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RFC 8586 / Forwarding without loops

Recognize a returning request

01

A Content Delivery Network (CDN) forwards requests on behalf of an origin service. If CDN A forwards to CDN B and a bad route sends B back to A, repeated forwarding can form a loop. CDN-Loop records CDN identifiers so a participant can notice an earlier visit. The names and stopping policy here are fictional examples.

DIAGRAMRead RFC section 2 ↗
RFC 8586 · § 2EXAMPLE
3 / 3
Recognize a returning requestA fictional request has reached CDN A. A first adds a.example and forwards to B. B preserves A and adds b.example, then an erroneous route returns the request to A. The final state keeps that arrived return distinct from a new A-to-B attempt, which stops under an explicitly illustrative local policy. There is no mandated one-visit threshold, HTTP error code, authenticated history or claim that all CDN requests must be forwarded.Example routing error · A → B → ACDN ACDN Breturn already arrivedanother forwardA sees its own a.example entryCDN-Loop: a.example, b.exampleExample policy: stop on this returnNot a universal RFC threshold or response code
03
Stop a further forwarding attempt

The returned request has already arrived at A. A sees a.example in the field; this example’s policy stops a new A-to-B forward. No particular response code or universal one-visit limit is implied.

Source for this drawing: RFC 8586 · Read RFC section 2 ↗

Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
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A message needs an address.

Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.

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Terms in this drawing
CDN
Content Delivery Network: an intermediary service that can handle and forward requests in front of an origin.
Origin
The service for which a CDN handles requests.
Forwarding loop
A routing cycle in which a request returns to an earlier forwarding participant.
Related RFCs and changes

“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.

KEY POINT

A CDN can inspect the recorded identifiers before forwarding again.

  • Conforming CDNs SHOULD add a cdn-info entry to CDN-Loop in all requests they generate or forward, creating the field when necessary.
  • The field is intended to help a CDN identify that an incoming request has already passed through its servers.
  • RFC8586 does not specify a universal visit threshold, detection algorithm or HTTP error status. Stopping on the first observed return is this example’s local policy, not a new RFC requirement.
  • A client can generate this field, so recorded identifiers are not authenticated proof of actual visits.
DETAILS

A request can arrive successfully yet be stopped from being forwarded again. A repeated identifier is a signal for a policy decision, not a cryptographic fact.

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