Constrained Application Protocol (CoAP) Block-Wise Transfer Options for Faster Transmission
draft-ietf-core-new-block-04
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 | 2021-01-06 | ||
| Replaces | draft-bosh-core-new-block | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-core-new-block-04
CORE M. Boucadair
Internet-Draft Orange
Intended status: Standards Track J. Shallow
Expires: July 10, 2021 January 6, 2021
Constrained Application Protocol (CoAP) Block-Wise Transfer Options for
Faster Transmission
draft-ietf-core-new-block-04
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 July 10, 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
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Alternative CoAP Block-Wise Transfer Options . . . . . . 3
1.2. CoAP Response Code (4.08) Usage . . . . . . . . . . . . . 5
1.3. Applicability Scope . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . 8
3.3. Using the Q-Block1 Option . . . . . . . . . . . . . . . . 8
3.4. Using the Q-Block2 Option . . . . . . . . . . . . . . . . 11
3.5. Using Observe and Q-Block2 Options . . . . . . . . . . . 13
3.6. Using Size1 and Size2 Options . . . . . . . . . . . . . . 13
3.7. Using Q-Block1 and Q-Block2 Options Together . . . . . . 13
4. The Use of 4.08 (Request Entity Incomplete) Response Code . . 13
5. The Use of Tokens . . . . . . . . . . . . . . . . . . . . . . 15
6. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15
6.1. Confirmable (CON) . . . . . . . . . . . . . . . . . . . . 15
6.2. Non-confirmable (NON) . . . . . . . . . . . . . . . . . . 15
7. Caching Considerations . . . . . . . . . . . . . . . . . . . 18
8. HTTP-Mapping Considerations . . . . . . . . . . . . . . . . . 20
9. Examples of Selective Block Recovery . . . . . . . . . . . . 20
9.1. Q-Block1 Option: Non-Confirmable Example . . . . . . . . 20
9.2. Q-Block2 Option: Non-Confirmable Example . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10.1. New CoAP Options . . . . . . . . . . . . . . . . . . . . 24
10.2. New Media Type . . . . . . . . . . . . . . . . . . . . . 24
10.3. New Content Format . . . . . . . . . . . . . . . . . . . 25
11. Security Considerations . . . . . . . . . . . . . . . . . . . 26
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
13.1. Normative References . . . . . . . . . . . . . . . . . . 26
13.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Examples with Confirmable Messages . . . . . . . . . 29
A.1. Q-Block1 Option . . . . . . . . . . . . . . . . . . . . . 29
A.2. Q-Block2 Option . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
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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. [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 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 where each individual 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 (i.e., 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.,
[I-D.ietf-dots-telemetry]). As a reminder, [RFC7959] recommends the
use of Confirmable (CON) responses to handle potential packet loss.
However, such a recommendation does not work with a flooded pipe DDoS
situation.
1.1. 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.
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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 (Section 6.2).
o To reduce the transmission times for CON transmission of large
bodies, NSTART needs to be increased from 1, but this affects
congestion control where other parameters need to be tuned
(Section 4.7 of [RFC7252]). Such tuning is out of scope of this
document.
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 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 packet. Even if there is
asymmetrical packet loss, a body can still be sent and received by
the peer whether the body comprises of a single or multiple payloads
assuming no recovery is required.
Note that similar performance 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
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 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 sender
endpoint either successful receipt 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.
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 option is not supported by a peer, then
transmissions can fall back to using Block1 and Block2 Options,
respectively.
The deviations from Block1 and Block2 Options are specified in
Section 3. Pointers to appropriate [RFC7959] sections are provided.
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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].
Q-Block1 and Q-Block2 Options are designed to work with Non-
confirmable requests and responses, in particular.
1.2. 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 4 for more details.
1.3. 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
of Non-Confirmable transmission. 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 be 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. Using OSCORE [RFC8613] does provide a security context and,
hence, a trust of the source endpoint. However, using a NoSec
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security mode may still be inadequate for reasons discussed in
Section 11.
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
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 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.
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).
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Q-Block1 Option is useful with the payload-bearing POST, PUT, FETCH,
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].
