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Update from Peter Moody.

This commit is contained in:
Brett Cannon 2009-09-27 19:38:05 +00:00
parent 77fb82981e
commit 989165fe9e
1 changed files with 94 additions and 37 deletions
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pep-3144.txt
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@ -31,40 +31,63 @@ Motivation:
Rationale:
ipaddr was designed with the goal of abstracting out as much of the common
functionality as possible. As mentioned earlier, the similarities between
addresses and networks, IPV6 and IPV4 allows much code to be reused since
python allows for easy (and clean) multiple inheritance. Methods which are
specific to IPV4 or IPV6, addresses or networks are inherited from
appropriately named classes (Basev4, Basev6, BaseNet, BaseIP, etc) to
provide the full functionality of IPv4Address, IPv4Network, IPv6Address and
IPv6Network.
ipaddr was designed with a few basic principals in mind:
- IPv4 and IPv6 objects are distinct.
- IP addresses and IP networks are distinct.
- the library should be useful and the assumptions obvious to the network
programmer.
- IP networks should be treated as lists (as opposed to some other
python intrinsic) in so far as it makes sense.
- the library should be lightweight and fast without sacrificing
expected functionality.
- Distinct IPV4 and IPV6 objects.
While there are many similarities, IPV4 and IPV6 objects are fundamentally
different. The similarities allow for easy abstraction of certain
operations which affect the bits from both in the same manner, but their
differences mean attempts to combine them into one object would be like
trying to force a round peg into a square hole (or visa versa).
differences mean attempts to combine them into one object yield unexpected
results. According to Vint Cerf, "I have seen a substantial amount of
traffic about IPv4 and IPv6 comparisons and the general consensus is that
these are not comparable." (Vint Cerf [2]). For python versions >= 3.0,
this means that (<, >, <=, >=) comparison operations between IPv4 and IPv6
objects raise a TypeError per the Ordering Comparisons [3].
- Distinct network and address objects.
Many people think of IP addresses and IP networks as synonymous, while they
are however, distinct. An IPV4 address is a single 32 bit number while the
IPV4 address assigned to a networked computer is a 32 bit address and
associated network. Similarly, an IPV6 address is a 128 bit number while
an IPV6 address assigned to a networked computer is a 128 bit number and
associated network information. The similarities leads to easy abstraction
of some methods and properties, but there are obviously a number of
address/network specific properties which require they be distinct. For
instance, IP networks contain network address (the base address of the
network), broadcast addresses (the upper end of the network, also the
address to which every machine on a given network is supposed listen, hence
the name broadcast), supernetworks and subnetworks, etc. The individual
property addresses in an IP network obviously don't have the same
properties, they're simply 32 or 128 bit numbers.
An IPV4 address is a single 32 bit number while the IPV4 address assigned
to a networked computer is a 32 bit address and associated network.
Similarly, an IPV6 address is a 128 bit number while an IPV6 address
assigned to a networked computer is a 128 bit number and associated network
information. The similarities leads to easy abstraction of some methods
and properties, but there are obviously a number of address/network
specific properties which require they be distinct. For instance, IP
networks contain a network address (the base address of the network),
broadcast address (the upper end of the network, also the address to
which every machine on a given network is supposed listen, hence the name
broadcast), supernetworks and subnetworks, etc. The individual property
addresses in an IP network obviously don't have the same properties,
they're simply 32 or 128 bit numbers.
- Principal of least confusion for network programmers.
It should be understood that, above all, this module is designed with the
network administrator in mind. In practice, this means that a number of
assumptions are made with regards to common usage and the library prefers
the usefulness of accepted practice over strict adherence to RFCs. For
example, ipaddr accepts '192.168.1.1/24' as a network definition because
this is a very common way of describing an address + netmask despite the
fact that 192.168.1.1 is actually an IP address on the network
192.168.1.0/24. Strict adherence would require that networks have all of
the host bits masked to zero, which would require two objects to describe
that IP + network. In practice, a looser interpretation of a network is
a very useful if common abstraction, so ipaddr prefers to make this
available. For the developer who is concerned with strict adherence,
ipaddr provides an optional 'strict' boolean argument to the
IPv(4|6)Network constructors which guarantees that all host bits are masked
down.
