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SID Structure

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An SRv6 Segment Identifier (SID) is a 128-bit value, formatted as a standard IPv6 address. It encodes both the location of a node and the behavior to be executed.

Anatomy of an SRv6 SID

|<------------ 128 bits ------------->|
|  Locator      | Function | Arguments|
|<-- L bits --->|<-F bits->|<-A bits->|
Field Description Typical Size
Locator Routable prefix identifying the node 40-64 bits
Function Identifies the local behavior (End, End.DT4, etc.) 16-24 bits
Arguments Optional parameters for the function 0-48 bits

Decode a SID

Paste any SID into the SRv6 SID Decoder to see its block, node, function and argument bits for the lengths you choose, and whether it sits in 5f00::/16.

Example

SID:  5f00:0:1::100

Breakdown:
  Locator:  5f00:0:1::/48   (identifies the node)
  Function: ::100            (End.DT4 - decap and lookup in IPv4 table)
  Args:     (none)

Locator as an IGP route

The locator block is advertised as a regular IPv6 route in the IGP (IS-IS or OSPFv3). This ensures reachability to any SRv6-capable node in the network.

IANA-Allocated SRv6 SID Block (5f00::/16)

RFC 9602 (October 2024) allocates a dedicated IPv6 prefix for SRv6 Segment Identifiers. IANA assigned the block from the IPv6 Unicast Address Registry and recorded it in the IPv6 Special-Purpose Address Registry.

Property Value
Address Block 5f00::/16
Name Segment Routing (SRv6) SIDs
RFC RFC 9602
Allocation Date 2024-04
Source True
Destination True
Forwardable True
Globally Reachable False
Reserved-by-Protocol False

The 5f00::/16 block is forwardable within SR domains but is explicitly marked as not globally reachable: packets carrying these addresses as destination should not transit the public Internet.

A typical allocation hierarchy within this block:

5f00::/16                          <- IANA SRv6 SID block
  5f00:<domain>::/32               <- Operator / SR domain
    5f00:<domain>:<node>::/48      <- Node locator
      5f00:<domain>:<node>:<func>::/64  <- SID (locator + function)

Using Operator GUA Prefixes vs 5f00::/16

RFC 9602 does not mandate 5f00::/16. Operators can continue using their own globally unique address (GUA) prefixes as SRv6 locators. Both approaches are valid:

Approach Prefix Source Pros Cons
IANA SRv6 block 5f00::/16 Instantly recognizable in captures and ACLs; easy filtering; no GUA consumption Not globally routable; requires explicit IGP/BGP advertisement within the SR domain
Operator GUA Provider /32 or /48 Already routable in the operator's network; dual-use as IPv6 reachability Consumes GUA space; harder to distinguish SRv6 traffic from regular IPv6

Most early SRv6 deployments (China Mobile, SoftBank, and others) use operator-assigned GUA prefixes. RFC 9602 provides a complementary option, especially useful for greenfield deployments and inter-domain SRv6 where a common recognizable prefix simplifies filtering.

Filtering 5f00::/16 at Domain Boundaries

Whether you use 5f00::/16 or your own GUA, you must prevent SRv6 SIDs from leaking outside the SR domain.

prefix-set DENY-SRV6
  5f00::/16 le 128
end-set

route-policy EBGP-IN
  if destination in DENY-SRV6 then
    drop
  endif
end-policy

policy-options {
    prefix-list SRV6-SID-SPACE {
        5f00::/16;
    }
    policy-statement REJECT-SRV6 {
        term deny-srv6 {
            from prefix-list SRV6-SID-SPACE;
            then reject;
        }
    }
}

ip6tables -A FORWARD -d 5f00::/16 -j DROP

For deployments using operator GUA prefixes, apply the same principle with your locator prefix range (see also RFC 8754 Section 5.1).

Further Reading

References

  1. RFC 8986 - SRv6 Network Programming - Defines SID structure, locator, function, and argument fields
  2. RFC 9602 - SRv6 SIDs in the IPv6 Addressing Architecture - IANA allocation of 5f00::/16 for SRv6 SIDs
  3. RFC 8754 - IPv6 Segment Routing Header (SRH) - SRH specification and security considerations
  4. IANA IPv6 Special-Purpose Address Registry - Official registry entry for 5f00::/16
  5. Segment Routing - segment-routing.net - Community resource with tutorials, demos, and SRv6 deployment information