1 / 21

Datagram Encapsulation and Fragmentation in Internet Transmission Paradigm

This chapter delves into the encapsulation and transmission of datagrams across networks, exploring the concept of fragmentation and reassembly. It covers the steps involved in datagram transmission, encapsulation through frames, and the challenges posed by different Maximum Transmission Units (MTUs) in heterogeneous network technologies. The text illustrates how routers perform fragmentation when a datagram exceeds the MTU of a network, dividing it into fragments that are reassembled at the ultimate destination. The potential issues with fragmentation, such as fragment loss and the impact on data integrity, are also discussed in detail.

hlarson
Download Presentation

Datagram Encapsulation and Fragmentation in Internet Transmission Paradigm

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 21 Internetworking Part 2 (Datagram Encapsulation, Transmission, Fragmentation, Reassembly)

  2. Internet Transmission Paradigm(General Case) • Source host • Forms datagram • Includes destination address • Sends to nearest router • Intermediate routers • Forward datagram to next router • Final router • Delivers to destination host

  3. Datagram Transmission • Datagram sent across conventional network • From source host and router • Between intermediate routers • From final router to destination host • Network hardware does not recognize • Datagram format • IP addresses • Encapsulation needed

  4. Illustration of IP Encapsulation • Entire datagram treated like data • Frame type identifies contents as IP datagram • Frame destination address gives next hop

  5. Frame and Datagram Destination Addresses • Frame address • Hardware (MAC) address • Next hop • Datagram address • IP address • Ultimate destination

  6. Frame Address For Encapsulated Datagram A datagram is encapsulated in a frame for transmission across a physical network. The destination address in the frame is the address of the next hop to which the datagram should be sent; the address is obtained by translating the IP address of the next hop to an equivalent hardware address.

  7. Frames and Datagrams • Datagram survives entire trip across Internet • Frame only survives one hop

  8. Illustration of Frame HeadersUsed for Datagram Transmission • Each hop extracts datagram and discards frame

  9. Maximum Frame Size • Each network technology imposes maximum frame size • Called Maximum Transmission Unit (MTU) • MTUs differ • Internet • Can contain heterogeneous technologies • Must accommodate multiple MTUs

  10. Illustration of How Two MTUsCause a Problem for IP • Host 1 • Creates datagram for Host 2 • Chooses datagram size of 1500 octets • Transmits datagram across network 1 • Router R • Receives datagram over network 1 • Must send datagram over network 2 • Employs fragmentation

  11. Datagram Fragmentation • Performed by routers • Needed when datagram larger than MTU of network • Divides datagram into pieces called fragments • Each fragment has datagram header • Fragments sent separately • Ultimate destination reassembles fragments

  12. Illustration of Datagram Fragmentation • Each fragment has IP datagram header • Header fields • Identify original datagram • Indicate where fragment fits

  13. Datagram header – fields for fragments • Flags • 1 – reserved (0), 2 – Do not fragment, 3 – More Fragments • Fragment offset

  14. Example of Reassembly • Host H1 generates 1500-octet datagram • Router R1 fragments • Router R2 transmits fragments • Host H2 reassembles

  15. Multiple Fragmenting Points • Let MTUs along internet path be • 1500 • 1500 • 900 • 1500 • 576 • 1500 • Result: fragmentation can occur twice

  16. Fragmenting a Fragment • Needed when fragment too large for network MTU • Arbitrary subfragmentation possible • Router divides fragments into smaller pieces • All fragments at same “level” • Offset given with respect to original datagram • Destination cannot distinguish subfragments

  17. Fragment Loss • Receiver • Collects incoming fragments • Reassembles when all fragments arrive • Does not know identity of router that did fragmentation • Cannot request missing pieces • Consequence: loss of one fragment means entire datagram lost

  18. Summary • Internet transmission paradigm • Source host • Zero or more routers • Destination host • Datagram encapsulated in network frame for transmission

  19. Summary (continued) • Network hardware has maximum payload size • Called MTU • Datagram must be smaller than hardware MTU • Internet can have multiple MTUs

  20. Summary (continued) • Datagram fragmentation • Accommodates multiple MTUs • Performed by router • Divides datagram into pieces • Ultimate destination reassembles

  21. Summary (continued) • Fragments can be fragmented • Multiple levels possible • All offsets at one level • Loss of any fragment means loss of entire datagram

More Related