This article is rather technical and covers the way we connect to BT for broadband links.
What is an LNS
An LNS (L2TP Network Server) is a piece of equipment that connects to a carrier such as BT or TT and handles the sessions from broadband lines. They are also used for dialup and mobile links.
The other end of the link, in the carrier is an LAC and is usually the BRAS. This passes on the PPP (Point to Point Protocol) connection from your router to us over a protocol called L2TP.
The LNS handles authentication (logging in using your username and password) and routing of your IP addresses to you. It handles the negotiation of the link with your equipment and establishes a session.
We operate multiple LNSs connected to multiple links in to carriers. This is partly for redundancy (if one of the LNSs or links break) and partly for load (to handle multiple gigabits of traffic).
We currently have 14 LNSs in use (A-N), plus one spare and one 'test' LNS. This has grown over time with four in 2012, seven in April 2016, nine in January 2017, twelve in November 2017 and 14 in September 2018.
Normally we run with n-1 live LNSs and so this leaves one as a backup in case of failure.
Maintenance and LNS switches
From time to time there are upgrades to the LNSs. This is usually to add new features, or fix bugs. The features are tested by the developers and then deployed on test LNSs which customers and staff can use to confirm all is well. Once ready the new software can be deployed on the live LNSs.
Upgrading an LNS is very quick, and it is only off line for under a second. However, it does mean stopping all of the sessions on the LNS. If we simply turned off an LNS it may take several minutes for all of the sessions to reconnect to another.
What we do to manage the upgrade is an LNS switchover. This means we upgrade the backup LNS (which has no traffic on it), and then we arrange for one of the live LNSs to close sessions one by one so they reconnect cleanly to the newly upgraded LNS. Each session is normally off for a few seconds, but this depends on the equipment you have (how quickly it retries) and the carrier network, so can be minutes in some cases.
Once all of the sessions are moved, the LNS is then ready to be upgraded, and swapped with one of the other live LNSs.
Normal procedure is to do this over night, swapping out one LNS per night. We even have a setting on the control pages to indicate when, during the night, you would prefer the LNS switch over.
Multiple line bonding
We offer multiple line bonding as a standard service, and this only works if the lines go to the same LNS. The system we have in place steers each connection to a preferred LNS. Whilst the load is, overall, spread over three (or more) live LNSs, you will consistently go to the same LNS for your login, and multiple lines on the same site will go to the same LNS. This ensures graphs are consistent and that bonding works.
Where you have multiple lines and we do an LNS switch over night, they will be switched based on the preferred time you have selected. However, once one line on a multi-line site has been switched, then next line will be switched within a few minutes of the first line coming up. This means that you do not have a long period where the lines are split between LNSs. This happens even if the other lines have a later time during the night preferred, but only after at least one line has come on line.
Choosing the Time
You can set the preferred time for the reconnect to happen to your own line(s), the default is 1AM. There is a setting on the control pages for this:
We have a test LNS. You can log in to this by prefixing your login with test- which causes your session to go to the test LNS.
Apart from helping us with testing of new features, the test LNS also have firewalling systems, and this can be useful where you need a temporary network side firewall. Support staff can advise if this is needed.
The only downside is that we do reload the test LNS with no notice.
What we mean by a PPP Kill is that the session, the connection, that joins your router to our LNS is stopped (killed). Your router then reconnects. Some routers reconnect in under 5 seconds, but some can take longer. In some rare cases a router will sulk and need manually restarting and we would recommend changing the router if this happens. Generally the process means that a line will be off from 5 to 10 seconds and then reconnect (slightly longer for 20CN lines). We can do a PPP kill, such as when moving lines between LNSs for a software upgrade. You can also do a PPP kill, either telling your router to reconnect via its user interface or from our control pages. A PPP kill does not mean your line re-syncs (unless your router is particularly strange) and so will have no impact on sync rates and other settings which can be affected by a re-sync.
We keep graphs for packet loss, latency and throughput and these come from the LNS. This means if any any time you change from one LNS to another the graphs are split. During the day the current graph is always from the LNS you last logged in to. Over night the graphs are archived. When viewing previous days the graphs from multiple LNS are merged.
Usually this creates a good graph for the previous day. However, in some cases the previous day on one of the LNSs may have been deleted before archive. In some cases the two LNS may operate on a different scale and so the combined graph looks messy and confusing. In some cases we change the graph format slightly and this can also lead to a messy graph.