The Men & Mice Blog

The ABCs of DNS: a select glossary from the Men & Mice training archives - Part 4

Posted by Men & Mice on 8/16/19 8:43 AM

Continuing our glossary of DNS tips & tricks, we’re covering the letters J, K, and L this time.

J is for “jumbogram”

Ever wondered what the largest (internet-layer) packet you can send? It’s 4,294,967,295 bytes. (One byte less than 4 GB.) Theoretically. Let’s break down the math (and the tech).

IPv6, among other things, has an extension that allows for a 32-bit length field. Jumbogram is the term for IPv6 packets taking advantage of it, capable of carrying more than the 65,535 octets of the limit of IPv4’s 16-bit length field.

However, transport layers such as TCP and UDP are limited to 16 bits. (TCP doesn’t have a length field, but the TCP MSS option and TCP Urgent field are both limited to 16 bits.) To make transporting larger payloads possible, these transport layers need a redesign to include 32-bit length fields.

RFC 1883, which first described the IPv6 standard, contained these modifications but was superseded by RFC 2460 which no longer did. RFC 2147 described the TCP and UDP enhancements but was obsoleted by RFC 2675 which merged the relevant parts from 1883 and 2147 into one document.

This is all theoretical, of course. RFC 2675 is listed as ‘informational’ and the practicality of jumbograms are debatable. But, as networking becomes more and more ubiquitous with larger and larger data transport needs, it may very well become everyday practice soon enough. Especially because larger payloads mean speedier delivery and less overhead - on the other hand, networks need better reliability to handle them. If just a small bit gets lost, scrambled, or corrupted, the whole payload has to be re-sent.

K is for “Kea”

In addition to DNS, an essential component of any network is DHCP. Just like DNS, development in DHCP doesn’t stop, to the extent of completely new software emerging to replace the old one: as is in the case of the Kea DHCP server.

Kea is the successor to ISC DHCP. While mature and robust, ISC DHCP is also old. It started in 1995, a time when networks were a lot smaller. Since then, network management became a lot more complex and mission-critical.

Kea is a modern DHCP server developed for the challenges of modern times. It's more scalable and offer better performance, with a different architecture. Kea also brings a somewhat different feature set, such as hooks and a rich API to configure users and subnets, Radius integration, and support for several database backends.

As is the case with any software no longer in widespread deployment, the development of ISC DHCP will cease in favor of Kea. ISC already recommends, particularly for new deployments, to use Kea instead of ISC DHCP.

To learn more about Kea and how to migrate from ISC DHCP take a look at its website.

L is for “labels”

As we discussed earlier, domain names are made up of three or more parts. These are called labels. 

A typical fully qualified domain name (FQDN) will look like this:

  • root (the trailing dot at the end)
  • top-level domain or “TLD” (such as .com, .net, etc.)
  • domain (such as menandmice)
  • host (such as www, info, etc.)

Labels can contain 1 to 63 octets. (An octet is a unit consisting of 8 bits. While technically the same as a byte, the latter is usually used to describe storage unit sizes.) Put it simpler, labels can be between 1 and 63 characters. The null label (length zero) is reserved for the root zone and is represented by the label terminating in the trailing dot.

Labels were initially restricted to ASCII, but in 2003 ICANN approved the IDNA (internationalized domain name) system. The IDNA maps Unicode characters to valid DNS characters via Punycode. For example, Þórsmörk.is (lovely place, you should visit!) would become xn--rsmrk-ztay3d.is 

Because domain names can have a maximum of 253 characters, the theoretical limit of a domain is 127 levels. (127 1-character labels + 126 dots separating them.)

Want to learn more?

This series is byte-sized (or, well, octet-sized) — but a lot more can be said and done. To learn more in-depth about DNS specifically, we offer a comprehensive DNS training program. 

You can enroll in different groups depending on your skill level:

  • If you’re new to DNS, we offer the DNS & BIND Fundamentals (DNSB-F) course. It’s part of the DNS & BIND Week (DNSB-W) and serves as a shorter introduction to the world of DNS and BIND.
  • If you’re already familiar with the basics, the full five-day DNS & BIND Week (DNSB-W) course takes you deeper into DNS, including a heavy emphasis on security, stopping just short of DNSSEC (for which we offer a separate course).
  • And if you're looking for even more, we offer the DNS & BIND Advanced (DNSB-A) program, getting into the deep end of things.

Check out our training calendar for 2019, and reach out to us with any questions.



Topics: IPv6, DNS training, Kea, domain name

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