Draft: draft-ietf-tls-ticketrequests

Commentary: Approved pending minor changes
TLS session tickets enable stateless connection resumption for
clients without server-side per-client state.  Servers vend session
tickets to clients, at their discretion, upon connection
establishment.  Clients store and use tickets when resuming future
connections.  Moreover, clients should use tickets at most once for
session resumption, especially if such keying material protects early
application data.  Single-use tickets bound the number of parallel
connections a client may initiate by the number of tickets received
from a given server.  To address this limitation, this document
describes a mechanism by which clients may specify the desired number
of tickets needed for future connections.
As per RFC5077, and as described in RFC8446, TLS servers send clients
session tickets at their own discretion in NewSessionTicket messages.
Clients are in complete control of how many tickets they may use when
establishing future and subsequent connections.  For example, clients
may open multiple TLS connections to the same server for HTTP, or may
race TLS connections across different network interfaces.  The latter
is especially useful in transport systems that implement Happy
Eyeballs RFC8305.  Since connection concurrency and resumption is
controlled by clients, a standard mechanism to request more than one
ticket is desirable.
This document specifies a new TLS extension - ticket_request - that
may be used by clients to express their desired number of session
tickets.  Servers may use this extension as a hint of the number of
NewSessionTicket messages to vend.  This extension is only applicable
to TLS 1.3 RFC8446 and future versions of TLS.
TLS session tickets enable stateless connection resumption for
clients without server-side, per-client state.  Servers vend an
arbitrary number of session tickets to clients, at their discretion,
upon connection establishment.  Clients store and use tickets when
resuming future connections.  This document describes a mechanism by
which clients may specify the desired number of tickets needed for
future connections.  This extension aims to provide a means for
servers to determine the number of tickets to generate in order to
reduce ticket waste, while simultaneously priming clients for future
connection attempts.
As per RFC5077, and as described in RFC8446, TLS servers send clients
an arbitrary number of session tickets at their own discretion in
NewSessionTicket messages.  There are two limitations with this
design.  First, servers choose some (often hard-coded) number of
tickets vended per connection.  Second, clients do not have a way of
expressing their desired number of tickets, which may impact future
connection establishment.  For example, clients may open multiple TLS
connections to the same server for HTTP, or may race TLS connections
across different network interfaces.  The latter is especially useful
in transport systems that implement Happy Eyeballs RFC8305.  Since
clients control connection concurrency and resumption, a standard
mechanism for requesting more than one ticket is desirable.
This document specifies a new TLS extension - "ticket_request" - that
may be used by clients to express their desired number of session
tickets.  Servers may use this extension as a hint of the number of
NewSessionTicket messages to vend.  This extension is only applicable
to TLS 1.3 RFC8446, DTLS 1.3 I-D.ietf-tls-dtls13, and future versions
thereof.
IANA is requested to Create an entry, ticket_requests(TBD), in the
existing registry for ExtensionType (defined in RFC8446), with "TLS
1.3" column values being set to "CH", and "Recommended" column being
set to "Yes".
Ticket re-use is a security and privacy concern.  Moreover, ticket
pooling as a means of avoiding or amortizing handshake costs must be
used carefully.  If servers do not rotate session ticket encryption
keys frequently, clients may be encouraged to obtain and use tickets
beyond common lifetime windows of, e.g., 24 hours.  Despite ticket
lifetime hints provided by servers, clients SHOULD dispose of pooled
tickets after some reasonable amount of time that mimics the ticket
rotation period.
IANA is requested to Create an entry, ticket_request(TBD), in the
existing registry for ExtensionType (defined in RFC8446), with "TLS
1.3" column values being set to "CH", and "Recommended" column being
set to "Yes".
Ticket re-use is a security and privacy concern.  Moreover, clients
must take care when pooling tickets as a means of avoiding or
amortizing handshake costs.  If servers do not rotate session ticket
encryption keys frequently, clients may be encouraged to obtain and
use tickets beyond common lifetime windows of, e.g., 24 hours.
Despite ticket lifetime hints provided by servers, clients SHOULD
dispose of pooled tickets after some reasonable amount of time that
mimics the ticket rotation period.