by
the peer whether the body comprises a single or multiple payloads,
assuming no recovery is required.
A CoAP endpoint can acknowledge all or a subset of the blocks.
Concretely, the receiving CoAP endpoint either informs the CoAP
sender endpoint of successful reception or reports on all blocks in
the body that have not yet been received. The CoAP sender endpoint
will then retransmit only the blocks that have been lost in
transmission.
Note that similar transmission rate benefits can be applied to
Confirmable messages if the value of NSTART is increased from 1
(Section 4.7 of [RFC7252]). However, the use of Confirmable messages
will not work effectively if there is asymmetrical packet loss. Some
examples with Confirmable messages are provided in Appendix A.
There is little, if any, benefit of using these options with CoAP
running over a reliable connection [RFC8323]. In this case, there is
no differentiation between CON and NON as they are not used. Some
examples using a reliable transport are provided in Appendix B.
Q-Block1 and Q-Block2 Options are similar in operation to the CoAP
Block1 and Block2 Options, respectively. They are not a replacement
for them, but have the following benefits:
o They can operate in environments where packet loss is highly
asymmetrical.
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o They enable faster transmissions of sets of blocks of data with
fewer packet interchanges.
o They support faster recovery should any of the blocks get lost in
transmission.
o They support sending an entire body using NON messages without
requiring that an intermediate response be received from the peer.
There are the following disadvantages over using CoAP Block1 and
Block2 Options:
o Loss of lock-stepping so payloads are not always received in the
correct (block ascending) order.
o Additional congestion control measures need to be put in place for
NON messages (Section 7.2).
o To reduce the transmission times for CON transmission of large
bodies, NSTART needs to be increased from 1, but this affects
congestion control and incurs a requirement to re-tune other
parameters (Section 4.7 of [RFC7252]). Such tuning is out of
scope of this document.
o Mixing of NON and CON during requests/responses using Q-Block is
not supported.
o The Q-Block Options do not support stateless operation/random
access.
o Proxying of Q-Block is limited to caching full representations.
o There is no multicast support.
Q-Block1 and Q-Block2 Options can be used instead of Block1 and
Block2 Options when the different transmission properties are
required. If the new options are not supported by a peer, then
transmissions can fall back to using Block1 and Block2 Options
(Section 4.1).
The deviations from Block1 and Block2 Options are specified in
Section 4. Pointers to appropriate [RFC7959] sections are provided.
The specification refers to the base CoAP methods defined in
Section 5.8 of [RFC7252] and the new CoAP methods, FETCH, PATCH, and
iPATCH introduced in [RFC8132].
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The No-Response Option [RFC7967] was considered but was abandoned as
it does not apply to Q-Block2 responses. A unified solution is
defined in the document.
3.1. CoAP Response Code (4.08) Usage
This document adds a media type for the 4.08 (Request Entity
Incomplete) response defining an additional message format for
reporting on payloads using the Q-Block1 Option that are not received
by the server.
See Section 5 for more details.
3.2. Applicability Scope
The block-wise transfer specified in [RFC7959] covers the general
case using Confirmable messages, but falls short in situations where
packet loss is highly asymmetrical or there is no need for an
acknowledgement. In other words, there is a need for Non-confirmable
support.
The mechanism specified in this document provides roughly similar
features to the Block1/Block2 Options. It provides additional
properties that are tailored towards the intended use case of Non-
confirmable transmission. Concretely, this mechanism primarily
targets applications such as DDoS Open Threat Signaling (DOTS) that
cannot use CON requests/responses because of potential packet loss
and that support application-specific mechanisms to assess whether
the remote peer is not overloaded and thus is able to process the
messages sent by a CoAP endpoint (e.g., DOTS heartbeats in
Section 4.7 of [RFC8782]). Other use cases are when an application
sends data but has no need for an acknowledgement of receipt and, any
data transmission loss is not critical.
The mechanism includes guards to prevent a CoAP agent from
overloading the network by adopting an aggressive sending rate.
These guards MUST be followed in addition to the existing CoAP
congestion control as specified in Section 4.7 of [RFC7252]. See
Section 7 for more details.
Any usage outside the primary use case of Non-confirmable with block
transfers should be carefully weighed against the potential loss of
interoperability with generic CoAP applications (See the
disadvantages listed in Section 3). It is hoped that the experience
gained with this mechanism can feed future extensions of the block-
wise mechanism that will both be generally applicable and serve this
particular use case.
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It is not recommended that these options are used in a NoSec security
mode (Section 9 of [RFC7252]) as the source endpoint needs to be
trusted. Using OSCORE [RFC8613] does provide a security context and,
hence, a trust of the source endpoint that prepared the inner OSCORE
content. However, even with OSCORE, using a NoSec security mode with
these options may still be inadequate, for reasons discussed in
Section 11.
4. The Q-Block1 and Q-Block2 Options
4.1. Properties of Q-Block1 and Q-Block2 Options
The properties of the Q-Block1 and Q-Block2 Options are shown in
Table 1. The formatting of this table follows the one used in
Table 4 of [RFC7252] (Section 5.10). The C, U, N, and R columns
indicate the properties Critical, UnSafe, NoCacheKey, and Repeatable
defined in Section 5.4 of [RFC7252]. Only Critical and UnSafe
columns are marked for the Q-Block1 Option. Critical, UnSafe, and
Repeatable columns are marked for the Q-Block2 Option. As these
options are UnSafe, NoCacheKey has no meaning and so is marked with a
dash.
+--------+---+---+---+---+--------------+--------+--------+---------+
| Number | C | U | N | R | Name | Format | Length | Default |
+========+===+===+===+===+==============+========+========+=========+
| TBA1 | x | x | - | | Q-Block1 | uint | 0-3 | (none) |
| TBA2 | x | x | - | x | Q-Block2 | uint | 0-3 | (none) |
+--------+---+---+---+---+--------------+--------+--------+---------+
Table 1: CoAP Q-Block1 and Q-Block2 Option Properties
The Q-Block1 and Q-Block2 Options can be present in both the request
and response messages. The Q-Block1 Option pertains to the request
payload and the Q-Block2 Option pertains to the response payload.
When the Content-Format Option is present together with the Q-Block1
or Q-Block2 Option, the option applies to the body not to the payload
(i.e., it must be the same for all payloads of the same body).
The Q-Block1 Option is useful with the payload-bearing, e.g., POST,
PUT, FETCH, PATCH, and iPATCH requests and their responses.
The Q-Block2 Option is useful, e.g., with GET, POST, PUT, FETCH,
PATCH, and iPATCH requests and their payload-bearing responses
(response codes 2.01, 2.02, 2.04, and 2.05) (Section 5.5 of
[RFC7252]).
A CoAP endpoint (or proxy) MUST support either both or neither of the
Q-Block1 and Q-Block2 Options.
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If the Q-Block1 Option is present in a request or the Q-Block2 Option
is returned in a response, this indicates a block-wise transfer and
describes how this specific block-wise payload forms part of the
entire body being transferred. If it is present in the opposite
direction, it provides additional control on how that payload will be
formed or was processed.
To indicate support for Q-Block2 responses, the CoAP client MUST
include the Q-Block2 Option in a GET or similar request (FETCH, for
example), the Q-Block2 Option in a PUT or similar request (POST, for
example), or the Q-Block1 Option in a PUT or similar request so that
the server knows that the client supports this Q-Block functionality
should it need to send back a body that spans multiple payloads.
Otherwise, the server would use the Block2 Option (if supported) to
send back a message body that is too large to fit into a single IP
packet [RFC7959].
How a client decides whether it needs to include a Q-Block1 or
Q-Block2 Option can be driven by a local configuration parameter,
triggered by an application (DOTS, for example), etc. Such
considerations are out of the scope of the document.
Implementation of the Q-Block1 and Q-Block2 Options is intended to be
optional. However, when it is present in a CoAP message, it MUST be
processed (or the message rejected). Therefore, Q-Block1 and
Q-Block2 Options are identified as Critical options.
With CoAP over UDP, the way a request message is rejected for
critical options depends on the message type. A Confirmable message
with an unrecognized critical option is rejected with a 4.02 (Bad
Option) response (Section 5.4.1 of [RFC7252]). A Non-confirmable
message with an unrecognized critical option is either rejected with
a Reset message or just silently ignored (Sections 5.4.1 and 4.3 of
[RFC7252]). To reliably get a rejection message, it is therefore
REQUIRED that clients use a Confirmable message for determining
support for Q-Block1 and Q-Block2 Options. This CON message can be
sent under the base CoAP congestion control setup specified in
Section 4.7 of [RFC7252] (that is, NSTART does not need to be
increased (Section 7.1)).
The Q-Block1 and Q-Block2 Options are unsafe to forward. That is, a
CoAP proxy that does not understand the Q-Block1 (or Q-Block2) Option
must reject the request or response that uses either option (See
Section 5.7.1 of [RFC7252]).
The Q-Block2 Option is repeatable when requesting retransmission of
missing blocks, but not otherwise. Except that case, any request
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carrying multiple Q-Block1 (or Q-Block2) Options MUST be handled
following the procedure specified in Section 5.4.5 of [RFC7252].
The Q-Block1 and Q-Block2 Options, like the Block1 and Block2
Options, are of both class E and class U for OSCORE processing
(Table 2). The Q-Block1 (or Q-Block2) Option MAY be an Inner or
Outer option (Section 4.1 of [RFC8613]). The Inner and Outer values
are therefore independent of each other. The Inner option is
encrypted and integrity protected between clients and servers, and
provides message body identification in case of end-to-end
fragmentation of requests. The Outer option is visible to proxies
and labels message bodies in case of hop-by-hop fragmentation of
requests.
