[spring] Question of SRH.Last_entry=0 in Reduced mode

Huzhibo <huzhibo@huawei.com> Wed, 29 June 2022 01:09 UTC

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From: Huzhibo <huzhibo@huawei.com>
To: "spring@ietf.org" <spring@ietf.org>
Thread-Topic: Question of SRH.Last_entry=0 in Reduced mode
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Date: Wed, 29 Jun 2022 01:09:44 +0000
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Subject: [spring] Question of SRH.Last_entry=0 in Reduced mode
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I have a simple question: Do we allow to have empty Segment list in SRH?   For instance, only carry common SRH header(8 Bytes) or carry the Common header with option TLV without any SID.

According to the definition of RFC8754[1], "TLVs are present when the Hdr Ext Len is greater than (Last Entry+1)*2".

In reduced mode, if we only have one single SRv6 SID or one single CSID container, the single SRv6 SID/CSID container will be put in the DA and no SID will be carried in the SRH.

In this case, we will meet some errors:

1.       the node will not process the TLV if the length of TLV is longer that (0+1)*2.

2.       the node cannot locate the correct beginning of the TLV because no matter 1 SID or 0 SID in the SRH, the last entry will be 0 in both cases.

We may suggest to disable reduced mode and encode the SID in the SRH when only one SID/CSID container in the SRH, so that the node can process the TLV(HMAC, etc.) correctly.



[1]. https://datatracker.ietf.org/doc/html/rfc8754#section-2