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Network Working Group                                    H. BalakrishnanRequest for Comments: 3124                                       MIT LCSCategory: Standards Track                                      S. Seshan                                                                     CMU                                                               June 2001The Congestion ManagerStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This document describes the Congestion Manager (CM), an end-system   module that:   (i) Enables an ensemble of multiple concurrent streams 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.1. 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 inRFC-2119 [Bradner97].   STREAM      A group of packets that all share the same source and destination      IP address, IP type-of-service, transport protocol, and source and      destination transport-layer port numbers.Balakrishnan, et. al.       Standards Track                     [Page 1]

RFC 3124                 The Congestion Manager                June 2001   MACROFLOW      A group of CM-enabled streams that all use the same congestion      management and scheduling algorithms, and share congestion state      information.  Currently, streams destined to different receivers      belong to different macroflows.  Streams destined to the same      receiver MAY belong to different macroflows.  When the Congestion      Manager is in use, streams that experience identical congestion      behavior 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.   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.Balakrishnan, et. al.       Standards Track                     [Page 2]

RFC 3124                 The Congestion Manager                June 2001   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.      We emphasize that all the API functions described in this document      are "abstract" calls and that conformant CM implementations may      differ in specific implementation details.2. Introduction   The framework described in this document integrates congestion   management across all applications and transport protocols.  The CM   maintains congestion parameters (available aggregate and per-stream   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.  However, the receiving   application must provide feedback to the sending application about   received packets and losses, and the latter is expected to use the CM   API to update CM state.  This document does not address networks with   reservations or service differentiation.   The CM is an end-system module that enables an ensemble of multiple   concurrent streams to perform stable 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-stream   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.  When the CM is used, all   data transmissions subject to the CM must be done with the explicit   consent of the CM via this API to ensure proper congestion behavior.   Systems MAY choose to use CM, and if so they MUST follow this   specification.   This document focuses on applications and networks where the   following conditions hold:Balakrishnan, et. al.       Standards Track                     [Page 3]

RFC 3124                 The Congestion Manager                June 2001   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 streams 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 streams 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 streams 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,   Allman99]), the ensemble of streams 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],Balakrishnan, et. al.       Standards Track                     [Page 4]

RFC 3124                 The Congestion Manager                June 2001   [Padmanabhan98], and [Eggert00] 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, and algorithms, see [Balakrishnan99]; for a   description of an implementation and performance results, see   [Andersen00].   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 [Allman99].   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, the network has to be protected against compromises   to the CM and the policer at the end hosts, a task that requires   router machinery [Floyd99a].  We do not address this issue further in   this document.Balakrishnan, et. al.       Standards Track                     [Page 5]

RFC 3124                 The Congestion Manager                June 2001   |--------| |--------| |--------| |--------|       |--------------|   |  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, and (iii) the scheduler.  The API is (in part)   motivated by the requirements of application-level framing (ALF)   [Clark90], and is described inSection 4.  The CM internals (Section5) include a congestion controller (Section 5.1) and a scheduler to   orchestrate data transmissions between concurrent streams 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 apportions available bandwidth amongst the different   streams within each macroflow and notifies applications when they are   permitted to send data.  This document focuses on well-behaved   applications; a future one will describe the sender-receiver protocol   and header formats that will handle applications that do not   incorporate their own feedback to the CM.3. CM API   By convention, the IETF does not treat Application Programming   Interfaces as standards track.  However, it is considered important   to have the CM API and CM algorithm requirements in one coherent   document.  The following section on the CM API uses the terms MUST,Balakrishnan, et. al.       Standards Track                     [Page 6]

