- Văn bản
- Lịch sử
The two high-order bits of each opt
The two high-order bits of each option TYPE field specify how a host or router
should dispose of the datagram if it does not understand the option:
Bits In Type Meaning
00 Skip this option
01 Discard datagram; do not send ICMP message
10 Discard datagram; send ICMP message to source
11 Discard datagram; send ICMP for non-multicast
In addition, the third bit in the TYPE field specifies whether the option can change
in transit. Having such information is important for authentication-the contents of an
option that can change in transit are treated as zeroes for purposes of authentication.
33.16 Size Of The IPv6 Address Space
In IPv6 each address occupies 16 octets, four times the size of an IPv4 address.
The large address space guarantees that IPv6 can tolerate any reasonable address assignment scheme. In fact, if the designers decide to change the addressing scheme later, the
address space is sufficiently large to accommodate a reassignment.
It is difficult to comprehend the size of the IPv6 address space. One way to look
at it relates the magnitude to the size of the population: the address space is so large
that every person on the planet can have sufficient addresses to have their own internet
as large as the current Internet. A second way to think of IPv6 addressing relates it to
the physical space available: the earth's surface has approximately 5.1x 10' square kilometers, meaning that there are over addresses per square meter of the earth's surface. Another way to understand the size relates it to address exhaustion. For example,
consider how long it would take to assign all possible addresses. A 16-octet integer can
hold 2'28values. Thus, the address space is greater than 3.4 x 10". If addresses are assigned at the rate of one million addresses every microsecond, it would take over 1020
years to assign all possible addresses.
33.17 IPv6 Colon Hexadecimal Notation
Although it solves the problem of having insufficient capacity, the large address
size poses an interesting new problem: humans who maintain internets must read, enter,
and manipulate such addresses. Obviously, binary notation is untenable. However, the
dotted decimal notation used for IPv4 does not make such addresses sufficiently compact either. To understand why, consider an example 128-bit number expressed in dotted decimal notation:
Sec. 33.17 IPv6 Colon Hexadecimal Notation 61 1
To help make address slightly more compact and easier to enter, the IPv6 designers
propose using colon hexadecimal notation (abbreviated colon hex) in which the value of
each 16-bit quantity is represented in hexadecimal separated by colons. For example,
when the value shown above in dotted decimal notation has been translated to colon hex
nqtation and printed using the same spacing, it becomes:
Colon hex notation has the obvious advantage of requiring fewer digits and fewer
separator characters than dotted decimal. In addition, colon hex notation includes two
techniques that make it extremely useful. First, colon hex notation allows zero
compression in which a string of repeated zeros is replaced by a pair of colons. For example, the address:
FF05:0:0:0:0:0:0:B3
can be written:
To ensure that zero compression produces an unambiguous interpretation, the proposal specifies that it can be applied only once in any address. Zero compression is
especially useful when used with the proposed address assignment scheme because
many addresses will contain contiguous strings of zeros. Second, colon hex notation incorporates dotted decimal suffies; we will see that such combinations are intended to
be used during the transition from IPv4 to IPv6. For example, the following string is
valid colon hex notation:
Note that although the numbers separated by colons each specify the value of a
16-bit quantity, numbers in the dotted decimal portion each specify the value of one octet. Of course, zero compression can be used with the number above to produce an
equivalent colon hex string that looks quite similar to an IPv4 address:
Finally, IPv6 extends CIDR-like notation by allowing an address to be followed by
a slash and an integer that specifies a number of bits. For example,
specifies the first 60 bits of the address or 12ABOOOOOOOOCD3in hexadecimal.
612 The Future Of TCP/IP (IF'v6) Chap. 33
33.18 Three Basic IPv6 Address Types
Like IPv4, IPv6 associates an address with a specific network connection, not with
a specific computer. Thus, address assignments are similar to IPv4: an IPv6 router has
two or more addresses, and an IPv6 host with one network connection needs only one
address. IPv6 also retains (and extends) the IPv4 address hierarchy in which a physical
network is assigned a prefix. However, to make address assignment and modification
easier, IPv6 permits multiple prefixes to be assigned to a given network, and allows a
computer to have multiple, simultaneous addresses assigned to a given interface.
In addition to permitting multiple, simultaneous addresses per network connection,
IPv6 expands, and in some cases unifies, IPv4 special addresses. In general, a destination address on a datagram falls into one of three categories:
Unicast The destination address specifies a single computer (host
or router); the datagram should be routed to the destination
along a shortest path.
Anycast The destination is a set of computers, possibly at different
locations, that all share a single address; the datagram
should be routed along a shortest path and delivered to exactly one member of the group (i.e., the closest member)?.
Multicast The destination is a set of computers, possibly at multiple
locations. One copy of the datagram will be delivered to
each member of the group using hardware multicast or
broadcast if viable.
33.19 The Duality Of Broadcast And Multicast
IPv6 does not use the terms broadcast or directed broadcast to refer to delivery to
all computers on a physical network or to a logical IP subnet. Instead, it uses the term
multicast, and treats broadcast as a special form of multicast. The choice may seem odd
to anyone who understands network hardware because more hardware technologies support broadcast than support multicast. In fact, a hardware engineer is likely to view
multicasting as a restricted form of broadcasting- the hardware sends a multicast
packet to all computers on the network exactly like a broadcast packet, and the interface
hardware on each computer filters all multicast packets except those that software has
instructed the interface hardware to accept.
In theory, the choice between multicast and limited forms of broadcast is irrelevant
because one can be simulated with the other. That is, broadcasting and multicasting are
duals of one another that provide the same functionality. To understand why, consider
how to simulate one with the other. If broadcast is available, a packet can be delivered
to a group by sending it to all machines and arranging for software on each computer to
decide whether to accept or discard the incoming packet. If multicast is available, a
?Anycast addresses were formerly known as cluster addresses.
