tls E. Sy
Internet-Draft University of Hamburg
Intended status: Experimental March 01, 2019
Expires: September 2, 2019
TLS Resumption across Server Name Indications for TLS 1.3
draft-sy-tls-resumption-group-00
Abstract
This document defines a mechanism for resuming a TLS 1.3 session
across different Server Name Indications.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Overview on Resumptions across SNI values . . . . . . . . . . 3
4. The "resumption_group" Extension . . . . . . . . . . . . . . 3
4.1. Client Behavior . . . . . . . . . . . . . . . . . . . . . 3
4.2. Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
5. Expectations on Certificates . . . . . . . . . . . . . . . . 5
6. Compatibility Issues with Middleboxes . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Most web transactions are short transfers that are significantly
delayed by the TLS connection establishment. To accelerate the
connection establishment, TLS 1.3 [RFC8446] and its predecessors
provide session resumption mechanisms. They abbreviate the TLS
handshake based on a shared secret exchanged during a prior TLS
session between client and server. In total, these resumption
handshakes significantly reduce computational overhead for
cryptographic operations and save up to one round-trip compared to
the full TLS connection establishment.
TLS 1.3 [RFC8446] allows resumption handshakes across Server Name
Indications (SNIs) when they share the same TLS certificate.
However, TLS 1.3 recommends not to use TLS resumptions across SNIs to
avoid loosing a single-use ticket in case of a failed resumption
attempt. This practice requires costly full TLS connection
establishments in situations where a performance-optimized resumption
handshake across SNI values would be possible. To illustrate this
performance limitation, we describe the common situation of a
redirected web request. We assume that the hostname example.com
redirects to www.example.com and both hostnames are operated by the
same entity and use the same certificate for their authentication. A
client requesting www.example.com via this redirect requires two full
TLS handshakes following the recommendation of TLS 1.3 [RFC8446].
Using resumption across SNI values, the later full handshake can be
converted to a performance-optimized resumed handshake. A
comprehensive study of the performance benefits of resumptions across
SNI values for popular websites can be found in [PERF].
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This document defines a mechanism to inform the client in between
which SNI values TLS resumptions are supported. This information
enables the client to use resumption across SNI values only in
situations where the chance of a successful resumption handshake is
high.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Overview on Resumptions across SNI values
When a client wants to form a TLS connection to a server, it
indicates support for the "resumption_group" extension in the
ClientHello message. To signal its support for this extension type,
the server returns the "resumption_group" extension with an empty
data field.
The client is now aware, that all SNI values for which the presented
server certificate is valid, form a TLS resumption group. Thus,
resumption tickets issued by a group member are designated to be used
to establish resumed connections to any member of the same group.
4. The "resumption_group" Extension
This extension carries no data as defined in the following
ResumptionGroup structure:
struct {
} ResumptionGroup;
4.1. Client Behavior
To indicate support for the "resumption_group" extension, the client
sends this extension type within the initial ClientHello message to
the server.
Upon receiving the server's response, the client checks whether the
"resumption_group" extension is present in the extension list of the
server's CertificateEntry (see Section 4.2.2 of [RFC6066]).
If this extension type is not included in the response of the server,
then the client reasons that the server is not configured to support
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the "resumption_group" extension and proceeds with a normal
handshake.
Otherwise, the client proceeds with a normal connection establishment
and associates all retrieved resumption tickets to the corresponding
resumption group. This resumption group is formed of all SNI values
that are valid for the presented server certificate.
To establish a resumed connection to any SNI value included in a
resumption group, the client uses a resumption ticket associated to
the same group. The Client Hello of a resumed handshake MUST NOT
include the "resumption_group" extension.
Tickets received during a resumed connection MUST be associated to
the same resumption group of the ticket that was used during the
establishment of this connection.
If a SNI value is a member of multiple resumption groups, then the
client is recommended to use the freshest valid ticket for a
resumption handshake. It is assumed, that fresher resumption tickets
are more likely to be accepted by the respective server.
According to [RFC8446], clients MUST NOT cache tickets longer than
seven days.
Note, that TLS resumption enables a server to link resumed
connections to the same client. A study on the feasibility of this
tracking mechanism can be found in [TRAC]. To protect the client's
privacy against tracking via this mechanism, it is RECOMMENDED to
cache resumption tickets only for ten minutes.
4.2. Server Behavior
Upon receiving an initial Client Hello message, the server validates
if the client provided an extension of the type "resumption_group".
If the "resumption_group" extension is not listed by the client, then
the server's response MUST NOT include an entry for this extension
type. Otherwise, the server includes the "resumption_group"
extension in the extension list of the server's CertificateEntry, to
signal support for resumptions across SNI values. Subsequently, the
server proceeds with a normal handshake.
This extension type does not affect the server behavior for resumed
connection establishments.
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5. Expectations on Certificates
This "resumption_group" extension forms the resumption group based on
the SNI values that are valid for the server's certificate. To
optimize the performance benefit of this extension, the server's
certificate is RECOMMENDED to only include SNI values that mutually
support the resumption of their TLS connections. Otherwise, the
client's resumption attempt across SNI values will fail if the server
does not support this practice. Note, that each failed resumption
handshake uses up a single-use resumption ticket. As a result, these
failed attempts might use up all cached single-use tickets, which
hinders the client to establish performance-optimized resumption
handshakes to legitimate SNI values.
6. Compatibility Issues with Middleboxes
[RFC8446]; Section 9.3 requires MITM proxies to remove any extensions
they do not understand. If a conformant MITM proxy does not support
this extension, it will remove this extension type from the Client
Hello. As a result, the server reacts as if it is not supporting
this extension type.
7. Security Considerations
Clients MUST only resume to a new SNI value if this SNI value is
valid for the server certificate presented in the original
connection. To facilitate a correct implementation of this
requirement, the resumption group is identical to the list of SNI
values valid for a specific server certificate. Note, that the
security of TLS resumptions across different SNI values is also
discussed in Section 4.6.1 of [RFC8446].
8. IANA Considerations
TODO IANA needs to be requested to create an entry, resumption_group,
in the existing registry for ExtensionType (defined in [RFC8446]),
with "TLS 1.3" column values being set to "CH, EE", and "Recommended"
column being set to "Yes".
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
9.2. Informative References
[PERF] Sy, E., Moennich, M., Mueller, T., Federrath, H., and M.
Fischer, "Enhanced Performance for the encrypted Web
through TLS Resumption across Hostnames", 2019,
<https://arxiv.org/pdf/1902.02531.pdf>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[TRAC] Sy, E., Burkert, C., Federrath, H., and M. Fischer,
"Tracking Users across the Web via TLS Session
Resumption", 2018, <https://arxiv.org/pdf/1810.07304.pdf>.
Acknowledgments
Tobias Mueller and Christian Burkert provided ideas for this
document.
Author's Address
Erik Sy
University of Hamburg
Email: tls@erik-sy.de
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