IPsec latching: IPsec Channels: Connection Latching

Network Working Group                                        N. Williams
Request for Comments: 5660                                           Sun
Category: Standards Track                                   October 2009


                  IPsec Channels: Connection Latching

Abstract

   This document specifies, abstractly, how to interface applications
   and transport protocols with IPsec so as to create "channels" by
   latching "connections" (packet flows) to certain IPsec Security
   Association (SA) parameters for the lifetime of the connections.
   Connection latching is layered on top of IPsec and does not modify
   the underlying IPsec architecture.

   Connection latching can be used to protect applications against
   accidentally exposing live packet flows to unintended peers, whether
   as the result of a reconfiguration of IPsec or as the result of using
   weak peer identity to peer address associations.  Weak association of
   peer ID and peer addresses is at the core of Better Than Nothing
   Security (BTNS); thus, connection latching can add a significant
   measure of protection to BTNS IPsec nodes.

   Finally, the availability of IPsec channels will make it possible to
   use channel binding to IPsec channels.

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................4
   2. Connection Latching .............................................4
      2.1. Latching of Quality-of-Protection Parameters ...............8
      2.2. Connection Latch State Machine .............................9
      2.3. Normative Model: ULP Interfaces to the Key Manager ........12
           2.3.1. Race Conditions and Corner Cases ...................17
           2.3.2. Example ............................................18
      2.4. Informative Model: Local Packet Tagging ...................19
      2.5. Non-Native Mode IPsec .....................................21
      2.6. Implementation Note Regarding Peer IDs ....................22
   3. Optional Features ..............................................22
      3.1. Optional Protection .......................................22
   4. Simultaneous Latch Establishment ...............................23
   5. Connection Latching to IPsec for Various ULPs ..................23
      5.1. Connection Latching to IPsec for TCP ......................24
      5.2. Connection Latching to IPsec for UDP with
           Simulated Connections .....................................24
      5.3. Connection Latching to IPsec for UDP with
           Datagram-Tagging APIs .....................................25
      5.4. Connection Latching to IPsec for SCTP .....................25
      5.5. Handling of BROKEN State for TCP and SCTP .................26
   6. Security Considerations ........................................27
      6.1. Impact on IPsec ...........................................27
      6.2. Impact on IPsec of Optional Features ......................28
      6.3. Security Considerations for Applications ..................28
      6.4. Channel Binding and IPsec APIs ............................29
      6.5. Denial-of-Service Attacks .................................29
   7. Acknowledgements ...............................................30
   8. References .....................................................30
      8.1. Normative References ......................................30
      8.2. Informative References ....................................30

1.  Introduction

   IPsec protects packets with little or no regard for stateful packet
   flows associated with upper-layer protocols (ULPs).  This exposes
   applications that rely on IPsec for session protection to risks
   associated with changing IPsec configurations, configurations that
   allow multiple peers access to the same addresses, and/or weak
   association of peer IDs and their addresses.  The latter can occur as
   a result of "wildcard" matching in the IPsec Peer Authorization
   Database (PAD), particularly when Better Than Nothing Security (BTNS)
   [RFC5387] is used.

   Applications that wish to use IPsec may have to ensure that local
   policy on the various end-points is configured appropriately
   [RFC5406] [USING-IPSEC].  There are no standard Application
   Programming Interfaces (APIs) to do this (though there are non-
   standard APIs, such as [IP_SEC_OPT.man]) -- a major consequence of
   which, for example, is that applications must still use hostnames
   (and, e.g., the Domain Name System [RFC1034]) and IP addresses in
   existing APIs and must depend on an IPsec configuration that they may
   not be able to verify.  In addition to specifying aspects of required
   Security Policy Database (SPD) configuration, application
   specifications must also address PAD/SPD configuration to strongly
   bind individual addresses to individual IPsec identities and
   credentials (certificates, public keys, etc.).

   IPsec is, then, quite cumbersome for use by applications.  To address
   this, we need APIs to IPsec.  Not merely APIs for configuring IPsec,
   but also APIs that are similar to the existing IP APIs (e.g., "BSD
   Sockets"), so that typical applications making use of UDP [RFC0768],
   TCP [RFC0793], and Stream Control Transmission Protocol (SCTP)
   [RFC4960] can make use of IPsec with minimal changes.

   This document describes the foundation for IPsec APIs that UDP and
   TCP applications can use: a way to bind the traffic flows for, e.g.,
   TCP connections to security properties desired by the application.
   We call these "connection latches" (and, in some contexts, "IPsec
   channels").  The methods outlined below achieve this by interfacing
   ULPs and applications to IPsec.

   If widely adopted, connection latching could make application use of
   IPsec much simpler, at least for certain classes of applications.

   Connection latching, as specified herein, is primarily about watching
   updates to the SPD and Security Association Database (SAD) to detect
   changes that are adverse to an application's requirements for any
   given packet flow, and to react accordingly (such as by synchronously
   alerting the ULP and application before packets can be sent or
   received under the new policy).  Under no circumstance are IPsec
   policy databases to be modified by connection latching in any way
   that can persist beyond the lifetime of the related packet flows, nor
   reboots.  Under no circumstance is the PAD to be modified at all by
   connection latching.  If all optional features of connection latching
   are excluded, then connection latching can be implemented as a
   monitor of SPD and SAD changes that intrudes in their workings no
   more than is needed to provide synchronous alerts to ULPs and
   applications.

   We assume the reader is familiar with the IPsec architecture
   [RFC4301] and Internet Key Exchange Protocol version 2 (IKEv2)
   [RFC4306].

   Note: the terms "connection latch" and "IPsec channel" are used
   interchangeably below.  The latter term relates to "channel binding"
   [RFC5056].  Connection latching is suitable for use in channel
   binding applications, or will be, at any rate, when the channel
   bindings for IPsec channels are defined (the specification of IPsec
   channel bindings is out of scope for this document).

   Note: where this document mentions IPsec peer "ID" it refers to the
   Internet Key Exchange (IKE) peer ID (e.g., the ID derived from a
   peer's cert, as well as the cert), not the peer's IP address.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Abstract function names are all capitalized and denoted by a pair of
   parentheses.  In their descriptions, the arguments appear within the
   parentheses, with optional arguments surrounded by square brackets.
   Return values, if any, are indicated by following the function
   argument list with "->" and a description of the return value.  For
   example, "FOO(3-tuple, [message])" would be a function named "FOO"
   with two arguments, one of them optional, and returning nothing,
   whereas "FOOBAR(handle) -> state" would be a function with a single,
   required argument that returns a value.  The values' types are
   described in the surrounding text.

2.  Connection Latching

   An "IPsec channel" is a packet flow associated with a ULP control
   block, such as a TCP connection, where all the packets are protected
   by IPsec SAs such that:

   o  the peer's identity is the same for the lifetime of the packet
      flow;

   o  the quality of IPsec protection used for the packet flow's
      individual packets is the same for all of them for the lifetime of
      the packet flow.

