Re: [Softwires] PSID format as described in rfc 7597 vs. rfc 7598

yannis nikolopoulos <yanodd@otenet.gr> Tue, 26 July 2016 07:52 UTC

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From: yannis nikolopoulos <yanodd@otenet.gr>
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Date: Tue, 26 Jul 2016 10:52:27 +0300
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Subject: Re: [Softwires] PSID format as described in rfc 7597 vs. rfc 7598
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hello,

On 07/26/2016 02:59 AM, Xing Li wrote:
> otroan@employees.org 写道:
>> Normen,
>>
>>> I just want to make sure that I'm not misunderstanding this:
>>>
>>> RFC 7597, section 6 describes the last 16 Bits of the construction 
>>> of the Ipv6 interface identifier as follows:
>>>
>>>    The PSID field is left-padded with zeros to create a 16-bit field.
>>>
>>> RFC7598 Section 5.1. describes the format for provision the PSID to 
>>> clients with the following text:
>>>
>>>    o  PSID: 16 bits long.  The PSID value algorithmically identifies 
>>> a set of ports assigned to a CE. The first k bits on the left of 
>>> this field contain the PSID binary value.  The remaining (16 - k) 
>>> bits on the right are padding zeros.
>>>
>>> To me this seems that the two RFCs use two different formats to 
>>> express the same information in a field with the same name.
>>>
>>> Lets assume an example where the port split ratio is 6 (=k), slicing 
>>> the IPv4 address up into up 2**6 = 64 slices, each segment having 
>>> 1024 ports.
>>>
>>> In RFC 7597, to select the third port-range, the parameters become 
>>> OFFSET 0, PSID-LEN 6, PSID 0x2 (left padded with zeros to 16 bits)
>>> In RFC 7598, to select the third port-range, the parameters become 
>>> OFFSET 0, PSID-LEN 6, PSID 0x800 (leftmost k-bits on this field 
>>> contain the PSID binary value, which is right padded with zeros to 
>>> fit 16 bits)
>>>
>>> So,
>>> 1, Is my understanding of the two RFC's PSID formats correct?
>>
>> I believe so.
>>
>>> 2, What's the reason for the difference in the formats?
>>
>> Good question. The 7597 PSID in the IID is mainly there for pretty 
>> printing / troubleshooting, and it makes sense to left pad it.
>> I can only guess about the 7598 format, possibly to keep the PSID 
>> field consistent with the other fields (prefix) which are all right 
>> padded.
>

this might create some confusion about the way the lwB4 will construct 
the IID for the tunnel endpoint (use "2" or "800" for the last 16 bits?)

> I agree with Ole. For your information, our implementation has a 
> configuration function to select the format of PSID. Regards, xing

where is this function implemented?

best regards,
Yannis

>
>> Best regards,
>> Ole
>
>
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-- 
Yannis Nikolopoulos		OTE S.A
e-mail: yanodd@otenet.gr	IP Network Planning & Engineering
tel: +302106116293
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