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EXPERIMENTS.md

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SCION Experiments Guide

Hands-on experiments to explore SCION's capabilities.

Experiment 1: Path Discovery

Goal: Understand how SCION discovers multiple paths.

Steps

# 1. Start the network
make up && sleep 20

# 2. View paths from AS111 to AS211
make paths

Expected Output

╔══════════════════════════════════════════════════════════════╗
║           SCION Path Discovery                               ║
╚══════════════════════════════════════════════════════════════╝

  Source:      1-ff00:0:111
  Destination: 2-ff00:0:211

[0] Hops: [1-ff00:0:111#2 > 2-ff00:0:211#2] MTU: 1472 Latency: ~1ms
    (Direct peer link)

[1] Hops: [1-ff00:0:111#1 > 1-ff00:0:110#2 > 1-ff00:0:110#1 > 
          2-ff00:0:210#1 > 2-ff00:0:210#2 > 2-ff00:0:211#1]
    MTU: 1472 Latency: ~3ms
    (Via core ASes)

Analysis

  • Path 0 is shorter (direct peer link)
  • Path 1 traverses both ISD cores
  • Both paths are valid and usable

Experiment 2: Link Failure & Failover

Goal: Observe how SCION handles link failures.

Steps

# 1. Verify current paths
make paths

# 2. Break the core link (stop AS110's router)
make break-link ROUTER=router-110

# 3. Wait for path updates (~10 seconds)
sleep 10

# 4. Check available paths again
make paths

Expected Result

  • Before: 2+ paths available
  • After: Only the PEER path remains (111 → 211 direct)
  • The core path through AS110 is no longer valid

Restore

make restore-link ROUTER=router-110
sleep 20
make paths  # Core path should return

Experiment 3: Latency Comparison

Goal: Compare latency across different paths.

Steps

# Measure latency on all paths
make measure

Analysis

  • Direct peer link: ~1-2ms (fewer hops)
  • Core path: ~3-5ms (more hops)
  • Latency difference is small in Docker (same host)
  • In real networks, geographic distance matters more

Experiment 4: Interactive Path Selection

Goal: Manually select a path and observe PCFS.

Steps

make interactive

Walk-through

  1. Script shows all available paths
  2. You select a path by index (0, 1, ...)
  3. SCMP echo requests are sent over that specific path
  4. Observe: the path you chose is embedded in every packet

Experiment 5: Multi-Path Communication

Goal: Demonstrate using multiple paths simultaneously.

Steps

Open two terminal windows:

Terminal 1 (via core path):

make shell-111
# Inside container:
scion ping 2-ff00:0:211 -c 50

Terminal 2 (observe traffic):

docker logs -f scion-router-110
# You should see packets being forwarded

Advanced: Split Traffic

A real SCION application could:

  1. Discover all paths via daemon
  2. Measure latency on each
  3. Distribute traffic based on capacity
  4. Failover when a path breaks

Experiment 6: Topology Modification

Goal: Add a new AS to the network.

Steps

  1. Edit topology/topology.topo:
ASes:
  # Add new AS in ISD 1
  "1-ff00:0:112":
    cert_issuer: 1-ff00:0:110
    mtu: 1472

links:
  # Connect to existing leaf
  - {a: "1-ff00:0:111#3", b: "1-ff00:0:112#1", linkAtoB: PEER}

  1. Update docker-compose.yml (add services for AS112)

  2. Update scripts/setup.sh (generate AS112 configs)

  3. Regenerate and restart:

make clean
make setup
make up

Experiment 7: Traffic Analysis

Goal: Observe SCION packet structure.

Steps

# 1. Shell into a router container
docker exec -it scion-router-111 /bin/bash

# 2. Capture packets (if tcpdump is available)
tcpdump -i any -n port 50001 or port 50002

# 3. In another terminal, generate traffic
make ping

Observation

SCION packets contain:

  • Source/destination addresses with AS identifiers
  • Complete path in the header
  • Hop-by-hop forwarding information

Experiment Summary

# Experiment SCION Concept
1 Path Discovery Beaconing, Path Service
2 Link Failure Isolation, Failover
3 Latency Comparison Path Metrics
4 Interactive Selection PCFS, Sender Control
5 Multi-Path Path Diversity
6 Topology Modification AS Configuration
7 Traffic Analysis Packet Structure

Troubleshooting

No paths available?

# Check control service logs
docker logs scion-cs-111

# Ensure all services are running
make status

Ping fails?

# Verify daemon is reachable
docker exec scion-host-111 scion showpaths 2-ff00:0:211 --no-probe

Services not starting?

# Check for config errors
docker compose logs | grep -i error