Internet Engineering Task Force (IETF) M. Larsen

Internet Engineering Task Force (IETF) M. Larsen
Request for Comments: 6056 Tieto
BCP: 156 F. Gont
Category: Best Current Practice UTN/FRH
ISSN: 2070-1721 January 2011

Recommendations for Transport-Protocol Port Randomization

Abstract

During the last few years, awareness has been raised about a number
of "blind" attacks that can be performed against the Transmission
Control Protocol (TCP) and similar protocols. The consequences of
these attacks range from throughput reduction to broken connections
or data corruption. These attacks rely on the attacker's ability to
guess or know the five-tuple (Protocol, Source Address, Destination
Address, Source Port, Destination Port) that identifies the transport
protocol instance to be attacked. This document describes a number
of simple and efficient methods for the selection of the client port
number, such that the possibility of an attacker guessing the exact
value is reduced. While this is not a replacement for cryptographic
methods for protecting the transport-protocol instance, the
aforementioned port selection algorithms provide improved security
with very little effort and without any key management overhead. The
algorithms described in this document are local policies that may be
incrementally deployed and that do not violate the specifications of
any of the transport protocols that may benefit from them, such as
TCP, UDP, UDP-lite, Stream Control Transmission Protocol (SCTP),
Datagram Congestion Control Protocol (DCCP), and RTP (provided that
the RTP application explicitly signals the RTP and RTCP port
numbers).

Status of This Memo

This memo documents an Internet Best Current Practice.

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). Further information on
BCPs is available in 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
http://www.rfc-editor.org/info/rfc6056.

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Copyright Notice

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
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
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Without obtaining an adequate license from the person(s) controlling
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outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Ephemeral Ports . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Traditional Ephemeral Port Range . . . . . . . . . . . . . 5
2.2. Ephemeral Port Selection . . . . . . . . . . . . . . . . . 6
2.3. Collision of instance-ids . . . . . . . . . . . . . . . . 7
3. Obfuscating the Ephemeral Port Selection . . . . . . . . . . . 8
3.1. Characteristics of a Good Algorithm for the
Obfuscation of the Ephemeral Port Selection . . . . . . . 8
3.2. Ephemeral Port Number Range . . . . . . . . . . . . . . . 10
3.3. Algorithms for the Obfuscation of the Ephemeral Port
Selection . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Algorithm 1: Simple Port Randomization Algorithm . . . 11
3.3.2. Algorithm 2: Another Simple Port Randomization
Algorithm . . . . . . . . . . . . . . . . . . . . . . 13
3.3.3. Algorithm 3: Simple Hash-Based Port Selection
Algorithm . . . . . . . . . . . . . . . . . . . . . . 14
3.3.4. Algorithm 4: Double-Hash Port Selection Algorithm . . 16
3.3.5. Algorithm 5: Random-Increments Port Selection
Algorithm . . . . . . . . . . . . . . . . . . . . . . 18
3.4. Secret-Key Considerations for Hash-Based Port
Selection Algorithms . . . . . . . . . . . . . . . . . . . 19
3.5. Choosing an Ephemeral Port Selection Algorithm . . . . . . 20
4. Interaction with Network Address Port Translation (NAPT) . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.1. Normative References . . . . . . . . . . . . . . . . . . . 24
7.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. Survey of the Algorithms in Use by Some Popular
Implementations . . . . . . . . . . . . . . . . . . . 28
A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.5. OpenSolaris . . . . . . . . . . . . . . . . . . . . . . . 28

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1. Introduction

Recently, awareness has been raised about a number of "blind" attacks
(i.e., attacks that can be performed without the need to sniff the
packets that correspond to the transport protocol instance to be
attacked) that can be performed against the Transmission Control
Protocol (TCP) [RFC0793] and similar protocols. The consequences of
these attacks range from throughput reduction to broken connections
or data corruption [RFC5927] [RFC4953] [Watson].

All these attacks rely on the attacker's ability to guess or know the
five-tuple (Protocol, Source Address, Source port, Destination
Address, Destination Port) that identifies the transport protocol
instance to be attacked.

Services are usually located at fixed, "well-known" ports [IANA] at
the host supplying the service (the server). Client applications
connecting to any such service will contact the server by specifying
the server IP address and service port number. The IP address and
port number of the client are normally left unspecified by the client
application and thus are chosen automatically by the client
networking stack. Ports chosen automatically by the networking stack
are known as ephemeral ports [Stevens].

While the server IP address, the well-known port, and the client IP
address may be known by an attacker, the ephemeral port of the client
is usually unknown and must be guessed.

This document describes a number of algorithms for the selection of
ephemeral port numbers, such that the possibility of an off-path
attacker guessing the exact value is reduced. They are not a
replacement for cryptographic methods of protecting a transport-
protocol instance such as IPsec [RFC4301], the TCP MD5 signature
option [RFC2385], or the TCP Authentication Option [RFC5925]. For
example, they do not provide any mitigation in those scenarios in
which the attacker is able to sniff the packets that correspond to
the transport protocol instance to be attacked. However, the
proposed algorithms provide improved resistance to off-path attacks
with very little effort and without any key management overhead.

The mechanisms described in this document are local modifications
that may be incrementally deployed, and that do not violate the
specifications of any of the transport protocols that may benefit
from them, such as TCP [RFC0793], UDP [RFC0768], SCTP [RFC4960], DCCP
[RFC4340], UDP-lite [RFC3828], and RTP [RFC3550] (provided the RTP
application explicitly signals the RTP and RTCP port numbers with,
e.g., [RFC3605]).

