Network Protocols

The Internet is comprised of a number of heterogeneous networks communicating within a shared framework. Although many frameworks have been proposed and implemented, the Transmission Control Protocol (TCP) and IP (TCP/IP) protocol suite [26] dominate today’s computer networks. The ability to accommodate arbitrary network systems and topologies is derived from the fact that the TCP/IP suite provides no specification regard- ing hardware layers. Concerned only with transporting data, the TCP/IP suite begins at layer 3 of the OSI model. Today the bulk of traffic found on the Internet is comprised of the layer 3 IP protocol and two layer 4 transport protocols [27] [28] [29] (TCP and User Datagram Protocol (UDP)). At the base, the IP protocol provides a data-orientated connectionless means of routing packets from one network to another. As the Internet has developed a number of changes to the IP protocol have been required, with IPv6 currently

1_{Although the traditional Transmission Control Protocol (TCP)/IP model does not utilise a strict layered}

separation such as the OSI model, the OSI model is used as a conceptual framework for the remainder of this thesis

being rolled out as a replacement to the existing IPv4 platform. At a router level, the major aspect of the change (aside from the new IP header) is the massive expansion in the number of routable addresses provided by the IP specification. Whereas IPv4 utilises a 32-bit address scheme, the newer IPv6 protocol uses 128-bit source and destination ad- dresses. Because the IP protocol does not guarantee delivery and cannot be used to mul- tiplex data from multiple network applications it is common for a higher level transport protocol to be encapsulated within an IP packet. For applications requiring a session- based connection, the TCP protocol can guarantee reliable transmission, creates unique application-based sessions and provides mechanisms for congestion avoidance. For sys- tems requiring multiplexing but without the overhead associated with a state-maintained connection like TCP, the UDP protocol can be used to encapsulate data from the appli- cation layer within an IP packet. In addition to these three protocols a number of other protocols are used within the TCP/IP stack, providing functionality such as routing table updates and secure communication.

At a router level, the dominance of IP, TCP and UDP present an advantage by limiting the complexity of the network traffic. Applications such as session-based firewalling, re- quiring 5-tuple classification (identification using Source Address, Destination Address, Source Port, Destination Port and Protocol), can be efficiently implemented using the knowledge that TCP-based data represents over 80% of the traffic [28] found on the In- ternet. At a purely engineering level, protocol improvements such as IPv6 present a chal- lenge in two ways. Firstly, routers must be able to support both protocol versions while legacy systems are replaced. Secondly, packet processing techniques such as packet for- warding and classification are optimised for 32-bit address and 32-bit architectures and may not be directly compatible as the number of unique addresses is expanded.

2.2.2 Network Technologies

Lacking any specification of the hardware or link layer details of a network, the TCP/IP suite allows arbitrary communication technologies to be incorporated within the existing

framework. In Table 2.1 some of the communication standards currently in operation are summarised. For local networks, communication can be achieved using both a wired and wireless solution. Wired network technologies such as Ethernet 100 Mbps (100BASE-T) and the state of the art Gigabit Ethernet provide a cheap method of transferring data over short distances. For greater flexibility, wireless equivalents such as IEEE 802.11x can be implemented. For bulk data transfer, optical technologies such as SONET/SDH [30][31] allow network providers to transfer large quantities of data over optical fiber.

Table 2.1: Communication Technologies Technology Standard Bit Rate (Mbps)

Wireless IEEE 802.11b 11 IEEE 802.11g 54 IEEE 802.11n 600 Ethernet 10BASE-T 10 100BASE-T 100 1000BASE-T 1000 10000BASE-T 10000 SONET/SDH OC-1 51.84 OC-3 155.52 OC-12 622.32 OC-48 2,488 OC-192 9,953 OC-768 39,813

In Figure 2.1, we use the network layering methodology outlined in [32] to describe a sample WAN. At the access layer, residential and corporate networks use technologies such as Ethernet, Wireless LAN and PPPoE to connect users. To connect various access networks, the edge or distribution layer allows fast connection between regional networks operating inside the WAN. From a router perspective, access layer data is multiplexed before being transferred over high speed Gigabit Ethernet and OC-3/OC-12 data links. Finally, connection between distribution networks is provided via a core switching net- work. In this example, the core switching network might represent the network used to connect large metropolitan cities. As in the distribution layer, data is again multiplexed

Figure 2.1: Network Layer Topology [32]

before being transferred over high speed links such as 10 Gigabit Ethernet or OC-48/OC- 192 connections.

Although a network is divided into three layers in the example outlined above, the separation of layers is not definite and can be altered to accommodate even higher level connections. For example, the TAT-14 OC-768 [33] optical link between the United States of America and Europe provides a means of connecting two large WANs.

Again, from a router design perspective this layer model allows routers to be designed which target specific network topologies. The core network provides maximal switching speed with minimal packet processing. Services provided within the core network are primarily data plane functions such as forwarding, while for routers located within the distribution network, the services provided can be argued to be both data-plane functions such as packet switching, as well as control plane tasks such as congestion avoidance, fire- wall security and load balancing (e.g. domain name resolution). Finally, routers located within the access network provide services such as web switching, metering, detection

Figure 2.2: Router Line Card [34]

and prevention of network intrusion. The difficulty with such a layered model is that the topology of the Internet is not static. Services such as QoS require routers to be able to differentiate between certain types of traffic at both the access and distribution layer.