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Versions: 00 01 02 03 04 draft-sun-dhc-port-set-option

Network WG                                                  Gabor Bajko
Internet Draft                                         Teemu Savolainen
Intended Status: Proposed Standard                                Nokia
Expires: August 3, 2009                                    M. Boucadair
                                                               P. Levis
                                                         France Telecom
                                                       February 3, 2009


                 Port Restricted IP Address Assignment
                     draft-bajko-pripaddrassign-00


Status of this Memo

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

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   This Internet-Draft will expire on August 3, 2009.

   Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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Abstract

   When IPv6 was designed, the assumption was that the transition from
   IPv4 to IPv6 will occur way before the exhaustion of the available

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   IPv4 address pool. The unexpected growth of the IPv4 Internet and
   the hesitation and technical difficulties to deploy IPv6 indicates
   that the transition may take much longer than originally
   anticipated.
   It is expected that communication using IPv6 addresses will increase
   during the next few years to come at the expense of communication
   using IPv4 addresses. The Internet should reach a safety point in
   the future, where the number of IPv4 public addresses in use at a
   given time begins decreasing. It is very likely that the IPv4 public
   address pool currently available at IANA will be exhausted before
   the internet reaches this safety point. This creates a need to
   prolong the lifetime of the available IPv4 addresses.

   This document defines methods to allocate the same IPv4 address to
   multiple hosts, with the aim to prolong the availability of public
   IPv4 addresses, possibly for as long as it takes for IPv6 to take
   over the demand for IPv4.

Conventions used in this document

   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].

Terminology and abbreviations used in this document

   Port restricted IPv4 address: an IP address which can only be used
   in conjunction with the specified ports. Port restriction refers to
   all known transport protocols (UDP, TCP, SCTP, DCCP).

   CGN          Carrier Grade Network Address Translator
   CPE          Consumer Premises Equipment, a device that resides
                between internet service provider's network and
                consumers' home network.
   PRA          Port Restricted IPv4 Address

















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Table of Content

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .4
   2. Port Randomization . . . . . . . . . . . . . . . . . . . . . . .5
   3. DHCPv4 Option for allocating port restricted public IPv4
      address . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4. Port Mask sub-option usage . . . . . . . . . . . . . . . . . . .8
   4.1 Illustration Examples . . . . . . . . . . . . . . . . . . . . .9
   5. Random Port delegation function . . . . . . . . . . . . . . . .10
   6. Option Usage . . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.1 Client Behaviour . . . . . . . . . . . . . . . . . . . . . . .12
   6.2 Server Behaviour . . . . . . . . . . . . . . . . . . . . . . .13
   7. Applicability . . . . . . . . . . . . . . . . . . . . . . . . .14
   7.1 ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
   7.2 6to4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
   7.3 Protocols not supported by multiplexing gateway . . . . . . . 15
   8. IANA considerations . . . . . . . . . . . . . . . . . . . . . .15
   9. Security considerations . . . . . . . . . . . . . . . . . . . .15
   10. Normative References . . . . . . . . . . . . . . . . . . . . .16
   11. Informative References . . . . . . . . . . . . . . . . . . . .16
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . .17
   13. Editor's Addresses . . . . . . . . . . . . . . . . . . . . . .17































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1. Introduction

   There are a number of possible solutions to deal with the problem of
   transitioning from IPv4 to IPv6; however none of them is a one fits
   all solution. Different solutions fit different deployment scenarios
   (see also [ARKK2008]). See [WING2008] for comparison.

   As a possible additional and complementary solution for the IPv4-
   IPv6 coexistence period, this document describes a method, using
   newly defined DHCPv4 [RFC2131] option that allows servers to assign
   port restricted IPv4 addresses to clients. By assigning the same
   IPv4 address to multiple clients, the availability of IPv4 addresses
   can be, hopefully, prolonged for as long as it takes for IPv6 to
   take over the demand for IPv4.

