33.22 Embedded IPv4 Addresses And T

33.22 Embedded IPv4 Addresses And Transition
Although the prefm 0000 0000 is labeled Resewed in the figure, the designers plan
to use a small fraction of addresses in that section to encode IPv4 addresses. In particular, any address that begins with 80 zero bits followed by 16 bits of all ones or 16 bits
of all zeros contains an Wv4 address in the low-order 32 bits. The value of the 16-bit
field indicates whether the node also has a conventional IPv6 unicast address. Figure
33.9 illustrates the two forms.
Sec. 33.22 Embedded IPv4 Addresses And Transition 615
1-80 zero bits-116 bitsl-32 bits-I
10000. . . . . . . . . . . . . . . . . .W O O I 0000 I lPv4 Address 1
0000. . . . . . . . . . . . . . . . ..WOO 1 FFFF I IPv4 Address
Figure 33.9 The encoding of an IPv4 address in an IPv6 address. The 16-bit
field contains 0000 if the node also has a conventional IPv6 address, and FFFF if it does not.
The encoding will be needed during a transition from IPv4 to IPv6 for two reasons.
First, a computer may choose to upgrade from IPv4 to IPv6 software before it has been
assigned a valid IPv6 address. Second, a computer running IPv6 software may need to
communicate with a computer that runs only IPv4 software.
Having a way to encode an IPv4 address in an IPv6 address does not solve the
problem of making the two version interoperate. In addition to address encoding, translation is needed. To use a translator, an IPv6 computer generates a datagram that contains the IPv6 encoding of the IPv4 destination address. The IPv6 computer sends the
datagram to a translator, which uses IPv4 to communicate with the destination. When
the translator receives a reply from the destination, it translates the IPv4 datagram to
IPv6 and sends it back to the IPv6 source.
It may seem that translating protocol addresses could fail because higher layer protocols verify address integrity. In particular, TCP and UDP, use a pseudo header in
their checksum computation. The pseudo header includes both the source and destination protocol addresses, so changing such addresses could affect the computation. However, the designers planned carefully to allow TCP or UDP on an IPv4 machine to communicate with the corresponding transport protocol on an IPv6 machine. To avoid
checksum mismatch, the IPv6 encoding of an IPv4 address has been chosen so that the
16-bit 1's complement checksum for both an IPv4 address and the IPv6 encoding of the
address are identical. The point is:
In addition to choosing technical details of a new Internet Protocol,
the IETF work on IPv6 has focused on finding a way to transition
from the current protocol to the new protocol. In particular, the
current proposal for IPv6 allows one to encode an IPv4 address inside an IPv6 address such that address translation does not change
the pseudo header checksum.
616 The Future Of TCPlIP (IPv6) Chap. 33
33.23 Unspecified And LoopbackAddresses
As in IPv4, a few IPv6 addresses have been assigned special meaning. For example, the all 0's address:
is an unspecified address which cannot be assigned to any computer or used as a destination. It is only used as a source address during bootstrap by a computer that has not
yet learned its address.
Like IPv4, IPv6 also has a loopback address that is used for testing software. The
IPv6 loopback address is:
Any datagram sent to the loopback address will be delivered to the local machine; it
must never be used as a destination address on an outgoing datagram.
33.24 Unicast Address Hierarchy
One of the most important changes between IPv4 and IPv6 arises from the allocation strategy used for unicast addresses and the resulting address hierarchy. Recall that
the original IPv4 addressing scheme used a two-level hierarchy in which an address is
divided into a globally unique prefix and a suffi. IPv6 extends the concept by adopting an address hierarchy with three conceptual levels as Figure 33.10 illustrates.
Level Purpose
1 Globally-known public topology
2 Individual site
3 Individual network interface
Figure 33.10 The three conceptual levels of the Pv6 unicast address hierarchy. In practice, an address has additional structure.
The two lowest levels of the conceptual hierarchy are easiest to understand because
they correspond to identifiable entities. The lowest level corresponds to a single attachment between a computer and a network. The middle level of the hierarchy
corresponds to a set of computers and networks located at a site, which implies both
contiguous physical co~ectivityand a single organization that owns and operates the
equipment. We will see that the addressing scheme accommodates both large and small
sites, and allows a site to have complex internal structure.
