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

otroan@employees.org Mon, 25 July 2016 19:05 UTC

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From: otroan@employees.org
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Date: Mon, 25 Jul 2016 21:05:22 +0200
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To: normen.kowalewski@telekom.de
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Cc: softwires@ietf.org, draft-ietf-softwire-map@ietf.org, draft-ietf-softwire-map-dhcp@ietf.org
Subject: Re: [Softwires] PSID format as described in rfc 7597 vs. rfc 7598
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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.

Best regards,
Ole