pppd sample working on Ubuntu 18.04

client_output

using channel 47
Using interface ppp0
Connect: ppp0 <--> /dev/pts/3
sent [LCP ConfReq id=0x1 <magic 0x552f41bf>]
sent [LCP ConfReq id=0x1 <magic 0x552f41bf>]
rcvd [LCP ConfReq id=0x1 <magic 0x82dcfce2>]
sent [LCP ConfAck id=0x1 <magic 0x82dcfce2>]
sent [LCP ConfReq id=0x1 <magic 0x552f41bf>]
rcvd [LCP ConfAck id=0x1 <magic 0x552f41bf>]
sent [LCP EchoReq id=0x0 magic=0x552f41bf]
sent [IPCP ConfReq id=0x1 <addr 192.168.11.4>]
rcvd [LCP EchoReq id=0x0 magic=0x82dcfce2]
sent [LCP EchoRep id=0x0 magic=0x552f41bf]
rcvd [IPCP ConfReq id=0x1 <addr 192.168.15.3>]
sent [IPCP ConfAck id=0x1 <addr 192.168.15.3>]
rcvd [LCP EchoRep id=0x0 magic=0x82dcfce2]
rcvd [IPCP ConfAck id=0x1 <addr 192.168.11.4>]
not replacing default route to eth0 [10.0.2.2]
local  IP address 192.168.11.4
remote IP address 192.168.15.3
Script /etc/ppp/ip-up started (pid 16153)
Script /etc/ppp/ip-up finished (pid 16153), status = 0x0

# client terminated by Ctrl-C
Terminating on signal 2
Connect time 0.2 minutes.
Sent 0 bytes, received 0 bytes.
Script /etc/ppp/ip-down started (pid 16161)
sent [LCP TermReq id=0x2 "User request"]
rcvd [LCP TermAck id=0x2]
Connection terminated.
Script /etc/ppp/ip-down finished (pid 16161), status = 0x0

pppd_sample.md

$ sudo apt-get install -y ppp xl2tpd socat

# Create two pseudo terminals. Use them as serial devices.
$ socat -d -d pty,raw,echo=0 pty,raw,echo=0

# Run pppd server
# ref. http://www11442uo.sakura.ne.jp/evirog/ppp-serial/
$ sudo pppd -detach debug noauth local nobsdcomp nodeflate noaccomp nopcomp novj default-asyncmap /dev/pts/2 192.168.15.3: 115200

# Run ppd client in another terminal
$ sudo pppd -detach debug noauth /dev/pts/3 115200 defaultroute noipdefault nobsdcomp nodeflate noaccomp nopcomp novj default-asyncmap 192.168.11.4:

# We can send data by IP once ppp is established.
$ ip address
...
40: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 3
    link/ppp 
    inet 192.168.15.3 peer 192.168.11.4/32 scope global ppp0
       valid_lft forever preferred_lft forever
41: ppp1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 3
    link/ppp 
    inet 192.168.11.4 peer 192.168.15.3/32 scope global ppp1
       valid_lft forever preferred_lft forever

$ ping -c 3 192.168.15.3
$ ping -c 3 192.168.11.4

$ ping -c 3 192.168.15.3
PING 192.168.15.3 (192.168.15.3) 56(84) bytes of data.
64 bytes from 192.168.15.3: icmp_seq=1 ttl=64 time=0.035 ms
64 bytes from 192.168.15.3: icmp_seq=2 ttl=64 time=0.087 ms
64 bytes from 192.168.15.3: icmp_seq=3 ttl=64 time=0.105 ms

$ ping -c 3 192.168.11.5
PING 192.168.11.4 (192.168.11.4) 56(84) bytes of data.
64 bytes from 192.168.11.4: icmp_seq=1 ttl=64 time=0.036 ms
64 bytes from 192.168.11.4: icmp_seq=2 ttl=64 time=0.162 ms
64 bytes from 192.168.11.4: icmp_seq=3 ttl=64 time=0.047 ms

server_output

using channel 48
Using interface ppp1
Connect: ppp1 <--> /dev/pts/2
sent [LCP ConfReq id=0x1 <magic 0x82dcfce2>]
rcvd [LCP ConfAck id=0x1 <magic 0x82dcfce2>]
rcvd [LCP ConfReq id=0x1 <magic 0x552f41bf>]
sent [LCP ConfAck id=0x1 <magic 0x552f41bf>]
sent [LCP EchoReq id=0x0 magic=0x82dcfce2]
sent [IPCP ConfReq id=0x1 <addr 192.168.15.3>]
rcvd [LCP EchoReq id=0x0 magic=0x552f41bf]
sent [LCP EchoRep id=0x0 magic=0x82dcfce2]
rcvd [LCP EchoRep id=0x0 magic=0x552f41bf]
rcvd [IPCP ConfReq id=0x1 <addr 192.168.11.4>]
sent [IPCP ConfAck id=0x1 <addr 192.168.11.4>]
rcvd [IPCP ConfAck id=0x1 <addr 192.168.15.3>]
local  IP address 192.168.15.3
remote IP address 192.168.11.4
Script /etc/ppp/ip-up started (pid 16155)
Script /etc/ppp/ip-up finished (pid 16155), status = 0x0

# client terminated
rcvd [LCP TermReq id=0x2 "User request"]
LCP terminated by peer (User request)
Connect time 0.2 minutes.
Sent 0 bytes, received 0 bytes.
Script /etc/ppp/ip-down started (pid 16162)
sent [LCP TermAck id=0x2]
Script /etc/ppp/ip-down finished (pid 16162), status = 0x0
Connection terminated.
Modem hangup

Adding -x option to socat is helpful to debug.
This shows a data stream.

$ socat -x -d -d pty,raw,echo=0 pty,raw,echo=0