Routing Headers are defined in RFC 8200. The Segment Routing has a Routing Type value 4 to be assigned to identify SR header. The Segment Routing Header (SRH) is defined as follows:
Few of the important fields in the SRH header are described below,
Segment list – Segment list represent segment id (SID) which is 128 bit IPv6 address .The Segment list is encoded starting from the last segment of the SID list , means , the first element of the segment list (index 0) contains the last SID in the a list , the second element contains the penultimate segment in the SID list and so on.
Each Segment id in the list is combination of locator & function as explained in the earlier Post.
Last Entry – It contains the index value of last element in the segment list.
Next header – It Identifies the header immediately following the routing header .
Routing Type– It identifies the particular routing header type , here in this case , value will be 4 , representing Segment Routing header.
Segments Left – It identifies number of explicitly listed intermediate nodes still to be visited before reaching the final destination.
TLVs– Optional TLVs are used to carry additional Meta-data information.
In the next Post we will discuss forwarding mechanism of SRv6 . I hope this article is useful .