Traffic Engineering

Question 1

Recall that the three steps of traffic engineering: Measure, Model, and Control.
a. What are the two things that need to be measured, and how could each be measured?

Answer 1

We need to measure the topology, including not only the connectivity but also the
capacity of each link and router. This could be done by routers self-reporting, similar to
how they exchange information in a Link State protocol, but in practice is probably more
often simply entered as data by a network engineer.
We also need to measure the traffic, or offered load. This can be done using the “simple
counters” measurement technique that we learned about earlier, since we want to know how much traffic is on each part of the network but don’t necessarily need the
details of specific flows

Question 2

What are two ways that control could be implemented?

Answer 2

The “traditional” way to implement control is by adjusting link weights, which indirectly
affects the routes calculated by the routing protocol. In practice, link weights are more
often used this way than to represent any “real” property of the network, like
bandwidth or link latency. Another way to implement control is by using SDN to directly
control the routes that are used on the network.

Question 3

In intra-AS multipath, traffic can be split among paths with the same path length (i.e., sum
of link weights along that path). In inter-AS multipath, what properties of the paths (i.e., of
the BGP advertisements) need to be equal in order to allow multipath over those paths?
(You can limit your answer to the BGP properties that we learned about in class – there are
couple that we didn’t learn about that also apply here, but don’t worry about those.)

Answer 3

LOCAL_PREF, the local preference parameter
AS_PATH length, as determined by counting the number of ASes in the AS_PATH
MULTI_EXIT_DISC, the MED value IGP metric to the NEXT_HOP, i.e., equal “hot potato” routing
distance

Question 4

How does using pods and pseudo-MACs improve the scalability of a Layer 2 network?

Answer 4

This changes the flat layer 2 addressing (MAC addresses) into a hierarchical addressing
(pseudo-MAC addresses). This means that switches only need to store a forwarding entry for
each host in the same pod plus one for each other pod, rather than needing an entry for each
host on the entire network. (Notice that hierarchical addressing is the same thing that allows IP
to scalable at layer 3, so the idea is to push that concept down into layer 2.)

Question 5

10. What are the advantages of using a Jellyfish topology over a traditional hierarchical data
    center topology?

Answer 5

• Network load balancing – prevents bottleneck links and heavily loaded aggregation or core
switches
• Higher capacity – since the network is balanced, more hosts can reasonably be hosted on a
network with the same number of switches
• Shorter paths – shorter average number of hops between any two hosts results in faster
network performance
• Incremental expansion – allows adding switches to the network without reconfiguring the
existing network infrastructure or adding additional “higher-level” switches

Question 6

What are the drawbacks or problems with using a Jellyfish topology?

Answer 6

Does not handle heterogeneous switch devices well, except when expanding the network
with switches larger than those originally used.
• Long cable runs between random switch pairs may be necessary, but are inconvenient and
difficult to install