Internet DRAFT - draft-amsuess-core-repeat-request-tag
draft-amsuess-core-repeat-request-tag
CoRE Working Group C. Amsuess
Internet-Draft Energy Harvesting Solutions
Updates: RFC7959 (if approved) J. Mattsson
Intended status: Standards Track G. Selander
Expires: January 2, 2018 Ericsson AB
July 01, 2017
Repeat And Request-Tag
draft-amsuess-core-repeat-request-tag-00
Abstract
This document defines two optional extensions to the Constrained
Application Protocol (CoAP): the Repeat option and the Request-Tag
option. Each of these options when integrity protected, such as with
DTLS or OSCOAP, protects against certain attacks on CoAP message
exchanges.
The Repeat option enables a CoAP server to verify the freshness of a
request by requiring the CoAP client to make another request and
include a server-provided challenge. The Request-Tag option allows
the CoAP server to match message fragments belonging to the same
request message, fragmented using the CoAP Block-Wise Transfer
mechanism. This document also specifies additional processing
requirements on Block1 and Block2 options.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 2, 2018.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Request Freshness . . . . . . . . . . . . . . . . . . . . 3
1.2. Fragmented Message Body Integrity . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. The Repeat Option . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Repeat Processing . . . . . . . . . . . . . . . . . . . . 5
2.3. Applications . . . . . . . . . . . . . . . . . . . . . . 7
3. The Request-Tag Option . . . . . . . . . . . . . . . . . . . 8
3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Request-Tag Processing . . . . . . . . . . . . . . . . . 9
3.3. Applications . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1. Body Integrity Based on Payload Integrity . . . . . . 10
3.3.2. Multiple Concurrent Blockwise Operations . . . . . . 11
4. Block2 / ETag Processing . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Performance Impact When Using the Repeat Option . . 14
Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The initial CoAP suite of specifications ([RFC7252], [RFC7641],
[RFC7959]) was designed with the assumption that security could be
provided on a separate layer, in particular by using DTLS
([RFC6347]). However, for some use cases, additional functionality
or extra processing is needed to support secure CoAP operations.
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This document specifies two server-oriented CoAP options, the Repeat
option and the Request-Tag option, addressing the security features
request freshness and fragmented message body integrity,
respectively. These options in themselves do not replace the need
for a security protocol; they specify the format and processing of
data which, when integrity protected in a message, e.g. using DTLS
([RFC6347]) or OSCOAP ([I-D.ietf-core-object-security]), provide
those security features. The Request-Tag option and also the ETag
option are mandatory to use with Block1 and Block2, respectively, to
secure blockwise operations with multiple representations of a
particular resource as is specified in this document.
1.1. Request Freshness
A CoAP server receiving a request may not be able to verify when the
request was sent by the CoAP client. This remains true even if the
request was protected with a security protocol, such as DTLS. This
makes CoAP requests vulnerable to certain delay attacks which are
particularly incriminating in the case of actuators
([I-D.mattsson-core-coap-actuators]). Some attacks are possible to
mitigate by establishing fresh session keys (e.g. performing the DTLS
handshake) for each actuation, but in general this is not a solution
suitable for constrained environments.
A straightforward mitigation of potential delayed requests is that
the CoAP server rejects a request the first time it appears and asks
the CoAP client to prove that it intended to make the request at this
point in time. The Repeat option, defined in this document,
specifies such a mechanism which thereby enables the CoAP server to
verify the freshness of a request. This mechanism is not only
important in the case of actuators, or other use cases where the CoAP
operations require freshness of requests, but also in general for
synchronizing state between CoAP client and server.
1.2. Fragmented Message Body Integrity
CoAP was designed to work over unreliable transports, such as UDP,
and include a lightweight reliability feature to handle messages
which are lost or arrive out of order. In order for a security
protocol to support CoAP operations over unreliable transports, it
must allow out-of-order delivery of messages using e.g. a sliding
replay window such as described in Section 4.1.2.6 of DTLS
([RFC6347]).
The Block-Wise Transfer mechanism [RFC7959] extends CoAP by defining
the transfer of a large resource representation (CoAP message body)
as a sequence of blocks (CoAP message payloads). The mechanism uses
a pair of CoAP options, Block1 and Block2, pertaining to the request
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and response payload, respectively. The blockwise functionality does
not support the detection of interchanged blocks between different
message bodies to the same endpoint having the same block number.
This remains true even when CoAP is used together with a security
protocol such as DTLS or OSCOAP, within the replay window
([I-D.amsuess-core-request-tag]), which is a vulnerability of CoAP
when using RFC7959.
