One document matched: draft-ietf-softwire-map-08.xml
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<rfc category="std" docName="draft-ietf-softwire-map-08" ipr="trust200902">
<front>
<title abbrev="MAP">Mapping of Address and Port with Encapsulation
(MAP)</title>
<author fullname="Ole Troan" initials="O" surname="Troan" role="editor">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>Philip Pedersens vei 1</street>
<city>Lysaker</city>
<code>1366</code>
<country>Norway</country>
</postal>
<email>ot@cisco.com</email>
</address>
</author>
<author fullname="Wojciech Dec" initials="W" surname="Dec">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>Haarlerbergpark Haarlerbergweg 13-19</street>
<city>Amsterdam, NOORD-HOLLAND</city>
<code>1101 CH</code>
<country>Netherlands</country>
</postal>
<phone></phone>
<email>wdec@cisco.com</email>
</address>
</author>
<author fullname="Xing Li" initials="X" surname="Li">
<organization abbrev="">CERNET Center/Tsinghua University</organization>
<address>
<postal>
<street>Room 225, Main Building, Tsinghua University</street>
<city>Beijing 100084</city>
<country>CN</country>
</postal>
<email>xing@cernet.edu.cn</email>
</address>
</author>
<author fullname="Congxiao Bao" initials="C" surname="Bao">
<organization abbrev="">CERNET Center/Tsinghua University</organization>
<address>
<postal>
<street>Room 225, Main Building, Tsinghua University</street>
<city>Beijing 100084</city>
<country>CN</country>
</postal>
<email>congxiao@cernet.edu.cn</email>
</address>
</author>
<author fullname="Satoru Matsushima" initials="S" surname="Matsushima">
<organization abbrev="">SoftBank Telecom</organization>
<address>
<postal>
<street>1-9-1 Higashi-Shinbashi, Munato-ku</street>
<city>Tokyo</city>
<country>Japan</country>
</postal>
<email>satoru.matsushima@g.softbank.co.jp</email>
</address>
</author>
<author fullname="Tetsuya Murakami" initials="T" surname="Murakami">
<organization abbrev="">IP Infusion</organization>
<address>
<postal>
<street>1188 East Arques Avenue</street>
<city>Sunnyvale</city>
<country>USA</country>
</postal>
<email>tetsuya@ipinfusion.com</email>
</address>
</author>
<author fullname="Tom Taylor" initials="T" surname="Taylor" role="editor">
<organization abbrev="">Huawei Technologies</organization>
<address>
<postal>
<street></street>
<city>Ottawa</city>
<country>Canada</country>
</postal>
<email>tom.taylor.stds@gmail.com</email>
</address>
</author>
<date year="2013" />
<area>Internet</area>
<workgroup>Network Working Group</workgroup>
<!-- SECTION 0: Abstract -->
<abstract>
<t>This document describes a mechanism for transporting IPv4 packets
across an IPv6 network using IP encapsulation, and a generic mechanism
for mapping between IPv6 addresses and IPv4 addresses and transport
layer ports.</t>
</abstract>
</front>
<middle>
<!-- SECTION 1: Introduction -->
<section title="Introduction">
<t>Mapping of IPv4 addresses in IPv6 addresses has been described in
numerous mechanisms dating back to 1996 <xref target="RFC1933"></xref>.
The Automatic tunneling mechanism described in RFC1933, assigned a
globally unique IPv6 address to a host by combining the host's IPv4
address with a well-known IPv6 prefix. Given an IPv6 packet with a
destination address with an embedded IPv4 address, a node could
automatically tunnel this packet by extracting the IPv4 tunnel end-point
address from the IPv6 destination address.</t>
<t>There are numerous variations of this idea, described in 6over4 <xref
target="RFC2529"></xref>, 6to4 <xref target="RFC3056"></xref>, ISATAP
<xref target="RFC5214"></xref>, and 6rd <xref
target="RFC5969"></xref>.</t>
<t>The commonalities of all these IPv6 over IPv4 mechanisms are: <list
style="symbols">
<t>Automatically provisions an IPv6 address for a host or an IPv6
prefix for a site</t>
<t>Algorithmic or implicit address resolution of tunnel end point
addresses. Given an IPv6 destination address, an IPv4 tunnel
endpoint address can be calculated.</t>
<t>Embedding of an IPv4 address or part thereof into an IPv6
address.</t>
</list></t>
<t>In phases of IPv4 to IPv6 migration, IPv6 only networks will be
common, while there will still be a need for residual IPv4 deployment.
This document describes a generic mapping of IPv4 to IPv6, and a
mechanism for encapsulating IPv4 over IPv6.</t>
<t>Just as the IPv6 over IPv4 mechanisms referred to above, the residual
IPv4 over IPv6 mechanism must be capable of:</t>
<t><list style="symbols">
<t>Provisioning an IPv4 prefix, an IPv4 address or a shared IPv4
address.</t>
<t>Algorithmically map between an IPv4 prefix, IPv4 address or a
shared IPv4 address and an IPv6 address.</t>
</list></t>
<t>The mapping scheme described here supports encapsulation of
IPv4 packets in IPv6 in both mesh and hub and spoke topologies,
including address mappings with full independence between IPv6
and IPv4 addresses.</t>
<t>This document describes delivery of IPv4 unicast service across an
IPv6 infrastructure. IPv4 multicast is not considered further in this
document.</t>
<t>The A+P (Address and Port) architecture of sharing an IPv4 address by
distributing the port space is described in <xref
target="RFC6346"></xref>. Specifically section 4 of <xref
target="RFC6346"></xref> covers stateless mapping. The corresponding
stateful solution DS-lite is described in <xref
target="RFC6333"></xref>. The motivation for the work is described in
<xref target="I-D.ietf-softwire-stateless-4v6-motivation"></xref>.</t>
<t>A companion document defines a DHCPv6 option for provisioning of MAP
<xref target="I-D.ietf-softwire-map-dhcp"></xref>. Other means of
provisioning is possible. Deployment considerations are described in
<xref target="I-D.ietf-softwire-map-deployment"/>.</t>
<t>MAP relies on IPv6 and is designed to deliver production-quality
dual-stack service while allowing IPv4 to be phased out within the SP
network. The phasing out of IPv4 within the SP network is independent of
whether the end user disables IPv4 service or not. Further,
"Greenfield"; IPv6-only networks may use MAP in order to deliver IPv4 to
sites via the IPv6 network.</t>
</section>
<!-- SECTION 2: REQUIREMENTS LANGUAGE -->
<section anchor="conventions" title="Conventions">
<t>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 <xref
target="RFC2119"></xref>.</t>
</section>
<!-- conventions -->
<section title="Terminology">
<t><list hangIndent="24" style="hanging">
<t hangText="MAP domain:">One or more MAP CEs and BRs connected to
the same virtual link. A service provider may deploy a single MAP
domain, or may utilize multiple MAP domains.</t>
<t hangText="MAP Rule">A set of parameters describing the mapping
between an IPv4 prefix, IPv4 address or shared IPv4 address and an
IPv6 prefix or address. Each domain uses a different mapping rule
set.</t>
<t hangText="MAP node">A device that implements MAP.</t>
<t hangText="MAP Border Relay (BR):">A MAP enabled router managed by
the service provider at the edge of a MAP domain. A Border Relay
router has at least an IPv6-enabled interface and an IPv4 interface
connected to the native IPv4 network. A MAP BR may also be referred
to simply as a "BR" within the context of MAP.</t>
<t hangText="MAP Customer Edge (CE):">A device functioning as a
Customer Edge router in a MAP deployment. A typical MAP CE adopting
MAP rules will serve a residential site with one WAN side interface,
and one or more LAN side interfaces. A MAP CE may also be referred
to simply as a "CE" within the context of MAP.</t>
<t hangText="Port-set:">The separate part of the transport layer
port space; denoted as a port-set.</t>
