Constrained Application Protocol (CoAP) Block-Wise Transfer Options for Faster Transmission
draft-ietf-core-new-block-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9177.
|
|
|---|---|---|---|
| Authors | Mohamed Boucadair , Jon Shallow | ||
| Last updated | 2020-12-07 (Latest revision 2020-10-29) | ||
| Replaces | draft-bosh-core-new-block | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-core-new-block-02
CORE M. Boucadair
Internet-Draft Orange
Intended status: Standards Track J. Shallow
Expires: May 2, 2021 October 29, 2020
Constrained Application Protocol (CoAP) Block-Wise Transfer Options for
Faster Transmission
draft-ietf-core-new-block-02
Abstract
This document specifies alternative Constrained Application Protocol
(CoAP) Block-Wise transfer options: Q-Block1 and Q-Block2 Options.
These options are similar to the CoAP Block1 and Block2 Options, not
a replacement for them, but do enable faster transmission rates for
large amounts of data with less packet interchanges as well as
supporting 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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 2, 2021.
Copyright Notice
Copyright (c) 2020 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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Existing CoAP Block-Wise Transfer Options . . . . . . . . 3
1.2. Alternative CoAP Block-Wise Transfer Options . . . . . . 3
1.3. Updated CoAP Response Code (4.08) . . . . . . . . . . . . 4
1.4. Applicability Scope . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The Q-Block1 and Q-Block2 Options . . . . . . . . . . . . . . 6
3.1. Properties of Q-Block1 and Q-Block2 Options . . . . . . . 6
3.2. Structure of Q-Block1 and Q-Block2 Options . . . . . . . 7
3.3. Using the Q-Block1 Option . . . . . . . . . . . . . . . . 8
3.4. Using the Q-Block2 Option . . . . . . . . . . . . . . . . 9
3.5. Working with Observe and Q-Block2 Options . . . . . . . . 11
3.6. Working with Size1 and Size2 Options . . . . . . . . . . 11
3.7. Use of Q-Block1 and Q-Block2 Options Together . . . . . . 11
4. The Use of 4.08 (Request Entity Incomplete) Response Code . . 11
5. The Use of Tokens . . . . . . . . . . . . . . . . . . . . . . 13
6. Congestion Control . . . . . . . . . . . . . . . . . . . . . 13
7. Caching Considerations . . . . . . . . . . . . . . . . . . . 14
8. HTTP-Mapping Considerations . . . . . . . . . . . . . . . . . 15
9. Examples of Selective Block Recovery . . . . . . . . . . . . 15
9.1. Q-Block1 Option: Non-Confirmable Example . . . . . . . . 16
9.2. Q-Block2 Option: Non-Confirmable Example . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10.1. New CoAP Options . . . . . . . . . . . . . . . . . . . . 19
10.2. New Content Format . . . . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Examples with Confirmable Messages . . . . . . . . . 22
A.1. Q-Block1 Option . . . . . . . . . . . . . . . . . . . . . 22
A.2. Q-Block2 Option . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
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1.1. Existing CoAP Block-Wise Transfer Options
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. [RFC7959]
introduced the CoAP Block1 and Block2 Options to handle data records
that cannot fit in a single IP packet, so not having to rely on IP
fragmentation and 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 where each block has to be requested and can only ask
for (or send) the next block when the request for the previous block
has completed. Packet, and hence block transmission rate, is
controlled by Round Trip Times (RTTs).
There is a requirement for these blocks of data to be transmitted at
higher rates under network conditions where there may be asymmetrical
transient packet loss. 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., [I-D.ietf-dots-telemetry]). As a reminder, [RFC7959]
recommends use of Confirmable (CON) responses to handle potential
packet loss; which does not work with a flooded pipe DDoS situation.
1.2. Alternative CoAP Block-Wise Transfer Options
This document introduces the CoAP Q-Block1 and Q-Block2 Options.
These 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.
o They enable faster transmissions of sets of blocks of data with
less 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-confirmable (NON)
without requiring a response from the peer.
There are the following disadvantages over using CoAP Block 1 and
Block2 Options:
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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.
Using NON messages, the faster transmissions occur as all the Blocks
can be transmitted serially (as are IP fragmented packets) without
having to wait for an acknowledgement 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 packet. Even if
there is asymmetrical packet loss, a body can still be sent and
received by the peer whether the body compromises of a single or
multiple payloads assuming no recovery is required.
