ofb_dl is the Data Link layer of OpenFibre (ECSS-E-ST-50-11C clause 5.7). It transfers the N-Chars of up to 32
virtual channels and broadcast messages over the link in frames, controls the flow of every virtual channel with FCTs,
recovers from errors with ACK, NACK and RETRY, and resets the link with the far end.
The module consists of the blocks of the architecture (sections 7.2 and 7.3):
Medium access, transmit scheduler (precedence), frame assembly, sequence numbers and CRC-8
DT-7
ofb_dl_erb
Error recovery buffer, ACK and NACK processing
DR-1, DR-2, DR-4
ofb_dl_rx_check
Data word identification, CRC-8 and sequence checks, decoding of control words
DR-3
ofb_dl_rx_err
Receive error state machine, ACK and NACK requests
DR-5
ofb_dl_rx_buf
Frame buffer: commit or drop of received data frames, distribution to the input VC buffers
DR-6
ofb_dl_vc_in (x NumVc_g)
Input VC buffer, FCT requests
DR-7
ofb_dl_bc_in
Broadcast input buffer
DC-1
ofb_dl_link_reset
Link reset state machine
DC-2
in ofb_dl
Status and event outputs for the MIB
This issue covers phases 2 to 4 of the roadmap: up to 32 virtual channels, the quality of service of ECSS 5.7.4
(priority, bandwidth reservation, schedule; section 2.13), the continuous mode, and rows of NumLanes_g words for a
Multi-Lane layer with 1 to 4 lanes (section 2.14). With NumLanes_g = 1 a row is one word and the FCT multiplier
and the data segment multiplier are 1.
Clock domains: the user side of the VC and broadcast buffers runs on UserClk, everything else on Clk (the core
clock). The rows to and from the Multi-Lane layer are on Clk; the crossing to the lane clock is in the core top
level.
For every VC the Data Link layer shall accept beats of NumLanes_g words of four N-Chars or Fills (32 bits per word, one K flag per character, word 0 first) from the Network layer with a valid / ready handshake on UserClk, and pass received beats to the Network layer with a valid / ready handshake.
5.7.1b, c, 5.7.2.1
DL-IF-02
The Data Link layer shall accept broadcast messages (8 data bytes, broadcast channel, B_TYPE, DELAYED flag) with a valid / ready handshake and pass received broadcast messages (with DELAYED and LATE flags) with a valid strobe on UserClk.
5.7.1b, c, 5.3.8.4
DL-IF-03
The Data Link layer shall pass rows to the Multi-Lane layer with a valid / ready handshake and receive rows with a valid strobe and a CRC-16 error flag, as defined by the interface of ofb_multilane.
5.7.1e, f
DL-IF-04
The Data Link layer shall drive link reset, LaneReset and the near-end capability (INIT3LinkResetFlag, DataScrambled) of the Multi-Lane layer, and use its far-end capability event and its lane active indication.
5.7.1g to i
DL-IF-05
The Data Link layer shall accept the management parameters DataScrambled, Link Reset, Interface Reset and the per-VC parameters, and provide the status of Table 5-37 for the Data Link layer as levels and one-cycle events.
5.7.1j, k, 5.9.3, 5.9.4
2.2 Output VC buffer and flow control (DT-1, DT-2)¶
ID
Requirement
ECSS
DL-VO-01
Every VC shall have an output VC buffer of VcOutDepth_g beats (at least 64 words, 256 N-Chars), which accepts beats while it is not full and keeps their order.
5.7.2.2a to f
DL-VO-02
On link reset the output VC buffer shall be flushed; if the last character written by the Network layer was not an EOP, EEP or Fill, all new words shall be discarded up to and including the next word with an EOP or EEP.
5.7.2.2g, 5.7.10a.1 to 3
DL-VO-03
The output VC buffer shall indicate a data segment ready when its FCT credit counter allows at least one row and it contains a full segment, a word with an EOP or EEP, or is full.
5.7.3.1b
DL-VO-04
A data segment shall have at most min(segment size, credit, words in the buffer, free space of the error recovery buffer) words in whole rows, the segment size being 64 x P words (DL-RW-03), so that the input VC buffer at the far end has room for it.
5.7.3.1d, 5.7.4.1c, 5.3.8.2c
DL-CR-01
Every VC shall have an FCT credit counter of CreditWidth_g bits (at least four FCTs) that is increased by M x 64 words for every received FCT of the VC, decreased by the words sent, set to zero on link reset and not changed on LaneReset.
5.7.3.1a, e to h, j, k
DL-CR-02
An FCT that would overflow the credit counter shall leave it unchanged at its maximum value and raise the credit overflow status of the VC.
