Change:
Inspired by NAME . We can go further and just completely define EncodedClientHelloInner first.

Discussion:
5.1. <del> Some TLS 1.3 extensions can be quite large, thus repeating them in the ClientHelloInner and ClientHelloOuter can lead to an excessive overall size.  One pathological example is "key_share" with post- quantum algorithms.  To reduce the impact of duplicated extensions, the client may use the "ech_outer_extensions" extension. OuterExtensions consists of one or more ExtensionType values, each of which reference an extension in ClientHelloOuter.  The extensions in OuterExtensions MUST appear in ClientHelloOuter in the same relative order, however, there is no requirement that they be continguous. For example, OuterExtensions may contain extensions A, B, C, while ClientHelloOuter contains extensions A, D, B, C, E, F. The "ech_outer_extensions" extension is only used for compressing the ClientHelloInner.  It can only be included in EncodedClientHelloInner, and MUST NOT be sent in either ClientHelloOuter or ClientHelloInner. When sending ClientHello, the client first computes ClientHelloInner, including any PSK binders.  It then computes a new value, the EncodedClientHelloInner, which is the following structure: </del> <ins> Before encrypting, the client pads and optionally compresses ClientHelloInner into a EncodedClientHelloInner structure, defined below: </ins> The "client_hello" field is computed by first making a copy of ClientHelloInner and setting the "legacy_session_id" field to the

Handshake structure's four byte header.  The "zeros" field MUST be all zeroes. <del> The client then MAY substitute extensions which it knows will be duplicated in ClientHelloOuter.  To do so, the client removes and replaces extensions from EncodedClientHelloInner with a single "ech_outer_extensions" extension.  Removed extensions MUST be ordered consecutively in ClientHelloInner.  The list of outer extensions, OuterExtensions, includes those which were removed from EncodedClientHelloInner, in the order in which they were removed. </del> <ins> Repeating large extensions, such as "key_share" with post-quantum algorithms, between ClientHelloInner and ClientHelloOuter can lead to excessive size.  To reduce the size impact, the client MAY substitute extensions which it knows will be duplicated in ClientHelloOuter.  It does so by removing and replacing extensions from EncodedClientHelloInner with a single "ech_outer_extensions" extension, defined as follows: OuterExtensions contains the removed ExtensionType values.  Each value references the matching extension in ClientHelloOuter.  The values MUST be ordered contiguously in ClientHelloInner, and the "ech_outer_extensions" extension MUST be inserted in the corresponding position in EncodedClientHelloInner.  Additionally, the extensions MUST appear in ClientHelloOuter in the same relative order.  However, there is no requirement that they be contiguous. For example, OuterExtensions may contain extensions A, B, C, while ClientHelloOuter contains extensions A, D, B, C, E, F. The "ech_outer_extensions" extension can only be included in EncodedClientHelloInner, and MUST NOT appear in either ClientHelloOuter or ClientHelloInner. </ins> Finally, the client pads the message by setting the "zeros" field to a byte string whose contents are all zeros and whose length is the