Experiment report 1. Actions taken - Inspected local addressing and routing: - Ran `ip addr show lo && ip addr show User-eth0 && ip route show`. - Found stable loopback address `128.173.10.1/32`. - Found point-to-point link to Uni on `User-eth0`: - Local: `10.0.6.1/30` - Peer: `10.0.6.2/30` - Confirmed default route: - `default via 10.0.6.2 dev User-eth0 src 128.173.10.1` - No route changes were made, because the default route was already correctly installed. - Tested DNS resolution for the reported site: - Ran `getent hosts acm.org`. - Result: `acm.org` resolved to `198.82.0.1`. - Tested local connectivity to Uni: - Ran `ping -c 3 -I 128.173.10.1 10.0.6.2`. - Result: 3/3 replies, about 4 ms latency. - This confirmed the direct link to Uni was working when sourced from the stable loopback. - Tested network-layer reachability to acm.org: - Ran `ping -c 3 -I 128.173.10.1 198.82.0.1`. - Result: 3/3 replies, about 98 ms latency. - This showed that DNS and IP routing to the destination were functional. - Tested HTTP access: - Ran `curl -v --max-time 10 --interface 128.173.10.1 http://acm.org/`. - TCP connection to `198.82.0.1:80` succeeded. - Server responded with: - `HTTP/1.1 503 Service Unavailable` - `Server: nginx/1.18.0` - Body: `503 Service Unavailable` - Tested HTTPS access: - Ran `curl -vk --max-time 10 --interface 128.173.10.1 https://acm.org/`. - TCP connection to `198.82.0.1:443` succeeded. - TLS 1.3 handshake completed. - Certificate was for `CN=acm.org`, `O=ACM Digital Library`, issued by `Testbed CA`. - Server again responded with: - `HTTP/1.1 503 Service Unavailable` - `Server: nginx/1.18.0` - Re-tested HTTPS later with timing details: - Ran: - `curl -sS -o /tmp/acm.out -w 'dns=%{time_namelookup} connect=%{time_connect} tls=%{time_appconnect} firstbyte=%{time_starttransfer} total=%{time_total} http=%{http_code} remote=%{remote_ip}\n' --max-time 10 --interface 128.173.10.1 https://acm.org/ && cat /tmp/acm.out` - Result: - DNS: 0.025 s - TCP connect: 0.123 s - TLS: 0.258 s - First byte: 0.356 s - HTTP status: 503 - Remote IP: `198.82.0.1` - Body: `503 Service Unavailable` - Reported the final result to the laptop owner and closed the task with `report_done`. 2. Justification behind each decision - I first checked loopback and routing because the node instructions said that the loopback address is the stable address to advertise and to use as the source for diagnostic traffic to non-adjacent nodes. This avoided misleading results caused by sourcing traffic from point-to-point infrastructure addresses. - I tested DNS resolution before deeper connectivity checks because the owner reported a website loading failure, and DNS failure is a common cause of browser errors. - I pinged Uni first to confirm that the directly connected university neighbor was reachable and that the default next hop was operational. - I pinged `198.82.0.1` from `128.173.10.1` to distinguish network reachability problems from application-layer problems. - I tested both HTTP and HTTPS because a browser may try either protocol or redirect between them, and the failure mode could differ between ports 80 and 443. - I used `curl -v` and `curl -vk` to capture objective protocol-level evidence: TCP connection status, TLS handshake status, HTTP response code, server identity, and response body. - I did not change any routes because the default route via Uni was present and functional. There was no evidence of a local route problem. - I contacted Uni/KP only after collecting local evidence. This followed the instruction not to simply relay the owner’s vague report, but to reproduce the problem and provide technical observations. - I waited for KP confirmation before reporting back to the owner because the issue appeared to be remote service-side, and the KP could investigate beyond my local network view. 3. What I discovered about the network - The User node’s stable loopback address is `128.173.10.1/32`. - The User node is directly connected to Uni over `User-eth0`: - User link address: `10.0.6.1/30` - Uni peer address: `10.0.6.2/30` - User’s default route already pointed to Uni: - `default via 10.0.6.2 dev User-eth0 src 128.173.10.1` - Uni advertised its own stable loopback as `128.173.0.1/32`. - Uni provides upstream/default connectivity for campus via `10.0.6.2`. - Point-to-point infrastructure prefixes are not advertised network-wide. - The path from User to `acm.org` was operational at several layers: - DNS worked: `acm.org` resolved to `198.82.0.1`. - ICMP worked: pings to `198.82.0.1` succeeded. - TCP worked: connections to ports 80 and 443 succeeded. - TLS worked: HTTPS handshake completed successfully. - The failure was at the HTTP application layer: the nginx server returned `503 Service Unavailable`. - The Knowledge Plane confirmed that Uni’s forwarding, routes, firewall, and NAT were not the cause. Uni had upstream routes via AS1, forwarding was enabled, packet filter policies were ACCEPT, and only outbound MASQUERADE NAT was present. - The problem was reproduced outside the university network by AS1, which ruled out a university-local network failure. - ACM confirmed that the ACM Digital Library endpoint was reachable at network, TCP, and TLS layers but was returning HTTP/HTTPS 503 from the service itself. - The final diagnosis was `CANNOT pending ACM administrator action`: no local network fix was available, and ACM administrators had already been notified. 4. Coordination with other agents - Received a route update from Uni: - Uni advertised stable loopback `128.173.0.1/32`. - Uni stated it provides upstream/default connectivity via link peer `10.0.6.2`. - Uni stated no point-to-point infrastructure prefixes are advertised. - Received a route request from Uni asking me to advertise my stable loopback and any reachable attached prefixes/devices, and to confirm my default route. - Sent Uni a route update: - Advertised my stable loopback: `128.173.10.1/32`. - Stated I had no additional directly attached campus prefixes/devices to advertise. - Confirmed my default route: - `default via 10.0.6.2 dev User-eth0 src 128.173.10.1` - Sent Uni/KP an investigation request with my local findings: - Owner reported browser failure for `acm.org`. - DNS resolved to `198.82.0.1`. - ICMP succeeded. - TCP to ports 80 and 443 succeeded. - HTTPS TLS handshake succeeded. - Both HTTP and HTTPS returned nginx `503 Service Unavailable`. - Sent follow-up messages to Uni/KP while waiting for a diagnosis, including a later repeated HTTPS test that again returned HTTP 503. - Received final KP diagnosis from Uni: - `CANNOT pending ACM administrator action`. - Uni, AS1, AS2, and ACM had investigated. - ACM had already been notified. - No routing, DNS, TCP, TLS, firewall, NAT, or local configuration change was indicated.