One document matched: draft-amit-quick-start-04.txt
Differences from draft-amit-quick-start-03.txt
Internet Engineering Task Force A. Jain
INTERNET-DRAFT F5 Networks
draft-amit-quick-start-04.txt S. Floyd
Expires: August 2005 M. Allman
ICIR
P. Sarolahti
Nokia / Univ. Helsinki
20 February 2005
Quick-Start for TCP and IP
Status of this Memo
By submitting this Internet-Draft, we certify that any applicable
patent or other IPR claims of which we are aware have been
disclosed, or will be disclosed, and any of which we become aware
will be disclosed, in accordance with RFC 3668 (BCP 79).
By submitting this Internet-Draft, we accept the provisions of
Section 3 of RFC 3667 (BCP 78).
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This draft specifies an optional Quick-Start mechanism for transport
protocols, in cooperation with routers, to determine an allowed
sending rate at the start and at times in the middle of a data
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transfer. While Quick-Start is designed to be used by a range of
transport protocols, in this document we describe its use with TCP.
By using Quick-Start, a TCP host, say, host A, would indicate its
desired sending rate in bytes per second, using a Quick Start
Request option in the IP header of a TCP packet. A Quick-Start
request for a higher sending rate would be sent in a TCP packet.
Each router along the path could, in turn, either approve the
requested rate, reduce the requested rate, or indicate that the
Quick-Start request is not approved. If the Quick-Start request is
not approved, then the sender would use the default congestion
control mechanisms. The Quick-Start mechanism can determine if
there are routers along the path that do not understand the Quick-
Start Request option, or have not agreed to the Quick-Start rate
request. TCP host B communicates the final rate request to TCP host
A in a transport-level Quick-Start Response in an answering TCP
packet. Quick-Start is designed to allow connections to use higher
sending rates when there is significant unused bandwidth along the
path, and all of the routers along the path support the Quick-Start
Request.
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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
Changes from draft-amit-quick-start-03.txt:
* Added a citation to the paper on "Evaluating Quick-Start for
TCP", and added pointers to the work in that paper.
This work includes:
- Discussions of router algorithms.
- Discussions of sizing Quick-Start requests.
* Added sections on "Misbehaving Middleboxes", and on "Attacks on
Quick-Start".
Changes from draft-amit-quick-start-02.txt:
* Added a discussion on Using Quick-Start in the Middle of a
Connection. The request would be on the total rate,
not on the additional rate.
* Changed name "Initial Rate" to "Rate Request", and changed
the units from packets per second to bytes per second.
* The following sections are new:
- The Quick-Start Request Option for IPv6
- Quick-Start in IP Tunnels
- When to Use Quick-Start
- TCP: Responding to a Loss of a Quick-Start Packet
- TCP: A Quick-Start Request for a Larger Initial Window
- TCP: A Quick-Start Request after an Idle Period
- The Quick-Start Mechanisms in DCCP and other Transport
Protocols
- Quick-Start with DCCP
- Implementation and Deployment Issues
- Design Decisions
* Added a discussion of Kunniyur's Anti-ECN proposal.
* Added a section on simulations, with a brief discussion of the
simulations by Srikanth Sundarrajan.
Changes from draft-amit-quick-start-01.txt:
* Added a discussion in the related work section about the
possibility of optimistically sending a large initial window,
without explicit permission of routers.
* Added a discussion in the related work section about the
tradeoffs of XCP vs. Quick-Start.
* Added a section on "The Quick-Start Request: Packets or Bytes?"
Changes from draft-amit-quick-start-00.txt:
* The addition of a citation to [KHR02].
* The addition of a Related Work section.
* Deleted the QS Nonce, in favor of a random initial value for the
QS TTL.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Assumptions and General Principles. . . . . . . . . . . . . . 7
2.1. Overview of Quick-Start. . . . . . . . . . . . . . . . . 8
3. The Quick-Start Request in IP . . . . . . . . . . . . . . . . 11
3.1. The Quick-Start Request Option for IPv4. . . . . . . . . 11
3.2. The Quick-Start Request Option for IPv6. . . . . . . . . 13
3.3. Processing the Quick-Start Request at
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.4. Deciding the Permitted Rate Request at a
Router. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.5. Quick-Start in IP Tunnels. . . . . . . . . . . . . . . . 15
4. The Quick-Start Mechanisms in TCP . . . . . . . . . . . . . . 17
4.1. When to Use Quick-Start. . . . . . . . . . . . . . . . . 18
4.2. The Quick-Start Response Option in the TCP
header. . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3. TCP: Sending the Quick-Start Response. . . . . . . . . . 20
4.4. TCP: Receiving and Using the Quick-Start
Response Packet . . . . . . . . . . . . . . . . . . . . . . . 21
4.5. TCP: Responding to a Loss of a Quick-Start
Packet. . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.6. TCP: A Quick-Start Request for a Larger Ini-
tial Window . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.7. TCP: A Quick-Start Request after an Idle
Period. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.8. An Example Quick-Start Scenario with TCP . . . . . . . . 25
5. The Quick-Start Mechanism in other Transport Pro-
tocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1. Quick-Start with DCCP. . . . . . . . . . . . . . . . . . 27
6. Evaluation of Quick-Start . . . . . . . . . . . . . . . . . . 28
6.1. Benefits of Quick-Start. . . . . . . . . . . . . . . . . 29
6.2. Costs of Quick-Start . . . . . . . . . . . . . . . . . . 29
6.3. Protection against Misbehaving Nodes . . . . . . . . . . 31
6.4. Quick-Start with QoS-enabled Traffic . . . . . . . . . . 33
6.5. Limitations of Quick-Start . . . . . . . . . . . . . . . 34
6.6. Attacks on Quick-Start . . . . . . . . . . . . . . . . . 34
6.7. Simulations with Quick-Start . . . . . . . . . . . . . . 34
7. Related Work. . . . . . . . . . . . . . . . . . . . . . . . . 35
7.1. Fast Start-ups without Explicit Information
from Routers. . . . . . . . . . . . . . . . . . . . . . . . . 35
7.2. Optimistic Sending without Explicit Informa-
tion from Routers . . . . . . . . . . . . . . . . . . . . . . 36
7.3. Fast Start-ups with other Information from
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7.4. Fast Start-ups with more Fine-Grained Feed-
back from Routers . . . . . . . . . . . . . . . . . . . . . . 38
8. Implementation and Deployment Issues. . . . . . . . . . . . . 38
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8.1. Implementation Issues for Sending Quick-
Start Requests. . . . . . . . . . . . . . . . . . . . . . . . 39
8.2. Implementation Issues for Processing Quick-
Start Requests. . . . . . . . . . . . . . . . . . . . . . . . 39
8.3. Possible Deployment Scenarios. . . . . . . . . . . . . . 40
8.4. Would QuickStart packets take the slow path
in routers? . . . . . . . . . . . . . . . . . . . . . . . . . 41
8.5. A Comparison with the Deployment Problems of
ECN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
9. Security Considerations . . . . . . . . . . . . . . . . . . . 41
10. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . 41
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42
A. Design Decisions. . . . . . . . . . . . . . . . . . . . . . . 42
A.1. Alternate Mechanisms for the Quick-Start
Request: ICMP and RSVP. . . . . . . . . . . . . . . . . . . . 42
A.1.1. ICMP. . . . . . . . . . . . . . . . . . . . . . . . 42
A.1.2. RSVP. . . . . . . . . . . . . . . . . . . . . . . . 43
A.2. Alternate Encoding Functions . . . . . . . . . . . . . . 44
A.3. The Quick-Start Request: Packets or Bytes? . . . . . . . 45
A.4. Quick-Start Semantics: Total Rate or Addi-
tional Rate?. . . . . . . . . . . . . . . . . . . . . . . . . 47
A.5. Alternate Responses to the Loss of a Quick-
Start Packet. . . . . . . . . . . . . . . . . . . . . . . . . 47
A.6. Why Not Include More Functionality?. . . . . . . . . . . 48
A.7. A QuickStart Nonce?. . . . . . . . . . . . . . . . . . . 51
Normative References . . . . . . . . . . . . . . . . . . . . . . 51
Informative References . . . . . . . . . . . . . . . . . . . . . 52
IANA Considerations. . . . . . . . . . . . . . . . . . . . . . . 55
AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . . . 55
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 55
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 55
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1. Introduction
Each TCP connection begins with a question: "What is the appropriate
sending rate for the current network path?" The question is not
answered explicitly for TCP, but each TCP connection determines the
sending rate by probing the network path and altering the congestion
window (cwnd) based on perceived congestion. Each connection starts
with a pre-configured initial congestion window (ICW). Currently,
TCP allows an initial window of between one and four MSS-sized
segments [RFC2581,RFC3390]. The TCP connection then probes the
network for available bandwidth using the slow-start procedure
[Jac88,RFC2581], doubling cwnd during each congestion-free round-
trip time (RTT).
The slow-start algorithm can be time-consuming --- especially over
networks with large bandwidth or long delays. It may take a number
of RTTs in slow-start before the TCP connection begins to fully use
the available bandwidth of the network. For instance, it takes
log_2(N) - 2 round-trip times to build cwnd up to N segments,
assuming an initial congestion window of 4 segments. This time in
slow-start is not a problem for large file transfers, where the
slow-start stage is only a fraction of the total transfer time.
However, in the case of moderate-sized web transfers the connection
might carry out its entire transfer in the slow-start phase, taking
many round-trip times, where one or two RTTs might have been
sufficient.
A fair amount of work has already been done to address the issue of
choosing the initial congestion window for TCP, with RFC 3390
allowing an initial window of up to four segments based on the MSS
used by the connection [RFC3390]. Our underlying premise is that
explicit feedback from all of the routers along the path would be
required, in the current architecture, for best-effort connections
to use initial windows significantly larger than those allowed by
[RFC3390], in the absence of other information about the path.
The Congestion Manager [RFC3124] and TCP control block sharing
[RFC2140] both propose sharing congestion information among multiple
TCP connections with the same endpoints. With the Congestion
Manager, a new TCP connection could start with a high initial cwnd
if it was sharing the path and the cwnd with a pre-existing TCP
connection to the same destination that had already obtained a high
congestion window. RFC 2140 discusses ensemble sharing, where an
established connection's congestion window could be `divided up' to
be shared with a new connection to the same host. However, neither
of these approaches addresses the case of a connection to a new
destination, with no existing or recent connection (and therefore
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congestion control state) to that destination.
Quick-Start would not be the first mechanism for explicit
communication from routers to transport protocols about sending
rates. Explicit Congestion Notification (ECN) gives explicit
congestion control feedback from routers to transport protocols,
based on the router detecting congestion before buffer overflow
[RFC3168]. In contrast, routers do not use Quick-Start to get
congestion information, but instead use Quick-Start as an optional
mechanism to give permission to transport protocols to use higher
sending rates, based on the ability of all the routers along the
path to determine if their respective output links are significantly
underutilized.
2. Assumptions and General Principles
This section describes the assumptions and general principles behind
the design of the Quick-Start mechanism.
Assumptions:
* The data transfer in the two directions of a connection traverses
different queues, and possibly even different routers. Thus, any
mechanism for determining the allowed sending rate would have to be
used independently for each direction.
* The path between the two endpoints is relatively stable, such that
the path used by the Quick-Start request is generally the same path
used by the Quick-Start packets one round-trip time later.
* Any new mechanism must be incrementally deployable, and might not
be supported by all of the routers and/or end-hosts. Thus, any new
mechanism must be able to accommodate non-supporting routers or end-
hosts without disturbing the current Internet semantics.
General Principles:
* Our underlying premise is that explicit feedback from all of the
routers along the path would be required, in the current
architecture, for best-effort connections to use initial windows
significantly larger than those allowed by [RFC3390], in the absence
of other information about the path.
* A router should only approve a request for a higher sending rate
if the output link is underutilized. Any other approach will result
in either per-flow state at the router, or the possibility of a
(possibly transient) queue at the router.
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* No per-flow state should be required at the router.
There are also a number of questions regarding the Quick-Start
mechanism that are discussed later in this document.
Open Questions:
* Would the benefits of the Quick-Start mechanism be worth the added
complexity?
