//
// FILE: seg.h

// Description: This file contains segment definitions and interfaces to send and receive segments. 
// The prototypes support snp_sendseg() and snp_rcvseg() for sending to the network layer.
//
// Date: April 18, 2008
//       April 21, 2008 **Added more detailed description of prototypes fixed ambiguities** ATC
//       April 26, 2008 **Added checksum descriptions**
//

#ifndef SEG_H
#define SEG_H

#include "constants.h"

//Segment type definition. Used by SRT.
#define	SYN 0
#define	SYNACK 1
#define	FIN 2
#define	FINACK 3
#define	DATA 4
#define	DATAACK 5

//segment header definition. 

typedef struct srt_hdr {
	unsigned int src_port;        //source port number
	unsigned int dest_port;       //destination port number
	unsigned int seq_num;         //sequence number
	unsigned int ack_num;         //ack number
	unsigned short int length;    //segment data length
	unsigned short int  type;     //segment type
	unsigned short int  rcv_win;  //currently not used
	unsigned short int checksum;  //checksum for this segment
} srt_hdr_t;

//segment definition

typedef struct segment {
	srt_hdr_t header;
	char data[MAX_SEG_LEN];
} seg_t;

//This is the data structure exchanged between the SNP process and the SRT process.
//It contains a node ID and a segment. 
//For snp_sendseg(), the node ID is the destination node ID of the segment.
//For snp_recvseg(), the node ID is the source node ID of the segment.
typedef struct sendsegargument {
	int nodeID;		//node ID 
	seg_t seg;		//a segment 
} sendseg_arg_t;

//SRT process uses this function to send a segment and its destination node ID in a sendseg_arg_t structure to SNP process to send out. 
//Parameter network_conn is the TCP descriptor of the connection between the SRT process and the SNP process. 
//Return 1 if a sendseg_arg_t is succefully sent, otherwise return -1.
int snp_sendseg(int network_conn, int dest_nodeID, seg_t* segPtr);

//SRT process uses this function to receive a  sendseg_arg_t structure which contains a segment and its src node ID from the SNP process. 
//Parameter network_conn is the TCP descriptor of the connection between the SRT process and the SNP process. 
//When a segment is received, use seglost to determine if the segment should be discarded, also check the checksum.  
//Return 1 if a sendseg_arg_t is succefully received, otherwise return -1.
int snp_recvseg(int network_conn, int* src_nodeID, seg_t* segPtr);

//SNP process uses this function to receive a sendseg_arg_t structure which contains a segment and its destination node ID from the SRT process.
//Parameter tran_conn is the TCP descriptor of the connection between the SRT process and the SNP process. 
//Return 1 if a sendseg_arg_t is succefully received, otherwise return -1.
int getsegToSend(int tran_conn, int* dest_nodeID, seg_t* segPtr); 

//SNP process uses this function to send a sendseg_arg_t structure which contains a segment and its src node ID to the SRT process.
//Parameter tran_conn is the TCP descriptor of the connection between the SRT process and the SNP process. 
//Return 1 if a sendseg_arg_t is succefully sent, otherwise return -1.
int forwardsegToSRT(int tran_conn, int src_nodeID, seg_t* segPtr); 

// for seglost(seg_t* segment):
// a segment has PKT_LOST_RATE probability to be lost or invalid checksum
// with PKT_LOST_RATE/2 probability, the segment is lost, this function returns 1
// If the segment is not lost, return 0. 
// Even the segment is not lost, the packet has PKT_LOST_RATE/2 probability to have invalid checksum
// We flip  a random bit in the segment to create invalid checksum
int seglost(seg_t* segPtr); 
//
//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

//This function calculates checksum over the given segment.
//The checksum is calculated over the segment header and segment data.
//You should first clear the checksum field in segment header to be 0.
//If the data has odd number of octets, add an 0 octets to calculate checksum.
//Use 1s complement for checksum calculation.
unsigned short checksum(seg_t* segment);

//Check the checksum in the segment,
//return 1 if the checksum is valid,
//return -1 if the checksum is invalid
int checkchecksum(seg_t* segment);

#endif