Internet DRAFT - draft-williams-exp-tcp-host-id-opt
draft-williams-exp-tcp-host-id-opt
Network Working Group B. Williams
Internet-Draft Akamai, Inc.
Intended status: Experimental M. Boucadair
Expires: May 8, 2016 France Telecom
D. Wing
Cisco Systems, Inc.
November 5, 2015
Experimental Option for TCP Host Identification
draft-williams-exp-tcp-host-id-opt-07
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 8, 2016.
Copyright Notice
Copyright (c) 2015 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
Williams, et al. Expires May 8, 2016 [Page 1]
Internet-Draft Experimental TCP Host ID Option November 2015
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Important Use Cases . . . . . . . . . . . . . . . . . . . 3
1.2. Experiment Goals . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Option Format . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Option Use . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Option Values . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Sending Host Requirements . . . . . . . . . . . . . . . . 7
4.2.1. Alternative SYN Cookie Support . . . . . . . . . . . 8
4.2.2. Persistent TCP Connections . . . . . . . . . . . . . 8
4.2.3. Packet Fragmentation . . . . . . . . . . . . . . . . 9
4.3. Multiple In-Path HOST_ID Senders . . . . . . . . . . . . 9
4.4. Option Interpretation . . . . . . . . . . . . . . . . . . 10
5. Interaction with Other TCP Options . . . . . . . . . . . . . 11
5.1. Multipath TCP (MPTCP) . . . . . . . . . . . . . . . . . . 11
5.2. Authentication Option (TCP-AO) . . . . . . . . . . . . . 11
5.3. TCP Fast Open (TFO) . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13
8. Pervasive Monitoring Considerations . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 18
A.1. Changes from version 06 to 07 . . . . . . . . . . . . . . 18
A.2. Changes from version 05 to 06 . . . . . . . . . . . . . . 18
A.3. Changes from version 04 to 05 . . . . . . . . . . . . . . 18
A.4. Changes from version 03 to 04 . . . . . . . . . . . . . . 19
A.5. Changes from version 02 to 03 . . . . . . . . . . . . . . 19
A.6. Changes from version 01 to 02 . . . . . . . . . . . . . . 20
A.7. Changes from version 00 to 01 . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
A broad range of issues associated with address sharing have been
well documented in [RFC6269] and [RFC7620]. In addition, [RFC6967]
provides analysis of various solutions to the problem of revealing
Williams, et al. Expires May 8, 2016 [Page 2]
Internet-Draft Experimental TCP Host ID Option November 2015
the sending host's identifier (HOST_ID) information to the receiver,
indicating that a solution using a TCP [RFC0793] option for this
purpose is among the possible approaches that could be applied with
limited performance impact and a high success ratio. The purpose of
this 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.
Multiple recent Internet Drafts define TCP options for the purpose of
host identification: [I-D.wing-nat-reveal-option],
[I-D.abdo-hostid-tcpopt-implementation], and
[I-D.williams-overlaypath-ip-tcp-rfc]. Specification of multiple
option formats to serve the purpose of host identification increases
the burden for potential implementers and presents interoperability
challenges as well. This document defines a common TCP option format
that supersedes all three of the above proposals.
The option defined in this document uses the TCP experimental option
codepoint sharing mechanism defined in [RFC6994] and is intended to
allow broad deployment of the mechanism on the public Internet. In
addition, one of the referenced specifications,
[I-D.williams-overlaypath-ip-tcp-rfc], is associated with
unauthorized use of a TCP option kind number, and moving to the TCP
experimental option codepoint allows the authors of that document to
correct the error.
Section 5 of this document discusses compatibility between this new
TCP option and existing commonly deployed TCP options.
1.1. Important Use Cases
This memo focuses primarily on the following address-sharing
scenarios where this mechanism is currently in use:
Carrier Grade NAT (CGN): As defined in [RFC6888], [RFC6333], and
other sources, a CGN allows multiple hosts connected to the public
Internet to share a single Internet routable IPv4 address. One
important characteristic of the CGN use case is that it modifies
IP packets in-path, but does not serve as the end point for the
associated TCP connections.
