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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Version Number        |         Global Flags          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Message Type  |   Reply mode  |  Return Code  | Return S.code |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sender's Handle                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                              TLVs                             ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          

Figure 1: Overlay OAM Ping format

The format of the Echo Request/Echo Reply control packet is to support ping and traceroute functionality in overlay networks Figure 1 resembles the format of MPLS LSP Ping [RFC4379] with some exceptions.

The interpretation of the fields is as following:

• The Version reflects the current version. The version number is to be incremented whenever a change is made that affects the ability of an implementation to correctly parse or process control packet.
• The Global Flags is a bit vector field
• The Message Type filed reflects the type of the packet. Value TBA2 identifies Echo Request and TBA3 - Echo Reply
• The Reply Mode defines the type of the return path requested by the sender of the Echo Request.
• Return Codes and Subcodes can be used to inform the sender about result of processing its request.
• The Sender's Handle is filled in by the sender, and returned unchanged by the receiver in the Echo Reply.
• The Sequence Number is assigned by the sender and can be (for example) used to detect missed replies.
• TLVs (Type-Length-Value tuples) have the two octets long Type field, two octets long Length field that is length of the Value field in octets.

2.1. Overlay Echo Request Transmission

Overlay Echo Request control packet MUST use the appropriate encapsulation of the monitored overlay network. Overlay network encpsulation MUST identify Echo Request as OAM packet. Overlay encapsulation uses different methods to identify OAM payload [I-D.ietf-nvo3-vxlan-gpe], [I-D.ietf-nvo3-gue], [I-D.ietf-nvo3-geneve], [I-D.ietf-sfc-nsh],[I-D.ietf-bier-mpls-encapsulation]. Overlay network's header MUST be immediately followed by the Overlay OAM Header [I-D.ooamdt-rtgwg-ooam-header]. Message Type field in the Overlay OAM Header MUST be set to Overlay Echo Request value (TBA2).

Value of the Reply Mode field MAY be set to:

• Do Not Reply (TBA4) if one-way monitoring is desired. If Echo Request is used to measure synthetic packet loss, the receiver MAY report loss measurement results to a remote node.
• Reply via an IPv4/IPv6 UDP Packet (TBA5) value likely will be the most used.
• Reply via Application Level Control Channel (TBA6) value if the overlay network MAY have bi-directional paths.
• Reply via Specified Path (TBA7) value in order to enforce use of the particular return path specified in the included TLV to verify bi-directional continuity and also increase robustness of the monitoring by selecting more stable path.

2.2. Overlay Echo Request Reception

2.3. Overlay Echo Reply Transmission

The Reply Mode field directs whether and how the Echo Reply message should be sent. The sender of the Echo Request MAY use TLVs to request that corresponding Echo Reply be sent using the specified path. Value TBA3 is referred as "Do not reply" mode and suppresses transmission of Echo Reply packet. Default value (TBA5) for the Reply mode field requests the responder to send the Echo Reply packet out-of-band as IPv4 or IPv6 UDP packet. [Selection of destination and source IP addresses and UDP port numbers to be provided in the next update.]

2.4. Overlay Echo Reply Reception

3. IANA Considerations

3.1. Overlay Echo Request/Echo Reply Type

IANA is requested to assign new type from the Overlay OAM Protocol Types registry as follows: