Cisco ASR 1000 Series Configuration Guide

the next unacknowledged packet, or wait for the retransmission timer to start slowly. This delay could lead
to performance issues.
Implementation of RFC 2581 and RFC 3782 addresses the modifications to the Fast-Recovery algorithm that
incorporates a response to partial acknowledgments received during Fast Recovery, improving performance
in situations where multiple packets are lost in a single window of data.
This feature is an enhancement to the existing Fast Recovery algorithm. No commands are used to enable or
disable this feature.
The output of the debug ip tcp transactions command has been enhanced to monitor acknowledgment packets
by showing the following conditions:
•
TCP entering Fast Recovery mode.
•
Duplicate acknowledgments being received during Fast Recovery mode.
•
Partial acknowledgments being received.
TCP Explicit Congestion Notification
The TCP Explicit Congestion Notification (ECN) feature allows an intermediate router to notify end hosts of
impending network congestion. It also provides enhanced support for TCP sessions associated with applications,
such as Telnet, web browsing, and transfer of audio and video data that are sensitive to delay or packet loss.
The benefit of this feature is the reduction of delay and packet loss in data transmissions. Use the ip tcp ecn
command in global configuration mode to enable TCP ECN.
TCP MSS Adjustment
The TCP MSS Adjustment feature enables the configuration of the maximum segment size (MSS) for transient
packets that traverse a device, specifically TCP segments with the SYN bit set. Use the ip tcp adjust-mss
command in interface configuration mode to specify the MSS value on the intermediate device of the SYN
packets to avoid truncation.
When a host (usually a PC) initiates a TCP session with a server, the host negotiates the IP segment size by
using the MSS option field in the TCP SYN packet. The value of the MSS field is determined by the MTU
configuration on the host. The default MSS value for a PC is 1500 bytes.
The PPP over Ethernet (PPPoE) standard supports a Maximum Transmission Unit (MTU) of only 1492 bytes.
The disparity between the host and PPPoE MTU size can cause the device in between the host and the server
to drop 1500-byte packets and terminate TCP sessions over the PPPoE network. Even if the path MTU (which
detects the correct MTU across the path) is enabled on the host, sessions may be dropped because system
administrators sometimes disable ICMP error messages that must be relayed from the host for path MTU to
work.
The ip tcp adjust-mss command helps prevent TCP sessions from being dropped by adjusting the MSS value
of the TCP SYN packets.
The ip tcp adjust-mss command is effective only for TCP connections passing through the device.
In most cases, the optimum value for the max-segment-size argument of the ip tcp adjust-mss command is
1452 bytes. This value plus the 20-byte IP header, the 20-byte TCP header, and the 8-byte PPPoE header add
up to a 1500-byte packet that matches the MTU size for the Ethernet link.
IP Application Services Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
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Configuring TCP
TCP Explicit Congestion Notification

Cisco ASR 1000 Series Specifications

General

Brand	Cisco
Model	ASR 1000 Series
Category	Network Router
Language	English