Application Proxy: As defined in [RFC1919], an application proxy
splits a TCP connection into two segments, serving as an endpoint
for each of the connections and relaying data flows between the
connections.
Williams, et al. Expires May 8, 2016 [Page 3]
Internet-Draft Experimental TCP Host ID Option November 2015
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.
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 Section 4 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.
The receiver-side 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 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 [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 has been selected as an
effective way to satisfy these specific use cases.
Williams, et al. Expires May 8, 2016 [Page 4]
Internet-Draft Experimental TCP Host ID Option November 2015
1.2. Experiment Goals
The testing effort documented in
[I-D.abdo-hostid-tcpopt-implementation] 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.
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:
o Differentiate between attack and non-attack traffic.
o Enforce per-client policies.
o Assist load-balancing decision-making.
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.
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 Section 7 for
additional discussion.
Continued experimentation on the public Internet is also expected to
support improved guidance on TCP option interoperability, especially
in the context of Multipath TCP [RFC6824] and TCP Fast Open
[RFC7413]. See Section 5 for additional discussion.
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. Expires May 8, 2016 [Page 5]
Internet-Draft Experimental TCP Host ID Option November 2015
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. As noted in Section 1.2,
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.
Williams, et al. Expires May 8, 2016 [Page 6]
Internet-Draft Experimental TCP Host ID Option November 2015
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.
IP Address/Subnet: An inserting host/device could simply populate
the option value with the IP address value in use behind the host/
device. In the case of IPv6 addresses, it could be difficult to
include the full address due to TCP option space constraints, so
the value would likely need to provide only a portion of the
address (e.g. the first 64 bits).
IP Address and TCP Port: Some networks share public IP addresses
among multiple subscribers with a portion of the TCP port number
space being assigned to each subscriber [RFC6346]. When such a
system is behind an address sharing host/device, inclusion of both
the IP address and the TCP port number will more uniquely identify
the sending host than just the IP address on its own.
When multiple host identifiers are necessary (e.g. an IP address and
a port number), the HOST_ID option is included multiple times within
the packet, once for each identifier. While this approach
significantly increases option space utilization when multiple
identifiers are included, cases where only a single identifier is
included are expected to be more common and thus it is beneficial to
optimize for those cases.
See Section 7 below for discussion of privacy considerations related
to selection of HOST_ID values.
4.2. Sending Host Requirements
The HOST_ID option MUST only be added by the sending host or any
device involved in the forwarding path that changes IP addresses and/
or TCP port numbers (e.g., NAT44 [RFC3022], Layer-2 Aware NAT, DS-
Lite AFTR [RFC6333], NPTv6 [RFC6296], NAT64 [RFC6146], Dual-Stack
Extra Lite [RFC6619], TCP Proxy, etc.). The HOST_ID option MUST NOT
Williams, et al. Expires May 8, 2016 [Page 7]
Internet-Draft Experimental TCP Host ID Option November 2015
be added or modified en-route by any device that does not modify IP
addresses and/or TCP port numbers.
The sending host or intermediary device cannot determine whether the
option value is used in a stateful manner by the receiver, nor can it
determine whether SYN cookies are in use by the receiver. For this
reason, the option MUST be included in all segments, both SYN and
non-SYN segments, until return segments from the receiver positively
indicate that the TCP connection is fully established on the receiver
(e.g. the return segment either includes or acknowledges data).
4.2.1. Alternative SYN Cookie Support
The authors have also considered an alternative approach to SYN
cookie support in which the receiving host (i.e. the host that
accepts the TCP connection) to echo the option back to the sender in
the SYN/ACK segment when a SYN cookie is being sent. This would
allow the host sending HOST_ID to determine whether further inclusion
of the option is necessary. This approach would have the benefit of
not requiring inclusion of the option in non-SYN segments if SYN
cookies had not been used. Unfortunately, this approach fails if the
responding host itself does not support the option, since an
intermediate node would have no way to determine that SYN cookies had
been used.
4.2.2. Persistent TCP Connections
Some types of middleboxes (e.g. application proxy) open and maintain
persistent TCP connections to regularly visited destinations in order
to minimize connection establishment burden. Such middleboxes might
use a single persistent TCP connection for multiple different client
hosts over the life of the persistent connection.
