CoRE Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track Z. Shelby, Ed.
Expires: August 18, 2012 Sensinode
February 15, 2012
Blockwise transfers in CoAP
draft-ietf-core-block-08
Abstract
CoAP is a RESTful transfer protocol for constrained nodes and
networks. Basic CoAP messages work well for the small payloads we
expect from temperature sensors, light switches, and similar
building-automation devices. Occasionally, however, applications
will need to transfer larger payloads -- for instance, for firmware
updates. With HTTP, TCP does the grunt work of slicing large
payloads up into multiple packets and ensuring that they all arrive
and are handled in the right order.
CoAP is based on datagram transports such as UDP or DTLS, which
limits the maximum size of resource representations that can be
transferred without too much fragmentation. Although UDP supports
larger payloads through IP fragmentation, it is limited to 64 KiB
and, more importantly, doesn't really work well for constrained
applications and networks.
Instead of relying on IP fragmentation, this specification extends
basic CoAP with a pair of "Block" options, for transferring multiple
blocks of information from a resource representation in multiple
request-response pairs. In many important cases, the Block options
enable a server to be truly stateless: the server can handle each
block transfer separately, with no need for a connection setup or
other server-side memory of previous block transfers.
In summary, the Block options provide a minimal way to transfer
larger representations in a block-wise fashion.
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 http://datatracker.ietf.org/drafts/current/.
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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 August 18, 2012.
Copyright Notice
Copyright (c) 2012 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
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Block-wise transfers . . . . . . . . . . . . . . . . . . . . . 6
2.1. The Block Options . . . . . . . . . . . . . . . . . . . . 6
2.2. Using the Block Options . . . . . . . . . . . . . . . . . 10
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4. The Size Option . . . . . . . . . . . . . . . . . . . . . . . 20
5. HTTP Mapping Considerations . . . . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.1. Mitigating Resource Exhaustion Attacks . . . . . . . . . . 25
7.2. Mitigating Amplification Attacks . . . . . . . . . . . . . 26
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. Normative References . . . . . . . . . . . . . . . . . . . 28
9.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. Historical Note . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
The CoRE WG is tasked with standardizing an Application Protocol for
Constrained Networks/Nodes, CoAP. This protocol is intended to
provide RESTful [REST] services not unlike HTTP [RFC2616], while
reducing the complexity of implementation as well as the size of
packets exchanged in order to make these services useful in a highly
constrained network of themselves highly constrained nodes.
This objective requires restraint in a number of sometimes
conflicting ways:
o reducing implementation complexity in order to minimize code size,
o reducing message sizes in order to minimize the number of
fragments needed for each message (in turn to maximize the
probability of delivery of the message), the amount of
transmission power needed and the loading of the limited-bandwidth
channel,
o reducing requirements on the environment such as stable storage,
good sources of randomness or user interaction capabilities.
CoAP is based on datagram transports such as UDP, which limit the
maximum size of resource representations that can be transferred
without creating unreasonable levels of IP fragmentation. In
addition, not all resource representations will fit into a single
link layer packet of a constrained network, which may cause
adaptation layer fragmentation even if IP layer fragmentation is not
required. Using fragmentation (either at the adaptation layer or at
the IP layer) to enable the transport of larger representations is
possible up to the maximum size of the underlying datagram protocol
(such as UDP), but the fragmentation/reassembly process burdens the
lower layers with conversation state that is better managed in the
application layer.
This specification defines a pair of CoAP options to enable _block-
wise_ access to resource representations. The Block options provide
a minimal way to transfer larger resource representations in a block-
wise fashion. The overriding objective is to avoid creating
conversation state at the server for block-wise GET requests. (It is
impossible to fully avoid creating conversation state for POST/PUT,
if the creation/replacement of resources is to be atomic; where that
property is not needed, there is no need to create server
conversation state in this case, either.)
In summary, this specification adds a pair of Block options to CoAP
that can be used for block-wise transfers. Benefits of using these
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options include:
o Transfers larger than can be accommodated in constrained-network
link-layer packets can be performed in smaller blocks.
o No hard-to-manage conversation state is created at the adaptation
layer or IP layer for fragmentation.
o The transfer of each block is acknowledged, enabling
retransmission if required.
o Both sides have a say in the block size that actually will be
used.
o The resulting exchanges are easy to understand using packet
analyzer tools and thus quite accessible to debugging.
o If needed, the Block options can also be used as is to provide
random access to power-of-two sized blocks within a resource
representation.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119, BCP 14
[RFC2119] and indicate requirement levels for compliant CoAP
implementations.
