A protocol data unit (PDU) is the unit of data that two peer protocol entities exchange at a particular layer of a protocol stack. It consists of protocol control information (PCI) plus user data. The exact fields and the unit’s name depend on the layer and the protocol, so “PDU” is a general category, not one universal packet format.
The definition in the standards’ own words
RFC 1208, the 1991 networking glossary, defines the term this way: “A PDU is a data object exchanged by protocol machines (entities) within a given layer.” The same entry says PDUs consist of both protocol control information and user data.
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The key idea is the word peer. A PDU is defined by who exchanges it: the same-layer entities on the sending and receiving sides. It is not defined by size or by a particular header layout.
How a PDU is built: SDU plus PCI
In OSI terminology, a layer receives data from the layer above. Seen from the receiving layer’s service interface, that data is a service data unit (SDU). The layer then adds its own protocol control information (PCI), and the combination is that layer’s PDU. The PDU is then passed down, where it becomes the SDU of the next layer, and so on.
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For example, suppose an upper layer hands some data to a lower layer. The lower layer adds the control fields its protocol requires, such as fields that identify or manage the exchange, and sends the combined unit to its peer. That combined unit is the lower layer’s PDU. The receiving peer reads and removes the PCI and passes the remaining data up as an SDU.
Key terms compared
| Term | Meaning | Relation to the PDU |
|---|---|---|
| PDU | The unit exchanged between peer protocol entities at one layer | The whole unit: PCI plus user data |
| SDU | Data handed to a layer by the layer above | The payload the layer wraps |
| PCI | Control information added by an OSI entity to the data passed down | The layer’s own addition; with the data, it forms the PDU |
| TPDU | A transport-layer PDU | A layer-specific PDU |
The practical difference between a PDU and an SDU is perspective. The SDU is what a layer is asked to carry for the layer above. The PDU is what the layer actually exchanges with its peer. One layer’s PDU becomes the next lower layer’s SDU.
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Layer-specific names are more precise
“PDU” is a generic label. Specifications usually use a more specific name.
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- Transport: RFC 892 (the 1984 ISO transport protocol specification) discusses transport protocol data units (TPDUs). It distinguishes data TPDUs, which transfer transport service data, from control TPDUs, which carry protocol control.
- Network: RFC 926 (1984, connectionless-mode network service) describes a particular network-layer PDU and its fields.
Terminology also varies by model and protocol. Not every layer has a single, universally agreed PDU name, so name the protocol when precision matters.
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Is a PDU the same as a packet?
Informally, often. RFC 1392, the 1993 Internet Users’ Glossary, glosses PDU as standards terminology for “packet.” But “packet” is loose. If you need to be exact, use the unit name that belongs to the specific layer and protocol.
Segmentation: one example, not a rule
A larger service unit can end up as several protocol units. In the network protocol of RFC 926, an initial PDU carries the whole user data, and segmentation can produce derived PDUs. These rules belong to that protocol. Do not assume every protocol segments, or reassembles, in the same way.
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What to compare when two units are both called PDUs
- Layer and protocol: which entities exchange the unit?
- Contents and format: which control information and user data does it carry?
- Purpose: does it carry user data, control information, or both?
- Segmentation and reassembly: can a larger unit become several, and under what rules?
Scope of the sources
The RFCs cited here are historical OSI-era documents. They are good evidence for what the terminology means and how those specific protocols define their units. They do not show that current networks use those protocols, and one protocol’s header fields should never be presented as universal PDU fields.
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