This package is the congestion aware fetcher, used in the traffic generator.
It implements a consumer that follows the TCP CUBIC congestion control algorithm, simulating traffic patterns similar to bulk file transfer.
It requires at least one thread, running the FetchThread_Run function.
TaskDef defines a fetch task that retrieves one segmented object. A segmented object is a group of NDN packets, which have a common name prefix and have SegmentNameComponent as the last component. The TaskDef contains these fields:
- Prefix: a name prefix except the last SegmentNameComponent.
- Importantly, if you are retrieving from the file server, this field must end with the VersionNameComponent.
- InterestLifetime
- HopLimit
- SegmentRange: retrieve a consecutive subset of the available segments.
- If the fetcher encounters a Data packet whose FinalBlockId equals its last name component, the fetching will terminate at this segment, even if the upper bound of SegmentRange has not been reached.
Normally, a fetch task generates traffic similar to bulk file transfer, in which contents of the received packets are discarded. It is however possible to write the received payload into a file. In this case, the TaskDef additionally contains these fields:
- Filename: output file name.
- FileSize: total file size.
- SegmentLen: the payload length in every segment; the last segment may be shorter.
A worker is a thread running the FetchThread_Run function.
It can simultaneously process zero or more fetch tasks, which are arranged in an RCU-protected linked list.
It has an io_uring handle in order to write payload to files when requested.
A TaskContext stores information of an ongoing fetch task, which can be initialized from a TaskDef. It includes a taskSlot (aka C.FetchTask) used by C code, along with several Go objects. It is responsible for opening and closing the file, if the TaskDef requests to write payload to a file. Each taskSlot has an index number that used as the PIT token for its Interests, which allows the reply Data packets to come back to the same taskSlot.
FetchLogic contained with the taskSlot implements the algorithmic part of the fetch procedure. It includes an RTT estimator, a CUBIC-like congestion control implementation, and a retransmission queue. It makes decisions on when to transmit an Interest for a certain segment number, and gets notified about when the Data arrives with or without a congestion mark. Nack packets are not considered in the congestion aware fetcher.
Fetcher is the top level. It controls one or more workers and owns one or more task slots. A incoming TaskDef is placed into an unused task slot, and then added to the worker with the least number of ongoing fetch tasks.