Subject: [PWE3] Reserved bits in control word: CESoPSN (RFC 5086) and RFC 3931 (L2TPv3)

From: Javi <[email-masked]>
Date: Tue, 15 Dec 2009 13:13:09 +0100

Any idea why this?

Default L2-Specific Sublayer Format (or Control Word CW) proposed by RFC
3931 does not set its first four bits to zero, so that it does not
satisfy RFC 5086 (it does not provide the necessary MPLS payload
discrimination). Why?

I summary RFCs involved:

1) RFC 5086: CESoPSN

   *The difference in the CESoPSN packet formats for IP PSN using
   UDP-based demultiplexing and the rest of the PSN and demultiplexing
   combinations* *(including L2TPv3)*, is based on the following
   considerations:

   2.  Compliance with the common PWE3 mechanisms for keeping PWs Equal
       Cost Multipath (ECMP)-safe for the MPLS PSN by providing for PW-
       IP packet discrimination (see [RFC3985], Section 5.4.3).  This
       *requires placing the PWE3 control word immediately after the PW
       label*.

   3.  Commonality of the CESoPSN packet formats for MPLS networks and
       IPv4/IPv6 networks with Layer 2 Tunneling Protocol Version 3
       (L2TPv3) demultiplexing facilitates smooth stitching of *L2TPv3-
       based* and MPLS-based segments of CESoPSN PWs (see [PWE3-MS]).

2) RFC 4385: CW for MPLS PW

.  Generic PW MPLS Control Word

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0|          Specified by PW Encapsulation                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The PW set-up protocol or configuration mechanism determines whether
   a PW uses a PWMCW.  *Bits 0..3 differ from the first four bits of an
   IP packet* [BCP] and hence provide the necessary MPLS payload
   discrimination.

3) RFC 4553:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0|L|R|RSV|FRG|   LEN     |       Sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 3.  Structure of the SAToP Control Word

   The use of *Bits 0 to 3* is described in [RFC4385].  These bits *MUST
   be set to zero *unless they are being used to indicate the start of an
   Associated Channel Header (ACH).  An ACH is needed if the state of
   the SAToP PW is being monitored using Virtual Circuit Connectivity
   Verification [RFC 5085].

4) RFC 3931: L2TPv3

Default L2-Specific Sublayer Format

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |x|S|x|x|x|x|x|x|              Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *The S (Sequence) bit is set to 1* when ...

Javi
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---

From: Yaakov Stein <[email-masked]>
Date: Tue, 15 Dec 2009 14:15:55 +0200

Because 3931 is for L2TPv3 which runs over IP.

You only need the initial nibble value when it occurs right after the
bottom label on an MPLS stack, to differentiate between a PW
and IPv4 or IPv6 over MPLS.

Y(J)S

-----Original Message-----
From: Javi [mailto:[email-masked]] 
Sent: Tuesday, December 15, 2009 14:13
To: [email-masked]
Cc: Alexander Vainshtein; Yaakov Stein
Subject: Reserved bits in control word: CESoPSN (RFC 5086) and RFC 3931 (L2TPv3)

Any idea why this?

I summary RFCs involved:

1) RFC 5086: CESoPSN

    0                   1                   2                   3

3) RFC 4553:

    0                   1                   2                   3

4) RFC 3931: L2TPv3

    0                   1                   2                   3

Javi