Internet Engineering Task Force (IETF) C. Bormann Request for Comments: 7959 Universitaet Bremen TZI Updates: 7252 Z. Shelby, Ed. Category: Standards Track ARM ISSN: 2070-1721 August 2016 Block-Wise Transfers in the Constrained Application Protocol (CoAP)
AbstractThe Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred. 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. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252.
Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7959. Copyright Notice Copyright (c) 2016 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 (http://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 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.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Block-Wise Transfers . . . . . . . . . . . . . . . . . . . . 6 2.1. The Block2 and Block1 Options . . . . . . . . . . . . . . 7 2.2. Structure of a Block Option . . . . . . . . . . . . . . . 8 2.3. Block Options in Requests and Responses . . . . . . . . . 10 2.4. Using the Block2 Option . . . . . . . . . . . . . . . . . 12 2.5. Using the Block1 Option . . . . . . . . . . . . . . . . . 14 2.6. Combining Block-Wise Transfers with the Observe Option . 15 2.7. Combining Block1 and Block2 . . . . . . . . . . . . . . . 16 2.8. Combining Block2 with Multicast . . . . . . . . . . . . . 16 2.9. Response Codes . . . . . . . . . . . . . . . . . . . . . 17 2.9.1. 2.31 Continue . . . . . . . . . . . . . . . . . . . . 17 2.9.2. 4.08 Request Entity Incomplete . . . . . . . . . . . 17 2.9.3. 4.13 Request Entity Too Large . . . . . . . . . . . . 17 2.10. Caching Considerations . . . . . . . . . . . . . . . . . 18 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.1. Block2 Examples . . . . . . . . . . . . . . . . . . . . . 19 3.2. Block1 Examples . . . . . . . . . . . . . . . . . . . . . 23 3.3. Combining Block1 and Block2 . . . . . . . . . . . . . . . 25 3.4. Combining Observe and Block2 . . . . . . . . . . . . . . 26 4. The Size2 and Size1 Options . . . . . . . . . . . . . . . . . 29 5. HTTP-Mapping Considerations . . . . . . . . . . . . . . . . . 31 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 7.1. Mitigating Resource Exhaustion Attacks . . . . . . . . . 33 7.2. Mitigating Amplification Attacks . . . . . . . . . . . . 34 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 8.1. Normative References . . . . . . . . . . . . . . . . . . 34 8.2. Informative References . . . . . . . . . . . . . . . . . 35 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 36 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
RFC7228]) and networks (such as IPv6 over Low- Power Wireless Personal Area Networks (6LoWPANs) [RFC4944]) [RFC7252]. The CoAP protocol is intended to provide RESTful [REST] services not unlike HTTP [RFC7230], 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 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 (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. Because CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS), the maximum size of resource representations that can be transferred without too much fragmentation is limited. 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) for the transport of larger representations would be 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. The present 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 the need for 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.)
Block-wise transfers are realized as combinations of exchanges, each of which is performed according to the CoAP base protocol [RFC7252]. Each exchange in such a combination is governed by the specifications in [RFC7252], including the congestion control specifications (Section 4.7 of [RFC7252]) and the security considerations (Section 11 of [RFC7252]; additional security considerations then apply to the transfers as a whole, see Section 7). The present specification minimizes the constraints it adds to those base exchanges; however, not all variants of using CoAP are very useful inside a block-wise transfer (e.g., using Non-confirmable requests within block-wise transfers outside the use case of Section 2.8 would escalate the overall non-delivery probability). To be perfectly clear, the present specification also does not remove any of the constraints posed by the base specification it is strictly layered on top of. For example, back-to-back packets are limited by the congestion control described in Section 4.7 of [RFC7252] (NSTART as a limit for initiating exchanges, PROBING_RATE as a limit for sending with no response); block-wise transfers cannot send/solicit more traffic than a client could be sending to / soliciting from the same server without the block-wise mode. In some cases, the present specification will RECOMMEND that a client perform a sequence of block-wise transfers "without undue delay". This cannot be phrased as an interoperability requirement, but is an expectation on implementation quality. Conversely, the expectation is that servers will not have to go out of their way to accommodate clients that take considerable time to finish a block-wise transfer. For example, for a block-wise GET, if the resource changes while this proceeds, the entity-tag (ETag) for a further block obtained may be different. To avoid this happening all the time for a fast-changing resource, a server MAY try to keep a cache around for a specific client for a short amount of time. The expectation here is that the lifetime for such a cache can be kept short, on the order of a few expected round-trip times, counting from the previous block transferred. In summary, this specification adds a pair of Block options to CoAP that can be used for block-wise transfers. Benefits of using these options include: o Transfers larger than what 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 individual 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 (without changes) to provide random access to power-of-two sized blocks within a resource representation. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, see Section 4.6 of [RFC7252]. Even though the options are Critical, a server may decide to start using them in an unsolicited way in a response. No effort was expended to provide a capability indication mechanism supporting that decision: since the block-wise transfer mechanisms are so fundamental to the use of CoAP for representations larger than about a kilobyte, there is an expectation that they are very widely implemented. 