Many Internet protocols were designed 30 years ago.
The capacities, properties and prices of different components
constituting the Internet were vastly different from now. Memory was
scarce, is now abundant and cheap. Reading data from peripheral devices
was an expensive operation. With the advent of solid state disks
(re)reading parts of a file has become a rather cheap and fast
operation. Speeds of the links have skyrocketed. Ongoing trials
experiment with 100 Gb/s network interface cards. The energy footprint
of ICT and the Internet are increasingly becoming under pressure. This
spawns new infrastructure models as virtualisation and cloud computing
that promise more efficient use of resources that are located at
optimal places where environmental and cost parameters are optimal.
This development, however, also necessitates the transport of massive
amounts of data and computing from the place of the user to/from the
place of the cloud centers.
Internet protocols designed 30 years ago, such
as TCP, are still widely used and it is amazing that those still work.
However, more and more it becomes clear that in networks with 100 Gb/s
speed and 100 ms RTT packet loss and other congestion avoidance
protocols can cause a very inefficient behavior of the transport. The
current TCP protocol keeps a buffer in memory on both the sending and
receiving side to allow retransmission for lost data. The protocol
makes sure that when received data is successfully processed on
the receiving side this data is also removed from the buffer on the
sending side and makes room for new data. This so called sliding window
protocol becomes very inefficient in extreme situations with high speed
and round trip time. Lost packets cause the retransmission of data but
also block the sending side from removing old data and filling its
buffer with new data. Due to the round trip time it will take a while
before the sender is notified from success or failure. One of the
reasons for this buffering system is the inability or high cost for the
sending side protocol to re-read the data that got lost. Obviously in
interactive applications (telnet alike) asking the user to type the
last line again is a no-go. In case of file transfer one could propose
to re-read data from the source media (disk or tape). That was
considered an expensive operation as the device needs to mechanically
reposition itself what takes considerable time compared with the speeds
in the network. However, solid state disks do not have moving parts and
read operations have no seek time.
Given the low cost of (re-)reading data from
ssd's make a file transfer protocol that reads data, that is lost in
the network, again from the source media. In essence this places the
sliding window in TCP completely on the file. If we also assume or know
that we are operating inside a private lightpath or vlan environment
with dedicated capacity one could also study network behavior where the
congestion avoidance is completely absent. The senders always transmit
at maximum rate and don't care when packets get dropped in the network
under the assumption that retransmission is a low cost operation.