Compressed SID (CSID) for SRv6 SFC

In SRv6, an SRv6 SID is a 128-bit value. When too many 128-bit SRv6 SIDs are included in an SRH, the introduced overhead will affect the transmission efficiency of payload. In order to address this problem, Compressed SID(CSID) is proposed. This document defines new behaviors for service segments with REPLACE-CSID and NEXT-CSID flavors to enable compressed SRv6 service programming.