Liaison statement
LS/r on TLS and DTLS terminology – Follow-up comments and further questions for clarification (Ref: IETF No. 1379) [to IETF, 3GPP]

ITU-T Q3/16 thanks you for your reply on our LS request (IETF No. 1379, our
TD284/Gen). We would like to summarize our understanding by providing
definitions and indicating some follow-up questions, which are related to
(D)TLS protocol handling from the perspective of a decomposed gateway.

The text in italics relates to your reply. Follow-up questions are inserted:

Thanks for your questions about (D)TLS.  What follows is a response to your
questions; questions proceeded by Q# and answers by A#.  If you have additional
question please let us know.

Q1.

[What is] the distinction between (D)TLS session and (D)TLS connection (which
implies a definition for each term, beyond the available descriptions /
glossary from RFC side).

A1.

A TLS session is shared cryptographic state established by a full TLS handshake
between two peers.  The session's cryptographic state is used to establish the
cryptographic key material for a TLS connection.  A TLS connection is a
transport relationship established between two peers that contains the
cryptographic state to cryptographically protect data sent and received on the
connection established through a full or abbreviated (session resumption) TLS
handshake.  A TLS session may span one or more TLS connections.  Every TLS
connection is associated with one TLS session.  If session resumption
(abbreviated handshake) is not used then the TLS session and TLS connection
will essentially be the same. A TLS session is identified by a Session ID in
the client and server hellos. A TLS connection is usually identified by a host
addresses and port numbers.

Reply 1 (ITU-T):

a) TLS session: Thus, there is a "TLS session cryptographic state" object,
which is shared by the two TLS session endpoints at client and server side. In
our understanding the TLS client session endpoint state and the TLS server
session endpoint state information largely overlaps (i.e., identical
parameter-value pairs), but there is one additional parameter (the "server
address") at client side. Is this correct?

Our understanding is that based on your description above "resumable TLS
session" could be defined as follows

Resumable TLS session: The pair of a resumable TLS client session endpoint
state and a resumable TLS server session endpoint state, coupled by the TLS
full handshake procedure which was executed across the associated TLS
connection.

b) TLS connection: the TLS connection endpoints at client and server side could
be described by a n-tuple if the TLS connection represents a "transport
relationship" according to your description.

c) TLS connection identifier: the following definition could be established
based on your explanation ("A TLS connection is usually identified by a host
addresses and port numbers"):

TLS connection endpoint identifier: The local IP transport address and
indication of "TLS/L4" protocol stack, i.e., the 4-tuple of {AL, PL, T, "TLS"}
from the TLS connection endpoint.

NOTE 1 – Parameter "L4" is required because TLS is a L4 independent protocol.

d) Ratio between TLS session to TLS connection: there is a ratio of 1:N.

e) TLS session resumption: according to your explanation, resumption is a)
correlated with an abbreviated handshake and b) affecting the TLS connection,
hence we might define:

TLS session resumption: A TLS session level concept which represents the
execution of a TLS abbreviated handshake procedure on an existing TLS session,
i.e., a semi-permanent TLS session, for the purpose of either deriving a
further TLS connection or updating of an existing TLS connection.

Q2.

The DTLS association concept, e.g., is it equivalent to a DTLS session or DTLS
connection or something in addition?

A2.

The DTLS association is the same as a DTLS connection

Reply 2 (ITU-T):

Thanks for confirmation. We had the same interpretation based on the TLS
related RFCs.

Q3.

The TLS renegotiation procedure: what is the definition and at which level (TLS
session or TLS connection level) does this procedure occur?

Q4.

The TLS resumption procedure: what is the definition and relation to TLS
renegotiation?

A3 and A4:

Resumption refers to the use of the state from an existing TLS session to
establish a new TLS connection using an abbreviated handshake. During
resumption the cryptographic parameters (algorithms etc.) remain the same.  TLS
renegotiation is the process of executing a new TLS handshake to establish new
cryptographic parameters for a TLS connection (effectively a new TLS connection
using the same host addresses and ports as the previous one). If the handshake
is a full handshake then both a new session and a new connection are
established and the renegotiated session may have different parameters.

Please note that session resumption and renegotiation are optional features;
though TLS 1.2 recommended support for renegotiation; renegotiation was also
updated by RFC 5746.  Please note that TLS 1.3 is currently under development
and these features are being reviewed.

Reply 3/4 (ITU-T):

In order to emphasize the difference between TLS session resumption and TLS
session renegotiation, we would summarize your description as follows:

TLS session renegotiation: A TLS session level concept which leads, - by the
execution of a TLS handshake procedure (full or abbreviated) -, to the update
of TLS protocol status information of an already established TLS connection,
for the purpose of the establishment of new cryptographic parameters.

Could you please confirm or clarify our understanding? Q3/16 is appreciative
for your cooperation.