**A New Model for TCP Packet Loss.**

__Daniel Genin__ and Kevin Mills

ITL/ANTD, NIST Gaithersburg, MD 20899.

TCP is the dominant protocol
used in today's Internet to reliably transmit data between end-systems. Studies
have shown that TCP packets account for over 80% of all Internet traffic by
volume. The current version of TCP attempts to minimize congestion in and
maximize utilization of a network by reducing transmission speed when packets
are lost (due to buffer overflow) and increasing transmission speed when
packets are delivered successfully. Because this *congestion control*
mechanism is executed independently by all end-systems the resulting collective
behavior is very hard to predict even for simple networks, let alone for the
entire Internet with its thousands of routers and millions of end-systems.
Nevertheless, stable and efficient operation of the Internet under increasing
user and application demands depends upon thorough understanding of the network
traffic dynamics.

Clearly, what is really needed are faithful and simultaneously computationally tractable mathematical models. Simulation, while permitting limited exploration of the network dynamics, cannot reveal the relationships between network parameters and quantities of interest such as throughput, packet loss, etc. In spite of the great efforts devoted to modeling network dynamics under TCP congestion control in the last 15 years the results are still very meager. For instance, there is still no mathematical model relating throughput of a TCP connection and such fundamental network parameters as bandwidth, delay, and buffer size. This unsatisfactory state of affairs is an indication of the magnitude and difficulty of the problem.

The bulk of research on the
topic has focused on *fluid approximation* models. Fluid approximation
models are continuous deterministic limits of discrete stochastic packet level
models, applicable when the number of packets sent in a round trip (and hence
the bandwidth-delay product) is large. A major stumbling point for the fluid
approximation models is the stochastic nature of packet loss. While packet loss
is sensitive to higher moments of the inter-arrival time distribution these
statistics are not captured by the simplistic fluid approximation models.
Consequently, since throughput of a TCP connection is essentially determined by
packet loss the accuracy of equilibrium prediction of the resulting models is
mediocre at best and their power of prediction of network dynamics is
non-existent.

Our project focuses on understanding the relationship between packet loss and the fundamental network parameters -- bandwidth, delay and buffer size. By studying the statistics of the packet arrival process we derive a model relating packet loss to these parameters. In the future, our packet loss model should lead to more accurate models of TCP network dynamics by improving the accuracy of existing fluid approximation models.

**CATEGORY:
Mathematics**

**Mentor****'s
Name: Kevin Mills**

**Email: kmills at
nist dot gov**

**Address: Bldg.
222, Rm. 218, MS8920**

**Division: ANTD,
Lab: ITL**

**Tel:(301) 975-3618**

**Author's Name:
Daniel Genin**

**Email: dgenin at
nist dot gov**

**Division: ANTD,
Lab: ITL**

**Address: Bldg.
222, Rm. 214, MS 8920**

**Tel:(301) 975-6353**

**Fax:(301) 975-6238**

**Is your mentor a Sigma Xi Member? No**