path: root/fs/dlm/midcomms.c

// SPDX-License-Identifier: GPL-2.0-only
/******************************************************************************
*******************************************************************************
**
**  Copyright (C) Sistina Software, Inc.  1997-2003  All rights reserved.
**  Copyright (C) 2004-2021 Red Hat, Inc.  All rights reserved.
**
**
*******************************************************************************
******************************************************************************/

/*
 * midcomms.c
 *
 * This is the appallingly named "mid-level" comms layer. It takes care about
 * deliver an on application layer "reliable" communication above the used
 * lowcomms transport layer.
 *
 * How it works:
 *
 * Each nodes keeps track of all send DLM messages in send_queue with a sequence
 * number. The receive will send an DLM_ACK message back for every DLM message
 * received at the other side. If a reconnect happens in lowcomms we will send
 * all unacknowledged dlm messages again. The receiving side might drop any already
 * received message by comparing sequence numbers.
 *
 * How version detection works:
 *
 * Due the fact that dlm has pre-configured node addresses on every side
 * it is in it's nature that every side connects at starts to transmit
 * dlm messages which ends in a race. However DLM_RCOM_NAMES, DLM_RCOM_STATUS
 * and their replies are the first messages which are exchanges. Due backwards
 * compatibility these messages are not covered by the midcomms re-transmission
 * layer. These messages have their own re-transmission handling in the dlm
 * application layer. The version field of every node will be set on these RCOM
 * messages as soon as they arrived and the node isn't yet part of the nodes
 * hash. There exists also logic to detect version mismatched if something weird
 * going on or the first messages isn't an expected one.
 *
 * Termination:
 *
 * The midcomms layer does a 4 way handshake for termination on DLM protocol
 * like TCP supports it with half-closed socket support. SCTP doesn't support
 * half-closed socket, so we do it on DLM layer. Also socket shutdown() can be
 * interrupted by .e.g. tcp reset itself. Additional there exists the othercon
 * paradigm in lowcomms which cannot be easily without breaking backwards
 * compatibility. A node cannot send anything to another node when a DLM_FIN
 * message was send. There exists additional logic to print a warning if
 * DLM wants to do it. There exists a state handling like RFC 793 but reduced
 * to termination only. The event "member removal event" describes the cluster
 * manager removed the node from internal lists, at this point DLM does not
 * send any message to the other node. There exists two cases:
 *
 * 1. The cluster member was removed and we received a FIN
 * OR
 * 2. We received a FIN but the member was not removed yet
 *
 * One of these cases will do the CLOSE_WAIT to LAST_ACK change.
 *
 *
 *