Network Working Group                                        B. Williams
Internet-Draft                                              Akamai, Inc.
Intended status: Informational                              M. Boucadair
Expires: December 16, 2016                                France Telecom
                                                                 D. Wing
                                                     Cisco Systems, Inc.
                                                           June 14, 2016

           An Experimental TCP Option for Host Identification


   Recent RFCs have discussed issues with host identification in IP
   address sharing systems, such as shared address/prefix sharing
   devices and application-layer proxies.  Potential solutions for
   revealing a host identifier in shared address deployments have also
   been discussed.  This memo describes the design, deployment, and
   privacy considerations for one such solution in operational use on
   the Internet today that uses a TCP option to transmit a host

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 16, 2016.

Copyright Notice

   Copyright (c) 2016 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
   ( in effect on the date of

Williams, et al.        Expires December 16, 2016               [Page 1]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Important Use Cases . . . . . . . . . . . . . . . . . . .   3
     1.2.  Document Goals  . . . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Option Format . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Option Use  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Option Values . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Sending Host Requirements . . . . . . . . . . . . . . . .   8
       4.2.1.  Alternative SYN Cookie Support  . . . . . . . . . . .   8
       4.2.2.  Persistent TCP Connections  . . . . . . . . . . . . .   8
       4.2.3.  Packet Fragmentation  . . . . . . . . . . . . . . . .   9
     4.3.  Multiple In-Path HOST_ID Senders  . . . . . . . . . . . .   9
   5.  Option Interpretation . . . . . . . . . . . . . . . . . . . .  10
   6.  Interaction with Other TCP Options  . . . . . . . . . . . . .  11
     6.1.  Multipath TCP (MPTCP) . . . . . . . . . . . . . . . . . .  11
     6.2.  Authentication Option (TCP-AO)  . . . . . . . . . . . . .  11
     6.3.  TCP Fast Open (TFO) . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
   9.  Pervasive Monitoring Considerations . . . . . . . . . . . . .  14
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     12.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Change History . . . . . . . . . . . . . . . . . . .  18
     A.1.  Changes from version 07 to 08 . . . . . . . . . . . . . .  18
     A.2.  Changes from version 06 to 07 . . . . . . . . . . . . . .  18
     A.3.  Changes from version 05 to 06 . . . . . . . . . . . . . .  19
     A.4.  Changes from version 04 to 05 . . . . . . . . . . . . . .  19
     A.5.  Changes from version 03 to 04 . . . . . . . . . . . . . .  20
     A.6.  Changes from version 02 to 03 . . . . . . . . . . . . . .  20
     A.7.  Changes from version 01 to 02 . . . . . . . . . . . . . .  20
     A.8.  Changes from version 00 to 01 . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

Williams, et al.        Expires December 16, 2016               [Page 2]

Internet-Draft       Experimental TCP Host ID Option           June 2016

1.  Introduction

   A broad range of issues associated with address sharing have been
   documented in [RFC6269] and [RFC7620].  In addition, [RFC6967]
   provides analysis of various solutions to the problem of revealing
   the sending host's identifier (HOST_ID) information to the receiver,
   indicating that a solution using a TCP [RFC0793] option for this
   purpose is among the possible approaches that could be applied with
   limited performance impact and a high success ratio.  The purpose of
   this memo is to describe a TCP HOST_ID option that is currently
   deployed on the public Internet using the TCP experimental option
   codepoint, including discussion of related design, deployment, and
   privacy considerations.

   Multiple Internet Drafts have defined TCP options for the purpose of
   host identification: [I-D.wing-nat-reveal-option],
   [I-D.abdo-hostid-tcpopt-implementation], and
   [I-D.williams-overlaypath-ip-tcp-rfc].  Specification of multiple
   option formats to serve the purpose of host identification increases
   the burden for potential implementers and presents interoperability
   challenges as well, so the authors of those drafts have worked
   together to define a common TCP option that supersedes the formats
   from those three drafts.  This memo describes a version of that
   common TCP option format that is currently in use on the public

   The option defined in this memo uses the TCP experimental option
   codepoint sharing mechanism defined in [RFC6994].  One of the earlier
   draft specifications, [I-D.williams-overlaypath-ip-tcp-rfc], is
   associated with unauthorized use of a TCP option kind number, and
   moving to the TCP experimental option code-point has allowed the
   authors of that draft to correct their error.

