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The Congestion Manager
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC-2026 [Bradner96].
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-
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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.
1. Abstract
This document describes the Congestion Manager (CM), an end-system
module that (i) enables an ensemble of multiple concurrent flows
from a sender destined to the same receiver and sharing the same
congestion properties to perform proper congestion avoidance and
control, and (ii) allows applications to easily adapt to network
congestion. This framework integrates congestion management across
all applications and transport protocols. The CM maintains
congestion parameters (available aggregate and per-flow bandwidth,
per-receiver round-trip times, etc.) and exports an API that
enables applications to learn about network characteristics, pass
information to the CM, share congestion information with each
other, and schedule data transmissions. This document focuses on
applications and transport protocols with their own independent
per-byte or per-packet sequence number information, and does not
require modifications to the receiver protocol stack. The
receiving application must provide feedback to the sending
application about received packets and losses, and the latter uses
the CM API to update the CM state. This document does not address
networks with reservations or service discrimination.
2. Conventions used in this document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119
[Bradner97].
FLOW
A stream of packets that all share the same source and
destination IP address, IP type-of-service, transport
protocol, and source and destination transport port numbers.
MACROFLOW
A group of flows that uses the same congestion management and
scheduling algorithms, and shares congestion state
information. Flows destined to different receivers MUST belong
to different macroflows. Flows destined to the same receiver
MAY belong to different macroflows. Flows that experience
identical congestion behavior in the Internet and use the
same congestion control algorithm SHOULD belong to the same
macroflow.
APPLICATION Any software module that uses the CM. This includes
user-level applications such as Web servers or audio/video
servers, as well as in-kernel protocols such as TCP [Postel81]
that use the CM for congestion control.
WELL-BEHAVED APPLICATION
An application that only transmits when allowed by the CM and
accurately accounts for all data that it has sent to the
receiver by informing the CM using the CM API.
STREAM
A logical sequence of packets generated by an application that
directly corresponds (one-to-one) with a network-layer FLOW.
PATH MAXIMUM TRANSMISSION UNIT (PMTU)
The size of the largest packet that the sender can transmit
without it being fragmented en route to the receiver. It
includes the sizes of all headers and data except the IP
header.
CONGESTION WINDOW (cwnd)
A CM state variable that modulates the amount of outstanding
data between sender and receiver.
OUTSTANDING WINDOW (ownd)
The number of bytes that has been transmitted by the source,
but not known to have been either received by the destination
or lost in the network.
INITIAL WINDOW (IW)
The size of the sender's congestion window at the beginning of
a macroflow.
DATA TYPE SYNTAX
We use "u64" for unsigned 64-bit, "u32" for unsigned 32- bit,
"u16" for unsigned 16-bit, "u8" for unsigned 8-bit, "i32" for
signed 32-bit, "i16" for signed 16-bit quantities, "float" for
IEEE floating point values. The type "void" is used to
indicate that no return value is expected from a
call. Pointers are referred to using "*" syntax, following C
language convention.
3. Introduction
The CM is an end-system module that enables an ensemble of multiple
concurrent flows to perform proper congestion avoidance and
control, and allows applications to easily adapt their
transmissions to prevailing network conditions. It integrates
congestion management across all applications and transport
protocols. It maintains congestion parameters (available aggregate
and per-flow bandwidth, per-receiver round-trip times, etc.) and
exports an API that enables applications to learn about network
characteristics, pass information to the CM, share congestion
information with each other, and schedule data transmissions. All
data transmissions MUST be done with the explicit consent of the CM
via this API to ensure proper congestion behavior.
This document focuses on applications and networks where the
following conditions hold:
1. Applications are well-behaved with their own independent
per-byte or per-packet sequence number information, and use the
CM API to update internal state in the CM.
2. Networks are best-effort without service discrimination or
reservations. In particular, it does not address situations
where different flows between the same pair of hosts traverse
paths with differing characteristics.