In this document, the term "byte" is used in its now customary sense
as a synonym for "octet".
Where bit arithmetic is explained, this document uses the notation
familiar from the programming language C, except that the operator
"**" stands for exponentiation.
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2. Block-wise transfers
As discussed in the introduction, there are good reasons to limit the
size of datagrams in constrained networks:
o by the maximum datagram size (~ 64 KiB for UDP)
o by the desire to avoid IP fragmentation (MTU of 1280 for IPv6)
o by the desire to avoid adaptation layer fragmentation (60-80 bytes
for 6LoWPAN [RFC4919])
When a resource representation is larger than can be comfortably
transferred in the payload of a single CoAP datagram, a Block option
can be used to indicate a block-wise transfer. As payloads can be
sent both with requests and with responses, this specification
provides two separate options for each direction of payload transfer.
In the following, the term "payload" will be used for the actual
content of a single CoAP message, i.e. a single block being
transferred, while the term "body" will be used for the entire
resource representation that is being transferred in a block-wise
fashion.
In most cases, all blocks being transferred for a body will be of the
same size. The block size is not fixed by the protocol. To keep the
implementation as simple as possible, the Block options support only
a small range of power-of-two block sizes, from 2**4 (16) to 2**10
(1024) bytes. As bodies often will not evenly divide into the power-
of-two block size chosen, the size need not be reached in the final
block (but even for the final block, the chosen power-of-two size
will still be indicated in the block size field of the Block option).
2.1. The Block Options
+------+----------+--------+--------+--------+---------------+
| Type | C/E | Name | Format | Length | Default |
+------+----------+--------+--------+--------+---------------+
| 19 | Critical | Block1 | uint | 1-3 B | 0 (see below) |
| | | | | | |
| 17 | Critical | Block2 | uint | 1-3 B | 0 (see below) |
+------+----------+--------+--------+--------+---------------+
Table 1: Block Option Numbers
Both Block1 and Block2 options can be present both in request and
response messages. In either case, the Block1 Option pertains to the
request payload, and the Block2 Option pertains to the response
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payload.
Hence, for the methods defined in [I-D.ietf-core-coap], Block1 is
useful with the payload-bearing POST and PUT requests and their
responses. Block2 is useful with GET, POST, and PUT requests and
their payload-bearing responses (2.01, 2.02, 2.04, 2.05 -- see
section "Payload" of [I-D.ietf-core-coap]).
(As a memory aid: Block_1_ pertains to the payload of the _1st_ part
of the request-response exchange, i.e. the request, and Block_2_
pertains to the payload of the _2nd_ part of the request-response
exchange, i.e. the response.)
Where Block1 is present in a request or Block2 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 block-wise payload forms
part of the entire body being transferred ("descriptive usage").
Where it is present in the opposite direction, it provides additional
control on how that payload will be formed or was processed ("control
usage").
Implementation of either Block option is intended to be optional.
However, when it is present in a CoAP message, it MUST be processed
(or the message rejected); therefore it is identified as a critical
option. It MUST NOT occur more than once.
Three items of information may need to be transferred in a Block
option:
o The size of the block (SZX);
o whether more blocks are following (M);
o the relative number of the block (NUM) within a sequence of blocks
with the given size.
The value of the option is a 1-, 2- or 3-byte integer which encodes
these three fields, see Figure 1.
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0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| NUM |M| SZX |
+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NUM |M| SZX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NUM |M| SZX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Block option value
The block size is encoded as a three-bit unsigned integer (0 for 2**4
to 6 for 2**10 bytes), which we call the "SZX" (size exponent); the
actual block size is then "2**(SZX + 4)". SZX is transferred in the
three least significant bits of the option value (i.e., "val & 7"
where "val" is the value of the option).
The fourth least significant bit, the M or "more" bit ("val & 8"),
indicates whether more blocks are following or the current block-wise
transfer is the last block being transferred.
The option value divided by sixteen (the NUM field) is the sequence
number of the block currently being transferred, starting from zero.
The current transfer is therefore about the "size" bytes starting at
byte "NUM << (SZX + 4)". (Note that, as an implementation
convenience, "(val & ~0xF) << (val & 7)", i.e. the option value with
the last 4 bits masked out, shifted to the left by the value of SZX,
gives the byte position of the block.)
The default value of both the Block1 and the Block2 Option is zero,
indicating that the current block is the first and only block of the
transfer (block number 0, M bit not set); however, there is no
explicit size implied by this default value.
