All IP addresses within a 10.0.0.0/16 VPC are fully utilized with application servers across two Availability Zones. The application servers need to…

A single NAT gateway translates source IP addresses and ports for outbound traffic. For traffic destined to the same destination IP address, destination port, and protocol, a NAT gateway can support up to 55,000 simultaneous connections. When many application servers send frequent UDP probes to a single authentication server, they can collectively exhaust the available source ports on the NAT gateway for that specific destination. This results in new connection attempts from some servers being dropped, causing the observed communication failures. This is a known limitation called port allocation exhaustion.

A. NAT gateways fully support TCP, UDP, and ICMP traffic. The issue is not protocol incompatibility but resource exhaustion.
B. While the authentication server could be the issue, the symptom of some servers failing intermittently points towards a bottleneck like the NAT gateway.
D. A NAT gateway must be launched in a public subnet with a route to an internet gateway to function correctly for internet-bound traffic.

1. AWS Documentation, VPC User Guide, "NAT gateways". Under the "NAT gateway basics" section, it states, "A NAT gateway supports 5 Gbps of bandwidth and automatically scales up to 100 Gbps... For each unique combination of destination IP address, destination port, and protocol, a NAT gateway supports up to 55,000 simultaneous connections or flows."

2. AWS Knowledge Center, "Why can't my instances access the internet using a NAT gateway?". This article addresses troubleshooting and mentions port exhaustion as a potential cause: "If you have a large number of connections to a single destination IP address and port, then you might be limited by the number of available source ports."