1. Data Communications and Networking DNS
CMPT 371
Data Communications and Networking
DNS

2. jpl.nasa.gov. . edu com gov us ca uk arpa fr In-addr sun ny hp nasa ca
sfu bc
jpl
fraser
fraser cs
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3. Zone An administrative division of the domain name tree
Each zone is the responsibility of one administrative authority
A zone may include hosts and sub-domains
Sub domains in a zone may or may not have authority delegated to other administrative authorities. Any subset of sub-domains may be delegated
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4. DNS Name Tree: zones .ca bc ab sk on qc .ca domain .ca zone qc.ca zone
sk.ca zone
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5. Authority for the DNS namespace
A particular DNS name server will service a zone. Its database of zone information will contain
entries for any hosts or subdomains in the zone
delegation information for subdomains or zones that have been delegated to other authorities (form their own zones)
Includes the address of (pointer to) the DNS servers for the delegated domains or zones
excludes information about further delegation of authority in delegated zones or hosts in delegated domains
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6. Operation of a DNS server
A DNS name server is initialized knowing the contents of its zone information database.
The zone information database includes
Address (A) and possible alias (CNAME) records for each host in the serviced zones
Name Server (NS) records for each DNS server in the zone
Mail server (MX) records specify hosts that will process and forward mail for the zone and their priority
Each record includes a time to live (TTL)
Lifetime (TTL) of each record is set by administrator
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7. Operation of a DNS server
When a request is made to a DNS server the answer it supplies consists of some of the records in the zone information database
The TTL of each supplied record tells the requestor how long the information in that record will remain valid
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8. Caching and TTL Each DNS server uses a cache to improve its efficiency
At initialization time the cache is empty
Each time a DNS query is made by the DNS server, the records in the resulting response are cached
Over time the size of the cache increases as more information from varied queries is added.
To keep the size of the cache manageable and the contents of the cache up to date (information is dynamic and changes over time) each entry in the cache must eventually (after the TTL has expired) be removed from the cache
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9. Authoritative responses
Each time a DNS query is received by the server
The records in the cache are searched
The zone database is searched
The longest match is found (either from the database or the cache) and returned to the source of the query
If the answer is returned from the zone database the answer is known as an authoritative response
If the answer is returned from the cache it is NOT authoritative
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10. DNS There are two approaches to answering a query
Iterative: the name server receiving the query responds with either the IP address of the host or the name of the next server it would consult (next higher server in the tree)
Recursive: the name server will, if necessary, directly query the next name server, and will return the final answer
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11. A host submitting a query (1)
An application or user on host Drab, in domain cs.sfu.ca requests IP address for ftp.isc.org
The application or user or the user expects to receive the IP address of ftp.isc.org without making additional queries.
The application will make a request by calling a function ( gethostbyname() ) OR
the user will make a request using a resolver (resolving software such as dig or nslookup)
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12. A host submitting a query (2)
An application or user on host Drab, in domain cs.sfu.ca sends a request to the local DNS server for the IP address for ftp.isc.org
This request may require the local DNS server (may or may not be on host drab) to
Make an additional request or requests.
Analyze the reply or replies to the request/s
Supply the resulting IP address and potentially other related information to the requesting process or user.
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13. Query from local DNS server
Assume that we begin with a cache containing only the addresses of the root servers.
The local DNS server must then determine the desired IP address. It will make a series of iterative requests for information on the address of ftp.isc.org.
The local DNS server will send a request to one of the root servers. The longest match the root server can make will be to the TLD .org (as .org has been delegated)
The root server will send back a response with the IP address and name of an authoritative server for the .org domain (plus other information)
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14. Query from the local DNS server: 2
The local DNS server will process the returned data, add the record for the DNS server for the .org domain to the cache, and formulate a request to the DNS server for the .org domain
The local DNS server will send a request to one of the DNS servers for the domain .org
The DNS server for the domain .org will send back a response with the IP address and name (plus other information) of an authoritative server for the isc.org domain. The isc.org domain has been delegated by the .org DNS server to the ISC, so no longer domain name match can be made.
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15. Query from the local DNS server: 3
The local DNS server will process the returned data, add the DNS server for the isc.org domain to the cache, and formulate a request to the DNS server for the isc.org domain
The local DNS server will send a request to one of the DNS servers for the domain isc.org
The DNS server for the domain isc.org will send back a response with the IP address and name (plus other information) of ftp.isc.org.
The local DNS server will process the returned data,
add an entry for the ftp.isc.org to the cache
formulate and send a reply to the original request from host Drab
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16. DNS Query Local DNS server Root DNS server DNS server for .org
Iterative query
Application or user
Local DNS server
Root DNS server
DNS server for .org
DNS server for isc.org
Referred to .org
Referred to isc.org
IP Address of ftp.isc.org
Recursive reply
Recursive query
all queries/replies are for the address of ftp.isc.org
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17. Using the Cache: subsequent queries (1)
A later query to ftp.isc.org will find the IP address available in the local DNS servers cache. The DNS server will send back the results without making further queries
A later query to ftp2.isc.org will find the entry for isc.org DNS server in the cache of the local DNS server. A single query to the isc.org DNS server will provide the needed information
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18. Using the Cache: subsequent queries (2)
A later query to fbt.nas.org will find the entry for org DNS server in the cache of the local DNS server. A query to the org DNS server will provide the address of the nas.org server and a request tho the nas.org server will supply the needed information
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19. Recursive Requests
In the example above the process or user on the host made a recursive request, and the DNS server made only iterative requests.
