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Internet Control Protocols

In addition to IP, which is used for data transfer, the Internet has several control protocols used in the network layer, including ICMP, ARP, RARP, and BOOTP. In this section we will look at each of these in turn.

The Internet Control Message Protocol

The operation of the Internet is monitored closely by the routers. When something unexpected occurs, the event is reported by the ICMP (Internet Control Message Protocol), which is also used to test the Internet. About a dozen types of ICMP messages are defined. The most important ones are listed in table below. Each ICMP message type is encapsulated in an IP packet.

The DESTINATION UNREACHABLE message is used when the subnet or a router cannot locate the destination, or a packet with the DF bit cannot be delivered because a "small - packet" network stands in the way.

The TIME EXCEEDED message is sent when a packet is dropped due to its counter reaching zero. This event is a symptom that packets are looping, that there is enormous congestion, or that the timer values are being set too low.

Message TypeDescription
Destination UnreachablePacket could not be delivered
Time ExceededTime to live field hit 0
Parameter ProblemInvalid header field
Source QuenchChoke Packet
RedirectTeach a router about geography
Echo RequestAsk a machine if it is alive
Echo ReplyYes, I am alive
Timestamp RequestSame as Echo request, but with timestamp
Timestamp ReplySame as Echo reply, but with timestamp

The PARAMETER PROBLEM message indicates that an illegal value has been detected in a header field. This problem indicates a bug in the sending host’s IP software, or possibly in the software of a router transited.

The SOURCE QUENCH message was formerly used to throttle hosts that were sending too many packets. When a host received this message, it was expected to slow down. It is rarely used any more because when congestion occurs, these packets tend to add more fuel to the fire.

The REDIRECT message is used when a router notices that a packet seems to be routed wrong. It is used by the router to tell the sending host about the probable error.

The ECHO REQUEST and ECHO REPLY messages are used to see if a given destination is reachable and alive. Upon receiving the ECHO message, the destination is expected to send an ECHO REPLY message back. The TIMESTAMP REQUEST and TIMESTAMP REPLY messages are similar, except that the arrival time of the message and the departure time of the reply are recorded in the reply. This facility is used to measure network performance.

In addition to these messages, there are four others that deal with Internet addressing, to allow hosts to discover their network numbers and to handle the case of multiple LANs sharing a single IP address. ICMP is defined in RFC 792.

The Address Resolution Protocol

Although every machine on the Internet has one (or more) IP addresses, these cannot actually be used for sending packets because the data link layer hardware does not understand Internet addresses. Nowadays, most hosts are attached to a LAN by an interface card that only understands LAN addresses. For example, every Ethernet card ever manufactured comes equipped with a 48-bit Ethernet address. Manufacturers of Ethernet cards request a block of addresses from a central authority to ensure that no two cards have the same address (to avoid conflicts should the two cards ever appear on the same LAN). The cards send and receive frames based on 48-bit Ethernet addresses. They know nothing at all about 32-bit IP addresses.

The question now, arises: How do IP addresses get mapped onto data link layer addresses, such as Ethernet? To explain how this works, let us use the example of figure below, in which a small university with several class C networks is illustrated. Here we have two Ethernets, one in the Computer Science department, with IP address and one in Electrical Engineering, with IP address These are connected by a campus FDDI ring with IP address Each machine on an Ethernet has a unique Ethernet address, labeled E1 through E6, and each machine on the FDDI ring has an FDDI address, labeled F1 through F3.

Let us start out by seeing how a user on host 1 sends a packet to a user on host 2. Let us assume the sender knows the name of the intended receiver, possibly something like The first step is to find the IP address for host 2, known as This lookup is performed by the Domain Name System. A DNS server will be consulted which returns the IP address for host 2 (

The upper layer software on host 1 now builds a packet with in the Destination address field and gives it to the IP software to transmit. The IP software can look at the address and see that the destination is on its own network, but it needs a way to find the destination's Ethernet address. One solution is to have a configuration file somewhere in the system that maps IP addresses onto Ethernet addresses. This solution is certainly possible, but for organizations with thousands of machines, keeping these files up to date is an error-prone, time-consuming job.