   An IPsec channel is created when the associated packet flow is
   created.  This can be the result of a local operation (e.g., a
   connect()) that causes the initial outgoing packet for that flow to
   be sent, or it can be the result of receiving the first/initiating
   packet for that flow (e.g., a TCP SYN packet).

   An IPsec channel is destroyed when the associated packet flow ends.
   An IPsec channel can also be "broken" when the connection latch
   cannot be maintained for some reason (see below), in which case the
   ULP and application are informed.

   IPsec channels are created by "latching" various parameters listed
   below to a ULP connection when the connections are created.  The
   REQUIRED set of parameters bound in IPsec channels is:

   o  Type of protection: confidentiality and/or integrity protection;

   o  Transport mode versus tunnel mode;

   o  Quality of protection (QoP): cryptographic algorithm suites, key
      lengths, and replay protection (see Section 2.1);

   o  Local identity: the local ID asserted to the peer, as per the
      IPsec processing model [RFC4301] and BTNS [RFC5386];

   o  Peer identity: the peer's asserted and authorized IDs, as per the
      IPsec processing model [RFC4301] and BTNS [RFC5386].

   The SAs that protect a given IPsec channel's packets may change over
   time in that they may expire and be replaced with equivalent SAs, or
   they may be re-keyed.  The set of SAs that protect an IPsec channel's
   packets need not be related by anything other than the fact that they
   must be congruent to the channel (i.e., the SAs' parameters must
   match those that are latched into the channel).  In particular, it is
   desirable that IPsec channels survive the expiration of IKE_SAs and
   child SAs because operational considerations of the various key
   exchange protocols then cannot affect the design and features of
   connection latching.

   When a situation arises in which the SPD is modified, or an SA is
   added to the SAD, such that the new policy and/or SA are not
   congruent to an established channel (see previous paragraph), then we
   consider this a conflict.  Conflict resolution is addressed below.

   Requirements and recommendations:

   o  If an IPsec channel is desired, then packets for a given
      connection MUST NOT be sent until the channel is established.

   o  If an IPsec channel is desired, then inbound packets for a given
      connection MUST NOT be accepted until the channel is established.
      That is, inbound packets for a given connection arriving prior to
      the establishment of the corresponding IPsec channel must be
      dropped or the channel establishment must fail.

   o  Once an IPsec channel is established, packets for the latched
      connection MUST NOT be sent unprotected nor protected by an SA
      that does not match the latched parameters.

   o  Once an IPsec channel is established, packets for the latched
      connection MUST NOT be accepted unprotected nor protected by an SA
      that does not match the latched parameters.  That is, such packets
      must either be dropped or cause the channel to be terminated and
      the application to be informed before data from such a packet can
      be delivered to the application.

   o  Implementations SHOULD provide programming interfaces for
      inquiring the values of the parameters latched in a connection.

   o  Implementations that provide such programming interfaces MUST make
      available to applications all relevant and available information
      about a peer's ID, including authentication information.  This
      includes the peer certificate, when one is used, and the trust
      anchor to which it was validated (but not necessarily the whole
      certificate validation chain).

   o  Implementations that provide such programming interfaces SHOULD
      make available to applications any information about local and/or
      remote public and private IP addresses, in the case of NAT-
      traversal.

   o  Implementations that provide such programming interfaces SHOULD
      make available to applications the inner and outer local and peer
      addresses whenever the latched connection uses tunnel-mode SAs.

   o  Implementations SHOULD provide programming interfaces for setting
      the values of the parameters to be latched in a connection that
      will be initiated or accepted, but these interfaces MUST limit
      what values applications may request according to system policy
      (i.e., the IPsec PAD and SPD) and the application's local
      privileges.

      (Typical system policy may not allow applications any choices
      here.  Policy extensions allowing for optional protection are
      described in Section 3.1.)

   o  Implementations SHOULD create IPsec channels automatically by
      default when the application does not explicitly request an IPsec
      channel.  Implementations MAY provide a way to disable automatic
      creation of connection latches.

   o  The parameters latched in an IPsec channel MUST remain unchanged
      once the channel is established.

   o  Timeouts while establishing child SAs with parameters that match
      those latched into an IPsec channel MUST be treated as packet loss
      (as happens, for example, when a network partitions); normal ULP
      and/or application timeout handling and retransmission
      considerations apply.

   o  Implementations that have a restartable key management process (or
      "daemon") MUST arrange for existing latched connections to either
      be broken and disconnected, or for them to survive the restart of
      key exchange processes.  (This is implied by the above
      requirements.)  For example, if such an implementation relies on
      keeping some aspects of connection latch state in the restartable
      key management process (e.g., values that potentially have large
      representations, such as BTNS peer IDs), then either such state
      must be restored on restart of such a process, or outstanding
      connection latches must be transitioned to the CLOSED state.

   o  Dynamic IPsec policy (see Section 3.1) related to connection
      latches, if any, MUST be torn down when latched connections are
      torn down, and MUST NOT survive reboots.

   o  When IKE dead-peer detection (DPD) concludes that the remote peer
      is dead or has rebooted, then the system SHOULD consider all
      connection latches with that peer to be irremediably broken.

   We describe two models, one of them normative, of IPsec channels for
   native IPsec implementations.  The normative model is based on
   abstract programming interfaces in the form of function calls between
   ULPs and the key management component of IPsec (basically, the SAD,
   augmented with a Latch Database (LD)).  The second model is based on
   abstract programming interfaces between ULPs and the IPsec
   (Encapsulating Security Payload / Authentication Header (ESP/AH))
   layer in the form of meta-data tagging of packets within the IP
   stack.

   The two models given below are not, however, entirely equivalent.
   One model cannot be implemented with Network Interface cards (NICs)
   that offload ESP/AH but that do not tag incoming packets passed to
   the host processor with SA information, nor allow the host processor
   to so tag outgoing packets.  That same model can be easily extended
   to support connection latching with unconnected datagram "sockets",
   while the other model is rigidly tied to a notion of "connections"
   and cannot be so extended.  There may be other minor differences
   between the two models.  Rather than seek to establish equivalency
   for some set of security guarantees, we instead choose one model to
   be the normative one.

   We also provide a model for non-native implementations, such as bump-
   in-the-stack (BITS) and Security Gateway (SG) implementations.  The
   connection latching model for non-native implementations is not full-
   featured as it depends on estimating packet flow state, which may not
   always be possible.  Nor can non-native IPsec implementations be
   expected to provide APIs related to connection latching
   (implementations that do could be said to be native).  As such, this
   third model is not suitable for channel binding applications
   [RFC5056].

2.1.  Latching of Quality-of-Protection Parameters

   In IPsec, the assumption of IKE initiator/responder roles is non-
   deterministic.  That is, sometimes an IKE SA and child SAs will be
   initiated by the "client" (e.g., the caller of the connect() BSD
   sockets function) and sometimes by the "server" (e.g., the caller of
   the accept() BSD Sockets function).  This means that the negotiation
   of quality of protection is also non-deterministic unless one of the
   peers offers a single cryptographic suite in the IKE negotiation.

