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rfc:rfc7974

Independent Submission B. Williams Request for Comments: 7974 Akamai, Inc. Category: Informational M. Boucadair ISSN: 2070-1721 Orange

                                                               D. Wing
                                                          October 2016
         An Experimental TCP Option for Host Identification

Abstract

 Recent RFCs have discussed issues with host identification in IP
 address-sharing systems, such as 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 identifier.

Independent Submissions Editor Note

 This Informational document specifies an experimental TCP HOST_ID
 option that is already fairly widely deployed.  It discusses that
 option's privacy considerations in considerable detail and highlights
 the care providers need to exercise in any actual deployment.  The
 Independent Submissions Editor has chosen to publish this document in
 the Independent Stream so that potential deployers and implementors
 can understand all its details, so as to produce implementations that
 will interwork properly with other (existing) deployments.

IESG Note

 This proposal was previously proposed for adoption by the TCPM
 working group and rejected as being an undesirable technical design
 for both transport and privacy reasons.  This document specifies a
 new TCP option that uses the shared experimental options format.  The
 use of experimental TCP options is specified in [RFC6994] for TCP
 options "that are not yet eligible for assigned codepoints".  As this
 proposal has been rejected by the IETF community, it is not eligible
 for the registration of a TCP option codepoint.  It should be further
 noted that for experimental TCP options, it "is only appropriate to
 use these values in explicitly-configured experiments; they MUST NOT
 be shipped as defaults in implementations" [RFC4727].  The IESG also
 carried out a review as described in [RFC5742] and concluded that
 this proposal violates IETF principles expressed in [RFC7258] about
 pervasive monitoring as an attack and should therefore not be
 published without IETF review and IESG approval.  (The process

Williams, et al. Informational [Page 1] RFC 7974 Experimental TCP HOST_ID Option October 2016

 described in [RFC5742] nonetheless allows the Independent Submissions
 Editor to publish, as has been chosen in this case.)  Deployments of
 this proprietary TCP option may be widely viewed as undermining
 privacy and are likely to encounter issues with reliability of
 transport.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7974.

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
 (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.

Williams, et al. Informational [Page 2] RFC 7974 Experimental TCP HOST_ID Option October 2016

Table of Contents

 1. Introduction ....................................................4
    1.1. Important Use Cases ........................................4
    1.2. Document Goals .............................................6
 2. Terminology .....................................................6
 3. Option Format ...................................................7
 4. Option Use ......................................................7
    4.1. Option Values ..............................................7
    4.2. Sending Host Requirements ..................................9
         4.2.1. Alternative SYN Cookie Support ......................9
         4.2.2. Persistent TCP Connections ..........................9
         4.2.3. Packet Fragmentation ...............................10
    4.3. Multiple In-Path HOST_ID Senders ..........................10
 5. Option Interpretation ..........................................11
 6. Interaction with Other TCP Options .............................12
    6.1. Multipath TCP (MPTCP) .....................................12
    6.2. Authentication Option (TCP-AO) ............................12
    6.3. TCP Fast Open (TFO) .......................................13
 7. Security Considerations ........................................13
 8. Privacy Considerations .........................................14
 9. Pervasive Monitoring (PM) Considerations .......................15
 10. IANA Considerations ...........................................16
 11. References ....................................................16
    11.1. Normative References .....................................16
    11.2. Informative References ...................................17
 Acknowledgements ..................................................20
 Authors' Addresses ................................................20

Williams, et al. Informational [Page 3] RFC 7974 Experimental TCP HOST_ID Option October 2016

1. Introduction

 A broad range of issues associated with address sharing have been
 documented in [RFC6269] and [RFC7620].  In addition, [RFC6967]
 provides an 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 [RFC793] 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 documents have defined TCP options for the purpose of host
 identification: [REVEAL], [HOSTID], and [OVERLAYPATH].  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
 documents have worked together to define a common TCP option that
 supersedes the formats from those three documents.  This memo
 describes a version of that common TCP option format that is
 currently in use on the public Internet.
 The option defined in this memo uses the TCP experimental option
 codepoint sharing mechanism defined in [RFC6994].  One of the earlier
 specifications, [OVERLAYPATH], is associated with unauthorized use of
 a TCP option kind number, and moving to the TCP experimental option
 codepoint has allowed the authors of that document to correct their
 error.