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 retransmission 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 (Table 2). The Q-Block1 (or Q-Block2) Option MAY be an
Inner or Outer option (see 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
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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
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
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, 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. It is also used to identify a particular payload
of a body that needs to be retransmitted. The Request-Tag is opaque,
the server still treats it as opaque but the client SHOULD 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
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be randomly generated and then subsequently incremented by the
client whenever a new body of data is being transmitted between
peers.
Section 3.6 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 or, if some of the
payloads are sent as Non-confirmable and have not arrived, a
retransmit missing payloads response is needed.
Each individual payload of the body is treated as a new request (see
Section 5).
The client MUST send the payloads with the block numbers increasing,
starting from zero, until the body is complete (subject to any
congestion control (Section 6)). Any missing payloads requested by
the server must in addition be separately transmitted with increasing
block numbers.
The following Response Codes are used:
2.01 (Created)
This Response Code indicates successful receipt of the entire body
and the resource was created.
2.04 (Changed)
This Response Code indicates successful receipt of the entire body
and the resource was updated.
2.31 (Continue)
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) in the response have been successfully received.
A response using this Response Code SHOULD NOT be generated for
every received Q-Block1 Option request. It SHOULD only be
generated when all the payload requests are Non-confirmable and
MAX_PAYLOADS payloads have been received by the server
(Section 6.2).
It SHOULD NOT be generated for CON.
4.00 (Bad Request)
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This Response Code MUST be returned if the request does not
include both a Request-Tag Option and a Size1 Option but does
include a Q-Block1 option.
4.02 (Bad Option)
Either this Response Code or a reset message MUST be returned if
the server does not support the Q-Block1 Option.
4.08 (Request Entity Incomplete)
This Response Code returned without Content-Type "application/
missing-blocks+cbor-seq" (Section 10.3) 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
missing payloads using the same Request-Tag, Size1 and Q-Block1 to
specify the block number, SZX, and M bit as appropriate.
The Request-Tag value to use is determined from the token in the
4.08 (Request Entity Incomplete) response and then finding the
matching client request which contains the Request-Tag that is
being used for this Q-Block1 body.
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.
If the server has not received all the payloads of a body, but one or
more NON payloads have been received, it SHOULD wait for up to
NON_RECEIVE_TIMEOUT (Section 6.2) before sending a 4.08 (Request
Entity Incomplete) response. Further considerations related to the
transmission timings of 4.08 (Request Entity Incomplete) and 2.31
(Continue) Response Codes are discussed in Section 6.2.
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.
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If the client elects to stop the transmission of a complete body, it
SHOULD "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 SHOULD ignore the payload of the packet, but MUST still respond as
if the block was received for the first time.
A server SHOULD only maintain a partial body (missing payloads) for
up to NON_PARTIAL_TIMEOUT (Section 6.2).
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 that block and for all of the remaining
blocks within the body. If the request includes multiple Q-Block2
Options and these options overlap (e.g., combination of M being set
(this and all the 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, the client still treats it as opaque but the
server SHOULD 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 3.6 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.
The client SHOULD wait for up to NON_RECEIVE_TIMEOUT (Section 6.2)
after the last received payload for NON payloads before issuing a
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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. Further considerations related to the transmission timing for
missing requests are discussed in Section 6.2.
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) to requests not
adhering to this behavior.
For Confirmable responses, the client continues to acknowledge each
packet. The server will detect failure to send a packet, 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.
If the client receives a duplicate block with the same ETag, it
SHOULD silently ignore the packet.
A client SHOULD only maintain a partial body (missing payloads) for
up to NON_PARTIAL_TIMEOUT (Section 6.2) or as defined by the Max-Age
Option (or its default), whichever is the less.
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 currently cached body response.
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 body response SHOULD be restarted with
a different ETag Option value. Any subsequent missing block requests
MUST be responded to using the latest ETag Option value.
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.
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 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.
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3.5. Using 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 and the Observe Option MUST NOT be included. If the ETag
value in the response changes, then the previously received partial
body should be considered as not fresh and the whole body re-
requested.
3.6. 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.
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 preserve the same value in each of those
payloads.
3.7. 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. 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.
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.