Note that the same performance benefits can be applied to Confirmable
messages if the value of NSTART is increased from 1 (Section 4.7 of
[RFC7252]). However, the asymmetrical packet loss is not a benefit
here. Some sample 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 Confirmable and NON as they are not used.
A CoAP endpoint can acknowledge all or a subset of the blocks.
Concretely, the receiving CoAP endpoint informs the CoAP endpoint
sender either successful receipt or reports on all blocks in the body
that have been not yet been received. The CoAP endpoint sender will
then retransmit only the blocks that have been lost in transmission.
Q-Block1 and Q-Block2 Options can be used instead of Block1 and
Block2 Options respectively when the different transmission semantics
are required. If the option is not supported by a peer, then
transmissions can fall back to using Block1 and Block2 respectively.
The deviations from Block1 and Block2 Options are specified in
Section 3. 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].
1.3. Updated CoAP Response Code (4.08)
This document updates the 4.08 (Request Entity Incomplete) by
defining an additional message format for reporting on payloads using
the Q-Block1 Option that are not received by the server.
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See Section 4 for more details.
1.4. Applicability Scope
The block-wise transfer specified in [RFC7959] covers the general
case, but falls short in situations where packet loss is highly
asymmetrical. 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. Concretely, this mechanism primarily targets applications such
as DDoS Open Threat Signaling (DOTS) that can't use Confirmable (CON)
responses to handle potential packet loss and that support
application-specific mechanisms to assess whether the remote peer is
able to handle the messages sent by a CoAP endpoint (e.g., DOTS
heartbeats in Section 4.7 of [RFC8782]).
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 6 for more details.
This mechanism is not intended for general CoAP usage, and any use
outside the intended use case should be carefully weighed against the
loss of interoperability with generic CoAP applications. It is hoped
that the experience gained with this mechanism can feed future
extensions of the block-wise mechanism that will both generally
applicable and serve this particular use case.
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.
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].
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
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used for the entire resource representation that is being transferred
in a block-wise fashion.
3. The Q-Block1 and Q-Block2 Options
3.1. Properties of Q-Block1 and Q-Block2 Options
The properties of 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 C and U columns are marked for the
Q-Block1 Option. C, U, and R columns are marked for the Q-Block2
Option.
+--------+---+---+---+---+--------------+--------+--------+---------+
| 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.
The Content-Format Option applies to the body, not to the payload
(i.e., it must be the same for all payloads of the same body).
Q-Block1 Option is useful with the payload-bearing POST, PUT, PATCH,
and iPATCH requests and their responses (2.01 and 2.04).
Q-Block2 Option is useful with GET, POST, PUT, FETCH, PATCH, and
iPATCH requests and their payload-bearing responses (2.01, 2.03,
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.
To indicate support for Q-Block2 responses, the CoAP client MUST
include the Q-Block2 Option in a GET or similar request, the Q-Block2
Option in a PUT or similar request, or the Q-Block1 Option in a PUT
or similar so that the server knows that the client supports this
Q-Block2 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].
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If Q-Block1 Option is present in a request or Q-Block2 Option in a
response (i.e., in that message to the payload of which it pertains),
it 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.
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.
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.
The Q-Block2 Option is repeatable when requesting re-transmission of
missing Blocks, but not otherwise. Except that case, any request
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 both a class E and a class U in terms of OSCORE
processing (see Section 4.1 of [RFC8613]): The Q-Block1 (or Q-Block2)
Option MAY be an Inner or Outer option. 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.
3.2. 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).
3.3. 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, 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 single
Request-Tag Option [I-D.ietf-core-echo-request-tag]. The Request-Tag
value MUST be the same for all of the blocks in the body of data that
is being transferred. It is also used to identify a particular block
of a body that needs to be re-transmitted. The Request-Tag is opaque
in nature, but it is RECOMMENDED that the client treats it 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 server still treats it as an opaque entity. The Request-Tag
value MUST be different for distinct bodies or sets of blocks of data
and SHOULD be incremented whenever a new body of data is being
transmitted for a CoAP session between peers. The initial Request-
Tag value SHOULD be randomly generated by the client.
For Confirmable transmission, the server MUST continue to acknowledge
each packet. NSTART will also need to be increased from the default
(1) to get faster transmission rates.
