Date: Mon, 23 Nov 1998 12:41:07 -0800 From: bmah@CA.Sandia.GOV (Bruce A. Mah) To: "Nick A. Fikouras" <nick@dcs.shef.ac.uk> Cc: bmah@california.sandia.gov, Andrey Tchoritch <andy@moldsat.md>, "freebsd-questions@FreeBSD.ORG" <freebsd-questions@FreeBSD.ORG> Subject: Re: Timestamps and nonces in IP packets Message-ID: <199811232041.MAA19687@stennis.ca.sandia.gov> In-Reply-To: Your message of "Mon, 23 Nov 1998 18:41:18 GMT." <3659AC4E.376CB6A@dcs.shef.ac.uk>
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--==_Exmh_547062568P Content-Type: text/plain; charset=us-ascii If memory serves me right, "Nick A. Fikouras" wrote: > Bruce A. Mah wrote: > > The timestamp option in the TCP header allows to sender of a TCP segment to > > compute the round-trip time (RTT) for every segment sent and acknowledged. > It > > places a timestamp in every outgoing segment it sends...the receiver (assum > ing > > it support this option) copies this timestamp back to an appropriate field > in > > the ACK it sends back for that segment. > > > > Why can't the TCP sequence number be used instead. TCP uses that sequence num > bers > anyway to acknowledge the safe receipt of data. Yes, TCP uses sequence numbers to ensure that data has been *received*, but the timestamps are used to compute the round-trip *time* of a particular segment. > > Without the use of the timestamp extension, the sending TCP can only estima > te > > the RTT once per round trip. > > > > I thought it always requires a round trip to measure the Round Trip Time. It does. But consider that a sender can have multiple segments in the network at once. Normally, the TCP algorithms can only measure the RTT once per round-trip time. With the timestamp option, the RTT can be measured once per TCP segment sent and acknowledged. At least in theory, this allows a more accurate measurement of the RTT, but I haven't seen any references to say "how much better" (anyone?). (Why can you only measure the RTT once per round trip without the timestamp option? I'm not sure.) > Thank you for your response Bruce. 8) My pleasure. I had to look up a couple of things to write a coherent response, so I learned something too. Bruce. --==_Exmh_547062568P Content-Type: application/pgp-signature -----BEGIN PGP MESSAGE----- Version: 2.6.2 iQCVAwUBNlnIY6jOOi0j7CY9AQEY3AP/URd4H2tDh3eMl0RoDrb/MIiyfuCT5xkm fXgwpKKP3giDlphmcDcWfK1tpi4V8KP0VI1vQbZ9T+w6zQ1Mscjj/IHddtyr74jk ZZkQFplFKbzFT+Y5sWbKhatA6QevSxGTz+oT0TtFZvYWgGNTsWPCAQM4ESNgBMmQ LyRULTJry14= =3NXb -----END PGP MESSAGE----- --==_Exmh_547062568P-- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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