Encrypted Signaling Transport Modes for the Host Identity Protocol
RFC 6261

Internet Engineering Task Force (IETF)                        A. Keranen
Request for Comments: 6261                                      Ericsson
Category: Experimental                                          May 2011
ISSN: 2070-1721

                Encrypted Signaling Transport Modes for
                       the Host Identity Protocol

Abstract

   This document specifies two transport modes for Host Identity
   Protocol (HIP) signaling messages that allow them to be conveyed over
   encrypted connections initiated with the Host Identity Protocol.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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 a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6261.

Copyright Notice

   Copyright (c) 2011 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 Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Transport Mode Negotiation . . . . . . . . . . . . . . . . . .  3
     3.1.  Mode Negotiation in the HIP Base Exchange  . . . . . . . .  3
     3.2.  Mode Negotiation after the HIP Base Exchange . . . . . . .  5
     3.3.  Error Notifications  . . . . . . . . . . . . . . . . . . .  5
   4.  HIP Messages on Encrypted Connections  . . . . . . . . . . . .  5
     4.1.  ESP Mode . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  ESP-TCP Mode . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Recovering from Failed Encrypted Connections . . . . . . . . .  7
   6.  Host Mobility and Multihoming  . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2. Informational References . . . . . . . . . . . . . . . . . 10
   Appendix A.  Mobility and Multihoming Examples . . . . . . . . . . 11

1.  Introduction

   Host Identity Protocol (HIP) [RFC5201] signaling messages can be
   exchanged over plain IP using the protocol number reserved for this
   purpose, or over UDP using the UDP port reserved for HIP NAT
   traversal [RFC5770].  When two hosts perform a HIP base exchange,
   they set up an encrypted connection between them for data traffic,
   but continue to use plain IP or UDP for HIP signaling messages.

   This document defines how the encrypted connection can be used also
   for HIP signaling messages.  Two different modes are defined: HIP
   over Encapsulating Security Payload (ESP) and HIP over TCP.  The
   benefit of sending HIP messages over ESP is that all signaling
   traffic (including HIP headers) will be encrypted.  If HIP messages
   are sent over TCP (which in turn is transported over ESP), TCP can
   handle also message fragmentation where needed.

2.  Terminology

   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 RFC 2119 [RFC2119].

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3.  Transport Mode Negotiation

   This section defines how support for different HIP signaling message
   transport modes is indicated and how the use of different modes is
   negotiated.

3.1.  Mode Negotiation in the HIP Base Exchange

   A HIP host implementing this specification SHOULD indicate the modes
   it supports, and is willing to use, in the base exchange.  The HIP
   signaling message transport mode negotiation is similar to HIP NAT
   traversal mode negotiation: first the Responder lists the supported
   modes in a HIP_TRANSPORT_MODE parameter (see Figure 1) in the R1
   packet.  The modes are listed in priority order, the more preferred
   mode(s) first.  If the Initiator supports, and is willing to use, any
   of the modes proposed by the Responder, it selects one of the modes
   by adding a HIP_TRANSPORT_MODE parameter containing the selected mode
   to the I2 packet.  Finally, if the Initiator selected one of the
   modes and the base exchange succeeds, hosts MUST use the selected
   mode for the following HIP signaling messages sent between them for
   the duration of the HIP association or until another mode is
   negotiated.

   If the Initiator cannot, or will not, use any of the modes proposed
   by the Responder, the Initiator SHOULD include an empty
   HIP_TRANSPORT_MODE parameter to the I2 packet to signal that it
   supports this extension but will not use any of the proposed modes.
   Depending on local policy, the Responder MAY either abort the base
   exchange or continue HIP signaling without using an encrypted
   connection, if there was no HIP_TRANSPORT_MODE parameter in I2 or the
   parameter was empty.  If the Initiator selects a mode that the
   Responder does not support (and hence was not included in R1), the
   Responder MUST abort the base exchange.  If the base exchange is
   aborted due to (possibly lack of) HIP_TRANSPORT_PARAMETER, the
   Responder SHOULD send a NO_VALID_HIP_TRANSPORT_MODE notification (see
   Section 3.3) to the Initiator.