The Congestion Manager framework can be extended to support
applications that do not provide their own feedback and to
differentially served networks. These extensions will be addressed
in later documents.
The CM is motivated by two main goals:
(i) Enable efficient multiplexing. Increasingly, the trend on the
Internet is for unicast data senders (e.g., Web servers) to
transmit heterogeneous types of data to receivers, ranging from
unreliable real-time streaming content to reliable Web pages and
applets. As a result, many logically different flows share the
same path between sender and receiver. For the Internet to remain
stable, each of these streams must incorporate control protocols
that safely probe for spare bandwidth and react to
congestion. Unfortunately, these concurrent flows typically compete
with each other for network resources, rather than share them
effectively. Furthermore, they do not learn from each other about
the state of the network. Even if they each independently implement
congestion control (e.g., a group of TCP connections each
implementing the algorithms in [Jacobson88, Stevens97]), the
ensemble of flows tends to be more aggressive in the face of
congestion than a single TCP connection implementing standard TCP
congestion control and avoidance [Balakrishnan98].
(ii) Enable application adaptation to congestion. Increasingly
popular real-time streaming applications run over UDP using their
own user-level transport protocols for good application
performance, but in most cases today do not adapt or react properly
to network congestion. By implementing a stable control algorithm
and exposing an adaptation API, the CM enables easy application
adaptation to congestion. Applications adapt the data they
transmit to the current network conditions.
The CM framework builds on recent work on TCP control block sharing
[Touch97], integrated TCP congestion control (TCP-Int)
[Balakrishnan98] and TCP sessions [Padmanabhan98]. [Touch97]
advocates the sharing of some of the state in the TCP control block
to improve transient transport performance and describes sharing
across an ensemble of TCP connections. [Balakrishnan98] and
[Padmanabhan98] describe several experiments that quantify the
benefits of sharing congestion state, including improved stability
in the face of congestion and better loss recovery. Integrating
loss recovery across concurrent connections significantly improves
performance because losses on one connection can be detected by
noticing that later data sent on another connection has been
received and acknowledged. The CM framework extends these ideas in
two significant ways: (i) it extends congestion management to
non-TCP streams, which are becoming increasingly common and often
do not implement proper congestion management, and (ii) it provides
an API for applications to adapt their transmissions to current
network conditions. For an extended discussion of the motivation
for the CM, its architecture, API, algorithms and performance, see
[Balakrishnan99].
The resulting end-host protocol architecture at the sender is shown
in Figure 1. The CM helps achieve network stability by
implementing stable congestion avoidance and control algorithms
that are "TCP-friendly" [Mahdavi98] based on algorithms described
in [Stevens97]. However, it does not attempt to enforce proper
congestion behavior for all applications (but it does not preclude
a policer on the host that performs this task). Note that while
the policer at the end-host can use CM, protecting the network
against compromises to the CM and policer requires router machinery
[Floyd99a]. We do not address this issue further in this document.
|--------| |--------| |--------| |--------| |--------------|
| HTTP | | FTP | | RTP 1 | | RTP 2 | | |
|--------| |--------| |--------| |--------| | |
| | | ^ | ^ | |
| | | | | | | Scheduler |
| | | | | | |---| | |
| | | |-------|--+->| | | |
| | | | | |<--| |
v v v v | | |--------------|
|--------| |--------| |-------------| | | ^
| TCP 1 | | TCP 2 | | UDP 1 | | A | |
|--------| |--------| |-------------| | | |
^ | ^ | | | | |--------------|
| | | | | | P |-->| |
| | | | | | | | |
|---|------+---|--------------|------->| | | Congestion |
| | | | I | | |
v v v | | | Controller |
|-----------------------------------| | | | |
| IP |-->| | | |
|-----------------------------------| | | |--------------|
|---|
Figure 1
The key components of the CM framework are (i) the API, (ii) the
congestion controller, (iii) the scheduler. The API is (in part)
motivated by the ideas of application-level framing (ALF) [Clark90]
and is described in Section 4. The CM internals (Section 5)
include a congestion controller (Section 5.1) and a scheduler to
orchestrate data transmissions between concurrent flows in a
macroflow (Section 5.2). The congestion controller adjusts the
aggregate transmission rate between sender and receiver based on
its estimate of congestion in the network. It obtains feedback
about its past transmissions from applications themselves via the
API. The scheduler allocates available bandwidth amongst the
different flows within each macroflow and notifies applications
when they are permitted to send data. A future document will
describe the sender-receiver protocol and header formats that will
handle applications that do not incorporate their own feedback to
the CM. (This document focuses on well-behaved applications.)
