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ethernet flow control wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 description toggle description subsection 1 1 pause frame 1 2 issues 2 subsequent efforts toggle subsequent efforts subsection 2 1 congestion management 2 2 priority flow control 3 see also 4 references 5 external links toggle the table of contents ethernet flow control 4 languages 日本語 한국어 македонски 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia redirected from ieee 802 1qbb technique to suspend transmission to avoid congestion wireshark screenshot of an ethernet pause frame ethernet flow control is a mechanism for temporarily stopping the transmission of data on the ethernet family of computer networks the goal of this mechanism is to avoid packet loss in the presence of network congestion the first flow control mechanism the pause frame was defined by the ieee 802 3x standard the follow on priority based flow control as defined in the ieee 802 1qbb standard provides a link level flow control mechanism that can be controlled independently for each class of service cos as defined by ieee p802 1p and is applicable to data center bridging dcb networks and to allow for prioritization of voice over ip voip video over ip and database synchronization traffic over default data traffic and bulk file transfers description edit a sending station computer or network switch may be transmitting data faster than the other end of the link can accept it using flow control the receiving station can signal the sender requesting suspension of transmissions until the receiver catches up flow control on ethernet can be implemented at the data link layer the first flow control mechanism the pause frame was defined by the institute of electrical and electronics engineers ieee task force that defined full duplex ethernet link segments the ieee standard 802 3x was issued in 1997 1 pause frame edit an overwhelmed network node can send a pause frame which halts the transmission of the sender for a specified period of time a media access control mac frame ethertype 0x8808 is used to carry the pause command with the control opcode set to 0x0001 hexadecimal 1 only stations configured for full duplex operation may send pause frames when a station wishes to pause the other end of a link it sends a pause frame to either the unique 48 bit destination address of this link or to the 48 bit reserved multicast address of 01 80 c2 00 00 01 2 annex 31b 3 3 the use of a well known address makes it unnecessary for a station to discover and store the address of the station at the other end of the link another advantage of using this multicast address arises from the use of flow control between network switches the particular multicast address used is selected from a range of addresses that have been reserved by the ieee 802 1d standard which specifies the operation of switches used for bridging normally a frame with a multicast destination sent to a switch will be forwarded out to all other ports of the switch however this range of multicast addresses is special and will not be forwarded by an 802 1d compliant switch instead frames sent to this range are understood to be frames meant to be acted upon only within the switch a pause frame includes the period of pause time being requested in the form of a two byte 16 bit unsigned integer 0 through 65535 this number is the requested duration of the pause the pause time is measured in units of pause quanta where each quanta is equal to 512 bit times by 1999 several vendors supported receiving pause frames but fewer implemented sending them 3 4 issues edit one original motivation for the pause frame was to handle network interface controllers nics that did not have enough buffering to handle full speed reception this problem has become less common with advances in bus speeds and memory sizes a more likely scenario is network congestion within a switch for example a flow can come into a switch on a higher speed link than the one it goes out or several flows can come in over two or more links that total more than an output link s bandwidth these will eventually exhaust any amount of buffering in the switch however blocking the sending link will cause all flows over that link to be delayed even those that are not causing any congestion this situation is a case of head of line hol blocking and can happen more often in core network switches due to the large numbers of flows generally being aggregated many switches use a technique called virtual output queues to eliminate the hol blocking internally so they will never send pause frames 4 subsequent efforts edit congestion management edit another effort began in march 2004 and in may 2004 it became the ieee p802 3ar congestion management task force in may 2006 the objectives of the task force were revised to specify a mechanism to limit the transmitted data rate at about 1 granularity the request was withdrawn and the task force was disbanded in 2008 5 priority flow control edit ethernet flow control disturbs the ethernet class of service defined in ieee 802 1p as the data of all priorities is stopped to clear the existing buffers which might also consist of low priority data as a remedy to this problem cisco systems defined its own priority flow control extension to the standard protocol this mechanism uses 14 bytes of the 42 byte padding in a regular pause frame the mac control opcode for a priority pause frame is 0x0101 unlike the original pause priority pause indicates the pause time in quanta for each of the eight priority classes separately 6 the extension was subsequently standardized by the priority based flow control pfc project authorized on march 27 2008 as ieee 802 1qbb 7 draft 2 3 was proposed on june 7 2010 claudio desanti of cisco was the editor 8 the effort was part of the data center bridging task group which developed fibre channel over ethernet 9 see also edit explicit congestion notification phy level collision avoidance references edit 1 2 ieee standards for local and metropolitan area networks supplements to carrier sense multiple access with collision detection csma cd access method and physical layer specifications specification for 802 3 full duplex operation and physical layer specification for 100 mb s operation on two pairs of category 3 or better balanced twisted pair cable 100base t2 institute of electrical and electronics engineers 1997 doi 10 1109 ieeestd 1997 95611 isbn 978 1 55937 905 2 cite book cs1 maint deprecated archival service link ieee standard for ethernet pdf ieee standards association 2018 08 31 doi 10 1109 ieeestd 2018 8457469 isbn 978 1 5044 5090 4 retrieved 2022 11 29 cite book website ignored help dead link ann sullivan greg kilmartin scott hamilton september 13 1999 switch vendors pass interoperability tests network world pp 81 82 retrieved may 10 2011 1 2 vendors on flow control network world fusion september 13 1999 archived from the original on 2012 02 07 vendor comments on flow control in the 1999 test ieee p802 3ar congestion management task force december 18 2008 retrieved may 10 2011 priority flow control build reliable layer 2 infrastructure pdf white paper cisco systems june 2009 retrieved may 10 2011 ieee 802 1qbb ieee 802 1q priority based flow control institute of electrical and electronics engineers june 7 2010 retrieved may 10 2011 data center bridging task group institute of electrical and electronics engineers june 7 2010 retrieved may 10 2011 external links edit ethernet media access control pause frames techfest ethernet technical summary 1999 archived from the original on 2012 02 04 retrieved may 10 2011 tim higgins november 7 2007 when flow control is not a good thing small net builder retrieved january 6 2020 linux tool for generating flow control pause frames archived 2012 05 24 at the wayback machine python tool to generate pfc frames ethernet flow control topics in high performance messaging archived from the original on 2007 12 08 v t e ethernet family of local area network technologies speeds 10 mbit s 100 mbit s 1 gbit s 2 5 and 5 gbit s 10 gbit s 25 and 50 gbit s 40 and 100 gbit s 200 400 800 and 1600 gbit s general physical layer autonegotiation ethertype flow control frames jumbos organizations ieee 802 3 ethernet alliance mef forum media fiber twisted pair coaxial first mile 10g epon historic csma cd starlan 10broad36 10base fb 10base fl 10base5 10base2 mau foirl 100basevg lattisnet long reach applications audio automotive carrier data center energy efficiency industrial metro power synchronous transceivers gbic sfp sfp qsfp qsfp osfp xenpak x2 xfp cfp interfaces aui ead mdi mii gmii xgmii xaui category commons retrieved from https en wikipedia org w index php title ethernet_flow_control oldid 1353729149 categories ieee 802 ethernet standards flow control data hidden categories articles with short description short description is different from wikidata cs1 maint deprecated archival service cs1 errors periodical ignored all articles with dead external links articles with dead external links from july 2024 webarchive template wayback links this page was last edited on 12 may 2026 at 01 03 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license 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