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 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. 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 naming these options (for block-wise transfers as well as in Section 4), we use the number 1 ("Block1", "Size1") to refer to the transfer of the resource representation that pertains to the request, and the number 2 ("Block2", "Size2") to refer to the transfer of the resource representation for the response. 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. The Content-Format Option applies to the body, not to the payload; in particular, the boundaries between the blocks may be in places that are not separating whole units in terms of the structure, encoding, or content-coding used by the Content-Format. (Similarly, 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.) In most cases, all blocks being transferred for a body (except for the last one) will be of the same size. (If the first request uses a bigger block size than the receiver prefers, subsequent requests will use the preferred block 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). +-----+---+---+---+---+--------+--------+--------+---------+ | No. | C | U | N | R | Name | Format | Length | Default | +-----+---+---+---+---+--------+--------+--------+---------+ | 23 | C | U | - | - | Block2 | uint | 0-3 | (none) | | | | | | | | | | | | 27 | C | U | - | - | Block1 | uint | 0-3 | (none) | +-----+---+---+---+---+--------+--------+--------+---------+ Table 1: Block Option Numbers Both Block1 and Block2 Options can be present in both the request and response messages. In either case, the Block1 Option pertains to the request payload, and the Block2 Option pertains to the response payload.
Hence, for the methods defined in [RFC7252], 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, and 2.05 -- see Section 5.5 of [RFC7252]). 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 specific 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. Either Block option MUST NOT occur more than once in a single message. Section 3.2 of [RFC7252]). This integer value encodes these three fields, see Figure 1. (Due to the CoAP uint-encoding rules, when all of NUM, M, and SZX happen to be zero, a zero-byte integer will be sent.)
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 using a three-bit unsigned integer (0 for 2**4 bytes 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 if 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)". Implementation note: 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 first byte of the block being transferred. More specifically, within the option value of a Block1 or Block2 Option, the meaning of the option fields is defined as follows: NUM: Block Number, indicating the block number being requested or provided. Block number 0 indicates the first block of a body (i.e., starting with the first byte of the 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 represented as 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. There is no default value for the Block1 and Block2 Options. Absence of one of these options is equivalent to an option value of 0 with respect to the value of NUM and 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 0, M bit not set). 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.)
down to the size given in the response. (The effect is that, if the server uses the smaller of (1) its preferred block size and (2) the block size 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. * 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 a non-zero block number). 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 from the same endpoint and plans to implement the request atomically (e.g., acts only upon reception of the last block of payload). In this case, the response MUST NOT carry a Block2 Option. + 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. (Note that the resource is now in a partially updated state; this approach is only appropriate where exposing such an
intermediate state is acceptable. The client can reduce the window by quickly continuing to update the resource, or, in case of failure, restarting the update.) * 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. Using one or both Block options, a single REST operation can be split into multiple CoAP message exchanges. As specified in [RFC7252], each of these message exchanges uses their own CoAP Message ID. The Content-Format Option sent with the requests or responses MUST reflect the Content-Format of the entire body. If blocks of a response body arrive with different Content-Format Options, it is up to the client how to handle this error (it will typically abort any ongoing block-wise transfer). If blocks of a request arrive at a server with mismatching Content-Format Options, the server MUST NOT assemble them into a single request; this usually leads to a 4.08 (Request Entity Incomplete, Section 2.9.2) error response on the mismatching block.
using 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 Block2 Option in the response.) The server uses 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 whose representations 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 ([RFC7252], Section 5.10.6), 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, 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 server may identify the sequence by the combination of the requesting endpoint and the URI being the same in each block-wise request.) 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. Clients that want to retrieve all blocks of a resource SHOULD strive to do so without undue delay. Servers can fully expect to be free to discard any cached state after a period of EXCHANGE_LIFETIME ([RFC7252], Section 4.8.2) after the last access to the state, however, there is no requirement to always keep the state for as long. The Block2 Option provides no way for a single endpoint to perform multiple concurrently proceeding block-wise response payload transfer (e.g., GET) operations to the same resource. This is rarely a requirement, but as a workaround, a client may vary the cache key (e.g., by using one of several URIs accessing resources with the same semantics, or by varying a proxy-safe elective option).
