Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)
RFC 8974

Document Type RFC - Proposed Standard (January 2021; No errata)
Updates RFC 7252, RFC 8323
Authors Klaus Hartke  , Michael Richardson 
Last updated 2021-01-08
Replaces draft-hartke-core-stateless
Stream Internet Engineering Task Force (IETF)
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Internet Engineering Task Force (IETF)                         K. Hartke
Request for Comments: 8974                                      Ericsson
Updates: 7252, 8323                                        M. Richardson
Category: Standards Track                                      Sandelman
ISSN: 2070-1721                                             January 2021

                 Extended Tokens and Stateless Clients
             in the Constrained Application Protocol (CoAP)

Abstract

   This document provides considerations for alleviating Constrained
   Application Protocol (CoAP) clients and intermediaries of keeping
   per-request state.  To facilitate this, this document additionally
   introduces a new, optional CoAP protocol extension for extended token
   lengths.

   This document updates RFCs 7252 and 8323 with an extended definition
   of the "TKL" field in the CoAP message header.

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
   https://www.rfc-editor.org/info/rfc8974.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Extended Tokens
     2.1.  Extended Token Length (TKL) Field
     2.2.  Discovering Support
       2.2.1.  Extended-Token-Length Capability Option
       2.2.2.  Trial and Error
     2.3.  Intermediaries
   3.  Stateless Clients
     3.1.  Serializing Client State
     3.2.  Using Extended Tokens
     3.3.  Transmitting Messages
   4.  Stateless Intermediaries
     4.1.  Observing Resources
     4.2.  Block-Wise Transfers
     4.3.  Gateway Timeouts
     4.4.  Extended Tokens
   5.  Security Considerations
     5.1.  Extended Tokens
     5.2.  Stateless Clients and Intermediaries
   6.  IANA Considerations
     6.1.  CoAP Signaling Option Number
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Updated Message Formats
     A.1.  CoAP over UDP
     A.2.  CoAP over TCP/TLS
     A.3.  CoAP over WebSockets
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a RESTful
   application-layer protocol for constrained environments [RFC7228].
   In CoAP, clients (or intermediaries in the client role) make requests
   to servers (or intermediaries in the server role), which satisfy the
   requests by returning responses.

   While a request is ongoing, a client typically needs to keep some
   state that it requires for processing the response when that arrives.
   Identification of this state is done in CoAP by means of a token: an
   opaque sequence of bytes that is chosen by the client and included in
   the CoAP request and that is returned by the server verbatim in any
   resulting CoAP response (Figure 1).

          +-----------------+     request with     +------------+
          |        |        |   state identifier   |            |
          |        |        |       as token       |            |
          |    .-<-+->------|--------------------->|------.     |
          |   _|_           |                      |      |     |
          |  /   \ stored   |                      |      |     |
          |  \___/ state    |                      |      |     |
          |    |            |                      |      |     |
          |    '->-+-<------|<---------------------|------'     |
          |        |        |     response with    |            |
          |        v        |   token echoed back  |            |
          +-----------------+                      +------------+
                Client                                 Server

             Figure 1: Token as an Identifier for Request State

   In some scenarios, it can be beneficial to reduce the amount of state
   that is stored at the client at the cost of increased message sizes.
   A client can opt into this by serializing (parts of) its state into
   the token itself and then recovering this state from the token in the
   response (Figure 2).

          +-----------------+     request with     +------------+
          |        |        |   serialized state   |            |
          |        |        |       as token       |            |
          |        +--------|=====================>|------.     |
          |                 |                      |      |     |
          |    look ma,     |                      |      |     |
          |    no state!    |                      |      |     |
          |                 |                      |      |     |
          |        +--------|<=====================|------'     |
          |        |        |     response with    |            |
          |        v        |   token echoed back  |            |
          +-----------------+                      +------------+
                Client                                 Server

             Figure 2: Token as Serialization of Request State

   Section 3 of this document provides considerations for clients
   becoming "stateless" in this way.  (The term "stateless" is in quotes
   here, because it's a bit oversimplified.  Such clients still need to
   maintain per-server state and other kinds of state.  So it would be
   more accurate to just say that the clients are avoiding per-request
   state.)

