Communications Between the Local and Remote Hosts (System Administration Guide, Volume 3)

System Administration Guide, Volume 3

Communications Between the Local and Remote Hosts

At this point, PPP communications start, as the link between local and remote hosts is now established.

The first lines in this part of the session constitute a configuration request (Config-Req). This is the first PPP packet sent to the remote host. The configuration request is one example of a Link Control Protocol (LCP) packet. It requests that configuration be set up and then sets up the PPP link between endpoint machines. The following example shows a sample configuration request.

Example 24-3 Configuration Request

11:54:20 004298 ipdptp0 SEND PPP ASYNC 29 Octets LCP Config-Req 
ID=4c LEN=24 MRU=1500 ACCM=00000000 MAG#=69f4f5b2 ProtFCOMP
AddrCCOMP

Here is a description of the configuration request.

11:54:20 - Time stamp field, indicating the time when the packet was sent
004298 - Number of the packet
ipdptp0 - Network interface used
SEND PPP ASYNC - Indicates that the modem is sending asynchronous PPP
29 Octets - Amount of data the host sent
LCP - Packet type to send
ID=4c - Identifier associated with the packet; it is actually part of the packet
LEN=24 - Length of the LCP part of the packet

The remaining items are a list of options to be negotiated between hosts.

MRU=1500 - Maximum receive unit (MRU), the largest packet size the calling host can receive from the remote host
ACCM=00000000 - Asynchronous Character Map (ACCM), the mask sent to the remote host that tells what control characters to escape on transmission
MAG#=69f4f5b2 - Magic number field; used for loopback-detection mechanism
ProtFCOMP AddrCCOMP - Asks for the remote host to compress certain parts of the frame header (protocol field, address field)

The next lines are reporting invalid PPP packets. They come from the remote host, which is sending out UNIX text. This does not indicate a problem with PPP.

11:54:20 004299 ipdptp0 RECEIVE {Invalid ppp packet}PPP ASYNC 7
Octets [BAD FCS] {Unrecognized protocol:    1}

11:54:20 004299 ipdptp0 RECEIVE PPP ASYNC 73 Octets [BAD FCS]
{Unrecognized protocol:    880a}

In these packets, the local host receives the remote host's request for configuration, then sends out another configuration request. The packets are identical except for their ID fields. The ID field helps to distinguish between the two packets.

11:54:21 004301 ipdptp0 RECEIVE PPP ASYNC 29 Octets LCP Config-
Req  ID=35 LEN=24 MRU=1500 ACCM=00000000 MAG#=a8562e5f ProtFCOMP
AddrCCOMP

11:54:21 004302 ipdptp0 SEND PPP ASYNC 29 Octets LCP Config-Req 
ID=4d LEN=24 MRU=1500 ACCM=00000000 MAG#=69f4f5b2 ProtFCOMP
AddrCCOMP

In this packet, the local host acknowledges the remote request by sending it a configuration acknowledgment (Config-ACK).

11:54:21 004303 ipdptp0 SEND PPP ASYNC 29 Octets LCP Config-ACK 
ID=35 LEN=24 MRU=1500 ACCM=00000000 MAG#=a8562e5f ProtFCOMP
AddrCCOMP

The local host receives a configuration request (Config-Req) from the remote host.

11:54:21 004304 ipdptp0 RECEIVE PPP ASYNC 29 Octets LCP Config-
Req  ID=36 LEN=24 MRU=1500 ACCM=00000000 MAG#=a8562e5f ProtFCOMP
AddrCCOMP

In these packets, the local host acknowledges the second packet sent by the remote host and receives the remote host's acknowledgment.

11:54:21 004305 ipdptp0 SEND PPP ASYNC 29 Octets LCP Config-ACK 
ID=36 LEN=24 MRU=1500 ACCM=00000000 MAG#=a8562e5f ProtFCOMP
AddrCCOMP

11:54:21 004306 ipdptp0 RECEIVE PPP ASYNC 29 Octets LCP Config-
ACK  ID=4d LEN=24 MRU=1500 ACCM=00000000 MAG#=69f4f5b2 ProtFCOMP
AddrCCOMP

Here the local host negotiates parameters about IP transmission. LEN=16 gives the packet size. VJCOMP indicates Van Jacobsen header compression. IPADDR is followed by the calling host's IP address.

11:54:21 004307 ipdptp0 SEND PPP ASYNC 21 Octets IP_NCP Config-
Req  ID=4e LEN=16 VJCOMP MAXSID=15 Sid-comp-OK IPADDR=192.9.68.70

This packet indicates that the local host has received IP configuration from the remote host, including its IP address.

11:54:22 004308 ipdptp0 RECEIVE PPP ASYNC 21 Octets IP_NCP
Config-Req  ID=37 LEN=16 VJCOMP MAXSID=15 Sid-comp-OK
IPADDR=192.9.68.71

The local host sends this ACK to the remote host and receives an ACK from the remote host.

11:54:22 004309 ipdptp0 SEND PPP ASYNC 21 Octets IP_NCP Config-
ACK  ID=37 LEN=16 VJCOMP MAXSID=15 Sid-comp-OK IPADDR=192.9.68.71

11:54:22 004310 ipdptp0 RECEIVE PPP ASYNC 21 Octets IP_NCP
Config-ACK  ID=4e LEN=16 VJCOMP MAXSID=15 Sid-comp-OK
IPADDR=192.9.68.70

The first message that follows indicates that IP has started on the link. The next message indicates that the local host is sending IP traffic over the link.

11:54:22 start_ip: IP up on interface ipdptp0, timeout set for
120 seconds

11:54:24 004311 ipdptp0 SEND PPP ASYNC 89 Octets IP_PROTO

In the first message that follows, the local host receives IP traffic from the remote host. The subsequent messages indicate that the interface was disconnected because of an idle timeout.

11:54:25 004312 ipdptp0 RECEIVE PPP ASYNC 89 Octets IP_PROTO
11:56:25 process_ipd_msg: interface ipdptp0 has disconnected
11:56:25 disconnect: disconnected connection from  ipdptp0

The next messages begin the termination sequence. The first message indicates that the remote host has sent a packet to terminate the IP layer. The second is the local host's acknowledgment of the request to terminate.

11:56:25 004313 ipdptp0 RECEIVE PPP ASYNC 9 Octets IP_NCP Term-
REQ  ID=38 LEN=4

11:56:25 004314 ipdptp0 SEND PPP ASYNC 9 Octets IP_NCP Term-ACK 
ID=38 LEN=4

The local host receives a request to terminate the LCP layer. The second message is an acknowledgment of the request, causing a graceful shutdown.

11:56:25 004315 ipdptp0 RECEIVE PPP ASYNC 9 Octets LCP Term-REQ 
ID=39 LEN=4

11:56:25 004316 ipdptp0 SEND PPP ASYNC 9 Octets LCP Term-ACK 
ID=39 LEN=4

This message indicates that the link has closed.

11:56:29 004317 ipdptp0 PPP DIAG CLOSE