Introduction

On Monday, we finished talking about switches and bridges.
We talked about source routing through a switched Ethernet.
We then talked gave a distributed algorithm for bridges to learn bridge tables. This involves learning a spanning tree. Then each bridge, for frames whose source address is not in the table, adds a new entry according the port the frame came in on. For frames for which no destination entry exists, the bridge sends the frame on all outgoing ports that are part of the spanning tree. If the destination has an entry, the bridge just uses it.
We gave a brief description of how VLANs work to conclude our discussion on switched ethernets.
After talking about bridges, we then started talking about internetworks, arbitrary collections of networks interconnected by some kind of host-to-host packet delivery service.
We said packet delivery is best-effort. I.e., no guarantee of delivery.
We began looking at the most common internetwork protocol: IP, and looked at the format for IP packets.
We said that IP packets might cross delivery networks with different maximum transmission units when going from source to destination. So packets can be fragmented, and then refragmented again multiple times in the journey from source to destination. The destination hosts then reconstructs the original packet.
We begin today by looking at addressing in IP, focusing on IPv4 -- later we will talk about IPv6.

Global Addresses

Ethernet addresses are flat -- that is they don't say anything about the network structure.
IP addresses are hierarchical -- they are made of several parts which do say something about the structure of the internetwork.
For IPv4, addresses are 32 bit numbers, typically written as four 8-bit decimal numbers with periods in between them.
For routers each interface might have its own IP address.
Originally, IP networks were divided into three classes: A, B, and C.
If the higher-order bit of the address was 0 it was a class A network, 7 bits would be used for the network identifier and such a network could have up to 2²⁴-2 (about 16 million) hosts. For example, IP addresses whose high order byte is 73 (01001001 in binary) might be all part of the Comcast network.
If the high-order two bits were 10, the network was a class B Network, 14 bits would be used for the network identifier and such a network could have up to 65,535 hosts.
Lastly, if the high-order three bits were 110, 21 bits were used to specify the network and the remaining 8 bits could be used to say the host.
This original approach turned out to be somewhat inflexible and something called classless inter-domain routing (CIDR) took its place.
We will look at this in a moment, but first it is helpful to understand how IP packets are forwarded.

Datagram Forwarding and IP

We next look at how IP packets are forwarded in a network.
In order to understand this it is useful to recall the following things about IP datagrams:
- Every IP packet contains the IP address of the destination host.
- The "network part" of an IP address uniquely identifies a single physical network that is part of a larger Internet.
- All hosts and routers that share the same network part of their address are connected to the same physical network and thus can communicate with each other by sending frames over that network.
- The Internet is connected. Further every physical network that is part of the Internet has at least one router that is connected to at least one other physical network; this router can exchange packets with hosts or routers on either network.
To forward a packet a router looks at the network address of the destination and compares it to the network addresses of its interfaces. If it finds a match then it forwards the packet over that interface. Otherwise, it checks if the network address exists in its forwarding tables. If yes, then it forwards according to the table entry. If not, it sends the packet to a default router.
For a host with only one interface and only a default router in its forward table this reduces to: If the network number of the destination is my network number then deliver the packet directly; otherwise deliver the packet to the default router.
Notice that IP is allowing us to be more scalable because we now have a two level hierarchy to get a packet to its destination: we route to the correct network and then use switches in that network to get to the correct host.

Subnetting

The original intent of IP addresses was that the network part would uniquely identify exactly one physical network.
The problem is a class C network can only have 255 nodes and it is very easy for a small company to exceed this.
On the other hand, there are far too few class B network addresses (only 2¹⁴, about 16000 ).
If we assign many network numbers to an organization (i.e., make an organization up out of many Class C networks) then router forwarding tables will start to grow too quickly and so the forwarding algorithms will tend to get slower.
To solve this problem, the idea of subnetting was introduced.
In subnetting, one takes a single IP network number and allocates the IP addresses with that number to several physical networks referred to as subnets.
It should be the case that these subnets are physically close to each other.
All nodes on the same subnet must be configured with the same subnet mask. This mask indicates which bits of the IP address will be kept when dealing with the subnet. So for instance, 255.255.255.0 could be bit-wise AND'd to the IP address of a host on a Class C network to get the relevant subnet (i.e., its network address).

More on Subnetting

Subnetting effectively allows us to split up a large network like a Class B network into smaller subnets which are still bigger than a Class C network.
Class B Network
Network number Host Number

Subnet Mask 255.255.192
111111111111111111 00000000000000

Subnetting the Class B Network
Network number Subnet ID Host Number
To send when subnetting is in use, a host ANDs the destination IP address with the host's own IP mask. If it results in the hosts own subnet, then it knows it can deliver the packet over the subnet.
Otherwise, if the result is not equal, the packet needs to be sent to a router to be forwarded to another subnet.
To handle subnetting a router's forwarding table now has entries (subnet address, subnet mask, outgoing line)
When a packet arrives at a router. Its destination IP address is extracted, this is matched to a router entry, by first ANDing it with the entry mask then comparing to the subnet address. If it matches the corresponding line is used.

