--- title: "RFC and IPv4/IPv6 fragmentation" subject: "Internet and Intranet" chapter: 2 key: "semester_8/internet_and_intranet:rfc-fragmentation-ipv4-ipv6" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/rfc-fragmentation-ipv4-ipv6 site: BCT Engineering --- # RFC and IPv4/IPv6 fragmentation Internet and Intranet · Chapter 2: Internet Protocol Overview - Asked in: 2072 Asoj, 2+8 marks - Video: [IPv6 and IPv6 Fragmentation](https://www.youtube.com/watch?v=-QuSJ31VJKo) — Network Insight: The Visual Age - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/rfc-fragmentation-ipv4-ipv6 - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-2 ## Question Define internet RFC. How differently IP datagram fragmentation is carried out in IPv4 and IPv6? Explain each with example. ## Short answer An RFC is a numbered permanent public document specifying Internet protocols. In IPv4, source or routers fragment using base-header fields; a 4000-byte datagram becomes 1480-byte fragments. In IPv6, only the source fragments using a Fragment extension header; a 4000-byte payload becomes 1448-byte fragments, and routers send Packet Too Big. ## Answer #### Internet RFC A **Request for Comments (RFC)** is a numbered document published by the RFC Editor that describes Internet protocols, standards, procedures and research. It is freely available, is never modified once published (a new RFC replaces or updates it), and comes from the IETF, IAB, IRTF or independent streams. Examples: RFC 791 (IPv4), RFC 8200 (IPv6), RFC 793 (TCP). #### Why fragmentation is needed Every link has a **Maximum Transmission Unit (MTU)**, the largest packet it can carry (Ethernet: 1500 bytes). If a datagram is larger than the MTU of the link, it must be split into smaller pieces, called fragments, and reassembled at the destination. #### Fragmentation in IPv4 - **Who fragments:** the source host **or any router** on the path whose outgoing link has a smaller MTU. - **Header fields used:** *Identification* (16 bits, same for all fragments of one datagram), *Flags* (DF = don't fragment, MF = more fragments) and *Fragment Offset* (13 bits, position of the data in units of 8 bytes). Each fragment gets its own IP header (20 bytes) and its own checksum. - **Reassembly:** done only at the **destination host**, using identification, offset and the MF bit. If one fragment is lost, the whole datagram is discarded after a timeout. - If DF = 1 and the packet is too big, the router drops it and sends ICMP "Fragmentation needed" (used by Path MTU Discovery). **Example:** a 4000-byte datagram (20-byte header + 3980 data), identification 777, goes onto a link with MTU 1500. Data per fragment = 1500 - 20 = 1480 bytes (a multiple of 8). | Fragment | Total length | Data | MF | Offset (units of 8 B) | |---|---|---|---|---| | 1 | 1500 | 1480 | 1 | 0 | | 2 | 1500 | 1480 | 1 | 185 | | 3 | 1040 | 1020 | 0 | 370 | Check: 1480 + 1480 + 1020 = 3980 data bytes; offsets 1480/8 = 185 and 2960/8 = 370. #### Fragmentation in IPv6 - **Who fragments:** **only the source host.** Routers never fragment IPv6 packets. If a packet is too large for a link, the router drops it and returns an ICMPv6 **"Packet Too Big"** message that contains the MTU. The source uses **Path MTU Discovery** (strongly recommended by RFC 8200) to find the smallest MTU on the path; a source that does not use it must send packets no larger than 1280 bytes, the minimum link MTU. - **Header used:** the **Fragment extension header** (Next Header = 44, 8 bytes) containing Next Header, Fragment Offset (13 bits, units of 8 bytes), M flag (more fragments) and a 32-bit Identification. The base IPv6 header has no fragmentation fields. - The packet is divided into an **unfragmentable part** (base header + hop-by-hop / routing headers) that is copied into every fragment and a **fragmentable part** that is split. - Reassembly is only at the destination. **Example:** source sends 4000 bytes of payload and learns that path MTU = 1500. Each fragment: 40 (base header) + 8 (fragment header) + data. Data per fragment = 1500 - 48 = 1452, reduced to 1448 to be a