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sending rate
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... Each connection begins with a question: "What is the appropriate
sending rate for the current network path?" The question is not
answered explicitly, but each TCP connection ...
... network path?" The question is not
answered explicitly, but each TCP connection determines the sending
rate by probing the network path and altering the congestion window
...
... host (say, host A) would indicate its
desired sending rate in bytes per second, using a Quick-Start Option
...
... queues, and possibly even different routers. Thus, any
mechanism for determining the allowed sending rate would have to be
used independently for each direction.
...
... used in the middle of a connection, e.g., after an idle or
underutilized period, as well as for the initial sending rate; these
uses of Quick-Start are discussed later in the document.
...
... routers to work together, with
end-points requesting a higher sending rate in the Quick-Start (QS)
...
... When sent as a request, the Quick-Start Option includes a request for
a sending rate in bits per second, and a Quick-Start Time to Live ...
...
The sender initializes the Rate Request to the desired sending rate,
including an estimate of the transport and IP header ...
... connection could already have an established
congestion window or sending rate. The Rate Request is the requested
total rate for the connection, including the current rate of the
...
... connection; the Rate Request is *not* a request for an additional
sending rate over and above the current sending rate. If the Rate
Request is denied, or lowered to a value below the connection ...
... connection; the Rate Request is *not* a request for an additional
sending rate over and above the current sending rate. If the Rate
Request is denied, or lowered to a value below the connection's
...
... Request is denied, or lowered to a value below the connection's
current sending rate, then the sender ignores the request, and
reverts to the default congestion control ...
...
The Quick-Start Request does not report the current sending rate of
the connection sending the request; in the default case of a router ...
... router makes the conservative assumption that the
flow's current sending rate is zero. Each participating router can
either terminate or approve the Quick-Start Request ...
... TCP
sender puts the requested sending rate in bits per second,
appropriately formatted, in the Quick-Start Option ...
... Quick-Start's Rate Requests are not meant to be used for
fine-grained control of the transport's sending rate. Rather, the
transport MAY issue a Rate Request when no information about the
...
... transport MAY issue a Rate Request when no information about the
appropriate sending rate is available, and the default congestion
control mechanisms might be significantly underestimating the
appropriate sending rate ...
... sending rate is available, and the default congestion
control mechanisms might be significantly underestimating the
appropriate sending rate.
The following are potential points where Quick-Start ...
... transport has concrete
knowledge of a changing network path, then the current sending
rate may not be appropriate, and the transport sender may use
...
... As a general guideline, a TCP sender SHOULD NOT request a sending
rate larger than it is able to use over the next round-trip time of
the connection ...
... connection (or over 100 ms, if the round-trip time is not known),
except as required to round up the desired sending rate to the next-
highest allowable request.
...
... connection initiation, and Section 4.8 discusses issues that arise
when Quick-Start is used to request a larger sending rate after an
idle period.
...
... sender to translate the approved Quick-Start sending rate in bytes
per second into a congestion window that is larger than needed, with
...
... congestion window from R*T/B to approximately R*T/(B*2)
packets, the actual number of packets that were needed to fill the
pipe at a sending rate of R KBps. (Note: this is just an
illustration; the congestion window ...
... TCP receiver can restrict its
sending rate for pure acknowledgment traffic to at most 100 pure ACK
...
... TCP
receiver could limit its increase in the sending rate for pure ACK
packets by at most doubling the sending rate ...
... sending rate for pure ACK
packets by at most doubling the sending rate for pure ACK packets
from one round-trip time ...
... congestion window that the TCP connection achieved since the last
packet drop and translate this to a sending rate to get the
maximum allowed request rate. If the request is granted, then
...
... round-trip time, and translates
the Quick-Start Response into an allowed window or allowed sending
rate. The sender sends a Report of the Approved Rate. The sender
...
... starts sending Quick-Start packets, rate-paced out at the approved
sending rate.
* After the sender ...
... sender adjusts its current congestion window or sending rate to be
consistent with the actual amount of data that was transmitted in
that round-trip time ...
... traffic policers to
check that the sender's sending rate does not exceed the rate in the
Report of Approved Rate.
...
...
Section 4.7 discusses some of the issues with deciding the initial
sending rate to request. Quick-Start raises additional issues about
the communication between the transport protocol ...
... it has a much better chance of being appropriate than an end-host
trying to *estimate* an appropriate sending rate using "block
box" probing techniques of the entire path.
...
... bandwidth estimation techniques and for the
first packet of the initial window to contain information, such as
the size or sending rate of the initial window. The proposal would
be that congested routers would use this information in the first
...
... connections
wishing to start up with a higher initial sending rate should be
encouraged to use other mechanisms, such as the explicit reservation
...
... window of 4380 bytes allowed by [RFC3390] (given appropriate packet
sizes) would translate to an initial sending rate of 35 Kbps. Thus,
for TCP flows ...
... TCP connection with
a long, 1 second round-trip time, this would give a maximum sending
rate of 1.07 Gbps.
...
... increase in the total rate for that connection, over and above
the current sending rate.
When the Quick-Start Request ...
... Quick-Start Request is sent after an idle period, the
current sending rate is zero, and there is no difference between (1)
and (2) above. However, a Quick-Start Request can also be sent in
...
... if the router is granting an *additional* rate with each rate
request, over and above the current sending rate, then it is in a
connection's interest to send as many rate requests as possible, even
...
... if very few of them are, in fact, granted.