RFC 3124                 The Congestion Manager                June 2001   SHOULD, etc., but the terms are meant to apply within the context of   an implementation of the CM API.  The section does not apply to   congestion control implementations in general, only to those   implementations offering the CM API.   Using the CM API, streams 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 path bandwidth 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.   We emphasize that these API functions are abstract calls and   conformant CM implementations may differ in specific details, as long   as equivalent functionality is provided.   When a new stream is created by an application, it passes some   information to the CM via the cm_open(stream_info) API call.   Currently, stream_info consists of the following information: (i) the   source IP address, (ii) the source port, (iii) the destination IP   address, (iv) the destination port, and (v) the IP protocol number.3.1 State maintenance   1. Open: All applications MUST call cm_open(stream_info) before      using the CM API.  This returns a handle, cm_streamid, for the      application to use for all further CM API invocations for that      stream.  If the returned cm_streamid is -1, then the cm_open()      failed and that stream cannot use the CM.      All other calls to the CM for a stream use the cm_streamid      returned from the cm_open() call.   2. Close: When a stream terminates, the application SHOULD invoke      cm_close(cm_streamid) to inform the CM about the termination      of the stream.   3. Packet size: cm_mtu(cm_streamid) returns the estimated PMTU of      the path between sender and receiver.  Internally, this      information SHOULD be obtained via path MTU discovery      [Mogul90].  It MAY be statically configured in the absence of      such a mechanism.Balakrishnan, et. al.       Standards Track                     [Page 7]

RFC 3124                 The Congestion Manager                June 20013.2 Data transmission   The CM accommodates two types of adaptive senders, enabling   applications to dynamically adapt their content based on prevailing   network conditions, and supporting ALF-based applications.   1. 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.  Thus, 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.   The CM must implement a cm_request(i32 cm_streamid) call for streams   wishing to send data in this style.  After some time, depending on   the rate, the CM MUST invoke a callback using cmapp_send(), which is   a grant for the stream to send up to PMTU bytes.  The callback-style   API is the recommended choice for ALF-based streams.  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.  The CM MAY provide an alternate interface,   cm_request(int k).  The cmapp_send callback for this request is   granted the right to send up to k PMTU sized segments.Section 4.3   discusses the time duration for which the transmission grant is   valid, whileSection 5.2 describes how these requests are scheduled   and callbacks made.   2. Synchronous-style.  The above callback-based API accommodates a   class of ALF streams that are "asynchronous."  Asynchronous   transmitters do not transmit based on a periodic clock, but do so   triggered by asynchronous events like file reads or captured frames.   On the other hand, there are many streams that are "synchronous"   transmitters, which transmit periodically based on their own internal   timers (e.g., an audio senders that sends at a constant sampling   rate).  While CM callbacks could be configured to periodically   interrupt such transmitters, the transmit loop of such applications   is less affected if they retain their original timer-based loop.  In   addition, it complicates the CM API to have a stream express the   periodicity and granularity of its callbacks.  Thus, the CM MUST   export 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 stream, srtt is the current smoothed round trip time   estimate in microseconds, and rttdev is the smoothed linear deviation   in the round-trip time estimate calculated using the same algorithm   as in TCP [Paxson00].  The newrate value reports an instantaneous   rate calculated, for example, by taking the ratio of cwnd and srtt,   and dividing by the fraction of that ratio allocated to the stream.Balakrishnan, et. al.       Standards Track                     [Page 8]

RFC 3124                 The Congestion Manager                June 2001   In response, the stream MUST adapt its packet size or change its   timer interval to conform to (i.e., not exceed) the allowed rate.  Of   course, it may choose not to use all of this rate.  Note that the CM   is not on the data path of the actual transmission.   To avoid unnecessary cmapp_update() callbacks that the application   will only ignore, the CM MUST provide a cm_thresh(float   rate_downthresh, float rate_upthresh, float rtt_downthresh, float   rtt_upthresh) function that a stream can use at any stage in its   execution.  In response, the CM SHOULD invoke the callback only when   the rate decreases to less than (rate_downthresh * lastrate) or   increases to more than (rate_upthresh * lastrate), where lastrate is   the rate last notified to the stream, or when the round-trip time   changes correspondingly by the requisite thresholds.  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.   The CM MUST implement a cm_query(i32 cm_streamid, u64* rate, u32*   srtt, u32* rttdev) to allow an application to query the current CM   state.  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.  If the CM does not have valid estimates for the   macroflow, it fills in negative values for the rate, srtt, and   rttdev.   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 inSection 6.1.1, where   the benefit of the cm_request() callback API for TCP will become   apparent.   The reader will notice that the basic CM API does not provide an   interface for buffered congestion-controlled transmissions.  This is   intentional, since this transmission mode can be implemented using   the callback-based primitive.Section 6.1.2 describes how   congestion-controlled UDP sockets may be implemented using the CM   API.3.3 Application notification   When a stream receives feedback from receivers, it MUST use   cm_update(i32 cm_streamid, u32 nrecd, u32 nlost, u8 lossmode, i32   rtt) to inform the CM about events such as congestion losses,Balakrishnan, et. al.       Standards Track                     [Page 9]

