Tcp Window Scaling; Tcp Sliding Window; Tcp Outgoing Queue Size; Tcp Congestion Avoidance - Cisco ASR 1000 Series Configuration Manual

Configuring TCP

TCP Window Scaling

The TCP Window Scaling feature adds support for the Window Scaling option in RFC 1323,TCP Extensions
for High Performance. A larger window size is recommended to improve TCP performance in network paths
with large bandwidth-delay product characteristics that are called Long Fat Networks (LFNs). The TCP
Window Scaling enhancement provides LFN support.
The window scaling extension expands the definition of the TCP window to 32 bits and then uses a scale
factor to carry this 32-bit value in the 16-bit window field of the TCP header. The window size can increase
to a scale factor of 14. Typical applications use a scale factor of 3 when deployed in LFNs.
The TCP Window Scaling feature complies with RFC 1323. The maximum window size was increased to
1,073,741,823 bytes. The larger scalable window size will allow TCP to perform better over LFNs. Use the
ip tcp window-size command in global configuration mode to configure the TCP window size.

TCP Sliding Window

A TCP sliding window provides an efficient use of network bandwidth because it enables hosts to send multiple
bytes or packets before waiting for an acknowledgment.
In TCP, the receiver specifies the current window size in every packet. Because TCP provides a byte-stream
connection, window sizes are expressed in bytes. A window is the number of data bytes that the sender is
allowed to send before waiting for an acknowledgment. Initial window sizes are indicated at connection setup,
but might vary throughout the data transfer to provide flow control. A window size of zero means "Send no
data." The default TCP window size is 4128 bytes. We recommend that you keep the default value unless
your device is sending large packets (greater than 536 bytes). Use the ip tcp window-size command to change
the default window size.
In a TCP sliding-window operation, for example, the sender might have a sequence of bytes to send (numbered
1 to 10) to a receiver who has a window size of five. The sender then places a window around the first five
bytes and transmits them together. The sender then waits for an acknowledgment.
The receiver responds with an ACK = 6, indicating that it has received bytes 1 to 5 and is expecting byte 6
next. In the same packet, the receiver indicates that its window size is 5. The sender then moves the sliding
window five bytes to the right and transmits bytes 6 to 10. The receiver responds with an ACK = 11, indicating
that it is expecting sequenced byte 11 next. In this packet, if the receiver indicates that its window size is 0,
the sender cannot send any more bytes until the receiver sends another packet with a window size greater than
0.

TCP Outgoing Queue Size

The default TCP outgoing queue size per connection is five segments if the connection has a TTY associated
with it (such as a Telnet connection). If no TTY connection is associated with a connection, the default queue
size is 20 segments. Use the ip tcp queuemax command to change the five-segment default value.

TCP Congestion Avoidance

The TCP Congestion Avoidance feature enables the monitoring of acknowledgment packets to the TCP sender
when multiple packets are lost in a single window of data. Previous to introduction of this feature, the sender
would exit Fast-Recovery mode, wait for three or more duplicate acknowledgment packets before retransmitting
IP Application Services Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
TCP Window Scaling
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