The benefits and drawbacks of Quick-Start are discussed in more
detail in Section 6 on "Evaluation of Quick-Start".
* A practical consideration is that packets with known and unknown
IP options are often dropped in the current Internet [MAF04].
This does not preclude using Quick-Start in Intranets. Further,
[MAF04] also shows that over time the blocking of packets
negotiating ECN has dropped, and therefore an incremental deployment
story for Quick-Start based on IP Options is not out of the
question. Appendix A.1 on "Alternate Mechanisms for the Quick-Start
Request" discusses the possibility of using RSVP or ICMP instead of
IP Options for carrying Quick-Start Requests to routers.
* Apart from the merits and shortcomings of the Quick-Start
mechanism, is there likely to be a compelling need to add explicit
congestion-related feedback from routers over and above the one-bit
feedback from ECN?
If the answer to the question above is yes, should we be considering
mechanisms that, while more complex, are also sufficiently more
powerful than Quick-Start? This is discussed further in Appendix
A.6 on "Why Not Include More Functionality".
2.1. Overview of Quick-Start
In this section we give an overview of the use of Quick-Start with
TCP, used to request a higher congestion window. The description in
this section is non-normative; the normative description of Quick-
Start with IP and TCP follows in Sections 3 and 4. Quick-Start can
be 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.
Quick-Start requires end-points and routers to work together, with
end-points requesting a higher sending rate in the Quick-Start
Request (QSR) option in IP, and routers along the path approving,
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modifying, discarding or ignoring (and therefore disallowing) the
Quick-Start Request. The receiver uses reliable, transport-level
mechanisms to inform the sender of the status of the Quick-Start
Request. In addition, Quick-Start assumes a unicast, congestion-
controlled transport protocol; we do not consider the use of Quick-
Start for multicast traffic.
The Quick-Start Request Option includes a request for a sending rate
in bytes per second, and a Quick-Start TTL (QS TTL) to be
decremented by every router along the path that understands the
option and approves the request. The Quick-Start TTL is initialized
by the sender to a random value. The transport receiver returns the
rate and information about the TTL to the sender using transport-
level mechanisms. In particular, the receiver computes the
difference between the Quick-Start TTL and the TTL in the IP header
of the Quick-Start request packet, and returns this in the Quick-
Start response. The sender uses this information to determine if
all of the routers along the path decremented the Quick-Start TTL,
approving the Quick-Start Request.
If the request is approved by all of the routers along the path,
then the TCP sender combines this allowed rate with the measurement
of the round-trip time, and ends up with an allowed TCP window.
This window is sent rate-paced over the round-trip time, or until an
ACK packet is received.
Figure 1 shows a successful use of Quick-Start, with both routers
along the path approving the Quick-Start Request. In this example,
Quick-Start is used by TCP to establish the initial congestion
window.
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Sender Router 1 Router 2 Receiver
------ -------- -------- --------
| Quick-Start Request
| in SYN or SYN/ACK -->
|
| Decrement
| QS TTL
| to approve
| request -->
|
| Decrement
| QS TTL
| to approve
| request -->
|
| Return Quick-Start
| info to sender in
| <-- TCP ACK packet.
|
| Quick-Start approved,
| translate to cwnd.
V Send cwnd paced over one RTT. -->
Figure 1: A successful Quick-Start Request.
Figure 2 shows an unsuccessful use of Quick-Start, with one of the
routers along the path not approving the Quick-Start Request. If
the Quick-Start Request is not approved, then the sender uses the
default congestion control mechanisms for that transport protocol,
including the default initial congestion window, response to idle
periods, etc.
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Sender Router 1 Router 2 Receiver
------ -------- -------- --------
| Quick-Start Request
| in SYN or SYN/ACK -->
|
| Decrement
| QS TTL
| to approve
| request -->
|
| Forward packet
| without modifying
| Quick-Start Option. -->
|
| Return Quick-Start
| info to sender in
| <-- TCP ACK packet.
|
| Quick-Start not approved.
V Use default initial cwnd. -->
Figure 2: An unsuccessful Quick-Start Request.
3. The Quick-Start Request in IP
3.1. The Quick-Start Request Option for IPv4
The Quick-Start Request for IPv4 is defined as follows:
0 1 2 3
+----------+----------+----------+----------+
| Option | Length=4 | QS TTL | Rate |
| | | | Request |
+----------+----------+----------+----------+
Figure 1. The Quick-Start Request Option for IPv4.
The first byte contains the option field, which includes the one-bit
copy flag, the 2-bit class field, and the 5-bit option number (to be
assigned by IANA).
The second byte contains the length field, indicating an option
length of four bytes.
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The third byte contains the Quick-Start TTL (QS TTL) field. The
sender sets the QS TTL field to a random value. Routers that
approve the Quick-Start Request decrement the QS TTL (mod 256). The
QS TTL is used by the sender to detect if all of the routers along
the path understood and approved the Quick-Start option.
The transport sender also calculates and remembers the TTL Diff, the
difference between the IP TTL value and the QS TTL value in the
Quick-Start request packet, as follows:
TTL Diff = ( IP TTL - QS TTL ) mod 256. (1)
The fourth byte is the Rate Request field. The sender initializes
the Rate Request to the desired sending rate, including an estimate
of the transport and IP header overhead.
Our current proposal for an encoding function uses only the first
four bits of the fourth byte, leaving the other four bits reserved
for future use. The encoding function sets the request rate to
K*2^N bps, for N the value in the Rate Request field, and for K set
to 40,000. For N=0, the rate request would be set to zero,
regardless of the encoding function. This is illustrated in the
table below. For a four-bit Rate Request field, the request range
would be from 80 Kbps to 1.3 Gbps. Alternate encodings for the Rate
Request are given in Appendix A.2.
N Rate Request (in Kbps)
--- -------------------
0 0
1 80
2 160
3 320
4 640
5 1,280
6 2,560
7 5,120
8 10,240
9 20,480
10 40,960
11 81,920
12 163,840
13 327,680
14 655,360
15 1,310,720
Mapping from the Rate Request field to the rate request in Kbps.
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Routers can approve the Quick-Start Request for a lower rate by
decreasing the Rate Request in the Quick-Start Request.
We note that unlike a Quick-Start Request sent at the beginning of a
connection, when a Quick-Start Request is sent in the middle of a
connection, the 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's current sending rate, then the sender can ignore the
request, and revert to the default congestion control mechanisms of
the transport protocol.
In IPv4, a change in IP options at routers requires recalculating
the IP header checksum.
3.2. The Quick-Start Request Option for IPv6
The Quick-Start Request Option for IPv6 is placed in the Hop-by-Hop
Options extension header that is processed at every network node
along the communication path [RFC 2460]. The option format following
the generic Hop-by-Hop Options header is similar to the IPv4 format
with the exception that the Length field should exclude the common
type and length fields in the option format and be set to 2.
0 1 2 3
+----------+----------+----------+----------+
| Option | Length=2 | QS TTL | Rate |
| | | | Request |
+----------+----------+----------+----------+
Figure 2. The Quick-Start Request Option for IPv6.
The transport receiver compares the Quick-Start TTL with the IPv6
Hop Limit field in order to calculate the TTL Diff. (The Hop Limit
in IPv6 is the equivalent of the TTL in IPv6.) That is, TTL Diff is
calculated as follows:
TTL Diff = ( IPv6 Hop Limit - QS TTL ) mod 256.
(1)
Unlike IPv4, modifying or deleting the Quick-Start Request IPv6
Option does not require checksum re-calculation, because the IPv6
header does not have a checksum field, and modifying the Quick-Start
Request in the IPv6 Hop-by-Hop options header does not affect the
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IPv6 pseudo-header checksum used in upper-layer checksum
calculations.
Note that [RFC2460] specifies that when a specific flow label has
been assigned to packets, the contents of the Hop-by-Hop options,
excluding the next header field, must originate with the same
contents throughout the IP flow lifetime. This requirement would
have to be modified to implement Quick-Start on an IPv6
implementation that uses flow labels, because the Quick-Start
Request option would be included in only a small fraction of the
packets during a flow lifetime.
3.3. Processing the Quick-Start Request at Routers
Each participating router can either terminate or forward the Quick-
Start Request. The router terminates the Quick-Start Request if the
router is not underutilized, and therefore has decided not to grant
the Quick-Start Request.
The preferable method for a router to terminate the Quick-Start
Request is to delete the Quick-Start Request from the IP header. A
less preferable but possibly more efficient method is to simply
forward the packet with the Quick-Start Request unchanged, or with
the Rate Request set to zero.
If the participating router has decided to approve the Quick-Start
Request, it does the following:
* It decrements the QS TTL by one.
* If the router is only willing to approve an Rate Request less than
that in the Quick-Start Request, then the router puts the smaller
Rate Request in that field of the Quick-Start Request. The router
MUST NOT increase the Rate Request in the Quick-Start Request.
* In IPv4, it updates the IP header checksum.
A non-participating router forwards the Quick-Start Request
unchanged, without decrementing the QS TTL. Of course, the non-
participating router still decrements the TTL field in the IP
header, as is required for all routers [RFC1812]. As a result, the
TCP sender will be able to detect that the Quick-Start Request had
not been understood or approved by all of the routers along the
path.
A router that modifies or deletes the Quick-Start Request in the
IPv4 header also has to update the IPv4 Header checksum. For IPv6,
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no checksum updates are needed.
3.4. Deciding the Permitted Rate Request at a Router
In this section we briefly outline how a router might decide whether
or not to approve a Quick-Start Request. As an example, the router
could ask the following questions:
* Has the router's output link been underutilized for some time
(e.g., several seconds).
* Would the output link remain underutilized if the arrival rate was
to increase by the aggregate rate requests that the router has
approved over the last fraction of a second?
Answering this question requires that the router have some knowledge
of the available bandwidth on the output link for that output queue.
It also requires that the router keep two counters, one indicating
the total aggregate Rate Requests that have been approved over the
recent interval of time, and one for the total aggregate Rate
Requests approved over the previous interval of time. Thus, if an
underutilized router experiences a SYN flood, then the router would
begin to deny Rate Request requests, even if the router remains
underutilized.
* If the router's output link has been underutilized and the
aggregate Quick Start Request Rate options granted is low enough to
prevent a near-term bandwidth shortage, then the router could
approve the Quick-Start Request.
Section 8.2 discusses some of the implementation issues in
processing Quick-Start requests at routers. [SAF05] discusses the
range of possible Quick-Start algorithms at the router for deciding
whether to approve a Quick-Start request. In order to explore the
limits of the possible functionality at routers, [SAF05] also
discusses Extreme Quick-Start mechanisms at routers, where the
router would keep per-flow state concerning approved Quick-Start
requests.
3.5. Quick-Start in IP Tunnels
In this section we consider the effect of IP tunnels on Quick-Start.
In the discussion, we use TTL Diff, defined earlier as the
difference between the IP TTL and the Quick-Start TTL, mod 256.
Recall that the sender considers the Quick-Start request approved if
the value of TTL Diff for the packet entering the network is the
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same as the value of TTL Diff for the packet exiting the network.
There are two legitimate ways for handling the Quick-Start Request
with IP tunnels:
(1) The tunnel ingress node does not support Quick-Start, or does
not approve the Quick-Start request. The node could strip the Quick-
Start Request option from the IP header before encapsulation.
Alternately, the ingress node can decrement the IP TTL before
encapsulation, while leaving the Quick-Start TTL unchanged, changing
TTL Diff. This is the assumed behavior of current IP tunnels that
are not aware of Quick-Start.
For a tunnel ingress node that does not support Quick-Start,
problems with a Quick-Start Request could still occur if a tunnel
discards the outer header at egress and does not decrement the inner
IP TTL at the ingress. In this case, if both the inner IP TTL and
the Quick-Start TTL are decremented after decapsulation at a Quick-
Start aware egress, or if neither is decremented at the egress, then
TTL Diff would be the same after egress as it was before ingress, so
that it would wrongly appear that all the routers in the tunnel had
approved the Quick-Start request. Fortunately, we are not aware of
tunnel technologies that operate this way; to the best of our
knowledge, all tunnels decrement the IP TTL either at the ingress
before encapsulation, or at the egress router after decapsulation,
thus changing TTL Diff.