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Since these mechanisms are obfuscation techniques, focus has been on
a reasonable compromise between the level of obfuscation and the ease
of implementation. Thus, the algorithms must be computationally
efficient and not require substantial state.

We note that while the technique of mitigating "blind" attacks by
obfuscating the ephemeral port selection is well-known as "port
randomization", the goal of the algorithms described in this document
is to reduce the chances of an attacker guessing the ephemeral ports
selected for new transport protocol instances, rather than to
actually produce mathematically random sequences of ephemeral ports.

Throughout this document, we will use the term "transport-protocol
instance" as a general term to refer to an instantiation of a
transport protocol (e.g., a "connection" in the case of connection-
oriented transport protocols) and the term "instance-id" as a short-
handle to refer to the group of values that identify a transport-
protocol instance (e.g., in the case of TCP, the five-tuple
{Protocol, IP Source Address, TCP Source Port, IP Destination
Address, TCP Destination Port}).

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].

2. Ephemeral Ports

2.1. Traditional Ephemeral Port Range

The Internet Assigned Numbers Authority (IANA) assigns the unique
parameters and values used in protocols developed by the Internet
Engineering Task Force (IETF), including well-known ports [IANA].
IANA has reserved the following use of the 16-bit port range of TCP
and UDP:

o The Well-Known Ports, 0 through 1023.

o The Registered Ports, 1024 through 49151

o The Dynamic and/or Private Ports, 49152 through 65535

The dynamic port range defined by IANA consists of the 49152-65535
range, and is meant for the selection of ephemeral ports.

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2.2. Ephemeral Port Selection

As each communication instance is identified by the five-tuple
{protocol, local IP address, local port, remote IP address, remote
port}, the selection of ephemeral port numbers must result in a
unique five-tuple.

Selection of ephemeral ports such that they result in unique
instance-ids (five-tuples) is handled by some implementations by
having a per-protocol global "next_ephemeral" variable that is equal
to the previously chosen ephemeral port + 1, i.e., the selection
process is:

/* Initialization at system boot time. Could be random */
next_ephemeral = min_ephemeral;

/* Ephemeral port selection function */
count = max_ephemeral - min_ephemeral + 1;

do {
port = next_ephemeral;
if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}

if (check_suitable_port(port))
return port;

count--;

} while (count > 0);

return ERROR;

Traditional BSD Port Selection Algorithm

Note:
check_suitable_port() is a function that checks whether the
resulting port number is acceptable as an ephemeral port. That
is, it checks whether the resulting port number is unique and may,
in addition, check that the port number is not in use for a
connection in the LISTEN or CLOSED states and that the port number
is not in the list of port numbers that should not be allocated as
ephemeral ports. In BSD-derived systems, the
check_suitable_port() would correspond to the in_pcblookup_local()
function, where all the necessary checks would be performed.

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This algorithm works adequately provided that the number of
transport-protocol instances (for each transport protocol) that have
a lifetime longer than it takes to exhaust the total ephemeral port
range is small, so that collisions of instance-ids are rare.

However, this method has the drawback that the "next_ephemeral"
variable and thus the ephemeral port range is shared between all
transport-protocol instances, and the next ports chosen by the client
are easy to predict. If an attacker operates an "innocent" server to
which the client connects, it is easy to obtain a reference point for
the current value of the "next_ephemeral" variable. Additionally, if
an attacker could force a client to periodically establish, e.g., a
new TCP connection to an attacker-controlled machine (or through an
attacker-observable path), the attacker could subtract consecutive
source port values to obtain the number of outgoing TCP connections
established globally by the target host within that time period (up
to wrap-around issues and instance-id collisions, of course).

2.3. Collision of instance-ids

While it is possible for the ephemeral port selection algorithm to
verify that the selected port number results in a instance-id that is
not currently in use by that system, the resulting five-tuple may
still be in use at a remote system. For example, consider a scenario
in which a client establishes a TCP connection with a remote web
server, and the web server performs the active close on the
connection. While the state information for this connection will
disappear at the client side (that is, the connection will be moved
to the fictional CLOSED state), the instance-id will remain in the
TIME-WAIT state at the web server for 2*MSL (Maximum Segment
Lifetime). If the same client tried to create a new incarnation of
the previous connection (that is, a connection with the same
instance-id as the one in the TIME_WAIT state at the server), an
instance-id "collision" would occur. The effect of these collisions
range from connection-establishment failures to TIME-WAIT state
assassination (with the potential of data corruption) [RFC1337]. In
scenarios in which a specific client establishes TCP connections with
a specific service at a server, these problems become evident.
Therefore, an ephemeral port selection algorithm should ideally
minimize the rate of instance-id collisions.

A simple approach to minimize the rate of these collisions would be
to choose port numbers incrementally, so that a given port number
would not be reused until the rest of the port numbers in the
ephemeral port range have been used for a transport protocol
instance. However, if a single global variable were used to keep
track of the last ephemeral port selected, ephemeral port numbers
would be trivially predictable, thus making it easier for an off-path

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attacker to "guess" the instance-id in use by a target transport-
protocol instance. Sections 3.3.3 and 3.3.4 describe algorithms that
select port numbers incrementally, while still making it difficult
for an off-path attacker to predict the ephemeral ports used for
future transport-protocol instances.

