Subject: [mpls] New Comments on Segment Routing(6): Challenge of Loose Explicit Path for SR-TE Path

From: Lizhenbin <[email-masked]>
Date: Mon, 26 Jan 2015 11:40:57 +0000

Hi all authors of segment routing,

This is the sixth comments on segment routing. It is another issue related with SR-TE path.

                 S4---S5
                /       \
    S1---S2---S3         S8---S9---S10
                \       /
                 S6---S7

For MPLS TE, there are two types of explicit paths: strict and loose. Even for the loose hop, the ingress node will

calculate all the hops to the loose hop. And the MPLS TE path is independent from the routing, this means even if

the route change, the MPLS TE path to the loose hop can be kept. For example, as shown in the above figure, if the

ingress S1 configures the loose hop S8 to the destination S10, it will calculate the path S1->S2->S3->S4->S5->S8->S9->S10.

If the metric changes for the link between S3 and S4 and make the shortest route path to S8 from the S3->S4->S5->S8 to
S3->S6->S7->S8. But the exiting MPLS TE path will not change accordingly.

For SR-BE Path, if the loose hop is configured, there will be two possible choices:
Option 1. Same as the existing MPLS TE.

If the option is adopted, there will be following challenges:
1) This means the loose hop cannot reduce the label stack for the SR-TE path. It need more labels to be encapsulated after

calculating the whole path and representing each hop with node label or adjacency label. The deep depth of the label statck

 may exceeds the hardware limitation or cause low efficiency payload.
2) More challenges can be proposed from the inter-area bahavior. If PCE is not adopted, if the ingress node has to only

specify the ABRs as the loose hop to the destination for inter-area, for MPLS TE, the ingress node can acquire the end-to-end

path info through RRO.  But since there is no RRO-like mechanism in IGP, it is impossible for SR-TE path to get the end-to-end

path info. This means the ingress node cannot get the whole label stack info to determine the whole SR-TE path.

Option 2. Limited labels for the loose hop. For example, since only loose hop S8 is specified, then the label stack for the

SR-TE path to the S10 is [S8 S10]. That is, two layers of labels is enough.

This method can avoid the deep label stack issue. But it will introduce other issues. In fact, for the SR-TE path, it mixes

the route path and the TE path together. There may be following issues:

1) The route change will have effect on the SR-TE path. If the routes to the S8 is changes, the SR-TE path to the S10 will

change accordingly.
2) For the ingress node, from the point of the network maintenance view, it loses the control on the end-to-end path. That is,

the end-to-end path is unknown to the ingress node.
3) If there is load balance for the route, for example, there are two equal paths to S8 as the above topology: S3->S4->S5->S8,

S3->S6->S7->S8.If BFD is enabled for the SR-TE path, the BFD packets and the real traffic may go through different path.

There will be the possibility that BFD will detect the wrong path and may cause furthe problem such as the wrong traffic switch.

I wonder if the issue has been taken into account and which option you prefer.

Regards,

Robin