5.7.3.1i
2.3 Broadcast output and broadcast flow control (DT-3)¶
ID
Requirement
ECSS
DL-BO-01
Broadcast messages shall be buffered in a broadcast output buffer of BcOutDepth_g messages.
5.7.1b
DL-BO-02
A Broadcast Bandwidth Credit Counter shall be decreased by one for every broadcast frame sent and increased by one after each interval of 4 / (Normalised Expected Broadcast Bandwidth) words, saturate at 256, never be negative, and be set to zero on link reset; no broadcast frame shall be sent while it is zero.
5.7.5a to k
DL-BO-03
A broadcast message that cannot be sent immediately because no lane is active or because of error recovery shall be sent with the LATE flag set; the DELAYED flag shall be passed from the Network layer unchanged.
Only VCs with a data segment ready (DL-VO-03) shall compete for sending the next data segment; the selection follows the quality of service of section 2.13.
The words of the Data Link layer shall be sent in the precedence order RETRY, broadcast frame to be resent, broadcast frame, ACK or NACK, FCT to be resent, FCT, FULL, data frame to be resent, data frame, idle frame.
5.3.10c6 to c15
DL-TS-02
A RETRY shall be able to interrupt a broadcast, data or idle frame; a broadcast frame shall be able to be inserted within a data frame; ACK, NACK, FCT and FULL shall be able to be inserted within a data or idle frame. A frame that is interrupted by a RETRY is not completed.
5.3.10i to n
DL-TS-03
An idle frame shall be sent only when there is no data frame, broadcast frame or control word to send; it shall end on a word boundary as soon as a data or broadcast frame is to be sent and after 64 PRBS words.
5.3.10o, 5.3.8.3j to l, 5.7.6.6a
DL-TS-04
While broadcast frames of the error recovery buffer are resent no new broadcast frame shall be sent; while broadcast frames or FCTs are resent no new FCT; while broadcast frames, FCTs or data frames are resent no new data frame.
5.3.10q to s, 5.7.7.1e
DL-TS-05
An ACK shall be sent as soon as possible after it is requested, with at least 15 words between two ACKs; a pending ACK or NACK is replaced by a newer request of the other kind.
5.7.7.2.1b to f, 5.7.7.2.2c, e
DL-TS-06
A FULL shall be sent when the error recovery buffer becomes full and every 64 words while it stays full; a FULL shall also be sent after an RXERR or a CRC error when all output VC buffers are empty, nothing else is to be sent and the error recovery buffer is not empty.
5.7.7.1q to s
DL-FA-01
A data segment shall be sent as a data frame: SDF with the VC number, the data words, EDF with the sequence number (the CRC-16 field is filled by the Multi-Lane layer).
5.7.6.1a, 5.3.5.1.3, 5.3.5.1.4, 5.3.8.2
DL-FA-02
A broadcast message shall be sent as a broadcast frame: SBF with broadcast channel and B_TYPE, two data words, EBF with STATUS (DELAYED, LATE), sequence number and CRC-8.
5.7.6.1b, 5.3.5.1.5, 5.3.5.1.6, 5.3.8.4
DL-FA-03
An idle frame shall consist of an SIF with the current sequence number and CRC-8, followed by PRBS words of the ECSS generator (seed 0xFFFF after link reset, continued from one idle frame to the next, paused while control words are embedded).
5.7.6.2.3, 5.7.6.6, 5.3.5.1.7, 5.3.8.3
DL-FA-04
FCT, ACK, NACK, FULL and RETRY shall be built as specified, FCT with the multiplier M - 1 and the VC number.
5.3.5.2, 5.3.5.3
DL-FA-05
On link reset, the frame being sent shall be stopped and the idle frame PRBS generator set to its seed.
5.7.6.1c, 5.7.10b.1, b.3
2.6 Sequence numbers and CRC-8 on transmission (DT-8)¶
ID
Requirement
ECSS
DL-SQ-01
A 7-bit Transmit Sequence Counter (modulo 128) and a Transmit Polarity Flag shall be kept; immediately before an EDF, EBF or FCT is passed on, the counter shall be incremented and the new value with the flag placed in its SEQ_NUM; SIF and FULL carry the current value.
5.7.6.3.1a to c, g to i, 5.3.5.1.2
DL-SQ-02
On link reset the counter and the flag shall be cleared. When a valid NACK is processed the counter shall be set to the sequence count of the NACK before the RETRY is sent, and the flag inverted after the RETRY.
