Forwarding
Relates IP to Lab 3. Covers the principles of end-to-end datagram delivery in IP networks.
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Delivery of an IP datagram
View at the data link layer layer:
Internetwork is a collection of LANs or point-to-point links or switched networks that are connected by routers
R1
Point-to-point link
R2
Point-to-point link
H2
Network of Ethernet switches
Ethernet
IP
H1 R3
Token Ring LAN
R4
Ethernet
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Delivery of an IP datagram
View at the IP layer:
An IP network is a logical entity with a network number We represent an IP network as a "cloud" The IP delivery service takes the view of clouds, and ignores the data link layer view
R1
10.2.1.0/24
R2
20.2.1.0/28
H2
IP
10.1.0.0/24
10.1.2.0/24
20.1.0.0/16
10.3.0.0/16
H1
R3
R4
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Tenets of end-to-end delivery of datagrams
The following conditions must hold so that an IP datagram can be successfully delivered 1. The network prefix of an IP destination address must 1. The network prefix of an IP destination address must correspond to a unique data link layer network (=LAN or correspond to a unique data link layer network (=LAN or point-to-point link or switched network). point-to-point link or switched network). (The reverse need not be true!) (The reverse need not be true!) 2. Routers and hosts that have a common network prefix 2. Routers and hosts that have a common network prefix must be able to exchange IP dagrams using a data link must be able to exchange IP dagrams using a data link protocol (e.g., Ethernet, PPP) protocol (e.g., Ethernet, PPP) 3. Every data link layer network must be connected to at least 3. Every data link layer network must be connected to at least one other data link layer network via a router. one other data link layer network via a router.
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Routing tables
Each router and each host keeps a routing table which tells the router how to process an outgoing packet Main columns:
1. 2. 3. Destination address: where is the IP datagram going to? Next hop: how to send the IP datagram? Interface: what is the output port?
Next hop and interface column can often be summarized as one column Routing tables are set so that datagrams gets closer to the its destination
Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Next interface Hop direct direct R4 direct R4 R4 eth0 eth0 serial0 eth1 eth0 eth0
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Routing table of a host or router
IP datagrams can be directly delivered ("direct") or is sent to a router ("R4")
Delivery with routing tables
Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.2.0.0/16 30.1.1.0/28 Next Hop R3 direct direct R3 R2 R2 Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Next Hop R1 R1 direct R4 direct direct Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Next Hop R2 R2 R2 R2 R2 direct
R1
10.2.1.0/24
R2
20.2.1.0/28
H2
10.1.2.0/24 20.1.0.0/16 20.2.1.2/28
to: 20.2.1.2
H1
Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Next Hop direct R3 R3 R3 R3 R3
10.1.0.0/24
10.3.0.0/16
R3
Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Next Hop direct direct R4 direct R4 R4
R4
Destination 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Next Hop R3 R3 R2 direct direct R2
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Delivery of IP datagrams
There are two distinct processes to delivering IP datagrams: 1. Forwarding: How to pass a packet from an input interface to the output interface? 2. Routing: How to find and setup the routing tables?
Forwarding must be done as fast as possible: on routers, is often done with support of hardware on PCs, is done in kernel of the operating system Routing is less time-critical On a PC, routing is done as a background process
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Processing of an IP datagram in IP
Routing Protocol Static routing UDP TCP
Demultiplex Yes routing table Lookup next hop Yes IP forwarding enabled? No No Destination address local?
IP module
Send datagram
Discard
Input queue
Data Link Layer
IP router: IP forwarding enabled Host: IP forwarding disabled 8
Processing of an IP datagram in IP
Processing of IP datagrams is very similar an on IP router and a host Main difference: "IP forwarding" is enabled on router and disabled on host IP forwarding enabled if a datagram is received, but it is not for the local system, the datagram will be sent to a different system IP forwarding disabled if a datagram is received, but it is not for the local system, the datagram will be dropped
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Processing of an IP datagram at a router
Receive an IP datagram 1. 2. 3. 4. 5. 6. 7. 8. 9. IP header validation Process options in IP header Parsing the destination IP address Routing table lookup Decrement TTL Perform fragmentation (if necessary) Calculate checksum Transmit to next hop Send ICMP packet (if necessary)
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Routing table lookup
When a router or host need to transmit an IP datagram, it performs a routing table lookup Routing table lookup: Use the IP destination address as a key to search the routing table. Result of the lookup is the IP address of a next hop router, and/or the name of a network interface
Destination address network prefix or host IP address or loopback address or default route
Next hop/ interface IP address of next hop router or Name of a network interface
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Type of routing table entries
Network route Destination addresses is a network address (e.g., 10.0.2.0/24) Most entries are network routes Host route Destination address is an interface address (e.g., 10.0.1.2/32) Used to specify a separate route for certain hosts Default route Used when no network or host route matches The router that is listed as the next hop of the default route is the default gateway (for Cisco: "gateway of last resort) Loopback address Routing table for the loopback address (127.0.0.1) The next hop lists the loopback (lo0) interface as outgoing interface
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Routing table lookup: Longest Prefix Match
Longest Prefix Match: Search for the routing table entry that has the longest match with the prefix of the destination = IP address
128.143.71.21
Destination address 10.0.0.0/8 128.143.0.0/16 128.143.64.0/20 128.143.192.0/20 128.143.71.0/24 128.143.71.55/32 default
Next hop R1 R2 R3 R3 R4 R3 R5
1. Search for a match on all 32 bits 2. Search for a match for 31 bits ..... 32. Search for a mach on 0 bits
Host route, loopback entry 32-bit prefix match Default route is represented as 0.0.0.0/0 0-bit prefix match
The longest prefix match for 128.143.71.21 is for 24 bits with entry 128.143.71.0/24 Datagram will be sent to R4
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Route Aggregation
Longest prefix match algorithm permits to aggregate prefixes with identical next hop address to a single entry This contributes significantly to reducing the size of routing tables of Internet routers
Destination
10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.2.0.0/16 30.1.1.0/28
Next Hop
R3 direct direct R3 R2 R2
Destination
10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.0.0.0/8
Next Hop
R3 direct direct R3 R2
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How do routing tables get updated?
Adding an interface: Configuring an interface eth2 with 10.0.2.3/24 adds a routing table entry: Adding a default gateway: Configuring 10.0.2.1 as the default gateway adds the entry: Static configuration of network routes or host routes Update of routing tables through routing protocols ICMP messages
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Destination
10.0.2.0/24
Next Hop/ interface
eth2
Destination
0.0.0.0/0
Next Hop/ interface
10.0.2.1
Routing table manipulations with ICMP
When a router detects that an IP datagram should have gone to a different router, the router (here R2) forwards the IP datagram to the correct router sends an ICMP redirect message to the host Host uses ICMP message to update its routing table
R1
(2) IP datagram
(1) IP datagram
R2
Destination Next Hop 10.1.0.0/24 R1 ...
(3) ICMP redirect
Ethernet
H1
Destination Next Hop 10.1.0.0/24 R2 R1 ...
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ICMP Router Solicitation ICMP Router Advertisement
After bootstrapping a host broadcasts an ICMP router R1 solicitation. In response, routers send an ICMP router advertisement message Also, routers periodically broadcast ICMP router advertisement
R2
ICMP router advertisement ICMP router solicitation ICMP router advertisement
Ethernet
This is sometimes called the Router Discovery Protocol
H1
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