Sec. 33.24 Unicast Address Hierarchy 617
To provide flexibility, the top level of the hierarchy, which is labeled public topology, is not precisely defined. In general, one can think of the public topology as a
"section" of the global Internet that is available for public access. Two types of public
topology are envisioned. The first type corresponds to a major Internet Service Provider (ISP)that provides long-haul service to customers, which are known as subscribers.
The second type, which is called an exchange, is a newly envisioned organization. According to the designers, exchanges will provide two functions. First, an exchange will
operate like a NAP to intercomect major ISPs and pass traffic among them. Second,
unlike current NAPS, exchanges will also service individual subscribers, which means
that the exchange will assign the subscriber an address. The chief advantage of an address assigned by an exchange is that the address will not specify an ISP. Thus, a subscriber will be free to move from one ISP to another.
33.25 Aggregatable Global Unicast Address Structure
Authority for IPv6 address assignment flows down the hierarchy. Each top-level
organization (e.g., an ISP or exchange) is assigned a unique prefm. When an organization becomes a subscriber of a top-level ISP, the organization is assigned a unique
number for its site. Finally, a manager must assign a number to each network comection. To make routing efficient, successive sets of bits in the address are reserved for
each assignment. Figure 33.11 illustrates the format, which is known as a aggregatable
global unicast address format.
top ,,site +. third
level level level
. ,
Figure 33.11 The division of an IPV6 aggregatable global unicast address into
separate fields along with an indication of how those fields
correspond to the three-level hierarchy.
The 3-bit field labeled P in the figure corresponds to the fonnat prefi, which is
001 for an aggregatable global unicast address. The &bit RES field is reserved for the
future and contains zeroes. Remaining fields in the address are arranged to make routing efficient. In particular, fields that correspond to the highest level of the hierarchy
are grouped together to comprise the most significant bits of the address. Field TLA ID
contains an identifier used for Top-Level Aggregation (i.e., a unique identifier assigned
to the ISP or exchange that owns the address). The owner of the address uses field
N U to provide Next-Level Aggregation (e.g., to identify a particular subscriber).
TLA
ID
SLA
RES NLAID ID INTERFACE ID
618 The Future Of TCPIIP (IPV6) Chap. 33
The 16-bit field labeled SLA ID (Site-LevelAggregation) is available for a specific
site to use. The designers envision it being used much like an IPv4 subnet field. Thus,
a site with only a few networks can choose to treat the field as a network identifier, and
a site that has many networks can use the field to partition networks into groups which
can then be arranged in a hierarchy. To create a one-level hierarchy at the site, the organization must use a prefm to identify the group and a suffvr to identify a particular
network in the group. As with IPv4 subnetting, the division into groups improves routing efficiency because a routing table only contains routes to each of the other groups
rather than to each individual network.
33.26 Interface Identifiers
As Figure 33.1 1 shows, the low-order 64 bits of an IPv6 aggregatable unicast address identifies a specific network interface. Unlike IPv4, however, the IPV6 suffix was
chosen to be large enough to accommodate a direct encoding of the interface hardware
address. Encoding a hardware address in an IP address has two consequences. First,
IPv6 does not use ARP to resolve an IP address to a hardware address. Instead, IPv6
uses a neighbor discovery protocol available with a new version of ICMP (ICMPV6)to
allow a node to determine which computers are its directly c o ~ e c t e dneighbors.
Second, to guarantee interoperability, all computers must use the same encoding for a
hardware address. Consequently, the IPv6 standards specify exactly how to encode
various forms of hardware address. In the simplest case, the hardware address is placed
directly in the IPv6 address; some formats use more complex transformations.
Two example encodings will help clarify the concept. For example, IEEE defines
a standard 64-bit globally unique address format known as EUI-64. The only change
needed when encoding an EUI-64 address in an IPv6 address consists of inverting bit 6
in the high-order octet of the address, which indicates whether the address is known to
be globally unique.
A more complex change is required for a conventional 48-bit Ethernet address.
Figure 33.12 illustrates the encoding. As the figure shows, bits from the original address are not contiguous in the encoded form. Instead, 16 bits with hexadecimal value
OXFFFE are inserted in the middle. In addition, bit 6, which indicates whether the address has global scope, is changed from 0 to 1. Remaining bits of the address, including the group bit (labeled g), the ID of the company that manufactured the interface (labeled c),and the manufacturer's extension are copied as shown.
Sec. 33.26 Interface Identifiers
0 8 24 47
cccccOgcccccccccc