Detailed Network Communication Path with OSI Layers

Request Path

Request Path:
1. **Client Device (Iran)**: Initiates the request from the local machine or application.
2. **Local ISP Router**: Routes the request to the Internet Service Provider’s network.
3. **Undersea Cable Router**: Transmits the request across the undersea cable network connecting different continents.
4. **US Gateway Router**: Routes the request to the appropriate entry point in the US network.
5. **Firewall and DMZ**: The request passes through security measures to ensure safe access into the internal network.
6. **Destination Server (USA)**: Receives and processes the request.
    

Return Path

Return Path:
1. **Destination Server (USA)**: Generates and sends a response.
2. **Firewall and DMZ**: The response passes through security measures to ensure it is secure.
3. **US Gateway Router**: Routes the response to the appropriate exit point in the US network.
4. **Undersea Cable Router**: Transmits the response across the undersea cable network back to the originating continent.
5. **Local ISP Router**: Routes the response to the Internet Service Provider’s network in Iran.
6. **Client Device (Iran)**: Receives and processes the response.
    

Request Path Diagram

Return Path Diagram

Routing Tables Example

Networking Protocols and Models

Interior Gateway Protocols (IGP)

Interior Gateway Protocols (IGP) are used for routing within an Autonomous System (AS). These protocols help routers find the best paths within a large network. The main IGP protocols include:

• RIP (Routing Information Protocol):
  • Uses Distance-Vector routing algorithm.
  • Supports a maximum of 15 hops.
  • Updates routing tables every 30 seconds.
  • Suitable for small to medium-sized networks.
• OSPF (Open Shortest Path First):
  • Uses Link-State routing algorithm.
  • Divides the network into Areas for scalability and efficiency.
  • Uses Incremental Updates to reduce network traffic.
  • Suitable for large and complex networks.
• EIGRP (Enhanced Interior Gateway Routing Protocol):
  • Hybrid protocol combining Distance-Vector and Link-State features.
  • Uses DUAL (Diffusing Update Algorithm) to compute the best paths.
  • Faster convergence than RIP and OSPF.
  • Suitable for large and complex networks with high demands.

Exterior Gateway Protocol (EGP)

Exterior Gateway Protocols (EGP) are used for routing between Autonomous Systems (AS). The primary EGP protocol is:

• BGP (Border Gateway Protocol):
  • Uses Path-Vector routing algorithm.
  • Maintains path information using AS path lists to prevent routing loops.
  • Supports complex routing policies.
  • Used as the main routing protocol between ASes on the Internet.

// Pseudocode for CSMA/CA
do {
    listen();
    if (channel is clear) {
        send RTS();
        if (CTS received) {
            send data();
            if (ACK received) {
                data transmission successful;
            }
        }
    } else {
        wait for a random backoff time;
    }
} while (data to send);
        

// Pseudocode for CSMA/CD
do {
    listen();
    if (channel is idle) {
        send data();
        if (collision detected) {
            send jam signal();
            wait for a random backoff time;
        }
    }
} while (data to send);
        

TCP/IP and UDP/IP

TCP/IP and UDP/IP are protocol suites used for data transfer over internet networks:

• TCP/IP:
  • TCP (Transmission Control Protocol):
    • Connection-oriented.
    • Provides reliability, ensuring data is delivered correctly and in order.
    • Supports flow control and congestion control.
  • IP (Internet Protocol):
    • Responsible for addressing and routing data packets in the network.
    • Has two main versions: IPv4 and IPv6.
• UDP/IP:
  • UDP (User Datagram Protocol):
    • Connectionless.
    • Provides no guarantee of data delivery.
    • Suitable for applications requiring high speed and low latency, such as live streaming and online gaming.

OSI Model

The OSI model is a seven-layer framework that divides network communication into manageable parts:

• Physical Layer: Handles the raw transmission of bits over physical media like cables and radio waves.
• Data Link Layer: Manages frame transfer between devices on the same physical medium and provides error detection and correction.
• Network Layer: Responsible for routing packets between devices across different networks using protocols like IP.
• Transport Layer: Ensures reliable and error-free data transfer between hosts, with TCP and UDP operating at this layer.
• Session Layer: Manages and controls communication sessions between applications, including session establishment and synchronization.
• Presentation Layer: Translates, encrypts, and compresses data for application layer services.
• Application Layer: Provides network services directly to end-user applications, including protocols like HTTP, FTP, and SMTP.

