INTAREA WG M. Boucadair
Internet-Draft France Telecom
Intended status: Informational J. Touch
Expires: October 20, 2012 USC/ISI
P. Levis
France Telecom
R. Penno
Cisco
April 18, 2012
Analysis of Solution Candidates to Reveal a Host Identifier (HOST_ID) in
Shared Address Deployments
draft-ietf-intarea-nat-reveal-analysis-02
Abstract
This document analyzes a set of solution candidates to mitigate some
of the issues encountered when address sharing is used. In
particular, this document focuses on means to reveal a host
identifier (HOST_ID) when a Carrier Grade NAT (CGN) or application
proxies are involved in the path. This host identifier must be
unique to each host under the same shared IP address.
Requirements Language
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 RFC 2119 [RFC2119].
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 20, 2012.
Copyright Notice
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Copyright (c) 2012 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Purpose and Scope . . . . . . . . . . . . . . . . . . . . 4
2. Problem to Be Solved . . . . . . . . . . . . . . . . . . . . . 5
2.1. IPv6 May Also Be Concerned . . . . . . . . . . . . . . . . 6
3. Solutions Analysis . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Recommendation . . . . . . . . . . . . . . . . . . . . . . 9
4. HOST_ID and Privacy . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Detailed Solutions Analysis . . . . . . . . . . . . . 14
A.1. Use the Identification Field of IP Header (IP-ID) . . . . 14
A.1.1. Description . . . . . . . . . . . . . . . . . . . . . 14
A.1.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 14
A.2. Define an IP Option . . . . . . . . . . . . . . . . . . . 14
A.2.1. Description . . . . . . . . . . . . . . . . . . . . . 14
A.2.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 15
A.3. Assign Port Sets . . . . . . . . . . . . . . . . . . . . . 15
A.3.1. Description . . . . . . . . . . . . . . . . . . . . . 15
A.3.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 15
A.4. Use ICMP . . . . . . . . . . . . . . . . . . . . . . . . . 16
A.4.1. Description . . . . . . . . . . . . . . . . . . . . . 16
A.4.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 16
A.5. Define a TCP Option . . . . . . . . . . . . . . . . . . . 17
A.5.1. Description . . . . . . . . . . . . . . . . . . . . . 17
A.5.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 17
A.6. PROXY Protocol . . . . . . . . . . . . . . . . . . . . . . 18
A.6.1. Description . . . . . . . . . . . . . . . . . . . . . 18
A.6.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 19
A.7. Host Identity Protocol (HIP) . . . . . . . . . . . . . . . 19
A.7.1. Description . . . . . . . . . . . . . . . . . . . . . 19
A.7.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 19
A.8. Inject Application Headers . . . . . . . . . . . . . . . . 19
A.8.1. Description . . . . . . . . . . . . . . . . . . . . . 19
A.8.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
1.1. Context
As reported in [RFC6269], several issues are encountered when an IP
address is shared among several subscribers. Examples of such issues
are listed below:
o Implicit identification (Section 13.2 of [RFC6269])
o SPAM (Section 13.3 of [RFC6269])
o Blacklisting a mis-behaving host (Section 13.1 of [RFC6269])
o Redirect users with infected machines to a dedicated portal
(Section 5.1 of [RFC6269])
The sole use of the IPv4 address is not sufficient to uniquely
distinguish a host. As a mitigation, it is tempting to investigate
means which would help in disclosing an information to be used by the
remote server as a means to uniquely disambiguate packets of hosts
using the same IPv4 address.
The risk of not mitigating these issues are: OPEX increase for IP
connectivity service providers (costs induced by calls to a hotline),
revenue loss for content providers (loss of users audience),
customers unsatisfaction (low quality of experience, service
segregation, etc.).
1.2. Purpose and Scope
The purpose of this document is to analyze a set of alternative
channels to convey a host identifier and to assess to what extent
they solve the problem described in Section 2. Below are listed the
candidates analyzed in the document:
o Use the Identification field of IP header (denoted as IP-ID,
Appendix A.1).
o Define a new IP option (Appendix A.2).
o Assign port sets (Appendix A.3).
o Use ICMP (Appendix A.3).
o Define a new TCP Option (Appendix A.5).
o Enable Proxy Protocol ( (Appendix A.6)).
o Activate HIP (Appendix A.7).
o Inject application headers (Appendix A.8).