More specifically, within the option value of a Block1 or Block2
Option, the meaning of the option fields is defined as follows:
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NUM: Block Number. The block number is a variable-size (4, 12, or
20 bit) unsigned integer (uint, see Appendix A of
[I-D.ietf-core-coap]) indicating the block number being requested
or provided. Block number 0 indicates the first block of a body.
M: More Flag (not last block). For descriptive usage, this flag, if
unset, indicates that the payload in this message is the last
block in the body; when set it indicates that there are one or
more additional blocks available. When a Block2 Option is used in
a request to retrieve a specific block number ("control usage"),
the M bit MUST be sent as zero and ignored on reception. (In a
Block1 Option in a response, the M flag is used to indicate
atomicity, see below.)
SZX: Block Size. The block size is a three-bit unsigned integer
indicating the size of a block to the power of two. Thus block
size = 2**(SZX + 4). The allowed values of SZX are 0 to 6, i.e.,
the minimum block size is 2**(0+4) = 16 and the maximum is
2**(6+4) = 1024. The value 7 for SZX (which would indicate a
block size of 2048) is reserved, i.e. MUST NOT be sent and MUST
lead to a 4.00 Bad Request response code upon reception in a
request.
The Block options are used in one of three roles:
o In descriptive usage, i.e. a Block2 Option in a response (e.g., a
2.05 response for GET), or a Block1 Option in a request (e.g., PUT
or POST):
* The NUM field in the option value describes what block number
is contained in the payload of this message.
* The M bit indicates whether further blocks are required to
complete the transfer of that body.
* The block size given by SZX MUST match the size of the payload
in bytes, if the M bit is set. (SZX does not govern the
payload size if M is unset). For Block2, if the request
suggested a larger value of SZX, the next request MUST move SZX
down to the size given here. (The effect is that, if the
server uses the smaller of its preferred block size and the one
requested, all blocks for a body use the same block size.)
o A Block2 Option in control usage in a request (e.g., GET):
* The NUM field in the Block2 Option gives the block number of
the payload that is being requested to be returned in the
response.
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* In this case, the M bit has no function and MUST be set to
zero.
* The block size given (SZX) suggests a block size (in the case
of block number 0) or repeats the block size of previous blocks
received (in the case of block numbers other than 0).
o A Block1 Option in control usage in a response (e.g., a 2.xx
response for a PUT or POST request):
* The NUM field of the Block1 Option indicates what block number
is being acknowledged.
* If the M bit was set in the request, the server can choose
whether to act on each block separately, with no memory, or
whether to handle the request for the entire body atomically,
or any mix of the two. If the M bit is also set in the
response, it indicates that this response does not carry the
final response code to the request, i.e. the server collects
further blocks and plans to implement the request atomically
(e.g., acts only upon reception of the last block of payload).
Conversely, if the M bit is unset even though it was set in the
request, it indicates the block-wise request was enacted now
specifically for this block, and the response carries the final
response to this request (and to any previous ones with the M
bit set in the response's Block1 Option in this sequence of
block-wise transfers); the client is still expected to continue
sending further blocks, the request method for which may or may
not also be enacted per-block.
* Finally, the SZX block size given in a control Block1 Option
indicates the largest block size preferred by the server for
transfers toward the resource that is the same or smaller than
the one used in the initial exchange; the client SHOULD use
this block size or a smaller one in all further requests in the
transfer sequence, even if that means changing the block size
(and possibly scaling the block number accordingly) from now
on.
2.2. Using the Block Options
Using one or both Block options, a single REST operation can be split
into multiple CoAP message exchanges. As specified in
[I-D.ietf-core-coap], each of these message exchanges uses their own
CoAP Message ID.
When a request is answered with a response carrying a Block2 Option
with the M bit set, the requester may retrieve additional blocks of
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the resource representation by sending further requests with the same
options and a Block2 Option giving the block number and block size
desired. In a request, the client MUST set the M bit of a Block2
Option to zero and the server MUST ignore it on reception.
To influence the block size used in a response, the requester also
uses the Block2 Option, giving the desired size, a block number of
zero and an M bit of zero. A server MUST use the block size
indicated or a smaller size. Any further block-wise requests for
blocks beyond the first one MUST indicate the same block size that
was used by the server in the response for the first request that
gave a desired size using a Block2 Option.
Once the Block2 Option is used by the requester, all requests in a
single block-wise transfer MUST ultimately use the same size, except
that there may not be enough content to fill the last block (the one
returned with the M bit not set). (Note that the client may start
using the Block2 Option in a second request after a first request
without a Block2 Option resulted in a Block option in the response.)