A simple but inefficient approach to minimize the rate of collisions
of instance-ids would be, e.g., in the case of TCP, for both
endpoints of a TCP connection to keep state about recent connections
(e.g., have both endpoints end up in the TIME-WAIT state).

3. Obfuscating the Ephemeral Port Selection

3.1. Characteristics of a Good Algorithm for the Obfuscation of the
Ephemeral Port Selection

There are several factors to consider when designing an algorithm for
selecting ephemeral ports, which include:

o Minimizing the predictability of the ephemeral port numbers used
for future transport-protocol instances.

o Minimizing collisions of instance-ids.

o Avoiding conflict with applications that depend on the use of
specific port numbers.

Given the goal of improving the transport protocol's resistance to
attack by obfuscation of the instance-id selection, it is key to
minimize the predictability of the ephemeral ports that will be
selected for new transport-protocol instances. While the obvious
approach to address this requirement would be to select the ephemeral
ports by simply picking a random value within the chosen port number
range, this straightforward policy may lead to collisions of
instance-ids, which could lead to the interoperability problems
(e.g., delays in the establishment of new connections, failures in
connection establishment, or data corruption) discussed in
Section 2.3. As discussed in Section 1, it is worth noting that
while the technique of mitigating "blind" attacks by obfuscating the
ephemeral port selection is well-known as "port randomization", the
goal of the algorithms described in this document is to reduce the
chances that an attacker will guess the ephemeral ports selected for
new transport-protocol instances, rather than to actually produce
sequences of mathematically random ephemeral port numbers.

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It is also worth noting that, provided adequate algorithms are in
use, the larger the range from which ephemeral ports are selected,
the smaller the chances of an attacker are to guess the selected port
number.

In scenarios in which a specific client establishes transport-
protocol instances with a specific service at a server, the problems
described in Section 2.3 become evident. A good algorithm to
minimize the collisions of instance-ids would consider the time a
given five-tuple was last used, and would avoid reusing the last
recently used five-tuples. A simple approach to minimize the rate of
collisions would be to choose port numbers incrementally, so that a
given port number would not be reused until the rest of the port
numbers in the ephemeral port range have been used for a transport-
protocol instance. However, if a single global variable were used to
keep track of the last ephemeral port selected, ephemeral port
numbers would be trivially predictable.

It is important to note that a number of applications rely on binding
specific port numbers that may be within the ephemeral port range.
If such an application were run while the corresponding port number
were in use, the application would fail. Therefore, ephemeral port
selection algorithms avoid using those port numbers.

Port numbers that are currently in use by a TCP in the LISTEN state
should not be allowed for use as ephemeral ports. If this rule is
not complied with, an attacker could potentially "steal" an incoming
connection to a local server application in at least two different
ways. Firstly, an attacker could issue a connection request to the
victim client at roughly the same time the client tries to connect to
the victim server application [CPNI-TCP] [TCP-SEC]. If the SYN
segment corresponding to the attacker's connection request and the
SYN segment corresponding to the victim client "cross each other in
the network", and provided the attacker is able to know or guess the
ephemeral port used by the client, a TCP "simultaneous open" scenario
would take place, and the incoming connection request sent by the
client would be matched with the attacker's socket rather than with
the victim server application's socket. Secondly, an attacker could
specify a more specific socket than the "victim" socket (e.g.,
specify both the local IP address and the local TCP port), and thus
incoming SYN segments matching the attacker's socket would be
delivered to the attacker, rather than to the "victim" socket (see
Section 10.1 of [CPNI-TCP]).

It should be noted that most applications based on popular
implementations of the TCP API (such as the Sockets API) perform
"passive opens" in three steps. Firstly, the application obtains a
file descriptor to be used for inter-process communication (e.g., by

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issuing a socket() call). Secondly, the application binds the file
descriptor to a local TCP port number (e.g., by issuing a bind()
call), thus creating a TCP in the fictional CLOSED state. Thirdly,
the aforementioned TCP is put in the LISTEN state (e.g., by issuing a
listen() call). As a result, with such an implementation of the TCP
API, even if port numbers in use for TCPs in the LISTEN state were
not allowed for use as ephemeral ports, there is a window of time
between the second and the third steps in which an attacker could be
allowed to select a port number that would be later used for
listening to incoming connections. Therefore, these implementations
of the TCP API should enforce a stricter requirement for the
allocation of port numbers: port numbers that are in use by a TCP in
the LISTEN or CLOSED states should not be allowed for allocation as
ephemeral ports [CPNI-TCP] [TCP-SEC].

The aforementioned issue does not affect SCTP, since most SCTP
implementations do not allow a socket to be bound to the same port
number unless a specific socket option (SCTP_REUSE_PORT) is issued on
the socket (i.e., this behavior needs to be explicitly allowed
beforehand). An example of a typical SCTP socket API can be found in
[SCTP-SOCKET].

DCCP is not affected by the exploitation of "simultaneous opens" to
"steal" incoming connections, as the server and the client state
machines are different [RFC4340]. However, it may be affected by the
vector involving binding a more specific socket. As a result, those
tuples {local IP address, local port, Service Code} that are in use
by a local socket should not be allowed for allocation as ephemeral
ports.

3.2. Ephemeral Port Number Range

As mentioned in Section 2.1, the dynamic ports consist of the range
49152-65535. However, ephemeral port selection algorithms should use
the whole range 1024-65535.

This range includes the IANA Registered Ports; thus, some of these
port numbers may be needed for providing a particular service at the
local host, which could result in the problems discussed in
Section 3.1. As a result, port numbers that may be needed for
providing a particular service at the local host SHOULD NOT be
included in the pool of port numbers available for ephemeral port
randomization. If the host does not provide a particular service,
the port can be safely allocated to ordinary processes.

A possible workaround for this potential problem would be to maintain
a local list of the port numbers that should not be allocated as
ephemeral ports. Thus, before allocating a port number, the

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ephemeral port selection function would check this list, avoiding the
allocation of ports that may be needed for specific applications.
Rather than naively excluding all the registered ports,
administrators should identify services that may be offered by the
local host and SHOULD exclude only the corresponding registered
ports.

Ephemeral port selection algorithms SHOULD use the largest possible
port range, since this reduces the chances of an off-path attacker of
guessing the selected port numbers.

3.3. Algorithms for the Obfuscation of the Ephemeral Port Selection

Ephemeral port selection algorithms SHOULD obfuscate the selection of
their ephemeral ports, since this helps to mitigate a number of
attacks that depend on the attacker's ability to guess or know the
five-tuple that identifies the transport-protocol instance to be
attacked.

The following subsections describe a number of algorithms that could
be implemented in order to obfuscate the selection of ephemeral port
numbers.

3.3.1. Algorithm 1: Simple Port Randomization Algorithm

In order to address the security issues discussed in Sections 1 and
2.2, a number of systems have implemented simple ephemeral port
number randomization, as follows:

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/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count = num_ephemeral;

do {
if(check_suitable_port(port))
return next_ephemeral;

if (next_ephemeral == max_ephemeral) {
next_ephemeral = min_ephemeral;
} else {
next_ephemeral++;
}

count--;
} while (count > 0);

return ERROR;

Algorithm 1

Note:
random() is a function that returns a 32-bit pseudo-random
unsigned integer number. Note that the output needs to be
unpredictable, and typical implementations of POSIX random()
function do not necessarily meet this requirement. See [RFC4086]
for randomness requirements for security.

All the variables (in this and all the algorithms discussed in
this document) are unsigned integers.

Since the initially chosen port may already be in use with IP
addresses and server port that are identical to the ones being used
for the socket for which the ephemeral port is to be selected, the
resulting five-tuple might not be unique. Therefore, multiple ports
may have to be tried and verified against all existing transport-
protocol instances before a port can be chosen.

Web proxy servers, Network Address Port Translators (NAPTs)
[RFC2663], and other middleboxes aggregate multiple peers into the
same port space and thus increase the population of used ephemeral
ports, and hence the chances of collisions of instance-ids. However,
[Allman] has shown that at least in the network scenarios used for
measuring the collision properties of the algorithms described in
this document, the collision rate resulting from the use of the
aforementioned middleboxes is nevertheless very low.

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Since this algorithm performs port selection without taking into
account the port numbers previously chosen, it has the potential of
reusing port numbers too quickly, thus possibly leading to collisions
of instance-ids. Even if a given instance-id is verified to be
unique by the port selection algorithm, the instance-id might still
be in use at the remote system. In such a scenario, a connection
request could possibly fail ([Silbersack] describes this problem for
the TCP case).

However, this algorithm is biased towards the first available port
after a sequence of unavailable port numbers. If the local list of
registered port numbers that should not be allocated as ephemeral
ports (as described in Section 3.2) is significant, an attacker may
actually have a significantly better chance of guessing a port
number.

This algorithm selects ephemeral port numbers randomly and thus
reduces the chances that an attacker will guess the ephemeral port
selected for a target transport-protocol instance. Additionally, it
prevents attackers from obtaining the number of outgoing transport-
protocol instances (e.g., TCP connections) established by the client
in some period of time.

3.3.2. Algorithm 2: Another Simple Port Randomization Algorithm

The following pseudo-code illustrates another algorithm for selecting
a random port number, in which in the event a local instance-id
collision is detected, another port number is selected randomly:

/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count = num_ephemeral;

do {
if(check_suitable_port(port))
return next_ephemeral;

next_ephemeral = min_ephemeral + (random() % num_ephemeral);
count--;
} while (count > 0);

return ERROR;

Algorithm 2

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When there are a large number of port numbers already in use for the
same destination endpoint, this algorithm might be unable (with a
very small remaining probability) to select an ephemeral port (i.e.,
it would return "ERROR"), even if there are still a few port numbers
available that would result in unique five-tuples. However, the
results in [Allman] have shown that in common scenarios, one port
choice is enough, and in most cases where more than one choice is
needed, two choices suffice. Therefore, in those scenarios this
would not be problem.

3.3.3. Algorithm 3: Simple Hash-Based Port Selection Algorithm

We would like to achieve the port-reuse properties of the traditional
BSD port selection algorithm (described in Section 2.2), while at the
same time achieve the unpredictability properties of Algorithm 1 and
Algorithm 2.