+--------+-----------------+---+---+
| Number | Name | E | U |
+========+=================+===+===+
| TBA1 | Q-Block1 | x | x |
| TBA2 | Q-Block2 | x | x |
+--------+-----------------+---+---+
Table 2: OSCORE Protection of Q-Block1 and Q-Block2 Options
Note that if Q-Block1 or Q-Block2 Options are included in a packet as
Inner options, Block1 or Block2 Options MUST NOT be included as Inner
options. Similarly, there MUST NOT be a mix of Q-Block and Block for
the Outer options. Messages that do not adhere with this behavior
MUST be rejected with 4.02 (Bad Option). Q-Block and Block Options
can be mixed across Inner and Outer options as these are handled
independently of each other. For clarity, if OSCORE is not being
used, there MUST NOT be a mix of Q-Block and Block Options in the
same packet.
4.2. Structure of Q-Block1 and Q-Block2 Options
The structure of Q-Block1 and Q-Block2 Options follows the structure
defined in Section 2.2 of [RFC7959].
There is no default value for the Q-Block1 and Q-Block2 Options.
Absence of one of these options is equivalent to an option value of 0
with respect to the value of block number (NUM) and more bit (M) that
could be given in the option, i.e., it indicates that the current
block is the first and only block of the transfer (block number is
set to 0, M is unset). However, in contrast to the explicit value 0,
which would indicate a size of the block (SZX) of 0, and thus a size
value of 16 bytes, there is no specific explicit size implied by the
absence of the option -- the size is left unspecified. (As for any
uint, the explicit value 0 is efficiently indicated by a zero-length
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option; this, therefore, is different in semantics from the absence
of the option).
4.3. Using the Q-Block1 Option
The Q-Block1 Option is used when the client wants to send a large
amount of data to the server using the POST, PUT, FETCH, PATCH, or
iPATCH methods where the data and headers do not fit into a single
packet.
When Q-Block1 Option is used, the client MUST include a Request-Tag
Option [I-D.ietf-core-echo-request-tag]. The Request-Tag value MUST
be the same for all of the requests for the body of data that is
being transferred. The Request-Tag is opaque, but the client MUST
ensure that it is unique for every different body of transmitted
data.
Implementation Note: It is suggested that the client treats the
Request-Tag as an unsigned integer of 8 bytes in length. An
implementation may want to consider limiting this to 4 bytes to
reduce packet overhead size. The initial Request-Tag value should
be randomly generated and then subsequently incremented by the
client whenever a new body of data is being transmitted between
peers.
Section 4.6 discusses the use of Size1 Option.
For Confirmable transmission, the server continues to acknowledge
each packet, but a response is not required (whether separate or
piggybacked) until successful receipt of the body by the server. For
Non-confirmable transmission, no response is required until either
the successful receipt of the body by the server or a timer expires
with some of the payloads having not yet arrived. In the latter
case, a "retransmit missing payloads" response is needed. For
reliable transports (e.g., [RFC8323]), a response is not required
until successful receipt of the body by the server.
Each individual message that carries a block of the body is treated
as a new request (Section 6).
The client MUST send the payloads in order of increasing block
number, starting from zero, until the body is complete (subject to
any congestion control (Section 7)). Any missing payloads requested
by the server must in addition be separately transmitted with
increasing block numbers.
The following Response Codes are used:
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2.01 (Created)
This Response Code indicates successful receipt of the entire body
and that the resource was created. The token to use MUST be one
of the tokens that were received in a request for this block-wise
exchange. However, it is desirable to provide the one used in the
last received request, since that will aid any troubleshooting.
The client should then release all of the tokens used for this
body. Note that the last received payload might not be the one
with the highest block number.
2.02 (Deleted)
This Response Code indicates successful receipt of the entire body
and that the resource was deleted when using POST (Section 5.8.2
[RFC7252]). The token to use MUST be one of the tokens that were
received in a request for this block-wise exchange. However, it
is desirable to provide the one used in the last received request.
The client should then release all of the tokens used for this
body.
2.04 (Changed)
This Response Code indicates successful receipt of the entire body
and that the resource was updated. The token to use MUST be one
of the tokens that were received in a request for this block-wise
exchange. However, it is desirable to provide the one used in the
last received request. The client should then release all of the
tokens used for this body.
2.05 (Content)
This Response Code indicates successful receipt of the entire
FETCH request body (Section 2 of [RFC8132]) and that the
appropriate representation of the resource is being returned. The
token to use MUST be one of the tokens that were received in a
request for this block-wise exchange. However, it is desirable to
provide the one used in the last received request.
If the FETCH request includes the Observe Option, then the server
MUST use the same token as used for the 2.05 (Content) response
for returning any Observe triggered responses so that the client
can match them up.
The client should then release all of the tokens used for this
body apart from the one used for tracking an observed resource.
2.31 (Continue)
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This Response Code can be used to indicate that all of the blocks
up to and including the Q-Block1 Option block NUM (all having the
M bit set) have been successfully received. The token to use MUST
be one of the tokens that were received in a request for this
latest MAX_PAYLOADS_SET block-wise exchange. However, it is
desirable to provide the one used in the last received request.
The client should then release all of the tokens used for this
MAX_PAYLOADS_SET.
A response using this Response Code MUST NOT be generated for
every received Q-Block1 Option request. It SHOULD only be
generated when all the payload requests are Non-confirmable and a
MAX_PAYLOADS_SET has been received by the server. More details
about the motivations for this optimization are discussed in
Section 7.2.
This Response Code SHOULD NOT be generated for CON as this may
cause duplicated payloads to unnecessarily be sent.
4.00 (Bad Request)
This Response Code MUST be returned if the request does not
include a Request-Tag Option or a Size1 Option but does include a
Q-Block1 option.
4.02 (Bad Option)
This Response Code MUST be returned for a Confirmable request if
the server does not support the Q-Block Options. Note that a
reset message may be sent in case of Non-confirmable request.
4.08 (Request Entity Incomplete)
As a reminder, this Response Code returned without Content-Type
"application/missing-blocks+cbor-seq" (Section 12.3) is handled as
in Section 2.9.2 [RFC7959].
This Response Code returned with Content-Type "application/
missing-blocks+cbor-seq" indicates that some of the payloads are
missing and need to be resent. The client then retransmits the
individual missing payloads using the same Request-Tag, Size1,
and, Q-Block1 Option to specify the same NUM, SZX, and M bit as
sent initially in the original, but not received, packet.
The Request-Tag value to use is determined by taking the token in
the 4.08 (Request Entity Incomplete) response, locating the
matching client request, and then using its Request-Tag.
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The token to use in the 4.08 (Request Entity Incomplete) response
MUST be one of the tokens that were received in a request for this
block-wise body exchange. However, it is desirable to provide the
one used in the last received request. See Section 5 for further
information.
If the server has not received all the blocks of a body, but one
or more NON payloads have been received, it SHOULD wait for
NON_RECEIVE_TIMEOUT (Section 7.2) before sending a 4.08 (Request
Entity Incomplete) response.
4.13 (Request Entity Too Large)
This Response Code can be returned under similar conditions to
those discussed in Section 2.9.3 of [RFC7959].
This Response Code can be returned if there is insufficient space
to create a response PDU with a block size of 16 bytes (SZX = 0)
to send back all the response options as appropriate. In this
case, the Size1 Option is not included in the response.
Further considerations related to the transmission timings of 4.08
(Request Entity Incomplete) and 2.31 (Continue) Response Codes are
discussed in Section 7.2.
If a server receives payloads with different Request-Tags for the
same resource, it should continue to process all the bodies as it has
no way of determining which is the latest version, or which body, if
any, the client is terminating the transmission for.
If the client elects to stop the transmission of a complete body, and
absent any local policy, the client MUST "forget" all tracked tokens
associated with the body's Request-Tag so that a reset message is
generated for the invalid token in the 4.08 (Request Entity
Incomplete) response. The server on receipt of the reset message
SHOULD delete the partial body.
If the server receives a duplicate block with the same Request-Tag,
it MUST ignore the payload of the packet, but MUST still respond as
if the block was received for the first time.
A server SHOULD maintain a partial body (missing payloads) for
NON_PARTIAL_TIMEOUT (Section 7.2).
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4.4. Using the Q-Block2 Option
In a request for any block number, the M bit unset indicates the
request is just for that block. If the M bit is set, this has
different meanings based on the NUM value:
NUM is zero: This is a request for the entire body.
'NUM modulo MAX_PAYLOADS' is zero, while NUM is not zero: This is
used to confirm that the current MAX_PAYLOADS_SET (the latest
block having block number NUM-1) has been successfully received
and that, upon receipt of this request, the server can continue to
send the next MAX_PAYLOADS_SET (the first block having block
number NUM). This is the 'Continue' Q-Block-2 and conceptually
has the same usage (i.e., continue sending the next set of data)
as the use of 2.31 (Continue) for Q-Block1.
Any other value of NUM: This is a request for that block and for all
of the remaining blocks in the current MAX_PAYLOADS_SET.
If the request includes multiple Q-Block2 Options and these options
overlap (e.g., combination of M being set (this and later blocks) and
being unset (this individual block)) resulting in an individual block
being requested multiple times, the server MUST only send back one
instance of that block. This behavior is meant to prevent
amplification attacks.
The payloads sent back from the server as a response MUST all have
the same ETag (Section 5.10.6 of [RFC7252]) for the same body. The
server MUST NOT use the same ETag value for different representations
of a resource.
The ETag is opaque, but the server MUST ensure that it is unique for
every different body of transmitted data.
Implementation Note: It is suggested that the server treats the
ETag as an unsigned integer of 8 bytes in length. An
implementation may want to consider limiting this to 4 bytes to
reduce packet overhead size. The initial ETag value should be
randomly generated and then subsequently incremented by the server
whenever a new body of data is being transmitted between peers.
Section 4.6 discusses the use of Size2 Option.
The client may elect to request any detected missing blocks or just
ignore the partial body. This decision is implementation specific.
