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Administering TCP/IP Networks, IPMP, and IP Tunnels in Oracle® Solaris 11.4

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Updated: November 2020

Using the TCP ECN Feature

In Oracle Solaris 11.4, TCP actively includes Explicit Congestion Notification (ECN) negotiation with a peer when initiating a connection. ECN negotiation enables TCP to more accurately detect congestion on the network and react appropriately.

This feature requires that you use routers and switches that support ECN and allow ECN-enabled packets to pass through the network. Otherwise, remote connectivity might become problematic and you would need to adjust the ECN setting.

    To detect whether a connection problem with a remote host is related to ECN, use the following approach:

  1. Identify the target's IP address.

    For this example, assume from the host source-server you are connecting to old-server with the IP address

  2. Use the snoop command as follows:

    $ snoop -V from src tcp
    Using device net3 (promiscuous mode)
    old-server.example.com -> source-server ETHER Type=0800 (IP), size=60 bytes
    old-server.example.com -> source-server IP  D= S= LEN=40, ID=30421, TOS=0x0, TTL=25
    old-server.example.com -> source-server TCP D=60726 S=2007 Rst Ack=3966642163 Win=0
    $ snoop -v -I net3 from src tcp
    output elided
    IP:   ----- IP Header -----
    IP:   Version = 4
    IP:   Header length = 20 bytes
    IP:   Type of service = 0x00
    IP:         xxx. .... = 0 (precedence)
    IP:         ...0 .... = normal delay
    IP:         .... 0... = normal throughput
    IP:         .... .0.. = normal reliability
    IP:         .... ..0. = not ECN capable transport
    IP:         .... ...0 = no ECN congestion experienced
    IP:   Total length = 40 bytes
    IP:   Identification = 22213
    IP:   Flags = 0x0
    IP:         .0.. .... = may fragment
    IP:         ..0. .... = last fragment
    IP:   Fragment offset = 0 bytes
    IP:   Time to live = 25 seconds/hops
    IP:   Protocol = 6 (TCP)
    IP:   Header checksum = 6157
    IP:   Source address =, old-server.example.com
    IP:   Destination address =, source-server.example.com
    IP:   No options
    output elided

    The output's IP Header indicates that ECN is not supported.

  3. Change the default ECN property setting and then retest connections to the target.

    root:~$ ipadm set-prop -p ecn=passive tcp