Xref header: wrong article number

"Martin Trautmann" <[email protected]>
Newsgroups gmane.network.slrn.user
Message-ID <[email protected]>
Hi all,

my news admin fixed a problem that caused annoying malfunctions for ages: slrn tried to obtain an article where the server claimed that the article was not available.

The headers had been received before. The article could be obtained e.g. by message id. How did it happen that the server could not return the article when slrn asked for it?

The answer is kind of a surprise: slrn takes the article number from the Xref headers.

These numbers where incorrect on occasions, especially for the initial posting of a thread.

The solution by the news admin was: future postings will no longer show those xref headers.

Do we need xref?
rfc2980 names xref as strongly recommended while xover is available - but it does not explain what should be within?

rfc3977 names examples only, but neither content nor requirement

The slrn FAQs name that marking crossposted articles as read is based on xref headers. 
"Unfortunately, Xref is optional under NOV so many systems do not automatically provide it even though it is one of the recommended headers."
Where do I find this recommendation? rfc2980 only? Or is there better information what should be within?

What else is lost without xref?
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