Router Proxying ICMP Echo Replies for Firewalled Devices
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Solution Overview
Problem
Devices behind firewalls often disable ICMP echo replies, making remote diagnostics difficult due to non-compliance with RFC 793, especially with the rise of Windows XP Service Pack 2 and personal firewalls, which hinders network troubleshooting.
Innovation Solution
A routing device maintains an ARP table and sends periodic ICMP echo requests to all listed devices, tracks non-responding devices, and acts as a proxy by sending ARP requests and replies to determine the round trip time, allowing it to send a proxy ICMP echo reply to the requesting device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If devices disable ICMP echo replies for security purposes, then security is improved, but network diagnostics capability deteriorates
Solution Approach 1:
The router acts as an intermediary by maintaining ARP table information and generating ICMP echo replies on behalf of firewalled devices. When the router receives an ICMP echo request for a device that has disabled direct replies, it uses its cached ARP information to determine the device's presence and generates a proxy reply, thus maintaining diagnostics capability without requiring devices to expose themselves directly.
2Measurement precision
If routers send periodic ICMP echo requests to all devices, then network monitoring capability is improved, but network traffic increases
Solution Approach 1:
The router performs preliminary actions by maintaining continuous ARP table information and pre-caching device presence data. This allows the router to respond to ICMP echo requests without needing to actively query each device periodically, reducing unnecessary traffic while maintaining monitoring capability through the cached ARP information.
3Reliability
If routers act as proxy for ICMP echo requests, then network connectivity detection is improved, but response time may increase
Solution Approach 1:
The router maintains pre-cached ARP table information that includes device presence and timing data. When an ICMP echo request arrives, the router can immediately query its cached information and generate a proxy reply without needing to perform real-time device queries, thus minimizing additional delay while maintaining accurate connectivity detection.
Data Source
AI summary
A method and apparatus for determining reachability of a device connected to a computer network by receiving from a requesting device at least one ICMP echo request, the at least one ICMP echo request having a destination address associated with a non-echoing device; in response to the received ICMP echo request, sending a ping to the non-echoing device; receiving from the non-echoing device a reply to the sent ping; issuing from a routing device a subsequent at least one ICMP echo request to the non-echoing device and determining that no response is received to the issued at least one ICMP echo request; receiving at least one subsequent ICMP echo request from a requesting device, the at least one subsequent having a destination address associated with the non-echoing device; and in response to the received reply to the additional ping, sending a proxy ICMP echo reply to the requesting device.