Network Working Group 後藤ゆき (Y. Goto) Internet-Draft independent Intended status: Informational 29 September 2026 Expires: 2 April 2027 Cases of Protocol Ossification on the Internet draft-yuki-ossification-cases-00 Abstract This document catalogues cases of protocol ossification. Protocol ossification is a phenomenon in which a new protocol, version, or extension cannot traverse an existing Internet path; such problems have been discovered and reported during protocol standardization and deployment. This document summarizes the reported observations and sources for those cases, together with case-specific responses. Discussion Venues This note is to be removed before publishing as an RFC. Source for this draft and an issue tracker can be found at https://github.com/flano-yuki/draft-yuki-ossification-cases. 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 https://datatracker.ietf.org/drafts/current/. 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 2 April 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Goto Expires 2 April 2027 [Page 1] Internet-Draft Ossification Cases September 2026 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Communication Failure Classifications . . . . . . . . . . . . 4 4. Cases: Issues, Reported Observations, and Mitigations . . . . 4 4.1. IP Layer . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1.1. Dropping IPv6 Extension Headers . . . . . . . . . . . 5 4.2. Transport Layer . . . . . . . . . . . . . . . . . . . . . 5 4.2.1. Reachability of New IP Transport Protocols . . . . . 5 4.2.2. TCP Fast Open and TCP Option Intolerance . . . . . . 6 4.2.3. TCP Sequence Translation and SACK Inconsistency . . . 6 4.2.4. MPTCP and ECN . . . . . . . . . . . . . . . . . . . . 6 4.2.5. UDP Options: Mismatch between UDP Length and IP Length . . . . . . . . . . . . . . . . . . . . . . . 7 4.2.6. Unsafe UDP Options and Endpoint Ossification . . . . 8 4.3. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.3.1. Version Intolerance . . . . . . . . . . . . . . . . . 8 4.3.2. ClientHello Extension Intolerance and TLS 1.3 Compatibility Mode . . . . . . . . . . . . . . . . . 9 4.3.3. Large PQC ClientHello Messages and TLS Inspection Intolerance . . . . . . . . . . . . . . . . . . . . . 9 4.3.4. Encrypted Client Hello and Middleboxes Assuming Plaintext SNI . . . . . . . . . . . . . . . . . . . . 10 4.4. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.4.1. No Response or FORMERR to EDNS(0) OPT Records . . . . 10 4.4.2. Individual EDNS Option Intolerance and UDP Fragmentation . . . . . . . . . . . . . . . . . . . . 11 4.5. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.5.1. HTTP Fields and WAF Allowlists . . . . . . . . . . . 11 4.6. QUIC and HTTP/3 . . . . . . . . . . . . . . . . . . . . . 12 4.6.1. Blocking UDP/443 . . . . . . . . . . . . . . . . . . 12 4.6.2. Middlebox Ossification on Google QUIC Public Flags . 12 4.6.3. IETF QUIC Versions and Visible Invariants . . . . . . 13 4.7. NTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.7.1. NTPv4 Extension Field Intolerance . . . . . . . . . . 14 4.7.2. Why NTPv5 Negotiation Cannot Use Extension Fields . . 14 5. Security Considerations . . . . . . . . . . . . . . . . . . . 14 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.1. Normative References . . . . . . . . . . . . . . . . . . 15 7.2. Informative References . . . . . . . . . . . . . . . . . 17 Goto Expires 2 April 2027 [Page 2] Internet-Draft Ossification Cases September 2026 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 1. Introduction Protocol specifications can contain extension points for future versions, options, extension fields, and other additions. Such extension points normally define rules intended to preserve interoperability in the presence of unknown parameters. However, even when an extension point is permitted by the specification, network devices or middleware on the path can make assumptions about packet formats and values based on what they already recognize. A new value or structure can violate those assumptions and disrupt communication. This phenomenon--where deploying an extension or a new version of an existing protocol over the Internet is impeded--is called _protocol ossification_. Protocol ossification has been observed in a range of protocols, including TCP, TLS 1.3, and QUIC. This document summarizes such cases and their mitigations. Each mitigation distinguishes specified measures, implemented measures, and observed deployment actions recorded by a source, as applicable. 