RSTP協議——第6章(6.3)

6.3         Quality of Service maintenance

Quality of Service comprises the following:

 

a) Service availability

b) Frame loss

c) Frame misordering

d) Frame duplication

e) Frame transit delay

f) Frame lifetime

g) Undetected frame error rate

h) Maximum service data unit size supported

i) Frame priority

j) Throughput

 

服務質量的指標以下：

a) 服務可用性

b) 丟包

c) 包亂序

d) 包重複

e) 包傳輸延時

f) 包生存時間

g) 漏檢錯誤率

h) 所支持的最大服務數據單元尺寸

i) 幀優先級

j) 吞吐量

6.3.1   Service availability

Service availability is measured as a fraction of some total time during which the MAC Service is provided. The operation of a Bridge can increase or lower the service availability.

服務可用性是在給定的時間段內，可以使用業務功能的時間所佔百分比。對網橋的操做能夠提高或下降服務可用性。

 

Service availability can be increased by automatic reconfiguration of the network (see Clause 17) to avoid the use of a failed component (e.g., repeater, cable, or connector) in the data path. Service availability can be lowered by the failure of a Bridge or by a Bridge filtering frames.

服務可用性能夠經過對網絡的自動重配置進行提高，其方式爲避免使用數據路徑上有故障的部件（例如：中繼器、電纜或接口）。服務可用性可能會因爲網橋的故障或網橋過濾數據包而下降。

 

A Bridge can discard frames (6.3.2) to preserve other aspects of the MAC Service (6.3.3 and 6.3.4) during reconfiguration, lowering service availability for end stations that do not benefit from the reconfiguration. If an end station moves, it can then be unable to receive frames from other end stations until the filtering information held by the Bridges and used to localize traffic is updated. To minimize service denial, filtering information that has been dynamically learned can be modified when reconfiguration takes place (17.11). However, filtering information that is statically configured cannot be modified in this way.

在重配置期間，網橋可以丟棄數據包(6.3.2)以保留MAC服務(6.3.3 and 6.3.4)的其它方面，這下降了沒有從這種重配置獲得好處的終端工做站的服務可用性。若是一臺終端工做站移動了位置，除非網橋保存的用來定位的過濾信息被刷新，不然該終端工做站就不能接收到其它終端工做站發送的數據包。爲了儘可能減小拒絕服務，動態學習的過濾信息在重配置發生時進行修改。不過靜態配置的過濾信息不能採用這種方式修改。

 

A Bridge may deny service and discard frames to prevent network access by unauthorized devices.

網橋能夠拒絕服務而且丟棄數據包，以阻止未受權設備對網絡的訪問。

 

To maximize the service availability, no loss of service or delay in service provision is caused by Bridges, except as a consequence of a failure, removal, or insertion of a network component, or as a consequence of the movement of an end station, or as a consequence of an attempt to perform unauthorized access. These are regarded as extraordinary events. The operation of any additional protocol necessary to maintain the quality of the MAC Service is thus limited to the configuration of the Bridged Local Area Network, and is independent of individual instances of service provision.

爲了儘量提升服務可用性，網橋不能致使服務丟失或延時，故障、移動、插入新的網絡部件、終端工做站移動位置、試圖進行非受權訪問等緣由除外。這些被認爲是特殊事件。任何用於維持MAC服務質量的額外的協議操做被限制在橋接LAN的配置範圍內，不依賴於具體的服務。

 

NOTE—This is true only in the absence of admission control mechanisms, i.e., where the Bridges provide a 「best effort」 service. The specification and applicability of admission controls in Bridges is outside the scope of this standard.

注意：只有缺少接納控制機制纔會這樣，即，網橋提供「盡力而爲」的服務。接納控制機制的規定和可用性不在本標準討論範圍以內。

6.3.2   Frame loss

The MAC Service does not guarantee the delivery of Service Data Units. Frames transmitted by a source station arrive, uncorrupted, at the destination station with high probability. The operation of a Bridge introduces minimal additional frame loss.

