Date: Wed, 11 Sep 2002 19:13:30 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: net@freebsd.org Cc: justin@mac.com Subject: Re: computing the Ack Seq. No. Message-ID: <200209120213.g8C2DUZL032515@vashon.polstra.com> In-Reply-To: <5A3F3774-C5B2-11D6-A7E3-00306544D642@mac.com> References: <5A3F3774-C5B2-11D6-A7E3-00306544D642@mac.com>
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In article <5A3F3774-C5B2-11D6-A7E3-00306544D642@mac.com>, Justin C. Walker <justin@mac.com> wrote: > > On Wednesday, September 11, 2002, at 10:50 AM, John Polstra wrote: > > > In article <F6467yRLVbNDN4CxpAO000022f6@hotmail.com>, > > soheil h <soheil_h_y@hotmail.com> wrote: > >> hi list > >> I wrote the code below and it doesn't work correctly > >> please tell me what is wrong > >> int len, tlen; > >> tcpiphdr ti, ti_send; > >> int hlen; > >> int acklen; > >> /* > >> the hlen is ip header hlen > >> */ > >> .... > >> /* the ip->ip_len is ntohs'ed by NTOHS in io_input() */ > >> len = ti->ti_len; > >> tlen = ti->ti_off << 2; > >> acklen = len - hlen - tlen; > >> ti_send->ti_ack = htonl(ntohl(ti->ti_seq) + acklen ); > >> /* this field is incorrect and i don't know why */ > > > > If the TH_SYN flag is set in the received packet, you must increment > > the ack by 1. Likewise, if the TH_FIN flag is set, you must increment > > the ack by 1. In other words, each of those flags counts the same as > > a byte of data, as far as the ack calculation is concerned. > > Also, it's not clear to me why you include the IP header length in the > count of acknowledged TCP bytes. Perhaps I missunderstand your code... I think that part is OK. "len" is the total packet length including the IP and TCP headers. He takes that and subtracts out the lengths of the headers to get the number of bytes to ack (acklen). That's correct: you ack only the payload, not the headers. John -- John Polstra John D. Polstra & Co., Inc. Seattle, Washington USA "Disappointment is a good sign of basic intelligence." -- Chögyam Trungpa To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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