|Title||Issues with IP Address Sharing
|Author||M. Ford, Ed., M. Boucadair, A.
Durand, P. Levis, P. Roberts
Internet Engineering Task Force (IETF) M. Ford, Ed.
Request for Comments: 6269 Internet Society
Category: Informational M. Boucadair
ISSN: 2070-1721 France Telecom
Issues with IP Address Sharing
The completion of IPv4 address allocations from IANA and the Regional
Internet Registries (RIRs) is causing service providers around the
world to question how they will continue providing IPv4 connectivity
service to their subscribers when there are no longer sufficient IPv4
addresses to allocate them one per subscriber. Several possible
solutions to this problem are now emerging based around the idea of
shared IPv4 addressing. These solutions give rise to a number of
issues, and this memo identifies those common to all such address
sharing approaches. Such issues include application failures,
additional service monitoring complexity, new security
vulnerabilities, and so on. Solution-specific discussions are out of
Deploying IPv6 is the only perennial way to ease pressure on the
public IPv4 address pool without the need for address sharing
mechanisms that give rise to the issues identified herein.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2011 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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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Shared Addressing Solutions . . . . . . . . . . . . . . . . . 4
3. Analysis of Issues as They Relate to First and Third
Parties . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Content Provider Example . . . . . . . . . . . . . . . . . . . 8
5. Port Allocation . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Outgoing Ports . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Incoming Ports . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Port Negotiation . . . . . . . . . . . . . . . . . . . 11
5.2.2. Connection to a Well-Known Port Number . . . . . . . . 12
5.2.3. Port Discovery Mechanisms . . . . . . . . . . . . . . 12
6. Impact on Applications . . . . . . . . . . . . . . . . . . . . 12
7. Geo-location and Geo-proximity . . . . . . . . . . . . . . . . 14
8. Tracking Service Usage . . . . . . . . . . . . . . . . . . . . 15
9. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10. MTU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 16
12. Traceability . . . . . . . . . . . . . . . . . . . . . . . . . 17
13. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
13.1. Abuse Logging and Penalty Boxes . . . . . . . . . . . . . 18
13.2. Authentication . . . . . . . . . . . . . . . . . . . . . . 19
13.3. Spam . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.4. Port Randomization . . . . . . . . . . . . . . . . . . . . 19
13.5. IPsec . . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.6. Policing Forwarding Behavior . . . . . . . . . . . . . . . 20
14. Transport Issues . . . . . . . . . . . . . . . . . . . . . . . 20
14.1. Parallel Connections . . . . . . . . . . . . . . . . . . . 20
14.2. Serial Connections . . . . . . . . . . . . . . . . . . . . 20
14.3. TCP Control Block Sharing . . . . . . . . . . . . . . . . 21
15. Reverse DNS . . . . . . . . . . . . . . . . . . . . . . . . . 21
16. Load Balancing . . . . . . . . . . . . . . . . . . . . . . . . 21
17. IPv6 Transition Issues . . . . . . . . . . . . . . . . . . . . 21
18. Introduction of Single Points of Failure . . . . . . . . . . . 22
19. State Maintenance Reduces Battery Life . . . . . . . . . . . . 22
20. Support of Multicast . . . . . . . . . . . . . . . . . . . . . 22
21. Support of Mobile-IP . . . . . . . . . . . . . . . . . . . . . 22
22. Security Considerations . . . . . . . . . . . . . . . . . . . 23
23. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
24. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
25. Informative References . . . . . . . . . . . . . . . . . . . . 23
Appendix A. Classes of Address Sharing Solution . . . . . . . . . 27
Appendix B. Address Space Multiplicative Factor . . . . . . . . . 27
Allocations of IPv4 addresses from the Internet Assigned Numbers
Authority (IANA) were completed on February 3, 2011 [IPv4_Pool].
Allocations from Regional Internet Registries (RIRs) are anticipated
to be complete around a year later, although the exact date will vary
from registry to registry. This is causing service providers around
the world to start to question how they will continue providing IPv4
connectivity service to their subscribers when there are no longer
sufficient IPv4 addresses to allocate them one per subscriber.
Several possible solutions to this problem are now emerging based
around the idea of shared IPv4 addressing. These solutions give rise
to a number of issues, and this memo identifies those common to all
such address sharing approaches and collects them in a single
Deploying IPv6 is the only perennial way to ease pressure on the
public IPv4 address pool without the need for address sharing
mechanisms that give rise to the issues identified herein. In the
short term, maintaining growth of IPv4 services in the presence of
IPv4 address depletion will require address sharing. Address sharing
will cause issues for end-users, service providers, and third parties
such as law enforcement agencies and content providers. This memo is
intended to highlight and briefly discuss these issues.
Increased IPv6 deployment should reduce the burden being placed on an
address sharing solution, and should reduce the costs of operating
that solution. Increasing IPv6 deployment should cause a reduction
in the number of concurrent IPv4 sessions per subscriber. If the
percentage of end-to-end IPv6 traffic significantly increases, so
that the volume of IPv4 traffic begins decreasing, then the number of
IPv4 sessions will decrease. The smaller the number of concurrent
IPv4 sessions per subscriber, the higher the number of subscribers
able to share the same IPv4 public address, and consequently, the
lower the number of IPv4 public addresses required. However, this
effect will only occur for subscribers who have both an IPv6 access
and a shared IPv4 access. This motivates a strategy to
systematically bind a shared IPv4 access to an IPv6 access. It is
difficult to foresee to what extent growing IPv6 traffic will reduce
the number of concurrent IPv4 sessions, but in any event, IPv6
deployment and use should reduce the pressure on the available public
IPv4 address pool.
