UDP Broadcast Flooding

剑心通明

A broadcast is a data packet that is destined for multiple hosts. Broadcasts can occur at the data link layer and the network layer. Data-link broadcasts are sent to all hosts attached to a p
articular physical network. Network layer broadcasts are sent to all hosts attached to a particular logical network. The Transmission Control Protocol/Internet Protocol (TCP/IP) supports the following types of broadcast packets:

• All ones—By setting the broadcast address to all ones (255.255.255.255), all hosts on the network receive the broadcast.
  
• Network—By setting the broadcast address to a specific network number in the network portion of the IP address and setting all ones in the host portion of the broadcast address, all hosts on the specified network receive the broadcast. For example, when a broadcast packet is sent with the broadcast address of 131.108.255.255, all hosts on network number 131.108 receive the broadcast.
  
• Subnet—By setting the broadcast address to a specific network number and a specific subnet number, all hosts on the specified subnet receive the broadcast. For example, when a broadcast packet is set with the broadcast address of 131.108.4.255, all hosts on subnet 4 of network 131.108 receive the broadcast.
  

Because broadcasts are recognized by all hosts, a significant goal of router configuration is to control unnecessary proliferation of broadcast packets. Cisco routers support two kinds of broadcasts: directed and flooded. A directed broadcast is a packet sent to a specific network or series of networks, whereas a flooded broadcast is a packet sent to every network. In IP internetworks, most broadcasts take the form of User Datagram Protocol (UDP) broadcasts.
Although current IP implementations use a broadcast address of all ones, the first IP implementations used a broadcast address of all zeros. Many of the early implementations do not recognize broadcast addresses of all ones and fail to respond to the broadcast correctly. Other early implementations forward broadcasts of all ones, which causes a serious network overload known as a broadcast storm. Implementations that exhibit these problems include systems based on versions of BSD UNIX prior to Version 4.3.
In the brokerage community, applications use UDP broadcasts to transport market data to the desktops of traders on the trading floor. This case study gives examples of how brokerages have implemented both directed and flooding broadcast schemes in an environment that consists of Cisco routers and Sun workstations.
Figure 19-1
illustrates a typical topology. Note that the addresses in this network use a 10-bit netmask of 255.255.255.192.
Figure 19-1: Topology that requires UDP broadcast forwarding.

In
Figure 19-1
, UDP broadcasts must be forwarded from a source segment (Feed network) to many destination segments that are connected redundantly. Financial market data, provided, for example, by Reuters, enters the network through the Sun workstations connected to the Feed network and is disseminated to the TIC servers. The TIC servers are Sun workstations running Teknekron Information Cluster software. The Sun workstations on the trader networks subscribe to the TIC servers for the delivery of certain market data, which the TIC servers deliver by means of UDP broadcasts. The two routers in this network provide redundancy so that if one router becomes unavailable, the other router can assume the load of the failed router without intervention from an operator. The connection between each router and the Feed network is for network administration purposes only and does not carry user traffic.
Two different approaches can be used to configure Cisco routers for forwarding UDP broadcast traffic: IP helper addressing and UDP flooding. This case study analyzes the advantages and disadvantages of each approach.
Note Regardless of whether you implement IP helper addressing or UDP flooding, you must use the ip forward-protocol udp global configuration command to enable the UDP forwarding. By default, the ip forward-protocol udp command enables forwarding for ports associated with the following protocols: Trivial File Transfer Protocol, Domain Name System, Time service, NetBIOS Name Server, NetBIOS Datagram Server, Boot Protocol, and Terminal Access Controller Access Control System. To enable forwarding for other ports, you must specify them as arguments to the ip forward-protocol udp command.

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