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[PWE3] Proposed change to 5..4.1 of PWE3 architecture
After discussions with Yaakov Stein regarding the use of UDP/IP
as a PW mechanism (used by both TDM approaches), it is proposed
that 5.4.1 be amended as shown below.
Is this acceptable to the WG?
- Stewart
New text:
5.4.1. PWE3 over an IP PSN
The protocol definition of PWE3 over an IP PSN should employ existing
IETF protocols where possible.
+---------------------+ +-------------------------+
| Payload |------------->| Raw payload if possible |
/=====================\ +-------------------------+
H Payload Convergence H-----------+->| Flags, seq # etc |
H---------------------H / +-------------------------+
H Timing H---------/--->| RTP |
H---------------------H / +-------------+ |
H Sequencing H----one of | |
\=====================/ \ | +-----------+
| PW Demultiplexer |---------+--->| L2TP, MPLS etc. |
+---------------------+ +-------------------------+
| PSN Convergence |------------->| Not needed |
+---------------------+ +-------------------------+
| PSN |------------->| IP |
+---------------------+ +-------------------------+
| Data-link |------------->| Data-link |
+---------------------+ +-------------------------+
| Physical |------------->| Physical |
+---------------------+ +-------------------------+
Figure 10: PWE3 over an IP PSN
Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a
rule, the payload should be carried as received from the NSP, with
the Payload Convergence Layer provided when needed. However in
certain circumstances it may be justifiable to transmit the payload
in some processed form. The reasons for this must be documented in
the Encapsulation Layer definition for that payload type.
Where appropriate, explicit timing is provided by RTP [RFC3550],
which when used also provides a sequencing service. When the PSN is
UDP/IP, the RTP header follows the UDP header and precedes the PW
control field, while for all other cases the RTP header follows the
PW control header.
The encapsulation layer may additionally carry a sequence number;
sequencing is to be provided either by RTP, or by the PW
encapsulation layer but not both.
PW Demultiplexing is provided by the PW label, which may take the
form specified in a number of IETF protocols, e.g. an MPLS label, an
L2TP session ID, or a UDP port number.
When PWs are carried over IP the PSN Convergence Layer will not be
needed.
As a special case, if the PW Demultiplexer is an MPLS label, the
protocol architecture of section 5.4.2 can be used instead of the
protocol architecture of this section.
=======================================================================
For reference the following is the text that appears in the -06 draft.
5.4.1. PWE3 over an IP PSN
The protocol definition of PWE3 over an IP PSN SHOULD employ existing
IETF protocols where possible.
+---------------------+ +-------------------------+
| Payload |------------->| Raw payload if possible |
/=====================\ +-------------------------+
H Payload Convergence H-----------+->| As Needed |
H---------------------H / +-------------------------+
H Timing H---------/--->| RTP |
H---------------------H / +-------------+ |
H Sequencing H----one of | |
\=====================/ \ | +-----------+
| PW Demultiplexer |---------+--->| L2TP, MPLS etc. |
+---------------------+ +-------------------------+
| PSN Convergence |------------->| Not needed |
+---------------------+ +-------------------------+
| PSN |------------->| IP |
+---------------------+ +-------------------------+
| Data-link |------------->| Data-link |
+---------------------+ +-------------------------+
| Physical |------------->| Physical |
+---------------------+ +-------------------------+
Figure 10: PWE3 over an IP PSN
Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a
rule, the payload SHOULD be carried as received from the NSP, with
the Payload Convergence Layer provided when needed. (It is accepted
that there MAY sometimes be good reason not to follow this rule, but
the exceptional circumstances need to be documented in the
Encapsulation Layer definition for that payload type).
Where appropriate, timing is provided by RTP [RFC3550], which when
used also provides a sequencing service. PW Demultiplexing may be
provided by a number of existing IETF tunnel protocols. Some of
these tunnel protocols provide an optional sequencing service.
(Sequencing is provided either by RTP, or by the PW Demultiplexer
Layer, but not both).
RTP is normally carried over UDP, however the tunnel protocols that
are capable of carrying a PW, provide sufficient functionality to
carry RTP without an intervening transport layer. UDP MAY therefore
be omitted from the protocol stack.
A PSN Convergence Layer is not needed, because all the tunnel
protocols shown above are designed to operate directly over an IP
PSN.
As a special case, if the PW Demultiplexer is an MPLS label, the
protocol architecture of section 5.4.2 can be used instead of the
protocol architecture of this section.
=========================================================================
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