   The solution described in this document is intended to be used by
   large ISPs, who as of the date of writing this document, have a
   large enough IPv4 address pool to be able to allocate one public
   IPv4 address for each and every client. They expect though that the
   situation is unsustainable and they will soon not be able to provide
   every client with a public IPv4 address. Such ISPs have two
   possibilities to choose from:
   - deploy Network Address Translators (NAT), which can be a
   significant investment for ISPs not having NATs yet. The address
   space limitations of [RFC1918] may even force these large ISPs to
   deploy double NATs, which come with all the harmful behaviour of
   Carrier Grade NATs (CGN), as described in [MAEN2008]; or
   - allocate fragments of the same public IPv4 address directly to
   multiple clients (which can be CPEs or end hosts), thus avoid the
   cost of deploying multiple layers of NATs or carrier grade NATs. It
   is however assumed, that the demand for IPv4 addresses will decrease
   in the not so distant future, being taken over by IPv6, as the
   proposal in this draft is not by any means a permanent solution for
   the IPv4 address exhaustion problem. In fact, some presented
   deployment scenarios require existence of IPv6 access network.

   For ISPs not having NATs yet, a solution not requiring NATs would
   probably be preferred. For some other ISPs, who already have NATs in
   place, increasing the capacity of their NATs might be a viable
   alternative.

   In other deployment scenarios, allocation of shared addresses to
   devices at the edge of the network would result in distribution of
   NAT functionality to the edges, in some cases even to CPEs [APLUSP].

   This document proposes to use new DHCPv4 option to allocate port-
   restricted IPv4 addresses to the clients. This method is meant to be
   an IPv4 to IPv6 transition tool, to be only temporarily used during
   the period when the demand for public IPv4 addresses will exceed the
   availability of them.



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   The port restricted IPv4 address option described in this document
   can be used in various deployment scenarios, some of which are
   described in [BOUCADAIRARCH], [APLUSP], and [DSLITE].


















































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2. Port Randomization

   It is well documented that attackers can perform "blind" attacks
   against transport 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. Most of these attacks can be prevented by
   randomly selecting the client source port number such that the
   possibility of an attacker guessing the exact value is reduced.
   [RANDOMPORT] defines a few algorithms which can select a random port
   from the available port range. Clients usually have the (1024,
   65535) port range at their disposal to select a random, not yet used
   port.

   When an IP address is allocated to multiple clients, the source port
   range has to be divided between the clients. The smaller the port
   range, the easier is for an attacker to guess the next port the
   client is going to use. Therefore, it is imperative to divide the
   port range between clients sharing the same IP address in such a way
   that random selection is preserved. This document proposes two
   different methods for port allocation, which preserves partly or
   completely the randomness of the source ports:

      o The first mechanism uses a port mask with a bit locator to
        communicate a range or multiple ranges of ports to a client.
        Randomness is preserved when the client is able to select a
        port randomly across all the available port ranges. The
        algorithms described in [RANDOMPORT] can be used to select a
        random port from one port range, but implementations may find
        it difficult to select random ports across port ranges.

      o The second mechanism uses a cryptographic function to
        preallocate random ports from the entire port range. The key
        and other input parameters are communicated to the client,
        which can calculate the ports it can use. The 'side effect' of
        this mechanism is that the client is forced to use random
        ports, as a number of random ports allowed to be used by the
        client are preallocated by the server. When this mode is used,
        the network equipments in charge of routing the inbound packets
        towards the clients may require more processing resources.










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3. DHCPv4 Option for allocating port restricted public IPv4 address

   This section defines new DHCPv4 option, which allows allocation of
   port restricted IPv4 addresses.

   The option layout is depicted below:

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   | Option Code   |    length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Sub-Option 1                              |
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ...                                     |
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Sub-Option n                              |
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Code
        Option Code
          OPTION-IPv4-PRA (TBD) - 1     byte

        Length
          An 8-bit field indicating the length of the option excluding
          the 'Option Code' and the 'Length' fields

        Sub-options
          A series of DHCPv4 sub-options.

   The sub-option layout is depicted below:

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Sub-opt Type  |    length     |              DATA             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                 DATA                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The sub-option types defined in this document are:
   1    port mask
   2    random port delegation function

   These two options are exclusive with each other (if one is used, the
   other one is not).

