Module 2 — Lesson 4: Encapsulation & PDUs (Formal Treatment)

Flashcards covering the formal definitions, terminology, PDU naming conventions, and header field details for encapsulation, de-encapsulation, and fragmentation based on the Lesson 4 transcript.

Encapsulation

The process by which each layer of a protocol stack adds its own control information (a header, and at the Data Link layer, a trailer) to the data received from the layer above, before passing the resulting unit down to the layer below.

De-encapsulation

The reverse process where each layer at the receiving end strips off the header and trailer corresponding to its own layer, reads the control information, and passes the remaining contents up to the layer above.

PDU (Protocol Data Unit)

The generic technical term for whatever a chunk of data is called at any given layer of the protocol stack.

Segment

The specific name for a Layer 4 (Transport) PDU when the TCP protocol is used.

Datagram

The specific name for a Layer 4 (Transport) PDU when the UDP protocol is used.

Packet

The PDU name used at Layer 3 (Network layer in OSI; Internet layer in TCP/IP).

Frame

The PDU name used at Layer 2 (Data Link layer).

Bits

The PDU name used at Layer 1 (Physical layer).

EtherType

A $2\,byte$ field in the Ethernet header that tells the receiving device which Layer 3 protocol (IPv4, IPv6, or ARP) to hand the payload up to during de-encapsulation.

FCS (Frame Check Sequence)

A $4\,byte$ trailer at the Data Link layer containing a CRC checksum used to detect transmission errors.

MTU (Maximum Transmission Unit)

The upper bound constraint on the maximum size of the data link payload, typically set at $1500\,bytes$ for Ethernet.

Ethernet Minimum Payload

A requirement that frames must have a minimum payload of $46\,bytes$ for collision detection timing; smaller data units are padded with filler bytes.

TTL (Time to Live)

A $1\,byte$ field in the IP header that starts at a value (like $64$ or $128$) and decrements at every router hop to prevent infinite loops.

Protocol Field

A $1\,byte$ field in the IP header (Layer 3) that identifies whether the payload is a TCP segment, a UDP datagram, or an ICMP message.

Sequence Number

A TCP header field that allows the receiving end to reassemble segments in the correct order even if they arrive out of sequence.

Acknowledgment Number

A TCP header field that confirms receipt of data, enabling the protocol's reliability guarantee.

TCP Flags

Single-bit indicators (such as SYN, ACK, and FIN) used to control connection states and initiate or close the three-way handshake.

Fragmentation

The process of splitting a single unit of data into multiple smaller pieces because it exceeds the MTU, with each piece requiring its own complete header for independent transmission.

Overhead

The extra bytes added by headers and trailers (the cost of addressing, routing, and reliability) that are not part of the actual application data.

Socket

The combination of an IP address and a port number, forming a unique application-level addressing point.