IBM SC41-5420-04 manual Dead Gateway Processing, Negative Advice from TCP or the Data Link Layer

Figure 46. Example of ICMP Redirect

To see routing changes due to ICMP redirect messages, select NETSTAT menu 2 or NETSTAT *RTE and then press PF11. Comparing the next hop in this display with the next hop present in the routing table, you can verify whether a route has been dynamically changed.

Dead Gateway Processing

RFC-1122,Requirements For Internet Hosts - Communication Layers, requires the IP layer to include a dead gateway algorithm to manage suspected gateway failures. This section is intended to give you an overview of dead gateway processing.

Two types of gateway failures can occur:

vFailure of a first-hop gateway. A first-hop gateway is the gateway that is specified in an IP route. First-hop gateways must be on a directly-connected network. This type of failure can be detected by either TCP or the data link layer.

vFailure of a gateway other than the first-hop gateway. The path between source and destination TCP/IP hosts can traverse multiple gateways. This type of failure can be detected only by TCP.

Dead gateway processing is initiated when IP receives a negative advice indicator from either TCP or the data link layer. These indicators from TCP and the data link layer are referred to as advice since they may result from transient conditions as well as from a serious gateway failure.

Negative Advice from TCP or the Data Link Layer

Retransmissions on a TCP connection occur as a result of transient or non-transient problems somewhere along the path to a destination host. When TCP notices excessive retransmissions on a TCP connection, a TCP negative advice indicator is sent to IP.

Chapter 2. TCP/IP: Operation, Management, and Advanced Topics 55