VCCV-ping application

VCCV effectively creates an IP control channel within the pseudowire between PE1 and PE2. PE2 should be able to distinguish on the receive side VCCV control messages from user packets on that VLL. There are three possible methods of encapsulating a VCCV message in a VLL which translates into three types of control channels:

Use of a Router Alert Label immediately above the VC label. This method has the drawback that if ECMP is applied to the outer LSP label (for example, transport label), the VCCV message does not follow the same path as the user packets. This effectively means it does not troubleshoot the appropriate path. This method is supported by the 7450 ESS, 7750 SR, and 7950 XRS routers.
Use of the OAM control word as shown:
```
                                                           
      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 1| FmtID |   Reserved    |         Channel Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```
The first nibble is set to 0x1. The Format ID and the reserved fields are set to 0 and the channel type is the code point associated with the VCCV IP control channel as specified in the PWE3 IANA registry (RFC 4446). The channel type value of 0x21 indicates that the Associated Channel carries an IPv4 packet.

The use of the OAM control word assumes that the draft-martini control word is also used on the user packets. This means that if the control word is optional for a VLL and is not configured, the PE node only advertises the router alert label as the CC capability in the Label Mapping message. This method is supported by the 7450 ESS, 7750 SR and 7950 XRS routers.
Set the TTL in the VC label to 1 to force PE2 control plane to process the VCCV message. This method is not guaranteed to work under all circumstances. For instance, the draft mentions some implementations of penultimate hop popping overwrite the TTL field. This method is not supported by the 7450 ESS, 7750 SR, and 7950 XRS routers.

When sending the label mapping message for the VLL, PE1 and PE2 must indicate which of the above OAM packet encapsulation methods (for example, which control channel type) they support. This is accomplished by including an optional VCCV TLV in the pseudowire FEC Interface Parameter field. The format of the VCCV TLV is shown below:

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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0x0c     |       0x04    |   CC Types    |   CV Types    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note that the absence of the optional VCCV TLV in the Interface parameters field of the pseudowire FEC indicates the PE has no VCCV capability.

The Control Channel (CC) Type field is a bitmask used to indicate if the PE supports none, one, or many control channel types.

0x00 None of the following VCCV control channel types are supported
0x01 PWE3 OAM control word
0x02 MPLS Router Alert Label
0x04 MPLS inner label TTL = 1

If both PE nodes support more than one of the CC types, then the router PE uses of the one with the lowest type value. For instance, OAM control word is used in preference to the MPLS router alert label.

The Connectivity Verification (CV) bitmask field is used to indicate the specific type of VCCV packets to be sent over the VCCV control channel. The valid values are:

0x00 None of the below VCCV packet type are supported.

0x01 icmp ping. Not applicable to a VLL over a MPLS or GRE SDP and is therefore not supported by the 7450 ESS, 7750 SR, and 7950 XRS routers.

0x02 LSP ping. This is used in VCCV ping application and applies to a VLL over an MPLS or a GRE SDP. This is supported by the 7450 ESS, 7750 SR, and 7950 XRS routers.

A VCCV ping is an LSP echo request message as defined in RFC 8029. It contains an L2 FEC stack TLV which must include within the sub-TLV type 10 ‟FEC 128 Pseudowire”. It also contains a field which indicates to the destination PE which reply mode to use. There are four reply modes defined in RFC 8029:

Reply mode, meaning:

Do not reply. This mode is supported by the routers.
Reply via an IPv4/IPv6 UDP packet. This mode is supported by the routers.
Reply with an IPv4/IPv6 UDP packet with a router alert. This mode sets the router alert bit in the IP header and is not be confused with the CC type which makes use of the router alert label. This mode is not supported by the routers.
Reply via application level control channel. This mode sends the reply message inband over the pseudowire from PE2 to PE1. PE2 encapsulates the Echo Reply message using the CC type negotiated with PE1. This mode is supported by the routers.

The reply is an LSP echo reply message as defined in RFC 8029. The message is sent as per the reply mode requested by PE1. The return codes supported are the same as those supported in the router LSP ping capability.

The VCCV ping feature is in addition to the service ping OAM feature which can be used to test a service between router nodes. The VCCV ping feature can test connectivity of a VLL with any third party node which is compliant to RFC 5085.

Figure: VCCV-ping application