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Port              |           Mode ID #1          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Mode ID #2           |           Mode ID #3          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Mode ID #n           |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type     7680
     Port     transport layer port number (or zero if not used)
     Length   length in octets, excluding Type, Length, and Padding
     Mode ID  defines the proposed or selected transport mode(s)

     The following HIP Transport Mode IDs are defined:

         ID name   Value
         RESERVED    0
         DEFAULT     1
         ESP         2
         ESP-TCP     3

           Figure 1: Format of the HIP_TRANSPORT_MODE Parameter

   The mode DEFAULT indicates that the same transport mode (e.g., plain
   IP or UDP) that was used for the base exchange should be used for
   subsequent HIP signaling messages.  In the ESP mode, the messages are
   sent as such on the encrypted ESP connection; in the ESP-TCP mode,
   TCP is used within the ESP tunnel.  If a mode that uses a transport
   layer connection within the ESP tunnel (e.g., ESP-TCP) is offered,
   the Port field MUST contain the local port number the host will use
   for the connection.  If none of the modes utilize a transport layer
   protocol, the Port field SHOULD be set to zero when the parameter is
   sent and ignored when received.  The Port and Mode ID fields are
   encoded as unsigned integers using network byte order.

   The HIP_TRANSPORT_MODE parameter resides on the signed part of the
   HIP packets, and hence it is covered by the signatures of the R1, I2,
   and UPDATE packets.

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3.2.  Mode Negotiation after the HIP Base Exchange

   If a HIP host wants to change to a different transport mode (or start
   using a transport mode) some time after the base exchange, it sends a
   HIP UPDATE packet with a HIP_TRANSPORT_MODE parameter containing the
   mode(s) it would prefer to use.  The host receiving the UPDATE SHOULD
   respond with an UPDATE packet containing the mode that is selected as
   in the negotiation during the base exchange.  If the receiving host
   does not support, or is not willing to use, any of the listed modes,
   it SHOULD respond with an UPDATE packet where the HIP_TRANSPORT_MODE
   parameter contains only the currently used transport mode (even if
   that was not included in the previous UPDATE packet) and continue
   using that mode.

   Since the HIP_TRANSPORT_MODE parameter's type is not critical (as
   defined in Section 5.2.1 of [RFC5201]), a host not supporting this
   extension would simply reply with an acknowledgement UPDATE packet
   without a HIP_TRANSPORT_MODE parameter.  In such a case, depending on
   local policy as in mode negotiation during the base exchange, the
   host that requested the new transport mode MAY close the HIP
   association.  If the association is closed, the host closing the
   association SHOULD send a NO_VALID_HIP_TRANSPORT_MODE NOTIFY packet
   to the other host before closing the association.

3.3.  Error Notifications

   During a HIP signaling transport mode negotiation, if a
   HIP_TRANSPORT_MODE parameter does not contain any mode that the
   receiving host is willing to use, or a HIP_TRANSPORT_MODE parameter
   does not exist in a HIP packet where the receiving host expected to
   see it, the receiving host MAY send back a NOTIFY packet with a
   NOTIFICATION parameter [RFC5201] error type
   NO_VALID_HIP_TRANSPORT_MODE (value 100).  The Notification Data field
   for the error notifications SHOULD contain the HIP header of the
   rejected packet.

4.  HIP Messages on Encrypted Connections

   This specification defines two different transport modes for sending
   HIP packets over encrypted ESP connections.  These modes require that
   the ESP transport format [RFC5202] is negotiated to be used between
   the hosts.  If the ESP transport format is not used, these modes MUST
   NOT be offered in the HIP_TRANSPORT_MODE parameter.  If a
   HIP_TRANSPORT_MODE parameter containing an ESP transport mode is
   received but the ESP transport format is not used, a host MUST NOT
   select such a mode but act as specified in Section 3.1 (if performing
   a base exchange) or Section 3.2 (if performing an UPDATE) when no
   valid mode is offered.

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   The ESP mode provides simple protection for all the signaling traffic
   and can be used as a generic replacement for the DEFAULT mode in
   cases where all signaling traffic should be encrypted.  If the HIP
   messages may become so large that they would need to be fragmented,
   e.g., because of HIP certificates [RFC6253] or DATA messages
   [RFC6078], it is RECOMMENDED to use the ESP-TCP mode that can handle
   message fragmentation at the TCP level instead of relying on IP-level
   fragmentation.

