P2P-CI: Peer-to-Peer System with Centralized Index

CSC/ECE 573 – Internet Protocols | Project #1 | Spring 2026

Overview

This project implements a Peer-to-Peer system with a Centralized Index (P2P-CI) for downloading RFC (Request for Comments) documents. Peers share RFC files directly with each other, while a central server maintains an index of which peer has which RFC.

All communication happens over TCP using two custom application-layer protocols:

P2S (Peer-to-Server): ADD, LOOKUP, LIST
P2P (Peer-to-Peer): GET

File Structure

p2p-ci/
├── server.py          # Centralized index server
├── peer.py            # Peer node (upload server + client)
├── Makefile           # Build and run instructions
├── README.md          # This file
├── screenshots/       # Demo screenshots
├── peer1/
│   └── rfcs/
│       ├── rfc2345.txt
│       └── rfc3457.txt
└── peer2/
    └── rfcs/
        ├── rfc123.txt
        └── rfc3457.txt

Requirements

Python 3.x
No external libraries (uses Python standard library only)

How to Run

Step 1 — Start the Server

make run-server
# or:
python3 server.py

Listens on well-known port 7734.

Step 2 — Start Peer 1 (new terminal)

make run-peer1 HOST=localhost
# or:
cd peer1 && python3 ../peer.py localhost

Step 3 — Start Peer 2 (new terminal)

make run-peer2 HOST=localhost
# or:
cd peer2 && python3 ../peer.py localhost

Each peer automatically:

Starts an upload server on a random OS-assigned port
Connects to the server at port 7734
Registers all RFCs found in its local rfcs/ directory

Application Protocols

P2S Protocol (Peer ↔ Server)

Request format:

METHOD RFC <number> P2P-CI/1.0\r\n
Host: <hostname>\r\n
Port: <upload-port>\r\n
Title: <rfc-title>\r\n
\r\n

Method	Purpose
ADD	Register a locally available RFC with the server
LOOKUP	Find all peers that have a specific RFC
LIST	Get the full RFC index from the server

Response format:

P2P-CI/1.0 <status-code> <phrase>\r\n
\r\n
RFC <number> <title> <hostname> <port>\r\n
...\r\n
\r\n

P2P Protocol (Peer ↔ Peer)

Request format:

GET RFC <number> P2P-CI/1.0\r\n
Host: <hostname>\r\n
OS: <operating-system>\r\n
\r\n

Response format:

P2P-CI/1.0 <status-code> <phrase>\r\n
Date: <date>\r\n
OS: <os>\r\n
Last-Modified: <date>\r\n
Content-Length: <bytes>\r\n
Content-Type: text/plain\r\n
\r\n
<file contents>

Status Codes

Code	Phrase	Meaning
200	OK	Success
400	Bad Request	Malformed request
404	Not Found	RFC not found
505	P2P-CI Version Not Supported	Wrong protocol version

Peer Menu

1. ADD    — manually register an RFC with the server
2. LOOKUP — find which peers have a specific RFC
3. LIST   — view all RFCs registered across all peers
4. GET    — download an RFC from another peer (LOOKUP + GET + ADD)
5. EXIT   — disconnect from server and quit

Download Flow (Choice 4)

User selects 4
    │
    ├─► LOOKUP  → asks server "who has RFC X?"
    │           ← server replies with peer hostnames + upload ports
    │
    ├─► GET     → opens NEW TCP connection directly to peer's upload port
    │           ← peer sends headers + file over same connection
    │             connection closed after transfer
    │
    └─► ADD     → tells server "I now have RFC X too"
                  server updates its index

Demo Scenarios

Scenario 1 — Server Startup and Peer Registration

The server starts on port 7734. When peers connect, they register their RFCs using ADD requests. The server updates its peer list and RFC index and responds 200 OK.

