/* @(#) $Header: tcphdr.c,v 1.7 94/10/06 16:15:36 deyke Exp $ */

/* TCP header conversion routines
 * Copyright 1991 Phil Karn, KA9Q
 */
#include "global.h"
#include "mbuf.h"
#include "tcp.h"
#include "ip.h"
#include "internet.h"

/* Convert TCP header in host format into mbuf ready for transmission,
 * link in data (if any). If ph != NULL, compute checksum, otherwise
 * take checksum from tcph->checksum
 */
struct mbuf *
htontcp(
register struct tcp *tcph,
struct mbuf *bp,
struct pseudo_header *ph)
{
	uint16 hdrlen;
	register char *cp;

	hdrlen =  TCPLEN;
	if(tcph->optlen > 0 && tcph->optlen <= TCP_MAXOPT){
		hdrlen += tcph->optlen;
	} else if(tcph->mss != 0){
		hdrlen += MSS_LENGTH;
	}
	bp = pushdown(bp,hdrlen);
	cp = bp->data;
	cp = put16(cp,tcph->source);
	cp = put16(cp,tcph->dest);
	cp = put32(cp,tcph->seq);
	cp = put32(cp,tcph->ack);
	*cp++ = hdrlen << 2;    /* Offset field */
	*cp = 0;
	if(tcph->flags.congest)
		*cp |= 64;
	if(tcph->flags.urg)
		*cp |= 32;
	if(tcph->flags.ack)
		*cp |= 16;
	if(tcph->flags.psh)
		*cp |= 8;
	if(tcph->flags.rst)
		*cp |= 4;
	if(tcph->flags.syn)
		*cp |= 2;
	if(tcph->flags.fin)
		*cp |= 1;
	cp++;
	cp = put16(cp,tcph->wnd);
	if(ph == NULLHEADER){
		/* Use user-supplied checksum */
		cp = put16(cp,tcph->checksum);
	} else {
		/* Zero out checksum field for later recalculation */
		*cp++ = 0;
		*cp++ = 0;
	}
	cp = put16(cp,tcph->up);

	/* Write options, if any */
	if(hdrlen > TCPLEN){
		if(tcph->optlen > 0)
			memcpy(cp,tcph->options,tcph->optlen);
		else if(tcph->mss != 0){
			*cp++ = MSS_KIND;
			*cp++ = MSS_LENGTH;
			cp = put16(cp,tcph->mss);
		}
	}
	/* Recompute checksum, if requested */
	if(ph != NULLHEADER)
		put16(&bp->data[16],cksum(ph,bp,ph->length));

	return bp;
}
/* Pull TCP header off mbuf */
int
ntohtcp(
register struct tcp *tcph,
struct mbuf **bpp)
{
	int hdrlen,i,optlen,kind;
	register int flags;
	char hdrbuf[TCPLEN],*cp;

	i = pullup(bpp,hdrbuf,TCPLEN);
	/* Note that the results will be garbage if the header is too short.
	 * We don't check for this because returned ICMP messages will be
	 * truncated, and we at least want to get the port numbers.
	 */
	tcph->source = get16(&hdrbuf[0]);
	tcph->dest = get16(&hdrbuf[2]);
	tcph->seq = get32(&hdrbuf[4]);
	tcph->ack = get32(&hdrbuf[8]);
	hdrlen = (hdrbuf[12] & 0xf0) >> 2;
	flags = hdrbuf[13];
	tcph->flags.congest = flags & 64;
	tcph->flags.urg = flags & 32;
	tcph->flags.ack = flags & 16;
	tcph->flags.psh = flags & 8;
	tcph->flags.rst = flags & 4;
	tcph->flags.syn = flags & 2;
	tcph->flags.fin = flags & 1;
	tcph->wnd = get16(&hdrbuf[14]);
	tcph->checksum = get16(&hdrbuf[16]);
	tcph->up = get16(&hdrbuf[18]);
	tcph->mss = 0;
	tcph->optlen = hdrlen - TCPLEN;

	/* Check for option field. Only space for one is allowed, but
	 * since there's only one TCP option (MSS) this isn't a problem
	 */
	if(i < TCPLEN || hdrlen < TCPLEN)
		return -1;      /* Header smaller than legal minimum */
	if(tcph->optlen == 0)
		return (int)hdrlen;     /* No options, all done */

	if(tcph->optlen > len_p(*bpp)){
		/* Remainder too short for options length specified */
		return -1;
	}
	pullup(bpp,tcph->options,tcph->optlen); /* "Can't fail" */
	/* Process options */
	for(cp=tcph->options,i=tcph->optlen; i > 0;){
		kind = *cp++;
		/* Process single-byte options */
		switch(kind){
		case EOL_KIND:
			i--;
			cp++;
			return (int)hdrlen;     /* End of options list */
		case NOOP_KIND:
			i--;
			cp++;
			continue;       /* Go look for next option */
		}
		/* All other options have a length field */
		optlen = uchar(*cp++);

		/* Process valid multi-byte options */
		switch(kind){
		case MSS_KIND:
			if(optlen == MSS_LENGTH){
				tcph->mss = get16(cp);
			}
			break;
		}
		optlen = max(2,optlen); /* Enforce legal minimum */
		i -= optlen;
		cp += optlen - 2;
	}
	return (int)hdrlen;
}