Sec. 33.19 The Duality Of BroadcastAnd Multicast 613
packet can be delivered to all machines by arranging for all machines to listen to one
multicast group similar to the all hosts group discussed in Chapter 17.
33.20 An Engineering Choice And Simulated Broadcast
Knowing that broadcasting and multicasting are theoretical duals of one another
does not help choose between them. To see why the designers of IPv6 chose multicasting as the central abstraction instead of broadcasting, consider applications instead of
looking at the underlying hardware. An application either needs to communicate with a
single application or with a group of applications. Direct communication is handled
best via unicast; group communication is handled best by multicast or broadcast. To
provide the most flexibility, group membership should not be determined by network
connections, because group members can reside at arbitrary locations. Using broadcast
for all group communication does not scale to handle an internet as large as the global
Internet.
Not surprisingly, the designers pre-define some multicast addresses that can be
used in place of an IPv4 network broadcast address. Thus, in addition to its own unicast address, each router is required to accept packets addressed to the A11 Routers multicast groups for its local environment.
33.21 Proposed IPv6 Address Space Assignment
The question of how to partition the IPv6 address space has generated much discussion. There are two central issues: how to manage address assignment and how to
map an address to a route. The first issue focuses on the practical problem of devising
a hierarchy of authority. Unlike the current Internet, which uses a two-level hierarchy
of network prefix (assigned by the Internet authority) and host suffix (assigned by the
organization), the large address space in IPv6 permits a multi-level hierarchy or multiple hierarchies. The second issue focuses on computational efficiency. Independent of
the hierarchy of authority that assigns addresses, a router must examine each datagram
and choose a path to the destination. To keep the cost of high-speed routers low, the
processing time required to choose a path must be kept small.
As Figure 33.8 shows, the designers of IPv6 propose assigning address classes in a
way similar to the scheme used for Pv4. Although the first 8 bits of an address are
sufficient to identify its type, the address space is not partitioned into sections of equal
size.
614 The Future Of TCPIIP (IF'v6) Chap. 33
Binary Prefix Type Of Address Part Of Address Space
Resewed (IPv4 compatibility)
Unassigned
NSAP Addresses
IPX Addresses
Unassigned
Unassigned
Unassigned
Aggregatable Global Unicast
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Link-Local Unicast Addresses
Site-Local Unicast Addresses
Multicast Addresses
should dispose of the datagram if it does not understand the option:
Bits In Type Meaning
00 Skip this option
01 Discard datagram; do not send ICMP message
10 Discard datagram; send ICMP message to source
11 Discard datagram; send ICMP for non-multicast
In addition, the third bit in the TYPE field specifies whether the option can change
in transit. Having such information is important for authentication-the contents of an
option that can change in transit are treated as zeroes for purposes of authentication.
33.16 Size Of The IPv6 Address Space
In IPv6 each address occupies 16 octets, four times the size of an IPv4 address.
The large address space guarantees that IPv6 can tolerate any reasonable address assignment scheme. In fact, if the designers decide to change the addressing scheme later, the
address space is sufficiently large to accommodate a reassignment.
It is difficult to comprehend the size of the IPv6 address space. One way to look
at it relates the magnitude to the size of the population: the address space is so large
that every person on the planet can have sufficient addresses to have their own internet
as large as the current Internet. A second way to think of IPv6 addressing relates it to
the physical space available: the earth's surface has approximately 5.1x 10' square kilometers, meaning that there are over addresses per square meter of the earth's surface. Another way to understand the size relates it to address exhaustion. For example,
consider how long it would take to assign all possible addresses. A 16-octet integer can
hold 2'28values. Thus, the address space is greater than 3.4 x 10". If addresses are assigned at the rate of one million addresses every microsecond, it would take over 1020
years to assign all possible addresses.
33.17 IPv6 Colon Hexadecimal Notation
Although it solves the problem of having insufficient capacity, the large address
size poses an interesting new problem: humans who maintain internets must read, enter,
and manipulate such addresses. Obviously, binary notation is untenable. However, the
dotted decimal notation used for IPv4 does not make such addresses sufficiently compact either. To understand why, consider an example 128-bit number expressed in dotted decimal notation:
Sec. 33.17 IPv6 Colon Hexadecimal Notation 61 1
To help make address slightly more compact and easier to enter, the IPv6 designers
propose using colon hexadecimal notation (abbreviated colon hex) in which the value of
each 16-bit quantity is represented in hexadecimal separated by colons. For example,
when the value shown above in dotted decimal notation has been translated to colon hex
nqtation and printed using the same spacing, it becomes:
Colon hex notation has the obvious advantage of requiring fewer digits and fewer
separator characters than dotted decimal. In addition, colon hex notation includes two
techniques that make it extremely useful. First, colon hex notation allows zero
compression in which a string of repeated zeros is replaced by a pair of colons. For example, the address:
FF05:0:0:0:0:0:0:B3
can be written:
To ensure that zero compression produces an unambiguous interpretation, the proposal specifies that it can be applied only once in any address. Zero compression is
especially useful when used with the proposed address assignment scheme because
many addresses will contain contiguous strings of zeros. Second, colon hex notation incorporates dotted decimal suffies; we will see that such combinations are intended to
be used during the transition from IPv4 to IPv6. For example, the following string is
valid colon hex notation:
Note that although the numbers separated by colons each specify the value of a
16-bit quantity, numbers in the dotted decimal portion each specify the value of one octet. Of course, zero compression can be used with the number above to produce an
equivalent colon hex string that looks quite similar to an IPv4 address:
Finally, IPv6 extends CIDR-like notation by allowing an address to be followed by
a slash and an integer that specifies a number of bits. For example,
specifies the first 60 bits of the address or 12ABOOOOOOOOCD3in hexadecimal.