A better solution is for host 1 to output a broadcast packet onto the Ethernet asking: "Who owns IP address" The broadcast will arrive at every machine on Ethernet, and each one will check its IP address. Host 2 alone will respond with its Ethernet address (E2). In this way host 1 learns that IP address is on the host with Ethernet address E2. The protocol for asking this question and getting the reply is called ARP (Address Resolution Protocol). Almost every machine on the Internet runs it. It is defined in RFC 826.

The advantage of using ARP over configuration files is the simplicity. The system manager does not have to do much except assign each machine an lP address and decide about subnet masks. ARP does the rest.

At this point, the IP software on host 1 builds an Ethernet frame addressed to E2, puts the IP packet (addressed to in the payload field, and dumps it onto the Ethernet. The Ethernet card of host 2 detects this frame, recognizes it as a frame for itself, scoops it up, and causes an interrupt. The Ethernet driver extracts the IP packet from the payload and passes it to the IP software, which sees that it is correctly addressed, and processes it.

Various optimizations are possible to make ARP more efficient. To start with, once a machine has run ARP, it caches the result in case it needs to contact the same machine shortly. Next time it will find the mapping in its own cache, thus eliminating the need for a second broadcast. In many cases host 2 will need to send back a reply, forcing it, too, to run ARP to determine the sender's Ethernet address. This ARP broadcast can be avoided by having host 1 include its IP to Ethernet mapping in the ARP packet. When ARP broadcast arrives at host 2, the pair (, E1) is entered into host 2's ARP cache for future use. In fact, all machines on the Ethernet can enter this mapping into their ARP caches.

Yet another optimization is to have every machine broadcast its mapping when it boots. This broadcast is generally done in the form of an ARP looking for its own IP address. There should not be a response, but a side effect of the broadcast is to make an entry in everyone's ARP cache. If a response does arrive, two machines have been assigned the same IP address. The new one should inform the system manager and not boot.

To allow mappings to change, for example, when an Ethernet card breaks and is replaced with a new one (and thus a new Ethernet address), entries in the ARP cache should time out after a few minutes.

Now let us look at figure above again. This time host 1 wants to send a packet to host 6 ( Using ARP will fail because host 4 will not see the broadcast (routers do not forward Ethernet-level broadcasts). There are two solutions. First, the CS router could be configured to respond to ARP requests for network (and possibly other local networks). In this case, host 1 will make an ARP cache entry of (, E3) and happily send all traffic for host 4 to the local router. This solution is called proxy ARP. The second solution is to have host 1 immediately see that the destination is on a remote network and just send all such traffic to a default Ethernet address that handles all remote traffic, in this case E3. This solution does not require having the CS router know which remote networks it is serving.

Either way, what happens is that host 1 packs the IP packet into the payload field of an Ethernet frame addressed to E3. When the CS router gets the Ethernet frame, it removes the IP packet from the payload field and looks up the IP address in routing tables. It discovers that packets for network are supposed to go to router If it does not already know the FDDI address of, it broadcasts an ARP packet onto the ring and learns that its ring address is F3. It then inserts the packet into the payload field of an FDDI frame addressed to F3 and puts it on the ring.

At the EE router, the FDDI driver removes the packet from the payload field and gives it to the IP software, which sees that it needs to send the packet to If this IP address is not in its ARP cache, it broadcasts an ARP request on the EE Ethernet and learns that the destination address is E6. So it builds an Ethernet frame addressed to E6, puts the packet in the payload field, and is extracted from the frame and passed to the IP software for processing.

Going from host 1 to a distant network over a WAN works essentially the same way, except that this time the CS router’s tables tell it to use the WAN router whose FDDI address is F2.

The Reverse Address Resolution Protocol

ARP solves the problem of finding out which Ethernet address corresponds to a given IP address. Sometimes the reverse problem has to be solved: Given an Ethernet address, what is the corresponding IP address? In particular, this problem occurs when booting a diskless workstation. Such a machine will normally get the binary image of its operating system from a remote file server. But how does it learn its IP address?

The solution is to use the RARP (Reverse Address Resolution Protocol) defined in RFC 903. This protocol allows a newly - booted workstation to broadcast its Ethernet address and say: "My 48 - bit Ethernet address is Does anyone out there know my IP address?" The RARP server sees this request, looks up the Ethernet address in its configuration files, and sends back the corresponding IP address.