2. Terminology Endpoint: A host, application, or service that initiates or terminates communication. Middlebox: A device or function between endpoints that does more than forward packets, including state tracking, inspection, transformation, or policy enforcement. NATs, firewalls, proxies, and load balancers are examples. Ossification: Loss of interoperability for a valid extension when an endpoint or an on-path implementation fails to handle an unknown value or new structure as required by the specification. It might reject, drop, rewrite, or misinterpret a version, code point, option, extension field, or header field reserved or defined for future use. Intentional blocking: A deliberate decision not to permit an unknown feature because of a security policy, regulation, or operational requirement. It can produce the same connection failure as ossification, but its cause is different. This document distinguishes the two. Goto Expires 2 April 2027 [Page 3] Internet-Draft Ossification Cases September 2026 GREASE: A technique that sends reserved values with no semantic meaning during normal operation, exposing implementations that cannot ignore unknown values and preserving extensibility. 3. Communication Failure Classifications Communication failures caused by protocol ossification can take different forms. This document assigns a *communication manifestation* to each case. Initial reachability failure: The first packet, request, or response does not pass, so that a connection or its first transaction cannot start. Negotiation failure: A peer or path does not respond to, rejects, or incorrectly responds to a proposed version, capability, option, or extension. In-flight dropping: Packets, including those after initial exchange, are dropped on the path. A case identifies whether this is one- way or two-way when known. Field or option removal/rewrite: Communication might continue, but a middlebox removes, adds, or changes a header field, option, or payload. Semantic or state failure: Packets arrive, but rewriting, incomplete interpretation, or endpoint state disagreement breaks a function or subsequent communication. Silent fallback or feature degradation: Basic communication continues, but the new capability is not used and an older mechanism is used instead. Failure of broad deployment: In addition to individual failures, the protocol or feature cannot safely be assumed available on the general Internet. Intentional policy blocking: An operator explicitly does not permit a packet or feature under a security policy or similar policy. 4. Cases: Issues, Reported Observations, and Mitigations For each case, this document identifies the reported observation, its sources, and the associated response. 4.1. IP Layer Goto Expires 2 April 2027 [Page 4] Internet-Draft Ossification Cases September 2026 4.1.1. Dropping IPv6 Extension Headers *Issue:* Firewalls and other middleboxes drop packets containing IPv6 Extension Headers (EH), including standard Fragment Headers. Variable location of the Layer 4 header, arbitrary EH chains, and fragment reassembly are common causes. *Communication manifestation:* *Initial reachability failure*, *in- flight dropping* (one-way or two-way), and sometimes *intentional policy blocking*. *Report category:* *Measurement or operational observation.* *Reported observations and sources:* [RFC7872] measured dropping of EH-containing packets on the Internet. [RFC7045] records widely deployed firewalls that did not recognize standardized EHs or handle Fragment Headers. [RFC9098] also summarizes operational problems and reports of intentional dropping. 4.2. Transport Layer 4.2.1. Reachability of New IP Transport Protocols *Issue:* NATs and stateful firewalls often implement and permit flow state only for TCP and UDP. An endpoint cannot assume that a new IP transport protocol, such as SCTP or DCCP, will traverse the general Internet. *Communication manifestation:* *Initial reachability failure*, *failure of broad deployment*, and sometimes *intentional policy blocking*. *Report category:* *Measurement or operational observation.* *Reported observations and sources:* Edeline et al. [Edeline] report RIPE Atlas and comparison traffic measurements showing that middleboxes impede new transports and extensions to existing transports. WebRTC data channels specify SCTP over DTLS over UDP rather than native SCTP [RFC8841]. *Mitigation:* *Specified measure:* WebRTC data channels specify SCTP over DTLS over UDP rather than native SCTP [RFC8841]. Goto Expires 2 April 2027 [Page 5] Internet-Draft Ossification Cases September 2026 4.2.2. TCP Fast Open and TCP Option Intolerance *Issue:* TCP Fast Open (TFO) cookies and data in SYN packets change assumptions made by middlebox TCP state machines. Operational deployments observed post-handshake black holes and one-way data drops. *Communication manifestation:* *Field or option removal/rewrite*, *negotiation failure*, and *silent fallback or feature degradation*. *Report category:* *Measurement or operational observation.