The server SHOULD use the block size indicated in the request option
or a smaller size, but the requester MUST take note of the actual
block size used in the response it receives to its initial request
and proceed to use it in subsequent requests. The server behavior
MUST ensure that this client behavior results in the same block size
for all responses in a sequence (except for the last one with the M
bit not set, and possibly the first one if the initial request did
not contain a Block2 Option).
Block-wise transfers can be used to GET resources the representations
of which are entirely static (not changing over time at all, such as
in a schema describing a device), or for dynamically changing
resources. In the latter case, the Block2 Option SHOULD be used in
conjunction with the ETag Option, to ensure that the blocks being
reassembled are from the same version of the representation: The
server SHOULD include an ETag option in each response. If an ETag
option is available, the client's reassembler, when reassembling the
representation from the blocks being exchanged, MUST compare ETag
Options. If the ETag Options do not match in a GET transfer, the
requester has the option of attempting to retrieve fresh values for
the blocks it retrieved first. To minimize the resulting
inefficiency, the server MAY cache the current value of a
representation for an ongoing sequence of requests. The client MAY
facilitate identifying the sequence by using the Token Option with a
non-default value. Note well that this specification makes no
requirement for the server to establish any state; however, servers
that offer quickly changing resources may thereby make it impossible
for a client to ever retrieve a consistent set of blocks.
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In a request with a request payload (e.g., PUT or POST), the Block1
Option refers to the payload in the request (descriptive usage).
In response to a request with a payload (e.g., a PUT or POST
transfer), the block size given in the Block1 Option indicates the
block size preference of the server for this resource (control
usage). Obviously, at this point the first block has already been
transferred by the client without benefit of this knowledge. Still,
the client SHOULD heed the preference and, for all further blocks,
use the block size preferred by the server or a smaller one. Note
that any reduction in the block size may mean that the second request
starts with a block number larger than one, as the first request
already transferred multiple blocks as counted in the smaller size.
To counter the effects of adaptation layer fragmentation on packet
delivery probability, a client may want to give up retransmitting a
request with a relatively large payload even before MAX_RETRANSMIT
has been reached, and try restating the request as a block-wise
transfer with a smaller payload. Note that this new attempt is then
a new message-layer transaction and requires a new Message ID.
(Because of the uncertainty whether the request or the
acknowledgement was lost, this strategy is useful mostly for
idempotent requests.)
In a blockwise transfer of a request payload (e.g., a PUT or POST)
that is intended to be implemented in an atomic fashion at the
server, the actual creation/replacement takes place at the time the
final block, i.e. a block with the M bit unset in the Block1 Option,
is received. If not all previous blocks are available at the server
at this time, the transfer fails and error code 4.08 (Request Entity
Incomplete) MUST be returned. The error code 4.13 (Request Entity
Too Large) can be returned at any time by a server that does not
currently have the resources to store blocks for a block-wise request
payload transfer that it would intend to implement in an atomic
fashion. (Note that a 4.13 response to a request that does not
employ Block1 is a hint for the client to try sending Block1, and a
4.13 response with a smaller SZX in the Block1 than requested is a
hint to try a smaller SZX.)
If multiple concurrently proceeding block-wise request payload
transfer (e.g., PUT or POST) operations are possible, the requester
SHOULD use the Token Option to clearly separate the different
sequences. In this case, when reassembling the representation from
the blocks being exchanged to enable atomic processing, the
reassembler MUST compare any Token Options present (and, as usual,
taking an absent Token Option to default to the empty Token). If
atomic processing is not desired, there is no need to process the
Token Option (but it is still returned in the response as usual).
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3. Examples
This section gives a number of short examples with message flows for
a block-wise GET, and for a PUT or POST. These examples demonstrate
the basic operation, the operation in the presence of
retransmissions, and examples for the operation of the block size
negotiation.
In all these examples, a Block option is shown in a decomposed way
separating the kind of Block option (1 or 2), block number (NUM),
more bit (M), and block size exponent (2**(SZX+4)) by slashes. E.g.,
a Block2 Option value of 33 would be shown as 2/2/0/32), or a Block1
Option value of 59 would be shown as 1/3/1/128.
The first example (Figure 2) shows a GET request that is split into
three blocks. The server proposes a block size of 128, and the
client agrees. The first two ACKs contain 128 bytes of payload each,
and third ACK contains between 1 and 128 bytes.
