TCP enhancements

In this writeup, we summarize the extensions made to TCP (relative to what I teach in the Internet architecture/protocols course). The list includes: 1. Larger window sizes accommodated through a window scale option is proposed for LFNs (Networks with Long Fat Pipes) [1], which are networks that have large values of the delay bandwidth product (DBP). TCP performance depends upon this product. The example cited is satellite networks in which round-trip times are at least 558 ms [8]. In optical networks, the bandwidth is high; therefore even if propagation delay is not that high, the total DBP can be high. Therefore much of the extensions made to TCP for satellite networks is applicable to optical networks. Reference [1] additionally proposes a RTTM (Round Trip Time Measurement) option and a PAWS (Protect Against Wrapped Sequences) for LFNs. 2. Even with fast retransmit/recovery, if multiple packets are dropped within one window, the system will go into Slow Start [1]. This is explained in [10]. In the absence of SACKs, it says that when multiple packets are lost in one window, then the key difference between retransmits that occur after an RTO vs. after a triple duplicate kicks in. After an RTO, all packets are retransmitted following the one that was lost. Whereas after a TD loss detection, only the lost packet is retransmitted. The fast recovery scheme increases cwnd by three because it assumes that three packets were successfully received, which led to the three duplicate ACKs. This is called " inflating " the window. After retransmitting the lost packet, for every duplicate ACK, the cwnd is " inflated " by 1 on the assumption that the duplicate ACK was generated every time a new packet was successfully received. Now, if multiple packets were lost in the window, this will not be recognized at the sender until it receives the ACK for the retransmitted packet. When this arrives, it will see that the ACK is not for all packets sent subsequent to the lost packet; instead it asks for some other packet. I assume by the time this happens the RTO for the lost packet will expire causing a drop of cwnd to 1 and Slow Start recovery. Reference [10] calls this a " partial ACK, " and proposes a modification to Fast Recovery that prevents this dropping off to Slow Start recovery. This is called NewReno. It is an experimental RFC-not …