132 Software Networks
To separate the flow control from the congestion control, LS-STCP
uses two different sequence numbers. The first is the ASN
(Association Sequence Number), which is the sequence number for
the active connection. It is used to order all the chunks received by the
buffer on the connection to the receiver. The second is the sequence
number of data chunks for each path, called the PSN (Path Sequence
Number). It ensures that each path is reliable and controls the path
congestion. In the LS-SCTP architecture, the Path Assignment module
takes care of the distribution of the data between the available paths.
This distribution is based on the availability of bandwidth. This
module distributes chunks depending on the cwnd/RTT ratio of each
path. Once the path is selected, the chunk provides a PID, indicating
the path and a PSN.
Another difference between LS-SCTP and SCTP is the
retransmission mechanism. To improve the likelihood of a
retransmitted data chunk reaching the receiver, the retransmission uses
a different path to that used for the previous attempted transmission.
The Path Monitor module is responsible for overseeing the available
paths and updating the list of active paths. This module updates the
active path list when it obtains information from the network relating
to the failure of an existing path or the availability of a new path.
When the transmitter detects that a path is unavailable, it changes the
status of that path to “inactive”. Inactive paths still remain associated
with the connection. The transmitter continues to monitor them by
means of the HeartBeat mechanism. As soon as an inactive path
becomes active once more, Path Monitor adds it to the list of active
paths and uses it for transmission.
5.8.2. CMT (Concurrent Multipath Transfer)
Multihoming is incompletely supported by SCTP. The only reason
for having multiple paths available is to have redundancy in place.
SCTP transmits via the primary path. The secondary paths are used
only if the primary path fails. An extension of SCTP, called CMT
(Concurrent Multipath Transfer), has been put forward for
transmission via multiple paths.
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To separate the flow control from the congestion control, LS-STCP
uses two different sequence numbers. The first is the ASN
(Association Sequence Number), which is the sequence number for
the active connection. It is used to order all the chunks received by the
buffer on the connection to the receiver. The second is the sequence
number of data chunks for each path, called the PSN (Path Sequence
Number). It ensures that each path is reliable and controls the path
congestion. In the LS-SCTP architecture, the Path Assignment module
takes care of the distribution of the data between the available paths.
This distribution is based on the availability of bandwidth. This
module distributes chunks depending on the cwnd/RTT ratio of each
path. Once the path is selected, the chunk provides a PID, indicating
the path and a PSN.
Another difference between LS-SCTP and SCTP is the
retransmission mechanism. To improve the likelihood of a
retransmitted data chunk reaching the receiver, the retransmission uses
a different path to that used for the previous attempted transmission.
The Path Monitor module is responsible for overseeing the available
paths and updating the list of active paths. This module updates the
active path list when it obtains information from the network relating
to the failure of an existing path or the availability of a new path.
When the transmitter detects that a path is unavailable, it changes the
status of that path to “inactive”. Inactive paths still remain associated
with the connection. The transmitter continues to monitor them by
means of the HeartBeat mechanism. As soon as an inactive path
becomes active once more, Path Monitor adds it to the list of active
paths and uses it for transmission.
5.8.2. CMT (Concurrent Multipath Transfer)
Multihoming is incompletely supported by SCTP. The only reason
for having multiple paths available is to have redundancy in place.
SCTP transmits via the primary path. The secondary paths are used
only if the primary path fails. An extension of SCTP, called CMT
(Concurrent Multipath Transfer), has been put forward for
transmission via multiple paths.
www.it-ebooks.info