Blockwise transfers in CoAP
draft-ietf-core-block-00

Abstract

CoAP is a RESTful transfer protocol for constrained nodes and networks. CoAP is based on datagram transport, which limits the maximum size of resource representations that can be transferred without too much fragmentation. The Block option provides a minimal way to transfer larger representations in a block-wise fashion.

Status of this Memo

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Table of Contents

1.  Introduction
2.  Block-wise transfers
    2.1.  The Block Option
    2.2.  Using the Block Option
3.  IANA Considerations
4.  Security Considerations
    4.1.  Mitigating Amplification Attacks
5.  Acknowledgements
6.  References
    6.1.  Normative References
    6.2.  Informative References
§  Authors' Addresses

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] (Fielding, R., “Architectural Styles and the Design of Network-based Software Architectures,” 2000.) services not unlike HTTP [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), 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:

• reducing implementation complexity in order to minimize code size,
• 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,
• 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 fragmentation. The Block option provides a minimal way to transfer larger resource representations in a block-wise fashion.

In this document, the key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” are to be interpreted as described in BCP 14 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) and indicate requirement levels for compliant CoAP implementations.

2.  Block-wise transfers

Not all resource representations will fit into a single link layer packet of a constrained network. 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 loads the lower layers with conversation state that is better managed in the application layer.

This specification proposes an option to enable block-wise access to resource representations. 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.)

Implementation of the Block option is intended to be optional. However, when it is present in a CoAP message, it MUST be processed; therefore it is identified as a critical option.

The size of the blocks should not be fixed by the protocol. On the other hand, implementation should be as simple as possible. The Block option therefore supports a small range of power-of-two block sizes, from 2^4 (16) to 2^11 (2048) bytes. One of these eight values can be encoded in three bits (0 for 2^4 to 7 for 2^11 bytes), the SZX (size exponent); the actual block size is then 1 << (SZX + 4).

2.1.  The Block Option

When a representation is larger than can be comfortably transferred in a single UDP datagram, the Block option can be used to indicate a block-wise transfer. Block is a 1-, 2- or 3-byte integer, the four least significant bits of which indicate the size and whether the current block-wise transfer is the last block being transferred (M or “more” bit). The value divided by sixteen is the number of the block currently being transferred, starting from zero, i.e., the current transfer is about the size bytes starting at block number << (SZX + 4). The default value of the Block Option is zero, indicating that the current block is the first (block number 0) and only (M bit not set) block of the transfer; however, there is no explicit size implied by this default value.

        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 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

(Note that the option with the last 4 bits masked out, shifted to the left by the value of SZX, gives the byte position of the block.)

NUM:

Block Number. The block number is a variable 4-20 bit unsigned integer indicating the block number being requested or provided. Block number 0 indicates the first block of a representation.

M:

More Flag. This flag indicates if this block is the last in a representation when set. When not set it indicates that there are one or more blocks available. When the block option is used to retrieve a specific block number the M bit MUST be sent as zero and ignored on reception.

SZX:

Block Size. The block size indicates the size of a block to the power of two. Thus block size = 2^(SZX + 4). Thus the minimum block size is 16 and the maximum is 2048.

The Block option is used in one of three roles:

• In the request for a GET, the Block option gives the block number requested and suggests a block size (block number 0) or echoes the block size of previous blocks received (block numbers other than 0).
• In the response for a GET or in the request for a PUT or POST, the Block option describes what block number is contained in the payload, and whether further blocks are part of that body (M bit). If the M bit is set, the size of the payload body in bytes MUST indeed be the power of two given by the block size. All blocks for a REST transaction MUST use the same block size, except for the last block (M bit not set).
• In the response for a PUT or POST, the Block option indicates what block number is being acknowledged. In this case, the M bit is set to indicate that this response does not carry the final response to the request; this can occur when the M bit was set in the request and the server implements PUT/POST atomically (i.e., acts only upon reception of the last block).

2.2.  Using the Block Option

Using the Block option, a single REST operation can be split into multiple CoAP message transactions. Each of these message transactions uses their own CoAP transaction ID.

When a GET is answered with a response carrying a Block option with the M bit set, the requestor may retrieve additional blocks of the resource representation by sending requests with a Block option giving the block number desired. In such a Block option, the M bit MUST be sent as zero and ignored on reception.

To influence the block size used in response to a GET request, the requestor uses the Block option, giving the desired size, a block number of zero and an M bit of zero. A server SHOULD 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 already used in the response for the first one.

If the Block option is used by the requestor, all GET requests in a single transaction (except for the last one with the M bit not set) MUST ultimately use the same size. The server SHOULD use the block size indicated in the request option or a smaller size, but the requestor MUST take note of the actual block size used in the response it receives to its initial GET and proceed to use it in subsequent GETs; 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).

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 Block 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. When reassembling the representation from the blocks being exchanged, the reassembler MUST compare Etag options. If the Etag options do not match in a GET transfer, the requestor 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, but there is no requirement for the server to establish any state. The client MAY facilitate identifying the sequence by using the Token option.

In a PUT or POST transfer, the Block option refers to the body in the request, i.e., there is no way to perform a block-wise retrieval of the body of the response. Servers that do need to supply large bodies in response to PUT/POST SHOULD therefore be employing redirects.

In a PUT or POST transfer that is intended to be implemented in an atomic fashion at the server, the actual creation/replacement takes place at the time a block with the M bit unset is received. If not all previous blocks are available at the server at this time, the transfer fails and error code 4__ (TBD) MUST be returned. The error code 4__ can also be returned at any time by a server that does not currently have the resources to store blocks for a block-wise PUT or POST transfer that it would intend to implement in an atomic fashion.

If multiple concurrently proceeding block-wise PUT or POST operations are possible, the requestor 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 (taking an absent Token option to default to the empty Token). If atomic processing is not desired, there is no need to check the Token option.

In summary, this specification:

Adds a Block Option that can be used for block-wise transfers.

Benefits:

Transfers larger than can be accommodated in constrained-network link-layer packets can be performed in smaller blocks.

No hard-to-manage conversation state is created at the adaptation layer or IP layer for fragmentation.

The transfer of each block is acknowledged, enabling retransmission if required.

Both sides have a say in the block size that actually will be used.

TBD:

Give examples with detailed message flows for a block-wise GET, PUT and POST.

3.  IANA Considerations

This draft adds the following option number to Table 2 of [I‑D.ietf‑core‑coap] (Shelby, Z., Frank, B., and D. Sturek, “Constrained Application Protocol (CoAP),” September 2010.):

4.  Security Considerations

TBD. (Weigh the security implications of application layer block-wise transfer against those of adaptation-layer or IP-layer fragmentation.)

4.1.  Mitigating Amplification Attacks

TBD. (This section discusses how CoAP nodes could become implicated in DoS attacks by using the amplifying properties of the protocol, as well as mitigations for this threat.)

A CoAP server can reduce the amount of amplification it provides to an attacker by offering large resource representations only in relatively small blocks. 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.

5.  Acknowledgements

Of course, much of the content of this draft is the result of discussions with the [I‑D.ietf‑core‑coap] (Shelby, Z., Frank, B., and D. Sturek, “Constrained Application Protocol (CoAP),” September 2010.) authors. Tokens were suggested by Gilman Tolle and refined by Klaus Hartke.

6.  References

6.1. Normative References

6.2. Informative References

Authors' Addresses