# Packet Switching Fabric

## Overall Design

This implementation relies upon a packet-switching fabric that applies business logic to each packet in order to ensure that packets meet various preconditions and can be properly routed. At a high level, this fabric looks like this:

![Packets enter the switching fabric on an inbound link, are processed, and then forwarded out on an outbound link. Responses traverse the fabric in the opposite direction.](https://2074555819-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ll81BPRiYKSquNEk5qa%2F-LlbkXaKl9jh0grYtocx%2F-Llbm2gG_Cf-SCZA44lC%2Fimage.png?alt=media\&token=b284661a-11fd-4312-a314-d5ecf8c7d39c)

### Packet Filters

A packet filter applies business logic to a particular ILPv4 Prepare packet. The contract allows the filter to modify an in-flight packet, forward it onward to the next filter, or reject the packet outright. Read more about the contract [here](https://github.com/sappenin/java-ilpv4-connector/blob/aa84b650426817d9db322f9d0889ab80505f3c90/ilpv4-connector-service-api/src/main/java/com/sappenin/interledger/ilpv4/connector/packetswitch/filters/PacketSwitchFilter.java). 

### Packet Filter Chain

Packet filters are assembled together into a filter chain that is applied to every incoming packet. This implementation has a filter chain that looks like this:

![Each incoming ILPv4 packet is "filtered" according to the business logic of each Packet Filter.](https://2074555819-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ll81BPRiYKSquNEk5qa%2F-LlbmIjqQX7foPb3iTbc%2F-LlbnPzjU7MCKAoTFNc9%2Fimage.png?alt=media\&token=44bc49f4-06a6-4562-97e0-1394ba16339e)

### Link Filters

A Link Filter is similar to a Packet Filter in that it applies business logic to a particular ILPv4 Prepare packet. However, these filters are only engaged before sending packets into an Outbound Link.

Like Packet Filters, the contract allows the filter to modify an in-flight packet, forward it onward, or reject the packet outright without sending it into the Outbound link. Read more about the contract [here](https://github.com/sappenin/java-ilpv4-connector/blob/aa84b650426817d9db322f9d0889ab80505f3c90/ilpv4-connector-service-api/src/main/java/com/sappenin/interledger/ilpv4/connector/links/filters/LinkFilter.java). 

### Link Filter Chain

Link filters are assembled together into a filter chain that is applied to every outgoing packet. This implementation has a filter chain that looks like this:

![Each outgoing ILPv4 packet is "filtered" according to the business logic of each Link Filter.](https://2074555819-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ll81BPRiYKSquNEk5qa%2F-LlbmIjqQX7foPb3iTbc%2F-LlboM4uzV8_dXsIs-kh%2Fimage.png?alt=media\&token=d40faab2-92d1-4a11-9841-68f1affde0d7)