2. Shared Addressing Solutions
In many networks today, a subscriber is provided with a single public
IPv4 address at their home or small business. For instance, in fixed
broadband access, an IPv4 public address is assigned to each CPE
(Customer Premises Equipment). CPEs embed a NAT function that is
responsible for translating private IPv4 addresses ([RFC1918]
addresses) assigned to hosts within the local network, to the public
IPv4 address assigned by the service provider (and vice versa).
Therefore, devices located with the LAN share the single public IPv4
address and they are all associated with a single subscriber account
and a single network operator.
A number of proposals currently under consideration in the IETF rely
upon the mechanism of multiplexing multiple subscribers' connections
over a smaller number of shared IPv4 addresses. This is a
significant change. These proposals include Carrier Grade NAT (CGN
a.k.a. LSN for Large Scale NAT) [LSN-REQS], Dual-Stack Lite
[DS-Lite], NAT64 [RFC6145] [RFC6146], Address+Port (A+P) proposals
[A+P] [PORT-RANGE], and Stateless Address Mapping [SAM]. Appendix A
and Appendix B provide a classification of these different types of
solutions and discuss some of the design considerations to be borne
in mind when deploying large-scale address sharing. Whether we're
talking about DS-Lite, A+P, NAT64, or CGN isn't especially important
-- it's the view from the outside that matters, and given that, most
of the issues identified below apply regardless of the specific
address sharing scenario in question.
In these new proposals, a single public IPv4 address would be shared
by multiple homes or small businesses (i.e., multiple subscribers),
so the operational paradigm described above would no longer apply.
In this document, we refer to this new paradigm as large-scale
address sharing. All these proposals extend the address space by
adding port information; they differ in the way they manage the port
Security issues associated with NAT have long been documented (see
[RFC2663] and [RFC2993]). However, sharing IPv4 addresses across
multiple subscribers by any means, either moving the NAT
functionality from the home gateway to the core of the service
provider network or restricting the port choice in the subscriber's
NAT, creates additional issues for subscribers, content providers,
and network operators. Many of these issues are created today by
public Wi-Fi hotspot deployments. As such large-scale address
sharing solutions become more widespread in the face of IPv4 address
depletion, these issues will become both more severe and more
commonly felt. NAT issues in the past typically only applied to a
single legal entity; as large-scale address sharing becomes more
prevalent, these issues will increasingly span across multiple legal
All large-scale address sharing proposals share technical and
operational issues, and these are addressed in the sections that
follow. These issues are common to any service-provider NAT,
enterprise NAT, and also non-NAT solutions that share individual IPv4
addresses across multiple subscribers. This document is intended to
bring all of these issues together in one place.
3. Analysis of Issues as They Relate to First and Third Parties
In this section, we present an analysis of whether the issues
identified in the remainder of this document are applicable to the
organization deploying the shared addressing mechanism (and by
extension their subscribers) and/or whether these issues impact third
parties (e.g., content providers, law enforcement agencies, etc.).
In this analysis, issues that affect end-users are deemed to affect
first parties, as end-users can be expected to complain to their
service provider when problems arise. Where issues can expect to be
foreseen and addressed by the party deploying the shared addressing
solution, they are not attributed.
In Figure 1, we have also tried to indicate (with 'xx') where issues
are newly created in addition to what could be expected from the
introduction of a traditional NAT device. Issues marked with a
single 'x' are already present today in the case of typical CPE NAT;
however, they can be expected to be more severe and widespread in the
case of large-scale address sharing.
| Issue | 1st | 3rd |
| | party | parties |
| Restricted allocations of outgoing | x | |
| ports will impact performance for end-users | | |
| | | |
| Incoming port negotiation mechanisms may fail | xx | |
| | | |
| Incoming connections to Assigned Ports will | x | |
| not work | | |
| | | |
| Port discovery mechanisms will not work | x | |
| | | |
| Some applications will fail to operate | x | x |
| | | |
| Assumptions about parallel/serial connections | x | x |
| may fail | | |
| | | |
| Issue | 1st | 3rd |
| | party | parties |
| TCP control block sharing will be affected | x | x |
| | | |
| Reverse DNS will be affected | x | x |
| | | |
| Inbound ICMP will fail in many cases | x | x |
| | | |
| Amplification of security issues will occur | xx | xx |
| | | |
| Fragmentation will require special handling | x | |
| | | |
| Single points of failure and increased | x | |
| network instability may occur | | |
| | | |
| Port randomization will be affected | x | |
| | | |
| Service usage monitoring and abuse logging | xx | xx |
| will be impacted for all elements in the chain | | |
| between service provider and content provider | | |
| | | |
| Penalty boxes will no longer work | xx | xx |
| | | |
| Spam blacklisting will be affected | xx | xx |
| | | |
| Geo-location services will be impacted | xx | xx |
| | | |
| Geo-proximity mechanisms will be impacted | xx | xx |
| | | |
| Load balancing algorithms may be impacted | | xx |
| | | |
| Authentication mechanisms may be impacted | | x |
| | | |
| Traceability of network usage and abusage will | | xx |
| be affected | | |
| | | |
| IPv6 transition mechanisms will be affected | xx | |
| | | |
| Frequent keep-alives will reduce battery life | x | |
| | | |
Figure 1: Shared addressing issues for first and third parties
As can be seen from Figure 1, deployment of large-scale address
sharing will create almost as many problems for third parties as it
does for the service provider deploying the address sharing
mechanism. Several of these issues are specific to the introduction
of large-scale address sharing as well. All of these issues are
discussed in further detail below.