* The case was observed in an Apple service deployment on iOS 9 and OS X 10.11. *Reported observations and sources:* Paasch [Paasch] shows a 30-second post-handshake black hole caused by middleboxes at some ISPs, and one-way data dropping. [RFC9065] also notes middleboxes that remove unknown TCP options, and [RFC7413] specifies fallback behavior for TFO. *Mitigation:* *Specified measure:* Treat TFO as opportunistic and promptly retry with an ordinary TCP handshake. *Observed deployment action:* The Apple deployment used aggressive client timeouts to blacklist a network for TFO and whitelisted a network after a successful TFO connection. It used TCP keepalive and receive sequence state to detect one-way loss. 4.2.3. TCP Sequence Translation and SACK Inconsistency *Issue:* A middlebox that inserts or removes TCP payload can translate the fixed sequence and acknowledgment fields but fail to translate SACK ranges. Endpoints then receive invalid SACK information and loss recovery can degrade. *Communication manifestation:* *Field or option removal/rewrite* and *semantic or state failure*. *Report category:* *Documented implementation behavior.* *Reported observations and sources:* [RFC9065] identifies middleboxes that rewrite only the fixed TCP header and not SACK information as an example of TCP ossification. 4.2.4. MPTCP and ECN *Issue:* Paths that do not preserve MPTCP options such as MP_CAPABLE, MP_JOIN, and DSS cause capability negotiation to fail and fall back to regular TCP. ECN-capable SYNs and ECE/CWR handling have similar compatibility risks. Goto Expires 2 April 2027 [Page 6] Internet-Draft Ossification Cases September 2026 *Communication manifestation:* *Negotiation failure*, *field or option removal/rewrite*, and *silent fallback or feature degradation*. *Report category:* *Mixed.* MPTCP includes *documented implementation and operational behavior* in [RFC8041]. ECN-capable SYN fallback in [RFC3168] is a *specification consideration or design constraint*. *Reported observations and sources:* [RFC8684] specifies MPTCP fallback for middlebox compatibility and [RFC8041] records operational heuristics. [RFC3168] specifies fallback for middleboxes that drop ECN-capable SYNs. *Mitigation:* *Specified measure:* For MPTCP, fall back to TCP without MPTCP options when a middlebox prevents connection establishment. For ECN, retransmit the SYN without ECN negotiation if an ECN-capable SYN receives no response. 4.2.5. UDP Options: Mismatch between UDP Length and IP Length *Issue:* UDP Options use the surplus area after the user data indicated by the UDP Length field and before the end of the IP payload. The UDP Length and IP payload length therefore intentionally differ. Some implementations and inspection devices classify this difference as anomalous or an attack. *Communication manifestation:* *Initial reachability failure* or *in- flight dropping*. An IDS alert alone does not necessarily cause failure. *Report category:* *Measurement or operational observation* and *documented implementation behavior.* *Reported observations and sources:* [RFC9868], Section 18, records interoperability tests on Linux, macOS, Windows Cygwin, and NATs that delivered only user data, but also reports embedded devices that delivered the entire IP datagram to a UDP application. It records the default configuration of the Alcatel-Lucent "Brick" IDS, which reported a UDP/IP length mismatch as an attack. *Mitigation:* *Specified measure:* Initially use only SAFE Options so that legacy receivers retain the meaning of UDP user data. Goto Expires 2 April 2027 [Page 7] Internet-Draft Ossification Cases September 2026 4.2.6. Unsafe UDP Options and Endpoint Ossification *Issue:* UNSAFE Options for compression, encryption, fragmentation, and similar functions can break semantics when a legacy receiver ignores the option and processes user data. UDP provides no standard stateful negotiation, so a sender cannot know in advance that an endpoint supports such an option. *Communication manifestation:* *Negotiation failure*, *semantic or state failure*, or *silent fallback or feature degradation*. *Report category:* *Specification consideration or design constraint.* *Reported observations and sources:* [RFC9868] defines SAFE Options as those that do not change user-data meaning if ignored. It deliberately defines incompatible behavior for UNSAFE Options, for example by making user data empty and placing payload in a FRAG Option. The RFC also explains that endpoint support cannot be known in advance. *Mitigation:* *Specified measure:* Make a new option SAFE where possible. For UNSAFE Options, use capability exchange, fallback, and reordering/loss handling at a higher layer, and do not permit in- transit modification. 