Segment Routing (SRv6)

The Paradigm Shift: $\\text{Source Routing}$

Imagine you are going on a road trip.Traditional Routing is like driving and stopping at every intersection to ask a local for directions — each router consults its $\\text{FIB}$ and makes an independent decision based on the destination address alone. Segment Routing is like having a $\\text{GPS}$ that prints a list of waypoints on your windshield before you leave: "Turn right at Node A, then left at Node B, detour through the firewall at Node C." The source knows the full path; intermediate nodes simply follow the instructions.

This "source routing" principle eliminates the need for routers in the middle of the network to maintain per-flow state, dramatically simplifying the core while providing the source with unprecedented traffic engineering control.

$\\text{SRv6}$ Packet Structure

Unlike $\\text{SR-MPLS}$, which uses a stack of $\\text{MPLS}$ labels prepended before the $\\text{IP}$ header, $\\text{SRv6}$ uses the standard $\\text{IPv6}$ Routing Header (Type 4) — also called the Segment Routing Header ($\\text{SRH}$). This means $\\text{SRv6}$ packets are valid $\\text{IPv6}$ packets and can cross any $\\text{IPv6-capable}$ network without modification.

• Segment List: An array of $\\text{IPv6 SIDs}$ representing the desired path, stored in reverse order (destination first).
• Segments Left: A pointer (decremented at each hop) that indicates which $\\text{SID}$ in the list is currently active — analogous to the $\\text{MPLS}$ label at the top of the stack.

Why $\\text{SRv6}$? (Eliminating Complexity)

1. No $\\text{LDP/RSVP}$: Eliminates two of the most complex protocols in the Service Provider core. $\\text{LDP}$ distributes label bindings for every $\\text{IP}$ prefix; $\\text{RSVP-TE}$ maintains per-tunnel state signaling on every node in the path. $\\text{SRv6}$ eliminates both with a single control-plane extension to $\\text{IS-IS}$ or $\\text{OSPF}$.
2. $\\text{TI-LFA (Topology Independent Loop-Free Alternate)}$:Provides guaranteed $<50\\, \\text{ms}$ protection against link or node failures for any arbitrary network topology — without requiring a pre-provisioned backup $\\text{LSP}$. The backup path is computed using post-convergence $\\text{SPF}$ results.
3. Service Chaining: Forces traffic through specific security (firewall), optimization ($\\text{WAN}$ accelerator), or $\\text{NAT VNFs}$ simply by including their $\\text{SIDs}$ in the segment list. No $\\text{MPLS VPN}$ tunnels or $\\text{PBR}$ required.
4. Network Slicing: Enables distinct "Network Slices" where high-priority traffic (video conferencing, industrial control) takes a low-latency path, while bulk data (backups, analytics) takes a cheaper, high-bandwidth path — all on the same physical infrastructure.

Conclusion

$\\text{SRv6}$ is more than a routing protocol; it is a Network Programming Language. It treats the entire global network as a single programmable entity, allowing path steering, service insertion, and traffic engineering to be expressed as a simple list of $\\text{IPv6}$ addresses. For operators seeking to simplify their $\\text{WAN}$, enable $\\text{5G}$ network slicing, or build cloud-scale fabrics, $\\text{SRv6}$ is the architecture that makes it operationally feasible.