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NE40E V800R010C10SPC500 Feature Description - QoS 01

This is NE40E V800R010C10SPC500 Feature Description - QoS
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
CAR

CAR

What Is CAR

In traffic policing, committed access rate (CAR) is used to control traffic. CAR uses token buckets to measure traffic and determines whether a packet is conforming to the specification.

CAR has the following two functions:

  • Rate limit: Only packets allocated enough tokens are allowed to pass so that the traffic rate is restricted.

  • Traffic classification: Packets are marked internal priorities, such as the scheduling precedence and drop precedence, based on the measurement performed by token buckets.

CAR Process

Figure 7-5 CAR process

  • When a packet arrives, the device matches the packet against matching rules. If the packet matches a rule, the router uses token buckets to meter the traffic rate.

  • The router marks the packet red, yellow, or green based on the metering result. Red indicates that the traffic rate exceeds the specifications. Yellow indicates that the traffic rate exceeds the specifications but is within an allowed range. Green indicates that the traffic rate is conforming to the specifications.

  • The device drops packets marked red, re-marks and forwards packets marked yellow, and forwards packets marked green.

Marking Process of CAR

NE40Es conform to relevant standards to implement CAR.

CAR supports srTCM with single bucket, srTCM with two buckets, and trTCM. This section provides examples of the three marking methods in Color-Blind mode. The implementation in Color-Aware mode is similar to that in Color-Blind mode.

  • SrTCM with Single Bucket

    This example uses the CIR 1 Mbit/s, the committed burst size (CBS) 2000 bytes, and the excess burst size (EBS) 0. The EBS 0 indicates that only bucket C is used. Bucket C is initially full of tokens.

    • If the first arriving packet is 1500 bytes long, the packet is marked green because the number of tokens in bucket C is greater than the packet length. The number of tokens in bucket C then decreases by 1500 bytes, with 500 bytes remaining.
    • Assume that the second packet arriving at the interface after a delay of 1 ms is 1500 bytes long. Additional 125-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 1 ms = 1000 bits = 125 bytes). Bucket C now has 625-byte tokens, which are not enough for the 1500-byte second packet. Therefore, the second packet is marked red.
    • Assume that the third packet arriving at the interface after a delay of 1 ms is 1000 bytes long. Additional 125-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 1 ms = 1000 bits = 125 bytes). Bucket C now has 750-byte tokens, which are not enough for the 1000-byte third packet. Therefore, the third packet is marked red.
    • Assume that the fourth packet arriving at the interface after a delay of 20 ms is 1500 bytes long. Additional 2500-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 20 ms = 20000 bits = 2500 bytes). This time 3250-byte tokens are destined for bucket C, but the excess 1250-byte tokens over the CBS (2000 bytes) are dropped. Therefore, bucket C has 2000-byte tokens, which are enough for the 1500-byte fourth packet. The fourth packet is marked green, and the number of tokens in bucket C decreases by 1500 bytes to 500 bytes.

    The following table illustrates this process:

    No. Time Packet Length Delay Token Addition Tokens in Bucket C Before Packet Processing Tokens in Bucket C After Packet Processing Marking
    - - - - - 2000 2000 -
    1 0 1500 0 0 2000 500 Green
    2 1 1500 1 125 625 625 Red
    3 2 1000 1 125 750 750 Red
    4 22 1500 20 2500 2000 500 Green
  • SrTCM with Two Buckets

    This example uses the CIR 1 Mbit/s and the CBS and EBS both 2000 bytes. Buckets C and E are initially full of tokens.

