Edge delivery, at a glance.
A static reference page for the mechanics behind CDN and ADC traffic routing - session persistence, cache offload, and edge compute placement. Deployed as a container so it can run natively on a vK8s workload at a Regional Edge site.
Session persistence
Why a client keeps landing on the same backend.
Cookie-based stickiness
The edge picks an origin once, then injects a cookie recording that choice. Future requests carrying a valid cookie skip re-selection and route straight there.
F5 XC HTTP LBInjection trigger
A new cookie is only written when no valid one exists on the inbound request - new client, expired TTL, tampered value, or a failover reassignment after the sticky origin goes unhealthy.
edge logicTTL / Max-Age
Duration is explicit and configurable - e.g. a 3600-second value keeps a client bound to one origin for exactly one hour before a fresh selection is made.
configLegacy cookie insert
Classic BIG-IP LTM cookie insert encodes the real server IP and port directly in the cookie value - functional, but a known pentest finding since it leaks backend topology.
BIG-IP LTMDelivery mechanics
Routing, caching, and DNS fundamentals underneath the edge.
DNS resolver
Translates a hostname into an IP before any connection exists. Recursive resolvers walk root -> TLD -> authoritative nameserver, then cache the answer for its TTL.
Resource path redirect
Redirect decision keyed on the URL path itself - exact, prefix, or regex match - rather than a cookie or geography. Common for deprecating an old URL structure.
Honoring origin headers
The edge trusts and applies origin's own Cache-Control, Expires, and Vary values instead of overriding them with edge-defined defaults - directly determines cache correctness.
Offload percentage
Share of traffic served from edge cache vs. origin. High offload means origin barely sees traffic; a sudden drop usually flags a cache-control regression.
XC HTTP LB vs. BIG-IP LTM
Same persistence problem, two generations of the F5 stack.
| Capability | BIG-IP LTM (classic) | F5 Distributed Cloud (XC) |
|---|---|---|
| Mechanism | Persistence profile attached to a virtual server | HTTP Load Balancer, cookie-based stickiness |
| Enable path | Persistence profile (Cookie, Source Address, SIP, MSRDP, etc.) | LB "Other Settings" -> Load Balancing Algorithm |
| TTL control | Cookie expiration set in the persistence profile | Explicit TTL in milliseconds -> sets Max-Age |
| Failover | Pool member health monitors | Health checks against origin pool members |
| Edge compute | iRules on the appliance | vK8s on Regional Edge / Customer Edge |
| Cookie contents | Encodes real server IP/port (encoded, not encrypted by default) | Opaque stickiness cookie, HttpOnly + Secure configurable |
Uncommon status codes
Ones you'll actually run into in CDN troubleshooting, beyond 200/301/404/500.
Glossary
Expand a term.