Delivery, Forwarding, and Unicast Routing Protocols

1. Direct vs. Indirect Packet Delivery

Packet handling at the network layer is classified into two distinct delivery modes:

 Direct Delivery:
 [ Host A ] =============================> [ Host B ]
            (Both on Subnet 192.168.1.0/24)
            (Resolved via ARP; direct Layer 2 frame transmission)

 Indirect Delivery:
 [ Host A ] ──> [ Router R1 ] ──> [ Router R2 ] ──> [ Host C ]
 Subnet A                                           Subnet C
 |<--- Indirect 1 ------>|<--- Indirect 2 ---->|<--- Direct --->|

Direct Delivery:
- Occurs when the destination host is on the same physical network as the deliverer (either the source host or the final router).
- The sender computes Dest IP AND Subnet Mask. If it matches its own Network ID, the destination MAC is resolved via ARP and the packet is delivered directly.
Indirect Delivery:
- Occurs when the destination host resides on a different network.
- The packet is delivered to the Default Gateway (Router). The packet traverses intermediate routers hop-by-hop (indirect deliveries) until reaching the final router on the destination subnet, which performs the final direct delivery.

2. Router Forwarding & Longest Prefix Match

Routers maintain forwarding tables to direct incoming packets to appropriate outbound interfaces.

Forwarding Techniques:

Next-Hop Method: Stores only the IP address of the next router ( $R_{\text{next}}$ ) rather than the complete end-to-end path.
Network-Specific Routing: Stores one route entry per destination network rather than per host.
Default Route (0.0.0.0/0): Captures all packets with no explicit entry in the table.

The Longest Prefix Match Rule:

When multiple routes in a CIDR routing table match the destination IP address, the router forwards the packet using the route with the Longest Prefix (Highest mask length $/n$ ), because it represents the most specific route.

 Routing Table:
 1.  192.168.0.0 /16  ──> Interface eth0
 2.  192.168.1.0 /24  ──> Interface eth1
 3.  0.0.0.0     /0   ──> Interface eth2 (Default)

 Packet with Destination IP 192.168.1.55 matches both /16 and /24.
 Rule: /24 has a longer prefix than /16 (24 > 16) ===> Forward to eth1!

3. Unicast Routing Protocols Architecture

Unicast routing protocols discover network topology and compute loop-free optimal paths dynamically.

                            ROUTING PROTOCOLS
                                    |
         +--------------------------+--------------------------+
         |                                                     |
  INTERIOR GATEWAY PROTOCOLS (IGP)              EXTERIOR GATEWAY PROTOCOLS (EGP)
  (Routing WITHIN an Autonomous System)         (Routing BETWEEN Autonomous Systems)
         |                                                     |
  +------+------+                                              |
  |             |                                              |
Distance      Link-State                                  Path-Vector
 Vector         (OSPF)                                       (BGP)
 (RIP)

Autonomous System (AS): A collection of routers and subnets under single administrative authority presenting a unified routing policy.

4. Distance Vector Routing: RIP

Distance Vector Routing uses the Bellman-Ford Algorithm:

Philosophical Principle: "Each router tells its immediate neighbors about the entire world."
Routing Information Protocol (RIP):
- Metric: Hop Count (traversed routers). Max hops = 15; 16 = infinity (unreachable).
- Updates: Transmits full routing tables periodically every 30 seconds over UDP Port 520.
- Count-to-Infinity Problem: Link failure can cause looping metric increments up to 16.
- Remedies: Split Horizon (never advertise routes back out the interface they were learned), Poison Reverse (advertise failed route with metric 16), and Hold-down Timers.

5. Link-State Routing: OSPF

Link-State Routing uses Dijkstra's Shortest Path First (SPF) Algorithm:

Philosophical Principle: "Each router tells the entire world about its immediate neighbors."
Open Shortest Path First (OSPF):
- Link State Advertisements (LSAs): Routers flood link status packets throughout the area.
- Link State Database (LSDB): Every router builds an identical mathematical topological graph of the network and computes the shortest-path tree using Dijkstra's algorithm.
- Metric: Cost, inversely proportional to bandwidth:

\text{Cost} = \frac{10^8\text{ bps}}{\text{Bandwidth}}

Hierarchical Design: Divides networks into Areas, connected via Area 0 (Backbone Area) through Area Border Routers (ABRs).
- Designated Router (DR): Multi-access networks elect a DR and BDR to reduce adjacency overhead.

6. Path-Vector Routing: BGP

Path-Vector Routing powers the core inter-domain Internet through BGP-4 (Border Gateway Protocol Version 4):

EGP Protocol: Operates across independent Autonomous Systems over TCP Port 179.
AS-Path Attribute: BGP advertises reachable network prefixes along with the full sequence of ASNs (Autonomous System Numbers) traversed:

\text{Path for Net X} = [ \text{AS 400}, \text{AS 200}, \text{AS 100} ]

Loop Prevention: If a router sees its own ASN in an incoming path advertisement, it discards the route, eliminating loops.
Policy Routing: Routes are chosen based on commercial, political, and peering rules rather than simple shortest distance.

7. Comprehensive Unicast Routing Protocol Comparison

Feature	RIP (v2)	OSPF (v2/v3)	BGP (BGP-4)
Category	Distance Vector (IGP)	Link-State (IGP)	Path Vector (EGP)
Algorithm	Bellman-Ford	Dijkstra (SPF)	Path Vector Policy
Routing Metric	Hop Count (Max 15)	Cost ( $\propto \frac{1}{\text{Bandwidth}}$ )	AS-Path & Policy Attributes
Underlying Transport	UDP Port 520	IP Protocol 89	TCP Port 179
Convergence Speed	Slow	Very Fast	Moderate
Loop Prevention	Split Horizon & Poison Reverse	SPF Tree & Area Hierarchy	AS-Path attribute inspection
Scope	Small, legacy LANs	Enterprise campus backbones	Global Internet Core

8. Key Exam Questions & Takeaways

Explain the Longest Prefix Match rule with an example.
- If an incoming IP matches both a /16 route and a /24 route, the router selects the /24 route because it represents a more specific destination network.
Why does RIP define 16 as infinity?
- To bound the execution time of the Count-to-Infinity problem and ensure routing loops terminate.
What transport protocol and port does BGP use, and why?
- BGP uses TCP Port 179. Because BGP handles critical inter-domain Internet routing, using TCP guarantees reliable, connection-oriented message delivery without requiring internal retransmission or windowing logic.

Delivery, Forwarding and Unicast Routing Protocols