Server — receives peer1's ADD request:

New TCP connection logged immediately
ADD request received in correct protocol format
Peer registered: [Server] Added Mac.lan:63235
RFC added to index: [Server] Added RFC 2345 from Mac.lan
Response sent: P2P-CI/1.0 200 OK with echoed RFC data line

Peer 1 — connects and registers RFCs:

Upload server starts on random port 63235
Connects to server at port 7734
Auto-registers RFC 2345 and RFC 3457
Full ADD request printed in protocol format
Server's 200 OK response with RFC data displayed

Server — receives peer2's ADD requests:

Server handles both peers concurrently in separate threads
Peer 2 registers RFC 3457 and RFC 123 independently

Scenario 2 — LIST ALL

A peer queries the server for the complete RFC index across all active peers.

Request sent:

LIST ALL P2P-CI/1.0
Host: Mac.lan
Port: 63235

Response received:

P2P-CI/1.0 200 OK

RFC 123 A Proferred Official ICP Mac.lan 63237
RFC 3457 Requirements for IPsec Remote Access Scenarios Mac.lan 63237
RFC 2345 Domain Names and Company Name Retrieval Mac.lan 63235

All 3 RFCs from both peers returned — RFC number, title, hostname, and upload port for each entry.

Scenario 3 — LOOKUP

A peer looks up which peers have a specific RFC.

Request sent:

LOOKUP RFC 3457 P2P-CI/1.0
Host: Mac.lan
Port: 63235
Title:

Response received:

P2P-CI/1.0 200 OK

RFC 3457 Requirements for IPsec Remote Access Scenarios Mac.lan 63237

Server returns hostname and upload port of every peer that has RFC 3457.

Scenario 4 — GET (200 OK) — Peer-to-Peer File Download

Peer 1 downloads RFC 3457 directly from Peer 2 over a new TCP connection to Peer 2's upload port.

Peer 1 — downloader side:

Full flow on Peer 1:

LOOKUP → server returns Peer 2's upload port (63237)
GET request sent directly to Peer 2:

GET RFC 3457 P2P-CI/1.0
Host: Mac.lan
OS: Darwin 24.3.0

Peer 2 responds with all 5 required headers + file:

P2P-CI/1.0 200 OK
Date: Sun, 26 Apr 2026 21:30:16 GMT
OS: Darwin 24.3.0
Last-Modified: Fri, 17 Apr 2026 16:08:18 GMT
Content-Length: 1839
Content-Type: text/plain

File saved: rfcs/rfc3457.txt (1839 bytes)
ADD sent to server → server now knows Peer 1 also has RFC 3457

Peer 2 — uploader side:

Peer 2's upload server (background thread) receives the GET and serves the file:

[UPLOAD] Request from ('192.168.1.241', 63245):
GET RFC 3457 P2P-CI/1.0
Host: Mac.lan
OS: Darwin 24.3.0
[UPLOAD] Sent RFC 3457 to ('192.168.1.241', 63245) (1839 bytes)

Scenario 5 — 404 Not Found

A peer requests an RFC that does not exist in the system.

LOOKUP RFC 4342 P2P-CI/1.0
Host: Mac.lan
Port: 63237

Response: P2P-CI/1.0 404 Not Found

Server returns 404 Not Found when no active peer has the requested RFC.

Scenario 6 — 400 Bad Request

A client sends a request with an invalid/unrecognized method.

printf "BLAH RFC 123 P2P-CI/1.0\r\nHost: localhost\r\nOS: Mac\r\n\r\n" | nc localhost 63237

P2P-CI/1.0 400 Bad Request

The peer's upload server rejects any request that does not conform to the protocol format.

Scenario 7 — 505 P2P-CI Version Not Supported

A client sends a request with an unsupported protocol version.

printf "GET RFC 123 P2P-CI/2.0\r\nHost: localhost\r\nOS: Mac\r\n\r\n" | nc localhost 63237

P2P-CI/1.0 505 P2P-CI Version Not Supported

Only P2P-CI/1.0 is accepted. Any other version is rejected immediately.

Scenario 8 — Peer Removal on Disconnect

When a peer exits, the server removes all associated records from both the peer list and the RFC index.

