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

State Posted
Posted Date 2015-03-04
From Group ITU-T-SG-16-Q3
From Contact Christian Groves
To Group tls
To Contacts spt
Joseph Salowey
CcStephen Farrell
Kathleen Moriarty
Transport Layer Security Discussion List
Response Contact Christian.Groves@nteczone.com
Purpose For action
Deadline 2015-06-08 Action Needed
Attachments LS/r on TLS and DTLS terminology – Follow-up comments and further questions for clarification (Ref: IETF No. 1379) [to IETF, 3GPP]
Body
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.