| CORE | M. Boucadair |
| Internet-Draft | Orange |
| Intended status: Standards Track | J. Shallow |
| Expires: November 25, 2020 | May 24, 2020 |
Constrained Application Protocol (CoAP) Block-Wise Transfer Options for Faster Transmission
draft-bosh-core-new-block-01
This document specifies new Constrained Application Protocol (CoAP) Block-Wise transfer options: Block3 and Block4 Options.
These options are similar to the CoAP Block1 and Block2 Options, but 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.
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/.
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This Internet-Draft will expire on November 25, 2020.
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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.
The CoAP Block1 and Block2 Options work well in environments where there are no or minimal packet losses. These options operate synchronously where each block has to be requested and can only ask for (or send) the next block when the request for the previous block has completed. Packet, and hence block transmission rate, is controlled by Round Trip Times (RTTs).
There is a requirement for these blocks of data to be transmitted under network conditions where there may be transient packet loss. An example is when a network is subject to a Distributed Denial of Service (DDoS) attack and there is a need for DDoS mitigation agents relying upon CoAP to communicate with each other (e.g., [I-D.ietf-dots-telemetry]). As a reminder, [RFC7959] recommends use of Confirmable (CON) responses to handle potential packet loss; which does not work with a flooded pipe DDoS situation.
This document introduces the CoAP Block3 and Block4 Options. These options are similar in operation to the CoAP Block1 and Block2 Options respectively, but enable faster transmissions of sets of blocks of data with less packet interchanges as well as supporting faster recovery should any of the Blocks get lost in transmission.
Using Non-confirmable (NON) messages, the faster transmissions occur as all the Blocks can be transmitted serially (as are IP fragmented packets) without having to wait for an acknowledgement or next request from the remote CoAP peer. Recovery of missing Blocks is faster in that multiple missing Blocks can be requested in a single CoAP packet.
Note that the same performance benefits can be applied to Confirmable messages if the value of NSTART is increased from 1 (Section 4.7 of [RFC7252]). Some sample examples with Confirmable messages are provided in Appendix A.
A CoAP endpoint can acknowledge all or a subset of the blocks. Concretely, the receiving CoAP endpoint informs the CoAP endpoint sender either successful receipt or reports on all blocks in the body that have been not yet been received. The CoAP endpoint sender will then retransmit only the blocks that have been lost in transmission.
Alternative CoAP options may be considered to cover the extra functionality of the Block3 and Block4 Options using Block1 and Block2 Options, but then the semantic and usage of the Block1 and Block2 Options would have to change.
The deviation from Block1 and Block2 Options are specified in Section 3. Pointers to appropriate [RFC7959] sections are provided.
The specification refers to the base CoAP methods defined in Section 5.8 of [RFC7252] and the new CoAP methods, FETCH, PATCH, and iPATCH introdcued in [RFC8132].
This document defines a new CoAP Response Code (Section 5.9 of [RFC7252]), called TBA3 (Missing payloads), to report on payloads using the Block3 Option that are not received by the server.
See Section 4 for more details.
The mechanism specified in the document 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].
This mechanism primally 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 [I-D.ietf-dots-signal-channel]).
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.
The properties of Block3 and Block4 Options are shown in Table 1. The formatting of this table follows the one used in Table 4 of [RFC7252] (Section 5.10). The C, U, N, and R columns indicate the properties Critical, Unsafe, NoCacheKey, and Repeatable defined in Section 5.4 of [RFC7252]. Only C column is marked for the Block3 Option. Only C and R columns are marked for the Block4 Option.
+--------+---+---+---+---+-----------+--------+--------+---------+
| Number | C | U | N | R | Name | Format | Length | Default |
+========+===+===+===+===+===========+========+========+=========+
| TBA1 | x | | | | Block3 | uint | 0-7 | (none) |
| TBA2 | x | | | x | Block4 | uint | 0-7 | (none) |
+--------+---+---+---+---+-----------+--------+--------+---------+
Table 1: CoAP Block3 and Block4 Option Properties
The Block3 and Block4 Options can be present in both the request and response messages. The Block3 Option pertains to the request payload and the Block4 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).