4. Content Provider Example
Taking a content provider as an example of a third party, and
focusing on the issues that are created specifically by the presence
of large-scale address sharing, we identify the following issues as
being of particular concern:
o Degraded geo-location for targeted advertising and licensed
content restrictions (see Section 7).
o Additional latency due to indirect routing and degraded geo-
proximity (see Section 7).
o Exposure to new amplification attacks (see Section 13).
o Service usage monitoring is made more complicated (see Section 8).
o Incoming port negotiation mechanisms may fail (see Section 5.2.1).
o IP blocking for abuse/spam will cause collateral damage (see
o Load balancing algorithms may be impacted (see Section 16).
o Traceability of network usage and abuse will be impacted (see
5. Port Allocation
When we talk about port numbers, we need to make a distinction
between outgoing connections and incoming connections. For outgoing
connections, the actual source port number used is usually
irrelevant. (While this is true today, in a port-range solution, it
is necessary for the source port to be within the allocated range.)
But for incoming connections, the specific port numbers allocated to
subscribers matter because they are part of external referrals (used
by third parties to contact services run by the subscribers).
The total number of subscribers able to share a single IPv4 address
will depend upon assumptions about the average number of ports
required per active subscriber, and the average number of
simultaneously active subscribers. It is important to realize that
the TCP design makes it undesirable for clients to re-use ports while
they remain in the TIME-WAIT state (typically 4 minutes after the
connection has concluded). TIME-WAIT state removes the hazard of old
duplicates for "fast" or "long" connections, in which clock-driven
Initial Sequence Number selection is unable to prevent overlap of the
old and new sequence spaces. The TIME-WAIT delay allows all old
duplicate segments time enough to die in the Internet before the
connection is reopened [RFC1337]. Therefore, ports in this state
must be included in calculations concerning port usage per
Most of the time the source port selected by a client application
will be translated (unless there is direct knowledge of a port-range
restriction in the client's stack), either by a NAT in the
subscriber's device, or by a CPE NAT, or by a CPE NAT and a CGN.
[RFC1700] (which was replaced by an online database, as described by
[RFC3232]) defines the Assigned Ports (both System and User). IANA
has further classified the whole port space into three categories, as
defined in [IANA_Ports]:
o The Well-Known Ports are those from 0 through 1023.
o The Registered Ports are those from 1024 through 49151.
o The Dynamic and/or Private Ports are those from 49152 through
[RFC4787] notes that current NATs have different policies with regard
to this classification; some NATs restrict their translations to the
use of dynamic ports, some also include registered ports, some
preserve the port if it is in the well-known range. [RFC4787] makes
it clear that the use of port space (1024-65535) is safe: "mapping a
source port to a source port that is already registered is unlikely
to have any bad effects". Therefore, for all address sharing
solutions, there is no reason to only consider a subset of the port
space (1024-65535) for outgoing source ports.
5.1. Outgoing Ports
According to measurements, the average number of outgoing ports
consumed per active subscriber is much, much smaller than the maximum
number of ports a subscriber can use at any given time. However, the
distribution is heavy-tailed, so there are typically a small number
of subscribers who use a very high number of ports [CGN_Viability].
This means that an algorithm that dynamically allocates outgoing port
numbers from a central pool will typically allow more subscribers to
share a single IPv4 address than algorithms that statically divide
the resource by pre-allocating a fixed number of ports to each
subscriber. Similarly, such an algorithm should be more able to
accommodate subscribers wishing to use a relatively high number of
It is important to note here that the desire to dynamically allocate
outgoing port numbers will make a service provider's job of
maintaining records of subscriber port number allocations
considerably more onerous (see Section 12). The number of records
per subscriber will increase from 1 in a scheme where ports are
statically allocated, to a much larger number equivalent to the total
number of outgoing ports consumed by that subscriber during the time
period for which detailed logs must be kept.
A potential problem with dynamic allocation occurs when one of the
subscriber devices behind such a port-shared IPv4 address becomes
infected with a worm, which then quickly sets about opening many
outbound connections in order to propagate itself. Such an infection
could rapidly exhaust the shared resource of the single IPv4 address
for all connected subscribers. It is therefore necessary to impose
limits on the total number of ports available to an individual
subscriber to ensure that the shared resource (the IPv4 address)
remains available in some capacity to all the subscribers using it.
However, static schemes for ports assignment may introduce security
issues [RFC6056] when small contiguous port ranges are statically
assigned to subscribers. Another way to mitigate this issue is to
kill off (randomly) established connections when the port space runs
out. A user with many connections will be proportionally more likely
to get impacted.
Session failure due to NAT state overflow or timeout (when the NAT
discards session state because it's run out of resource) can be
experienced when the configured quota per user is reached or if the
NAT is out of resources.
5.2. Incoming Ports
It is desirable to ensure that incoming ports remain stable over
time. This is challenging as the network doesn't know anything in
particular about the applications that it is supporting, and
therefore has no real notion of how long an application/service
session is still ongoing and therefore requiring port stability.
Early measurements [CGN_Viability] also seem to indicate that, on
average, only very few ports are used by subscribers for incoming
connections. However, a majority of subscribers accept at least one
This means that it is not necessary to pre-allocate a large number of
incoming ports to each subscriber. It is possible to either pre-
allocate a small number of ports for incoming connections or do port
allocation on demand when the application wishing to receive a
connection is initiated. The bulk of incoming ports can be reserved
as a centralized resource shared by all subscribers using a given
public IPv4 address.