   When creating narrow child SAs with traffic selectors matching the
   connection latch's 5-tuple, it is possible to constrain the IKE
   Quality-of-Protection negotiation to a single cryptographic suite.
   Therefore, implementations SHOULD provide an API for requesting the
   use of such child SAs.  Implementors SHOULD consider an application
   request for a specific QoP to imply a request for narrow child SAs.

   When using SAs with traffic selectors encompassing more than just a
   single flow, then the system may only be able to latch a set of
   cryptographic suites, rather than a single cryptographic suite.  In
   such a case, an implementation MUST report the QoP being used as
   indeterminate.

2.2.  Connection Latch State Machine

   Connection latches can exist in any of the following five states:

   o  LISTENER

   o  ESTABLISHED

   o  BROKEN (there exist SAs that conflict with the given connection
      latch, conflicting SPD changes have been made, or DPD has been
      triggered and the peer is considered dead or restarted)

   o  CLOSED (by the ULP, the application or administratively)

   and always have an associated owner, or holder, such as a ULP
   transmission control block (TCB).

   A connection latch can be born in the LISTENER state, which can
   transition only to the CLOSED state.  The LISTENER state corresponds
   to LISTEN state of TCP (and other ULPs) and is associated with IP
   3-tuples, and can give rise to new connection latches in the
   ESTABLISHED state.

   A connection latch can also be born in the ESTABLISHED and BROKEN
   states, either through the direct initiative of a ULP or when an
   event occurs that causes a LISTENER latch to create a new latch (in
   either ESTABLISHED or BROKEN states).  These states represent an
   active connection latch for a traffic flow's 5-tuple.  Connection
   latches in these two states can transition to the other of the two
   states, as well as to the CLOSED state.

   Connection latches remain in the CLOSED state until their owners are
   informed except where the owner caused the transition, in which case
   this state is fleeting.  Transitions from ESTABLISHED or BROKEN
   states to the CLOSED state should typically be initiated by latch
   owners, but implementations SHOULD provide administrative interfaces
   through which to close active latches.

   Connection latches transition to the BROKEN state when there exist
   SAs in the SAD whose traffic selectors encompass the 5-tuple bound by
   the latch, and whose peer and/or parameters conflict with those bound
   by the latch.  Transitions to the BROKEN state also take place when
   SPD changes occur that would cause the latched connection's packets
   to be sent or received with different protection parameters than
   those that were latched.  Transitions to the BROKEN state are also
   allowed when IKEv2 DPD concludes that the remote peer is dead or has
   rebooted.  Transitions to the BROKEN state always cause the
   associated owner to be informed.  Connection latches in the BROKEN
   state transition back to ESTABLISHED when all SA and/or SPD conflicts
   are cleared.

   Most state transitions are the result of local actions of the latch
   owners (ULPs).  The only exceptions are: birth into the ESTABLISHED
   state from latches in the LISTENER state, transitions to the BROKEN
   state, transitions from the BROKEN state to ESTABLISHED, and
   administrative transitions to the CLOSED state.  (Additionally, see
   the implementation note about restartable key management processes in
   Section 2.)

   The state diagram below makes use of conventions described in
   Section 1.1 and state transition events described in Section 2.3.

      <CREATE_LISTENER_LATCH(3-tuple, ...)>
                     :
                     v    <CREATE_CONNECTION_LATCH(5-tuple, ...)>
                /--------\           :   :
         +------|LISTENER|......     :   :
         |      \--------/     :     :   :   +--------------------+
         |        :            :     :   :   |Legend:             |
         |        :            :     :   :   | dotted lines denote|
         |  <conn. trigger event>    :   :   |    latch creation  |
         |      (e.g., TCP SYN :     :   :   |                    |
         |       received,     :     :   :   | solid lines denote |
         |       connect()     :     :   :   |    state transition|
         |       called, ...)  v     v   :   |                    |
         |        :        /-----------\ :   | semi-solid lines   |
         |        :        |ESTABLISHED| :   |    denote async    |
         |    <conflict>   \-----------/ :   |    notification    |
         |        :         ^       |    :   +--------------------+
         |        :         |      <conflict
         |        :    <conflict    or DPD>
         |        :     cleared>    |    :
         |        :         |       |    :
         |        :         |       v    v
         |        :      /----------------\
         |        :.....>|     BROKEN     |.-.-.-.-.-> <ALERT()>
         |               \----------------/
         |                       |
      <RELEASE_LATCH()>   <RELEASE_LATCH()>
         |                       |
         |                       v
         |                    /------\
         +------------------->|CLOSED|
                              \------/

                Figure 1: Connection Latching State Machine

   The details of the transitions depend on the model of connection
   latching followed by any given implementation.  See the following
   sections.

2.3.  Normative Model: ULP Interfaces to the Key Manager

   This section describes the NORMATIVE model of connection latching.

   In this section, we describe connection latching in terms of a
   function-call interface between ULPs and the "key manager" component
   of a native IPsec implementation.  Abstract interfaces for creating,
   inquiring about, and releasing IPsec channels are described.

   This model adds a service to the IPsec key manager (i.e., the
   component that manages the SAD and interfaces with separate
   implementations of, or directly implements, key exchange protocols):
   management of connection latches.  There is also a new IPsec
   database, the Latch Database (LD), that contains all connection latch
   objects.  The LD does not persist across system reboots.

   The traditional IPsec processing model allows the concurrent
   existence of SAs with different peers but overlapping traffic
   selectors.  Such behavior, in this model, directly violates the
   requirements for connection latching (see Section 2).  We address
   this problem by requiring that connection latches be broken (and
   holders informed) when such conflicts arise.

   The following INFORMATIVE figure illustrates this model and API in
   terms that are familiar to many implementors, though not applicable
   to all:

      +--------------------------------------------+
      |                       +--------------+     |
      |                       |Administrator |     |
      |                       |apps          |     |
      |                       +--------------+     |
      |                            ^      ^        |
      |                            |      |        | user mode
      |                            v      v        |
      | +--------------+      +-------++--------+  |
      | |App           |      |IKEv2  ||        |  |
      | |              |      | +---+ || +----+ |  |
      | |              |      | |PAD| || |SPD | |  |
      | |              |      | +---+ || +--^-+ |  |
      | +--------------+      +-+-----++----+---+  |
      |   ^                     |           |      |
      +---|---------------------|-----------|------+  user/kernel mode
      |   |syscalls             |  PF_KEY   |      |  interface
      |   |                     | [RFC2367] |      |
      +---|---------------------|-----------|------+
      |   v                     |           |      |
      |+-------+   +------------|-----------|-----+|
      ||ULP    |   | IPsec   key|manager    |     ||
      |+-------+   |            |  +--------v----+||
      | ^  ^       |            |  | Logical SPD |||
      | |  |       |            |  +-----------^-+||
      | |  |       |            +-------+      |  ||  kernel mode
      | |  |       |                    |      |  ||
      | |  |       | +----------+    +--v--+   |  ||
      | |  +-------->| Latch DB |<-->| SAD |   |  ||
      | |          | +----------+    +--^--+   |  ||
      | |          +--------------------|------|--+|
      +-|-------------------------------v------v---+
      | | IPsec Layer  (ESP/AH)                    |
      | |                                          |
      +-v------------------------------------------+
      |   IP Layer                                 |
      +--------------------------------------------+