Even the extreme case when the tunnel ingress is at the TCP sender
and the tunnel egress is at the TCP receiver, our assumption is that
the IP TTL will be decremented either at the tunnel ingress or at
the tunnel egress, changing TTL Diff and preventing the end-nodes
from wrongly inferring that the Quick-Start Request was approved by
all of the routers along the path. If there are tunnels where the
IP TTL in not decremented, perhaps for PPP over SSH, then additional
attention will have to be paid to the robustness of Quick-Start in
these environments.
A Quick-Start aware egress must also make sure that the Quick-Start
Request is not approved if for some reason the inner header includes
the Quick-Start Request option, but the outer header does not. In
this case the egress node should remove the Quick-Start Request
option from the inner header after decapsulation. Alternately, the
egress node could decrement the Rate Request in the Quick-Start
Request option to zero.
(2) The tunnel ingress node may choose to support Quick-Start, and
locally approve the Quick-Start Request. In this case the IP TTL
and Quick-Start option must be copied from the inner IP header to
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the outer header at the tunnel ingress. Upon decapsulation, the IP
TTL and the Quick-Start option in the outer IP header must be copied
back to the inner header. If the ingress router decrements the IP
TTL in the inner header before encapsulation, or in the outer header
after encapsulation, then if the ingress router wishes to approve
the Quick-Start request, it must decrement the Quick-Start TTL at
the same time, so as not to change TTL Diff. Similarly, if the
egress router wishes to approve the Quick-Start request, then when
it decrements the IP TTL in the outer header before decapsulation,
or in the inner header after decapsulation, it must decrement the
Quick-Start TTL at the same time.
A tunnel ingress node can support a Quick-Start request without
explicitly verifying that the tunnel egress also supports Quick-
Start. All that the ingress node has to do is to decrement the IP
TTL, but not the Quick-Start TTL, in the inner header after
encapsulation. In this case, if the egress node simply discards the
outer header at the egress point, TTL Diff will be different after
the tunnel egress than it was at the tunnel ingress, and the Quick-
Start will not be considered by the end-nodes as having been
approved in the network. Thus, the tunnel ingress node on its own
can provide protection against egress nodes that might discard the
outer header at the egress point.
4. The Quick-Start Mechanisms in TCP
This section describes how the Quick-Start mechanism would be used
in TCP. We first sketch the procedure and then tightly define it in
the subsequent subsections.
If a TCP sender, say host A, would like to use Quick-Start, the TCP
sender puts the requested sending rate in bytes per second,
appropriately formatted, in the Quick-Start Request option in the IP
header of the TCP packet, called the Quick-Start request packet.
(We will be somewhat loose in our use of "packet" vs. "segment" in
this section.) For initial start-up, the Quick-Start request packet
can be either the SYN or SYN/ACK packet, as described above. The
requested rate includes an estimate for the transport and IP header
overhead. The TCP receiver, say host B, returns the Quick-Start
Response option in the TCP header in the responding SYN/ACK packet
or ACK packet, called the Quick-Start response packet, informing
host A of the results of their request.
If the acknowledging packet does not contain a Quick-Start Response,
or contains a Quick-Start Response with the wrong value for the TTL
Diff, then host A MUST assume that its Quick-Start request failed.
In this case, host A uses TCP's default congestion control
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procedure. For initial start-up, host A uses the default initial
congestion window.
If the returning packet contains a valid Quick-Start Response, then
host A uses the information in the response, along with its
measurement of the round-trip time, to determine the Quick-Start
congestion window (QS-cwnd). Quick-Start packets are defined as
packets sent as the result of a successful Quick-Start request, up
to the time when the first Quick-Start packet is acknowledged. In
order to use Quick-Start, the TCP host is also required to use rate-
based pacing to pace out Quick-Start packets at the rate indicated
in the Quick-Start Response.
The two TCP end-hosts can independently decide whether to request
Quick-Start. For example, host A could sent a Quick-Start Request
in the SYN packet, and host B could also send a Quick-Start Request
in the SYN/ACK packet.
4.1. When to Use Quick-Start
In addition to the use of Quick-Start when a connection is
established, there are several additional points in a connection
when a transport protocol may want to issue a Rate Request. We
first re-iterate the notion that Quick-Start is a coarse-grained
mechanism. That is, Quick-Start's Rate Requests are not meant to be
used for fine-grained control of the transport's sending rate.
Rather, the transport can 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.
The following are the potential points where Quick-Start may be
useful:
(1) At connection initiation when the transport has no idea of
the capacity of the network, as discussed above. (A transport
that uses TCP Control Block sharing, the Congestion Manager, or
the like may not need Quick-Start to determine an appropriate
rate.)
(2) After a lengthy idle period when the transport no longer has
a validated estimate of the available bandwidth for this flow.
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(3) After a host has been explicitly informed that a link in the
path has gone down and has come back up. In this case, the
network has changed in unknown ways and the sender has lost its
validated assessment of the appropriate sending rate.
(4) After a host has received explicit indications that one of
the endpoints has moved its point of network attachment. This
can happen due to some underlying mobility mechanism like Mobile
IP [RFC3344,RFC3775]. Some transports, such as SCTP [RFC2960],
may associate with multiple IP addresses and can switch
addresses (and, therefore network paths) in mid-connection. If
the transport has concrete knowledge of a changing network path
then the current sending rate may not be appropriate and the
transport sender may use Quick-Start to probe the network for
the appropriate rate at which to send. (Alternatively,
traditional slow start should be used in this case when Quick-
Start is not available.)
(5) After an application-limited period when the sender has been
using only a small amount of its appropriate share of the
network capacity, and has no valid estimate for its fair share.
In this case, Quick-Start may be an appropriate mechanism to
assess the available capacity on the network path.
Of the above, this document recommends that a TCP MAY attempt to use
Quick-Start in cases (1) and (2). Cases (3) and (4) require
external notifications not presently defined for TCP or other
transport protocols. Case (5) requires further thought and
investigation with regard to how the transport protocol could detect
it was in a situation that would warrant transmitting a Quick-Start
Rate Request.
Section 4.6 discusses some of the issues of using Quick-Start at
connection initiation, and Section 4.7 discusses issues that arise
when Quick-Start is used to request a larger sending rate after an
idle period.
4.2. The Quick-Start Response Option in the TCP header
TCP's Quick-Start Response option is defined as follows:
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0 1 2 3
+----------+----------+----------+----------+
| Kind | Length=4 | Rate | TTL |
| | | Request | Diff |
+----------+----------+----------+----------+
Figure 2. The Quick-Start Response option in the TCP header.
The first byte of the Quick-Start Response option contains the
option kind, identifying the TCP option (to be assigned by IANA).
The second byte of the Quick-Start Response option contains the
option length in bytes. The length field is set to four bytes.
The third byte of the Quick-Start Response option contains the
allowed Rate Request, formatted as in the Quick-Start Request
option.
The fourth byte of the TCP option contains the TTL Diff. The TTL
Diff contains the difference between the IP TTL and QS TTL fields in
the received Quick-Start request packet, as calculated in equation
(1).
4.3. TCP: Sending the Quick-Start Response
An end host, say host B, that receives a TCP packet containing a
Quick-Start Request passes the Quick-Start Request, along with the
value in the IP TTL field, to the receiving TCP layer.
If the TCP host is willing to permit the Quick-Start Request, then a
Quick-Start Response option is included in the TCP header of the
corresponding acknowledgement packet. The Rate Request in the
Quick-Start Response option is set to the received value of the Rate
Request in the Quick-Start Request option, or to a lower value if
the TCP receiver is only willing to allow a lower Rate Request. The
TTL Diff in the Quick-Start Response is set to the difference
between the IP TTL value and the QS TTL value as given in equation
(1).
When the Quick-Start Response is being sent on the SYN/ACK, in
response to a Quick-Start Request on the SYN, then the Quick-Start
Response will be resent if the SYN/ACK is retransmitted. When the
Quick-Start Response is being sent on an ACK, for example in
response to the Quick-Start Request on the SYN/ACK, then the Quick-
Start Response MUST be resent on data packets until that TCP host
receives an acknowledgement from the other endpoint.
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4.4. TCP: Receiving and Using the Quick-Start Response Packet
A TCP host, say TCP host A, that sent a Quick-Start Request and
receives an answering Quick-Start Response in the acknowledgement
first checks that the Quick-Start Response is valid. The Quick-
Start Response is valid if it contains the correct value for the TTL
Diff, and an equal or lesser value for the Rate Request than that
transmitted in the Quick-Start Request. If this check is not
successful, then the Quick-Start request failed, and the TCP host
MUST use the default TCP congestion window that it would have used
without Quick-Start.
If the checks of the TTL Diff and the Rate Request are successful,
then the TCP host sets its Quick-Start congestion window (in terms
of MSS-sized segments), QS-cwnd, as follows:
QS-cwnd = (R * T) / (MSS + H) (2)
where R the Rate Request in bytes per second, T the measured round-
trip time in seconds, and H the estimated header size in bytes
(e.g., 40 bytes). [Derivation: the sender gets R bytes per second
including packet headers, but only R*MSS/(MSS+H) bytes per second,
or equivalently R*T*MSS/(MSS+H) bytes per round-trip time, of
application data.] The TCP host sets its congestion window cwnd to
QS-cwnd only if QS-cwnd is greater than cwnd; otherwise QS-cwnd is
ignored. If QS-cwnd is used, the TCP host sets a flag that it is in
Quick-Start mode, and while in Quick-Start mode the TCP sender uses
rate-based pacing, pacing out Quick-Start packets at the specified
Rate Request. Quick-Start mode ends when the TCP host receives an
ACK for one of the Quick-Start packets.
Because the Quick-Start request packet might not have used the fast
path in routers, the round-trip time measurement for the Quick-Start
request might be unnecessarily large. If the congestion window has
not been fully used when the first ack arrives ending the Quick-
Start mode, then the congestion window is decreased to the amount
that has actually been used so far. This should address the problem
of an overly-large congestion window from an overly-large
measurement of the round-trip time.
If the Quick-Start mode ends with all Quick-Start packets being
successfully acknowledged, the TCP sender returns to using the
default congestion control mechanisms. After all the packets are
acknowledged from a Quick-Start request for an initial window, for
example, the TCP sender remains in slow-start, if permitted by
ssthresh, continuing to increase its congestion window rather
aggressively from one round-trip time to the next. To add
robustness, the TCP sender is required to use Limited Slow-Start
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along with Quick-Start. With Limited Slow-Start, the TCP sender
limits the number of packets by which the congestion window is
increased for one window of data during slow-start [F04].
4.5. TCP: Responding to a Loss of a Quick-Start Packet
For TCP, we have defined a ``Quick-Start packet'' as one of the
packets sent in the window immediately following a successful Quick-
Start request. After detecting the loss of a Quick-Start packet,
TCP MUST revert to the default congestion control procedures that
would have been used if the Quick-Start request had not been
approved. For example, if Quick-Start is used for setting the
initial window, and a packet from the initial window is lost, then
the TCP sender must then slow-start with the default initial window
that would have been used if Quick-Start had not been used. In
addition to reverting to the default congestion control mechanisms,
the sender must take into account that the Quick-Start congestion
window was too large. Thus, the sender should decrease ssthresh to
at most half the number of Quick-Start packets that were
successfully transmitted. Section A.5 discusses possible
alternatives in responding to the loss of a Quick-Start packet.
4.6. TCP: A Quick-Start Request for a Larger Initial Window
Some of the issues of using Quick-Start are related to the specific
scenario in which Quick-Start is used. This section discusses the
following issues that arise when Quick-Start is used by TCP to
request a larger initial window: (1) determining the rate to
request; (2) interactions with Path MTU Discovery; and (3) Quick-
Start request packets that are eaten by middleboxes.
(1) Determining the rate to request:
As discussed in [SAF05], the data sender does not necessarily have
information about the size of the data transfer at connection
initiation; for example, in request-response protocols such as HTTP,
the server doesn't know the size or name of the requested object
during connection initiation. [SAF05] explores some of the
performance implications of overly-large Quick-Start requests, and
discusses heuristics that end-nodes could use to size their requests
appropriately.