   Section 4 of this document extends the considerations for clients to
   intermediaries, which may want to avoid keeping state for not only
   the requests they send to servers but also the requests they receive
   from clients.

   The serialization of state into tokens is limited by the fact that
   both CoAP over UDP [RFC7252] and CoAP over reliable transports
   [RFC8323] restrict the maximum token length to 8 bytes.  To overcome
   this limitation, Section 2 of this document introduces a CoAP
   protocol extension for extended token lengths.

   While the use case (avoiding per-request state) and the mechanism
   (extended token lengths) presented in this document are closely
   related, each can be used independently of the other.  Some
   implementations may be able to fit their state in just 8 bytes; some
   implementations may have other use cases for extended token lengths.

1.1.  Terminology

   In this document, the term "stateless" refers to an implementation
   strategy for a client (or intermediary in the client role) that does
   not require it to keep state for the individual requests it sends to
   a server (or intermediary in the server role).  The client still
   needs to keep state for each server it communicates with (e.g., for
   token generation, message retransmission, and congestion control).

   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.

2.  Extended Tokens

   This document updates the message formats defined for CoAP over UDP
   [RFC7252] and CoAP over TCP, TLS, and WebSockets [RFC8323] with a new
   definition of the "TKL" field.

2.1.  Extended Token Length (TKL) Field

   The definition of the "TKL" field is updated as follows:

   Token Length (TKL):  4-bit unsigned integer.  A value between 0 and
      12, inclusive, indicates the length of the variable-length "Token"
      field in bytes.  The other three values are reserved for special
      constructs:

      13:   An 8-bit unsigned integer directly precedes the "Token"
            field and indicates the length of the "Token" field minus
            13.

      14:   A 16-bit unsigned integer in network byte order directly
            precedes the "Token" field and indicates the length of the
            "Token" field minus 269.

      15:   Reserved.  This value MUST NOT be sent and MUST be processed
            as a message-format error.

   All other fields retain their definitions.

   The updated message formats are illustrated in Appendix A.

   The new definition of the "TKL" field increases the maximum token
   length that can be represented in a message to 65804 bytes.  However,
   the maximum token length that sender and recipient implementations
   support may be shorter.  For example, a constrained node of Class 1
   [RFC7228] might support extended token lengths only up to 32 bytes.

   In CoAP over UDP, it is often beneficial to keep CoAP messages small
   enough to avoid IP fragmentation.  The maximum practical token length
   may therefore also be influenced by the Path MTU (PMTU).  See
   Section 4.6 of [RFC7252] for details.

2.2.  Discovering Support

   Extended token lengths require support from server implementations.
   Support can be discovered by a client implementation in one of two
   ways:

   *  Where Capabilities and Settings Messages (CSMs) are available,
      such as in CoAP over TCP, support can be discovered using the
      Extended-Token-Length Capability Option defined in Section 2.2.1.

   *  Otherwise, such as in CoAP over UDP, support can only be
      discovered by trial and error, as described in Section 2.2.2.

2.2.1.  Extended-Token-Length Capability Option

   A server can use the elective Extended-Token-Length Capability Option
   to indicate the maximum token length it can accept in requests.

   +=+=+===+=========+=======================+========+========+=======+
   |#|C| R | Applies | Name                  | Format | Length | Base  |
   | | |   | to      |                       |        |        | Value |
   +=+=+===+=========+=======================+========+========+=======+
   |6| |   | CSM     | Extended-Token-Length | uint   | 0-3    | 8     |
   +-+-+---+---------+-----------------------+--------+--------+-------+

            Table 1: The Extended-Token-Length Capability Option

   C=Critical, R=Repeatable

   As per Section 3 of [RFC7252], the base value (and the value used
   when this option is not implemented) is 8.

   The active value of the Extended-Token-Length Option is replaced each
   time the option is sent with a modified value.  Its starting value is
   its base value.