      Length
        An 8-bit field indicating the length of the sub-option
        excluding the 'Sub-opt Type' and the 'Length' fields. The value


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        of the length field is 8 when the Sub-opt Type equals 1 and 26
        when the sub-opt Type equals 2.
   The format of the DATA field when the sub-opt type indicates port
   mask (value = 1):

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         IPv4 address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Port Range Value          |       Port Range Mask         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 address
        Public IPv4 address

   Port Range Value and Port Range Mask
        Port Range Value indicates the value of the mask to be applied
        and Port Range Mask indicates the position of the bits which
        are used to build the mask.

   Section 4 describes how the client derives the allocated port range
   from the Port Range Value and Port Range Mask values.

   The format of the DATA field when the sub-opt type indicates random
   port delegation function (value = 2):

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         IPv4 address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        function               |         starting point        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    number of delegated ports  |         key K               ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...                                                           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...                                                           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...                                                           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ... key K                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   IP address
        Public IPv4 address

   Function
        A 16bit field whose value is associated with predefined
        encryption functions. This specification associates value 1
        with the predefined function described in section 5.


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   Starting Point
        A 16bit value used as an input to the specified function


   Number of delegated ports
        A 16bit value specifying the number of ports delegated to the
        client for use as source port values

   Key K
        A 128 bit key used as input to the predefined function for
        delegated port calculation

4. Port Mask sub-option usage

   The port mask sub-option is used to specify one or multiple range of
   ports pertaining to the given IP address.

   Concretely, this option is used to notify a remote DHCP client about
   the Port Mask to be applied when selecting a port value as a source
   port. The Port Mask option is used to infer a set of allowed port
   values. A Port Mask defines a set of ports that all have in common a
   subset of pre-positioned bits. This ports set is also called Port
   Range. Two port numbers are said to belong to the same Port Range if
   and only if, they have the same Port Mask.

   A Port Mask contains two fields: Port Range Value and Port Range
   Mask.

   - The 'Port Range Value' field indicates the value of the
   significant bits of the Port Mask. The .Port Range Value. is coded
   as follows:
           - The significant bits are those where "1" values are set in
   the Port Range Mask. These bits may take a value of "0" or "1 ".
           - All the other bits (non significant ones) are set to "0".

   - The 'Port Range Mask' field indicates the position of the
   significant bits identified by the bit(s) set to "1".

   The Port Range Value field indicates the value of the mask to be
   applied and the Port Range Mask field indicates the position of the
   bits which are used to build the mask. The "1" values in the Port
   Range Mask field indicate by their position the significant bits of
   the Port Range Value (the pattern of the Port Range Value).

   For example:
        - A Port Range Mask field equal to 1000000000000000 indicates
        that the first bit (the most significant one) is used as a
        pattern of the Port Range Value field;

        - A Port Range Mask field equal to 0000101000000000 indicates
        that the 5th and the 7th most significant bits are used as a
        pattern of the Port Range Value.

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   The pattern of the Port Range Value is all the fixed bits in the
   Port Range Value. All the ports the CPE is allowed to use as source
   ports must have their number in accordance with the pattern.

   The Port Range Value is coded as follows:
        - The pattern bits of the Port Range Value are those where "1"
        values are set in the Port Range Mask.  These bits may take a
        value of 0 or 1.
        - All the other bits are set to "0".

4.1 Illustration Examples

   In each of the three examples below allocation of 2048 ports is done
   differently. In all examples it is possible for 32 hosts to share
   the same public IPv4 address. The 4th example illustrates the
   ability of the procedure to enforce a balanced distribution of port
   numbers including the well-known-port values.

   a) the following Port Range Mask and Port Range Value are conveyed
   using DHCP to assign a Port Range (from 2048 to 4095) to a given
   device:
        - Port Range Value: 0000100000000000 (2048)
        - Port Range Mask: 1111100000000000 (63488)

   b) Unlike the previous example, this one illustrates the case where
   a non Continuous Port Range is assigned to a given customer's
   device. In this example, the Port Range Value defines 128 Continuous
   Port Ranges, each one with a length of 16 port values.  Note that
   the two first Port Ranges are both in the well-known ports span
   (i.e. 0-1023) but these two ranges are not adjacent.

