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Transcript of Routing
IP Packets Routing On the board, at a high-level, explain:
Access method, physical addressing
Switching at layer 2 (switches within WANS)
Switching at layer 3 (routers between WANS and LANS)
ARP – getting the physical address
Logical addressing and routing Processing at the destination computer Services provided at the source computer Processing at each router An imaginary part of the Internet Forwarding process in a connection-oriented network Internet as a combination of LANs and WANs connected together Will review Exam 2 Lecture #12 Chapter 4 & Chapter 6
Delivery & Forwarding of IP Packets
(Part 1 of 2) Error checking at the data link layer Forwarding process in a connectionless network Internet as a block box At the conceptual level, we can think of the global Internet as a black box network that connects millions (if not billions) of computers in the world together. At this level, we are only concerned that a message from the application layer in one computer reaches the application layer in another computer. A connection-oriented packet switched network Router switches label from 14 to 66 (Tx) Set up path or virtual-circuit by first sending request packet
Rx sends an ACK packet in setting up virtual-circuit Flow of one packet in an established virtual circuit Delay in a connectionless network A connectionless packet-switched network Delay in a connection-oriented network When the router is looking for the route, it:
First check for direct delivery
Then host-specific delivery,
The network-specific delivery, and
Finally, default delivery
This order can be organized with in the routing table Router’s Table Logistics Simplified forwarding module in classless address Router R1 receives 500 packets for destination 126.96.36.199 - how does Router R1 uses it’s routing table ??? A routing example Host or router uses a routing table
Table can be either static or dynamic in nature
A static routing table contains information entered manually.
A dynamic routing table is updated periodically using one of the dynamic routing protocols such as RIP, OSPF, or BGP
Regarding dynamic routing table: if fiber cut or router failure, the tables are updated STATIC VERSUS DYNAMIC ROUTING Recall for the Classful case, subnetting is done within the organization Simplified forwarding module in classful address with subnetting For the Classful case, per router, a table was needed for each class – this made the searching simple Simplified forwarding module in classful address without subnetting There are various routing methods:
Next-Hop Routing – table only holds the address of the next hop (instead info regarding the entire route) – routing table for each host
Network-Specific Routing – instead of an entry for each host (on the same network), only one entry for the network is defined
Host-Specific Routing – for a specific destination host, you might want to control the exact route – in this case, the actual Rx is listed in the routing table and the desired next hop is listed
Default Routing – instead of listing all of the various networks in the Internet, Tx host would use one entry called the Default (network address 0.0.0.0) ROUTING METHODS Delivery and
IP Packets Chapter 6 With the classless approach, routing tables increased – in reducing the size of some tables, use a router to represent multiple blocks – address aggregation Address aggregation Routing Table for R1 in the Illustrated Configuration – Classless Case Doesn’t know what network is connected to router here Configuration for the Classful and Subnetting Case Configuration for routing for R1, Classful Case In this case, you want every packet traveling to Host B to traverse through R3. For the other hosts on N2 and N3, the Network-specific routing approach is used. Host-Specific Routing – for a specific destination host, you might want to control the exact route – in this case, the actual Rx is listed in the routing table and the desired next hop is listed Host-specific Routing Network-Specific Routing – instead of an entry for each host on the same physical network, only one entry for the network is defined Network-specific routing Show more routers in better illustrating the routing table Next-Hop Routing – table only holds the address of the next hop (instead info regarding the entire route) Next-hop routing Project 3 due today at 6:30pm
Project 4 posted on website (due 10/31/12) Lecture #13 Chapter 4 & Chapter 6
Delivery & Forwarding of IP Packets
(Part 2 of 2) Cross Point Crossbar Switching Fabric Components The topics discussed in this section include: We represent a router as a black box that accepts incoming packets from one of the input ports (next hop), uses a routing table to find the departing output port, and sends the packet from this output port (interface). STRUCTURE OF A ROUTER In this case, R1 sends to a specific network however, R2 sends to the remainder of the Internet (default) Default Routing – instead of listing all of the various networks in the Internet, Tx host would use one entry called the Default (network address 0.0.0.0) Default Routing This is where delay is incurred Performs layer 1 and 2 functions: bits to signal, packet encapsulated into frame, error control overhead added Performs layer 1 and 2 functions: signal to bits, packet decapsulated from frame, error control performed on bits, buffers packets before going to the switching fabric Router components U
UG Mask Destination Next Hop I.
255.255.0.0 188.8.131.52 -- m0
255.255.0.0 184.108.40.206 220.127.116.11 m1
255.255.255.0 18.104.22.168 22.214.171.124 m1
0.0.0.0 0.0.0.0 126.96.36.199 m0 Make the routing table for router R1 in the Figure Example 2 Router stops when match is made U case
UG case Host-specific
188.8.131.52 & 255.255.255.255 184.108.40.206 no match
220.127.116.11 & 255.255.255.0 18.104.22.168 match Mask Dest. Next Hop I.
255.0.0.0 22.214.171.124 -- m0
255.255.255.224 126.96.36.199 - m2
255.255.255.224 188.8.131.52 - m1
---------------------------------------------------------------------------255.255.255.255 184.108.40.206 220.127.116.11 m0
255.255.255.0 18.104.22.168 22.214.171.124 m0
255.255.255.0 126.96.36.199 188.8.131.52 m0
0.0.0.0 0.0.0.0 184.108.40.206 m0 Reference count: # of users using this route at any moment
Use: # of packets transmitted through this router for the corresponding Rx
Interface: name of the interface Mask: used to extract the net id of the Rx. For Host-Specific Routing - the mask is 255.255.255.255 and for Default Routing – the mask is 0.0.0.0.
Destination Address: either the destination host address or destination network address
Next-hop Address: next hop router address Flags
U - The router is up and running. If router is down, the packet discarded
G - The destination is in another network. If G flag present, indirect delivery (if not, direct delivery)
H – If H flag present, destination field contains Host-specific address (if not present, network address)
D – If D flag present, routing info added to host routing table via ICMP’s redirection (cover later)
M - If M flag present, routing info was modified via ICMP’s redirection (cover later) Routing Table Given a packet came in on port 1 and needed to go out of port 6, the binary string of 110 will be used – explain this Example Uses a binary string to route across the switch A banyan switch