VOICE & MESSAGING TRACK | How SMS Works - Scott Maher (Twilio)

Summary

Scott Mayer explains the complex world of SMS messaging, focusing on how Twilio connects to carriers using SMTP and SMPP protocols. He highlights the asynchronous, bi-directional nature of SMS transmission and the use of stateful protocols that ensure message reliability. Mayer delves into the technical details, including commands like 'Submit SM' and 'Deliver SM,' and the importance of Delivery Receipts (DLRs) in message tracking. He also introduces the actor model for concurrency, which ensures high performance and reliability in Twilio’s system. Ultimately, Mayer emphasizes Twilio's ability to simplify the SMS infrastructure for developers, providing multiple interfaces to facilitate global messaging.

Takeaways

• 😀 SMS is a complex network that predates the internet and is like a 'Jurassic Internet' connecting carriers using protocols like SMTP.
• 😀 Twilio acts as an SMTP gateway for carriers, simplifying the process of sending SMS messages through REST APIs and other methods.
• 😀 SMTP (Short Message Peer-to-Peer) is the protocol used to communicate between carriers and devices, and is similar to HTTP but differs in that it is binary, asynchronous, and bi-directional.
• 😀 The basic unit of SMTP communication is called a PDU (Protocol Data Unit), which can represent various actions like submitting or delivering messages.
• 😀 SMTP is stateful, meaning it maintains persistent connections between clients and servers, unlike HTTP, which is stateless.
• 😀 In SMTP, messages are transmitted asynchronously, allowing for efficient delivery even without waiting for responses to every message sent.
• 😀 The protocol supports sliding windows, meaning a set number of messages can be sent without needing a response, which varies based on the carrier.
• 😀 Twilio's infrastructure hides the complexities of working with different versions and implementations of SMTP by providing a unified, easy-to-use interface for developers.
• 😀 Mobile Originated (MO) and Mobile Terminated (MT) messages in SMTP represent the direction of message flow, from handset to server or vice versa, using different commands.
• 😀 Delivery Receipts (DLR) are used to confirm message delivery, but their format is inconsistent across carriers, making it difficult to rely on them for standardized feedback.
• 😀 Twilio normalizes DLRs and translates them into status callbacks for easier interpretation, ensuring a consistent experience across different carriers and platforms.

Q & A

What is the significance of SMTP in the context of SMS messaging?

SMTP is a protocol that allows carriers to communicate with each other when sending SMS messages. It acts as a gateway for SMS traffic between different carriers, allowing messages to traverse networks despite the fact that each carrier may implement their own variations of the protocol.

What is the role of Twilio in SMS message delivery?

Twilio serves as an SMTP gateway to carriers, providing a unified interface for users to send and receive SMS messages. Twilio handles the complexities of communicating with different carriers, enabling users to send messages through a simplified REST API without worrying about the details of carrier-specific protocols.

Can you explain the difference between SMSC and ESME?

SMSC stands for Short Message Service Center, which is the server responsible for routing and delivering SMS messages. ESME stands for External Short Message Entity, which is a client application or system that interacts with the SMSC to send and receive SMS messages.

What is a PDU in the context of SMS messaging?

PDU stands for Protocol Data Unit. In SMS, it refers to the data unit that contains a message and its related information, like sender, receiver, and status. PDUs are essential in the communication between the ESME and the SMSC and can contain commands for message submission or delivery.

What is the difference between 'Mobile Originated' and 'Mobile Terminated' messages?

'Mobile Originated' (MO) refers to messages sent from a mobile device to an SMSC, typically sent using the 'deliver SM' command. 'Mobile Terminated' (MT) refers to messages that are delivered from an SMSC to a mobile device, typically sent using the 'submit SM' command. These terms reflect the direction of the message flow.

How does the statefulness of SMTP affect message delivery?

SMTP is a stateful protocol, meaning that once a client connects to an SMSC, a persistent connection is maintained. This allows the client and server to track message transactions and ensures reliability, though it complicates load balancing, as the server needs to keep track of each session’s state.

What is a sliding window in the context of SMTP?

A sliding window is a limit on the number of messages a client can send to the server without receiving a response. It is a way to control flow and prevent congestion. Different carriers have different sliding window sizes, which means the number of messages a client can send varies.

How do delivery receipts (DLRs) work in SMS messaging?

Delivery Receipts (DLRs) are used to confirm whether an SMS message has been successfully delivered to the recipient. However, DLRs can vary significantly across carriers, both in terms of format and timing. Twilio normalizes DLR data and transforms it into a consistent status callback format.

What is the advantage of using the actor model in SMS message processing?

The actor model allows for safe concurrency and high availability in SMS processing. By treating each actor as an independent entity that processes messages asynchronously, the system can handle large volumes of messages while ensuring reliability, fault tolerance, and scalability.

How does Twilio integrate different messaging platforms through its super network?

Twilio's super network provides a unified interface to interact with various messaging platforms, such as SMS, Facebook, and IoT devices. This allows users to send messages to any endpoint, regardless of the underlying protocol or platform, by leveraging Twilio’s extensive carrier and application integrations.