UCEN 3133 Advanced Computer Networks - Tutorial 3 - Year 2012

Tutorial 3 – MPLS Operation

1. A smaller autonomous system (AS) is defined inside the boundaries of a larger AS, and both of them employ MPLS to forward packets. A label switched path (LSP) has already been established from end-to-end between edge Label Switched Routers (LSRs) from the larger AS. This LSP tunnels through the smaller AS. In this scenario, what mechanism does MPLS provide in order to minimize routing information maintenance in the internal LSRs of the smaller AS ? Describe in detail how this mechanism works as a labelled packet is forwarded through the smaller AS.

MPLS uses a stack of labels.

1. When a transit packet arrives with its own label corresponding to an external LSP, the ingress LSR of the smaller AS pushes a new label onto the label stack.
2. This new label corresponds to an internal LSP within the smaller AS
3. The internal LSRs of the smaller AS switch the packet through to the egress LSR, working only on the topmost label (the new label)
4. At the egress LSR of the smaller AS, the new label is popped off the stack restoring original label.
5. The packet resumes its travel using the original external LSP for the larger AS.

2. What is the motivation behind the use of Penultimate Hop Popping in an MPLS network ? Describe briefly how it works.

It provides a form of optimization to minimize the performance penalty incurred when the egress LSR of a MPLS network has to perform two lookup operations when processing a label stack. When a packet with a stack of two labels arrives at the penultimate LSR (the LSR before the egress LSR), it pops the top stack before forwarding it onwards to the last stack.

3. Distinguish between the independent and ordered control approach to exchanging labels in the Label Distribution Protocol (LDP).

Independent control - Each LSR advertises label mappings at will. In an unsolicited mode, LSRs will generally advertise new labels whenever the IP routing information changes.

Ordered control - In this mode,  all path advertisements originate at the egress router, and the label advertisement messages travel upstream to the ingress router. For advertisement on demand, a request from the ingress router triggers a chain of requests downstream to the egress router

4. Explain, with the aid of a diagram, how the liberal approach to label retention works in the LDP ? What advantage does the liberal approach offer over the conservative approach, and what is its corresponding disadvantage ?

It retains all label mappings even if they don’t correspond to an existing LSP.

• Advantage: If the next hop path changes for the LSP, the given LSR  may already have a new LSP ready, so it can react to the routing change almost immediately. Whereas in conservative approach, the LSR will need to request label mappings first from alternative downstream LSRs in the neighbourhood.
• Disadvantage: Larger mapping table space required to keep all the labels, as well as network overhead in label exchange when the number of neighbouring LSRs is very large.

5. Explain two approaches that could be employed to address the potential problem of loops in the label distribution protocol (LDP)

·         The hop count field in the label request message is incremented by all LSRs that the message passes through. If the hop count ever reaches a maximum value,  a loop has occurred and the LSR sends a LDP notification message to the originator of the message.

• Path vectors - Each router inserts its own LSR ID in the path vector field of the message, and if it ever finds its own ID here in a newly received message, it breaks the loop

6. Name the two situations in which a LSR in the middle of a LSP may need to return an Internet Control Message Protocol (ICMP) error message to a datagram’s source. What is the problem with using ICMP in a Virtual Private Network (VPN) deployed using MPLS, and how is this addressed by the LSR that wishes to create the ICMP message ?

Two situations:

• The datagrams hop limit may run out
• The packet may encounter a problem in a link between routers (link broken, or datagram too large to be fragmented and sent over link)

In a VPN, only the egress router at the boundary of the VPN is able to access the source IP address. Other intermediate routers cannot access the source IP address (which is encrypted using IPSec), and hence cannot route the appropriate ICMP message back to the source.

This is addressed by creating a ICMP message and adding the original stack label from the packet that caused the error. This ICMP datagram will follow the route of the original packet to the egress router, who can then successfully route it back to its source.

7. How might a session initialization deadlock occur during the LDP of a MPLS ? Explain how this can be overcome using the LDP hello message.

As part of LDP, two LSRs may not be able to agree on parameters during session initialization. The propose / reject cycle could then continue indefinitely, causing a deadlock. This can be overcome by having the network administrator intervene and make the passive LSR accept the proposed session. To make the active LSR propose again, the passive LSR increments the configuration sequence number in its hello messages to the active LSR.