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The data payload of the 4.08 (Request Entity Incomplete) response is
encoded as a CBOR Sequence [RFC8742]. There are one or more missing
CBOR encoded missing block numbers. The missing block numbers MUST
be unique in each 4.08 (Request Entity Incomplete) response when
created by the server; the client SHOULD drop 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" (see Section 10.3).
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 = [+ missing-block-number]
; A unique block number not received:
missing-block-number = uint
Figure 1: Structure of the Missing Blocks Payload
The token to use for the response SHOULD be the token that was used
in the highest block number received payload. The Q-Block1 Option
from the same packet SHOULD be included.
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 3.2.2 of [RFC8949]), or alternatively limit the
array count to 23 so that the initial byte with the array major
type and data length in the additional information can be updated
with the overall count once the payload count is confirmed.
Further restricting the count to MAX_PAYLOADS means that
congestion control is less likely to be invoked on the server.
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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.
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
The transmission of the payloads of a body either SHOULD all be
Confirmable or all be Non-confirmable. This is meant to simplify the
congestion control procedure.
6.1. Confirmable (CON)
Congestion control for CON requests and responses is specified in
Section 4.7 of [RFC7252]. For 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 out of scope of this document as it is expected that all
requests and responses using Q-Block1 and Q-Block2 will be Non-
confirmable.
It is implementation specific as to whether there should be any
further requests for missing data as there will have been significant
transmission failure as individual payloads will have failed after
MAX_TRANSMIT_SPAN.
6.2. Non-confirmable (NON)
This document introduces new parameters MAX_PAYLOADS, NON_TIMEOUT,
NON_RECEIVE_TIMEOUT, NON_PROBING_WAIT, and NON_PARTIAL_TIMEOUT
primarily for use with NON.
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MAX_PAYLOADS should be configurable with a default value of 10. Both
CoAP endpoints SHOULD 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].
NON_TIMEOUT is the maximum period of delay between sending sets of
MAX_PAYLOADS payloads for the same body. NON_TIMEOUT has the same
value as ACK_TIMEOUT (Section 4.8 of [RFC7252]).
NON_RECEIVE_TIMEOUT is the maximum time to wait for a missing payload
before requesting retransmission. NON_RECEIVE_TIMEOUT has a value of
twice NON_TIMEOUT.
NON_PROBING_WAIT is used to limit the potential wait needed
calculated when using PROBING_WAIT. NON_PROBING_WAIT has the same
value as computed for EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]).
NON_PARTIAL_TIMEOUT is used for expiring partially received bodies.
NON_PARTIAL_TIMEOUT has the same value as computed for
EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]).
+---------------------+---------------+
| Parameter Name | Default Value |
+=====================+===============|
| MAX_PAYLOADS | 10 |
| NON_TIMEOUT | 2 s |
| NON_RECEIVE_TIMEOUT | 4 s |
| NON_PROBING_WAIT | 247 s |
| NON_PARTIAL_TIMEOUT | 247 s |
+---------------------+---------------+
Table 3: Derived Protocol Parameters
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 single body of blocks 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
gap 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
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not negotiated, the DOTS application uses customized defaults as
discussed in Section 4.5.2 of [RFC8782].
Each NON 4.08 (Request Entity Incomplete) response is subject to
PROBING_RATE.
Each NON GET or FETCH request using Q-Block2 Option is subject 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
NON_TIMEOUT before sending the next set of payloads to manage
potential congestion issues.
If the CoAP peer reports at least one payload has not arrived for
each body for at least a 24 hour period and it is known that there
are no other network issues over that period, then the value of
MAX_PAYLOADS can be reduced by 1 at a time (to a minimum of 1) and
the situation re-evaluated for another 24 hour period until there is
no report of missing payloads under normal operating conditions.
Note that the CoAP peer will not know about the MAX_PAYLOADS change
until it is reconfigured. As a consequence, the peer may indicate
that there are some missing payloads prior to the actual payload
being transmitted as all of its MAX_PAYLOADS payloads have not
arrived.
The sending of a set of missing payloads of a body is subject to
MAX_PAYLOADS set of payloads.
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 set of payloads without any 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 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) or 4.08 (Request Entity Incomplete) Response Code on
receipt of the last of the MAX_PAYLOADS payloads to prevent the
client unnecessarily delaying. If the last of the MAX_PAYLOADS
payloads does not arrive (or the final payload where the M bit is not
set does not arrive), then the server SHOULD delay for
NON_RECEIVE_TIMEOUT before sending the 4.08 (Request Entity
Incomplete) Response Code.
