TCP/IP Tutorial and Technical Overview - IBM Redbooks

IP address(es) One or more IP addresses that are associated with the
virtual router.
Authentication data The authentication string is currently only used for
simple text authentication.
24.7 Round-robin DNS
Early solutions to address load balancing were often located at the point where
host names are translated into actual IP addresses: the Domain Name System
(see 12.1, “Domain Name System (DNS)” on page 426). By rotating through a
table of alternate IP addresses for a specific service, some degree of load
balancing is achieved. This approach is often called round-robin DNS. The
advantages of this approach are that it is protocol-compliant and transparent
both to the client and the destination host. Also, it is performed only once at the
start of the transaction.
Unfortunately, this approach is sometimes defeated because intermediate name
servers and client software (including some of the most popular browsers) cache
the IP address returned by the DNS service and ignore an expressly specified
time-to-live (TTL) value (see 3.1, “Internet Protocol (IP)” on page 68), particularly
if the TTL is short or zero. As a result, the balancing function provided by the
DNS is bypassed, because the client continues to use a cached IP address
instead of resolving it again. Even if a client does not cache the IP address, basic
round-robin DNS still has limitations:
► It does not provide the ability to differentiate by port.
► It has no awareness of the availability of the servers.
► It does not take into account the workload on the servers.
RFC 1794 discusses DNS support for load balancing and mentions round-robin
DNS.
24.8 Alternative solutions to load balancing
There are many vendors currently offering load balancing hardware or software
products. The techniques used vary widely and have advantages and
disadvantages.
Chapter 24. Availability, scalability, and load balancing 921