Transport protocols

A transport protocol refers to the protocol at the bottom of the protocol stack which is responsible for sending and receiving messages to/from the network. There are a number of transport protocols in JavaGroups. They are discussed in the following sections.

A typical protocol stack configuration using UDP is^5.3:

  "UDP(mcast_addr=224.0.0.35;mcast_port=45566;ip_ttl=32;" +
      "mcast_send_buf_size=150000;mcast_recv_buf_size=80000):" +
  "PING(timeout=2000;num_initial_members=3):" +
  "MERGE2(min_interval=5000;max_interval=10000):" +
  "FD_SOCK:" +
  "VERIFY_SUSPECT(timeout=1500):" +
  "pbcast.STABLE(desired_avg_gossip=20000):" +
  "pbcast.NAKACK(gc_lag=50;retransmit_timeout=300,600,1200,2400,4800):" +
  "UNICAST(timeout=5000;min_wait_time=2000):" +
  "FRAG(frag_size=4096;down_thread=false;up_thread=false):" +
  "pbcast.GMS(join_timeout=5000;join_retry_timeout=2000;" +
              "shun=false;print_local_addr=true)"

In a nutshell the properties of the protocols are:

UDP

Uses IP multicast for group messages and UDP packets for messages to individual members

PING

Uses IP multicast (by default) to find initial members. Once found, the current coordinator can be determined and a unicast JOIN request will be sent to it

MERGE2

Will merge subgroups back into one group

FD_SOCK

Failure detection based on sockets (in a ring form between members). Generates notification if a member fails

VERIFY_SUSPECT

Double-checks whether suspected member is really dead, otherwise suspicion generated from protocol below is discarded

pbcast.STABLE

Deletes messages that have been seen by all members (distributed message garbage collection)

pbcast.NAKACK

Ensures (a) message reliability and (b) FIFO. Message reliability guarantees that a message will be received. If not, receiver will request retransmission. FIFO guarantees that all messages from sender P will be received in the order P sent them

UNICAST

Same as NAKACK for unicast messages: messages from sender P will not be lost (retransmission if necessary) and will be in FIFO order (essentially the same as TCP in TCP/IP, without the flow control)

FRAG

Fragments large messages into smaller ones and reassembles them back at the receiver side. For both multicast and unicast messages

pbcast.GMS

Membership protocol. Responsible for joining/leaving members and installing new views.