RFC 2001 (rfc2001) - Page 2 of 6

TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms

RFC 2001                          TCP                       January 1997


   The algorithm to avoid this is called slow start.  It operates by
   observing that the rate at which new packets should be injected into
   the network is the rate at which the acknowledgments are returned by
   the other end.

   Slow start adds another window to the sender's TCP:  the congestion
   window, called "cwnd".  When a new connection is established with a
   host on another network, the congestion window is initialized to one
   segment (i.e., the segment size announced by the other end, or the
   default, typically 536 or 512).  Each time an ACK is received, the
   congestion window is increased by one segment.  The sender can
   transmit up to the minimum of the congestion window and the
   advertised window.  The congestion window is flow control imposed by
   the sender, while the advertised window is flow control imposed by
   the receiver.  The former is based on the sender's assessment of
   perceived network congestion; the latter is related to the amount of
   available buffer space at the receiver for this connection.

   The sender starts by transmitting one segment and waiting for its
   ACK.  When that ACK is received, the congestion window is incremented
   from one to two, and two segments can be sent.  When each of those
   two segments is acknowledged, the congestion window is increased to
   four.  This provides an exponential growth, although it is not
   exactly exponential because the receiver may delay its ACKs,
   typically sending one ACK for every two segments that it receives.

   At some point the capacity of the internet can be reached, and an
   intermediate router will start discarding packets.  This tells the
   sender that its congestion window has gotten too large.

   Early implementations performed slow start only if the other end was
   on a different network.  Current implementations always perform slow
   start.

2.  Congestion Avoidance

   Congestion can occur when data arrives on a big pipe (a fast LAN) and
   gets sent out a smaller pipe (a slower WAN).  Congestion can also
   occur when multiple input streams arrive at a router whose output
   capacity is less than the sum of the inputs.  Congestion avoidance is
   a way to deal with lost packets.  It is described in [2].

   The assumption of the algorithm is that packet loss caused by damage
   is very small (much less than 1%), therefore the loss of a packet
   signals congestion somewhere in the network between the source and
   destination.  There are two indications of packet loss:  a timeout
   occurring and the receipt of duplicate ACKs.




Stevens                     Standards Track