5.2.1. Port Negotiation
In current deployments, one important and widely used feature of many
CPE devices is the ability to open incoming ports (port forwarding)
either manually, or with a protocol such as the Universal Plug and
Play Internet Gateway Device (UPnP IGD) [UPnP-IGD]. If a CGN is
present, the port must also be opened in the CGN. CGN makes
subscribers dependent on their service provider for this
CPE and CGN will need to cooperate in order for port forwarding
functionality to work. UPnP, or NAT-PMP proxy could be a solution if
there is a direct link (or tunnel) between the CPE and the CGN. An
alternative solution is a web interface to configure the incoming
port mapping on the CGN. Protocol development is underway in the
IETF to provide a generalized, automated solution via the Port
Control Protocol [PCP].
Note that such an interface effectively makes public what was
previously a private management interface and this raises security
concerns that must be addressed.
For port-range solutions, port forwarding capabilities may still be
present at the CPE, with the limitation that the open incoming port
must be within the allocated port range (for instance, it is not
possible to open port 5002 for incoming connections if port 5002 is
not within the allocated port range).
18.104.22.168. Universal Plug and Play (UPnP)
Using the UPnP semantic, an application asks "I want to use port
number X, is that OK?", and the answer is yes or no. If the answer
is no, the application will typically try the next port in sequence,
until it either finds one that works or gives up after a limited
number of attempts. UPnP IGD 1.0 has no way to redirect the
application to use another port number. UPnP IGD 2.0 improves this
situation and allows for allocation of any available port.
22.214.171.124. NAT Port Mapping Protocol (NAT-PMP)
NAT-PMP enables the NAT to redirect the requesting application to a
port deemed to be available for use by the NAT state mapping table.
5.2.2. Connection to a Well-Known Port Number
Once an IPv4-address sharing mechanism is in place, inbound
connections to well-known port numbers will not work in the general
case. Any application that is not port-agile cannot be expected to
work. Some workaround (e.g., redirects to a port-specific URI) could
be deployed given sufficient incentives. There exist several
proposals for 'application service location' protocols that would
provide a means of addressing this problem, but historically these
proposals have not gained much deployment traction.
For example, the use of DNS SRV records [RFC2782] provides a
potential solution for subscribers wishing to host services in the
presence of a shared-addressing scheme. SRV records make it possible
to specify a port value related to a service, thereby making services
accessible on ports other than the well-known ports. It is worth
noting that this mechanism is not applicable to HTTP and many other
5.2.3. Port Discovery Mechanisms
Port discovery using a UDP port to discover a service available on a
corresponding TCP port, either through broadcast, multicast, or
unicast, is a commonly deployed mechanism. Unsolicited inbound UDP
will be dropped by address sharing mechanisms as they have no live
mapping to enable them to forward the packet to the appropriate host.
Address sharing thereby breaks this service discovery technique.
6. Impact on Applications
Address sharing solutions will have an impact on the following types
o Applications that establish inbound communications - These
applications will have to ensure that ports selected for inbound
communications are either within the allocated range (for port-
range solutions) or are forwarded appropriately by the CGN (for
CGN-based solutions). See Section 5.2 for more discussion.
o Applications that carry address and/or port information in their
payload - Where translation of port and/or address information is
performed at the IP and transport layers by the address sharing
solution, an ALG will also be required to ensure application-layer
data is appropriately modified. Note that ALGs are required in
some cases, and in many other cases end-to-end protocol mechanisms
have developed to work around a lack of ALGs in address
translators, to the point of it being preferable to avoid any
support in the NAT.
o Applications that use fixed ports - See Section 5.2.2 for more
o Applications that do not use any port (e.g., ICMP echo) - Such
applications will require special handling -- see Section 9 for
o Applications that assume the uniqueness of source addresses (e.g.,
IP address as identifier) - Such applications will fail to operate
correctly in the presence of multiple, discrete, simultaneous
connections from the same source IP address.
o Applications that explicitly prohibit concurrent connections from
the same address - Such applications will fail when multiple
subscribers sharing an IP address attempt to use them
o Applications that do not use TCP or UDP for transport - All IP-
address sharing mechanisms proposed to date are limited to TCP,
UDP, and ICMP, thereby preventing end-users from fully utilizing
the Internet (e.g., SCTP, DCCP, RSVP, protocol 41 (IPv6-over-
IPv4), protocol 50 (IPsec ESP)).
Applications already frequently implement mechanisms in order to
circumvent the presence of NATs (typically CPE NATs):
o Application Layer Gateways (ALGs): Many CPE devices today embed
ALGs that allow applications to behave correctly despite the
presence of NAT on the CPE. When the NAT belongs to the
subscriber, the subscriber has flexibility to tailor the device to
his or her needs. For CGNs, subscribers will be dependent on the
set of ALGs that their service provider makes available. For
port-range solutions, ALGs will require modification to deal with
the port-range restriction, but will otherwise have the same
capabilities as today. Note that ALGs are required in some cases,
and in many other cases end-to-end protocol mechanisms have
developed to work around a lack of ALGs, to the point of it being
preferable to avoid any support in the NAT.
o NAT Traversal Techniques: There are several commonly deployed
mechanisms that support operating servers behind a NAT by
forwarding specific TCP or UDP ports to specific internal hosts
([UPnP-IGD], [NAT-PMP], and manual configuration). All of these
mechanisms assume the NAT's WAN address is a publicly routable IP
address, and fail to work normally when that assumption is wrong.