RFC 3124                 The Congestion Manager                June 2001   successful receptions, type of loss (timeout event, Explicit   Congestion Notification [Ramakrishnan99], etc.) and round-trip time   samples.  The nrecd parameter indicates how many bytes were   successfully received by the receiver since the last cm_update call,   while the nrecd parameter identifies how many bytes were received   were lost during the same time period.  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 application.  The lossmode parameter provides an   indicator of how a loss was detected.  A value of CM_NO_FEEDBACK   indicates that the application has received no feedback for all its   outstanding data, and is reporting this to the CM.  For example, a   TCP that has experienced a timeout would use this parameter to inform   the CM of this.  A value of CM_LOSS_FEEDBACK indicates that the   application has experienced some loss, which it believes to be due to   congestion, but not all outstanding data has been lost.  For example,   a TCP segment loss detected using duplicate (selective)   acknowledgments or other data-driven techniques fits this category.   A value of CM_EXPLICIT_CONGESTION indicates that the receiver echoed   an explicit congestion notification message.  Finally, a value of   CM_NO_CONGESTION indicates that no congestion-related loss has   occurred.  The lossmode parameter MUST be reported as a bit-vector   where the bits correspond to CM_NO_FEEDBACK, CM_LOSS_FEEDBACK,   CM_EXPLICIT_CONGESTION, and CM_NO_CONGESTION.  Note that over links   (paths) that experience losses for reasons other than congestion, an   application SHOULD inform the CM of losses, with the CM_NO_CONGESTION   field set.   cm_notify(i32 cm_streamid, u32 nsent) MUST be called when data is   transmitted from the host (e.g., in the IP output routine) to inform   the CM that nsent bytes were just transmitted on a given stream.   This allows the CM to update its estimate of the number of   outstanding bytes for the macroflow and for the stream.   A cmapp_send() grant from the CM to an application is valid only for   an expiration time, equal to the larger of the round-trip time and an   implementation-dependent threshold communicated as an argument to the   cmapp_send() callback function.  The application MUST NOT send data   based on this callback after this time has expired.  Furthermore, if   the application decides not to send data after receiving this   callback, it SHOULD call cm_notify(stream_info, 0) to allow the CM to   permit other streams in the macroflow to transmit data.  The CM   congestion controller MUST be robust to applications forgetting to   invoke cm_notify(stream_info, 0) correctly, or applications that   crash or disappear after having made a cm_request() call.Balakrishnan, et. al.       Standards Track                    [Page 10]

RFC 3124                 The Congestion Manager                June 20013.4 Querying   If applications wish to learn about per-stream available bandwidth   and round-trip time, they can use the CM's cm_query(i32 cm_streamid,   i64* rate, i32* srtt, i32* rttdev) call, which fills in the desired   quantities.  If the CM does not have valid estimates for the   macroflow, it fills in negative values for the rate, srtt, and   rttdev.3.5 Sharing granularity   One of the decisions the CM needs to make is the granularity at which   a macroflow is constructed, by deciding which streams belong to the   same macroflow and share congestion information.  The API provides   two functions that allow applications to decide which of their   streams ought to belong to the same macroflow.   cm_getmacroflow(i32 cm_streamid) returns a unique i32 macroflow   identifier.  cm_setmacroflow(i32 cm_macroflowid, i32 cm_streamid)   sets the macroflow of the stream cm_streamid 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 stream, should one exist.   The default suggested aggregation method is to aggregate by   destination IP address; i.e., all streams to the same destination   address are aggregated to a single macroflow by default.  The   cm_getmacroflow() and cm_setmacroflow() calls can then be used to   change this as needed.  We do note that there are some cases where   this may not be optimal, even over best-effort networks.  For   example, when a group of receivers are behind a NAT device, the   sender will see them all as one address.  If the hosts behind the NAT   are in fact connected over different bottleneck links, some of those   hosts could see worse performance than before.  It is possible to   detect such hosts when using delay and loss estimates, although the   specific mechanisms for doing so are beyond the scope of this   document.   The objective of this interface is to set up sharing of groups not   sharing policy of relative weights of streams in a macroflow.  The   latter requires the scheduler to provide an interface to set sharing   policy.  However, because we want to support many different   schedulers (each of which may need different information to set   policy), we do not specify a complete API to the scheduler (but seeBalakrishnan, et. al.       Standards Track                    [Page 11]

