Security protocol survey: A Survey of the Interaction between Security Protocols and Transport Services

Internet Engineering Task Force (IETF)                       T. Enghardt
Request for Comments: 8922                                     TU Berlin
Category: Informational                                         T. Pauly
ISSN: 2070-1721                                               Apple Inc.
                                                              C. Perkins
                                                   University of Glasgow
                                                                 K. Rose
                                               Akamai Technologies, Inc.
                                                                 C. Wood
                                                              Cloudflare
                                                            October 2020


  A Survey of the Interaction between Security Protocols and Transport
                                Services

Abstract

   This document provides a survey of commonly used or notable network
   security protocols, with a focus on how they interact and integrate
   with applications and transport protocols.  Its goal is to supplement
   efforts to define and catalog Transport Services by describing the
   interfaces required to add security protocols.  This survey is not
   limited to protocols developed within the scope or context of the
   IETF, and those included represent a superset of features a Transport
   Services system may need to support.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8922.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Goals
     1.2.  Non-goals
   2.  Terminology
   3.  Transport Security Protocol Descriptions
     3.1.  Application Payload Security Protocols
       3.1.1.  TLS
       3.1.2.  DTLS
     3.2.  Application-Specific Security Protocols
       3.2.1.  Secure RTP
     3.3.  Transport-Layer Security Protocols
       3.3.1.  IETF QUIC
       3.3.2.  Google QUIC
       3.3.3.  tcpcrypt
       3.3.4.  MinimaLT
       3.3.5.  CurveCP
     3.4.  Packet Security Protocols
       3.4.1.  IPsec
       3.4.2.  WireGuard
       3.4.3.  OpenVPN
   4.  Transport Dependencies
     4.1.  Reliable Byte-Stream Transports
     4.2.  Unreliable Datagram Transports
       4.2.1.  Datagram Protocols with Defined Byte-Stream Mappings
     4.3.  Transport-Specific Dependencies
   5.  Application Interface
     5.1.  Pre-connection Interfaces
     5.2.  Connection Interfaces
     5.3.  Post-connection Interfaces
     5.4.  Summary of Interfaces Exposed by Protocols
   6.  IANA Considerations
   7.  Security Considerations
   8.  Privacy Considerations
   9.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   Services and features provided by transport protocols have been
   cataloged in [RFC8095].  This document supplements that work by
   surveying commonly used and notable network security protocols, and
   identifying the interfaces between these protocols and both transport
   protocols and applications.  It examines Transport Layer Security
   (TLS), Datagram Transport Layer Security (DTLS), IETF QUIC, Google
   QUIC (gQUIC), tcpcrypt, Internet Protocol Security (IPsec), Secure
   Real-time Transport Protocol (SRTP) with DTLS, WireGuard, CurveCP,
   and MinimaLT.  For each protocol, this document provides a brief
   description.  Then, it describes the interfaces between these
   protocols and transports in Section 4 and the interfaces between
   these protocols and applications in Section 5.

   A Transport Services system exposes an interface for applications to
   access various (secure) transport protocol features.  The security
   protocols included in this survey represent a superset of
   functionality and features a Transport Services system may need to
   support both internally and externally (via an API) for applications
   [TAPS-ARCH].  Ubiquitous IETF protocols such as (D)TLS, as well as
   non-standard protocols such as gQUIC, are included despite
   overlapping features.  As such, this survey is not limited to
   protocols developed within the scope or context of the IETF.  Outside
   of this candidate set, protocols that do not offer new features are
   omitted.  For example, newer protocols such as WireGuard make unique
   design choices that have implications for and limitations on
   application usage.  In contrast, protocols such as secure shell (SSH)
   [RFC4253], GRE [RFC2890], the Layer 2 Tunneling Protocol (L2TP)
   [RFC5641], and Application Layer Transport Security (ALTS) [ALTS] are
   omitted since they do not provide interfaces deemed unique.

   Authentication-only protocols such as the TCP Authentication Option
   (TCP-AO) [RFC5925] and the IPsec Authentication Header (AH) [RFC4302]
   are excluded from this survey.  TCP-AO adds authentication to long-
   lived TCP connections, e.g., replay protection with per-packet
   Message Authentication Codes.  (TCP-AO obsoletes TCP MD5 "signature"
   options specified in [RFC2385].)  One primary use case of TCP-AO is
   for protecting BGP connections.  Similarly, AH adds per-datagram
   authentication and integrity, along with replay protection.  Despite
   these improvements, neither protocol sees general use and both lack
   critical properties important for emergent transport security
   protocols, such as confidentiality and privacy protections.  Such
   protocols are thus omitted from this survey.

