Subject: [PWE3] RFC4717 Figure 12

From: Stewart Bryant <[email-masked]>
Date: Fri, 05 Aug 2011 14:50:58 +0100

I have been dealing with Errata 999

I got down to issue 11(a) which I have moved into
Errata 2921

http://www.rfc-editor.org/errata_search.php?eid=2921

I concerns Fig 12 and order of the U.E.C bits

   The AAL5 transparent VCC service is OPTIONAL.

    0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               PSN Transport Header (As Required)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Pseudowire Header                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0| Resvd |   Optional Sequence Number    |M|V| Res |U|E|C|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             "                                 |
    |                        AAL5 CPCS-PDU                          |
    |                      (n * 48 bytes)                           |
    |                             "                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 12: AAL5 transparent service encapsulation

I then looked at Figure 8.1/Y1412 AAL type 5 PDU encapsulation
which looks as follows

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    Transport Label (4 Octets)
    +-+-+-+-+-+-+-+-+
    Interworking Label (4 Octets)
    +-+-+-+-+-+-+-+-+
     0 0| Leng Ind		
    +-+-+-+-+-+-+-+-+
    Sequence Number (2 Octets)
    +-+-+-+-+-+-+-+-+
     M V| res |A E C
    +-+-+-+-+-+-+-+-+

where A=AUU, E = EFCI, C=CLP

Originally I was going to confirm that the bits were in the
right order. However I am now confused by the contradiction
between the 0000/Reserved in Fig 12 of RFC4717 and the 00/Len in
Figure 8.1/Y1412

Am I comparing the correct figures?

If so which is correct?

Does anyone have a shipping implementation of PDU mode as
defined in either of these specifications, if so what do you
think the bit order should be?

- Stewart

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From: "Andrew G. Malis" <[email-masked]>
Date: Fri, 5 Aug 2011 10:51:32 -0400

Stewart,

I believe that all is well.

Errata 2921 complains about the apparent inconsistency between the
control words in figures 10 and 12 in RFC 4717. However, there really
is no inconsistency, because the two figures are encapsulating
different things.

Let's look at Figure 12 first. Figure 12 describes the encapsulation
of AAL5 PDUs. When you compare figure 12 (as you did) with Figure
8.1/Y.1412, you'll see that the "|M|V| Res |U|E|C|" bits in figure 12
exactly matches the octet in the AAL5 header just before the payload.
So when encapsulating, once can just copy the octet from the AAL5 PDU
to the control word. So that's all fine.

By comparison, figure 10 describes the encapsulation of ATM cells. The
errata complains that the control words in figures 12 and 10 are
inconsistent. Well, they're not meant to be consistent, since the PTI
and C fields in the control word in figure 10 are from the ATM cell
header, not the AAL5 header, which is the source of the bits in the
control word in figure 12. What's more, figure 10 is also consistent
with figures 5, 7, 8, and 9.

So in my opinion, this errata is unconfirmed.

Cheers,
Andy

On Fri, Aug 5, 2011 at 9:50 AM, Stewart Bryant <[email-masked]> wrote:
> _______________________________________________
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---

From: Giles Heron <[email-masked]>
Date: Fri, 05 Aug 2011 16:18:06 +0100

Looks like a strange one to me.

The length field in the original PWE control word was added so that PWEs
could carry packets of less than 46 bytes (including the label stack and
control word) which get padded when carried over MPLSoE trunks.  Without the
length field there would be no way to know what was PWE payload and what was
padding (this issue doesn't arise with IPoMPLS because the IP header has a
length field).  The reason for the 6 bit length field is simply because 2^5
< 46 < 2^6.

In the case of ATM AAL5 PDUs (as opposed to SDUs) this problem doesn't
occur, as the PDU is N * 48 bytes and so the packet must always be at least
56 bytes long (PW label plus CW plus PDU with N==1).

On that basis I'd say Y1412 is incorrect.

Not that the PDU mode ever made much sense to me ;)

Giles