Change:
If they don't enforce a limit, clients can cause them to do a lot of work.
Fixed by .
Approved pending

Discussion:
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 <del> which clients may specify the desired number of tickets needed for </del> <ins> which clients can specify the desired number of tickets needed for </ins> 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

1. <del> As per RFC5077, and as described in RFC8446, TLS servers send clients </del> <ins> As per RFC5077, and as described in RFC8446, TLS servers vend clients </ins> 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 <del> 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 </del> <ins> 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 </ins> 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 <del> 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 </del> <ins> 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 </ins> 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 <del> connections may race across interfaces and address families.  In </del> <ins> connections can race across interfaces and address families.  In </ins> 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

distinct tickets will be needed to prevent premature ticket invalidation by racing. <del> Connection priming: In some systems, connections may be primed or </del> <ins> Connection priming: In some systems, connections can be primed or </ins> bootstrapped by a centralized service or daemon for faster connection establishment.  Requesting tickets on demand allows such services to vend tickets to clients to use for accelerated

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 <del> 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. </del> <ins> 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. </ins> <del> Decline resumption: Clients may indicate they have no intention of </del> <ins> Decline resumption: Clients can indicate they have no intention of </ins> resuming connections by sending a ticket request with count of zero. 3. <del> Clients may indicate to servers their desired number of tickets for a </del> <ins> Clients can indicate to servers their desired number of tickets for a </ins> single connection via the following "ticket_request" extension: <del> Clients may send this extension in ClientHello.  It contains the </del> <ins> Clients MAY send this extension in ClientHello.  It contains the </ins> following structure: The number of tickets desired by the client. <del> 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. </del> <ins> 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. </ins> Servers that support ticket requests MUST NOT echo "ticket_request" in the EncryptedExtensions message.  A client MUST abort the connection with an "illegal_parameter" alert if the "ticket_request" extension is present in the EncryptedExtensions message. <del> Clients MUST NOT change the value of TicketRequestContents.count in second ClientHello messages sent in response to a HelloRetryRequest. </del> <ins> 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. </ins> 4.

dispose of pooled tickets after some reasonable amount of time that mimics the ticket rotation period. <ins> 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. </ins> 6. The authors would like to thank David Benjamin, Eric Rescorla, Nick <del> Sullivan, Martin Thomson, and other members of the TLS Working Group for discussions on earlier versions of this draft. </del> <ins> Sullivan, Martin Thomson, Hubert Kario, and other members of the TLS Working Group for discussions on earlier versions of this draft. </ins>