This specification does not attempt to support the use of persistent
TCP connections for multiple client hosts due to the perceived
complexity of providing such support. Instead, the HOST_ID option is
only allowed to be used at connection initiation. An inserting host/
device that supports both the HOST_ID option and multi-client
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.
Williams, et al. Expires May 8, 2016 [Page 8]
Internet-Draft Experimental TCP Host ID Option November 2015
4.2.3. Packet Fragmentation
In order to avoid the overhead associated with in-path IP
fragmentation, it is desirable for the inserting host/device to avoid
including the HOST_ID option when IP fragmentation might be required.
This is not a firm requirement, though, because the HOST_ID option is
only included in the first few packets of a TCP connection and thus
associated IP fragmentation will have minimal impact. The option
SHOULD NOT be included in packets if the resulting packet would
require local fragmentation.
It can be difficult to determine whether local fragmentation would be
required. For example, in cases where multiple interfaces with
different MTUs are in use, a local routing decision has to be made
before the MTU can be determined and in some systems this decision
could be made after TCP option handling is complete. Additionally,
it could be true that inclusion of the option causes the packet to
violate the path's MTU but that the path's MTU has not been learned
yet on the sending host/device.
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.
4.3. Multiple In-Path HOST_ID Senders
The possibility exists that there could be multiple in-path hosts/
devices configured to insert the HOST_ID option. For example, the
client's TCP packets might first traverse a CGN device on their way
to the edge of a public Internet overlay network. In order for the
HOST_ID value to most uniquely identify the sender, it needs to
represent both the identity observed by the CGN device (the
subscriber's internal IP address, e.g. [RFC6598]) and the identity
observed by the overlay network (the shared address of the CGN
device). The mechanism for handling the received HOST_ID value could
vary depending upon the nature of the new HOST_ID value to be
inserted, as described below.
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.
Williams, et al. Expires May 8, 2016 [Page 9]
Internet-Draft Experimental TCP Host ID Option November 2015
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.
4.4. 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 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.
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.
Williams, et al. Expires May 8, 2016 [Page 10]
Internet-Draft Experimental TCP Host ID Option November 2015
5. Interaction with Other TCP Options
This section details how the HOST_ID option functions in conjunction
with other TCP options.
5.1. Multipath TCP (MPTCP)
TCP provides for a maximum of 40 octets for TCP options. As
discussed in Appendix A of MPTCP [RFC6824], a typical SYN from
modern, popular operating systems contains several TCP options (MSS,
window scale, SACK permitted, and timestamp) which consume 19-24
octets depending on word alignment of the options. The initial SYN
from a multipath TCP client would consume an additional 16 octets.
HOST_ID needs at least 6 octets to be useful, so 9-21 octets are
sufficient for many scenarios that benefit from HOST_ID. However, 4
octets are not enough space for the HOST_ID option. Thus, a TCP SYN
containing all the typical TCP options (MSS, window Scale, SACK
permitted, timestamp), and also containing multipath capable or
multipath join, and also being word aligned, has insufficient space
to accommodate HOST_ID. This means something has to give. The
choices are either to avoid word alignment in that case (freeing 5
octets) or avoid adding the HOST_ID option. 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.
5.2. Authentication Option (TCP-AO)
The TCP-AO option [RFC5925] is incompatible with address sharing due
to the fact that it provides integrity protection of the source IP
address. For this reason, the only use cases where it makes sense to
combine TCP-AO and HOST_ID are those where the TCP-AO-NAT extension
[RFC6978] is in use. Injecting a HOST_ID TCP option does not
interfere with the use of TCP-AO-NAT because the TCP options are not
included in the MAC calculation.
5.3. TCP Fast Open (TFO)
The TFO option [RFC7413] uses a zero length cookie (total option
length 2 bytes) to request a TFO cookie for use on future
connections. The server-generated TFO cookie is required to be at
least 4 bytes long and allowed to be as long as 16 bytes (total
option length 6 to 18 bytes). The cookie request form of the option
leaves enough room available in a SYN packet with the most commonly
used options to accommodate the HOST_ID option, but a valid TFO
cookie length of any longer than 13 bytes would prevent even the
minimal 6 byte HOST_ID option from being included in the header.
