# How to know for sure that a router is congested?

Suppose a router X is dropping packets. Another router A will investigate whether X is dropping packets due to congestion. What are the procedures that A should follow?

A already knows X's buffer capacity. Please explain the procedures and assume an arbitrary threshold T (if needed).

• Knowing X's buffer capacity shouldn't make a difference for checking for congestion because you can't know the offset of the packet you'd send to check for congestion in the buffer's sliding window. Should be as simple as, send a packet from $A$ to $X$ and wait for $T$. If there is no answer, assume $X$ must have dropped the packet. – Francesco Gramano May 4 '15 at 7:11
• Yeah but there might be two reason behind X dropping the packets.. 1. Congestion 2. Malicious attack .. I am trying to figure out the congestion conditions .. Conversely, any other conditions will be malicious.. – SrvSsG May 4 '15 at 8:32
• So, to be clear, is $X$ dropping all incoming packets, or only a proper subset? – Francesco Gramano May 4 '15 at 8:51
• It drops every packets. – SrvSsG May 4 '15 at 9:11
• Why would A need to distinguish between a router that's extremely congested and one that isn't transmitting any packet? From A's perspective, it's useless either way. Unless A is the only entity talking to X (in which case, where is X routing to?), how is it useful for A to know X's sliding window? – Gilles May 5 '15 at 8:03