My question relates to TCP Segment-creation on the sending device.
My understanding of TCP is that it will buffer "non-urgent" bytes of traffic until either some kind of internal timeout is reached...or the MSS is reached. Then the segment is finished and transmitted onto the wire.
My question: If TCP has been buffering "normal/non-urgent" bytes, and then receives a string of "urgent" bytes from the upper-layer process will it:
- Terminate buffering of "non-urgent" bytes, send the non-urgent segment, and start creation of a new TCP segment, beginning with the "urgent" bytes...or...
- Continue building the currently-buffered, partial-segment, placing the urgent bytes somewhere in the middle of the segment after the normal bytes.
RFC 1122 (section 4.2.2.4) indicates that the Urgent Pointer points to the LAST BYTE of urgent data in a segment (inferring that non-urgent data could follow the urgent data within the same segment). It does not clarify if a segment must BEGIN with urgent data...or, if the urgent data might be "in the middle".
This question concerns a possible TCP segment with the "urgent" bit set but NOT the "push" bit. My understanding of RFC 793 is that they are mutually exclusive of each other (although typically set together).
Thanks!
My understanding of TCP is that it will buffer "non-urgent" bytes of traffic until either some kind of internal timeout is reached
If the Nagle algorithm is enabled, which it is by default. Otherwise it will just send the data immediately, subject to windowing etc.
...or the MSS is reached. Then the segment is finished and transmitted onto the wire.
Not really. It will transmit as and when it can, subject only to the Nagle algorithm, windowing, congestion control, etc.
My question: If TCP has been buffering "normal/non-urgent" bytes, and then receives a string of "urgent" bytes from the upper-layer process will it:
- Terminate buffering of "non-urgent" bytes, send the non-urgent segment, and start creation of a new TCP segment, beginning with the "urgent" bytes...or...
- Continue building the currently-buffered, partial-segment, placing the urgent bytes somewhere in the middle of the segment after the normal bytes.
Neither. See above. From the point of view of actually sending, there is nothing 'urgent' about urgent data.
RFC 1122 (section 4.2.2.4) indicates that the Urgent Pointer points to the LAST BYTE of urgent data in a segment (inferring that non-urgent data could follow the urgent data within the same segment).
Correct, except that you mean 'implying'.
It does not clarify if a segment must BEGIN with urgent data...or, if the urgent data might be "in the middle".
It doesn't require that the segment must begin with urgent data, so it needn't.
This question concerns a possible TCP segment with the "urgent" bit set but NOT the "push" bit.
Why? The PUSH bit is basically meaningless to modern TCP implementations, and ignored, and there is no way you can detect segment boundaries, so why do you care?
My understanding of RFC 793 is that they are mutually exclusive of each other (although typically set together).
Why? Please explain.
