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RFC 5595, "The Datagram Congestion Control Protocol (DCCP) Service Codes", September 2009

Source of RFC: dccp (tsv)

Errata ID: 1891
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT

Reported By: Alfred Hoenes
Date Reported: 2009-09-22
Held for Document Update by: Lars Eggert
Date Held: 2011-03-01

Section 2.7, pg.10 says:

OLD:
 o  It preserves the 4 lowest bits of the final bytes of the Service
    Code, which allows many common series of Service Codes to be
    mapped to a set of adjacent port numbers, e.g., Foo1, and Foo2;
    Fooa and Foob would be assigned adjacent ports.

It should say:

NEW:
 o  It preserves the 3 lowest bits of the final bytes of the Service
                     ^^
    Code, which allows many common series of Service Codes to be
    mapped to a set of adjacent port numbers, e.g., Foo1, and Foo2;
    Fooa and Foob would be assigned adjacent ports.

Notes:

Simply trying the full example shows that the statement is not
entirely correct. The quoted formula reveals that only the least
significant *3* bits are left unchanged (due to the '<<3'
operation on sc[2]). It turns out that ranges of *8* contiguous
SC values are mapped to contiguous 8 port numbers but, depending
on the two least significant bits of sc[2], groups of 4 adjacent
ranges are shuffled around. More complicated patterns arise for
the next higher level of 4-clusters of range-groups, etc.
Thus, the situation is not hopeless for the indicated purpose,
but much more complicated than indicated in the RFC text.

I leave it as an exercise to the interested reader to figure
out the details for the size of range she is interested in.

A future revision of the RFC however should fix the text.

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