Class B Network
Network number	Host Number

Subnet Mask 255.255.192
111111111111111111	00000000000000

Subnetting the Class B Network
Network number	Subnet ID	Host Number

In-Class Exercise

Consider the routing table:

SubnetNumber SubnetMask NextHop

128.96.39.0 255.255.255.128 Interface 0

128.96.39.128 255.255.255.128 Interface 1

128.96.40.0 255.255.255.128 R2

192.4.153.0 255.255.255.192 R3

(default) R4
For each of the following destination addresses, how would this router route a packet:
1. 128.96.39.10
2. 128.96.40.12
3. 128.96.40.151
4. 192.4.153.17
5. 192.4.153.90
Please post your solution to the Mar 1 In-Class Exercise Thread.

SubnetNumber	SubnetMask	NextHop
128.96.39.0	255.255.255.128	Interface 0
128.96.39.128	255.255.255.128	Interface 1
128.96.40.0	255.255.255.128	R2
192.4.153.0	255.255.255.192	R3
(default)		R4

Classless Interdomain Routing (CIDR)

CIDR is a technique that address two scaling concerns in the internet: the growth of backbone routing tables as more and more network numbers need to be stored in them, and the potential for the 32 bit IP address space to be exhausted well before the 4 billionth host is attached to the internet.
Rather than use classes, in CIDR, network addresses are often written as Address/#lead on bits in the mask. Masks are 32 bits with the lead bits on, the rest off. For example, we might write the IP address as: 194.24.0.0/21
This number can then be used to refer to a whole a network and gives us a finer control over the size of such networks than could be had using a classful approach.

Address Translation (ARP)

So far we have explained how IP packets are sent to the correct network.
We still need to explain how they get to the correct host.
The IP address of the destination host is typically different from the kinds of addresses such as Ethernet addresses that the particular destination network interface cards understand.
To do this a protocol called ARP (Address Resolution Protocol) is used by each host on a network to map between IP addresses and link-level addresses.
The set of mappings held by a given host is called an ARP table or ARP cache.
Since mappings may change over time, entries in these tables have timers associated with them and get timed-out and removed periodically (on the order of 15 minutes).
ARP relies on the fact that many link-layer protocols support broadcast.
If a host wants to send an IP datagram to a host that it knows is on the same network, it first checks for an entry in its table, if it finds an entry it uses it.
Otherwise, the host broadcasts an ARP query on the network.
Each host on the network receives the query and checks to see if it matches its IP address. If it does match, it sends a response message that contains its link-layer address back to the originator of the query. The originator can then add the appropriate entry to its table, and also send the IP datagram.
It should be noted that when a host broadcasts a query message, each other host on the network can learn the sender's link-layer and IP addresses and if they don't have these in their tables they can add them.

Host Configuration

Ethernet addresses are globally unique but do not reflect the internetwork structure.
IP addresses need to be globally unique and reflect this structure. So they can't be pre-configured into a host because a manufacturer can't in general know where you want to connect a host into the Internet.
For this reason, IP addresses need to be reconfigurable. Most OS's provide a means to manually configure the IP information.
If every machine had to be manually configured, this would be a lot of work, and error prone.

DHCP

To solve the problem of manually configured IP addresses, an automated method is typically used. DHCP (dynamic host configuration protocol) is the most common such approach.
DHCP relies on the existence of a DHCP server that is responsible for providing configuration information to hosts.
To contact a DHCP server, a newly booted or attached machine sends a DHCPDISCOVER message (this uses UDP) to 255.255.255.255. This is a broadcast IP address.
On each network (making use of DHCP) there is at least one relay agent or DHCP server. Relay agents forward DHCPDISCOVER message to a designated DHCP server.
The DHCPDISCOVER message has the host's Ethernet address. The DHCP server responds by sending the message back to the host with its yiaddr (your internet address) field filled. The server adds the Ethernet IP address pair to its table.
In DHCP, IP addresses are leased for a given period of time, after which the lease expires and the server can remove the entry from its tables. Hosts must periodically either renew their leases or get new addresses.

Error Reporting (ICMP)

IP is perfectly willing to drop datagrams -- for instance, if the router does not know how to forward the datagram or when one fragment of a datagram fails to arrive at the destination.
IP does not always fail silently.
It is configured with a companion protocol known as ICMP (Internet Control Message Protocol) that defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully.
For example, ICMP defines error messages indicating that the destination host is unreachable, that the reassembly process failed, that the TTL has reached 0, that the IP header checksum failed, and so on.
ICMP can also send some control messages from a router back to a host. One example of this is an ICMP-Redirect, which tells the host that a better route to a destination exists.