Using RARP is better than embedding an IP address in the memory image because it allows the same image to be used on all machines. If the IP address were buried inside the image, each workstation would need its own image.

A disadvantage of RARP is that it uses a destination address of all 1s (limited broadcasting) to reach the RARP server. However, such broadcasts are not forwarded by routers, so a RARP server is needed on each network. To get around this problem, an alternative bootstrap protocol called BOOTP has been invented. Unlike RARP, it uses UDP messages, which are forwarded over routers. It also provides a diskless workstation with additional information, including the IP address of the file server holding the memory image, the IP address of the default router, and the subnet mask to use. BOOTP is described in RFC 951.

The Interior Gateway Routing Protocol: OSPF

The Internet is made up of a large number of autonomous systems. Each AS is operated by a different organization and can use its own routing algorithm inside. For example, the internal networks of companies X, Y, and Z would usually be seen as three ASes if all three were on the Internet. All three may use different routing algorithms internally. Nevertheless, having standards, even for internal routing, simplifies the implementation at the boundaries between ASes and allows reuse of code. In this section we will study routing within an AS. In the next one, we will look at routing between ASes. A routing algorithm within an AS is called an interior gateway protocol; an algorithm for routing between ASes is called an exterior gateway protocol.

The original Internet interior gateway protocol was a distance vector protocol based on the Bellrnan - Ford algorithm. It worked well in small systems, but less well as ASes got larger. So it was replaced in 1979 by a link state protocol. In 1988, the Internet Engineering Task Force began work on a successor. That successor, called OSPF (Open Shortest Path First) became a standard in 1990. It received support from router vendors making it the main interior gateway protocol. The OSPF is specified in RFC 1247.

Given the long experience with other routing protocols, the group designing the new protocol had a long list of requirements that had to be met. First, the algorithm had to be published in the open literature, hence the "O" in OSPF. A proprietary solution owned by one company would not do. Second, the new protocol had to support a variety of distance metrics, including physical distance, delay, and so on. Third, it had to be a dynamic algorithm, one that adapted to changes in the topology automatically and quickly.

Fourth, and new for OSPF, it had to support routing based on type of service. The new protocol had to be able to route real-time traffic one way and other traffic a different way. The IP protocol has a Type of Service field, but no existing routing protocol used it.

Fifth, and related to the above, the new protocol had to do load balancing, splitting the load over multiple lines. Most previous protocols sent all packets over the best route. The second - best route was not used at all. In many cases, splitting the load over multiple lines gives better performance.

Sixth, support for hierarchical systems was needed. By 1988, the Internet had grown so large that no router could be expected to know the entire topology. The new routing protocol had to be designed so that no router would have to.

Seventh, some modicum of security was required to prevent fun Ioving students from spoofing routers by sending them false routing information. Finally, provision was needed for dealing with routers that were connected to the Internet via a tunnel. Previous protocols did not handle this well.

OSPF supports three kinds of connections and networks:

  1. Point-to-point lines between exactly two routers.
  2. Multi-access networks with broadcasting (e.g., most LANs).
  3. Multi-access networks without broadcasting (e.g., most packet switched WANs).

A multi-access network is one that can have multiple routers on it, each of which can directly communicate with all the others. All LANs and WANs have this property.

OSPF distinguishes four classes of routers:

  1. Internal routers that are wholly within one area.
  2. Area border routers that connect two or more areas.
  3. Backbone routers are on the backbone.
  4. AS boundary routers talk to routers in other ASes.

The Exterior Gateway Routing Protocol: BGP

Within a single AS, the recommended routing protocol on the Internet is OSPF (although it is certainly not the only one in use). Between ASes, a different protocol, BGP (Border Gateway Protocol), is used. A different protocol is needed between ASes because the goals of an interior gateway protocol and an exterior gateway protocol are not the same. All that an interior gateway protocol has to do is move packets as efficiently as possible from the source to the destination. It does not have to worry about politics.

Exterior gateway protocol routers have to worry about politics a great deal. For example, a corporate AS might want the ability to send packets to any Internet site and receive packets from any Internet site. However, it might be unwilling to carry transit packets originating in a foreign AS and ending in a different foreign AS, even if its own AS was on the shortest path between the two foreign ASes ("That’s their problem, not ours"). On the other hand, it might be willing to carry transit traffic for its neighbors, or even for specific other ASes that paid it for this service. Telephone companies, for example, might be happy to act as a carrier for their customers, but not for others. Exterior gateway protocols in general, and BGP in particular, have been designed to allow many kinds of routing policies to be enforced in the inter-AS traffic.

