(4.0) The following diagram has routers A, B, C, D (4.0) and E; E is the “border
ID: 3588124 • Letter: #
Question
(4.0) The following diagram has routers A, B, C, D (4.0) and E; E is the “border router” connecting the site to the Internet. All router-to-router connections are via Ethernet-LAN /24 subnets with addresses of the form 200.0.x. Give forwarding tables for each of A, B, C and D. Each table should include each of the listed subnets and also a default default default entry that routes traffic toward router E. Directly connected subnets may be listed with a next_hop of “direct”. 200.0.5A200.0.6B200.0.7D200.0.8EInternet 200.0.9 C 200.0.10
Explanation / Answer
The Header Checksum field is the “Internet checksum” applied to the header only, not the body. Its only purpose is to allow the discarding of packets with corrupted headers. When the TTL value is decremented the router must update the header checksum. This can be done “algebraically” by adding a 1 in the correct place to compensate, but it is not hard simply to re-sum the 8 halfwords of the average header. The header checksum must also be updated when an IPv4 packet header is rewritten by a NAT router.
The Source and Destination Address fields contain, of course, the IPv4 addresses. These would normally be updated only by NAT firewalls.
The source-address field is supposed to be the sender’s IPv4 address, but hardly any ISP checks that traffic they send out has a source address matching one of their customers, despite the call to do so in RFC 2827. As a result, IP spoofing – the sending of IP packets with a faked source address – is straightforward.
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