Distributed Denial-of-Service Open Threat Signaling (DOTS) Signal Channel Configuration Attributes for Robust Block Transmission

Introduction The Constrained Application Protocol (CoAP) , although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. The block-wise transfer introduced the CoAP Block1 and Block2 options to handle data records that cannot fit in a single IP packet, to avoid having to rely on IP fragmentation. The block-wise transfer was further updated by for use over TCP, TLS, and WebSockets. The CoAP Block1 and Block2 options work well in environments where there are no or minimal packet losses. These options operate synchronously where each individual block has to be requested and can only ask for (or send) the next block when the request for the previous block has completed. Packet rates, and hence block transmission rates, are controlled by Round-Trip Times (RTTs). There is a requirement for these blocks of data to be transmitted at higher rates under network conditions where there may be asymmetrical transient packet loss (e.g., responses may get dropped). An example is when a network is subject to a Distributed Denial of Service (DDoS) attack and there is a need for DDoS mitigation agents relying upon CoAP to communicate with each other (e.g., ). As a reminder, recommends the use of Confirmable (CON) responses to handle potential packet loss. However, such a recommendation does not work with a "flooded pipe" DDoS situation because the returning ACK packets may not get through. The block-wise transfer specified in covers the general case but falls short in situations where packet loss is highly asymmetrical. The mechanism specified in provides features roughly similar to the Block1/Block2 options but also provides additional properties that are tailored towards the intended DDoS Open Threat Signaling (DOTS) transmission. Concretely, primarily targets applications such as DOTS that can't use Confirmable responses to handle potential packet loss and that support application-specific mechanisms to assess whether the remote peer is able to handle the messages sent by a CoAP endpoint (e.g., DOTS heartbeats as discussed in ). includes guards to prevent a CoAP agent from overloading the network by adopting an aggressive sending rate. These guards are followed in addition to the existing CoAP congestion control as specified in (mainly PROBING_RATE). lists the additional CoAP parameters that are used for the guards (). Note that NON in this table refers to Non-confirmable. Congestion Control Parameters

Parameter Name	Default Value
MAX_PAYLOADS	10
NON_MAX_RETRANSMIT	4
NON_TIMEOUT	2 s
NON_TIMEOUT_RANDOM	between 2-3 s
NON_RECEIVE_TIMEOUT	4 s
NON_PROBING_WAIT	between 247-248 s
NON_PARTIAL_TIMEOUT	247 s

PROBING_RATE and other transmission parameters are negotiated between DOTS peers as discussed in . Nevertheless, negotiating the parameters listed in is not supported in . This document defines new DOTS signal channel attributes, corresponding to the parameters in , that are used to customize the configuration of robust block transmission in a DOTS context.

Terminology 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 when, and only when, they appear in all capitals, as shown here. Readers should be familiar with the terms and concepts defined in and . The terms "payload" and "body" are defined in . The term "payload" is thus used for the content of a single CoAP message (i.e., a single block being transferred), while the term "body" is used for the entire resource representation that is being transferred in a block-wise fashion. The meanings of the symbols in YANG tree diagrams are defined in and .

DOTS Attributes for Robust Block Transmission defines the following parameters that are used for congestion control purposes:

MAX_PAYLOADS:: This parameter represents the maximum number of payloads that can be transmitted at any one time.
NON_MAX_RETRANSMIT:: This parameter represents the maximum number of times a request for the retransmission of missing payloads can occur without a response from the remote peer. By default, NON_MAX_RETRANSMIT has the same value as MAX_RETRANSMIT ().
NON_TIMEOUT:: This parameter represents the maximum period of delay between sending sets of MAX_PAYLOADS payloads for the same body. NON_TIMEOUT has the same value as ACK_TIMEOUT ().
NON_TIMEOUT_RANDOM:: This parameter represents the initial actual delay between sending the first two MAX_PAYLOADS_SETs of the same body. It is a random duration between NON_TIMEOUT and (NON_TIMEOUT * ACK_RANDOM_FACTOR).
NON_RECEIVE_TIMEOUT:: This parameter represents the maximum time to wait for a missing payload before requesting retransmission. By default, NON_RECEIVE_TIMEOUT has a value of twice NON_TIMEOUT.
NON_PROBING_WAIT:: This parameter is used to limit the potential wait needed when using PROBING_RATE.
NON_PARTIAL_TIMEOUT:: This parameter is used for expiring partially received bodies.