5.7.6.3.1d to f, 5.7.7.2.4c.2 to 4
DL-SQ-03
The CRC-8 (x^8 + x^2 + x + 1, seed 0) of SIF, FCT, ACK, NACK and FULL shall cover their first three characters; the CRC-8 of a broadcast frame shall cover the SBF, the two data words and the first three characters of the EBF.
Data segments, FCTs and broadcast messages shall be stored in the error recovery buffer when they are sent, referenced by their sequence number, with room for at least one FCT and one broadcast message regardless of the data stored.
5.7.7.1a to d
DL-ER-02
A valid ACK (CRC-8 correct, polarity equal to the Transmit Polarity Flag) shall delete all items sent with a sequence count less than or equal to its sequence count (modulo 128).
5.7.7.2.3a to c
DL-ER-03
A valid NACK shall delete the items up to its sequence count, set the Transmit Sequence Counter to its count, cause a RETRY, invert the polarity and resend the remaining items with new sequence numbers: broadcast frames first, then FCTs, then data frames, each in their original order. A NACK with the other polarity shall be ignored.
5.7.7.2.4a to d, 5.7.7.1e, g
DL-ER-04
A valid ACK or NACK with a sequence count different from that of the previous valid ACK or NACK and no item with that count in the buffer shall cause a link reset and raise the Link Reset Caused by Protocol Error status.
5.7.7.2.3d, 5.7.7.2.4e
DL-ER-05
The error recovery buffer shall be full when it cannot store an item of a kind (data word, FCT, broadcast message) or 127 items are waiting for acknowledgement; no new item of that kind shall then be accepted.
5.7.7.1n, o
DL-ER-06
On link reset the error recovery buffer shall be emptied; on LaneReset it shall not.
5.7.7.1h, i, 5.7.10b.8
DL-ER-07
The number of error recovery attempts (RETRYs sent) shall be counted, reported, and cleared by reset, interface reset or the MIB. The empty state of the buffer shall be reported.
5.7.7.1j to l, Table 5-37
2.8 Data word identification and frame checks (DR-1, DR-2, DR-4)¶
ID
Requirement
ECSS
DL-WI-01
The data word identification state machine shall implement RxNothing, RxDataFrame, RxBroadcastFrame, RxBroadcast&DataFrame and RxIdleFrame with the transitions, word counters, maximum lengths (data and idle frame 64 words, broadcast frame 2 words) and frame error conditions of the standard, evaluated in the order given; FCT, ACK, NACK, FULL and RETRY shall be accepted in any state, unknown control words ignored.
5.7.8
DL-WI-02
A data frame shall be accepted when its EDF arrives in RxDataFrame with a correct CRC-16 (from the Multi-Lane layer) and a correct sequence number; its data segment shall then be written into the input VC buffer of its VC (a VC number that the core does not implement: the segment is discarded). Otherwise it shall be discarded.
5.7.6.7a, b
DL-WI-03
A broadcast frame shall be accepted when its EBF has a correct CRC-8 and sequence number and the frame has two data words; the message shall then be passed to the broadcast input buffer.
5.7.6.7c, d
DL-WI-04
An FCT with correct CRC-8 and sequence number shall be passed to the credit counter of its VC.
5.7.6.7e, f
DL-SR-01
A 7-bit Receive Sequence Counter shall be kept, cleared on link reset and not changed on LaneReset; EDF, EBF and FCT shall be in sequence when their count is the counter plus one and their polarity equals the Receive Polarity Flag, SIF and FULL when their count equals the counter and the polarity matches. An accepted data frame, broadcast frame or FCT shall increment the counter; a frame or FCT out of sequence shall be discarded.
5.7.6.3.2, 5.3.5.1.2
DL-SR-02
The CRC-8 of received SIF, FCT, ACK, NACK, FULL and broadcast frames shall be checked; a word with a CRC error shall not be acted upon.
5.7.6.5g to j
DL-SR-03
Valid ACKs and NACKs (CRC-8 correct) shall be passed to the error recovery buffer with their sequence number.
The receive error state machine shall implement Valid Positive, Valid Negative, Error Positive and Error Negative with the transitions of the standard and determine the Receive Polarity Flag; it shall enter Valid Positive on link reset. Every received word shall be checked against the flag as updated by the previous word, also when the words follow in consecutive cycles.
5.7.7.3, 5.7.6.3.2i
DL-RE-02
An ACK shall be requested when a data frame, broadcast frame, FCT or FULL is received without sequence or CRC error. ACKs carry the Receive Sequence Counter and the Receive Polarity Flag.
