Ethernet flow control is a mechanism for temporarily stopping the transmission of data on

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

family

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

s. The goal of this mechanism is to avoid

packet loss Packet loss occurs when one or more packets of data travelling across a computer network fail to reach their destination. Packet loss is either caused by errors in data transmission, typically across wireless networks, or network congestion.Kur ...

in the presence of

network congestion Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...

. 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 Class of service (COS or CoS) is a parameter used in data and voice protocols to differentiate the types of payloads contained in the packet being transmitted. The objective of such differentiation is generally associated with assigning priorities ...

(CoS), as defined by IEEE P802.1p and is applicable to

data center bridging Data center bridging (DCB) is a set of enhancements to the Ethernet local area network communication protocol for use in data center environments, in particular for use with clustering and storage area networks. Motivation Ethernet is the primary ...

(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

A sending station (computer or

network switch A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A netw ...

) 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 data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

. The first flow control mechanism, the ''pause frame'', was defined by the

Institute of Electrical and Electronics Engineers The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operation ...

(IEEE) task force that defined

full duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

Ethernet link segments. The IEEE standard 802.3x was issued in 1997.

Pause frame

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 In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...

(MAC)

frame A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...

(

EtherType EtherType is a two-Octet (computing), octet field in an Ethernet frame. It is used to indicate which Communications protocol, protocol is Encapsulation (networking), encapsulated in the payload of the frame and is used at the receiving end by th ...

0x8808) is used to carry the pause command, with the Control opcode set to 0x0001 (

hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...

). 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 The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

destination address of this link or to the 48-bit reserved

multicast address A multicast address is a logical identifier for a group of hosts in a computer network that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the link laye ...

of . 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 address which have been reserved by the

IEEE 802.1D IEEE 802.1D is the Ethernet MAC bridges standard which includes bridging, Spanning Tree Protocol and others. It is standardized by the IEEE 802.1 working group. It includes details specific to linking many of the other 802 projects including t ...

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 address 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 The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

(16-bit), unsigned

integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...

(0 through 65535). This number is the requested duration of the pause. The pause time is measured in units of pause "quanta", where each unit is equal to 512

bit time Bit time is a concept in computer networking. It is defined as the time it takes for one bit to be ejected from a network interface controller (NIC) operating at some predefined standard speed, such as 10 Mbit/s. The time is measured betwe ...

s. By 1999, several vendors supported receiving pause frames, but fewer implemented sending them. Vendor comments on flow control in the 1999 test.

Issues

One original motivation for the pause frame was to handle

network interface controller A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network. Ear ...

s (NICs) that did not have enough buffering to handle full-speed reception. This problem is not as common with advances in bus speeds and memory sizes. A more likely scenario is

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 blocking Head-of-line blocking (HOL blocking) in computer networking is a performance-limiting phenomenon that occurs when a line of packets is held up in a queue by a first packet. Examples include input buffered network switches, out-of-order delivery a ...

(HOL), and can happen more often in

core network A backbone or core network is a part of a computer network which interconnects networks, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building ...

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 will never send pause frames.

Subsequent efforts

Congestion management

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.

Priority flow control

Ethernet flow control disturbs the Ethernet class of service (defined in IEEE 802.1p), as the data of all priorities are stopped to clear the existing buffers which might also consist of low priority data. As a remedy to this problem,

Cisco Systems Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational corporation, multinational digital communications technology conglomerate (company), conglomerate corporation headquartered in San Jose, California. Cisco develo ...

defined their 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 eight priority classes separately. The extension was subsequently standardized by the Priority-based Flow Control (PFC) project authorized on March 27, 2008, as IEEE 802.1Qbb. Draft 2.3 was proposed on June 7, 2010. Claudio DeSanti of Cisco was editor. The effort was part of the

task group, which developed

Fibre Channel over Ethernet Fibre Channel over Ethernet (FCoE) is a computer network technology that encapsulates Fibre Channel frames over Ethernet networks. This allows Fibre Channel to use 10 Gigabit Ethernet networks (or higher speeds) while preserving the Fibre Channel ...

References

External links

* *
Linux Tool for generating flow control PAUSE framesPython Tool to Generate PFC Frames
* {{DEFAULTSORT:Ethernet Flow Control IEEE 802 Ethernet standards Flow control (data)