CLIENT SERVER
| |
| CON [MID=1234], GET, /status ------> |
| |
| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |
| |
| CON [MID=1235], GET, /status, 2/1/0/128 ------> |
| |
| <------ ACK [MID=1235], 2.05 Content, 2/1/1/128 |
| |
| CON [MID=1236], GET, /status, 2/2/0/128 ------> |
| |
| <------ ACK [MID=1236], 2.05 Content, 2/2/0/128 |
Figure 2: Simple blockwise GET
In the second example (Figure 3), the client anticipates the
blockwise transfer (e.g., because of a size indication in the link-
format description [I-D.ietf-core-link-format]) and sends a size
proposal. All ACK messages except for the last carry 64 bytes of
payload; the last one carries between 1 and 64 bytes.
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CLIENT SERVER
| |
| CON [MID=1234], GET, /status, 2/0/0/64 ------> |
| |
| <------ ACK [MID=1234], 2.05 Content, 2/0/1/64 |
| |
| CON [MID=1235], GET, /status, 2/1/0/64 ------> |
| |
| <------ ACK [MID=1235], 2.05 Content, 2/1/1/64 |
: :
: ... :
: :
| CON [MID=1238], GET, /status, 2/4/0/64 ------> |
| |
| <------ ACK [MID=1238], 2.05 Content, 2/4/1/64 |
| |
| CON [MID=1239], GET, /status, 2/5/0/64 ------> |
| |
| <------ ACK [MID=1239], 2.05 Content, 2/5/0/64 |
Figure 3: Blockwise GET with early negotiation
In the third example (Figure 4), the client is surprised by the need
for a blockwise transfer, and unhappy with the size chosen
unilaterally by the server. As it did not send a size proposal
initially, the negotiation only influences the size from the second
message exchange onward. Since the client already obtained both the
first and second 64-byte block in the first 128-byte exchange, it
goes on requesting the third 64-byte block ("2/0/64"). None of this
is (or needs to be) understood by the server, which simply responds
to the requests as it best can.
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CLIENT SERVER
| |
| CON [MID=1234], GET, /status ------> |
| |
| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |
| |
| CON [MID=1235], GET, /status, 2/2/0/64 ------> |
| |
| <------ ACK [MID=1235], 2.05 Content, 2/2/1/64 |
| |
| CON [MID=1236], GET, /status, 2/3/0/64 ------> |
| |
| <------ ACK [MID=1236], 2.05 Content, 2/3/1/64 |
| |
| CON [MID=1237], GET, /status, 2/4/0/64 ------> |
| |
| <------ ACK [MID=1237], 2.05 Content, 2/4/1/64 |
| |
| CON [MID=1238], GET, /status, 2/5/0/64 ------> |
| |
| <------ ACK [MID=1238], 2.05 Content, 2/5/0/64 |
Figure 4: Blockwise GET with late negotiation
In all these (and the following) cases, retransmissions are handled
by the CoAP message exchange layer, so they don't influence the block
operations (Figure 5, Figure 6).
CLIENT SERVER
| |
| CON [MID=1234], GET, /status ------> |
| |
| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |
| |
| CON [MID=1235], GE///////////////////////// |
| |
| (timeout) |
| |
| CON [MID=1235], GET, /status, 2/2/0/64 ------> |
| |
| <------ ACK [MID=1235], 2.05 Content, 2/2/1/64 |
: :
: ... :
: :
| CON [MID=1238], GET, /status, 2/5/0/64 ------> |
| |
| <------ ACK [MID=1238], 2.05 Content, 2/5/0/64 |
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Figure 5: Blockwise GET with late negotiation and lost CON
CLIENT SERVER
| |
| CON [MID=1234], GET, /status ------> |
| |
| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |
| |
| CON [MID=1235], GET, /status, 2/2/0/64 ------> |
| |
| //////////////////////////////////tent, 2/2/1/64 |
| |
| (timeout) |
| |
| CON [MID=1235], GET, /status, 2/2/0/64 ------> |
| |
| <------ ACK [MID=1235], 2.05 Content, 2/2/1/64 |
: :
: ... :
: :
| CON [MID=1238], GET, /status, 2/5/0/64 ------> |
| |
| <------ ACK [MID=1238], 2.05 Content, 2/5/0/64 |
Figure 6: Blockwise GET with late negotiation and lost ACK
The following examples demonstrate a PUT exchange; a POST exchange
looks the same, with different requirements on atomicity/idempotence.
To ensure that the blocks relate to the same version of the resource
representation carried in the request, the client in Figure 7 sets
the Token to "v17" in all requests. Note that, similar to GET, the
responses to the requests that have a more bit in the request Block1
Option are provisional; only the final response tells the client that
the PUT succeeded.