There have been attempts to avoid that problem by automatically
disabling the NAT function and bridging traffic instead upon
assignment of a private IP address to the WAN interface (as is
required for [Windows-Logo] certification). Bridging (rather than
NATting) has other side effects (DHCP requests are served by an
upstream DHCP server that can increase complexity of in-home
7. Geo-location and Geo-proximity
IP addresses are frequently used to indicate, with some level of
granularity and some level of confidence, where a host is physically
located. Using IP addresses in this fashion is a heuristic at best,
and is already challenged today by other deployed capabilities, e.g.,
tunnels. Geo-location services are used by content providers to
allow them to conform with regional content licensing restrictions,
to target advertising at specific geographic areas, or to provide
customized content. Geo-location services are also necessary for
emergency services provision. In some deployment contexts (e.g.,
centralized CGN), shared addressing will reduce the level of
confidence and level of location granularity that IP-based geo-
location services can provide. Viewed from the content provider, a
subscriber behind a CGN geo-locates to wherever the prefix of the CGN
appears to be; very often that will be in a different city than the
IP addresses are also used as input to geo-location services that
resolve an IP address to a physical location using information from
the network infrastructure. Current systems rely on resources such
as RADIUS databases and DHCP lease tables. The use of address
sharing will prevent these systems from resolving the location of a
host based on IP address alone. It will be necessary for users of
such systems to provide more information (e.g., TCP or UDP port
numbers), and for the systems to use this information to query
additional network resources (e.g., Network Address Translation -
Protocol Translation (NAT-PT) binding tables). Since these new data
elements tend to be more ephemeral than those currently used for geo-
location, their use by geo-location systems may require them to be
cached for some period of time.
Other forms of geo-location will still work as usual.
A slightly different use of an IP address is to calculate the
proximity of a connecting host to a particular service delivery
point. This use of IP address information impacts the efficient
delivery of content to an end-user. If a CGN is introduced in
communications and it is far from an end-user connected to it,
application performance may be degraded insofar as IP-based geo-
proximity is a factor.
8. Tracking Service Usage
As large-scale address sharing becomes more commonplace, monitoring
the number of unique users of a service will become more complex than
simply counting the number of connections from unique IP addresses.
While this is a somewhat inexact methodology today due to the
widespread use of CPE NAT, it will become a much less useful approach
in the presence of widespread large-scale address sharing solutions.
In general, all elements that monitor usage or abusage in the chain
between a service provider that has deployed address sharing and a
content provider will need to be upgraded to take account of the port
value in addition to IP addresses.
ICMP does not include a port field and is consequently problematic
for address sharing mechanisms. Some ICMP message types include a
fragment of the datagram that triggered the signal to be sent, which
is assumed to include port numbers. For some ICMP message types, the
Identifier field has to be used as a de-multiplexing token. Sourcing
ICMP echo messages from hosts behind an address sharing solution does
not pose problems, although responses to outgoing ICMP echo messages
will require special handling, such as making use of the ICMP
Identifier value to route the response appropriately.
For inbound ICMP there are two cases. The first case is that of ICMP
sourced from outside the network of the address sharing solution
provider. Where ICMP messages include a fragment of an outgoing
packet including port numbers, it may be possible to forward the
packet appropriately. In addition to these network signaling
messages, several applications (e.g., peer-to-peer applications) make
use of ICMP echo messages that include no hints that could be used to
route the packet correctly. Measurements derived by such
applications in the presence of an address sharing solution will be
erroneous or fail altogether. The second case is that of ICMP
sourced from within the network of the address sharing solution
provider (e.g., for network management, signaling, and diagnostic
purposes). In this case, ICMP can be routed normally for CGN-based
solutions owing to the presence of locally unique private IP
addresses for each CPE device. For port-range solutions, ICMP echo
messages will not be routable without special handling, e.g., placing
a port number in the ICMP Identifier field, and having port-range
routers make routing decisions based upon that field.
Considerations related to ICMP message handling in NAT-based
environments are specified in [RFC5508].
In applications where the end hosts are attempting to use path MTU
Discovery [RFC1191] to optimize transmitted packet size with
underlying network MTU, shared addressing has a number of items that
must be considered. As covered in Section 9, ICMP "Packet Too Big"
messages must be properly translated through the address sharing
solution in both directions. However, even when this is done
correctly, MTU can be a concern. Many end hosts cache information
that was received via Path MTU Discovery (PMTUD) for a certain period
of time. If the MTU behind the address sharing solution is
inconsistent, the public end host may have the incorrect MTU value
cached. This may cause it to send packets that are too large,
causing them to be dropped if the DF (Don't Fragment) bit is set, or
causing them to be fragmented by the network, increasing load and
overhead. Because the host eventually will reduce MTU to the lowest
common value for all hosts behind a given public address, it may also
send packets that are below optimal size for the specific connection,
increasing overhead and reducing throughput.
This issue also generates a potential attack vector -- a malevolent
user could send an ICMP "Packet Too Big" (Type 3, Code 4) message
indicating a Next-Hop MTU of anything down to 68 octets. This value
will be cached by the off-net server for all subscribers sharing the
address of the malevolent user. This could lead to a denial of
service (DoS) against both the remote server and the large-scale NAT
device itself (as they will both have to handle many more packets per
When a packet is fragmented, transport-layer port information (either
UDP or TCP) is only present in the first fragment. Subsequent
fragments will not carry the port information and so will require
special handling. In addition, the IP Identifier may no longer be
unique as required by the receiver to aid in assembling the fragments
of a datagram.
In many jurisdictions, service providers are legally obliged to
provide the identity of a subscriber upon request to the appropriate
authorities. Such legal requests have traditionally included the
source IPv4 address and date (and usually the time), which is
sufficient information when subscribers are assigned IPv4 addresses
for a long duration.
However, where one public IPv4 address is shared between several
subscribers, the IPv4 address no longer uniquely identifies a
subscriber. There are two solutions to this problem:
o The first solution is for servers to additionally log the source
port of incoming connections and for the legal request to include
the source port. The legal request should include the
information: [Source IP address, Source Port, Timestamp] (and
possibly other information). Accurate time-keeping (e.g., use of
NTP or Simple NTP) is vital because port assignments are dynamic.