Each individual payload of the body is treated as a new request (see
Section 5).
A 2.01 (Created) or 2.04 (Changed) Response Code indicates successful
receipt of the entire body.
The 2.31 (Continue) Response is not used in the current version of
the specification.
A 4.00 (Bad Request) Response Code MUST be returned if the request
does not include a Request-Tag Option but does include a Q-Block1
option.
A 4.02 (Bad Option) Response Code MUST be returned if the server does
not support the Q-Block1 Option.
A 4.13 (Request Entity Too Large) Response Code can be returned under
similar conditions to those discussed in Section 2.9.3 of [RFC7959].
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A 4.08 (Request Entity Incomplete) Response Code returned without
Content-Type "application/missing-blocks+cbor-seq" (Section 10.2) is
handled as in Section 2.9.2 [RFC7959].
A 4.08 (Request Entity Incomplete) 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 re-transmits the missing payloads using the Request-Tag and
Q-Block1 to specify the block number, SZX, and M bit as appropriate.
The Request-Tag value to use is determined from the payload of the
4.08 (Request Entity Incomplete) Response Code. If the client does
not recognize the Request-Tag, the client can ignore this response.
If the server has not received all the payloads of a body, but one or
more payloads have been received, it SHOULD wait for up to
MAX_TRANSMIT_SPAN (Section 4.8.2 of [RFC7252]) before sending the
4.08 (Request Entity Incomplete) Response Code. However, this time
MAY be reduced to two times ACK_TIMEOUT before sending a 4.08
(Request Entity Incomplete) Response Code to cover the situation
where MAX_PAYLOADS has been triggered by the client causing a break
in transmission.
If the client transmits a new body of data with a new Request-Tag to
the same resource on a server, the server MUST remove any partially
received body held for a previous Request-Tag for that resource.
If the server receives a duplicate block with the same Request-Tag,
it SHOULD silently ignore the packet.
A server SHOULD only maintain a partial body (missing payloads) for
up to EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]).
3.4. 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 indicates
that this is a request for this block and for all of the remaining
blocks within the body. If the server receives multiple requests
(implied or otherwise) for the same block, it MUST only send back one
instance of that block.
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 in nature, but it is RECOMMENDED that the server
treats it as an unsigned integer of 8 bytes in length. An
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implementation may want to consider limiting this to 4 bytes to
reduce packet overhead size. The client still treats it as an opaque
entity. The ETag value MUST be different for distinct bodies or sets
of blocks of data and SHOULD be incremented whenever a new body of
data is being transmitted for a CoAP session between peers. The
initial ETag value SHOULD be randomly generated by the server.
If the client detects that some of the payloads are missing, the
missing payloads are requested by issuing a new GET, POST, PUT,
FETCH, PATCH, or iPATCH request that contains one or more Q-Block2
Options that define the missing blocks with the M bit unset.
The requested missing block numbers 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) if this is the case.
The ETag Option MUST NOT be used in the request as the server could
respond with a 2.03 (Valid Response) with no payload. If the server
responds with a different ETag Option value (as the resource
representation has changed), then the client SHOULD drop all the
payloads for the current body that are no longer valid.
The client may elect to request the missing blocks or just ignore the
partial body. It SHOULD wait for up to MAX_TRANSMIT_SPAN
(Section 4.8.2 of [RFC7252]) before issuing a GET, POST, PUT, FETCH,
PATCH, or iPATCH request for the missing blocks. However, this time
MAY be reduced to two times ACK_TIMEOUT before sending the request to
cover the situation where MAX_PAYLOADS has been triggered by the
server causing a break in transmission.
With NON transmission, the client only needs to indicate that some of
the payloads are missing by issuing a GET, POST, PUT, FETCH, PATCH,
or iPATCH request for the missing blocks.
For Confirmable transmission, the client SHOULD continue to
acknowledge each packet as well as issuing a separate GET, POST, PUT,
FETCH, PATCH, or iPATCH for the missing blocks.
If the server transmits a new body of data (e.g., a triggered
Observe) with a new ETag to the same client as an additional
response, the client MUST remove any partially received body held for
a previous ETag.
If the client receives a duplicate block with the same ETag, it
SHOULD silently ignore the packet.
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A client SHOULD only maintain a partial body (missing payloads) for
up to EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]) or as defined by
the Max-Age Option whichever is the less.
