MPLS Tutorial header image

MPLS Tutorial

≡ Menu

Basic Network QoS Technologies

This section explains basic QoS technologies that are avail-able which are IntServ (RSVP), DiffServ,MPLS.

IntServ Networks

IntServ is the architecture that re-serves the flow all along the network end-to-end as requested by the application or user and thus, provides guaranteed QoS. RSVP is the signalling protocol that requests bandwidth and other resources for the
IntServ architecture. Intserv defines 2 service models: Guaranteed Service and Controlled load. Guaranteed Service is for applications requiring strict QoS and provides the mathematical upper limit on the queuing delay. Control load controls load using multiplex statistics and for applications with higher flexibility that guaranteed service. [Wan] gives a good overview on Intserv and RSVP. More details on IntServ and RSVP can be found out from [RCS], [Bea], [Wrob], [Wroa] and [SPG].

DiffServ Networks

There is a difficulty in implementing and deploying Integrated Services and RSVP and there are critical issues like scalablity in IntServ. And to overcome these issues and to assure simplicity, Differentiated Services (DS) was introduced.
Customers can mark DS fields of individual packets to indicate the desired service or have them marked by the leaf router based on MF classification.At the ingress of the ISP networks, packets are classified, policed and bpossibly shaped. The
classification, policing and shaping rules used at the ingress routers are derived from the SLAs. The amount of buffering space needed for these operations is also derived from the SLAs. When a packet enters one domain from another domain, its DS field may be re-marked, as determined by the SLA between the two domains.

Thus, the DiffServ network lets an application or flow packets to be marked at the edge router which thus, can be differentiated from other packet flows and thus, differentiated service can be provided. These marked packets are then provided forwarding priority preferences in the core routers depending on the mark put on them by the edge routers.

MPLS (Multi-Protocol Label Swithcing)

It is a forward-ing scheme. It evolved from Ciscos Tag Switching. In the OSI seven-layer model, it is between Layer 2 (L2, link layer)and Layer 3 (L3, network layer).

Each MPLS packet has a header containing a 20-bit label, a 3-bit Class of Service (COS) field, an 1-bit label stackindicator and an 8-bit TTL field. The MPLS header is encapsulated between the link layer header and the network layer
header. A MPLS capable router, which is known as Label Switched Router (LSR), examines only the label in forwarding the packet. The network protocol can be IP or anything.

MPLS sets up labels across the LSRs by using some protocol known as Label Distribution Protocol (LDP), which may not be the same in all LSRs. This LDP is used to set upLabel Switched Paths (LSPs). A LSP is similar to ATM virtual circuit and is uni-directional from sender to reciever. LSP set-up can be control driven or data driven. In control-driven LSP setup, label distributions are triggered by control-driven traffic like routing updates. In data-driven set up, the label distribution is triggered by request of a flow or an aggregation of flows (known as traffic trunk). LSP set-up between 2 hosts can be done explicitly. Then the path is known as explicit path (EP). This is one of the most useful
feature of MPLS and is heavily used in Traffic Engineering (TE). Thus, by using the labels as a result of label distribution during set-up of a LSP, forwarding table is used to forward or process packets based on their MPLS labels.

Normal packets are classified at the ingress router of the MPLS enabled network. MPLS header is put on the packets depending on the kind of routing and service, we want from the packet. These labeled packets are then used by the LSRs to forward the packets. This is done by looking into the forwarding table which was constructed when the labels were distributed during LSP set-up. This process of forwarding using labels is faster than the forwading done by normal IP routing tables. When an incoming packet reaches the LSR, the incoming label is swapped by the outgoing label and the packet is forwared to the next LSR. This label-switching process is similar to ATm VCI/VPI processing. Inside the MPLS domain, packet classification, forwarding and QoS service are determined using labels and COS. When the packet leaves the domain, the MPLS header is removed.

Summarizing, MPLS provides following advatages:
1. Faster routing and processing.

2. Efficient tunneling mechanism.

Both these features make it effective for TE (Traffic Engineer-ing). Details can found in [ea01] and [ea99].
MPLS can be used with DiffServ to provide effective QoS as discussed in next section.

DiffServ over MPLS
[17] explains DiffServ over MPLS as:

Although traffic engineering is realized by MPLS as discussed above, the DiffServ is required for scalable QoS control. However, the MPLS encapsulates IP packets using the shim header having the label, and the core router cannot refer to the DSCP. Incompatibility with the DiffServ posed a problem. The IETF has proposed the DiffServ
over MPLS [ea00] to solve this problem.

The DiffServ over MPLS can map multiple BAs of DiffServ to a single LSP of MPLS. By this, traffic on the LSP can be forwarded based on the PHB of BA. The E-LSP to allow assigning multiple BAs to a single LSP using the EXP field and the L-LSP to allow assigning a single LSP to a single BA (displays multiple packet discarding
priorities) can be used for LSP and BA mapping.

E-LSP: The E-LSP shows the PHB of a packet using the EXP field of MPLS shim header. Up to eight BAs can be mapped in the EXP field.

L-LSP: The L-LSP determines the packet scheduling characteristics based on the MPLS label and the packet discarding priority based on the shim header or layer-2 packet discarding mechanism. The native ATM uses the L-LSP as it cannot use the EXP field. As the NEs replace packet labels hop by hop, the label and DSCP mapping is difficult to
manage. While the E-LSP is easier to control than the L-LSP as it can previously determine the mapping between the EXP field and DSCP of each packet on the entire network.