[Ns-bugs] [Bug 579] TCP congestion window is not updated whent segment size chages

[code@nsnam.ece.gatech.edu]

http://www.nsnam.org/bugzilla/show_bug.cgi?id=579


Craig Dowell <craigdo at ee.washington.edu> changed:

           What    |Removed                     |Added
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                 CC|                            |craigdo at ee.washington.edu




--- Comment #3 from Craig Dowell <craigdo at ee.washington.edu>  2009-06-29 16:26:47 EDT ---
I took a look at this and it is a bit more complicated, I think.

There are two Attribute setters that interact when setting a third (initial)
congestion window variable.  We have to worry about consistency of: m_cWnd,
m_initialCWnd and m_segmentSize.

The only time that the segment size can be changed is from (socket) object
creation up to the SYN of a connection which can provide an MSS option.  TCP
specs say no, can't do it afterward.  Ns-3 TCP seems to ignore the MSS in the
SYN, so as it stands we just have to deal with the static-initialization-by
attribute problem. 

To make this work, to a first approximation all we have to do is to add:

    m_cWnd = m_initialCWnd * m_segmentSize;

to the Attribute setters for both "InitialCwnd" and "SegmentSize."  Then
setting either attribute will set the initial m_cWnd correctly.

However, the TCP protocol is in charge of changing m_cWnd and it isn't going to
expect some other entity to mess with it, so I think we must not allow changes
to m_segmentSize or m_initialCWnd after a connection leaves the CLOSED state
unless TCP does it (modifies m_Cwnd according to the protocol, or sets
m_segmentSize according to the MSS in a SYN).

So I think the right thing to do is to also change the attribute setters to
restrict the times (states) during which they can be used to preclude having a
user shoot him or herself in the foot:

void
TcpSocketImpl::SetSegSize (uint32_t size)
{
  m_segmentSize = size;
  NS_ABORT_MSG_UNLESS (m_state == CLOSED, 
    "TcpSocketImpl::SetSegSize(): Cannot change segment size dynamically.");
  m_cWnd = m_initialCWnd * m_segmentSize;
}

and 

void
TcpSocketImpl::SetInitialCwnd (uint32_t cwnd)
{
  m_initialCWnd = cwnd;
  NS_ABORT_MSG_UNLESS (m_state == CLOSED, 
    "TcpSocketImpl::SetInitialCwnd(): Cannot change initial cwnd
dynamically.");
  m_cWnd = m_initialCWnd * m_segmentSize;
}

If we ever teach ns-3 TCP to repect an initial MSS, it will need to do:

  m_segmentSize = segmentSizeFromSynOption;
  m_cWnd = m_initialCWnd * m_segmentSize;

outside of the Attribute setters.  I think this covers all of the bases.

Does this sound right?  










As it stands now, you can set m_segmentSize to anything you want at any time
you want.  


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