Access, Distribution, and Core Layers

These layers are used in large network designs such as enterprise and campus networks:

• Access Layer:
  • Closest layer to users and end devices.
  • Manages connections to the network and resource access.
  • Includes switches and access points.
• Distribution Layer:
  • Aggregates traffic from the access layer and forwards it to the core layer.
  • Provides routing, security, and policy enforcement services.
  • Includes routers and intermediate switches.
• Core Layer:
  • The backbone of the network, handling high-speed and high-capacity traffic.
  • Includes high-speed, powerful switches and routers.
  • Optimized for performance and reliability.

Additional Networking Concepts

ARP (Address Resolution Protocol)

ARP is a protocol used to map IP addresses to MAC addresses within a local network. When a device wants to communicate with another device on the same local network, it uses ARP to find the MAC address associated with the destination IP address.

• ARP Request: Sent by a device to all devices on the local network asking "Who has IP address X.X.X.X? Please send your MAC address."
• ARP Reply: Sent by the device with the requested IP address, providing its MAC address.

RARP (Reverse Address Resolution Protocol)

RARP is a protocol used to map MAC addresses to IP addresses. It is used by devices that do not know their own IP address when they first boot up, such as diskless workstations.

• RARP Request: Sent by a device that knows its MAC address but needs to learn its IP address. It asks, "What is my IP address?"
• RARP Reply: Sent by a RARP server, providing the IP address associated with the device's MAC address.

ARP Tables

ARP tables store mappings between IP addresses and MAC addresses for devices on the local network. These tables are maintained by network devices to speed up the process of resolving IP addresses to MAC addresses.

• Dynamic Entries: Added automatically through ARP requests and replies.
• Static Entries: Manually configured and do not expire.

Layer 2 and Layer 3 Routing Tables

Routing tables at different layers of the OSI model manage the routing of data:

• Layer 2 Routing Tables:
  • Used by Layer 2 devices like switches.
  • Contain MAC addresses and the associated ports on the switch.
  • Manage how frames are forwarded within a local network segment.
• Layer 3 Routing Tables:
  • Used by Layer 3 devices like routers.
  • Contain IP addresses and the associated next-hop addresses or interfaces.
  • Manage how packets are routed between different networks.

ISIS (Intermediate System to Intermediate System)

ISIS is a link-state routing protocol used in large networks to exchange routing information within a single AS. It is known for its scalability and robustness.

• IS-IS: Operates in both IPv4 and IPv6 environments.
• Link-State Advertisements (LSAs): Used to disseminate information about network topology.
• Levels of Operation: Supports hierarchical routing with Level 1 (intra-area) and Level 2 (inter-area) routes.

Gateway

A gateway is a network device that acts as an entry and exit point between different networks, often performing protocol translation between different network architectures.

• Default Gateway: The router or device a computer uses to access a network outside its local subnet.
• Gateway Functions: Include network address translation (NAT), firewall protection, and protocol conversion.

DNS (Domain Name System)

DNS is a hierarchical system used to translate human-readable domain names into IP addresses, allowing users to access websites and services using easily remembered names.

• DNS Resolver: A server that queries DNS records on behalf of a client.
• DNS Records: Include A (address), CNAME (canonical name), MX (mail exchange), and more.
• Recursive vs. Iterative Queries: Recursive queries involve the resolver querying multiple DNS servers until it finds the answer, while iterative queries return the best answer available at that moment.

DHCP (Dynamic Host Configuration Protocol)

DHCP is a network management protocol used to dynamically assign IP addresses and other network configuration parameters to devices on a network, simplifying IP address management.

• DHCP Discover: Sent by a client to locate a DHCP server.
• DHCP Offer: Sent by the server with an IP address and configuration information.
• DHCP Request: Sent by the client to accept the offered configuration.
• DHCP Acknowledgment: Sent by the server to confirm the assignment of the IP address and configuration parameters.

ICMP and Ping

ICMP (Internet Control Message Protocol) is a network layer protocol used for sending error messages and operational information indicating success or failure when communicating with another IP address, such as network unreachable, host unreachable, or protocol unreachable. It is an integral part of IP networks, helping with diagnostics and network management.