Peer 1 exits cleanly:

Peer 1 selects option 5 → [PEER] Disconnecting... → TCP connection closed.

Server removes Peer 1's records:

[SERVER] Peer Mac.lan:63235 left. Removed 1 peer(s), 2 RFC(s).

Peer 2 runs LIST — only Peer 2's RFCs remain:

P2P-CI/1.0 200 OK

RFC 123 A Proferred Official ICP Mac.lan 63237
RFC 3457 Requirements for IPsec Remote Access Scenarios Mac.lan 63237

Peer 1's RFC 2345 is gone — server correctly cleaned up all records on disconnect.

Concurrency

The server uses Python threading — each peer connection runs in a dedicated thread. A threading.Lock protects the shared peer list and RFC index from race conditions.

The peer's upload server also runs in a background thread, allowing it to serve simultaneous download requests from other peers while the user interacts with the menu.

To test: start 3 or more peers simultaneously — all connect, register, and interact with the server independently without blocking each other.

Testing Error Cases

# 404 Not Found — RFC does not exist on that peer
printf "GET RFC 9999 P2P-CI/1.0\r\nHost: localhost\r\nOS: Mac\r\n\r\n" | nc localhost <upload_port>

# 400 Bad Request — malformed/unrecognized method
printf "BLAH RFC 123 P2P-CI/1.0\r\nHost: localhost\r\nOS: Mac\r\n\r\n" | nc localhost <upload_port>

# 505 Version Not Supported — wrong protocol version
printf "GET RFC 123 P2P-CI/2.0\r\nHost: localhost\r\nOS: Mac\r\n\r\n" | nc localhost <upload_port>

<upload_port> is printed at peer startup: [UPLOAD] Upload server listening on port XXXXX

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Repository files navigation

P2P-CI: Peer-to-Peer System with Centralized Index

Overview

File Structure

Requirements

How to Run

Step 1 — Start the Server

Step 2 — Start Peer 1 (new terminal)

Step 3 — Start Peer 2 (new terminal)

Application Protocols

P2S Protocol (Peer ↔ Server)

P2P Protocol (Peer ↔ Peer)

Status Codes

Peer Menu

Download Flow (Choice 4)

Demo Scenarios

Scenario 1 — Server Startup and Peer Registration

Scenario 2 — LIST ALL

Scenario 3 — LOOKUP

Scenario 4 — GET (200 OK) — Peer-to-Peer File Download

Scenario 5 — 404 Not Found

Scenario 6 — 400 Bad Request

Scenario 7 — 505 P2P-CI Version Not Supported

Scenario 8 — Peer Removal on Disconnect

Concurrency

Testing Error Cases

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Name		Name	Last commit message	Last commit date
Latest commit History 2 Commits
peer1/rfcs		peer1/rfcs
peer2/rfcs		peer2/rfcs
peer3/rfcs		peer3/rfcs
screenshots		screenshots
.DS_Store		.DS_Store
Makefile		Makefile
README.md		README.md
peer.py		peer.py
server.py		server.py

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Repository files navigation

P2P-CI: Peer-to-Peer System with Centralized Index

Overview

File Structure

Requirements

How to Run

Step 1 — Start the Server

Step 2 — Start Peer 1 (new terminal)

Step 3 — Start Peer 2 (new terminal)

Application Protocols

P2S Protocol (Peer ↔ Server)

P2P Protocol (Peer ↔ Peer)

Status Codes

Peer Menu

Download Flow (Choice 4)

Demo Scenarios

Scenario 1 — Server Startup and Peer Registration

Scenario 2 — LIST ALL

Scenario 3 — LOOKUP

Scenario 4 — GET (200 OK) — Peer-to-Peer File Download

Scenario 5 — 404 Not Found

Scenario 6 — 400 Bad Request

Scenario 7 — 505 P2P-CI Version Not Supported

Scenario 8 — Peer Removal on Disconnect

Concurrency

Testing Error Cases

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