One document matched: draft-williams-exp-tcp-host-id-opt-06.xml
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<rfc category="exp" ipr="trust200902" docName="draft-williams-exp-tcp-host-id-opt-06">
<front>
<title abbrev="Experimental TCP Host ID Option">Experimental Option for
TCP Host Identification</title>
<author fullname="Brandon Williams" initials="B." surname="Williams">
<organization>Akamai, Inc.</organization>
<address>
<postal>
<street>8 Cambridge Center</street>
<city>Cambridge</city>
<region>MA</region>
<code>02142</code>
<country>USA</country>
</postal>
<email>brandon.williams@akamai.com</email>
</address>
</author>
<author fullname="Mohamed Boucadair" initials="M." surname="Boucadair">
<organization>France Telecom</organization>
<address>
<postal>
<street></street>
<city>Rennes</city>
<region>35000</region>
<code></code>
<country>Fance</country>
</postal>
<email>mohamed.boucadair@orange.com</email>
</address>
</author>
<author fullname="Dan Wing" initials="D." surname="Wing">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>dwing@cisco.com</email>
</address>
</author>
<date year="2015" />
<abstract>
<t>Recent proposals discussed in the IETF have identified benefits to
more distinctly identifying the hosts that are hidden behind a shared
address/prefix sharing device or application-layer proxy. Analysis
indicates that the use of a TCP option for this purpose can be
successfully applied to some use cases. This document discusses
design, deployment, and privacy considerations for such a TCP option
that is in operational use on the Internet today.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>A broad range of issues associated with address sharing have been
well documented in <xref target="RFC6269"></xref> and <xref
target="RFC7620"></xref>. In addition, <xref target="RFC6967"></xref>
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 <xref target="RFC0793"></xref>
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 document is to describe a TCP HOST_ID option that is
currently deployed on the Internet using the TCP experimental option
codepoint, including discussion of related design, deployment, and
privacy considerations.</t>
<t>Multiple recent Internet Drafts define TCP options for the purpose of
host identification: <xref target="I-D.wing-nat-reveal-option"></xref>,
<xref target="I-D.abdo-hostid-tcpopt-implementation"></xref>, and <xref
target="I-D.williams-overlaypath-ip-tcp-rfc"></xref>. Specification of
multiple option formats to serve the purpose of host identification
increases the burden for potential implementers and presents
interoperability challenges as well. This document defines a common TCP
option format that supersedes all three of the above proposals.</t>
<t>The option defined in this document uses the TCP experimental option
codepoint sharing mechanism defined in <xref target="RFC6994"></xref>
and is intended to allow broad deployment of the mechanism on the public
Internet. In addition, one of the referenced specifications, <xref
target="I-D.williams-overlaypath-ip-tcp-rfc"></xref>, is associated
with unauthorized use of a TCP option kind number, and moving to the TCP
experimental option codepoint allows the authors of that document to
correct the error.</t>
<t><xref target="interaction"></xref> of this document discusses
compatibility between this new TCP option and existing commonly deployed
TCP options.</t>
<section anchor="use_cases" title="Important Use Cases">
<t>This memo focuses primarily on the following address-sharing
scenarios where this mechanism is currently in use:
<list style="hanging">
<t hangText="Carrier Grade NAT (CGN):">As defined in
<xref target="RFC6888"/>, <xref target="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.</t>
<t hangText="Application Proxy:">As defined in
<xref target="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.</t>
<t hangText="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
<xref target="RFC3135"></xref> 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.</t>
</list></t>
<t>With this set of scenarios, the TCP option can either be 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 <xref target="use"></xref> below for additional
details about the types of devices that add the option to a TCP
packet, as well as limitations on use of the option when it is to be
inserted by an address-sharing middlebox, including issues related
to packet fragmentation.</t>
<t>The receiver-side use cases considered by this memo include the
following:
<list style="symbols">
<t>Differentiating between attack and non-attack traffic when the
source of the attack is sharing an address with non-attack
traffic.</t>
<t>Application of per-subscriber policies for resource
utilization, etc. when multiple subscribers are sharing a common
address.</t>
<t>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.</t>
</list></t>
<t>In all of the above cases, differentiation between address-sharing
clients commonly needs to be 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 cannot fully meet the receiver-side requirements.