In the absence of other information, there could be a configured
value for the Quick-Start Rate Request. Quick-Start will be more
effective if Quick-Start requests are not larger than necessary;
every Quick-Start request that is approved but not used takes away
from the bandwidth pool available for granting successive Quick-
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Start requests. Therefore, it is recommended that the request for
the initial sending rate be somewhat conservative, in order to
improve the chances for more Quick-Start requests to be approved.
(2) Interactions with Path MTU Discovery:
A second issue when Quick-Start is used to request a large initial
window concerns the interactions between the large initial window
and Path MTU Discovery. Some of the issues are discussed in RFC
3390:
"When larger initial windows are implemented along with Path MTU
Discovery [RFC1191], alternatives are to set the "Don't
Fragment" (DF) bit in all segments in the initial window, or to
set the "Don't Fragment" (DF) bit in one of the segments. It is
an open question as to which of these two alternatives is best."
Unfortunately, the sender doesn't necessarily know the Path MTU when
it sends packets in the initial window. The sender should be
conservative in the packet size used. Sending a large number of
overly-large packets with the DF bit set is not desirable, but
sending a large number of packets that are fragmented in the network
can be equally undesirable.
One possibility would be for the sender to delay using the approved
rate request for one round-trip time, while it sends a small number
of packets to do Path MTU Discovery. While delaying the use of an
approved rate request indefinitely is not acceptable, delaying the
use for one round-trip time is within the bounds of acceptable
behavior.
In the future, it might be possible for the TCP SYN packet to do a
probe about the Path MTU. For example, [W03] has proposed an IP
Option that queries routers for their MTU before starting a Path MTU
Discovery process.
(3) Quick-Start request packets that are eaten by middleboxes:
It is always possible for a TCP SYN packet carrying a Quick-Start
request to be dropped in the network due to congestion, or to be
blocked due to interactions with middleboxes. Measurement studies
of interactions between transport protocols and middleboxes [MAF04]
show that for 70% of the web servers investigated, no connection is
established if the TCP SYN packet contains an unknown IP option (and
for 43% of the web servers, no connection is established if the TCP
SYN packet contains an IP TimeStamp Option). In both cases, this is
presumably due to middleboxes along that path.
If the TCP sender doesn't receive a response to the SYN or SYN/ACK
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packet containing the Quick-Start Request, then the TCP sender
SHOULD resend the SYN or SYN/ACK packet without the Quick-Start
Request. Similarly, if the TCP sender receives a TCP reset in
response to the SYN or SYN/ACK packet containing the Quick-Start
Request, then the TCP sender SHOULD resend the SYN or SYN/ACK packet
without the Quick-Start Request [RFC3360].
While RFC 1122 and 2988 recommend that the sender should set the
initial RTO to three seconds, many TCP implementations set the
initial RTO to one second. For a TCP SYN packet sent with a Quick-
Start request, we recommend an RTO of one second, so that the sender
can retransmit the SYN packet reasonably promptly if the original
TCP SYN packet is dropped by a middlebox in the network.
We note that if the TCP SYN packet is using the IP Quick-Start
Option for a Quick-Start request, and it also using bits in the TCP
header to negotiate ECN-capability with the TCP host at the other
end, then the drop of a TCP SYN packet could be due to congestion,
to a middlebox dropping the packet because of the IP Option, or
because of a middlebox dropping the packet because of the
information in the TCP header negotiating ECN. In this case, the
sender could resend the dropped packet without either the Quick-
Start or the ECN requests. Alternately, the sender could resend the
dropped packet with only the ECN request in the TCP header,
resending the TCP SYN packet without either the Quick-Start or the
ECN requests if the second TCP SYN packet is dropped. The second
choice seems reasonable to us, given that a TCP SYN packet today is
more likely to be blocked due to IP Options than due to an ECN
request in the TCP header.
4.7. TCP: A Quick-Start Request after an Idle Period
This section discusses the following issues that arise when Quick-
Start is used by TCP to request a larger window after an idle
period: (1) determining the rate to request; and (2) the response if
Quick-Start packets are dropped;
(1) Determining the rate to request:
After an idle period, an easy rule of thumb would be for the TCP
sender to determine the largest congestion window that the TCP
connection achieved since the last packet drop, to translate this
congestion window to a sending rate, and use this rate in the Quick-
Start request after the idle period. If the request is granted,
then the sender essentially restarts with its old congestion window
from before the idle period.
The sender should not use Quick-Start if the idle period has been
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less than an RTO, and the congestion window has not decayed down to
less than half of its value at the start of the idle period.
(2) Response if Quick-Start packets are dropped:
If Quick-Start packets are dropped after an idle period, then the
sender should revert to half of the Quick-Start window, or to the
congestion window that the sender would have used if the Quick-Start
request had not been approved, whichever is smaller.
A technical question is whether a Quick-Start Request sent in the
middle of a connection could carry a data payload. For example, for
TCP, a Quick-Start Request in the middle of a connection could carry
a data payload, or could be in a TCP acknowledgement packet. Is
there any advice in this regard that should be offered to the
transport protocol?
4.8. An Example Quick-Start Scenario with TCP
The following is an example scenario in the case when both hosts
request Quick-Start for setting their initial windows:
* The TCP SYN packet from Host A contains a Quick-Start Request in
the IP header.
* Routers along the forward path modify the Quick-Start Request as
appropriate.
* Host B receives the Quick-Start Request in the SYN packet, and
calculates the TTL Diff. If Host B approves the Quick-Start
Request, then Host B sends a Quick-Start Response in the TCP header
of the SYN/ACK packet. Host B also sends a Quick-Start Request in
the IP header of the SYN/ACK packet.
* Routers along the reverse path modify the Quick-Start Request as
appropriate.
* Host A receives the Quick-Start Response in the SYN/ACK packet,
and checks the TTL Diff and Rate Request for validity. If they are
valid, then Host A sets its initial congestion window appropriately,
and sets up rate-based pacing to be used with the initial window.
If the Quick-Start Response is not valid, then Host A uses TCP's
default initial window.
Host A also calculates the TTL Diff for the Quick-Start Request in
the incoming SYN/ACK packet, and sends a Quick-Start Response in the
TCP header of the ACK packet.
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* Host A repeats sending the Quick-Start Response in data packets at
least once per round-trip time until it receives an acknowledgement
from Host B for one of those data packets.
* Host B receives the Quick-Start Response in an ACK packet, and
checks the TTL Diff and Rate Request for validity. If the Quick-
Start Response is valid, then Host B sets its initial congestion
window appropriately, and sets up rate-based pacing to be used with
its initial window. If the Quick-Start Response is not valid, then
Host B uses TCP's default initial window.
5. The Quick-Start Mechanism in other Transport Protocols
The section earlier specified the use of Quick-Start in TCP. In
this section, we generalize this to give guidelines for the use of
Quick-Start with other transport protocols. We also discuss briefly
how Quick-Start could be specified for other transport protocols.
The general guidelines for Quick-Start in transport protocols are as
follows:
* Quick-Start is only specified for unicast transport protocols with
appropriate congestion control mechanisms.
* A transport-level mechanism is needed for the Quick-Start response
from the receiver to the sender. This response contains the Rate
Request and the TTL Diff. The Quick-Start response should ideally
be sent reliably.
* The sender checks the validity of the Quick-Start response.
* The sender has an estimate of the round-trip time, and translates
the Quick-Start response into an allowed window or allowed sending
rate. The sender starts sending Quick-Start packets, rate-paced out
at the approved sending rate.
* After the sender receives the first acknowledgement packet for a
Quick-Start packet, no more Quick-Start packets are sent. The
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.
* When the last Quick-Start packet is acknowledged, the sender
continues using the standard congestion control mechanisms of that
protocol.
* If one of the Quick-Start packets is lost, then the sender reverts
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to the standard congestion control method of that protocol that
would have been used if the Quick-Start request had not been
approved. In addition, the sender takes into account the
information that the Quick-Start congestion window was too large
(e.g., by decreasing ssthresh in TCP).
5.1. Quick-Start with DCCP
DCCP is a new transport protocol for congestion-controlled,
unreliable datagrams, intended for applications such as streaming
media, Internet telephony, and on-line games. In DCCP, the
application has a choice of congestion control mechanisms, with the
currently-specified Congestion Control Identifiers (CCIDs) being
CCID 2 for TCP-like congestion control, and CCID 3 for TFRC, an
equation-based form of congestion control. We refer the reader to
[KHF04] for a more detailed description of DCCP, and of the
congestion control mechanisms.
Because CCID 3 uses a rate-based congestion control mechanism, it
raises some new issues about the use of Quick-Start with transport
protocols. In this document we don't attempt to specify the use of
Quick-Start with DCCP. However, we do discuss some of the issues
that might arise.
In considering the use of Quick-Start with CCID 3 for requesting a
higher initial sending rate, the following questions arise: (1) how
does the sender respond if a Quick-Start packet is dropped; and (2)
when does the sender determine that there has been no feedback from
the receiver, and reduce the sending rate?
(1) How does the sender respond if a Quick-Start packet is dropped:
As in TCP, if an initial Quick-Start packet is dropped, the CCID 3
sender should revert to the congestion control mechanisms it would
have used if the Quick-Start request had not been approved.
(2) When does the sender decide there has been no feedback from the
receiver:
Unlike TCP, CCID 3 does not use acknowledgements for every packet,
or for every other packet. In contrast, the CCID 3 receiver sends
feedback to the 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-trip times (when the sender is
sending at least one packet every two round-trip times). When a
Quick-Start request is used, it would seem prudent to use a smaller
time interval, e.g., to reduce the sending rate if no feedback is
received from the receiver in at least two round-trip times.
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The question also arises of how the sending rate should be reduced
after a period of no feedback from the receiver. The default CCID 3
response of halving the sending rate might not be sufficient; an
alternative would be to reduce the sending rate to the sending rate
that would have been used if no Quick-Start request had been
approved. That is, if a CCID 3 sender uses a Quick-Start request,
special rules might be required to handle the sender's response to a
period of no feedback from the receiver regarding the Quick-Start
packets.
Similarly, in considering the use of Quick-Start with CCID 3 for
requesting a higher sending rate after an idle period, the following
questions arise: (1) what rate does the sender request; (2) what is
the response to a loss; and (3) when does the sender determine that
there has been no feedback from the receiver, and the sending rate
must be reduced?
(1) What rate does the sender request:
As in TCP, there is a straightforward answer to the rate request
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, 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 that should
be requested by the CCID 3 sender. A Quick-Start request is useful
with CCID 3 when the sender is coming out of an idle or
underutilized period, because in standard operation CCID 3 does not
allow the sender to send more that twice as fast as the receiver has
reported received in the most recent feedback message.
(2) What is the response to loss:
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.
(3) When does the sender decide there has been no feedback from the
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 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 approved.
6. Evaluation of Quick-Start
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6.1. Benefits of Quick-Start
The main benefit of Quick-Start is the faster start-up for the
transport connection itself. For a small TCP transfer of one to
five packets, Quick-Start is probably of very little benefit; at
best, it might shorten the connection lifetime from three to two
round-trip times (including the round-trip time for connection
establishment). Similarly, for a very large transfer, where the
slow-start phase would have been only a small fraction of the
connection lifetime, Quick-Start would be of limited benefit.
Quick-Start would not significantly shorten the connection lifetime,
but it might eliminate or at least shorten the start-up phase.
However, for moderate-sized connections in a well-provisioned
environment, Quick-Start could allow the entire transfer of M
packets to be completed in one round-trip time (after the initial
round-trip time for the SYN exchange), instead of the log_2(M)-2
round-trip times that it would normally for the data transfer, in an
uncongested environments (assuming an initial window of four
packets).
6.2. Costs of Quick-Start
This section discusses the costs of Quick-Start for the connection
and for the routers along the path.
The cost of having a Quick-Start packet dropped:
For the sender the biggest risk in using Quick-Start lies in the
possibility of suffering from congestion-related losses of the
Quick-Start packets. This should be an unlikely situation because
routers are expected to approve Quick-Start Requests only when they
are significantly underutilized. However, a transient increase in
cross-traffic in one of the routers, a sudden decrease in available
bandwidth on one of the links, or congestion at a non-IP queue could
result in packet losses even when the Quick-Start Request was
approved by all of the routers along the path. If a Quick-Start
packet is dropped, then the sender reverts to the congestion control
mechanisms it would have used if the Quick-Start request has not
been approved, so the performance cost to the connection of having a
Quick-Start packet dropped is small, compared to the performance
without Quick-Start. (On the other hand, the performance difference
between Quick-Start with a Quick-Start packet dropped and Quick-
Start with no Quick-Start packet dropped can be considerable.)