<t hangText="Port-set ID (PSID):">Algorithmically identifies a set
of ports exclusively assigned to a CE.</t>
<t hangText="Shared IPv4 address:">An IPv4 address that is shared
among multiple CEs. Only ports that belong to the assigned port-set
can be used for communication. Also known as a Port-Restricted IPv4
address.</t>
<t hangText="End-user IPv6 prefix:">The IPv6 prefix assigned
to an End-user CE by other means than MAP
itself. E.g. Provisioned using DHCPv6 PD <xref
target="RFC3633"></xref>, assigned via SLAAC <xref
target="RFC4862"/>, or configured manually. It is unique for
each CE.</t>
<t hangText="MAP IPv6 address:">The IPv6 address used to reach the
MAP function of a CE from other CEs and from BRs.</t>
<t hangText="Rule IPv6 prefix:">An IPv6 prefix assigned by a Service
Provider for a mapping rule.</t>
<t hangText="Rule IPv4 prefix:">An IPv4 prefix assigned by a Service
Provider for a mapping rule.</t>
<t hangText="Embedded Address (EA) bits:">The IPv4 EA-bits in the
IPv6 address identify an IPv4 prefix/address (or part thereof) or a
shared IPv4 address (or part thereof) and a port-set identifier.</t>
</list></t>
</section>
<!-- SECTION 3: DESCRIPTION -->
<section title="Architecture">
<t>In accordance with the requirements stated above, the MAP
mechanism can operate with shared IPv4 addresses, full IPv4
addresses or IPv4 prefixes. Operation with shared IPv4 addresses
is described here, and the differences for full IPv4 addresses
and prefixes are described below.</t>
<t>The MAP mechanism uses existing standard building blocks.
The existing NAPT on the CE is used with additional support for
restricting transport protocol ports, ICMP identifiers and
fragment identifiers to the configured port set. For packets
outbound from the private IPv4 network, the CE NAPT MUST
translate transport identifiers (e.g. TCP and UDP port numbers)
so that they fall within the CE's assigned port-range.</t>
<t>The NAPT MUST in turn be connected to a MAP aware forwarding
function, that does encapsulation/ decapsulation of IPv4 packets
in IPv6. MAP supports the encapsulation mode specified in <xref
target="RFC2473"/>. In addition MAP specifies an algorithm to
do "address resolution" from an IPv4 address and port to an IPv6
address. This algorithmic mapping is specified in <xref
target="mapping_algorithm"/>.</t>
<t>The MAP architecture described here, restricts the use of the
shared IPv4 address to only be used as the global address
(outside) of the NAPT <xref target="RFC2663"/> running on the
CE. A shared IPv4 address MUST NOT be used to identify an
interface. While it is theoretically possible to make host
stacks and applications port-aware, that is considered too
drastic a change to the IP model <xref target="RFC6250"/>.</t>
<t>For full IPv4 addresses and IPv4 prefixes, the architecture
just described applies with two differences. First, a full IPv4
address or IPv4 prefix can be used as it is today, e.g., for
identifying an interface or as a DHCP pool,
respectively. Secondly, the NAPT is not required to restrict the
ports used on outgoing packets.</t>
<t>This architecture is illustrated in <xref target="topology"/>.</t>
<figure align="center" anchor="topology" title="Network Topology">
<preamble></preamble>
<artwork align="center"><![CDATA[
User N
Private IPv4
| Network
|
O--+---------------O
| | MAP CE |
| +-----+--------+ |
| NAPT44| MAP | |
| +-----+ | | |\ ,-------. .------.
| +--------+ | \ ,-' `-. ,-' `-.
O------------------O / \ O---------O / Public \
/ IPv6 only \ | MAP | / IPv4 \
( Network --+ Border +- Network )
\ (MAP Domain) / | Relay | \ /
O------------------O \ / O---------O \ /
| MAP CE | /". ,-' `-. ,-'
| +-----+--------+ | / `----+--' ------'
| NAPT44| MAP | |/
| +-----+ | |
| | +--------+ |
O---.--------------O
|
User M
Private IPv4
Network
]]></artwork>
</figure>
<t>The MAP BR is responsible for connecting external IPv4 networks to
the IPv4 nodes in one or more MAP domains.</t>
</section>
<section anchor="mapping_algorithm" title="Mapping Algorithm">
<t>A MAP node is provisioned with one or more mapping rules.</t>
<t>Mapping rules are used differently depending on their function. Every
MAP node must be provisioned with a Basic mapping rule. This is used by
the node to configure its IPv4 address, IPv4 prefix or shared IPv4
address. This same basic rule can also be used for forwarding, where an
IPv4 destination address and optionally a destination port is mapped
into an IPv6 address. Additional mapping rules are specified to allow
for multiple different IPv4 sub-nets to exist within the domain and
optimize forwarding between them.</t>
<t>Traffic outside of the domain (i.e. When the destination IPv4 address
does not match (using longest matching prefix) any Rule IPv4 prefix in
the Rules database) is forwarded to the BR.</t>
<t>There are two types of mapping rules: <list style="numbers">
<t>Basic Mapping Rule (BMR) - mandatory. A CE can be provisioned
with multiple End-user IPv6 prefixes. There can only be one
Basic Mapping Rule per End-user IPv6 prefix. However all CE's
having End-user IPv6 prefixes within (aggregated by) the same
Rule IPv6 prefix may share the same Basic Mapping Rule. In
combination with the End-user IPv6 prefix, the Basic Mapping
Rule is used to derive the IPv4 prefix, address, or shared
address and the PSID assigned to the CE.</t>
<t>Forwarding Mapping Rule (FMR) - optional, used for
forwarding. The Basic Mapping Rule is also a Forwarding Mapping
Rule. Each Forwarding Mapping Rule will result in an entry in
the Rules table for the Rule IPv4 prefix. Given a destination
IPv4 address and port within the MAP domain, a MAP node can use
the matching FMR to derive the End-user IPv6 address of the
interface through which that IPv4 destination address and port
combination can be reached.</t>
</list></t>
<t>Both mapping rules share the same parameters:<list style="symbols">
<t>Rule IPv6 prefix (including prefix length)</t>
<t>Rule IPv4 prefix (including prefix length)</t>
<t>Rule EA-bits length (in bits)</t>
<!-- <t>Rule Port Parameters (optional)</t>-->
</list></t>
<t>A MAP node finds its Basic Mapping Rule by doing a longest match
between the End-user IPv6 prefix and the Rule IPv6 prefix in the Mapping
Rules table. The rule is then used for IPv4 prefix, address or shared
address assignment.</t>
<t>A MAP IPv6 address is formed from the BMR Rule IPv6 prefix. This
address MUST be assigned to an interface of the MAP node and is used to
terminate all MAP traffic being sent or received to the node.</t>
<t>Port-aware IPv4 entries in the Rules table are installed for
all the Forwarding Mapping Rules and an default route to the MAP
BR (see section <xref target="outside-domain"/>.</t>
<t>Forwarding rules are used to allow direct communication
between MAP CEs, known as mesh mode. In hub and spoke mode,
there are no forwarding rules, all traffic MUST be forwarded
directly to the BR.</t>
<section title="Port mapping algorithm" anchor="portmap">
<t>The port mapping algorithm is used in domains whose rules allow
IPv4 address sharing.</t>
<t>The simplest way to represent a port range is using a
notation similar to CIDR <xref target="RFC4632"/>. For example
the first 256 ports are represented as port prefix 0.0/8. The
last 256 ports as 255.0/8. In hexadecimal, 0x0000/8 (PSID = 0)
and 0xFF00/8 (PSID = 0xFF). Using this technique, but wishing
to avoid allocating the system ports <xref
target="I-D.ietf-tsvwg-iana-ports"/> to the user, one would
have to exclude the use of one or more PSIDs (e.g., PSIDs 0 to
3 in the example just given).
