IPD Calculation and Source
IPD Calculation
The time to wait before transmitting a subsequent packet through the same local CA port to the same DLID address is based on the time it takes to transmit the current packet.
The second packet isn't forwarded to the local CA Port until Ts has elapsed since the previous packet was forwarded to the port for transmission. Ts is calculated as:
where IPD is an 8-bit value defined as per Table 24-1 on page 594, and the time it takes to transmit a packet is calculated as:
where Lr = the local CA port speed calculated using its LinkWidthActive and LinkSpeedActive attribute elements. The PktLen value is multiplied by four to yield the number of bytes in the packet.
IPD Source
The IPD value to be used for a path between the source port and a given destination port (as defined by its DLID address) is based upon a value obtained from the SA's PathRecord (refer to “PathRecord” on page 975 for more information) that defines the speed of the overall path (as determined by the SM during subnet discovery and configuration). The 6-bit PathRecord.Rate field defines the path's overall speed as follows:
1 = 1Gb/s. The IPD to be used depends on the speed of the source port:
- If the source port speed is 2.5Gb/s, the IPD = 2. This would yield a source port transmit speed of 2.5Gb/s ÷ 3 (IPD + 1) = 833Mb/s.
- If the source port speed is 10Gb/s, the IPD = 9. This would yield a source port transmit speed of 10Gb/s ÷ 10 (IPD + 1) = 1Gb/s.
- If the source port speed is 30Gb/s, the IPD = 29. This would yield a source port transmit speed of 30Gb/s ÷ 30 (IPD + 1) = 1Gb/s.
2 = 2.5Gb/s. The IPD to be used depends on the speed of the source port:
- If the source port speed is 10Gb/s, the IPD = 3. This would yield a source port transmit speed of 10Gb/s ÷ 3 (IPD + 1) = 2.5Gb/s.
- If the source port speed is 30Gb/s, the IPD = 11. This would yield a source port transmit speed of 30Gb/s ÷ 12 (IPD + 1) = 2.5Gb/s.
3 = 10 Gb/s. The IPD to be used depends on the speed of the source port:
- If the source port speed is 10Gb/s, the IPD = 0. This would yield a source port transmit speed of 10Gb/s ÷ 1 (IPD + 1) = 10Gb/s.
- If the source port speed is 30Gb/s, the IPD = 2. This would yield a source port transmit speed of 30Gb/s ÷ 3 (IPD + 1) = 10Gb/s.
4 = 30 Gb/s. In this case, every link between the source and destination port are 12x links, so the IPD is 0 (i.e., no delay is necessary between the transmission of packets to that destination through the local CA port).
5-64 = Reserved.
While all 256 possible IPD values are valid, Table 24-1 on page 594 defines the minimum number of values that must be supported by a CA design. The specification contains the following statement:
“If a CA is requested to use a value that it doesn't support, the CA will use a supported value, and return that value in the appropriate MADs or verbs.”
Relative to this statement, it should be noted that the actual IPD the QP or EEC will use is not returned as an output parameter from the Create Address Handle, Modify QP, or Modify EEC verbs, nor is it returned in the communications reply (REP) message.
| IPD | Rate | Comment |
|---|---|---|
| 0 | 100% | Suited for matched links. Required by all CAs. |
| 2 | 33% | Suited for 30Gb/s to 10 Gb/s conversion. Required by CAs that support 30Gb/s or higher link rate. |
| 3 | 25% | Suited for 10Gb/s to 2.5 Gb/s conversion. Required by CAs that support 10Gb/s or higher link rate. |
| 11 | 8% | Suited for a 30Gb/s to 2.5 Gb/s conversion. Required by CAs that support 30Gb/s or higher link rate. |