   The following Port Range Mask and Port Range Value are conveyed in
   DHCP messages:
        - Port Range Value : 0000000001010000 (80)
        - Port Range Mask : 0000000111110000 (496)

   This means that the 128 following Continuous Port Ranges are
   assigned to the same device:
        - from 80 to 95
        - from 592 to 607
        - ...
        - from 65104 to 65119

   c) In this example, the Port Range Value defines two Continuous Port
   Ranges, each one being 1024 ports long:

        - Port Range Value : 0000000000000000 (0)
        - Port Range Mask : 1111010000000000 (62464)

   This means that the two following Continuous Port Ranges are
   assigned to the same device:

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        - from 0 to 1023, and
        - from 2048 to 3071

   d) In this example, 64 continuous Port Ranges are allocated to each
   CPE (among a set of 4 CPEs sharing the same IPv4 address).

   Among the 64 continuous Port Ranges to each CPE, there is always one
   within the span of the first 1024 well-known port values. Hereafter
   is given the Port Range Value and Port Range Mask assigned to 2 CPEs
   (CPE#0 and CPE#3, CPE#1 and CPE#2 being not represented here):

   1.  CPE#0

        - Port Range Value: 0000000000000000 (0)
        - Port Range Mask:  0000001100000000 (768)

   The CPE#0 has therefore the 64 following Continuous Port Ranges:
        - 1st range: 0-255
        - ...
        - 64th range: 64512-64767

   2.  CPE#3

        - Port Range Value: 0000001100000000 (768)
        - Port Range Mask:  0000001100000000 (768)

   The CPE#2 has therefore the 64 following Continuous Port Ranges:
        - 1st range: 768-1023
        - ...
        - 64th range: 65280-65535

5. Random Port delegation function

   Delegating random ports can be achieved by defining a function which
   takes as input a key 'k' and an integer 'x' within the range (1024,
   65535) and produces an output 'y' also within the range (1024,
   65535).

   The server uses a cryptographical mechanism (described below) to
   select the random ports for each host. Instead of assigning a range
   of ports using port mask to the client, the server sends the inputs
   of a predefined cryptographic mechanism: a key, an initial value,
   and the number of ports assigned to this host. The client can then
   calculate the full list of assigned ports itself.

   The cryptographical mechanism ensures that the entire 64k port range
   can be efficiently distributed to multiple hosts in a way that when
   hosts calculate the ports, the results will never overlap with ports
   other hosts have calculated (property of permutation), and ports in
   the reserved range (smaller than 1024) are not used. As the
   randomization is done crypthographically, an attacker seeing a host


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   using some port X cannot determine which other ports the host may be
   using (as the attacker does not know the key).

   Calculation of the random port list is done as follows:

   The cryptographic mechanism uses an encryption function y = E(K,x)
   that takes as input a key K (for example, 128 bits) and an integer x
   (the plaintext) in range (1024, 65535), and produces an output y
   (the ciphertext), also an integer in range (1024, 65535). This
   section describes one such encryption function, but others are also
   possible.

   The server will select the key K. When server wants to allocate e.g.
   2048 random ports, it selects a starting point 'a' (1024 <= a <=
   65536-2048) in a way that the range (a, a+2048) does not overlap
   with any other active client, and calculates the values E(K,a),
   E(K,a+1), E(K,a+2), ..., E(K,a+2046), E(K,a+2047). These are the
   port numbers allocated for this host. Instead of sending the port
   numbers individually, the server just sends the values 'K', ' a',
   and '2048'. The client will then repeat the same calculation.

   The server SHOULD use different K for each IPv4 address it allocates
   to make attacks as difficult as possible. This way, learning the K
   used in IPv4 address IP1 would not help in attacking IPv4 address
   IP2 that is allocated by the same server to different hosts.

   With typical encryption functions (such as AES and DES), the input
   (plaintext) and output (ciphertext) are blocks of some fixed size;
   for example, 128 bits for AES, and 64 bits for DES. For port
   randomization, we need an encryption function whose input and output
   is an integer in range (1024, 65535).