4. CM API
Using the CM API, flows can determine their share of the available
bandwidth, request and have their data transmissions scheduled,
inform the CM about successful transmissions, and be informed when
the CM's estimate of a path's characteristics changes. Thus, the CM frees
applications from having to maintain information about the state of
congestion and available bandwidth along any path.
The function prototypes below follow standard C language
convention.
4.1 State maintenance
1. Open: All applications MUST call cm_open(u32 dst) before using
the CM API. dst is the 32-bit IPv4 address. This returns an
i32 handle, cm_flowid, for the application to use for all
further CM API invocations for that flow. If cm_flowid is -1,
then the cm_open() failed and the flow cannot use the CM.
2. Close: When a flow terminates, the application SHOULD invoke
cm_close(i32 cm_flowid) to inform the CM about the termination
of the flow. This allows CM to clean up its internal state in a
timely manner. The CM also cleans up its internal state for the
flow (and if necessary the associated macroflow) when there has
been no activity for CM_CLEANUP seconds (the current value is 75
seconds).
3. Packet size: cm_mtu(i32 cm_flowid) returns the estimated PMTU of
the path between sender and receiver. Internally, this
information may either be statically configured, or obtained via
path MTU discovery [Mogul90].
4.2 Data transmission
The CM accommodates three types of senders, including streams that
use ALF to dynamically adapt their content based on prevailing
network conditions.
1. Buffered transmission. A sender stream can call
cm_send(i32 cm_flowid, (u8*) data, u32 length) to transmit data via
this API. Here, the CM is on the data path and buffers the data for
eventual transmission, which in turn occurs at a time determined by
its congestion controller and scheduler. The data buffer MUST
contain a raw IP datagram (excluding the IP header) ready to be
sent, and length MUST be the length of the entire IP payload (i.e,
excluding the IP header). A disadvantage of this method is that
ALF-based applications are not accommodated, because the sender does
not get to revisit and change its prior transmission decisions once
data is buffered in the CM.
2. Callback-based transmission. The callback-based transmission API
puts the stream in firm control of deciding what to transmit at
each point in time. To achieve this, the CM does not buffer any
data; instead, it allows streams the opportunity to adapt to
unexpected network changes at the last possible instant. This
enables streams to "pull out" and repacketize data upon learning
about any rate change, which is hard to do once the data has been
buffered. A stream wishing to send data in this style can call
cm_request(i32 cm_flowid). After some time, depending on the rate,
the CM invokes a callback using cmapp_send(), which is a grant for
the stream to send up to PMTU bytes. Note that cm_request()
does not take the number of bytes or MTU-sized units as an
argument; each call to cm_request() is an implicit request for
sending up to PMTU bytes. Section 5.2 describes how these requests
are scheduled and callbacks made. The callback-style API is the
recommended choice for ALF-based streams.
3. Synchronous-style. The above callback-based API accommodates a
class of ALF streams that are "asynchronous." Asynchronous stream
transmissions are triggered by asynchronous events like file reads.
