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Versions: (draft-larsen-tsvwg-port-randomization) 00 01 02 03 04 05 06 07 08 09 RFC 6056

Transport Area Working Group                                   M. Larsen
(tsvwg)                                                      TietoEnator
Internet-Draft                                                   F. Gont
Intended status: BCP                                             UTN/FRH
Expires: October 14, 2010                                 April 12, 2010


         Transport Protocol Port Randomization Recommendations
                 draft-ietf-tsvwg-port-randomization-07

Abstract

   During the las 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 described
   port number obfuscation algorithms provide improved security/
   obfuscation 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, SCTP, DCCP, and RTP (provided
   the RTP application explicitly signals the RTP and RTCP port
   numbers).

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on October 14, 2010.

Copyright Notice

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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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 . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Ephemeral Ports  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Traditional Ephemeral Port Range . . . . . . . . . . . . .  7
     2.2.  Ephemeral port selection . . . . . . . . . . . . . . . . .  7
     2.3.  Collision of instance-id's . . . . . . . . . . . . . . . .  8
   3.  Obfuscating the Ephemeral Ports  . . . . . . . . . . . . . . . 10
     3.1.  Characteristics of a good ephemeral port obfuscation
           algorithm  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Ephemeral port number range  . . . . . . . . . . . . . . . 12
     3.3.  Ephemeral Port Obfuscation Algorithms  . . . . . . . . . . 12
       3.3.1.  Algorithm 1: Simple port randomization algorithm . . . 12
       3.3.2.  Algorithm 2: Another simple port randomization
               algorithm  . . . . . . . . . . . . . . . . . . . . . . 14
       3.3.3.  Algorithm 3: Simple hash-based algorithm . . . . . . . 15
       3.3.4.  Algorithm 4: Double-hash obfuscation algorithm . . . . 17
       3.3.5.  Algorithm 5: Random-increments port selection
               algorithm  . . . . . . . . . . . . . . . . . . . . . . 19
     3.4.  Secret-key considerations for hash-based port
           obfuscation algorithms . . . . . . . . . . . . . . . . . . 21
     3.5.  Choosing an ephemeral port obfuscation algorithm . . . . . 22
   4.  Port obfuscation and Network Address Port Translation
       (NAPT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Survey of the algorithms in use by some popular
                implementations . . . . . . . . . . . . . . . . . . . 31
     A.1.  FreeBSD  . . . . . . . . . . . . . . . . . . . . . . . . . 31
     A.2.  Linux  . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     A.3.  NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     A.4.  OpenBSD  . . . . . . . . . . . . . . . . . . . . . . . . . 31
     A.5.  OpenSolaris  . . . . . . . . . . . . . . . . . . . . . . . 31
   Appendix B.  Changes from previous versions of the draft (to
                be removed by the RFC Editor before publication
                of this document as a RFC . . . . . . . . . . . . . . 32
     B.1.  Changes from draft-ietf-tsvwg-port-randomization-06  . . . 32
     B.2.  Changes from draft-ietf-tsvwg-port-randomization-05  . . . 32
     B.3.  Changes from draft-ietf-tsvwg-port-randomization-04  . . . 32
     B.4.  Changes from draft-ietf-tsvwg-port-randomization-03  . . . 32
     B.5.  Changes from draft-ietf-tsvwg-port-randomization-02  . . . 32
     B.6.  Changes from draft-ietf-tsvwg-port-randomization-01  . . . 32
     B.7.  Changes from draft-ietf-tsvwg-port-randomization-00  . . . 33
     B.8.  Changes from draft-larsen-tsvwg-port-randomization-02  . . 33



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     B.9.  Changes from draft-larsen-tsvwg-port-randomization-01  . . 33
     B.10. Changes from draft-larsen-tsvwg-port-randomization-00  . . 33
     B.11. Changes from draft-larsen-tsvwg-port-randomisation-00  . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35















































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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 [I-D.ietf-tcpm-icmp-attacks] [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 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 and 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
   [I-D.ietf-tcpm-tcp-auth-opt].  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
   obfuscation 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].



























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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.

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-id's (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:




















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       /* 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 (five-tuple is unique)
               return port;

           count--;

       } while (count > 0);

       return ERROR;

                                 Figure 1

   This algorithm works adequately provided that the number of
   transport-protocol instances (for a each transport protocol) that
   have a life-time longer than it takes to exhaust the total ephemeral
   port range is small, so that collisions of instance-id's 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 routing 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-id's

   While it is possible for the ephemeral port selection algorithm to
   verify that the selected port number results in a instance-id that is



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   not currently in use by that system, the resulting instance-id 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 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 attacker
   to "guess" the instance-id in use by a target transport-protocol
   instance.  Section 3.3.3 and Section 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-id's would be, e.g. in the case of TCP, for both end-
   points of a TCP connection to keep state about recent connections
   (e.g., have both end-points end up in the TIME-WAIT state).















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3.  Obfuscating the Ephemeral Ports

3.1.  Characteristics of a good ephemeral port obfuscation algorithm

   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-id's

   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, 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-id's, 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
   election 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 sequences of
   mathematically random ephemeral port numbers.