</t>
<t>When the PSID is embedded in the End-user IPv6 prefix, then
to minimise dependencies between the End-user IPv6 prefix and
the assigned port set, it is desirable to minimize the
restrictions of possible PSID values. This is achieved by
using an infix representation of the port value. Using such a
representation, the well-known ports are excluded by
restrictions on the value of the high-order bitfield (A)
rather than the PSID.</t>
<t>The infix algorithm allocates ports to a given CE as a series of
contiguous ranges spaced at regular intervals throughout the
complete range of possible port set values.</t>
<t><figure align="left" anchor="psid-fig"
title="Structure of a port-restricted port field">
<preamble></preamble>
<artwork align="left"><![CDATA[
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-----------+-----------+-------+
Ports in | A | PSID | M |
the CE port set | > 0 | | |
+-----------+-----------+-------+
| a bits | k bits |m bits |
]]></artwork>
</figure></t>
<t><list style="hanging">
<t hangText="a-bits">The number of offset bits. The default
Offset bits (a) are 6, this excludes the system ports
(0-1023).</t>
<t hangText="A">Selects the range of the port number. For a
> 0, A MUST be larger than 0. This ensures that the
algorithm excludes the system ports. For this value of a,
the system ports, but no others, are excluded by requiring
that A be greater than 0. For smaller values of a, A still
has to be greater than 0, but this excludes ports above
1023. For larger values of a, the minimum value of A has to
be higher to exclude all the system ports. The interval
between successive contiguous ranges assigned to the same
user is 2^a.</t>
<t hangText="PSID">The Port Set Identifier. Different
Port-Set Identifiers (PSID) guarantee non-overlapping
port-sets.</t>
<t hangText="k-bits">The length in bits of the PSID
field. The sharing ratio is 2^k. The number of ports
assigned to the user is 2^(16-k) - 2^m (excluded ports)</t>
<t hangText="M">Selects the specific port within the
particular range specified by the concatenation of A and the
PSID.</t>
<t hangText="m bits">The size contiguous ports. The
number of contiguous ports is given by 2^m.</t> </list></t>
</section>
<section title="Basic mapping rule (BMR)">
<t>The Basic Mapping Rule is mandatory, used by the CE to
provision itself with an IPv4 prefix, IPv4 address or shared
IPv4 address. Recall from <xref target="mapping_algorithm"/>
that the BMR consists of the following parameters:
<list style="symbols">
<t>Rule IPv6 prefix (including prefix length)</t>
<t>Rule IPv4 prefix (including prefix length)</t>
<t>Rule EA-bits length (in bits)</t>
</list>
</t>
<t><xref target="addressallocation-fig"/> shows the structure
of the complete MAP IPv6 address as specified in this document.
</t>
<t><figure align="center" anchor="addressallocation-fig"
title="MAP IPv6 Address Format">
<preamble></preamble>
<artwork align="center"><![CDATA[
| n bits | o bits | s bits | 128-n-o-s bits |
+--------------------+-----------+---------+-----------------------+
| Rule IPv6 prefix | EA bits |subnet ID| interface ID |
+--------------------+-----------+---------+-----------------------+
|<--- End-user IPv6 prefix --->|
]]></artwork>
</figure></t>
<t>The Rule IPv6 prefix is the part of the End-user IPv6 prefix that
is common among all CEs using the same Basic Mapping Rule within the
MAP domain. The EA bits encode the CE specific IPv4 address and port
information. The EA bits, which are unique for a given Rule IPv6
prefix, can contain a full or part of an IPv4 address and, in the
shared IPv4 address case, a Port-Set Identifier (PSID). An EA-bit
length of 0 signifies that all relevant MAP IPv4 addressing
information is passed directly in the BMR, and not derived from
the End-user IPv6 prefix.</t>
<t>The MAP IPv6 address is created by concatenating the
End-user IPv6 prefix with the MAP subnet identifier (if the End-user
IPv6 prefix is shorter than 64 bits) and the interface identifier as
specified in <xref target="interface-id"></xref>.</t>
<t>The MAP subnet identifier is defined to be the first subnet (all
bits set to zero).</t>
<t>Define:
<list style="empty">
<t>r = length of the IPv4 prefix given by the BMR;</t>
<t>o = length of the EA bit field as given by the BMR;</t>
<t>p = length of the IPv4 suffix contained in the EA bit field.</t>
</list></t>
<t>The length r MAY be zero, in which case the complete IPv4
address or prefix is encoded in the EA bits. If only a part of the
IPv4 address/prefix is encoded in the EA bits, the Rule IPv4 prefix is
provisioned to the CE by other means (e.g. a DHCPv6 option). To create
a complete IPv4 address (or prefix), the IPv4 address suffix (p) from
the EA bits, is concatenated with the Rule IPv4 prefix (r bits).</t>
<t>The offset of the EA bits field in the IPv6 address is
equal to the BMR Rule IPv6 prefix length. The length of the EA
bits field (o) is given by the BMR Rule EA-bits length, and
can be between 0 and 48. A length of 48 means that the
complete IPv4 address and port is embedded in the End-user
IPv6 prefix (a single port is assigned). A length of 0 means
that no part of the IPv4 address or port is embedded in the
address. The sum of the Rule IPv6 Prefix length and the Rule
EA-bits length MUST be less or equal than the End-user IPv6
prefix length.</t>
<t>If o + r < 32 (length of the IPv4 address in bits), then an IPv4
prefix is assigned. This case is shown in <xref target="addressallocation4-fig"/>.</t>
<t><figure align="center" anchor="addressallocation4-fig"
title="IPv4 prefix">
<preamble>IPv4 prefix:</preamble>
<artwork align="center"><![CDATA[
| r bits | p bits |
+-------------+---------------------+
| Rule IPv4 | IPv4 Address suffix |
+-------------+---------------------+
| < 32 bits |
]]></artwork>
</figure></t>
<t>If o + r is equal to 32, then a full IPv4 address is to be
assigned. The address is created by concatenating the Rule IPv4 prefix
and the EA-bits. This case is shown in <xref target="addressallocation3-fig"/>.</t>
<t><figure align="center" anchor="addressallocation3-fig"
title="Complete IPv4 address">
<preamble>Complete IPv4 address:</preamble>
<artwork align="center"><![CDATA[
| r bits | p bits |
+-------------+---------------------+
| Rule IPv4 | IPv4 Address suffix |
+-------------+---------------------+
| 32 bits |
]]></artwork>
</figure></t>
<t>If o + r is > 32, then a shared IPv4 address is to be assigned.
