I Loop Detection and Loop Prevention in NRF Forwarding Scenarios

NRF forwarding may involve multiple NRFs. Since nrfHostConfig defines the NRF peers used for forwarding, NRF also uses it together with the Via header (forwarding history) to ensure that forwarded requests do not circulate between NRFs.

The following sections describes the behavior of loop detection and loop prevention for NRF forwarding in the following modes:

• NRF forwarding : URI- {apiRoot}/nrf-configuration/v1/forwardingOptions
• NRF Growth forwarding: URI- {apiRoot}/nrf-configuration/v1/nrfGrowth/nrfForwardingOptions

Loop detection

Loop detection helps to detect if a request has been forwarded to the same NRF causing an infinite loop. To detect, NRF evaluates the Via header in the incoming requests. If it contains its own host and port, NRF declares a loop has been detected and returns an error response as configured under errorResponses for errorCondition=NRF_Forwarding_Loop_Detection.

Example:

• NRF forwarding:
  The following image describes the high-level view of NRF forwarding across Segment 1 and Segment 2, where the request is forwarded from one NRF to another and then incorrectly returned to the originating NRF, resulting in loop detection.

  Figure I-1 Loop Detection -NRF Forwarding

  
  

  

  
  

  NRF-A of Segment 1 forwards a request to NRF-E of Segment 2. Due to an incorrect routing or forwarding configuration, the request is routed back to NRF-A. Upon receiving the request, NRF-A detects that the Via header already contains its own configured host:port (ocnrfHost:ocnrfPort in {apiRoot}/nrf-configuration/v1/generalOptions URI) entry. NRF identifies this as a forwarding loop and returns the configured error response (for example, HTTP 508 “Loop Detected”).

• NRF Growth forwarding:
  The following image describes the high-level view of NRF Growth forwarding across Segment 1 and Segment 2, where each segment contains Set 1 and Set 2, and a returned request reaches the originating NRF again, resulting in loop detection.

  Figure I-2 Loop Detection_NRF Growth Forwarding

  
  

  

  
  

  NRF-A in NRF Set-1 of Segment 1 forwards a request to NRF-E in NRF Set-2 of Segment 2 based on the NRF Growth forwarding configuration. Due to an incorrect routing or forwarding configuration, the request is forwarded back to NRF-A in NRF Set-1 of Segment 1. Upon receiving the request, NRF-A in NRF Set-1 of Segment 1 detects that the Via header already contains its own configured ocnrfHost:ocnrfPort entry. NRF identifies this as a forwarding loop and returns the configured error response (for example, HTTP 508 Loop Detected).

Loop Prevention

Loop prevention prevents creation of loops by checking the forwarding history before forwarding a request. Before forwarding a request, NRF compares the entries in the Via header with the configured NRF identities in nrfHostConfig. If the selected destination NRF already appears in the Via header, NRF determines that the destination NRF has already processed the request and skips forwarding to that NRF to prevent a forwarding loop.

Example

• NRF forwarding:
  The following image describes the high-level view of NRF forwarding across Segment 1 and Segment 2, where the destination NRF checks the Via header before forwarding and skips sending the request back to an NRF that already processed it.

  Figure I-3 Loop Prevention_NRF Forwarding

  
  

  

  
  

  NRF-A in Segment 1 forwards a request to NRF-E in Segment 2. NRF-E in Segment 2 determines that the next forwarding destination is NRF-A in Segment 1. Before forwarding the request, NRF-E in Segment 2 compares the Via header with the configured NRF identities in nrfHostConfig and finds that NRF-A in Segment 1's host:port already exists in the Via header. Since the destination NRF has already processed the request, NRF-E in Segment 2 does not forward the request to NRF-A in Segment 1, thereby preventing a forwarding loop.

• NRF Growth forwarding:
  The following image describes the high-level view of NRF Growth forwarding across Segment 1 and Segment 2, where each segment contains Set 1 and Set 2, and the destination NRF prevents a loop by skipping forwarding to an NRF already present in the Via header.

  Figure I-4 Loop Prevention_NRF Growth Forwarding

  
  

  

  
  

  NRF-A in Segment 1 of NRF Set1 forwards a request to NRF-E in Segment 2 of NRF Set 2. NRF-E in Segment 2 of NRF Set 2 determines that the next forwarding destination is NRF-A in Segment 1 of NRF Set1. Before forwarding the request, NRF-E in Segment 2 of NRF Set 2 compares the Via header with the configured NRF identities under nrfHostConfig.staticNrfConfigList[].nrfHostConfigList[] and finds that NRF-A in Segment 1 of NRF Set 1's host:port already exists in the Via header. Since the destination NRF has already processed the request, NRF-E in Segment 2 of NRF Set 2 does not forward the request to NRF-A in Segment 1 of NRF Set1, thereby preventing a forwarding loop.