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For NON payloads, the client SHOULD wait NON_RECEIVE_TIMEOUT
(Section 7.2) after the last received payload before requesting
retransmission of any missing blocks. Retransmission is requested by
issuing a GET, POST, PUT, FETCH, PATCH, or iPATCH request that
contains one or more Q-Block2 Options that define the missing
block(s). Generally the M bit on the Q-Block2 Option(s) SHOULD be
unset, although the M bit MAY be set to request this and later blocks
from this MAX_PAYLOADS_SET, see Section 10.2.4 for an example of this
in operation. Further considerations related to the transmission
timing for missing requests are discussed in Section 7.2.
The missing block numbers requested by the client MUST have an
increasing block number in each additional Q-Block2 Option with no
duplicates. The server SHOULD respond with a 4.00 (Bad Request) to
requests not adhering to this behavior. Note that the ordering
constraint is meant to force the client to check for duplicates and
remove them. This also helps with troubleshooting.
If the client receives a duplicate block with the same ETag, it MUST
silently ignore the payload.
A client SHOULD maintain a partial body (missing payloads) for
NON_PARTIAL_TIMEOUT (Section 7.2) or as defined by the Max-Age Option
(or its default of 60 seconds (Section 5.6.1 of [RFC7252])),
whichever is the less. On release of the partial body, the client
should then release all of the tokens used for this body apart from
the token that is used to track a resource that is being observed.
The ETag Option should not be used in the request for missing blocks
as the server could respond with a 2.03 (Valid) response with no
payload. It can be used in the request if the client wants to check
the freshness of the locally cached body response.
The server SHOULD maintain a cached copy of the body when using the
Q-Block2 Option to facilitate retransmission of any missing payloads.
If the server detects part way through a body transfer that the
resource data has changed and the server is not maintaining a cached
copy of the old data, then the transmission is terminated. Any
subsequent missing block requests MUST be responded to using the
latest ETag and Size2 Option values with the updated data.
If the server responds during a body update with a different ETag
Option value (as the resource representation has changed), then the
client should treat the partial body with the old ETag as no longer
being fresh. The client may then request all of the new data by
specifying Q-Block2 with block number '0' and the M bit set.
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If the server transmits a new body of data (e.g., a triggered Observe
notification) with a new ETag to the same client as an additional
response, the client should remove any partially received body held
for a previous ETag for that resource as it is unlikely the missing
blocks can be retrieved.
If there is insufficient space to create a response PDU with a block
size of 16 bytes (SZX = 0) to send back all the response options as
appropriate, a 4.13 (Request Entity Too Large) is returned without
the Size1 Option.
For Confirmable traffic, the server typically acknowledges the
initial request using an ACK with a piggybacked payload, and then
sends the subsequent payloads of the MAX_PAYLOADS_SET as CON
responses. These CON responses are individually ACKed by the client.
The server will detect failure to send a packet and SHOULD terminate
the body transfer, but the client can issue, after a
MAX_TRANSMIT_SPAN delay, a separate GET, POST, PUT, FETCH, PATCH, or
iPATCH for any missing blocks as needed.
4.5. Using Observe Option
For a request that uses Q-Block1, the Observe value [RFC7641] MUST be
the same for all the payloads of the same body. This includes any
missing payloads that are retransmitted.
For a response that uses Q-Block2, the Observe value MUST be the same
for all the payloads of the same body. This is different from Block2
usage where the Observe value is only present in the first block
(Section 3.4 of [RFC7959]). This includes payloads transmitted
following receipt of the 'Continue' Q-Block2 Option (Section 4.4) by
the server. If a missing payload is requested by a client, then both
the request and response MUST NOT include the Observe Option.
4.6. Using Size1 and Size2 Options
Section 4 of [RFC7959] defines two CoAP options: Size1 for indicating
the size of the representation transferred in requests and Size2 for
indicating the size of the representation transferred in responses.
For Q-Block1 and Q-Block2 Options, the Size1 or Size2 Option values
MUST exactly represent the size of the data on the body so that any
missing data can easily be determined.
The Size1 Option MUST be used with the Q-Block1 Option when used in a
request and MUST be present in all payloads of the request,
preserving the same value. The Size2 Option MUST be used with the
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Q-Block2 Option when used in a response and MUST be present in all
payloads of the response, preserving the same value.
4.7. Using Q-Block1 and Q-Block2 Options Together
The behavior is similar to the one defined in Section 3.3 of
[RFC7959] with Q-Block1 substituted for Block1 and Q-Block2 for
Block2.
4.8. Using Q-Block2 Option With Multicast
Servers MUST ignore multicast requests that contain the Q-Block2
Option. As a reminder, Block2 Option can be used as stated in
Section 2.8 of [RFC7959].
5. The Use of 4.08 (Request Entity Incomplete) Response Code
4.08 (Request Entity Incomplete) Response Code has a new Content-Type
"application/missing-blocks+cbor-seq" used to indicate that the
server has not received all of the blocks of the request body that it
needs to proceed. Such messages must not be treated by the client as
a fatal error.
Likely causes are the client has not sent all blocks, some blocks
were dropped during transmission, or the client has sent them
sufficiently long ago that the server has already discarded them.
The new data payload of the 4.08 (Request Entity Incomplete) response
with Content-Type set to "application/missing-blocks+cbor-seq" is
encoded as a CBOR Sequence [RFC8742]. It comprises one or more
missing block numbers encoded as CBOR unsigned integers [RFC8949].
The missing block numbers MUST be unique in each 4.08 (Request Entity
Incomplete) response when created by the server; the client MUST
ignore any duplicates in the same 4.08 (Request Entity Incomplete)
response.
The Content-Format Option (Section 5.10.3 of [RFC7252]) MUST be used
in the 4.08 (Request Entity Incomplete) response. It MUST be set to
"application/missing-blocks+cbor-seq" (Section 12.3).
The Concise Data Definition Language [RFC8610] (and see Section 4.1
[RFC8742]) for the data describing these missing blocks is as
follows:
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; This defines an array, the elements of which are to be used
; in a CBOR Sequence:
payload = [+ missing-block-number]
; A unique block number not received:
missing-block-number = uint
Figure 1: Structure of the Missing Blocks Payload
This CDDL syntax MUST be followed.
It is desirable that the token to use for the response is the token
that was used in the last block number received so far with the same
Request-Tag value. Note that the use of any received token with the
same Request-Tag would be acceptable, but providing the one used in
the last received payload will aid any troubleshooting. The client
will use the token to determine what was the previously sent request
to obtain the Request-Tag value that was used.
If the size of the 4.08 (Request Entity Incomplete) response packet
is larger than that defined by Section 4.6 [RFC7252], then the number
of reported missing blocks MUST be limited so that the response can
fit into a single packet. If this is the case, then the server can
send subsequent 4.08 (Request Entity Incomplete) responses containing
the missing other blocks on receipt of a new request providing a
missing payload with the same Request-Tag.
The missing blocks MUST be reported in ascending order without any
duplicates. The client SHOULD silently drop 4.08 (Request Entity
Incomplete) responses not adhering with this behavior.
Implementation Note: Consider limiting the number of missing
payloads to MAX_PAYLOADS to minimize congestion control being
needed. The CBOR sequence does not include any array wrapper.
The 4.08 (Request Entity Incomplete) with Content-Type "application/
missing-blocks+cbor-seq" SHOULD NOT be used when using Confirmable
requests or a reliable connection [RFC8323] as the client will be
able to determine that there is a transmission failure of a
particular payload and hence that the server is missing that payload.
6. The Use of Tokens
Each new request generally uses a new Token (and sometimes must, see
Section 4 of [I-D.ietf-core-echo-request-tag]). Additional responses
to a request all use the token of the request they respond to.
Implementation Note: By using 8-byte tokens, it is possible to
easily minimize the number of tokens that have to be tracked by
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clients, by keeping the bottom 32 bits the same for the same body
and the upper 32 bits containing the current body's request number
(incrementing every request, including every re-transmit). This
allows the client to be alleviated from keeping all the per-
request-state, e.g., in Section 3 of [RFC8974]. However, if using
NoSec, Section 5.2 of [RFC8974] needs to be considered for
security implications.
7. Congestion Control for Unreliable Transports
The transmission of all the blocks of a single body over an
unreliable transport MUST either all be Confirmable or all be Non-
confirmable. This is meant to simplify the congestion control
procedure.
As a reminder, there is no need for CoAP-specific congestion control
for reliable transports [RFC8323].
7.1. Confirmable (CON)
Congestion control for CON requests and responses is specified in
Section 4.7 of [RFC7252]. In order to benefit from faster
transmission rates, NSTART will need to be increased from 1.
However, the other CON congestion control parameters will need to be
tuned to cover this change. This tuning is not specified in this
document, given that the applicability scope of the current
specification assumes that all requests and responses using Q-Block1
and Q-Block2 will be Non-confirmable (Section 3.2) apart from the
initial Q-Block functionality negotiation.
Following the failure to transmit a packet due to packet loss after
MAX_TRANSMIT_SPAN time (Section 4.8.2 of [RFC7252]), it is
implementation specific as to whether there should be any further
requests for missing data.
7.2. Non-confirmable (NON)
This document introduces new parameters MAX_PAYLOADS, NON_TIMEOUT,
NON_TIMEOUT_RANDOM, NON_RECEIVE_TIMEOUT, NON_MAX_RETRANSMIT,
NON_PROBING_WAIT, and NON_PARTIAL_TIMEOUT primarily for use with NON
(Table 3).
Note: Randomness may naturally be provided based on the traffic
profile, how PROBING_RATE is computed (as this is an average), and
when the peer responds. Randomness is explicitly added for some
of the congestion control parameters to handle situations where
every thing is in sync when retrying.
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MAX_PAYLOADS should be configurable with a default value of 10. Both
CoAP endpoints MUST have the same value (otherwise there will be
transmission delays in one direction) and the value MAY be negotiated
between the endpoints to a common value by using a higher level
protocol (out of scope of this document). This is the maximum number
of payloads that can be transmitted at any one time.