A straightforward mitigation of mixing up blocks from different
messages is to use unique identifiers for different message bodies,
which would provide equivalent protection to the case where the
complete body fits into a single payload. The ETag option [RFC7252],
set by the CoAP server, identifies a response body fragmented using
the Block2 option. This document defines the Request-Tag option for
identifying the request body fragmented using the Block1 option,
similar to ETag, but ephemeral and set by the CoAP client.
1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Unless otherwise specified, the terms "client" and "server" refers to
"CoAP client" and "CoAP server", respectively, as defined in
[RFC7252].
The terms "payload" and "body" of a message are used as in [RFC7959].
The complete interchange of a request and a response body is called a
(REST) "operation", while a request and response message (as matched
by their tokens) is called an "exchange". An operation fragmented
using [RFC7959] is called a "blockwise operation". A blockwise
operation which is fragmenting the request body is called a
"blockwise request operation". A blockwise operation which is
fragmenting the response body is called a "blockwise response
operation".
Two blockwise operations between the same endpoint pair on the same
resource are said to be "concurrent" if a block of the second request
is exchanged even though the client still intends to exchange further
blocks in the first operation. (Concurrent blockwise request
operations are impossible with the options of [RFC7959] because the
second operation's block overwrites any state of the first
exchange.).
The Repeat and Request-Tag options are defined in this document. The
concept "Request-Tag value" is defined in Section 3.1.
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2. The Repeat Option
The Repeat option is a server-driven challenge-response mechanism for
CoAP. The Repeat option value is a challenge from the server to the
client included in a CoAP response and echoed in a CoAP request.
2.1. Option Format
The Repeat Option is elective, safe-to-forward, not part of the
cache-key, and not repeatable, see Figure 1. Note that the Repeat
option has nothing to do with the property of an option being
repeatable, i.e. be allowed to occur more than once in a message, as
defined in Section 5.4.5 of [RFC7252].
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| No. | C | U | N | R | Name | Format | Length | Default | E |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| TBD | | | | | Repeat | opaque | 8-40 | (none) | x |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
E=Encrypt and Integrity Protect (when using OSCOAP)
Figure 1: Repeat Option Summary
The value of the Repeat option MUST be a (pseudo-)random bit string
of a length of at least 64 bits. A new (pseudo-)random bit string
MUST be generated by the server for each use of the Repeat option.
2.2. Repeat Processing
It is important to identify under what conditions a CoAP request to a
resource is required to be fresh. These conditions can for example
include what resource is requested, the request method and other data
in the request, and conditions in the environment such as the state
of the server or the time of the day.
A server MAY include the Repeat option in a response. The Repeat
option MUST NOT be used with empty CoAP requests. If the server
receives a request which has freshness requirements, and the request
does not contain the Repeat option, the server SHOULD send a 4.03
Forbidden response with a Repeat option. The server SHOULD cache the
transmitted Repeat option value and the response transmit time (here
denoted t0).
Upon receiving a response with the Repeat option within the
EXCHANGE_LIFETIME ([RFC7252]) of the original request, the client
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SHOULD echo the Repeat option with the same value in a new request to
the server. Upon receiving a 4.03 Forbidden response with the Repeat
option in response to a request within the EXCHANGE_LIFETIME of the
original request, the client SHOULD resend the original request. The
client MAY send a different request compared to the original request.
If the server receives a request which has freshness requirements,
and the request contains the Repeat option, the server MUST verify
that the option value equals a cached value; otherwise the request is
not processed further. The server MUST calculate the round-trip time
RTT = (t1 - t0), where t1 is the request receive time. The server
MUST only accept requests with a round-trip time below a certain
threshold T, i.e. RTT < T, otherwise the request is not processed
further, and an error message MAY be sent. The threshold T is
application specific, its value depends e.g. on the freshness
requirements of the request. An example message flow is illustrated
in Figure 2.
When used to serve freshness requirements, CoAP messages containing
the Repeat option MUST be integrity protected, e.g. using DTLS or
OSCOAP ([I-D.ietf-core-object-security]).
If the server loses time synchronization, e.g. due to reboot, it MUST
delete all cached Repeat option values and response transmission
times.
