How does a network that uses a protocol with a 3-bit EXP field carry traffic transparently between customer sites that may need up to 11 different classes of service? VPN service offerings limit the number of classes of service available to customers, and MPLS is no exception. PE routers map customer-to-service provider traffic classes. However, the simple fact of crossing a service provider network should not impact how traffic is treated after it arrives at the remote site. In other words, the original traffic class information must not be lost by the MPLS network. To deal with this situation, MPLS has three different modes to carry customer DSCP settings in what are called pipes. Before looking at pipes in more detail, we should address how DSCP values are mapped to MPLS EXP. The simplest way is to use the same model as for IP Precedence or 802.1p given in Table 10-1. MPLS vendors often have their own suggestions, such as AF31 to EXP3 and so on, but there is no actual standard to follow. Each MPLS network administrator is free to come up with whichever mapping works best for his applications. Tunnels and PipesWhen IP packets are encapsulated in MPLS, the DSCP class settings can be copied to the EXP bits. The PHB across label switching routers (LSRs) is processed according to these EXP settings. Because of penultimate hop popping (PHP), the last LSR on a label switched path (LSP) does not see the MPLS EXP field, so the QoS settings must be signaled in another way. MPLS support for DiffServ (RFC 3270) adds different models to account for the various places that labels can be popped and how to relay marking information between domains. The following list qualifies the different kinds of LSPs used with the DiffServ model:
Three different modes are found in MPLS networks. Each mode specifies the relationship between IP DSCP and MPLS EXP information:
Note For further information, see Appendix B, "MPLS QoS, Traffic Engineering and Guaranteed Bandwidth," which contains an expanded explanation of MPLS QoS tunnel and pipe modes, traffic engineering, and guaranteed bandwidth (including examples). |