If there is insufficient space to create a response PDU with a block
size of 16 bytes (SZX = 0) to reflect back all the request options as
appropriate, a 4.13 (Request Entity Too Large) is returned without
the Size2 Option.
3.5. Working with Observe and Q-Block2 Options
As the blocks of the body are sent without waiting for
acknowledgement of the individual blocks, the Observe value [RFC7641]
MUST be the same for all the blocks of the same body.
If the client requests missing blocks, this is treated as a new
request. The Observe value may change but MUST still be reported.
If the ETag value changes then the previously received partial body
should be destroyed and the whole body re-requested.
3.6. Working with 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.
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. The Size2 Option MUST be used with the Q-Block2 Option when
used in a response.
If Size1 or Size2 Options are used, they MUST be used in all payloads
of the body and MUST have the same value.
3.7. Use of 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. 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
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server has not received all of the blocks of the request body that it
needs to proceed.
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 data payload of the 4.08 (Request Entity Incomplete) Response
Code is encoded as a CBOR Sequence [RFC8742]. First is CBOR encoded
Request-Tag followed by 1 or more missing CBOR encoded missing block
numbers. The missing block numbers MUST be unique in each 4.08
(Request Entity Incomplete) when created by the server; the client
SHOULD drop any duplicates in the same 4.08 (Request Entity
Incomplete) message.
The Content-Format Option (Section 5.10.3 of [RFC7252]) MUST be used
in the 4.08 (Request Entity Incomplete) Response Code. It MUST be
set to "application/missing-blocks+cbor-seq" (see Section 10.2).
The Concise Data Definition Language [RFC8610] for the data
describing these missing blocks is as follows:
; This defines an array, the elements of which are to be used
; in a CBOR Sequence:
payload = [request-tag, + missing-block-number]
request-tag = bstr
; A unique block number not received:
missing-block-number = uint
Figure 1: Structure of the Missing Blocks Payload
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 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 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: Updating the payload without overflowing the
overall packet size as each block number can be of varying length
needs consideration. It is possible to use Indefinite-Length
Arrays (Section 2.2.1 of [RFC7049]), limit the array count to 23
(Undefined value) so that the array data byte can be updated with
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the overall length once the payload length is confirmed or limited
to MAX_PAYLOADS count. Limiting the count to MAX_PAYLOADS means
that Congestion Control is less likely to be invoked on the
server.
5. The Use of Tokens
Each new request MUST use a unique Token (Section 4 of
[I-D.ietf-core-echo-request-tag]). Additional responses may use the
same Token.
Implementation Note: To minimize on the number of tokens that have
to be tracked by clients, it is recommended that the bottom 32
bits is kept the same for the same body and the upper 32 bits
contains the individual payload number.
Servers continue to treat the token as a unique opaque entity. If
an individual payload has to be resent (e.g., requested upon
packet loss), then the retransmitted packet is treated as a new
request (i.e., the bottom 32 bits must change).
6. Congestion Control
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. The body of blocks will be subjected to
PROBING_RATE (Section 4.7 of [RFC7252]).
Each NON 4.08 (Request Entity Incomplete) Response Codes is subjected
to PROBING_RATE.
Each NON GET or similar request using Q-Block2 Option is subjected to
PROBING_RATE.
As the sending of many payloads of a single body may itself cause
congestion, it is RECOMMENDED that after transmission of every set of
MAX_PAYLOADS payloads of a single body, a delay is introduced of
ACK_TIMEOUT (Section 4.8.2 of [RFC7252]) before the next set of
payload transmissions to manage potential congestion issues.
MAX_PAYLOADS should be configurable with a default value of 10.
Note: The default value is chosen for reasons similar to those
discussed in Section 5 of [RFC6928].
For NON transmissions, it is permissible, but not required, to send
the ultimate payload of a MAX_PAYLOADS set as a Confirmable packet.
If a Confirmable packet is used, then the transmitting peer MUST wait
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for the ACK to be returned before sending the next set of payloads,
which can be in time terms less than the ACK_TIMEOUT delay.
Also, for NON transmissions, it is permissible, but not required, to
send a Confirmable packet for the final payload of a body transfer
(that is, M bit unset). If a Confirmable packet is used, then the
transmitting peer MUST wait for the appropriate response to be
returned for successful transmission, or respond to requests for the
missing blocks (if any).
