Connection Draining

Only supported in Classic Load Balancer (CLB)
gracefully disconnect the connections (ASG/EC2)
allows in-flight request to complete
timeout value: 1s - 3600s (default: 300s)
Timeout does not apply to failed health check
applies when instance taken out of ASG - manually or ASG activities
- InService: Instance deregistraiton currently in progress
- AS waiting for all connections to complete of timeout - whichever occur first

Registration Delay

Supported in ALB, NLB, GWLB - defined on the target group
stop sending requests to deregistration targtes
existing connections can continue until they complete naturally
enabled by default with value 300s
0-3600s

X-Forwarded-For and Proxy protocol are two alternative versions of gaining visibility of original client IP address when using proxy servers or load balancers

X-Forwarded-For

Set of http/https headers (layer 7)
Header is appended or added by proxies/LB
Http Header -> X-Forwarded-For: client,proxy1,proxy2
Client is the left most in the list
Supported in CLB and ALB but NOT SUPPORTED IN NLB

PROXY protocol

Proxy protocol works at Layer 4
TCP header works with layer 4
- also works with http/https
v1 (human readable) - CLB || v2 (binary encoded) - NLB
Not supported in ALB
Use Case -> unbroken https encryption between client and server (tcp listener)