Datagram Forwarding - Module 11: Networking

Datagram Forwarding

Module Overview

• Subject: Networking

• Focus: Fundamental communication service in the Internet

• Key Topics:

  • Formats of packets sent across the internet

  • Datagram encapsulation, forwarding, fragmentation, and reassembly

  • Types of Services: Connection-oriented vs Connectionless

Connectionless Service

• Definition:

  • Data communication between two nodes where the sender sends data without ensuring the receiver's availability.

  • Packets are referred to as datagrams.

• Application Implications:

  • Programs remain unaware of underlying physical networks.

  • Data can be sent and received without knowledge of the land or destination network.

Types of Network Services

• Connection-Oriented Services:

  • Requires a connection to be established before communication.

  • Analogy: Telephone call – you dial, they answer, connection is established.

• Connectionless Services:

  • Broadcasting information without confirmation of receipt.

  • Analogy: Megaphone - broadcasting without certainty of the audience.

• TCP/IP Protocols:

  • Includes protocols for both services.

  • Provides an unreliable connectionless delivery service (low overhead) and a connection-oriented service using underlying connectionless service.

How Datagrams Travel Across the Internet

1. Packet Creation: Host creates a packet and includes the destination address in the packet header.

2. Router Role:

   • The nearby router receives the packet.

   • Looks up the destination address and selects the next router on the path.

3. Delivery to Final Destination:

   • Process continues until the packet reaches the final intended router or host.

• Virtual Packets:

  • Packets are hardware independent.

  • Each host/router has protocol software recognizing internet packets.

Datagram Structure and Headers

IPv4 Datagram Header Format

• Each field has a fixed size, ensuring efficient processing by routers.

• Fields in IPv4 Header:

  • Identification (16 bits): Sequential number assigned to the datagram for reassembly.

  • Flags (3 bits): Specifies fragmentation status and order.

  • Fragment Offset (13 bits): Indicates position of this fragment in the original datagram.

  • Time to Live (8 bits): Initialized by sender, decremented by each router; discarded if it reaches zero.

  • Type (8 bits): Specifies payload type.

  • Header Checksum (16 bits): Checks for errors in header fields via a complement algorithm.

  • Source IP Address (32 bits): Address of original sender, unchanging.

  • Destination IP Address (32 bits): Address of the final destination, also unchanging.

  • IP Options: Optional controls for routing or processing, most datagrams omit this field.

  • Padding: Used to ensure header aligns on 32-bit boundaries.

Transition to IPv6 Datagram Header

• IPv6 New Header Format:

  • Consists of a base header plus optional extension headers.

  • Version (4 bits): Indicates IPv6.

  • Traffic Class (8 bits): Similar to service type in IPv4.

  • Flow Label (20 bits): Designates a datagram with a label switched path (rarely used).

  • Payload Length (16 bits): Specifies size of the payload in octets.

  • Next Header (8 bits): Identifies information type that follows the current header.

  • Hop Limit (8 bits): Functions like Time to Live in IPv4.

  • Source Address (128 bits): Address of original sender.

  • Destination Address (128 bits): Address of recipient.

Next Hop Forwarding

• Process: Each router uses the destination address to decide the next hop, forwarding the datagram accordingly.

• Forwarding Table:

  • Initialized on router startup, updated with topology changes.

  • Contains entries indicating destinations and their corresponding next hops.

• Example:

  • Each router has multiple IP addresses for different interfaces with CIDR (Classless Inter-Domain Routing) notation.

Forwarding Process

1. Router receives a datagram (e.g., destined for 192.40.10.3).

2. Checks entries in the forwarding table:

   • Matches destination prefix using subnet mask to find next hop.

3. Longest Prefix Match: Resolves overlaps in entries to choose the most specific route.

4. If no explicit entry exists, a default entry is used.

Best Effort Delivery

• Definition: A service that does not guarantee the delivery or quality of data transmission.

• IP Protocol:

  • Makes a best effort to deliver datagrams without error corrections or guarantees of delivery.

Encapsulation Technique

• Encapsulation:

  • When datagrams are placed into a frame's payload area without alterations.

  • Frame headers differ between various protocols.

  • Analogy: Like a vacuum tube system used in banking; contents remain unchanged during transmission.

Frame Processing

• Receiver identifies payload type through frame type field agreement.

• Upon arrival, routers strip the old frame and encapsulate the datagram into a new frame, repeating this until the final destination is reached.

• Maximum Transmission Unit (MTU):

  • Defines maximum data size a frame can carry; datagrams must fit within the MTU limits.

• Fragmentation:

  • Necessary for transporting larger datagrams over networks with smaller MTUs.

  • Routers divide datagrams into smaller fragments when needed.

Fragmentation and Reassembly

• Fragmentation Process:

  • Each fragment retains the original datagram’s header format with identification and offset fields.

• Reassembly Process:

  • Performed at the final destination using identification number and offsets to place each fragment correctly.

• Out-of-Order Fragments:

  • The reassembly process anticipates and accommodates out-of-order arrival of fragments.

Conclusion

• Summary of Key Points:

  • Datagram structure (IP and IPv6).

  • Forwarding mechanisms via next hop routing and the use of forwarding tables.

  • Importance of best effort delivery and encapsulation in IP protocol operations.

  • Handling fragmentation and reassembly for varying MTU sizes.

Final Notes

• Encourage questions in class and to test knowledge on the subject.