The sending of the set of missing blocks is subject to MAX_PAYLOADS.
Note: A delay of ACK_TIMEOUT after every transmission of
MAX_PAYLOADS blocks may be observed even if the peer agent is able
to handle more blocks without experiencing an overload. This
delay can be reduced by using CON for the MAX_PAYLOADS packet to
trigger sending the next set of data when the ACK is received.
Nevertheless, this behavior is likely to create other timeout
issues in a lossy environment (e.g., unidirectional loss as in
DDoS pipe flooding). The use of NON is thus superior but requires
an additional signal in the MAX_PAYLOADS packet to seek for a 2.31
(Continue) from the peer if it is ready to receive the next set of
blocks.
7. Caching Considerations
Caching block based information is not straight forward in a proxy.
For Q-Block1 and Q-Block2 Options, 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. 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 initial CoAP client (Q-Block1) or
ultimate CoAP server (Q-Block2) is implementation specific.
As the entire body is being cached in the proxy, the Q-Block1 and
Q-Block2 Options are not part of the cache key.
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,
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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.
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.
A proxy that supports Q-Block2 Option MUST be prepared to receive a
GET or similar message 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-Block2s. If the cache key matching body is not 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).
8. 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.
9. Examples of Selective Block Recovery
This section provides some sample flows to illustrate the use of
Q-Block1 and Q-Block2 Options. Figure 2 lists the conventions that
are used in the following subsections.
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T: Token value
O: Observe Option value
M: Message ID
RT: Request-Tag
ET: ETag
QB1: Q-Block1 Option values NUM/More/SZX
QB2: Q-Block2 Option values NUM/More/SZX
\: Trimming long lines
[[]]: Comments
-->X: Message loss
X<--: Message loss
Figure 2: Notations Used in the Figures
9.1. Q-Block1 Option: Non-Confirmable 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:0x01 T:0xf0 RT=10 QB1:0/1/1024
+--------->| NON PUT /path M:0x02 T:0xf1 RT=10 QB1:1/1/1024
+--------->| NON PUT /path M:0x03 T:0xf2 RT=10 QB1:2/1/1024
+--------->| NON PUT /path M:0x04 T:0xf3 RT=10 QB1:3/0/1024
|<---------+ NON 2.04 M:0xf1 T:0xf3
...
Figure 3: Example of NON Request with Q-Block1 Option (Without Loss)
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 4.
CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x05 T:0xe0 RT=11 QB1:0/1/1024
+--->X | NON PUT /path M:0x06 T:0xe1 RT=11 QB1:1/1/1024
+--->X | NON PUT /path M:0x07 T:0xe2 RT=11 QB1:2/1/1024
+--------->| NON PUT /path M:0x08 T:0xe3 RT=11 QB1:3/0/1024
| |
...
Figure 4: Example of NON Request with Q-Block1 Option (With Loss)
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The server realizes that some blocks are missing and asks for the
missing ones in one go (Figure 5). It does so by indicating which
blocks have been received in the data portion of the response.
CoAP CoAP
Client Server
| |
...
|<---------+ NON 4.08 M:0xf2 T:0xe3 [Missing 1,2 for RT=11]
+--------->| NON PUT /path M:0x09 T:0xe4 RT=11 QB1:1/1/1024
+--->X | NON PUT /path M:0x0a T:0xe5 RT=11 QB1:2/1/1024
| |
|<---------+ NON 4.08 M:0xf3 T:0xe4 [Missing 2 for RT=11]
+--------->| NON PUT /path M:0x0b T:0xe6 RT=11 QB1:2/1/1024
|<---------+ NON 2.04 M:0xf4 T:0xe6
| |
...
Figure 5: Example of NON Request with Q-Block1 Option (Blocks
Recovery)
Under high levels of traffic loss, the client can elect not to retry
sending missing blocks of data. This decision is implementation
specific.
9.2. Q-Block2 Option: Non-Confirmable Example
Figure 6 illustrates the example of Q-Block2 Option. The client
sends a NON GET carrying an Observe and a Q-Block2 Options. The
Q-Block2 Option indicates a size hint (1024 bytes). This request is
replied by the server using four (4) blocks that are transmitted to
the client without any loss. Each of these blocks carries a Q-Block2
Option. The same process is repeated when an Observe is triggered,
but no loss is experienced by any of the notification blocks.
