Re: [netmod] 6991bis: address-with-prefix-length

Kristian Larsson <kristian@spritelink.net> Thu, 18 April 2019 20:34 UTC

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From: Kristian Larsson <kristian@spritelink.net>
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Subject: Re: [netmod] 6991bis: address-with-prefix-length
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On 2019-04-18 14:18, Martin Bjorklund wrote:
> Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> wrote:
>> On Thu, Apr 18, 2019 at 10:41:11AM +0200, Ladislav Lhotka wrote:
>>>>>
>>>>> I am not in favour of adding this type. Having ip-prefix next to
>>>>> ip-address-and-prefix is confusing.
>>>>
>>>> Confusing or not, they are NOT interchangeable and actually do different
>>>> things, which is why both are needed. There's plenty of precedence to
>>>
>>> I actually agree with you. It is a historical accident that these
>>> two different things got mixed up (and some vendors contributed to
>>> this). I would argue that
>>>
>>> - IP prefix is a set of IP addresses, and as such can be thought of
>>>    as a single entity.
>>>
>>> - IP address and subnet mask/prefix are two separate things, the
>>>    latter being an instruction for routing to *other* destination
>>>    addresses.
>>
>> I think we should be pragmatic. There are other common types that are
>> in fact constructed out of simpler types, date-and-time is a prime
>> example of a type constructed out of a date value and a time value.
> 
> I think that date-and-time represents one thing - a single point in
> time.

Convenient for users to enter a single point in time in terms of year, 
month, day, hours, minutes and seconds, perhaps. But not as convenient 
for a program that needs to compare two date-and-times. Clearly for a 
program comparing times against each other we must represent a point in 
time as the number of vibrations of cesium since an arbitrarily chosen 
epoch.


>> It
>> is sometimes convenient to treat something that is in fact constructed
>> as an atomic value.
> 
> Convenient for users that enter these values, perhaps.  But not as
> convenient for a program (or a filter) that needs one of the combined
> values.

Really? Are you using a text representation of IP addresses when you 
handle them in your program?

If you are to deal with IP addresses, prefixes etc in a robust way in 
your program, you need an internal datatype that understands what an 
address is - it needs to handle it as bits and massage it to any other 
presentation you want. It needs to understand relevant comparisons and 
operations, like is prefix A contained in prefix B?

Or if we are dealing with time, then a class that understands leap 
years, leap seconds, time zones etc can be fairly useful so you don't 
have to fall in any of those pitfalls.

I don't think we choose a format or representation in our YANG models 
primarily to suit the algorithmic needs of a computer program, in that 
case an IPv4 address would just be a uint32 and not the dotted quad 
format we have today.


>  For example, suppose I want to find all entries with a given
> prefix; that is non-trivial with a combined ip-address-and-prefix
> type.

This seems like a very weird example since it doesn't support your case; 
it is not easier with two separate leaves!?

The alternative to using ip-address-and-prefix-length would be to use 
two leaves; one for the address and the other for the subnet mask / 
prefix-length.

combined:
ip-address-and-prefix-length: 1.2.3.4/24

split:
address: 1.2.3.4
prefix-length: 24

Say we have another interface with address '1.2.3.5' (prefix-length 24 
still). In what way is it easier to determine these are part of the same 
IP prefix / subnetwork by having the values split in two leaves? There 
is no text operation that can easily do this for us - we need to parse 
the values with some class / type in our programming language that helps 
us make this comparison so in what way is ip-address-and-prefix-length 
worse?

Let us look at some examples how this is typically done. Again, 
postgresql has the 'inet' type. From the docs:

"The input format for this type is address/y where address is an IPv4 or 
IPv6 address and y is the number of bits in the netmask. If the /y 
portion is missing, the netmask is 32 for IPv4 and 128 for IPv6, so the 
value represents just a single host. On display, the /y portion is 
suppressed if the netmask specifies a single host."

It wants it combined, which means the two leaves need to be formatted 
into something that looks like 1.2.3.4/24.

Python ipaddress.IPv4, from example:

   interface = IPv4Interface('192.0.2.5/24')

Same thing. Rust ipaddress? Same thing. Go net? Same. Our internal 
classes that compute IP addressing? Same thing. It seems most of the 
datatypes that natively handle this kind of information takes a text 
format like 1.2.3.4/24 as input (and not as separate fields), which is 
what is being suggested we have a datatype for.

   kll