612 The Future Of TCP/IP (IF'v6) Chap. 33
33.18 Three Basic IPv6 Address Types
Like IPv4, IPv6 associates an address with a specific network connection, not with
a specific computer. Thus, address assignments are similar to IPv4: an IPv6 router has
two or more addresses, and an IPv6 host with one network connection needs only one
address. IPv6 also retains (and extends) the IPv4 address hierarchy in which a physical
network is assigned a prefix. However, to make address assignment and modification
easier, IPv6 permits multiple prefixes to be assigned to a given network, and allows a
computer to have multiple, simultaneous addresses assigned to a given interface.
In addition to permitting multiple, simultaneous addresses per network connection,
IPv6 expands, and in some cases unifies, IPv4 special addresses. In general, a destination address on a datagram falls into one of three categories:
Unicast The destination address specifies a single computer (host
or router); the datagram should be routed to the destination
along a shortest path.
Anycast The destination is a set of computers, possibly at different
locations, that all share a single address; the datagram
should be routed along a shortest path and delivered to exactly one member of the group (i.e., the closest member)?.
Multicast The destination is a set of computers, possibly at multiple
locations. One copy of the datagram will be delivered to
each member of the group using hardware multicast or
broadcast if viable.
33.19 The Duality Of Broadcast And Multicast
IPv6 does not use the terms broadcast or directed broadcast to refer to delivery to
all computers on a physical network or to a logical IP subnet. Instead, it uses the term
multicast, and treats broadcast as a special form of multicast. The choice may seem odd
to anyone who understands network hardware because more hardware technologies support broadcast than support multicast. In fact, a hardware engineer is likely to view
multicasting as a restricted form of broadcasting- the hardware sends a multicast
packet to all computers on the network exactly like a broadcast packet, and the interface
hardware on each computer filters all multicast packets except those that software has
instructed the interface hardware to accept.
In theory, the choice between multicast and limited forms of broadcast is irrelevant
because one can be simulated with the other. That is, broadcasting and multicasting are
duals of one another that provide the same functionality. To understand why, consider
how to simulate one with the other. If broadcast is available, a packet can be delivered
to a group by sending it to all machines and arranging for software on each computer to
decide whether to accept or discard the incoming packet. If multicast is available, a
?Anycast addresses were formerly known as cluster addresses.
Sec. 33.19 The Duality Of BroadcastAnd Multicast 613
packet can be delivered to all machines by arranging for all machines to listen to one
multicast group similar to the all hosts group discussed in Chapter 17.
33.20 An Engineering Choice And Simulated Broadcast
Knowing that broadcasting and multicasting are theoretical duals of one another
does not help choose between them. To see why the designers of IPv6 chose multicasting as the central abstraction instead of broadcasting, consider applications instead of
looking at the underlying hardware. An application either needs to communicate with a
single application or with a group of applications. Direct communication is handled
best via unicast; group communication is handled best by multicast or broadcast. To
provide the most flexibility, group membership should not be determined by network
connections, because group members can reside at arbitrary locations. Using broadcast
for all group communication does not scale to handle an internet as large as the global
Internet.
Not surprisingly, the designers pre-define some multicast addresses that can be
used in place of an IPv4 network broadcast address. Thus, in addition to its own unicast address, each router is required to accept packets addressed to the A11 Routers multicast groups for its local environment.
33.21 Proposed IPv6 Address Space Assignment
The question of how to partition the IPv6 address space has generated much discussion. There are two central issues: how to manage address assignment and how to
map an address to a route. The first issue focuses on the practical problem of devising
a hierarchy of authority. Unlike the current Internet, which uses a two-level hierarchy
of network prefix (assigned by the Internet authority) and host suffix (assigned by the
organization), the large address space in IPv6 permits a multi-level hierarchy or multiple hierarchies. The second issue focuses on computational efficiency. Independent of
the hierarchy of authority that assigns addresses, a router must examine each datagram
and choose a path to the destination. To keep the cost of high-speed routers low, the
processing time required to choose a path must be kept small.
As Figure 33.8 shows, the designers of IPv6 propose assigning address classes in a
way similar to the scheme used for Pv4. Although the first 8 bits of an address are
sufficient to identify its type, the address space is not partitioned into sections of equal
size.