         Figure 2: Informative Implementation Architecture Diagram

   The ULP interfaces to the IPsec LD are as follows:

   o  CREATE_LISTENER_LATCH(3-tuple, [type and quality-of-protection
      parameters]) -> latch handle | error
         If there is no conflicting connection latch object in the
         LISTENER state for the given 3-tuple (local address, protocol,
         and local port number), and local policy permits it, then this
         operation atomically creates a connection latch object in the
         LISTENER state for the given 3-tuple.

         When a child SA is created that matches a listener latch's
         3-tuple, but not any ESTABLISHED connection latch's 5-tuple
         (local address, remote address, protocol, local port number,
         and remote port number), then the key manager creates a new
         connection latch object in the ESTABLISHED state.  The key
         manager MUST inform the holder of the listener latch of
         connection latches created as a result of the listener latch;
         see the "ALERT()" interface below.

   o  CREATE_CONNECTION_LATCH(5-tuple, [type and quality-of-protection
      parameters], [peer ID], [local ID]) -> latch handle | error

         If a) the requested latch does not exist (or exists, but is in
         the CLOSED state), b) all the latch parameters are provided, or
         if suitable SAs exist in the SAD from which to derive them, and
         c) if there are no conflicts with the SPD and SAD, then this
         creates a connection latch in the ESTABLISHED state.  If the
         latch parameters are not provided and no suitable SAs exist in
         the SAD from which to derive those parameters, then the key
         manager MUST initiate child SAs, and if need be, IKE_SA, from
         which to derive those parameters.

         The key manager MAY delay the child SA setup and return
         immediately after the policy check, knowing that the ULP that
         requested the latch will subsequently output a packet that will
         trigger the SA establishment.  Such an implementation may
         require an additional, fleeting state in the connection latch
         state machine, a "LARVAL" state, so to speak, that is not
         described herein.

         If the connection latch ultimately cannot be established,
         either because of conflicts or because no SAs can be
         established with the peer at the destination address, then an
         error is returned to the ULP.  (If the key manager delayed SA
         establishment, and SA establishment ultimately fails, then the
         key manager has to inform the ULP, possibly asynchronously.
         This is one of several details that implementors who use a
         LARVAL state must take care of.)

   o  RELEASE_LATCH(latch object handle)

         Changes the state of the given connection latch to CLOSED; the
         connection latch is then deleted.

         The key manager MAY delete any existing child SAs that match
         the given latch if it had been in the ESTABLISHED states.  If
         the key manager does delete such SAs, then it SHOULD inform the
         peer with an informational Delete payload (see IKEv2
         [RFC4306]).

   o  FIND_LATCH(5-tuple) -> latch handle | error

         Given a 5-tuple returns a latch handle (or an error).

   o  INQUIRE_LATCH(latch object handle) -> {latch state, latched
      parameters} | error

         Returns all available information about the given latch,
         including its current state (or an error).

   The IPsec LD interface to the ULP is as follows:

   o  ALERT(latch object handle, 5-tuple, new state, [reason])

         Alerts a ULP as to an asynchronous state change for the given
         connection latch and, optionally, provides a reason for the
         change.

      This interface is to be provided by each ULP to the key manager.
      The specific details of how this interface is provided are
      implementation details, thus not specified here (for example, this
      could be a "callback" function or "closure" registered as part of
      the CREATE_LISTENER_LATCH() interface, or it could be provided
      when the ULP is loaded onto the running system via a registration
      interface provided by the key manager).

   Needless to say, the LD is updated whenever a connection latch object
   is created, deleted, or broken.

   The API described above is a new service of the IPsec key manager.
   In particular, the IPsec key manager MUST prevent conflicts amongst
   latches, and it MUST prevent conflicts between any latch and existing
   or proposed child SAs as follows:

   o  Non-listener connection latches MUST NOT be created if there exist
      conflicting SAs in the SAD at the time the connection latch is
      requested or would be created (from a listener latch).  A child SA
      conflicts with another, in view of a latch, if and only if: a) its
      traffic selectors and the conflicting SA's match the given
      latch's, and b) its peer, type-of-protection, or quality-of-
      protection parameters differ from the conflicting SA.

   o  Child SA proposals that would conflict with an extant connection
      latch and whose traffic selectors can be narrowed to avoid the
      conflict SHOULD be narrowed (see Section 2.9 of [RFC4306]);
      otherwise, the latch MUST be transitioned to the BROKEN state.

   o  Where child SA proposals that would conflict with an extant
      connection latch cannot be narrowed to avoid the conflict, the key
      manager MUST break the connection latch and inform the holder
      (i.e., the ULP) prior to accepting the conflicting SAs.

   Finally, the key manager MUST protect latched connections against SPD
   changes that would change the quality of protection afforded to a
   latched connection's traffic, or which would bypass it.  When such a
   configuration change takes place, the key manager MUST respond in
   either of the following ways.  The REQUIRED to implement behavior is
   to transition into the BROKEN state all connection latches that
   conflict with the given SPD change.  An OPTIONAL behavior is to
   logically update the SPD as if a PROTECT entry had been added at the
   head of the SPD-S with traffic selectors matching only the latched
   connection's 5-tuple, and with processing information taken from the
   connection latch.  Such updates of the SPD MUST NOT survive system
   crashes or reboots.

   ULPs create latched connections by interfacing with IPsec as follows:

   o  For listening end-points, the ULP will request a connection latch
      listener object for the ULP listener's 3-tuple.  Any latching
      parameters requested by the application MUST be passed along.

   o  When the ULP receives a packet initiating a connection for a
      5-tuple matching a 3-tuple listener latch, then the ULP will ask
      the key manager whether a 5-tuple connection latch was created.
      If not, then the ULP will either reject the new connection or
      accept it and inform the application that the new connection is
      not latched.

   o  When initiating a connection, the ULP will request a connection
      latch object for the connection's 5-tuple.  Any latching
      parameters requested by the application MUST be passed along.  If
      no latch can be created, then the ULP MUST either return an error
      to the application or continue with the new connection and inform
      the application that the new connection is not latched.

   o  When a connection is torn down and no further packets are expected
      for it, then the ULP MUST request that the connection latch object
      be destroyed.

   o  When tearing down a listener, the ULP MUST request that the
      connection latch listener object be destroyed.

   o  When a ULP listener rejects connections, the ULP will request the
      destruction of any connection latch objects that may have been
      created as a result of the peer's attempt to open the connection.

   o  When the key manager informs a ULP that a connection latch has
      transitioned to the BROKEN state, then the ULP MUST stop sending
      packets and MUST drop all subsequent incoming packets for the
      affected connection until it transitions back to ESTABLISHED.
      Connection-oriented ULPs SHOULD act as though the connection is
      experiencing packet loss.

   o  When the key manager informs a ULP that a connection latch has
      been administratively transitioned to the CLOSED state, then
      connection-oriented ULPs MUST act as though the connection has
      been reset by the peer.  Implementations of ULPs that are not
      connection-oriented, and which have no API by which to simulate a
      reset, MUST drop all inbound packets for that connection and MUST
      NOT send any further packets -- the application is expected to
      detect timeouts and act accordingly.