The number of IPv4 address suffix bits (p) in the EA bits is given by
32 - r bits. The PSID bits are used to create a port-set. The length
of the PSID bit field within EA bits is: q = o - p.</t>
<t><figure align="center" anchor="addressallocation2-fig"
title="Shared IPv4 address">
<preamble>Shared IPv4 address:</preamble>
<artwork align="center"><![CDATA[
| r bits | p bits | | q bits |
+-------------+---------------------+ +------------+
| Rule IPv4 | IPv4 Address suffix | |Port-Set ID |
+-------------+---------------------+ +------------+
| 32 bits |
]]></artwork>
</figure></t>
<t>The length of r MAY be 32, with no part of the IPv4 address
embedded in the EA bits. This results in a mapping with no
dependence between the IPv4 address and the IPv6 address. In
addition the length of o MAY be zero (no EA bits embedded in
the End-User IPv6 prefix), meaning that also the PSID is
provisioned using e.g. the DHCP option.</t>
<t>See <xref target="appendixA"/> for an example of the Basic
Mapping Rule.</t>
</section>
<section title="Forwarding mapping rule (FMR)">
<t>The Forwarding Mapping Rule is optional, and used in mesh
mode to enable direct CE to CE connectivity.</t>
<t>On adding an FMR rule, an IPv4 route is installed in the Rules
table for the Rule IPv4 prefix.</t>
<t>On forwarding an IPv4 packet, a best matching prefix look up is
done in the Rules table and the correct FMR is chosen.</t>
<t><figure align="left" anchor="aplusptoipv6-fig"
title="Deriving of MAP IPv6 address">
<preamble></preamble>
<artwork align="left"><![CDATA[
| 32 bits | | 16 bits |
+--------------------------+ +-------------------+
| IPv4 destination address | | IPv4 dest port |
+--------------------------+ +-------------------+
: : ___/ :
| p bits | / q bits :
+----------+ +------------+
|IPv4 sufx| |Port-Set ID |
+----------+ +------------+
\ / ____/ ________/
\ : __/ _____/
\ : / /
| n bits | o bits | s bits | 128-n-o-s bits |
+--------------------+-----------+---------+------------+----------+
| Rule IPv6 prefix | EA bits |subnet ID| interface ID |
+--------------------+-----------+---------+-----------------------+
|<--- End-user IPv6 prefix --->|
]]></artwork>
</figure></t>
<t>See <xref target="appendixA"/> for an example of the
Forwarding Mapping Rule.</t>
</section>
<section anchor="outside-domain" title="Destinations outside the MAP domain">
<t>IPv4 traffic between MAP nodes that are all within one MAP
domain is encapsulated in IPv6, with the senders MAP IPv6
address as the IPv6 source address and the receiving MAP
node's MAP IPv6 address as the IPv6 destination address. To
reach IPv4 destinations outside of the MAP domain, traffic is
also encapsulated in IPv6, but the destination IPv6 address is
set to the configured IPv6 address of the MAP BR.</t>
<t>On the CE, the path to the BR can be represented as a point
to point IPv4 over IPv6 tunnel <xref target="RFC2473"/> with
the source address of the tunnel being the CE's MAP IPv6
address and the BR IPv6 address as the remote tunnel
address. When MAP is enabled, a typical CE router will install
a default route to the BR.</t>
<t>The BR forwards traffic received from the outside to CE's
using the normal MAP forwarding rules.</t>
</section>
</section>
<section anchor="interface-id" title="The IPv6 Interface Identifier">
<t>The Interface identifier format of a MAP node is described
below.</t>
<t><figure align="left" anchor="interfaceid2-fig" title="">
<preamble></preamble>
<artwork align="left"><![CDATA[
| 128-n-o-s bits |
| 16 bits| 32 bits | 16 bits|
+--------+----------------+--------+
| 0 | IPv4 address | PSID |
+--------+----+-----------+--------+
]]></artwork>
</figure></t>
<t>In the case of an IPv4 prefix, the IPv4 address field is right-padded
with zeroes up to 32 bits. The PSID field is left-padded to create a 16
bit field. For an IPv4 prefix or a complete IPv4 address, the PSID field
is zero.</t>
<t>If the End-user IPv6 prefix length is larger than 64, the
most significant parts of the interface identifier is
overwritten by the prefix.</t>
</section>
<section title="MAP Configuration">
<t>For a given MAP domain, the BR and CE MUST be configured with the
following MAP elements. The configured values for these elements are
identical for all CEs and BRs within a given MAP domain.</t>
<t><list style="symbols">
<t>The Basic Mapping Rule and optionally the Forwarding Mapping
Rules, including the Rule IPv6 prefix, Rule IPv4 prefix, and Length
of EA bits</t>
<t>Hub and spoke mode or Mesh mode. (If all traffic should be sent
to the BR, or if direct CE to CE traffic should be supported).</t>
</list></t>
<t>In addition the MAP CE MUST be configured with the IPv6
address(es) of the MAP BR (<xref
target="outside-domain"/>).</t>
<section title="MAP CE">
<t>The MAP elements are set to values that are the same across
all CEs within a MAP domain. The values may be configured in a
variety of manners, including provisioning methods such as the
Broadband Forum's "TR-69" Residential Gateway management
interface, an XML-based object retrieved after IPv6
connectivity is established, or manual configuration by an
administrator. This document focuses on how to configure the
necessary parameters via IPv6 DHCP. A CE that allows IPv6
configuration by DHCP SHOULD implement this option. Other
configuration and management methods may use the format
described by this option for consistency and convenience of
implementation on CEs that support multiple configuration
methods.</t>
<t>The only remaining provisioning information the CE requires
in order to calculate the MAP IPv4 address and enable IPv4
connectivity is the IPv6 prefix for the CE. The End-user IPv6
prefix is configured as part of obtaining IPv6 Internet
access.</t>
<t>The MAP provisioning parameters, and hence the IPv4 service
itself, is tied to the End-user IPv6 prefix lease; thus, the
MAP service is also tied to this in terms of authorization,
accounting, etc. The MAP IPv4 address, prefix or shared IPv4
address and port set has the same lifetime as its associated
End-user IPv6 prefix.</t>