A densely populated CGN could mean even very small amounts of
clock skew between a third party's server and the CGN operator
will result in ambiguity about which customer was using a specific
port at a given time.
o The second solution considers it unrealistic to expect all servers
to log the source port number of incoming connections. To deal
with this, service providers using IPv4 address sharing may need
to log IP destination addresses.
Destination logging is imperfect if multiple subscribers are
accessing the same (popular) server at nearly the same time; it can
be impossible to disambiguate which subscriber accessed the server,
especially with protocols that involve several connections (e.g.,
HTTP). Thus, logging the destination address on the NAT is inferior
to logging the source port at the server.
If neither solution is used (that is, the server is not logging
source port numbers and the NAT is not logging destination IP
addresses), the service provider cannot trace a particular activity
to a specific subscriber. In this circumstance, the service provider
would need to disclose the identity of all subscribers who had active
sessions on the NAT during the time period in question. This may be
a large number of subscribers.
Address sharing solutions must record and store all mappings
(typically during 6-12 months, depending on the local jurisdiction)
that they create. If we consider one mapping per session, a service
provider should record and retain traces of all sessions created by
all subscribers during one year (if the legal storage duration is one
year). This may be challenging due to the volume of data requiring
storage, the volume of data to repeatedly transfer to the storage
location, and the volume of data to search in response to a query.
Address sharing solutions may mitigate these issues to some extent by
pre-allocating groups of ports. Then only the allocation of the
group needs to be recorded, and not the creation of every session
binding within that group. There are trade-offs to be made between
the sizes of these port groups, the ratio of public addresses to
subscribers, whether or not these groups timeout, and the impact on
logging requirements and port randomization security [RFC6056].
Before noting some specific security-related issues caused by large-
scale address sharing, it is perhaps worth noting that, in general,
address sharing creates a vector for attack amplification in numerous
ways. See Section 10 for one example.
13.1. Abuse Logging and Penalty Boxes
When an abuse is reported today, it is usually done in the form: IPv4
address X has done something bad at time T0. This is not enough
information to uniquely identify the subscriber responsible for the
abuse when that IPv4 address is shared by more than one subscriber.
Law enforcement authorities may be particularly impacted because of
this. This particular issue can be fixed by logging port numbers,
although this will increase logging data storage requirements.
A number of services on the network today log the IPv4 source
addresses used in connection attempts to protect themselves from
certain attacks. For example, if a server sees too many requests
from the same IPv4 address in a short period of time, it may decide
to put that address in a penalty box for a certain time during which
requests are denied, or it may require completion of a CAPTCHA
(Completely Automated Public Turing test to tell Computers and Humans
Apart) for future requests. If an IPv4 address is shared by multiple
subscribers, this would have unintended consequences in a couple of
ways. First it may become the natural behavior to see many login
attempts from the same address because it is now shared across a
potentially large number of subscribers. Second and more likely is
that one user who fails a number of login attempts may block out
other users who have not made any previous attempts but who will now
fail on their first attempt. In the presence of widespread large-
scale address sharing, penalty box solutions to service abuse simply
will not work.
In addition, there are web tie-ins into different blacklists that web
administrators subscribe to in order to redirect users with infected
machines (e.g., detect the presence of a worm) to a URL that says
"Hey, your machine is infected!". With address sharing, someone
else's worm can interfere with the ability to access the service for
other subscribers sharing the same IP address.
Simple address-based identification mechanisms that are used to
populate access control lists will fail when an IP address is no
longer sufficient to identify a particular subscriber. Including
port numbers in access control list definitions may be possible at
the cost of extra complexity, and may also require the service
provider to make static port assignments, which conflicts with the
requirement for dynamic assignments discussed in Section 5.1.
Address or DNS-name-based signatures (e.g., some X.509 signatures)
may also be affected by address sharing as the address itself is now
a shared token, and the name to address mapping may not be current.
Another case of identifying abusers has to do with spam blacklisting.
When a spammer is behind a CGN or using a port-shared address,
blacklisting of their IP address will result in all other subscribers
sharing that address having their ability to source SMTP packets
restricted to some extent.
13.4. Port Randomization
A blind attack that can be performed against TCP relies on the
attacker's ability to guess the 5-tuple (Protocol, Source Address,
Destination Address, Source Port, Destination Port) that identifies
the transport protocol instance to be attacked. [RFC6056] describes
a number of methods for the random selection of the source port
number, such that the ability of an attacker to correctly guess the
5-tuple is reduced. With shared IPv4 addresses, the port selection
space is reduced. Preserving port randomization is important and may
be more or less difficult depending on the address sharing solution
and the size of the port space that is being manipulated. Allocation
of non-contiguous port ranges could help to mitigate this issue.
It should be noted that guessing the port information may not be
sufficient to carry out a successful blind attack. An in-window TCP
Sequence Number (SN) should also be known or guessed. A TCP segment
is processed only if all previous segments have been received, except
for some Reset segment implementations that immediately process the
Reset as long as it is within the Window. If SN is randomly chosen,
it will be difficult to guess it (SN is 32 bits long); port
randomization is one protection among others against blind attacks.
There is more detailed discussion of improving TCP's robustness to
Blind In-Window Attacks in [RFC5961].