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CoAP CoAP
Client Server
| |
+--------->| NON GET /path M:0x01 T:0xf0 O:0 QB2:0/0/1024
|<---------+ NON 2.05 M:0xf1 T:0xf0 O:1234 ET=21 QB2:0/1/1024
|<---------+ NON 2.05 M:0xf2 T:0xf0 O:1234 ET=21 QB2:1/1/1024
|<---------+ NON 2.05 M:0xf3 T:0xf0 O:1234 ET=21 QB2:2/1/1024
|<---------+ NON 2.05 M:0xf4 T:0xf0 O:1234 ET=21 QB2:3/0/1024
...
[[Observe triggered]]
|<---------+ NON 2.05 M:0xf5 T:0xf0 O:1235 ET=22 QB2:0/1/1024
|<---------+ NON 2.05 M:0xf6 T:0xf0 O:1235 ET=22 QB2:1/1/1024
|<---------+ NON 2.05 M:0xf7 T:0xf0 O:1235 ET=22 QB2:2/1/1024
|<---------+ NON 2.05 M:0xf8 T:0xf0 O:1235 ET=22 QB2:3/0/1024
...
Figure 6: Example of NON Notifications with Q-Block2 Option (Without
Loss)
Figure 7 shows the example of an Observe that is triggered but for
which some notification blocks are lost. The client detects the
missing blocks and request their retransmission. It does so by
indicating the blocks that were successfully received.
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CoAP CoAP
Client Server
| |
...
[[Observe triggered]]
|<---------+ NON 2.05 M:0xf9 T:0xf0 O:1236 ET=23 QB2:0/1/1024
| X<---+ NON 2.05 M:0xfa T:0xf0 O:1236 ET=23 QB2:1/1/1024
| X<---+ NON 2.05 M:0xfb T:0xf0 O:1236 ET=23 QB2:2/1/1024
|<---------+ NON 2.05 M:0xfc T:0xf0 O:1236 ET=23 QB2:3/0/1024
| |
[[Client realizes blocks are missing and asks for the missing
ones in one go]]
+--------->| NON GET /path M:0x02 T:0xf1 QB2:1/0/1024\
| | QB2:2/0/1024
| X<---+ NON 2.05 M:0xfd T:0xf1 ET=23 QB2:1/1/1024
|<---------+ NON 2.05 M:0xfe T:0xf1 ET=23 QB2:2/1/1024
| |
[[Get the final missing block]]
+--------->| NON GET /path M:0x03 T:0xf2 QB2:1/0/1024
|<---------+ NON 2.05 M:0xff T:0xf2 ET=23 QB2:1/1/1024
...
Figure 7: Example of NON Notifications with Q-Block2 Option (Blocks
Recovery)
Under high levels of traffic loss, the client can elect not to retry
getting missing blocks of data. This decision is implementation
specific.
10. IANA Considerations
10.1. New CoAP Options
IANA is requested to add the following entries to the "CoAP Option
Numbers" sub-registry [Options]:
+--------+------------------+-----------+
| Number | Name | Reference |
+========+==================+===========+
| TBA1 | Q-Block1 | [RFCXXXX] |
| TBA2 | Q-Block2 | [RFCXXXX] |
+--------+------------------+-----------+
Table 2: CoAP Q-Block1 and Q-Block2 Option Numbers
This document suggests 19 (TBA1) and 51 (TBA2) as a values to be
assigned for the new option numbers.
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10.2. New Content Format
This document requests IANA to register the CoAP Content-Format ID
for the "application/missing-blocks+cbor-seq" media type in the "CoAP
Content-Formats" registry [Format]:
o Media Type: application/missing-blocks+cbor-seq
o Encoding: -
o Id: TBD3
o Reference: [RFCXXXX]
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. In particular, it is NOT
RECOMMENDED that the NoSec security mode is used if the Q-Block1 and
Q-Block2 Options are to be used.
Security considerations related to the use of Request-Tag are
discussed in Section 5 of [I-D.ietf-core-echo-request-tag].
12. Acknowledgements
Thanks to Achim Kraus, Jim Schaad, and Michael Richardson for the
comments on the mailing list.
Special thanks to Christian Amsuess and Carsten Bormann for their
suggestions and several reviews, which improved this specification
significantly.