However, many applications use "synchronous" streams, which
transmit periodically based on their own internal timers (e.g., an
audio sender that sends at a constant sampling rate). While CM
callbacks could be configured to periodically interrupt such
transmitters, the structure of such applications is less affected
if they retain their original timer-based loop. Thus, the CM
exports an API that allows such streams to be informed of changes
in rates using the cmapp_update(u64 newrate, u32 srtt, u32 rttdev)
callback function, where newrate is the new rate in bits per second
for this flow, srtt is the current smoothed round trip time
estimate in microseconds, and rttdev is the mean linear
deviation of the round-trip time estimate. In response, the stream
MUST adapt its packet size or change its timer interval to not
exceed the allowed rate. Of course, it may choose not to use all
of this rate.
To avoid unnecessary cmapp_update() callbacks that the application
will only ignore, the stream can use the cm_thresh(float
downthresh, float upthresh) function at any stage in its execution.
In response, the CM will invoke the callback when the rate
decreases to less than (downthresh * lastrate) or increases to more
than (upthresh * lastrate), where lastrate is the rate last
notified to the stream. This information is used as a hint by the
CM, in the sense the cmapp_update() can be called even if these
conditions are not met. (At this point, the API does not include a
callback when the round-trip time or variation changes
significantly; this may be changed in the future.)
An application can query the current CM state by using cm_query(u32
cm_flowid, u64* rate, u32* srtt, u32* rttdev). This sets the rate
variable to the current rate estimate in bits per second, the srtt
variable to the current smoothed round-trip time estimate in
microseconds, and rttdev to the mean linear deviation of the
round-trip time in microseconds.
Note that a stream can use more than one of the above transmission
APIs at the same time. In particular, the knowledge of sustainable
rate is useful for asynchronous streams as well as synchronous
ones; e.g., an asynchronous Web server disseminating images using
TCP may use cmapp_send() to schedule its transmissions and
cmapp_update() to decide whether to send a low- resolution or
high-resolution image. A TCP implementation using the CM is
described in Section 6.1.1, where the benefit of the cm_request()
API for TCP will become apparent.
4.3 Application notification
When a stream receives feedback from receivers, it MUST use
cm_update(i32 cm_flowid, u32 nsent, u32 nrecd, u8 lossmode, i32
rtt) to inform the CM about events such as congestion losses,
successful receptions, type of loss (timeout event, Explicit
Congestion Notification [Ramakrishnan97], etc.) and round-trip time
samples. The nsent parameter indicates how many bytes were sent,
the nrecd parameter identifies how many of those bytes were
received. The rtt value indicates the round-trip time measured
during the transmission of these bytes. The rtt value must be set
to -1 if no valid round-trip sample was obtained by the stream.
The lossmode parameter provides an indicator of how a loss was
detected. A value of CM_PERSISTENT indicates that the application
believes congestion to be severe, e.g., a TCP that has experienced
a timeout. A value of CM_TRANSIENT indicates that the application
believes that the congestion is not severe, e.g., a TCP loss
detected using duplicate (selective) acknowledgements or other
data-driven techniques. A value of CM_ECN indicates that the
receiver echoed an explicit congestion notification
[Ramakrishnan98] message. Finally, a value of CM_NOLOSS indicates
that no congestion-related loss has occurred.
cm_notify(i32 cm_flowid, u32 nsent) MUST be called when data is
transmitted from the host (e.g., by the IP output routine) to
inform the CM that nsent bytes were just transmitted on the
specified flow. This allows the CM to update its estimate of ownd
for the macroflow and for the flow. If a stream does not transmit
any data upon a cmapp_send() callback invocation, it SHOULD call
cm_notify(flowid, 0) to allow the CM to permit other flows in the
macroflow to transmit data.
