M ULTIPLE S TRIPES /T REES

The concept of multiple stripes has been brought up in previous research [2], [3]. Each message transported from source peer will be divided into several stripes. In tree building step, we not only construct one tree, we construct as many different trees as the number of stripes that one message is divided into. And each tree transmits different stripe of the message to subscribers. Hence, the peer receives different stripes of the message through different transmission paths. Figure 4 present an example of multiple stripes. By this approach, we get a lot of advantages mentioned before. Moreover, we enhance multiple stripes by combining with IDA approach. The description is in following content.

Figure 4: An example that a message is divided into 2 stripes

3.3.1 Combine with IDA

The combination of multiple stripes and IDA mean that one message will be divided into several stripes by IDA approach. And each stripe will be transmitted through different trees.

The purpose of multiple stripes/trees is to prevent losing a whole message when one link broken. The purpose of IDA is to promise peers will restore the original message even some stripes lose.

When we use (n, m) IDA technology, every peer joining in the ALM service will have n different transmission paths (actually, it is mostly less than n, we will explain about it in next chapter) that the origination is source peer and destination is itself. Each transmission path of the peer transmits different stripe and peers only need to receive any m stripes of the total n

stripes, then those stripes can be restored to the original message. In other words, the transmission process can tolerate to loss at most n-m stripes.

Therefore, we must make sure that any peer leave the ALM service will not cause the remand peers who are still in the ALM service can not restore the original message. This situation happens only when the leaving peer is in more than n-m of n transmission paths of any peer. When such peer leaves the ALM service, it will cause more than n-m transmission paths of the peer break down and the peer can only receive less than m stripes. Hence, the peer can not restore the original message. This situation can be prevented by building disjoint transmission paths.

3.3.2 Building Disjoint Paths

Base on the description before, we know that we must prevent any peer is in more than n-m transmission path of other peers. The solution for building this kind of transmission paths

is to restrict that every peer become interior node in constant number of trees. And the number must not bigger than n-m. Therefore, there are two different choices to decide the number of being interior node for each peer. The first choice is every peer becomes interior node in more than one tree and the second choice is to restrict every peer can become interior node in only one tree. The advantage of first choice is that some peers with high upstream bandwidth will contribute their bandwidth averagely in multiple trees without contributing whole upstream bandwidth in only one tree. It is good for Trickle because in transmission process with IDA, the importance of all stripes is the same. However, there is a critical disadvantage to contribute bandwidth in multiple trees. It will cause the height of trees in ALM service much bigger and the end to end delay will be higher because the degree of nodes in each tree are smaller.

Therefore we decide to pick the second choice that every peer becomes interior node in only one tree. This method restrict that any peer at most in one of n transmission paths of other subscribers. And then, we have to use a method to decide peers become interior node in

which tree. In SplitStream [2], it provides a method that can establish interior-node-disjoint trees. Each tree root has different prefix of groupId. Due to the tree building method, groupId and nodeId of peers will cause which peers become interior node. Therefore, it will restrict that every peer will play the role of interior node in no more than one tree and being leaf node in the remand trees. This method is only fit for the tree building algorithms that base on DHT.

In out hypothesis, we hope Trickle can be applied to any tree building algorithm. Therefore, we use different method to decide peers become interior node in which tree.

3.3.3 Become Interior Node

Each peer decides it will become interior node in which tree by itself. When a new peer is in the process of joining service, there are two decision methods to decide which tree it should be interior node. One, in order to prevent violating the DHT concept, if the tree building algorithm is based on DHT, it decides to be interior node in the tree whose root has the same prefix hash id. It is approximately the same as SplitStream, but the only difference is the peer already decides the tree it wants to be interior node by itself in the beginning of joining service. The second method is the peer randomly decides which tree it should be interior node and be leaf node in other trees.

The advantage of random solution is that we don’t need a server to do resource (upstream bandwidth) management. However, it may probably cause large number of peer being interior node in the same tree and the resource distributed unfairness. It will cause that the upstream bandwidth of some trees is not enough. We solve this problem by using the concept of helper. Those trees with poor resource will ask helper for help. The helper will share their upstream bandwidth to those trees that need it. Therefore, when the number of helper is nearly infinite, the upstream bandwidth won’t be exhausted. And the concept of helper will be described in next section.