In an SD-WAN network, when a Versa Operating SystemTM (VOSTM) device has two or more paths to a destination, you can use packet replication to duplicate, or mirror, packets and transmit them over multiple paths. If a packet is lost on one link, the mirrored packet is delivered on one or more secondary links. If the remote device receives more than one copy of the packet, it sends the first received packet towards the LAN and drops the subsequent ones. Packet replication provides a way to improve the quality of voice traffic and other mission-critical application traffic. It is recommended that you enable packet replication for sites whose audio calls are not clear.

You can enable packet replication for a specific application or for forward error correction (FEC) parity packets. You can configure packet replication to turn on automatically when paths become non-compliant with configured SLA metrics and to stop automatically when link utilization exceeds a configured threshold, which are both useful for sites that have limited link capacity.

Note:

Packet replication does not work on single-access circuit. Packet replication is generally used for links that are experiencing packet loss, if we allow packet replication on already congested links it will not be useful.

Use Case Scenario:

Explain how traffic is replicated in a forwarding profile with replication factor 2 and link priority as mentioned below:

Consider all links are in same transport domain and all SLAs are up. Replicated packets will take priority 2 combination or priority4.

Solution:

To replicate traffic in a forwarding profile with replication factor 2 and link priority between Local and Remote nodes (as explained in the table above):

• Find a path on the alternate circuit in the highest priority (P1).
• If there is no alternate path in the highest priority then look in the next priority (P2) and so on.

In the example above, 

• The traffic replicates on "WAN_LINK1 - WAN_LINK2" path and not on "WAN_LINK2 - WAN_LINK2" path.
• If all the paths are in the same priority, then replication will work on "WAN_LINK2 - WAN_LINK1" because it is a different local circuit. 

When picking a path within the same priority, you can pick the next available path with a different local circuit. 

Frequently Asked Questions:

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If there are 3 WAN interfaces and the replication factor is 3 can we configure the interfaces in a manner so that original packet egress on the first interface, the first and second duplicate packets egress on the 2nd and 3rd WAN interfaces?

Ans: Yes, replication factor 3 will send first packet through first WAN interface and subsequent two packets on the other two WAN interfaces.

When a CPE receives packets, how it differentiates between original and replicated packets? Whether sequence number of the original and replicated packet remain same?

Ans:Versa FlexVNF tag every packet with sequence number when replication is enabled. Original and replicated packets have same sequence number.

How is FEC better than packet retransmission as even in FEC destination FlexVNF regenerates lost packet?

Ans:For UDP traffic (RTP: Audio and Video), there is no retransmission from the sender and any packet lost will result in distorted video or audio at receiving end. FEC regeneration help as end host does not see packet loss.

 For TCP traffic, the sender can retransmit the packet, but packet loss will indicate congestion and the window will be reduced which will impact the throughput/performance. With FEC regeneration, sender will not see a loss and congestion will not be detected and hence end host will see better throughput.