Added complexity at routers:
The main cost of Quick-Start at routers concerns the costs of added
complexity. The added complexity at the end-points is moderate, and
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might easily be outweighed by the benefit of Quick-Start to the end
hosts. The added complexity at the routers is also somewhat
moderate; it involves estimating the unused bandwidth on the output
link over the last several seconds, processing the Quick-Start
request, and keeping a counter of the aggregate Quick-Start rate
approved over the last fraction of a second. However, this added
complexity at routers adds to the development cycle, and could
prevent the addition of other competing functionality to routers.
Thus, careful thought would have to be given to the addition of
Quick-Start to IP.
The slow path in routers:
Another drawback of Quick-Start is that packets containing the
Quick-Start Request message might not take the fast path in routers.
This would mean extra delay for the end hosts, and extra processing
burden for the routers. This extra burden is mitigated somewhat by
the following factors: only very few packets would carry the Quick-
Start Request option; very small flows of, say, one to five packets
would receive little benefit from Quick-Start, and presumably would
not use the Quick-Start Request; flows from end hosts with low-
bandwidth access links would receive little benefit from Quick-
Start, and hopefully could be configured not to use the Quick-Start
Request. In addition, in typical environments where most of the
packets belong to large flows, the burden of the Quick-Start Option
on routers would be considerably reduced. Nevertheless, it is still
conceivable, in the worst case, that up to 10% of the packets were
Quick-Start packets, and this could slow down the processing of
Quick-Start packets in routers considerably. In particular, because
many Quick-Start packets are likely to be TCP SYN or SYN/ACK
packets, the slow processing of Quick-Start packets would slow down
the establishment of the corresponding TCP connections.
Multiple paths:
One limitation of Quick-Start is that it presumes that the data
packets of a connection will follow the same path as the Quick-Start
request packet. If this is not the case, then the connection could
be sending the Quick-Start packets, at the approved rate, along a
path that was already congested, or that became congested as a
result of this connection. This is, however, similar to what would
happen if the connection's path was changed in the middle of the
connection, when the connection had already established the allowed
initial rate.
Non-IP queues:
A problem of any mechanism for feedback from routers at the IP level
is that there can be queues and bottlenecks in the end-to-end path
that are not in IP-level routers. As an example, these include
queues in layer-two Ethernet or ATM networks. One possibility would
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be that an IP-level router adjacent to such a non-IP queue or
bottleneck would be configured to reject Quick-Start requests if
that was appropriate.
6.3. Protection against Misbehaving Nodes
In this section we discuss the protection against receivers lying
about the Quick-Start Request, and against other possible
misbehaviors regarding Quick-Start. First, we note that it is not
necessarily in the receiver's interest to lie about the Quick-Start
Request. If the sender sends at too-high of an initial rate, and
has a packet dropped, this does not improve the performance of the
connection, relative to the case when the Quick-Start Request was
not approved.
Receivers lying about whether the request was approved:
The use of the Quick-Start TTL initialized by the sender to a random
value makes it difficult for the receiver to lie to the sender about
whether the request has been approved by all of the routers along
the path. If a router that understands the Quick-Start Request
deletes the Request, or zeroes the QS TTL in the request, then the
chances of a downstream router or misbehaving receiver guessing the
value of the QS TTL is at most 1/256.
In particular, if a router deletes the Quick-Start Request, it is
unlikely that the receiver would be able to send a valid Quick-Start
Response back to the sender. Similarly, if there are routers along
the path that do not understand or approve of the Quick-Start
Request, and that forward the Quick-Start Request unchanged, it
would be not be easy for a downstream router or the receiver to
cheat and modify the QS TTL field so that the request was considered
valid, because the downstream routers do not know the initial value
for the QS TTL.
Receivers lying about the rate request:
The receiver could lie to the sender about the Rate Request in the
received Quick-Start Request. However, the receiver doesn't know
the Rate Request in the original Quick-Start Request sent by the
sender, and a higher Rate Request reported by the receiver will only
be considered valid by the sender if it is no higher than the Rate
Request originally requested by the sender. This limits the ability
of the receiver to cheat. For example, if the sender sends a Quick-
Start Request with an Rate Request of X, and the receiver reports
receiving a Quick-Start Request with an Rate Request of Y > X, then
the sender knows that either some router along the path
malfunctioned (increasing the Rate Request inappropriately), or the
receiver is lying about the Rate Request in the received packet.
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However, if the sender sends a Quick-Start Request with an Rate
Request of Z, the receiver receives the Quick-Start Request with an
approved Rate Request of X, and reports an Rate Request of Y, for X
< Y < Z, then the receiver succeeds in lying to the sender about the
approved rate.
One protection against such misbehavior from the receiver would be
for a router decreasing a Rate Request in a Quick-Start Request to
report the decrease directly to the sender. However, it is
hopefully sufficient protection that the receiver does not know the
Rate Request in the original Quick-Start Request.
One way to add additional protection would be for senders to use
some degree of randomization in the requested Rate Request, so that
it is difficult for receivers to guess the original value for the
Rate Request. However, this is more difficult if there is fairly
coarse granularity in the set of rate requests available to the
sender.
Similarly, a router could attempt to cheat and increase the rate
request, but this would only be effective if there were no
downstream routers that denied the Rate Request.
Misbehaving routers:
In addition to protecting against misbehaving receivers, it is
necessary also to protect against misbehaving routers. Consider
collusion between an ingress router and an egress router belonging
to the same Intranet. The ingress router could decrement the Rate
Request at the ingress, with the egress router increasing it again
at the egress. The routers between the ingress and egress that
approved the decremented rate request might not have been willing to
approve the larger, original request.
Another form of collusion would be for the ingress router to inform
the egress router out-of-band of the IP TTL and QS TTL in the
request packet at the ingress. This would enable the egress router
to modify the QS TTL so that it appeared that all of the routers
along the path had approved the request. We would note that in the
extreme case, there does not appear to be any protection against a
colluding ingress and egress router. Even if an intermediate router
had deleted the Quick-Start Request Option from the packet, the
ingress router could have sent the Quick-Start Request Option to the
egress router out-of-band, with the egress router inserting the
Quick-Start Request Option, with a modified QS TTL field, back in
the packet.
However, unlike ECN, there is somewhat less incentive for
cooperating ingress and egress routers to collude to falsely modify
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the Quick-Start Request so that it appears to have been approved by
all of the routers along the path. With ECN, a colluding ingress
router could falsely mark a packet as ECN-capable, with the
colluding egress router returning the ECN field in the IP header to
its original non-ECN-capable codepoint, and congested routers along
the path could have been fooled into not dropping that packet. This
collusion would give an unfair competitive advantage to the traffic
protected by the colluding ingress and egress routers.
In contrast, with Quick-Start, the ingress and egress routers
colluding to make it falsely appear that a Quick-Start request was
approved does not necessarily give an advantage to the traffic
covered by that collusion. If some router along the path really
does not have enough available bandwidth to approve the Quick-Start
request, then the Quick-Start packets sent as a result of the
falsely-approved request could be dropped in the network, to the
resulting disadvantage of the connection. Thus, while the ingress
and egress routers could collude to prevent intermediate routers
from denying a Quick-Start request, it would generally not be to the
connection's advantage for this to happen.
Of course, if the congested router was ECN-capable, and the
colluding ingress and egress routers were lying about ECN-capability
as well as about Quick-Start, then the result could be that the
Quick-Start request falsely appears to the sender to have been
approved, the Quick-Start packets falsely appear to the congested
router to be ECN-capable, and the colluding routers succeed in
giving a competitive advantage to the traffic protected by their
collusion.
Misbehaving middleboxes:
A separate possibility is that of traffic normalizers or other
middleboxes along that path that re-write IP TTLs, in order to foil
other kinds of attacks in the network. If such a traffic normalizer
re-wrote the IP TTL, but did not adjust the Quick-Start TTL by the
same amount, then the sender's mechanism for determining if the
request was approved by all routers along the path would no longer
be reliable. Re-writing the IP TTL could result in false positives
(with the sender incorrectly believing that the Quick-Start request
was approved) as well as false negatives (with the sender
incorrectly believing that the Quick-Start request was denied).
6.4. Quick-Start with QoS-enabled Traffic
The discussion in this paper has largely been of Quick-Start with
default, best-effort traffic. However, Quick-Start could also be
used by traffic using some form of differentiated services, and
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routers could take the traffic class into account when deciding
whether or not to grant the Quick-Start request. We don't address
this context further in this paper, since it is orthogonal to the
specification of Quick-Start. However, we note that routers should
be discouraged from granting Quick-Start requests for higher-
priority traffic when this is likely to result in significant packet
loss for lower-priority traffic.
6.5. Limitations of Quick-Start
The Quick-Start proposal, taken together with the recent proposal
for HighSpeed TCP [F03], could go a significant way towards
extending the range of performance for best-effort traffic in the
Internet. However, there are many things that the Quick-Start
proposal would not accomplish. Quick-Start is not a congestion
control mechanism, and would not help in making more precise use of
the available bandwidth, that is, of achieving the goal of very high
throughput with very low delay and very low packet loss rates.
Quick-Start would not give routers more control over the decrease
rates of active connections. One of the open questions addressed
later in this document is whether the limited capabilities of Quick-
Start are sufficient to warrant standardization and deployment, or
whether more work is needed to explore the space of potential
mechanisms.
6.6. Attacks on Quick-Start
As discussed in [SAF05], Quick-Start is vulnerable to two kinds of
Quick-Start attacks: (1) attacks to increase the routers'
processing and state load; and (2) attacks with bogus Quick-Start
requests to temporarily tie up available Quick-Start bandwidth,
preventing routers from approving Quick-Start requests from other
connections. Routers can protect against the first kind of attack
by applying a simple limit on the rate at which Quick-Start requests
will be considered by the router. The second kind of attack, which
is more difficult to defend against, is discussed in more detail in
[SAF05].
6.7. Simulations with Quick-Start
Quick-Start was added to the NS simulator [SH02] by Srikanth
Sundarrajan, and additional functionality was added by Pasi
Sarolahti. The validation test is at `test-all-quickstart' in the
'tcl/test' directory in NS. The initial simulation studies from
[SH02] show a significant performance improvement using Quick-Start
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for moderate-sized flows (between 4KB and 128KB) in under-utilized
environments. These studies are of file transfers, with the
improvement measured as the relative increase in the overall
throughput for the file transfer. The study shows that potential
improvement from Quick-Start is proportional to the delay-bandwidth
product of the path.
The Quick-Start simulations in [SAF05] explore the following: the
potential benefit of Quick-Start for the connection; the relative
benefits of different router-based algorithms for approving Quick-
Start requests; and the effectiveness of Quick-Start as a function
of the senders' algorithms for choosing the size of the rate
request. [SAF05] also consideres the potential of Extreme Quick-
Start algorithms at routers, which keep per-flow state at routers
for Quick-Start connections, in protecting the availability of
Quick-Start bandwidth in the face of frequent overly-larqe Quick-
Start requests.
7. Related Work
Any evaluation of Quick-Start must include a discussion of the
relative benefits of approaches that use no explicit information
from routers, and of approaches that use more fine-grained feedback
from routers as part of a larger congestion control mechanism. We
discuss three classes of proposals (no explicit feedback from
routers; explicit feedback about the initial rate; and more fine-
grained feedback from routers) in the sections below.
7.1. Fast Start-ups without Explicit Information from Routers
One possibility would be for senders to use information from the
packet streams to learn about the available bandwidth, without
explicit information from routers. These techniques would not allow
a start-up as fast as that available from Quick-Start, in an
underutilized environment; one has to have sent some packets
already to use the packet stream to learn about available bandwidth.
However, these techniques could allow a start-up considerably faster
than the current slow-start. While it seems clear that approaches
*without* explicit feedback from the routers will be strictly less
powerful that is possible *with* explicit feedback, it is also
possible that approaches that are more aggressive than slow-start
are possible without explicit feedback from routers.