614 The Future Of TCPIIP (IF'v6) Chap. 33
Binary Prefix Type Of Address Part Of Address Space
Resewed (IPv4 compatibility)
Unassigned
NSAP Addresses
IPX Addresses
Unassigned
Unassigned
Unassigned
Aggregatable Global Unicast
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Link-Local Unicast Addresses
Site-Local Unicast Addresses
Multicast Addresses
0/5000
Hai bit cao thứ tự của mỗi trường loại tùy chọn xác định làm thế nào một máy chủ lưu trữ hoặc bộ định tuyếnnên vứt bỏ của datagram nếu không hiểu tùy chọn:Bit trong kiểu ý nghĩa00 bỏ qua tùy chọn này01 huỷ datagram; không gửi tin nhắn ICMP10 huỷ datagram; gửi tin nhắn ICMP nguồn11 huỷ datagram; gửi ICMP cho phòng không phát đa hướngNgoài ra, các bit thứ ba trong trường loại chỉ định cho dù các tùy chọn có thể thay đổiquá cảnh. Có thông tin đó là quan trọng đối với các nội dung the xác thực của mộttùy chọn có thể thay đổi trên đường vận chuyển được coi là zeroes cho mục đích xác thực.33.16 kích thước không gian địa chỉ IPv6Ở IPv6 từng địa chỉ chiếm 16 octet, bốn lần kích thước của một địa chỉ IPv4.Không gian lớn địa chỉ đảm bảo rằng IPv6 có thể chịu đựng được bất kỳ chương trình gán địa chỉ hợp lý. Trong thực tế, nếu các nhà thiết kế quyết định thay đổi địa chỉ các lược đồ sau đó, cáckhông gian địa chỉ là đủ lớn để chứa một tái bố trí.Nó rất khó để thấu hiểu kích thước không gian địa chỉ IPv6. Một cách khác để xemtại nó liên quan tầm quan trọng để kích thước của dân số: không gian địa chỉ là rất lớnmỗi người trên hành tinh có thể có đủ địa chỉ có internet của riêng của họlớn như Internet hiện tại. Một cách thứ hai để nghĩ về địa chỉ IPv6 liên quan đểkhông gian vật lý có sẵn: bề mặt của trái đất có khoảng 5,1 x 10' km², có nghĩa là rằng không có trên địa chỉ cho mỗi mét vuông của bề mặt trái đất. Một cách khác để hiểu kích thước liên quan đến địa chỉ kiệt sức. Ví dụ,Hãy xem xét làm thế nào lâu để gán tất cả địa chỉ có thể. Một số nguyên 16-octet có thểGiữ 2' 28values. Vì vậy, không gian địa chỉ là lớn hơn 3,4 x 10". Nếu địa chỉ được chỉ định ở mức trong một triệu địa chỉ mỗi microsecond, nó sẽ mất hơn 1020năm để chỉ định tất cả địa chỉ có thể.33.17 ký hiệu hệ thập lục phân IPv6 ColonMặc dù nó giải quyết vấn đề là có không đủ năng lực, địa chỉ lớnKích thước đặt ra một vấn đề mới thú vị: con người duy trì internets phải đọc, nhập,và thao tác các địa chỉ. Rõ ràng, ký hiệu nhị phân là không thể bảo vệ. Tuy nhiên, cácchấm ký hiệu thập phân được sử dụng cho IPv4 không làm cho các địa chỉ đầy đủ nhỏ gọn một trong hai. Để hiểu lý do tại sao, hãy xem xét một số 128-bit ví dụ thể hiện trong chấm ký hiệu thập phân:SEC. 33.17 IPv6 Colon ký hiệu hệ thập lục phân 61 1Để giúp làm cho địa chỉ hơn một chút nhỏ gọn và dễ dàng hơn để nhập, các nhà thiết kế IPv6đề nghị sử dụng ký hiệu hệ thập lục phân đại tràng (viết tắt ruột hex) trong đó giá trị củamỗi số lượng 16-bit được đại diện trong hệ thập lục phân cách nhau bằng dấu hai chấm. Ví dụ,Khi giá trị Hiển thị ở trên trong chấm ký hiệu thập phân đã được dịch sang đại tràng hexnqtation và in bằng cách sử dụng các khoảng cách tương tự, nó sẽ trở thành:Ký hiệu hex Colon có lợi thế rõ ràng của đòi hỏi phải có chữ số ít hơn và ít hơntách các nhân vật hơn chấm thập phân. Ngoài ra, ký hiệu hex ruột bao gồm haikỹ thuật mà làm cho nó cực kỳ hữu ích. Đầu tiên, ký hiệu hex colon cho phép zeronén trong đó một chuỗi lặp đi lặp lại Zero được thay bằng một cặp dấu hai chấm. Ví dụ: địa chỉ:FF05:0:0:0:0:0:0:B3có thể được viết:Để đảm bảo rằng nén không tạo ra một giải thích rõ ràng, các đề nghị xác định rằng nó có thể được áp dụng chỉ một lần trong bất kỳ địa chỉ. Không nén làđặc biệt hữu ích khi được sử dụng với các đề án chuyển nhượng địa chỉ được đề xuất bởi vìnhiều địa chỉ sẽ chứa chuỗi tiếp giáp của Zero. Thứ hai, ký hiệu hex ruột kết hợp chấm thập phân suffies; chúng ta sẽ thấy rằng kết hợp như vậy được dự định đểđược sử dụng trong quá trình chuyển đổi IPv4 và IPv6. Ví dụ, Chuỗi sau đây làhợp lệ ký hiệu đại tràng hex:Lưu ý rằng mặc dù những con số cách nhau bằng dấu hai chấm mỗi chỉ định giá trị của mộtsố lượng 16-bit, các con số ở chấm thập phân phần mỗi chỉ định giá trị của một octet. Tất nhiên, không nén có thể được sử dụng với số ở trên để sản xuất mộttương đương đại tràng hex chuỗi trông khá giống với một địa chỉ IPv4:Cuối cùng, IPv6 kéo dài giống như CIDR ký hiệu bằng cách cho phép một địa chỉ để được theo sau bởimột dấu gạch chéo và một số nguyên xác định một số bit. Ví dụ,chỉ định 60 bit đầu tiên của địa chỉ hoặc 12ABOOOOOOOOCD3in hệ thập lục phân.612 tương lai TCP/IP (nếu ' v6) chap 3333.18 ba loại địa chỉ IPv6 cơ bảnNhư IPv4, IPv6 kết hợp một địa chỉ với các kết nối mạng cụ thể, không phải vớimột máy tính cụ thể. Do đó, địa chỉ bài tập là tương tự như IPv4: một bộ định tuyến IPv6 cóhai hoặc nhiều địa chỉ, và một loạt IPv6 với một mạng kết nối nhu cầu chỉ có mộtđịa chỉ. IPv6 cũng giữ lại (và kéo dài) hệ thống phân cấp địa chỉ IPv4 trong đó một thể chấtmạng được gán một tiền tố. Tuy nhiên, để thực hiện chuyển nhượng địa chỉ và sửa đổidễ dàng hơn, IPv6 cho phép nhiều tiền tố được gán cho một mạng nhất định, và cho phép mộtCác máy tính có nhiều, đồng thời địa chỉ được gán cho một giao diện nhất định.Ngoài việc cho