RFC 3124                 The Congestion Manager                June 2001Section 5.2).  A later guideline document is expected to describe a   few simple schedulers (e.g., weighted round-robin, hierarchical   scheduling) and the API they export to provide relative   prioritization.4. CM internals   This section describes the internal components of the CM.  It   includes a Congestion Controller and a Scheduler, with well-defined,   abstract interfaces exported by them.4.1 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   network path used by the macroflow.   The congestion controller MUST implement a "TCP-friendly" [Mahdavi98]   congestion control algorithm.  Several macroflows MAY (and indeed,   often will) use the same congestion control algorithm but each   macroflow maintains state about the network used by its streams.   The congestion control module MUST implement the following abstract   interfaces.  We emphasize that these are not directly visible to   applications; they are within the context of a macroflow, and are   different from the CM API functions ofSection 4.   - void query(u64 *rate, u32 *srtt, u32 *rttdev): This function     returns the estimated rate (in bits per second) and smoothed     round trip time (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 an     application.  The nsent parameter indicates the number of bytes     just sent by the application.   - void update(u32 nsent, u32 nrecd, u32 rtt, u32 lossmode): This     function is called whenever any of the CM streams 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 theBalakrishnan, et. al.       Standards Track                    [Page 12]

RFC 3124                 The Congestion Manager                June 2001     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).   Although these interfaces are not visible to applications, the   congestion controller MUST implement these abstract interfaces to   provide for modular inter-operability with different separately-   developed schedulers.   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.4.2 Scheduler   While it is the responsibility of the congestion control module to   determine when and how much data can be transmitted, it is the   responsibility of a macroflow's scheduler module to determine which   of the streams should get the opportunity to transmit data.   The Scheduler MUST implement the following interfaces:   - void schedule(u32 num_bytes): When the congestion control module     determines that data can be sent, the schedule() routine MUST be     called with no more than the number of bytes that can be sent.     In turn, the scheduler MAY call the cmapp_send() function that CM     applications must provide.   - float query_share(i32 cm_streamid): This call returns the     described stream's share of the total bandwidth available to the     macroflow.  This call combined with the query call of the     congestion controller provides the information to satisfy an     application's cm_query() request.   - void notify(i32 cm_streamid, u32 nsent): This interface is used     to notify the scheduler module whenever data is sent by a CM     application.  The nsent parameter indicates the number of bytes     just sent by the application.     The Scheduler MAY implement many additional interfaces.  As     experience with CM schedulers increases, future documents may     make additions and/or changes to some parts of the scheduler     API.Balakrishnan, et. al.       Standards Track                    [Page 13]

RFC 3124                 The Congestion Manager                June 20015. Examples5.1 Example applications   This section describes three possible uses of the CM API by   applications.  We describe two asynchronous applications---an   implementation of a TCP sender and an implementation of congestion-   controlled UDP sockets, and a synchronous application---a streaming   audio server.  More details of these applications and CM   implementation optimizations for efficient operation are described in   [Andersen00].   All applications that use the CM MUST incorporate feedback from the   receiver.  For example, it must periodically (typically 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 results   in the CM updating ownd and may result in the CM changing its   estimates and calling cmapp_update() of the streams of the macroflow.   The protocols in this section are examples and suggestions for   implementation, rather than requirements for any conformant   implementation.5.1.1 TCP   A TCP implementation that uses CM should use the cmapp_send()   callback API.  TCP only identifies which data it should send upon the   arrival of an acknowledgement or expiration of a timer.  As a result,   it requires tight control over when and if new data or   retransmissions are sent.   When TCP either connects to or accepts a connection from another   host, it performs a cm_open() call to associate the TCP connection   with a cm_streamid.   Once a connection is established, the CM is used to control the   transmission 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.  Loss recovery is   still performed by TCP based on fast retransmissions and recovery as   well as timeouts.  In addition, TCP is also modified to have its own   outstanding window (tcp_ownd) estimate.  Whenever data segments are   sent from its cmapp_send() callback, TCP updates its tcp_ownd value.   The ownd variable is also updated after each cm_update() call.  TCP   also maintains a count of the number of outstanding segments   (pkt_cnt).  At any time, TCP can calculate the average packet size   (avg_pkt_size) as tcp_ownd/pkt_cnt.  The avg_pkt_size is used by TCPBalakrishnan, et. al.       Standards Track                    [Page 14]