   One possible way to do this is to use the 'Generalized-Feistel
   Cipher' [CIPHERS] construction by Black and Rogaway, with AES as the
   underlying round function.

   This would look as follows (using pseudo-code):

        def E(k, x):
            y = Feistel16(k, x)
            if y >= 1024:
                  return y
            else:
                  return E(k, y)

   Note that although E(k,x) is recursive, it is guaranteed to
   terminate. The average number of iterations is just slightly over 1.

   Feistel16 is a 16-bit block cipher:

        def Feistel16(k, x):
            left = x & 0xff

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            right = x >> 8
            for round = 1 to 3:
                temp = (left + FeistelRound(k, round, right)) & 0xff
                left = right
                right = temp
            return (right << 8) | left

   The Feistel round function uses:

        def FeistelRound(k, round, x):
            msg[0] = round
            msg[1] = x >> 8
            msg[2] = x & 0xff
            msg[3...15] = 0
            return AES(k, msg)

   Performance: To generate list of 2048 port numbers, about 6000 calls
   to AES are required (i.e., encrypting 96 kilobytes). Thus, it will
   not be a problem for any device that can do, for example, HTTPS (web
   browsing over SSL/TLS).

   Other port generator functions may be predefined in Standards Track
   documents and allocated a not yet allocated 'function' value within
   the corresponding sub-option type field.

6. Option Usage

6.1 Client Behaviour

   A DHCP client which supports the option defined in this document
   MUST support both sub-option types.

   A DHCP client which supports the extensions defined in this
   document, SHOULD insert the option OPTION-IPv4-PRA with both sub-
   option types into DHCPDISCOVER message to explicitly let the server
   know that it supports port restricted IPv4 addresses.
      o In the port mask sub-option type, the client SHALL set the IPv4
        address and Mask Locator fields to all zeros. The client MAY
        indicate the number of desired ports in Port Range Value-field,
        or set that to all zeroes.
      o In the random port delegation sub-option type, the client SHALL
        set the IPv4 address field, key field and starting point field
        to all zeros. The client MAY indicate in function field which
        encryption function it prefers, and in the number of delegated
        ports field the number of ports the client would desire.

   When a client, which supports the option defined in this document,
   receives a DHCPOFFER with the 'yiaddr' (client IP address) field set
   to 0.0.0.0, it SHOULD check for the presence of OPTION-IPv4-PRA
   option. If such an option is present, the client MAY send a
   DHCPREQUEST message and insert the option OPTION-IPv4-PRA with the
   corresponding sub-options received in the OPTION-IPv4-PRA option of

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   the previous DHCPOFFER. The client MUST NOT include a 'Requested IP
   Address' DHCP option (code 50) into this DHCPREQUEST.

   The client MUST NOT insert the IP address received in OPTION-IPv4-
   PRA into the 'Requested IP Address' DHCP option (code 50). When the
   client receives a DHCPACK message with an OPTION-IPv4-PRA option, it
   MAY start using the specified IP address in conjunction with the
   source ports specified by the mechanism chosen by DHCP server. The
   client MUST NOT use the IP address with different source port
   numbers, as that may result in a conflict, since the same IP address
   with a different source port group may be assigned to a different
   client. Furthermore, the client MUST notice the situation where an
   outgoing IP packet has the same IP address as destination address
   than the client itself has, but the port number is not belonging to
   the allocated set. In this case the client MUST detect that the
   packet is not destined for itself, and it MUST send it forward.

   In case the initial port set received by the client from the server
   is exhausted and the client needs additional ports, it MAY request
   so by sending a new DHCPDISCOVER message.

   In some deployment scenarios the DHCP client may also act as a DHCP
   server for a network behind it, in which case the host may further
   split the allocated set for other hosts.

6.2 Server Behaviour

   When a server, which supports the option defined in this document,
   receives a DHCPDISCOVER message, it SHOULD check the presence of the
   OPTION-IPv4-PRA option.

   If OPTION-IPv4-PRA is not present in DHCPDISCOVER, the server SHOULD
   allocate full unrestricted public or private [RFC1918] IPv4 address
   to the client, if available, by generating a DHCPOFFER as described
   in [RFC2131].