MAC服務沒有保證服務數據單元的傳輸。數據包從源工做站發送，有很大機率能夠沒有損壞地到達目的地。網橋的操做盡量減小數據包丟失。

 

A frame transmitted by a source station can fail to reach its destination station as a result of the following:

數據包從源發送，而不能到達目的地的緣由以下：

 

a) Frame corruption during physical layer transmission or reception.

a)  在物理層發送或接收時數據包發生損壞。

 

b) Frame discard by a Bridge because

b)  由於以下緣由網橋丟棄數據包：

 

1) It is unable to transmit the frame within some maximum period of time, and is required to discard the frame to prevent the maximum frame lifetime (6.3.6) from being exceeded.

1)  在最大週期內沒法發送數據包，須要丟棄該包以免超出數據包最大生存時間(6.3.6)。

 

2) It is unable to continue to store the frame due to exhaustion of internal buffering capacity as frames continue to arrive at a rate in excess of that at which they can be transmitted.

2)  接收數據包的速率超過發送的速率，致使緩衝區空間衰竭。

 

3) The size of the service data unit carried by the frame exceeds the maximum supported by the MAC procedures employed on the LAN to which the frame is to be relayed.

3)  數據包攜帶的服務數據單元的尺寸超出了MAC所支持的最大數量。

 

4) Changes in the physical topology of the network necessitate frame discard for a limited period of time to maintain other aspects of QoS (see 17.10).

4) 網絡物理拓撲的改變致使在有限的時間內丟棄數據包，以維持QoS的其它方面功能(see 17.10)。

 

5) The device attached to the Port is not authorized for access to the network.

5)  鏈接到端口的設備沒有獲得網絡訪問受權。

 

6) The configuration of Static Filtering Entries in the Filtering Database (7.9.1) disallows the forwarding of frames with particular destination addresses on specific Ports.

6) 過濾數據庫中靜態過濾記錄的配置不容許在指定端口向指定目的地址轉發數據包。

 

NOTE—As Static Filtering Entries are associated with particular Ports or combinations of Ports, there is a possibility that mis-configuration of Static Filtering Entries will lead to unintended frame discard during or following automatic reconfiguration of the Bridged Local Area Network.

注意：由於靜態過濾記錄是與特殊端口或端口組合相關的，所以靜態過濾記錄配置錯誤可能會致使橋接LAN自動重配置期間或隨後的時間內丟棄不想丟棄的數據包。

6.3.3   Frame misordering

The MAC Service (9.2 of ISO/IEC 15802-1) permits a negligible rate of reordering of frames with a given user priority for a given combination of destination address and source address. MA_UNITDATA.indication service primitives corresponding to MA_UNITDATA.request primitives, with the same requested priority and for the same combination of destination and source addresses, are received in the same order as the request primitives were processed.

對於給定用戶優先級、給定目的和源地址的組合，MAC服務(9.2 of ISO/IEC 15802-1)容許很低的數據包重排序比率。對應於MA_UNITDATA.request原語的MA_UNITDATA.indication服務原語，且具備相同的請求優先級和相同的目的與源地址的組合，則按照請求原語一樣的順序接收並處理。

 

NOTE 1—The Forwarding Process in Bridges (7.7) does not misorder or duplicate frames.

注意1：網橋內的轉發處理(7.7)不會致使數據包的亂序或重複。

 

Where Bridges are capable of connecting the individual MACs in such a way that multiple paths between any source station–destination station pairs exist, the operation of a protocol is required to ensure that a single path is used.

當網橋可以把獨立的MAC鏈接起來，在任意源和目的工做站之間提供多條路徑時，協議的操做要求確保使用單條路徑。

 

NOTE 2—Frame misordering and duplication (6.3.4) does not occur during normal operation. When RSTP is configuring or reconfiguring the network (see Clause 17), there is an increased and implementation-dependent probability that frames that are in transit will be misordered or duplicated as network paths change, since a Bridge can buffer frames awaiting transmission through its Ports. Since the probability of duplication or misordering occurring as a result of reconfiguration is small, and the frequency of physical network failures leading to reconfiguration is also generally small, the degradation of the properties of the MAC service is considered to be negligible. Some LAN protocols, for example, LLC Type 2, are particularly sensitive to frame misordering and duplication; to support these the Force Protocol Version parameter (17.13.4) can be used to delay the transition of ports to a Forwarding state and further reduce the probability of such events. A more detailed discussion of misordering and duplication in RSTP can be found in Annex K (informative).