   The option value MUST NOT be less than 8 or greater than 65804.  If
   an option value less than 8 is received, the option MUST be ignored.
   If an option value greater than 65804 is received, the option value
   MUST be set to 65804.

   Any option value greater than 8 implies support for the new
   definition of the "TKL" field specified in Section 2.1.  Indication
   of support by a server does not oblige a client to actually make use
   of token lengths greater than 8.

   If a server receives a request with a token of a length greater than
   what it indicated in its Extended-Token-Length Option, it MUST handle
   the request as a message-format error.

   If a server receives a request with a token of a length less than, or
   equal to, what it indicated in its Extended-Token-Length Option but
   is unwilling or unable to handle the token at that time, it MUST NOT
   handle the request as a message-format error.  Instead, it SHOULD
   return a 5.03 (Service Unavailable) response.

   The Extended-Token-Length Capability Option does not apply to
   responses.  The sender of a request is simply expected not to use a
   token of a length greater than it is willing to accept in a response.

2.2.2.  Trial and Error

   A server implementation that does not support the updated definition
   of the "TKL" field specified in Section 2.1 will consider a request
   with a "TKL" field value outside the range 0 to 8 to be a message-
   format error and reject it (Section 3 of [RFC7252]).  A client can
   therefore determine support by sending a request with an extended
   token length and checking whether or not it is rejected by the
   server.

   In CoAP over UDP, the way a request message is rejected depends on
   the message type.  A Confirmable message with a message-format error
   is rejected with a Reset message (Section 4.2 of [RFC7252]).  A Non-
   confirmable message with a message-format error is either rejected
   with a Reset message or just silently ignored (Section 4.3 of
   [RFC7252]).  To reliably get a Reset message, it is therefore
   REQUIRED that clients use a Confirmable message for determining
   support.

   As per RFC 7252, Reset messages are empty and do not contain a token;
   they only return the Message ID (Figure 3).  They also do not contain
   any indication of what caused a message-format error.  To avoid any
   ambiguity, it is therefore RECOMMENDED that clients use a request
   that has no potential message-format error other than the extended
   token length.

          +-----------------+   request message    +------------+
          |        |        |    with extended     |            |
          |        |        |     token length     |            |
          |    .-<-+->------|--------------------->|------.     |
          |   _|_           |                      |      |     |
          |  /   \ stored   |                      |      |     |
          |  \___/ state    |                      |      |     |
          |    |            |                      |      |     |
          |    '->-+-<------|<---------------------|------'     |
          |        |        |     Reset message    |            |
          |        v        |   with only message  |            |
          +-----------------+    ID echoed back    +------------+
                Client                                 Server

         Figure 3: A Confirmable Request with an Extended Token Is
     Rejected with a Reset Message If the Server Does Not Have Support

   An example of a suitable request is a GET request in a Confirmable
   message that includes only an If-None-Match option and a token of the
   greatest length that the client intends to use.  Any response with
   the same token echoed back indicates that tokens up to that length
   are supported by the server.

   Since network addresses may change, a client SHOULD NOT assume that
   extended token lengths are supported by a server for an unlimited
   duration.  Unless additional information is available, the client
   should assume that addresses (and therefore extended token lengths)
   are valid for a minimum of 1800 s and a maximum of 86400 s (1 day).
   A client may use additional forms of input into this determination.
   For instance, a client may assume a server that is in the same subnet
   as the client has a similar address lifetime as the client.  The
   client may use DHCP lease times or Router Advertisements to set the
   limits.  For servers that are not local, if the server was looked up
   using DNS, then the DNS resource record will have a Time To Live
   (TTL), and the extended token length should be kept for only that
   amount of time.