   The server SHOULD offer the port restricted IPv4 address when the
   server has support the extensions specified in this document and
   when:
      o DHCP client has included OPTION-IPv4-PRA option and server.s
        policy indicates saving unrestricted IPv4 addresses for clients
        that do not support the extensions defined in this document
      o server receives a DHCPDISCOVER message and server can only
        offer port restricted IP address to the client
      o server receives a DHCPDISCOVER message from a client without
        the OPTION-IPv4-PRA, but knows by means outside the scope of
        this document that the client supports the usage of port-
        restricted IPv4 addresses (or it is only entitled to be
        provisioned with such addresses)

   When server chooses to offer port restricted IPv4 address for
   clients with OPTION-IPv4-PRA, it MUST:

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      o set the 'yiaddr' (client IP address) field of the DHCPOFFER
        message to 0.0.0.0
      o choose the port allocation mechanisms, if it is not statically
        configured
      o select a port restricted IPv4 address to be allocated for the
        client
      o generate parameters required for the chosen port allocation
        mechanism in case of using sub-option type '1', or including
        the key K for random port generation in case of using the sub-
        option type '2'

   When the server receives a DHCPREQUEST message from the client with
   an OPTION-IPv4-PRA option field containing the IP address and port
   allocation mechanism parameters it has previously offered to the
   client, the server MUST send a DHCPACK, where the 'yiaddr' (client
   IP address) field is set to 0.0.0.0 and the OPTION-IPv4-PRA option
   including the IPv4 address and parameters required for the used
   allocation mechanism.

   When the server receives a DHCPREQUEST message from the client with
   an OPTION-IPv4-PRA option field containing an IPv4 address and port
   set it has previously not offered to the client, the server MUST
   send a DHCPNAK to the client.

   When the server detects that a client (by eg having a specific
   hardware address), has already been allocated with a port restricted
   IPv4 address, sent another DHCPDISCOVER, it MAY, based on local
   policy, offer the client with additional port restricted IPv4
   address.

   If the server is deployed in a cascaded DHCP server scenario, the
   host MAY both act as a DHCP client for another server and DHCP
   server for other DHCP clients.

   A server SHOULD ensure the client is residing on an access link
   where usage of port-restricted addresses is not causing problems,
   before allocating it a port restricted IPv4 address.

7. Applicability

   The multiplexing of IP flows in gateway is based on the port numbers
   used by transport layer protocols such as TCP, UDP, SCTP, and DCCP.
   However, the protocols not containing port numbers need special
   handling in order to be multiplexed correctly.





7.1 ICMP



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   Those ICMP messages that embed the IP packet that triggered sending
   of ICMP message, such as ICMP error, can be multiplexed based on the
   port number present in the embedded original packet.

   ICMP messages not containing embedded packets, like ICMP echo, are
   TBD.

7.2 6to4

   A host utilizing 6to4 [RFC3056] with port restricted IPv4 addresses
   MUST pick the 16-bit .SLA ID. value for the 6to4 prefix(es)
   construction from the pool of allocated port values. The
   multiplexing gateway MUST then multiplex 6to4 traffic based on .SLA
   ID. value as it would multiplex plain IPv4 traffic based on port
   values. I.e. for incoming packets the gateway shall look at the
   destination IPv4 address and the .SLA ID.-field from tunneled IPv6
   packet.s destination IPv6 address, and then select the right route
   as it would have picked the port number from a transport layer
   header.

7.3 Protocols not supported by multiplexing gateway

   The case where port range router is not able to multiplex a protocol
   is similar to a case where middle box, such as firewall or NAT,
   blocks traffic it is not able or willing to pass trough. The
   application is recommended to fallback to UDP encapsulation often
   used for NAT traversal, for which gateway is able to perform
   multiplexing.

8. IANA considerations

   This document defines new DHCPv4 option as described in section 3:
   Port Restricted IP Address Option for DHCPv4 (OPTION-IPv4-PRA) TBD.

9. Security considerations

   The solution is generally vulnerable to DoS when used in shared
   medium or when access network authentication is not a prerequisite
   to IP address assignment. The solution SHOULD only be used on point-
   to-point links, tunnels, and/or in environments where authentication
   at link layer is performed before IP address assignment, and not
   shared medium.