1.1.  Important Use Cases

   The authors' implementations have primarily focussed on the following
   address-sharing use cases in which currently deployed systems insert
   the HOST_ID option:

   Carrier Grade NAT (CGN):  As defined in [RFC6888], [RFC6333], and
      other sources, a CGN allows multiple hosts connected to the public
      Internet to share a single Internet routable IPv4 address.  One
      important characteristic of the CGN use case is that it modifies
      IP packets in-path, but does not serve as the end point for the
      associated TCP connections.

   Application Proxy:  As defined in [RFC1919], an application proxy
      splits a TCP connection into two segments, serving as an endpoint

Williams, et al.        Expires December 16, 2016               [Page 3]

Internet-Draft       Experimental TCP Host ID Option           June 2016

      for each of the connections and relaying data flows between the

   Overlay Network:  An overlay network is an Internet based system
      providing security, optimization, or other services for data flows
      that transit the system.  A network-layer overlay will sometimes
      act much like a CGN, in that packets transit the system with NAT
      being applied at the edge of the overlay.  A transport-layer or
      application-layer overlay [RFC3135] will typically act much like
      an application proxy, in that the TCP connection will be segmented
      with the overlay network serving as an endpoint for each of the
      TCP connections.

   In this set of sender use cases, the TCP option is either applied to
   an individual TCP packet at the connection endpoint (e.g. an
   application proxy or a transport layer overlay network) or at an
   address-sharing middle box (e.g. a CGN or a network layer overlay
   network).  See Section 4 below for additional details about the types
   of devices that add the option to a TCP packet, as well as existing
   limitations on use of the option when it is inserted by an address-
   sharing middlebox, including issues related to packet fragmentation.

   The existing receiver use cases considered by this memo include the

   o  Differentiating between attack and non-attack traffic when the
      source of the attack is sharing an address with non-attack

   o  Application of per-subscriber policies for resource utilization,
      etc. when multiple subscribers are sharing a common address.

   o  Improving server-side load-balancing decisions by allowing the
      load for multiple clients behind a shared address to be assigned
      to different servers, even when session-affinity is required at
      the application layer.

   In all of the above cases, differentiation between address-sharing
   clients is performed by a network function that does not process the
   application layer protocol (e.g.  HTTP) or the security protocol
   (e.g.  TLS), because the action needs to be performed prior to
   decryption or parsing the application layer.  Due to this, a solution
   implemented within the application layer or security protocol was
   considered unable to fully meet the receiver-side requirements.  At
   the same time, as noted in [RFC6967], use of an IP option for this
   purpose has a low success rate.  For these reasons, using a TCP
   option to deliver the host identifier was deemed by the authors to be

Williams, et al.        Expires December 16, 2016               [Page 4]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   an effective way to satisfy these specific use cases.  See Section 5
   below for details about receiver-side interpretation of the option.

1.2.  Document Goals

   Publication of this memo is intended to serve multiple purposes.

   First and foremost, the document intends to inform readers about a
   mechanism that is in broad use on the public Internet.  The authors
   are each affiliated with companies that have implemented and/or
   deployed systems that use the HOST_ID option on the public Internet.
   Other systems might encounter packets that contain this TCP option,
   and this document is intended to help others understand the nature of
   the TCP option when it is encountered so they can make informed
   decisions about how to handle it.

   The testing effort documented in
   [I-D.abdo-hostid-tcpopt-implementation] indicated that a TCP option
   could be used for host identification purposes without significant
   disruption of TCP connectivity to legacy servers and networks that do
   not support the option.  It also showed how mechanisms available in
   existing TCP implementations could make use of such a TCP option for
   diagnostics and/or packet filtering.  The authors' uses of the TCP
   option on the public Internet has confirmed that it can be used
   effectively for our use cases, but it has also uncovered some
   interoperability issues associated with the option's use on the
   public Internet, especially regarding interactions with other TCP
   options that support new transport capability being specified within
   the IETF.  Section 6 discusses those interactions and limitations and
   our systems' handling of associated issues.

   Discussions within the IETF have raised privacy concerns about the
   option's use, especially as regards pervasive monitoring risks.
   Existing uses of the option limit the nature of the HOST_ID values
   that are used and the systems that insert them in order to mitigate
   pervasive monitoring risks.  Section 8 and Section 9 discuss the
   authors' assessments of the privacy and monitoring impact of this TCP
   option in its current uses and suggest behavior for some external
   systems when the option is encountered.  Continued discussion
   following publication of this memo is expected to allow further
   refinement of requirements related to the values used to populate the
   option and how those values can be interpreted by the receiver.
   There is a trade-off between providing the expected functionality to
   the receiver and protecting the privacy of the sender, and continued
   assessment will be necessary in order to find the right balance.