   When HIP NAT traversal [RFC5770] is used, the ESP and HIP packets are
   sent UDP encapsulated.  The use of different NAT traversal modes, and
   in particular UDP encapsulation, is independent of the transport mode
   (as specified in this document) of HIP packets.  However, when HIP
   packets are sent over an ESP connection, no additional UDP
   encapsulation (i.e., within the ESP connection) for the HIP packets
   is needed and MUST NOT be used since the ESP packets are already UDP
   encapsulated, if needed for NAT traversal.  For example, if UDP
   encapsulation is used as defined in [RFC5770], and the ESP-TCP
   transport mode is used as defined in this document, the HIP packets
   are sent over IP, UDP, ESP, and TCP (in that order).

   HIP messages that result in changing or generating new keying
   material, i.e., the base exchange and re-keying UPDATE messages, MUST
   NOT be sent over the encrypted connection that is created using the
   keying material that is being changed, nor over an encrypted
   connection using the newly created keying material.

   It should be noted that when HIP messages are sent using an encrypted
   connection, on-path network elements (e.g., firewalls and HIP-aware
   NATs) that would normally see the HIP headers and contents of the
   unencrypted parameters, cannot see any part of the messages unless
   they have access to the encryption keying material.  The original HIP
   design made an explicit decision to expose some of this information
   to HIP-aware NATs.  If an encrypted transport mode is used, only the
   base exchange or update without encryption is visible to such NATs.

4.1.  ESP Mode

   If the ESP mode is selected in the base exchange, both hosts MUST
   listen for incoming HIP signaling messages and send outgoing messages
   on the encrypted connection.  The ESP header's next header value for
   HIP messages sent over ESP MUST be set to HIP (139).

4.2.  ESP-TCP Mode

   If the ESP-TCP mode is selected, the host with the larger HIT
   (calculated as defined in Section 6.5 of [RFC5201]) MUST start to
   listen for an incoming TCP connection on the encrypted connection

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   (i.e., to the HIT of the host) on the port it used in the Port field
   of the transport mode parameter.  The other host MUST create a TCP
   connection to that port and the host MAY use the port it sent in the
   transport mode parameter as the source port for the connection.  Once
   the TCP connection is established, both hosts MUST listen for
   incoming HIP signaling messages and send the outgoing messages using
   the TCP connection.  The ESP next header value for messages sent
   using the ESP-TCP mode TCP connections MUST be set to TCP (6).

   If the hosts are unable to create the TCP connection, the host that
   initiated the mode negotiation MUST restart the negotiation with the
   UPDATE message and SHOULD NOT propose the ESP-TCP mode.  If local
   policy does not allow use of any mode other than ESP-TCP, the HIP
   association SHOULD be closed.  The UPDATE or CLOSE message MUST be
   sent using the same transport mode that was used for negotiating the
   use of the ESP-TCP mode.

   Since TCP provides reliable transport, the HIP messages sent over TCP
   MUST NOT be retransmitted.  Instead, a host SHOULD wait to detect
   that the TCP connection has failed to retransmit the packet
   successfully in a timely manner (such detection is platform- and
   policy-specific) before concluding that there is no response.

5.  Recovering from Failed Encrypted Connections

   If the encrypted connection fails for some reason, it can no longer
   be used for HIP signaling and the hosts SHOULD re-establish the
   connection using HIP messages that are sent outside of the encrypted
   connection.  Hence, while listening for incoming HIP messages on the
   encrypted connection, hosts MUST still accept incoming HIP messages
   using the same transport method (e.g., UDP or plain IP) that was used
   for the base exchange.  When responding to a HIP message sent outside
   of the encrypted connection, the response MUST be sent using the same
   transport method as the original message used.  If encryption was
   previously used, hosts SHOULD send outside of the encrypted
   connection only HIP messages that are used to re-establish the
   encrypted connection.  In particular, when the policy requires that
   only encrypted messages (e.g., DATA messages using an encrypted
   transport mode) be sent, they MUST be sent using an encrypted
   connection.  Note that a policy MUST NOT prevent sending unencrypted
   UPDATE messages used for re-establishing the encrypted connection,
   since that would prevent recovering from failed encrypted
   connections.