注意2：正常操做時不會發生數據包亂序和重複(6.3.4)。當RSTP進行配置或重配置時（參見第17章），依賴於RSTP的具體實現，有可能會因爲網絡路徑改變致使數據包亂序或重複，由於網橋能夠在端口緩衝數據包並等待發送。因爲重配置致使的數據包重複或亂序的程度很小，而物理網絡故障致使重配置的機率也是至關的低，所以MAC服務的退化就微不足道了。一些LAN協議，例如，LLC Type 2，對於包亂序和丟失特別敏感，爲了支持這些協議，可使用強制協議版本參數(17.13.4)使得端口延時到轉發態再發送，以減小這種狀況發生的可能性。RSTP協議可能致使亂序和重複的細節參見附件K。

6.3.4   Frame duplication

The MAC Service (9.2 of ISO/IEC 15802-1) permits a negligible rate of duplication of frames. Bridges do not duplicate user data frames.

MAC服務(9.2 of ISO/IEC 15802-1) 容許很低的數據包重複率。網橋不會致使用戶數據包重複。

 

The potential for frame duplication in a Bridged Local Area Network arises through the possibility of multiple paths between source and destination end stations. Where Bridges can connect individual LANs to provide multiple paths, the operation of a protocol is required to ensure that a single path is used.

在源和目的工做站之間存在多條路徑的時候，橋接LAN內數據包重複的可能性就增長了。當網橋把多個獨立的LAN鏈接在一塊兒，提供了多條路徑，則協議的操做要求確保使用單條路徑。

6.3.5   Frame transit delay

The MAC Service introduces a variable frame transit delay that is dependent on media types and media access control methods. Frame transit delay is the elapsed time between an MA_UNITDATA.request primitive and the corresponding MA_UNITDATA.indication primitive. Elapsed time values are calculated only on Service Data Units that are successfully transferred.

MAC服務引入了一個可變的包傳輸延時，其依賴於介質類型和介質訪問控制方法。數據包傳輸延時是MA_UNITDATA.request原語與對應的MA_UNITDATA.indication原語之間的時間。改時間僅根據成功傳輸的服務數據單元進行計算。

 

Since the MAC Service is provided at an abstract interface within an end station, it is not possible to specify the total frame transit delay precisely. It is, however, possible to measure the media access and frame transmission and reception, and the transit delay introduced by an intermediate system, in this case a Bridge.

因爲MAC服務在一臺終端工做站內以抽象接口的方式提供，所以它不可能指定合適的總數據包傳輸延時。而測量介質訪問、數據包發送和接收、中間系統引入的傳輸延時是有可能的，在本案例中，就是網橋。

 

The minimum additional transit delay introduced by a Bridge is the time taken to receive a frame plus that taken to access the media onto which the frame is to be relayed. The frame is completely received before it is relayed as the Frame Check Sequence (FCS) is to be calculated and the frame discarded if in error.

網橋引入的最小的額外傳輸延時爲數據包接收時間加上訪問數據包要傳輸的介質的時間。數據包被轉發前須要被徹底接收下來，而且計算幀校驗序列，該序列值錯誤的包將會被丟棄。

6.3.6   Frame lifetime

The MAC Service mandates an upper bound to the transit delay experienced for a particular instance of communication. This maximum frame lifetime is necessary to ensure the correct operation of higher layer protocols. The additional transit delay introduced by a Bridge is discussed in 6.3.5.

MAC服務根據經驗爲特定的通訊肯定了傳輸延時的上限。這就是最大數據包生存時間，用於確保高層協議的正確操做。網橋引入的額外的傳輸延時參見6.3.5。

 

Since the information provided by the MAC Sublayer to a Bridge does not include the transit delay already experienced by any particular frame, Bridges discard frames to enforce a maximum delay in each Bridge. A recommended and an absolute maximum value are specified in Table 7-3.