<t>A single MAP CE MAY be connected to more than one MAP domain, just
as any router may have more than one IPv4-enabled service provider
facing interface and more than one set of associated addresses
assigned by DHCP. Each domain a given CE operates within would require
its own set of MAP configuration elements and would generate its own
IPv4 address.</t>
<t>The MAP DHCP option is specified in <xref
target="I-D.ietf-softwire-map-dhcp"></xref>.</t>
</section>
<section title="MAP BR">
<t>The MAP BR MUST be configured with the same MAP elements as the MAP
CEs operating within the same domain.</t>
<t>For increased reliability and load balancing, the BR IPv6 address
MAY be an anycast address shared across a given MAP domain. As MAP is
stateless, any BR may be used at any time. If the BR IPv6 address is
anycast the relay MUST use this anycast IPv6 address as the source
address in packets relayed to CEs.</t>
<t>Since MAP uses provider address space, no specific routes need to
be advertised externally for MAP to operate, neither in IPv6 nor IPv4
BGP. However, if anycast is used for the MAP IPv6 relays, the anycast
addresses must be advertised in the service provider's IGP.</t>
</section>
<section title="Backwards compatibility">
<t>A MAP-E CE provisioned with only the IPv6 address of the
BR, and with no IPv4 address and port range configured by
other means, MUST disable its NAT44 functionality. This
characteristic makes a MAP CE compatible with DS-Lite <xref
target="RFC6333"></xref> AFTRs, whose addresses are configured
as the MAP BR.</t>
</section>
<!--
<section title="Address Independence">
<t>The MAP solution supports use and configuration of domains in so
called 1:1 mode (meaning 1 mapping rule set per CE), which allows
complete independence between the IPv6 prefix assigned to the CE and
the IPv4 address and/or port-range it uses. This is achieved in all
cases when the EA-bit length is set to 0.</t>
<t>The constraint imposed is that each such MAP domain be
composed of just 1 MAP CE which has a predetermined IPv6
prefix, i.e. The BR would be configured with a rule-set per
CPE, where the FMR would uniquely describe the IPv6 prefix of
a given CE. Each CE would have a distinct BMR, that would
fully describe that CE's IPv4 address, and PSID if any.</t>
<section title="1:1 mode with no address sharing">
<t>A domain rule (BMR or FMR) with a setting of EA-bit length of 0,
and a sharing ratio of 1, indicates that in that domain no part of
the IPv4 address is derived from the IPv6 prefix. Instead the IPv4
address is entirely derived from the Rule IPv4-prefix conveyed in
the BMR for a CE. In other words, with an EA-bit length of 0 and a
sharing ratio of 1, the CE would form its IPv4 address entirely out
of the IPv4-prefix received in the BMR (which would be 32 bits
long).</t>
<t>Appendix A gives an example of this configuration.</t>
</section>
<section title="1:1 mode with address sharing">
<t>A domain rule (BMR or FMR) with a setting of EA-bit length of 0,
and a sharing ratio >1, indicates that in that domain no part of
the IPv4 address is derived from the IPv6 prefix. Instead the IPv4
address and PSID are respectively entirely derived from the Rule
IPv4-prefix and the Rule port parameters, both conveyed in the BMR
to a CE. In other words, with an EA-bit length of 0 and a sharing
ratio > 1, the CE would form its IPv4 address entirely out of the
IPv4-prefix received in the BMR (which could be 32 bits long) and
derive its port set from the PSID also conveyed in the BMR.</t>
<t>Appendix A gives an example of this configuration.</t>
</section>
</section>
-->
</section>
<section title="Forwarding Considerations">
<t>Figure 1 depicts the overall MAP architecture with IPv4 users (N and
M) networks connected to a routed IPv6 network.</t>
<t>MAP supports Encapsulation mode as specified in <xref
target="RFC2473"></xref>.</t>
<t>For a shared IPv4 address, a MAP CE forwarding IPv4 packets
from the LAN performs NAT44 functions first and creates
appropriate NAT44 bindings. The resulting IPv4 packets MUST
contain the source IPv4 address and source transport identifiers
defined by MAP. The IPv4 packet is forwarded using the CE's MAP
forwarding function. The IPv6 source and destination addresses
MUST then be derived as per <xref
target="mapping_algorithm"></xref> of this draft.</t>
<section title="Receiving Rules">
<t>A MAP CE receiving an IPv6 packet to its MAP IPv6 address
sends this packet to the CE's MAP function where it is
decapsulated. All other IPv6 traffic is forwarded as per the
CE's IPv6 routing rules. The resulting IPv4 packet is then
forwarded to the CE’s NAT44 function where the destination
port number MUST be checked against the stateful port mapping
session table and the destination port number MUST be mapped to
its original value.</t>
<t>A MAP BR receiving IPv6 packets selects a best matching MAP
domain rule based on a longest address match of the packets'
source address against the BR's configured MAP BMR prefix(es),
as well as a match of the packet destination address against the
configured BR IPv6 address or FMR prefix(es). The selected MAP
rule allows the BR to determine the EA-bits from the source IPv6
address. The BR MUST perform a validation of the consistency of
the source IPv6 address and source port number for the packet
using BMR. If the packets source port number is found to be
outside the range allowed for this CE and the BMR, the BR MUST
drop the packet and respond with an ICMPv6 "Destination
Unreachable, Source address failed ingress/egress policy" (Type
1, Code 5).</t>
<t>In order to prevent spoofing of IPv4 addresses, the MAP node
MUST validate the embedded IPv4 source address and transport
layer port of the encapsulated IPv6 packet with the IPv4 source
address and transport layer port it is encapsulated by according
to the parameters of the matching mapping rule. If the two
source addresses and transport layer ports do not match, the
packet MUST be silently discarded and a counter incremented to
indicate that a potential spoofing attack may be underway.
