Understanding Layer 4 Protocols


To appreciate the part that a content switch plays in the lifecycle of a user session, it is important to understand the component parts that make up such a session. Many protocols can be considered as Layer 4. Routing protocols such as OSPF, proprietary ones such as EIGRP, redundancy protocols such as the Virtual Router Redundancy Protocol (VRRP), and a host of others such as ICMP, IGMP, and IP itself can all be identified by a unique protocol number in the IP header (see Figure 2-5).

Figure 2-5. Different IP protocol numbers identify which Layer 4 protocol is being used.

graphics/02fig05.gif

The list of IP protocol numbers is administered and controlled by the Internet Assigned Numbers Authority (IANA), and a comprehensive list can be found at www.iana.org/. Table 2-1 lists some of the more common IP protocol numbers.

Table 2-1. Some Examples of Common IP Protocol Numbers

IP P ROTOCOL N UMBER

L AYER 4 P ROTOCOL

1

ICMP ” Internet Control Message Protocol

6

TCP ” Transmission Control Protocol

17

UDP ” User Datagram Protocol

112

VRRP ” Virtual Router Redundancy Protocol

Some Layer 4 protocols effectively operate at this layer alone. VRRP, for example, uses Layer 4 headers to transport all information between a series of participating routers in an IP subnet and consequently has no need for upper layer protocol information. Its payload is simply the information contained at Layer 4. Other routing protocols, such as the Border Gateway Protocol (BGP), will use the reliable Layer 4 Transport layer protocol with the BGP routing information and updates carried in the upper layer payloads.

In terms of content switching, the two most commonly understood Layer 4 protocols are TCP and UDP. The majority of the standard Application layer protocols are carried either within TCP or UDP depending on whether there is a requirement for a reliable end-to-end connection. Taking a Web user example, the browser application needs to ensure that all packets are successfully delivered when presenting the user with the desired Web page. The HTTP protocol will therefore rely on TCP as its Transport layer protocol, to guarantee delivery, which in turn will use IP as its delivery mechanism.



Optimizing Network Performance with Content Switching
Optimizing Network Performance with Content Switching: Server, Firewall and Cache Load Balancing
ISBN: 0131014684
EAN: 2147483647
Year: 2003
Pages: 85

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