Williams, et al.        Expires December 16, 2016               [Page 5]

Internet-Draft       Experimental TCP Host ID Option           June 2016

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Option Format

   When used for host identification, the TCP experimental option uses
   the experiment identification mechanism described in [RFC6994] and
   has the following format and content.

    0          1          2          3
    01234567 89012345 67890123 45678901
   |  Kind  | Length |       ExID      |
   |  Host ID ...

   Kind:  The option kind value is 253

   Length:  The length of the option is variable, based on the required
      size of the host identifier (e.g. a 2 octet host ID will require a
      length of 6, while a 4 octet host ID will require a length of 8).

   ExID:  The experiment ID value is 0x0348 (840).

   Host ID:  The host identifier is a value that can be used to
      differentiate among the various hosts sharing a common public IP
      address.  See below for further discussion of this value.

4.  Option Use

   This section describes requirements associated with the use of the
   option, including: expected option values, which hosts are allowed to
   include the option, and segments that include the option.

4.1.  Option Values

   The information conveyed in the HOST_ID option is intended to
   uniquely identify the sending host to the best capability of the
   machine that adds the option to the segment, while at the same time
   avoiding inclusion of information that does not assist this purpose.
   In addition, the option is not intended to be used to expose
   information about the sending host that could not be discovered by
   observing segments in transit on some portion of the Internet path
   between the sender and the receiver.  Existing use cases have

Williams, et al.        Expires December 16, 2016               [Page 6]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   different requirements for receiver side functionality, so this
   document attempts to provide a high degree of flexibility for the
   machine that adds the option to TCP segments.

   The HOST_ID option value MUST correlate to IP addresses and/or TCP
   port numbers that were changed by the inserting host/device (i.e.,
   some of the IP address and/or port number bits are used to generate
   the HOST_ID).  Example values that satisfy this requirement include
   the following:

   Unique ID:  An inserting host/device could maintain a pool of locally
      unique ID values that are dynamically mapped to the unique source
      IP address values in use behind the host/device as a result of
      address sharing.  This ID value would be meaningful only within
      the context of a specific shared IP address due to the local
      uniqueness characteristic.  Such an ID value could be smaller than
      an IP address (e.g. 16-bits) in order to conserve TCP option
      space.  This option is preferred because it does not increase IP
      address visibility on the forward side of the address sharing
      system, and it SHOULD be used in cases where receiver side
      requirements can be met without direct inclusion of the original
      IP address (e.g. some load balancing uses).

   IP Address/Subnet:  An inserting host/device could simply populate
      the option value with the IP address value in use behind the host/
      device.  In the case of IPv6 addresses, it could be difficult to
      include the full address due to TCP option space constraints, so
      the value would likely need to provide only a portion of the
      address (e.g. the first 64 bits).

   IP Address and TCP Port:  Some networks share public IP addresses
      among multiple subscribers with a portion of the TCP port number
      space being assigned to each subscriber [RFC6346].  When such a
      system is behind an address sharing host/device, inclusion of both
      the IP address and the TCP port number will more uniquely identify
      the sending host than just the IP address on its own.

   When multiple host identifiers are necessary (e.g.  an IP address and
   a port number), the HOST_ID option is included multiple times within
   the packet, once for each identifier.  While this approach
   significantly increases option space utilization when multiple
   identifiers are included, cases where only a single identifier is
   included are expected to be more common and thus it is beneficial to
   optimize for those cases.  Note that some middleboxes might reorder
   TCP options, so this method could be problematic if such a middlebox
   is in-path between the address sharing system and the receiver.  This
   has not proven to be a problem for existing use cases.

Williams, et al.        Expires December 16, 2016               [Page 7]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   See Section 8 below for discussion of privacy considerations related
   to selection of HOST_ID values.

4.2.  Sending Host Requirements

   The HOST_ID option MUST only be added by the sending host or any
   device involved in the forwarding path that changes IP addresses and/
   or TCP port numbers (e.g., NAT44 [RFC3022], Layer-2 Aware NAT, DS-
   Lite AFTR [RFC6333], NPTv6 [RFC6296], NAT64 [RFC6146], Dual-Stack
   Extra Lite [RFC6619], TCP Proxy, etc.).  The HOST_ID option MUST NOT
   be added or modified en-route by any device that does not modify IP
   addresses and/or TCP port numbers.

