diff --git a/i2c.cpp b/i2c.cpp
new file mode 100644
index 0000000..0a33b65
--- /dev/null
+++ b/i2c.cpp
@@ -0,0 +1,323 @@
+#include "i2c.hpp"
+
+#include <fcntl.h>
+#include <linux/i2c-dev.h>
+#include <linux/i2c.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+#include <array>
+#include <cerrno>
+#include <cstring>
+#include <host-ipmid/iana.hpp>
+#include <host-ipmid/oemopenbmc.hpp>
+#include <memory>
+
+namespace oem
+{
+namespace i2c
+{
+
+// Instance object.
+std::unique_ptr<I2c> globalOemI2c;
+
+// Block read (I2C_M_RECV_LEN) reads count byte, then that many bytes,
+// then possibly a checksum byte. The specified byte count limit,
+// I2C_SMBUS_BLOCK_MAX, is 32, but it seems intractible to prove
+// every I2C implementation uses that limit. So to prevent overflow,
+// allocate buffers based on the largest possible count byte of 255.
+constexpr size_t maxRecvLenBuf = 1 + 255 + 1;
+typedef std::array<uint8_t, maxRecvLenBuf> BlockBuf;
+
+struct ParsedStep
+{
+    const uint8_t* reqData;
+    size_t length;
+    DevAddr devAddr;
+    bool isRead;
+    // When nonzero, device supplies count for this step, as in SMBUS block
+    // mode. Value will tell driver how many extra bytes to read for entire
+    // block: 1 for count byte w/o PEC, 2 if there is also a PEC byte.
+    uint16_t blockExtra;
+    bool noStart;
+};
+
+struct ParsedReq
+{
+    BusId localbus;
+    bool usePec;
+    size_t numSteps;
+    std::array<ParsedStep, maxSteps> step;
+};
+
+static ipmi_ret_t parseReqHdr(const uint8_t* reqBuf, size_t reqLen,
+                              size_t* bytesUsed, i2c::ParsedReq* req)
+{
+    // Request header selects bus & flags for operation;
+    // additional bytes beyond are to be interpreted as steps.
+    if (reqLen < *bytesUsed + requestHeaderLen)
+    {
+        std::fprintf(stderr, "i2c::parse reqLen=%zu?\n", reqLen);
+        return IPMI_CC_REQ_DATA_LEN_INVALID;
+    }
+    // Deserialize request header bytes.
+    req->localbus = reqBuf[requestHeaderBus];
+    auto reqFlags = reqBuf[requestHeaderFlags];
+    *bytesUsed += requestHeaderLen;
+
+    // Decode flags.
+    req->usePec = !!(reqFlags & requestFlagsUsePec);
+    return IPMI_CC_OK;
+}
+
+static ipmi_ret_t parseReqStep(const uint8_t* reqBuf, size_t reqLen,
+                               size_t* bytesUsed, i2c::ParsedReq* req)
+{
+    size_t bytesLeft = reqLen - *bytesUsed;
+    if (req->numSteps >= maxSteps || bytesLeft < stepHeaderLen)
+    {
+        std::fprintf(stderr, "i2c::parse[%zu] bytesLeft=%zu?\n", req->numSteps,
+                     bytesLeft);
+        return IPMI_CC_REQ_DATA_LEN_INVALID;
+    }
+    const uint8_t* stepHdr = reqBuf + *bytesUsed;
+    auto step = &req->step[req->numSteps++];
+
+    // Deserialize request step header bytes.
+    uint8_t devAndDir = stepHdr[stepHeaderDevAndDir];
+    uint8_t stepFlags = stepHdr[stepHeaderFlags];
+    step->length = stepHdr[stepHeaderParm];
+    bytesLeft -= stepHeaderLen;
+    *bytesUsed += stepHeaderLen;
+
+    // Decode device addr & direction.
+    step->devAddr = devAndDir >> 1;
+    step->isRead = !!(devAndDir & 1);
+
+    // Decode step flags.
+    step->noStart = !!(stepFlags & stepFlagsNoStart);
+
+    if (step->isRead)
+    {
+        // Read could select blockExtra.