At the same time, as noted in <xref target="RFC6967"></xref>, 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 has been
selected as an effective way to satisfy these specific use
cases.</t>
</section>
<section anchor="experiment_goals" title="Experiment Goals">
<t>The testing effort documented in
<xref target="I-D.abdo-hostid-tcpopt-implementation"></xref>
confirmed 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 improved diagnostics and/or
packet filtering.</t>
<t>Specification of the TCP option described in this memo will enable
additional activity to assess the viability of the option for the
receiver-side use cases discussed above:
<list style="symbols">
<t>Differentiate between attack and non-attack traffic.</t>
<t>Enforce per-client policies.</t>
<t>Assist load-balancing decision-making.</t>
</list>
In particular, documentation of the mechanism is expected to provide
opportunities for engagement with a broader range of both
application and middleware implementations in order to develop a
more complete picture of how well the option meets the use-case
requirements.</t>
<t>Continued experimentation on the public Internet 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
tradeoff between providing the expected functionality to the
receiver and protecting the privacy of the sender, and additional
work is necessary in order to find the right balance. See
<xref target="privacy"></xref> for additional discussion.</t>
<t>Continued experimentation on the public Internet is also expected
to support improved guidance on TCP option interoperability,
especially in the context of Multipath TCP
<xref target="RFC6824"></xref> and TCP Fast Open
<xref target="RFC7413"/>. See <xref target="interaction"></xref> for
additional discussion.</t>
</section>
</section>
<section title="Terminology">
<t>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 <xref
target="RFC2119"></xref>.</t>
</section>
<section anchor="format" title="Option Format">
<t>When used for host identification, the TCP experimental option uses
the experiment identification mechanism described in
<xref target="RFC6994"></xref> and has the following format and
content.</t>
<figure>
<artwork><![CDATA[
0 1 2 3
01234567 89012345 67890123 45678901
+--------+--------+--------+--------+
| Kind | Length | ExID |
+--------+--------+--------+--------+
| Host ID ...
+--------+---
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="Kind:">The option kind value is 253</t>
<t hangText="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).</t>
<t hangText="ExID:">The experiment ID value is 0x0348 (840).</t>
<t hangText="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.</t>
</list></t>
</section>
<section anchor="use" title="Option Use">
<t>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.</t>
<section anchor="opt_values" title="Option Values">
<t>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. As noted in
<xref target="experiment_goals"></xref>, identifying the optimal set
of values to use for this purpose is one of the experimental goals for
this document. For this reason, the document attempts to provide a
high degree of flexibility for the machine that adds the option to TCP
segments.</t>
<t>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:
<list style="hanging">
<t hangText="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.</t>
<t hangText="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).</t>
<t hangText="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
<xref target="RFC6346"></xref>. 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.</t>
</list></t>
<t>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.</t>
<t>See <xref target="privacy"></xref> below for discussion of
privacy considerations related to selection of HOST_ID values.</t>
</section>
<section anchor="host_reqs" title="Sending Host Requirements">
<t>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 <xref target="RFC3022"></xref>,
Layer-2 Aware NAT, DS-Lite AFTR <xref target="RFC6333"></xref>,
NPTv6 <xref target="RFC6296"></xref>, NAT64
<xref target="RFC6146"></xref>, Dual-Stack Extra Lite
<xref target="RFC6619"></xref>, 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.</t>
<t>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).</t>
<section anchor="alt_cookie_support" title="Alternative SYN Cookie Support">
<t>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.</t>
</section>
<section anchor="pconns" title="Persistent TCP Connections">
<t>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.</t>
<t>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 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.</t>
</section>
<section anchor="frag_reqs" title="Packet Fragmentation">
<t>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 have
minimal impact. The option SHOULD NOT be included in packets if
the resulting packet would require local fragmentation.</t>
<t>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.</t>
<t>Due to the difficulty of avoiding IP fragmentation entirely, an
important experimental goal for this document is to evaluate the
impact of IP fragmentation that results from use of the
option.</t>
</section>
</section>
<section anchor="multi_id" title="Multiple In-Path HOST_ID Senders">
<t>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.
<xref target="RFC6598"></xref>) 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.</t>
<t>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.</t>
<t>An inserting host/device that sends a unique ID (as described in
<xref target="opt_values"></xref>) has two options for how to handle
the HOST_ID value(s) from the received packet.
<list style="numbers">
<t>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.</t>
<t>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.</t>
</list>
An inserting host/device that sends a unique ID MUST use one of the
above two mechanisms.</t>
</section>
<section anchor="interpretation" title="Option Interpretation">
<t>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.</t>
<t>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 deployments 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.</t>
<t>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.</t>
</section>
</section>
<section anchor="interaction" title="Interaction with Other TCP Options">
<t>This section details how the HOST_ID option functions in conjunction
with other TCP options.</t>
<section title="Multipath TCP (MPTCP)">
<t>TCP provides for a maximum of 40 octets for TCP options. As
discussed in Appendix A of MPTCP <xref target="RFC6824"></xref>, 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.</t>
<t>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. Although option packing
seems like the best approach, we expect to learn from deployment
experience during the experiment which of these options is most
viable in practice.</t>
</section>
<section title="Authentication Option (TCP-AO)">
<t>The TCP-AO option <xref target="RFC5925"></xref> 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 <xref target="RFC6978"></xref> 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.</t>
</section>
<section title="TCP Fast Open (TFO)">
<t>The TFO option <xref target="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.</t>
<t>There are multiple possibilities for allowing TFO and HOST_ID to be
supported for the same connection, including:
<list style="symbols">
<t>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.</t>
<t>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.</t>
</list>
We expect to learn from deployment experience during the experiment
whether one of these options is workable, or whether the two
mechanisms (TFO and HOST_ID) will be deemed mutually exclusive. In
particular, reducing the TFO cookie size in order to include the
HOST_ID option could have unacceptable security implications.</t>
<t>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 issue could also lead to the conclusion that TFO and HOST_ID
are mutually exclusive.</t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>Security (including privacy) considerations common to all HOST_ID
solutions are discussed in <xref target="RFC6967"></xref>.</t>
<t>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 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.</t>
<t>Note that the above trust requirement applies equally to HOST_ID
option values propagated forward from a previous in-path host as
described in <xref target="multi_id"></xref>. 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.</t>
<t>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.</t>
</section>
<section anchor="privacy" title="Privacy Considerations">
<t>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.</t>
<t>A NAT device used to provide interoperability between a local area
network (LAN) using private <xref target="RFC1918"></xref> 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.</t>
<t>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
value, but any geographic or network location information derived from
the client's IP address MUST NOT be used as the HOST_ID value.</t>
<t>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.</t>
<t>The HOST_ID option MUST be stripped from IP packets traversing middle
boxes that provide network-based anonymity services.</t>
</section>
<section title="Pervasive Monitoring Considerations">
<t><xref target="RFC7258"></xref> 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 guidance to implementors 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.