Section 2.9.1). If not all previous blocks are available at the server at the time of processing the final block, the transfer fails and error code 4.08 (Request Entity Incomplete, Section 2.9.2) MUST be returned. A server MAY also return a 4.08 error code for any (final or non-final) Block1 transfer that is not in sequence; therefore, clients that do not have specific mechanisms to handle this case SHOULD always start with block zero and send the following blocks in order. One reason that a client might encounter a 4.08 error code is that the server has already timed out and discarded the partial request body being assembled. Clients SHOULD strive to send all blocks of a request without undue delay. Servers can fully expect to be free to discard any partial request body when a period of EXCHANGE_LIFETIME
([RFC7252], Section 4.8.2) has elapsed after the most recent block was transferred; however, there is no requirement on a server to always keep the partial request body for as long. 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 its Block1 Option than requested is a hint to try a smaller SZX.) A block-wise transfer of a request payload that is implemented in a stateless fashion at the server is likely to leave the resource being operated on in an inconsistent state while the transfer is still ongoing or when the client does not complete the transfer. This characteristic is closer to that of remote file systems than to that of HTTP, where state is always kept on the server during a transfer. Techniques well known from shared file access (e.g., client-specific temporary resources) can be used to mitigate this difference from HTTP. The Block1 Option provides no way for a single endpoint to perform multiple concurrently proceeding block-wise request payload transfer (e.g., PUT or POST) operations to the same resource. Starting a new block-wise sequence of requests to the same resource (before an old sequence from the same endpoint was finished) simply overwrites the context the server may still be keeping. (This is probably exactly what one wants in this case -- the client may simply have restarted and lost its knowledge of the previous sequence.) RFC7641]. Resources observed by clients may be larger than can be comfortably processed or transferred in one CoAP message. The following rules apply to the combination of block-wise transfers with notifications. Observation relationships always apply to an entire resource; the Block2 Option does not provide a way to observe a single block of a resource. As with basic GET transfers, the client can indicate its desired block size in a Block2 Option in the GET request establishing or renewing the observation relationship. If the server supports block- wise transfers, it SHOULD take note of the block size and apply it as a maximum size to all notifications/responses resulting from the GET
request (until the client is removed from the list of observers or the entry in that list is updated by the server receiving a new GET request for the resource from the client). When sending a 2.05 (Content) notification, the server only sends the first block of the representation. The client retrieves the rest of the representation as if it had caused this first response by a GET request, i.e., by using additional GET requests with Block2 Options containing NUM values greater than zero. (This results in the transfer of the entire representation, even if only some of the blocks have changed with respect to a previous notification.) As with other dynamically changing resources, 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, and the reassembling client MUST compare the ETag Options (Section 2.4). Even more so than for the general case of Block2, clients that want to retrieve all blocks of a resource they have been notified about with a first block SHOULD strive to do so without undue delay. See Section 3.4 for examples.
In both cases, the client retrieves any further blocks using unicast exchanges; in the unicast requests, the client SHOULD heed any block size preferences indicated by the server in the response to the multicast request. Other uses of the Block options in conjunction with multicast messages are for further study. RFC7252], this specification defines two response codes and extends the meaning of one. Section 2.3. Implementation note: A server can reject a Block1 transfer with this code when NUM != 0 and a different Content-Format is indicated than expected from the current state of the resource. If it implements the transfer in a stateless fashion, it can match up the Content-Format of the block against that of the existing resource. If it implements the transfer in an atomic fashion, it can match up the block against the partially reassembled piece of representation that is going to replace the state of the resource.) Section 220.127.116.11 of [RFC7252], the response code 4.13 (Request Entity Too Large) is defined to be like HTTP 413 "Request Entity Too Large". [RFC7252] also recommends that this response SHOULD include a Size1 Option (Section 4) to indicate the maximum size of request entity the server is able and willing to handle, unless the server is not in a position to make this information available.
The present specification allows the server to return this response code at any time during a Block1 transfer to indicate that it does not currently have the resources to store blocks for a transfer that it would intend to implement in an atomic fashion. It also allows the server to return a 4.13 response to a request that does not employ Block1 as a hint for the client to try sending Block1. Finally, a 4.13 response to a request with a Block1 Option (control usage, see Section 2.3) where the response carries a smaller SZX in its Block1 Option is a hint to try that smaller SZX.