   The main benefit of this model of connection latching is that it
   accommodates IPsec implementations where ESP/AH handling is
   implemented in hardware (for all or a subset of the host's SAD), even
   where the hardware does not support tagging inbound packets with the
   indexes of SAD entries corresponding to the SAs that protected them.

2.3.1.  Race Conditions and Corner Cases

   ULPs MUST drop inbound packets and stop sending packets immediately
   upon receipt of a connection latch break message.  Otherwise, the ULP
   will not be able to distinguish inbound packets that were protected
   consistently with the connection's latch from inbound packets that
   were not.  This may include dropping inbound packets that were
   protected by a suitable SA; dropping such packets is no different,
   from the ULP's point of view, than packet loss elsewhere on the
   network at the IP layer or below -- harmless, from a security point
   of view as the connection fails safe, but it can result in
   retransmits.

   Another race condition is as follows.  A PROTECTed TCP SYN packet may
   be received and decapsulated, but the SA that protected it could have
   expired before the key manager creates the connection latch that
   would be created by that packet.  In this case, the key manager will
   have to initiate new child SAs so as to determine what the sender's
   peer ID is so it can be included in the connection latch.  Here,
   there is no guarantee that the peer ID for the new SAs will be the
   same as those of the peer that sent the TCP SYN packet.  This race
   condition is harmless: TCP will send a SYN+ACK to the wrong peer,
   which will then respond with a RST -- the connection latch will
   reflect the new peer however, so if the new peer is malicious it will
   not be able to appear to be the old peer.  Therefore, this race
   condition is harmless.

2.3.2.  Example

   Consider several systems with a very simple PAD containing a single
   entry like so:

                                               Child SA
      Rule Remote ID                          IDs allowed  SPD Search by
      ---- ---------                          -----------  -------------
      1   <any valid to trust anchor X> 192.0.2/24      by-IP

                           Figure 3: Example PAD

   And a simple SPD like so:

      Rule Local             Remote            Next  Action
            TS                TS               Proto
      ---- -----             ------            ----- ----------------
       1   192.0.2/24:ANY    192.0.2/24:1-5000 TCP   PROTECT(ESP,...)
       1   192.0.2/24:1-5000 192.0.2/24:ANY    TCP   PROTECT(ESP,...)
       1   ANY         ANY         ANY   BYPASS

                        Figure 4: [SG-A] SPD Table

   Effectively this says: for TCP ports 1-5000 in our network, allow
   only peers that have credentials issued by CA X and PROTECT that
   traffic with ESP, otherwise, bypass all other traffic.

   Now let's consider two hosts, A and B, in this network that wish to
   communicate using port 4000, and a third host, C, that is also in the
   same network and wishes to attack A and/or B.  All three hosts have
   credentials and certificates issued by CA X.  Let's also imagine that
   A is connected to its network via a wireless link and is dynamically
   addressed.

   B is listening on port 4000.  A initiates a connection from port
   32800 to B on port 4000.

   We'll assume no IPsec APIs, but that TCP creates latches where
   possible.

   We'll consider three cases: a) A and B both support connection
   latching, b) only A does, c) only B does.  For the purposes of this
   example, the SAD is empty on all three hosts when A initiates its TCP
   connection to B on port 4000.

   When an application running on A initiates a TCP connection to B on
   port 4000, A will begin by creating a connection latch.  Since the
   SAD is empty, A will initiate an IKEv2 exchange to create an IKE_SA
   with B and a pair of child SAs for the 5-tuple {TCP, A, 32800, B,
   4000}, then a new latch will be created in ESTABLISHED state.
   Sometime later, TCP will send a SYN packet protected by the A-to-B
   child SA, per the SPD.

   When an application running on B creates a TCP listener "socket" on
   port 4000, B will create a LISTENER connection latch for the 3-tuple
   {TCP, B, 4000}.  When B receives A's TCP SYN packet, it will then
   create a connection latch for {TCP, B, 4000, A, 32800}.  Since, by
   this point, child SAs have been created whose traffic selectors
   encompass this 5-tuple and there are no other conflicting SAs in the
   SAD, this connection latch will be created in the ESTABLISHED state.

   If C attempts to mount a man-in-the-middle attack on A (i.e.,
   pretends to have B's address(es)) any time after A created its
   connection latch, then C's SAs with A will cause the connection latch
   to break, and the TCP connection to be reset (since we assume no APIs
   by which TCP could notify the application of the connection latch
   break).  If C attempts to impersonate A to B, then the same thing
   will happen on B.

   If A does not support connection latching, then C will be able to
   impersonate B to A at any time.  Without having seen the cleartext of
   traffic between A and B, C will be limited by the TCP sequence
   numbers to attacks such as RST attacks.  Similarly, if B does not
   support connection latching, then C will be able to impersonate A to
   B.

2.4.  Informative Model: Local Packet Tagging

   In this section, we describe connection latching in terms of
   interfaces between ULPs and IPsec based on tagging packets as they go
   up and down the IP stack.

   This section is INFORMATIVE.

   In this model, the ULPs maintain connection latch objects and state,
   rather than the IPsec key manager, as well as effectively caching a
   subset of the decorrelated SPD in ULP TCBs.  Tagging packets, as they
   move up and down the stack, with SA identifiers then allows the ULPs
   to enforce connection latching semantics.  These tags, of course,
   don't appear on the wire.

   The interface between the ULPs and IPsec interface is as follows:

   o  The IPsec layer tags all inbound protected packets addressed to
      the host with the index of the SAD entry corresponding to the SA
      that protected the packet.

   o  The IPsec layer understands two types of tags on outbound packets:

      *  a tag specifying a set of latched parameters (peer ID, quality
         of protection, etc.) that the IPsec layer will use to find or
         acquire an appropriate SA for protecting the outbound packet
         (else IPsec will inform the ULP and drop the packet);

      *  a tag requesting feedback about the SA used to protect the
         outgoing packet, if any.

