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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pervasive monitoring as an attack and should therefore not be
published without IETF review and IESG approval.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
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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.
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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
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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.
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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.
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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].
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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.
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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.
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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
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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.
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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.
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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.
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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
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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
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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
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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>.
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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>.
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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
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