User Datagram Protocol is a simpler message-based connectionless protocol. Connectionless protocols do not set up a dedicated end-to-end connection. Communication is achieved by transmitting information in one direction from source to destination without verifying the readiness or state of the receiver. So how can the destination reorder packets ?

  • "So how can the destination reorder packets ?" That would be up to the application or application-layer protocol. – Ron Maupin Jan 8 at 21:01

Does UDP perform packet re-ordering at the destination?

No. UDP datagrams are transported to and received by the destination (application) as is. If their order is changed during transport they arrive out of order. Datagrams may also get lost on the way without a recovery process (unless the application provides some mechanism by itself).

Of course, an application can put a sequence number, identifier, ... in its user data as it is transported by UDP - that is regularly done, but without any effort from UDP itself.

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    Just to add a trivial example, Cisco routers are often configured to add sequence numbers to syslog messages, so you will be able to detect dropped and reordered messages if the logging was over UDP. – jonathanjo Jan 9 at 9:44

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