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    Understanding TFTP ALG Conversation

    By default TFTP servers listen for incoming requests from TFTP clients on port 69. A TFTP client chooses its source tunnel identifier (TID) port and sends its initial request to the server. In response, the server uses the TID chosen as the source port and sends a response to the client’s TID as the destination port. The two TIDs ports are then used for the rest of the data transfer.

    Read file conversation steps:

    1. Host A (client) sends an RRQ packet to host B (server) with A’s TID as source and port 69 as destination.
    2. Host B (server) sends a DATA packet to host A (client) with B’s TID as source and A’s TID as destination.
    3. Host A (client) sends an ACK packet to host B (server) with A’s TID as source and B’s TID as destination.
    4. DATA and ACK packets conversation continues until file data transferring is complete.

    Write file conversation steps:

    1. Host A (client) sends a WRQ packet to host B (server) with A’s TID as source and port 69 as destination.
    2. Host B (server) sends an ACK packet to host A (client) with B’s TID as source and A’s TID as destination.
    3. Host A (client) sends a DATA packet to host B (server) with A’s TID as source and B’s TID as destination.
    4. Host B (server) sends an ACK packet to host A (client) with B’s TID as source and A’s TID as destination.

    Modified: 2017-10-30