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Figure 52 Address sources for dial-ups
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NAT and Other Midboxes
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Registries assume that you will conserve address space by using NAT for appropriate customer topologies Indeed, whether you have considered the use of private address space and NAT is one of the specific questions in RIPE-NCC s address request template (see Representative Templates from RIPE-NCC later in this chapter) The most common midbox is network address translation (NAT) Other types include firewalls, load distributors, tunneling functions, and so on (see Figure 53)
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Troubleshooting Realities
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Many new technologies of the midbox flavor, especially NAT, have been introduced in an effort to solve the addressing shortage, but they generally introduce operational issues that must be considered Other address-conserving methods, such as dynamic address assignment with DHCP or the IPCP subprotocol of PPP, also introduce operational challenges How do service providers ping, traceroute, or use other standard diagnostics for enterprise address spaces to which they do not directly connect For enterprises themselves, when addresses are dynamically assigned, how does a troubleshooter find the specific address, at a specific moment in time, that has been assigned to the device having problems You must have some fixed identifier to find the device In campus networks, it can be the cubicle or jack identifier that is wired to a specific switch, from
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Figure 53 Midboxes
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Int 1 Tunneling Router Int 2 Transit Router 1 Transit Router 2
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which you can retrieve the active MAC address, which can be looked up in the DHCP log file If your DHCP and DNS are dynamically linked, you may not initially need to track down the physical location You could simply ping or traceroute to the DNS name A similar method works if you give DNS names to your dial-in ports and have the IPCP function of PPP associate addresses and names If midboxes or tunnels are present, you will either have to troubleshoot cooperatively with the user or to have access to both sides of the address-translating or address-encapsulating device For example, in Figure 54, the white interfaces are in the carrier s address space but the black interfaces are in the subscriber s address space Note the juxtaposed black and white interfaces at the ingress and egress to the tunnel If your administrative machinery is sufficiently reliable, you can print DNS name labels for hosts and stick them on the machines Your troubleshooting, however, will rely on the stickiness of glue, the ability of the user to read the identifier to you, and the promptness with which data bases are updated, not to mention the ability to keep creative users from changing things on their own, for reasons that may even seem perfectly valid to them Another aspect to conservation is not assigning globally routable addresses to single-homed enterprises (Figure 55) Reasonable exceptions may be made for well-known servers, but NAT with port translation will work for many hostbased applications It would be considered desirable if a customer multihomed to the same provider could still use NAT, but that realistically presents a difficult challenge of synchronization and failover among the multiple NATs (Figure 56) Remember that a wide range of technologies fall under the general heading of NAT (see [Berkowitz 2000] for details)
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Figure 54 Tunneling and traceroute
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Figure 55 Single-homed connection with NAT
4 5 6
"Inside" Realm 2
"Inside" Realm 1
7 8 9 10 11 12
"Outside" Public Internet
Rule 1 2 3 4 5 6 7 8 9 10 11 12
Owner P1 or DMZ P1 or DMZ P2 or DMZ P2 or DMZ P1 P1 P2 P2 P1 P1 ISP ISP
Source P1 DMZ DMZ P2 P1 P2 P1 P2 P1 Public P1 Public
Destination DMZ P1 P2 DMZ P2 P1 P2 P1 Public P1 Public P1
Figure 56 NAT types
5
NAT is one example of the broader concept of midboxes Midboxes will be discussed in more detail in 13, but, generically, they are devices, often serving useful purposes, that violate the original IP assumption that IP addresses are significant end-to-end
Addressing Aspects of Multihoming
Multihoming involves IP connectivity to more than one provider access point These points can be of the same or different providers In general, it is quite realistic to use PA space when multihoming to different locations of the same provider using PA space (Figure 57) In certain cases, such as when both customer connections go to the same provider router or firewall, this may be achievable, but multihoming defined here as BGP connectivity with two or more other autonomous systems is usually a justification for using registered address space (Figure 58) Another aspect of conservation is reducing the use of slots in the global routing table In an ideal world, this is done by allocating or assigning contiguous blocks of addresses, subnetting them as required inside the recipient s network but only announcing the aggregate to the world and the global routing tables In practice, many exceptions to ideal aggregation are needed Let s explore the mechanisms of aggregation now