   The sending host or intermediary device cannot determine whether the
   option value is used in a stateful manner by the receiver, nor can it
   determine whether SYN cookies are in use by the receiver.  For this
   reason, the option MUST be included in all segments, both SYN and
   non-SYN segments, until return segments from the receiver positively
   indicate that the TCP connection is fully established on the receiver
   (e.g. the return segment either includes or acknowledges data).

4.2.1.  Alternative SYN Cookie Support

   The authors have also considered an alternative approach to SYN
   cookie support in which the receiving host (i.e.  the host that
   accepts the TCP connection) to echo the option back to the sender in
   the SYN/ACK segment when a SYN cookie is being sent.  This would
   allow the host sending HOST_ID to determine whether further inclusion
   of the option is necessary.  This approach would have the benefit of
   not requiring inclusion of the option in non-SYN segments if SYN
   cookies had not been used.  Unfortunately, this approach fails if the
   responding host itself does not support the option, since an
   intermediate node would have no way to determine that SYN cookies had
   been used.

4.2.2.  Persistent TCP Connections

   Some types of middleboxes (e.g. application proxy) open and maintain
   persistent TCP connections to regularly visited destinations in order
   to minimize connection establishment burden.  Such middleboxes might
   use a single persistent TCP connection for multiple different client
   hosts over the life of the persistent connection.

   This specification does not attempt to support the use of persistent
   TCP connections for multiple client hosts due to the perceived
   complexity of providing such support.  Instead, the HOST_ID option is
   only allowed to be used at connection initiation.  An inserting host/
   device that supports both the HOST_ID option and multi-client

Williams, et al.        Expires December 16, 2016               [Page 8]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   persistent TCP connections MUST NOT apply the HOST_ID option to TCP
   connections that could be used for multiple clients over the life of
   the connection.  If the HOST_ID option was sent during connection
   initiation, the inserting host/device MUST NOT reuse the connection
   for data flows originating from a client that would require a
   different HOST_ID value.

4.2.3.  Packet Fragmentation

   In order to avoid the overhead associated with in-path IP
   fragmentation, it is desirable for the inserting host/device to avoid
   including the HOST_ID option when IP fragmentation might be required.
   This is not a firm requirement, though, because the HOST_ID option is
   only included in the first few packets of a TCP connection and thus
   associated IP fragmentation will generally have minimal impact.  The
   option SHOULD NOT be included in packets if the resulting packet
   would require local fragmentation.

   It can be difficult to determine whether local fragmentation would be
   required.  For example, in cases where multiple interfaces with
   different MTUs are in use, a local routing decision has to be made
   before the MTU can be determined and in some systems this decision
   could be made after TCP option handling is complete.  Additionally,
   it could be true that inclusion of the option causes the packet to
   violate the path's MTU but that the path's MTU has not been learned
   yet on the sending host/device.

   In existing deployed systems, the impact of IP fragmentation that
   results from use of the option has been minimal.

4.3.  Multiple In-Path HOST_ID Senders

   The possibility exists that there could be multiple in-path hosts/
   devices configured to insert the HOST_ID option.  For example, the
   client's TCP packets might first traverse a CGN device on their way
   to the edge of a public Internet overlay network.  In order for the
   HOST_ID value to most uniquely identify the sender, it needs to
   represent both the identity observed by the CGN device (the
   subscriber's internal IP address, e.g.  [RFC6598]) and the identity
   observed by the overlay network (the shared address of the CGN
   device).  The mechanism for handling the received HOST_ID value could
   vary depending upon the nature of the new HOST_ID value to be
   inserted, as described below.

   The problem of multiple in-path HOST_ID senders has not been observed
   in existing deployed systems.  For this reason, existing
   implementations do not consistently support this scenario.  Some

Williams, et al.        Expires December 16, 2016               [Page 9]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   systems do not propagate forward the received HOST_ID option value in
   any way, while other systems follow the guidance described below.

   An inserting host/device that uses the received packet's source IP
   address as the HOST_ID value (possibly along with the port) MUST
   propagate forward the HOST_ID value(s) from the received packet,
   since the source IP address and port only represent the previous in-
   path address sharing device and do not represent the original sender.
   In the CGN-plus-overlay example, this means that the overlay will
   include both the CGN's HOST_ID value(s) and a HOST_ID with the source
   IP address received by the overlay.

   An inserting host/device that sends a unique ID (as described in
   Section 4.1) has two options for how to handle the HOST_ID value(s)
   from the received packet.