   ULPs create latched connections by interfacing with IPsec as follows:

   o  When the ULP passes a connection's initiating packet to IP, the
      ULP requests feedback about the SA used to protect the outgoing
      packet, if any, and may specify latching parameters requested by
      the application.  If the packet is protected by IPsec, then the
      ULP records certain parameters of the SA used to protect it in the
      connection's TCB.

   o  When a ULP receives a connection's initiating packet, it processes
      the IPsec tag of the packet, and it records in the connection's
      TCB the parameters of the SA that should be latched.

   Once SA parameters are recorded in a connection's TCB, the ULP
   enforces the connection's latch, or binding, to these parameters as
   follows:

   o  The ULP processes the IPsec tag of all inbound packets for a given
      connection and checks that the SAs used to protect input packets
      match the connection latches recorded in the TCBs.  Packets that
      are not so protected are dropped (this corresponds to
      transitioning the connection latch to the BROKEN state until the
      next acceptable packet arrives, but in this model, this transition
      is imaginary) or cause the ULP to break the connection latch and
      inform the application.

   o  The ULP always requests that outgoing packets be protected by SAs
      that match the latched connection by appropriately tagging
      outbound packets.

   By effectively caching a subset of the decorrelated SPD in ULP TCBs
   and through its packet tagging nature, this method of connection
   latching can also optimize processing of the SPD by obviating the
   need to search it, both, on input and output, for packets intended
   for the host or originated by the host.  This makes implementation of
   the OPTIONAL "logical SPD" updates described in Sections 2.3 and 3.1
   an incidental side effect of this approach.

   This model of connection latching may not be workable with ESP/AH
   offload hardware that does not support the packet tagging scheme
   described above.

   Note that this model has no explicit BROKEN connection latch state.

   Extending the ULP/IPsec packet-tagging interface to the application
   for use with connection-less datagram transports should enable
   applications to use such transports and implement connection latching
   at the application layer.

2.5.  Non-Native Mode IPsec

   This section is INFORMATIVE.

   Non-native IPsec implementations, primarily BITS and SG, can
   implement connection latching, too.  One major distinction between
   native IPsec and BITS, bump-in-the-wire (BITW), or SG IPsec is the
   lack of APIs for applications at the end-points in the case of the
   latter.  As a result, there can be no uses of the latch management
   interfaces as described in Section 2.3: not at the ULP end-points.
   Therefore, BITS/BITW/SG implementations must discern ULP connection
   state from packet inspection (which many firewalls can do) and
   emulate calls to the key manager accordingly.

   When a connection latch is broken, a BITS/BITW/SG implementation may
   have to fake a connection reset by sending appropriate packets (e.g.,
   TCP RST packets), for the affected connections.

   As with all stateful middleboxes, this scheme suffers from the
   inability of the middlebox to interact with the applications.  For
   example, connection death may be difficult to ascertain.  Nor can
   channel binding applications work with channels maintained by proxy
   without being able to communicate (securely) about it with the
   middlebox.

2.6.  Implementation Note Regarding Peer IDs

   One of the recommendations for connection latching implementors is to
   make peer CERT payloads (certificates) available to the applications.

   Additionally, raw public keys are likely to be used in the
   construction of channel bindings for IPsec channels (see [IPSEC-CB]),
   and they must be available, in any case, in order to implement leap-
   of-faith at the application layer (see [RFC5386] and [RFC5387]).

   Certificates and raw public keys are large bit strings, too large to
   be reasonably kept in kernel-mode implementations of connection
   latching (which will likely be the typical case).  Such
   implementations should intern peer IDs in a user-mode database and
   use small integers to refer to them from the kernel-mode SAD and LD.
   Corruption of such a database is akin to corruption of the SAD/LD; in
   the event of corruption, the implementation MUST act as though all
   ESTABLISHED and BROKEN connection latches are administratively
   transitioned to the CLOSED state.  Implementations without IPsec APIs
   MAY hash peer IDs and use the hash to refer to them, preferably using
   a strong hash algorithm.

3.  Optional Features

   At its bare minimum, connection latching is a passive layer atop
   IPsec that warns ULPs of SPD and SAD changes that are incompatible
   with the SPD/SAD state that was applicable to a connection when it
   was established.

   There are some optional features, such as (abstract) APIs.  Some of
   these features make connection latching a somewhat more active
   feature.  Specifically, the optional logical SPD updates described in
   Section 2.3 and the optional protection/bypass feature described in
   the following sub-section.

3.1.  Optional Protection

   Given IPsec APIs, an application could request that a connection's
   packets be protected where they would otherwise be bypassed; that is,
   applications could override BYPASS policy.  Locally privileged
   applications could request that their connections' packets be
   bypassed rather than protected; that is, privileged applications
   could override PROTECT policy.  We call this "optional protection".

   Both native IPsec models of connection latching can be extended to
   support optional protection.  With the model described in
   Section 2.4, optional protection comes naturally: the IPsec layer
   need only check that the protection requested for outbound packets
   meets or exceeds (as determined by local or system policy) the
   quality of protection, if any, required by the SPD.  In the case of
   the model described in Section 2.3, enforcement of minimum protection
   requirements would be done by the IPsec key manager via the
   connection latch state machine.

   When an application requests, and local policy permits, either
   additional protection or bypassing protection, then the SPD MUST be
   logically updated such that there exists a suitable SPD entry
   protecting or bypassing the exact 5-tuple recorded by the
   corresponding connection latch.  Such logical SPD updates MUST be
   made at connection latch creation time, and MUST be made atomically
   (see the note about race conditions in Section 2.3).  Such updates of
   the SPD MUST NOT survive system crashes or reboots.

4.  Simultaneous Latch Establishment

   Some connection-oriented ULPs, specifically TCP, support simultaneous
   connections (where two clients connect to each other, using the same
   5-tuple, at the same time).  Connection latching supports
   simultaneous latching as well, provided that the key exchange
   protocol does not make it impossible.

   Consider two applications doing a simultaneous TCP connect to each
   other and requesting an IPsec channel.  If they request the same
   connection latching parameters, then the connection and channel
   should be established as usual.  Even if the key exchange protocol in
   use doesn't support simultaneous IKE_SA and/or child SA
   establishment, provided one peer's attempt to create the necessary
   child SAs succeeds, then the other peer should be able to notice the
   new SAs immediately upon failure of its attempts to create the same.

   If, however, the two peer applications were to request different
   connection latching parameters, then the connection latch must fail
   on one end or on both ends.

5.  Connection Latching to IPsec for Various ULPs

   The following sub-sections describe connection latching for each of
   three transport protocols.  Note that for TCP and UDP, there is
   nothing in the following sections that should not already be obvious
   from the remainder of this document.  The section on SCTP, however,
   specifies details related to SCTP multi-homing, that may not be as
   obvious.