4.4 Sharing granularity
The CM needs to decide which flows belong to a single macroflow and
share congestion information. The API provides two functions that
allow applications to group their streams into the same macroflows.
cm_getmacroflow(i32 cm_flowid) returns a unique i32 macroflow
identifier. cm_setmacroflow(i32 cm_macroflowid, i32 cm_flowid)
sets the macroflow of the flow cm_flowid to cm_macroflowid. If the
cm_macroflowid that is passed to cm_setmacroflow() is -1, then a
new macroflow is constructed and this is returned to the caller.
Each call to cm_setmacroflow() overrides the previous macroflow
association for the flow, should one exist.
The default aggregation method (i.e., within which macroflow should
CM place a flow if the application does not use cm_setmacroflow)
is as yet unresolved.
5. CM internals
This section describes the internal components of the CM. It
includes a congestion controller and a scheduler, with well-defined
interfaces exported by them.
5.1 Congestion controller
Associated with each macroflow is a congestion control algorithm;
the collection of all these algorithms comprises the congestion
controller of the CM. The control algorithm decides when and how
much data can be transmitted by a macroflow. It uses application
notifications (Section 4.3) from concurrent streams on the same
macroflow to build up information about the congestion state of the
different network paths.
The congestion controller MUST implement "TCP-friendly" [Mahdavi98]
congestion control algorithms. Several macroflows MAY (and indeed,
often will) use the same congestion control algorithm but each
macroflow maintains separate state about the network used by its
flows.
The congestion control module MUST implement the following
interfaces (these are not directly visible to applications; they
are within the context of a macroflow):
- void query(u64 *rate, u32 *srtt, u32 *rttdev): This function
returns the estimated rate (in bits per second), smoothed round
trip time (in microseconds) and mean linear deviation of the
round-trip time estimate (in microseconds) for the macroflow.
- void notify(u32 nsent): This function MUST be used to notify the
congestion control module whenever data is sent by a flow. The
nsent parameter indicates the number of bytes just sent.
- void update(u32 nsent, u32 nrecd, u32 rtt, u32 lossmode): This
function is called whenever any of the CM flows associated with a
macroflow identifies that data has reached the receiver or has
been lost en route. The nrecd parameter indicates the number of
bytes that have just arrived at the receiver. The nsent parameter
is the sum of the number of bytes just received and the number of
bytes identified as lost en route. The rtt parameter is the
estimated round trip time in microseconds during the transfer.
The lossmode parameter provides an indicator of how a loss was
detected (section 4.3).
The congestion control module MUST also call the associated
scheduler's schedule() function (section 5.2) when it believes that
the current congestion state allows an MTU-sized packet to be sent.
5.2 Scheduler
While it is the responsibility of a macroflow's congestion
controller to determine when and how much data can be transmitted,
it is the responsibility of a its scheduler to determine which of
its flows gets the opportunity to transmit data at any time.
The Scheduler MUST implement the following interfaces:
- void schedule(u32 num_bytes): When the congestion controller
determines that data can be sent, the schedule() routine MUST be
called with the number of bytes that can be sent. In turn, the
scheduler MAY call a stream's cmapp_send() function.
- float query_share(u32 cm_flowid): This call returns the described
flow's share of the total bandwidth available to the
macroflow. This call combined with the query call of the
congestion control provides the information to satisfy a stream's
cm_query() request.
- void notify(u32 cm_flowid, u32 nsent): This interface is used to
notify the scheduler whenever data is sent by a CM flow. The
nsent parameter indicates the number of bytes just sent by the
application.
- void delete_flow(u32 cm_flowid): This call notifies the scheduler
that a particular flow has been closed.
6. Examples
6.1 Example applications
This section describes the use of the CM API by an asynchronous
application (an implementation of a TCP sender) and a synchronous
application (an audio server).
6.1.1 TCP/CM
TCP/CM removes the congestion management functionality from TCP and
uses the CM to control the flow of outgoing data. The CM eliminates
the need for tracking and reacting to congestion in TCP, because
the CM and its transmission API ensure proper congestion behavior.
TCP/CM continues to perform all functions related to loss recovery
and flow control.