   1.  A host/device that sends a unique ID MAY strip the received
       HOST_ID option and insert its own option, provided that it uses
       the received HOST_ID value as a differentiator for selecting the
       unique ID.  What this means in the CGN-plus-overlay example above
       is that the overlay is allowed to drop the HOST_ID value inserted
       by the CGN provided that the HOST_ID value selected by the
       overlay represents both the CGN itself and the HOST_ID value
       inserted by the CGN.

   2.  A host/device that sends a unique ID MAY instead select a unique
       ID that represents only the previous in-path address-sharing
       host/device and propagate forward the HOST_ID value inserted by
       the previous host/device.  In the CGN-plus-overlay example, this
       means that the overlay would include both the CGN's HOST_ID value
       and a HOST_ID with a unique ID of its own that was selected to
       represent the CGN's shared address.

   An inserting host/device that sends a unique ID MUST use one of the
   above two mechanisms.

5.  Option Interpretation

   Due to the variable nature of the option value, it is not possible
   for the receiving machine to reliably determine the value type from
   the option itself.  For this reason, a receiving host/device SHOULD
   interpret the option value as an opaque identifier.

   This specification allows the inserting host/device to provide
   multiple HOST_ID options.  The order of appearance of TCP options
   could be modified by some middleboxes, so receivers SHOULD NOT rely
   on option order to provide additional meaning to the individual
   options.  Instead, when multiple HOST_ID options are present, their

Williams, et al.        Expires December 16, 2016              [Page 10]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   values SHOULD be concatenated together in the order in which they
   appear in the packet and treated as a single large identifier.

   For both of the receiver requirements discussed above, this
   specification uses SHOULD rather than MUST because reliable
   interpretation and ordering of options could be possible if the
   inserting host and the interpreting host are under common
   administrative control and integrity protect communication between
   the inserting host and the interpreting host.  Mechanisms for
   signaling the value type(s) and integrity protection are not provided
   by this specification, and in their absence the receiving host/device
   MUST interpret the option value(s) as a single opaque identifier.

6.  Interaction with Other TCP Options

   This section details how the HOST_ID option functions in conjunction
   with other TCP options.

6.1.  Multipath TCP (MPTCP)

   TCP provides for a maximum of 40 octets for TCP options.  As
   discussed in Appendix A of MPTCP [RFC6824], a typical SYN from
   modern, popular operating systems contains several TCP options (MSS,
   window scale, SACK permitted, and timestamp) which consume 19-24
   octets depending on word alignment of the options.  The initial SYN
   from a multipath TCP client would consume an additional 16 octets.

   HOST_ID needs at least 6 octets to be useful, so 9-21 octets are
   sufficient for many scenarios that benefit from HOST_ID.  However, 4
   octets are not enough space for the HOST_ID option.  Thus, a TCP SYN
   containing all the typical TCP options (MSS, window Scale, SACK
   permitted, timestamp), and also containing multipath capable or
   multipath join, and also being word aligned, has insufficient space
   to accommodate HOST_ID.  This means something has to give.  The
   choices are either to avoid word alignment in that case (freeing 5
   octets) or avoid adding the HOST_ID option.  Each of these approaches
   is used in existing implementations and has been deemed acceptable
   for the associated use case.

6.2.  Authentication Option (TCP-AO)

   The TCP-AO option [RFC5925] is incompatible with address sharing due
   to the fact that it provides integrity protection of the source IP
   address.  For this reason, the only use cases where it makes sense to
   combine TCP-AO and HOST_ID are those where the TCP-AO-NAT extension
   [RFC6978] is in use.  Injecting a HOST_ID TCP option does not
   interfere with the use of TCP-AO-NAT because the TCP options are not
   included in the MAC calculation.

Williams, et al.        Expires December 16, 2016              [Page 11]

Internet-Draft       Experimental TCP Host ID Option           June 2016

6.3.  TCP Fast Open (TFO)

   The TFO option [RFC7413] uses a zero length cookie (total option
   length 2 bytes) to request a TFO cookie for use on future
   connections.  The server-generated TFO cookie is required to be at
   least 4 bytes long and allowed to be as long as 16 bytes (total
   option length 6 to 18 bytes).  The cookie request form of the option
   leaves enough room available in a SYN packet with the most commonly
   used options to accommodate the HOST_ID option, but a valid TFO
   cookie length of any longer than 13 bytes would prevent even the
   minimal 6 byte HOST_ID option from being included in the header.