5.7.7.2.1a, e
DL-RE-03
A NACK shall be requested when an RXERR or a CRC error occurs while the data word identification state machine is in RxDataFrame, RxBroadcastFrame or RxBroadcast&DataFrame, or when a sequence error is detected in a control word with a valid CRC; not for unknown control words. NACKs carry the Receive Sequence Counter and the inverse of the Receive Polarity Flag.
5.7.7.2.2a, b, d, 5.7.7.3
2.10 Frame buffer and input VC buffers (DR-5, DR-6, DR-7)¶
ID
Requirement
ECSS
DL-VI-01
Every VC shall have an input VC buffer of VcInDepth_g words (at least 64 x NumLanes_g words) on UserClk, which stores the received words without gaps and keeps the order of the received N-Chars and Fills.
5.7.2.3a, b, 5.3.7.2d
DL-VI-02
After link reset an input VC buffer shall request one FCT (multiplier M = P, DL-RW-03) for every M x 64 words of space, and one further FCT every time the Network layer has read M x 64 words; FCT requests of several VCs shall be served in round-robin order.
5.7.3.2b to d
DL-VI-03
A word received for a full input VC buffer is an overflow: the link shall be reset and the input buffer overflow status of the VC raised.
5.7.3.2e
DL-VI-04
On link reset the input VC buffers shall be flushed; if the last character read by the Network layer was not an EOP, EEP or Fill, an EEP shall be the next character read.
5.7.2.3d, 5.7.10a.4 to 7
DL-BI-01
Accepted broadcast messages shall be passed to the Network layer through a broadcast input buffer of BcInDepth_g messages; a message that finds the buffer full shall be discarded and counted.
The link reset state machine shall implement Configuration Reset, Near-End Reset, Check Far-End Reset and Link Initialised with the transitions of the standard; reset and Interface Reset lead to Configuration Reset, the Link Reset parameter, a protocol error and an input buffer overflow to Near-End Reset.
5.7.9.1 to 5.7.9.5
DL-LR-02
In Near-End Reset link reset and LaneReset of all lanes shall be asserted for one cycle; in Check Far-End Reset the INIT3LinkResetFlag of the near-end capability shall be 1, in Link Initialised 0.
5.7.9.3b, 5.7.9.4b, 5.7.9.5b
DL-LR-03
The far-end capability shall be used as an event (not as a stored level): Check Far-End Reset moves to Link Initialised, and Link Initialised to Near-End Reset when no lane is active, on a capability event with INIT3LinkResetFlag set; the latter raises the Far-End Link Reset status.
5.7.9.4c, 5.7.9.5c, Table 5-37
DL-LR-04
Link reset shall perform the actions of 5.7.10b in all Data Link blocks: frame stopped, broadcast credit zero, PRBS seed, sequence counters and polarity flags cleared, error recovery buffer emptied, data word identification in RxNothing.
5.7.10b
DL-LR-05
Received words shall be passed to the receive checks only in Link Initialised: words that arrive while the near end is reset belong to the link before the reset (an ACK of a discarded frame would otherwise cause a protocol error and a second link reset).
The Data Link layer shall report, per VC, Has Credit, input buffer overflow and FCT credit counter overflow, and for the link 16-bit CRC error, frame error, CRC-8 error, sequence error, error recovery buffer empty, number of error recovery attempts, Link Reset Caused by Protocol Error and Far-End Link Reset; errors are one-cycle events, the MIB keeps them until read.
Table 5-37
2.13 Quality of service and continuous mode (phase 3)¶
ID
Requirement
ECSS
DL-QS-01
Every VC shall have a priority level (0 highest to NumPrio_g - 1 lowest, at least four levels), a Normalised Expected Bandwidth (as the factor 1 / bandwidth, 8.8 fixed point; factor 0 means bandwidth 0: the VC does not send) and 64 allocated time-slot bits.
5.7.4.2a, c, 5.7.4.3c, 5.7.4.6a to f, 5.7.4.5e, i, Table 5-36
DL-QS-02
A VC shall compete for the next data segment only when it has a segment ready, its bit of the current time-slot is set and its bandwidth is not zero.
5.7.4.1b to d, 5.7.4.3d to g, 5.7.4.7
DL-QS-03
The bandwidth credit of every VC shall be updated when a data segment is sent and otherwise every 66 words: + the words sent on the link since the last update, - the words of the data frame of that VC (including SDF and EDF) times its factor; it shall saturate at plus and minus the Bandwidth Credit Limit B (CreditLimit_g words) and be cleared on Interface Reset.
5.7.4.5a to k, q
DL-QS-04
The precedence of a VC shall be its priority precedence 2B(Q - 1 - R) + B plus its bandwidth credit; the priority precedence shall be zero while the credit is below the Minimum Bandwidth Credit Threshold (-0.9 B). The competing VC with the highest precedence shall send the next segment (the lowest VC number on equal precedence).