DNS servers can also make recursive requests. However, busy DNS servers are often configured to accept only iterative requests. (this way they do not need to process the returning results as well, this reduces load on the busy server). Therefore, the iterative approach is more commonly used by DNS servers
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20. Domain Server Message Messages exchanged between clients and servers
Comer 2000: fig 24.5
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21. Example using dig: 1
jregan15: dig ftp.isc.org ; <<>> DiG <<>> ftp.isc.org ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 5 ;; QUESTION SECTION: ;ftp.isc.org. IN A ;; ANSWER SECTION: ftp.isc.org IN A ;; AUTHORITY SECTION: isc.org IN NS ns-ext.lga1.isc.org. isc.org IN NS ns-ext.nrt1.isc.org. isc.org IN NS ns-ext.sth1.isc.org. isc.org IN NS ns-ext.isc.org.
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22. Example using dig: 2
;; ADDITIONAL SECTION: ns-ext.lga1.isc.org IN A ns-ext.nrt1.isc.org IN A ns-ext.sth1.isc.org IN A ns-ext.isc.org IN A ns-ext.isc.org IN AAAA 2001:4f8:0:2::13 ;; Query time: 1 msec ;; SERVER: #53( ) ;; WHEN: Fri Nov 5 06:21: ;; MSG SIZE rcvd: 236
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23. Data Communications and Networking P2P
CMPT 371
Data Communications and Networking
P2P

24. Comparison
All the application layer protocols considered so far use client server architecture
Now let’s consider peer to peer architecture
To understand why we would want to take this alternate approach lets consider transmitting a large file from one host to several others using both approaches.
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25. How long? One server to N clientsU1 US D1 U2
Internet
DR1 DN D2
DR2 UN D3 U5 D4 U3 … D5 U4
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26. How long to send to all clients
The server must send one copy of the file to each of the clients. The file has F bits and the rate of transmission from the server to the internet is Us.
Each copy will take F/ Us seconds to transmit into the internet.
Transmitting all N copies will take N*F/ Us seconds
But the clients also need to receive their copies
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27. How long to receive all copies
The longest time for any particular host to download the file from the internet is for the host with the slowest download rate Dmin.
This host will take F/Dmin seconds to download
If we send to each host at the rate that host can receive (alternating blocks of bits between hosts) then the maximum download time for all files is F/Dmin seconds
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28. How long 1 server - nclients
Consider that the server distributes its delivery of bits in proportion the download rate of each host.
If the host that has the longest download time receives bits at a minimum of the download rate then the time to distribute the file to all clients is F/Dmin seconds
If bits arrive more slowly that the download rate of the slowest downloading host then the time to distribute the file to all hosts will be N*F/ Us the time taken to transmit the N copies of the file
Download Time = max {F/Dmin , N*F/ Us }
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29. How long? P2P … U1 US U2 D1 DR1 DN Internet D2 DR2 UN D3 U5 D4 U3 D5
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30. How long to send using P2P
The server must send at least one copy of the file to the peers. This takes F/Us seconds
The slowest peer must have time to download all the bits in the file. This takes F/Dmin seconds
The fastest that any host can receive uploads is
so the fastest all hosts can receive the uploaded files is
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31. From Kurose and Ross
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32. Bit torrent: TRADE DATA CHUNKS
tracker: tracks peers
participating in torrent
NEWEST HOST IN TORRENT
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33. What is a bit torrent
A TORRENT is a group of peers exchanging portions (chunks) of a file
A Torrent is controlled by a TRAKKER, a host managing the peers trading chunks of file
The file being exchanged is divided into chunks (512K)
Peers taking part in the trading of chunks may join the group at and time and leave the group after obtaining all or any part of the file
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34. How does a torrent work (1)
A peer “Mew” joins a torrent to obtain a file
“Mew” starts with none of the chunks of the file
She registers with the tracker
She gets list of potential peers from the tracker. The trakker selects a subset of all available peers.
She tries to establish TCP connections with all of the peers from the list the tracker supplied
She establishes connection with a subset of the peers from the list the tracker provided. Call these peers the neighboring peers
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35. How does a torrent work (2)
As time passes
The peer “Mew” will periodically check with t he tracker to confirm she is still part of the torrent
The peer “Mew” will acquire chunks of the file by downloading them from other neighboring peers
The peer “Mew” will upload chunks of the file it already has to neighboring peers that need those chunks
Chunks are not necessarily acquired or supplied in order
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36. Acquiring Chunks
At a particular time different peers will each have a different subset of the chunks of the file
At any time a peer can obtain a list of which available chunks from each neighboring peer
The peer “Mew” can thus find the chunks she needs
The peer “Mew” can obtain the chunks she needs by sending requests to the neighboring peers that have those chunks
Peers will usually request the chunks that are hardest to find first (to equalize the availability of those chunks)
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37. Trading Algorithm (tit for tat)
Every 10 seconds “Mew” measures which four neighboring peers are supplying her data at the highest rate
“Mew” unchokes those four neighboring peers by sending them chunks they have requested.
Every 30 seconds she also optimistically unchokes one randomly chosen neighboring peer by sending that peer chunks
The optimistically unchoked neighboring peer may then become one of “Mew”s trading partners
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