Block3 is useful with the payload-bearing POST, PUT, PATCH, and iPATCH requests and their responses (2.01 and 2.04). Block4 Option is useful with GET, POST, PUT, and FETCH requests and their payload-bearing responses (2.01, 2.03, 2.04, and 2.05) (Section 5.5 of [RFC7252]).
To indicate support for Block4 responses, the CoAP client MUST include the Block4 Option in a GET or FETCH request so that the server knows that the client supports this Block4 functionality. Otherwise, the server would use the Block2 Option (if supported) to send back a message body that is larger than what can fit into a single IP packet [RFC7959].
If Block3 Option is present in a request or Block4 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 (referred to as "descriptive usage"). If it is present in the opposite direction, it provides additional control on how that payload will be formed or was processed (referred to as "control usage").
Implementation of Block3 (or Block4) Option is intended to be optional. However, when it is present in a CoAP message, it MUST be processed (or the message rejected). Therefore, Block3 and Block4 Options are identified as Critical options.
The Block3 and Block4 Options are safe to forward. That is, a CoAP proxy that does not understand the Block3 (or Block4) Option should forward the option on.
The Block4 Option is repeatable when requesting re-transmission of missing Blocks, but not otherwise. Except that case, any request carrying multiple Block3 (or Block4) Options MUST be handled following the procedure specified in Section 5.4.5 of [RFC7252].
PROBING_RATE parameter in CoAP indicates the average data rate that must not be exceeded by a CoAP endpoint in sending to a peer endpoint that does not respond. The body of blocks will be subjected to PROBING_RATE (Section 4.7 of [RFC7252]).
The Block3 and Block4 Options are of class E and U for OSCORE [RFC8613].
The structure of Block3 and Block4 Options follows the structure defined in Section 2.2 of [RFC7959] with two additional fields:
As such, five items of information may need to be transferred in a Block3 or Block4 Option:
The value of the Block3 or Block4 Option is a variable-size (0 to 7 byte) unsigned integer (uint) (Section 3.2 of [RFC7252]). This integer value encodes the aforementioned five fields as shown in Figure 1. Note that, due to the CoAP uint-encoding rules, when all of NUM, M, SZX, S, and BID happen to be zero, a zero-byte integer will be sent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BID |S| NUM |M| SZX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BID |S| NUM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NUM |M| SZX |
+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BID |S| NUM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NUM |M| SZX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BID |S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NUM |M| SZX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Structure of Block3 and Block4 Options
The twenty-fourth least significant bit is the S bit ("val & 0x1000000").
The option value shifted right by 25 (“(val >> 25)”)(the BID field) is the Block identifier that identifies which sequence of blocks this particular block is in.
The current transfer is about the "size" bytes starting at byte "NUM << (SZX + 4)".
Within the option value of a Block3 or Block4 Option, the meaning of the option fields is defined below. Note that the block size (SZX, size exponent), the M bit, and the NUM fields are defined in Section 2.2 of [RFC7959], but are provided below for the reader's convenience:
There is no default value for the Block3 and Block4 Options. Absence of one of these options is equivalent to an option value of 0 with respect to the value of NUM, M, S, and BID 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 and S bits are unset, and BID is set to 0). However, in contrast to the explicit value 0, which would indicate an 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).
The Block3 Options is used when the client wants to send a large amount of data to the server using the POST, PUT, PATCH, or iPATCH methods where the data and headers do not fit into a single packet.
The client sends all the individual payloads of the body using the same BID, only expecting a response when all the payloads have been sent. It is RECOMMENDED that after transmission of every set of MAX_PAYLOADS payloads of a single body, a delay is introduced of ACK_TIMEOUT (Section 4.8.2 of [RFC7252]) before the next set of payload transmissions to manage potential congestion issues. MAX_PAYLOADS should be configurable with a default value of 10.
For NON transmissions, it is permissible, but not required, to send the penultimate payload of a MAX_PAYLOADS set as a Confirmable packet. If a Confirmable packet is used, then the client MUST wait for the ACK to be returned before sending the next set of payloads, which can be in time terms less than the ACK_TIMEOUT delay.