5.7.4.4a to f, 5.7.4.5m, n, p, 5.7.4.6d, g
DL-QS-05
A VC whose credit reaches the Minimum Bandwidth Credit Threshold shall report bandwidth over use; a VC whose credit stays at +B for the Virtual Channel Idle Time Limit (words) shall report bandwidth under use.
5.7.4.5l, o, Table 5-37
DL-QS-06
The current time-slot shall be set by SCHEDULE.request (time-slot number from the Network layer); optionally (generic) by received broadcast messages of a configured broadcast type.
5.7.4.3a, b, 6.3.4
DL-QS-07
On Interface Reset: priority lowest, factor of VC0 for 10 % (10.0), of the other VCs the minimum bandwidth (factor 0xFFFF), all time-slots allocated, bandwidth credits zero.
5.7.4.5q, r, 5.7.4.6h, Table 5-36
DL-CM-01
In continuous mode (per VC), when the output VC buffer is about to become full or no lane is active while the buffer is not empty, the buffer shall be flushed, an EEP placed in it, and the rest of an incomplete packet discarded up to and including its EOP or EEP; the VC then accepts words without back-pressure.
N is NumLanes_g. A row is the unit of the transmit and receive row streams of the Multi-Lane layer (DL-IF-03).
ID
Requirement
ECSS
DL-RW-01
A data row shall carry N words of one data frame; the words of a data segment shall be the beats of the output VC buffer in order. Every other word (control words, words of broadcast and idle frames) shall be a row of one word with the replicate flag.
5.6.4.1, 5.6.4.2a, 5.6.4.3a, 5.6.4.4a
DL-RW-02
The maximum number of data-sending lanes P (1 to N, management parameter) shall be taken over only while the link reset state machine resets the link.
Table 5-36
DL-RW-03
A data segment and an idle frame shall have at most 64 x P words, the FCTs sent shall carry the multiplier M = P, and a received data frame or idle frame with more than 64 x P words shall be a frame error.
5.6.4.2i, 5.3.8.2c, 5.7.3.2c, 5.7.8, Table 5-36
DL-RW-04
The FCT credit counter shall be decreased by N words for every data row sent.
5.7.3.1f
DL-RW-05
A received data row may hold 1 to N words (mask, word 0 first); every word shall count for the frame length and be stored in the input VC buffer.
5.6.5c, 5.7.8
DL-RW-06
The Network layer shall read the input VC buffer in beats of N words; a beat shall end after a word with an EOP or EEP, its remaining words being words of four Fills, or when N words are available.
5.7.2.3, 5.7.2.2 note
2.15 Uncorrectable errors in the buffers (fault tolerance)¶
ID
Requirement
ECSS
DL-ED-01
An uncorrectable error (DED) in a word read from an output VC buffer, the broadcast output buffer, the error recovery buffer (reads for sending, not the scrubber) or the frame buffer shall reset the link; a row of the frame buffer with a DED shall not be written into an input VC buffer.
none (fault tolerance, P5); 5.7.10 (link reset)
DL-ED-02
A beat of an input VC buffer with a DED shall be replaced by an EEP followed by Fills, and the rest of its packet shall be discarded up to and including the next word with an EOP or EEP; a broadcast message with a DED shall be discarded.
A VC competes when its credit is greater than zero; the segment is limited to the credit, so the far-end buffer always has room
FULL (5.7.7.1n to q)
The error recovery buffer is managed per kind of item; a kind that has no room blocks only new items of that kind, and FULL is sent (note 2 of 5.7.7.1q). Resending stored items is always allowed
Frame interrupted by RETRY
Every item is complete in the error recovery buffer before it is sent (store and forward), so an interrupted frame is resent completely
Frame error
Not a NACK condition by itself (5.7.7.2.2a); the missing frame is detected by the sequence number of the next frame, SIF or FULL
NACK polarity
NACKs carry the inverse of the Receive Polarity Flag (notes of 5.7.7.3.2c to 5.7.7.3.5b)
FCT multiplier and data segment multiplier (Table 5-36)
Both equal the maximum number of data-sending lanes P, taken over at link reset; both ends of a link use the same P (the frame length check of 5.7.8 uses the own parameter)
Beats of the VC ports with several lanes
A beat is a row of the data frame; the user fills a beat after the end of a packet with Fill words (note of 5.7.2.2d: more Fills can be needed to keep the packet cargo word alignment with a wider port). Received beats end at the end of a packet