   The UPDATE messages used for re-establishing the encrypted connection
   MUST contain a HIP_TRANSPORT_MODE parameter and the negotiation
   proceeds as described in Section 3.2.

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6.  Host Mobility and Multihoming

   If a host obtains a new address, a new Security Association (SA) pair
   may be created for (or an existing SA pair may be moved to) the new
   address, as described in [RFC5206].  If the ESP or ESP-TCP transport
   mode is used, HIP signaling continues using the (new) SA pair and the
   same transport mode as before.

   With the ESP mode, the first mobility UPDATE message SHOULD be sent
   using the old SA, and the following messages, including the response
   to the first UPDATE, SHOULD be sent using the new SAs.
   Retransmissions of the UPDATE messages use the same SA as the
   original message.  If the ESP-TCP mode is used, the HIP signaling TCP
   connection is moved to the new SA pair like any other TCP connection.
   However, the mobility UPDATE messages SHOULD NOT be sent over the TCP
   connection, but using plain ESP as in the ESP mode, and consequently
   hosts MUST be prepared to receive UPDATE messages over plain ESP even
   if the ESP-TCP mode is used.

   In some cases, the host may not be able to send the mobility UPDATE
   messages using the encrypted connection before it breaks.  This
   results in a similar situation as if the encrypted connection had
   failed and the hosts need to renegotiate the new addresses using
   unencrypted UPDATE messages and possibly rendezvous [RFC5204] or HIP
   relay [RFC5770] servers.  Also, these UPDATE messages MUST contain
   the HIP_TRANSPORT_MODE parameter and perform the transport mode
   negotiation.

   Examples of the signaling flows with mobility and multihoming are
   shown in Appendix A.

7.  Security Considerations

   By exchanging the HIP messages over an ESP connection, all HIP
   signaling data (after the base exchange but excluding keying material
   (re)negotiation and some of the mobility UPDATE messages) will be
   encrypted, but only if NULL encryption is not used.  Thus, a host
   requiring confidentiality for the HIP signaling messages must check
   that encryption is negotiated for use on the ESP connection.
   Moreover, the level of protection provided by the ESP transport modes
   depends on the selected ESP transform; see [RFC5202] and [RFC4303]
   for security considerations of the different ESP transforms.

   While this extension to HIP allows for negotiation of security
   features, there is no risk of downgrade attacks since the mode
   negotiation happens using signed (R1/I2 or UPDATE) packets and only
   after both hosts have been securely identified in the base exchange.
   If an attacker would attempt to change the modes listed in the

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   HIP_TRANSPORT_MODE parameter, that would break the signatures and the
   base exchange (or update) would not complete.  Furthermore, since
   both "secure" modes (ESP and ESP-TCP) defined in this document are
   equally secure, the only possible downgrade attack would be to make
   both hosts accept the DEFAULT mode.  If the local policy (of either
   host) requires using a secure mode, the base exchange or update would
   again simply fail (as described in Section 3.1).

8.  Acknowledgements

   Thanks to Gonzalo Camarillo, Kristian Slavov, Tom Henderson, Miika
   Komu, Jan Melen, and Tobias Heer for reviews and comments.

9.  IANA Considerations

   This section is to be interpreted according to [RFC5226].

   This document updates the IANA maintained "Host Identity Protocol
   (HIP) Parameters" registry [RFC5201] by assigning a new HIP Parameter
   Type value (7680) for the HIP_TRANSPORT_MODE parameter (defined in
   Section 3.1).

   The HIP_TRANSPORT_MODE parameter has 16-bit unsigned integer fields
   for different modes, for which IANA has created and now maintains a
   new sub-registry entitled "HIP Transport Modes" under the "Host
   Identity Protocol (HIP) Parameters" registry.  Initial values for the
   transport mode registry are given in Section 3.1; future assignments
   are to be made through IETF Review or IESG Approval [RFC5226].
   Assignments consist of a transport mode identifier name and its
   associated value.

