books/architecture: add Monero oddities (#343)

* add `oddities/`

* swap `Expected`, `Why`

books/architecture/src/SUMMARY.md

@@ -157,6 +157,11 @@
 
 ---
 
+- [🟢 Monero oddities](oddities/intro.md)
+    - [🟡 Little-endian IPv4 addresses](oddities/le-ipv4.md)
+
+---
+
 - [⚪️ Appendix](appendix/intro.md)
     - [🟢 Crates](appendix/crates.md)
     - [🔴 Contributing](appendix/contributing.md)

books/architecture/src/oddities/intro.md

@@ -0,0 +1,37 @@
+# Monero oddities
+This section is a list of any peculiar, interesting,
+or non-standard behavior that Monero has that is not
+planned on being changed or deprecated.
+
+This section exists to hold all the small yet noteworthy knowledge in one place,
+instead of in any single contributor's mind.
+
+These are usually behaviors stemming from implementation rather than protocol/cryptography.
+
+## Formatting
+This is the markdown formatting for each entry in this section.
+
+If applicable, consider using this formatting when adding to this section.
+
+```md
+# <concise_title_of_the_behavior>
+
+## What
+A detailed description of the behavior.
+
+## Expected
+The norm or standard behavior that is usually expected.
+
+## Why
+The reasoning behind why this behavior exists and/or
+any links to more detailed discussion on the behavior.
+
+## Affects
+A (potentially non-exhaustive) list of places that this behavior can/does affect.
+
+## Example
+An example link or section of code where the behavior occurs.
+
+## Source
+A link to original `monerod` code that defines the behavior.
+```

books/architecture/src/oddities/le-ipv4.md

@@ -0,0 +1,24 @@
+# Little-endian IPv4 addresses
+
+## What
+Monero encodes IPv4 addresses in [little-endian](https://en.wikipedia.org/wiki/Endianness) byte order.
+
+## Expected
+In general, [networking-related protocols/code use _networking order_ (big-endian)](https://en.wikipedia.org/wiki/Endianness#Networking).
+
+## Why
+TODO
+
+- <https://github.com/monero-project/monero/issues/3826>
+- <https://github.com/monero-project/monero/pull/5544>
+
+## Affects
+Any representation and (de)serialization of IPv4 addresses must keep little
+endian in-mind, e.g. the P2P wire format or `int` encoded IPv4 addresses in RPC.
+
+For example, [the `ip` field in `set_bans`](https://www.getmonero.org/resources/developer-guides/daemon-rpc.html#set_bans).
+
+For Cuprate, this means Rust's [`Ipv4Addr::from_bits/from`](https://doc.rust-lang.org/1.82.0/src/core/net/ip_addr.rs.html#1182) cannot be used in these cases as [it assumes big-endian encoding](https://doc.rust-lang.org/1.82.0/src/core/net/ip_addr.rs.html#540).
+
+## Source
+- <https://github.com/monero-project/monero/blob/893916ad091a92e765ce3241b94e706ad012b62a/contrib/epee/include/net/net_utils_base.h#L97>