   If a server supports extended token lengths but receives a request
   with a token of a length it is unwilling or unable to handle, it MUST
   NOT reject the message, as that would imply that extended token
   lengths are not supported at all.  Instead, if the server cannot
   handle the request at the time, it SHOULD return a 5.03 (Service
   Unavailable) response; if the server will never be able to handle the
   request (e.g., because the token is too large), it SHOULD return a
   4.00 (Bad Request) response.

      |  Design Note: The requirement to return an error response when a
      |  token cannot be handled might seem somewhat contradictory, as
      |  returning the error response requires the server also to return
      |  the token it cannot handle.  However, processing a request
      |  usually involves a number of steps from receiving the message
      |  to passing it to application logic.  The idea is that a server
      |  implementing this extension supports large tokens at least in
      |  its first few processing steps, enough to return an error
      |  response rather than a Reset message.

      |  Design Note: To prevent the trial-and-error-based discovery
      |  from becoming too complicated, no effort is made to indicate
      |  the maximum supported token length.  A client implementation
      |  would probably already choose the shortest token possible for
      |  the task (such as being stateless, as described in Section 3),
      |  so it would probably not be able to reduce the length any
      |  further anyway should a server indicate a lower limit.

2.3.  Intermediaries

   Tokens are a hop-by-hop feature: if there are one or more
   intermediaries between a client and a server, every token is scoped
   to the exchange between a node in the client role and the node in the
   server role that it is immediately interacting with.

   When an intermediary receives a request, the only requirement is that
   it echoes the token back in any resulting response.  There is no
   requirement or expectation that an intermediary passes a client's
   token on to a server or that an intermediary uses extended token
   lengths itself in its request to satisfy a request with an extended
   token length.  Discovery needs to be performed for each hop where
   extended token lengths are to be used.

3.  Stateless Clients

   A client can be alleviated of keeping per-request state as follows:

   1.  The client serializes (parts of) its per-request state into a
       sequence of bytes and sends those bytes as the token of its
       request to the server.

   2.  The server returns the token verbatim in the response to the
       client, which allows the client to recover the state and process
       the response as if it had kept the state locally.

   As servers are just expected to return any token verbatim to the
   client, this implementation strategy for clients does not impact the
   interoperability of client and server implementations.  However,
   there are a number of significant, nonobvious implications (e.g.,
   related to security and other CoAP protocol features) that client
   implementations need take into consideration.

   The following subsections discuss some of these considerations.

3.1.  Serializing Client State

   The format of the serialized state is generally an implementation
   detail of the client and opaque to the server.  However, serialized
   state information is an attractive target for both unwanted nodes
   (e.g., on-path attackers) and wanted nodes (e.g., any configured
   forward proxy) on the path.  The serialization format therefore needs
   to include security measures such as the following:

   *  A client SHOULD protect the integrity of the state information
      serialized in a token.

   *  Even when the integrity of the serialized state is protected, an
      attacker may still replay a response, making the client believe it
      sent the same request twice.  For this reason, the client SHOULD
      implement replay protection (e.g., by using sequence numbers and a
      replay window).  For replay protection, integrity protection is
      REQUIRED.

   *  If processing a response without keeping request state is
      sensitive to the time elapsed since sending the request, then the
      client SHOULD include freshness information (e.g., a timestamp) in
      the serialized state and reject any response where the freshness
      information is insufficiently fresh.

   *  Information in the serialized state may be privacy sensitive.  A
      client SHOULD encrypt the serialized state if it contains privacy-
      sensitive information that an attacker would not get otherwise.

   *  When a client changes the format of the serialized state, it
      SHOULD prevent false interoperability with the previous format
      (e.g., by changing the key used for integrity protection or
      changing a field in the serialized state).

3.2.  Using Extended Tokens

   A client that depends on support for extended token lengths
   (Section 2) from the server to avoid keeping request state needs to
   perform a discovery of support (Section 2.2) before it can be
   stateless.

   This discovery MUST be performed in a stateful way, i.e., keeping
   state for the request (Figure 4).  If the client was stateless from
   the start, and the server does not support extended tokens, then no
   error message could be processed, since the state would neither be
   present at the client nor returned in the Reset message (Figure 5).