Note: The default value of 10 is chosen for reasons similar to
those discussed in Section 5 of [RFC6928], especially given the
target application discussed in Section 3.2.
NON_TIMEOUT is used to compute the delay between sending
MAX_PAYLOADS_SET for the same body. By default, NON_TIMEOUT has the
same value as ACK_TIMEOUT (Section 4.8 of [RFC7252]).
NON_TIMEOUT_RANDOM is the initial actual delay between sending the
first two MAX_PAYLOADS_SETs of the same body. The same delay is then
used between the subsequent MAX_PAYLOADS_SETs. It is a random
duration (not an integral number of seconds) between NON_TIMEOUT and
(NON_TIMEOUT * ACK_RANDOM_FACTOR). ACK_RANDOM_FACTOR is set to 1.5
as discussed in Section 4.8 of [RFC7252].
NON_RECEIVE_TIMEOUT is the initial time to wait for a missing payload
before requesting retransmission for the first time. Every time the
missing payload is re-requested, the time to wait value doubles. The
time to wait is calculated as:
Time-to-Wait = NON_RECEIVE_TIMEOUT * (2 ** (Re-Request-Count - 1))
NON_RECEIVE_TIMEOUT has a default value of twice NON_TIMEOUT.
NON_RECEIVE_TIMEOUT MUST always be greater than NON_TIMEOUT_RANDOM by
at least one second so that the sender of the payloads has the
opportunity to start sending the next MAX_PAYLOADS_SET before the
receiver times out.
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. After this occurs, the local endpoint SHOULD
consider the body stale, remove any body, and release Tokens and
Request-Tag on the client (or the ETag on the server). By default,
NON_MAX_RETRANSMIT has the same value as MAX_RETRANSMIT (Section 4.8
of [RFC7252]).
NON_PROBING_WAIT is used to limit the potential wait needed when
using PROBING_RATE. By default, NON_PROBING_WAIT is computed in a
similar way as EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]) but
with ACK_TIMEOUT, MAX_RETRANSMIT, and PROCESSING_DELAY substituted
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with NON_TIMEOUT, NON_MAX_RETRANSMIT, and NON_TIMEOUT_RANDOM,
respectively:
NON_PROBING_WAIT = NON_TIMEOUT * ((2 ** NON_MAX_RETRANSMIT) - 1) *
ACK_RANDOM_FACTOR + (2 * MAX_LATENCY) + NON_TIMEOUT_RANDOM
NON_PARTIAL_TIMEOUT is used for expiring partially received bodies.
By default, NON_PARTIAL_TIMEOUT is computed in the same way as
EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]) but with ACK_TIMEOUT
and MAX_RETRANSMIT substituted with NON_TIMEOUT and
NON_MAX_RETRANSMIT, respectively:
NON_PARTIAL_TIMEOUT = NON_TIMEOUT * ((2 ** NON_MAX_RETRANSMIT) -
1) * ACK_RANDOM_FACTOR + (2 * MAX_LATENCY) + NON_TIMEOUT
+---------------------+-------------------+
| Parameter Name | Default Value |
+=====================+===================+
| MAX_PAYLOADS | 10 |
| NON_MAX_RETRANSMIT | 4 |
| NON_TIMEOUT | 2 s |
| NON_TIMEOUT_RANDOM | between 2-3 s |
| NON_RECEIVE_TIMEOUT | 4 s |
| NON_PROBING_WAIT | between 247-248 s |
| NON_PARTIAL_TIMEOUT | 247 s |
+---------------------+-------------------+
Table 3: Congestion Control Parameters
The PROBING_RATE parameter in CoAP indicates the average data rate
that must not be exceeded by a CoAP endpoint in sending to a peer
endpoint that does not respond. A single body will be subjected to
PROBING_RATE (Section 4.7 of [RFC7252]), not the individual packets.
If the wait time between sending bodies that are not being responded
to based on PROBING_RATE exceeds NON_PROBING_WAIT, then the wait time
is limited to NON_PROBING_WAIT.
Note: For the particular DOTS application, PROBING_RATE and other
transmission parameters are negotiated between peers. Even when
not negotiated, the DOTS application uses customized defaults as
discussed in Section 4.5.2 of [RFC8782]. Note that MAX_PAYLOADS,
NON_MAX_RETRANSMIT, NON_TIMEOUT, NON_PROBING_WAIT, and
NON_PARTIAL_TIMEOUT can be negotiated between DOTS peers, e.g., as
per [I-D.bosh-dots-quick-blocks]. When explicit values are
configured for NON_PROBING_WAIT and NON_PARTIAL_TIMEOUT, these
values are used without applying any jitter.
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Each NON 4.08 (Request Entity Incomplete) response is subject to
PROBING_RATE.
Each NON GET or FETCH request using a Q-Block2 Option is subject to
PROBING_RATE.
As the sending of many payloads of a single body may itself cause
congestion, after transmission of every MAX_PAYLOADS_SET of a single
body, a delay MUST be introduced of NON_TIMEOUT_RANDOM before sending
the next MAX_PAYLOADS_SET unless a 'Continue' is received from the
peer for the current MAX_PAYLOADS_SET, in which case the next
MAX_PAYLOADS_SET MAY start transmission immediately.
Note: Assuming 1500-byte packets and the MAX_PAYLOADS_SET having
10 payloads, this corresponds to 1500 * 10 * 8 = 120 Kbits. With
a delay of 2 seconds between MAX_PAYLOADS_SET, this indicates an
average speed requirement of 60 Kbps for a single body should
there be no responses. This transmission rate is further reduced
by being subject to PROBING_RATE.
The sending of a set of missing blocks of a body is restricted to
those in a MAX_PAYLOADS_SET at a time. In other words, a
NON_TIMEOUT_RANDOM delay is still observed between each
MAX_PAYLOAD_SET.
For Q-Block1 Option, if the server responds with a 2.31 (Continue)
Response Code for the latest payload sent, then the client can
continue to send the next MAX_PAYLOADS_SET without any further delay.
If the server responds with a 4.08 (Request Entity Incomplete)
Response Code, then the missing payloads SHOULD be retransmitted
before going into another NON_TIMEOUT_RANDOM delay prior to sending
the next set of payloads.
For the server receiving NON Q-Block1 requests, it SHOULD send back a
2.31 (Continue) Response Code on receipt of all of the
MAX_PAYLOADS_SET to prevent the client unnecessarily delaying. If
not all of the MAX_PAYLOADS_SET were received, the server SHOULD
delay for NON_RECEIVE_TIMEOUT (exponentially scaled based on the
repeat request count for a payload) before sending the 4.08 (Request
Entity Incomplete) Response Code for the missing payload(s). If all
of the MAX_PAYLOADS_SET were received and a 2.31 (Continue) had been
sent, but no more payloads were received for NON_RECEIVE_TIMEOUT
(exponentially scaled), the server SHOULD send a 4.08 (Request Entity
Incomplete) response detailing the missing payloads after the block
number that was indicated in the sent 2.31 (Continue). If the
repeated response count of the 4.08 (Request Entity Incomplete)
exceeds NON_MAX_RETRANSMIT, the server SHOULD discard the partial
body and stop requesting the missing payloads.
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It is likely that the client will start transmitting the next
MAX_PAYLOADS_SET before the server times out on waiting for the last
of the previous MAX_PAYLOADS_SET. On receipt of a payload from the
next MAX_PAYLOADS_SET, the server SHOULD send a 4.08 (Request Entity
Incomplete) Response Code indicating any missing payloads from any
previous MAX_PAYLOADS_SET. Upon receipt of the 4.08 (Request Entity
Incomplete) Response Code, the client SHOULD send the missing
payloads before continuing to send the remainder of the
MAX_PAYLOADS_SET and then go into another NON_TIMEOUT_RANDOM delay
prior to sending the next MAX_PAYLOADS_SET.
For the client receiving NON Q-Block2 responses, it SHOULD send a
'Continue' Q-Block2 request (Section 4.4) for the next
MAX_PAYLOADS_SET on receipt of all of the MAX_PAYLOADS_SET, to
prevent the server unnecessarily delaying. Otherwise the client
SHOULD delay for NON_RECEIVE_TIMEOUT (exponentially scaled based on
the repeat request count for a payload), before sending the request
for the missing payload(s). If the repeat request count for a
missing payload exceeds NON_MAX_RETRANSMIT, the client SHOULD discard
the partial body and stop requesting the missing payloads.
The server SHOULD recognize the 'Continue' Q-Block2 request as a
continue request and just continue the transmission of the body
(including Observe Option, if appropriate for an unsolicited
response) rather than as a request for the remaining missing blocks.
It is likely that the server will start transmitting the next
MAX_PAYLOADS_SET before the client times out on waiting for the last
of the previous MAX_PAYLOADS_SET. Upon receipt of a payload from the
new MAX_PAYLOADS_SET, the client SHOULD send a request indicating any
missing payloads from any previous MAX_PAYLOADS_SET. Upon receipt of
such request, the server SHOULD send the missing payloads before
continuing to send the remainder of the MAX_PAYLOADS_SET and then go
into another NON_TIMEOUT_RANDOM delay prior to sending the next
MAX_PAYLOADS_SET.
The client does not need to acknowledge the receipt of the entire
body.
Note: If there is asymmetric traffic loss causing responses to
never get received, a delay of NON_TIMEOUT_RANDOM after every
transmission of MAX_PAYLOADS_SET will be observed. The endpoint
receiving the body is still likely to receive the entire body.
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8. Caching Considerations
Caching block based information is not straight forward in a proxy.