    • If the first packet arriving at the interface is 1500 bytes long, the packet is marked green because the number of tokens in bucket C is greater than the packet length. The number of tokens in bucket C then decreases by 1500 bytes, with 500 bytes remaining. The number of tokens in bucket E remains unchanged.
    • Assume that the second packet arriving at the interface after a delay of 1 ms is 1500 bytes long. Additional 125-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 1 ms = 1000 bits = 125 bytes). Bucket C now has 625-byte tokens, which are not enough for the 1500-byte second packet. Bucket E has 2000-byte tokens, which are enough for the second packet. Therefore, the second packet is marked yellow, and the number of tokens in bucket E decreases by 1500 bytes, with 500 bytes remaining. The number of tokens in bucket C remains unchanged.
    • Assume that the third packet arriving at the interface after a delay of 1 ms is 1000 bytes long. Additional 125-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 1 ms = 1000 bits = 125 bytes). Bucket C now has 750-byte tokens and bucket E has 500-byte tokens, neither of which is enough for the 1000-byte third packet. Therefore, the third packet is marked red. The number of tokens in buckets C and E remain unchanged.
    • Assume that the fourth packet arriving at the interface after a delay of 20 ms is 1500 bytes long. Additional 2500-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 20 ms = 20000 bits = 2500 bytes). This time 3250-byte tokens are destined for bucket C, but the excess 1250-byte tokens over the CBS (2000 bytes) are put into bucket E instead. Therefore, bucket C has 2000-byte tokens, and bucket E has 1750-byte tokens. Tokens in bucket C are enough for the 1500-byte fourth packet. Therefore, the fourth packet is marked green, and the number of tokens in bucket C decreases by 1500 bytes, with 500 bytes remaining. The number of tokens in bucket E remains unchanged.

    The following table illustrates the preceding process:

    No. Time Packet Length Delay Token Addition Tokens in Buckets Before Packet Processing Tokens in Buckets After Packet Processing Marking
    Bucket C Bucket E Bucket C Bucket E
    - - - - - 2000 2000 2000 2000 -
    1 0 1500 0 0 2000 2000 500 2000 Green
    2 1 1500 1 125 625 2000 625 500 Yellow
    3 2 1000 1 125 750 500 750 500 Red
    4 22 1500 20 2500 2000 1750 500 1750 Green
  • TrTCM

    This example uses the CIR 1 Mbit/s, the PIR 2 Mbit/s, and the CBS and EBS both 2000 bytes. Buckets C and P are initially full of tokens.

    • If the first packet arriving at the interface is 1500 bytes long, the packet is marked green because the number of tokens in both buckets P and C is greater than the packet length. Then the number of tokens in both buckets P and C decreases by 1500 bytes, with 500 bytes remaining.
    • Assume that the second packet arriving at the interface after a delay of 1 ms is 1500 bytes long. Additional 250-byte tokens are put into bucket P (PIR x time period = 2 Mbit/s x 1 ms = 2000 bits = 250 bytes) and 125-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 1 ms = 1000 bits = 125 bytes). Bucket P now has 750-byte tokens, which are not enough for the 1500-byte second packet. Therefore, the second packet is marked red, and the number of tokens in buckets P and C remain unchanged.
    • Assume that the third packet arriving at the interface after a delay of 1 ms is 1000 bytes long. Additional 250-byte tokens are put into bucket P (PIR x time period = 2 Mbit/s x 1 ms = 2000 bits = 250 bytes) and 125-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 1 ms = 1000 bits = 125 bytes). Bucket P now has 1000-byte tokens, which equals the third packet length. Bucket C has only 750-byte tokens, which are not enough for the 1000-byte third packet. Therefore, the third packet is marked yellow. The number of tokens in bucket P decreases by 1000 bytes, with 0 bytes remaining. The number of tokens in bucket C remains unchanged.
    • Assume that the fourth packet arriving at the interface after a delay of 20 ms is 1500 bytes long. Additional 5000-byte tokens are put into bucket P (PIR x time period = 2 Mbit/s x 20 ms = 40000 bits = 5000 bytes), but excess tokens over the PBS (2000 bytes) are dropped. Bucket P has 2000-byte tokens, which are enough for the 1500-byte fourth packet. Bucket C has 750-byte tokens left, and additional 2500-byte tokens are put into bucket C (CIR x time period = 1 Mbit/s x 20 ms = 2000 bits = 250 bytes). This time 3250-byte tokens are destined for bucket C, but excess tokens over the CBS (2000 bytes) are dropped. Bucket C then has 2000-byte tokens, which are enough for the 1500-byte fourth packet. Therefore, the fourth packet is marked green. The number of tokens in both buckets P and C decreases by 1500 bytes, with 500 bytes remaining.