These parameters are used together with the PROBING_RATE parameter, which in CoAP indicates the average data rate that must not be exceeded by a CoAP endpoint in sending to a peer endpoint that does not respond. The single body of blocks will be subjected to PROBING_RATE (), not the individual packets. If the wait time between sending bodies that are not being responded to based on PROBING_RATE exceeds NON_PROBING_WAIT, then the wait time is limited to NON_PROBING_WAIT. This document augments the "ietf-dots-signal-channel" DOTS signal YANG module defined in with the following additional attributes that can be negotiated between DOTS peers to enable robust and faster transmission:

max-payloads:: This attribute echoes the MAX_PAYLOADS parameter defined in . This is an optional attribute. If the attribute is supplied in both 'idle-config' and 'mitigating-config', then it MUST convey the same value. If the attribute is only provided as part of 'idle-config' (or 'mitigating-config'), then the other definition (i.e., 'mitigating-config' (or 'idle-config')) MUST be updated to the same value.
non-max-retransmit:: This attribute echoes the NON_MAX_RETRANSMIT parameter defined in . The default value of this attribute is 'max-retransmit'. Note that DOTS uses a default value of '3' instead of '4' (which is used generically by CoAP for 'max-transmit'; see and ). This is an optional attribute.
non-timeout:: This attribute, expressed in seconds, echoes the NON_TIMEOUT parameter defined in . The default value of this attribute is 'ack-timeout'. This attribute is also used to compute the NON_TIMEOUT_RANDOM parameter. This is an optional attribute.
non-receive-timeout:: This attribute, expressed in seconds, echoes the NON_RECEIVE_TIMEOUT parameter defined in . The default value of this attribute is twice 'non-timeout'. This is an optional attribute.
non-probing-wait:: This attribute, expressed in seconds, echoes the NON_PROBING_WAIT parameter defined in . This is an optional attribute.
non-partial-timeout:: This attribute, expressed in seconds, echoes the NON_PARTIAL_TIMEOUT parameter defined in . The default value of this attribute is 247 seconds. This is an optional attribute.

The tree structure of the "ietf-dots-robust-trans" module () is shown in .

DOTS Fast Block Transmission Tree Structure These attributes are mapped to Concise Binary Object Representation (CBOR) types as specified in and in . DOTS clients follow the procedure specified in to negotiate, configure, and retrieve the DOTS signal channel session behavior (including Q-Block parameters) with DOTS peers.

Implementation Note 1:: 'non-probing-wait' ideally should be left having some jitter and so should not be hard-coded with an explicit value. It is suggested to use a base value (using NON_TIMEOUT instead of NON_TIMEOUT_RANDOM); the jitter (ACK_RANDOM_FACTOR - 1) is then added to each time the value is checked.
Implementation Note 2:: If any of the signal channel session configuration parameters is updated, the 'non-probing-wait' and 'non-partial-timeout' values should be recalculated according to the definition algorithms provided in unless explicit values are provided as part of the negotiated configuration.

An example of a PUT message to configure Q-Block parameters is depicted in . In this example, a non-default value is configured for the 'max-payloads' attribute, while default values are used for 'non-max-retransmit', 'non-timeout', and 'non-receive-timeout' in both idle and mitigation times. Given that 'non-probing-wait' and 'non-partial-timeout' are not explicitly configured in this example, these attributes will be computed following the algorithms provided in . The meanings of the other attributes are detailed in .

