Draft: draft-ietf-tls-ticketrequests

Commentary: If they do n't enforce a limit , clients can cause them to do a lot of work . 
 Fixed by . 
 Approved pending
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.
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 can 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.
As per RFC5077, and as described in RFC8446, TLS servers vend 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 can impact future
connection establishment.  For example, clients can open multiple TLS
connections to the same server for HTTP, or 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
can be used by clients to express their desired number of session
tickets.  Servers can 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.
Connection racing: Happy Eyeballs V2 RFC8305 describes techniques
for performing connection racing.  The Transport Services
Architecture implementation from TAPS also describes how
connections may race across interfaces and address families.  In
cases where clients have early data to send and want to minimize
or avoid ticket re-use, unique tickets for each unique connection
attempt are useful.  Moreover, as some servers may implement
single-use tickets (and even session ticket encryption keys),
distinct tickets will be needed to prevent premature ticket
invalidation by racing.
Connection racing: Happy Eyeballs V2 RFC8305 describes techniques
for performing connection racing.  The Transport Services
Architecture implementation from TAPS also describes how
connections can race across interfaces and address families.  In
cases where clients have early data to send and want to minimize
or avoid ticket re-use, unique tickets for each unique connection
attempt are useful.  Moreover, as some servers may implement
single-use tickets (and even session ticket encryption keys),
distinct tickets will be needed to prevent premature ticket
invalidation by racing.
Less ticket waste: Currently, TLS servers use application-
specific, and often implementation-specific, logic to determine
how many tickets to issue.  By moving the burden of ticket count
to clients, servers do not generate wasteful tickets for clients.
Moreover, as ticket generation may involve expensive computation,
e.g., public key cryptographic operations, avoiding waste is
desirable.
Decline resumption: Clients may indicate they have no intention of
resuming connections by sending a ticket request with count of
zero.
A supporting server MAY vend TicketRequestContents.count
NewSessionTicket messages to a requesting client, and SHOULD NOT send
more than TicketRequestContents.count NewSessionTicket messages to a
requesting client.  Servers SHOULD place a limit on the number of
tickets they are willing to vend to clients.  Thus, the number of
NewSessionTicket messages sent should be the minimum of the server's
self-imposed limit and TicketRequestContents.count.  Servers MUST NOT
send more than 255 tickets to clients.
Clients MUST NOT change the value of TicketRequestContents.count in
second ClientHello messages sent in response to a HelloRetryRequest.
Less ticket waste: Currently, TLS servers use application-
specific, and often implementation-specific, logic to determine
how many tickets to issue.  By moving the burden of ticket count
to clients, servers do not generate wasteful tickets.  As an
example, clients might only request one ticket during resumption.
Moreover, as ticket generation might involve expensive
computation, e.g., public key cryptographic operations, avoiding
waste is desirable.
Decline resumption: Clients can indicate they have no intention of
resuming connections by sending a ticket request with count of
zero.
A supporting server MAY use TicketRequestContents.count when
determining how many NewSessionTicket messages to send to a
requesting client, and SHOULD place a limit on the number of tickets
sent.  The number of NewSessionTicket messages sent SHOULD be the
minimum of the server's self-imposed limit and
TicketRequestContents.count.
If a client receives a HelloRetryRequest, the presence (or absence)
of the "ticket_request" extension MUST be maintained in the second
ClientHello message.  Moreover, if this extension is present, a
client MUST NOT change the value of TicketRequestContents.count in
the second ClientHello message.
Servers that do not enforce a limit on the number of NewSessionTicket
messages sent in response to a "ticket_request" extension could leave
themselves open to DoS attacks, especially if ticket creation is
expensive.