Original text:
extension has been negotiated.  Recall that (as discussed previously) each peer chooses the CID value it will receive and use to identify the connection, so an implementation can choose to <del> always recieve CIDs of a fixed length.  If, however, an </del>  implementation chooses to receive different lengths of CID, the assigned CID values must be self-delineating since there is no other mechanism available to determine what connection (and thus, 5.1. The following MAC algorithm applies to block ciphers that do not use <del> the with Encrypt-then-MAC processing described in RFC7366. </del>  The rationale behind this construction is to separate the MAC input for DTLS without the connection ID from the MAC input with the update mechanism in DTLS 1.2 makes this extension unsuitable for mobility scenarios where correlation must be considered.  Deployments that use DTLS in multi-homing environments and are concerned about <del> this aspects SHOULD refuse to use CIDs in DTLS 1.2 and switch to DTLS 1.3 where a CID update mechanism is provided and sequence number </del>  encryption is available. The specification introduces record padding for the CID-enhanced

Reviewer comments:
Tom Petch's review: URL
URL Some editorial quirks s.2 lacks the boiler plate of RFC8174 s.3 I found this unclear until I had understood it all (or perhaps I do not understand it) "URL again, alternately, to use a zero-length CID)." This suggests that a zero length CID is valid in Application Data which later text seems to contradict; otherwise I cannot see what this is saying. " If DTLS peers have negotiated the use of a CIDs using the ClientHello and the ServerHello messages " arguably sending a zero CID and receiving a zero CID is a successful Hello negotiation perhaps " If DTLS peers have negotiated the use of a non-zero CID in at least one direction, using the ClientHello and the ServerHello messages" "The DTLS peers determine whether incoming and outgoing messages need.." seems not to cater for unidirectional CIDs; perhaps "The DTLS peers determine whether incoming or outgoing, or both, messages need.. " s.4 /always recieve CIDs/always receive CIDs/ s.5.1 "the with Encrypt-then-MAC processing described in [RFC7366]." I do not understand why 'with' is needed s.5.2 ditto s.8 /this aspects SHOULD refuse/these aspects SHOULD refuse/ s.10 I would find this clearer as three sections for the three IANA actions new column for ExtensionType new value for ExtensionType new value for ContentType " IANA is requested to allocate an entry to the existing TLS "ExtensionType Values" registry, defined in [RFC5246],.." well no; whatever you think of RFC8447 the name has changed " IANA is requested to allocate an entry to the existing "TLS ExtensionType Values" registry, defined in [RFC5246],.." or, if you are picky (which I am not), IANA is requested to allocate an entry to the existing "TLS "ExtensionType Values" registry, defined in [RFC5246], and renamed by RFC8447 An extra column is added but I cannot see what value should be placed in that column for existing entries. "The tls12_cid ContentType is only applicable to DTLS 1.2." Good information but I struggle to see what IANA will do with it; I see nowhere for it to go. Tom Petch >

Revised text:
extension has been negotiated.  Recall that (as discussed previously) each peer chooses the CID value it will receive and use to identify the connection, so an implementation can choose to  <ins> always receive CIDs of a fixed length.  If, however, an </ins> implementation chooses to receive different lengths of CID, the assigned CID values must be self-delineating since there is no other mechanism available to determine what connection (and thus, 5.1. The following MAC algorithm applies to block ciphers that do not use  <ins> the Encrypt-then-MAC processing described in RFC7366. </ins> The rationale behind this construction is to separate the MAC input for DTLS without the connection ID from the MAC input with the update mechanism in DTLS 1.2 makes this extension unsuitable for mobility scenarios where correlation must be considered.  Deployments that use DTLS in multi-homing environments and are concerned about  <ins> these aspects SHOULD refuse to use CIDs in DTLS 1.2 and switch to DTLS 1.3 where a CID update mechanism is provided and sequence number </ins> encryption is available. The specification introduces record padding for the CID-enhanced