📦 Transport Layer: TCP & UDP

Role of the Transport Layer

The transport layer (Layer 4 in the OSI model) is responsible for end‑to‑end communication between applications. It provides two main services: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Medical engineers must understand the trade‑offs to choose the right protocol for devices, image transfer, or real‑time monitoring.

TCP – Reliable, Connection‑Oriented

Properties: Connection setup (three‑way handshake), guaranteed delivery, in‑order packets, flow control, error checking, retransmission.

Use in healthcare: Transferring MRI/CT images (DICOM), accessing electronic health records (EHR), database transactions – where every byte must arrive correctly.

Advantages: Reliability, congestion control, automatic retransmission.

Disadvantages: Higher overhead, latency due to acknowledgements, slower than UDP.

UDP – Fast, Connectionless

Properties: No connection establishment, no guarantees of delivery or ordering, low overhead, checksum (optional).

Use in healthcare: Real‑time patient vitals streaming, VoIP for telemedicine, live video during surgery – where speed matters more than occasional packet loss.

Advantages: Low latency, minimal overhead, supports broadcast/multicast.

Disadvantages: No reliability; applications must handle lost packets if needed.

Port Numbers

Ports identify specific applications or services on a device. They are 16‑bit numbers (0–65535) divided into three ranges:

Well‑known ports (0–1023): Assigned to common services (e.g., HTTP, HTTPS, FTP).
Registered ports (1024–49151): Used by applications that are not standard (e.g., DICOM 104, 11112).
Dynamic/private ports (49152–65535): Temporary ports used by clients.

Common Ports in Medical / General IT

⚠️ Use secure ports whenever possible. Insecure ports could cause data risk or theft.

Port	Protocol	Service	Usage
20/21	TCP	FTP	File transfer (legacy)
22	TCP	SSH	Secure remote access
23	TCP	Telnet	Insecure remote access
25	TCP	SMTP	Insecure Email sending
53	TCP/UDP	DNS	Domain name resolution
67	UDP	DHCP	DHCP Server
68	UDP	DHCP	DHCP Client
69	UDP	TFTP	Trivial FTP
80	TCP	HTTP	Insecure Web browsing
104	TCP	DICOM	Medical image transfer (standard)
110	TCP	POP3	Insecure Email retrieval
123	UDP	NTP	Time synchronization (critical for logs)
143	TCP	IMAP	Insecure Email retrieval
443	TCP	HTTPS	Secure web browsing
465	TCP	SMTPS	Secure email sending (SMTP over SSL)
587	TCP	SMTP Submission	SMTP submission port
993	TCP	IMAPS	Secure IMAP email retrieval
995	TCP	POP3S	Secure POP3 email retrieval
514	UDP	Syslog	Logging from medical devices
11112	TCP	DICOM	Alternative DICOM port
3389	TCP	RDP	Remote desktop

When configuring medical devices or firewalls, always verify the required port numbers from the manufacturer.

Practical Tips for Medical Engineers

Use TCP for non‑real‑time data (reports, images, records) – reliability is paramount.
Use UDP for real‑time streaming (vitals, video) – but add application‑level sequence numbers if needed.
Firewall rules must allow the correct ports; restrict unnecessary ones to reduce attack surface.
Port conflicts can occur if two services try to use the same port; use dynamic ports for clients.
Document all port usage for your medical network to simplify troubleshooting and security audits.

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