Re: [PWE3] draft-manhoudt-pwe3-tsop-00.txt and IP

[Gert Manhoudt <[e-mail]>]

Stewart, Yaacov and Alexander,

Thanks for your comments on draft-manhoudt-pwe3-tsop-00. It is our 
intent to request the PWE3 WG to accept this document as a working group 
draft. At this stage I want to address those points that could be 
interpreted to  imply that the TSoP approach as such is not acceptable. 
The other comments are of a less principal nature and can be addressed 
later (and can probably be fixed by adapting the text).

1. TSoP does not require the termination of any STM-N overhead:
Yaakov's e-mail contains the statement: "this function is at least a 
section (in SDH terminology a regenerator section) termination". I 
believe that this statement is not true: There is no reason why STM-N 
signals can not be carried transparently over some server layer with 
minimal interaction between client and server (as should always be the 
goal).

The correct architectural analogue would be to compare a TSoP PW section 
to an OTN network that transports an STM-N client signal. The mapping of 
an STM-16/OC-48 in an ODU1 does not require any processing of STM-N 
overhead or even framing for alignment purposes: All STM-16 bits are 
directly written into the OPU1 payload area. This is described in G.709 
section 17.2.1. In terms of functional blocks this is described in G.798 
section 14.3.1. Note that Appendix I.1 of this same Recommendation 
describes the mapping of STM-N in OPU-k using OPTIONAL RS-monitoring, 
just like TSoP.

Because a TSoP PW section (STM-N over PSN) is modeled to behave itself 
in the same way as an STM-N over OTN section, the TSoP decapsulation 
function inserts Generic AIS as defined in G.709, rather than SDH 
multiplex section AIS as defined in G.707, in case the client signal is 
interrupted.

As a side remark: Yaakov's mail also contains the statement: "(since it 
does not divide up STM-Ns, it is not a line termination)". Note that 
this directly contradicts the remark that the S1-byte must be processed. 
If that were really necessary, it would not be sufficient to limit the 
required SDH functionality to a regenerator section termination, but 
rather a full multiplex section termination (SONET: line termination) 
would be necessary, since the S1-byte is located in the MS part of the 
STM-N overhead and not in the RS part: SDH regenerators pass the S1-byte 
transparently.

2. TSoP passes the S1-byte transparently:
The S1-byte in the STM-N multiplex section overhead carries the 
"traceability information" of the STM-N signal, i.e. the quality level 
of the master clock at the beginning of a chain of slaved STM-N systems. 
Normally this quality level is PRC/PRS, but during fault situations, it 
may lower to SSU/ST2 or even SEC/ST3. Downstream STM-N systems can use 
this information to select another reference signal or to go in 
hold-over mode.

The question is now whether the passage of an STM-N signal over a 
TSoP/PSN section should affect the value of the S1-byte. We believe that 
this is not the case: The nature of a passage of an STM-N signal through 
a TSoP Pseudowire is such that the traceability does not change, since 
the (long term average of the) frequency of the STM-N signals before 
entering the TSoP PW and after leaving the TSoP PW is the same: No bits 
are created or deleted from the STM-N client signal. This means that if 
the long term frequency accuracy of an STM-N signal is 1E-11 (i.e. it is 
traceable to PRC/PRS), that this will not change by passing the TSoP PW, 
i.e. the traceability is invariant.

Of course, during a failure, the TSoP decapsulation function substitutes 
the G-AIS signal which is no longer frequency synchronous with the input 
STM-N. In this case the downstream STM-N node will immediately detect 
Loss of Frame and will no longer be able to read the S1-byte. According 
to standard SDH reference selection procedures (see G.781), this node 
will no longer consider such a signal to be an acceptable 
synchronization reference.

3. A TSoP PWE3 can be carried over UDP/IP:
This option has been included in the TSoP draft purely because it is 
present in the SAToP standard and to keep the encapsulation options 
aligned. If experience has shown that transport over an UDP/IP PSN of 
higher bitrate circuit emulation PW's is not recommendable, the authors 
have no problem to remove this option.


Regards,
Gert Manhoudt.



On 10 jul 2012 11:01, Alexander Vainshtein wrote:
> Hi all,
> Several comments on the draft in question:
>
> 1. The draft seems to follow closely RFC 4553, including support of IPv4/IPv6 as the PSN, congestion handling etc. I echo Stewart's question regarding usage of IPv4/IPv6 PSN (with limited TE capabilities) for carrying such high-rate CBR flows as STM-1/OC-3 and higher.
>
> 2. The draft correctly mentions RFC 4842 as an alternative (and approved by the IETF) method for emulating SONET/SDH over PSN. However, it is worth noting that one of the reasons this method has been preferred to transparent passing of the entire SONET/SDDH bit stream (e.g., as proposed in one of the early precursors of RFC 4553 - see  http://www.watersprings.org/pub/id/draft-vainshtein-cesopsn-01.txt, Annex B) to was elimination of the need to manipulate the S1 byte in the SONET/SDH overhead. The lower nibble of this byte contains encoded quality of the system clock of the SONET/SDH mux that has been used to drive the outgoing SONET/SDH bit stream. Simply passing tits value transparently thru the PSN does not make any sense (and, indeed, potentially violates integrity of clock distribution). The IWF proposed in the draft cannot access this byte because it is unaware of the SONET/SDH boundaries, and neither can this be done by the  OSn_TT_So & OSn/RSn_A_So/  OSn_TT_Sk & OSn/RSn_A_Sk  functional blocks to which the IWF is connected. IMHO the draft should explain in some detail how this issue is resolved. Alternatively, it should state that the SONET/SDH bit stream that has passed thru the TSoP PW MUST NOT be used as the clock source by the CE device.
>
> 3. The draft defines only LOS as the mandatory SONET/SDH AC fault to affect setting of the L bit in the PW control word. However, if Loss of Frame defect is encountered (dLOF),  de-scrambling of the incoming SONET/SDH stream cannot be completed, and the payload of the TSoP would become junk; hence IMO LOF must be added as a mandatory trigger for asserting the SF condition in Section 3.1.  (And, while the reference to the ITU-T G.783 functional blocks implies that, it would be nice to explain that the incoming SONET/SDH bit stream is segmented and encapsulated after de-scrambling - for the sake of the readers that do not parse the ITU-T-ish easily:-)
>
> 4. The draft seems to imply that TSoP requires, as a pre-requisite, availability of common high-quality clock for the pair of PEs terminating a TSoP PW by both mandating the use of the minimal RTP header in the encapsulation and not recommending usage of adaptive clock recovery for these PWs. However, this important restriction is not reflected in the Applicability Statement section of the draft.
>
> 5. The draft mentions the need not to exceed the path MTU between the pair of PEs in the case of IPv4/IPv6 PSN but not in the case of the MPLS PSN (where it obviously applies).
>
> Hopefully these notes will be useful.
>
> Regards,
>       Sasha
>
>> -----Original Message-----
>> From: [e-mail] [mailto:[e-mail]] On Behalf
>> Of Stewart Bryant
>> Sent: Tuesday, July 10, 2012 10:57 AM
>> To: pwe3
>> Subject: [PWE3] draft-manhoudt-pwe3-tsop-00.txt and IP
>>
>> Do the authors think that running transparent
>> SONET emulation over IP is likely in practice, or is
>> this included because SAToP included it?
>>
>> Stewart
>> (Speaking as an individual)
>>
>>
>> _______________________________________________
>> pwe3 mailing list
>> [e-mail]
>> https://www.ietf.org/mailman/listinfo/pwe3
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