Also, for NON transmissions, it is permissible, but not required, to send a Confirmable packet for the final payload of a body (that is, M bit unset). If a Confirmable packet is used, then the client MUST wait for the 2.01 (Created) or 2.04 (Changed) Response Codes to be returned for successful transmission, or TBA3 to then resend the missing blocks (if any).
With NON transmission, the server acknowledges receipt of all of the payloads that make up the body or respond at any time during the receipt of the payloads to acknowledge that some of the payloads have arrived, but others are missing. It is RECOMMENDED that, unless there are receipt issues, the server only responds when the final payload (i.e., M bit unset) is received.
For Confirmable transmission, the server MUST continue to acknowledge each packet. NSTART will also need to be increased from the default (1) to get faster transmission rates.
Non-empty Tokens MUST be included. Each individual payload of the body MUST have a different Token.
A 2.01 (Created) or 2.04 (Changed) Response Code indicates successful receipt of the entire body. The 2.31 (Continue) Response Code MUST NOT be used.
A 4.02 (Bad Option) Response Code MUST be returned if the server does not support the Block3 Option.
Use of 4.08 (Request Entity Incomplete) Response Code is discouraged when using Block3 Option because packets may arrive out of sequence; TBA3 (Missing Payloads) Response Code (Section 4) SHOULD be used instead. However, 4.08 (Request Entity Incomplete) Response Code is still valid to reject a Content-Format mismatch.
A 4.13 (Request Entity Too Large) Response Code can be returned under similar conditions to those discussed in Section 2.9.3 of [RFC7959].
A TBA3 (Missing Payloads) Response Code indicates that some of the payloads are missing and need to be resent. The client then re-transmits the missing payloads using Block3 to specify the BID, M bit, block number, SZX, and M bit unset. As discussed above, the sending of the list of missing blocks is subject to MAX_PAYLOAD.
If the server has not received the final payload (i.e., a block with M bit unset), but one or other payloads have been received, it SHOULD wait for up to MAX_TRANSMIT_SPAN (Section 4.8.2 of [RFC7252]) before sending the TBA3 (Missing Payloads) Response Code. However, this timer MAY be reduced to two times ACK_TIMEOUT before sending a TBA3 (Missing Payloads) Response Code to cover the situation where MAX_PAYLOADS has been triggered by the client causing a break in transmission.
In all cases, multiple TBA3 (Missing Payloads) Response Codes are traffic limited by PROBING_RATE.
If the client transmits a new body of data with a new BID to the same server, the server MUST remove any partially received body held for a previous BID for that resource.
If the server receives a duplicate block with the same BID, it SHOULD silently ignore the packet.
A server SHOULD only maintain a partial body (missing payloads) for up to EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]).
Support for the receipt of Block4 Option by the client is indicated by using the Block4 Option in the GET or FETCH request. If the Block4 Option is not included in the request, the server MUST NOT send data using the Block4 Option.
The initiating request that contains Block4 Option MUST set both BID and 'M' to 0. All other Block4 Options in a request MUST have the M bit unset.
The payloads sent back from the server as a response MUST all have the same BID which MUST NOT be 0. The sending of the payloads is subject to MAX_PAYLOADS. If MAX_PAYLOADS is exceeded, the server MUST introduce an ACK_TIMEOUT delay before transmitting the next set of payloads.
For NON transmission, it is permissible, but not required, to send the penultimate payload of a MAX_PAYLOADS set as a Confirmable packet. If a Confirmable packet is used, then the server MUST wait for the ACK to be received before sending the next set of payloads, which can be in time terms less than the ACK_TIMEOUT delay.
Also, for NON transmission, it is permissible, but not required, to send a Confirmable packet for the final payload of a body (i.e., M bit unset). If a Confirmable packet is used, the server MUST wait for the ACK to be returned for successful transmission.
If the client detects that some of the payloads are missing, the missing payloads are requested by issuing a new GET or FETCH request that contains one or more Block4 Options that define the missing blocks. A new Token MUST be used for this request. The rate of requests for missing blocks is subject to PROBING_RATE.
The client may elect to request the missing blocks or just ignore the partial body.
