{
  "slug": "buffer-to-buffer-credit",
  "term": "Buffer-to-buffer credit",
  "aliases": [
    "BB_credit",
    "BB credits",
    "Fibre Channel buffer credit"
  ],
  "definition": "A buffer-to-buffer credit permits a Fibre Channel sender to transmit a frame into advertised receive-buffer space on the adjacent link endpoint.",
  "note": "Credits are direction-specific and count frames rather than bytes. Transmission spends a credit; in basic R_RDY flow control, a receiver returns credit when a buffer becomes available. Zero credits pauses transmission. The cited implementation documents credit loss after corrupted frames or credit-return primitives. Its BB_SC_N recovery feature on E ports supports R_RDY mode, not ER_RDY mode; verify the actual mode before selecting a recovery procedure. [credits]\n\nThis is link flow control, not command completion. FCP carries SCSI status separately. Treating a returned credit as proof of a successful or durable write confuses these layers. [fcp-response]",
  "related": [
    "fibre-channel",
    "fc-frame",
    "fc-exchange",
    "credit-starvation",
    "buffer-cycle-deadlock"
  ],
  "citations": [
    {
      "id": "credits",
      "title": "Cisco MDS 9000 Interfaces Configuration Guide, Release 9.x: Buffer-to-Buffer Credits and Credit Recovery",
      "url": "https://www.cisco.com/c/en/us/td/docs/dcn/mds9000/sw/9x/configuration/interfaces/cisco-mds-9000-nx-os-interfaces-configuration-guide-9x/configuring_interface_buffers.html",
      "accessed": "2026-10-19"
    },
    {
      "id": "fcp-response",
      "title": "Linux v6.12: fc_fcp.h, fcp_resp and SCSI status",
      "url": "https://github.com/torvalds/linux/blob/v6.12/include/scsi/fc/fc_fcp.h",
      "accessed": "2026-10-19"
    }
  ],
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-19",
  "provenance": [
    "Source-backed protocol definition; no throughput, failure-rate or durability measurement is implied."
  ]
}