Which factors contribute to the effect of Congestion in application response time?

Congestion occurs when the demand for a network resource, typically bandwidth, exceeds its available capacity. This leads to the formation of queues in network devices (like routers and switches), increased delay, and potential packet loss. The primary factors causing this state are the presence of competing traffic flows from other applications or users ("Other traffic on the network") and network designs where the aggregate potential traffic from endpoints is greater than the capacity of a shared link ("link oversubscription"). Both factors directly contribute to the network overload that defines congestion, thereby degrading application response time.

A. TCP Behavior and non-optimal TCP settings: TCP's mechanisms (e.g., slow start, congestion avoidance) are designed to react to and manage congestion, not cause it. While poor settings can worsen the situation, the root cause is excess traffic, not the protocol itself.

B. Physical distances between tiers and chattiness: Physical distance contributes to propagation delay (latency), a constant factor based on the speed of light. Chattiness exacerbates the impact of latency, but neither directly causes the network overload characteristic of congestion.

D. Delay caused by transmission and serialization: This delay is a function of packet size and link bandwidth (Packet Size / Bandwidth). It is a fixed component of latency for a given packet and link, existing even on an empty network, and is not the variable delay caused by network overload.

1. Official Vendor Documentation: Riverbed Technology, "SteelHead Deployment Guide - Version 9.8," Section: "About Network Congestion," pp. 55-56. The guide explains that congestion happens when links, particularly oversubscribed WAN links, are saturated with traffic from multiple sources, leading to queuing and packet loss. This directly supports the concepts of "other traffic" and "oversubscription."

2. University Courseware: J. Kurose and K. Ross, "Computer Networking: A Top-Down Approach, 7th Edition," Pearson, 2017. Chapter 3, Section 3.7, "Congestion Control in TCP." The text defines the causes of congestion as scenarios where multiple hosts are contending for limited bandwidth on a shared link, causing router queues to overflow. This aligns with "Other traffic on the network" and is the fundamental problem that "link oversubscription" creates. (This textbook is standard curriculum at numerous reputable universities).

3. University Courseware: MIT OpenCourseWare, "6.829 Computer Networks, Fall 2002," Lecture 15 Notes. The lecture notes describe congestion as a state where "the aggregate demand for a resource (link, buffer) exceeds its capacity." This directly supports option C as the fundamental cause.