All the payload responses to a specific GET or FETCH request MUST have the same Token as in the request.
With NON transmission, the client only needs to indicate that some of the payloads are missing by issuing a GET or FETCH request for the missing blocks.
For Confirmable transmission, the client SHOULD continue to acknowledge each packet as well as issuing a separate GET or FETCH for the missing blocks. NSTART will also need to be increased from the default (1) to get faster transmission rates.
If the server transmits a new body of data with a new BID to the same client with the same Token, the client MUST remove any partially received body held for a previous BID for that Token.
If the client receives a duplicate block with the same BID, it SHOULD silently ignore the packet.
A client SHOULD only maintain a partial body (missing payloads) for up to EXCHANGE_LIFETIME (Section 4.8.2 of [RFC7252]) or as defined by the Max-Age Option whichever is the less.
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.
Likewise, the Tokens MUST all have the same value for all the blocks of the same body. This is so that if any of the blocks gets lost during transmission (including the first one), the receiving CoAP endpoint can take the appropriate decisions as to how to continue (implementation specific).
If the client requests missing blocks, the client MUST use a different Token; all repeated missing blocks for that new request MUST use the new Token.
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.
It is RECOMMENDED that the Size1 Option is used with the Block3 Option and that the Size2 Option is used with the Block4 Option.
If Size1 or Size2 Options are used, they MUST be used in all payloads of the body and MUST have the same value.
If the Size1 Option is not specified, there is support for streaming data to the server by always setting the S and M bits in the payload. The server can still indicate missing blocks with the TBA3 (Missing Payloads) Response Code but is NOT RECOMMENDED. By unsettling the M bit, the client can indicate that this is the last payload and hence the end of the streaming of data.
Likewise, if the Size2 Option is not specified, there is support for streaming data to the client by always setting the S and M bits in the payload. The client can request missing blocks but is NOT RECOMMENDED. The client needs to indicate to the server that it can handle streamed data by setting the S bit in the initial request. If the client no longer wants to receive data, it can send a GET request to the same resource with the same BID, but both M and S bits unset.
When the Block Number (NUM) reaches the maximum(2**20 -1), it simply wraps around to 0 and then continues to be incremented for each payload.
As a reminder, the use of MAX_PAYLOADS has still to be followed so that the network is not overloaded with too much traffic.
The ETag Option defined in Section 5.10.6 of [RFC7252] applies to the whole representation of the resource, and thus to the body of the response.
If ETag Option is used, it MUST be used in all payloads of the body and MUST have the same value.
If the ETag changes, but the BID does not, then the payload MUST be ignored as it is invalid and cannot be included in the partial body.
If ETag is used in the request, then Section 5.10.6.2 of [RFC7252] still applies. As such, 2.03 (Valid) Response Code can be returned.
The behavior is similar to the one defined in Section 3.3 of [RFC7959] with Block3 substituted for Block1 and Block4 for Block2.
TBA3 (Missing Payloads) Response Code is a new client error status code (Section 5.9.2 of [RFC7252]) used to indicate that the server has not received 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 long enough ago that the server has already discarded them.
The body of the TBA3 (Missing Payloads) Response Code contains a count of the missing block numbers followed by set of 1 or more missing block numbers encoded in CBOR format (0 unsigned) for compactness. If the size of the TBA3 (Missing Payloads) 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 necessary, multiple TBA3 (Missing Payloads) Response Codes can be sent back; each covering a unique set of missing blocks.
The Content-Format Option (Section 5.10.3 of [RFC7252]) MUST be used in the TBA3 (Missing Payloads) Response Code. It MUST be set to "application/cbor".
The Block3 and Block4 Options are part of the cache key. As such, a CoAP proxy that does not understand the Block3 and Block4 Options must follow the recommendations in Section 5.7.1 of [RFC7252] for caching.
If the S bit is set, the data SHOULD NOT be cached.
This specification does not require a proxy to obtain the complete representation before it serves parts of it to the client. Otherwise, the considerations discussed in Section 2.10 of [RFC7959] apply for the Block3 and Block4 Options (with Block3 substituted for Block1 and Block4 substituted for Block2) for proxies that support Block3 and Block4 Options.
