Re: TCP Urgent Pointer Usage

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Dave,

In my test app I am not setting the SO_OOBINLINE socket option. Because of
this, the byte indicated by the urgent pointer field in the packet is
removed. In the app, I am reading the network stream using the class
NetworkStream (System.IO.Stream.NetworkStream) and the blocking read method.
Using this class, how would I read this removed byte of data or be notified
that an urgent byte of data is available?

As a follow-up, after reading the article you referenced, I am left
wondering whether the urgent flag is widely used. It appears that it would
not be. Is this an accurate statement?

Thanks again,
Nate



"Dave Sexton" wrote:

Hi N. Spiker,

From MSDN for OOB data:
Arrival of a TCP segment with the URG (for urgent) flag set indicates the
existence of a single byte of OOB data within the TCP data stream. The "OOB
data block is one byte in size. The urgent pointer is a positive offset from
the current sequence number in the TCP header that indicates the location of
the "OOB data block (ambiguously, as noted in the preceding). It might,
therefore, point to data that has not yet been received.

If SO_OOBINLINE is disabled (the default) when the TCP segment containing
the byte pointed to by the urgent pointer arrives, the OOB data block (one
byte) is removed from the data stream and buffered. If a subsequent TCP
segment arrives with the urgent flag set (and a new urgent pointer), the OOB
byte currently queued can be lost as it is replaced by the new OOB data
block (as occurs in Berkeley Software Distribution). It is never replaced in
the data stream, however.

With SO_OOBINLINE enabled, the urgent data remains in the data stream. As a
result, the OOB data block is never lost when a new TCP segment arrives
containing urgent data. The existing OOB data "mark" is updated to the new
position.

Link to article that contains info about OOB and non-OOB data flaged as
urgent; watch for wrapping:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winsock/winsock/protocol_independent_out_of_band_data_2.asp

HTH

"N. Spiker" <N. Spiker@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:DF39B93C-8E9F-430A-A41A-EC04D2D39042@xxxxxxxxxxxxxxxx
I am attempting to receive a single TCP packet with some text ending with
carriage return and line feed characters. When the text is send and the
packet has the urgent flag set, the text read from the socket is missing
the
last character (line feed). When the same text is sent without the urgent
flag set, all of the characters are read.

I'm reading the data using the blocking read call of the network stream
class. The .NET documentation does not discuss a special method needed to
support reading urgent data from a network stream.

I verified that in both cases the data is arriving correctly using a
packet
sniffer (Ethereal). I also verified that both receiving and sending
parties
are interpreting the urgent pointer according to the BSD implementation.

Is the urgent flag supported?



.

Relevant Pages

  • Re: TCP Urgent Pointer Usage
    ... Arrival of a TCP segment with the URG (for urgent) flag set indicates the ... existence of a single byte of OOB data within the TCP data stream. ...
    (microsoft.public.dotnet.general)
  • Re: TCP Urgent Pointer Usage
    ... Try reading the out-of-band data using the managed Socket class by passing ... wondering whether the urgent flag is widely used. ... although OOB data could be used as well without the BsdUrgent flag. ...
    (microsoft.public.dotnet.general)
  • Re: commit 64ff3b938ec6782e6585a83d5459b98b0c3f6eb8 breaks rlogin
    ... Our TCP stack does not set the urgent flag if the urgent pointer ... Our current behaviour may in fact delay the urgent notification ...
    (Linux-Kernel)
  • Re: commit 64ff3b938ec6782e6585a83d5459b98b0c3f6eb8 breaks rlogin
    ... Our TCP stack does not set the urgent flag if the urgent pointer ... Our current behaviour may in fact delay the urgent notification ...
    (Linux-Kernel)
  • Re: TCP out of band data and read()
    ... TCP has no mechanism to transport ... but the sockets API automatically treats the byte that the urgent ... Which is what the sockets API does. ... what happens to the byte that the urgent pointer ...
    (comp.unix.programmer)