Hi Check if the issue is similar to one mentioned below
This syslog is seen on the ASA:
%ASA-6-722036: Group <ac_users_group> User <vpn> IP <10.1.75.111>
Transmitting large packet 1418 (threshold 1347).
Causes
The cause of this issue is the failure to build a Datagram Transport Layer Security (DTLS) tunnel. This could be because of two reasons:
- DTLS is blocked somewhere in the path
- Use of a non-default DTLS port
DTLS is Blocked Somewhere in the Path
As of ASA Release 9.x and AnyConnect Release 3.x, an optimization has been introduced in the form of distinct Maximum Transition Units (MTUs) that are negotiated for TLS/DTLS between the client/ASA. Previously, the client derived a rough estimate MTU which covered both TLS/DTLS and was obviously less than optimal. Now, the ASA computes the encapsulation overhead for both TLS/DTLS and derives the MTU values accordingly.
As long as DTLS is enabled, the client applies the DTLS MTU (in this case 1418) on the VPN adapter (which is enabled before the DTLS tunnel is established and is needed for routes/filters enforcement), to ensure optimum performance. If the DTLS tunnel cannot be established or it is dropped at some point, the client fails over to TLS and adjusts the MTU on the virtual adapter (VA) to the TLS MTU value (this requires a session level reconnect).
Resolution
In order to eliminate this visible transition of DTLS > TLS, the administrator can configure a separate tunnel group for TLS only access for users that have trouble with the establishment of the DTLS tunnel (such as due to firewall restrictions).
- The best option is to set the AnyConnect MTU value to be lower than the TLS MTU, which is then negotiated.
group-policy ac_users_group attributes
webvpn
anyconnect mtu 1300
This makes TLS and DTLS MTU values equal. Reconnections are not seen in this case.
- The second option is to allow fragmentation.
group-policy ac_users_group attributes
webvpn
anyconnect ssl df-bit-ignore enable
With fragmentation, large packets (whose size exceeds the MTU value) can be fragmented and sent through the TLS tunnel.
- The third option is to set the Maximum Segment Size (MSS) to 1460 as follows:
sysopt conn tcpmss 1460
In this case, the TLS MTU will be 1427 (RC4/SHA1) which is larger than the DTLS MTU 1418 (AES/SHA1/LZS). This should resolve the issue with TCP from the ASA to the AnyConnect client (thanks to MSS), but large UDP traffic from the ASA to the AnyConnect client might suffer from this as it will be dropped by the AnyConnect client due to the lower AnyConnect client MTU 1418. If sysopt conn tcpmss is modified, it might affect other features such as LAN-to-LAN (L2L) IPSec VPN tunnels.
HTH
Abaji.