+        if (stepFlags & stepFlagsRecvLen)
+        {
+            step->blockExtra = req->usePec ? 2 : 1;
+        }
+    }
+    else
+    {
+        // For write, requested byte count must follow.
+        if (bytesLeft < step->length)
+        {
+            std::fprintf(stderr, "i2c::parse[%zu] bytesLeft=%zu, parm=%zu?\n",
+                         req->numSteps, bytesLeft, step->length);
+            return IPMI_CC_REQ_DATA_LEN_INVALID;
+        }
+        step->reqData = reqBuf + *bytesUsed;
+        *bytesUsed += step->length;
+    }
+    return IPMI_CC_OK;
+}
+
+// Parse i2c request.
+static ipmi_ret_t parse(const uint8_t* reqBuf, size_t reqLen,
+                        i2c::ParsedReq* req)
+{
+    size_t bytesUsed = 0;
+    auto rc = parseReqHdr(reqBuf, reqLen, &bytesUsed, req);
+    if (rc != IPMI_CC_OK)
+    {
+        return rc;
+    }
+    do
+    {
+        rc = parseReqStep(reqBuf, reqLen, &bytesUsed, req);
+        if (rc != IPMI_CC_OK)
+        {
+            return rc;
+        }
+    } while (bytesUsed < reqLen);
+    return IPMI_CC_OK;
+}
+
+// Convert parsed request to I2C messages.
+static ipmi_ret_t buildI2cMsgs(const i2c::ParsedReq& req,
+                               std::unique_ptr<i2c::BlockBuf> rxBuf[],
+                               struct i2c_msg msgs[],
+                               struct i2c_rdwr_ioctl_data* msgset)
+{
+    size_t minReplyLen = 0;
+
+    for (size_t i = 0; i < req.numSteps; ++msgset->nmsgs, ++i)
+    {
+        const auto& step = req.step[i];
+        auto* msg = &msgs[i];
+        msg->addr = step.devAddr;
+
+        if (!step.isRead)
+        {
+            msg->flags = 0;
+            msg->len = step.length;
+            msg->buf = const_cast<uint8_t*>(step.reqData);
+            continue;
+        }
+        rxBuf[i] = std::make_unique<i2c::BlockBuf>();
+        msg->buf = rxBuf[i]->data();
+
+        if (step.blockExtra == 0)
+        {
+            msg->flags = I2C_M_RD;
+            msg->len = step.length;
+            minReplyLen += msg->len;
+        }
+        else
+        {
+            // Special buffer setup needed for block read:
+            // . 1. msg len must allow for maximum possible transfer,
+            // . 2. blockExtra must be preloaded into buf[0]
+            // The internal i2c_transfer API is slightly different;
+            // the rdwr ioctl handler adapts by moving blockExtra
+            // into msg.len where the driver will expect to find it.
+            //
+            // References:
+            //  drivers/i2c/i2c-dev.c: i2cdev_ioctl_rdwr()
+            msg->flags = I2C_M_RD | I2C_M_RECV_LEN;
+            msg->len = maxRecvLenBuf;
+            msg->buf[0] = step.blockExtra;
+            minReplyLen += step.blockExtra;
+        }
+    }
+
+    if (minReplyLen > i2c::largestReply)
+    {
+        std::fprintf(stderr, "I2c::transfer minReplyLen=%zu?\n", minReplyLen);
+        return IPMI_CC_RESPONSE_ERROR; // Won't fit in response message
+    }
+
+#ifdef __IPMI_DEBUG__
+    for (size_t i = 0; i < req.numSteps; ++i)
+    {
+        auto* msg = &msgs[i];
+        std::fprintf(stderr, "I2c::transfer msg[%zu]: %02x %04x %d\n", i,
+                     msg->addr, msg->flags, msg->len);
+    }
+#endif
+
+    return IPMI_CC_OK;
+}
+
+static int openBus(BusId localbus)
+{
+    char busCharDev[16];
+    std::snprintf(busCharDev, sizeof(busCharDev) - 1, "/dev/i2c-%d", localbus);
+    int busFd = open(busCharDev, O_RDWR);
+    if (busFd < 0)
+    {
+        std::fprintf(stderr,
+                     "NetFn:[0x2E], OEM:[0x002B79], Cmd:[0x02], "
+                     "I2C Bus Open(\"%s\"): \"%s\"\n",
+                     busCharDev, strerror(-busFd));
+    }
+    return busFd;
+}
+
+} // namespace i2c
+
+ipmi_ret_t I2c::transfer(ipmi_cmd_t cmd, const uint8_t* reqBuf,
+                         uint8_t* replyBuf, size_t* dataLen)
+{
+    // Parse message header.