   This document only surveys point-to-point protocols; multicast
   protocols are out of scope.

1.1.  Goals

   This survey is intended to help identify the most common interface
   surfaces between security protocols and transport protocols, and
   between security protocols and applications.

   One of the goals of the Transport Services effort is to define a
   common interface for using transport protocols that allows software
   using transport protocols to easily adopt new protocols that provide
   similar feature sets.  The survey of the dependencies security
   protocols have upon transport protocols can guide implementations in
   determining which transport protocols are appropriate to be able to
   use beneath a given security protocol.  For example, a security
   protocol that expects to run over a reliable stream of bytes, like
   TLS, restricts the set of transport protocols that can be used to
   those that offer a reliable stream of bytes.

   Defining the common interfaces that security protocols provide to
   applications also allows interfaces to be designed in a way that
   common functionality can use the same APIs.  For example, many
   security protocols that provide authentication let the application be
   involved in peer identity validation.  Any interface to use a secure
   transport protocol stack thus needs to allow applications to perform
   this action during connection establishment.

1.2.  Non-goals

   While this survey provides similar analysis to that which was
   performed for transport protocols in [RFC8095], it is important to
   distinguish that the use of security protocols requires more
   consideration.

   It is not a goal to allow software implementations to automatically
   switch between different security protocols, even where their
   interfaces to transport and applications are equivalent.  Even
   between versions, security protocols have subtly different guarantees
   and vulnerabilities.  Thus, any implementation needs to only use the
   set of protocols and algorithms that are requested by applications or
   by a system policy.

   Different security protocols also can use incompatible notions of
   peer identity and authentication, and cryptographic options.  It is
   not a goal to identify a common set of representations for these
   concepts.

   The protocols surveyed in this document represent a superset of
   functionality and features a Transport Services system may need to
   support.  It does not list all transport protocols that a Transport
   Services system may need to implement, nor does it mandate that a
   Transport Service system implement any particular protocol.

   A Transport Services system may implement any secure transport
   protocol that provides the described features.  In doing so, it may
   need to expose an interface to the application to configure these
   features.

2.  Terminology

   The following terms are used throughout this document to describe the
   roles and interactions of transport security protocols (some of which
   are also defined in [RFC8095]):

   Transport Feature:  a specific end-to-end feature that the transport
      layer provides to an application.  Examples include
      confidentiality, reliable delivery, ordered delivery, and message-
      versus-stream orientation.

   Transport Service:  a set of Transport Features, without an
      association to any given framing protocol, that provides
      functionality to an application.

   Transport Services system:  a software component that exposes an
      interface to different Transport Services to an application.

   Transport Protocol:  an implementation that provides one or more
      different Transport Services using a specific framing and header
      format on the wire.  A Transport Protocol services an application,
      whether directly or in conjunction with a security protocol.

   Application:  an entity that uses a transport protocol for end-to-end
      delivery of data across the network.  This may also be an upper
      layer protocol or tunnel encapsulation.

   Security Protocol:  a defined network protocol that implements one or
      more security features, such as authentication, encryption, key
      generation, session resumption, and privacy.  Security protocols
      may be used alongside transport protocols, and in combination with
      other security protocols when appropriate.

   Handshake Protocol:  a protocol that enables peers to validate each
      other and to securely establish shared cryptographic context.

   Record:  framed protocol messages.

   Record Protocol:  a security protocol that allows data to be divided
      into manageable blocks and protected using shared cryptographic
      context.

   Session:  an ephemeral security association between applications.

   Connection:  the shared state of two or more endpoints that persists
      across messages that are transmitted between these endpoints.  A
      connection is a transient participant of a session, and a session
      generally lasts between connection instances.

   Peer:  an endpoint application party to a session.

   Client:  the peer responsible for initiating a session.

   Server:  the peer responsible for responding to a session initiation.

3.  Transport Security Protocol Descriptions

   This section contains brief transport and security descriptions of
   various security protocols currently used to protect data being sent
   over a network.  These protocols are grouped based on where in the
   protocol stack they are implemented, which influences which parts of
   a packet they protect: Generic application payload, application
   payload for specific application-layer protocols, both application
   payload and transport headers, or entire IP packets.

