Basic WAN Ethernet model
- Customer connects to an Ethernet link using a router interface
- The Ethernet link leaves the customer building and connects to some nearby SP location called Point Of Presence (PoP).
- SP uses an Ethernet switch, Inside the SP’s network, the SP uses any technology that it wants to create the specific Ethernet WAN services
Ethernet WAN Behavior (Logically & Physically)
Logically, behaves like a point to point connection between two routers
Physically, behaves as if a physical fiber Ethernet link existed between the two routers
Ethernet terminology
Ethernet WAN: A generic name to differentiate it from an Ethernet LAN
Ethernet Line Service (E-Line): A term from the Metro Ethernet Forum (MEF) for the kind of point to point Ethernet WAN service shown throughout this book
Ethernet emulation: A term emphasizing that the link is not a literal Ethernet link from end to end
Ethernet over MPLS (EoMPLS): A term that refers to Multiprotocol Label Switching (MPLS), a technology that can be used to create the Ethernet service for the customer
Routing In EoMPLS WAN
The EoMPLS link uses Ethernet for both Layer 1 and 2 functions.
- To send the IP packet to Router R1 next, PC1 encapsulates the IP packet in an Ethernet frame that has the destination MAC address of R1
- Router R1 de-encapsulates the IP packet from the Ethernet frame and encapsulates the packet into a new Ethernet frame, with a new Ethernet header and trailer
- Router R2 de-encapsulates the IP packet from the Ethernet frame, encapsulates the packet into an Ethernet frame, encapsulates the packet into an Ethernet frame that has the destination MAC address of PC2 and forwards the Ethernet frame to PC2
IP routing
IP focuses on the job of routing data, in the form of IP packets, from the source host to the destination host.
Does not concern itself with the physical transmission of data, instead relying on the lower TCP/IP layers to do the physical transmission of the data
Network layer specifies how packet travel end to end over a TCP/IP network, even when the packet crosses different LAN & WAN
Routing Logic
PC1 analyzes the destination address:
- If PC2’s address is not on the same LAN as PC1, send packet to default gateway
- Else forward to switch to forward packet to respective destination
IP address grouping
IP subnetting, Uses a routing table to determine the location as to where the packet should route to
Network Layer & Data Link Layer Routing (LAN & WAN)
Network layer focuses on transmitting data packets from end to end
Data link layer focuses on transmitting data packets from point to point
Network layer passes off to data link layer to decide how point to point data transmission should occur
The process of point to point transmission involves encapsulation and decapsulation of Ethernet Header and Trailer and observing the IP header to decide the ultimate destination
Process:
1. Use the data link frame check sequence field to ensure that the frame had no errors, if error occurred discard the frame
- Assuming that the frame was not discarded, discard the old data link header and trailer, leaving the IP packet
- Compare the IP packet’s destination IP address to the routing table, and find the route that best matches the destination address. Route identifies the outgoing interface of the router and possibly the next hop router IP address
- Encapsulate the IP packet inside a new data-link header and trailer, appropriate for the outgoing interface and forward the frame
IP header
4 Bytes Wide (Ordered)
- Version
- Length
- DS Field
- Packet Length
- ID
- Flag
- Fragment offset
- TTL
- Protocol
- Header checksum
- Source Address
- Destination Address
Principle of IP routing protocol
- Each router, independent of the routing protocol, adds a route to its routing table for each subnet directly connected to the router
- Each router’s routing protocol tells its neighbors abt the routes in its routing table, including the directly connected routes and routes learned from other routers
- After learning a new route from neighbor, the router’s routing protocol adds a route to its IP routing table, with the next hop router of that route typically being the neighbor from which the route was learned
IP routing, IP updating & Subnet routing (IP advertising)
- Subnet 150.150.4.0 exists as a subnet at the bottom of the figure, connected to Router R3
- R3 adds a connected route for 150.150.4.0 to its IP routing table; this happens without help from the routing protocol
- R3 sends a routing protocol message called a routing update, to R2, causing R2 to learn about subnet 150.150.4.0
- R2 adds a route for subnet 150.150.4.0 to its routing table
- R2 sends a similar routing update to R1, causing R1 to learn about subnet 150.150.4.0
- R1 adds a route for subnet 150.150.4.0 to its routing table. The route lists R1’s own serial0 as the outgoing interface and R2 as the next hop router IP address
Domain Name System (DNS)
A TCP/IP concept that defines a way to use hostname to identify other computers, a means to resolve names into the matching IP address
DNS names and IP address are distributed over all servers and DNS server communicates to get information from one another
Workings of DNS
- Host connect to a server
- Host types a name to refer to the server
- Host sends a DNS message, a DNS query to the DNS server using UDP
- DNS server replies back a DNS reply that lists the server’s IP address
- Host can now send an IP packet to destination address
Address Resolution Protocol (ARP)
A protocol which resolves MAC address that a destination host uses. Identifies MAC address of a host via IP address
Problem: On Ethernet LANS, whenever a host or router needs to encapsulate an IP packet in a new Ethernet frame, the host or routers knows all facts except destination MAC address
Components of ARP
ARP defines a protocol that includes:
- ARP request, which is a message that makes the simple request “if this is your IP address, please reply with your MAC address”
- ARP reply, which indeed lists both the original IP address and the matching MAC address
Working of ARP
- Host/Router sends an ARP broadcast request to all connected devices on a LAN
- All devices on the LAN will then process the received frame
- Devices which are not associated with the IP address, then discards the request
- Device which is associated with the IP address, then returns an ARP reply (Consist of device’s IP address and MAC address)
IP addresses storing & ARP Table Purpose
Noe that hosts and routers remember the ARP results, keeping the information in their ARP cache or ARP table
Each time a host or router needs to send a packet encapsulated in an Ethernet frame, it first checks its ARP cache for the correct IP address and matching MAC address. Hosts and routers will let ARP cache entries time out to clean up the table, so occasional ARP requests can be seen
ARP Table mechanism
ARP tables on routers and switches flushed after a certain amount of time to conserve resources and prevent inaccurate entries
Internet Control Message Protocol
A protocol used to report problems or issues with IP packets on a network. When a problem is experienced with an IP packet, the IP packet is destroyed and an ICMP message is generated and sent to the host that originated the packet
ICMP delivers messages inside IP packets
Most popular use - Ping command
Ping command
A ping (Packet Internet Groper) command issued from a network device generates an echo request packet that is sent to the destination device. Upon receiving the echo request, the destination device generates an echo reply
Consist of TTL field, give a good indication of network latency (delay)
Response notations:
- ! - One exclamation mark per response
- . - One period for each timeout
- U - Destination unreachable message
- N - Network unreachable message
- P - Protocol unreachable message
- Q - Source quench message
- M - Could not fragment
- ? - Unknown packet type
