Understanding Overlay ping and traceroute Packet Support

Overlay ping and traceroute Functionality

Overlay ping and traceroute packets are sent as User Datagram Protocol (UDP) echo requests and replies and are encapsulated in the VXLAN header. VTEPs, which initiate and terminate overlay tunnels, send and receive overlay OAM packets. Overlay ping and traceroute are supported only in VXLAN overlay networks in which the sending and receiving VTEPs are both Juniper Networks devices.

The overlay ping functionality validates both the data plane and the MAC address and IP address of the VTEPs. This additional validation is different from the more commonly known IP ping functionality where the actual destination replies to the echo request without the overlay segment context.

While tracing a route in a VXLAN overlay network, Juniper Networks devices that are along the route that support overlay traceroute additionally provide a timestamp. Third-party devices and Juniper Networks devices that do not support overlay traceroute do not provide this timestamp.

Overlay OAM Packet Format for UDP Payloads

The format of overlay OAM packets depends on the type of payload that is carried in the tunnel. In the case of VXLAN tunnels, the inner packet is a Layer 2 packet.

Figure 1 shows complete headers on a VXLAN-encapsulated overlay OAM packet.

Figure 1: VXLAN-Encapsulated Overlay OAM Packet

• Outer Ethernet header—Contains the source MAC (SMAC) and destination MAC (DMAC) addresses of directly connected nodes in the physical network. These addresses change at every hop.

• Outer IP header—Contains the source and destination IP addresses of the Juniper Networks devices that function as the VTEPs that initiate and terminate the tunnel.

• Outer UDP header—Contains the source port associated with the flow entropy and destination port. The source port is an internally calculated hash value. The destination port is the standard UDP port (4789) used for VXLAN.

• VXLAN header—Contains the VXLAN Network Identifier (VNI) or the segment ID of the VXLAN, and new router alert (RA) flag bits.

• Inner Ethernet header—Contains a control MAC address (00-00-5E-90-xx-xx) for both the SMAC and DMAC. This address is not forwarded out of the VTEP. Alternatively, the SMAC can be set to a non-control MAC address. However, if a non-control MAC address is used, the VTEP must not learn the SMAC from the overlay OAM packets.

• Inner IP header—Contains the source IP address that can be set to the IP address of the endpoint or source VTEP. The destination IP address can be set to the 127/8 address, which ensures that the overlay OAM packet is not forwarded out of the ports of the Juniper Networks device that is configured as a VTEP.

• Inner UDP header—Contains a new reserved value used in the destination port field in the inner UDP header. This value identifies the incoming UDP packet as an overlay OAM packet.

• Inner UDP payload—Contains all of the overlay OAM-specific message format and type, length, and value (TLV) definitions.

  The Inner UDP payload format is as follows:

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   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Message Type  | Reply mode    | Return Code   | Return Subcode|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Originator Handle                                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Sequence Number                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | TimeStamp Sent (seconds)                                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | TimeStamp Sent (microseconds)                                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | TimeStamp Received (seconds)                                  |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | TimeStamp Received (microseconds)                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | TLVs ...                                                      |
  .                                                               .
  .                                                               .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  

  The OAM-specific message type is one of the following:

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  Value What it means
  ----- -------------
  1     Echo Request
  2     Echo Reply
  
  Reply Mode Values:-
  Value What it means
  ----- ---------------------------------
  1     Do not reply
  2     Reply via an IPv4/IPv6 UDP Packet
  3     Reply via Overlay Segment
  

  The TLV definition for VXLAN ping is as follows:

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  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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Type = 1(VXLAN ping IPv4)   | Length                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | VXLAN VNI                                      | Reserved     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | IPv4 Sender Address                                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+