   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-id's 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



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   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 ports range.
   If such an application was run while the corresponding port number
   was 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] [I-D.gont-tcp-security].  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
   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] [I-D.gont-tcp-security].




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   The aforementioned issue do 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 explititly allowed
   beforehand).  An example of a typical SCTP socket API can be found in
   [I-D.ietf-tsvwg-sctpsocket].

   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.

   Since this range includes ports numbers assigned by IANA, this may
   not always be possible, though.  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 ephemeral port selection
   function would check this list, avoiding the allocation of ports that
   may be needed for specific applications.

   Ephemeral port selection algorithms SHOULD use the largest possible
   port range, since this improves obfuscation.

3.3.  Ephemeral Port Obfuscation Algorithms

   Ephemeral port selection algorithms SHOULD obfuscate the allocation
   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 Section 1 and
   Section 2.2, a number of systems have implemented simple ephemeral



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   port number randomization, as follows:


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

       do {
           if(resulting five-tuple is unique)
                   return next_ephemeral;

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

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

       return ERROR;

                                 Figure 2

   We will refer to this algorithm as '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 identical
   IP addresses and server port, 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, NAPTs [RFC2663], and other middle-boxes 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-id's.  However, [Allman] has shown that at
   least in the network scenarios used for measuring the collision



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   properties of the algorithms described in this document, the
   collision rate resulting from the use of the aforementioned middle-
   boxes is nevertheless very low.

   Since this algorithm performs a completely random port selection
   (i.e., 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-id's.  Even if a
   given five-tuple is verified to be unique by the port selection
   algorithm, the five-tuple 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).

   This algorithm selects ephemeral port numbers randomly and thus
   reduces the chances of an attacker of guessing 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(resulting five-tuple is unique)
                   return next_ephemeral;

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

       return ERROR;

                                 Figure 3

   We will refer to this algorithm as 'Algorithm 2'.  This algorithm
   might be unable to select an ephemeral port (i.e., return "ERROR")
   even if there are port numbers that would result in unique five-
   tuples, when there are a large number of port numbers already in use.



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   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 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 obfuscation 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 would thus separate the ephemeral
   port space of the transport-protocol instances on a "per destination
   end-point" basis (this "separation of the ephemeral port space" means
   that transport-protocol instances with different remote end-points
   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(resulting five-tuple is unique)
               return port;

           count--;

       } while (count > 0);

       return ERROR;

                                 Figure 4

   We will refer to this algorithm as 'Algorithm 3'.

   In other words, the function F() provides a "per destination end-
   point" 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 the 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 simply 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 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 an obfuscation
   technique.  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 from 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 most likely will
   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
   to use this as input to the offset function F() and ignore hostnames
   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
   connections to an attacker controlled machine (or through an attacker
   observable routing 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 5-tuple collisions, of course).

3.3.4.  Algorithm 4: Double-hash obfuscation algorithm

   A tradeoff 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(resulting five-tuple is unique)
             return port;

        count--;

     } while (count > 0);

     return ERROR;

                                 Figure 5

   We will refer to this algorithm as '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 as "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 end-point 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 tranport-protocol instance with some
   remote end-point will not necessarily cause the 'next_ephemeral'
   variable corresponding to other end-points 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



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   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
   result in an improvement over Algorithm 3.  Further increasing the
   TABLE_LENGTH will increase the obfuscation, and possibly further
   decrease the collision rate.

   An attacker can perform traffic analysis for any "increment space"
   into which the attacker has "visibility", namely that 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 obfuscation algorithm, which offers
   a middle ground between the algorithms that select ephemeral ports
   randomly (such as those described in Section 3.3.1 and
   Section 3.3.2), and those that offer obfuscation but no randomization
   (such as those described in Section 3.3.3 and Section 3.3.4).  We
   will refer to this algorithm as 'Algorithm 5'.

























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

      /* 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(resulting five-tuple is unique)
                  return port;

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

      return ERROR;

                                 Figure 6

   This algorithm aims at at producing a monotonically-increasing
   sequence to prevent the collision of instance-id's, while avoiding
   the use of fixed increments, which would lead to trivially-
   predictable sequences.  The value "N" allows for direct control of
   the tradeoff between the level of obfuscation 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-id's.  Therefore, "N" should be selecting according to the
   following criteria:

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

   o  It should minimize collisions of instance-id's

   o  It should maximize obfuscation

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



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3.4.  Secret-key considerations for hash-based port obfuscation
      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 Section 3.3.3 and Section 3.3.4)
   used for each destination end-point 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 end-point.

   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 has been used N times (i.e. we consider it insecure).

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

   o  There is little traffic (the performance overhead of collisions is
      tolerated).

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








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3.5.  Choosing an ephemeral port obfuscation 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-id's.  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).

   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:





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


                                 Figure 7

   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() would be available, and
   thus 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.  Port obfuscation and Network Address Port Translation (NAPT)

   Network Address Port Translation (NAPT) translate 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 end-
   points of transport-protocol instance, the obfuscation of the
   ephemeral ports (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 ephemeral ports by
   means of any of the algorithms discussed in this document.  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].