   Note that not all security protocols can be easily categorized, e.g.,
   as some protocols can be used in different ways or in combination
   with other protocols.  One major reason for this is that channel
   security protocols often consist of two components:

   *  A handshake protocol, which is responsible for negotiating
      parameters, authenticating the endpoints, and establishing shared
      keys.

   *  A record protocol, which is used to encrypt traffic using keys and
      parameters provided by the handshake protocol.

   For some protocols, such as tcpcrypt, these two components are
   tightly integrated.  In contrast, for IPsec, these components are
   implemented in separate protocols: AH and the Encapsulating Security
   Payload (ESP) are record protocols, which can use keys supplied by
   the handshake protocol Internet Key Exchange Protocol Version 2
   (IKEv2), by other handshake protocols, or by manual configuration.
   Moreover, some protocols can be used in different ways: While the
   base TLS protocol as defined in [RFC8446] has an integrated handshake
   and record protocol, TLS or DTLS can also be used to negotiate keys
   for other protocols, as in DTLS-SRTP, or the handshake protocol can
   be used with a separate record layer, as in QUIC [QUIC-TRANSPORT].

3.1.  Application Payload Security Protocols

   The following protocols provide security that protects application
   payloads sent over a transport.  They do not specifically protect any
   headers used for transport-layer functionality.

3.1.1.  TLS

   TLS (Transport Layer Security) [RFC8446] is a common protocol used to
   establish a secure session between two endpoints.  Communication over
   this session prevents "eavesdropping, tampering, and message
   forgery."  TLS consists of a tightly coupled handshake and record
   protocol.  The handshake protocol is used to authenticate peers,
   negotiate protocol options such as cryptographic algorithms, and
   derive session-specific keying material.  The record protocol is used
   to marshal and, once the handshake has sufficiently progressed,
   encrypt data from one peer to the other.  This data may contain
   handshake messages or raw application data.

3.1.2.  DTLS

   DTLS (Datagram Transport Layer Security) [RFC6347] [DTLS-1.3] is
   based on TLS, but differs in that it is designed to run over
   unreliable datagram protocols like UDP instead of TCP.  DTLS modifies
   the protocol to make sure it can still provide equivalent security
   guarantees to TLS with the exception of order protection/non-
   replayability.  DTLS was designed to be as similar to TLS as
   possible, so this document assumes that all properties from TLS are
   carried over except where specified.

3.2.  Application-Specific Security Protocols

   The following protocols provide application-specific security by
   protecting application payloads used for specific use cases.  Unlike
   the protocols above, these are not intended for generic application
   use.

3.2.1.  Secure RTP

   Secure RTP (SRTP) is a profile for RTP that provides confidentiality,
   message authentication, and replay protection for RTP data packets
   and RTP control protocol (RTCP) packets [RFC3711].  SRTP provides a
   record layer only, and requires a separate handshake protocol to
   provide key agreement and identity management.

   The commonly used handshake protocol for SRTP is DTLS, in the form of
   DTLS-SRTP [RFC5764].  This is an extension to DTLS that negotiates
   the use of SRTP as the record layer and describes how to export keys
   for use with SRTP.

   ZRTP [RFC6189] is an alternative key agreement and identity
   management protocol for SRTP.  ZRTP Key agreement is performed using
   a Diffie-Hellman key exchange that runs on the media path.  This
   generates a shared secret that is then used to generate the master
   key and salt for SRTP.

3.3.  Transport-Layer Security Protocols

   The following security protocols provide protection for both
   application payloads and headers that are used for Transport
   Services.

3.3.1.  IETF QUIC

   QUIC is a new standards-track transport protocol that runs over UDP,
   loosely based on Google's original proprietary gQUIC protocol
   [QUIC-TRANSPORT] (See Section 3.3.2 for more details).  The QUIC
   transport layer itself provides support for data confidentiality and
   integrity.  This requires keys to be derived with a separate
   handshake protocol.  A mapping for QUIC of TLS 1.3 [QUIC-TLS] has
   been specified to provide this handshake.