Additionally, a CE MUST allow forwarding of packets sourced by
the configured BR IPv6 address.</t>
<t>By default, the CE router MUST drop packets received on the
MAP virtual interface (i.e., after decapsulation of IPv6) for
IPv4 destinations not for its own IPv4 shared address, full IPv4
address or IPv4 prefix.</t>
</section>
<section title="ICMP">
<t>ICMP message should be supported in MAP domain. Hence, the NAT44 in
MAP CE must implement the behavior for ICMP message conforming to the
best current practice documented in <xref target="RFC5508"></xref>.</t>
<t>If a MAP CE receives an ICMP message having ICMP identifier field in
ICMP header, NAT44 in the MAP CE must rewrite this field to a specific
value assigned from the port-set. BR and other CEs must handle this
field similar to the port number in the TCP/UDP header upon receiving
the ICMP message with ICMP identifier field.</t>
<t>If a MAP node receives an ICMP error message without the ICMP
identifier field for errors that is detected inside a IPv6 tunnel, a
node should relay the ICMP error message to the original source. This
behavior should be implemented conforming to the section 8 of <xref
target="RFC2473"></xref>.</t>
</section>
<section title="Fragmentation and Path MTU Discovery">
<t>Due to the different sizes of the IPv4 and IPv6 header, handling the
maximum packet size is relevant for the operation of any system
connecting the two address families. There are three mechanisms to
handle this issue: Path MTU discovery (PMTUD), fragmentation, and
transport-layer negotiation such as the TCP Maximum Segment Size (MSS)
option <xref target="RFC0897"></xref>. MAP uses all three mechanisms to
deal with different cases.</t>
<section title="Fragmentation in the MAP domain">
<t>Encapsulating an IPv4 packet to carry it across the MAP domain will
increase its size (40 bytes). It is strongly recommended that the MTU
in the MAP domain is well managed and that the IPv6 MTU on the CE WAN
side interface is set so that no fragmentation occurs within the
boundary of the MAP domain.</t>
<t>Fragmentation on MAP domain entry is described in section 7.2 of
<xref target="RFC2473"></xref></t>
<t>The use of an anycast source address could lead to any ICMP
error message generated on the path being sent to a different
BR. Therefore, using dynamic tunnel MTU Section 6.7 of <xref
target="RFC2473"></xref> is subject to IPv6 Path MTU
black-holes. A MAP BR SHOULD NOT by default use Path MTU
discovery across the MAP domain.</t>
<t>Multiple BRs using the same anycast source address could
send fragmented packets to the same CE at the same time. If
the fragmented packets from different BRs happen to use the
same fragment ID, incorrect reassembly might occur. See <xref
target="RFC4459"/> for an analysis of the problem. Section 3.4
suggests solving the problem by fragmenting the inner
packet.</t>
</section>
<section title="Receiving IPv4 Fragments on the MAP domain borders">
<t>Forwarding of an IPv4 packet received from the outside of the MAP
domain requires the IPv4 destination address and the transport
protocol destination port. The transport protocol information is only
available in the first fragment received. As described in section
5.3.3 of <xref target="RFC6346"></xref> a MAP node receiving an IPv4
fragmented packet from outside has to reassemble the packet before
sending the packet onto the MAP link. If the first packet received
contains the transport protocol information, it is possible to
optimize this behavior by using a cache and forwarding the fragments
unchanged. A description of this algorithm is outside the scope of
this document.</t>
</section>
<section title="Sending IPv4 fragments to the outside">
<t>If two IPv4 host behind two different MAP CE's with the same IPv4
address sends fragments to an IPv4 destination host outside the
domain. Those hosts may use the same IPv4 fragmentation identifier,
resulting in incorrect reassembly of the fragments at the destination
host. Given that the IPv4 fragmentation identifier is a 16 bit field,
it could be used similarly to port ranges. A MAP CE SHOULD rewrite the
IPv4 fragmentation identifier to be within its allocated port set.</t>
</section>
</section>
</section>
<section title="NAT44 Considerations">
<t>The NAT44 implemented in the MAP CE SHOULD conform with the
behavior and best current practice documented in <xref
target="RFC4787"></xref>, <xref target="RFC5508"></xref>, and
<xref target="RFC5382"></xref>. In MAP address sharing mode
(determined by the MAP domain/rule configuration parameters) the
operation of the NAT44 MUST be restricted to the available port
numbers derived via the basic mapping rule.</t>
</section>
<!-- SECTION 4: IANA Considerations -->
<section title="IANA Considerations">
<t>This specification does not require any IANA actions.</t>
</section>
<!-- SECTION 5: Security Considerations -->
<section title="Security Considerations">
<t><list style="hanging">
<t hangText="Spoofing attacks:">With consistency checks between IPv4
and IPv6 sources that are performed on IPv4/IPv6 packets received by
MAP nodes, MAP does not introduce any new opportunity for spoofing
attacks that would not already exist in IPv6.</t>
<t hangText="Denial-of-service attacks:">In MAP domains
where IPv4 addresses are shared, the fact that IPv4 datagram
reassembly may be necessary introduces an opportunity for
DOS attacks. This is inherent to address sharing, and is
common with other address sharing approaches such as DS-Lite
and NAT64/DNS64. The best protection against such attacks is
to accelerate IPv6 deployment, so that, where MAP is
supported, it is less and less used.</t>
<t hangText="Routing-loop attacks:">This attack may exist in some
automatic tunneling scenarios are documented in <xref
target="RFC6324"></xref>. They cannot exist with MAP because each
BRs checks that the IPv6 source address of a received IPv6 packet is
a CE address based on Forwarding Mapping Rule.</t>
<t
hangText="Attacks facilitated by restricted port set:">From
hosts that are not subject to ingress filtering of <xref
target="RFC2827"></xref>, some attacks are possible by an attacker
injecting spoofed packets during ongoing transport connections
(<xref target="RFC4953"></xref>, <xref target="RFC5961"></xref>,
<xref target="RFC6056"></xref>. The attacks depend on guessing which
ports are currently used by target hosts, and using an unrestricted
port set is preferable, i.e. Using native IPv6 connections that are
not subject to MAP port range restrictions. To minimize this type of
attacks when using a restricted port set, the MAP CE's NAT44
filtering behavior SHOULD be "Address-Dependent Filtering".
