# HTTP IN trunk HTTP IN SMS trunks allow you to forward incoming SMS messages from your Direct Inward Dialing (DID) numbers to a specified web server or application endpoint. Each message is delivered as an HTTP request. Use this trunk type when an application or web server should receive the messages sent to your DID numbers. To receive messages by email instead, use an [SMS to Email trunk](../sms-to-email/sms-to-email-trunk.html). To exchange messages over SMPP, use an [SMPP ESME trunk](../smpp-esme/smpp-esme-trunk.html) or an [SMPP SMSC trunk](../smpp-smsc/smpp-smsc-trunk.html). An HTTP IN trunk receives inbound messages. To send messages over HTTP, use an [HTTP OUT trunk](../http-out/http-out-trunk.html). ## How inbound SMS reaches your HTTP endpoint When someone sends an SMS to one of your DID numbers, DIDWW uses the SMS trunk assigned to that number to determine where to deliver the message. For an HTTP IN trunk, DIDWW sends an HTTP request to the trunk's **Request URL**. Variables in the request carry data from the received message, such as the sender's number and the message text. ``` %%{init: { "theme": "base", "themeVariables": { "fontSize": "14px", "primaryColor": "#e0f2fe", "primaryBorderColor": "#38bdf8", "primaryTextColor": "#1f2d3d", "secondaryColor": "#ccfbf1", "secondaryBorderColor": "#2dd4bf", "secondaryTextColor": "#1f2d3d", "lineColor": "#0066cc", "textColor": "#1f2d3d", "nodeTextColor": "#1f2d3d", "edgeLabelBackground": "#ffffff" } }}%% flowchart LR sender["Sender"] --> |sends SMS| did["DID number"] did --> |assigned trunk| trunk["HTTP IN trunk"] trunk --> |HTTP request| app["Your HTTP endpoint"] app -.-> |2xx response| trunk classDef endpoint fill:#e0f2fe,stroke:#0066cc,color:#1f2d3d,stroke-width:1.5px classDef process fill:#ccfbf1,stroke:#2dd4bf,color:#1f2d3d,stroke-width:1.5px class sender,app endpoint class did,trunk process linkStyle default stroke:#0066cc,stroke-width:1.5px ``` ## HTTP IN trunks and DID numbers A DID number receives the inbound SMS, while the HTTP IN trunk defines the endpoint to which DIDWW delivers it. To receive messages, [assign the HTTP IN trunk or an SMS trunk group containing it](../../../phone-numbers/my-numbers/how-to-guides/assign-sms-trunk.html#assigning-sms-trunk) to one or more DID numbers that support inbound SMS. Creating a trunk does not assign it to a DID number. Multiple DID numbers can use the same HTTP IN trunk. The `{SMS_DST_ADDR}` variable adds the DID number that received each message to the request, so that your endpoint can tell the numbers apart. See [Request variables](http-in-trunk-settings-reference.html#http-in-trunk-settings-variables). ## Integration choices You define the request that DIDWW sends to your endpoint. The **HTTP Method** can be `GET`, `POST`, or `PUT`. Optional **Query Parameters** and **Headers** add custom key-value pairs and HTTP headers to each request. For `POST` and `PUT`, the **Body type** sets the format of the request body, such as JSON. With `GET`, message data can be sent only in the **Request URL**, **Query Parameters**, and **Headers**. Variables insert data from the received message into the **Request URL**, **Query Parameters**, **Headers**, and request body. The body format must match the format that your endpoint expects. See [Request variables](http-in-trunk-settings-reference.html#http-in-trunk-settings-variables) and [Body type values](http-in-trunk-settings-reference.html#http-in-trunk-settings-body-types). HTTPS is recommended for the **Request URL**, and HTTP is also supported. With **Validate HTTPS Certificate** enabled, DIDWW validates the certificate of your server. With **Follow redirect** enabled, DIDWW follows HTTP 3xx redirect status codes. DIDWW sends the requests from DIDWW-defined source IP addresses. If your endpoint is behind a firewall, the firewall must allow these addresses. See [HTTP request details from DIDWW](../technical-data/http-specification.html#sms-in-spec-request). For all fields, see [HTTP IN trunk settings reference](http-in-trunk-settings-reference.html). ## Delivery and retries If your endpoint does not respond with a `2xx` status code, DIDWW re-attempts to deliver the message until its time to live (TTL) of 3600 seconds (1 hour) expires. See [Inbound SMS technical details](../technical-data/http-specification.html#sms-in-spec-technical). [Inbound SMS