   There are multiple possibilities for allowing TFO and HOST_ID to be
   supported for the same connection, including:

   o  If the TFO implementation allows the cookie size to be
      configurable, the configured cookie size can be specifically
      selected to leave enough option space available in a typical TFO
      SYN packet to allow inclusion of the HOST_ID option.

   o  If the TFO implementation provides explicit support for the
      HOST_ID option, it can be designed to use a shorter cookie length
      when the HOST_ID option is present in the TFO cookie request SYN.

   Reducing the TFO cookie size in order to include the HOST_ID option
   could have unacceptable security implications, and so existing
   deployed systems that use the HOST_ID option consider TFO and HOST_ID
   to be mutually exclusive and do not support the use of both options
   on the same TCP connection.

   It should also be noted that the presence of data in a TFO SYN
   increases the likelihood that there will be no space available in the
   SYN packet to support inclusion of the HOST_ID option without IP
   fragmentation, even if there is enough room in the TCP option space.
   This is an additional reason existing system consider TFO and HOST_ID
   to be mutually exclusive.

7.  Security Considerations

   Security (including privacy) considerations common to all HOST_ID
   solutions are discussed in [RFC6967].

   The content of the HOST_ID option SHOULD NOT be used for purposes
   that require a trust relationship between the sender and the receiver
   (e.g. billing and/or subscriber policy enforcement).  This
   requirement uses SHOULD rather than MUST because reliable
   interpretation of options could be possible if the inserting host and
   the interpreting host are under common administrative control and

Williams, et al.        Expires December 16, 2016              [Page 12]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   integrity protect communication between the inserting host and the
   interpreting host.  Mechanisms for signaling the value type(s) and
   integrity protection are not provided by this specification, and in
   their absence the receiving host/device MUST NOT use the HOST_ID
   value for purposes that require a trust relationship.

   Note that the above trust requirement applies equally to HOST_ID
   option values propagated forward from a previous in-path host as
   described in Section 4.3.  In other words, if the trust mechanism
   does not apply to all option values in the packet, then none of the
   HOST_ID values can be considered trusted and the receiving host/
   device MUST NOT use any of the HOST_ID values for purposes that
   require a trust relationship.  An inserting host/device that has such
   a trust relationship MUST NOT propagate forward an untrusted HOST_ID
   in such a way as to allow it to be considered trusted.

   When the receiving network uses the values provided by the option in
   a way that does not require trust (e.g. maintaining session affinity
   in a load-balancing system), then use of a mechanism to enforce the
   trust relationship is OPTIONAL.

8.  Privacy Considerations

   Sending a TCP SYN across the public Internet necessarily discloses
   the public IP address of the sending host.  When an intermediate
   address sharing device is deployed on the public Internet, anonymity
   of the hosts using the device will be increased, with hosts
   represented by multiple source IP addresses on the ingress side of
   the device using a single source IP address on the egress side.  The
   HOST_ID TCP option removes that increased anonymity, taking
   information that was already visible in TCP packets on the public
   Internet on the ingress side of the address sharing device and making
   it available on the egress side of the device as well.  In some
   cases, an explicit purpose of the address sharing device is
   anonymity, in which case use of the HOST_ID TCP option would be
   incompatible with the purpose of the device.

   A NAT device used to provide interoperability between a local area
   network (LAN) using private [RFC1918] IP addresses and the public
   Internet is sometimes specifically intended to provide anonymity for
   the LAN clients as described in the above paragraph.  For this
   reason, address sharing devices at the border between a private LAN
   and the public Internet MUST NOT insert the HOST_ID option.

   The HOST_ID option MUST NOT be used to provide client geographic or
   network location information that was not publicly visible in IP
   packets for the TCP flows processed by the inserting host.  For
   example, the client's IP address MAY be used as the HOST_ID option

Williams, et al.        Expires December 16, 2016              [Page 13]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   value, but any geographic or network location information derived
   from the client's IP address MUST NOT be used as the HOST_ID value.

   The HOST_ID option MAY provide differentiating information that is
   locally unique such that individual TCP flows processed by the
   inserting host can be reliably identified.  The HOST_ID option MUST
   NOT provide client identification information that was not publicly
   visible in IP packets for the TCP flows processed by the inserting
   host, such as subscriber information linked to the IP address.

   The HOST_ID value MUST be changed whenever the subscriber IP address
   changes.  This requirement ensures that the HOST_ID option does not
   introduce a new globally unique identifier that persists across
   subscriber IP address changes.

   The HOST_ID option MUST be stripped from IP packets traversing middle
   boxes that provide network-based anonymity services.

