Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

The basic design philosophy of DTLS is to construct "TLS over datagram transport".  Datagram transport does not require nor provide reliable or in-order delivery of data.  The DTLS protocol preserves <del> this property for application data.  Applications such as media </del> <ins> this property for application data.  Applications, such as media </ins> streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data.  The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. <ins> Note that while low-latency streaming and gaming use DTLS to protect data (e.g. for protection of a WebRTC data channel), telephony utilizes DTLS for key establishment, and Secure Real-time Transport Protocol (SRTP) for protection of data RFC5763. </ins> TLS cannot be used directly in datagram environments for the following five reasons:


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

thus avoiding concerns about interleaving multiple HelloRetryRequests. <ins> For more detail on timeouts and retransmission, see timeout- retransmissions. </ins> 3.2. In DTLS, each handshake message is assigned a specific sequence


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

DTLS record MUST fit within a single datagram.  In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from <del> the record layer. </del> <ins> the record layer.  For more information about PMTU issues see pmtu- issues. </ins> Multiple DTLS records MAY be placed in a single datagram.  Records are encoded consecutively.  The length field from DTLS records


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

probes the implementation to see how it responds to various types of error.  Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service <del> (DoS) attacks because UDP forgery is so easy.  Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. </del> <ins> (DoS) attacks because UDP forgery is so easy.  Thus, the practice of generating fatal alerts is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. </ins> If DTLS is being carried over a transport that is resistant to forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

5.8.2. <del> Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if many instances of a DTLS time out early and retransmit too quickly on a congested link.  Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 RFC6298) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum).  Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values.  Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications.  Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. </del> <ins> The configuration of timer settings varies with implementations and certain deployment environments require timer value adjustments. Mishandling of the timer can lead to serious congestion problems, for example if many instances of a DTLS time out early and retransmit too quickly on a congested link.  Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 RFC6298) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum).  Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values.  Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time-sensitive applications.  Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. </ins> Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted,


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

be quite large.  If all the messages in a large flight are sent at once, this can result in network congestion.  A better strategy is to send out only part of the flight, sending more when messages are <del> acknowledged.  DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension RFC7924 and certificate compression RFC8879. </del> <ins> acknowledged.  Several extensions have been standardized to reduce the size of the certificate message, for example the cached information extension RFC7924, certificate compression RFC8879 and RFC6066, which defines the "client_certificate_url" extension allowing DTLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. </ins> 5.8.4.


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

first.  Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before.  In contrast, implementations MUST <del> NOT have send KeyUpdate, NewConnectionId or RequestConnectionId </del> <ins> NOT have sent KeyUpdate, NewConnectionId or RequestConnectionId </ins> message if an earlier message of the same type has not yet been acknowledged.


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

retransmissions.  Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, <del> which are needed to encrypt to decrypt messages found in Record 1. </del> <ins> which are needed to encrypt or decrypt messages found in Record 1. </ins> ack-msg provides the necessary background details for this interaction.


Question:
This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than 60 seconds (the RFC 6298 maximum). Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that a 100 msec timer is recommended rather than the 3-second RFC 6298 default in order to improve latency for time- sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal. Implementations SHOULD retain the current timer value until a message is transmitted and acknowledged without having to be retransmitted, at which time the value may be reset to the initial value. [BA] Is it always possible to distinguish a retransmission from a late arrival of an original packet? This seems like it could result in wrongly resetting the timer in some situations. Large Flight Sizes DTLS does not have any built-in congestion control or rate control; in general this is not an issue because messages tend to be small. However, in principle, some messages - especially Certificate - can be quite large. If all the messages in a large flight are sent at once, this can result in network congestion. A better strategy is to send out only part of the flight, sending more when messages are acknowledged. DTLS offers a number of mechanisms for minimizing the size of the certificate message, including the cached information extension [RFC7924] and certificate compression [RFC8879]. [BA] How does the implementation know how much of the flight to send? Not sure how prevalent large certs are for DTLS (e.g. compared with the self-signed certs of WebRTC), but in EAP-TLS deployments, large certs have caused problems. The EAP-TLS cert document draft-ietf-emu-eaptlscert cites some additional mechanisms for reducing certificate sizes, such as draft-ietf-tls-ctls and [RFC6066] which defines the "clientcertificateurl" extension which allows TLS clients to send a sequence of Uniform Resource Locators (URLs) instead of the client certificate. Alert Messages Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS. For instance in the flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? The use of the ACK for the second case is mandatory for the proper functioning of the protocol. For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket indefinitely until its transmission cap is reached. [BA] Do you mean "maximum retransmission timemout value"?
Addressed in

peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS.  For instance in the <del> flow shown in Figure 11 if the client does not send the ACK message when it received record 1 indicating loss of record 0, the entire flight would be retransmitted.  When DTLS 1.3 is used in deployments with lossy networks, such as low-power, long range radio networks as well as low-power mesh networks, the use of ACKs is recommended. </del> <ins> flow shown in dtls-key-update if the client does not send the ACK message when it received record 1 indicating loss of record 0, the entire flight would be retransmitted.  When DTLS 1.3 is used in deployments with lossy networks, such as low-power, long range radio networks as well as low-power mesh networks, the use of ACKs is recommended. </ins> The use of the ACK for the second case is mandatory for the proper functioning of the protocol.  For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket <del> indefinitely until its transmission cap is reached. </del> <ins> indefinitely until its maximum retransmission timemout value is reached. </ins> 8.