Ping

Ping is a utility that uses ICMP to test the reachability of a host on an IP network. It sends ICMP Echo Request messages to the target host and waits for an ICMP Echo Reply. The primary purpose of Ping is to check if the target host is reachable and measure the round-trip time for messages sent from the origin to a destination computer.

• ICMP Echo Request: Sent by the source host to the target host.
• ICMP Echo Reply: Sent by the target host back to the source host if the Echo Request is received.

First Ping Timeout

When you send a Ping request to a host for the first time, you might experience a timeout for the initial request. This is often because the ARP (Address Resolution Protocol) process needs to resolve the target host's MAC address from its IP address before the ICMP Echo Request can be sent. Here’s the sequence of events:

1. The source device sends an ARP request to determine the MAC address of the target device associated with its IP address.
2. The ARP request is broadcasted on the local network, asking, "Who has IP address X.X.X.X? Please send your MAC address."
3. The target device responds with an ARP reply, providing its MAC address.
4. With the MAC address obtained, the source device can now send the ICMP Echo Request to the target device.
5. The target device receives the ICMP Echo Request and sends back an ICMP Echo Reply.

Due to the time taken for the ARP process to complete, the first Ping request might timeout. Subsequent Ping requests are usually successful as the MAC address of the target device is cached in the ARP table, allowing direct communication without needing to perform ARP again.

Autonomous System (AS)

An Autonomous System (AS) is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. Each AS is assigned a unique AS number (ASN) which is used in both interior and exterior routing protocols to maintain routing information and ensure proper data flow across the Internet.

Characteristics of an AS

• Unified Control: All the IP networks and routers within an AS operate under the same administrative domain.
• Unique ASN: Each AS is identified by a unique ASN assigned by the Internet Assigned Numbers Authority (IANA) or a regional Internet registry (RIR).
• Routing Policies: The AS has its own routing policies which determine how data is routed within the AS and to other ASes.

Types of AS

• Single-Homed AS: An AS connected to only one other AS.
• Multi-Homed AS: An AS connected to multiple ASes for redundancy and load balancing, but does not allow traffic to pass through it from one AS to another.
• Transit AS: An AS that provides transit services to other ASes, allowing data to pass through it from one AS to another.

Routing Within and Between ASes

Routing within an AS is managed using Interior Gateway Protocols (IGPs) such as OSPF, EIGRP, or RIP. Routing between ASes is managed using an Exterior Gateway Protocol (EGP), with the Border Gateway Protocol (BGP) being the most commonly used EGP.

Importance of AS in Internet Routing

ASes are crucial for the scalability and management of the Internet. They allow for the division of the global Internet into manageable chunks, each with its own set of routing policies and administrative control. This hierarchical structure helps to maintain the stability and efficiency of the global routing system.

eBGP (External BGP) and iBGP (Internal BGP)

eBGP (External BGP)

eBGP is used for exchanging routing information between routers in different Autonomous Systems (ASes). This protocol ensures that each AS is aware of the best paths to reach networks outside its own AS.

• Inter-AS Communication: eBGP is designed for communication between different ASes, providing the means for inter-network routing.
• AS Path Information: eBGP includes the AS path in its routing updates, which helps prevent routing loops by allowing routers to see the sequence of ASes a route has traversed.
• Routing Policy Enforcement: eBGP allows for the implementation of complex routing policies, ensuring that traffic flows according to the administrative preferences of each AS.

iBGP (Internal BGP)

iBGP is used for exchanging routing information between routers within the same Autonomous System. This helps maintain a consistent view of the network within the AS.

• Intra-AS Communication: iBGP operates within a single AS, ensuring that all routers within the AS have a unified view of external routes.
• Full Mesh Topology: iBGP typically requires a full mesh topology, where each router running iBGP is peered with every other iBGP router in the AS. This ensures all routers have complete and consistent routing information.
• Routing Information Distribution: iBGP prevents the re-advertisement of routes learned from other iBGP peers to avoid routing loops within the AS.

Differences Between eBGP and iBGP

• Scope: eBGP is used for routing between different ASes, while iBGP is used within a single AS.
• Routing Updates: eBGP includes the AS path in its updates to prevent loops, while iBGP requires a full mesh to ensure consistent routing information.
• Routing Policies: eBGP can implement complex routing policies for inter-AS traffic, whereas iBGP focuses on maintaining internal routing consistency.