Furthermore, the MAP CEs SHOULD use a DNS transport proxy function
to handle DNS traffic, and source such traffic from IPv6 interfaces
not assigned to MAP.</t>
</list></t>
<t><xref target="RFC6269"></xref> outlines general issues with IPv4
address sharing.</t>
</section>
<!-- SECTION 6: Contributors -->
<section title="Contributors">
<t>This document is the result of the IETF Softwire MAP design team
effort and numerous previous individual contributions in this area:</t>
<t><figure><artwork><![CDATA[
Chongfeng Xie (China Telecom)
Room 708, No.118, Xizhimennei Street Beijing 100035 CN
Phone: +86-10-58552116
Email: xiechf@ctbri.com.cn
]]></artwork></figure></t>
<t><figure><artwork><![CDATA[
Qiong Sun (China Telecom)
Room 708, No.118, Xizhimennei Street Beijing 100035 CN
Phone: +86-10-58552936
Email: sunqiong@ctbri.com.cn
]]></artwork></figure></t>
<t><figure><artwork><![CDATA[
Gang Chen (China Mobile)
53A,Xibianmennei Ave. Beijing 100053 P.R.China
Email: chengang@chinamobile.com
]]></artwork></figure></t>
<t><figure><artwork><![CDATA[
Yu Zhai
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing 100084
CN
Email: jacky.zhai@gmail.com
]]></artwork></figure></t>
<t><figure><artwork><![CDATA[
Wentao Shang (CERNET Center/Tsinghua University)
Room 225, Main Building, Tsinghua University Beijing 100084
CN
Email: wentaoshang@gmail.com
]]></artwork></figure></t>
<t><figure><artwork><![CDATA[
Guoliang Han (CERNET Center/Tsinghua University)
Room 225, Main Building, Tsinghua University Beijing 100084
CN
Email: bupthgl@gmail.com
]]></artwork></figure></t>
<t><figure><artwork><![CDATA[
Rajiv Asati (Cisco Systems)
7025-6 Kit Creek Road Research Triangle Park NC 27709 USA
Email: rajiva@cisco.com
]]></artwork></figure></t>
</section>
<!-- SECTION 7: Acknowledgements -->
<section title="Acknowledgements">
<t>This document is based on the ideas of many, including
Masakazu Asama, Mohamed Boucadair, Gang Chen, Maoke Chen,
Wojciech Dec, Xiaohong Deng, Jouni Korhonen, Tomasz Mrugalski,
Jacni Qin, Chunfa Sun, Qiong Sun, and Leaf Yeh. The authors want
in particular to recognize Remi Despres, who has tirelessly
worked on generalized mechanisms for stateless address
mapping.</t>
<t>The authors would like to thank Guillaume Gottard, Dan Wing,
Jan Zorz, Necj Scoberne, Tina Tsou, Kristian Poscic, and
especially Tom Taylor for the thorough review and comments of
this document.</t>
</section>
</middle>
<back>
<!-- SECTION 8.1: Normative References -->
<references title="Normative References">
&rfc2119;
&rfc2473;
&I-D.ietf-softwire-map-dhcp;
</references>
<!-- SECTION 8.2: Informative References -->
<references title="Informative References">
&rfc6052;
&rfc6346;
&I-D.ietf-softwire-stateless-4v6-motivation;
&I-D.ietf-tsvwg-iana-ports;
&rfc6333;
&rfc1933;
&rfc5969;
&rfc3056;
&rfc2529;
&rfc5214;
<!-- &rfc4380;-->
<!-- &rfc2766;-->
<!-- &rfc6146;-->
&rfc3633;
&rfc6269;
&rfc6250;
&rfc2663;
&rfc0897;
&rfc6324;
&rfc2827;
&rfc4953;
&rfc5961;
&rfc6056;
&rfc4787;
&rfc5508;
&rfc5382;
<!-- &I-D.dec-stateless-4v6;-->
&rfc4862;
&rfc4632;
&rfc4459;
&I-D.ietf-softwire-map-deployment;
</references>
<section title="Examples" anchor="appendixA">
<t><figure>
<preamble>Example 1 - Basic Mapping Rule</preamble>
<artwork align="left"><![CDATA[
Given the MAP domain information and an IPv6 address of
an endpoint:
End-user IPv6 prefix: 2001:db8:0012:3400::/56
Basic Mapping Rule: {2001:db8:0000::/40 (Rule IPv6 prefix),
192.0.2.0/24 (Rule IPv4 prefix),
16 (Rule EA-bits length)}
PSID length: (16 - (32 - 24) = 8. (Sharing ratio of 256)
PSID offset: 6 (default)
A MAP node (CE or BR) can via the BMR, or equivalent FMR,
determine the IPv4 address and port-set as shown below:
EA bits offset: 40
IPv4 suffix bits (p) Length of IPv4 address (32) -
IPv4 prefix length (24) = 8
IPv4 address: 192.0.2.18 (0xc0000212)
PSID start: 40 + p = 40 + 8 = 48
PSID length: o - p = (56 - 40) - 8 = 8
PSID: 0x34
Available ports (63 ranges) : 1232-1235, 2256-2259, ...... ,
63696-63699, 64720-64723
The BMR information allows a MAP CE to determine (complete)
its IPv6 address within the indicated IPv6 prefix.
IPv6 address of MAP CE: 2001:db8:0012:3400:0000:c000:0212:0034
]]></artwork>
</figure></t>
<t><figure>
<preamble>Example 2 - BR:</preamble>
<artwork align="left"><![CDATA[
Another example can be made of a MAP BR,
configured with the following FMR when receiving a packet
with the following characteristics:
IPv4 source address: 1.2.3.4 (0x01020304)
IPv4 source port: 80
IPv4 destination address: 192.0.2.18 (0xc0000212)
IPv4 destination port: 1232
Configured Forwarding Mapping Rule: {2001:db8::/40 (Rule IPv6 prefix),
192.0.2.0/24 (Rule IPv4 prefix),
16 (Rule EA-bits length)}
IPv6 address of MAP BR: 2001:db8:ffff::1
The above information allows the BR to derive as follows
the mapped destination IPv6 address for the corresponding
MAP CE, and also the mapped source IPv6 address for
the IPv4 source address.
IPv4 suffix bits (p): 32 - 24 = 8 (18 (0x12))
PSID length: 8
PSID: 0x34 (1232)
The resulting IPv6 packet will have the following key fields:
IPv6 source address: 2001:db8:ffff::1
IPv6 destination address: 2001:db8:0012:3400:0000:c000:0212:0034
]]></artwork>
</figure></t>
<t><figure>
<preamble>Example 3 - FMR:</preamble>
<artwork align="left"><![CDATA[
An IPv4 host behind the MAP CE (addressed as per the previous
examples) corresponding with IPv4 host 1.2.3.4 will have its
packets encapsulated by IPv6 using the IPv6 address of the BR
configured on the MAP CE as follows:
IPv6 address of BR used by MAP CE: 2001:db8:ffff::1
IPv4 source address: 192.0.2.18
IPv4 destination address: 1.2.3.4
IPv4 source port: 1232
IPv4 destination port: 80
IPv6 source address of MAP CE: 2001:db8:0012:3400:0000:c000:0212:0034
IPv6 destination address: 2001:db8:ffff::1
]]></artwork>
</figure><figure>
<preamble>Example 4 - Rule with no embedded address bits and no address sharing</preamble>
<artwork><![CDATA[
End-User IPv6 prefix: 2001:db8:0012:3400::/56
Basic Mapping Rule: {2001:db8:0012:3400::/56 (Rule IPv6 prefix),
192.0.2.1/32 (Rule IPv4 prefix),
0 (Rule EA-bits length)}
PSID length: 0 (Sharing ratio is 1)
PSID offset: n/a
A MAP node (CE or BR) can via the BMR or equivalent FMR, determine
the IPv4 address and port-set as shown below:
EA bits offset: 0
IPv4 suffix bits (p): Length of IPv4 address (32) -
IPv4 prefix length (32) = 0
IPv4 address: 192.0.2.1 (0xc0000201)
PSID start: 0
PSID length: 0
PSID: null
The BMR information allows a MAP CE also to determine (complete)
its full IPv6 address by combining the IPv6 prefix with the MAP
interface identifier (that embeds the IPv4 address).
IPv6 address of MAP CE: 2001:db8:0012:3400:0000:c000:0201:0000
]]></artwork>
</figure><figure>
<preamble>Example 5 - Rule with no embedded address bits and address sharing (sharing
ratio 256)</preamble>
<artwork><![CDATA[
End-User IPv6 prefix: 2001:db8:0012:3400::/56
Basic Mapping Rule: {2001:db8:0012:3400::/56 (Rule IPv6 prefix),
192.0.2.1/32 (Rule IPv4 prefix),
0 (Rule EA-bits length)}
PSID length: 8. (Provisioned with DHCP. Sharing ratio of 256)
PSID offset: 6 (Default)
PSID : 0x20 (Provisioned with DHCP.)