- Treat network elements as lists (in so far as it's possible).
@ -84,7 +107,7 @@ Rationale:
While some network programmers will undoubtedly want more than this library
provides, keeping the functionality to strictly what's required from a IP
address manipulation module is critical to keeping the code fast, easily
comprehensible and extensible. I've tried to provide enough options in
comprehensible and extensible. It is a goal to provide enough options in
terms of functionality to allow the developer to easily do their work
without needlessly cluttering the library. Finally, It's important to note
that this design doesn't prevent subclassing or otherwise extending to meet
@ -114,15 +137,15 @@ Specification:
addresses and networks, both IPv4 and IPv6. In short, there is common
functionality shared between (ipaddr class names in parentheses):
1. all IP addresses and networks, both IPv4 and IPv6. (IPAddrBase)
1. all IP addresses and networks, both IPv4 and IPv6. (_IPAddrBase)
2. all IP addresses of both versions. (BaseIP)
2. all IP addresses of both versions. (_BaseIP)
3. all IP networks of both version. (BaseNet)
3. all IP networks of both version. (_BaseNet)
4. all IPv4 objects, both addresses and networks. (BaseV4)
4. all IPv4 objects, both addresses and networks. (_BaseV4)
5. all IPv6 objects, both addresses and networks. (BaseV6)
5. all IPv6 objects, both addresses and networks. (_BaseV6)
Seeing this as a clear hierarchy is important for recognizing how much
code is common between the four main classes. For this reason, ipaddr uses
@ -137,9 +160,9 @@ Specification:
might guess, return the appropriately typed address or network objects for
the given argument.
Finally, there is no meaningful natural ordering between IPv4 and IPv6
addresses ("these protocols are ships-in-the-night"), so rather than invent
a standard, ipaddr follows Ordering Comparisons [2] and returns a TypeError
Finally, as mentioned earlier, there is no meaningful natural ordering
between IPv4 and IPv6 addresses and networks [2]. Rather than invent a
standard, ipaddr follows Ordering Comparisons and returns a TypeError
when asked to compare objects of differing IP versions. In practice, there
are many ways a programmer may wish to order the addresses, so this this
shouldn't pose a problem for the developer who can easily write:
@ -167,7 +190,9 @@ Specification:
In [1]: IPNetwork('1.1.1.1').with_hostmask
Out[1]: '1.1.1.1/0.0.0.0'
the same applies to IPv6
the same applies to IPv6. It should be noted that netmasks and hostmasks
are not commonly used in IPv6, the methods exist for compatibility with
IPv4.
- Lazy evaluation combined with aggressive caching of network elements.
@ -308,17 +333,49 @@ Specification:
(the same methods exist for IPv4 networks and addresses, but they're
just stubs for returning the normal __str__ representation).
- Most other common operations.
It is a design goal to support all of the common operation expected from
an IP address manipulation module. As such, finding supernets, subnets,
address and network containment etc are all supported.
Reference Implementation:
A reference implementation is available at:
http://ipaddr-py.googlecode.com/svn/branches/2.0.x
http://ipaddr-py.googlecode.com/svn/trunk
References:
[1] http://bugs.python.org/issue3959
[2] http://docs.python.org/dev/3.0/whatsnew/3.0.html#ordering-comparisons
[2] Appealing to authority is a logical fallacy, but Vint Cerf is an
an authority who can't be ignored. Full text of the email follows:
"""
I have seen a substantial amount of traffic about IPv4 and IPv6
comparisons and the general consensus is that these are not comparable.
If we were to take a very simple minded view, we might treat these as
pure integers in which case there is an ordering but not a useful one.
In the IPv4 world, "length" is important because we take longest (most
specific) address first for routing. Length is determine by the mask,
as you know.
Assuming that the same style of argument works in IPv6, we would have
to conclude that treating an IPv6 value purely as an integer for
comparison with IPv4 would lead to some really strange results.
All of IPv4 space would lie in the host space of 0::0/96 prefix of
IPv6. For any useful interpretation of IPv4, this is a non-starter.
I think the only sensible conclusion is that IPv4 values and IPv6 values
should be treated as non-comparable.
Vint
"""
[3] http://docs.python.org/dev/3.0/whatsnew/3.0.html#ordering-comparisons
Copyright: