---
title: UDP DNS transfer with retry
summary: Sends a DNS UDP datagram, validates the reply's transaction id, and retries on timeout.
status: draft
owner: SAIF Documentation
last_reviewed: 2026-07-30
library: SAIF.Framework.DNS
reference_type: package
operation_id: OP-DNS-004
trigger: Resolver.Lookup invokes Resolver.UdpTransfer
---

# UDP DNS transfer with retry

This operation owns all of the library's network transport behavior. It stamps a fresh transaction identifier into the outbound request on every attempt, sends the datagram to the selected DNS server, waits up to one second for a reply, and retries on timeout before finally giving up and raising `NoResponseException`.

| Field | Value |
| --- | --- |
| **Operation ID** | OP-DNS-004 |
| **Trigger** | Generic DNS lookup needs to send a serialized request and receive a reply |
| **Inputs** | Target `IPEndPoint` (`server`), request `byte[]` (`requestMessage`) |
| **Outputs** | Reply `byte[]`; throws `NoResponseException` once retries are exhausted |
| **Primary components** | `Resolver.UdpTransfer`, `Socket` |
| **Entry point** | `SAIF.Framework.DNS\Resolver.cs` |

## Steps

1. Initialize `attempts` to zero and enter a loop that continues while `attempts <= _udpRetryAttempts`; with `_udpRetryAttempts` set to 2, this allows the initial send plus two additional timed-out retries ([BR-005](../business-rules.md#br-005-udp-requests-are-retried-up-to-two-additional-times-before-failing)).
2. On each iteration, stamp the current `_uniqueId` into the first two bytes of `requestMessage`, giving the outbound datagram a fresh transaction identifier.
3. Create a UDP `Socket` configured for IPv4 datagram transport.
4. Set the socket's receive timeout to 1000 milliseconds.
5. Send the request bytes to the supplied server endpoint.
6. Allocate a 512-byte buffer to receive the reply.
7. Attempt to receive a reply into that buffer.
   - If the first two reply bytes match the transaction identifier just stamped, accept the datagram and return it immediately ([BR-006](../business-rules.md#br-006-udp-replies-are-only-accepted-when-the-transaction-id-matches)).
   - If the reply identifier does not match, discard the datagram and continue to another loop iteration ([BR-006](../business-rules.md#br-006-udp-replies-are-only-accepted-when-the-transaction-id-matches)).
8. If `socket.Receive` throws `SocketException` (typically the 1-second timeout elapsing), increment `attempts` and let the loop retry the send/receive cycle ([BR-005](../business-rules.md#br-005-udp-requests-are-retried-up-to-two-additional-times-before-failing)).
9. In a `finally` block for every iteration, increment `_uniqueId` and close the socket, regardless of whether the attempt succeeded, timed out, or received a mismatched reply.
10. If the loop exhausts all allowed attempts without returning a matching datagram, throw `NoResponseException` ([BR-005](../business-rules.md#br-005-udp-requests-are-retried-up-to-two-additional-times-before-failing)).

## Process flow

The following diagram visualizes this operation's control flow.

```mermaid
flowchart TD
	Start([UdpTransfer called]) --> Stamp[Stamp fresh transaction id]
	Stamp --> Send[Send UDP datagram]
	Send --> Wait[Wait up to 1 second for a reply]
	Wait --> Match{Reply id matches?}
	Match -->|Yes| EndOk([Return reply bytes])
	Match -->|No, or timeout| Retry{Attempts remaining?}
	Retry -->|Yes| Stamp
	Retry -->|No| EndFail([Throw NoResponseException])
```

## Failure and edge-case behavior

- A receive timeout surfaces internally as `SocketException`, which this operation converts into a retry attempt rather than an immediate failure.
- A reply with a mismatched transaction identifier is discarded silently; the caller has no way to observe that a stray or unrelated packet was received.
- `_uniqueId` advances after every iteration, including timed-out and mismatched attempts, so consecutive attempts never reuse the same transaction identifier.
- A new `Socket` is created and torn down on every iteration; no socket is reused across attempts.

## Related documentation
- [Operations and processes](../operations-processes.md)
- [Business rules](../business-rules.md)

## Sources
- SAIF.Framework.DNS\Resolver.cs