A proxy that supports Block4 Option MUST be prepared to receive a GET message indicating one or more missing blocks. The proxy can serve from its cache missing blocks that are available in its cache in a set as a server would send all the Block4s. If one or more requested blocks are not available in the cache, the proxy SHOULD update the GET request with the blocks that it can serve from the cache, and then forward on the request, after modifying it to remove the missing block references it has in cache, to the next hop. Alternatively, the original request unmodified (from the missing block perspective) MAY be forwarded on to the server. All the responses are then passed back to the client with the cache getting updated.
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).
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.
This section provides some sample flows to illustrate the use of Block3 and Block4 Options. These conventions are used in the following subsections:
T: Token value O: Observe Option value M: Message ID B3: Block3 Option values BID/S/NUM/More/SZX B4: Block3 Option values BID/S/NUM/More/SZX \: Trimming long lines [[]]: Comments -->X: Message loss X<--: Message loss
Figure 2 depicts an example of a NON PUT request conveying Block3 Option. All the blocks are received by the server.
CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x01 T:0xf0 B3:10/0/0/1/1024
+--------->| NON PUT /path M:0x02 T:0xf1 B3:10/0/1/1/1024
+--------->| NON PUT /path M:0x03 T:0xf2 B3:10/0/2/1/1024
+--------->| NON PUT /path M:0x04 T:0xf3 B3:10/0/3/0/1024
|<---------+ NON 2.04 M:0xf1 T:0xf3
...
Figure 2: Example of NON Request with Block3 Option (Without Loss)
Consider now a scenario where a new body of data is to be sent by the client, but some blocks are dropped in transmission as illustrated in Figure 3.
CoAP CoAP
Client Server
| |
+--------->| NON PUT /path M:0x05 T:0xe0 B3:11/0/0/1/1024
+--->X | NON PUT /path M:0x06 T:0xe1 B3:11/0/1/1/1024
+--->X | NON PUT /path M:0x07 T:0xe2 B3:11/0/2/1/1024
+--------->| NON PUT /path M:0x08 T:0xe3 B3:11/0/3/0/1024
| |
...
Figure 3: Example of NON Request with Block3 Option (With Loss)
The server realizes that some blocks are missing and asks for the missing ones in one go (Figure 4). It does so by indicating which blocks have been received in the data portion of the response.
CoAP CoAP
Client Server
| |
...
|<---------+ NON TBA3 M:0xf2 T:0xe3 [Missing 1,2]
+--------->| NON PUT /path M:0x09 T:0xe4 B3:11/0/1/1/1024
+--->X | NON PUT /path M:0x0a T:0xe5 B3:11/0/2/1/1024
| |
|<---------+ NON TBA3 M:0xf3 T:0xe4 [Missing 2]
+--------->| NON PUT /path M:0x0b T:0xe6 B3:11/0/2/1/1024
|<---------+ NON 2.04 M:0xf4 T:0xe6
| |
...
Figure 4: Example of NON Request with Block3 Option (Blocks Recovery)
Under high levels of traffic loss, the client can elect not to retry sending missing blocks of data. This decision is implementation specific.
Figure 5 illustrates the example of Block4 Option. The client sends a NON GET carrying an Observe and a Block4 Options. The Block4 Option indicates a size hint (1024 bytes). This request is replied by the serer using four (4) blocks that are transmitted to the client without any loss. Each of these blocks carries a Block4 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 B4:0/0/0/0/1024
|<---------+ NON 2.05 M:0xf1 T:0xf0 O:1234 B4:21/0/0/1/1024
|<---------+ NON 2.05 M:0xf2 T:0xf0 O:1234 B4:21/0/1/1/1024
|<---------+ NON 2.05 M:0xf3 T:0xf0 O:1234 B4:21/0/2/1/1024
|<---------+ NON 2.05 M:0xf4 T:0xf0 O:1234 B4:21/0/3/0/1024
...