Virtual Networks and Tunnels

So far, we have been interested in making it possible for nodes on different networks to communicate.
Sometimes it is useful to be able to only allow a subset of the nodes in the Internet to communicate in what is called a virtual private network (VPN).
The key building block for a VPN is an IP Tunnel. This is a virtual point-to-point link between a pair of nodes that are actually separated by an arbitrary number of networks.
Such a link has a router at the entrance to the tunnel and a router at the exit of the tunnel.
Whenever the entrance router wants to send data over the link it encapsulates it in an IP packet with a destination IP address of the router on the other end of the tunnel.
So if the original data was an IP datagram, we might encapsulate this datagram in a IP datagram destined for the end of the tunnel.
The routing table entry for a VPN link looks like a standard routing table entry.
Several tunnels can be stuck together to build up a virtual network.

Why bother to use tunnels?

Tunnels can be combined with encryption so that the traffic over the VPN cannot be read by everyone on the internet.
The VPN routers might be able to support protocols (MBone multicast) that standard routers do not.
It allows one to carry packets from different protocols across an IP network.

Routing

We now look at algorithms that enable routers and switches to acquire the information that is in their forwarding tables.
Forwarding is the process of looking at a packets destination address in a table, and sending a packet in a direction determined by that table.
Routing is the process by which forwarding tables are built.
A forwarding table is used when a packet is being forwarded and so must contain enough information to accomplish this function.
A routing table is the table that is built-up by the routing algorithm as a precursor to building the forwarding table.
The two are often kept separate so that the forwarding table can be as fast as possible.
The first algorithms we will consider scale to networks of moderate size.
They are used for intradomain routing protocols (aka interior gateway protocols (IGPs)).
Here a domain is an internetwork in which all the routers are under the same administrative control. For instance, a university campus network or a network of a single ISP.

Network as a Graph

We view a domain as a graph where the nodes are either hosts, switches, routers, or networks and the edges are network links.
Each edge has an associated cost, which indicates how desirable it is to send data over that link.
The basic problem with routing is to find the lowest-cost path between any two nodes in such a graph.
For a simple network, you could imagine statically computing this and then hard-coding the information into each node.
This approach is not taken in practice because:
- It does not deal with node or link failures
- It does not consider the addition of new nodes or links
- It implies that the edge costs cannot change, even though we might reasonably wish to temporarily assign a high cost to congested links.
For these reasons, routing is typically achieved by running routing protocols among the nodes.
These protocols provide a distributed, dynamic way to solve the problem of finding the lowest-cost path.

Distance Vector/Routing Information Protocol (RIP)

The distance-vector (aka Bellman-Ford) algorithm is one such distributed routing algorithm.
It was published by Bellman (in 1958) and Ford (in 1956), but was first proposed at a conference by Alfonso Shimbel 1955.
The starting assumption is that each node knows the costs of its directly connected neighbors.
The routing table has entries of the form (Destination, Cost, Next Hop) and is initialized to just these connected neighbors.
Each router then sends its routing table to each of its directly connected neighbors.
Using this information, the receiving router updates its own table. Here are a couple of cases where a receiving router might have to update its table:
- The sending router sends information for a host that the receiving router has never heard of before
- The path to some host through the sending router might be cheaper than some existing path.
This process is then repeated until hopefully the routing table converges to a table with routing information for the whole domain.
Routers following distance vector send periodic updates to make sure the routing information is current.
They also send triggered updates whenever a node receives information that causes the nodes tables to be updated.

Problems with Distance Vector

Although distance vector routing does converge to the correct answer, it tends to do it slowly.
As an example, consider the situations below in a linear network where on the left A comes alive at some time t and on right where it fails at some later time. On the left it takes a significant amount of time for all machines to get the correct distance to A. On the right the machines keep incrementing the distance to A forever (count to infinity problem):
One approach to solve this is to use a split horizon: You don't send routes you learned from neighbors back to them.
Another problem with Distance Vector routing is that it does not take line bandwidth into account.

More Internetworks

Outline

Introduction

Global Addresses

Datagram Forwarding and IP

Subnetting

More on Subnetting

In-Class Exercise

Classless Interdomain Routing (CIDR)

Address Translation (ARP)

Host Configuration

DHCP

Error Reporting (ICMP)

Virtual Networks and Tunnels

Why bother to use tunnels?

Routing

Network as a Graph

Distance Vector/Routing Information Protocol (RIP)

Problems with Distance Vector

More on RIP