Common Network Protocols Format

Network protocols define how data is formatted and exchanged. This article summarizes the message or packet formats of HTTP, SMTP, UDP, and TCP, from application-layer text messages to transport-layer binary headers.

Protocol Layers

HTTP and SMTP are application-layer protocols. They usually send their messages over TCP, which in turn is carried by IP. UDP is an alternative transport-layer protocol.

Application data:  HTTP message / SMTP message
Transport segment: TCP header + application data
IP packet:         IP header + transport segment
Link frame:        link-layer header + IP packet + trailer

Multi-byte integers in TCP and UDP headers use network byte order, which is big-endian. Header sizes are measured in bytes unless stated otherwise.

HTTP

HTTP uses a request-response model. HTTP/1.1 messages are text-based, while HTTP/2 and HTTP/3 use binary framing. The following format describes HTTP/1.1.

Request Format

METHOD request-target HTTP-version\r\n
Header-Name: value\r\n
Header-Name: value\r\n
\r\n
optional message body

Example:

GET /index.html HTTP/1.1
Host: example.com
Accept: text/html

The request line contains:

• Method: the requested operation, such as GET, POST, PUT, or DELETE.
• Request target: the path or resource being requested.
• HTTP version: commonly HTTP/1.1.

Response Format

HTTP-version status-code reason-phrase\r\n
Header-Name: value\r\n
Header-Name: value\r\n
\r\n
optional message body

Example:

HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: 5

Hello

The blank line separates the header section from the body. A body may be delimited by Content-Length, chunked transfer encoding, or the end of the connection, depending on the message.

SMTP

                 Email
                   |
          +--------+--------+
          |                 |
       Sending           Receiving
          |                 |
       SMTP            IMAP / POP3
          |
    +-----+-----+
    |     |     |
   25    587   465
 relay   submission
          |      |
      STARTTLS implicit TLS

SMTP transfers email using text commands and three-digit replies over TCP. Commands and replies end with \r\n.

Command Format

COMMAND optional-arguments\r\n

Common commands include:

• EHLO domain: introduces the client and requests server capabilities.
• MAIL FROM:<address>: specifies the envelope sender.
• RCPT TO:<address>: specifies an envelope recipient.
• DATA: begins the message content.
• QUIT: closes the SMTP session.

Reply Format

status-code text\r\n

Reply codes beginning with 2 indicate success, 3 request more input, 4 indicate temporary failure, and 5 indicate permanent failure. A hyphen after the code begins a multiline reply; the final line uses a space.

Example Exchange

S: 220 mail.example.com ESMTP ready
C: EHLO client.example.com
S: 250-mail.example.com
S: 250 SIZE 10485760
C: MAIL FROM:<[email protected]>
S: 250 OK
C: RCPT TO:<[email protected]>
S: 250 OK
C: DATA
S: 354 End data with <CRLF>.<CRLF>
C: From: [email protected]
C: To: [email protected]
C: Subject: Hello
C:
C: Message body.
C: .
S: 250 Message accepted

The SMTP envelope (MAIL FROM and RCPT TO) is distinct from the headers inside the email message. During DATA, a line containing only . ends the message. A body line that begins with . is escaped by adding another dot.

UDP

UDP is a connectionless, message-oriented transport protocol. Each datagram has a fixed eight-byte header followed by its payload.

  0               15 16              31
 +------------------+------------------+
 |   Source Port    | Destination Port |
 +------------------+------------------+
 |      Length      |     Checksum     |
 +------------------+------------------+
 |                Data ...             |
 +-------------------------------------+

UDP preserves datagram boundaries but does not guarantee delivery, ordering, duplicate protection, or congestion control. The checksum is optional in IPv4 when set to zero but mandatory in IPv6.

TCP

TCP is a connection-oriented byte-stream protocol. It provides reliable, ordered delivery using sequence numbers, acknowledgments, retransmission, flow control, and congestion control.

  0               15 16              31
 +------------------+------------------+
 |   Source Port    | Destination Port |
 +------------------+------------------+
 |            Sequence Number          |
 +-------------------------------------+
 |         Acknowledgment Number        |
 +----+---+---------+------------------+
 |DOFF|Res|  Flags  |   Window Size    |
 +----+---+---------+------------------+
 |     Checksum     |  Urgent Pointer  |
 +------------------+------------------+
 |       Options and Padding ...        |
 +-------------------------------------+
 |                Data ...              |
 +-------------------------------------+

A TCP header is 20 to 60 bytes long. TCP exposes an ordered stream of bytes, not a sequence of application messages, so protocols such as HTTP and SMTP must define their own message boundaries.

Summary