# Quality-of-Service-Linked Privileged Content-Caching Mechanism for Named Data Networks
## 3 Literature Survey
### PIT Management  
A PIT dictates the traffic flows in an NDN by providing a path from a producer to a consumer who has requested the content, and is the cornerstone of stateful forwarding. The magnitude of a PIT determines the maximum number of NDN traffic flows that can be managed. If there is no coherence between the PIT of NDN nodes along the path between the producer and consumer, then the traffic path may terminate and result in a wastage of the forwarding resources of predecessor NDN nodes. A PIT records an entry when an interest packet for the content is received by the NDN node, but it is neither the producer of the content nor can its CS satisfy the request; therefore, it forwards the interest packet to the next NDN node. The entry remains in a PIT until either the content request is satisfied by the NDN node, by receiving the requested content routed through its path, or the entry is pre-emptively removed due to a predefined policy, such as time expiry, resource allocation to high-priority traffic, etc. PIT saturation occurs when more interest entries are required to be recorded than what is allowed by the PIT's size. In the scenarios where a PIT reaches its maximum capacity, incoming interests which cannot be satisfied by the local CS may be dropped. This remedies the event of active traffic flow path termination for existing entries. Ignoring the interest packet entries due to PIT saturation may increase the transmission delay, if that NDN node is located along the shortest path between the content producer/supplier and the consumer. To mitigate this delay and re-transmission efforts for QoS traffic, strategies need to be developed for maintaining the coherence between the PITs of NDN nodes along the path of the QoS traffic. There is a need for a PIT entry replacement policy when full and new interests arrive at NDN nodes. HLLR [24] identifies an entry which has a long lifetime and satisfies the least number of content request interfaces and replaces it with the new one. The algorithm calculates a value called \"price\", which is the ratio of the PIT entry's lifetime to the number of request interfaces. The entry whose price is higher is evicted. An intelligent forwarding strategy [25] identifies PIT entries which contribute to network congestion based on the number of interests that are satisfied for a particular entry. When the number of interest entries increases in a PIT, the entry which has been pending for a long time and that is not satisfied by returning data is judged to be a congestion contributor and evicted. PRP-MC [26] does not remove a satisfied entry from the PIT, as in the case of a standard NDN. Even after satisfaction, the interest entry remains active, securing the path for content with same prefix names. The proposal takes a proactive approach of calculating a \"contribution\" value, which is an aggregation of the number of content interests satisfied by a group of interest entries with the same prefix names. This approach mimics the popularity-driven methods for interest entry eviction in PITs. A PIT lifetime mechanism [27] recognizes the problem of the PIT memory exhaustion phenomenon, even after measures such as interest entry eviction, after assigning a small time to a live value, and proposes a bounded hop-count method. The interest packet with more than the threshold distance indicated by the hop count is dropped, preferring, therefore, the interests from the nearest locations. This strategy demonstrates the reduction in packet-drop events. EQPR [28] manages the QoS traffic through PIT entry strategies based on packet priorities and estimated turn-around times. If the PIT is full, then non-priority entries are removed. Moreover, if the estimated turn-around time of a packet is greater than remaining lifetime of the PIT entry, then the given PIT entry is evicted. The PRWR [29] allows PIT entries, regardless of their QoS priority, when the PIT memory space is not full. Once the PIT is saturated, priority-interest packet entries replace non-priority-interest packet entries.