1.1. Important Use Cases

 The authors' implementations have primarily focused 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 endpoint 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
    for each of the connections and relaying data flows between the
    connections.

Williams, et al. Informational [Page 4] RFC 7974 Experimental TCP HOST_ID Option October 2016

 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 applied to an
 individual TCP packet either at the connection endpoint (e.g., an
 application proxy or a transport-layer overlay network) or at an
 address-sharing middlebox (e.g., a CGN or a network-layer overlay
 network).  See Section 4 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
 following:
 o  Differentiating between attack and non-attack traffic when the
    source of the attack is sharing an address with non-attack
    traffic.
 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
 an effective way to satisfy these specific use cases.  See Section 5
 for details about receiver-side interpretation of the option.

Williams, et al. Informational [Page 5] RFC 7974 Experimental TCP HOST_ID Option October 2016

1.2. Document Goals

 Publication of this memo is intended to serve multiple purposes.
 First and foremost, this 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, tested,
 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 [HOSTID] 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' use 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
 explains how our systems handle associated issues.
 Discussions within the IETF have raised privacy concerns about the
 option's use, especially in regard to 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.  Sections 8 and 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.

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 [RFC2119].

Williams, et al. Informational [Page 6] RFC 7974 Experimental TCP HOST_ID Option October 2016

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
 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.

Williams, et al. Informational [Page 7] RFC 7974 Experimental TCP HOST_ID Option October 2016

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

Williams, et al. Informational [Page 8] RFC 7974 Experimental TCP HOST_ID Option October 2016

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], L2-Aware NAT, DS-Lite
 Address Family Transition Router (AFTR) [RFC6333], IPv6-to-IPv6
 Network Prefix Translation (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) echoes 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 the burden of connection establishment.  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
 persistent TCP connections MUST NOT apply the HOST_ID option to TCP
 connections that could be used for multiple clients over the life of

Williams, et al. Informational [Page 9] RFC 7974 Experimental TCP HOST_ID Option October 2016

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

Williams, et al. Informational [Page 10] RFC 7974 Experimental TCP HOST_ID Option October 2016

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

Williams, et al. Informational [Page 11] RFC 7974 Experimental TCP HOST_ID Option October 2016

 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
 (Maximum Segment Size), window scale, SACK (selective acknowledgment)
 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, and timestamp) and also containing multipath capable or
 multipath join as well as 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 Authentication Option (TCP-AO) [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 Message Authentication Code (MAC)
 calculation.

Williams, et al. Informational [Page 12] RFC 7974 Experimental TCP HOST_ID Option October 2016

6.3. TCP Fast Open (TFO)

 The TFO option [RFC7413] uses a zero-length cookie (total option
 length is 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 is 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 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, 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 that the existing system considers 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. Informational [Page 13] RFC 7974 Experimental TCP HOST_ID Option October 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

Williams, et al. Informational [Page 14] RFC 7974 Experimental TCP HOST_ID Option October 2016

 example, the client's IP address MAY be used as the HOST_ID option
 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
 middleboxes that provide network-based anonymity services.

9. Pervasive Monitoring (PM) Considerations

 [RFC7258] provides the following guidance: "Those developing IETF
 specifications need to be able to describe how they have considered
 PM, 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 include 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 Sections
 8 and 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

Williams, et al. Informational [Page 15] RFC 7974 Experimental TCP HOST_ID Option October 2016

 values that do not match this description are prohibited by
 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 the 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
 end to end.
 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 (HOST_ID) that uses the
 shared experimental options format [RFC6994], with ExID in network-
 standard byte order.  IANA has registered HOST_ID (0x0348) in the
 "TCP Experimental Option Experiment Identifiers (TCP ExIDs)"
 registry.

11. References

11.1. Normative References

 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, DOI 10.17487/RFC0793, September 1981,
            <http://www.rfc-editor.org/info/rfc793>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.

