1.1.c Explain general network challenges

1.1.c [ii] Out of order packets

1.1.c [iv] Impact of micro burst

Cause Unnecessary Retransmission

Limits Transmission Speed

Reduce Receiver’s Efficiency

http://en.wikipedia.org/wiki/Micro-bursting_%28networking%29

In computer networking, micro-bursting is a behavior seen on fast packet-switched networks, where rapid bursts of data packets are sent in quick succession, leading to periods of full line-rate transmission that can overflow packet buffers of the network stack, both in network endpoints and routers and switches inside the network. It can be mitigated by the network scheduler. In particular, micro-bursting is often caused by the use of the TCP protocol on such a network.[

A Study of Internet Packet Reordering
Yi Wang1, Guohan Lu2, Xing Li3

Abstract. Packet reordering is a well-known phenomenon that the order of
packets is inverted in the Internet. Previous research indicates reordering can
affect the performance of both the network and the packets receiver.

1 Introduction
In the architecture of TCP/IP, the IP layer provides a “best effort” datagram service.
Although TCP is a reliable higher-layer protocol, packet reordering can affect its
performance and the efficiency of packet receiver:
(1) Causes Unnecessary Retransmission: When the TCP receiver gets packets out
of order, it sends duplicate ACKs to trigger fast retransmit algorithm at the sender.
These ACKs makes the TCP sender infer a packet has been lost and retransmit it. If
the temporary sequence number gap is caused by reordering, then the duplicate ACKs
and the fast retransmission are unnecessary and a waste of bandwidth.
(2) Limits Transmission Speed: When fast retransmission is triggered by duplicate
ACKs, the TCP sender assumes it is an indication of network congestion. It reduces
its congestion window (cwnd) to limit the transmission speed, which needs to grow
larger from a “slow start” again. If reordering happens frequently, the congestion
window is at a small size and can hardly grow larger. As a result, the TCP connection
has to transmit packets at a limited speed and can not efficiently utilize the bandwidth.
(3) Reduce Receiver’s Efficiency: TCP receiver has to hand in data to the upper
layer in order. When reordering happens, TCP has to buffer all the out-of-order pack-
ets until getting all packets in order. Meanwhile, the upper layer gets data in burst
rather than smoothly, which also reduce the system efficiency as a whole.