3.3.2.  Google QUIC

   Google QUIC (gQUIC) is a UDP-based multiplexed streaming protocol
   designed and deployed by Google following experience from deploying
   SPDY, the proprietary predecessor to HTTP/2.  gQUIC was originally
   known as "QUIC"; this document uses gQUIC to unambiguously
   distinguish it from the standards-track IETF QUIC.  The proprietary
   technical forebear of IETF QUIC, gQUIC was originally designed with
   tightly integrated security and application data transport protocols.

3.3.3.  tcpcrypt

   Tcpcrypt [RFC8548] is a lightweight extension to the TCP protocol for
   opportunistic encryption.  Applications may use tcpcrypt's unique
   session ID for further application-level authentication.  Absent this
   authentication, tcpcrypt is vulnerable to active attacks.

3.3.4.  MinimaLT

   MinimaLT [MinimaLT] is a UDP-based transport security protocol
   designed to offer confidentiality, mutual authentication, DoS
   prevention, and connection mobility.  One major goal of the protocol
   is to leverage existing protocols to obtain server-side configuration
   information used to more quickly bootstrap a connection.  MinimaLT
   uses a variant of TCP's congestion control algorithm.

3.3.5.  CurveCP

   CurveCP [CurveCP] is a UDP-based transport security that, unlike many
   other security protocols, is based entirely upon public key
   algorithms.  CurveCP provides its own reliability for application
   data as part of its protocol.

3.4.  Packet Security Protocols

   The following protocols provide protection for IP packets.  These are
   generally used as tunnels, such as for Virtual Private Networks
   (VPNs).  Often, applications will not interact directly with these
   protocols.  However, applications that implement tunnels will
   interact directly with these protocols.

3.4.1.  IPsec

   IKEv2 [RFC7296] and ESP [RFC4303] together form the modern IPsec
   protocol suite that encrypts and authenticates IP packets, either for
   creating tunnels (tunnel-mode) or for direct transport connections
   (transport-mode).  This suite of protocols separates out the key
   generation protocol (IKEv2) from the transport encryption protocol
   (ESP).  Each protocol can be used independently, but this document
   considers them together, since that is the most common pattern.

3.4.2.  WireGuard

   WireGuard [WireGuard] is an IP-layer protocol designed as an
   alternative to IPsec for certain use cases.  It uses UDP to
   encapsulate IP datagrams between peers.  Unlike most transport
   security protocols, which rely on Public Key Infrastructure (PKI) for
   peer authentication, WireGuard authenticates peers using pre-shared
   public keys delivered out of band, each of which is bound to one or
   more IP addresses.  Moreover, as a protocol suited for VPNs,
   WireGuard offers no extensibility, negotiation, or cryptographic
   agility.

3.4.3.  OpenVPN

   OpenVPN [OpenVPN] is a commonly used protocol designed as an
   alternative to IPsec.  A major goal of this protocol is to provide a
   VPN that is simple to configure and works over a variety of
   transports.  OpenVPN encapsulates either IP packets or Ethernet
   frames within a secure tunnel and can run over either UDP or TCP.
   For key establishment, OpenVPN can either use TLS as a handshake
   protocol or use pre-shared keys.

4.  Transport Dependencies

   Across the different security protocols listed above, the primary
   dependency on transport protocols is the presentation of data: either
   an unbounded stream of bytes, or framed messages.  Within protocols
   that rely on the transport for message framing, most are built to run
   over transports that inherently provide framing, like UDP, but some
   also define how their messages can be framed over byte-stream
   transports.

4.1.  Reliable Byte-Stream Transports

   The following protocols all depend upon running on a transport
   protocol that provides a reliable, in-order stream of bytes.  This is
   typically TCP.

   Application Payload Security Protocols:

   *  TLS

   Transport-Layer Security Protocols:

   *  tcpcrypt

4.2.  Unreliable Datagram Transports

   The following protocols all depend on the transport protocol to
   provide message framing to encapsulate their data.  These protocols
   are built to run using UDP, and thus do not have any requirement for
   reliability.  Running these protocols over a protocol that does
   provide reliability will not break functionality but may lead to
   multiple layers of reliability if the security protocol is
   encapsulating other transport protocol traffic.

   Application Payload Security Protocols:

   *  DTLS

   *  ZRTP

   *  SRTP

   Transport-Layer Security Protocols:

   *  QUIC

   *  MinimaLT

   *  CurveCP

   Packet Security Protocols:

   *  IPsec

   *  WireGuard

   *  OpenVPN

4.2.1.  Datagram Protocols with Defined Byte-Stream Mappings

   Of the protocols listed above that depend on the transport for
   message framing, some do have well-defined mappings for sending their
   messages over byte-stream transports like TCP.