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CLIENT SERVER
| |
| CON [MID=1234], PUT, /options, v17, 1/0/1/128 ------> |
| |
| <------ ACK [MID=1234], 2.04 Changed, 1/0/1/128 |
| |
| CON [MID=1235], PUT, /options, v17, 1/1/1/128 ------> |
| |
| <------ ACK [MID=1235], 2.04 Changed, 1/1/1/128 |
| |
| CON [MID=1236], PUT, /options, v17, 1/2/0/128 ------> |
| |
| <------ ACK [MID=1236], 2.04 Changed, 1/2/0/128 |
Figure 7: Simple atomic blockwise PUT
A stateless server that simply builds/updates the resource in place
(statelessly) may indicate this by not setting the more bit in the
response (Figure 8); in this case, the response codes are valid
separately for each block being updated. This is of course only an
acceptable behavior of the server if the potential inconsistency
present during the run of the message exchange sequence does not lead
to problems, e.g. because the resource being created or changed is
not yet or not currently in use.
CLIENT SERVER
| |
| CON [MID=1234], PUT, /options, v17, 1/0/1/128 ------> |
| |
| <------ ACK [MID=1234], 2.04 Changed, 1/0/0/128 |
| |
| CON [MID=1235], PUT, /options, v17, 1/1/1/128 ------> |
| |
| <------ ACK [MID=1235], 2.04 Changed, 1/1/0/128 |
| |
| CON [MID=1236], PUT, /options, v17, 1/2/0/128 ------> |
| |
| <------ ACK [MID=1236], 2.04 Changed, 1/2/0/128 |
Figure 8: Simple stateless blockwise PUT
Finally, a server receiving a blockwise PUT or POST may want to
indicate a smaller block size preference (Figure 9). In this case,
the client SHOULD continue with a smaller block size; if it does, it
MUST adjust the block number to properly count in that smaller size.
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CLIENT SERVER
| |
| CON [MID=1234], PUT, /options, v17, 1/0/1/128 ------> |
| |
| <------ ACK [MID=1234], 2.04 Changed, 1/0/1/32 |
| |
| CON [MID=1235], PUT, /options, v17, 1/4/1/32 ------> |
| |
| <------ ACK [MID=1235], 2.04 Changed, 1/4/1/32 |
| |
| CON [MID=1236], PUT, /options, v17, 1/5/1/32 ------> |
| |
| <------ ACK [MID=1235], 2.04 Changed, 1/5/1/32 |
| |
| CON [MID=1237], PUT, /options, v17, 1/6/0/32 ------> |
| |
| <------ ACK [MID=1236], 2.04 Changed, 1/6/0/32 |
Figure 9: Simple atomic blockwise PUT with negotiation
Block options may be used in both directions of a single exchange.
The following example demonstrates a blockwise POST request,
resulting in a separate blockwise response. The client in Figure 10
sets the Token to "37a" in all requests, which is echoed in all
response CONs in the separate response.
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CLIENT SERVER
| |
| CON [MID=1234], POST, /soap, 37a, 1/0/1/128 ------> |
| |
| <------ ACK [MID=1234], 2.01 Created, 1/0/1/128 |
| |
| CON [MID=1235], POST, /soap, 37a, 1/1/1/128 ------> |
| |
| <------ ACK [MID=1235], 2.01 Created, 1/1/1/128 |
| |
| CON [MID=1236], POST, /soap, 37a, 1/2/0/128 ------> |
| |
| <------ ACK [MID=1236], 0, 1/2/0/128 |
| |
| <------ CON [MID=4712], 2.01 Created, 37a, 2/0/1/128 |
| |
| ACK [MID=4712], 0, 2/0/1/128 ------> |
| |
| <------ CON [MID=4713], 2.01 Created, 37a, 2/1/1/128 |
| |
| ACK [MID=4713], 0, 2/1/1/128 ------> |
| |
| <------ CON [MID=4714], 2.01 Created, 37a, 2/2/1/128 |
| |
| ACK [MID=4714], 0, 2/2/1/128 ------> |
| |
| <------ CON [MID=4715], 2.01 Created, 37a, 2/3/0/128 |
| |
| ACK [MID=4715], 0, 2/3/0/128 ------> |
Figure 10: Atomic blockwise POST with separate blockwise response
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4. The Size Option
In many cases when transferring a large resource representation block
by block, it is advantageous to know the total size early in the
process. Some indication may be available from the maximum size
estimate attribute "sz" provided in a resource description
[I-D.ietf-core-link-format]. However, the size may vary dynamically,
so a more up-to-date indication may be useful.
The Size Option may be used for three purposes:
o in a request, to ask the server to provide a size estimate in the
response ("size request"). For this usage, the value MUST be set
to 0.
o in a response carrying a Block2 Option, to indicate the current
estimate the server has of the total size of the resource
representation.
o in a request carrying a Block1 Option, to indicate the current
estimate the client has of the total size of the resource
representation.