          +-----------------+    dummy request     +------------+
          |        |        |    with extended     |            |
          |        |        |        token         |            |
          |    .-<-+->------|=====================>|------.     |
          |   _|_           |                      |      |     |
          |  /   \ stored   |                      |      |     |
          |  \___/ state    |                      |      |     |
          |    |            |                      |      |     |
          |    '->-+-<------|<=====================|------'     |
          |        |        |     response with    |            |
          |        |        |    extended token    |            |
          |        |        |      echoed back     |            |
          |        |        |                      |            |
          |        |        |                      |            |
          |        |        |     request with     |            |
          |        |        |   serialized state   |            |
          |        |        |       as token       |            |
          |        +--------|=====================>|------.     |
          |                 |                      |      |     |
          |    look ma,     |                      |      |     |
          |    no state!    |                      |      |     |
          |                 |                      |      |     |
          |        +--------|<=====================|------'     |
          |        |        |     response with    |            |
          |        v        |   token echoed back  |            |
          +-----------------+                      +------------+
                Client                                 Server

      Figure 4: Depending on Extended Tokens for Being Stateless First
            Requires a Successful Stateful Discovery of Support

          +-----------------+    dummy request     +------------+
          |        |        |    with extended     |            |
          |        |        |        token         |            |
          |        +--------|=====================>|------.     |
          |                 |                      |      |     |
          |                 |                      |      |     |
          |                 |                      |      |     |
          |                 |                      |      |     |
          |              ???|<---------------------|------'     |
          |                 |     Reset message    |            |
          |                 |   with only message  |            |
          +-----------------+    ID echoed back    +------------+
                Client                                 Server

           Figure 5: Stateless Discovery of Support Does Not Work

   In environments where support can be reliably discovered through some
   other means, the discovery of support is OPTIONAL.  An example for
   this is the Constrained Join Protocol (CoJP) in a 6TiSCH network
   [6TISCH-MIN-SEC], where support for extended tokens is required from
   all relevant parties.

3.3.  Transmitting Messages

   In CoAP over UDP [RFC7252], a client has the choice between
   Confirmable and Non-confirmable messages for requests.  When using
   Non-confirmable messages, a client does not have to keep any message-
   exchange state, which can help in the goal of avoiding state.  When
   using Confirmable messages, a client needs to keep message-exchange
   state for performing retransmissions and handling Acknowledgement and
   Reset messages, however.  Non-confirmable messages are therefore
   better suited for avoiding state.  In any case, a client still needs
   to keep congestion-control state, i.e., maintain state for each node
   it communicates with and enforce limits like NSTART.

   As per Section 5.2 of [RFC7252], a client must be prepared to receive
   a response as a piggybacked response, a separate response, or a Non-
   confirmable response, regardless of the message type used for the
   request.  A stateless client MUST handle these response types as
   follows:

   *  If a piggybacked response passes the checks for token integrity
      and freshness (Section 3.1), the client processes the message as
      specified in RFC 7252; otherwise, it processes the acknowledgement
      portion of the message as specified in RFC 7252 and silently
      discards the response portion.

   *  If a separate response passes the checks for token integrity and
      freshness, the client processes the message as specified in RFC
      7252; otherwise, it rejects the message as specified in
      Section 4.2 of [RFC7252].

   *  If a Non-confirmable response passes the checks for token
      integrity and freshness, the client processes the message as
      specified in RFC 7252; otherwise, it rejects the message as
      specified in Section 4.3 of [RFC7252].

4.  Stateless Intermediaries

   Tokens are a hop-by-hop feature.  If a client makes a request to an
   intermediary, that intermediary needs to store the client's token
   (along with the client's transport address) while it makes its own
   request towards the origin server and waits for the response.  When
   the intermediary receives the response, it looks up the client's
   token and transport address for the received request and sends an
   appropriate response to the client.

   An intermediary might want to be "stateless" not only in its role as
   a client but also in its role as a server, i.e., be alleviated of
   storing the client information for the requests it receives.

   Such an intermediary can be implemented by serializing the client
   information along with the request state into the token towards the
   origin server.  When the intermediary receives the response, it can
   recover the client information from the token and use it to satisfy
   the client's request; therefore, the intermediary doesn't need to
   store the information itself.