phép nhiều, đồng thời địa chỉ cho một kết nối mạng,IPv6 mở rộng, và trong một số trường hợp hợp nhất, địa chỉ IPv4 đặc biệt. Nói chung, một địa chỉ đích trên một datagram rơi vào một trong ba loại:Unicast địa chỉ đích xác định một máy vi tính (máy chủhoặc bộ định tuyến); datagram nên được chuyển đến đíchcùng một con đường ngắn nhất.Anycast đích là một tập hợp các máy tính, có thể tại khác nhauđịa điểm, tất cả chia sẻ một địa chỉ duy nhất; datagramnên được chuyển cùng một con đường ngắn nhất và chuyển giao cho chính xác một thành viên của nhóm (tức là, các thành viên gần nhất)?.Phát đa hướng đích là một tập hợp các máy tính, có thể tại nhiềuđịa điểm. Một bản sao của datagram sẽ được gửi đếnmỗi thành viên của nhóm bằng cách sử dụng phần cứng phát đa hướng hoặcphát sóng nếu khả thi.33.19 nhị nguyên của phát sóng và phát đa hướngIPv6 không sử dụng phát sóng điều khoản hoặc đạo diễn phát sóng để chỉ gửi đếnTất cả máy tính trên một mạng vật lý hoặc một mạng con IP hợp lý. Thay vào đó, nó sử dụng thuật ngữphát đa hướng, và xử lý phát sóng như là một hình thức đặc biệt của multicast. Sự lựa chọn có vẻ lẻcho bất cứ ai những người hiểu được phần cứng mạng vì thêm phần cứng công nghệ hỗ trợ phát sóng hơn hỗ trợ multicast. Trong thực tế, một kỹ sư phần cứng có khả năng để xemmulticasting như là một hình thức hạn chế của phát sóng - phần cứng sẽ gửi một phát đa hướnggói cho tất cả các máy tính trên mạng chính xác giống như một gói phát sóng, và giao diệnCác phần cứng trên mỗi máy tính bộ lọc tất cả các gói dữ liệu phát đa hướng ngoại trừ những người mà phần mềm cóhướng dẫn phần cứng giao diện để chấp nhận.Trong lý thuyết, sự lựa chọn giữa phát đa hướng và giới hạn các hình thức phát sóng là không thích hợpbởi vì một trong những có thể được mô phỏng với người kia. Đó là, phát sóng và multicasting làduals của nhau mà cung cấp chức năng tương tự. Để hiểu lý do tại sao, hãy xem xétlàm thế nào để mô phỏng một với người kia. Nếu phát sóng có sẵn, một gói có thể được gửiđể một nhóm bằng cách gửi nó cho tất cả các máy và bố trí cho các phần mềm trên mỗi máy tính đểquyết định chấp nhận hoặc loại bỏ các gói. Nếu phát đa hướng có sẵn, một?Anycast địa chỉ đã được trước đây gọi là địa chỉ cụm.SEC. 33.19 Duality BroadcastAnd Multicast 613gói có thể được gửi đến tất cả các máy bằng cách sắp xếp cho tất cả các máy để lắng nghe mộtphát đa hướng nhóm tương tự như tất cả máy chủ nhóm thảo luận trong chương 17.33,20 một sự lựa chọn kỹ thuật và mô phỏng phát sóngBiết rằng phát sóng và multicasting là lý thuyết duals của nhaukhông giúp lựa chọn giữa chúng. Để xem tại sao các nhà thiết kế của IPv6 đã chọn multicasting là trừu tượng Trung tâm thay vì phát sóng, hãy xem xét các ứng dụng thay vìnhìn vào phần cứng nằm bên dưới. Một ứng dụng hoặc cần phải giao tiếp với mộtmột ứng dụng hoặc với một nhóm các ứng dụng. Giao tiếp trực tiếp được xử lýtốt nhất thông qua unicast; Nhóm giao tiếp được giải quyết tốt nhất bởi phát đa hướng hoặc phát sóng. Đểcung cấp nhiều tính linh hoạt, thành viên nhóm không nên được xác định bởi mạngkết nối, vì thành viên của nhóm có thể cư trú tại địa điểm tùy ý. Bằng cách sử dụng phát sóngcho tất cả các nhóm giao tiếp không quy mô để xử lý một internet lớn như toàn cầuInternet.Không ngạc nhiên, các nhà thiết kế trước xác định một số địa chỉ phát đa hướng có thểsử dụng thay cho một địa chỉ IPv4 mạng phát sóng. Vì vậy, ngoài địa chỉ unicast riêng của mình, mỗi router là cần thiết để chấp nhận các gói dữ liệu gửi cho nhóm phát đa hướng A11 router cho môi trường địa phương.33.21 đề nghị chuyển nhượng Space địa chỉ IPv6Các câu hỏi về làm thế nào để phân vùng không gian địa chỉ IPv6 đã tạo ra nhiều cuộc thảo luận. Có hai vấn đề Trung tâm: làm thế nào để quản lý địa chỉ chuyển nhượng và làm thế nào đểánh xạ địa chỉ cho một tuyến đường. Vấn đề đầu tiên tập trung vào vấn đề thực tế của đặt ramột hệ thống phân cấp của chính quyền. Không giống như Internet hiện tại, sử dụng một hệ thống phân cấp hai cấpcủa mạng tiền tố (được chỉ định bởi các cơ quan Internet) và máy chủ lưu trữ hậu tố (được chỉ định bởi cáctổ chức), không gian địa chỉ lớn trong IPv6 cho phép một hệ thống phân cấp đa cấp hoặc nhiều phân cấp. Vấn đề thứ hai tập trung vào tính toán hiệu quả. Độc lập củaHệ thống phân cấp thẩm quyền chỉ định địa chỉ, một router phải kiểm tra mỗi datagramvà chọn một con đường đến đích. Để giữ cho chi phí của bộ định tuyến tốc độ cao thấp, cácchế biến thời gian cần thiết để lựa chọn một con đường phải được giữ nhỏ.Như con số 33,8 cho thấy, các nhà thiết kế của IPv6 đề xuất gán địa chỉ các lớp học trong mộttheo cách tương tự như các đề án được sử dụng cho Pv4. Mặc dù 8 bit đầu tiên của một địa chỉđủ để xác định loại của nó, không gian địa chỉ không phân chia thành nhiều phần bằng nhauKích thước.614 tương lai TCPIIP (nếu ' v6) chap 33Tiền tố nhị phân loại địa chỉ một phần của không gian địa chỉResewed (IPv4 tương thích)UnassignedĐịa chỉ NSAPĐịa chỉ IPXUnassignedUnassignedUnassignedAggregatable toàn cầu UnicastUnassignedUnassignedUnassignedUnassignedUnassignedUnassignedUnassignedUnassignedUnassignedUnassignedĐịa chỉ liên kết địa phương UnicastĐịa chỉ trang web-Local UnicastĐịa chỉ phát đa hướng
đang được dịch, vui lòng đợi..