Williams, et al. Informational [Page 16] RFC 7974 Experimental TCP HOST_ID Option October 2016

 [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
            ICMPv6, UDP, and TCP Headers", RFC 4727,
            DOI 10.17487/RFC4727, November 2006,
            <http://www.rfc-editor.org/info/rfc4727>.
 [RFC5742]  Alvestrand, H. and R. Housley, "IESG Procedures for
            Handling of Independent and IRTF Stream Submissions",
            BCP 92, RFC 5742, DOI 10.17487/RFC5742, December 2009,
            <http://www.rfc-editor.org/info/rfc5742>.
 [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
            RFC 6994, DOI 10.17487/RFC6994, August 2013,
            <http://www.rfc-editor.org/info/rfc6994>.

11.2. Informative References

 [HOSTID]   Abdo, E., Boucadair, M., and J. Queiroz, "HOST_ID TCP
            Options: Implementation & Preliminary Test Results", Work
            in Progress, draft-abdo-hostid-tcpopt-implementation-03,
            July 2012.
 [OVERLAYPATH]
            Williams, B., "Overlay Path Option for IP and TCP", Work
            in Progress, draft-williams-overlaypath-ip-tcp-rfc-04,
            June 2013.
 [REVEAL]   Yourtchenko, A. and D. Wing, "Revealing hosts sharing an
            IP address using TCP option", Work in Progress,
            draft-wing-nat-reveal-option-03, 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,
            <http://www.rfc-editor.org/info/rfc1918>.
 [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",
            RFC 1919, DOI 10.17487/RFC1919, March 1996,
            <http://www.rfc-editor.org/info/rfc1919>.
 [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
            Address Translator (Traditional NAT)", RFC 3022,
            DOI 10.17487/RFC3022, January 2001,
            <http://www.rfc-editor.org/info/rfc3022>.

Williams, et al. Informational [Page 17] RFC 7974 Experimental TCP HOST_ID Option October 2016

 [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,
            <http://www.rfc-editor.org/info/rfc3135>.
 [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
            Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
            June 2010, <http://www.rfc-editor.org/info/rfc5925>.
 [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, <http://www.rfc-editor.org/info/rfc6146>.
 [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,
            <http://www.rfc-editor.org/info/rfc6269>.
 [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
            Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
            <http://www.rfc-editor.org/info/rfc6296>.
 [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,
            <http://www.rfc-editor.org/info/rfc6333>.
 [RFC6346]  Bush, R., Ed., "The Address plus Port (A+P) Approach to
            the IPv4 Address Shortage", RFC 6346,
            DOI 10.17487/RFC6346, August 2011,
            <http://www.rfc-editor.org/info/rfc6346>.
 [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, <http://www.rfc-editor.org/info/rfc6598>.
 [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,
            <http://www.rfc-editor.org/info/rfc6619>.
 [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,
            <http://www.rfc-editor.org/info/rfc6824>.

Williams, et al. Informational [Page 18] RFC 7974 Experimental TCP HOST_ID Option October 2016

 [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, <http://www.rfc-editor.org/info/rfc6888>.
 [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,
            <http://www.rfc-editor.org/info/rfc6967>.
 [RFC6978]  Touch, J., "A TCP Authentication Option Extension for NAT
            Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013,
            <http://www.rfc-editor.org/info/rfc6978>.
 [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
            Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
            2014, <http://www.rfc-editor.org/info/rfc7258>.
 [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
            Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
            <http://www.rfc-editor.org/info/rfc7413>.
 [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, <http://www.rfc-editor.org/info/rfc7620>.

Williams, et al. Informational [Page 19] RFC 7974 Experimental TCP HOST_ID Option October 2016

Acknowledgements

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

Authors' Addresses

 Brandon Williams
 Akamai, Inc.
 8 Cambridge Center
 Cambridge, MA  02142
 United States of America
 Email: brandon.williams@akamai.com
 Mohamed Boucadair
 Orange
 Email: mohamed.boucadair@orange.com
 Dan Wing
 Email: dwing-ietf@fuggles.com

Williams, et al. Informational [Page 20]

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