태터데스크 관리자

도움말
닫기
적용하기   첫페이지 만들기

태터데스크 메시지

저장하였습니다.

지니넷

Pause Frame

2007.09.12 12:14 : 네떡

3.2.1 PAUSE Frames

The addition of full-duplex mode to the Ethernet standard included an optional flow control operation known as "PAUSE" frames. PAUSE frames permit one end station to temporarily stop all traffic from the other end station (except MAC Control frames).

For example, assume a full-duplex link that connects two devices called "Station A" and "Station B". Suppose Station A transmits frames at a rate that causes Station B to enter into a state of congestion (i.e. no buffer space remaining to receive additional frames). Station B may transmit a PAUSE frame to Station A requesting that Station A stop transmitting frames for a specified period of time. Upon receiving the PAUSE frame, Station A will suspend further frame transmission until the specified time period has elapsed. This will allow Station B time to recover from the congestion state. At the end of the specified time period, Station A will resume normal transmission of frames.

Note that the PAUSE frame protocol is bi-directional. Station A may send frames to pause Station B, and Station B may send frames to pause Station A. A PAUSE frame is the one type of frame that a station is allowed to send even if it is currently in the paused state. Support for PAUSE frames is optional among devices that implement the full-duplex protocol (the use of PAUSE frames is not supported in a half-duplex environment). It is valid for a device to support only half of the protocol; i.e. it may transmit PAUSE frames without having the capability to decode them on the receive side, and vice-versa. Devices use the Auto-Negotiation protocol to discover the PAUSE frame capabilities of the device at the other end of the link. This permits interoperability between devices that do or do not support one or both halves of the protocol.

The format of a PAUSE frame is illustrated below. It conforms to the standard Ethernet frame format but includes a unique type field and other parameters as follows:

  • The destination address of the frame may be set to either the unique DA of the station to be paused, or to the globally assigned multicast address 01-80-C2-00-00-01 (hex). This multicast address has been reserved by the IEEE 802.3 standard for use in MAC Control PAUSE frames. It is also reserved in the IEEE 802.1D bridging standard as an address that will not be forward by bridges. This ensures the frame will not propagate beyond the local link segment.
  • The "Type" field of the PAUSE frame is set to 88-08 (hex) to indicate the frame is a MAC Control frame.
  • The MAC Control opcode field is set to 00-01 (hex) to indicate the type of MAC Control frame being used is a PAUSE frame. The PAUSE frame is the only type of MAC Control frame currently defined.
  • The MAC Control Parameters field contains a 16-bit value that specifies the duration of the PAUSE event in units of 512-bit times. Valid values are 00-00 to FF-FF (hex). If an additional PAUSE frame arrives before the current PAUSE time has expired, its parameter replaces the current PAUSE time, so a PAUSE frame with parameter zero allows traffic to resume immediately.
  • A 42-byte reserved field (transmitted as all zeros) is required to pad the length of the PAUSE frame to the minimum Ethernet frame size.

Preamble (7-bytes) Start Frame Delimiter (1-byte) Dest. MAC Address (6-bytes)
= (01-80-C2-
00-00-01)
or unique DA
Source MAC Address (6-bytes) Length/Type (2-bytes)
= 802.3 MAC Control
(88-08)
MAC Control
Opcode
(2-bytes)
= PAUSE
(00-01)
MAC Control
Parameters
(2-bytes)
= (00-00 to
FF-FF)
Reserved
(42-bytes)
= all zeros
Frame Check Sequence (4-bytes)

신고
Posted by 지니넷 Trackback 0 Comment 2


티스토리 툴바