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RFC 4717, "Encapsulation Methods for Transport of Asynchronous Transfer Mode (ATM) over MPLS Networks", December 2006

Source of RFC: pwe3 (int)

Errata ID: 2921
Status: Rejected
Type: Technical
Publication Format(s) : TEXT

Reported By: Alfred Hoenes
Date Reported: 2007-10-17
Rejected by: Stewart Bryant
Date Rejected: 2012-02-21


(11a)  danger of confusion - or a mis-specification?

The placement of the U and E bits in the Control Word shown in
Figure 12 (on page 29) comes to surprise.

Apparently, the two bits are exchanged relatively to their placement
within the PTI field in the Control Word shown in Figure 10
(on page 26).

If this has been done intentionally, it might surprise some
implementors, leading to interoperability problems.
In this case, a specific warning should have been added to the
RFC text in Section 11.1, somewhere below Figure 12, e.g.:

|  Warning to implementors: The order of the E and U bit is reversed
|  relative to their placement in the PTI field of the ATM cell header.
|  Section 11.2.2 below details the bit manipulations to be done by
|  the receiving PE.

But if this is an unintentional oversight, the Control Word part
of Figure 12,

                                                            [...]  |
|  |0 0 0 0| Resvd |   Optional Sequence Number    |M|V| Res |U|E|C|
   | [...]

should be corrected to say:

                                                            [...]  |
|  |0 0 0 0| Resvd |   Optional Sequence Number    |M|V| Res |E|U|C|
   | [...]
                                                              ^ ^
  In this case, the procedures laid out in Section 11.2.2
  could easily be collapsed to a simple bit field transfer!


This is element 11(a) of Errata 999

This needs to checked against the corresponding ITU specification for this mode.
RFC4717 and ITU-T Recommendation Y.1412 use the same bit definitions and the same bit ordering, and therefore one must conclude that the bit ordering was that which was intended by the authors and has thus implemented.

There have been no concerns expressed on the PWE3 list regarding the change of ordering between Figure 11 and Figure 12.

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