MPLS QoSUniform Tunnel and Pipe Modes

In uniform tunnel mode, any changes to a packet's differentiated services code points (DSCPs) are permanent, as shown in Figure B-1 and discussed in the following list:

1. A packet arrives at link switched router (LSR) A with a DSCP setting of 3. The switch policy determines that the appropriate Expedited bit (EXP) value for this class is also 3. The packet is forwarded across the core toward its destination.

2. LSR B reclassifies the packet into another class and sets the EXP to 4.

3. LSR C, the penultimate LSR, pops the MPLS label and egress LSR D sets the Type of Service (TOS) value to 4, to match the change in the MPLS core. If there is a label stack, the changes are propagated to the inner EXP field, and the next label would have its EXP set to 4.

4. Router E receives an IP packet with a DSCP setting of 4.

Figure B-1. MPLS Uniform Tunnel Mode

Pipe tunnel mode differs from uniform tunnels in two ways. First, changes to EXP are not copied to the internal DSCP. Second, the egress provider edge (PE) classification is done using the MPLS EXP value. Figure B-2 shows how pipe tunnel mode works, as described in the following steps:

1.	A packet arrives at LSR A with a DSCP setting of 3. The switch policy determines that the appropriate EXP value for this class is also 3.
2.	The packet is forwarded across the core toward its destination.
3.	LSR B reclassifies the packet into another class and sets the EXP to 4.
4.	When LSR D sends the packet, the DSCP setting is still 3. However, if there is a stack of MPLS labels, the changes are propagated to the inner EXP field. But, even in this case, the IP settings are never modified.

Figure B-2. MPLS Pipe Tunnel Mode

The pipe tunnel mode has a subtlety. LSR D ordinarily does not see the EXP values because of penultimate hop popping (PHP) on LSR C. However, because of the semantics of the pipe tunnel, LSR D needs EXP information so that it can apply the correct PHB for the packet. To reconcile this conflicting state of affairs, the egress LSR uses a concept called explicit null, which causes the penultimate-hop router to no longer pop the outer label when forwarding frames to its upstream neighbor. Explicit null labels are not needed if the egress LSR is going to receive a label anyway (for example, if there are two labels on the stack, even if the outer one is popped, the PE always receives a labeled packet).

A variation of the pipe tunnel mode is called short pipe mode; in this mode, the egress PE uses the packet's IP DSCP to apply per-hop behavior (PHB).