This is a fantastic project. Building a "toy" version is the best way to learn. Let's make a plan.

The key is to not build everything at once. We'll separate the project into two main parts: the Data Plane (getting a ping to work) and the Control Plane (managing the user and session).

We'll start with the Data Plane, and we'll "hard-code" all the connections first, just to make data flow.

Here is a 4-milestone plan.

Milestone 1: The "Core" Data Tunnel (S-GW + P-GW)

Your first goal is to build the "GTP-U Tunnel." This is two programs that shuffle data between a virtual "internet" and a virtual "radio" network.

• Component 1: The pgw (P-GW)

  • Job: This program is your "exit ramp" to the internet.
  • Task:
    1. Create a TUN/TAP device (e.g., tun0). This is your "internet."
    2. Listen on a UDP port (e.g., 2152) for "GTP-U" packets.
    3. Forwarding (Uplink): When a packet arrives, de-capsulate it (strip the GTP-U header) and write the inner IP packet to tun0.
    4. Forwarding (Downlink): When an IP packet arrives from tun0 (like a ping reply), en-capsulate it (wrap it in a GTP-U header) and send it over UDP to the S-GW's address.

• Component 2: The sgw (S-GW)

  • Job: For V1, this is a simple "data router."
  • Task:
    1. Listen on one UDP port for the eNodeB (e.g., 2153).
    2. Listen on another for the P-GW (e.g., 2154).
    3. Forwarding (Uplink): When a packet arrives from the eNodeB, forward it to the P-GW.
    4. Forwarding (Downlink): When a packet arrives from the P-GW, forward it to the eNodeB.

✅ End of Milestone 1: You can send a GTP-U packet to your sgw and see it come out the pgw's tun0 device.

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Milestone 2: The "Radio" Link (UE + eNodeB)

Now we build the two ends of the "radio" link. We'll use a simple UDP socket as our "simulated air."

• Component 3: The ue (Phone)

  • Job: This is the "client" machine.
  • Task:
    1. Create its own tun0 device. This is the "phone's" IP stack.
    2. Forwarding (Uplink): When an IP packet (like a ping) arrives from its tun0, send this raw IP packet over a UDP socket (our "air") to the eNodeB.
    3. Forwarding (Downlink): When a packet arrives from the "air," write it to tun0.

• Component 4: The enb (eNodeB)

  • Job: This is your "tower." It's the bridge between the "air" and the "core."
  • Task:
    1. Listen on a UDP port for the "air" (from the ue).
    2. Forwarding (Uplink): When a raw IP packet arrives from the ue, en-capsulate it into a GTP-U packet and send it to the sgw's UDP port.
    3. Forwarding (Downlink): When a GTP-U packet arrives from the sgw, de-capsulate it and send the inner IP packet to the ue over the "air" (UDP).

✅ End of Milestone 2: You can ping from the ue's tun0 device, and the ping will go all the way through enb -> sgw -> pgw -> pgw's tun0 and back!

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Milestone 3: The "Brains" (MME + HSS)

Right now, all the IP addresses are hard-coded. Now we add the "brains" to set up the session automatically. This is the "Control Plane."

• Component 5: The hss (Database)

  • Job: A simple user database.
  • Task: Create a simple server (e.g., TCP) that listens for a "user ID" and responds with "OK, here are their security keys." (We can skip the complex Diameter protocol and just invent our own simple JSON-based API).

• Component 6: The mme (Orchestrator)

  • Job: This is the most complex part. It manages the "Attach Procedure."
  • Task:
    1. Listen for connections from the enb (this is the S1-MME interface).
    2. When the ue connects, the enb will forward its "Attach Request" to the mme.
    3. The mme then performs the "login" sequence:
       • Asks the hss if the user is valid.
       • Tells the sgw and pgw to "Create Session" (using the GTP-C protocol—a new protocol you'll need to implement!).
       • Tells the enb to activate the "radio bearer" (the pipe) for the ue.

✅ End of Milestone 3: A user can "boot" the ue program, and it will automatically attach and get an IP address, without hard-coding.

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Milestone 4: The "Real" Radio Protocols (RLC, MAC, PDCP)

Finally, we make our "simulated air" (the UDP socket) imperfect.

• Job: Go back into the ue and enb programs.
• Task:
  1. Instead of just sending the IP packet over UDP, you first pass it to a PDCP "library" (which you write) to encrypt it.
  2. Then, you pass it to an RLC "library" (which you write) to segment it (slice it into smaller pieces).
  3. This is when you can use the tc (Traffic Control) tool we discussed. You'll run tc on your enb's "air" link to randomly drop 5% of packets.
  4. You can then test if your RLC layer's re-transmission logic kicks in and successfully re-sends the lost pieces.

✅ End of Milestone 4: You have a working, miniature LTE network that can handle packet loss, just like the real thing.

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This is a big, exciting, and very rewarding project.

Does it make sense to start with Milestone 1, just focusing on building that pgw and sgw data tunnel?