因爲MAC子層提供給網橋的信息沒有包含已經根據特殊數據包的經驗所提供的傳輸延時，網橋會丟棄這些包，以強制每一個網橋實施最大延時。Table 7-3提供了建議值。

6.3.7   Undetected frame error rate

The MAC Service introduces a very low undetected frame error rate in transmitted frames. Undetected errors are protected against by the use of an FCS that is appended to the frame by the MAC Sublayer of the source station prior to transmission, and checked by the destination station on reception.

MAC服務爲發送數據包引入了很是低的漏檢錯誤率。在源工做站發送數據包以前，MAC子層在包後附加了FCS，以防止漏檢錯誤，在接收側檢測FCS。

 

The FCS calculated for a given service data unit is dependent on the media access control method. It is therefore necessary to recalculate the FCS within a Bridge providing a relay function between IEEE 802 MACs of dissimilar types where there are differences in the method of calculation and/or the coverage of the FCS, or changes to the data that is within the coverage of the FCS. This introduces the possibility of additional undetected errors arising from the operation of a Bridge.

爲給定服務數據單元計算的FCS依賴於介質訪問控制方法。對於提供了在不一樣類型IEEE 802 MACs之間轉發的網橋來講，由於不一樣類型MAC的FCS計算方法和覆蓋範圍都是不同的，所以轉發時有必要從新計算FCS，或者把數據放到FCS的覆蓋範圍以內。這個操做增長了額外的漏檢錯誤發生的可能性。

 

NOTE—Application of the techniques described in Annex F (informative) allow an implementation to achieve an arbitrarily small increase in undetected frame error rate, even in cases where the data that is within the coverage of the FCS is changed.

注意：附件F描述的應用技術，可讓具體的實現得到很是低的漏檢錯誤率，即便覆蓋在FCS以內的數據發生了變化也是同樣。

6.3.8   Maximum Service Data Unit Size

The Maximum Service Data Unit Size that can be supported by an IEEE 802 LAN varies with the MAC method and its associated parameters (speed, electrical characteristics, etc.). It may be constrained by the owner of the LAN. The Maximum Service Data Unit Size supported by a Bridge between two LANs is the smaller of that supported by the LANs. No attempt is made by a Bridge to relay a frame to a LAN that does not support the size of Service Data Unit conveyed by that frame.

最大服務數據單元尺寸是IEEE 802 LAN經過MAC方法及其相應參數（速度、電特性，等等）來支持的。它能夠被LAN的全部者約束。網橋所支持的兩個LAN之間的最大服務數據單元尺寸是兩個LAN中較小的那一個。網橋不會轉發數據包到不支持該包的服務數據單元尺寸的LAN。

6.3.9   Frame priority

The MAC Service includes user priority as a QoS parameter. MA_UNITDATA.requests with a high priority may be given precedence over other request primitives made at the same station, or at other stations attached to the same LAN, and can give rise to earlier MA_UNITDATA.indications.

MAC服務包含了做爲QoS參數的用戶優先級。高優先級的MA_UNITDATA.requests原語在同一工做站或連到同一LAN上其它工做站的其它原語請求以前優先處理，而且更早的獲得MA_UNITDATA.indications。

 

The MAC Sublayer maps the requested user priorities onto the access priorities supported by the individual media access method. The requested user priority can be conveyed to the destination station with the transmitted frame, using the priority signaling mechanisms inherent in some media access methods. Since not all IEEE 802 LAN MACs can signal the user priority associated with a frame, Bridges regenerate user priority based upon a combination of signaled information and configuration information held in the Bridge.

MAC子層映射請求的用戶優先級到單獨的介質訪問方法所支持的訪問優先級上。請求的用戶優先級能夠經過發送包轉達給目的工做站，使用從某種介質訪問方法繼承來的優先級標記機制。由於不是全部的IEEE 802 LAN MACs都可以在數據包內標記用戶優先級，網橋根據已標記的信息和網橋內的配置信息，從新生成用戶優先級。

 

The transmission delay experienced by a frame in a Bridge comprises:

網橋內數據包的傳輸延時經驗值包括：

 

a) A queuing delay until the frame becomes first in line for transmission on the Port, in accordance with the procedure for selecting frames for transmission described in 7.7.4.

a)  隊列延時，直到隊列成爲端口發送隊列的第一條，參見7.7.4節。

 

b) The access delay for transmission of the frame.

b)  數據包發送的訪問延時。

 

Queueing delays can be managed using user priority. Access delays can be managed using user priority in media access methods that support more than one access priority.

