Define behavior when receiving TCP RST packet

[tleyden]

When receiving a TCP packet with the RST flag set, Couchbase Lite is currently emitting an Unparseable HTTP response from CBLWebSocket.mm

Here is an example packet from Sync Gateway (with a firewall / load balancer in the middle):

Transmission Control Protocol, Src Port: 4984, Dst Port: 57878, Seq: 1999, Len: 0
    Source Port: 4984
    Destination Port: 57878
    [Stream index: 0]
    [TCP Segment Len: 0]
    Sequence number: 1999    (relative sequence number)
    [Next sequence number: 1999    (relative sequence number)]
    Acknowledgment number: 0
    0101 .... = Header Length: 20 bytes (5)
    Flags: 0x004 (RST)
        000. .... .... = Reserved: Not set
        ...0 .... .... = Nonce: Not set
        .... 0... .... = Congestion Window Reduced (CWR): Not set
        .... .0.. .... = ECN-Echo: Not set
        .... ..0. .... = Urgent: Not set
        .... ...0 .... = Acknowledgment: Not set
        .... .... 0... = Push: Not set
        .... .... .1.. = Reset: Set
        .... .... ..0. = Syn: Not set
        .... .... ...0 = Fin: Not set
        [TCP Flags: ·········R··]
    Window size value: 0
    [Calculated window size: 0]
    [Window size scaling factor: 128]
    Checksum: 0xab8c [unverified]
    [Checksum Status: Unverified]
    Urgent pointer: 0
    [Timestamps]

What's the expected behavior in this case? Should it retry the connection in a backoff/retry style?

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• Version: CBL 2.0.1
• Client OS: iOS 11
• Server: SG 2.0.0

[pasin]

I think it should covert that (zero bytes with EOF) to a transient error. I will need to check what the error should be.

[snej]

RST just means the TCP connection is being aborted, likely by middleware. We should definitely retry. It's unfortunate that CFNetwork isn't treating this as an error.

[tleyden]

Related to #2140

(adding a link so that following the link to ticket #2140 in the comment might lead to this issue, and provide some more context on the change)