The impact of large-scale IP address sharing for IPsec operation
should be evaluated and assessed. [RFC3947] proposes a solution to
solve issues documented in [RFC3715]. [RFC5996] specifies Internet
Key Exchange (IKE) Protocol Version 2, which includes NAT traversal
mechanisms that are now widely used to enable IPsec to work in the
presence of NATs in many cases. Nevertheless, service providers may
wish to ensure that CGN deployments do not inadvertently block NAT
traversal for security protocols such as IKE (refer to [NAT-SEC] for
13.6. Policing Forwarding Behavior
[RFC2827] motivates and discusses a simple, effective, and
straightforward method for using ingress traffic filtering to
prohibit DoS attacks that use forged IP addresses. Following this
recommendation, service providers operating shared-addressing
mechanisms should ensure that source addresses, or source ports in
the case of port-range schemes, are set correctly in outgoing packets
from their subscribers or they should drop the packets.
If some form of IPv6 ingress filtering is deployed in the broadband
network and DS-Lite service is restricted to those subscribers, then
tunnels terminating at the CGN and coming from registered subscriber
IPv6 addresses cannot be spoofed. Thus, a simple access control list
on the tunnel transport source address is all that is required to
accept traffic on the internal interface of a CGN.
14. Transport Issues
14.1. Parallel Connections
One issue is systems that assume that multiple simultaneous
connections to a single IP address implies connectivity to a single
host -- such systems may experience unexpected results.
14.2. Serial Connections
Another issue is systems that assume that returning to a given IP
address means returning to the same physical host, as with stateful
transactions. This may also affect cookie-based systems.
14.3. TCP Control Block Sharing
[RFC2140] defines a performance optimization for TCP based on sharing
state between TCP control blocks that pertain to connections to the
same host, as opposed to maintaining state for each discrete
connection. This optimization assumes that an address says something
about the properties of the path between two hosts, which is clearly
not the case if the address in question is shared by multiple hosts
at different physical network locations. While CPE NAT today causes
problems for sharing TCP control block state across multiple
connections to a given IP address, large-scale address sharing will
make these issues more severe and more widespread.
15. Reverse DNS
Many service providers populate forward and reverse DNS zones for the
public IPv4 addresses that they allocate to their subscribers. In
the case where public addresses are shared across multiple
subscribers, such strings are, by definition, no longer sufficient to
identify an individual subscriber without additional information.
16. Load Balancing
Algorithms used to balance traffic load for popular destinations may
be affected by the introduction of address sharing. Where balancing
is achieved by deterministically routing traffic from specific source
IP addresses to specific servers, imbalances in load may be
experienced as address sharing is enabled for some of those source IP
addresses. This will require re-evaluation of the algorithms used in
the load-balancing design. In general, as the scale of address
sharing grows, load-balancing designs will need to be re-evaluated
and any assumptions about average load per source IP address
17. IPv6 Transition Issues
IPv4 address sharing solutions may interfere with existing IPv4 to
IPv6 transition mechanisms, which were not designed with IPv4
shortage considerations in mind. With port-range solutions, for
instance, incoming 6to4 packets should be able to find their way from
a 6to4 relay to the appropriate 6to4 CPE router, despite the lack of
direct port-range information (UDP/TCP initial source port did not
pass through the CPE port range translation process). One solution
would be for a 6to4 IPv6 address to embed not only an IPv4 address
but also a port range value.
Subscribers allocated with private addresses will not be able to
utilize 6to4 [RFC3056] to access IPv6, but may be able to utilize
Some routers enable 6to4 on their WAN link. 6to4 requires a publicly
routable IPv4 address. Enabling 6to4 when the apparently public IPv4
WAN address is in fact behind a NAT creates a disconnected IPv6
18. Introduction of Single Points of Failure
In common with all deployments of new network functionality, the
introduction of new nodes or functions to handle the multiplexing of
multiple subscribers across shared IPv4 addresses could create single
points of failure in the network. Any IP address sharing solution
should consider the opportunity to add redundancy features in order
to alleviate the impact on the robustness of the offered IP
connectivity service. The ability of the solution to allow hot
swapping from one machine to another should be considered. This is
especially important where the address sharing solution in question
requires the creation of per-flow state in the network.
19. State Maintenance Reduces Battery Life
In order for hosts to maintain network state in the presence of NAT,
keep-alive messages have to be sent at frequent intervals. For
battery-powered devices, sending these keep-alive messages can result
in significantly reduced battery performance than would otherwise be
the case [Mobile_Energy_Consumption].
20. Support of Multicast
[RFC5135] specifies requirements for a NAT that supports Any Source
IP Multicast or Source-Specific IP Multicast. Port-range routers
that form part of port-range solutions will need to support similar
requirements if multicast support is required.
21. Support of Mobile-IP
IP address sharing within the context of Mobile IP deployments (in
the home network and/or in the visited network) will require Home
Agents and/or Foreign Agents to be updated so as to take into account
the relevant port information. There may also be issues raised when
an additional layer of encapsulation is required thereby causing, or
increasing the need for, fragmentation and reassembly.
Issues for Mobile-IP in the presence of NAT are discussed in
22. Security Considerations
This memo does not define any protocol and therefore creates no new
security issues. Section 13 discusses some of the security and
identity-related implications of IP address sharing.
This document is based on sources co-authored by J.L. Grimault and A.
Villefranque of France Telecom.
This memo was partly inspired by conversations that took place as
part of Internet Society (ISOC) hosted roundtable events for
operators and content providers deploying IPv6. Participants in
those discussions included John Brzozowski, Leslie Daigle, Tom
Klieber, Yiu Lee, Kurtis Lindqvist, Wes George, Lorenzo Colliti, Erik
Kline, Igor Gashinsky, Jason Fesler, Rick Reed, Adam Bechtel, Larry
Campbell, Tom Coffeen, David Temkin, Pete Gelbman, Mark Winter, Will
Charnock, Martin Levy, Greg Wood, and Christian Jacquenet.
