Ethernet flow control
Encyclopedia
Ethernet flow control is a mechanism for temporarily stopping the transmission of data on Ethernet
family computer network
s.
is a popular family of computer network
protocols. Flow control
can be implemented at the data link layer
. A sending station (computer or network switch
) may be transmitting data faster than the other end of the link can accept it.
The first flow control mechanism, the PAUSE frame
, was defined by the Institute of Electrical and Electronics Engineers
(IEEE) task force that also defined full duplex Ethernet link segments. The IEEE standard 802.3x was issued in 1997.
Media Access Control
(MAC) frames to carry the PAUSE commands using Control opcode
0x0001 (hexadecimal
). 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 MAC Control frame to the 48-bit
destination reserved multicast address
of 01-80-C2-00-00-01. 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
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 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 two byte
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 unit is equal to 512 bit time
s.
By 1999, several vendors supported receiving pause frames, but fewer implemented sending them.
Pause frames have several disadvantages.
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
, 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 will never send pause frames.
), 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
came up with its own priority flow control extension of the standard protocol. This mechanism uses 14 bytes of the 42-byte stuffing 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 Priority-based Flow Control (PFC) project was 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 data center bridging
task group, which developed Fibre Channel over Ethernet
.
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
family computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s.
Description
EthernetEthernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
is a popular family of computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
protocols. Flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
can be implemented at the data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
. A sending station (computer or network switch
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
) may be transmitting data faster than the other end of the link can accept it.
The first flow control mechanism, the PAUSE frame
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
, was defined by the Institute of Electrical and Electronics Engineers
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...
(IEEE) task force that also defined full duplex Ethernet link segments. The IEEE standard 802.3x was issued in 1997.
Pause frame
The overwhelmed network element can send a PAUSE frame, which halts the transmission of the sender for a specified period of time.Media Access Control
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) frames to carry the PAUSE commands using Control opcode
Opcode
In computer science engineering, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question...
0x0001 (hexadecimal
Hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...
). 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 MAC Control frame to the 48-bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
destination reserved multicast address
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...
of 01-80-C2-00-00-01. 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
802.1D is the IEEE MAC Bridges standard which includes Bridging, Spanning Tree 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 the widely deployed 802.3 , 802.11 and 802.16 standards.VLANs are not...
standard which specifies the operation of switches used for bridging
Bridging (networking)
Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...
. Normally, a frame with a multicast destination sent to a switch will be forwarded out 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 two byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
unsigned integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
(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
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 Card operating at some predefined standard speed, such as 10 Mbit/s. The time is measured between the time the logical link control layer 2 sublayer receives the...
s.
By 1999, several vendors supported receiving pause frames, but fewer implemented sending them.
Pause frames have several disadvantages.
Issues
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 is not as common with advances in bus speeds and memory sizes. A more likely scenario is network congestionNetwork congestion
In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...
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
Head-of-line blocking is a performance-limiting phenomenon that occurs in buffered telecommunication network switches.-Description:...
, and can happen more often in core network
Core network
A core network, or network core, is the central part of a telecommunication network that provides various services to customers who are connected by the access network. One of the main functions is to route telephone calls across the PSTN....
switches due to the large numbers of flows generally being aggregated. Many switches use a technique called Virtual Output Queues
Virtual Output Queues
Virtual Output Queues are an input queuing strategy for use in telecommunications and computer network switches. It addresses a common problem known as Head-of-line blocking.-Description:...
to eliminate the HOL blocking internally, so will never send pause frames.
Congestion management
Another effort was begun in March 2004, and in May 2004 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 task force disbanded in 2008.Priority flow control
Ethernet Flow control disturbs the Ethernet class of service (defined in IEEE 802.1pIEEE 802.1p
IEEE P802.1p is the name of a task group active during 1995–98 responsible for adding traffic class expediting and dynamic multicast filtering to the IEEE 802.1D standard. Essentially, they provided a mechanism for implementing Quality of Service at the Media Access Control level...
), 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
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
came up with its own priority flow control extension of the standard protocol. This mechanism uses 14 bytes of the 42-byte stuffing 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 Priority-based Flow Control (PFC) project was 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 data center bridging
Data center bridging
Data center bridging refers to a set of enhancements to Ethernet local area networks for use in data center environments. Specifically, DCB goals are, for selected traffic, to eliminate loss due to queue overflow and to be able to allocate bandwidth on links. Essentially, DCB enables, to some...
task group, which developed Fibre Channel over Ethernet
Fibre Channel over Ethernet
Fibre Channel over Ethernet is an encapsulation of Fibre Channel frames over Ethernet networks. This allows Fibre Channel to use 10 Gigabit Ethernet networks while preserving the Fibre Channel protocol...
.