A MAP node can via the BMR determine the IPv4 address and port-set
as shown below:
EA bits offset: 0
IPv4 suffix bits (p): Length of IPv4 address (32) -
IPv4 prefix length (32) = 0
IPv4 address: 192.0.2.1 (0xc0000201)
PSID offset: 6
PSID length: 8
PSID: 0x20
Available ports (63 ranges) : 1536-1551, 2560-2575, ...... ,
64000-64015, 65024-65039
The BMR information allows a MAP CE also to determine (complete)
its full IPv6 address by combining the IPv6 prefix with the MAP
interface identifier (that embeds the IPv4 address and PSID).
IPv6 address of MAP CE: 2001:db8:0012:3400:0000:c000:0212:0034
Note that the IPv4 address and PSID is not derived from the IPv6
prefix assigned to the CE, but provisioned separately using
e.g. DHCP.
]]></artwork>
</figure></t>
</section>
<section title="A More Detailed Description of the Derivation of the Port Mapping Algorithm">
<t>This Appendix describes how the port mapping algorithm described in
<xref target="portmap"/> was derived. The algorithm is used in domains
whose rules allow IPv4 address sharing. </t>
<t>The basic requirement for a port mapping algorithm is
that the port sets it assigns to different MAP CEs MUST be non-overlapping.
A number of other requirements guided the choice of the algorithm:
<list style="symbols">
<t>In keeping with the general MAP algorithm the port set MUST be derivable
from a port set identifier (PSID) that can be embedded in the End-user IPv6
prefix.</t>
<t>The mapping MUST be reversible, such that, given the port number,
the PSID of the port set to which it belongs can be quickly derived.
</t>
<t>The algorithm MUST allow a broad range of address sharing ratios.</t>
<t>It SHOULD be possible to exclude subsets of the complete port numbering
space from assignment. Most operators would exclude the system ports
(0-1023). A conservative operator might exclude all but the
transient ports (49152-65535).
</t>
<t>The effect of port exclusion on the possible values of the End-user
IPv6 prefix (i.e., due to restrictions on the PSID value) SHOULD be minimized.</t>
<t>For administrative simplicity, the algorithm SHOULD allocate the
the same or almost the same number of ports to each CE sharing a given
IPv4 address.
</t>
</list>
</t>
<t>The two extreme cases that an algorithm satisfying those conditions
might support are: (1) the port
numbers are not contiguous for each PSID, but uniformly distributed
across the allowed port range;
(2) the port numbers are contiguous
in a single range for each PSID. The port mapping algorithm proposed
here is called the Generalized Modulus Algorithm (GMA) and supports
both these cases.</t>
<t>For a given IPv4 address sharing ratio (R) and the maximum number of contiguous
ports (M) in a port set, the GMA is defined as:
<list style="letters">
<t>The port numbers (P) corresponding to a given PSID are
generated by: <figure>
<artwork><![CDATA[
(1) ... P = (R * M) * i + M * PSID + j
]]></artwork>
</figure> where i and j are indices and the ranges
of i, j, and the PSID are discussed in a moment.</t>
<t>For any given port number P, the PSID is calculated as: <figure>
<artwork><![CDATA[
(2) ... PSID = trunc((P modulo (R * M)) / M)
]]></artwork>
</figure> where trunc() is the operation of rounding down to the
nearest integer.</t>
</list>
</t>
<t>Formula (1) can be interpreted as follows. First, the available
port space is divided into blocks of size R * M. Each block is
divided into R individual ranges of length M. The index i in
formula (1) selects a block, PSID selects a range within that
block, and the index j selects a specific port value within the
range. On the basis of this interpretation: <list style="symbols">
<t>i ranges from ceil(N / (R * M)) to trunc(65536/(R * M)) - 1,
where ceil is the operation of rounding up to the nearest integer
and N is the number of ports (e.g., 1024) excluded from the lower
end of the range. That is, any block containing excluded values is
discarded at the lower end, and if the
final block has fewer than R * M values it is discarded. This ensures
that the same number of ports is assigned to every PSID.
</t>
<t>PSID ranges from 0 to R - 1;</t>
<t>j ranges from 0 to M - 1.</t>
</list>
</t>
<section title="Bit Representation of the Algorithm">
<t> If R and M are powers of 2 (R = 2^k, M = 2^m), formula (1) translates
to a computationally convenient structure for any port number
represented as a 16-bit binary number. This structure is shown in
<xref target="bitrepresentation2-fig"/>.</t>
<t><figure align="left" anchor="bitrepresentation2-fig"
title="Bit Representation of a Port Number">
<preamble></preamble>
<artwork align="left"><![CDATA[
0 8 15
+---------------+----------+------+-------------------+
| P |
----------------+-----------------+-------------------+
| i | PSID | j |
+---------------+----------+------+-------------------+
|<----a bits--->|<-----k bits---->|<------m bits----->|
]]></artwork>
</figure></t>
<t>As shown in the figure, the index value i of formula (1)
is given by the first
a = 16 - k - m bits of the port number. The PSID value is given
by the next k bits, and the index value j is given by the last
m bits.</t>
<t>For any port number, the PSID can be obtained by a bit mask
operation.</t>
<t>Note that when M and R are powers of 2, 65536 divides evenly
by R * M. Hence the final block is complete and the upper bound
on i is exactly 65536/(R * M) - 1. The lower bound on i is still
the minimum required to ensure that the required set of ports is
excluded. No port numbers are wasted through discarding of blocks
at the lower end if block size R * M is a factor of N, the number
of ports to be excluded.
</t>
<t>As a final note, the number of blocks into which the range 0-65535
is being divided in the above representation is given by 2^a.
Hence the case where a = 0 can be interpreted as one where the
complete range has been divided into a single block, and individual
port sets are contained in contiguous ranges in that block. We cannot
throw away the whole block in that case, so port exclusion has to be
achieved by putting a lower bound equal to ceil(N / M) on the
allowed set of PSID values instead.
</t>
</section>
<section title="GMA examples">
<t><figure align="left" title="Example 1: with offset = 6 (a = 6)">
<preamble>For example, for R = 256, PSID = 0, offset: a = 6 and PSID
length: k = 8 bits</preamble>
<artwork align="left"><![CDATA[
Available ports (63 ranges) : 1024-1027, 2048-2051, ...... ,
63488-63491, 64512-64515
]]></artwork>
</figure></t>
<t><figure align="left" title="Example 2: with offset = 0 (a = 0) and N = 0">
<preamble>For example, for R = 64, PSID = 0, a = 0 (PSID offset = 0 and
PSID length = 6 bits), no port exclusion:</preamble>
<artwork align="left"><![CDATA[
Available ports (1 range) : 0-1023
]]></artwork>
</figure></t>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 19:28:35 |