Outline

• DNS resolution
• IPv6

DN Resolution -- Basic

• Simplest mechanism for resolution is to have all names and translations listed in a file, and search through the file.
• This is the original mechanism of gethostbyname: on Unix, the file is /etc/hosts. This file is still used, but only for the local host.
• This is impractical when names change frequently, as they do in a large, distributed internet.

DN Resolution -- Server

• Have gethostbyname contact a server to request the translation for a name.
• Each host must be configured with the IP address (not the domain name!!!!) of the server.

DN Resolution -- Distributed

• Have a distributed database. Each server manages a relatively small number of names, and knows who to contact for unknown names (delegation).
• This works well because each server only needs to be configured for the names in its administrative domain, and can dynamically resolve other names.
• Each host still needs to be configured with the IP address of a server. On most unixes, this list is in /etc/resolv.conf. Under windows, the control panel is used to specify the domain name servers.

DNS protocol overview

• a server is authoritative for names in its domain.
• for all other names, a server can either:
  • send the request to another server (proxy)
  • return the address of another server
• DNS can run on top of UDP:
  • overall time out
  • time out and retransmit
  • contact multiple servers
• DNS can run on top of TCP:
  • a field specifies the length of a message
• A name like ics.hawaii.edu is listed by two servers: ics.hawaii.edu and hawaii.edu.

DNS Database Objects

• {\bf A}: IP address for given name
• {\bf NS}: Name server for given name
• {\bf CNAME}: Canonical name for a given name
• {\bf MX}: Mail server for a given name
• {\bf KEY}: Public key for a given name

Distributed Database Management

• Each Resource Record carries a Time-To-Live (TTL) field.
• Unlike IP, this field is actually used as a time specifier. Any cached information is void after the TTL has expired.
• If I forward some information, I can either give it with my TTL, or with a lower TTL, never a larger TTL.
• Since distributing fake DNS data is a good way to have denial-of-service attacks, DNS now provides authentication and security.

DNS Database

• DNS is a fairly simple distributed database. Some reasons why:
  • no record locking is needed
  • records are not updated dynamically, so duplicate servers are easy
  • a record is always stored by the same authoritative server
  • data changes slowly
  • slow access (1 second) is OK.

Problems with IPv4

• Address space too small (32 bits).
• Insufficient structure in addresses.
• Support for real-time.
• Support for security.
• Support for auto-configuration.
• Support for mobile hosts.

IPv6

+++++++++++++++++++++++++++++++++
|Ver|Pri|      Flow Label       |
+++++++++++++++++++++++++++++++++
|Payload Length |Nxt Hdr|Hop Lmt|
+++++++++++++++++++++++++++++++++
|                               |
+                               +
|                               |
+         Source Address        +
|                               |
+                               +
|                               |
+++++++++++++++++++++++++++++++++
|                               |
+                               +
|                               |
+       Destination Address     +
|                               |
+                               +
|                               |
+++++++++++++++++++++++++++++++++

IPv6

• Header Length is missing
• Type of Service is now Priority
• Fragmentation (Id, flags, fragment offset) is in an extension header
• TTL is now Hop Limit
• Protocol is now Next Header (and substitutes for header length)
• Checksum is gone (and now required in higher-layer protocol)
• Options are in extension headers.

IPv6 Extension Headers

• Hop-by-Hop/Jumbo Payload (up to 2^{32} bytes/packet)
• Routing
• Fragmentation (source only)
• Destination Options
• Security/Authentication

IPv6 Address Classes

• Supersets of other standard addresses (NSAP, IPX).
• Provider-based addresses.
• Geographic-based addresses.
• Link local use addresses.
• Site local use addresses.
• Multicast.