The packet sent by a node to the source to inform it of congestion is called

Answer: Choke — a choke packet is the control message sent by a congested node to the source to request a reduction in sending rate.

• Definition: A choke packet is a control packet issued by a router or node that detects congestion; it informs the traffic source to slow down to help relieve the congestion.

• Typical source response: Upon receiving a choke packet, the source should reduce its transmission rate or shrink its sending window (throttle) until congestion eases.

• Why this is the correct choice compared to the others:

• Explicit: Refers to explicit congestion notification or marking (ECN), where routers mark packets or set header bits rather than sending a separate control packet.

• Backpressure: Describes a flow-control mechanism in which congestion signals propagate upstream; it is a general method, not the name of a specific control packet sent to the source.

• Retransmission: The act of resending lost or corrupted packets for reliability; it does not function as a congestion notification packet.