For Q-Block1 and Q-Block2 Options, for simplicity it is expected that
the proxy will reassemble the body (using any appropriate recovery
options for packet loss) before passing on the body to the
appropriate CoAP endpoint. This does not preclude an implementation
doing a more complex per payload caching, but how to do this is out
of the scope of this document. The onward transmission of the body
does not require the use of the Q-Block1 or Q-Block2 Options as these
options may not be supported in that link. This means that the proxy
must fully support the Q-Block1 and Q-Block2 Options.
How the body is cached in the CoAP client (for Q-Block1
transmissions) or the CoAP server (for Q-Block2 transmissions) is
implementation specific.
As the entire body is being cached in the proxy, the Q-Block1 and
Q-Block2 Options are removed as part of the block assembly and thus
do not reach the cache.
For Q-Block2 responses, the ETag Option value is associated with the
data (and onward transmitted to the CoAP client), but is not part of
the cache key.
For requests with Q-Block1 Option, the Request-Tag Option is
associated with the build up of the body from successive payloads,
but is not part of the cache key. For the onward transmission of the
body using CoAP, a new Request-Tag SHOULD be generated and used.
Ideally this new Request-Tag should replace the client's request
Request-Tag.
It is possible that two or more CoAP clients are concurrently
updating the same resource through a common proxy to the same CoAP
server using Q-Block1 (or Block1) Option. If this is the case, the
first client to complete building the body causes that body to start
transmitting to the CoAP server with an appropriate Request-Tag
value. When the next client completes building the body, any
existing partial body transmission to the CoAP server is terminated
and the new body representation transmission starts with a new
Request-Tag value. Note that it cannot be assumed that the proxy
will always receive a complete body from a client.
A proxy that supports Q-Block2 Option MUST be prepared to receive a
GET or similar request indicating one or more missing blocks. The
proxy will serve from its cache the missing blocks that are available
in its cache in the same way a server would send all the appropriate
Q-Block2 responses. If a body matching the cache key is not
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available in the cache, the proxy MUST request the entire body from
the CoAP server using the information in the cache key.
How long a CoAP endpoint (or proxy) keeps the body in its cache is
implementation specific (e.g., it may be based on Max-Age).
9. HTTP-Mapping Considerations
As a reminder, the basic normative requirements on HTTP/CoAP mappings
are defined in Section 10 of [RFC7252]. The implementation
guidelines for HTTP/CoAP mappings are elaborated in [RFC8075].
The rules defined in Section 5 of [RFC7959] are to be followed.
10. Examples with Non-confirmable Messages
This section provides some sample flows to illustrate the use of
Q-Block1 and Q-Block2 Options with NON. Examples with CON are
provided in Appendix A.
The examples in the following subsections assume MAX_PAYLOADS is set
to 10 and NON_MAX_RETRANSMIT is set to 4.
Figure 2 lists the conventions that are used in the following
subsections.
T: Token value
O: Observe Option value
M: Message ID
RT: Request-Tag
ET: ETag
QB1: Q-Block1 Option values NUM/More/Size
QB2: Q-Block2 Option values NUM/More/Size
Size: Actual block size encoded in SZX
\: Trimming long lines
[[]]: Comments
-->X: Message loss (request)
X<--: Message loss (response)
...: Passage of time
Payload N: Corresponds to the CoAP message that conveys
a block number (N-1) of a given block-wise exchange.
Figure 2: Notations Used in the Figures
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10.1. Q-Block1 Option
10.1.1. A Simple Example
Figure 3 depicts an example of a NON PUT request conveying Q-Block1
Option. All the blocks are received by the server.
CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x81 T:0xc0 RT=9 QB1:0/1/1024
+--------->| NON PUT /path M:0x82 T:0xc1 RT=9 QB1:1/1/1024
+--------->| NON PUT /path M:0x83 T:0xc2 RT=9 QB1:2/1/1024
+--------->| NON PUT /path M:0x84 T:0xc3 RT=9 QB1:3/0/1024
|<---------+ NON 2.04 M:0xf1 T:0xc3
| ... |
Figure 3: Example of NON Request with Q-Block1 Option (Without Loss)
10.1.2. Handling MAX_PAYLOADS Limits
Figure 4 depicts an example of a NON PUT request conveying Q-Block1
Option. The number of payloads exceeds MAX_PAYLOADS. All the blocks
are received by the server.
CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x01 T:0xf1 RT=10 QB1:0/1/1024
+--------->| NON PUT /path M:0x02 T:0xf2 RT=10 QB1:1/1/1024
+--------->| [[Payloads 3 - 9 not detailed]]
+--------->| NON PUT /path M:0x0a T:0xfa RT=10 QB1:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET receipt acknowledged by server]]
|<---------+ NON 2.31 M:0x81 T:0xfa
+--------->| NON PUT /path M:0x0b T:0xfb RT=10 QB1:10/0/1024
|<---------+ NON 2.04 M:0x82 T:0xfb
| ... |
Figure 4: Example of MAX_PAYLOADS NON Request with Q-Block1 Option
(Without Loss)
10.1.3. Handling MAX_PAYLOADS with Recovery
Consider now a scenario where a new body of data is to be sent by the
client, but some blocks are dropped in transmission as illustrated in
Figure 5.
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CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x11 T:0xe1 RT=11 QB1:0/1/1024
+--->X | NON PUT /path M:0x12 T:0xe2 RT=11 QB1:1/1/1024
+--------->| [[Payloads 3 - 8 not detailed]]
+--------->| NON PUT /path M:0x19 T:0xe9 RT=11 QB1:8/1/1024
+--->X | NON PUT /path M:0x1a T:0xea RT=11 QB1:9/1/1024
[[Some of MAX_PAYLOADS_SET have been received]]
| ... |
[[NON_TIMEOUT_RANDOM (client) delay expires]]
| [[Client starts sending next MAX_PAYLOAD_SET]]
+--->X | NON PUT /path M:0x1b T:0xeb RT=11 QB1:10/1/1024
+--------->| NON PUT /path M:0x1c T:0xec RT=11 QB1:11/1/1024
| |
Figure 5: Example of MAX_PAYLOADS NON Request with Q-Block1 Option
(With Loss)
On seeing a payload from the next MAX_PAYLOAD_SET, the server
realizes that some blocks are missing from the previous
MAX_PAYLOADS_SET and asks for the missing blocks in one go
(Figure 6). It does so by indicating which blocks from the previous
MAX_PAYLOADS_SET have not been received in the data portion of the
response (Section 5). The token used in the response should be the
token that was used in the last received payload. The client can
then derive the Request-Tag by matching the token with the sent
request.
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CoAP CoAP
Client Server
| |
|<---------+ NON 4.08 M:0x91 T:0xec [Missing 1,9]
| [[Client responds with missing payloads]]
+--------->| NON PUT /path M:0x1d T:0xed RT=11 QB1:1/1/1024
+--------->| NON PUT /path M:0x1e T:0xee RT=11 QB1:9/1/1024
| [[Client continues sending next MAX_PAYLOAD_SET]]
+--------->| NON PUT /path M:0x1f T:0xef RT=11 QB1:12/0/1024
| ... |
[[NON_RECEIVE_TIMEOUT (server) delay expires]]
| [[The server realizes a block is still missing and asks
| for the missing one]]
|<---------+ NON 4.08 M:0x92 T:0xef [Missing 10]
+--------->| NON PUT /path M:0x20 T:0xf0 RT=11 QB1:10/1/1024
|<---------+ NON 2.04 M:0x93 T:0xf0
| ... |
Figure 6: Example of NON Request with Q-Block1 Option (Blocks
Recovery)
10.1.4. Handling Recovery with Failure
Figure 7 depicts an example of a NON PUT request conveying Q-Block1
Option where recovery takes place, but eventually fails.
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CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x91 T:0xd0 RT=12 QB1:0/1/1024
+--->X | NON PUT /path M:0x92 T:0xd1 RT=12 QB1:1/1/1024
+--------->| NON PUT /path M:0x93 T:0xd2 RT=12 QB1:2/0/1024
| ... |
[[NON_RECEIVE_TIMEOUT (server) delay expires]]
| [[The server realizes a block is missing and asks
| for the missing one. Retry #1]]
|<---------+ NON 4.08 M:0x01 T:0xd2 [Missing 1]
| ... |
[[2 * NON_RECEIVE_TIMEOUT (server) delay expires]]
| [[The server realizes a block is still missing and asks
| for the missing one. Retry #2]]
|<---------+ NON 4.08 M:0x02 T:0xd2 [Missing 1]
| ... |
[[4 * NON_RECEIVE_TIMEOUT (server) delay expires]]
| [[The server realizes a block is still missing and asks
| for the missing one. Retry #3]]
|<---------+ NON 4.08 M:0x03 T:0xd2 [Missing 1]
| ... |
[[8 * NON_RECEIVE_TIMEOUT (server) delay expires]]
| [[The server realizes a block is still missing and asks
| for the missing one. Retry #4]]
|<---------+ NON 4.08 M:0x04 T:0xd2 [Missing 1]
| ... |
[[16 * NON_RECEIVE_TIMEOUT (server) delay expires]]
| [[NON_MAX_RETRANSMIT exceeded. Server stops requesting
| for missing blocks and releases partial body]]
| ... |
Figure 7: Example of NON Request with Q-Block1 Option (With Eventual
Failure)
10.2. Q-Block2 Option
These examples include the Observe Option to demonstrate how that
option is used. Note that the Observe Option is not required for
Q-Block2; the observe detail can thus be ignored.
10.2.1. A Simple Example
Figure 8 illustrates the example of Q-Block2 Option. The client
sends a NON GET carrying Observe and Q-Block2 Options. The Q-Block2
Option indicates a block size hint (1024 bytes). This request is
replied to by the server using four (4) blocks that are transmitted
to the client without any loss. Each of these blocks carries a
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Q-Block2 Option. The same process is repeated when an Observe is
triggered, but no loss is experienced by any of the notification
blocks.
