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Router solicitation message / Congestion and flow control messages

Router solicitation message 
8.2.7 This page will explain why router solicitation messages are used.
A host generates an ICMP router solicitation message in response to a missing default gateway.This message is sent using multicast and it is the first step in the router discovery process. A local router will respond with a router advertisement that identifies the default gateway for the local host. Figure identifies the frame format and Figure gives an explanation of each field.
The next page will discuss source quench messages.Congestion and flow control messages
8.2.8 This page will explain how source quench messages are used to solve problems related to network congestion.
If multiple computers try to access the same destination at the same time, the destination computer can be overwhelmed with traffic. Congestion can also occur when traffic from a high speed LAN reaches a slower WAN connection. Dropped packets occur when there is too much congestion on a network. ICMP source quench messages are used to reduce the amount of data lost. The source quench message asks senders to reduce the rate at which they transmit packets. Congestion will usually subside after a short period of time and the source will slowly increase the transmission rate if no other source quench messages are received. Most Cisco routers do not send source quench messages by default, because the source quench message may add to the network congestion.
A small office, home office (SOHO) is a scenario where ICMP source quench messages might be used effectively. A SOHO could consist of four computers that are networked with CAT-5 cable and have a shared Internet connection over a 56K modem. The 10-Mbps bandwidth of the SOHO LAN could quickly overwhelm the 56K bandwidth of the WAN link, which would result in data loss and retransmissions. The gateway host can use an ICMP source quench message to request that the other hosts reduce their transmission rates to prevent continued data loss. A network where congestion on the WAN link could cause communication problems is shown in Figure .
This page concludes this lesson. The next page will summarize the main points from this module.

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