4.3. TLS 4.3.1. Version Intolerance *Issue:* An old implementation can reject a ClientHello that advertises an unknown newer TLS version rather than selecting a supported lower version. *Communication manifestation:* *Negotiation failure* and *initial reachability failure*. *Report category:* *Documented implementation behavior.* *Reported observations and sources:* [RFC8446] records version intolerance. TLS 1.3 moves version preference to the supported_versions extension and fixes legacy_version to the TLS 1.2 value, 0x0303. *Mitigation:* *Specified measure:* Do not depend on putting a new version directly in an old version field; use a compatible negotiation extension. Goto Expires 2 April 2027 [Page 8] Internet-Draft Ossification Cases September 2026 4.3.2. ClientHello Extension Intolerance and TLS 1.3 Compatibility Mode *Issue:* Endpoints and middleboxes can reject unknown TLS extensions, cipher suites, lengths, or handshake ordering. Some middleboxes treated the TLS 1.3 wire image as anomalous relative to TLS 1.2. *Communication manifestation:* *Negotiation failure* and *initial reachability failure*; compatibility mode can instead produce *silent fallback or feature degradation*. *Report category:* *Measurement or operational observation.* *Reported observations and sources:* [RFC8446], Appendix D.4, relies on field measurements and specifies compatibility mode, including a dummy ChangeCipherSpec. Benjamin [Benjamin] reports a Chrome 63 rollout of TLS 1.3 draft 22 to 95% of stable users. A Canon PIXMA MX492 failed because BSAFE's private extended_random extension number 40 collided with TLS 1.3 key_share. Cisco Firepower in "Decrypt - Resign" mode improperly forwarded supported_versions, key_share, and the client random, breaking TLS 1.3 servers. [RFC8701] defines TLS GREASE. QUIC's use of TLS does not use the CCS compatibility mode [RFC9001]. *Mitigation:* *Specified measure:* Clients should continuously use GREASE. 4.3.3. Large PQC ClientHello Messages and TLS Inspection Intolerance *Issue:* Hybrid post-quantum key exchange enlarges supported_groups and key_share in ClientHello. The ClientHello can span multiple TCP segments. TLS inspection middleboxes that cannot reassemble and inspect it correctly can make the handshake fail. *Communication manifestation:* *Negotiation failure* and user-visible *initial reachability failure*; disabling PQC key exchange produces *silent fallback or feature degradation*. *Report category:* *Measurement or operational observation.* The sources identify concrete failures and fixes. *Reported observations and sources:* Chromium CECPQ2 [CECPQ2] records that larger TLS messages caused failures or timeouts in non- conformant middleware during a CECPQ2 rollout, including FortiGate and possibly Palo Alto Networks devices. CECPQ2 used X25519 and NTRU-HRSS and is not the same algorithm as current ML-KEM deployment, but is an operational precursor. Fortinet's technical note [FORTINET-MLKEM] documents ERR_SSL_PROTOCOL_ERROR and a fatal illegal_parameter alert for ML-KEM ClientHello with Flow-based TLS Goto Expires 2 April 2027 [Page 9] Internet-Draft Ossification Cases September 2026 Deep Inspection, with IPS Engine updates as the long-term resolution. Palo Alto Networks' technical note [PALOALTO-PQC] records SSL session failure when ClientHello arrives in multiple packets over an asymmetric path, with disabling the accumulation proxy or client PQC as workarounds. *Mitigation:* *Implemented measure:* Fortinet identifies an IPS Engine update for Flow-based TLS Deep Inspection as the long-term resolution [FORTINET-MLKEM]. 4.3.4. Encrypted Client Hello and Middleboxes Assuming Plaintext SNI *Issue:* ECH places the real SNI and related information in a ClientHelloInner and sends a ClientHelloOuter containing an encrypted_client_hello extension. A middlebox that rejects unknown TLS extensions, or an inspection/termination proxy that assumes visible SNI, can fail the handshake. Deliberately disabling ECH to restore plaintext SNI can produce the same result, but is *intentional policy blocking*, not ossification. *Communication manifestation:* *Negotiation failure*, *initial reachability failure*, or *silent fallback or feature degradation*. A deliberate ECH block is *intentional policy blocking*. *Report category:* *Specification consideration or design constraint.