   The following subsections discuss some considerations for this
   approach.

4.1.  Observing Resources

   One drawback of the approach is that an intermediary, without keeping
   request state, is unable to aggregate multiple requests for the same
   target resource, which can significantly reduce efficiency.  In
   particular, when clients observe [RFC7641] the same resource,
   aggregating requests is REQUIRED (Section 3.1 of [RFC7641]).  This
   requirement cannot be satisfied without keeping request state.

   Furthermore, an intermediary that does not keep track of the clients
   observing a resource is not able to determine whether these clients
   are still interested in receiving further notifications (Section 3.5
   of [RFC7641]) or want to cancel an observation (Section 3.6 of
   [RFC7641]).

   Therefore, an intermediary MUST NOT include an Observe Option in
   requests it sends without keeping both the request state for the
   requests it sends and the client information for the requests it
   receives.

4.2.  Block-Wise Transfers

   When using block-wise transfers [RFC7959], a server might not be able
   to distinguish blocks originating from different clients once they
   have been forwarded by an intermediary.  Intermediaries need to
   ensure that this does not lead to inconsistent resource state by
   keeping distinct block-wise request operations on the same resource
   apart, e.g., utilizing the Request-Tag Option [ECHO-REQUEST-TAG].

4.3.  Gateway Timeouts

   As per Section 5.7.1 of [RFC7252], an intermediary is REQUIRED to
   return a 5.04 (Gateway Timeout) response if it cannot obtain a
   response within a timeout.  However, if an intermediary does not keep
   the client information for the requests it receives, it cannot return
   such a response.  Therefore, in this case, the gateway cannot return
   such a response and as such cannot implement such a timeout.

4.4.  Extended Tokens

   A client may make use of extended token lengths in a request to an
   intermediary that wants to be "stateless".  This means that such an
   intermediary may have to serialize potentially very large client
   information into its token towards the origin server.  The tokens can
   grow even further when it progresses along a chain of intermediaries
   that all want to be "stateless".

   Intermediaries SHOULD limit the size of client information they are
   serializing into their own tokens.  An intermediary can do this, for
   example, by limiting the extended token lengths it accepts from its
   clients (see Section 2.2) or by keeping the client information
   locally when the client information exceeds the limit (i.e., not
   being "stateless").

5.  Security Considerations

5.1.  Extended Tokens

   Tokens significantly larger than the 8 bytes specified in RFC 7252
   have implications -- in particular, for nodes with constrained memory
   size -- that need to be mitigated.  A node in the server role
   supporting extended token lengths may be vulnerable to a denial of
   service when an attacker (either on-path or a malicious client) sends
   large tokens to fill up the memory of the node.  Implementations need
   to be prepared to handle such messages.

5.2.  Stateless Clients and Intermediaries

   Transporting the state needed by a client to process a response as
   serialized state information in the token has several significant and
   nonobvious security and privacy implications that need to be
   mitigated; see Section 3.1 for recommendations.

   In addition to the format requirements outlined there,
   implementations need to ensure that they are not vulnerable to
   maliciously crafted, delayed, or replayed tokens.

   It is generally expected that the use of encryption, integrity
   protection, and replay protection for serialized state is
   appropriate.

   In the absence of integrity and replay protection, an on-path
   attacker or rogue server/intermediary could return a state (either
   one modified in a reply, or an unsolicited one) that could alter the
   internal state of the client.

   It is for this reason that at least the use of integrity protection
   on the token is always recommended.

   It may be that in some very specific cases, as a result of a careful
   and detailed analysis of any potential attacks, it is decided that
   such cryptographic protections do not add value.  The authors of this
   document have not found such a use case as yet, but this is a local
   decision.

   It should further be emphasized that the encrypted state is created
   by the sending node and decrypted by the same node when receiving a
   response.  The key is not shared with any other system.  Therefore,
   the choice of encryption scheme and the generation of the key for
   this system is purely a local matter.