Ideally, we would like a "next_ephemeral" value for each set of
(local IP address, remote IP addresses, remote port), so that the
port-reuse frequency is the lowest possible. Each of these
"next_ephemeral" variables should be initialized with random values
within the ephemeral port range and, together, these would thus
separate the ephemeral port space of the transport-protocol instances
on a "per-destination endpoint" basis (this "separation of the
ephemeral port space" means that transport-protocol instances with
different remote endpoints will not have different sequences of port
numbers, i.e., will not be part of the same ephemeral port sequence
as in the case of the traditional BSD ephemeral port selection
algorithm). Since we do not want to maintain in memory all these
"next_ephemeral" values, we propose an offset function F() that can
be computed from the local IP address, remote IP address, remote
port, and a secret key. F() will yield (practically) different
values for each set of arguments, i.e.:

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/* Initialization at system boot time. Could be random. */
next_ephemeral = 0;

/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
offset = F(local_IP, remote_IP, remote_port, secret_key);
count = num_ephemeral;

do {
port = min_ephemeral +
(next_ephemeral + offset) % num_ephemeral;

next_ephemeral++;

if(check_suitable_port(port))
return port;

count--;

} while (count > 0);

return ERROR;

Algorithm 3

In other words, the function F() provides a "per-destination
endpoint" fixed offset within the global ephemeral port range. Both
the "offset" and "next_ephemeral" variables may take any value within
the storage type range since we are restricting the resulting port in
a similar way as in Algorithm 1 (described in Section 3.3.1). This
allows us to simply increment the "next_ephemeral" variable and rely
on the unsigned integer to wrap around.

The function F() should be a cryptographic hash function like MD5
[RFC1321]. The function should use both IP addresses, the remote
port, and a secret key value to compute the offset. The remote IP
address is the primary separator and must be included in the offset
calculation. The local IP address and remote port may in some cases
be constant and thus not improve the ephemeral port space separation;
however, they should also be included in the offset calculation.

Cryptographic algorithms stronger than, e.g., MD5 should not be
necessary, given that Algorithm 3 is simply a technique for the
obfuscation of the selection of ephemeral ports. The secret should
be chosen to be as random as possible (see [RFC4086] for
recommendations on choosing secrets).

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Note that on multiuser systems, the function F() could include user-
specific information, thereby providing protection not only on a
host-to-host basis, but on a user to service basis. In fact, any
identifier of the remote entity could be used, depending on
availability and the granularity requested. With SCTP, both
hostnames and alternative IP addresses may be included in the
association negotiation, and either of these could be used in the
offset function F().

When multiple unique identifiers are available, any of these can be
chosen as input to the offset function F() since they all uniquely
identify the remote entity. However, in cases like SCTP where the
ephemeral port must be unique across all IP address permutations, we
should ideally always use the same IP address to get a single
starting offset for each association negotiation with a given remote
entity to minimize the possibility of collisions. A simple numerical
sorting of the IP addresses and always using the numerically lowest
could achieve this. However, since most protocols will generally
report the same IP addresses in the same order in each association
setup, this sorting is most likely not necessary and the "first one"
can simply be used.

The ability of hostnames to uniquely define hosts can be discussed,
and since SCTP always includes at least one IP address, we recommend
using this as input to the offset function F() and ignoring hostname
chunks when searching for ephemeral ports.

It should be noted that, as this algorithm uses a global counter
("next_ephemeral") for selecting ephemeral ports, if an attacker
could, e.g., force a client to periodically establish a new TCP
connection to an attacker-controlled machine (or through an attacker-
observable path), the attacker could subtract consecutive source port
values to obtain the number of outgoing TCP connections established
globally by the target host within that time period (up to wrap-
around issues and five-tuple collisions, of course).

3.3.4. Algorithm 4: Double-Hash Port Selection Algorithm

A trade-off between maintaining a single global "next_ephemeral"
variable and maintaining 2**N "next_ephemeral" variables (where N is
the width of the result of F()) could be achieved as follows. The
system would keep an array of TABLE_LENGTH short integers, which
would provide a separation of the increment of the "next_ephemeral"
variable. This improvement could be incorporated into Algorithm 3 as
follows:

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/* Initialization at system boot time */
for(i = 0; i < TABLE_LENGTH; i++)
table[i] = random() % 65536;

/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;
offset = F(local_IP, remote_IP, remote_port, secret_key1);
index = G(local_IP, remote_IP, remote_port, secret_key2);
count = num_ephemeral;

do {
port = min_ephemeral + (offset + table[index]) % num_ephemeral;
table[index]++;

if(check_suitable_port(port))
return port;

count--;

} while (count > 0);

return ERROR;

Algorithm 4

"table[]" could be initialized with mathematically random values, as
indicated by the initialization code in pseudo-code above. The
function G() should be a cryptographic hash function like MD5
[RFC1321]. It should use both IP addresses, the remote port, and a
secret key value to compute a value between 0 and (TABLE_LENGTH-1).
Alternatively, G() could take an "offset" as input, and perform the
exclusive-or (XOR) operation between all the bytes in "offset".

The array "table[]" assures that successive transport-protocol
instances with the same remote endpoint will use increasing ephemeral
port numbers. However, incrementation of the port numbers is
separated into TABLE_LENGTH different spaces, and thus the port-reuse
frequency will be (probabilistically) lower than that of Algorithm 3.
That is, a new transport-protocol instance with some remote endpoint
will not necessarily cause the "next_ephemeral" variable
corresponding to other endpoints to be incremented.

It is interesting to note that the size of "table[]" does not limit
the number of different port sequences, but rather separates the
*increments* into TABLE_LENGTH different spaces. The port sequence
will result from adding the corresponding entry of "table[]" to the
variable "offset", which selects the actual port sequence (as in
Algorithm 3). [Allman] has found that a TABLE_LENGTH of 10 can

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result in an improvement over Algorithm 3. Further increasing the
TABLE_LENGTH will increase the unpredictability of the resulting port
number, and possibly further decrease the collision rate.

