virtual circuit connectivity verification

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Virtual circuit Connection Verification (VCCV) :
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VCCV provides connection verification services such as ping ,mpls ping regardless of underlying protocol such as MPLS ,IP tunnel. A network operator may use this to test the liveliness of the network.
Ping and other IP messages are encapsulated using the PWE3 encapsulation .These messages are referred to as VCCV messages.VCCV messages are exchanged after negotiation between PEs.

MPLS as PSN :
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VCCV creates control channel between PWE3 PES to exchange the IP monitoring tools. For more details of this , please refer PWE3 architecture.Packets sent across this channel are IP Packets ,allowing maximum flexibility.
When control word is present on VC , it is possible to indicate the control channel by setting the control channel header . this is referred as inband MPLS VCCV as the control channel would be in band.
When the control header is not in use , use of MPLS router alert label to indicate the IP control channel is also proposed.

IP probe traffic :
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Both ICMP and LDP ping are used for connectivity verification.
ICMP ping :
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When ICMP Packets are used , the source address is source address of LDP session and the destination address is destination address of LDP session.
The identifier and sequence number of fields of ICMP echo request and reply are used to track what VCs are tested.These fields are only intrepretted by sending PEs.

MPLS ping packet :
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The MPLS ping packet must contain the sub TLV 8 of PW circuits.This sub TLV contains VC id of circuit to be verified.
L2 circuit ID TLV for MPLS ping :
TLV contains the following fields
1 . Source and destination address of LDP session
2 . VC ID
3 . Encapsulation type
4 . PWID
FEC 128 VCID , FEC 129 attachment
5. PW parameters – interface parameters
L2TPV3 as PSN :
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When L2TPV3 is used as the underlying PSN , VCCV mechanism is needed . The L2TPV3 control connection employs the keep alive mechanism but it is not sufficient for data plane.
L2TPV3 messages are encoded in L2TPV3 session packet.VCCV mechanism is needed for verifying the session state at egress router.
Bidirectional forwarding detection protocol :
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BFD is a detection protocol is a detection protocol designed to provide fast forwarding path failure detection times for all media types,encapsulation ,topologies and routing protocols.Apart from this , BFD provides consistent failure detection method for network administrators.

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