   Section 3.5 provides guidance in choosing a port obfuscation
   algorithm.






























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5.  Security Considerations

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

   An eavesdropper, which 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.  Ephemeral port obfuscation does not
   provide any additional protection against this kind of attacks.  In
   such situations, proper authentication mechanisms such as those
   described in [RFC4301] should be used.

   If the local offset function F() 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].

   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.


















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6.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.














































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7.  Acknowledgements

   The offset function 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,
   Matthias Bethke, Stephane Bortzmeyer, Brian Carpenter, Vincent
   Deffontaines, Ralph Droms, Lars Eggert, Pasi Eronen, Gorry Fairhurst,
   Adrian Farrel, Guillermo Gont, Alfred Hoenes, Avshalom Houri, Charlie
   Kaufman, Amit Klein, Carlos Pignataro, Tim Polk, Kacheong Poon, Pasi
   Sarolahti, Randall Stewart, Joe Touch, Michael Tuexen, and Dan Wing
   for their valuable feedback on earlier versions 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 would like to thank Carolina Suarez for her love and
   support.
































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8.  References

8.1.  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.

   [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.

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

8.2.  Informative References

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

   [IANA]     "IANA Port Numbers",



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              <http://www.iana.org/assignments/port-numbers>.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-12 (work in progress),
              March 2010.

   [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.

   [I-D.ietf-tsvwg-sctpsocket]
              Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P.
              Lei, "Sockets API Extensions for Stream Control
              Transmission Protocol (SCTP)",
              draft-ietf-tsvwg-sctpsocket-22 (work in progress),
              March 2010.

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

   [CPNI-TCP]
              Gont, F., "CPNI Technical Note 3/2009: Security Assessment
              of the Transmission Control Protocol (TCP)",  UK Centre
              for the Protection of National Infrastructure, 2009.

   [I-D.gont-tcp-security]
              Gont, F., "Security Assessment of the Transmission Control
              Protocol (TCP)", draft-gont-tcp-security-00 (work in
              progress), February 2009.

   [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".



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   [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.

   [I-D.ietf-tcpm-tcp-auth-opt]
              Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-11
              (work in progress), March 2010.

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




































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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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Appendix B.  Changes from previous versions of the draft (to be removed
             by the RFC Editor before publication of this document as a
             RFC

B.1.  Changes from draft-ietf-tsvwg-port-randomization-06

   o  Fixes the writeo in the port number range.

   o  Fixes the requirements on the random() function.

   o  Other miscellaneous edits (resulting from IESG feedback.

B.2.  Changes from draft-ietf-tsvwg-port-randomization-05

   o  Addresses AD review feedback from Lars Eggert.

   o  Addresses AD review feedback from Lars Eggert.

B.3.  Changes from draft-ietf-tsvwg-port-randomization-04

   o  Fixes nits.

B.4.  Changes from draft-ietf-tsvwg-port-randomization-03

   o  Addresses WGLC comments from Mark Allman.  See:
      http://www.ietf.org/mail-archive/web/tsvwg/current/msg09149.html

B.5.  Changes from draft-ietf-tsvwg-port-randomization-02

   o  Added clarification of what we mean by "port randomization".

   o  Addresses feedback sent on-list and off-list by Mark Allman.

   o  Added references to [Allman] and [CPNI-TCP].

B.6.  Changes from draft-ietf-tsvwg-port-randomization-01

   o  Added Section 2.3.

   o  Added discussion of "lazy binding in Section 3.5.

   o  Added discussion of obtaining the number of outgoing connections.

   o  Miscellaneous editorial changes







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B.7.  Changes from draft-ietf-tsvwg-port-randomization-00

   o  Added Section 3.1.

   o  Changed Intended Status from "Standards Track" to "BCP".

   o  Miscellaneous editorial changes.

B.8.  Changes from draft-larsen-tsvwg-port-randomization-02

   o  Draft resubmitted as draft-ietf.

   o  Included references and text on protocols other than TCP.

   o  Added the second variant of the simple port randomization
      algorithm

   o  Reorganized the algorithms into different sections

   o  Miscellaneous editorial changes.

B.9.  Changes from draft-larsen-tsvwg-port-randomization-01

   o  No changes.  Draft resubmitted after expiration.

B.10.  Changes from draft-larsen-tsvwg-port-randomization-00

   o  Fixed a bug in expressions used to calculate number of ephemeral
      ports

   o  Added a survey of the algorithms in use by popular TCP
      implementations

   o  The whole document was reorganized

   o  Miscellaneous editorial changes

B.11.  Changes from draft-larsen-tsvwg-port-randomisation-00

   o  Document resubmitted after original document by M. Larsen expired
      in 2004

   o  References were included to current WG documents of the TCPM WG

   o  The document was made more general, to apply to all transport
      protocols





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   o  Miscellaneous editorial changes


















































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Authors' Addresses

   Michael Vittrup Larsen
   TietoEnator
   Skanderborgvej 232
   Aarhus  DK-8260
   Denmark

   Phone: +45 8938 5100
   Email: michael.larsen@tietoenator.com


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

   Phone: +54 11 4650 8472
   Email: fernando@gont.com.ar































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