This document discusses the specific scenario where the AnyConnect client might reconnect to the Adaptive Security Appliance (ASA) in exactly one minute. The users might not be able to receive traffic over the Transport Layer Security (TLS) tunnel until AnyConnect reconnects. This is dependent upon a few other factors which are discussed in this document.
In this example, the AnyConnect client is shown as it reconnects to the ASA.
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).
These Diagnostics and Reporting Tool (DART) logs are seen with this issue:
******************************************
Date : 11/16/2013
Time : 01:28:50
Type : Warning
Source : acvpnagent
Description : Reconfigure reason code 16:
New MTU configuration.
******************************************
Date : 11/16/2013
Time : 01:28:50
Type : Information
Source : acvpnagent
Description : The entire VPN connection is being reconfigured.
******************************************
Date : 11/16/2013
Time : 01:28:51
Type : Information
Source : acvpnui
Description : Message type information sent to the user:
Reconnecting to 10.1.1.2...
******************************************
Date : 11/16/2013
Time : 01:28:51
Type : Warning
Source : acvpnagent
Description : A new MTU needs to be applied to the VPN network interface.
Disabling and re-enabling the Virtual Adapter. Applications utilizing the
private network may need to be restarted.
******************************************
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
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).
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.
Another potential cause for the DTLS failure is enabling DTLS on a non-default port after the WebVPN is enabled (for example, when the webvpn enable outside command is entered). This is due to Cisco bug ID CSCuh61321 and has been seen in Release 9.x where the ASA pushes the non-default port to the client, but continues to listen to the default port. Consequently, the DTLS is not built and AnyConnect reconnects.
webvpn
port 444
enable outside
dtls port 444
anyconnect enable
ciscoasa(config-webvpn)# show asp table socket
Protocol Socket State Local Address Foreign Address
SSL 0001fc08 LISTEN 172.16.11.1:444 0.0.0.0:*
DTLS 00020dc8 LISTEN 172.16.11.1:443 0.0.0.0:*
After the TLS tunnel is established, the client attempts to establish the DTLS tunnel to port 444 as expected :
The order of the commands that lead to the problem and the accelerated security path (ASP) table sockets opened is:
Start with the WebVPN sockets not enabled.
ciscoasa(config)# show run webvpn
webvpn
anyconnect image disk0:/anyconnect-win-3.1.04066-k9.pkg 1
anyconnect enable
ciscoasa(config)# show asp table socket
Protocol Socket State Local Address Foreign Address
ciscoasa(config)#
Change TLS port to 444 and enable WebVPN.
ciscoasa(config-webvpn)# show run webvpn
webvpn
port 444
enable outside
anyconnect image disk0:/anyconnect-win-3.1.04066-k9.pkg 1
anyconnect enable
ciscoasa(config-webvpn)# show asp tabl socket
Protocol Socket State Local Address Foreign Address
SSL 0001fc08 LISTEN 172.16.11.1:444 0.0.0.0:*
DTLS 00020dc8 LISTEN 172.16.11.1:443 0.0.0.0:*
Change the DTLS port to 444.
ciscoasa(config-webvpn)# dtls port 444
ciscoasa(config-webvpn)#
ciscoasa(config-webvpn)# show run webvpn
webvpn
port 444
enable outside
dtls port 444
anyconnect image disk0:/anyconnect-win-3.1.04066-k9.pkg 1
anyconnect enable
ciscoasa(config-webvpn)# show asp table socket
Protocol Socket State Local Address Foreign Address
SSL 0001fc08 LISTEN 172.16.11.1:444 0.0.0.0:*
DTLS 00020dc8 LISTEN 172.16.11.1:443 0.0.0.0:*
The workaround for this problem is to follow the order of :
Disable the WebVPN.
Enter the DTLS port.
Enable the WebVPN.
This behaviour does not exist in Release 8.4.x versions, where the DTLS sockets get updated with the configured ports immediately after the configuration is entered:
ASA Release 8.4.6 :
ciscoasa(config-webvpn)# port 444
ciscoasa(config-webvpn)# enable outside
ciscoasa(config-webvpn)# show asp table socket
Protocol Socket Local Address Foreign Address State
SSL 0000bf2f 172.16.11.1:444 0.0.0.0:* LISTEN
DTLS 0000d5df 172.16.11.1:443 0.0.0.0:* LISTEN
ciscoasa(config-webvpn)# dtls port 444
ciscoasa(config-webvpn)#
ciscoasa(config-webvpn)# show asp table socket
Protocol Socket Local Address Foreign Address State
SSL 0000bf2f 172.16.11.1:444 0.0.0.0:* LISTEN
DTLS 0000eb5f 172.16.11.1:444 0.0.0.0:* LISTEN << changed immediately
Suppose that these ciphers are configured:
ssl encryption rc4-sha1 aes128-sha1 aes256-sha1
This sequence of events takes place in this case:
1380 - 5 (TLS header) - 8 (CSTP) - 0 (padding) - 20 (HASH) = 1347
%ASA-6-722036: Group <ac_users_group> User <vpn> IP <10.1.75.111>and drops packets with mp-svc-no-fragment-ASP drop reason.
Transmitting large packet 1418 (threshold 1347)
%ASA-6-602101: PMTU-D packet 1418 bytes greater than effective mtu 1347,
dest_addr=10.10.10.1, src_addr=10.48.66.200, prot=TCP
For more information on reconnect behavior and timers, see AnyConnect FAQ: Tunnels, Reconnect Behavior, and the Inactivity Timer
Cisco bug ID CSCuh61321 AC 3.1:ASA incorrectly handles alternate DTLS port,causes reconnect