Appendix E discusses a Report of Current Sending Rate as one possible
function in the Quick-Start Option. However, we have not
...
... Quick-Start is a rather coarse-grained mechanism
that would allow a connection to use a higher sending rate along
underutilized paths, but that does not attempt to provide a next-
generation transport protocol ...
... congestion control mechanism, but simply to get permission from
routers for a higher sending rate at start-up, or after an idle
period. Quick-Start ...
... underutilized, and addresses the single question of whether a higher
sending rate is allowed. New congestion control mechanisms with more
fine-grained feedback from routers ...
... mechanism, such as Quick-Start, for indicating an allowed initial
sending rate, or an allowed total sending rate after an idle or
underutilized period.
...
... Quick-Start, for indicating an allowed initial
sending rate, or an allowed total sending rate after an idle or
underutilized period.
...
... packet is about the relative increase or decrease in the rate or
window to take effect when that particular packet is acknowledged,
not about the allowed sending rate for the connection as a whole.
...
... Quick-Start sends a single Quick-Start Request, and the
answer to that request gives the allowed sending rate for an entire
window of data. As a result, Quick-Start could be problematic in an
...
... uses four bits of feedback in the rate request field to indicate the
allowed sending rate. XCP allocates a byte for per-packet feedback,
...
... bits of feedback, indicating that the sender should decrease,
maintain, or increase the sending rate or window when this packet is
acknowledged. However, the power of Quick-Start would be
...
... bits of
feedback; it seems likely that determining the initial sending rate
fundamentally requires more bits of feedback from routers ...
... state, per-packet feedback to increase or decrease the
sending rate.
On a more practical level, one difference between Quick-Start ...
... mechanism. Because Quick-Start is a mechanism for requesting an
initial sending rate in an underutilized environment, its fairness
issues are less severe than those of a general congestion control ...
... XCP; all that is needed is for the end nodes to report the
requested sending rate.
Table 3 provides a summary of the differences between Quick-Start ...
... +------------------+----------------------+----------------------+
Semantics: | Allowed sending rate | Change in rate/window,
| per connection. | per-packet ...
... Quick-Start with CCID 3 for requesting a
higher initial sending rate, the following questions arise:
(1) How does the sender ...
... sender roughly once per round-trip
time. In CCID 3, the allowed sending rate is halved if no
feedback is received from the receiver in at least four round-
...
... Quick-Start Request is used, it
would seem necessary to use a smaller time interval, e.g., to
reduce the sending rate if no feedback arrives from the receiver
in at least two round-trip times ...
... round-trip times.
The question also arises of how the sending rate should be reduced
after a period of no feedback from the receiver. As with TCP ...
... TCP, the
default CCID 3 response of halving the sending rate is not
necessarily a sufficient response to the absence of feedback; an
alternative is to reduce the sending rate ...
... sending rate is not
necessarily a sufficient response to the absence of feedback; an
alternative is to reduce the sending rate to the sending rate that
would have been used if no Quick-Start Request ...
... necessarily a sufficient response to the absence of feedback; an
alternative is to reduce the sending rate to the sending rate that
would have been used if no Quick-Start Request had been approved.
...
... Quick-Start with CCID 3 for
requesting a higher sending rate after an idle period, the following
questions arise:
...
... that the CCID 3 sender should use in requesting a higher sending
rate after an idle period. The sender knows the current loss
event rate ...
... loss
event rate, either from its own calculations or from feedback
from the receiver, and can determine the sending rate allowed by
that loss event rate. This is the upper bound on the sending
rate ...
... sending rate allowed by
that loss event rate. This is the upper bound on the sending
rate that should be requested by the CCID 3 sender. A Quick-
...
... The response to the loss of Quick-Start packets should be to
return to the sending rate that would have been used if Quick-
Start had not been requested.
...
... receiver?
As in the case of the initial sending rate, it would seem prudent
to reduce the sending rate if no feedback is received from the
...
... As in the case of the initial sending rate, it would seem prudent
to reduce the sending rate if no feedback is received from the
receiver in at least two round-trip times ...
... receiver in at least two round-trip times. It seems likely that,
in this case, the sending rate should be reduced to the sending
rate that would have been used if no Quick-Start Request had been
...
... round-trip times. It seems likely that,
in this case, the sending rate should be reduced to the sending
rate that would have been used if no Quick-Start Request had been
approved.
...
... Start Option.
Report of Current Sending Rate: A Quick-Start Request with the
`Report of Current Sending Rate ...
... Sending Rate: A Quick-Start Request with the
`Report of Current Sending Rate' codepoint set in the Function field
would be using the Rate Request field to report the current estimated
...
... codepoint set in the Function field
would be using the Rate Request field to report the current estimated
sending rate for that connection. This could accompany a second
Quick-Start Request ...
... connection that is using Quick-Start to increase its
current sending rate.
Request to Increase Sending Rate ...
... sending rate.
Request to Increase Sending Rate: A codepoint for `Request to
Increase Sending Rate ...
... Sending Rate: A codepoint for `Request to
Increase Sending Rate' in the Function field would indicate that the
connection is not idle or just starting ...
... starting up, but is attempting to use
Quick-Start to increase its current sending rate. This codepoint
would not change the semantics ...
... Pasi Sarolahti, Mark Allman, and Sally Floyd. Determining an Appropriate Sending Rate Over an Underutilized Network Path. February 2006. <http://www.icir.org/floyd/quickstart.html>. ...