Example of PUT to Convey the Configuration Parameters The payload of the message depicted in is CBOR-encoded as indicated by the Content-Format set to "application/dots+cbor" (). However, and for the sake of better readability, the example uses JSON encoding of YANG-modeled data following the mapping tables in and in : use the JSON names and types defined in . These conventions are inherited from .

YANG/JSON Mapping Parameters to CBOR The YANG/JSON mapping parameters to CBOR are listed in . Note: Implementers must check that the mapping output provided by their YANG-to-CBOR encoding schemes is aligned with the content of . YANG/JSON Mapping Parameters to CBOR

Parameter Name	YANG Type	CBOR Key	CBOR Major Type & Information	JSON Type
ietf-dots-robust-trans:max-payloads	container	32776	5 map	Object
ietf-dots-robust-trans:non-max-retransmit	container	32777	5 map	Object
ietf-dots-robust-trans:non-timeout	container	32778	5 map	Object
ietf-dots-robust-trans:non-receive-timeout	container	32779	5 map	Object
ietf-dots-robust-trans:non-probing-wait	container	32780	5 map	Object
ietf-dots-robust-trans:non-partial-timeout	container	32781	5 map	Object

DOTS Robust Block Transmission YANG Module This module uses the data structure extension defined in . WG List: Author: Mohamed Boucadair ; Author: Jon Shallow "; description "This module contains YANG definitions for the configuration of parameters that can be negotiated between a DOTS client and a DOTS server for robust block transmission. Copyright (c) 2023 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC 9362; see the RFC itself for full legal notices."; revision 2023-02-28 { description "Initial revision."; reference "RFC 9362: Distributed Denial-of-Service Open Threat Signaling (DOTS) Configuration Attributes for Robust Block Transmission"; } grouping robust-transmission-attributes { description "A set of DOTS signal channel session configuration parameters that are negotiated between DOTS agents when making use of Q-Block1 and Q-Block2 options."; container max-payloads { description "Indicates the maximum number of payloads that can be transmitted at any one time."; choice direction { description "Indicates the communication direction in which the data nodes can be included."; case server-to-client-only { description "These data nodes appear only in a message sent from the server to the client."; leaf max-value { type uint16; description "Maximum acceptable 'max-payloads' value."; } leaf min-value { type uint16; description "Minimum acceptable 'max-payloads' value."; } } } leaf current-value { type uint16; default "10"; description "Current 'max-payloads' value."; reference "RFC 9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission, Section 7.2"; } } container non-max-retransmit { description "Indicates the maximum number of times a request for the retransmission of missing payloads can occur without a response from the remote peer."; choice direction { description "Indicates the communication direction in which the data nodes can be included."; case server-to-client-only { description "These data nodes appear only in a message sent from the server to the client."; leaf max-value { type uint16; description "Maximum acceptable 'non-max-retransmit' value."; } leaf min-value { type uint16; description "Minimum acceptable 'non-max-retransmit' value."; } } } leaf current-value { type uint16; default "3"; description "Current 'non-max-retransmit' value."; reference "RFC 9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission, Section 7.2"; } } container non-timeout { description "Indicates the maximum period of delay between sending sets of MAX_PAYLOADS payloads for the same body."; choice direction { description "Indicates the communication direction in which the data nodes can be included."; case server-to-client-only { description "These data nodes appear only in a message sent from the server to the client."; leaf max-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Maximum 'ack-timeout' value."; } leaf min-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Minimum 'ack-timeout' value."; } } } leaf current-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; default "2.00"; description "Current 'ack-timeout' value."; reference "RFC 9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission, Section 7.2"; } } container non-receive-timeout { description "Indicates the time to wait for a missing payload before requesting retransmission."; choice direction { description "Indicates the communication direction in which the data nodes can be included."; case server-to-client-only { description "These data nodes appear only in a message sent from the server to the client."; leaf max-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Maximum 'non-receive-timeout' value."; } leaf min-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Minimum 'non-receive-timeout' value."; } } } leaf current-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; default "4.00"; description "Current 'non-receive-timeout' value."; reference "RFC 9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission, Section 7.2"; } } container non-probing-wait { description "Used to limit the potential wait needed when using 'probing-rate'."; choice direction { description "Indicates the communication direction in which the data nodes can be included."; case server-to-client-only { description "These data nodes appear only in a message sent from the server to the client."; leaf max-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Maximum 'non-probing-wait' value."; } leaf min-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Minimum 'non-probing-wait' value."; } } } leaf current-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Current 'non-probing-wait' value."; reference "RFC 9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission, Section 7.2"; } } container non-partial-timeout { description "Used for expiring partially received bodies."; choice direction { description "Indicates the communication direction in which the data nodes can be included."; case server-to-client-only { description "These data nodes appear only in a message sent from the server to the client."; leaf max-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Maximum 'non-partial-timeout' value."; } leaf min-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; description "Minimum 'non-partial-timeout' value."; } } } leaf current-value-decimal { type decimal64 { fraction-digits 2; } units "seconds"; default "247.00"; description "Current 'non-partial-timeout' value."; reference "RFC 9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission, Section 7.2"; } } } sx:augment-structure "/dots-signal:dots-signal" + "/dots-signal:message-type" + "/dots-signal:signal-config" + "/dots-signal:mitigating-config" { description "Indicates DOTS configuration attributes to use for robust transmission when a mitigation is active."; uses robust-transmission-attributes; } sx:augment-structure "/dots-signal:dots-signal" + "/dots-signal:message-type" + "/dots-signal:signal-config" + "/dots-signal:idle-config" { description "Indicates DOTS configuration parameters to use for robust transmission when no mitigation is active."; uses robust-transmission-attributes; } } ]]>