The two high-order bits of each option TYPE field specify how a host or router
should dispose of the datagram if it does not understand the option:
Bits In Type Meaning
00 Skip this option
01 Discard datagram; do not send ICMP message
10 Discard datagram; send ICMP message to source
11 Discard datagram; send ICMP for non-multicast
In addition, the third bit in the TYPE field specifies whether the option can change
in transit. Having such information is important for authentication-the contents of an
option that can change in transit are treated as zeroes for purposes of authentication.
33.16 Size Of The IPv6 Address Space
In IPv6 each address occupies 16 octets, four times the size of an IPv4 address.
The large address space guarantees that IPv6 can tolerate any reasonable address assignment scheme. In fact, if the designers decide to change the addressing scheme later, the
address space is sufficiently large to accommodate a reassignment.
It is difficult to comprehend the size of the IPv6 address space. One way to look
at it relates the magnitude to the size of the population: the address space is so large
that every person on the planet can have sufficient addresses to have their own internet
as large as the current Internet. A second way to think of IPv6 addressing relates it to
the physical space available: the earth's surface has approximately 5.1x 10' square kilometers, meaning that there are over addresses per square meter of the earth's surface. Another way to understand the size relates it to address exhaustion. For example,
consider how long it would take to assign all possible addresses. A 16-octet integer can
hold 2'28values. Thus, the address space is greater than 3.4 x 10". If addresses are assigned at the rate of one million addresses every microsecond, it would take over 1020
years to assign all possible addresses.
33.17 IPv6 Colon Hexadecimal Notation
Although it solves the problem of having insufficient capacity, the large address
size poses an interesting new problem: humans who maintain internets must read, enter,
and manipulate such addresses. Obviously, binary notation is untenable. However, the
dotted decimal notation used for IPv4 does not make such addresses sufficiently compact either. To understand why, consider an example 128-bit number expressed in dotted decimal notation:
Sec. 33.17 IPv6 Colon Hexadecimal Notation 61 1
To help make address slightly more compact and easier to enter, the IPv6 designers
propose using colon hexadecimal notation (abbreviated colon hex) in which the value of
each 16-bit quantity is represented in hexadecimal separated by colons. For example,
when the value shown above in dotted decimal notation has been translated to colon hex
nqtation and printed using the same spacing, it becomes:
Colon hex notation has the obvious advantage of requiring fewer digits and fewer
separator characters than dotted decimal. In addition, colon hex notation includes two
techniques that make it extremely useful. First, colon hex notation allows zero
compression in which a string of repeated zeros is replaced by a pair of colons. For example, the address:
FF05:0:0:0:0:0:0:B3
can be written:
To ensure that zero compression produces an unambiguous interpretation, the proposal specifies that it can be applied only once in any address. Zero compression is
especially useful when used with the proposed address assignment scheme because
many addresses will contain contiguous strings of zeros. Second, colon hex notation incorporates dotted decimal suffies; we will see that such combinations are intended to
be used during the transition from IPv4 to IPv6. For example, the following string is
valid colon hex notation:
Note that although the numbers separated by colons each specify the value of a
16-bit quantity, numbers in the dotted decimal portion each specify the value of one octet. Of course, zero compression can be used with the number above to produce an
equivalent colon hex string that looks quite similar to an IPv4 address:
Finally, IPv6 extends CIDR-like notation by allowing an address to be followed by
a slash and an integer that specifies a number of bits. For example,
specifies the first 60 bits of the address or 12ABOOOOOOOOCD3in hexadecimal.
612 The Future Of TCP/IP (IF'v6) Chap. 33
33.18 Three Basic IPv6 Address Types
Like IPv4, IPv6 associates an address with a specific network connection, not with
a specific computer. Thus, address assignments are similar to IPv4: an IPv6 router has
two or more addresses, and an IPv6 host with one network connection needs only one
address. IPv6 also retains (and extends) the IPv4 address hierarchy in which a physical
network is assigned a prefix. However, to make address assignment and modification
easier, IPv6 permits multiple prefixes to be assigned to a given network, and allows a
computer to have multiple, simultaneous addresses assigned to a given interface.
In addition to permitting multiple, simultaneous addresses per network connection,
IPv6 expands, and in some cases unifies, IPv4 special addresses. In general, a destination address on a datagram falls into one of three categories:
Unicast The destination address specifies a single computer (host
or router); the datagram should be routed to the destination
along a shortest path.
Anycast The destination is a set of computers, possibly at different
locations, that all share a single address; the datagram
should be routed along a shortest path and delivered to exactly one member of the group (i.e., the closest member)?.