RFC 3124                 The Congestion Manager                June 2001   to help estimate the amount of outstanding data.  Note that this is   not needed if the SACK option is used on the connection, since this   information is explicitly available.   The TCP output routines are modified as follows:      1. All congestion window (cwnd) checks are removed.      2. When application data is available.  The TCP output routines      perform all non-congestion checks (Nagle algorithm, receiver-      advertised window check, etc).  If these checks pass, the output      routine queues the data and calls cm_request() for the stream.      3. If incoming data or timers result in a loss being detected, the      retransmission is also placed in a queue and cm_request() is      called for the stream.      4. The cmapp_send() callback for TCP is set to an output routine.      If any retransmission is enqueued, the routine outputs the      retransmission.  Otherwise, the routine outputs as much new data      as the TCP connection state allows.  However, the cmapp_send()      never sends more than a single segment per call.  This routine      arranges for the other output computations to be done, such as      header and options computations.   The IP output routine on the host calls cm_notify() when the packets   are actually sent out.  Because it does not know which cm_streamid is   responsible for the packet, cm_notify() takes the stream_info as   argument (seeSection 4 for what the stream_info should contain).   Because cm_notify() reports the IP payload size, TCP keeps track of   the total header size and incorporates these updates.   The TCP input routines are modified as follows:      1. RTT estimation is done as normal using either timestamps 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 are      removed.      3. Upon the arrival of an ack for new data, TCP computes the value      of in_flight (the amount of data in flight) as snd_max-ack-1      (i.e., MAX Sequence Sent - Current Ack - 1).  TCP then calls      cm_update(streamid, tcp_ownd - in_flight, 0, CM_NO_CONGESTION,      rtt).Balakrishnan, et. al.       Standards Track                    [Page 15]

RFC 3124                 The Congestion Manager                June 2001      4. Upon the arrival of a duplicate acknowledgement, TCP must check      its dupack count (dup_acks) to determine its action.  If dup_acks      < 3, the TCP does nothing.  If dup_acks == 3, TCP assumes that a      packet was lost and that at least 3 packets arrived to generate      these duplicate acks.  Therefore, it calls cm_update(streamid, 4 *      avg_pkt_size, 3 * avg_pkt_size, CM_LOSS_FEEDBACK, rtt).  The      average packet size is used since the acknowledgments do not      indicate exactly how much data has reached the other end.  Most      TCP implementations interpret a duplicate ACK as an indication      that a full MSS has reached its destination.  Once a new ACK is      received, these TCP sender implementations may resynchronize with      TCP receiver.  The CM API does not provide a mechanism for TCP to      pass information from this resynchronization.  Therefore, TCP can      only infer the arrival of an avg_pkt_size amount of data from each      duplicate ack.  TCP also enqueues a retransmission of the lost      segment and calls cm_request().  If dup_acks > 3, TCP assumes that      a packet has reached the other end and caused this ack to be sent.      As a result, it calls cm_update(streamid, avg_pkt_size,      avg_pkt_size, CM_NO_CONGESTION, rtt).      5. Upon the arrival of a partial acknowledgment (one that does not      exceed the highest segment transmitted at the time the loss      occurred, as defined in [Floyd99b]), TCP assumes that a packet was      lost and that the retransmitted packet has reached the recipient.      Therefore, it calls cm_update(streamid, 2 * avg_pkt_size,      avg_pkt_size, CM_NO_CONGESTION, rtt).  CM_NO_CONGESTION is used      since the loss period has already been reported.  TCP also      enqueues a retransmission of the lost segment and calls      cm_request().   When the TCP retransmission timer expires, the sender identifies that   a segment has been lost and calls cm_update(streamid, avg_pkt_size,   0, CM_NO_FEEDBACK, 0) to signify that no feedback has been received   from the receiver and that one segment is sure to have "left the   pipe."  TCP also enqueues a retransmission of the lost segment and   calls cm_request().5.1.2 Congestion-controlled UDP   Congestion-controlled UDP is a useful CM application, which we   describe in the context of Berkeley sockets [Stevens94].  They   provide the same functionality as standard Berkeley UDP sockets, but   instead of immediately sending the data from the kernel packet queue   to lower layers for transmission, the buffered socket implementation   makes calls to the API exported by the CM inside the kernel and gets   callbacks from the CM.  When a CM UDP socket is created, it is bound   to a particular stream.  Later, when data is added to the packet   queue, cm_request() is called on the stream associated with theBalakrishnan, et. al.       Standards Track                    [Page 16]