Some text from [RFC7959] is reused for readers convenience.
13. References
13.1. Normative References
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", draft-ietf-core-echo-
request-tag-10 (work in progress), July 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[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>.
[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>.
[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>.
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13.2. Informative References
[Format] , <https://www.iana.org/assignments/core-parameters/core-
parameters.xhtml#content-formats>.
[I-D.ietf-dots-telemetry]
Boucadair, M., Reddy.K, T., Doron, E., chenmeiling, c.,
and J. Shallow, "Distributed Denial-of-Service Open Threat
Signaling (DOTS) Telemetry", draft-ietf-dots-telemetry-13
(work in progress), October 2020.
[Options] , <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>.
[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>.
[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>.
Appendix A. Examples with Confirmable Messages
These examples assume NSTART has been increased to at least 4.
The notations provided in Figure 2 are used in the following
subsections.
A.1. Q-Block1 Option
Let's now consider the use Q-Block1 Option with a CON request as
shown in Figure 8. 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
+--------->| CON PUT /path M:0x04 T:0xf3 RT=10 QB1:3/0/1024
|<---------+ ACK 0.00 M:0x01
|<---------+ ACK 0.00 M:0x02
|<---------+ ACK 0.00 M:0x03
|<---------+ ACK 2.04 M:0x04
Figure 8: Example of CON Request with Q-Block1 Option (Without Loss)
Now, suppose that a new body of data is to sent but with some blocks
dropped in transmission as illustrated in Figure 9. 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
+--------->| CON PUT /path M:0x08 T:0xf7 RT=11 QB1:3/1/1024
|<---------+ ACK 0.00 M:0x05
|<---------+ ACK 0.00 M:0x08
| |
[[The client retries sending packets not acknowledged]]
+--------->| 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
|<---------+ ACK 0.00 M:0x06
| |
[[The client retransmits messages not acknowledged
(exponential backoff)]]
+--->? | CON PUT /path M:0x07 T:0xf6 RT=11 QB1:2/1/1024
| |
[[Either transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 9: Example of CON Request with Q-Block1 Option (Blocks
Recovery)
It is implementation dependent as to whether a CoAP session is
terminated following acknowledge retry timeout, or whether the CoAP
session continues to be used under such adverse traffic conditions.
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If there is likely to be the possibility of network transient losses,
then the use of Non-confirmable traffic should be considered.
A.2. Q-Block2 Option
An example of the use of Q-Block2 Option with Confirmable messages is
shown in Figure 10.
Client Server
| |
+--------->| CON GET /path M:0x01 T:0xf0 O:0 QB2:0/0/1024
|<---------+ ACK 2.05 M:0x01 T:0xf0 O:1234 ET=21 QB2:0/1/1024
|<---------+ ACK 2.05 M:0xe1 T:0xf0 O:1234 ET=21 QB2:1/1/1024
|<---------+ ACK 2.05 M:0xe2 T:0xf0 O:1234 ET=21 QB2:2/1/1024
|<---------+ ACK 2.05 M:0xe3 T:0xf0 O:1234 ET=21 QB2:3/0/1024
...
[[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
|<---------+ CON 2.05 M:0xe7 T:0xf0 O:1235 ET=22 QB2:3/0/1024
|--------->+ ACK 0.00 M:0xe4
|--------->+ 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
|<---------+ CON 2.05 M:0xeb T:0xf0 O:1236 ET=23 QB2:3/0/1024
|--------->+ ACK 0.00 M:0xe8
|--------->+ ACK 0.00 M:0xeb
| |
[[Server retransmits messages not acknowledged]]
|<---------+ 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
|--------->+ ACK 0.00 M:0xe9
| |
[[Server retransmits messages not acknowledged
(exponential backoff)]]
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 ET=23 QB2:2/1/1024
| |
[[Either transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 10: Example of CON Notifications with Q-Block2 Option
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It is implementation-dependent as to whether a CoAP session is
terminated following acknowledge retry timeout, or whether the CoAP
session continues to be used under such adverse traffic conditions.
If there is likely to be the possibility of network transient losses,
then the use of Non-confirmable traffic should be considered.
Authors' Addresses
Mohamed Boucadair
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Jon Shallow
United Kingdom
Email: supjps-ietf@jpshallow.com
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