An attacker can perform traffic analysis for any "increment space"
into which the attacker has "visibility" -- namely, the attacker can
force the client to establish a transport-protocol instance whose
G(offset) identifies the target "increment space". However, the
attacker's ability to perform traffic analysis is very reduced when
compared to the traditional BSD algorithm (described in Section 2.2)
and Algorithm 3. Additionally, an implementation can further limit
the attacker's ability to perform traffic analysis by further
separating the increment space (that is, using a larger value for
TABLE_LENGTH).

3.3.5. Algorithm 5: Random-Increments Port Selection Algorithm

[Allman] introduced another port selection algorithm, which offers a
middle ground between the algorithms that select ephemeral ports
independently at random (such as those described in Sections 3.3.1
and 3.3.2), and those that offer obfuscation with less randomization
(such as those described in Sections 3.3.3 and 3.3.4).

/* Initialization code at system boot time. */
next_ephemeral = random() % 65536; /* Initialization value */
N = 500; /* Determines the trade-off */

/* Ephemeral port selection function */
num_ephemeral = max_ephemeral - min_ephemeral + 1;

count = num_ephemeral;

do {
next_ephemeral = next_ephemeral + (random() % N) + 1;
port = min_ephemeral + (next_ephemeral % num_ephemeral);

if(check_suitable_port(port))
return port;

count--;
} while (count > 0);

return ERROR;

Algorithm 5

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This algorithm aims at producing a monotonically increasing sequence
to prevent the collision of instance-ids, while avoiding the use of
fixed increments, which would lead to trivially predictable
sequences. The value "N" allows for direct control of the trade-off
between the level of unpredictability and the port-reuse frequency.
The smaller the value of "N", the more similar this algorithm is to
the traditional BSD port selection algorithm (described in
Section 2.2). The larger the value of "N", the more similar this
algorithm is to the algorithm described in Section 3.3.1 of this
document.

When the port numbers wrap, there is the risk of collisions of
instance-ids. Therefore, "N" should be selected according to the
following criteria:

o It should maximize the wrapping time of the ephemeral port space.

o It should minimize collisions of instance-ids.

o It should maximize the unpredictability of selected port numbers.

Clearly, these are competing goals, and the decision of which value
of "N" to use is a trade-off. Therefore, the value of "N" should be
configurable so that system administrators can make the trade-off for
themselves.

3.4. Secret-Key Considerations for Hash-Based Port Selection Algorithms

Every complex manipulation (like MD5) is no more secure than the
input values, and in the case of ephemeral ports, the secret key. If
an attacker is aware of which cryptographic hash function is being
used by the victim (which we should expect), and the attacker can
obtain enough material (e.g., ephemeral ports chosen by the victim),
the attacker may simply search the entire secret-key space to find
matches.

To protect against this, the secret key should be of a reasonable
length. Key lengths of 128 bits should be adequate.

Another possible mechanism for protecting the secret key is to change
it after some time. If the host platform is capable of producing
reasonably good random data, the secret key can be changed
automatically.

Changing the secret will cause abrupt shifts in the chosen ephemeral
ports, and consequently collisions may occur. That is, upon changing
the secret, the "offset" value (see Sections 3.3.3 and 3.3.4) used

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for each destination endpoint will be different from that computed
with the previous secret, thus leading to the selection of a port
number recently used for connecting to the same endpoint.

Thus, the change in secret key should be done with consideration and
could be performed whenever one of the following events occur:

o The system is being bootstrapped.

o Some predefined/random time has expired.

o The secret key has been used sufficiently often that it should be
regarded as insecure now.

o There are few active transport-protocol instances (i.e.,
possibility of a collision is low).

o System load is low (i.e., the performance overhead of local
collisions is tolerated).

o There is enough random data available to change the secret key
(pseudo-random changes should not be done).

3.5. Choosing an Ephemeral Port Selection Algorithm

[Allman] is an empirical study of the properties of the algorithms
described in this document, which has found that all the algorithms
described in this document offer low collision rates -- at most 0.3%.
That is, in those network scenarios assessed by [Allman], all of the
algorithms described in this document perform well in terms of
collisions of instance-ids. However, these results may vary
depending on the characteristics of network traffic and the specific
network setup.

The algorithm described in Section 2.2 is the traditional ephemeral
port selection algorithm implemented in BSD-derived systems. It
generates a global sequence of ephemeral port numbers, which makes it
trivial for an attacker to predict the port number that will be used
for a future transport protocol instance. However, it is very simple
and leads to a low port-reuse frequency.

Algorithm 1 and Algorithm 2 have the advantage that they provide
actual randomization of the ephemeral ports. However, they may
increase the chances of port number collisions, which could lead to
the failure of a connection establishment attempt. [Allman] found
that these two algorithms show the largest collision rates (among all
the algorithms described in this document).

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Algorithm 3 provides complete separation in local and remote IP
addresses and remote port space, and only limited separation in other
dimensions (see Section 3.4). However, implementations should
consider the performance impact of computing the cryptographic hash
used for the offset.

Algorithm 4 improves Algorithm 3, usually leading to a lower port-
reuse frequency, at the expense of more processor cycles used for
computing G(), and additional kernel memory for storing the array
"table[]".

Algorithm 5 offers middle ground between the simple randomization
algorithms (Algorithm 1 and Algorithm 2) and the hash-based
algorithms (Algorithm 3 and Algorithm 4). The upper limit on the
random increments (the value "N" in the pseudo-code included in
Section 3.3.5) controls the trade-off between randomization and port-
reuse frequency.

