Re: [Sigtran] Number of links towards a DPC (ITU)

My 2 cents.
The ITU spec may not say this, but why not do this.
1) Use the LSB to select the linkset within a combined linkset.
2) Once the Linkset is selected, use the full SLS value to select one of the links in the linkset.
In this way, we can still have 16 links in each linkset and combined linkset can have 32 links and still loadshare across all the links.
Each linkset will still have 16 of the SLS values so, there won't be any issues of un-even loadsharing even if some links go down in the route to the final destination.

Regards,
Satya

-----Original Message-----
From: sigtran-bounces@ietf.org [mailto:sigtran-bounces@ietf.org] On Behalf Of David Laight
Sent: 17 June 2013 10:11
To: bidulock@openss7.org; Arvind Kumar
Cc: sigtran@ietf.org
Subject: Re: [Sigtran] Number of links towards a DPC (ITU)

> ITU has a 4-bit SLS and a 4 bit SLC.  The SLS and SLC is shared under 
> ITU and ITU does not do SLS rotation.  Therefore, because 1 bit is 
> used for link-set selection, there can only be 8 links in a linkset 
> (16 links in a combined linkset).

In ITU each linkset can have 16 links.
Brian is right in that MTP3 only has a 4 bit SLS value to select the destination link, so traffic to a specific destination will only use a maximum of 16 links even if there are routes by 2 adjacents and there are 16 links in the linkset to each.

If you have two linksets with 16 links each, the routing rules for different destinations could be arranged to use different links in order to utilise all 32 links.

> ANSI has an 8-bit SLS and a 5-bit SLC.  The SLS and SLC are not shared 
> and ANSI does SLS rotation.  Therefore, it is possible to have 32 
> links in a linkset (64 links in a combined linkset).  Using the old 
> 5-bit SLS it was only possible to have 16 links in a linkset and 32 
> links in a combined linkset because 1 of the 5 bits was used for 
> linkset selection within a combined linkset.

ANSI only has 4 bits for the SLC (not 5), so a linkset is limited to 16 links.
However the larger SLS value (8 bits, 5 in some earlier versions) means that MTP3 can loadshare between more links.

It also isn't quite right to say that one of the bits is used for linkset selection. If some of the links are down, the sls that would be assigned to the 'down' links are evenly spread across all of the working links, not just assigned to the working links in their original linkset (this might be a config option since the ANSI and ITU specs differ here.)

An endsystem ITU MTP3 may have more SLS bits available, so could loadshare between more than 16 links.

	David

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