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It is possible that the client may start transmitting the next set of
MAX_PAYLOADS payloads before the server times out on waiting for the
last of the previous MAX_PAYLOADS payloads. On receipt of the first
payload from the new set of MAX_PAYLOADS payloads, the server SHOULD
send a 4.08 (Request Entity Incomplete) Response Code indicating any
missing payloads from any previous MAX_PAYLOADS payloads. 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 payloads and then go into another
NON_TIMEOUT delay prior to sending the next set of payloads.
For the client receiving NON Q-Block2 responses, it SHOULD send a
request for the next set of payloads or a request for the missing
payloads upon receipt of the last of the MAX_PAYLOADS payloads to
prevent the server unnecessarily delaying the transmission of the
body. If the last of the MAX_PAYLOADS payloads does not arrive (or
the final payload where the M bit is not set does not arrive), then
the client SHOULD delay for NON_RECEIVE_TIMEOUT before sending the
request for the missing payloads.
The request that the client sends to acknowledge the receipt of all
the current set of MAX_PAYLOADS payloads SHOULD contain a special
case Q-Block2 Option with NUM set to the first block of the next set
of MAX_PAYLOADS payloads and the M bit set to 1. The server SHOULD
recognize this special case 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
missing blocks.
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 after every
transmission of MAX_PAYLOADS blocks will be observed. The
endpoint receiving the body is still likely to receive the entire
body.
7. 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
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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-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).
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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.
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 (request)
X<--: Message loss (response)
...: Passage of time
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)
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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)
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. The
Token just needs to be one of those that have been received for this
Request-Tag, so the client can derive the Request-Tag.
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
| ... |
[[Server realizes a block is still missing and asks for the missing
one]]
|<---------+ 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 by "forgetting" all the tracked tokens
for this Request-Tag. 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 block size hint (1024 bytes). This
request is replied to by the server using four (4) blocks that are
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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.
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 requests 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.
Figure 8 shows the example of an Observe that is triggered but only
the first two notification blocks reaches 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:0x123 T:0xf0 O:1237 ET=24 QB2:0/1/1024
|<---------+ NON 2.05 M:0x124 T:0xf0 O:1237 ET=24 QB2:1/1/1024
| X<---+ NON 2.05 M:0x125 T:0xf0 O:1237 ET=24 QB2:2/1/1024
| X<---+ NON 2.05 M:0x126 T:0xf0 O:1237 ET=24 QB2:3/0/1024
| ... |
[[Client realizes blocks are missing and asks for the remaining missing
ones in one go by setting the M bit]]
+--------->| NON GET /path M:0x03 T:0xf3 QB2:2/1/1024
|<---------+ NON 2.05 M:0x127 T:0xf3 ET=24 QB2:2/1/1024
|<---------+ NON 2.05 M:0x128 T:0xf3 ET=24 QB2:3/0/1024
| ... |
Figure 8: Example of NON Notifications with Q-Block2 Option (Blocks
Recovery with M bit Set)
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 4: 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.
10.2. New Media Type
This document requests IANA to register the "application/missing-
blocks+cbor-seq" media type in the "Media Types" registry
[IANA-MediaTypes]:
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Type name: application
Subtype name: missing-blocks+cbor-seq
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations Section of
[This_Document].
Interoperability considerations: N/A
Published specification: [This_Document]
Applications that use this media type: Data serialization and deserialization.
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): 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
10.3. 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]:
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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.
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,
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. 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, Michael Richardson, and Marco
Tiloca for the comments.
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-11 (work in progress), November 2020.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
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[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] , <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-15
(work in progress), December 2020.
[IANA-MediaTypes]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[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>.
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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 9. All the blocks are acknowledged (ACK).
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 9: 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 10. The client will
retry sending blocks for which no ACK was received.
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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 body transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 10: Example of CON Request with Q-Block1 Option (Blocks
Recovery)
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 network transient 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 11.
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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 body transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 11: Example of CON Notifications with Q-Block2 Option
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 network transient losses,
then the use of NON should be considered.
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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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