RFC 3124                 The Congestion Manager                June 2001   socket.  When the CM schedules this stream for transmission, it calls   udp_ccappsend() in the UDP module.  This function transmits one MTU   from the packet queue, and schedules the transmission of any   remaining packets.  The in-kernel implementation of the CM UDP API   should not require any additional data copies and should support all   standard UDP options.  Modifying existing applications to use   congestion-controlled UDP requires the implementation of a new socket   option on the socket.  To work correctly, the sender must obtain   feedback about congestion.  This can be done in at least two ways:   (i) the UDP receiver application can provide feedback to the sender   application, which will inform the CM of network conditions using   cm_update(); (ii) the UDP receiver implementation can provide   feedback to the sending UDP.  Note that this latter alternative   requires changes to the receiver's network stack and the sender UDP   cannot assume that all receivers support this option without explicit   negotiation.5.1.3 Audio server   A typical audio application often has access to the sample in a   multitude of data rates and qualities.  The objective of the   application is then to deliver the highest possible quality of audio   (typically the highest data rate) its clients.  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, minimizing delaying   transmissions and reducing buffering before playback.  To meet these   requirements, this application can use the synchronous sender API   (Section 4.2).   When the source first starts, it uses the cm_query() call to get an   initial estimate of network bandwidth and delay.  If some other   streams on that macroflow have already been active, then it gets an   initial estimate that is valid; otherwise, it gets negative values,   which it ignores.  It then chooses an encoding that does not exceed   these estimates (or, in the case of an invalid estimate, uses   application-specific initial values) and begins transmitting data.   The application 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 application must change its choice   of audio encoding to ensure that it does not exceed these new   estimates.Balakrishnan, et. al.       Standards Track                    [Page 17]

RFC 3124                 The Congestion Manager                June 20015.2 Example congestion control module   To illustrate the responsibilities of a congestion control module,   the following describes some of the actions of a simple TCP-like   congestion control module 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.  It     returns the smoothed rtt estimate as srtt.   - 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_NO_FEEDBACK and to cwnd/2     (with a minimum of 1 MTU) if the loss_mode is CM_LOSS_FEEDBACK or     CM_EXPLICIT_CONGESTION.  AIMD_CC also sets its internal ssthresh     variable to cwnd/2.  If no loss had occurred, AIMD_CC mimics TCP     slow start and linear growth modes.  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.5.3 Example Scheduler Module   To clarify the responsibilities of a scheduler module, the following   describes some of the actions of a simple round robin scheduler   module (RR_sched):   - schedule(): RR_sched schedules as many streams as possible in round     robin fashion.   - query_share(): RR_sched returns 1/(number of streams in macroflow).   - notify(): RR_sched does nothing.  Round robin scheduling is not     affected by the amount of data sent.6. 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 andBalakrishnan, et. al.       Standards Track                    [Page 18]

