|We also believe that is critical that we maintain the tag and type as 4 byte lengths would would vigorously oppose any attempt to reduce their length.|
The simplest option (short of giving up on 4 octet alignment) may be #2: pad fields out to 4 octet boundaries.
chongo () /\oo/\
On 2009-Apr-27, at 19:08, Robert Lockhart wrote:
I see several reasons for ensuring that we keep both the Tag and type their current 4 byte lengths. We are still under 255 tags (156) but we are planning to define server to server at some point in the future (v2.0 and beyond) and there are extensions that we want to allow for. I wouldn't want to hamstring ourselves by not planning for it now. Plus we don't know what may change for the clients in later versions of the standard that might be useful to this type of device. I think we should set the expectation now before people potentially code themselves into a corner by accident.
The other issue we would have is that it would limit or remove the number of extensions that we might want to allow either via registration or assignment in the future for other standards that want to make use of KMIP.
I agree with the four byte boundaries but don't want to limit ourselves. If we do this I like Glen's suggestion in 1C and I don't know how soon we will hit 256 types but I am thinking it will take a little longer to generate them than it would to create another 99 new tags (thinking of server to server & extensions).
My 2 cents for the day,
Senior Solutions Architect THALES Information Systems Security ------------------------------------------------------- T: +1 408 457 7711 (Direct) Matt, I do see value in revising the TTLV protocol to align it to 4 byte boundaries, I think we just have to work through the details to make sure we find the best trade-off. Here are my thoughts: Your proposal devolves into 2 main ideas which are essentially separable:
As far as #1 above, I would suggest we consider 1C. As far as #2, I don't see how to improve on your proposal.
- You observe that the present composition is essentially a 4 byte Tag followed by a 1 byte Type field (i.e. (4-byte Tag = 0x42.00.00.YZ)) which immediately throws off the 4 byte alignment, and this is a first thing that needs to be addressed. There are various ways to address that:
- The specific proposal you illustrated on slide 6 is option 1.1 from slide 5 -- namely to drop the leading byte of the four Tag field, which is 0x42 in all cases today, making it effectively a 3 byte Tag. Then you transpose the Type and Tag fields, giving:
- (1-byte Type) | (3-byte Tag = 0x00.00.YZ)
- => Pro: this decreases the number of bytes which need to be communicated to give both Tag and Type by one
- => Con: in response to which a Tony and Jon observed that this eliminates the 0x42 which has proven useful in the testing so far
- Someone else (didn't catch the name) suggested that instead we just pad Tag out to 4 bytes, or effectively:
- (4-byte Tag = 0x42.00.00.YZ) | 4-byte Type)
- => Pro: this preserves the 0x42.00.00 that can be used to distinguish the start of a new Tag, and hence the start of a new TTLV message
- => Con: this increases the number of bytes which need to be communicated to give both Tag and Type by three
- A third option which occurs to me is a variation of option 1.2 from your slide 5, namely to cut a different byte from the Tag (0x00 instead of 0x42) and then not transpose or effectively:
- (4-byte Tag = 0x42.00.YZ) | 1-byte Type)
- => Pro: this decreases the number of bytes which need to be communicated to give both Tag and Type by one (i.e. same as 1A)
- => Pro: this preserves the first two of the distinguishable bytes (i.e. 0x42.00) that can be used to distinguish the start of a new Tag, and hence the start of a new TTLV message, though one could argue that is slightly less than the 3 bytes that exist today.
- Similarly make the Value modulo 4 bytes by:
- Padding binary fields out to the next 4 byte boundary
- Making BIGNUM fields an integer number of 4 byte fields long
IBM Distinguished Engineer
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If you have any preferences regarding the encoding choices from last
Friday's binary alignment presentation, please send them to me by
tomorrow so that I can incorporate them into the first revision of the
proposal. I plan to have the full proposal ready for discussion during
Thursday's meeting, if there is time available on the agenda.
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