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Tracing the flow of data through the TCP/IP/MPLS/MAC protocol stack and Carrier MPLS network:
Starting with the server on the right, which is downloading a file to the client on the left, the file transfer software takes a segment of the file and gives it to the TCP software on the server, which starts a timer, puts a sequence number, error check and source and destination application port number in a TCP header on the file segment, and passes this to the IP software on the server.
The IP software on the server creates an IP packet by adding the IP header, populating the source address with its IP address and the destination address with the IP address of the client on the left.
The IP packet is passed to the layer 2 LAN driver software on the server, which puts the packet in an LAN MAC frame, populating the source address with its MAC address and the destination as the MAC address of the right-hand Customer Edge router.
The frame is then signaled one bit at a time over the LAN cable connecting the server to the LAN switch, which directs the frame to the right-hand CE router.
Layer 2 software on that right-hand CE router receives the MAC frame from the sever via the switch, performs the error check, verifies that its MAC address is the destination, then extracts the IP packet and passes it to the IP routing software on the CE router.
The IP routing software on the CE router decides that the Provider Edge router is the next hop, puts the packet in a MAC frame, populates the destination MAC address with that of the PE, recalculates the error check and signals the frame one bit at a time over the circuit to the service provider's Provider Edge, which is the ingress to the carrier's MPLS network.
The Provider Edge is the ingress Label Edge Router. It receives the IP packet in a MAC frame over the fiber access circuit. This ingress device examines the destination IP address on the packet and along with other factors, decides what Forwarding Equivalence Class the packet belongs to, then implements its decision by pushing a label onto the packet.
It then does a table lookup in its Incoming Label Map to get the Next Hop Label Forwarding Entry, determines what LSR that items with this label are forwarded to, and transmits the labelled packet in a frame to that LSR on the appropriate circuit.
Each LSR in the middle of the network (not shown) receives a frame, extracts the packet then only looks at the label and performs a table lookup to determine where to forward the packet and possibly what relative priority it has.
Eventually, the labelled packet is delivered to the PE on the left, the network's egress Label Edge Router. This device pops the label off the packet and removes the MPLS header, then uses conventional IP routing to send the IP packet in an MAC frame over the access circuit to the CE router on the left.
The CE router on the left receives the IP packet in a MAC frame, passes it to its routing software, which must at this point determine which station on the LAN, which MAC address, has been assigned that IP address.
It first looks in its cache, which is a table relating IP addresses to MAC addresses. If it does not find an entry, it broadcasts an address resolution request to all stations on the LAN at the left, asking "who owns this IP address?"
The client responds with its MAC address. The premise router saves that information in the cache, then puts the packet in a MAC frame with the client's MAC address as the destination, then signals the frame one bit at a time to the left-hand LAN switch, which directs the frame to the client.
Layer 2 software on the client receives the frame over the LAN cable, extracts the IP packet and passes it to the IP software on the client.
Seeing that the destination IP address on the packet is its IP address, the client's IP software extracts the data out of the packet and passes it to the TCP software on the client.
The TCP software on the client performs the error check, and if it fails, discards the data.
Meanwhile, the TCP timer on the server times out, so the TCP software on the server retransmits and the whole process is repeated.
If the second time, the protocol data unit received by the TCP software on the client passes the error check, it sends an acknowledgement to the server so the server stops retransmitting.
The TCP software on the client extracts the data from the TCP protocol data unit and parks it in a memory space for the application identified by the destination port number on the TCP header… the file transfer application, which picks up the data shortly after.
Meanwhile, the file transfer application on the server is sending the next segment of the file.
Advantages of MPLS
MPLS-based carrier bandwidth-on-demand services bring a significant improvement to the user compared to previous technologies: the user-network interface.
The Customer Edge equipment required to connect to a carrier's MPLS service is an IP router.
This IP router required to connect to an MPLS service is no different than any other IP router. In the simplest case, a $20 edge router from Linksys normally used for residential Internet could be used.
The user-network interface to an MPLS-based service is IP. It requires no special equipment or knowledge on the part of the customer, and the customer does not have to configure or keep track of virtual circuits, LSPs, labels or anything of the sort.
This is a large advantage in terms of both cost and barrier to entry for the customer.
VPLS Services using MPLS
The "M" in MPLS stands for "Multiprotocol".
In this lesson, we have constantly referred to packets and the forwarding of labelled packets. While this is the most common use of MPLS, forwarding of labelled frames is also possible.
Carrier Virtual Private LAN Service (VPLS) is a service moving LAN MAC frames using MPLS labels.
With VPLS, the carrier appears like a giant, nationwide LAN switch to the customer, moving MAC frames between customer locations.
This can be implemented with Ethernet over MPLS (EoMPLS), where the customer's MAC frame has MPLS labels pasted on the front of the frame.
At this point, the labelled frame and a labelled packet can be treated the same way by LSRs in the MPLS network… encapsulated in a MAC frame for forwarding over a physical circuit.
TCP/IP over MPLS File Download