CoAP CoAP
Client Server
| |
+--------->| NON GET /path M:0x01 T:0xc0 O:0 QB2:0/1/1024
|<---------+ NON 2.05 M:0xf1 T:0xc0 O:1220 ET=19 QB2:0/1/1024
|<---------+ NON 2.05 M:0xf2 T:0xc0 O:1220 ET=19 QB2:1/1/1024
|<---------+ NON 2.05 M:0xf3 T:0xc0 O:1220 ET=19 QB2:2/1/1024
|<---------+ NON 2.05 M:0xf4 T:0xc0 O:1220 ET=19 QB2:3/0/1024
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0xf5 T:0xc0 O:1221 ET=20 QB2:0/1/1024
|<---------+ NON 2.05 M:0xf6 T:0xc0 O:1221 ET=20 QB2:1/1/1024
|<---------+ NON 2.05 M:0xf7 T:0xc0 O:1221 ET=20 QB2:2/1/1024
|<---------+ NON 2.05 M:0xf8 T:0xc0 O:1221 ET=20 QB2:3/0/1024
| ... |
Figure 8: Example of NON Notifications with Q-Block2 Option (Without
Loss)
10.2.2. Handling MAX_PAYLOADS Limits
Figure 9 illustrates the same as Figure 8 but this time has eleven
(11) payloads which exceeds MAX_PAYLOADS. There is no loss
experienced.
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CoAP CoAP
Client Server
| |
+--------->| NON GET /path M:0x01 T:0xf0 O:0 QB2:0/1/1024
|<---------+ NON 2.05 M:0x81 T:0xf0 O:1234 ET=21 QB2:0/1/1024
|<---------+ NON 2.05 M:0x82 T:0xf0 O:1234 ET=21 QB2:1/1/1024
|<---------+ [[Payloads 3 - 9 not detailed]]
|<---------+ NON 2.05 M:0x8a T:0xf0 O:1234 ET=21 QB2:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET acknowledged by client using
| 'Continue' Q-Block2]]
+--------->| NON GET /path M:0x02 T:0xf1 QB2:10/1/1024
|<---------+ NON 2.05 M:0x8b T:0xf0 O:1234 ET=21 QB2:10/0/1024
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0x91 T:0xf0 O:1235 ET=22 QB2:0/1/1024
|<---------+ NON 2.05 M:0x92 T:0xf0 O:1235 ET=22 QB2:1/1/1024
|<---------+ [[Payloads 3 - 9 not detailed]]
|<---------+ NON 2.05 M:0x9a T:0xf0 O:1235 ET=22 QB2:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET acknowledged by client using
| 'Continue' Q-Block2]]
+--------->| NON GET /path M:0x03 T:0xf2 QB2:10/1/1024
|<---------+ NON 2.05 M:0x9b T:0xf0 O:1235 ET=22 QB2:10/0/1024
[[Body has been received]]
| ... |
Figure 9: Example of NON Notifications with Q-Block2 Option (Without
Loss)
10.2.3. Handling MAX_PAYLOADS with Recovery
Figure 10 shows the example of an Observe that is triggered but for
which some notification blocks are lost. The client detects the
missing blocks and requests their retransmission. It does so by
indicating the blocks that are missing as one or more Q-Block2
Options.
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CoAP CoAP
Client Server
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0xa1 T:0xf0 O:1236 ET=23 QB2:0/1/1024
| X<---+ NON 2.05 M:0xa2 T:0xf0 O:1236 ET=23 QB2:1/1/1024
|<---------+ [[Payloads 3 - 9 not detailed]]
| X<---+ NON 2.05 M:0xaa T:0xf0 O:1236 ET=23 QB2:9/1/1024
[[Some of MAX_PAYLOADS_SET have been received]]
| ... |
[[NON_TIMEOUT_RANDOM (server) delay expires]]
| [[Server sends next MAX_PAYLOAD_SET]]
|<---------+ NON 2.05 M:0xab T:0xf0 O:1236 ET=23 QB2:10/0/1024
| [[On seeing a payload from the next MAX_PAYLOAD_SET,
| Client realizes blocks are missing and asks for the
| missing ones in one go]]
+--------->| NON GET /path M:0x04 T:0xf3 QB2:1/0/1024\
| | QB2:9/0/1024
| X<---+ NON 2.05 M:0xac T:0xf3 ET=23 QB2:1/1/1024
|<---------+ NON 2.05 M:0xad T:0xf3 ET=23 QB2:9/1/1024
| ... |
[[NON_RECEIVE_TIMEOUT (client) delay expires]]
| [[Client realizes block is still missing and asks for
| missing block]]
+--------->| NON GET /path M:0x05 T:0xf4 QB2:1/0/1024
|<---------+ NON 2.05 M:0xae T:0xf4 ET=23 QB2:1/1/1024
[[Body has been received]]
| ... |
Figure 10: Example of NON Notifications with Q-Block2 Option (Blocks
Recovery)
10.2.4. Handling Recovery using M-bit Set
Figure 11 shows the example of an Observe that is triggered but only
the first two notification blocks reach the client. In order to
retrieve the missing blocks, the client sends a request with a single
Q-Block2 Option with the M bit set.
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CoAP CoAP
Client Server
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0xb1 T:0xf0 O:1237 ET=24 QB2:0/1/1024
|<---------+ NON 2.05 M:0xb2 T:0xf0 O:1237 ET=24 QB2:1/1/1024
| X<---+ NON 2.05 M:0xb3 T:0xf0 O:1237 ET=24 QB2:2/1/1024
| X<---+ [[Payloads 4 - 9 not detailed]]
| X<---+ NON 2.05 M:0xb9 T:0xf0 O:1237 ET=24 QB2:9/1/1024
[[Some of MAX_PAYLOADS_SET have been received]]
| ... |
[[NON_TIMEOUT_RANDOM (server) delay expires]]
| [[Server sends next MAX_PAYLOAD_SET]]
| X<---+ NON 2.05 M:0xba T:0xf0 O:1237 ET=24 QB2:10/0/1024
| ... |
[[NON_RECEIVE_TIMEOUT (client) delay expires]]
| [[Client realizes blocks are missing and asks for the
| missing ones in one go by setting the M bit]]
+--------->| NON GET /path M:0x06 T:0xf5 QB2:2/1/1024
|<---------+ NON 2.05 M:0xbb T:0xf5 ET=24 QB2:2/1/1024
|<---------+ [[Payloads 3 - 9 not detailed]]
|<---------+ NON 2.05 M:0xc2 T:0xf5 ET=24 QB2:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET acknowledged by client using 'Continue'
| Q-Block2]]
+--------->| NON GET /path M:0x87 T:0xf6 QB2:10/1/1024
|<---------+ NON 2.05 M:0xc3 T:0xf0 O:1237 ET=24 QB2:10/0/1024
[[Body has been received]]
| ... |
Figure 11: Example of NON Notifications with Q-Block2 Option (Blocks
Recovery with M bit Set)
10.3. Q-Block1 and Q-Block2 Options
10.3.1. A Simple Example
Figure 12 illustrates the example of a FETCH using both Q-Block1 and
Q-Block2 Options along with an Observe Option. No loss is
experienced.
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CoAP CoAP
Client Server
| |
+--------->| NON FETCH /path M:0x10 T:0x90 O:0 RT=30 QB1:0/1/1024
+--------->| NON FETCH /path M:0x11 T:0x91 O:0 RT=30 QB1:1/1/1024
+--------->| NON FETCH /path M:0x12 T:0x93 O:0 RT=30 QB1:2/0/1024
|<---------+ NON 2.05 M:0x60 T:0x93 O:1320 ET=90 QB2:0/1/1024
|<---------+ NON 2.05 M:0x61 T:0x93 O:1320 ET=90 QB2:1/1/1024
|<---------+ NON 2.05 M:0x62 T:0x93 O:1320 ET=90 QB2:2/1/1024
|<---------+ NON 2.05 M:0x63 T:0x93 O:1320 ET=90 QB2:3/0/1024
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0x64 T:0x93 O:1321 ET=91 QB2:0/1/1024
|<---------+ NON 2.05 M:0x65 T:0x93 O:1321 ET=91 QB2:1/1/1024
|<---------+ NON 2.05 M:0x66 T:0x93 O:1321 ET=91 QB2:2/1/1024
|<---------+ NON 2.05 M:0x67 T:0x93 O:1321 ET=91 QB2:3/0/1024
| ... |
Figure 12: Example of NON FETCH with Q-Block1 and Q-Block2 Options
(Without Loss)
10.3.2. Handling MAX_PAYLOADS Limits
Figure 13 illustrates the same as Figure 12 but this time has eleven
(11) payloads in both directions which exceeds MAX_PAYLOADS. There
is no loss experienced.
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CoAP CoAP
Client Server
| |
+--------->| NON FETCH /path M:0x30 T:0xa0 O:0 RT=10 QB1:0/1/1024
+--------->| NON FETCH /path M:0x31 T:0xa1 O:0 RT=10 QB1:1/1/1024
+--------->| [[Payloads 3 - 9 not detailed]]
+--------->| NON FETCH /path M:0x39 T:0xa9 O:0 RT=10 QB1:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET acknowledged by server]]
|<---------+ NON 2.31 M:0x80 T:0xa9
+--------->| NON FETCH /path M:0x3a T:0xaa O:0 RT=10 QB1:10/0/1024
|<---------+ NON 2.05 M:0x81 T:0xaa O:1334 ET=21 QB2:0/1/1024
|<---------+ NON 2.05 M:0x82 T:0xaa O:1334 ET=21 QB2:1/1/1024
|<---------+ [[Payloads 3 - 9 not detailed]]
|<---------+ NON 2.05 M:0x8a T:0xaa O:1334 ET=21 QB2:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET acknowledged by client using
| 'Continue' Q-Block2]]
+--------->| NON FETCH /path M:0x3b T:0xab QB2:10/1/1024
|<---------+ NON 2.05 M:0x8b T:0xaa O:1334 ET=21 QB2:10/0/1024
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0x8c T:0xaa O:1335 ET=22 QB2:0/1/1024
|<---------+ NON 2.05 M:0x8d T:0xaa O:1335 ET=22 QB2:1/1/1024
|<---------+ [[Payloads 3 - 9 not detailed]]
|<---------+ NON 2.05 M:0x95 T:0xaa O:1335 ET=22 QB2:9/1/1024
[[MAX_PAYLOADS_SET has been received]]
| [[MAX_PAYLOADS_SET acknowledged by client using
| 'Continue' Q-Block2]]
+--------->| NON FETCH /path M:0x3c T:0xac QB2:10/1/1024
|<---------+ NON 2.05 M:0x96 T:0xaa O:1335 ET=22 QB2:10/0/1024
[[Body has been received]]
| ... |
Figure 13: Example of NON FETCH with Q-Block1 and Q-Block2 Options
(Without Loss)
Note that as 'Continue' was used, the server continues to use the
same token (0xaa) since the 'Continue' is not being used as a request
for a new set of packets, but rather is being used to instruct the
server to continue its transmission (Section 7.2).
