IEC: On-Line Education: WPF: Multiprotocol Label Switching (MPLS)

The source sends its data to the destination. In an MPLS domain, not all of the source traffic is necessarily transported through the same path. Depending on the traffic characteristics, different LSPs could be created for packets with different CoS requirements.

In Figure 6, LER1 is the ingress and LER4 is the egress router.

Figure 6. LSP Creation and Packet Forwarding through an MPLS Domain

Table 1 illustrates the step-by-step MPLS operations that occur on the data packets in an MPLS domain.

MPLS Actions Description

label creation and label distribution

Before any traffic begins the routers make the decision to bind a label to a specific FEC and build their tables.

In LDP, downstream routers initiate the distribution of labels and the label/FEC binding.

In addition, traffic-related characteristics and MPLS capabilities are negotiated using LDP.

A reliable and ordered transport protocol should be used for the signaling protocol. LDP uses TCP.

table creation

On receipt of label bindings each LSR creates entries in the label information base (LIB).

The contents of the table will specify the mapping between a label and an FEC.

mapping between the input port and input label table to the output port and output label table.

The entries are updated whenever renegotiation of the label bindings occurs.

label switched path creation As shown by the dashed blue lines in Figure 6, the LSPs are created in the reverse direction to the creation of entries in the LIBs.

label insertion/table-lookup

The first router (LER1 in Figure 6) uses the LIB table to find the next hop and request a label for the specific FEC.

Subsequent routers just use the label to find the next hop.

Once the packet reaches the egress LSR (LER4), the label is removed and the packet is supplied to the destination.

packet forwarding With reference to Figure 6 let us examine the path of a packet as it to its destination from LER1, the ingress LSR, to LER4, the egress LSR.

LER1 may not have any labels for this packet as it is the first occurence of this request. In an IP network, it will find the longest address match to find the next hop. Let LSR1 be the next hop for LER1.

LER1 will initiate a label request toward LSR1.

This request will propagate through the network as indicated by the broken green lines.

Each intermediary router will receive a label from its downstream router starting from LER2 and going upstream till LER1. The LSP setup is indicated by the broken blue lines using LDP or any other signaling protocol. If traffic engineering is required, CR–LDP will be used in determining the actual path setup to ensure the QoS/CoS requirements are complied with.

LER1 will insert the label and forward the packet to LSR1.

Each subsequent LSR, i.e., LSR2 and LSR3, will examine the label in the received packet, replace it with the outgoing label and forward it.

When the packet reaches LER4, it will remove the label because the packet is departing from an MPLS domain and deliver it to the destination.

The actual data path followed by the packet is indicated by the broken red lines.

Table 1. MPLS Actions

Table 2 shows a simple example of the LIB tables.

Input Port Incoming Port Label Output Port Outgoing Port Label

1 3 3 6

2 9 1 7

Table 2. Example LIB Table

It is interesting to consider the example of two streams of data packets entering an MPLS domain:

One packet stream is a regular data exchange between servers (e.g., file transfer protocol [FTP]).
The other packet stream is an intensive video stream, which requires the traffic engineering parameters of QoS (e.g., videoconferencing).
These packet streams are classified into 2 separate FECs at the ingress LSR.
The label mappings associated with the streams are 3 and 9, respectively.
The input ports at the LSR are 1 and 2, respectively.
The corresponding output interfaces are 3 and 1, respectively.
Label swapping must also be done, and the previous labels must be exchanged for 6 and 7, respectively.

Tunneling in MPLS

A unique feature of MPLS is that it can control the entire path of a packet without explicitly specifying the intermediate routers. It does this by creating tunnels through the intermediary routers that can span multiple segments. This concept is used in provisioning MPLS�based VPNs.

Consider the scenario in Figure 7. LERs (LER1, LER2, LER3, and LER4) all use BGP and create an LSP between them (LSP 1). LER1 is aware that its next destination is LER2, as it is transporting data for the source, which must go through two segments of the network. In turn, LER2 is aware that LER3 is its next destination, and so on. These LERs will use the LDP to receive and store labels from the egress LER (LER4 in this scenario) all the way to the ingress LER (LER1).

Figure 7. Tunneling in MPLS

However, for LER1 to send its data to LER2, it must go through several (in this case three) LSRs. Therefore, a separate LSP (LSP 2) is created between the two LERs (LER1 and LER2) that spans LSR1, LSR2, and LSR3. This, in effect, represents a tunnel between the two LERs. The labels in this path are different from the labels that the LERs created for LSP1. This holds true for LER3 and LER4, as well as for the LSRs in between them. LSP 3 is created for this segment.

To achieve this, the concept of a label stack is used when transporting the packet through two network segments. As a packet must travel through LSP 1, LSP 2, and LSP 3, it will carry two complete labels at a time. The pair used for each segment is (1) first segment, label for LSP 1 and LSP 2 and (2) second segment, label for LSP 1 and LSP 3.

When the packet exits the first network and is received by LER3, it will remove the label for LSP 2 and replace it with LSP 3 label, while swapping LSP 1 label within the packet with the next hop label. LER4 will remove both labels before sending the packet to the destination.

Multicast Operation

The multicast operation of MPLS is currently not defined. However, a general approach has been recommended whereby an incoming label is mapped to a set of outgoing labels. This can be constructed via a multicast tree. In this case, the incoming label will bind to the multicast tree and a set of output ports is used to transmit the packet. This operation is quite conducive to a local-area-network (LAN) environment. In a connection-oriented network such as ATM, the point-to-multipoint switched paths (VCCs) can be used for distributing multicast traffic.

5. MPLS Protocol Stack Architecture >>