    The following table illustrates this process:

    No. Time Packet Length Delay Token Addition Tokens in Buckets Before Packet Processing Tokens in Buckets After Packet Processing Marking
    Bucket C Bucket P Bucket C Bucket P Bucket C Bucket P Bucket C Bucket P
    - - - - - - - 2000 2000 2000 2000 -
    1 0 1500 0 0 0 0 2000 2000 500 500 Green
    2 1 1500 1 1 125 250 625 750 625 750 Red
    3 2 1000 1 1 125 250 750 1000 750 0 Yellow
    4 22 1500 20 20 2500 5000 2000 2000 500 500 Green

Usage Scenarios for the Three Marking Methods

The srTCM focus on the traffic burst size and have a simple token-adding method and packet processing mechanism. The trTCM focuses on the traffic burst rate and has a complex token-adding method and packet processing mechanism.

The srTCM and trTCM have their own advantages and disadvantages. They vary from each other in performance, such as the packet loss rate, burst traffic processing capability, hybrid packet forwarding capability, data forwarding smoothing capability. The three markers fit for traffic with different features as follows:

  • To control the traffic rate, use srTCM with single bucket.

  • To control the traffic rate and distinguish traffic marked differently and process them differently, use srTCM with two buckets. Note that traffic marked yellow must be processed differently from traffic marked green. Otherwise, the implementation outcome of srTCM with two buckets is the same as that of the srTCM with single bucket.

  • To control the traffic rate and check whether the traffic rate exceeds the CIR or PIR, use trTCM. Note that traffic marked yellow must be processed differently from traffic marked green. Otherwise, the implementation outcome of trTCM is the same as that of srTCM with single bucket.

CAR Parameter Setting

The CIR is the key to determine the volume of traffic allowed to pass through a network. The larger the CIR is, the higher the rate at which tokens are generated. The more the tokens allocated to packets, the greater the volume of traffic allowed to pass through. The CBS is also an important parameter. A larger CBS results in more accumulated tokens in bucket C and a greater volume of traffic allowed to pass through.

  • The CBS must be greater than or equal to the maximum packet length. For example, the CIR is 100 Mbit/s, and the CBS is 200 bytes. If a device receives 1500-byte packets, the packet length always exceeds the CBS, causing the packets to be marked red or yellow even if the traffic rate is lower than 100 Mbit/s. This leads to an inaccurate CAR implementation.

The Bucket depth (CBS, EBS or PBS) is set based on actual rate limit requirements. In principle, the bucket depth is calculated based on the following conditions:

  1. Bucket depth must be greater than or equal to the MTU.
  2. Bucket depth must be greater than or equal to the allowed burst traffic volume.

Condition 1 is easy to meet. Condition 2 is difficult to operate, and the following formula is introduced:

Bucket depth (bytes) = Bandwidth (kbit/s) x RTT (ms)/8. Note that RTT refers to round trip time and is set to 200 ms.

The following formulas are used for NE40Es:

  • When the bandwidth is lower than or equal to 100 Mbit/s: Bucket depth (bytes) = Bandwidth (kbit/s) x 1500 (ms)/8.
  • When the bandwidth is higher than 100 Mbit/s: Bucket depth (bytes) = 100,000 (kbit/s) x 1500 (ms)/8.

CAR calculates the bandwidth of packets based on the entire packet. For example, CAR counts the length of the frame header and CRC field but not the preamble, inter frame gap, or SFD of an Ethernet frame in the bandwidth. The following figure illustrates a complete Ethernet frame (bytes):



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Updated: 2019-01-03

Document ID: EDOC1100055046

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