Packet Data Setup

Transport Modes

FOU is recommended — offloads IP routing and NAT to a separate machine.

PDSN Configuration

FOU (Recommended)

{ "packet": { "transport": "fou", "fou_remote": "192.168.1.100:17010", "fou_local_port": 17011 } }

FOU-TCP with Docker Compose

The compose stack includes a fou-nat gateway container and a speedtest container. fou-nat runs the FOU-TCP relay, NAT, Unbound DNS, and an nginx proxy to the speed test.

Mobile packet-data clients can open:

• http://speed/
• http://speed.local.1xbts.org/

The default PDSN config advertises 10.55.0.1 as DNS so these names resolve on the packet-data link. Set UNBOUND_FORWARD_ADDRS to choose upstream DNS servers for other domains.

Use docker compose up -d --build after pulling updates so Compose rebuilds images that copy local files, including fou-nat and speedtest.

TUN

{ "packet": { "transport": "tun" } }

Creates a kernel TUN device per session with automatic IP config and NAT rules. Requires root.

FOU Remote Setup

On the Linux box handling IP routing (replace <pdsn_ip> with the PDSN/runtime host IP):

sudo modprobe fou
sudo ip fou add port 17010 ipproto 4
sudo ip link add fou0 type ipip \
    remote <pdsn_ip> local <linux_ip> \
    encap fou encap-dport 17011 encap-sport 17010
sudo ip addr add 10.55.0.1/24 dev fou0
sudo ip link set fou0 up
sudo sysctl -w net.ipv4.ip_forward=1
sudo iptables -t nat -A POSTROUTING -s 10.55.0.0/24 -o eth0 -j MASQUERADE

Verify

sudo ip fou show
ip addr show fou0
sudo tcpdump -i fou0 -n
sudo iptables -t nat -L -n -v | grep 10.55.0

Teardown

sudo ip link del fou0
sudo ip fou del port 17010
sudo iptables -t nat -D POSTROUTING -s 10.55.0.0/24 -o eth0 -j MASQUERADE

IP Pool

Lowest available address assigned per device/session key. Open PPP sessions are cached by IMSI/ESN across packet traffic-channel changes, and the cached session keeps its mobile IP reserved until pdsn.ppp_session_timeout_secs expires. The default timeout is 1800 seconds. Expired cached sessions are reaped periodically and release their reserved IPs.

Override packet.primary_dns and packet.secondary_dns in pdsn.json if mobiles should use external DNS directly instead of the gateway resolver.

Data Path

Uplink: Mobile → PPP/HDLC → RLP → MuxPDU → BTS → Abis bearer → BSC → A8 → PCF → A10 → PDSN PPP/IPCP → FOU/TUN/FOU-TCP → NAT/internet

Downlink: Internet/NAT → FOU/TUN/FOU-TCP → PDSN → A10 → PCF → A8 → BSC → Abis bearer → BTS → RLP/MuxPDU → mobile

Throughput

Troubleshooting