The algorithm suggested in RFC-793 for calculating the retransmission timeout is now known to be inadequate; see Section 220.127.116.11 below.
Recent work by Jacobson [TCP:7] on Internet congestion and TCP retransmission stability has produced a transmission algorithm combining "slow start" with "congestion avoidance". A TCP MUST implement this algorithm.
If a retransmitted packet is identical to the original packet (which implies not only that the data boundaries have not changed, but also that the window and acknowledgment fields of the header have not changed), then the same IP Identification field MAY be used (see Section 18.104.22.168).
Some TCP implementors have chosen to "packetize" the data stream, i.e., to pick segment boundaries when segments are originally sent and to queue these segments in a "retransmission queue" until they are acknowledged. Another design (which may be simpler) is to defer packetizing until each time data is transmitted or retransmitted, so there will be no segment retransmission queue.
In an implementation with a segment retransmission queue, TCP performance may be enhanced by repacketizing the segments awaiting acknowledgment when the first retransmission timeout occurs. That is, the outstanding segments that fitted would be combined into one maximum-sized segment, with a new IP Identification value. The TCP would then retain this combined segment in the retransmit queue until it was acknowledged. However, if the first two segments in the retransmission queue totalled more than one maximum- sized segment, the TCP would retransmit only the first segment using the original IP Identification field.