This section is intended to highlight some particularly important
aspects of this design and the related guidance that are relevant to
the pervasive monitoring discussion.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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).</t>
<t>The proposed 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.</t>
<t>Additional privacy-related considerations are discussed in
<xref target="privacy"></xref>.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>This document specifies a new TCP option that uses the shared
experimental options format <xref target="RFC6994"></xref>, with
ExID=0x0348 (840) in network-standard byte order. This ExID has already
been registered with IANA.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>Many thanks to W. Eddy, Y. Nishida, T. Reddy, M. Scharf, J. Touch,
A. Zimmermann, and A. Falk for their comments.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc0793;
&rfc2119;
&rfc6994;
</references>
<references title="Informative References">
&rfc1918;
&rfc1919;
&rfc3022;
&rfc3135;
&rfc5925;
&rfc6146;
&rfc6269;
&rfc6296;
&rfc6333;
&rfc6346;
&rfc6598;
&rfc6619;
&rfc6824;
&rfc6888;
&rfc6967;
&rfc6978;
&rfc7258;
&rfc7413;
&rfc7620;
&I-D.wing-nat-reveal-option;
&I-D.abdo-hostid-tcpopt-implementation;
&I-D.williams-overlaypath-ip-tcp-rfc;
</references>
<section anchor="change_history" title="Change History">
<t>[Note to RFC Editor: Please remove this section prior to
publication.]</t>
<section title="Changes from version 05 to 06">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Changes from version 04 to 05">
<t>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.</t>
<t>Add discussion of TCP Fast Open.</t>
<t>Correct some discussion of TCP-AO and TCP-AO-NAT.</t>
<t>Clarify exactly what the identifier is identifying.</t>
<t>Improve discussion on interpretation of multiple instances of the
option, including order of interpretation and set interpretation.</t>
<t>Evaluated whether use of multiple identifiers should be constrained.
This is unclear, and so left for the experiment to determine.</t>
<t>Discuss the possibility of the option value changing over the
life of the connection (spec now prohibits this).</t>
<t>Clarify use cases related to stripping and replacing the option.</t>
<t>Add discussion of non-local fragmentation.</t>
<t>Evaluate the reliability of attempts to exclude the option when
local fragmentation would be required.</t>
<t>Clarify the security requirements re: trust relationship.
Specifically calls out that common admin control and authentication
can allow additional uses.</t>
<t>Clarify privacy considerations regarding NATs that separate private
and public networks.</t>
<t>Remove restatement of requirements from other documents.</t>
<t>Justify use of SHOULD rather than MUST throughout.</t>
</section>
<section title="Changes from version 03 to 04">
<t>Improve discussion of RFC6967.</t>
<t>Don't use "message" to describe TCP segments.</t>
<t>Add reference to RFC6994 to section 3.</t>
<t>Clarify that this specifications supersedes earlier documents.</t>
<t>Improve discussion of SYN cookie handling.</t>
<t>Remove lower case uses of keywords (e.g. must, should, etc.)
throughout the document.</t>
<t>Some stronger privacy guidance, replacing SHOULD with MUST.</t>
<t>Add an experiment goal related to optimal option value.</t>
<t>Add text related to the identification goals of the option
value (still needs more work).</t>
</section>
<section title="Changes from version 02 to 03">
<t>Clarification of arguments in favor of this approach.</t>
<t>Add discussion of important use cases.</t>
<t>Clarification of experiment goals and earlier test results.</t>
</section>
<section title="Changes from version 01 to 02">
<t>Add note re: order of appearance.</t>
</section>
<section title="Changes from version 00 to 01">
<t>Add discussion of experiment goals.</t>
<t>Limit external references to the earlier specifications.</t>
<t>Add guidance to limit the types of device that add the option.</t>
<t>Improve/correct discussion of TCP-AO and security.</t>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 08:55:50 |