RFC 3124                 The Congestion Manager                June 2001   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 degrade the performance observed by applications.  For   example, a stream on a host can arbitrarily slow down any other   stream on the same macroflow, a form of denial of service.   The more dangerous form of attack occurs when an application gives   the CM a low estimate of congestion.  This would cause CM to be   overly aggressive and allow data to be sent much more quickly than   sound congestion control policies would allow.   [Touch97] describes a number of the security problems that arise with   congestion information sharing.  An additional vulnerability (not   covered by [Touch97])) occurs because applications have access   through the CM API to control shared state that will affect other   applications on the same computer.  For instance, a poorly designed,   possibly a compromised, or intentionally malicious UDP application   could misuse cm_update() to cause starvation and/or too-aggressive   behavior of others in the macroflow.7. References   [Allman99]        Allman, M. and Paxson, V., "TCP Congestion                     Control",RFC 2581, April 1999.   [Andersen00]      Balakrishnan, H., System Support for Bandwidth                     Management and Content Adaptation in Internet                     Applications, Proc. 4th Symp. on Operating Systems                     Design and Implementation, San Diego, CA, October                     2000.  Available fromhttp://nms.lcs.mit.edu/papers/cm-osdi2000.html   [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.Balakrishnan, et. al.       Standards Track                    [Page 19]

RFC 3124                 The Congestion Manager                June 2001   [Clark90]         Clark, D. and Tennenhouse, D., "Architectural                     Consideration for a New Generation of Protocols",                     Proc. ACM SIGCOMM, Philadelphia, PA, September                     1990.   [Eggert00]        Eggert, L., Heidemann, J., and Touch, J., "Effects                     of Ensemble TCP," ACM Computer Comm. Review,                     January 2000.   [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 T. Henderson,"The New Reno                     Modification to TCP's Fast Recovery Algorithm,"RFC2582, April 1999.   [Jacobson88]      Jacobson, V., "Congestion Avoidance and Control,"                     Proc. ACM SIGCOMM, Stanford, CA, August 1988.   [Mahdavi98]       Mahdavi, J. and Floyd, S., "The TCP Friendly                     Website,"http://www.psc.edu/networking/tcp_friendly.html   [Mogul90]         Mogul, J. and S. Deering, "Path MTU Discovery,"RFC1191, November 1990.   [Padmanabhan98]   Padmanabhan, V., "Addressing the Challenges of Web                     Data Transport," PhD thesis, Univ. of California,                     Berkeley, December 1998.   [Paxson00]        Paxson, V. and M. Allman, "Computing TCP's                     Retransmission Timer",RFC 2988, November 2000.   [Postel81]        Postel, J., Editor, "Transmission Control                     Protocol", STD 7,RFC 793, September 1981.   [Ramakrishnan99]  Ramakrishnan, K. and Floyd, S., "A Proposal to Add                     Explicit Congestion Notification (ECN) to IP,"RFC2481, January 1999.   [Stevens94]       Stevens, W., TCP/IP Illustrated, Volume 1.                     Addison-Wesley, Reading, MA, 1994.   [Touch97]         Touch, J., "TCP Control Block Interdependence",RFC2140, April 1997.Balakrishnan, et. al.       Standards Track                    [Page 20]

RFC 3124                 The Congestion Manager                June 20018. Acknowledgments   We thank David Andersen, Deepak Bansal, and Dorothy Curtis for their   work on the CM design and implementation.  We thank Vern Paxson for   his detailed comments, feedback, and patience, and Sally Floyd, Mark   Handley, and Steven McCanne for useful feedback on the CM   architecture.  Allison Mankin and Joe Touch provided several useful   comments on previous drafts of this document.9. Authors' Addresses   Hari Balakrishnan   Laboratory for Computer Science   200 Technology Square   Massachusetts Institute of Technology   Cambridge, MA 02139   EMail: hari@lcs.mit.edu   Web:http://nms.lcs.mit.edu/~hari/   Srinivasan Seshan   School of Computer Science   Carnegie Mellon University   5000 Forbes Ave.   Pittsburgh, PA 15213   EMail: srini@cmu.edu   Web:http://www.cs.cmu.edu/~srini/Balakrishnan, et. al.       Standards Track                    [Page 21]

RFC 3124                 The Congestion Manager                June 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Balakrishnan, et. al.       Standards Track                    [Page 22]