Initial estimate of RTT by TCP?
Gaurav Navlakha
Can anyone tell me with what initial estimate does TCP begin its
round-trip-time (RTT) estimate initially, when it has no samples to fall
back on? In other words, I'd like to know what's the value of the first
"timeout" that TCP sets for itself.
Thanks very much,
Gaurav.
--
Gaurav Navlakha
Department of Computer Science,
UIUC
IL, USA.
Eric A. Hall
> Can anyone tell me with what initial estimate does TCP begin its
> round-trip-time (RTT) estimate initially, when it has no samples
> to fall back on? In other words, I'd like to know what's the value
> of the first "timeout" that TCP sets for itself.
Assume you are asking about the default retransmission timer? RFC 1122
states that the default is 3000 ms (3 seconds), but compliance varies by
implementation.
High values mean that loss isn't detected very quickly, at least until a
smoothed RTT has been calculated. Conversely, those systems which have
tried to use lower values (some Solaris boxes used defaults of 200 ms)
ended up doing unnecessary retransmissions for hosts behind long slow
pipes (ie, dial-up clients connecting to a Solaris web server).
Many systems let you set the default if that's what you're getting at. If
you're writing your own stack, read RFC 1122.
--
Eric A. Hall http://www.ehsco.com/
Internet Core Protocols http://www.oreilly.com/catalog/coreprot/
Ender
One great document to read is TCP VEGAS: End to End Congestion
Avoidance on the Global Internet (written by Lawrence S. Brakmo). You can
find this document at the ACM Digital library http://www.acm.org . The
course-timeout(500ms) is the minimum counter used in timing the RTT so I
would guess that anything under 500ms is not counted (bear in mind that this
is a 6 year old document). Anyone know what the current timer is ?
"Gaurav Navlakha" <navl...@students.uiuc.edu> wrote in message
news:3B0928CB...@students.uiuc.edu...
Vernon Schryver
Gaurav Navlakha <navl...@students.uiuc.edu> wrote:
>Can anyone tell me with what initial estimate does TCP begin its
>round-trip-time (RTT) estimate initially, when it has no samples to fall
>back on? In other words, I'd like to know what's the value of the first
>"timeout" that TCP sets for itself.
It varies.
- see the recent I-D that describes the extension to a group of systems
pooling their experience of the old idea of a system remembering
previous connections to the same general area (e.g. keeping an RTT
with a route).
- `grep -i 'initial retransm' in a stack of RFC's find RFC 1123 saying:
] A measured RTT and variance (if available) should be used to
] calculate an initial retransmission interval. If this information
] is not available, a default of no less than 5 seconds should be
] used. Implementations may limit the retransmission interval,
] but this limit must exceed twice the Internet maximum segment
] lifetime plus service delay at the name server.
- section 4.2.3 of RFC 1122 says
] The following values SHOULD be used to initialize the
] estimation parameters for a new connection:
]
] (a) RTT = 0 seconds.
]
] (b) RTO = 3 seconds. (The smoothed variance is to be
] initialized to the value that will result in this RTO).
- Google has about 15,100 answers to the question
http://www.google.com/search?q=tcp+retransmission+initial including
words on how to change the configured value in some systems.
and pointers to some of the many research papers on the subject
Vernon Schryver v...@rhyolite.com
Rick (Cablenut)
Key: Tcpip\Parameters\Interfaces\interface
Value Type: REG_DWORD-number
Valid Range: 0-0xFFFF
Default: 3 seconds
"This parameter controls the initial time-out used for a TCP connection
request and initial data retransmission on a per-interface basis. Use
caution when tuning with this parameter because exponential backoff is used.
Setting this value to larger than 3 results in much longer time-outs to
nonexistent addresses."
--
Rick (Cablenut)
cablenut.com - Webmaster
Cablenut UBB Forum - http://www.networkranch.com/cgi-bin/ultimatebb.cgi
"Gaurav Navlakha" <navl...@students.uiuc.edu> wrote in message
news:3B0928CB...@students.uiuc.edu...