Williams, et al. Expires May 8, 2016 [Page 11]
Internet-Draft Experimental TCP Host ID Option November 2015
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.
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.
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.
6. 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
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
Williams, et al. Expires May 8, 2016 [Page 12]
Internet-Draft Experimental TCP Host ID Option November 2015
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.
7. Privacy Considerations
Sending a TCP SYN across the public Internet necessarily discloses
the public IP address of the sending host. When an intermediate
address sharing device is deployed on the public Internet, anonymity
of the hosts using the device will be increased, with hosts
represented by multiple source IP addresses on the ingress side of
the device using a single source IP address on the egress side. The
HOST_ID TCP option removes that increased anonymity, taking
information that was already visible in TCP packets on the public
Internet on the ingress side of the address sharing device and making
it available on the egress side of the device as well. In some
cases, an explicit purpose of the address sharing device is
anonymity, in which case use of the HOST_ID TCP option would be
incompatible with the purpose of the device.
A NAT device used to provide interoperability between a local area
network (LAN) using private [RFC1918] IP addresses and the public
Internet is sometimes specifically intended to provide anonymity for
the LAN clients as described in the above paragraph. For this
reason, address sharing devices at the border between a private LAN
and the public Internet MUST NOT insert the HOST_ID option.
The HOST_ID option MUST NOT be used to provide client geographic or
network location information that was not publicly visible in IP
packets for the TCP flows processed by the inserting host. For
example, the client's IP address MAY be used as the HOST_ID option
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
Williams, et al. Expires May 8, 2016 [Page 13]
Internet-Draft Experimental TCP Host ID Option November 2015
introduce a new globally unique identifier that persists across
subscriber IP address changes.
The HOST_ID option MUST be stripped from IP packets traversing middle
boxes that provide network-based anonymity services.
8. Pervasive Monitoring Considerations
[RFC7258] provides the following guidance: "those developing IETF
specifications need to be able to describe how they have considered
Pervasive Monitoring, and, if the attack is relevant to the work to
be published, be able to justify related design decisions."
Legitimate concerns about host identification have been raised within
the IETF. The authors of this memo have attempted to address those
concerns by providing 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.
When a generated identifier is used, this document prohibits the
address sharing device from using globally unique or permanent
identifiers. Only locally unique identifiers are allowed. As with
persistent IP addresses, persistent HOST_ID values could facilitate
user tracking and are therefore prohibited. The specific
requirements for permissible HOST_ID values are discussed in
Section 7 and Section 4.1.
This specification does not target exposing a host beyond what the
original packet, issued from that host, would have already exposed on
the public Internet without introduction of the option. The option
is intended only to carry forward information that was conveyed to
the address-sharing device in the original packet, and HOST_ID option
values that do not match this description are prohibited by
requirements discussed in Section 7. 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.
Provided that this document's guidance on option values is followed,
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 proposed TCP option allows network identification to a similar
level as the first 64 bits of an IPv6 address. That is, the server
Williams, et al. Expires May 8, 2016 [Page 14]
Internet-Draft Experimental TCP Host ID Option November 2015
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).
9. IANA Considerations
This document specifies a new TCP option that uses the shared
experimental options format [RFC6994], with ExID=0x0348 (840) in
network-standard byte order. This ExID has already been registered
with IANA.
10. Acknowledgements
Many thanks to W. Eddy, Y. Nishida, T. Reddy, M. Scharf, J.
Touch, A. Zimmermann, and A. Falk for their comments.
11. References
11.1. Normative References
[RFC0793] 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>.
[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
[I-D.abdo-hostid-tcpopt-implementation]
Abdo, E., Boucadair, M., and J. Queiroz, "HOST_ID TCP
Options: Implementation & Preliminary Test Results",
draft-abdo-hostid-tcpopt-implementation-03 (work in
progress), July 2012.