Typical policies involve political, security, or economic considerations. A few examples of routing constraints are

  1. No transit traffic through certain ASes.
  2. Never put Iraq on a route starting at the Pentagon.
  3. Do not use the United States to get from British Columbia to Ontario.
  4. Only transit Albania if there is no alternative to the destination.
  5. Traffic starting or ending at IBM should not transit Microsoft.

Policies are manually configured into each BGP router. They are not part of the protocol itself.

The current definition of BGP is in RFC 1654. Additional useful information can be found in RFC 1268.

Internet Muticasting

Normal IP communication is between one sender and one receiver. However, for some applications it is useful for a process to be able to send to large number of receivers simultaneously. Examples are updating replicated, distributed databases, transmitting stock quotes to multiple brokers, and handling digital conference (i.e., multi-party) telephone calls.

IP supports multicasting, using class D addresses. Each class D address identifies a group of hosts. Twenty - eight bits are available for identifying groups, so over 250 million groups can exist at the same time. When a process sends a packet to a class D address, a best - efforts attempt is made to deliver it to all the members of the group addressed, but no guarantees are given. Some members may not get the packet.

Two kinds of group addresses are supported: permanent addresses and temporary ones. A permanent group is always there and does not have to be set up. Each permanent group has a permanent group address. Some examples of permanent group addresses are - All system on a LAN - All routers on a LAN - All OSPF routers on a LAN

Temporary groups must be created before they can be used. A process can ask its host to join a specific group. It can also ask its host to leave the group.

Multicasting is implemented by special multicast routers, which may or may not be co-located with the standard routers. About once a minute, each multicast router sends a hardware (i.e., data link layer) multicast to the hosts on its LAN ( asking them to report back on the groups their processes currently belong to. Each host sends back responses for all the class D addresses it is interested in.

These query and response packets use a protocol called IGMP (Internet Group Management Protocol), which is vaguely analogous to ICMP. It has only two kinds of packets: query and response, each with a simple fixed format containing some control information in the first word of the payload field and a class D address in the second word. It is described in RFC 1112.

Multicast routing is done using spanning trees. Each multicast router exchanges information with its neighbors using a modified distance vector protocol in order for each one to construct a spanning tree per group covering all group members. Various optimizations are used to prune the tree to eliminate routers and networks not interested in particular groups. The protocol makes heavy use of tunneling to avoid bothering nodes not in a spanning tree.

Mobile IP

Many users of the Internet have portable computers and want to stay connected to the Internet when they visit a distant Internet site and even on the road in between. Unfortunately, the IP addressing system makes working far from home easier said than done. In this section we will examine the problem and the solution.

The real villain is the addressing scheme itself. Every IP address contains three fields: the class, the network number, and the host number. For example consider the machine with IP address The 160.80 gives the class (B) and network number (8272); the 40.20 is the host number (10260). Routers all over the world have routing tables telling which line to use to get to network 160.80. Whenever a packet comes in with a destination IP address of the form, it goes out on that line.

If all of a sudden, the machine with that address is carted off to some distant site, the packets for it will continue to be routed to its home LAN (or router). The owner will no longer get email, and so on. Giving the machine a new IP address corresponding to its new location is unattractive because large numbers of people, programs, and databases would have to be informed of the change.

Another approach is to have the routers use complete IP addresses for routing, instead of just the class and network. However, this strategy would require each router to have millions of table entries, at astronomical cost to the Internet.

When people began demanding the ability to have mobile hosts, the IETF set up a Working Group to find a solution.

The solution chosen would require the roaming users to create a home agent. Every site that wants to allow visitors has to create a foreign agent. When a mobile host shows up at a foreign site, it contacts the foreign host there and registers. The foreign host then contacts the user's home agent and gives it a care-of address, normally the foreign agent's own IP address.