[[Observe triggered]]
|<---------+ NON 2.05 M:0xf5 T:0xf0 O:1235 B4:22/0/0/1/1024
|<---------+ NON 2.05 M:0xf6 T:0xf0 O:1235 B4:22/0/1/1/1024
|<---------+ NON 2.05 M:0xf7 T:0xf0 O:1235 B4:22/0/2/1/1024
|<---------+ NON 2.05 M:0xf8 T:0xf0 O:1235 B4:22/0/3/0/1024
...
Figure 5: Example of NON Notifications with Block4 Option (Without Loss)
Figure 6 shows the example of an Observe that is triggered but for which some notification blocks are lost. The client detects the missing blocks and request their retransmission. It does so by indicating the blocks that were successfully received.
CoAP CoAP
Client Server
| |
...
[[Observe triggered]]
|<---------+ NON 2.05 M:0xf9 T:0xf0 O:1236 B4:23/0/0/1/1024
| X<---+ NON 2.05 M:0xfa T:0xf0 O:1236 B4:23/0/1/1/1024
| X<---+ NON 2.05 M:0xfb T:0xf0 O:1236 B4:23/0/2/1/1024
|<---------+ NON 2.05 M:0xfc T:0xf0 O:1236 B4:23/0/3/0/1024
| |
[[Client realises blocks are missing and asks for the missing
ones in one go]]
+--------->| NON GET /path M:0x02 T:0xf1 B4:23/0/1/0/1024\
| | B4:23/0/2/0/1024
| X<---+ NON 2.05 M:0xfd T:0xf1 B4:23/0/1/1/1024
|<---------+ NON 2.05 M:0xfe T:0xf1 B4:23/0/2/1/1024
| |
[[Get final missing block]]
+--------->| NON GET /path M:0x03 T:0xf2 B4:23/0/1/0/1024
|<---------+ NON 2.05 M:0xff T:0xf2 B4:23/0/1/1/1024
...
Figure 6: Example of NON Notifications with Block4 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.
IANA is requested to add the following entries to the "CoAP Option Numbers" sub-registry available at https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#option-numbers:
+--------+------------------+-----------+ | Number | Name | Reference | +========+==================+===========+ | TBA1 | Block3 | [RFCXXXX] | | TBA2 | Block4 | [RFCXXXX] | +--------+------------------+-----------+ Table 2: CoAP Block3 and Block4 Option Numbers
This document suggests 21 (TBA1) and 25 (TBA2) as a values to be assigned for the new option numbers.
IANA is requested to add the following entry to the "CoAP Response Codes" sub-registry available at https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#response-codes:
+------+------------------+-----------+ | Code | Description | Reference | +======+==================+===========+ | TBA3 | Missing Payloads | [RFCXXXX] | +------+------------------+-----------+ Table 3: New CoAP Response Code
This document suggests 4.18 (TBA3) as a value to be assigned for the new Response Code.
Security considerations discussed in Section 9 of [RFC7959] should be taken into account.
[[discuss if any security issues related to the incremental BID values. Lifetime of a BID (pointer to RFC8200)]]
Thanks to Achim Kraus, Christian Amsüss, Carsten Bormann, and Jim Schaad for the comments on the mailing list.
Some text from [RFC7959] is reused for readers convenience.
| [I-D.ietf-dots-signal-channel] | Reddy.K, T., Boucadair, M., Patil, P., Mortensen, A. and N. Teague, "Distributed Denial-of-Service Open Threat Signaling (DOTS) Signal Channel Specification", Internet-Draft draft-ietf-dots-signal-channel-41, January 2020. |
| [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", Internet-Draft draft-ietf-dots-telemetry-08, May 2020. |
| [RFC6928] | Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis, "Increasing TCP's Initial Window", RFC 6928, DOI 10.17487/RFC6928, April 2013. |
These examples assume NSTART has been increased to at least 4.
Let's now consider the use Block3 Option with a CON request as shown in Figure 7. All the blocks are acknowledged (ACK).