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Client Server
| |
+----->| Code: 0.03 (PUT)
| PUT | Token: 0x41
| | Uri-Path: lock
| | Payload: 0 (Unlock)
| |
|<-----+ t0 Code: 4.03 (Forbidden)
| 4.03 | Token: 0x41
| | Repeat: 0x6c880d41167ba807
| |
+----->| t1 Code: 0.03 (PUT)
| PUT | Token: 0x42
| | Uri-Path: lock
| | Repeat: 0x6c880d41167ba807
| | Payload: 0 (Unlock)
| |
|<-----+ Code: 2.04 (Changed)
| 2.04 | Token: 0x42
| |
Figure 2: Repeat option message flow
Constrained server implementations can use the mechanisms outlined in
Appendix A to minimize the memory impact of having many unanswered
Repeat responses.
2.3. Applications
1. Actuation requests often require freshness guarantees to avoid
accidental or malicious delayed actuator actions.
2. To avoid additional roundtrips for applications with multiple
actuator requests in rapid sequence between the same client and
server, the server may use the Repeat option (with a new value)
in response to a request containing the Repeat option. The
client then uses the Repeat option with the new value in the next
actuation request, and the server compares the receive time
accordingly.
3. If a server reboots during operation it may need to synchronize
state with requesting clients before continuing the interaction.
For example, with OSCOAP it is possible to reuse a persistently
stored security context by synchronizing the Partial IV (sequence
number) using the Repeat option.
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4. When a device joins a multicast/broadcast group the device may
need to synchronize state or time with the sender to ensure that
the received message is fresh. By synchronizing time with the
broadcaster, time can be used for synchronizing subsequent
broadcast messages. A server MUST NOT synchronize state or time
with clients which are not the authority of the property being
synchronized. E.g. if access to a server resource is dependent
on time, then the client MUST NOT set the time of the server.
5. A server that sends large responses to unauthenticated peers and
wants to mitigate the amplification attacks described in
Section 11.3 of [RFC7252] (where an attacker would put a victim's
address in the source address of a CoAP request) can ask a client
to Repeat its request to verify the source address. This needs
to be done only once per peer, and limits the range of potential
victims from the general Internet to endpoints that have been
previously in contact with the server. For this application, the
Repeat option can be used in messages that are not integrity
protected, for example during discovery.
3. The Request-Tag Option
The Request-Tag is intended for use as a short-lived identifier for
keeping apart distinct blockwise request operations on one resource
from one client. It enables the receiving server to reliably
assemble request payloads (blocks) to their message bodies, and, if
it chooses to support it, to reliably process simultaneous blockwise
request operations on a single resource. The requests must be be
integrity protected in order to protect against interchange of blocks
between different message bodies.
3.1. Option Format
The Request-Tag option has the same properties as the Block1 option:
it is critical, unsafe, not part of the cache-key, and not
repeatable, see Figure 3.
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| No. | C | U | N | R | Name | Format | Length | Default | E |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| TBD | x | x | - | | Request-Tag | opaque | 0-8 | (none) | * |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
E=Encrypt and Integrity Protect (when using OSCOAP)
Figure 3: Request-Tag Option Summary
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[Note to RFC editor: If this document is not released together with
OSCOAP but before it, the following paragraph and the "E" column
above need to move into OSCOAP.]
Request-Tag, like the Block1 option, is a special class E option in
terms of OSCOAP processing (see Section 4.3.1.2 of
[I-D.ietf-core-object-security]): The Request-Tag MAY be an inner or
outer option. The inner option is encrypted and integrity protected
between client and server, and provides message body identification
in case of end-to-end fragmentation of requests. The outer option is
visible to proxies and labels message bodies in case of hop-by-hop
fragmentation of requests.
Every message in which the Block1 option is set is considered to
carry a "Request-Tag value" that can be compared for equality with
the value of any other such message. Absence of the Request-Tag
option implies a value that is distinct from any value of a message
with the Request-Tag option set, and equal to that of any other
message without the Request-Tag option. Messages with the Request-
Tag option set have equal Request-Tag values if and only if their
option lengths and option values are equal.
The value of the Request-Tag option is generated by the client for
every blockwise request operation. Clients are encouraged to
generate compact Request-Tag values. It MUST be different from any
other Request-Tag value used in the same security context on the same
resource, but MAY be reused when all earlier operations with the same
value are concluded. What constitutes a concluded operation depends
on the application, and is outlined individually in Section 3.3.
3.2. Request-Tag Processing
A server MUST NOT act on any two blocks in the same blockwise request
operation that have different Request-Tag values. This also means
that a block cannot overwrite kept context when the Request-Tag does
not match (cf. [RFC7959] Section 2.5). The server is still under no
obligation to keep state of more than one transaction. When an
operation is in progress and a second one cannot be served at the
same time, the server MUST respond to the second request with a 5.03
(Service Unavailable) response code and SHOULD indicate the time it
is willing to wait for additional blocks in the first operation using
the Max-Age option, as specified in Section 5.9.3.4 of [RFC7252].