* [RFC9849] describes an incompatible TLS-terminating proxy as capable of retry or connection failure depending on the client's trust configuration. *Reported observations and sources:* [RFC9849], Section 6.2, specifies GREASE encrypted_client_hello for clients without an ECHConfig. Section 10.10.4 explicitly presents broad GREASE ECH deployment as a mitigation for network ossification. Section 8.1.2 explains how a conforming proxy ignores unknown parameters and connects to the public name, and how failure can result when the proxy certificate is not authoritative for that name. *Mitigation:* *Specified measure:* Clients should continue GREASE ECH. 4.4. DNS 4.4.1. No Response or FORMERR to EDNS(0) OPT Records *Issue:* An authoritative server, recursive resolver, or middlebox can give no response, return FORMERR, or remove an EDNS(0) OPT pseudo-RR. A resolver may be unable to distinguish packet loss from EDNS intolerance and falls back to plain DNS. Goto Expires 2 April 2027 [Page 10] Internet-Draft Ossification Cases September 2026 *Communication manifestation:* *Negotiation failure*, *in-flight dropping* (query or response), and *silent fallback or feature degradation*. *Report category:* *Measurement or operational observation.* *Reported observations and sources:* [RFC8906] records widespread non-response to EDNS queries, fallback to plain DNS, and possible DNSSEC validation failure. [RFC9170] treats DNS extension intolerance as a case requiring broad fallback. *Mitigation:* *Specified measure:* Resolvers need EDNS fallback. draft-ietf-dnsop-grease-03 [DNSOP-GREASE] proposes low-rate GREASE of DNS extension points to expose intolerance to unknown values early. 4.4.2. Individual EDNS Option Intolerance and UDP Fragmentation *Issue:* Implementations that return FORMERR for an unknown EDNS option do not distinguish lack of that option from lack of EDNS as a whole. Large DNS UDP responses that require IP fragmentation can time out when fragments are dropped, including for DNSSEC resolution. *Communication manifestation:* *Negotiation failure*, *in-flight dropping* (primarily one-way on the response path), and *silent fallback or feature degradation*. *Report category:* *Documented implementation and operational behavior.* *Reported observations and sources:* [RFC8906] describes incorrect handling of unknown EDNS options. [RFC8027] describes EDNS-size fallback for DNSSEC reachability, and [RFC10001] discusses oversized EDNS UDP payloads and fragmentation reachability. 4.5. HTTP 4.5.1. HTTP Fields and WAF Allowlists *Issue:* HTTP header fields, methods, status codes, and cache directives are extension points. A WAF, proxy, or cache that permits only known field names or values can block, remove, or misinterpret a new field or a permitted new value. *Communication manifestation:* *Field or option removal/rewrite*, *semantic or state failure*, or one-way *in-flight dropping* of a request or response. Goto Expires 2 April 2027 [Page 11] Internet-Draft Ossification Cases September 2026 *Report category:* *Specification consideration or design constraint.* *Reported observations and sources:* draft-nottingham-http-grease [HTTP-GREASE] identifies HTTP methods, status codes, header/trailer fields, cache directives, content codings, and range units as potentially ossified extension points and proposes GREASE. 4.6. QUIC and HTTP/3 4.6.1. Blocking UDP/443 *Issue:* In enterprise networks, access networks, or firewalls that block UDP/443, QUIC Initial packets cannot complete a round trip and an HTTP/3 connection cannot start. *Communication manifestation:* *Initial reachability failure*, *failure of broad deployment*, and sometimes *intentional policy blocking*. *Report category:* *Measurement or operational observation.* The available measurement is not limited to UDP/443. *Reported observations and sources:* The UDP reachability measurements in Edeline et al. [Edeline] show that UDP is not universally traversable. 4.6.2. Middlebox Ossification on Google QUIC Public Flags Google QUIC (GQUIC) in this section is the pre-IETF QUIC deployed and studied by Google from 2013 and in the 2017 paper cited below. It differs from IETF QUIC in wire format and cryptographic handshake. *Issue:* In October 2016, Google changed one public flag bit in a GQUIC packet header. A firewall product used that flag to identify and explicitly block GQUIC. Before the change, all GQUIC packets were blocked and the client could fall back to TCP. After the change, the classifier let the initial packet pass but blocked later packets, creating a black hole after connection start and preventing TCP fallback. *Communication manifestation:* *In-flight dropping* after the first packet, *semantic or state failure*, and *initial reachability failure* when fallback does not activate. *Report category:* *Measurement or operational observation.