[ns] Under what condition does the TCP/Tahoe sender go into slow start?

["Jingyi" <eehjy@ust.hk>]

Hi,

Usually a TCP/Tahoe sender goes into slow start after receiving
3 dup ACKs or a RTO. But are there any additional requirements?
I met a situation which I don't understand. This is a case of
multiple packet losses:

Packets lost: 28 30 32 34 36 38
Actions at the sender:
    ---------------------------------------------------
            recv            |        send (enque)
    ---------------------------------------------------
            ....            |        ....
            ACK 26          |        46
            ACK 27          |        47
            ACK 27          |                
            ACK 27          |
        *   ACK 27          |        28 (retx)
            ACK 27          |
            ....            |        ....
            ACK 27          |
            ACK 29          |        30,31 (retx)
            ACK 31          |        32,33,34 (retx)
            ACK 31          |
            ....            |        ....
            ACK 37          |        41,42,43 (retx)
            ACK 37          |
            ACK 47          |        48,49,50,51,52,53,54
            ACK 47          |
            ACK 47          |
        #   ACK 47          |
            ....            |        ....
            ACK 47          |
            ACK 48          |        55,56
            ....            |        ....

The first ACK 47 is generated by the receiver after it receives 
the retransmitted packet 38. Following retransmissions (up to 
packet 43) will generate the following dup ACK 47s.
Regarding the line with "#", which is the 3rd dup ACK, why 
doesn't the sender retransmit packet 48 (i.e., go into slow 
start)? 

Thanks a lot!

Jerry