Finally, a special case that may preclude the utilization of
Algorithm 3 and Algorithm 4 should be analyzed. There exist some
applications that contain the following code sequence:

s = socket();
bind(s, IP_address, port = *);

In some BSD-derived systems, the call to bind() will result in the
selection of an ephemeral port number. However, as neither the
remote IP address nor the remote port will be available to the
ephemeral port selection function, the hash function F() used in
Algorithm 3 and Algorithm 4 will not have all the required arguments,
and thus the result of the hash function will be impossible to
compute. Transport protocols implementing Algorithm 3 or Algorithm 4
should consider using Algorithm 2 when facing the scenario just
described.

An alternative to this behavior would be to implement "lazy binding"
in response to the bind() call. That is, selection of an ephemeral
port would be delayed until, e.g., connect() or send() are called.
Thus, at that point the ephemeral port is actually selected, all the
necessary arguments for the hash function F() are available, and
therefore Algorithm 3 and Algorithm 4 could still be used in this
scenario. This algorithm has been implemented by Linux [Linux].

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4. Interaction with Network Address Port Translation (NAPT)

Network Address Port Translation (NAPT) translates both the network
address and transport-protocol port number, thus allowing the
transport identifiers of a number of private hosts to be multiplexed
into the transport identifiers of a single external address
[RFC2663].

In those scenarios in which a NAPT is present between the two
endpoints of a transport-protocol instance, the obfuscation of the
ephemeral port selection (from the point of view of the external
network) will depend on the ephemeral port selection function at the
NAPT. Therefore, NAPTs should consider obfuscating the selection of
ephemeral ports by means of any of the algorithms discussed in this
document.

A NAPT that does not implement port preservation [RFC4787] [RFC5382]
SHOULD obfuscate selection of the ephemeral port of a packet when it
is changed during translation of that packet.

A NAPT that does implement port preservation SHOULD obfuscate the
ephemeral port of a packet only if the port must be changed as a
result of the port being already in use for some other session.

A NAPT that performs parity preservation and that must change the
ephemeral port during translation of a packet SHOULD obfuscate the
ephemeral ports. The algorithms described in this document could be
easily adapted such that the parity is preserved (i.e., force the
lowest order bit of the resulting port number to 0 or 1 according to
whether even or odd parity is desired).

Some applications allocate contiguous ports and expect to see
contiguous ports in use at their peers. Clearly, this expectation
might be difficult to accommodate at a NAPT, since some port numbers
might already be in use by other sessions, and thus an alternative
port might need to be selected, thus resulting in a non-contiguous
port number sequence (see Section 4.2.3 of [RFC4787]). A NAPT that
implements a simple port randomization algorithm (such as Algorithm
1, Algorithm 2, or Algorithm 5) is likely to break this assumption,
even if the endpoint selecting an ephemeral port does select
ephemeral ports that are contiguous. However, since a number of
different ephemeral port selection algorithms have been implemented
by deployed NAPTs, any application that relies on any specific
ephemeral port selection algorithm at the NAPT is likely to suffer
interoperability problems when a NAPT is present between the two
endpoints of a transport-protocol instance. Nevertheless, some of
the algorithms described in this document (namely Algorithm 3 and
Algorithm 4) select consecutive ephemeral ports such that they are

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RFC 6056 Port Randomization Recommendations January 2011

contiguous (except when one of the port numbers needed to produce a
contiguous sequence is already in use by some other NAPT session).
Therefore, a NAPT willing to produce sequences of contiguous port
numbers should consider implementing Algorithm 3 or Algorithm 4 of
this document. Section 3.5 provides further guidance in choosing a
port selection algorithm.

It should be noted that in some network scenarios, a NAPT may
naturally obscure ephemeral port selections simply due to the vast
range of services with which it establishes connections and to the
overall rate of the traffic [Allman].

5. Security Considerations

Obfuscating the ephemeral port selection is no replacement for
cryptographic mechanisms, such as IPsec [RFC4301], in terms of
protecting transport-protocol instances against blind attacks.

An eavesdropper that can monitor the packets that correspond to the
transport-protocol instance to be attacked could learn the IP
addresses and port numbers in use (and also sequence numbers, etc.)
and easily perform an attack. Obfuscation of the ephemeral port
selection does not provide any additional protection against this
kind of attack. In such situations, proper authentication mechanisms
such as those described in [RFC4301] should be used.

This specification recommends including the whole range 1024-65535
for the selection of ephemeral ports, and suggests that an
implementation maintains a list of those port numbers that should not
be made available for ephemeral port selection. If the list of port
numbers that are not available is significant, Algorithm 1 may be
highly biased and generate predictable ports, as noted in
Section 3.3.1. In particular, if the list of IANA Registered Ports
is accepted as the local list of port numbers that should not be made
available, certain ports may result with 500 times the probability of
other ports. Systems that support numerous applications resulting in
large lists of unavailable ports, or that use the IANA Registered
Ports without modification, MUST NOT use Algorithm 1.

If the local offset function F() (in Algorithm 3 and Algorithm 4)
results in identical offsets for different inputs at greater
frequency than would be expected by chance, the port-offset mechanism
proposed in this document would have a reduced effect.

If random numbers are used as the only source of the secret key, they
should be chosen in accordance with the recommendations given in
[RFC4086].

