G-SID format
As shown in Figure 37 , the locator portion of an SRv6 SID contains the Common Prefix and Node ID
portions. The Common Prefix portion represents the address of the common prefix. The Node ID portion identifies a node. G-SRv6 can compress all SIDs with the same common prefix into 32-bit
G-SIDs. A G-SID contains the Node ID and Function portions of a 128-bit SRv6 SID. A 128-bit SRv6
SID is formed by the Common Prefix portion, a 32-bit G-SID, and the 0 (Args&MBZ) portion.
Figure 37 Compressible SRv6 SID
Locator
Common Prefix Node ID Function Args&MBZ
G-SID
G-SRv6 packet
G-SRv6 packet format
As shown in Figure 38 , G-SRv6 can encapsulate both G-SIDs and 128-bit SRv6 SIDs in the segment
list of the SRH. It needs to encapsulate four G-SIDs in a group to the original location of a 128-bit
SRv6 SID. If the location contains fewer than four G-SIDs (less than 128 bits), G-SRv6 pads the remaining bits with 0s. Multiple consecutive G-SIDs form a compressed path, called a G-SID list. A
G-SID list can contain one or more groups of G-SIDs.
Figure 38 G-SRv6 packet format
Version
Next header
Last Entry
Traffic Class Flow Label
Payload Length Next Header=43
Source Address(128 bits)
Destination Address(128 bits)
Hdr Ext Len
Flags
Routing Type
Tag
Hop Limit
Segments Left
G-SID c0
128-bit SRv6 SID
G-SID c1 ( COC ) G-SID c2 ( COC ) G-SID c3 ( COC )
G-SID b0 ( COC ) G-SID b1 ( COC ) G-SID b2 ( COC ) G-SID b3 ( COC )
128-bit SRv6 SID
G-SID a0 G-SID a1 ( COC ) G-SID a2 ( COC ) G-SID a3 ( COC )
128-bit SRv6 SID ( COC )
128-bit SRv6 SID
NOTE:
If the SRv6 SID of the next node requires compression, the routing protocol adds the Continue of
Compression (COC) flag to the advertised SRv6 SID of the local node. The COC flag indicates that the next SRv6 SID is a G-SID. A COC flag only identifies the forwarding behavior of an SRv6 SID,
and is not actually carried in the packet. The COC flags in Figure 38
are for illustration purposes only.
The G-SIDs in the segment list are arranged as follows:
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•
The SRv6 SID before the G-SID list is a 128-bit SRv6 SID with the COC flag, indicating that the next SID is a 32-bit G-SID.
•
Except the last G-SID, all G-SIDs in the G-SID list must carry the COC flag to indicate that the next SID is a 32-bit G-SID.
•
The last G-SID in the G-SID list must be a 32-bit G-SID without the COC flag, indicating that the next SID is a 128-bit SRv6 SID.
•
The next SRv6 SID after the G-SID list is a 128-bit SRv6 SID that can carry the COC flag or does not carry the COC flag.
Calculating the destination address with G-SID
As shown in Figure 39 , G-SRv6 combines the G-SID and Common Prefix in the segment list to form
a new destination address.
•
Common Prefix —Common prefix address manually configured by the administrator.
•
G-SID —Compressed 32-bit SID obtained from the SRH.
•
SID Index (SI) —Index that identifies a G-SID in a group of G-SIDs. This field is the least significant two bits of the destination IPv6 address. The value range is 0 to 3. The SI value decreases by 1 at each node that performs SID compression. If the SI value becomes 0, the SL value decreases by 1. In a group of G-SIDs in the segment list, the G-SIDs are arranged from left to right based on SI values. The SI value is 0 for the leftmost G-SID, and is 3 for the rightmost G-SID.
•
0 —If the total length of the Common Prefix, G-SID, and SI portions is less than 128 bits, the deficient bits are padded with 0s before the SI portion.
Figure 39 Destination address calculated with G-SID
Destination Address(128 bits)
Common Prefix G-SID 0 SI
A:0:0:0: 1:1 0 3
Suppose the following conditions exist:
•
The Common Prefix deployed on the SRv6 node is A:0:0:0::/64.
•
The G-SID in the SRv6 packet is 1:1.
•
The SI value associated with the G-SID is 3.
Based on the previous conditions, the device calculates the destination address as A:0:0:0:1:1::3.
Upon receiving the G-SRv6 packet, the SRv6 node calculates the destination address for the packet as follows:
•
If the destination address of the packet is a 128-bit SRv6 SID with the COC flag in the segment list, the next SID is a G-SID. The device decreases the SI value by 1 and searches for the
G-SID group corresponding to [SI-1]. Then, the device calculates the destination address based on the 32-bit G-SID identified by SI value 3.
•
If the destination address of the packet is a 32-bit SRv6 SID with the COC flag in the segment list, the next SID is a G-SID.
If the SI value is larger than 0, the device decreases the SI value by 1 and searches for the
G-SID group corresponding to SL value of the packet. Then, the device calculates the destination address based on the 32-bit G-SID identified by [SI-1].
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