A size request can be easily distinguished from a size indication, as
the third case is not useful for a GET or DELETE, and an actual size
indication of 0 would either be overridden by the actual size of the
payload for a PUT or POST or would not be useful.
In the latter two cases ("size indication"), the value of the option
is the current estimate, measured in bytes.
The Size Option is "elective", i.e., a client MUST be prepared for
the server to ignore the size estimate request. The Size Option MUST
NOT occur more than once.
+------+----------+------+--------+--------+---------+
| Type | C/E | Name | Format | Length | Default |
+------+----------+------+--------+--------+---------+
| 18 | Elective | Size | uint | 0-4 B | (none) |
+------+----------+------+--------+--------+---------+
Implementation Notes:
o As a quality of implementation consideration, blockwise transfers
for which the total size considerably exceeds the size of one
block are expected to include size indications, whenever those can
be provided without undue effort (preferably with the first block
exchanged). If the size estimate does not change, the indication
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does not need to be repeated for every block.
o The end of a blockwise transfer is governed by the M bits in the
Block Options, _not_ by exhausting the size estimates exchanged.
o As usual for an option of type uint, the value 0 is best expressed
as an empty option (0 bytes). There is no default value.
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5. HTTP Mapping Considerations
In this subsection, we give some brief examples for the influence the
Block options might have on intermediaries that map between CoAP and
HTTP.
For mapping CoAP requests to HTTP, the intermediary may want to map
the sequence of block-wise transfers into a single HTTP transfer.
E.g., for a GET request, the intermediary could perform the HTTP
request once the first block has been requested and could then
fulfill all further block requests out of its cache. A constrained
implementation may not be able to cache the entire object and may use
a combination of TCP flow control and (in particular if timeouts
occur) HTTP range requests to obtain the information necessary for
the next block transfer at the right time.
For PUT or POST requests, there is more variation in how HTTP servers
might implement ranges. Some WebDAV servers do, but in general the
CoAP-to-HTTP intermediary will have to try sending the payload of all
the blocks of a block-wise transfer within one HTTP request. If
enough buffering is available, this request can be started when the
last CoAP block is received. A constrained implementation may want
to relieve its buffering by already starting to send the HTTP request
at the time the first CoAP block is received; any HTTP 408 status
code that indicates that the HTTP server became impatient with the
resulting transfer can then be mapped into a CoAP 4.08 response code
(similarly, 413 maps to 4.13).
For mapping HTTP to CoAP, the intermediary may want to map a single
HTTP transfer into a sequence of block-wise transfers. If the HTTP
client is too slow delivering a request body on a PUT or POST, the
CoAP server might time out and return a 4.08 response code, which in
turn maps well to an HTTP 408 status code (again, 4.13 maps to 413).
HTTP range requests received on the HTTP side may be served out of a
cache and/or mapped to GET requests that request a sequence of blocks
overlapping the range.
(Note that, while the semantics of CoAP 4.08 and HTTP 408 differ,
this difference is largely due to the different way the two protocols
are mapped to transport. HTTP has an underlying TCP connection,
which supplies connection state, so a HTTP 408 status code can
immediately be used to indicate that a timeout occurred during
transmitting a request through that active TCP connection. The CoAP
4.08 response code indicates one or more missing blocks, which may be
due to timeouts or resource constraints; as there is no connection
state, there is no way to deliver such a response immediately;
instead, it is delivered on the next block transfer. Still, HTTP 408
is probably the best mapping back to HTTP, as the timeout is the most
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likely cause for a CoAP 4.08. Note that there is no way to
distinguish a timeout from a missing block for a server without
creating additional state, the need for which we want to avoid.)
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6. IANA Considerations
This draft adds the following option numbers to the CoAP Option
Numbers registry of [I-D.ietf-core-coap]:
+--------+--------+-----------+
| Number | Name | Reference |
+--------+--------+-----------+
| 17 | Block2 | [RFCXXXX] |
| | | |
| 18 | Size | [RFCXXXX] |
| | | |
| 19 | Block1 | [RFCXXXX] |
+--------+--------+-----------+
Table 2: CoAP Option Numbers
This draft adds the following response code to the CoAP Response
Codes registry of [I-D.ietf-core-coap]:
+------+--------------------------------+-----------+
| Code | Description | Reference |
+------+--------------------------------+-----------+
| 136 | 4.08 Request Entity Incomplete | [RFCXXXX] |
+------+--------------------------------+-----------+
Table 3: CoAP Response Codes
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7. Security Considerations
Providing access to blocks within a resource may lead to surprising
vulnerabilities. Where requests are not implemented atomically, an
attacker may be able to exploit a race condition or confuse a server
by inducing it to use a partially updated resource representation.