A synthesis is provided in Section 3 while the detailed analysis is
elaborated in Appendix A.
Section 4 discusses privacy issues common to all HOST_ID solutions.
It is out of scope of this document to elaborate on privacy issues
specific to each solution.
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2. Problem to Be Solved
Observation: Today, some servers use the source IPv4 address as an
identifier to treat some incoming connections differently.
Tomorrow, due to the introduction of CGNs (e.g., NAT44
[RFC3022], NAT64 [RFC6146]), that address will be shared.
In particular, when a server receives packets from the same
source address, because this address is shared, the server
does not know which host is the sending host [RFC6269].
Objective: The server should be able to sort out the packets by
sending host.
Requirement: The server must have extra information than the source
IP address to differentiate the sending host. We call
HOST_ID this information.
For all solutions analyzed, we provide answers to the following
questions:
What is the HOST_ID? It must be unique to each host under the same
IP address. It does not need to be globally unique. Of course,
the combination of the (public) IP source address and the
identifier (i.e., HOST_ID) ends up being relatively unique. As
unique as today's 32-bit IPv4 addresses which, today, can change
when a host re-connects.
Where is the HOST_ID? (which protocol, which field): If the HOST_ID
is put at the IP level, all packets will have to bear the
identifier. If it is put at a higher connection-oriented level,
the identifier is only needed once in the session establishment
phase (for instance TCP three-way-handshake), then, all packets
received in this session will be attributed to the HOST_ID
designated during the session opening.
Who puts the HOST_ID? For almost all the analyzed solutions, the
address sharing function injects the HOST_ID. When there are
several address sharing functions in the data path, we describe
to what extent the proposed solution is efficient. Another
option to avoid potential performance degradation is to let the
host inject its HOST_ID but the address sharing function will
check its content (just like an IP anti-spoofing function).
What are the security considerations? Security considerations are
common to all analyzed solutions (see Section 6). Privacy-
related aspects are discussed in Section 4.
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2.1. IPv6 May Also Be Concerned
Some of the issues mentioned in Section 2 are independent of IPv4 vs.
IPv6. Even in IPv6, address sharing can be used for a variety of
reasons (e.g., to hide network topology, to defeat hosts from
offering network services directly, etc.).
A solution to reveal HOST_ID is also needed in IPv6 deployment.
3. Solutions Analysis
3.1. Requirements
Whatever the channel used to convey the HOST_ID, the following
requirements are to be met:
Uniqueness of identifiers in HOST_ID: It is RECOMMENDED that
HOST_IDs be limited to providing local uniqueness rather than
global uniqueness.
Refresh rate of HOST_ID: Address sharing function SHOULD NOT use
permanent HOST_ID values.
Manipulate HOST_IDs: Address sharing function SHOULD be able to
strip, re-write and add HOST_ID fields.
Interference between HOST_IDs: An address sharing function, able to
inject HOST_IDs in several layers, SHOULD reveal subsets of the
same information (e.g., full IP address, lower 16 bits of IP
address, etc.).
3.2. Synthesis
The following Table 1 summarizes the approaches analyzed in this
document.
o "Success ratio" indicates the ratio of successful communications
when the option is used. Provided figures are inspired from the
results documented in [Options].
o "Deployable today" indicates if the solution can be generalized
without any constraint on current architectures and practices.
o "Possible Perf Impact" indicates the level of expected performance
degradation. The rationale behind the indicated potential
performance degradation is whether the injection requires some
treatment at the IP level or not.
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o "OS TCP/IP Modif" indicates whether a modification of the OS
TCP/IP stack is required at the server side.