* Google observed the failure during global deployment, rolled back the change, and contacted the vendor. Goto Expires 2 April 2027 [Page 12] Internet-Draft Ossification Cases September 2026 *Reported observations and sources:* Section 7.5 of Langley et al. [Langley] records the one-bit change, pathological loss of later packets in a firewall, fallback failure, rollback, and a vendor classifier update. It also describes GQUIC's encryption of most transport-header information to reduce middlebox modification and ossification. *Mitigation:* *Observed deployment action:* In this case, client rollback and the vendor classifier update restored service. *Implemented measure:* GQUIC encrypted transport information that did not require middlebox interpretation. 4.6.3. IETF QUIC Versions and Visible Invariants IETF QUIC in this section is the standardized protocol based on [RFC9000]. It is different from the GQUIC described in the preceding section. The risk of middleboxes fixing on a visible wire image is common to both. *Issue:* A QUIC-aware middlebox can depend on its own interpretation of the version-1 first packet, connection ID, fixed bit, or other visible values and thereby interfere with version 2 or a future version. *Communication manifestation:* *Negotiation failure*, *initial reachability failure*, or *failure of broad deployment*. *Report category:* *Specification consideration or design constraint.* QUIC v2 exercises version negotiation to resist ossification. Chaos Protection is an endpoint testing practice. *Reported observations and sources:* [RFC9369] specifies QUIC v2 to exercise version negotiation and counter an ossification vector, including middlebox attention to the first packet. [RFC9000] limits version-independent properties. *Mitigation:* *Specified measure:* QUIC v2 exercises version negotiation to mitigate ossification around version-1 Initial packets [RFC9369]. *Implemented measure:* Chrome's QUIC implementation uses "Chaos Protection" that splits ClientHello across CRYPTO frames and varies PING, PADDING, and frame order; the quiche implementation [QUICHE-CHAOS] records this practice. 4.7. NTP Goto Expires 2 April 2027 [Page 13] Internet-Draft Ossification Cases September 2026 4.7.1. NTPv4 Extension Field Intolerance *Issue:* NTPv4 can append Extension Fields after its fixed header. Existing implementations that discard a packet with an unknown Extension Field cannot interoperate with a peer sending a new field, although a receiver that does not use the field would otherwise ignore it and continue time synchronization. *Communication manifestation:* *Negotiation failure* or one-way/two- way *in-flight dropping* of requests and responses; retry without the field can be *silent fallback or feature degradation*. *Report category:* *Documented implementation behavior.* *Reported observations and sources:* [RFC7822] says a host SHOULD ignore an unknown Extension Field, subject to policy. [RFC7821] explicitly states that its UDP Checksum Complement Extension Field cannot interoperate with legacy implementations that discard unknown Extension Fields. 4.7.2. Why NTPv5 Negotiation Cannot Use Extension Fields *Issue:* NTPv5 is not wire-compatible with NTPv4. Some widely deployed NTPv4 servers interpret a higher-version request as NTPv4 and copy its version number into the response, or do not respond to a request containing an unknown Extension Field. NTPv4 Extension Fields therefore cannot be used for NTPv5 capability negotiation. *Communication manifestation:* *Negotiation failure*, one-way *in- flight dropping* of requests, and *failure of broad deployment*. *Report category:* *Documented implementation behavior.* The source records existing implementation behavior as a design input. *Reported observations and sources:* draft-ietf-ntp-ntpv5-09, Section 12 [NTPV5-DRAFT] records these behaviors. *Mitigation:* *Specified measure:* NTPv5 places its negotiation signal in an existing reference-timestamp field. 5. Security Considerations This document introduces no new security considerations. Security considerations for each case are in the RFCs, Internet-Drafts, and other sources cited in the text. Goto Expires 2 April 2027 [Page 14] Internet-Draft Ossification Cases September 2026 6. IANA Considerations This document has no IANA actions. 