   Application Payload Security Protocols:

   *  DTLS when used as a handshake protocol for SRTP [RFC7850]

   *  ZRTP [RFC6189]

   *  SRTP [RFC4571][RFC3711]

   Packet Security Protocols:

   *  IPsec [RFC8229]

4.3.  Transport-Specific Dependencies

   One protocol surveyed, tcpcrypt, has a direct dependency on a feature
   in the transport that is needed for its functionality.  Specifically,
   tcpcrypt is designed to run on top of TCP and uses the TCP Encryption
   Negotiation Option (TCP-ENO) [RFC8547] to negotiate its protocol
   support.

   QUIC, CurveCP, and MinimaLT provide both transport functionality and
   security functionality.  They depend on running over a framed
   protocol like UDP, but they add their own layers of reliability and
   other Transport Services.  Thus, an application that uses one of
   these protocols cannot decouple the security from transport
   functionality.

5.  Application Interface

   This section describes the interface exposed by the security
   protocols described above.  We partition these interfaces into pre-
   connection (configuration), connection, and post-connection
   interfaces, following conventions in [TAPS-INTERFACE] and
   [TAPS-ARCH].

   Note that not all protocols support each interface.  The table in
   Section 5.4 summarizes which protocol exposes which of the
   interfaces.  In the following sections, we provide abbreviations of
   the interface names to use in the summary table.

5.1.  Pre-connection Interfaces

   Configuration interfaces are used to configure the security protocols
   before a handshake begins or keys are negotiated.

   Identities and Private Keys (IPK):  The application can provide its
      identity, credentials (e.g., certificates), and private keys, or
      mechanisms to access these, to the security protocol to use during
      handshakes.

      *  TLS

      *  DTLS

      *  ZRTP

      *  QUIC

      *  MinimaLT

      *  CurveCP

      *  IPsec

      *  WireGuard

      *  OpenVPN

   Supported Algorithms (Key Exchange, Signatures, and Ciphersuites)
   (ALG):  The application can choose the algorithms that are supported
      for key exchange, signatures, and ciphersuites.

      *  TLS

      *  DTLS

      *  ZRTP

      *  QUIC

      *  tcpcrypt

      *  MinimaLT

      *  IPsec

      *  OpenVPN

   Extensions (EXT):  The application enables or configures extensions
      that are to be negotiated by the security protocol, such as
      Application-Layer Protocol Negotiation (ALPN) [RFC7301].

      *  TLS

      *  DTLS

      *  QUIC

   Session Cache Management (CM):  The application provides the ability
      to save and retrieve session state (such as tickets, keying
      material, and server parameters) that may be used to resume the
      security session.

      *  TLS

      *  DTLS

      *  ZRTP

      *  QUIC

      *  tcpcrypt

      *  MinimaLT

   Authentication Delegation (AD):  The application provides access to a
      separate module that will provide authentication, using the
      Extensible Authentication Protocol (EAP) [RFC3748] for example.

      *  IPsec

      *  tcpcrypt

   Pre-Shared Key Import (PSKI):  Either the handshake protocol or the
      application directly can supply pre-shared keys for use in
      encrypting (and authenticating) communication with a peer.

      *  TLS

      *  DTLS

      *  ZRTP

      *  QUIC

      *  tcpcrypt

      *  MinimaLT

      *  IPsec

      *  WireGuard

      *  OpenVPN

5.2.  Connection Interfaces

   Identity Validation (IV):  During a handshake, the security protocol
      will conduct identity validation of the peer.  This can offload
      validation or occur transparently to the application.

      *  TLS

      *  DTLS

      *  ZRTP

      *  QUIC

      *  MinimaLT

      *  CurveCP

      *  IPsec

      *  WireGuard

      *  OpenVPN

   Source Address Validation (SAV):  The handshake protocol may interact
      with the transport protocol or application to validate the address
      of the remote peer that has sent data.  This involves sending a
      cookie exchange to avoid DoS attacks.  (This list omits protocols
      that depend on TCP and therefore implicitly perform SAV.)