5.1.  Connection Latching to IPsec for TCP

   IPsec connection latch creation/release for TCP [RFC0793] connections
   is triggered when:

   o  a TCP listener end-point is created (e.g., when the BSD Sockets
      listen() function is called on a socket).  This should cause the
      creation of a LISTENER connection latch.

   o  a TCP SYN packet is received on an IP address and port number for
      which there is a listener.  This should cause the creation of an
      ESTABLISHED or BROKEN connection latch.

   o  a TCP SYN packet is sent (e.g., as the result of a call to the BSD
      Sockets connect() function).  This should cause the creation of an
      ESTABLISHED or BROKEN connection latch.

   o  any state transition of a TCP connection to the CLOSED state will
      cause a corresponding transition for any associated connection
      latch to the CLOSED state as well.

   See Section 5.5 for how to handle latch transitions to the BROKEN
   state.

5.2.  Connection Latching to IPsec for UDP with Simulated Connections

   UDP [RFC0768] is a connection-less transport protocol.  However, some
   networking APIs (e.g., the BSD Sockets API) allow for emulation of
   UDP connections.  In this case, connection latching can be supported
   using either model given above.  We ignore, in this section, the fact
   that the connection latching model described in Section 2.4 can
   support per-datagram latching by extending its packet tagging
   interfaces to the application.

   IPsec connection latch creation/release for UDP connections is
   triggered when:

   o  an application creates a UDP "connection".  This should cause the
      creation of an ESTABLISHED or BROKEN connection latch.

   o  an application destroys a UDP "connection".  This should cause the
      creation of an ESTABLISHED or BROKEN connection latch.

   When a connection latch transitions to the BROKEN state and the
   application requested (or system policy dictates it) that the
   connection be broken, then UDP should inform the application, if
   there is a way to do so, or else it should wait, allowing the
   application-layer keepalive/timeout strategy, if any, to time out the
   connection.

   What constitutes an appropriate action in the face of administrative
   transitions of connection latches to the CLOSED state depends on
   whether the implementation's "connected" UDP sockets API provides a
   way for the socket to return an error indicating that it has been
   closed.

5.3.  Connection Latching to IPsec for UDP with Datagram-Tagging APIs

   Implementations based on either model of connection latching can
   provide applications with datagram-tagging APIs based on those
   described in Section 2.4.  Implementations UDP with of the normative
   model of IPsec connection latching have to confirm, on output, that
   the application provided 5-tuple agrees with the application-provided
   connection latch; on input, UDP can derive the tag by searching for a
   connection latch matching incoming datagram's 5-tuple.

5.4.  Connection Latching to IPsec for SCTP

   SCTP [RFC4960], a connection-oriented protocol is similar, in some
   ways, to TCP.  The salient difference, with respect to connection
   latching, between SCTP and TCP is that SCTP allows each end-point to
   be identified by a set of IP addresses, though, like TCP, each end-
   point of an SCTP connection (or, rather, SCTP association) can only
   have one port number.

   We can represent the multiplicity of SCTP association end-point
   addresses as a multiplicity of 5-tuples, each of which with its own
   connection latch.  Alternatively, we can extend the connection latch
   object to support a multiplicity of addresses for each end-point.
   The first approach is used throughout this document; therefore, we
   will assume that representation.

   Of course, this approach results in N x M connection latches for any
   SCTP associations (where one end-point has N addresses and the other
   has M); whereas the alternative requires one connection latch per
   SCTP association (with N + M addresses).  Implementors may choose
   either approach.

   IPsec connection latch creation/release for SCTP connections is
   triggered when:

   o  an SCTP listener end-point is created (e.g., when the SCTP sockets
      listen() function is called on a socket).  This should cause the
      creation of a LISTENER connection latch for each address of the
      listener.

   o  an SCTP INIT chunk is received on an IP address and port number
      for which there is a listener.  This should cause the creation of
      one or more ESTABLISHED or BROKEN connection latches, one for each
      distinct 5-tuple given the client and server's addresses.

   o  an SCTP INIT chunk is sent (e.g., as the result of a call to the
      SCTP sockets connect() function).  This should cause the creation
      of one or more ESTABLISHED or BROKEN connection latches.

   o  an SCTP Address Configuration Change Chunk (ASCONF) [RFC5061]
      adding an end-point IP address is sent or received.  This should
      cause the creation of one or more ESTABLISHED or BROKEN connection
      latches.

   o  any state transition of an SCTP association to the CLOSED state
      will cause a corresponding transition for any associated
      connection latches to the CLOSED state as well.

   o  an SCTP ASCONF chunk [RFC5061] deleting an end-point IP address is
      sent or received.  This should cause one or more associated
      connection latches to be CLOSED.

   See Section 5.5 for how to handle latch transitions to the BROKEN
   state.

5.5.  Handling of BROKEN State for TCP and SCTP

   There are several ways to handle connection latch transitions to the
   BROKEN state in the case of connection-oriented ULPs like TCP or
   SCTP:

   a.  Wait for a possible future transition back to the ESTABLISHED
       state, until which time the ULP will not move data between the
       two end-points of the connection.  ULP and application timeout
       mechanisms will, of course, be triggered in the event of too
       lengthy a stay in the BROKEN state.  SCTP can detect these
       timeouts and initiate failover, in the case of multi-homed
       associations.

   b.  Act as though the connection has been reset (RST message
       received, in TCP, or ABORT message received, in SCTP).

   c.  Act as though an ICMP destination unreachable message had been
       received (in SCTP such messages can trigger path failover in the
       case of multi-homed associations).

   Implementations SHOULD provide APIs that allow applications either 1)
   to be informed (asynchronously or otherwise) of latch breaks so that
   they may choose a disposition, and/or 2) to select a specific
   disposition a priori (before a latch break happens).  The options for
   disposition are wait, close, or proceed with path failover.

   Implementations MUST provide a default disposition in the event of a
   connection latch break.  Though (a) is clearly the purist default, we
   RECOMMEND (b) for TCP and SCTP associations where only a single path
   remains (one 5-tuple), and (c) for multi-homed SCTP associations.
   The rationale for this recommendation is as follows: a conflicting SA
   most likely indicates that the original peer is gone and has been
   replaced by another, and it's not likely that the original peer will
   return; thus, failing faster seems reasonable.

   Note that our recommended default behavior does not create off-path
   reset denial-of-service (DoS) attacks.  To break a connection latch,
   an attacker would first have to successfully establish an SA, with
   one of the connection's end-points, that conflicts with the
   connection latch and that requires multiple messages to be exchanged
   between that end-point and the attacker.  Unless the attacker's
   chosen victim end-point allows the attacker to claim IP address
   ranges for its SAs, then the attacker would have to actually take
   over the other end-point's addresses, which rules out off-path
   attacks.

6.  Security Considerations

6.1.  Impact on IPsec

   Connection latching effectively adds a mechanism for dealing with the
   existence, in the SAD, of multiple non-equivalent child SAs with
   overlapping traffic selectors.  This mechanism consists of, at
   minimum, a local notification of transport protocols (and, through
   them, applications) of the existence of such a conflict that affects
   a transport layer's connections.  Affected transports are also
   notified when the conflict is cleared.  The transports must drop
   inbound packets, and must not send outbound packets for connections
   that are affected by a conflict.  In this minimal form, connection
   latching is a passive, local feature layered atop IPsec.