+    auto reqLen = *dataLen;
+    *dataLen = 0;
+    i2c::ParsedReq req = {};
+    auto rc = parse(reqBuf, reqLen, &req);
+    if (rc != IPMI_CC_OK)
+    {
+        return rc;
+    }
+
+    // Build full msgset
+    std::unique_ptr<i2c::BlockBuf> rxBuf[i2c::maxSteps];
+    struct i2c_msg msgs[i2c::maxSteps] = {};
+    struct i2c_rdwr_ioctl_data msgset = {
+        .msgs = msgs,
+        .nmsgs = 0,
+    };
+    rc = buildI2cMsgs(req, rxBuf, msgs, &msgset);
+    if (rc != IPMI_CC_OK)
+    {
+        return rc;
+    }
+
+    // Try to open i2c bus
+    int busFd = i2c::openBus(req.localbus);
+    if (busFd < 0)
+    {
+        return IPMI_CC_UNSPECIFIED_ERROR;
+    }
+    int ioError = ioctl(busFd, I2C_RDWR, &msgset);
+
+    // Done with busFd, so close it immediately to avoid leaking it.
+    (void)close(busFd);
+    if (ioError < 0)
+    {
+        std::fprintf(stderr, "I2c::transfer I2C_RDWR ioError=%d?\n", ioError);
+        return IPMI_CC_UNSPECIFIED_ERROR; // I2C_RDWR I/O error
+    }
+
+    // If we read any data, append it, in the order we read it.
+    uint8_t* nextReplyByte = replyBuf;
+    size_t replyLen = 0;
+    for (size_t i = 0; i < req.numSteps; ++i)
+    {
+        const auto& step = req.step[i];
+        if (step.isRead)
+        {
+            const auto* msg = &msgs[i];
+            size_t lenRead =
+                step.blockExtra ? *msg->buf + step.blockExtra : msg->len;
+            replyLen += lenRead;
+            if (replyLen > i2c::largestReply)
+            {
+                std::fprintf(stderr, "I2c::transfer[%zu] replyLen=%zu?\n", i,
+                             replyLen);
+                return IPMI_CC_RESPONSE_ERROR; // Won't fit in response message
+            }
+            std::memcpy(nextReplyByte, msg->buf, lenRead);
+            nextReplyByte += lenRead;
+        }
+    }
+    *dataLen = replyLen;
+    return IPMI_CC_OK;
+}
+
+void I2c::registerWith(Router* oemRouter)
+{
+    Handler f = [this](ipmi_cmd_t cmd, const uint8_t* reqBuf, uint8_t* replyBuf,
+                       size_t* dataLen) {
+        return transfer(cmd, reqBuf, replyBuf, dataLen);
+    };
+
+    std::fprintf(stderr, "Registering OEM:[%#08X], Cmd:[%#04X] for I2C\n",
+                 googOemNumber, Cmd::i2cCmd);
+    oemRouter->registerHandler(googOemNumber, Cmd::i2cCmd, f);
+
+    std::fprintf(stderr, "Registering OEM:[%#08X], Cmd:[%#04X] for I2C\n",
+                 obmcOemNumber, Cmd::i2cCmd);
+    oemRouter->registerHandler(obmcOemNumber, Cmd::i2cCmd, f);
+}
+
+namespace i2c
+{
+// Currently ipmid dynamically loads providers such as these;
+// this creates our singleton upon load.
+void setupGlobalOemI2c() __attribute__((constructor));
+
+void setupGlobalOemI2c()
+{
+    globalOemI2c = std::make_unique<I2c>();
+    globalOemI2c->registerWith(oem::mutableRouter());
+}
+} // namespace i2c
+} // namespace oem