CoAP CoAP
Client Server
| |
+--------->| CON PUT /path M:0x01 T:0xf0 B3:10/0/0/1/1024
+--------->| CON PUT /path M:0x02 T:0xf1 B3:10/0/1/1/1024
+--------->| CON PUT /path M:0x03 T:0xf2 B3:10/0/2/1/1024
+--------->| CON PUT /path M:0x04 T:0xf3 B3:10/0/3/0/1024
|<---------+ ACK 0.00 M:0x01
|<---------+ ACK 0.00 M:0x02
|<---------+ ACK 0.00 M:0x03
|<---------+ ACK 0.00 M:0x04
Figure 7: Example of CON Request with Block3 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 8. The client will retry sending blocks for which no ACK was received.
CoAP CoAP
Client Server
| |
+--------->| CON PUT /path M:0x05 T:0xf4 B3:11/0/0/1/1024
+--->X | CON PUT /path M:0x06 T:0xf5 B3:11/0/1/1/1024
+--->X | CON PUT /path M:0x07 T:0xf6 B3:11/0/2/1/1024
+--------->| CON PUT /path M:0x08 T:0xf7 B3:11/0/3/1/1024
|<---------+ ACK 0.00 M:0x05
|<---------+ ACK 0.00 M:0x08
| |
[[The client retries sending packets not acknowledged]]
+--------->| CON PUT /path M:0x06 T:0xf5 B3:11/0/1/1/1024
+--->X | CON PUT /path M:0x07 T:0xf6 B3:11/0/2/1/1024
|<---------+ ACK 0.00 M:0x06
| |
[[The client retransmits messages not acknowledged
(exponential backoff)]]
+--->? | CON PUT /path M:0x07 T:0xf6 B3:11/0/2/1/1024
| |
[[Either transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 8: Example of CON Request with Block3 Option (Blocks Recovery)
If there is likely to be the possibility of network transient losses, then the use of Non-confirmable traffic should be considered.
An example of the use of Block4 Option with Confirmable messages is shown in Figure 9.
Client Server
| |
+--------->| CON GET /path M:0x01 T:0xf0 O:0 B4:0/0/0/0/1024
|<---------+ ACK 2.05 M:0x01 T:0xf0 O:1234 B4:21/0/0/1/1024
|<---------+ ACK 2.05 M:0xe1 T:0xf0 O:1234 B4:21/0/1/1/1024
|<---------+ ACK 2.05 M:0xe2 T:0xf0 O:1234 B4:21/0/2/1/1024
|<---------+ ACK 2.05 M:0xe3 T:0xf0 O:1234 B4:21/0/3/0/1024
...
[[Observe triggered]]
|<---------+ CON 2.05 M:0xe4 T:0xf0 O:1235 B4:22/0/0/1/1024
|<---------+ CON 2.05 M:0xe5 T:0xf0 O:1235 B4:22/0/1/1/1024
|<---------+ CON 2.05 M:0xe6 T:0xf0 O:1235 B4:22/0/2/1/1024
|<---------+ CON 2.05 M:0xe7 T:0xf0 O:1235 B4:22/0/3/0/1024
|--------->+ 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 B4:23/0/0/1/1024
| X<---+ CON 2.05 M:0xe9 T:0xf0 O:1236 B4:23/0/1/1/1024
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 B4:23/0/2/1/1024
|<---------+ CON 2.05 M:0xeb T:0xf0 O:1236 B4:23/0/3/0/1024
|--------->+ ACK 0.00 M:0xe8
|--------->+ ACK 0.00 M:0xeb
| |
[[Server retransmits messages not acknowledged]]
|?---------+ CON 2.05 M:0xe9 T:0xf0 O:1236 B4:23/0/1/1/1024
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 B4:23/0/2/1/1024
|--------->+ ACK 0.00 M:0xe9
| |
[[Server retransmits messages not acknowledged
(exponential backoff)]]
| X<---+ CON 2.05 M:0xea T:0xf0 O:1236 B4:23/0/2/1/1024
| |
[[Either transmission failure (acknowledge retry timeout)
or successfully transmitted.]]
Figure 9: Example of CON Notifications with Block4 Option
It is implementation-dependent as to whether a CoAP session is terminated following acknowledge retry timeout, or whether the CoAP session continues to be used under such adverse traffic conditions.
If there is likely to be the possibility of network transient losses, then the use of Non-confirmable traffic should be considered.