   The cryptographically random port delegation mechanism is vulnerable
   for blind attacks initiated by hosts located in the same
   administrative domain, served by the same DHCP server, and that are
   sharing the same public IPv4 address, and therefore have knowledge
   of the cryptographic key used for that particular public IPv4
   address.

10. Normative References


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   [RFC2119]    Bradner, S., .Key words for use in RFCs to Indicate
                Requirement Levels., March 1997

   [RFC2131]    Droms, R., "Dynamic Host Configuration Protocol",
                RFC2131, March 1997

   [RFC3056]    Carpenter, B., Moore, K., .Connection of IPv6 Domains
                via IPv4 Clouds., February 2001



11. Informative References

   [ARKK2008]   Arkko, J., Townsley, M., "IPv4 Run-Out and IPv4-IPv6
                Co-Existence Scenarios", September 2008, draft-arkko-
                townsley-coexistence-00

   [WING2008]   Wing, D., Ward, D., Durand, A., "A Comparison of
                Proposals to Replace NAT-PT", September 2008, draft-
                wing-nat-pt-replacement-comparison

   [RFC1918]    Rekhter, Y., Moskowitz, B., Karrenberg, D., J. de
                Groot, G., Lear, E., "Address Allocation for Private
                Internets", RFC1918, February 1996

   [MAEN2008]   Maennel, O., Bush, R., Cittadini, L., Bellovin, S., "A
                Better Approach than Carrier-Grade-NAT", 2008,
                Technical Report CUCS-041-08

   [RANDOMPORT] Larsen, M., Gont, F., .Port Randomization., August
                2008, draft-ietf-tsvwg-port-randomization-02

   [CIPHERS]    John Black and Phillip Rogaway: .Ciphers with Arbitrary
                Finite Domains., Topics in Cryptology - CT-RSA 2002,
                Lecture Notes in Computer Science vol. 2271, 2002

   [DSLITE]     A. Durand et al .Dual-stack lite broadband deployments
                post IPv4 exhaustion., November 2008, draft-durand-
                softwire-dual-stack-lite

   [BOUCADAIR]  Boucadair, M, Ed., Grimault, J-L., Levis, P.,
                Villefranque, A., .DHCP Options for Conveying Port Mask
                and Port Range Router IP Address., October 2008, draft-
                boucadair-dhc-port-range

   [BOUCADAIRARCH]  Boucadair, M., Ed., Levis, P., Bajko, G.,
                Savolainen, T., .IPv4 Connectivity Access in the
                Context of IPv4 Address Exhaustion., January 2009,
                draft-boucadair-port-range




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   [APLUSP]     Maennel, O., Bush, R., Cittadini, L., Bellovin, S., "The
                A+P Approach to the IPv4 Address Shortage", January
                2009, draft-ymbk-aplusp

12. Contributors

   The port range allocation using Port Range Value / Port Range Mask
   comes from [BOUCADAIR], authored by Mohamed Boucadair, Jean Luc
   Grimault and Pierre Lewis.

   The encryption function from section 5 was provided by Pasi Eronen.

   The text on 6to4 handling was proposed by Dave Thaler.

   The rest of the document was written and edited by Gabor Bajko and
   Teemu Savolainen.

   The authors would also like to thank Lars Eggert, Olaf Maenel, Randy
   Bush, Alain Durand, Jean-Luc Grimault, Alain Villefranque for their
   valuable comments.


13. Authors' Addresses

   Gabor Bajko
   gabor(dot)bajko(at)nokia(dot)com


   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   FI-33720 TAMPERE
   Finland

   Email: teemu.Savolainen@nokia.com


   Mohamed Boucadair
   France Telecom
   42 rue des Coutures
   BP 6243
   Caen Cedex 4  14066
   France

   Email: mohamed.boucadair@orange-ftgroup.com


   Pierre Levis
   France Telecom
   42 rue des Coutures
   BP 6243
   Caen Cedex 4  14066

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   France

   Email: pierre.levis@orange-ftgroup.com


















































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