A server receiving a Request-Tag MUST treat it as opaque and make no
assumptions about its content or structure.
Two messages arriving at the server with the same Request-Tag value
do not necessarily belong to the same operation. They can still be
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treated as independent messages by the server (e.g. when it sends
2.01/2.04 responses for every block), or initiate a new operation
(overwriting kept context) when sending the first block again.
The Request-Tag option is not used in responses.
If a request that uses Request-Tag is rejected with 4.02 Bad Option,
the client MAY retry the operation without it, but then it MUST
serialize all operations that affect the same resource. Security
requirements can forbid dropping the Request-Tag option.
3.3. Applications
3.3.1. Body Integrity Based on Payload Integrity
When a client fragments a request body into multiple message
payloads, even if the individual messages are integrity protected, it
is still possible for a man-in-the-middle to maliciously replace
later operation's blocks with earlier operation's blocks (see
Section 3.2 of [I-D.amsuess-core-request-tag]). Therefore, the
integrity protection of each block does not extend to the operation's
request body.
In order to gain that protection, use the Request-Tag mechanism as
follows:
o The message payloads MUST be integrity protected end-to-end
between client and server.
o The client MUST NOT reuse a Request-Tag value within a security
association unless all previous blockwise request operations on
the same resource that used the same Request-Tag value have
concluded.
Note that the server needs to verify that all blocks within an
operation come from the same security association, because the
security association is a part of the endpoint as per [RFC7252].
o The client MUST NOT regard a blockwise request operation as
concluded unless all of the messages the client previously sent in
the operation have been confirmed by the message integrity
protection mechanism, or are considered invalid by the server if
replayed.
Typically, in OSCOAP, these confirmations can result either from
the client receiving an OSCOAP response message matching the
request (an empty ACK is insufficient), or because the message's
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sequence number is old enough to be outside the server's receive
window.
In DTLS, this can only be confirmed if the request message was not
retransmitted, and was responded to.
Authors of other documents (e.g. [I-D.ietf-core-object-security])
are invited to mandate this behavior for clients that execute
blockwise interactions over secured transports. In this way, the
server can rely on a conforming client to set the Request-Tag option
when required, and thereby conclude on the integrity of the assembled
body.
Note that this mechanism is implicitly implemented when the security
layer guarantees ordered delivery (e.g. CoAP over TLS
[I-D.tschofenig-core-coap-tcp-tls]). This is because with each
message, any earlier operation can be regarded as concluded by the
client, so it never needs to set the Request-Tag option unless it
wants to perform concurrent operations.
3.3.2. Multiple Concurrent Blockwise Operations
CoAP clients, especially CoAP proxies, may initiate a blockwise
request operation to a resource, to which a previous one is already
in progress, and which the new request should not cancel. One
example is when a CoAP proxy fragments an OSCOAP messages using
blockwise (so-called "outer" blockwise, see Section 4.3.1. of
[I-D.ietf-core-object-security])), where the Uri-Path is hidden
inside the encrypted message, and all the proxy sees is the server's
"/" path.
When a client fragments a message as part of a blockwise request
operation, it can do so without a Request-Tag option set. For this
application, an operation can be regarded as concluded when a final
Block1 option has been sent and acknowledged, or when the client
chose not to continue with the operation (e.g. by user choice, or in
the case of a proxy when it decides not to take any further messages
in the operation due to a timeout). When another concurrent
blockwise request operation is made (i.e. before the operation is
concluded), the client can use a different Request-Tag value (as
specified in Section 3.1). The possible outcomes are:
o The server responds with a successful code.
The concurrent blockwise operations can then continue.
o The server responds 4.02 Bad Option.
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This can indicate that the server does not support Request-Tag.
The client should wait for the first operation to conclude, and
then try the same request without a Request-Tag option.
o The server responds 5.03 Service Unavailable with a Max-Age option
to indicate when it is likely to be available again.
This can indicate that the server supports Request-Tag, but still
is not prepared to handle concurrent requests. The client should
wait for as long as the response is valid, and then retry the
operation, which may not need to carry a Request-Tag option by
then any more.
In the cases where a CoAP proxy receives an error code, it can
indicate the anticipated delay by sending a 5.03 Service Unavailable
response itself. Note that this behavior is no different from what a
CoAP proxy would need to do were it unaware of the Request-Tag
option.
