AS2-Version: 1.0  - All implementations of this specification MUST support and
                       advertise "AS2-Version: 1.0". Versions in the range "1.0"
                       through "1.9" MAY be used. All implementations MUST interpret
                       any value in that range as conforming to this specification,
                       with no differences in baseline behavior. In other words, only
                       the major version digit ("1") defines compatibility for implementations
                       that do not support additional, non-AS2-specified functionality.

                       Implementations MAY use "1.1" through "1.9" to signal extensions of
                       this specification. Any such extensions MUST be fully transparent to
                       implementations that recognize only "AS2-Version: 1.0".

   AS2-Version: 1.1  - Designates those implementations that MUST support compression as
                       defined by RFC 3274.

   AS2-Version: 1.2  - Indicates those implementations that include an EDIINT-Features header
                       as defined in RFC 6017. The values in an EDIINT-Features header specify
                       the features supported by the AS2 implementation. Examples may include
                       CEM, AS2-Reliability and multiple-attachments, however others may also be included.
                       A receiving implementation MUST NOT fail if it does not support or understand
                       any of the supported values contained within an EDIINT-Features header.

   AS2-Version: 1.3  - Indicates those implementations that support the modernization defined
                       by this specification, including updated algorithm requirements (e.g.,
                       SHA-256 for MIC/signatures; AES as the encryption baseline per [RFC8551]),
                       alignment with MDN handling as specified in [RFC8098], and support for
                       multiple-recipient encryption as described in [Section 7.2](#encryption-algorithms){:format="none"}
                       of this specification.

                       When both partners are configured for AS2 version 1.3, weak algorithms (e.g., SHA-1,
                       3DES, RC2, MD5) MUST NOT be generated by conformant implementations. When interoperating
                       with a legacy partner that operates at AS2 version 1.2 or lower, implementations MAY apply
                       the legacy interoperability clarifications described in Appendix B (non-normative).

                       Future minor versions (1.x) may designate additional extensions or clarifications
                       that remain backward-compatible with AS2 version 1.0. A major version update
                       (2.0 or higher) would indicate a non-backward-compatible revision.