An application seeking to establish a connection contacts the local
HEC, which in turn initiates a three-pass
connection establishment procedure. Although we limit ourselves
to a source-initiated,
source-routed scenario, this scheme can be adapted to
destination-initiated or adaptive routing schemes as well. In the following,
we briefly describe the procedure. For a more detailed description, please
refer to [12].
Pass 1
The HEC at the sender host sends an establish request message,
which travels downstream to the destination processed by each SEC along the
route. The establish request message for the new
connection 
contains the worst-case message length of Connection
and information about connections that branch in at switches upstream.
This information, which is updated by each SEC along the path, is used in
Pass 2 to calculate the BI and BO values defined in
Section 3.
Pass 2
When the establish request message reaches the destination host, the
second pass is invoked by sending an establish reply message
upstream to the sender host. As the message is sent from 
to
, it contains the value 
. This value
is zero when the message is sent from the destination host to the next
switch upstream.
Upon receiving an establish reply message for from
, the local SEC at calculates the local value of 
and can now
determine the guard-based calculated worst-case blocking delay
. It then checks whether 
is
less than or equal to the local blocking guard, 
and
aborts the establishment process if not. Otherwise it continues the process
by locally computing 
and forwarding 
to the next SEC upstream.
Pass 3
When the establish reply message reaches the HEC at the source
host, the final acceptance test is performed by comparing the
value in the establish reply with the deadline requirements of .
If the test fails, the connection is rejected and the establishment procedure
aborted. Otherwise, the third and final pass is invoked by sending an
establish confirm message along the route of to the
destination host. When sent from 
to , this message
contains the entry 
and
the number of hops to the destination, k.
The initial value of X at the sender HEC is 
.
Upon receiving an establish confirm message, the SEC at
updates the blocking guard to be 
, where
.
The old blocking guard value is stored for use during teardown.
When is torn down, the SEC attempts to re-establish the old blocking
guard, as described in Section 4.2.
The new value for the proposed residual time 
between this and
the next switch is calculated,
The new connection is marked as accepted and the establish confirm
message is forwarded to with the updated entries 
and k-1.
When the establish confirm message reaches the SEC at the receiver host, the establishment process is completed.