Periodic packet streams:
[JD02] explores the use of periodic packet streams to estimate the
available bandwidth along a path. The idea is that the one-way
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delays of a periodic packet stream show an increasing trend when the
stream's rate is higher than the available bandwidth. While [JD02]
states that the proposed mechanism does not cause significant
increases in network utilization, losses, or delays when done by one
flow at a time, the approach could be problematic if conducted
concurrently by a number of flows. [JD02] also gives an overview of
some of the earlier work on inferring the available bandwidth from
packet trains.
Swift-Start:
The Swift Start proposal from [PRAKS02] combines packet-pair and
packet-pacing techniques, beginning with a four-segment burst of
packets to estimate the available bandwidth along the path.
While continued research on the limits of the ability of TCP and
other transport protocols to learn of available bandwidth without
explicit feedback from the router seems useful, we note that there
are several fundamental advantages of explicit feedback from
routers.
(1) Explicit feedback is faster than implicit feedback:
One advantage of explicit feedback from the routers is that it
allows the transport sender to reliably learn of available bandwidth
in one round-trip time.
(2) Explicit feedback is more reliable than implicit feedback:
A second advantage of explicit feedback from the routers is that the
available bandwidth along the path does not necessarily map to the
allowed sending rate for an individual flow. As an example, if the
TCP sender sends four packets back-to-back in the initial window,
and the TCP receiver reports that the data packets were received
with roughly the same spacing as they were transmitted, does this
mean that the flow can infer an underutilized path? And how fast
can the flow send in the next round-trip time? Do the results
depend on the level of statistical multiplexing at the congested
link, and on the number of flows attempting a faster start-up at the
same time?
7.2. Optimistic Sending without Explicit Information from Routers
Another possibility that has been suggested [S02] is for the sender
to start with a large initial window without explicit permission
from the routers and without 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 data packet to drop or delay many or all of the packets
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from that initial window. In this way a flow's optimistically-large
initial window would not force the router to drop packets from
competing flows in the network. Such an approach would seem to
require some mechanism for the sender to ensure that the routers
along the path understood the mechanism for marking the first packet
of a large initial window.
Obviously there would be a number of questions to consider about an
approach of optimistic sending.
(1) Incremental deployment:
One question would be the potential complications of incremental
deployment, where some of the routers along the path might not
understand the packet information describing the initial window.
(2) Congestion collapse:
There could also be concerns about congestion collapse if many flows
used large initial windows, many packets were dropped from
optimistic initial windows, and many congested links ended up
carrying packets that are only going to be dropped downstream.
(3) Distributed Denial of Service attacks:
A third key question would be the potential role of optimistic
sender in amplifying the damage done by a Distributed Denial of
Service (DDoS) attack.
(4) Performance hits if a packet is dropped:
A fourth issue would be to quantify the performance hit to the
connection when a packet is dropped from one of the initial windows.
7.3. Fast Start-ups with other Information from Routers
There have been several proposals somewhat similar to Quick-Start,
where the transport protocol collects explicit information from the
routers along the path.
An IP Option about the free buffer size:
In related work, Joon-Sang Park and John Heidemann investigated the
use of a slightly different IP option for TCP connections to
discover the available bandwidth along the path [P00]. In that
proposal, the IP option would query the routers along the path about
the smallest available free buffer size. Also, the IP option would
have been sent after the initial SYN exchange, when the TCP sender
already had an estimate of the round-trip time.
The Performance Transparency Protocol:
The Performance Transparency Protocol (PTP) includes a proposal for
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a single PTP packet that would collect information from routers
along the path from the sender to the receiver [W00]. For example,
a single PTP packet could be used to determine the bottleneck
bandwidth along a path.
ETEN:
Additional proposals for end nodes to collect explicit information
from routers include Explicit Transport Error Notification (ETEN),
which includes a cumulative mechanism to notify endpoints of
aggregate congestion statistics along the path [KAPS02].
7.4. Fast Start-ups with more Fine-Grained Feedback from Routers
Proposals for more fine-grained congestion-related feedback from
routers include XCP [KHR02] and AntiECN marking [K03]. Section A.6
discusses in more detail the relationship between Quick-Start and
proposals for more fine-grained per-packet feedback from routers.
XCP:
Proposals such as XCP for new congestion control mechanisms based on
more feedback from routers are more powerful than Quick-Start, but
also are more complex to understand and more difficult to deploy.
XCP routers maintain no per-flow state, but provide more fine-
grained feedback to end-nodes than the one-bit congestion feedback
of ECN. The per-packet feedback from XCP can be positive or
negative, and specifies the increase or decrease in the sender's
congestion window when this packet is acknowledged.
AntiECN:
The AntiECN proposal is for a single bit in the packet header that
routers could set to indicate that they are underutilized. For each
TCP ACK arriving at the sender indicating that a packet has been
received with the Anti-ECN bit set, the sender would be able to
increase its congestion window by one packet, as it would during
slow-start.
8. Implementation and Deployment Issues
This section discusses some of the implementation issues with Quick-
Start. This section also discusses some of the key deployment
issues, such as the chicken-and-egg deployment problems of
mechanisms that have to be deployed in both routers and end nodes in
order to work, and the problems posed by the wide deployment of
middleboxes today that block the use of known or unknown IP Options.
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8.1. Implementation Issues for Sending Quick-Start Requests
Section 4.6 has discussed some of the issues with deciding the
initial sending rate to request. Quick-Start raises additional
issues about the communication between the transport protocol and
the application, and about the use of the past history with Quick-
Start in the end node.
One possibility is that a protocol implementation could provide an
API for applications to indicate when they want to request Quick-
Start, and what rate they would like to request. In the
conventional socket API this could be a socket option that is set
before a connection is established. Some applications, such those
that use TCP for bulk transfers, do not have interest in the
transmission rate, but they might know the amount of data that can
be sent immediately. Based on this, the sender implementation could
decide whether Quick-Start would be useful, and what rate should be
requested. Datagram-based real-time streaming applications, on the
other hand, may have a specific preference on the transmission rate
and they could indicate the required rate explicitly to the
transport protocol to be used in the Quick-Start Request.
We note that when Quick-Start is used, the TCP sender is required to
implement an additional timer for the paced transmission of Quick-
Start packets.
8.2. Implementation Issues for Processing Quick-Start Requests
A router or other network host must be able to determine the
approximate bandwidth of its outbound network interfaces in order to
process incoming Quick-Start rate requests, including those that
originate from the host itself. One possibility would be for hosts
to rely on configuration information to determine link bandwidths;
this has the drawback of not being robust to errors in
configuration. Another possibility would be for network device
drivers to infer the bandwidth for the interface and to communicate
this to the IP layer.
Particular issues will arise for wireless links with variable
bandwidth, where decisions will have to be made about how frequently
the network host gets updates of the changing bandwidth. It seems
appropriate that Quick-Start Requests would be handled particularly
conservatively for links with variable bandwidth. to avoid cases
where Quick-Start Requests are approved, the link bandwidth is
reduced, and the data packets that are send end up being dropped.
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8.3. Possible Deployment Scenarios
Because of possible problems discussed above concerning using Quick-
Start over some network paths, the most realistic initial deployment
of Quick-Start would likely to take place in Intranets and other
controlled environments. Quick-Start is most useful on high
bandwidth-delay paths that are significantly underutilized. The
primary initial users of Quick-Start would likely be in
organizations that provide network services to their users and also
have control over a large portion of the network path.
Below are a few examples of networking environments where Quick-
Start would potentially be useful. These are the environments that
might consider an initial deployment of Quick-Start in the routers
and end-nodes, where the incentives for routers to deploy Quick-
Start might be the most clear.
* Centrally-administrated organizational Intranets often have large
network capacity and the networks are underutilized for most of the
time. with the network nodes along the path administrated by a
single organization. Such Intranets might also include high-
bandwidth and high-delay paths to remote sites. In such an
environment, Quick-Start would be of benefit to users, and there
would be a clear incentive for the deployment of Quick-Start in
routers.
* Quick-Start could also be useful in high-delay environments of
Cellular Wide-Area Wireless Networks such as the GPRS [BW97] and
their enhancements and next generations. For example, GPRS EDGE
(Enhanced Data for GSM Evolution) is expected to provide wireless
bandwidth of up to 384 Kbps (roughly 32 1500-byte packets per
second) while the GPRS round-trip times are typically up to one
second excluding any possible queueing delays in the network
[GPAR02]. In addition, these networks sometimes have variable
additional delays due to resource allocation that could be avoided
by keeping the connection path constantly utilized, starting from
initial slow start. Thus, Quick-Start could be of significant
benefit to users in these environments.
* Geostationary Orbit (GEO) satellite links have one-way propagation
delays on the order of 250 ms while the bandwidth is typically
measured in megabits per second [RFC2488]. Because of the
considerable bandwidth-delay product on the link, TCP's slow start
is a major performance limitation in the beginning of the
connection. A large initial congestion window would be useful to
users of such satellite links.
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8.4. Would QuickStart packets take the slow path in routers?
How much delay would the slow path add to the processing time for
this packet? Similarly, if QuickStart packets took the slow path,
how much stress would it add to routers for there to be many more
packets on the slow path, because of the number of packets using
QuickStart? These are both questions to be considered for the
deployment of Quick-Start in the Internet.
8.5. A Comparison with the Deployment Problems of ECN
For ECN, only one router along the path has to understand. For
Quick-Start, all of the routers along the path would have to
understand. Also, Quick-Start has the complicating factor of using
IP Options, while ECN uses a field in the IP header itself.
9. Security Considerations
One security consideration would be if Quick-Start resulted in the
sender using an Rate Request that was inappropriately large,
resulting in congestion along the path. Such congestion could
result in an unacceptable level of packet drops along the path.
Such congestion could also be part of a Denial of Service attack.
A misbehaving TCP sender could use a non-conformant initial
congestion window even without the use of Quick-Start, so we
restrict our attention to problems with Quick-Start with conformant
TCP senders. (We also note that if the TCP sender is a busy web
server, then the TCP sender has some incentive to be conformant in
this regard.) Section 6.3 discusses the dangers of receivers or
routers lying about the Quick-Start rate request, or about whether
the rate request was approved.
10. Conclusions
We are presenting the Quick-Start mechanism as a proposal for a
simple, understandable, and incrementally-deployable mechanism that
would be sufficient to allow connections to start up with large
initial rates, or large initial congestion windows, in
overprovisioned, high-bandwidth environments. We expect there will
be an increasing number of overprovisioned, high-bandwidth
environments where the Quick-Start mechanism, or another mechanism
of similar power, could be of significant benefit to a wide range of
traffic. We are presenting the Quick-Start mechanism as a request
for feedback from the Internet community in considering these
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issues.
11. Acknowledgements
The authors wish to thank Mark Handley for discussions of these
issues. The authors also thank the End-to-End Research Group, the
Transport Services Working Group, and members of IPAM's program on
Large Scale Communication Networks for both positive and negative
feedback on this proposal. We thank Srikanth Sundarrajan for the
initial implementation of Quick-Start in the NS simulator, and for
the initial simulation study. We also thank Mohammed Ashraf, John
Border, Tom Dunigan, John Heidemann, Paul Hyder, Dina Katabi, and
Vern Paxson for feedback. This draft builds upon the concepts
described in [RFC3390], [AHO98], [RFC2415], and [RFC3168].
This is a modification of a draft originally by Amit Jain for
Initial Window Discovery.
A. Design Decisions
A.1. Alternate Mechanisms for the Quick-Start Request: ICMP and RSVP
This document has proposed using an IP Option for the Quick-Start
Request from the sender to the receiver, and using transport
mechanisms for the Quick-Start Response from the receiver back to
the sender. In this section we discuss alternate mechanisms, and
consider whether ICMP [RFC792, RFC2463] or RSVP [RFC2205] protocols
could be used for delivering the Quick-Start Request.
