What is an inv message?

Question

I was reviewing the PR #25720. This is part of description for the PR:

However, if an INV for a block is received before our headers chain is caught up, we'll then start to sync headers from each peer announcing the block. This can result in doing a big headers sync with many (if not all) of our peers simultaneously, which wastes bandwidth.

What is an INV message? I tried to figure it out by searching it but found nothing useful. This link had a small information about it but it is still not clear for me what it is.

Answer 1

inv is short for inventory. As the Bitcoin wiki says:

Inventory vectors are used for notifying other nodes about objects they have or data which is being requested.

Inventory vectors consist of the following data format:

The inv message (for transactions) was explained in the notes of this PR review club in Feb 2021 hosted by Elle Mouton.

Transaction relay is a three step process: inv -> getdata -> tx:

1. the relaying node sends an inv message to the receiving node to announce a new transaction.
2. if the receiving node wants to retrieve the transaction that it learned about in the inv message then it will send a getdata message to the relaying node to request the full transaction. (The receiving node won't send a getdata message to a peer for transactions that it has already seen or if it has already sent a getdata message for the transaction to a different peer.)
3. the relaying node delivers a tx message to the receiving node. If the relaying node is no longer able to deliver the transaction, it responds with notfound instead of the tx.

Answer 2

Michael's answer was very helpful for me, but I think it is also helpful to take a look at the source code and implementation of inv messages while the question is tagged bitcoin-core-development.

This is the source code for inv messages. Link:

/** inv message data */
class CInv
{
public:
    CInv();
    CInv(uint32_t typeIn, const uint256& hashIn);

    SERIALIZE_METHODS(CInv, obj) { READWRITE(obj.type, obj.hash); }

    friend bool operator<(const CInv& a, const CInv& b);

    std::string GetCommand() const;
    std::string ToString() const;

    // Single-message helper methods
    bool IsMsgTx() const { return type == MSG_TX; }
    bool IsMsgBlk() const { return type == MSG_BLOCK; }
    bool IsMsgWtx() const { return type == MSG_WTX; }
    bool IsMsgFilteredBlk() const { return type == MSG_FILTERED_BLOCK; }
    bool IsMsgCmpctBlk() const { return type == MSG_CMPCT_BLOCK; }
    bool IsMsgWitnessBlk() const { return type == MSG_WITNESS_BLOCK; }

    // Combined-message helper methods
    bool IsGenTxMsg() const
    {
        return type == MSG_TX || type == MSG_WTX || type == MSG_WITNESS_TX;
    }
    bool IsGenBlkMsg() const
    {
        return type == MSG_BLOCK || type == MSG_FILTERED_BLOCK || type == MSG_CMPCT_BLOCK || type == MSG_WITNESS_BLOCK;
    }

    uint32_t type;
    uint256 hash;
};