Specifications

4-66
Cisco Internet Streamer CDS 2.0-2.3 Software Configuration Guide
OL-13493-04
Chapter 4 Configuring Devices
Configuring the Service Engine
(Initially, the value of the ssthresh variable is adjusted to the receiver’s maximum window size [RMSS].
However, when congestion occurs, the ssthresh variable is set to half the current value of the cwnd
variable, marking the point of the onset of network congestion for future reference.)
The starting value of the cwnd variable is set to that of the sender maximum segment size (SMSS), which
is the size of the largest segment that the sender can transmit. The sender sends a single data segment,
and because the congestion window is equal to the size of one segment, the congestion window is now
full. The sender then waits for the corresponding ACK from the receiving side of the transmission. When
the ACK is received, the sender increases its congestion window size by increasing the value of the cwnd
variable by the value of one SMSS. Now the sender can transmit two segments before the congestion
window is again full and the sender is once more required to wait for the corresponding ACKs for these
segments. The slow start algorithm continues to increase the value of the cwnd variable and therefore
increase the size of the congestion window by one SMSS for every ACK received. If the value of the
cwnd variable increases beyond the value of the ssthresh variable, then the TCP flow control algorithm
changes from the slow start algorithm to the congestion-avoidance algorithm.
TCP-Over-Satellite Extensions
The SE has the ability to turn on TCP-over-satellite extensions (as documented in RFC 1323) to
maximize performance and end-to-end throughput over satellite-type connections.
The large number of satellites available to network infrastructures has increased the amount of
bandwidth available in the air. Taking advantage of these connections through satellite-type connections
has created new challenges in the use of TCP transactions and acknowledgments:
• Latency—Round-trip times to satellites orbiting 24,000 miles above the earth are 550 milliseconds
for a single satellite hop. Buffer size must be set to prevent low-throughput connections.
• Bit errors—Packet loss can occur in a land-based device-to-satellite connection in addition to the
losses caused by regular network congestion.
• Asymmetric bandwidth—Return bandwidth from satellites can be narrower than receiving
bandwidth, thereby affecting performance.
Use the fields provided under the TCP Server Settings and TCP Client Settings sections to set the TCP
connection so that it complies with RFC 1323.
Configuring IP General Settings
The Path Maximum Transmission Unit (MTU) Discovery discovers the largest IP packet size allowable
between the various links along the forwarding path and automatically sets the correct value for the
packet size. By using the largest MTU the links will bear, the sending device can minimize the number
of packets it must send.
Note The Path MTU Discovery is a process initiated by the sending device. If a server does not support IP
Path MTU Discovery, the receiving device has no mechanism available to avoid fragmenting datagrams
generated by the server.
To enable Path MTU Discovery, do the following:
Step 1 Choose Devices > Devices > General Settings > Network > IP General Settings. The IP General
Settings page is displayed.
Step 2 Check Enable Path MTU Discovery.