When a packet arrives at the user's home LAN, it comes in at some router attached to the LAN. The router then tries to locate the host in the usual way, by broadcasting an ARP packet asking, for example: "What is the Ethernet address of" The home agent responds to this query by giving its own Ethernet address. The router then sends packets for to the home agent. It, in turn, tunnels them to the care-of address by encapsulating them in the payload field of an IP packet addressed to the foreign agent. The foreign agent then decapsulates and delivers them to the data link address of the mobile host. In addition, the home agent gives the care - of address to the sender, so future packets can be tunnelled directly to the foreign agent.

One small detail is probably worth mentioning. At the time the mobile host moves, the router probably has its (soon-to-be-invalid) Ethernet address cached. To replace that Ethernet address with the home agent's, a trick called gratuitious ARP is used. This is a special, unsolicited message to the router that causes it to replace a specific cache entry, in this case, that of the mobile host about to leave. When the mobile host returns later, the same trick is used to update the router's, cache again.

Nothing in the design prevents a mobile host from being its own foreign agent, but that approach only works if the mobile host (in its capacity as foreign agent) is logically connected to the internet at its current site. Also, it must be able to acquire a (temporary) care-of IP address to use. That IP address must belong to the LAN to which it is currently attached.

The IETF solution for mobile hosts solves a number of other problems not mentioned so far. For example, how are agents located? The solution is for each agent to periodically broadcast its address and the type of services it is willing to provide (e.g., home, foreign, or both). When a mobile host arrives somewhere, it can just listen for these broadcasts, called advertisements. Alternatively, it can broadcast a packet announcing its arrival and hope that the local foreign agent responds to it.

Another problem that had to be solved is what to do about impolite mobile hosts that leave without saying goodbye. The solution is to make registration valid only for a fixed time interval. If it is not refreshed periodically, it times out, so the foreign host can clear its tables.

A final point addressed by the Working Group relates to levels of mobility. Imagine an airplane with an on-board Ethernet used by the navigation and avionics computers. On this Ethernet is a standard router that talks to the wired Internet on the ground over a radio link. One fine day, some clever marketing executive gets the idea to install Ethernet connectors in all the arm rests so passengers with mobile computers can also plug in.

Now we have two levels of mobility: the aircraft's own computers, which are stationary with respect to the Ethernet, and the passengers’ computers, which are mobile with respect to it. In addition, the on-board router is mobile with respect to routers on the ground. Being mobile with respect to a system that is itself mobile can be handled using recursive tunneling.

CIDR - Classless Inter Domain Routing

IP has been in heavy use for over a decade. It has worked extremely well, as demonstrated by the exponential growth of the Internet. Unfortunately, IP is rapidly becoming a victim of its own popularity: it is running out of addresses. This looming disaster has sparked a great deal of discussion and controversy within the Internet community about what to do about it. In this section we will describe both the problem and several proposed solutions.

Back in 1987, a few visionaries predicted that some day the Internet might grow to 100,000 networks. Most experts pooh-poohed this as being decades in the future, if ever. The 100,000th network was connected in 1996. The problem, simply stated, is that the Internet is rapidly running out of IP addresses. In principle, over 2 billion addresses exist, but the practice of organizing the address space by classes, wastes millions of them. In particular, the real villain is the class B network. For most organizations, a class A network, with l6 million addresses is too big, and a class C network, with 256 addresses is too small. A class B network, with 65,536, is just right.

In reality, a class B address is far too large for most organizations. Studies have shown that more than half of all class B networks have fewer than 50 hosts. A class C network would have done the job, but no doubt every organization that asked for a class B address thought that one day it would outgrow the 8-bit host field. In retrospect, it might have been better to have had class C networks use 10 bits instead of eight for the host number, allowing 1022 hosts per network. Had this been the case, most organizations would have probably settled for a class C network, and there would have been half a million of them (versus only 16,384 class B networks).

However, then another problem would have emerged more quickly: the routing table explosion. From the point of view of the routers, the IP address space is a two-level hierarchy, with network numbers and host numbers. Routers do not have to know about all the hosts, but they do have to know about all the networks. If half a million class C networks were in use, every router in the entire Internet would need a table with half a million entries, one per network, telling which line to use to get to that network, as well as other information.

The actual physical storage of half a million entry tables is probably doable, although expensive for critical routers that keep the tables in static RAM on I/O boards. A more serious problem is that the complexity of various algorithms relating to management of the tables grows faster than linear. Worse yet, much of the existing router software and firmware were designed at a time when the Internet had 1000 connected networks and 10,000 networks seemed decades away. Design choices made then are far from optimal now.