This section describes the modifications necessary to make TCP
Reno-like transmitter use the CM. Other loss recovery techniques
such as NewReno [Floyd99b] and integrated loss recovery across an
ensemble of concurrent connections [Balakrishnan98] can be
accommodated as easily.
TCP using the CM MUST use the callback-based API (i.e. it must
implement a cmapp_send() callback function). TCP/CM identifies what
data it should send only upon the arrival of an acknowledgement
(ACK) or expiration of a timer. As a result, it requires tight
control of when and if new data or retransmissions are sent --
using the buffered or synchronous API is therefore undesirable.
When TCP/CM either connects to or accepts a connection from another
TCP (which can be any TCP that conforms to RFC793 [Postel81]), it
calls cm_open() to associate the TCP/CM connection with a
cm_flowid. TCP/CM is also modified to have its own outstanding
window (tcp_ownd) estimate. Whenever, data is sent from its
cmapp_send() callback, TCP/CM updates its tcp_ownd value. The value
of tcp_ownd is also updated after each cm_update() call. In
addition, TCP/CM maintains a count of the number of outstanding
segments (seg_cnt). At any time, TCP/CM can calculate the average
segment size (avg_seg_size) as tcp_ownd/seg_cnt.
TCP/CM makes the following key modifications to a conformant TCP
implementation's output routines:
1. All congestion window (cwnd) checks and computations are removed.
2. When application data is available, TCP/CM output routines
performs all non-congestion checks (related to flow control, such
as the Nagle algorithm, receiver advertised window check,
etc). If these checks pass, the output routine queues the data
and calls cm_request() for the flow.
3. If incoming duplicate or selective ACKs or a timeout result in
a loss being detected, the retransmission is also placed in a
queue and cm_request() is called for the flow.
4. The cmapp_send() callback for TCP/CM is set to a simple output
routine. If any retransmission is enqueued, the routine outputs
the lost segment. Otherwise, the routine outputs as much new data
as the TCP/CM connection state allows. However, cmapp_send()
never sends more than a single segment per call.
The IP output routine on the host calls cm_notify() when the data
is actually sent out.
The TCP/CM input routines are modified as follows:
1. RTT and RTO estimation is done as usual using either
timestamps [Jacobson92] or Karn's algorithm. Any RTT estimate
that is generated is passed to CM via the cm_update call.
2. All cwnd and slow start threshold (ssthresh) updates and
computations are removed.
3. Upon the arrival of an ACK (with value ack_seq) for new data,
TCP/CM computes the value of in_flight (the amount of data in
flight) as (snd_max - ack_seq) (i.e. maximum sequence sent -
current ACK). TCP/CM then calls cm_update(flowid, tcp_ownd -
in_flight, 0, CM_NOLOSS, rtt). However, if the ACK contains an
ECN notification, TCP/CM calls cm_update(flowid, tcp_ownd -
in_flight, 0, CM_ECN, rtt) instead.
4. Upon the arrival of a duplicate ACK, TCP/CM must check its
dupack count (dup_acks) to determine its action. If dup_acks < 3,
the TCP/CM does nothing. If dup_acks is 3, TCP/CM assumes that a
segment was lost and that at least 3 segments reached to generate
these duplicate ACKs. Therefore, it calls cm_update(flowid, 4 *
avg_seg_size, 3 * avg_seg_size, CM_TRANSIENT, rtt). TCP/CM also
enqueues a retransmission of the lost segment and calls
cm_request(). If dup_acks > 3, TCP/CM infers that a segment has
reached the other end and caused a duplicate ACK to be sent. As a
result, it calls cm_update(flowid, avg_seg_size, avg_seg_size,
CM_NOLOSS, rtt). This allows the CM to accurately track ownd.
