Re: [IPFIX] basicList clarification

[Paul Aitken <[e-mail]>]

Gerald,

> Hi Paul,
>
> Thanks for the responses. It clarifies for most part; while I would 
> also like to clarify few things on my question. (tagged [Gerald] inline)

See [PJ] inline...


> So it appears we would need to define Y as IE and mark X as basicList 
> of Y.

What you're proposing is possible if there's a good reason. However we 
want to avoid unnecessary duplication of existing IEs into new "list of" 
IEs as much as possible, so you should only create X if there's a strong 
justification (eg it models an existing object).


> Thanks again
>
> On Tue, Dec 15, 2015 at 6:44 PM, Paul Aitken <[e-mail] 
> <mailto:[e-mail]>> wrote:
>
>     Gerald,
>
>>     Hi all,
>>
>>     It is my understanding from the RFCs that basicList type is an
>>     encoding for list of IPFIX Information Elements.
>
>     Yes, where the list contains zero or more instances of a single IE.
>
>
>>     To create the template much more strongly typed, we have defined
>>     a custom (enterprise) Information Element with a new ID (say X),
>>     which is defined as a basicList of specific items (eg.,
>>     InterfaceName).
>>
>>     My question to you all is: Is it mandatory that we have a
>>     separate information element called InterfaceName (say ID Y) and
>>     define X as a basicList of Y ?
>
>     Yes, that's how the protocol is defined.
>
>
>>     For our need, it is enough for us, that we define X as a
>>     basicList of string (as X is already strongly defined). But
>>     string is a type, not info element.
>>
>>     We would like to retain X (instead of defining a template
>>     directly with a standard basicList). And we don't have any other
>>     need to define a new Information Element Y.
>
>     Y defines the list elements which tells the Collector that the
>     list contains strings, and it defines the semantics of those
>     strings - eg whether they contain interface names, user names, a
>     list of selectorNames, or a list of Application names. Then a
>     basicList of Y tells the Collector that you're exporting zero or
>     more of them.
>
> [Gerald] This is definitely a choice. However, as I mentioned, this 
> makes the template weakly typed (ie., the element is just #291 
> basicList). We would like to enforce stronger types at the template 
> itself (this was the motive behind defining X)

[PJ] This was an objection from the Collector side when we were writing 
the IPFIX Structured Data draft, because the collector can no longer 
tell what information it's going to receive ahead of time (just "a 
list"); it has to check each Data Record to know what information the 
list contains. On the other hand, since the basicList is essentially a 
wrapper around any other IE, it's not necessary to create new 
strongly-defined list IEs for each of the existing (and potentially all 
future) IEs.


>     Perhaps you should define Y as a single instance of X (so Y is
>     well defined), then redefine X as a basicList of Y. Or if you
>     don't want/need to define both X and Y, then use the basicList IE
>     #291 to export a basicList of (strongly defined) Y.
>
> [Gerald] Yes, this is also a choice; but we don't need Y for any other 
> reason except to fit into the basicList detail. So wanted to know if 
> that can be avoided. I got my answer: No (if strongly typed templates).

[PJ] Strongly typed templates would require a new "list of" IE for every 
existing (and future) IE that was to become a list, which is incredibly 
wasteful.


>>     I realize that the basicList encoding has a information-element
>>     ID of the item in payload -- is this mandatory?
>
>     Yes. The encoding would not be interoperable without it.
>

[PJ] Here I understood the question to be whether the IE ID field was 
required in the basicList encoding - ie, it seemed that you were 
proposing a new encoding where, since X is already strong defined as a 
basicList of Y, there's no need to include Y in the export.


>>     Or for a strongly typed item X, we could expect the IPFIX parser
>>     to be aware of contents (interface name) by contract?
>
>     No. Every collector which implements RFC 6313 (IPFIX Structured
>     Data) should be able to decode a basicList of Y. However, the
>     mechanism which you propose requires the collector to have special
>     knowledge of the internals of X, which is not scalable. In short,
>     such an Exporter would not be interoperable with all Collectors.
>
> [Gerald] X is not a different encoding. X is a basicList (sort of more 
> strongly typed. ie., X is a basicList of Y). So the encoding is just 
> the same as #291 standard basicList. But when the template declares a 
> element as X, it is clearly typed even without having to look into the 
> data (or it also doesn't let the exporter send X of Z on the same 
> template).

[PJ] What you're proposing is possible and may be useful in some 
circumstances. For example, for the MIB export draft, we've defined new 
IEs (#443, #444) which are subTemplateLists, in order to give strong 
typing information because the IE contains a row from a table.

(See https://tools.ietf.org/html/draft-ietf-ipfix-mib-variable-export-10 
and http://www.iana.org/assignments/ipfix/ipfix.xhtml)

P.

>
>>     Pls let me know if my question is unclear.
>>
>>     Thanks in advance.
>>     - Gerald Naveen
>
>
>     The custom IE which you've defined must contain meta-data about
>     the list such as an element count, element lengths, delimiters or
>     separators. It creates a new way of encoding list information
>     which the collector must also understand in addtion to the
>     basicList encoding, which is not useful. In short, you should not
>     create list type elements; instead, use the basicList mechanism.
>
> [Gerald] Agree. We aren't trying to redefine basicList encoding for sure.
>
>
>>
>>
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>