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+------+------+-------+-------+-------+------+-----+------+
| IP | TCP | IP-ID | HTTP | Proxy | Port | HIP | ICMP |
|Option|Option| | Header| | Set | | |
| | | | (XFF) | | | | |
----------+------+------+-------+-------+-------+------+-----+------+
UDP | Yes | No | Yes | No | No | Yes | | Yes |
----------+------+------+-------+-------+-------+------+-----+------+
TCP | Yes | Yes | Yes | No | Yes | Yes | | Yes |
----------+------+------+-------+-------+-------+------+-----+------+
HTTP | Yes | Yes | Yes | Yes | Yes | Yes | | Yes |
----------+------+------+-------+-------+-------+------+-----+------+
Encrypted | Yes | Yes | Yes | No | Yes | Yes | | Yes |
Traffic | | | | | | | | |
----------+------+------+-------+-------+-------+------+-----+------+
Success | 30% | 99% | 100% | 100% | Low | 100% |Low | ~100%|
Ratio | | | | | | | | (6) |
----------+------+------+-------+-------+-------+------+-----+------+
Possible | High | Med | Low | Med | High | No | N/A | High |
Perf | | to | to | to | | | | |
Impact | | High | Med | High | | | | |
----------+------+------+-------+-------+-------+------+-----+------+
OS TCP/IP | Yes | Yes | Yes | No | No | No | | Yes |
Modif | | | | | | | | |
----------+------+------+-------+-------+-------+------+-----+------+
Deployable| Yes | Yes | Yes | Yes | No | Yes | No | Yes |
Today | | | | | | | | |
----------+------+------+-------+-------+-------+------+-----+------+
Notes | | | (1) | (2) | | (1) | (4) | (7) |
| | | | | | (3) | (5) | |
----------+------+------+-------+-------+-------+------+-----+------+
Notes:
(1) Requires mechanism to advertise NAT is participating in this
scheme (e.g., DNS PTR record).
(2) This solution is widely deployed.
(3) When the port set is not advertised, the solution is less
efficient for third-party services.
(4) Requires the client and the server to be HIP-compliant and HIP
infrastructure to be deployed.
(5) If the client and the server are HIP-enabled, the address
sharing function does not need to insert a host-hint. If the
client is not HIP-enabled, designing the device that performs
address sharing to act as a UDP/TCP-HIP relay is not viable.
(6) Implementation-specific.
(7) The solution is inefficient in various scenarios as discussed
in Section 3.
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Figure 1: Table 1: Summary of analyzed solutions.
According to the above table and the analysis elaborated in
Appendix A:
o IP Option, IP-ID and Proxy Protocol proposals are broken;
o HIP is not largely deployed;
o The use of Port Set may contradict the port randomization
[RFC6056] requirement identified in [RFC6269]. This solution can
be used by a service provider for the delivery of its own service
offerings relying on implicit identification.
o XFF is de facto standard deployed and supported in operational
networks (e.g., HTTP Severs, Load-Balancers, etc.).
o From an application standpoint, the TCP Option is superior to XFF/
Forwarded-For since it is not restricted to HTTP. Nevertheless
XFF/Forwarded-For is compatible with the presence of address
sharing and load-balancers in the communication path. To provide
a similar functionality, the TCP Option may be extended to allow
conveying a list of IP addresses and port numbers to not lose the
source IP address in the presence of load-balancers. Another
alternative is to combine the usage of both the HOST_ID TCP Option
and XFF/Forwarded-For. Extending TCP is still possible as
analyzed in [ExtendTCP].
3.3. Recommendation
Taking into account the analysis above and [RFC6269] context, the
following recommendation is made to mitigate the problem formulated
in Section 2:
An address sharing function SHOULD support HOST_ID TCP Option
(Appendix A.5).
4. HOST_ID and Privacy
IP address sharing is motivated by a number of different factors.
For years, many network operators have conserved the use of public
IPv4 addresses by making use of Customer Premises Equipment (CPE)
that assigns a single public IPv4 address to all hosts within the
customer's local area network and uses NAT [RFC3022] to translate
between locally unique private IPv4 addresses and the CPE's public
address. With the exhaustion of IPv4 address space, address sharing
between customers on a much larger scale is likely to become much
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more prevalent. While many individual users are unaware of and
uninvolved in decisions about whether their unique IPv4 addresses get
revealed when they send data via IP, some users realize privacy
benefits associated with IP address sharing, and some may even take
steps to ensure that NAT functionality sits between them and the
public Internet. IP address sharing makes the actions of all users
behind the NAT function unattributable to any single host, creating
room for abuse but also providing some identity protection for non-
abusive users who wish to transmit data with reduced risk of being
uniquely identified.
The proposals considered in this document add a measure of uniqueness
back to hosts that share a public IP address. The extent of that
uniqueness depends on which information is included in the HOST_ID.
The volatility of the HOST_ID information is similar to the source IP
address: a distinct HOST_ID may be used by the address sharing
function when the host reboots or gets a new internal IP address. As
with persistent IP addresses, persistent HOST_IDs facilitate user
tracking over time.
As a general matter, the HOST_ID proposals do not seek to make hosts
any more identifiable than they would be if they were using a public,
non-shared IP address. However, depending on the solution proposal,
the addition of HOST_ID information may allow a device to be
fingerprinted more easily than it otherwise would be. Should
multiple solutions be combined (e.g., TCP Option and XFF) that
include different pieces of information in the HOST_ID,
fingerprinting may become even easier.
The trust placed in the information conveyed in the HOST_ID is likely
to be the same as for current practices with source IP addresses. In
that sense, a HOST_ID can be spoofed as this is also the case for
spoofing an IP address. Furthermore, users of network-based
anonymity services (like Tor) may be capable of stripping HOST_ID
information before it reaches its destination.
HOST_ID specification document(s) SHOULD explain the privacy impact
of the solutions they specify, including the extent of HOST_ID
uniqueness and persistence, assumptions made about the lifetime of
the HOST_ID, whether and how the HOST_ID can be obfuscated or
recycled, and the impact of the use of the HOST_ID on device or
implementation fingerprinting. [I-D.iab-privacy-considerations]
provides further guidance.
For more discussion about privacy, refer to [RFC6462].
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5. IANA Considerations
This document does not require any action from IANA.
6. Security Considerations
The same security concerns apply for the injection of an IP option,
TCP Option and application-related content (e.g., XFF) by the address
sharing device. If the server trusts the content of the HOST_ID
field, a third party user can be impacted by a misbehaving user to
reveal a "faked" HOST_ID (e.g., original IP address).
HOST_ID may be used to leak information about the internal structure
of a network behind an address sharing function. If this behavior is
undesired for the network administrator, the address sharing function
can be configured to strip any existing HOST_ID in received packets
from internal hosts.
HOST_ID specification documents SHOULD elaborate further on threats
inherent to each individual solution to convey the HOST_ID (e.g., use
of the IP-ID field to count hosts behind a NAT [Count]).
7. Acknowledgments
Many thanks to D. Wing and C. Jacquenet for their review, comments
and inputs.
Thanks also to P. McCann, T. Tsou, Z. Dong, B. Briscoe, T. Taylor, M.
Blanchet, D. Wing and A. Yourtchenko for the discussions in Prague.
Some of the issues related to defining a new TCP Option have been
raised by L. Eggert.
Privacy text is provided by A. Cooper.
8. References
8.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
January 2011.
8.2. Informative References
[Count] "A technique for counting NATted hosts",
<http://www.cs.columbia.edu/~smb/papers/fnat.pdf>.
[ExtendTCP]
Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M. and H. Tokuda,, "Is it still possible to
extend TCP?", November 2011,
<http://nrg.cs.ucl.ac.uk/mjh/tmp/mboxes.pdf>.
[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-02 (work in
progress), January 2012.
[]
Wu, Y., Ji, H., Chen, Q., and T. ZOU), "IPv4 Header Option
For User Identification In CGN Scenario",
draft-chen-intarea-v4-uid-header-option-00 (work in
progress), March 2011.
[I-D.iab-privacy-considerations]
Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., and
J. Morris, "Privacy Considerations for Internet
Protocols", draft-iab-privacy-considerations-02 (work in
progress), March 2012.
[I-D.ietf-appsawg-http-forwarded]
Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
draft-ietf-appsawg-http-forwarded-01 (work in progress),
March 2012.
[I-D.ietf-intarea-ipv4-id-update]
Touch, J., "Updated Specification of the IPv4 ID Field",
draft-ietf-intarea-ipv4-id-update-04 (work in progress),
September 2011.
[I-D.wing-nat-reveal-option]
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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.
[I-D.yourtchenko-nat-reveal-ping]
Yourtchenko, A., "Revealing hosts sharing an IP address
using ICMP Echo Request",
draft-yourtchenko-nat-reveal-ping-00 (work in progress),
March 2012.
[Not_An_Option]
R. Fonseca, G. Porter, R. Katz, S. Shenker, and I.
Stoica,, "IP options are not an option", 2005, <http://
www.eecs.berkeley.edu/Pubs/TechRpts/2005/
EECS-2005-24.html>.
[Options] Alberto Medina, Mark Allman, Sally Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
2005, <http://conferences.sigcomm.org/imc/2004/papers/
p336-medina.pdf>.
[Proxy] Tarreau, W., "The PROXY protocol", November 2010, <http://
haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt>.
[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
for Policy-based Admission Control", RFC 2753,
January 2000.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", RFC 6269,
June 2011.
[RFC6302] Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
"Logging Recommendations for Internet-Facing Servers",
BCP 162, RFC 6302, June 2011.
[RFC6462] Cooper, A., "Report from the Internet Privacy Workshop",
RFC 6462, January 2012.
[Trusted_ISPs]
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"Trusted XFF list", <http://meta.wikimedia.org/wiki/
XFF_project#Trusted_XFF_list>.
Appendix A. Detailed Solutions Analysis
A.1. Use the Identification Field of IP Header (IP-ID)
A.1.1. Description
IP-ID (Identification field of IP header) can be used to insert an
information which uniquely distinguishes a host among those sharing
the same IPv4 address. An address sharing function can re-write the
IP-ID field to insert a value unique to the host (16 bits are
sufficient to uniquely disambiguate hosts sharing the same IP
address). Note that this field is not altered by some NATs; hence
some side effects such as counting hosts behind a NAT as reported in
[Count].
A variant of this approach relies upon the format of certain packets,
such as TCP SYN, where the IP-ID can be modified to contain a 16 bit
HOST_ID. Address sharing devices performing this function would
require to indicate they are performing this function out of band,
possibly using a special DNS record.
A.1.2. Analysis
This usage is not compliant with what is recommended in
[I-D.ietf-intarea-ipv4-id-update].
A.2. Define an IP Option
A.2.1. Description
A solution alternative to convey the HOST_ID is to define an IP
option [RFC0791]. HOST_ID IP option can be inserted by the address
sharing function to uniquely distinguish a host among those sharing
the same IP address. An example of such option is documented in
[I-D.chen-intarea-v4-uid-header-option]. This IP option allows to
convey an IPv4 address, an IPv6 prefix, a GRE key, IPv6 Flow Label,
etc.
Another way for using IP option has been described in Section 4.6 of
[RFC3022].
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A.2.2. Analysis
Unlike the solution presented in Appendix A.5, this proposal can
apply for any transport protocol. Nevertheless, it is widely known
that routers (and other middleboxes) filter IP options. IP packets
with IP options can be dropped by some IP nodes. Previous studies
demonstrated that "IP Options are not an option" (Refer to
[Not_An_Option], [Options]).
As a conclusion, using an IP option to convey a host-hint is not
viable.
A.3. Assign Port Sets
A.3.1. Description
This solution does not require any action from the address sharing
function to disclose a host identifier. Instead of assuming all
transport ports are associated with one single host, each host under
the same external IP address is assigned a restricted port set.
These port sets are then advertised to remote servers using off-line
means. This announcement is not required for the delivery of
internal services (i.e., offered by the service provider deploying
the address sharing function) relying on implicit identification.
Port sets assigned to hosts may be static or dynamic.
Port set announcements to remote servers do not require to reveal the
identity of individual hosts but only to advertise the enforced
policy to generate non-overlapping port sets (e.g., the transport
space associated with an IP address is fragmented to contiguous
blocks of 2048 port numbers).
A.3.2. Analysis
The solution does not require defining new fields nor options; it is
policy-based.
The solution may contradict the port randomization as identified in
[RFC6269]. A mitigation would be to avoid assigning static port sets
to individual hosts.
The method is convenient for the delivery of services offered by the
service provider offering also the IP connectivity service.
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A.4. Use ICMP
A.4.1. Description
Another alternative is to convey the HOST_ID using a separate
notification channel than the packets issued to invoke the service.
An implementation example is defined in
[I-D.yourtchenko-nat-reveal-ping]. This solution relies on a
mechanism where the address sharing function encapsulates the
necessary differentiating information into an ICMP Echo Request
packet that it sends in parallel with the initial session creation
(e.g., SYN). The information included in the ICMP Request Data
portion describes the five-tuples as seen on both of the sides of the
address sharing function.