When the TCP/CM retransmission timer expires, the sender identifies
that a segment has been lost and calls cm_update(flowid,
avg_seg_size, 0, CM_PERSISTENT, 0) to signify the occurrence of
persistent congestion to the CM. TCP/CM also enqueues a
retransmission of the lost segment and calls cm_request().
Finally, when the connection is closed, TCP/CM performs a cm_close()
to allow CM to reclaim the associated state.
6.1.2 Audio server
A typical audio source often has access to an audio sample in a
multitude of data rates and qualities. The objective of this
application is then to deliver the highest possible quality of
audio (typically the highest data rate) to its client. The
selection of which version of audio to transmit should be based on
the current congestion state of the network. In addition, the
source will want audio delivered to its users at a consistent
sampling rate. As a result, it must send data a regular rate,
without delaying transmissions thereby reducing buffering before
playback at the client. To meet these requirements, this
application can use the synchronous sender API (Section 4.2).
When the source first starts, it uses cm_query() to get an initial
estimate of network bandwidth and delay. It then chooses an
encoding that does not exceed these estimates and begins
transmitting data. The source also implements the
cmapp_update() callback. When the CM determines that network
characteristics have changed, it calls the application's
cmapp_update() function and passes it a new rate and round-trip
time estimate. The source MUST change its choice of audio
encoding if its current transmission rate exceeds the new estimates.
To use the CM, the application must incorporate feedback from the
receiver. In this example, it must periodically (at least once or
twice per round trip time) determine how many of its packets
arrived at the receiver. When the source gets this feedback, it
MUST use cm_update() to inform the CM of this new information.
This allows in the CM to update ownd and may result in the CM
changing its estimates and calling cmapp_update() of the streams of
the macroflow.
6.2 Example congestion controller
To illustrate the responsibilities of a congestion controller, this
section describes some of the actions of a simple TCP-like
congestion controller that implements Additive Increase
Multiplicative Decrease congestion control (AIMD_CC):
- query(): AIMD_CC returns the current congestion window (cwnd)
divided by the smoothed rtt (srtt) as its bandwidth estimate for
the macroflow. It also returns the smoothed rtt estimate as srtt and
the mean linear deviation as rttdev. These are calculated
according to the TCP algorithm [Jac88].
- notify(): AIMD_CC adds the number of bytes sent to its
outstanding data window (ownd).
- update(): AIMD_CC subtracts nsent from ownd. If the value of rtt
is non-zero, AIMD_CC updates srtt using the TCP srtt calculation.
If the update indicates that data has been lost, AIMD_CC sets
cwnd to 1 MTU if the loss_mode is CM_PERSISTENT and to ownd/2
(with a minimum of 1 MTU) if the loss_mode is CM_TRANSIENT or
CM_ECN. AIMD_CC also sets its internal ssthresh variable to
ownd/2. If no loss had occurred, AIMD_CC mimics TCP slow start
and linear growth modes [Stevens97]. It increments cwnd by nsent
when cwnd < ssthresh (bounded by a maximum of ssthresh-cwnd) and
by nsent * MTU/cwnd when cwnd > ssthresh.
- When cwnd or ownd are updated and indicate that at least one MTU
may be transmitted, AIMD_CC calls the CM to schedule a
transmission.
6.3 Example Scheduler Module
To clarify the responsibilities of a scheduler module, this section
describes some of the actions of a simple round robin scheduler
module (RR_sched):
- schedule(): RR_sched schedules as many flows as possible in round
robin fashion.
- query_share(): RR_sched returns 1/(number of flows in macroflow).
- notify(): RR_sched does nothing. Round robin scheduling is not
affected by the amount of data sent.
- delete_flow(): RR_sched adjusts its round robin schedule to omit
the deleted flow.
7. Security considerations
The CM provides many of the same services that the congestion
control in TCP provides. As such, it is vulnerable to many of the
same security problems. For example, incorrect reports of losses
and transmissions will give the CM an inaccurate picture of the
network's congestion state. By giving CM a high estimate of
congestion, an attacker can reduce the performance observed by
applications. The more dangerous form of attack is giving CM a low
estimate. This would cause CM to be overly aggressive and allow
data to be sent much more quickly than sound congestion control
policies would allow.