A.1.1. ICMP
Being a control protocol used between Internet nodes, one could
argue that ICMP is the ideal method for requesting a permission for
faster startup from routers. The ICMP header is above the IP
header. Quick-Start would be done with ICMP as follows: If the ICMP
protocol is used to implement Quick-Start, the equivalent of the
Quick-Start IP option would be carried in the ICMP header of the
ICMP Quick-Start Request. The ICMP Quick-Start Request would have
to pass by the routers on the path to the receiver; for now, we
don't address the mechanisms that would be needed to accomplish
this. A router that approves the Quick-Start Request would take the
same actions as in the case with the Quick-Start IP Option, and
forward the packet to the next router along the path. A router that
does not approve the Quick-Start Request, even with a decreased
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value for the Requested Rate, would delete the ICMP Quick-Start
Request, and send an ICMP Reply to the sender that the request was
not approved. If the ICMP Reply was dropped in the network, and did
not reach the receiver, the sender would still know that the request
was not approved from the absence of feedback from the receiver. If
the ICMP Quick-Start request was dropped in the network due to
congestion, the sender would assume that the request was not
approved. If the ICMP Quick-Start Request reached the receiver, the
receiver would use transport-level mechanisms to send a response to
the sender, exactly as with the IP Option.
One benefit of using ICMP would be that the delivery of the TCP SYN
packet or other initial packet would not be delayed by IP option
processing at routers. A greater advantage is that if middleboxes
were blocking packets with Quick-Start Requests, using the Quick-
Start Request in a separate ICMP packet would mean that the
middlebox behavior would not affect the connection as a whole. (To
get this robustness to middleboxes with TCP using an IP Quick-Start
Option, one would have to have a TCP-level Quick-Start Request
packet that was sent concurrently but separately from the TCP SYN
packet.)
However, there are a number of disadvantages to using ICMP. Some
firewalls and middleboxes may not forward the ICMP Quick-Start
Request packets. (If the ICMP Reply packet is dropped in the
network, this is not a problem, as we stated above.) In addition, it
would be difficult, if not impossible, for a router in the middle of
an IP tunnel to deliver an ICMP Reply packet to the actual source,
for example when the inner IP header is encrypted as in IPsec tunnel
mode [RFC2401]. Again, however, the ICMP Reply packet would not be
essential to the correct operation of ICMP Quick-Start.
Unauthenticated out-of-band ICMP messages could enable some types of
attacks by third-party malicious hosts that are not possible when
the control information is carried in-band with the IP packets that
can only be altered by the routers on the connection path. Finally,
as a minor concern, using ICMP would cause a small amount of
additional traffic in the network, which is not the case when using
IP options.
A.1.2. RSVP
With some modifications RSVP [RFC2205] could be used as a bearer
protocol for carrying the Quick-Start Requests. Because routers are
expected to process RSVP packets more extensively than the normal
transport protocol IP packets, delivering a Quick-Start rate request
using an RSVP packet would seem an appealing choice. However, Quick-
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Start with RSVP would require a few differences from the
conventional usage of RSVP. Quick-Start would not require periodical
refreshing of soft state, because Quick-Start does not require per-
connection state in routers. Quick-Start Requests would be
transmitted downstream from the sender to receiver in the RSVP Path
messages, which is different from the conventional RSVP model where
the reservations originate from the receiver. Furthermore, the
Quick-Start Response would be sent using the transport-level
mechanisms instead of using the RSVP Resv message.
If RSVP was used for carrying a Quick-Start Request, a new "Quick-
Start Request" class object would be included in the RSVP Path
message that is sent from the sender to receiver. The object would
contain the rate request field in addition to the common length and
type fields. The Send_TTL field in the RSVP common header could be
used as the equivalent of the QS TTL field. The Quick-Start capable
routers along the path would inspect the Quick-Start Request object
in the RSVP Path message, decrement Send_TTL and adjust the rate
request field if needed. If an RSVP router did not understand the
Quick-Start Request object, it would reject the entire RSVP message
and send an RSVP PathErr message back to the sender. When an RSVP
message with the Quick-Start Request object reaches the receiver,
the receiver sends a Quick-Start Reply message in the corresponding
transport protocol header in the same way as described in the
context of IP options earlier. If the RSVP message with the Quick-
Start Request object was dropped along the path, the transport
sender would simply proceed with the normal congestion control
procedures.
Much of the discussion about benefits and drawbacks of using ICMP
for making the Quick-Start Request also applies to the RSVP case. If
the Quick-Start Request was transmitted in a separate packet instead
of as an IP option, the transport protocol packet delivery would not
be delayed due to IP option processing at the routers, and the
initial transport packets would reach their destination more
reliably. The possible disadvantages of using ICMP and RSVP are also
expected to be similar: middleboxes in the network may not be able
to forward the Quick-Start Request messages, and the IP tunnels
might cause problems for processing the Quick-Start Requests.
A.2. Alternate Encoding Functions
In this section we look at alternate encoding functions for the Rate
Request field in the Quick-Start Request. The main requirements for
this function is that it should have a sufficiently wide range for
the requested rate. There is no need for overly-fine-grained
precision in the requested rate. Similarly, while it would be
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attractive for the encoding function to be easily computable, it is
also possible for end-nodes and routers to simply store the
256-entry table giving the mapping between the value N in the Rate
Request field, and the actual rate request f(N).
Linear functions:
The Quick-Start Request contains an 8-bit field for the Rate
Request. One possible proposal would be for this field to be
formatted in bits per second, scaled so that one unit equals 80
Kbps. Thus, for the value N in the Rate Request field, the
requested rate is 80,000*N bps. This gives a request range between
80 Kbps and 20.48 Mbps. For 1500-byte packets, this corresponds to
a request range between 6 and 1706 packets per second.
Powers of two:
If a granularity of factors of two is sufficient for the Rate
Request, then the encoding function with the most range would be for
the requested rate to be K*2^N, for N the value in the Rate Request
field, and for K some constant. For N=0, the rate request would be
set to zero, regardless of the encoding function. For example, for
K=40,000, the request range would be from 80 Kbps to 40*2^256 Kbps.
This clearly would be an unnecessarily large request range.
For a four-bit Rate Request field, the upper limit on the rate
request is 1.3 Gbps. It is possible that an upper limit of 1.3 Gbps
would be fine for the Quick-Start rate request, and that connections
wishing to start up with a higher initial sending rate should be
encouraged to use other mechanisms, such as the explicit reservation
of bandwidth. If an upper limit of 1.3 Gbps is not acceptable, then
five bits could be used for the Rate Request field.
If the granularity of factors of two is too coarse, then the
encoding function could use a base less than two. An alternate form
for the encoding function would be to use a hybrid of linear and
exponential functions.
We note that the Rate Request also has to be constrained by the
abilities of the transport protocol. For example, for TCP with
Window Scaling, the maximum window is at most 2**30 bytes. For a
TCP connection with a long, 1 second round-trip time, this would
give a maximum sending rate of 1.07 Gbps.
A.3. The Quick-Start Request: Packets or Bytes?
One of the design questions is whether the Rate Request field should
be in bytes per second or in packets per second. We will discuss
this separately from the perspective of the transport, and from the
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perspective of the router.
For TCP, the results from the Quick-Start Request are translated
into a congestion window in bytes, using the measured round-trip
time and the MSS. This window applies only to the bytes of data
payload, and does not include the bytes in the TCP or IP packet
headers. Other transport protocols would conceivably use the Quick-
Start Request directly in packets per second, or could translate the
Quick-Start Request to a congestion window in packets.
The assumption of this draft is that the router only approves the
Quick-Start Request when the output link is significantly
underutilized. For this, the router could measure the available
bandwidth in bytes per second, or could convert between packets and
bytes by some mechanism.
If the Quick-Start Request was in bytes per second, and applied only
to the data payload, then the router would have to convert from
bytes per second of data payload, to bytes per second of packets on
the wire. If the Rate Request field was in bytes per second and the
sender ended up using very small packets, this could translate to a
significantly larger number in terms of bytes per second on the
wire. Therefore, for a Quick-Start Request in bytes per second, it
makes most sense for this to include the transport and IP headers as
well as the data payload. Of course, this will be at best a rough
approximation on the part of the sender; the transport-level sender
might not know the size of the transport and IP headers in bytes,
and might know nothing at all about the separate headers added in IP
tunnels downstream. This rough estimate seems sufficient, however,
given the overall lack of fine precision in Quick-Start
functionality.
It has been suggested that the router could possibly use information
from the MSS option in the TCP packet header of the SYN packet to
convert the Quick-Start Request from packets per second to bytes per
second, or vice versa. The MSS option is defined as the maximum MSS
that the TCP sender expects to receive, not the maximum MSS that the
TCP sender plans to send [RFC793]. However, it is probably often
the case that this MSS also applies as an upper bound on the MSS
used by the TCP sender in sending.
We note that the sender does not necessarily know the Path MTU when
the Quick-Start Request is sent, or when the initial window of data
is sent. Thus, with IPv4, packets from the initial window could end
up being fragmented in the network if the "Don't Fragment" (DF) bit
is not set [RFC1191]. A Rate Request in bytes per second is
reasonably robust to fragmentation. Clearly a Rate Request in
packets per second is less robust in the presence of fragmentation.
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Interactions between larger initial windows and Path MTU Discovery
are discussed in more detail in RFC 3390 [RFC3390].
For a Quick-Start Request in bytes per second, the transport senders
would have the additional complication of estimating the bandwidth
usage added by the packet headers.
We have chosen an Rate Request field in bytes per second rather than
in packets per second because it seems somewhat more robust,
particularly to routers.
A.4. Quick-Start Semantics: Total Rate or Additional Rate?
For a Quick-Start Request sent in the middle of a connection, there
are two possible semantics for the Rate Request field, as follows:
(1) Total Rate: The requested Rate Request is the requested total
rate for the connection, including the current rate; or
(2) Additional Rate: The requested Rate Request is the requested
increase in the total rate for that connection, over and above the
current sending rate.
In this section we consider briefly the tradeoffs between these two
options, and explain why we have chosen the `Total Rate' semantics.
The Total Rate semantics makes it easier for routers to ``allocate''
the same rate to all connections. This lends itself to fairness,
and improves convergence times between old and new connections.
The Additional Rate semantics lends itself to gaming by the
connection, with the sender sending frequent Quick-Start Requests in
the hope of gaining a higher rate.
For either of these alternatives, there would not be room to report
the current sending rate in the Quick-Start Option using the current
minimal format for the Quick-Start Request. Thus, either the Quick-
Start Option would have to take more than four bytes to include a
report of the current sending rate, or the current sending rate
would not be reported to the routers.
A.5. Alternate Responses to the Loss of a Quick-Start Packet
Section 4.5 discusses TCP's response to the loss of a Quick-Start
packet in the initial window. This section discusses several
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alternate responses.
One possible alternative to reverting to the default slow-start
after the loss of a Quick-Start packet from the initial window would
have been to halve the congestion window and continue in congestion
avoidance. However, we note that this would not have been a
desirable response for either the connection or for the network as a
whole. The packet loss in the initial window indicates that Quick-
Start failed in finding an appropriate congestion window, meaning
that the congestion window after halving may easily also be wrong.
A more moderate alternate would be to continue in congestion
avoidance from a window of (W-D)/2, where W is the Quick-Start
congestion window, and D is the number of packets dropped or marked
from that window.
A.6. Why Not Include More Functionality?
As Section 6.5 discussed, this proposal for Quick-Start is a rather
coarse-grained mechanism that would allow connections to use larger
initial windows along underutilized paths, but that does not attempt
to provide a next-generation transport protocol, and does not
attempt the goal of providing very high throughput with very low
delay. As Section 7.4 discusses, there are a number of proposals
such as XCP and AntiECN for more fine-grained per-packet feedback
from routers that the current congestion control mechanisms, that do
attempt these more ambitious goals.
Compared to proposals such as XCP and AntiECN, Quick-Start offers
much less control; Quick-Start does not attempt to provide a new
congestion control mechanism, but simply to get permission from
routers for a higher sending rate at start-up, or after an idle
period. At the same time, Quick-Start would allow larger initial
windows that would proposals such as AntiECN, requires less input to
routers than XCP, and would require less frequent feedback from
routers than any new congestion control mechanism. Thus, Quick-
Start is less powerful in general than proposals for new congestion
control mechanisms such as XCP and AntiECN, but as powerful or more
powerful in terms of the specific issue of allowing larger initial
windows, and (we think) more amenable to incremental deployment in
the current Internet.