隊列延時可使用用戶優先級進行管理。訪問延時可使用介質訪問方法（該方法支持多個優先級）內的用戶優先級進行管理。

 

The Bridge maps user priority onto one or more traffic classes; Bridges that support more than one traffic class are able to support expedited classes of traffic. The Forwarding Process, 7.7, describes the use of user priority and traffic classes in MAC Bridges. Given the constraints placed upon frame misordering in a Bridge, as expressed in 6.3.3, the mappings of priority and traffic class are static.

網橋映射用戶權限到一個或多個通訊類別上。支持多個通訊類別的網橋可以支持加速類別的流量。轉發處理，參見7.7，描述了在MAC網橋裏使用用戶優先級和通訊類別。網橋內對於數據包的亂序給出了限制，參見6.3.3，優先級的映射和通訊類別是靜態的。

 

NOTE 1—The term Traffic Class, as used in this standard, is used only in the context of the operation of the priority handling and queueing functions of the Forwarding Process, as described in 7.7. Any other meanings attached to this term in other contexts do not apply to the use of the term in this standard.

注意1：通訊類別這個詞，在本標準內使用，只用在優先級操做的上下文，以及轉發處理的隊列函數內，參見7.7。這個詞在其它上下文中的其它含義都不適用於本標準。

 

The ability to signal user priority in IEEE 802 LANs allows user priority to be carried with end-to-end significance across a Bridged Local Area Network. This, coupled with a consistent approach to the mapping of user priority to traffic classes and of user priority to access_priority, allows consistent use of priority information, according to the capabilities of the Bridges and MACs in the transmission path.

在IEEE 802 LANs內標記用戶優先級的能力使得用戶優先級透過橋接LAN進行端到端傳輸。

 

NOTE 2—IEEE Std 802.1Q™ defines a frame format and procedures that can be used to carry user priority across LAN MAC types that are not able to signal user priority. Use of the IEEE 802.1Q frame format allows the end-to-end significance of user priority to be maintained regardless of the ability of individual LAN MAC types to signal priority.

注意2：IEEE Std 802.1Q™定義了數據包格式和跨越不一樣LAN MAC類型承載用戶優先級的函數，其中有些LAN MAC類型原本是不能標記用戶優先級的。使用IEEE 802.1Q包格式能夠維護端到端的用戶優先級而不用管LAN MAC是否具備標記優先級的能力。

 

Under normal circumstances, user priority is not modified in transit through the relay function of a Bridge; however, network management can control how user priority is propagated. Table 7-1 provides the ability to map incoming user priority values on a per-Port basis. By default, the regenerated user priority is identical to the incoming user priority.

在正常狀況下，用戶優先級在經過網橋轉發時是不會被修改的，不過，網管能夠控制用戶優先級如何進行傳播。Table 7 -1提供了接收的用戶優先級到端口的映射能力。缺省狀況下，從新生成的用戶優先級等於接收的用戶優先級。

6.3.10       Throughput

The total throughput provided by a Bridged Local Area Network can be significantly greater than that provided by an equivalent single LAN. Bridges may localize traffic within the network by filtering frames. Filtering services are described in 6.6.

橋接LAN的總吞吐量顯然比單個LAN的吞吐量要大。網橋能夠經過過濾數據包把網絡內的流量本地化，過濾服務說明參見6.6。

 

The throughput between end stations communicating through a Bridge can be lowered by frame discard due to the Bridge’s inability to transmit on the LAN to the destination at the required rate for an extended period.

若是在至關長的時間內，網橋不能在LAN上以指定速率將數據包傳輸到目的，則經過丟棄數據包，可以下降在終端工做站之間經過網橋進行通訊所形成的吞吐量，