8. References
[Balakrishnan98] Balakrishnan, H., Padmanabhan, V., Seshan, S.,
Stemm, M., and Katz, R., "TCP Behavior of a Busy Web Server:
Analysis and Improvements," Proc. IEEE INFOCOM, San Francisco,
CA, March 1998.
[Balakrishnan99] Balakrishnan, H., Rahul, H., and Seshan, S., "An
Integrated Congestion Management Architecture for Internet
Hosts," Proc. ACM SIGCOMM, Cambridge, MA, September 1999.
[Bradner96] Bradner, S., "The Internet Standards Process ---
Revision 3", BCP 9, RFC-2026, October 1996.
[Bradner97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC-2119, March 1997.
[Clark82] Clark, D., "Window and Acknowledgement Strategy in TCP,"
RFC-813, July 1982.
[Clark90] Clark, D. and Tennenhouse, D., "Architectural
Consideration for a New Generation of Protocols", Proc. ACM
SIGCOMM, Philadelphia, PA, September 1990.
[Floyd99a] Floyd, S. and Fall, K.," Promoting the Use of End-to-End
Congestion Control in the Internet," IEEE/ACM Trans. on
Networking, 7(4), August 1999, pp. 458-472.
[Floyd99b] Floyd, S. and Henderson, T., "The NewReno Modification
to TCP's Fast Recovery Algorithm," RFC-2582, April
1999. (Experimental.)
[Jacobson88] Jacobson, V., "Congestion Avoidance and Control,"
Proc. ACM SIGCOMM, Stanford, CA, August 1988.
[Jacobson92] Jacobson, V., Braden, R. and Borman, D., "TCP
Extensions for High Performance," RFC1323, May 1992.
[Mahdavi98] Mahdavi, J. and Floyd, S., "The TCP Friendly Website,"
http://www.psc.edu/networking/tcp_friendly.html
[Mogul90] Mogul, J. and Deering, S., "Path MTU Discovery,"
RFC-1191, November 1990.
[Padmanabhan98] Padmanabhan, V., "Addressing the Challenges of Web
Data Transport," PhD thesis, Univ. of California, Berkeley,
December 1998.
[Postel81] Postel, J. (ed.), "Transmission Control Protocol",
RFC-793, September 1981.
[Ramakrishnan98] Ramakrishnan, K. and Floyd, S., "A Proposal to Add
Explicit Congestion Notification (ECN) to IP," RFC-2481, January
1999. (Experimental.)
[Stevens97] Stevens, W., "TCP Slow Start, Congestion Avoidance,
Fast Retransmit, and Fast Recovery Algorithms," RFC-2001,
January 1997.
[Touch97] Touch, J., "TCP Control Block Interdependence," RFC-2140,
April 1997. (Informational.)
9. Acknowledgments
We thank Sally Floyd, Mark Handley, Steve McCanne, and Vern Paxson
for useful feedback and suggestions on the CM architecture. We
also thank David Andersen, Deepak Bansal, Dorothy Curtis, and
Hariharan Rahul for their work on the CM design and implementation.
10. Authors' addresses
Hari Balakrishnan
Laboratory for Computer Science
545 Technology Square
Massachusetts Institute of Technology
Cambridge, MA 02139
Email: hari@lcs.mit.edu
Web: http://wind.lcs.mit.edu/~hari/
Srinivasan Seshan
IBM T.J. Watson Research Center
30 Saw Mill River Rd.
Hawthorne, NY 10532
Email: sseshan@us.ibm.com
Web: http://www.research.ibm.com/people/s/srini/
Full Copyright Statement
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| PAFTECH AB 2003-2026 | 2026-04-22 22:54:59 |