We do not discuss proposals such as XCP in detail, but simply note
that there are a number of open questions. One question concerns
whether there is a pressing need for more sophisticated congestion
control mechanisms such as XCP in the Internet. Quick-Start is
inherently a rather crude tool that does not deliver assurances
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about maintaining high link utilization and low queueing delay;
Quick-Start is designed for use in environments that are
significantly 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 could allow
faster startups even in environments with rather high link
utilization. Is this a pressing requirement? Are the other
benefits of more fine-grained congestion control feedback from
routers a pressing requirement?
We would argue that even if more fine-grained per-packet feedback
from routers was implemented, it is reasonable to have a separate
mechanism such as Quick-Start for indicating an allowed initial
sending rate, or an allowed total sending rate after an idle or
underutilized period.
One fundamental difference between Quick-Start and current proposals
for fine-grained per-packet feedback is that the feedback of Quick-
Start is per-connection, giving an allowed sending rate for the
connection as a whole, while the proposals for per-packet feedback
for congestion control are about the increase or decrease in the
rate or window per-packet, when a particular data packet is
acknowledged.
A second difference is that unlike per-packet feedback, Quick-Start
lends itself to more than just a few bits of feedback from routers
to indicate the initial sending rate allowed by the router. While
XCP also allocates a byte for per-packet feedback, there has been
discussion of variants of XCP with less per-packet feedback. This
would be more like other proposals such as anti-ECN that use a
single bit of feedback from routers to indicate that the sender can
increase as fast as slow-starting, in response to this particular
packet acknowledgement. In general, there is probably considerable
power in fine-grained proposals with only two 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 considerably limited if
it was restricted to only two bits of feedback; it seems likely that
determining the initial sending rate fundamentally requires more
bits of feedback from routers than does the everyday, per-packet
feedback to increase or decrease the sending rate.
On a more practical level, one difference between Quick-Start and
proposals for per-packet feedback is that there are fewer open
issues with Quick-Start than there would be with a new congestion
control mechanism. For example, for a mechanism for requesting a
initial sending rate, the fairness issues of a general congestion
control mechanism go away, and there is no need for the end nodes to
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tell the routers the round-trip time and congestion window, as is
done in XCP; all that is needed is for the end nodes to report the
requested sending rate.
Proposals for
Quick-Start Per-Packet Feedback
+------------------+----------------------+----------------------+
Semantics: | Allowed sending rate | Change in rate/window,
| per connection. | per-packet.
+------------------+----------------------+----------------------+
Relationship to | In addition. | Replacement.
congestion ctrl: | |
+------------------+----------------------+----------------------+
Frequency: | Start-up, or after | Every packet.
| an idle period. |
+------------------+----------------------+----------------------+
Limitations: | Only useful on | General congestion
| underutilized paths.| control mechanism.
+------------------+----------------------+----------------------+
Input to routers: | Rate request. | RTT, cwnd, request (XCP).
| | None (Anti-ECN).
+------------------+----------------------+----------------------+
Bits of feedback: | One byte. | A few bits would
| | suffice?
+------------------+----------------------+----------------------+
Differences between Quick-Start and Proposals for
Fine-Grained Per-Packet Feedback.
A separate question concerns whether mechanisms such as Quick-Start,
in combination with HighSpeed TCP and other changes in progress,
would make a significant contribution towards meeting some of these
needs for new congestion control mechanisms. This could be viewed
as a positive step of meeting some of the current needs with a
simple and reasonably deployable mechanism, or alternately, as a
negative step of unnecessarily delaying more fundamental changes.
Without answering this question, we would note that our own approach
tends to favor the incremental deployment of relatively simple
mechanisms, as long as the simple mechanisms are not short-term
hacks but mechanisms that lead the overall architecture in the
fundamentally correct direction.
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A.7. A QuickStart Nonce?
An earlier version of this document included a QuickStart Nonce that
was initialized by the sender to a non-zero, `random' eight-bit
number, along with a QS TTL that was initialized to the same value
as the TTL in the IP header. The QuickStart Nonce would have been
returned by the TCP receiver to the TCP sender in the Quick-Start
Response. A router could deny the Quick-Start request by failing to
decrement the QS TTL field, by zeroing the QS Nonce field, or by
deleting the Quick-Start Request from the packet header. The QS
Nonce was included to provide some protection against broken
downstream routers, or against misbehaving TCP receivers who might
be inclined to lie about the Rate Request. This protection is now
provided by the use of a random initial value for the QS TTL field.
With the old QuickStart Nonce, along with the QS TTL field set to
the same value as the TTL field in the IP header, the Quick-Start
Request mechanism would have been self-terminating; the Quick-Start
Request would terminate at the first participating router after a
non-participating router had been encountered on the path. This
would have minimized unnecessary overhead incurred by routers
because of option processing for the Quick-Start Request. Thus, one
disadvantage of the new approach with a random initial value for the
QS TTL field is that intermediate routers can no longer determine
when some upstream router has not understood the QuickStart option.
However, a disadvantage of the old approach was that it offered no
protection against downstream routers or the TCP receiver hiding
evidence of upstream routers that do not understand the QuickStart
option.
Normative References
[RFC793] J. Postel, Transmission Control Protocol, RFC 793,
September 1981.
[RFC1191] Mogul, J. and S. Deering, Path MTU Discovery, RFC 1191,
November 1990.
[RFC2460] S. Deering and R. Hinden. Internet Protocol, Version 6
(IPv6) Specification. RFC 2460, December 1998.
[RFC2581] M. Allman, V. Paxson, and W. Stevens. TCP Congestion
Control. RFC 2581. April 1999.
[RFC3168] Ramakrishnan, K.K., Floyd, S., and Black, D. The Addition
of Explicit Congestion Notification (ECN) to IP. RFC 3168, Proposed
Standard, September 2001.
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[RFC3390] M. Allman, S. Floyd, and C. Partridge. Increasing TCP's
Initial Window. RFC 3390, October 2002.
Informative References
[RFC792] J. Postel. Internet Control Message Protocol. RFC 792,
September 1981.
[RFC1812] F. Baker (ed.). Requirements for IP Version 4 Routers. RFC
1812, June 1995.
[RFC2140] J. Touch. TCP Control Block Interdependence. RFC 2140.
April 1997.
[RFC2205] R. Braden, et al. Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification. RFC 2205, September 1997.
[RFC2309] B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering,
D. Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge, L.
Peterson, K. Ramakrishnan, S. Shenker, J. Wroclawski, L. Zhang,
Recommendations on Queue Management and Congestion Avoidance in the
Internet, RFC 2309, April 1998.
[RFC2401] S. Kent and R. Atkinson. Security Architecture for the
Internet Protocol. RFC 2401, November 1998.
[RFC2415] K. Poduri and K. Nichols. Simulation Studies of Increased
Initial TCP Window Size. RFC 2415. September 1998.
[RFC2416] T. Shepard and C. Partridge. When TCP Starts Up With Four
Packets Into Only Three Buffers. RFC 2416. September 1998.
[RFC2463] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification.
RFC 2463, December 1998.
[RFC2488] M. Allman, D. Glover, and L. Sanchez. Enhancing TCP Over
Satellite Channels using Standard Mechanisms. RFC 2488. January
1999.
[RFC2960] R. Stewart, et. al. Stream Control Transmission Protocol.
RFC 2960, October 2000.
[RFC3124] H. Balakrishnan and S. Seshan. The Congestion Manager. RFC
3124. June 2001.
[RFC3344] C. Perkins (ed.). IP Mobility Support for IPv4. RFC 3344,
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August 2002.
[RFC3360] S. Floyd. Inappropriate TCP Resets Considered Harmful.
RFC 3360, August 2002.
[RFC3775] D. Johnson, C. Perkins, and J. Arkko. Mobility Support in
IPv6. RFC 3775, June 2004.
[AHO98] M. Allman, C. Hayes and S. Ostermann. An evaluation of TCP
with Larger Initial Windows. ACM Computer Communication Review, July
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[BW97] G. Brasche and B. Walke. Concepts, Services and Protocols of
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Congestion Control in the Internet, IEEE/ACM Transactions on
Networking, August 1999.
[F03] Floyd, S., HighSpeed TCP for Large Congestion Windows, RFC
3649, December 2003.
[F04] Floyd, S., Limited Slow-Start for TCP with Large Congestion
Windows, RFC 3742, Experimental, March 2004.
[GPAR02] A. Gurtov, M. Passoja, O. Aalto, and M. Raitola. Multi-
Layer Protocol Tracing in a GPRS Network. In Proceedings of the IEEE
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Bandwidth: Measurement Methodology, Dynamics, and Relation with TCP
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[KHR02] Dina Katabi, Mark Handley, and Charles Rohrs, Internet
Congestion Control for Future High Bandwidth-Delay Product
Environments. ACM Sigcomm 2002, August 2002. URL
"http://ana.lcs.mit.edu/dina/XCP/".
[KHF04] E. Kohler, M. Handley, and S. Floyd, Datagram Congestion
Control Protocol (DCCP), internet draft draft-ietf-dccp-spec-09.txt,
work in progress, November 2004.
[K03] S. Kunniyur, "AntiECN Marking: A Marking Scheme for High
Bandwidth Delay Connections", Proceedings, IEEE ICC '03, May 2003.
Jain/Floyd/Allman/Sarolahti [Page 53]
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URL "http://www.seas.upenn.edu/~kunniyur/".
[KAPS02] Rajesh Krishnan, Mark Allman, Craig Partridge, James P.G.
Sterbenz. Explicit Transport Error Notification (ETEN) for Error-
Prone Wireless and Satellite Networks. Technical Report No. 8333,
BBN Technologies, March 2002. URL
"http://roland.lerc.nasa.gov/~mallman/papers/".
[MAF04] Alberto Medina, Mark Allman, and Sally Floyd, Measuring
Interactions Between Transport Protocols and Middleboxes, Internet
Measurement Conference 2004, August 2004. URL
"http://www.icir.org/tbit/".
[PK98] Venkata N. Padmanabhan and Randy H. Katz, TCP Fast Start: A
Technique For Speeding Up Web Transfers, IEEE GLOBECOM '98, November
1998.
[P00] Joon-Sang Park, Bandwidth Discovery of a TCP Connection,
report to John Jeidemann, 2000, private communication. Citation for
acknowledgement purposes only.
[PRAKS02] Craig Partridge, Dennis Rockwell, Mark Allman, Rajesh
Krishnan, James P.G. Sterbenz. A Swifter Start for TCP. Technical
Report No. 8339, BBN Technologies, March 2002. URL
"http://roland.lerc.nasa.gov/~mallman/papers/".
[S02] Ion Stoica, private communication, 2002. Citation for
acknowledgement purposes only.
[SAF05] Pasi Sarolahti, Mark Allman, and Sally Floyd. Evaluating
Quick-Start for TCP. Under submission, February 2005. URL
"http://www.icir.org/floyd/quickstart.html".
[SH02] Srikanth Sundarrajan and John Heidemann. Study of TCP Quick
Start with NS-2. Class Project, December 2002. Not publically
available; citation for acknowledgement purposes only.
[W00] Michael Welzl: PTP: Better Feedback for Adaptive Distributed
Multimedia Applications on the Internet, IPCCC 2000 (19th IEEE
International Performance, Computing, And Communications
Conference), Phoenix, Arizona, USA, 20-22 February 2000. URL
"http://informatik.uibk.ac.at/users/c70370/research/publications/".
[W03] Michael Welzl, PMTU-Options: Path MTU Discovery Using Options,
expired internet-draft draft-welzl-pmtud-options-01.txt, work-in-
progress. February 2003.
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IANA Considerations
The only IANA Considerations would be the addition of an IP option
to the list of IP options, and the addition of a TCP option to the
list of TCP options.
AUTHORS' ADDRESSES
Amit Jain
F5 Networks
Email : a.jain@f5.com
Sally Floyd
Phone: +1 (510) 666-2989
ICIR (ICSI Center for Internet Research)
Email: floyd@icir.org
URL: http://www.icir.org/floyd/
Pasi Sarolahti
Nokia Research Center
P.O. Box 407
FI-00045 NOKIA GROUP
Finland
Phone: +358 50 4876607
Email: pasi.sarolahti@iki.fi
Full Copyright Statement
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