A.4.2. Analysis
o This ICMP proposal is valid for both UDP and TCP. Address sharing
function may be configurable with the transport protocol which is
allowed to trigger those ICMP messages.
o A hint should be provided to the ultimate server (or intermediate
nodes) an ICMP Echo Request conveys a HOST_ID. This may be
implemented using magic numbers.
o Even if ICMP packets are blocked in the communication path, the
user connection does not have to be impacted.
o Some implementations requiring to delay the establishment of a
session until receiving the companion ICMP Echo Request, may lead
to some user experience degradation.
o Because of the presence of load-balancers in the path, the
ultimate server receiving the SYN packet may not be the one which
may receive the ICMP message conveying the HOST_ID.
o Because of the presence of load-balancers in the path, the port
number assigned by address sharing may be lost. Therefore the
mapping information conveyed in the ICMP may not be sufficient to
associate a SYN packet with a received ICMP.
o The proposal is not compatible with the presence of cascaded NAT.
o The ICMP proposal will add a traffic overhead for both the server
and the address sharing device.
o The ICMP proposal is similar to other mechanisms (e.g., syslog,
netflow) for reporting dynamic mappings to a mediation platform
(mainly for legal traceability purposes). Performance degradation
are likely to be experienced by address sharing functions because
ICMP messages are to be sent in particular for each new
instantiated mapping (and also even if the mapping exists).
o In some scenarios (e.g., Fixed-Mobile Convergence, Open WiFi,
etc.), HOST_ID should be interpreted by intermediate devices which
embed Policy Enforcement Points (PEP, [RFC2753]) responsible for
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granting access to some services. These PEPs need to inspect all
received packets in order to find the companion (traffic) messages
to be correlated with ICMP messages conveying HOST_IDs. This
induces more complexity to these intermediate devices.
A.5. Define a TCP Option
A.5.1. Description
HOST_ID may be conveyed in a dedicated TCP Option. An example is
specified in [I-D.wing-nat-reveal-option] which defines a new TCP
Option called USER_HINT. This option encloses the TCP client's
identifier (e.g., the lower 16 bits of their IPv4 address, their VLAN
ID, VRF ID, subscriber ID). The address sharing device inserts this
TCP Option into the TCP SYN packet.
A.5.2. Analysis
Using a new TCP Option to convey the HOST_ID does not require any
modification to the applications but it is applicable only for TCP-
based applications. Applications relying on other transport
protocols are therefore left unsolved.
[I-D.wing-nat-reveal-option] discusses the interference with other
TCP Options.
The risk related to handling a new TCP Option is low as measured in
[Options]. [I-D.abdo-hostid-tcpopt-implementation] provides a
detailed implementation and experimentation report of HOST_ID TCP
Option. [I-D.abdo-hostid-tcpopt-implementation] investigated in
depth the impact of activation HOST_ID in host, address sharing
function and the enforcement of policies at the server side.
[I-D.abdo-hostid-tcpopt-implementation] reports a failure ratio of
0,103% among top 100000 websites.
Some downsides have been raised against defining a TCP Option to
reveal a host identity:
o Conveying an IP address in a TCP Option may be seen as a violation
of OSI layers but since IP addresses are already used for the
checksum computation, this is not seen as a blocking point.
Moreover, updated version of [I-D.wing-nat-reveal-option] does not
allow anymore to convey an IP address (the HOST_ID is encoded in
16bits).
o TCP Option space is limited, and might be consumed by the TCP
client. Earlier versions of [I-D.wing-nat-reveal-option] discuss
two approaches to sending the HOST_ID: sending the HOST_ID in the
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TCP SYN (which consumes more bytes in the TCP header of the TCP
SYN) and sending the HOST_ID in a TCP ACK (which consumes only two
bytes in the TCP SYN). Content providers may find it more
desirable to receive the HOST_ID in the TCP SYN, as that more
closely preserves the HOST_ID received in the source IP address as
per current practices. It is more complicated to implement
sending the HOST_ID in a TCP ACK, as it can introduce MTU issues
if the ACK packet also contains TCP data, or a TCP segment is
lost. The latest specification of the HOST_ID TCP Option,
documented at [I-D.wing-nat-reveal-option], allows only to enclose
the HOST_ID in the TCP SYN packet.
o When there are several NATs in the path, the original HOST_ID may
be lost. In such case, the procedure may not be efficient.
o Interference with current usages such as X-Forwarded-For (see
Appendix A.8) should be elaborated to specify the behavior of
servers when both options are used; in particular specify which
information to use: the content of the TCP Option or what is
conveyed in the application headers.
o When load-balancers or proxies are in the path, this option does
not allow to preserve the original source IP address and source
port. Preserving such information is required for logging
purposes for instance (e.g., [RFC6302]).