Multicast The destination is a set of computers, possibly at multiple
locations. One copy of the datagram will be delivered to
each member of the group using hardware multicast or
broadcast if viable.
33.19 The Duality Of Broadcast And Multicast
IPv6 does not use the terms broadcast or directed broadcast to refer to delivery to
all computers on a physical network or to a logical IP subnet. Instead, it uses the term
multicast, and treats broadcast as a special form of multicast. The choice may seem odd
to anyone who understands network hardware because more hardware technologies support broadcast than support multicast. In fact, a hardware engineer is likely to view
multicasting as a restricted form of broadcasting- the hardware sends a multicast
packet to all computers on the network exactly like a broadcast packet, and the interface
hardware on each computer filters all multicast packets except those that software has
instructed the interface hardware to accept.
In theory, the choice between multicast and limited forms of broadcast is irrelevant
because one can be simulated with the other. That is, broadcasting and multicasting are
duals of one another that provide the same functionality. To understand why, consider
how to simulate one with the other. If broadcast is available, a packet can be delivered
to a group by sending it to all machines and arranging for software on each computer to
decide whether to accept or discard the incoming packet. If multicast is available, a
?Anycast addresses were formerly known as cluster addresses.
Sec. 33.19 The Duality Of BroadcastAnd Multicast 613
packet can be delivered to all machines by arranging for all machines to listen to one
multicast group similar to the all hosts group discussed in Chapter 17.
33.20 An Engineering Choice And Simulated Broadcast
Knowing that broadcasting and multicasting are theoretical duals of one another
does not help choose between them. To see why the designers of IPv6 chose multicasting as the central abstraction instead of broadcasting, consider applications instead of
looking at the underlying hardware. An application either needs to communicate with a
single application or with a group of applications. Direct communication is handled
best via unicast; group communication is handled best by multicast or broadcast. To
provide the most flexibility, group membership should not be determined by network
connections, because group members can reside at arbitrary locations. Using broadcast
for all group communication does not scale to handle an internet as large as the global
Internet.
Not surprisingly, the designers pre-define some multicast addresses that can be
used in place of an IPv4 network broadcast address. Thus, in addition to its own unicast address, each router is required to accept packets addressed to the A11 Routers multicast groups for its local environment.
33.21 Proposed IPv6 Address Space Assignment
The question of how to partition the IPv6 address space has generated much discussion. There are two central issues: how to manage address assignment and how to
map an address to a route. The first issue focuses on the practical problem of devising
a hierarchy of authority. Unlike the current Internet, which uses a two-level hierarchy
of network prefix (assigned by the Internet authority) and host suffix (assigned by the
organization), the large address space in IPv6 permits a multi-level hierarchy or multiple hierarchies. The second issue focuses on computational efficiency. Independent of
the hierarchy of authority that assigns addresses, a router must examine each datagram
and choose a path to the destination. To keep the cost of high-speed routers low, the
processing time required to choose a path must be kept small.
As Figure 33.8 shows, the designers of IPv6 propose assigning address classes in a
way similar to the scheme used for Pv4. Although the first 8 bits of an address are
sufficient to identify its type, the address space is not partitioned into sections of equal
size.
614 The Future Of TCPIIP (IF'v6) Chap. 33
Binary Prefix Type Of Address Part Of Address Space
Resewed (IPv4 compatibility)
Unassigned
NSAP Addresses
IPX Addresses
Unassigned
Unassigned
Unassigned
Aggregatable Global Unicast
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Link-Local Unicast Addresses
Site-Local Unicast Addresses
Multicast Addresses
should dispose of the datagram if it does not understand the option:
Bits In Type Meaning
00 Skip this option
01 Discard datagram; do not send ICMP message
10 Discard datagram; send ICMP message to source
11 Discard datagram; send ICMP for non-multicast
In addition, the third bit in the TYPE field specifies whether the option can change
in transit. Having such information is important for authentication-the contents of an
option that can change in transit are treated as zeroes for purposes of authentication.
33.16 Size Of The IPv6 Address Space
In IPv6 each address occupies 16 octets, four times the size of an IPv4 address.
The large address space guarantees that IPv6 can tolerate any reasonable address assignment scheme. In fact, if the designers decide to change the addressing scheme later, the
address space is sufficiently large to accommodate a reassignment.
It is difficult to comprehend the size of the IPv6 address space. One way to look
at it relates the magnitude to the size of the population: the address space is so large
that every person on the planet can have sufficient addresses to have their own internet
as large as the current Internet. A second way to think of IPv6 addressing relates it to
the physical space available: the earth's surface has approximately 5.1x 10' square kilometers, meaning that there are over addresses per square meter of the earth's surface. Another way to understand the size relates it to address exhaustion. For example,
consider how long it would take to assign all possible addresses. A 16-octet integer can
hold 2'28values. Thus, the address space is greater than 3.4 x 10". If addresses are assigned at the rate of one million addresses every microsecond, it would take over 1020
years to assign all possible addresses.
