WUMP protocols

WUMP: Windowing UDP Message Protocols

My UDP Message Protocol comes in three flavors: BUMP, HUMP, and CHUMP.
    BUMP: Basic UDP Message Protocol
    HUMP: Handoff UDP Message Protocol
    CHUMP: Cookie-handling UDP Message Protocol
As a group, these are known as WUMP: Windowing UDP Message Protocol. These are three related protocols for requesting and downloading a file from a server. HUMP and CHUMP are basically theoretical attempts to resolve obscure technical issues; the practical protocol for implementation is BUMP, and WUMP without further clarification usually refers to BUMP. All three support sliding windows, although a windowsize of 1 is supported if the client requests it.

Every packet begins with two bytes for PROTOCOL and OPCODE. PROTOCOL is 1, 2, or 3 respectively for BUMP, HUMP, or CHUMP; OPCODEs are

REQ	1
DATA	2
ACK	3
ERROR/CANCEL	4
HANDOFF	5 (HUMP only)

These will generally be defined in an include/import file.

Each protocol begins with the client sending a REQ packet to the server, at port SERVERPORT. As detailed below, the REQ packet contains both filename and window size. Check the current header/import files for the numeric value of SERVERPORT.

The server will not transfer the file from SERVERPORT; the problem is that if multiple simultaneous transfers are in progress then the server would have a considerable amount of bookkeeping to do. Instead, the server will create a child process and will "hand off" the transfer to that process, which will send the data from some new port CHILDPORT. The main difference between the three protocols here is in how this handoff is accomplished. The client never changes its port number; each separate client instance has a (hopefully) new, unique port, and so there is no conflict.

The third protocol, CHUMP, contains a "cookie" sent by the client in its initial REQ and echoed back by the server in every subsequent packet. The cookie fields don't appear at all in the other protocols. The CHUMP here is more properly CHUMP8, in that it sends an 8-byte cookie; if 4-byte cookies were desired then a new protocol CHUMP4 would be defined to support that packet layout. CHUMP16, with 16-byte cookies, would be another possibility. By not making the cookie size variable, and specified in the request packet, we ensure that DATA always begins at the same offset for a given protocol. We use COOKIESIZE below as if it were a constant, nominally 8; separate packet layouts would have to be defined for other sizes.

Packet layouts

REQ packets:

        protocol        1 byte
        opcode          1 byte
        winsize         2 bytes: size of offered receive window
            cookie      COOKIESIZE bytes, CHUMP only!
        filename        N bytes (stored with NUL below as a C string)
        NUL byte        1 byte

DATA packets will hold a 4-byte sequence number, and 512 bytes of data:
        protocol        1 byte
        opcode          1 byte
        pad                2 bytes; used for alignment only.
        blocknum        4 bytes
            cookie      COOKIESIZE bytes; CHUMP only!
        data            DATASIZE bytes, or less, up to length of packet

ACK packets are like DATA, but with no data field:
        protocol        1 byte
        opcode          1 byte
        pad                2 bytes; used for alignment only.
        blocknum      4 bytes
            cookie      COOKIESIZE bytes; CHUMP only!

ERROR packets support error and status reporting and cancellation.
        protocol        1 byte
        opcode          1 byte
        errorcode       2 bytes
        cookie      COOKIESIZE bytes; CHUMP only!

HANDOFF packets are used by the server in reporting to the client the new port
it will use. Note that a HANDOFF packet, like any other, may be lost.
        protocol        1 byte
        opcode          1 byte
        newport         2 bytes

BUMP: Basic UDP Message Protocol

Client sends REQ to SERVERPORT. Server responds from CHILDPORT with the first windowful of data. Client first learns of the new CHILDPORT when this windowful of data arrives; note that the client should be prepared to "latch on" to CHILDPORT if it sees any of the data in the first window.

      client                 server
          |                       |
          |                           |
          |    --REQ-to-SERVERPORT--> |
          |                   |
          | <-DATA-from-CHILDPORT-- |
          |                           |

PROBLEMS:

When the client sees data coming in from CHILDPORT, it has no explicit assurance that this data is actually in response to the REQ. The "handoff" is unauthenticated, so to speak.

If the client sends two REQs, then the server will start up two CHILDPORTs. Whichever is the first to get data to the client will be the port chosen for the transfer; the protocol will need a mechanism for cancelling the transfer on the other port. The simplest such protocol is for the client to send a CANCEL message to any port from which it receives data, other than the port it initially "latches on" to.

HUMP: Handoff UDP Message Protocol

Client sends REQ to SERVERPORT. Server sets up CHILDPORT, and then
sends a HANDOFF packet from SERVERPORT which contains the new CHILDPORT.
The client then sends an ACK[0] to this CHILDPORT to start the data.

The advantage is that the client receives clear instructions as to
where the data is to come from.

        client                         server
          |                            |
          | -----REQ-to-SERVERPORT--> |
          |                            |
          | <-HANDOFF-from-SERVERPORT- |
          |                            |
          | --ACK[0]-to-CHILDPORT-->   |
          |                            |
          | <--DATA-from-CHILDPORT--- |
          |                            |

PROBLEMS:

If the HANDOFF is lost, the CHILDPORT waits forever. The server will need to be sure it times out promptly. When the REQ is eventually resent, then the server will create a new child process and CHILDPORT to handle what it sees as a new request.

The HANDOFF is not to be retransmitted on timeout. If it were, then if the REQ were retransmitted we'd again be in the situation with two separate child processes on two separate CHILDPORTs sending HANDOFFs, and the client would have to accept the first and CANCEL the second as in BUMP.

Note that if the REQ is lost the server never sees anything and so never creates any child processes.

CHUMP: Cookie-Handling UDP Message Protocol

Client sends a REQ to SERVERPORT. The REQ contains a "cookie"; some long hash of, say, the filename+timestamp. The server includes this cookie in each DATA packet. We now follow the strategy of BUMP; *however* the presence of the cookie makes it clear that the incoming DATA from CHILDPORT is coming as a direct result of the REQ sent to the SERVERPORT.

PROBLEMS:

The cookie consumes bandwidth. A 32-bit cookie doesn't consume too much, although 64 bits might be safer.

If someone eavesdrops on the unencrypted cookie, malicious attacks are still possible.

Note also that the server has no way to protect the client from chaos, if the client always chooses the same cookie.

Final packets

All three protocols will use the sliding-window protocol to send data. The end of the file will be marked by a packet with number of data bytes less than 512 (the number of data bytes may be zero, but the UDP portion of the packet will not have size zero due to the header.) This strategy comes from TFTP.

Lost-final-ACK problem

Note that if the client sends the final ACK and exits, and this ACK is lost, then the server will continue to retransmit the final DATA.

The usual solution is to have the client send the final ACK and then "linger", or "dally", continuing to listen for duplicate DATA. The client at this point is only prepared to retransmit the ACK; it has already received all the data. For this reason, and because the linger period should be several timeout intervals (eg 10 seconds), lingering is best done in the background.

Old-late-duplicates problem

Suppose we transfer data via one of the above; the client uses CPORT and the server uses CHILDPORT. During the transfer, an extra DATA[2] is generated. Then, we transfer another file, and happen to choose the same ports at both ends. Choosing the same ports makes it arguably the same "connection", so we'll call it a second incarnation of the connection. During this second transfer, at the point we're expecting DATA[2], we get the old late DATA[2] from the first incarnation of the connection!

Oops.

Solutions:

We can make sure at least one endpoint chooses a new port number each time. This is another justification for the use of the server CHILDPORT; this is a port choice under the server's control.

Note that CHUMP has some additional protection here: packets with the wrong cookie are presumably from an old incarnation of the connection, and can be ignored. It is the client's responsibility to choose non-duplicate cookies, but then the client is the one who receives all the data (that is, data does not flow bidirectionally), so this isn't necessarily inappropriate.