[I-D.abdo-hostid-tcpopt-implementation] defines a TCP Option which
allows to reveal various combinations of source information (e.g.,
source port, source port and source IP address, source IPv6
prefix, etc.).
More discussion about issues raised when extending TCP can be found
at [ExtendTCP].
A.6. PROXY Protocol
A.6.1. Description
The solution, referred to as Proxy Protocol [Proxy], does not require
any application-specific knowledge. The rationale behind this
solution is to prepend each connection with a line reporting the
characteristics of the other side's connection as shown in the
example depicted in Figure 2:
PROXY TCP4 192.0.2.1 192.0.2.15 56324 443\r\n
Figure 2: Example of PROXY conection report
Upon receipt of a message conveying this line, the server removes the
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line. The line is parsed to retrieve the transported protocol. The
content of this line is recorded in logs and used to enforce
policies.
A.6.2. Analysis
This solution can be deployed in a controlled environment but it can
not be deployed to all access services available in the Internet. If
the remote server does not support the Proxy Protocol, the session
will fail. Other complications will raise due to the presence of
firewalls for instance.
As a consequence, this solution is broken and can not be recommended.
A.7. Host Identity Protocol (HIP)
A.7.1. Description
[RFC5201] specifies an architecture which introduces a new namespace
to convey an identity information.
A.7.2. Analysis
This solution requires both the client and the server to support HIP
[RFC5201]. Additional architectural considerations are to be taken
into account such as the key exchanges, etc.
If the address sharing function is required to act as a UDP/TCP-HIP
relay, this is not a viable option.
A.8. Inject Application Headers
A.8.1. Description
Another option is to not require any change at the transport nor the
IP levels but to convey at the application payload the required
information which will be used to disambiguate hosts. This format
and the related semantics depend on its application (e.g., HTTP, SIP,
SMTP, etc.).
For HTTP, the X-Forwarded-For (XFF) or Forwarded-For
([I-D.ietf-appsawg-http-forwarded]) headers can be used to display
the original IP address when an address sharing device is involved.
Service Providers operating address sharing devices can enable the
feature of injecting the XFF header which will enclose the original
IPv4 address or the IPv6 prefix part (see the example shown in
Figure 3). The address sharing device has to strip all included XFF
headers before injecting their own. Servers may rely on the contents
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of this field to enforce some policies such as blacklisting
misbehaving users. Note that XFF can also be logged by some servers
(this is for instance supported by Apache).
Forwarded: for=192.0.2.1,for=[2001:db8::1]
Forwarded: proto=https;by=192.0.2.15
Figure 3: Example of Forwarded-For
A.8.2. Analysis
Not all applications impacted by the address sharing can support the
ability to disclose the original IP address. Only a subset of
protocols (e.g., HTTP) can rely on this solution.
For the HTTP case, to prevent users injecting invalid HOST_IDs, an
initiative has been launched to maintain a list of trusted ISPs using
XFF: See for example the list available at: [Trusted_ISPs] of trusted
ISPs as maintained by Wikipedia. If an address sharing device is on
the trusted XFF ISPs list, users editing Wikipedia located behind the
address sharing device will appear to be editing from their
"original" IP address and not from the NATed IP address. If an
offending activity is detected, individual hosts can be blacklisted
instead of all hosts sharing the same IP address.
XFF header injection is a common practice of load balancers. When a
load balancer is in the path, the original content of any included
XFF header should not be stripped. Otherwise the information about
the "origin" IP address will be lost.
When several address sharing devices are crossed, XFF header can
convey the list of IP addresses (e.g., Figure 3). The origin HOST_ID
can be exposed to the target server.
XFF also introduces some implementation complexity if the HTTP packet
is at or close to the MTU size.
It has been reported that some "poor" implementation may encounter
some parsing issues when injecting XFF header.
For encrypted HTTP traffic, injecting XFF header may be broken.
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Authors' Addresses
Mohamed Boucadair
France Telecom
Rennes, 35000
France
Email: mohamed.boucadair@orange.com
Joe Touch
USC/ISI
Email: touch@isi.edu
Pierre Levis
France Telecom
Caen, 14000
France
Email: pierre.levis@orange.com
Reinaldo Penno
Cisco
USA
Email: repenno@cisco.com
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