33.17 IPv6 Colon Hexadecimal Notation
Although it solves the problem of having insufficient capacity, the large address
size poses an interesting new problem: humans who maintain internets must read, enter,
and manipulate such addresses. Obviously, binary notation is untenable. However, the
dotted decimal notation used for IPv4 does not make such addresses sufficiently compact either. To understand why, consider an example 128-bit number expressed in dotted decimal notation:
Sec. 33.17 IPv6 Colon Hexadecimal Notation 61 1
To help make address slightly more compact and easier to enter, the IPv6 designers
propose using colon hexadecimal notation (abbreviated colon hex) in which the value of
each 16-bit quantity is represented in hexadecimal separated by colons. For example,
when the value shown above in dotted decimal notation has been translated to colon hex
nqtation and printed using the same spacing, it becomes:
Colon hex notation has the obvious advantage of requiring fewer digits and fewer
separator characters than dotted decimal. In addition, colon hex notation includes two
techniques that make it extremely useful. First, colon hex notation allows zero
compression in which a string of repeated zeros is replaced by a pair of colons. For example, the address:
FF05:0:0:0:0:0:0:B3
can be written:
To ensure that zero compression produces an unambiguous interpretation, the proposal specifies that it can be applied only once in any address. Zero compression is
especially useful when used with the proposed address assignment scheme because
many addresses will contain contiguous strings of zeros. Second, colon hex notation incorporates dotted decimal suffies; we will see that such combinations are intended to
be used during the transition from IPv4 to IPv6. For example, the following string is
valid colon hex notation:
Note that although the numbers separated by colons each specify the value of a
16-bit quantity, numbers in the dotted decimal portion each specify the value of one octet. Of course, zero compression can be used with the number above to produce an
equivalent colon hex string that looks quite similar to an IPv4 address:
Finally, IPv6 extends CIDR-like notation by allowing an address to be followed by
a slash and an integer that specifies a number of bits. For example,
specifies the first 60 bits of the address or 12ABOOOOOOOOCD3in hexadecimal.
612 The Future Of TCP/IP (IF'v6) Chap. 33
33.18 Three Basic IPv6 Address Types
Like IPv4, IPv6 associates an address with a specific network connection, not with
a specific computer. Thus, address assignments are similar to IPv4: an IPv6 router has
two or more addresses, and an IPv6 host with one network connection needs only one
address. IPv6 also retains (and extends) the IPv4 address hierarchy in which a physical
network is assigned a prefix. However, to make address assignment and modification
easier, IPv6 permits multiple prefixes to be assigned to a given network, and allows a
computer to have multiple, simultaneous addresses assigned to a given interface.
In addition to permitting multiple, simultaneous addresses per network connection,
IPv6 expands, and in some cases unifies, IPv4 special addresses. In general, a destination address on a datagram falls into one of three categories:
Unicast The destination address specifies a single computer (host
or router); the datagram should be routed to the destination
along a shortest path.
Anycast The destination is a set of computers, possibly at different
locations, that all share a single address; the datagram
should be routed along a shortest path and delivered to exactly one member of the group (i.e., the closest member)?.
Multicast The destination is a set of computers, possibly at multiple
locations. One copy of the datagram will be delivered to
each member of the group using hardware multicast or
broadcast if viable.
33.19 The Duality Of Broadcast And Multicast
IPv6 does not use the terms broadcast or directed broadcast to refer to delivery to
all computers on a physical network or to a logical IP subnet. Instead, it uses the term
multicast, and treats broadcast as a special form of multicast. The choice may seem odd
to anyone who understands network hardware because more hardware technologies support broadcast than support multicast. In fact, a hardware engineer is likely to view
multicasting as a restricted form of broadcasting- the hardware sends a multicast
packet to all computers on the network exactly like a broadcast packet, and the interface
hardware on each computer filters all multicast packets except those that software has
instructed the interface hardware to accept.
In theory, the choice between multicast and limited forms of broadcast is irrelevant
because one can be simulated with the other. That is, broadcasting and multicasting are
duals of one another that provide the same functionality. To understand why, consider
how to simulate one with the other. If broadcast is available, a packet can be delivered
to a group by sending it to all machines and arranging for software on each computer to
decide whether to accept or discard the incoming packet. If multicast is available, a
?Anycast addresses were formerly known as cluster addresses.
Sec. 33.19 The Duality Of BroadcastAnd Multicast 613
packet can be delivered to all machines by arranging for all machines to listen to one
multicast group similar to the all hosts group discussed in Chapter 17.
33.20 An Engineering Choice And Simulated Broadcast
Knowing that broadcasting and multicasting are theoretical duals of one another
does not help choose between them. To see why the designers of IPv6 chose multicasting as the central abstraction instead of broadcasting, consider applications instead of
looking at the underlying hardware. An application either needs to communicate with a
single application or with a group of applications. Direct communication is handled
best via unicast; group communication is handled best by multicast or broadcast. To
provide the most flexibility, group membership should not be determined by network
connections, because group members can reside at arbitrary locations. Using broadcast
for all group communication does not scale to handle an internet as large as the global
Internet.
Not surprisingly, the designers pre-define some multicast addresses that can be
used in place of an IPv4 network broadcast address. Thus, in addition to its own unicast address, each router is required to accept packets addressed to the A11 Routers multicast groups for its local environment.
33.21 Proposed IPv6 Address Space Assignment
The question of how to partition the IPv6 address space has generated much discussion. There are two central issues: how to manage address assignment and how to
map an address to a route. The first issue focuses on the practical problem of devising
a hierarchy of authority. Unlike the current Internet, which uses a two-level hierarchy
of network prefix (assigned by the Internet authority) and host suffix (assigned by the
organization), the large address space in IPv6 permits a multi-level hierarchy or multiple hierarchies. The second issue focuses on computational efficiency. Independent of
the hierarchy of authority that assigns addresses, a router must examine each datagram
and choose a path to the destination. To keep the cost of high-speed routers low, the
processing time required to choose a path must be kept small.
As Figure 33.8 shows, the designers of IPv6 propose assigning address classes in a
way similar to the scheme used for Pv4. Although the first 8 bits of an address are
sufficient to identify its type, the address space is not partitioned into sections of equal
size.
614 The Future Of TCPIIP (IF'v6) Chap. 33
Binary Prefix Type Of Address Part Of Address Space
Resewed (IPv4 compatibility)
Unassigned
NSAP Addresses
IPX Addresses
Unassigned
Unassigned
Unassigned
Aggregatable Global Unicast
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Unassigned
Link-Local Unicast Addresses
Site-Local Unicast Addresses
Multicast Addresses
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