Partial transfers may also make certain problematic data invisible to
intrusion detection systems; it is RECOMMENDED that an intrusion
detection system (IDS) that analyzes resource representations
transferred by CoAP implement the Block options to gain access to
entire resource representations. Still, approaches such as
transferring even-numbered blocks on one path and odd-numbered blocks
on another path, or even transferring blocks multiple times with
different content and obtaining a different interpretation of
temporal order at the IDS than at the server, may prevent an IDS from
seeing the whole picture. These kinds of attacks are well understood
from IP fragmentation and TCP segmentation; CoAP does not add
fundamentally new considerations.
Where access to a resource is only granted to clients making use of a
specific security association, all blocks of that resource MUST be
subject to the same security checks; it MUST NOT be possible for
unprotected exchanges to influence blocks of an otherwise protected
resource. As a related consideration, where object security is
employed, PUT/POST should be implemented in the atomic fashion,
unless the object security operation is performed on each access and
the creation of unusable resources can be tolerated.
Misleading size indications may be used by an attacker to induce
buffer overflows in poor implementations, for which the usual
considerations apply.
7.1. Mitigating Resource Exhaustion Attacks
Certain blockwise requests may induce the server to create state,
e.g. to create a snapshot for the blockwise GET of a fast-changing
resource to enable consistent access to the same version of a
resource for all blocks, or to create temporary resource
representations that are collected until pressed into service by a
final PUT or POST with the more bit unset. All mechanisms that
induce a server to create state that cannot simply be cleaned up
create opportunities for denial-of-service attacks. Servers SHOULD
avoid being subject to resource exhaustion based on state created by
untrusted sources. But even if this is done, the mitigation may
cause a denial-of-service to a legitimate request when it is drowned
out by other state-creating requests. Wherever possible, servers
should therefore minimize the opportunities to create state for
untrusted sources, e.g. by using stateless approaches.
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Performing segmentation at the application layer is almost always
better in this respect than at the transport layer or lower (IP
fragmentation, adaptation layer fragmentation), e.g. because there is
application layer semantics that can be used for mitigation or
because lower layers provide security associations that can prevent
attacks. However, it is less common to apply timeouts and keepalive
mechanisms at the application layer than at lower layers. Servers
MAY want to clean up accumulated state by timing it out (cf. response
code 4.08), and clients SHOULD be prepared to run blockwise transfers
in an expedient way to minimize the likelihood of running into such a
timeout.
7.2. Mitigating Amplification Attacks
[I-D.ietf-core-coap] discusses the susceptibility of CoAP end-points
for use in amplification attacks.
A CoAP server can reduce the amount of amplification it provides to
an attacker by offering large resource representations only in
relatively small blocks. With this, e.g., for a 1000 byte resource,
a 10-byte request might result in an 80-byte response (with a 64-byte
block) instead of a 1016-byte response, considerably reducing the
amplification provided.
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8. Acknowledgements
Much of the content of this draft is the result of discussions with
the [I-D.ietf-core-coap] authors, and via many CoRE WG discussions.
Tokens were suggested by Gilman Tolle and refined by Klaus Hartke.
Charles Palmer provided extensive editorial comments to a previous
version of this draft, some of which the authors hope to have covered
in this version. Esko Dijk reviewed a more recent version, leading
to a number of further editorial improvements.
Kepeng Li, Linyi Tian, and Barry Leiba wrote up an early version of
the Size Option, which has informed this draft.
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9. References
9.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-08 (work in progress), October 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
9.2. Informative References
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-11 (work in progress),
January 2012.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", 2000.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, August 2007.
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Appendix A. Historical Note
(This appendix to be deleted by the RFC editor.)
An earlier version of this draft used a single option:
+------+----------+-------+--------+--------+---------------+
| Type | C/E | Name | Format | Length | Default |
+------+----------+-------+--------+--------+---------------+
| 13 | Critical | Block | uint | 1-3 B | 0 (see below) |
+------+----------+-------+--------+--------+---------------+
Note that this option number has since been reallocated in
[I-D.ietf-core-coap]; no backwards compatibility is provided after
July 1st, 2011.
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Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Fax: +49-421-218-7000
Email: cabo@tzi.org
Zach Shelby (editor)
Sensinode
Kidekuja 2
Vuokatti 88600
Finland
Phone: +358407796297
Email: zach@sensinode.com
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