Question:
Yet another attempt to This builds on Chris's PR but just omits the epoch entirely from the AEAD nonce calculation. This is more consistent with TLS and doesn't require special case reasoning about why we only need to build in the bottom 16 bits. This relies entirely on the keys being different. Needs analysis.
This seems promising.
NAME this LGTM, but there's a conflict. Can you resolve prior to merging? NAME are you OK with the new rationale, and text that allows it to be relaxed in the future?
If I have understood and calculated correctly DTLS 1.3 limits the number of packets in a AES-GCM connection to 2^40.5 compared to 2^64 in DTLS 1.2. This is quite a severe limitation that is not mentioned explicitly. The AEAD limits in themselves are not a problem, but the combination of AEAD limits (2^24.5) and a small epoch (2^16) is a problem. I think this is major problem in some use cases of DTLS. E.g. most 3GPP 5G use cases where DTLS or DTLS/SCTP is used for semi-permanent connections (lasting years) and where the terminating the connection cause severe disturbances. I did not find any discussion about this problem (but I could easily have missed it) It is known that the limit of 2^48 packets in SRTP can be reached and 2^40.5 is significantly smaller than this... If the document was not in such a late stage, I would have suggested to increase the size of the epoch to 32 bits..... I think this and the other problems DTLS 1.3 has with semi-permanent connections has to be solved sooner than later.
NAME thanks for filing this. I hear you about the late stage, but IIRC we never represent the entire epoch on the wire in any case, so it seems like just changing it to 32 bits would be pretty easy, so why not do it now?
How do the chacha20 numbers compare to the AES-GCM ones?
NAME I don't think (D)TLS 1.3 puts any limits on ChaCha20 so my understanding is understanding is that the total number of packets can be protected in the current drafts is ChaCha20 2^64 (2^48 2^16) AES-GCM 2^40.5 (2^24.5 2^16) AES-GCM 2^39 (2^23 * 2^16)
NAME I am all for changing this now. I made a PR that change epoch from uint16 to uint32. As you say, it seems like a pretty easy change to do. Is uint32 the right size?
Unless you think we should do uint64. NAME NAME what would we have to do to make this change?
URL Magnus Westerlund thinks uint64 would be a safer bet.Michael Tüxen suggest to wrap the uint16 epoch instead. That might be an easier solution. I don't see a direct problem in DTLSPlaintext, ACK message, or recordsequencenumber with a uint16 epoch that is allowed to wrap like: 4, 5, ... 2^16-1, 4, 5, ... 2^16-1, 4, 5, ...
NAME NAME NAME since it's through the IESG we would need to at least raise that this change was made after they had approved it. I suspect that doing some kind of 2-week heads up to the WG/IESG that we want to make the change before landing it. But, if we do that or something more drastic that's really up to NAME
I want to point out that allowing wrapping per may not be the best solution if one considers to use the DTLS epoch as part of the input to the TLS exporter as suggested in . Then one want an input that doesn't wrap, and then to ensure that one doesn't practically run into any limits, then an uint64 would be better. An unit32 appears to be sufficient for a quite extreme use case with minimal messages size (which is one DTLS record with 4 byte user message) and high bandwidth (10 Gbps) sustained over a period of more than 5 years before the epoch would wrap. However, to my understanding the full epoch value are so rarely included in transmitted DTLS messages that its size have minimal implication on the overhead that it is not worth saving 4-bytes to be certain to have eliminated any practical limitation.
I think a 2-week WGLC and notifying the IESG will suffice.
FYI. We have now run into quite many problems with the interworking of semi-permanent DTLS/SCTP connections (lasting years) and DTLS 1.3: No Diffie-Hellman rekeying No post-handshake server authentication No rekeying of the exporter_secret 2^40.5 packets limit 2 minute MSL requirement 2^38.5 byte SCTP user message size limit Our initial plan was to just replace DTLS 1.2 with DTLS 1.3, leave DTLS/SCTP as is, but as the problems keep adding up we are now startnig to consider quite big changes to DTLS/SCTP 1.3 to make this work: URL Using new DTLS connections instead of KeyUpdate could potentially address the first five, but is a significant change.
Hi, Based on the problems long-lived DTLS/SCTP had with DTLS 1.3 (as well as DTLS 1.2 without renegotiation and respecting AEAD limits) we restructured DTLS/SCTP (RFC6083bis) to use parallel connections instead of relying on renegotiation and post-handshake messages. We think this quite easily solves all the problems we had without requiring any updates of DTLS 1.3. It also makes DTLS/SCTP less dependant on version specific DTLS mechanisms. It will hopefully work without changes with any future DTLS 1.4. URL We can now firmly RECOMMEND DTLS 1.3 which I think is very important. 5G is to a large degree replacing or complementing IPsec with TLS, DTLS, and DTLS/SCTP. I will do my best to transition 3GPP to DTLS 1.3 asap. FYI, 3GPP DTLS/SCTP associations have very long lifetimes (can be years), strive for five nines (99.999%) availability and cannot be teared down without causing major disruptions. With such long lifetimes we think mutual reauthentication, rekeying of the SCTP-AUTH key, and forcing attackers to dynamic key exfiltration [RFC7624] are required features. I still think should be done. 2^40.5 packets is quite limiting.
Seems OK.Thanks -- this LGTM. We'll need to do a brief consensus call given the type of change.

sequence number fields in the DTLSCiphertext structure have been reduced from those in previous versions. <ins> The DTLS epoch serialized in DTLSPlaintext is 2 octets long for compatibility with DTLS 1.2.  However, this value is set as the least significant 2 octets of the connection epoch, which is an 8 octet counter incremented on every KeyUpdate.  See seq-and-epoch for details.  The sequence number is set to be the low order 48 bits of the 64 bit sequence number.  Plaintext records MUST NOT be sent with sequence numbers that would exceed 2^48-1, so the upper 16 bits will always be 0. </ins> The DTLSCiphertext structure has a variable length header. DTLSPlaintext records are used to send unprotected records and