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If an attacker uses dynamically assigned IP addresses, the current
ephemeral port offset (Algorithm 3 and Algorithm 4) for a given five-
tuple can be sampled and subsequently used to attack an innocent peer
reusing this address. However, this is only possible until a re-
keying happens as described above. Also, since ephemeral ports are
only used on the client side (e.g., the one initiating the transport-
protocol communication), both the attacker and the new peer need to
act as servers in the scenario just described. While servers using
dynamic IP addresses exist, they are not very common, and with an
appropriate re-keying mechanism the effect of this attack is limited.

6. Acknowledgements

The offset function used in Algorithm 3 and Algorithm 4 was inspired
by the mechanism proposed by Steven Bellovin in [RFC1948] for
defending against TCP sequence number attacks.

The authors would like to thank (in alphabetical order) Mark Allman,
Jari Arkko, Matthias Bethke, Stephane Bortzmeyer, Brian Carpenter,
Vincent Deffontaines, Ralph Droms, Lars Eggert, Pasi Eronen, Gorry
Fairhurst, Adrian Farrel, Guillermo Gont, David Harrington, Alfred
Hoenes, Avshalom Houri, Charlie Kaufman, Amit Klein, Subramanian
Moonesamy, Carlos Pignataro, Tim Polk, Kacheong Poon, Pasi Sarolahti,
Robert Sparks, Randall Stewart, Joe Touch, Michael Tuexen, Magnus
Westerlund, and Dan Wing for their valuable feedback on draft
versions of this document.

The authors would like to thank Alfred Hoenes for his admirable
effort in improving the quality of this document.

The authors would like to thank FreeBSD's Mike Silbersack for a very
fruitful discussion about ephemeral port selection techniques.

Fernando Gont's attendance to IETF meetings was supported by ISOC's
"Fellowship to the IETF" program.

7. References

7.1. Normative References

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.

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RFC 6056 Port Randomization Recommendations January 2011

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.

[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP)
attribute in Session Description Protocol (SDP)",
RFC 3605, October 2003.

[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.,
and G. Fairhurst, "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, July 2004.

[RFC4086] Eastlake, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106,
RFC 4086, June 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
March 2006.

[RFC4787] Audet, F. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, January 2007.

[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.

[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP",
BCP 142, RFC 5382, October 2008.

7.2. Informative References

[Allman] Allman, M., "Comments On Selecting Ephemeral Ports",
ACM Computer Communication Review, 39(2), 2009.

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RFC 6056 Port Randomization Recommendations January 2011

[CPNI-TCP] Gont, F., "CPNI Technical Note 3/2009: Security
Assessment of the Transmission Control Protocol
(TCP)", 2009, <http://www.cpni.gov.uk/Docs/
tn-03-09-security-assessment-TCP.pdf>.

[FreeBSD] The FreeBSD Project, <http://www.freebsd.org>.

[IANA] "IANA Port Numbers",
<http://www.iana.org/assignments/port-numbers>.

[Linux] The Linux Project, <http://www.kernel.org>.

[NetBSD] The NetBSD Project, <http://www.netbsd.org>.

[OpenBSD] The OpenBSD Project, <http://www.openbsd.org>.

[OpenSolaris] OpenSolaris, <http://www.opensolaris.org>.

[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.

[RFC1948] Bellovin, S., "Defending Against Sequence Number
Attacks", RFC 1948, May 1996.

[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.

[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, July 2007.

[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.

[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
July 2010.

[SCTP-SOCKET] Stewart, R., Poon, K., Tuexen, M., Lei, P., and V.
Yasevich, V., "Sockets API Extensions for Stream
Control Transmission Protocol (SCTP)", Work in
Progress, January 2011.

[Silbersack] Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability",
EuroBSDCon 2005 Conference.

[Stevens] Stevens, W., "Unix Network Programming, Volume 1:
Networking APIs: Socket and XTI", Prentice Hall, 1998.

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RFC 6056 Port Randomization Recommendations January 2011

[TCP-SEC] Gont, F., "Security Assessment of the Transmission
Control Protocol (TCP)", Work in Progress,
February 2010.

[Watson] Watson, P., "Slipping in the Window: TCP Reset
Attacks", CanSecWest 2004 Conference.

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RFC 6056 Port Randomization Recommendations January 2011

Appendix A. Survey of the Algorithms in Use by Some Popular
Implementations

A.1. FreeBSD

FreeBSD 8.0 implements Algorithm 1, and in response to this document
now uses a "min_port" of 10000 and a "max_port" of 65535 [FreeBSD].

A.2. Linux

Linux 2.6.15-53-386 implements Algorithm 3, with MD5 as the hash
algorithm. If the algorithm is faced with the corner-case scenario
described in Section 3.5, Algorithm 1 is used instead [Linux].

A.3. NetBSD

NetBSD 5.0.1 does not obfuscate its ephemeral port numbers. It
selects ephemeral port numbers from the range 49152-65535, starting
from port 65535, and decreasing the port number for each ephemeral
port number selected [NetBSD].

A.4. OpenBSD

OpenBSD 4.2 implements Algorithm 1, with a "min_port" of 1024 and a
"max_port" of 49151. [OpenBSD]

A.5. OpenSolaris

OpenSolaris 2009.06 implements Algorithm 1, with a "min_port" of
32768 and a "max_port" of 65535 [OpenSolaris].

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Authors' Addresses

Michael Vittrup Larsen
Tieto
Skanderborgvej 232
Aarhus DK-8260
Denmark

Phone: +45 8938 5100
EMail: [email protected]

Fernando Gont
Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
EMail: [email protected]

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