logs](../../../logs-analytics/sms-logs/inbound-logs.html#inbound-logs) show the delivery status of each message and the reason for a failed delivery. ## Concatenated messages A long SMS can arrive as a concatenated message that consists of several parts. DIDWW reassembles concatenated messages using the User Data Header (UDH) before it delivers them to your endpoint. Information about fragmentation is not transmitted. The maximum size of an inbound SMS message is 64 KB. See [Inbound SMS technical details](../technical-data/http-specification.html#sms-in-spec-technical). For encoding limits, concatenation, and fragment billing, see [SMS encoding reference](../technical-data/sms-encoding-reference.html#sms-encoding-reference). To compare how each trunk type delivers multipart messages, see [Reassembly by transport](../technical-data/sms-encoding-reference.html#sms-encoding-reassembly). ## Failover with SMS trunk groups An HTTP IN trunk delivers messages to a single **Request URL**. You can add the trunk to an existing [SMS trunk group](../trunk-groups/sms-trunk-groups.html) to enable failover, and assign the group to your DID numbers. Within the trunk group, the trunk's **Priority** determines the order in which trunks will be used. Note The system attempts to contact the target SMS trunk with the highest-numbered priority first. --- ## Get started Create a trunk, assign it to your DID numbers, and test message delivery. **Create an HTTP IN trunk** Define the HTTP request that DIDWW sends to your endpoint and create the trunk. [Create an HTTP IN trunk](how-to-guides/create-http-in-trunk.html) **Assign an SMS trunk** Assign the HTTP IN trunk or its trunk group to one or more DID numbers. [Assign an SMS trunk](../../../phone-numbers/my-numbers/how-to-guides/assign-sms-trunk.html#assigning-sms-trunk) **Test and view SMS logs** Send an SMS to an assigned DID number and review the delivery in Inbound SMS logs. [Inbound SMS Logs](../../../logs-analytics/sms-logs/inbound-logs.html#inbound-logs) ## Manage trunks and requests Review existing trunks and the data that each request carries. **Manage HTTP IN trunks** Edit trunk settings, or delete trunks that are no longer required. [Manage HTTP IN trunks](how-to-guides/manage-http-in-trunk.html) **Request settings** Look up the HTTP method, Request URL, query parameter, header, and body settings. [General settings](http-in-trunk-settings-reference.html#http-in-trunk-settings-general) **Request variables** Insert message data, such as the sender's number and the message text, into each request. [Request variables](http-in-trunk-settings-reference.html#http-in-trunk-settings-variables) ## Trunk groups and priorities Use these guides when your HTTP IN trunk will share message delivery with other SMS trunks. **Create an SMS trunk group** Combine multiple SMS trunks into one group that you can assign to DID numbers. [Create an SMS trunk group](../trunk-groups/how-to-guides/create-sms-trunk-group.html) **Set priorities in an SMS trunk group** Set the order in which the trunks in a group are attempted. [Set priorities in an SMS trunk group](../trunk-groups/how-to-guides/set-sms-trunk-group-priorities.html) **SMS trunk groups** Understand how SMS trunk groups provide failover for inbound SMS. [SMS trunk groups](../trunk-groups/sms-trunk-groups.html) ## Technical references **HTTP IN trunk settings reference** Find field definitions, available values, and constraints for HTTP IN trunk settings. [HTTP IN trunk settings reference](http-in-trunk-settings-reference.html) **Inbound SMS specification** Review request details, variables, body formats, retries, and the handling of concatenated messages. [Inbound SMS Specification](../technical-data/http-specification.html#sms-in-specification) **SMS encoding reference** Look up encoding limits, concatenation, and fragment billing. [SMS encoding reference](../technical-data/sms-encoding-reference.html#sms-encoding-reference) ## Related resources Review inbound SMS considerations and compare SMS trunk types. **Inbound SMS guidelines** Review delivery restrictions, firewall configuration, and troubleshooting for inbound SMS. [Inbound SMS Guidelines](../technical-data/guidelines.html#sms-in-guidelines) **SMS trunks** Compare SMS trunk types for receiving and sending messages. [SMS trunks](../index.html) **HTTP OUT trunk** Send outbound SMS over HTTP. [HTTP OUT trunk](../http-out/http-out-trunk.html)