9.  Pervasive Monitoring Considerations

   [RFC7258] provides the following guidance: "those developing IETF
   specifications need to be able to describe how they have considered
   Pervasive Monitoring, and, if the attack is relevant to the work to
   be published, be able to justify related design decisions."
   Legitimate concerns about host identification have been raised within
   the IETF.  The authors of this memo have attempted to address those
   concerns by providing details about the nature of the HOST_ID values
   and the types of middleboxes that should and should not be including
   the HOST_ID option in TCP headers, which describes limitations
   already imposed by existing deployed systems.  This section is
   intended to highlight some particularly important aspects of this
   design and the related guidance/limitations that are relevant to the
   pervasive monitoring discussion.

   When a generated identifier is used, this document prohibits the
   address sharing device from using globally unique or permanent
   identifiers.  Only locally unique identifiers are allowed.  As with
   persistent IP addresses, persistent HOST_ID values could facilitate
   user tracking and are therefore prohibited.  The specific
   requirements for permissible HOST_ID values are discussed in
   Section 8 and Section 4.1.

   This specification does not target exposing a host beyond what the
   original packet, issued from that host, would have already exposed on
   the public Internet without introduction of the option.  The option
   is intended only to carry forward information that was conveyed to
   the address-sharing device in the original packet, and HOST_ID option
   values that do not match this description are prohibited by

Williams, et al.        Expires December 16, 2016              [Page 14]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   requirements discussed in Section 8.  This design does not allow the
   HOST_ID option to carry personally identifiable information,
   geographic location identifiers, or any other information that is not
   available in the wire format of the associated TCP/IP headers.

   This document's guidance on option values is followed in existing
   deployed system.  Thus, the volatility of the information conveyed in
   a HOST_ID option is similar to that of the public, subscriber IP
   address.  A distinct HOST_ID is used by the address-sharing function
   when the host reboots or gets a new public IP address from the
   subscriber network.

   The described TCP option allows network identification to a similar
   level as the first 64 bits of an IPv6 address.  That is, the server
   can use the bits of the TCP option to help identify a host behind an
   address-sharing device, in much the same way the server would use the
   host's IPv6 network address if the client and server were using IPv6

   Some address-sharing middleboxes on the public Internet have the
   express intention of providing originator anonymity.  Publication of
   this document can help such middleboxes recognize the associated risk
   and take action to mitigate it (e.g. by stripping or modifying the
   option value).

10.  IANA Considerations

   This document specifies a new TCP option that uses the shared
   experimental options format [RFC6994], with ExID=0x0348 (840) in
   network-standard byte order.  This ExID has already been registered
   with IANA.

11.  Acknowledgements

   Many thanks to W.  Eddy, Y.  Nishida, T.  Reddy, M.  Scharf, J.
   Touch, A.  Zimmermann, and A.  Falk for their comments.

12.  References

12.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

Williams, et al.        Expires December 16, 2016              [Page 15]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,

12.2.  Informative References

              Abdo, E., Boucadair, M., and J. Queiroz, "HOST_ID TCP
              Options: Implementation & Preliminary Test Results",
              draft-abdo-hostid-tcpopt-implementation-03 (work in
              progress), July 2012.

              Williams, B., "Overlay Path Option for IP and TCP", draft-
              williams-overlaypath-ip-tcp-rfc-04 (work in progress),
              June 2013.

              Yourtchenko, A. and D. Wing, "Revealing hosts sharing an
              IP address using TCP option", draft-wing-nat-reveal-
              option-03 (work in progress), December 2011.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",
              RFC 1919, DOI 10.17487/RFC1919, March 1996,

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135,
              DOI 10.17487/RFC3135, June 2001,

Williams, et al.        Expires December 16, 2016              [Page 16]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,

   [RFC6346]  Bush, R., Ed., "The Address plus Port (A+P) Approach to
              the IPv4 Address Shortage", RFC 6346,
              DOI 10.17487/RFC6346, August 2011,

   [RFC6598]  Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
              M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
              Space", BCP 153, RFC 6598, DOI 10.17487/RFC6598, April
              2012, <>.

   [RFC6619]  Arkko, J., Eggert, L., and M. Townsley, "Scalable
              Operation of Address Translators with Per-Interface
              Bindings", RFC 6619, DOI 10.17487/RFC6619, June 2012,

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,

   [RFC6888]  Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
              A., and H. Ashida, "Common Requirements for Carrier-Grade
              NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
              April 2013, <>.