   When encryption is used, the use of AES-CCM [RFC3610] with a 64-bit
   tag is recommended, combined with a sequence number and a replay
   window.  This choice is informed by available hardware acceleration
   of on many constrained systems.  If a different algorithm is
   available accelerated on the sender, with similar or stronger
   strength, then it SHOULD be preferred.  Where privacy of the state is
   not required, and encryption is not needed, HMAC-SHA-256 [RFC6234],
   combined with a sequence number and a replay window, may be used.

   This size of the replay window depends upon the number of requests
   that need to be outstanding.  This can be determined from the rate at
   which new ones are made and the expected time period during which
   responses are expected.

   For instance, given a CoAP MAX_TRANSMIT_WAIT of 93 s (Section 4.8.2
   of [RFC7252]), any request that is not answered within 93 s will be
   considered to have failed.  At a request rate of one request per 10
   s, at most 10 (ceil(9.3)) requests can be outstanding at a time, and
   any convenient replay window larger than 20 will work.  As replay
   windows are often implemented with a sliding window and a bit, the
   use of a 32-bit window would be sufficient.

   For use cases where requests are being relayed from another node, the
   request rate may be estimated by the total link capacity allocated
   for that kind of traffic.  An alternate view would consider how many
   IPv6 Neighbor Cache Entries (NCEs) the system can afford to allocate
   for this use.

   When using an encryption mode that depends on a nonce, such as AES-
   CCM, repeated use of the same nonce under the same key causes the
   cipher to fail catastrophically.

   If a nonce is ever used for more than one encryption operation with
   the same key, then the same key stream gets used to encrypt both
   plaintexts, and the confidentiality guarantees are voided.  Devices
   with low-quality entropy sources -- as is typical with constrained
   devices, which incidentally happen to be a natural candidate for the
   stateless mechanism described in this document -- need to carefully
   pick a nonce-generation mechanism that provides the above uniqueness
   guarantee.

   [RFC8613], Appendix B.1.1 ("Sender Sequence Number") provides a model
   for how to maintain nonrepeating nonces without causing excessive
   wear of flash memory.

6.  IANA Considerations

6.1.  CoAP Signaling Option Number

   The following entry has been added to the "CoAP Signaling Option
   Numbers" registry within the "CoRE Parameters" registry.

        +============+========+=======================+===========+
        | Applies to | Number | Name                  | Reference |
        +============+========+=======================+===========+
        | 7.01       |      6 | Extended-Token-Length | RFC 8974  |
        +------------+--------+-----------------------+-----------+

                   Table 2: CoAP Signaling Option Number

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

7.2.  Informative References

   [6TISCH-MIN-SEC]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-minimal-
              security-15, 10 December 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-minimal-
              security-15>.

   [ECHO-REQUEST-TAG]
              Amsüss, C., Mattsson, J. P., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", Work in Progress,
              Internet-Draft, draft-ietf-core-echo-request-tag-11, 2
              November 2020, <https://tools.ietf.org/html/draft-ietf-
              core-echo-request-tag-11>.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <https://www.rfc-editor.org/info/rfc3610>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

Appendix A.  Updated Message Formats

   In Section 2, this document updates the CoAP message formats by
   specifying a new definition of the "TKL" field in the message header.
   As an alternative presentation of this update, this appendix shows
   the CoAP message formats for CoAP over UDP [RFC7252] and CoAP over
   TCP, TLS, and WebSockets [RFC8323] with the new definition applied.