IANA Considerations

Registry for DOTS Signal Channel CBOR Mappings This specification registers the following parameters in the IANA "DOTS Signal Channel CBOR Key Values" registry . DOTS Robust Block Transmission CBOR Mappings

Parameter Name	CBOR Key Value	CBOR Major Type	Change Controller	Specification Document(s)
ietf-dots-robust-trans:max-payloads	32776	5	IESG	RFC 9362
ietf-dots-robust-trans:non-max-retransmit	32777	5	IESG	RFC 9362
ietf-dots-robust-trans:non-timeout	32778	5	IESG	RFC 9362
ietf-dots-robust-trans:non-receive-timeout	32779	5	IESG	RFC 9362
ietf-dots-robust-trans:non-probing-wait	32780	5	IESG	RFC 9362
ietf-dots-robust-trans:non-partial-timeout	32781	5	IESG	RFC 9362

DOTS Robust Block Transmission YANG Module IANA has registered the following URI in the "ns" subregistry within the "IETF XML Registry" :

URI:: urn:ietf:params:xml:ns:yang:ietf-dots-robust-trans
Registrant Contact:: The IESG.
XML:: N/A; the requested URI is an XML namespace.

IANA has registered the following YANG module in the "YANG Module Names" subregistry within the "YANG Parameters" registry.

Name:: ietf-dots-robust-trans
Namespace:: urn:ietf:params:xml:ns:yang:ietf-dots-robust-trans
Maintained by IANA?: N
Prefix:: dots-robust
Reference:: RFC 9362

Security Considerations The security considerations for the DOTS signal channel protocol are discussed in . CoAP-specific security considerations are discussed in . Consistent with , the "ietf-dots-robust-trans" module is not intended to be used via NETCONF/RESTCONF. It serves as an abstract representation in DOTS signal channel messages. The "ietf-dots-robust-trans" module does not introduce any new vulnerabilities beyond those specified above.

References Normative References Informative References DOTS Signal Channel CBOR Key Values IANA

Acknowledgements Thanks to , , and for the review. Thanks to for the yangdoctors review. Thanks to for shepherding the document, for the AD review, for the artart directorate review, for the Gen-ART review, and for the int-dir review. Thanks to , , , , , and for their comments during the IESG review.