Internet-Draft Plaintext Sequence Numbers for DTLS1.3 April 2023
Pismenny Expires 13 October 2023 [Page]
TLS Working Group
Intended Status:
Standards Track
B. Pismenny

Plaintext Sequence Numbers for Datagram Transport Security Layer 1.3


This document specifies a TLS 1.3 extension that enables DTLS 1.3 to negotiate the use of plaintext sequence numbers instead of protected sequence numbers. Plaintext sequence numbers are advantageous in closed networks where the benefits of lower latency outweigh the risk of ossification and reduced privacy.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 13 October 2023.

Table of Contents

1. Introduction

Datagram Transport Layer Security (DTLS) 1.3 [RFC9147] packet encryption protects not only record data, but also the record header's sequence number. The sequence number is encrypted by XORing it with a mask which is generated by encrypting the leading 16 bytes of the record's ciphertext with a sequence number key.

For high performance networking, sequence number encryption is a trade-off between ossification and privacy on the one hand and latency and complexity for hardware acceleration on the other hand. Sequence number encryption improves privacy by hiding the real ordering of packets from on-path observers. Sequence number encryption also prevents protocol ossification, when middleboxes manipulate packet delivery based on the sequence number. Sequence number encryption however adds latency to packet processing on both sender and receiver. Sequence number encryption also increases the complexity and cost of NIC encryption accelerators, which are crucial for enabling encryption in high performance computing systems that seek to maximize performance and lowest penalty possible for encryption.

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Sequence Number Encryption Extension

enum {
  default_cipher (0),
  plaintext (1),
} SeqNumEncAlgs;

struct {
  select (Handshake.msg_type) {
    case CH:
      SeqNumEncAlgs supported_algs<1..255>;

    case SH:
      SeqNumEncAlgs selected_alg;
} SupportedSequenceNumberEncryptionAlgorithms;

The "sequence_number_encryption_algorithms" extension is used by the client to specify the record sequence number encryption algorithms it supports and by the server to select the algorithm it prefers. The ClientHello message lists algorithms by the order of their preference, starting from the most preferred algorithm.

If this extension is not present, in either ClientHello or EncryptedExtensions, then both parties MUST fallback to the default record sequence number encryption algorithm.

4. Security Considerations

This document allows endpoints to disable the record sequence number encryption algorithm, which retracts the on-path tracking anti-ossification protection established in [RFC9147] record sequence number encryption. It is therefore RECOMMENDED that users limit the deployment of this extension to closed environments, such as data centers, where the risk of on-path observers is negligible.

5. IANA Considerations

IANA is requested to assign a new value from the TLS ExtensionType values registry:

6. Normative References

Nir, Y., "A Flags Extension for TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-tlsflags-11, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, , <>.


TODO acknowledge.

Author's Address

Boris Pismenny