The authors are also grateful to Christian Jacquenet, Iain Calder,
Joel Halpern, Brian Carpenter, Gregory Lebovitz, Bob Briscoe, Marcelo
Bagnulo, Dan Wing, and Wes George for their helpful comments and
suggestions for improving the document.
This memo was created using the xml2rfc tool.
25. Informative References
[A+P] Bush, R., "The A+P Approach to the IPv4 Address Shortage",
Work in Progress, February 2011.
Alcock, S., "Research into the Viability of Service-
Provider NAT", 2008, <http://www.wand.net.nz/~salcock/
[DS-Lite] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", Work in Progress, May 2011.
IANA, "Port Numbers", <http://www.iana.org>.
ICANN, "Available Pool of Unallocated IPv4 Internet
Addresses Now Completely Emptied", February 2011,
[LSN-REQS] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common requirements for IP address sharing
schemes", Work in Progress, March 2011.
Haverinen, H., Siren, J., and P. Eronen, "Energy
Consumption of Always-On Applications in WCDMA Networks",
April 2007, <http://research.nokia.com/node/5597>.
[NAT-PMP] Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)", Work
in Progress, April 2008.
[NAT-SEC] Gont, F. and P. Srisuresh, "Security implications of
Network Address Translators (NATs)", Work in Progress,
[NAT444] Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
and H. Ashida, "NAT444", January 2011.
Haddad, W. and C. Perkins, "A Note on NAT64 Interaction
with Mobile IPv6", Work in Progress, March 2011.
[PCP] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", Work
in Progress, May 2011.
Boucadair, M., Levis, P., Bajko, G., and T. Savolainen,
"IPv4 Connectivity Access in the Context of IPv4 Address
Exhaustion: Port Range based IP Architecture", Work
in Progress, July 2009.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004.
[RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5135] Wing, D. and T. Eckert, "IP Multicast Requirements for a
Network Address Translator (NAT) and a Network Address
Port Translator (NAPT)", BCP 135, RFC 5135, February 2008.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 2011.
[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.
[SAM] Despres, R., "Scalable Multihoming across IPv6 Local-
Address Routing Zones Global-Prefix/Local-Address
Stateless Address Mapping (SAM)", July 2009.
[UPnP-IGD] UPnP Forum, "Universal Plug and Play (UPnP) Internet
Gateway Device (IGD) V 2.0", December 2010,
Microsoft, "Windows Logo Program Requirements and
Policies", 2006, <http://www.microsoft.com/whdc/winlogo/
Appendix A. Classes of Address Sharing Solution
IP address sharing solutions fall into two classes. Either a
service-provider-operated NAT function is introduced and subscribers
are allocated addresses from [RFC1918] space, or public IPv4
addresses are shared across multiple subscribers by restricting the
range of ports available to each subscriber. These classes of
solution are described in a bit more detail below.
o CGN-based solutions: These solutions propose the introduction of a
NAPT function in the service provider's network, denoted also as
Carrier Grade NAT (CGN), or Large Scale NAT (LSN) [LSN-REQS], or
Provider NAT. The CGN is responsible for translating private
addresses to publicly routable addresses. Private addresses are
assigned to subscribers, a pool of public addresses is assigned to
the CGN, and the number of public addresses is smaller than the
number of subscribers. A public IPv4 address in the CGN pool is
shared by several subscribers at the same time. Solutions making
use of a service provider-based NAT include [NAT444] (two layers
of NAT) and [DS-Lite] (a single layer of NAT).
o Port-range solutions: These solutions avoid the presence of a CGN
function. A single public IPv4 address is assigned to several
subscribers at the same time. A restricted port range is also
assigned to each subscriber so that two subscribers with the same
IPv4 address have two different port ranges that do not overlap.
These solutions are called Address+Port [A+P], or Port Range
[PORT-RANGE], or Stateless Address Mapping [SAM].
Appendix B. Address Space Multiplicative Factor
The purpose of sharing public IPv4 addresses is to increase the
addressing space. A key parameter is the factor by which service
providers want or need to multiply their IPv4 public address space,
and the consequence is the number of subscribers sharing the same
public IPv4 address. We refer to this parameter as the address space
multiplicative factor; the inverse is called the compression ratio.
The multiplicative factor can only be applied to the subset of
subscribers that are eligible for a shared address. The reasons a
subscriber cannot have a shared address can be:
o It would not be compatible with the service to which they are
currently subscribed (for example, business subscriber).
o Subscriber CPE is not compatible with the address sharing solution
selected by the service provider (for example, it does not handle
port restriction for port-range solutions or it does not allow
IPv4 in IPv6 encapsulation for the DS-Lite solution), and its
replacement is not easy.
Different service providers may have very different needs. A long-
lived service provider, whose number of subscribers is rather stable,
may have an existing address pool that will only need a small
extension to cope with the next few years, assuming that this address
pool can be re-purposed for an address sharing solution (small
multiplicative factor, less than 10). A new entrant or a new line of
business will need a much bigger multiplicative factor (e.g., 1000).
A mobile operator may see its addressing needs grow dramatically as
the IP-enabled mobile handset market grows.
When the multiplicative factor is large, the average number of ports
per subscriber is small. Given the large measured disparity between
average and peak port consumption [CGN_Viability], this will create
service problems in the event that ports are allocated statically.
In this case, it is essential for port allocation to map to need as
closely as possible, and to avoid allocating ports for longer than
necessary. Therefore, the larger the multiplicative factor, the more
dynamic the port assignment has to be.
Mat Ford (editor)
42 rue des Coutures
Caen Cedex 4 14066