4. Block2 / ETag Processing
The same security properties as in Section 3.3.1 can be obtained for
blockwise response operations. The threat model here is not an
attacker (because the response is made sure to belong to the current
request by the security layer), but blocks in the client's cache.
Analogous rules to Section 3.2 are already in place for assembling a
response body in Section 2.4 of [RFC7959].
To gain equivalent protection to Section 3.3.1, a server MUST use the
Block2 option in conjunction with the ETag option ([RFC7252],
Section 5.10.6), and MUST NOT use the same ETag value for different
representations of a resource.
5. IANA Considerations
[TBD: Fill out the option templates for Repeat and Request-Tag]
6. Security Considerations
Servers that store a Repeat challenge per client can be attacked for
resource exhaustion, and should consider minimizing the state kept
per client, e.g. using a mechanism as described in Appendix A.
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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,
<http://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,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<http://www.rfc-editor.org/info/rfc7959>.
7.2. Informative References
[I-D.amsuess-core-request-tag]
Amsuess, C., "Request-Tag option", draft-amsuess-core-
request-tag-00 (work in progress), March 2017.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-ietf-core-
object-security-03 (work in progress), May 2017.
[I-D.mattsson-core-coap-actuators]
Mattsson, J., Fornehed, J., Selander, G., and F.
Palombini, "Controlling Actuators with CoAP", draft-
mattsson-core-coap-actuators-02 (work in progress),
November 2016.
[I-D.tschofenig-core-coap-tcp-tls]
Bormann, C., Lemay, S., Technologies, Z., and H.
Tschofenig, "A TCP and TLS Transport for the Constrained
Application Protocol (CoAP)", draft-tschofenig-core-coap-
tcp-tls-05 (work in progress), November 2015.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
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[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
Appendix A. Performance Impact When Using the Repeat Option
The Repeat option requires the server to keep some state in order to
later verify the repeated request.
Instead of caching Repeat option values and response transmission
times, the server MAY use the encryption of the response transmit
time t0 as Repeat option value. Such a scheme needs to ensure that
the server can detect a replay of a previous encrypted response
transmit time.
For example, the server MAY encrypt t0 with AES-CCM-128-64-64 using a
(pseudo-)random secret key k generated and cached by the server. A
unique IV MUST be used with each encryption, e.g. using a sequence
number. If the server loses time synchronization, e.g. due to
reboot, then k MUST be deleted and replaced by a new random secret
key. When using encrypted response transmit times, the Repeat
processing is modified in the following way: The verification of
cached option value in the server processing is replaced by the
verification of the integrity of the encrypted option value using the
cached key and IV (e.g. sequence number).
The two methods - (a) the list of cached values, and (b) the
encryption of transmit time - have different impact on the
implementation:
o size of cached data (list of cached values vs. key and IV)
o size of message (typically larger with encrypted time)
o computation (encryption + decryption vs. generation new nonce +
cache + lookup)
In general, the encryption of transmission times is most useful if
the number of concurrent requests is high.
A hybrid scheme is also possible: the first Repeat option values are
cached, and if the number of concurrent requests reach a certain
threshold, then encrypted times are used until there is space for
storing new values in the list. In that case, the server may need to
make both verifications - either that the Repeat value is in the
list, or that it verifies in decryption - and in either case that the
transmission time is valid.
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Internet-Draft Repeat And Request-Tag July 2017
Appendix B. Request-Tag Message Size Impact
In absence of concurrent operations, the Request-Tag mechanism for
body integrity (Section 3.3.1) incurs no overhead if no messages are
lost (more precisely: in OSCOAP, if no operations are aborted due to
repeated transmission failure; in DTLS, if no packages are lost), or
when blockwise request operations happen rarely (in OSCOAP, if only
one request operation with losses within the replay window).
In those situations, the Request-Tag value of no Request-Tag option
present can be reused over and over again.
When the "no-Request-Tag value" is used-up within a security context,
the Request-Tag value of a present but empty option can be used (1
Byte overhead), and when that is used-up, 256 values from one byte
long options (2 Bytes overhead) can be used.
In situations where those overheads are unacceptable (e.g. because
the payloads are known to be at a fragmentation threshold), the
absent Request-Tag value can be made usable again:
o In DTLS, a new session can be established.
o In OSCOAP, the sequence number can be artificially increased so
that all lost messages are outside of the replay window by the
time the first request of the new operation gets processed, and
all earlier operations can therefore be regarded as concluded.
Authors' Addresses
Christian Amsuess
Energy Harvesting Solutions
Email: c.amsuess@energyharvesting.at
John Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
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