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IPQoS Administration Guide

Packet Forwarding in a Diffserv Environment

The following figure shows part of an intranet at a company with a partially diffserv-enabled environment. In this scenario, all hosts on networks 10.10.0.0 and 10.14.0.0 are IPQoS-enabled, and the local routers on both networks are diffserv-aware. However, the interim networks are not configured for diffserv.

Figure 1–2 Packet Forwarding Across Diffserv-Aware Network Hops

The context follows the graphic, which is a flow diagram

The next steps trace the flow of the packet that is shown in the previous figure. The steps begin with the progress of a packet that originates at host ipqos1 and continues through several hops to ipqos2.

  1. The user on ipqos1 runs the ftp command to access host ipqos2, three hops away.

  2. ipqos-1 applies its QoS policy to the resulting packet flow and successfully classifies the ftp traffic.

    The system administrator has created a class for all outgoing ftp traffic that originates on the local network 10.10.0.0. Traffic for the ftp class is assigned the AF22 per-hop behavior: class two, medium-drop precedence. A traffic flow rate of 2Mbits per second is configured for the ftp class.

  3. ipqos-1 meters the ftp flow to determine if it exceeds the committed rate of 2 Mbits per second.

  4. The marker on ipqos1 marks the DS fields in the outgoing ftp packets with the 010100 DSCP, corresponding to the AF22 PHB.

  5. The router diffrouter1 receives the ftp packets and checks the DSCP. If diffrouter1 is congested, it drops the packets that are marked with AF22.

  6. ftp traffic is forwarded to the next hop in agreement with the per-hop behavior that is configured for AF22 in diffrouter1's files.

  7. The ftp traffic traverses network 10.12.0.0 to genrouter, which is not diffserv-aware. As a result, the traffic receives “best-effort” forwarding behavior.

  8. genrouter passes the ftp traffic to network 10.13.0.0, where the traffic is received by diffrouter2.

  9. diffrouter2 is diffserv-aware. Therefore, the router forwards the ftp packets to the network in agreement with the PHB that is defined in the router policy for AF22 packets.

  10. ipqos2 receives the ftp traffic and prompts the user on ipqos1 for a user name and password.