   This document also defines a new HIP NOTIFICATION message type
   [RFC5201] NO_VALID_HIP_TRANSPORT_MODE (100) in Section 3.3.

10.  References

10.1.  Normative References

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

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC5202]  Jokela, P., Moskowitz, R., and P. Nikander, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 5202, April 2008.

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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

10.2.  Informational References

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC5204]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 5204, April 2008.

   [RFC5206]  Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
              Host Mobility and Multihoming with the Host Identity
              Protocol", RFC 5206, April 2008.

   [RFC5770]  Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
              Keranen, "Basic Host Identity Protocol (HIP) Extensions
              for Traversal of Network Address Translators", RFC 5770,
              April 2010.

   [RFC6078]  Camarillo, G. and J. Melen, "Host Identity Protocol (HIP)
              Immediate Carriage and Conveyance of Upper-Layer Protocol
              Signaling (HICCUPS)", RFC 6078, January 2011.

   [RFC6253]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 6253, May 2011.

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Appendix A.  Mobility and Multihoming Examples

   When changing interfaces due to mobility or multihoming, the hosts
   use HIP messages to notify the other host about the new address and
   to check that the host with the new address is still reachable.  The
   following examples show the signaling performed during the address
   change in two different scenarios.  Note that not all HIP parameters
   nor all the content of the parameters is shown in the examples.  This
   section and the examples are not normative; for normative behavior,
   see previous sections.

   In the examples, host A uses two different addresses (a1 and a2)
   while host B has just a single address (b1).  In the first example,
   "Make before Break" (Figure 2), host A starts to use the new address
   but can still use the old address (due to multihoming) for signaling.
   In the second example, "Break before Make" (Figure 3), host A loses
   the first address before obtaining the second address (e.g., due to
   mobility), and the mobility HIP signaling is done without the
   encrypted connection.

   The following notations are used in the examples:

   o  ESPx(y): data y sent encapsulated in ESP with SA x; if ESP-
      encapsulation is not used, the data is sent over plain IP or UDP

   o  UPDATE(x,y,z): HIP UPDATE message [RFC5201] with parameters x,y,z

   o  LOCATOR(x): HIP LOCATOR parameter [RFC5206] with locator x

   o  ESP_INFO(x,y): HIP ESP_INFO parameter [RFC5202] with "old SPI"
      value x and "new SPI" value y

   o  ACK, ECHO_REQ, and ECHO_RSP: HIP ACK, ECHO_REQUEST, and
      ECHO_RESPONSE parameters [RFC5201]

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            A                                                  B
                                 ESP1(...)
           a1 <----------------------------------------------> b1

                ESP1(UPDATE(LOCATOR(a2), ESP_INFO(0,SPI2a)))
           a1 -----------------------------------------------> b1

                   (A and B create SAs a2 <-> b1 (ESP2)
                    retransmissions of the first UPDATE
                    happen over ESP1)

               ESP2(UPDATE(ACK, ESP_INFO(0,SPI2b), ECHO_REQ)))
           a2 <----------------------------------------------- b1

                         ESP2(UPDATE(ACK, ECHO_RSP))
           a2 -----------------------------------------------> b1

                                  ESP2(...)
           a2 <----------------------------------------------> b1

                        Figure 2: Make Before Break

            A                                                  B
                                  ESP1(...)
           a1 <----------------------------------------------> b1
                           (A moves from a1 to a2)

                 UPDATE(LOCATOR(a2), ESP_INFO(SPI1a, SPI1a))
           a2 -----------------------------------------------> b1

                 UPDATE(ACK, ECHO_REQ, ESP_INFO(SPI1b,SPI1b))
           a2 <----------------------------------------------- b1

                           UPDATE(ACK, ECHO_RSP)
           a2 -----------------------------------------------> b1
                    (A and B move ESP1 SAs to a2 <-> b1)

                                  ESP1(...)
           a2 <----------------------------------------------> b1

                        Figure 3: Break Before Make

   When the ESP-TCP mode is used, the signaling flows are similar since
   TCP is not used for the mobility UPDATE messages as described in
   Section 6.

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Author's Address

   Ari Keranen
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   EMail: ari.keranen@ericsson.com

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