Define the Value 255 in Last Entry field of Segment Routing Header

Segment Routing Header (SRH) is one type of Routing Header (RH) . A Segment Left field is defined in as the basic structure of any type of RH. A Last Entry field is defined in as the structure of SRH. The two fields are both related to the segments, behaving as pointer to one of the segments in the SRH. When the Segment Left field is set to one and the Last Entry field is set to zero, there is one SID present in the SRH. However, with current specification, there is no way to indicate an SRH without any SID present. For example, there is no need for any segment in the Segment List of SRH (the so-called SRv6-BE case), but there is need for HMAC in SRH as the enhanced security mechanism. This document proposes to use the value 255 of Last Entry field to represent that there is no SID in the Segment List part of the SRH. Accordingly, the Segment Left field MUST be zero in this case. This has an impact that, the number of Segments allowed in an SRH shrinks, from maximum 256 to maximum 255. Practically this is not a big problem, because the number of segments bigger than 255 is very impossible. Thus, changing the meaning of Last Entry value 255 can support incremental development and deployment.