   We achieve this by adding a new type of IPsec database, the Latch
   Database (LD), containing objects that represent a transport
   protocol's interest in protecting a given packet flow from such
   conflicts.  The LD is managed in conjunction with updates to the SAD
   and the SPD, so that updates to either that conflict with established
   connection latches can be detected.  For some IPsec implementations,
   this may imply significant changes to their internals.  However, two
   different models of connection latching are given, and we hope that
   most native IPsec implementors will find at least one model to be
   simple enough to implement in their stack.

   This notion of conflicting SAs and how to deal with the situation
   does not modify the basic IPsec architecture -- the feature of IPsec
   that allows such conflicts to arise remains, and it is up to the
   transport protocols and applications to select whether and how to
   respond to them.

   There are, however, interesting corner cases in the normative model
   of connection latching that implementors must be aware of.  The notes
   in Section 2.3.1 are particularly relevant.

6.2.  Impact on IPsec of Optional Features

   Section 3 describes optional features of connection latching where
   the key manager takes on a somewhat more active, though still local,
   role.  There are two such features: optional protect/bypass and
   preservation of "logical" SPD entries to allow latched connections to
   remain in the ESTABLISHED state in the face of adverse administrative
   SPD (but not SAD) changes.  These two features interact with
   administrative interfaces to IPsec; administrators must be made aware
   of these features, and they SHOULD be given a way to break
   ESTABLISHED connection latches.  Also, given recent trends toward
   centralizing parts of IPsec policy, these two features can be said to
   have non-local effects where they prevent distributed policy changes
   from taking effect completely.

6.3.  Security Considerations for Applications

   Connection latching is not negotiated.  It is therefore possible for
   one end of a connection to be using connection latching while the
   other does not; in which case, it's possible for policy changes local
   to the non-latched end to cause packets to be sent unprotected.  The
   end doing connection latching will reject unprotected packets, but if
   they bear sensitive data, then the damage may already be done.
   Therefore, applications SHOULD check that both ends of a connection
   are latched (such a check is implicit for applications that use
   channel binding to IPsec).

   Connection latching protects individual connections from weak peer
   ID<->address binding, IPsec configuration changes, and from
   configurations that allow multiple peers to assert the same
   addresses.  But connection latching does not ensure that any two
   connections with the same end-point addresses will have the same
   latched peer IDs.  In other words, applications that use multiple
   concurrent connections between two given nodes may not be protected
   any more or less by use of IPsec connection latching than by use of
   IPsec alone without connection latching.  Such multi-connection
   applications can, however, examine the latched SA parameters of each
   connection to ensure that all concurrent connections with the same
   end-point addresses also have the same end-point IPsec IDs.

   Connection latching protects against TCP RST attacks.  It does not
   help, however, if the original peer of a TCP connection is no longer
   available (e.g., if an attacker has been able to interrupt the
   network connection between the two peers).

6.4.  Channel Binding and IPsec APIs

   IPsec channels are a prerequisite for channel binding [RFC5056] to
   IPsec.  Connection latching provides such channels, but the channel
   bindings for IPsec channels (latched connections) are not specified
   herein -- that is a work in progress [IPSEC-CB].

   Without IPsec APIs, connection latching provides marginal security
   benefits over traditional IPsec.  Such APIs are not described herein;
   see [ABSTRACT-API].

6.5.  Denial-of-Service Attacks

   Connection latch state transitions to the BROKEN state can be
   triggered by on-path attackers and any off-path attackers that can
   attack routers or cause an end-point to accept an ICMP Redirect
   message.  Connection latching protects applications against on- and
   off-path attackers in general, but not against on-path denial of
   service specifically.

   Attackers can break latches if they can trigger DPD on one or both
   end-points and if they cause packets to not move between two end-
   points.  Such attacks generally require that the attacker be on-path;
   therefore, we consider it acceptable to break latches when DPD
   concludes that a peer is dead or rebooted.

   Attackers can also break latches if IPsec policy on a node allows the
   attacker to use the IP address of a peer of that node.  Such
   configurations are expected to be used in conjunction with BTNS in
   general.  Such attacks generally require that the attacker be on-
   path.

7.  Acknowledgements

   The author thanks Michael Richardson for all his help, as well as
   Stephen Kent, Sam Hartman, Bill Sommerfeld, Dan McDonald, Daniel
   Migault, and many others who've participated in the BTNS WG or who've
   answered questions about IPsec, connection latching implementations,
   etc.

8.  References

8.1.  Normative References

   [RFC0768]         Postel, J., "User Datagram Protocol", STD 6,
                     RFC 768, August 1980.

   [RFC0793]         Postel, J., "Transmission Control Protocol", STD 7,
                     RFC 793, September 1981.

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4301]         Kent, S. and K. Seo, "Security Architecture for the
                     Internet Protocol", RFC 4301, December 2005.

   [RFC4306]         Kaufman, C., "Internet Key Exchange (IKEv2)
                     Protocol", RFC 4306, December 2005.

   [RFC4960]         Stewart, R., "Stream Control Transmission
                     Protocol", RFC 4960, September 2007.

   [RFC5061]         Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and
                     M. Kozuka, "Stream Control Transmission Protocol
                     (SCTP) Dynamic Address Reconfiguration", RFC 5061,
                     September 2007.

   [RFC5386]         Williams, N. and M. Richardson, "Better-Than-
                     Nothing Security: An Unauthenticated Mode of
                     IPsec", RFC 5386, November 2008.

8.2.  Informative References

   [ABSTRACT-API]    Richardson, M., "An abstract interface between
                     applications and IPsec", Work in Progress,
                     November 2008.

   [IPSEC-CB]        Williams, N., "End-Point Channel Bindings for IPsec
                     Using IKEv2 and Public Keys", Work in Progress,
                     April 2008.

   [IP_SEC_OPT.man]  Sun Microsystems, Inc., "ipsec(7P) man page,
                     Solaris 10 Reference Manual Collection".

   [RFC1034]         Mockapetris, P., "Domain names - concepts and
                     facilities", STD 13, RFC 1034, November 1987.

   [RFC2367]         McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
                     Management API, Version 2", RFC 2367, July 1998.

   [RFC5056]         Williams, N., "On the Use of Channel Bindings to
                     Secure Channels", RFC 5056, November 2007.

   [RFC5387]         Touch, J., Black, D., and Y. Wang, "Problem and
                     Applicability Statement for Better-Than-Nothing
                     Security (BTNS)", RFC 5387, November 2008.

   [RFC5406]         Bellovin, S., "Guidelines for Specifying the Use of
                     IPsec Version 2", BCP 146, RFC 5406, February 2009.

   [USING-IPSEC]     Dondeti, L. and V. Narayanan, "Guidelines for using
                     IPsec and IKEv2", Work in Progress, October 2006.

Author's Address

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US

   EMail: Nicolas.Williams@sun.com