P2P Reliable File Transfer with Congestion Control

Background

Time: Dec 2025
Context: Group course project for SUSTech CS305 (Computer Networks).
Repository: Tsurumalu/Computer-Network-Project-P2P

Goal

Build a reliable peer-to-peer (P2P) file transfer application on top of UDP. All transport semantics—handshaking, reliable delivery, retransmission, and congestion control—are implemented at the application layer. The system uses a network simulator for testing under packet loss, varying topologies, and peer crashes.

System Overview

File & chunk model: Files are split into 512 KiB chunks; each chunk is identified by a SHA-1 hash (20 bytes).
Peer model: Each peer holds a fragment (subset) of chunks and can upload to or download from other peers concurrently.
Download flow: On a DOWNLOAD command, a peer discovers missing chunks via WHOHAS / IHAVE, requests them with GET, then reassembles completed chunks into a pickle-serialized fragment file.
Concurrency: Single-threaded event loop using select; multiple chunk downloads from different peers run in parallel, with per-session upload limits enforced via DENIED.

Packet Protocol

Custom UDP packets (max 1400 bytes) with header fields for type, length, and sequence/ACK number:

Type	Code	Role
WHOHAS	0	Query which peers hold requested chunks
IHAVE	1	Reply listing available chunks
GET	2	Request a specific chunk
DATA	3	Transfer chunk payload segments
ACK	4	Acknowledge received segments
DENIED	5	Reject when upload connection limit is reached

Implementation Highlights

Reliable data transfer (RDT): Per-packet sequence numbers and ACKs; dynamic timeout from RTT estimation (α = 0.15, β = 0.3); timeout-based and fast retransmit (3 duplicate ACKs).
Congestion control (Tahoe-style): Slow start and congestion avoidance with cwnd and ssthresh; loss triggers window halving and return to slow start. cwnd history is recorded and plotted with matplotlib for presentation.
Structured codebase: Refactored peer.py with PeerState, DownloadSession, UploadSession, and a Peer class driving a non-blocking I/O loop.
Robustness: Handles concurrent multi-peer downloads, connection limits, peer crashes, and out-of-order segment buffering.
Testing: Validated with course-provided pytest suites (handshaking, basic transfer, concurrency, crash scenarios, and advanced topology tests) via grader.py and the network simulator.

Tech Stack

Python 3.12
UDP (socket) with select for single-threaded concurrency
SHA-1 chunk hashing
matplotlib (congestion window visualization)
pytest & network simulator (grading and integration tests)
Linux / WSL (required runtime environment)