      *  DTLS

      *  QUIC

      *  IPsec

      *  WireGuard

5.3.  Post-connection Interfaces

   Connection Termination (CT):  The security protocol may be instructed
      to tear down its connection and session information.  This is
      needed by some protocols, e.g., to prevent application data
      truncation attacks in which an attacker terminates an underlying
      insecure connection-oriented protocol to terminate the session.

      *  TLS

      *  DTLS

      *  ZRTP

      *  QUIC

      *  tcpcrypt

      *  MinimaLT

      *  IPsec

      *  OpenVPN

   Key Update (KU):  The handshake protocol may be instructed to update
      its keying material, either by the application directly or by the
      record protocol sending a key expiration event.

      *  TLS

      *  DTLS

      *  QUIC

      *  tcpcrypt

      *  MinimaLT

      *  IPsec

   Shared Secret Key Export (SSKE):  The handshake protocol may provide
      an interface for producing shared secrets for application-specific
      uses.

      *  TLS

      *  DTLS

      *  tcpcrypt

      *  IPsec

      *  OpenVPN

      *  MinimaLT

   Key Expiration (KE):  The record protocol can signal that its keys
      are expiring due to reaching a time-based deadline or a use-based
      deadline (number of bytes that have been encrypted with the key).
      This interaction is often limited to signaling between the record
      layer and the handshake layer.

      *  IPsec

   Mobility Events (ME):  The record protocol can be signaled that it is
      being migrated to another transport or interface due to connection
      mobility, which may reset address and state validation and induce
      state changes such as use of a new Connection Identifier (CID).

      *  DTLS (version 1.3 only [DTLS-1.3])

      *  QUIC

      *  MinimaLT

      *  CurveCP

      *  IPsec [RFC4555]

      *  WireGuard

5.4.  Summary of Interfaces Exposed by Protocols

   The following table summarizes which protocol exposes which
   interface.

   +===========+===+====+=====+==+==+======+==+=====+==+==+======+==+==+
   | Protocol  |IPK|ALG | EXT |CM|AD| PSKI |IV| SAV |CT|KU| SSKE |KE|ME|
   +===========+===+====+=====+==+==+======+==+=====+==+==+======+==+==+
   | TLS       | x | x  |  x  |x |  |  x   |x |     |x |x |  x   |  |  |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | DTLS      | x | x  |  x  |x |  |  x   |x |  x  |x |x |  x   |  |x |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | ZRTP      | x | x  |     |x |  |  x   |x |     |x |  |      |  |  |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | QUIC      | x | x  |  x  |x |  |  x   |x |  x  |x |x |      |  |x |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | tcpcrypt  |   | x  |     |x |x |  x   |  |     |x |x |  x   |  |  |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | MinimaLT  | x | x  |     |x |  |  x   |x |     |x |x |  x   |  |x |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | CurveCP   | x |    |     |  |  |      |x |     |  |  |      |  |x |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | IPsec     | x | x  |     |  |x |  x   |x |  x  |x |x |  x   |x |x |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | WireGuard | x |    |     |  |  |  x   |x |  x  |  |  |      |  |x |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+
   | OpenVPN   | x | x  |     |  |  |  x   |x |     |x |  |  x   |  |  |
   +-----------+---+----+-----+--+--+------+--+-----+--+--+------+--+--+

                                  Table 1

   x = Interface is exposed
   (blank) = Interface is not exposed

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

   This document summarizes existing transport security protocols and
   their interfaces.  It does not propose changes to or recommend usage
   of reference protocols.  Moreover, no claims of security and privacy
   properties beyond those guaranteed by the protocols discussed are
   made.  For example, metadata leakage via timing side channels and
   traffic analysis may compromise any protocol discussed in this
   survey.  Applications using Security Interfaces should take such
   limitations into consideration when using a particular protocol
   implementation.

8.  Privacy Considerations

   Analysis of how features improve or degrade privacy is intentionally
   omitted from this survey.  All security protocols surveyed generally
   improve privacy by using encryption to reduce information leakage.
   However, varying amounts of metadata remain in the clear across each
   protocol.  For example, client and server certificates are sent in
   cleartext in TLS 1.2 [RFC5246], whereas they are encrypted in TLS 1.3
   [RFC8446].  A survey of privacy features, or lack thereof, for
   various security protocols could be addressed in a separate document.