10.3.3. Handling Recovery
Consider now a scenario where some blocks are lost in transmission as
illustrated in Figure 14.
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CoAP CoAP
Client Server
| |
+--------->| NON FETCH /path M:0x50 T:0xc0 O:0 RT=31 QB1:0/1/1024
+--->X | NON FETCH /path M:0x51 T:0xc1 O:0 RT=31 QB1:1/1/1024
+--->X | NON FETCH /path M:0x52 T:0xc2 O:0 RT=31 QB1:2/1/1024
+--------->| NON FETCH /path M:0x53 T:0xc3 O:0 RT=31 QB1:3/0/1024
| ... |
[[NON_RECEIVE_TIMEOUT (server) delay expires]]
Figure 14: Example of NON FETCH with Q-Block1 and Q-Block2 Options
(With Loss)
The server realizes that some blocks are missing and asks for the
missing blocks in one go (Figure 15). It does so by indicating which
blocks have not been received in the data portion of the response.
The token used in the response is the token that was used in the last
received payload. The client can then derive the Request-Tag by
matching the token with the sent request.
CoAP CoAP
Client Server
| |
|<---------+ NON 4.08 M:0xa0 T:0xc3 [Missing 1,2]
| [[Client responds with missing payloads]]
+--------->| NON FETCH /path M:0x54 T:0xc4 O:0 RT=31 QB1:1/1/1024
+--------->| NON FETCH /path M:0x55 T:0xc5 O:0 RT=31 QB1:2/1/1024
| [[Server received FETCH body,
| starts transmitting response body]]
|<---------+ NON 2.05 M:0xa1 T:0xc3 O:1236 ET=23 QB2:0/1/1024
| X<---+ NON 2.05 M:0xa2 T:0xc3 O:1236 ET=23 QB2:1/1/1024
|<---------+ NON 2.05 M:0xa3 T:0xc3 O:1236 ET=23 QB2:2/1/1024
| X<---+ NON 2.05 M:0xa4 T:0xc3 O:1236 ET=23 QB2:3/0/1024
| ... |
[[NON_RECEIVE_TIMEOUT (client) delay expires]]
| |
Figure 15: Example of NON Request with Q-Block1 Option (Server
Recovery)
The client realizes that not all the payloads of the response have
been returned. The client then asks for the missing blocks in one go
(Figure 16). Note that, following Section 2.7 of [RFC7959], the
FETCH request does not include the Q-Block1 or any payload.
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CoAP CoAP
Client Server
| |
+--------->| NON FETCH /path M:0x56 T:0xc6 RT=31 QB2:1/0/1024\
| | QB2:3/0/1024
| [[Server receives FETCH request for missing payloads,
| starts transmitting missing blocks]]
| X<---+ NON 2.05 M:0xa5 T:0xc6 ET=23 QB2:1/1/1024
|<---------+ NON 2.05 M:0xa6 T:0xc6 ET=23 QB2:3/0/1024
| ... |
[[NON_RECEIVE_TIMEOUT (client) delay expires]]
| [[Client realizes block is still missing and asks for
| missing block]]
+--------->| NON FETCH /path M:0x57 T:0xc7 RT=31 QB2:1/0/1024
| [[Server receives FETCH request for missing payload,
| starts transmitting missing block]]
|<---------+ NON 2.05 M:0xa7 T:0xc7 ET=23 QB2:1/1/1024
[[Body has been received]]
| ... |
| [[Observe triggered]]
|<---------+ NON 2.05 M:0xa8 T:0xc3 O:1337 ET=24 QB2:0/1/1024
| X<---+ NON 2.05 M:0xa9 T:0xc3 O:1337 ET=24 QB2:1/1/1024
|<---------+ NON 2.05 M:0xaa T:0xc3 O:1337 ET=24 QB2:2/0/1024
[[NON_RECEIVE_TIMEOUT (client) delay expires]]
| [[Client realizes block is still missing and asks for
| missing block]]
+--------->| NON FETCH /path M:0x58 T:0xc8 RT=31 QB2:1/0/1024
| [[Server receives FETCH request for missing payload,
| starts transmitting missing block]]
|<---------+ NON 2.05 M:0xa7 T:0xc8 ET=24 QB2:1/1/1024
[[Body has been received]]
| ... |
Figure 16: Example of NON Request with Q-Block1 Option (Client
Recovery)
11. Security Considerations
Security considerations discussed in Section 7 of [RFC7959] should be
taken into account.
Security considerations discussed in Sections 11.3 and 11.4 of
[RFC7252] should be taken into account.
OSCORE provides end-to-end protection of all information that is not
required for proxy operations and requires that a security context is
set up (Section 3.1 of [RFC8613]). It can be trusted that the source
endpoint is legitimate even if NoSec security mode is used. However,
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an intermediary node can modify the unprotected outer Q-Block1 and/or
Q-Block2 Options to cause a Q-Block transfer to fail or keep
requesting all the blocks by setting the M bit and, thus, causing
attack amplification. As discussed in Section 12.1 of [RFC8613],
applications need to consider that certain message fields and
messages types are not protected end-to-end and may be spoofed or
manipulated. Therefore, it is NOT RECOMMENDED to use the NoSec
security mode if either the Q-Block1 or Q-Block2 Options is used.
If OSCORE is not used, it is also NOT RECOMMENDED to use the NoSec
security mode if either the Q-Block1 or Q-Block2 Options is used.
If NoSec is being used, Section D.5 of [RFC8613] discusses the
security analysis and considerations for unprotected message fields
even if OSCORE is not being used.
Security considerations related to the use of Request-Tag are
discussed in Section 5 of [I-D.ietf-core-echo-request-tag].
12. IANA Considerations
RFC Editor Note: Please replace [RFCXXXX] with the RFC number to be
assigned to this document.
12.1. CoAP Option Numbers Registry
IANA is requested to add the following entries to the "CoAP Option
Numbers" sub-registry [Options] defined in [RFC7252] within the
"Constrained RESTful Environments (CoRE) Parameters" registry:
+--------+------------------+-----------+
| Number | Name | Reference |
+========+==================+===========+
| TBA1 | Q-Block1 | [RFCXXXX] |
| TBA2 | Q-Block2 | [RFCXXXX] |
+--------+------------------+-----------+
Table 4: CoAP Q-Block1 and Q-Block2 Option Numbers
This document suggests 19 (TBA1) and 31 (TBA2) as values to be
assigned for the new option numbers.
12.2. Media Type Registration
This document requests IANA to register the "application/missing-
blocks+cbor-seq" media type in the "Media Types" registry
[IANA-MediaTypes]. This registration follows the procedures
specified in [RFC6838]:
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Type name: application
Subtype name: missing-blocks+cbor-seq
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Must be encoded as a CBOR
sequence [RFC8742], as defined in Section 4 of [RFCXXXX].
Security considerations: See Section 10 of [RFCXXXX].
Interoperability considerations: N/A
Published specification: [RFCXXXX]
Applications that use this media type: Data serialization and
deserialization. In particular, the type is used by applications
relying upon block-wise transfers, allowing a server to specify
non-received blocks and request for their retransmission, as
defined in Section 4 of [RFCXXXX].
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information: IETF,
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: none
Author: See Authors' Addresses section.
Change controller: IESG
Provisional registration? No
12.3. CoAP Content-Formats Registry
This document requests IANA to register the following CoAP Content-
Format for the "application/missing-blocks+cbor-seq" media type in
the "CoAP Content-Formats" registry [Format], defined in [RFC7252],
within the "Constrained RESTful Environments (CoRE) Parameters"
registry:
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o Media Type: application/missing-blocks+cbor-seq
o Encoding: -
o Id: TBA3
o Reference: [RFCXXXX]
This document suggests 272 (TBA3) as a value to be assigned for the
new content format number.
13. References
13.1. Normative References
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J. P., and G. Selander, "CoAP:
Echo, Request-Tag, and Token Processing", draft-ietf-core-
echo-request-tag-12 (work in progress), February 2021.
[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>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[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>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.
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[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[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>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/info/rfc8742>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
13.2. Informative References
[Format] "CoAP Content-Formats", <https://www.iana.org/assignments/
core-parameters/core-parameters.xhtml#content-formats>.
[I-D.bosh-dots-quick-blocks]
Boucadair, M. and J. Shallow, "Distributed Denial-of-
Service Open Threat Signaling (DOTS) Signal Channel
Configuration Attributes for Faster Block Transmission",
draft-bosh-dots-quick-blocks-01 (work in progress),
January 2021.
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[I-D.ietf-dots-telemetry]
Boucadair, M., Reddy, T., Doron, E., Chen, M., and J.
Shallow, "Distributed Denial-of-Service Open Threat
Signaling (DOTS) Telemetry", draft-ietf-dots-telemetry-15
(work in progress), December 2020.
