When a device boots up with the startup configuration, the nginx process will be running. NGINX is an internal webserver that acts as a proxy webserver. It provides Transport Layer Security (TLS)-based HTTPS. RESTCONF request sent via HTTPS is first received by the NGINX proxy web server, and the request is transferred to the confd web server for further syntax/semantics check.
Reference:
ios/prog/configuration/168/b_168_programmability_cg/RESTCONF.html
The https-based protocol-RESTCONF (RFC 8040), which is a stateless protocol, uses secure HTTP methods to provide CREATE, READ, UPDATE and DELETE (CRUD) operations on a conceptual datastore containing YANG-defined data -> RESTCONF only uses HTTPs.

Explanation
R3 advertises BGP updates to R1 with multiple AS 100 so R3 believes the path to reach AS 200 via R3 is farther than R2 so R3 will choose R2 to forward traffic to AS 200.

The IP protocol was designed for use on a wide variety of transmission links. Although the maximum length of an IP datagram is 65535, most transmission links enforce a smaller maximum packet length limit, called an MTU. The value of the MTU depends on the type of the transmission link. The design of IP accommodates MTU differences since it allows routers to fragment IP datagrams as necessary. The receiving station is responsible for the reassembly of the fragments back into the original full size IP datagram.
Fragmentation and Path Maximum Transmission Unit Discovery (PMTUD) is a standardized technique to determine the maximum transmission unit (MTU) size on the network path between two hosts, usually with the goal of avoiding IP fragmentation. PMTUD was originally intended for routers in IPv4. However, all modern operating systems use it on endpoints.
The TCP Maximum Segment Size (TCP MSS) defines the maximum amount of data that a host is willing to accept in a single TCP/IP datagram. This TCP/IP datagram might be fragmented at the IP layer. The MSS value is sent as a TCP header option only in TCP SYN segments. Each side of a TCP connection reports its MSS value to the other side. Contrary to popular belief, the MSS value is not negotiated between hosts. The sending host is required to limit the size of data in a single TCP segment to a value less than or equal to the MSS reported by the receiving host.
TCP MSS takes care of fragmentation at the two endpoints of a TCP connection, but it does not handle the case where there is a smaller MTU link in the middle between these two endpoints.
PMTUD was developed in order to avoid fragmentation in the path between the endpoints. It is used to dynamically determine the lowest MTU along the path from a packet's source to its destination.
Reference:
25885-pmtud-ipfrag.html

Explanation
https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-1/configurationguide/b_cg81/b_cg81_chapter_0100