Williams, et al.        Expires December 16, 2016              [Page 17]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   [RFC6967]  Boucadair, M., Touch, J., Levis, P., and R. Penno,
              "Analysis of Potential Solutions for Revealing a Host
              Identifier (HOST_ID) in Shared Address Deployments",
              RFC 6967, DOI 10.17487/RFC6967, June 2013,

   [RFC6978]  Touch, J., "A TCP Authentication Option Extension for NAT
              Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7620]  Boucadair, M., Ed., Chatras, B., Reddy, T., Williams, B.,
              and B. Sarikaya, "Scenarios with Host Identification
              Complications", RFC 7620, DOI 10.17487/RFC7620, August
              2015, <>.

Appendix A.  Change History

   [Note to RFC Editor: Please remove this section prior to

A.1.  Changes from version 07 to 08

   Changed document category from experimental to informational.

   Updated text throughout the document to further document that the
   option is in use on the public Internet and high-lighted specifics of
   how the option is used in existing implementations, especially when
   those implementations deviate from the document's recommendations.

   Added text to further clarify that the document does not represent
   IETF consensus, especially due to concerns about privacy and
   pervasive monitoring.

A.2.  Changes from version 06 to 07

   Clarified pervasive monitoring considerations and added back-pointers
   to where the requirements are more clearly called out.

Williams, et al.        Expires December 16, 2016              [Page 18]

Internet-Draft       Experimental TCP Host ID Option           June 2016

A.3.  Changes from version 05 to 06

   Re-write the introduction to clarify that this document describes a
   practice that is in use on the public Internet today, and that the
   purpose of the document is publish design, deployment, and privacy
   considerations related to its use.

   Correct wording in the abstract to clarify that the IETF has not
   indicated support for host identification, but rather than proposals
   discussed within the IETF have done so.

   Add a section that summarizes the authors' understanding of the
   impact on pervasive monitoring to re-enforce the importance of
   following the document's related guidance.

A.4.  Changes from version 04 to 05

   Make this document self-contained, rather than referring readers to
   use-cases and requirements contained in other I.D.s that were never
   published as RFCs.

   Add discussion of TCP Fast Open.

   Correct some discussion of TCP-AO and TCP-AO-NAT.

   Clarify exactly what the identifier is identifying.

   Improve discussion on interpretation of multiple instances of the
   option, including order of interpretation and set interpretation.

   Evaluated whether use of multiple identifiers should be constrained.
   This is unclear, and so left for the experiment to determine.

   Discuss the possibility of the option value changing over the life of
   the connection (spec now prohibits this).

   Clarify use cases related to stripping and replacing the option.

   Add discussion of non-local fragmentation.

   Evaluate the reliability of attempts to exclude the option when local
   fragmentation would be required.

   Clarify the security requirements re: trust relationship.
   Specifically calls out that common admin control and authentication
   can allow additional uses.

Williams, et al.        Expires December 16, 2016              [Page 19]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   Clarify privacy considerations regarding NATs that separate private
   and public networks.

   Remove restatement of requirements from other documents.

   Justify use of SHOULD rather than MUST throughout.

A.5.  Changes from version 03 to 04

   Improve discussion of RFC6967.

   Don't use "message" to describe TCP segments.

   Add reference to RFC6994 to section 3.

   Clarify that this specifications supersedes earlier documents.

   Improve discussion of SYN cookie handling.

   Remove lower case uses of keywords (e.g. must, should, etc.)
   throughout the document.

   Some stronger privacy guidance, replacing SHOULD with MUST.

   Add an experiment goal related to optimal option value.

   Add text related to the identification goals of the option value
   (still needs more work).

A.6.  Changes from version 02 to 03

   Clarification of arguments in favor of this approach.

   Add discussion of important use cases.

   Clarification of experiment goals and earlier test results.

A.7.  Changes from version 01 to 02

   Add note re: order of appearance.

A.8.  Changes from version 00 to 01

   Add discussion of experiment goals.

   Limit external references to the earlier specifications.

   Add guidance to limit the types of device that add the option.

Williams, et al.        Expires December 16, 2016              [Page 20]

Internet-Draft       Experimental TCP Host ID Option           June 2016

   Improve/correct discussion of TCP-AO and security.

Authors' Addresses

   Brandon Williams
   Akamai, Inc.
   8 Cambridge Center
   Cambridge, MA  02142


   Mohamed Boucadair
   France Telecom
   Rennes, 35000


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134


Williams, et al.        Expires December 16, 2016              [Page 21]