9.  Informative References

   [ALTS]     Ghali, C., Stubblefield, A., Knapp, E., Li, J., Schmidt,
              B., and J. Boeuf, "Application Layer Transport Security",
              <https://cloud.google.com/security/encryption-in-transit/
              application-layer-transport-security/>.

   [CurveCP]  Bernstein, D., "CurveCP: Usable security for the
              Internet", <https://curvecp.org/>.

   [DTLS-1.3] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-38, 29 May 2020,
              <https://tools.ietf.org/html/draft-ietf-tls-dtls13-38>.

   [MinimaLT] Petullo, W., Zhang, X., Solworth, J., Bernstein, D., and
              T. Lange, "MinimaLT: minimal-latency networking through
              better security", DOI 10.1145/2508859.2516737,
              <https://dl.acm.org/citation.cfm?id=2516737>.

   [OpenVPN]  OpenVPN, "OpenVPN cryptographic layer",
              <https://openvpn.net/community-resources/openvpn-
              cryptographic-layer/>.

   [QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-31,
              24 September 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-tls-31>.

   [QUIC-TRANSPORT]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-31, 24 September 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-transport-
              31>.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, DOI 10.17487/RFC2385, August
              1998, <https://www.rfc-editor.org/info/rfc2385>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
              <https://www.rfc-editor.org/info/rfc3748>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
              <https://www.rfc-editor.org/info/rfc4555>.

   [RFC4571]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets over Connection-
              Oriented Transport", RFC 4571, DOI 10.17487/RFC4571, July
              2006, <https://www.rfc-editor.org/info/rfc4571>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5641]  McGill, N. and C. Pignataro, "Layer 2 Tunneling Protocol
              Version 3 (L2TPv3) Extended Circuit Status Values",
              RFC 5641, DOI 10.17487/RFC5641, August 2009,
              <https://www.rfc-editor.org/info/rfc5641>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,
              <https://www.rfc-editor.org/info/rfc6189>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC7850]  Nandakumar, S., "Registering Values of the SDP 'proto'
              Field for Transporting RTP Media over TCP under Various
              RTP Profiles", RFC 7850, DOI 10.17487/RFC7850, April 2016,
              <https://www.rfc-editor.org/info/rfc7850>.

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", RFC 8095,
              DOI 10.17487/RFC8095, March 2017,
              <https://www.rfc-editor.org/info/rfc8095>.

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <https://www.rfc-editor.org/info/rfc8229>.

   [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>.

   [RFC8547]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., and E.
              Smith, "TCP-ENO: Encryption Negotiation Option", RFC 8547,
              DOI 10.17487/RFC8547, May 2019,
              <https://www.rfc-editor.org/info/rfc8547>.

   [RFC8548]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic Protection of TCP Streams
              (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
              <https://www.rfc-editor.org/info/rfc8548>.

   [TAPS-ARCH]
              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
              Perkins, C., Tiesel, P. S., and C. A. Wood, "An
              Architecture for Transport Services", Work in Progress,
              Internet-Draft, draft-ietf-taps-arch-08, 13 July 2020,
              <https://tools.ietf.org/html/draft-ietf-taps-arch-08>.

   [TAPS-INTERFACE]
              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P. S., Wood, C. A.,
              and T. Pauly, "An Abstract Application Layer Interface to
              Transport Services", Work in Progress, Internet-Draft,
              draft-ietf-taps-interface-09, 27 July 2020,
              <https://tools.ietf.org/html/draft-ietf-taps-interface-
              09>.

   [WireGuard]
              Donenfeld, J., "WireGuard: Next Generation Kernel Network
              Tunnel", <https://www.wireguard.com/papers/wireguard.pdf>.

Acknowledgments

   The authors would like to thank Bob Bradley, Frederic Jacobs, Mirja
   Kühlewind, Yannick Sierra, Brian Trammell, and Magnus Westerlund for
   their input and feedback on this document.

Authors' Addresses

   Theresa Enghardt
   TU Berlin
   Marchstr. 23
   10587 Berlin
   Germany

   Email: ietf@tenghardt.net


   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: tpauly@apple.com


   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow
   G12 8QQ
   United Kingdom

   Email: csp@csperkins.org


   Kyle Rose
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA 02144
   United States of America

   Email: krose@krose.org


   Christopher A. Wood
   Cloudflare
   101 Townsend St
   San Francisco,
   United States of America

   Email: caw@heapingbits.net