TCP Time Stamp
The TCP time-stamp option provides improved TCP round-trip time measurements. Because the time stamps
are always sent and echoed in both directions and the time-stamp value in the header is always changing, TCP
header compression will not compress the outgoing packet. To allow TCP header compression over a serial
link, the TCP time-stamp option is disabled. Use the ip tcp timestamp command to enable the TCP time-stamp
option.
Refer to RFC 1323 for more details on TCP time stamps.
TCP Maximum Read Size
The maximum number of characters that TCP reads from the input queue for Telnet and relogin at one time
is very large (the largest possible 32-bit positive number) by default. To change the TCP maximum read size
value, use the ip tcp chunk-size command in global configuration mode.
We do not recommend that you change this value.Note
TCP Path MTU Discovery
Path MTU Discovery is a method for maximizing the use of the available bandwidth in the network between
endpoints of a TCP connection, which is described in RFC 1191. IP Path MTU Discovery allows a host to
dynamically discover and cope with differences in the maximum allowable maximum transmission unit (MTU)
size of the various links along the path. Sometimes a device is unable to forward a datagram because it requires
fragmentation (the packet is larger than the MTU that you set for the interface with the interface configuration
command), but the “do not fragment” (DF) bit is set. The intermediate gateway sends a “Fragmentation needed
and DF bit set” Internet Control Message Protocol (ICMP) message to the sending host, alerting the host to
the problem. On receiving this message, the host reduces its assumed path MTU and consequently sends a
smaller packet that will fit the smallest packet size of all links along the path.
By default, TCP Path MTU Discovery is disabled. Existing connections are not affected irrespective of whether
this feature is enabled or disabled.
Customers using TCP connections to move bulk data between systems on distinct subnets would benefit most
by enabling this feature. Customers using remote source-route bridging (RSRB) with TCP encapsulation,
serial tunnel (STUN), X.25 Remote Switching (also known as XOT or X.25 over TCP), and some protocol
translation configurations might also benefit from enabling this feature.
Use the ip tcp path-mtu-discovery global configuration command to enable Path MTU Discovery for
connections initiated by the device when the device is acting as a host.
For more information about Path MTU Discovery, refer to the “Configuring IP Services” module of the IP
Application Services Configuration Guide.
IP Application Services Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
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Configuring TCP
TCP Time Stamp
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