Williams, et al. Expires May 8, 2016 [Page 15]
Internet-Draft Experimental TCP Host ID Option November 2015
[I-D.williams-overlaypath-ip-tcp-rfc]
Williams, B., "Overlay Path Option for IP and TCP", draft-
williams-overlaypath-ip-tcp-rfc-04 (work in progress),
June 2013.
[I-D.wing-nat-reveal-option]
Yourtchenko, A. and D. Wing, "Revealing hosts sharing an
IP address using TCP option", draft-wing-nat-reveal-
option-03 (work in progress), December 2011.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<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>.
[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>.
Williams, et al. Expires May 8, 2016 [Page 16]
Internet-Draft Experimental TCP Host ID Option November 2015
[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>.
[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>.
Williams, et al. Expires May 8, 2016 [Page 17]
Internet-Draft Experimental TCP Host ID Option November 2015
[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>.
Appendix A. Change History
[Note to RFC Editor: Please remove this section prior to
publication.]
A.1. Changes from version 06 to 07
Clarified pervasive monitoring considerations and added back-pointers
to where the requirements are more clearly called out.
A.2. Changes from version 05 to 06
Re-write the introduction to clarify that this document describes a
practice that is in use on the public Internet today, and that the
purpose of the document is publish design, deployment, and privacy
considerations related to its use.
Correct wording in the abstract to clarify that the IETF has not
indicated support for host identification, but rather than proposals
discussed within the IETF have done so.
Add a section that summarizes the authors' understanding of the
impact on pervasive monitoring to re-enforce the importance of
following the document's related guidance.
A.3. Changes from version 04 to 05
Make this document self-contained, rather than referring readers to
use-cases and requirements contained in other I.D.s that were never
published as RFCs.
Add discussion of TCP Fast Open.
Correct some discussion of TCP-AO and TCP-AO-NAT.
Clarify exactly what the identifier is identifying.
Improve discussion on interpretation of multiple instances of the
option, including order of interpretation and set interpretation.
Evaluated whether use of multiple identifiers should be constrained.
This is unclear, and so left for the experiment to determine.
Williams, et al. Expires May 8, 2016 [Page 18]
Internet-Draft Experimental TCP Host ID Option November 2015
Discuss the possibility of the option value changing over the life of
the connection (spec now prohibits this).
Clarify use cases related to stripping and replacing the option.
Add discussion of non-local fragmentation.
Evaluate the reliability of attempts to exclude the option when local
fragmentation would be required.
Clarify the security requirements re: trust relationship.
Specifically calls out that common admin control and authentication
can allow additional uses.
Clarify privacy considerations regarding NATs that separate private
and public networks.
Remove restatement of requirements from other documents.
Justify use of SHOULD rather than MUST throughout.
A.4. Changes from version 03 to 04
Improve discussion of RFC6967.
Don't use "message" to describe TCP segments.
Add reference to RFC6994 to section 3.
Clarify that this specifications supersedes earlier documents.
Improve discussion of SYN cookie handling.
Remove lower case uses of keywords (e.g. must, should, etc.)
throughout the document.
Some stronger privacy guidance, replacing SHOULD with MUST.
Add an experiment goal related to optimal option value.
Add text related to the identification goals of the option value
(still needs more work).
A.5. Changes from version 02 to 03
Clarification of arguments in favor of this approach.
Add discussion of important use cases.
Williams, et al. Expires May 8, 2016 [Page 19]
Internet-Draft Experimental TCP Host ID Option November 2015
Clarification of experiment goals and earlier test results.
A.6. Changes from version 01 to 02
Add note re: order of appearance.
A.7. Changes from version 00 to 01
Add discussion of experiment goals.
Limit external references to the earlier specifications.
Add guidance to limit the types of device that add the option.
Improve/correct discussion of TCP-AO and security.
Authors' Addresses
Brandon Williams
Akamai, Inc.
8 Cambridge Center
Cambridge, MA 02142
USA
Email: brandon.williams@akamai.com
Mohamed Boucadair
France Telecom
Rennes, 35000
Fance
Email: mohamed.boucadair@orange.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
Williams, et al. Expires May 8, 2016 [Page 20]