A.1.  CoAP over UDP

                   0   1   2   3   4   5   6   7
                 +-------+-------+---------------+
                 |       |       |               |
                 |  Ver  |   T   |      TKL      |   1 byte
                 |       |       |               |
                 +-------+-------+---------------+
                 |                               |
                 |             Code              |   1 byte
                 |                               |
                 +-------------------------------+
                 |                               |
                 |                               |
                 |                               |
                 +-         Message ID          -+   2 bytes
                 |                               |
                 |                               |
                 |                               |
                 +-------------------------------+
                 \                               \
                 /              TKL              /   0-2 bytes
                 \          (extended)           \
                 +-------------------------------+
                 \                               \
                 /             Token             /   0-65804 bytes
                 \                               \
                 +-------------------------------+
                 \                               \
                 /                               /
                 \                               \
                 /            Options            /   0 or more bytes
                 \                               \
                 /                               /
                 \                               \
                 +---------------+---------------+
                 |               |               |
                 |      15       |       15      |   1 byte (if payload)
                 |               |               |
                 +---------------+---------------+
                 \                               \
                 /                               /
                 \                               \
                 /            Payload            /   0 or more bytes
                 \                               \
                 /                               /
                 \                               \
                 +-------------------------------+

A.2.  CoAP over TCP/TLS

                   0   1   2   3   4   5   6   7
                 +---------------+---------------+
                 |               |               |
                 |      Len      |      TKL      |   1 byte
                 |               |               |
                 +---------------+---------------+
                 \                               \
                 /              Len              /   0-4 bytes
                 \          (extended)           \
                 +-------------------------------+
                 |                               |
                 |             Code              |   1 byte
                 |                               |
                 +-------------------------------+
                 \                               \
                 /              TKL              /   0-2 bytes
                 \          (extended)           \
                 +-------------------------------+
                 \                               \
                 /             Token             /   0-65804 bytes
                 \                               \
                 +-------------------------------+
                 \                               \
                 /                               /
                 \                               \
                 /            Options            /   0 or more bytes
                 \                               \
                 /                               /
                 \                               \
                 +---------------+---------------+
                 |               |               |
                 |      15       |       15      |   1 byte (if payload)
                 |               |               |
                 +---------------+---------------+
                 \                               \
                 /                               /
                 \                               \
                 /            Payload            /   0 or more bytes
                 \                               \
                 /                               /
                 \                               \
                 +-------------------------------+

A.3.  CoAP over WebSockets

                   0   1   2   3   4   5   6   7
                 +---------------+---------------+
                 |               |               |
                 |       0       |      TKL      |   1 byte
                 |               |               |
                 +---------------+---------------+
                 |                               |
                 |             Code              |   1 byte
                 |                               |
                 +-------------------------------+
                 \                               \
                 /              TKL              /   0-2 bytes
                 \          (extended)           \
                 +-------------------------------+
                 \                               \
                 /             Token             /   0-65804 bytes
                 \                               \
                 +-------------------------------+
                 \                               \
                 /                               /
                 \                               \
                 /            Options            /   0 or more bytes
                 \                               \
                 /                               /
                 \                               \
                 +---------------+---------------+
                 |               |               |
                 |      15       |       15      |   1 byte (if payload)
                 |               |               |
                 +---------------+---------------+
                 \                               \
                 /                               /
                 \                               \
                 /            Payload            /   0 or more bytes
                 \                               \
                 /                               /
                 \                               \
                 +-------------------------------+

Acknowledgements

   This document is based on the requirements of, and work on,
   "Constrained Join Protocol (CoJP) for 6TiSCH" (January 2020) by
   Mališa Vučinić, Jonathan Simon, Kris Pister, and Michael Richardson.

   Thanks to Christian Amsüss, Carsten Bormann, Roman Danyliw, Christer
   Holmberg, Benjamin Kaduk, Ari Keränen, Erik Kline, Murray Kucherawy,
   Warren Kumari, Barry Leiba, David Mandelberg, Dan Romascanu, Jim
   Schaad, Göran Selander, Mališa Vučinić, Éric Vyncke, and Robert
   Wilton for helpful comments and discussions that have shaped the
   document.

   Special thanks to John Mattsson for his contributions to the security
   considerations of the document, and to Thomas Fossati for his in-
   depth review, copious comments, and suggested text.

Authors' Addresses

   Klaus Hartke
   Ericsson
   Torshamnsgatan 23
   SE-16483 Stockholm
   Sweden

   Email: klaus.hartke@ericsson.com

   Michael C. Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/