In addition, various routing algorithms require each router to transmit its tables periodically. The larger the tables, the more likely some parts will get lost underway, leading to incomplete data at the other end and possibly routing instabilities.

The routing table problem could have been solved by going to a deeper hierarchy. For example, having each IP address contain a country, state, city, network, and host field might work. Then each router would only need to know how to get to each country, the states or provinces in its own country, the cities in its state or province, and the networks in its city. Unfortunately, this solution would require considerably more than 32 bits for IP addresses and would use addresses inefficiently.

In short most solutions solve one problem but create a new one. One solution that is now implemented and which gives the Internet a bit of extra breathing room is CIDR (Classless Inter Domain Routing). The basic idea behind CIDR, which is described in RFC 1519, is to allocate the remaining class C networks, of which there are almost two million, in variable-sized blocks. If a site needs, say, 2000 addresses, it is given a block of 2048 addresses (eight contiguous class C networks), and not a full class B address.

In addition to using blocks of contiguous class C networks as units, the allocation rules for the class C addresses were also changed in RFC 1519. The world was partitioned into four zones, and each one given a portion of the class C address space. The allocation was as follows:

  • Addresses to are for Europe
  • Addresses to are for North America
  • Addresses to are for Central and South America
  • Addresses to are for Asia and the Pacific

In this way, each region was given about 32 million addresses to allocate, with another 320 million class C addresses, from through held in reserve for the future. The advantage of this allocation is that now any router outside of Europe that gets a packet addressed to 194.xx.yy.zz or 195.xx.yy.zz can just send it to its standard European gateway. In effect 32 million addresses have now been compressed into one routing table entry. Similarly, for the other regions.

Of course, once a 194.xx.yy.zz packet gets to Europe, more detailed routing tables are needed. One possibility is to have 131,072 entries for networks 194.0.0.xx through 195.255.255.xx, but this is precisely this routing table explosion that we are trying to avoid. Instead, each routing table entry is extended by giving it a 32-bit mask. When a packet comes in, its destination address is first extracted. Then (conceptually) the routing table is scanned entry by entry, masking the destination address and comparing it to the table entry looking for a match.

To make this comparison process clearer, let us consider an example. Suppose that Cambridge University needs 2048 addresses and is assigned the addresses through, along with mask Next, Oxford University asks for 4096 addresses. Since a block of 4096 addresses must lie on a 4096-byte boundary, they cannot be given addresses starting at Instead they get through along with mask Now the University of Edinburgh asks for 1024 addresses and is assigned addresses through and mask

The routing tables all over Europe are now updated with three entries, each one containing a base address and a mask. These entries (in binary) are:

11000010 00011000 00000000 0000000011111111 11111111 11111000 00000000
11000010 00011000 00010000 0000000011111111 11111111 11110000 00000000
11000010 00011000 00001000 0000000011111111 11111111 11111100 00000000

Now consider what happens when a packet comes in addressed to, which in binary is

11000010 00011000 00010001 00000100

First it is Boolean ANDed with the Cambridge mask to get

11000010 00011000 00010000 00000000

This value does not match the Cambridge base address, so the original address is next ANDed with the Oxford mask to get

11000010 00011000 00010000 00000000

This value does match the Oxford mask, so the packet is sent to the Oxford router. In practice, the router entries are not tried sequentially; indexing tricks are used to speed up the search. Also, it is possible for two entries to match, in which case the one whose mask has the most 1 bits wins. Finally, the same idea can be applied to all addresses, not just the new class C addresses, so with CIDR, the old class A, B, and C networks are no longer used for routing. This is why CIDR is called classless routing.

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About the Author
Rajeev Kumar
CEO, Computer Solutions
Jamshedpur, India

Rajeev Kumar is the primary author of How2Lab. He is a B.Tech. from IIT Kanpur with several years of experience in IT education and Software development. He has taught a wide spectrum of people including fresh young talents, students of premier engineering colleges & management institutes, and IT professionals.

Rajeev has founded Computer Solutions & Web Services Worldwide. He has hands-on experience of building variety of websites and business applications, that include - SaaS based erp & e-commerce systems, and cloud deployed operations management software for health-care, manufacturing and other industries.

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