|
Another plus for Rendezvous was its ability to dynamically replace application components, such as DLLs (Dynamic Link Libraries), ActiveX processes and Java classes. Only Castanet did as well with the updating of "live" software modules.
Barry Nance, a computer analyst and consultant for 25 years, is the author of Introduction to Networking, 4th Edition (Que, 1997), Using OS/2 Warp (Que, 1994), and Client/Server LAN Programming (Que, 1994). He can reached at barryn@bix.com.
|
 |
The Current State Of IP Multicast
A multicast is a single data stream intended only for network nodes that have joined the appropriate multica
st group. The sender sends only a single data stream that all clients receive, thus reducing bandwidth usage dramatically. You would think from this description that the combination of push middleware and IP Multicast sounds like a marriage made in heaven. Unfortunately, the behavior of some routers, switches and firewalls, as well as the lack of a protocol for interdomain multicast routing (IDMR), may signal difficult early years in the marriage.
A multicast-enabled router forwards a multicast to a particular network only if that network has multicast receivers. When the last multicast receiver node on a network segment leaves a multicast group, the router prunes the multicast data stream for that group by no longer forwarding multicast data to nodes on that segment. As a result, network segments with no multicast receivers can avoid having to carry unnecessary multicast traffic. When routers and switches behave properly, according to the specification, multicasting is a boon to networks--members of multi
cast groups get efficient transmission of data, while nonmembers can use the network without fear of traffic jams and snarls.
Most network hardware products (adapters, routers and switches), as well as current TCP/IP protocol stacks, claim to support IP Multicast. However, the support is minimal, incorrect or not well-tested, in some cases. For example, a recent version of Digital Equipment Corp.'s GigaSwitch product turned multicast traffic into broadcast traffic. Granted, broadcasting at least lets the information flow to its destination. But why enhance applications that formerly broadcast data to use IP Multicast instead, if network touch points don't properly handle the messages?
Comprehensive testing of IP Multicast hasn't yet happened because most companies haven't yet facilitated the multicast feature on their networks. And many have not updated their network infrastructure with firmware versions that are multicast-compliant. One of the reasons for the omission is the lack of applications that c
an appropriately take advantage of multicasting. Forget push middleware for a moment and think of the predominant network applications--Web, e-mail, database and file access software products simply do not need multicast technology.
Because IP Multicast messages are based on the connectionless User Datagram Protocol (UDP), firewalls are another obstacle for IP Multicast to overcome. Multicast uses UDP, which doesn't guarantee delivery and thus doesn't acknowledge each transmission, to avoid crushing the network (not to mention the sender) with "Yes, I received it" responses from all the multicast receivers. The most commonly used type of firewall, the application gateway, cannot secure connectionless protocols and, unhappily for multicast, most organizations configure their firewalls to drop such messages. The alternative of establishing generic service passes for all packets through certain ports has the nasty side effect of defeating much of the security at that firewall.
Last but not least, the lack
of a protocol for IDMR has left a bad taste in the mouths of the companies that supply Internet access or carry Internet traffic. The most popular multicast routing protocols, PIM (Protocol-Independent Multicast), MOSPF (Multicast Open Shortest Path First) and DVMRP (Distance Vector Multicast Routing Protocol), do not allow for multiple autonomous networks that may not want to exchange all their routing information. PIM, MOSPF and DVMRP cannot limit the propagation of routing information based on policies and rules that administrators might want to use. They blindly forward all routing information to all known routers. And though the BGP (Border Gateway Protocol) does indeed give IP interdomain routing, BGP doesn't work for IP Multicast.
|
|
RSS