multiple of 8. | Fragment | Packet size | Data | M | Offset (units of 8 B) | |---|---|---|---|---| | 1 | 1496 | 1448 | 1 | 0 | | 2 | 1496 | 1448 | 1 | 181 | | 3 | 1152 | 1104 | 0 | 362 | Check: 1448 + 1448 + 1104 = 4000; offset 1448/8 = 181, 2896/8 = 362. #### Summary of differences | Basis | IPv4 | IPv6 | |---|---|---| | Who fragments | Source or any router | Source only | | Fragmentation info | In base header (ID, flags, offset) | In Fragment extension header | | Header per fragment | 20 bytes | 40 + 8 bytes | | Router load | Routers do the work | Routers only forward | | DF bit | Present | Not present; routers never fragment | | Path MTU Discovery | Optional | Strongly recommended (otherwise send at most 1280 bytes) | | Minimum MTU | 68 bytes (every host must accept 576-byte datagrams) | 1280 bytes | | Checksum | Recomputed per fragment | No header checksum | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which best describes an Internet RFC? A. A numbered, permanent public document from the RFC Editor describing Internet protocols and practices B. A private vendor manual that can be edited at any time C. A hardware certification issued by ISPs D. A temporary draft deleted after one year Answer: A. An RFC is numbered, freely available and never modified once published; new RFCs replace or update it. (Key point: Definition of RFC) 2. Where are IPv4 fragments reassembled, and what happens if one fragment is lost? A. At each router; only the missing fragment is resent B. At the destination host; the whole datagram is discarded after a timeout C. At the destination host; the missing fragment is requested by the router D. At the next router; the datagram is delivered incomplete Answer: B. Reassembly is only at the destination; a lost fragment makes the whole datagram be discarded after a timeout. (Key point: IPv4 fragmentation process) 3. In the 4000-byte IPv4 example with MTU 1500, what is the offset field of the second fragment? A. 1480 B. 185 C. 1500 D. 370 Answer: B. The second fragment starts at data byte 1480, and offset is in units of 8 bytes: 1480/8 = 185. (Key point: IPv4 example offsets) 4. What does an IPv6 router do with a packet larger than the next link's MTU? A. Fragments it using the Fragment header B. Drops it and sends ICMPv6 Packet Too Big containing the MTU C. Sets the DF bit and forwards it D. Forwards it unchanged and lets the destination fragment Answer: B. Routers never fragment IPv6; the source learns the MTU from Packet Too Big (Path MTU Discovery) and fragments itself. (Key point: IPv6 packet too big) ## Related in Internet and Intranet - [IPv4 vs IPv6 datagram fragmentation](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv4-ipv6-fragmentation) — Internet and Intranet, chapter 2, asked 2× - [TCP/IP applications; IPv4 vs IPv6 routing](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Atcpip-application-ipv4-ipv6-routing) — Internet and Intranet, chapter 2 - [RFC role and IPv6 addressing](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Arfc-development-ipv6-addressing) — Internet and Intranet, chapter 2 - [IPv6 features, header format, RFC streams](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv6-features-header-rfc) — Internet and Intranet, chapter 2 - [IPv6 migration; IPv4 vs IPv6 packet structure](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv6-migration-router-packet) — Internet and Intranet, chapter 2 ## Related in other subjects - [IPv6 datagram format](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-datagram-format) — Computer Networks and Security, chapter 7, asked 2× - [IPv4 drawbacks solved by IPv6](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv4-drawbacks-ipv6-advantages) — Computer Networks and Security, chapter 7, asked 2× - [Factors for IPv6; IPv4 to IPv6 transition](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-factors-transition) — Computer Networks and Security, chapter 7, asked 2× - [Compare IPv6/IPv4 headers; migration](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-ipv4-header-migration) — Computer Networks and Security, chapter 7, asked 2×