[IANA-MediaTypes]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[Options] "CoAP Option Numbers", <https://www.iana.org/assignments/
core-parameters/core-parameters.xhtml#option-numbers>.
[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>.
[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>.
[RFC8782] Reddy.K, T., Ed., Boucadair, M., Ed., Patil, P.,
Mortensen, A., and N. Teague, "Distributed Denial-of-
Service Open Threat Signaling (DOTS) Signal Channel
Specification", RFC 8782, DOI 10.17487/RFC8782, May 2020,
<https://www.rfc-editor.org/info/rfc8782>.
[RFC8974] Hartke, K. and M. Richardson, "Extended Tokens and
Stateless Clients in the Constrained Application Protocol
(CoAP)", RFC 8974, DOI 10.17487/RFC8974, January 2021,
<https://www.rfc-editor.org/info/rfc8974>.
Appendix A. Examples with Confirmable Messages
The following examples assume NSTART has been increased to 3.
The notations provided in Figure 2 are used in the following
subsections.
A.1. Q-Block1 Option
Let's now consider the use of Q-Block1 Option with a CON request as
shown in Figure 17. All the blocks are acknowledged (ACK).
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CoAP CoAP
Client Server
| |
+--------->| CON PUT /path M:0x01 T:0xf0 RT=10 QB1:0/1/1024
+--------->| CON PUT /path M:0x02 T:0xf1 RT=10 QB1:1/1/1024
+--------->| CON PUT /path M:0x03 T:0xf2 RT=10 QB1:2/1/1024
[[NSTART(3) limit reached]]
|<---------+ ACK 0.00 M:0x01
+--------->| CON PUT /path M:0x04 T:0xf3 RT=10 QB1:3/0/1024
|<---------+ ACK 0.00 M:0x02
|<---------+ ACK 0.00 M:0x03
|<---------+ ACK 2.04 M:0x04
| |
Figure 17: Example of CON Request with Q-Block1 Option (Without Loss)
Now, suppose that a new body of data is to be sent but with some
blocks dropped in transmission as illustrated in Figure 18. The
client will retry sending blocks for which no ACK was received.
CoAP CoAP
Client Server
| |
+--------->| CON PUT /path M:0x05 T:0xf4 RT=11 QB1:0/1/1024
+--->X | CON PUT /path M:0x06 T:0xf5 RT=11 QB1:1/1/1024
+--->X | CON PUT /path M:0x07 T:0xf6 RT=11 QB1:2/1/1024
[[NSTART(3) limit reached]]
|<---------+ ACK 0.00 M:0x05
+--------->| CON PUT /path M:0x08 T:0xf7 RT=11 QB1:3/1/1024
|<---------+ ACK 0.00 M:0x08
| ... |
[[ACK TIMEOUT (client) for M:0x06 delay expires]]
| [[Client retransmits packet]]
+--------->| CON PUT /path M:0x06 T:0xf5 RT=11 QB1:1/1/1024
[[ACK TIMEOUT (client) for M:0x07 delay expires]]
| [[Client retransmits packet]]
+--->X | CON PUT /path M:0x07 T:0xf6 RT=11 QB1:2/1/1024
|<---------+ ACK 0.00 M:0x06
| ... |
[[ACK TIMEOUT exponential backoff (client) delay expires]]
| [[Client retransmits packet]]
+--->X | CON PUT /path M:0x07 T:0xf6 RT=11 QB1:2/1/1024
| ... |
[[Either body transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 18: Example of CON Request with Q-Block1 Option (Blocks
Recovery)
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It is up to the implementation as to whether the application process
stops trying to send this particular body of data on reaching
MAX_RETRANSMIT for any payload, or separately tries to initiate the
new transmission of the payloads that have not been acknowledged
under these adverse traffic conditions.
If there is likely to be the possibility of transient network losses,
then the use of NON should be considered.
A.2. Q-Block2 Option
An example of the use of Q-Block2 Option with Confirmable messages is
shown in Figure 19.
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Client Server
| |
+--------->| CON GET /path M:0x01 T:0xf0 O:0 QB2:0/1/1024
|<---------+ ACK 2.05 M:0x01 T:0xf0 O:1234 ET=21 QB2:0/1/1024
|<---------+ CON 2.05 M:0xe1 T:0xf0 O:1234 ET=21 QB2:1/1/1024
|<---------+ CON 2.05 M:0xe2 T:0xf0 O:1234 ET=21 QB2:2/1/1024
|<---------+ CON 2.05 M:0xe3 T:0xf0 O:1234 ET=21 QB2:3/0/1024
|--------->+ ACK 0.00 M:0xe1
|--------->+ ACK 0.00 M:0xe2
|--------->+ ACK 0.00 M:0xe3
| ... |
| [[Observe triggered]]
|<---------+ CON 2.05 M:0xe4 T:0xf0 O:1235 ET=22 QB2:0/1/1024
|<---------+ CON 2.05 M:0xe5 T:0xf0 O:1235 ET=22 QB2:1/1/1024
|<---------+ CON 2.05 M:0xe6 T:0xf0 O:1235 ET=22 QB2:2/1/1024
[[NSTART(3) limit reached]]
|--------->+ ACK 0.00 M:0xe4
|<---------+ CON 2.05 M:0xe7 T:0xf0 O:1235 ET=22 QB2:3/0/1024
|--------->+ ACK 0.00 M:0xe5
|--------->+ ACK 0.00 M:0xe6
|--------->+ ACK 0.00 M:0xe7
| ... |
| [[Observe triggered]]
|<---------+ CON 2.05 M:0xe8 T:0xf0 O:1236 ET=23 QB2:0/1/1024
| X<---+ CON 2.05 M:0xe9 T:0xf0 O:1236 ET=23 QB2:1/1/1024
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 ET=23 QB2:2/1/1024
[[NSTART(3) limit reached]]
|--------->+ ACK 0.00 M:0xe8
|<---------+ CON 2.05 M:0xeb T:0xf0 O:1236 ET=23 QB2:3/0/1024
|--------->+ ACK 0.00 M:0xeb
| ... |
[[ACK TIMEOUT (server) for M:0xe9 delay expires]]
| [[Server retransmits packet]]
|<---------+ CON 2.05 M:0xe9 T:0xf0 O:1236 ET=23 QB2:1/1/1024
[[ACK TIMEOUT (server) for M:0xea delay expires]]
| [[Server retransmits packet]]
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 ET=23 QB2:2/1/1024
|--------->+ ACK 0.00 M:0xe9
| ... |
[[ACK TIMEOUT exponential backoff (server) delay expires]]
| [[Server retransmits packet]]
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 ET=23 QB2:2/1/1024
| ... |
[[Either body transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 19: Example of CON Notifications with Q-Block2 Option
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It is up to the implementation as to whether the application process
stops trying to send this particular body of data on reaching
MAX_RETRANSMIT for any payload, or separately tries to initiate the
new transmission of the payloads that have not been acknowledged
under these adverse traffic conditions.
If there is likely to be the possibility of transient network losses,
then the use of NON should be considered.
Appendix B. Examples with Reliable Transports
The notations provided in Figure 2 are used in the following
subsections.
B.1. Q-Block1 Option
Let's now consider the use of Q-Block1 Option with a reliable
transport as shown in Figure 20. There is no acknowledgment of
packets at the CoAP layer, just the final result.
CoAP CoAP
Client Server
| |
+--------->| PUT /path T:0xf0 RT=10 QB1:0/1/1024
+--------->| PUT /path T:0xf1 RT=10 QB1:1/1/1024
+--------->| PUT /path T:0xf2 RT=10 QB1:2/1/1024
+--------->| PUT /path T:0xf3 RT=10 QB1:3/0/1024
|<---------+ 2.04
| |
Figure 20: Example of Reliable Request with Q-Block1 Option
If there is likely to be the possibility of transient network losses,
then the use of unreliable transport with NON should be considered.
B.2. Q-Block2 Option
An example of the use of Q-Block2 Option with a reliable transport is
shown in Figure 21.
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Client Server
| |
+--------->| GET /path T:0xf0 O:0 QB2:0/1/1024
|<---------+ 2.05 T:0xf0 O:1234 ET=21 QB2:0/1/1024
|<---------+ 2.05 T:0xf0 O:1234 ET=21 QB2:1/1/1024
|<---------+ 2.05 T:0xf0 O:1234 ET=21 QB2:2/1/1024
|<---------+ 2.05 T:0xf0 O:1234 ET=21 QB2:3/0/1024
| ... |
| [[Observe triggered]]
|<---------+ 2.05 T:0xf0 O:1235 ET=22 QB2:0/1/1024
|<---------+ 2.05 T:0xf0 O:1235 ET=22 QB2:1/1/1024
|<---------+ 2.05 T:0xf0 O:1235 ET=22 QB2:2/1/1024
|<---------+ 2.05 T:0xf0 O:1235 ET=22 QB2:3/0/1024
| ... |
Figure 21: Example of Notifications with Q-Block2 Option
If there is likely to be the possibility of network transient losses,
then the use of unreliable transport with NON should be considered.
Acknowledgements
Thanks to Achim Kraus, Jim Schaad, and Michael Richardson for their
comments.
Special thanks to Christian Amsuess, Carsten Bormann, and Marco
Tiloca for their suggestions and several reviews, which improved this
specification significantly. Thanks to Francesca Palombini for the
AD review.
Thanks to Pete Resnick for the Gen-ART review, Colin Perkins for the
Tsvart review, and Emmanuel Baccelli for the Iotdir review. Thanks
to Martin Duke, Eric Vyncke, Benjamin Kaduk, Roman Danyliw, John
Scudder, and Lars Eggert for the IESG review.
Some text from [RFC7959] is reused for readers convenience.
Authors' Addresses
Mohamed Boucadair
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
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Jon Shallow
United Kingdom
Email: supjps-ietf@jpshallow.com
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