Readme.md

P2P-BitTorrent

Note:
This document is written to describe my code design and implementation in general.
For more specific details, please look over the document Design.pdf.
by ssjjcao [email protected]

1. additional files written for the project

file	description
packet.h	define the structure of a packet and its header
list.[h|c]	implement data structure of single linked list
store.[h|c]	store information of the chunk(s) this peer has; simulate tracker, manage information of all peers' chunks.
handler.[h|c]	handle packets received; send and receive chunk data reliably; implement congestion control.
task_get.[h|c]	initialize and manage GET task from user's cmd
conn.[h|c]	simulate connection of TCP; define structure(pool) to manage simultaneous downloads.

2. some structures

(1) GET task

    typedef struct task_get_s {  
      int get_num; //chunks number of this GET task
      int *status; //0: not start; 1: done; 2: processing.
      bt_peer_t **providers; //senders for each chunk
      chunk_t *get_chunks; //chunks to get
      char **chunk_data; //chunk data for each chunk
      char output[BT_FILENAME_LEN]; //output file
    } task_get_t;

(2) chunk and chunk buffer

    typedef struct chunk_s {
      int id;
      char chunk_hash[SHA1_HASH_SIZE]; //chunk hash(20 bytes) after shahash
    } chunk_t;

    typedef struct chunk_buffer_s {
      char chunk_hash[SHA1_HASH_SIZE];
      char *data_buf; //chunk data of chunk_hash
    } chunk_buffer_t;

(3) download and upload connection

    typedef struct down_conn_s {
      int from_here; //used when caching data into chunk_buf->data_buf
      int next_ack; //next ack expected, range form 1 to 512
      bt_peer_t *sender;
      chunk_buffer_t *chunk_buf; //used to cache data
    } down_conn_t;

    typedef struct up_conn_s {
      int last_ack;
      int to_send; //index of packet to send, range from 0 to (512 - 1)
      int available; //index(of last available packet in sender's window) + 1
      int dup_times; //duplicate times of last_ack
      int cwnd; //(dynamic)congestion window size
      int ssthresh; //slow start threshold
      int rtt_flag; //represent the end of a rtt
      long begin_time;
      packet_t **pkts; //cache all packets to send
      bt_peer_t *receiver;
    } up_conn_t;

(4) download and upload pool

    typedef struct down_pool_s {
      down_conn_t **conns;
      int conn_num; //current download connection number of peer
      int max_conn; //maxmium download connection number of peer
    } down_pool_t;

    typedef struct up_pool_s {
      up_conn_t **conns;
      int conn_num;
      int max_conn;
    } up_pool_t;

In fact, I only implement simultaneous download from other peers.
up_pool_t is useless in my implement, just in consistence with down_pool_t.

3. methods used to implement reliable transmission and congestion control

function	method
reliable transmission	GBN
simultaneous transmission	connection pool(a structure)
timer	signal: SIGALRM
congestion control	TCP Tahoe

Specific implementation of reliable transmission and congestion control: please look over Design