7. References 7.1. Normative References [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, . [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, . [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing of IPv6 Extension Headers", RFC 7045, DOI 10.17487/RFC7045, December 2013, . [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, . [RFC7821] Mizrahi, T., "UDP Checksum Complement in the Network Time Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March 2016, . [RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4 (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822, March 2016, . [RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu, "Observations on the Dropping of Packets with IPv6 Extension Headers in the Real World", RFC 7872, DOI 10.17487/RFC7872, June 2016, . [RFC8027] Hardaker, W., Gudmundsson, O., and S. Krishnaswamy, "DNSSEC Roadblock Avoidance", BCP 207, RFC 8027, DOI 10.17487/RFC8027, November 2016, . Goto Expires 2 April 2027 [Page 15] Internet-Draft Ossification Cases September 2026 [RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and Operational Experience with Multipath TCP", RFC 8041, DOI 10.17487/RFC8041, January 2017, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C. Paasch, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March 2020, . [RFC8701] Benjamin, D., "Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility", RFC 8701, DOI 10.17487/RFC8701, January 2020, . [RFC8841] Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo, "Session Description Protocol (SDP) Offer/Answer Procedures for Stream Control Transmission Protocol (SCTP) over Datagram Transport Layer Security (DTLS) Transport", RFC 8841, DOI 10.17487/RFC8841, January 2021, . [RFC8906] Andrews, M. and R. Bellis, "A Common Operational Problem in DNS Servers: Failure to Communicate", BCP 231, RFC 8906, DOI 10.17487/RFC8906, September 2020, . [RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, . [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, . [RFC9065] Fairhurst, G. and C. Perkins, "Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols", RFC 9065, DOI 10.17487/RFC9065, July 2021, . Goto Expires 2 April 2027 [Page 16] Internet-Draft Ossification Cases September 2026 [RFC9098] Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston, G., and W. Liu, "Operational Implications of IPv6 Packets with Extension Headers", RFC 9098, DOI 10.17487/RFC9098, September 2021, . [RFC9170] Thomson, M. and T. Pauly, "Long-Term Viability of Protocol Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170, December 2021, . [RFC9288] Gont, F. and W. Liu, "Recommendations on the Filtering of IPv6 Packets Containing IPv6 Extension Headers at Transit Routers", RFC 9288, DOI 10.17487/RFC9288, August 2022, . [RFC9369] Duke, M., "QUIC Version 2", RFC 9369, DOI 10.17487/RFC9369, May 2023, . [RFC9769] Lichvar, M. and A. Malhotra, "NTP Interleaved Modes", RFC 9769, DOI 10.17487/RFC9769, May 2025, . [RFC9849] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS Encrypted Client Hello", RFC 9849, DOI 10.17487/RFC9849, March 2026, . [RFC9868] Touch, J. and C. Heard, Ed., "Transport Options for UDP", RFC 9868, DOI 10.17487/RFC9868, October 2025, . [RFC10001] Momoka and T. Fiebig, "Operational Guidelines for DNS Transport in Mixed IPv4/IPv6 Environments", BCP 91, RFC 10001, DOI 10.17487/RFC10001, August 2026, . 7.2. Informative References [Benjamin] Benjamin, D., "Additional TLS 1.3 results from Chrome", TLS Working Group mailing list, December 2017, . [CECPQ2] Chromium, "CECPQ2", n.d., . Goto Expires 2 April 2027 [Page 17] Internet-Draft Ossification Cases September 2026 [DNSOP-GREASE] Huque, S. and M. P. Andrews, "Greasing Protocol Extension Points in the DNS", Internet-Draft, draft-ietf-dnsop- grease-03, n.d., . [Edeline] Edeline, K., Kühlewind, M., Trammell, B., Aben, E., and B. Donnet, "Using UDP for Internet Transport Evolution", arXiv:1612.07816, 2016, . [FORTINET-MLKEM] Fortinet, "Technical Tip: ERR_SSL_PROTOCOL_ERROR when using Flow-based Deep Inspection due to ML-KEM post- quantum TLS key exchange (Known Issue)", n.d., . [HTTP-GREASE] Nottingham, M., "Greasing HTTP", Internet-Draft, draft- nottingham-http-grease, n.d., . [Langley] "The QUIC Transport Protocol: Design and Internet-Scale Deployment", SIGCOMM 2017, n.d., . [NTPV5-DRAFT] Lichvar, M. and T. Mizrahi, "Network Time Protocol Version 5", Internet-Draft, draft-ietf-ntp-ntpv5-09, Section 12, n.d., . [Paasch] Paasch, C., "Deploying TCP Fast Open in the wild", IETF 94 TCPM presentation, n.d., . [PALOALTO-PQC] Palo Alto Networks, "Palo Alto Networks technical note on fragmented ClientHello inspection", n.d., . Goto Expires 2 April 2027 [Page 18] Internet-Draft Ossification Cases September 2026 [QUICHE-CHAOS] Chromium, "QUIC Chaos Protector implementation", n.d., . Acknowledgments The author thanks the authors and editors of the IETF specifications, operational documents, and measurement studies cited in this document. References Author's Address Yuki Goto independent Email: minami.hiroy@gmail.com Additional contact information: 後藤ゆき independent Goto Expires 2 April 2027 [Page 19]