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csc chp2 flashcards
topics = email, smtp, imap > p2p applications
| Question | Answer |
|---|---|
| what are the three major components of an email | user agents / mail servers / simple mail transfer protocol (SMTP) |
| what is the purpose of a user agent | (mail reader) used for composing, editing, reading mail messages (outlook/iphone mail client), outdoing, incoming messages stored on server |
| what are the three main characteristics of a mail server | mailbox (contains incoming messages for user) / message queue (of outgoing mail messages) and smtp protocol (between mail servers to send email messages) |
| what is the purpose of a client on a mail servers | sending mail server |
| what is the purpose of a server on a mail servers | receiving mail server |
| why does the RFC use TCP for emails | reliable transfer email messages from client to server |
| what is a direct transfer | sending server (acting like client) to receiving server |
| what are the three phases of a transfer | handshaking (greeting) > transfer of message > closure |
| what is a command interaction like HTTP | ASCII text |
| what is a response interaction like HTTP | status code and phrase |
| what format must a message be in | 7-bit ASCI |
| example for SMTP on slide 54 | try it |
| is HTTP push or pull | pull |
| is SMTP push or pull | push |
| what do both SMTP both have in common | ASCII command/response interaction and status code |
| what is the procedure for each object being sent in HTTP | each object is encapsulated in its own response message |
| how are objects sent/used in SMTP | multiple objects sent in multipart message |
| what type of connection does SMTP use | persistent |
| what format is the message required to be in using SMTP | 7-bit ASCII (header&body) |
| how does the SMTP determine the end of a message | CRLF |
| what is the format of a mail message | SMTP (protocol for exchanging emails) > RFC defines syntax for email message > header lines > body/message (ASCII characters) |
| what are mail access protocols used for | retrieval from server (IMAP > internet mail access protocol, messages are stored on server) |
| what does IMAP provide on servers | retrieval, deletion, folders of stored messages |
| what does HTTP provide on a server | web-based interface on top of SMTP (to send), IMAP or POP to retrieve emails`\ |
| what does SMTP provide on a server | deliver/storage of email messages to receiver's server |
| what does DNS stand for | domain name system |
| what's the purpose of a distributed database in a DNS | implemented in hierarchy of many name servers |
| what are application-layer protocols for in DNS | hosts, name servers communicate to resolve names (address/name translation) / core internet function is implemented as application-layer protocol |
| what are internet hosts/routers for in DNS | IP address (32-bit) used for addressing datagrams |
| what are some examples of a DNS service | hostname to IP address translation / host aliasing (alias names) / mail server aliasing / load distribution |
| what is load distribution used for in DNS | replication web servers, this helps regarding many IP addresses correspond to one name |
| when a client wants the IP address for a site (amazon.com) what happens | client queries root server to find ".com" DNS server > sites with ".com" are brought up in results from DNS server > when found IP address is shown |
| what are DNS root name servers used for | contact-of--last resort by name servers that can't resolve name |
| why are DNS root name servers important internet functions | DNSSEC provides security (authentication and message integrity) |
| what does ICANN stand for | internet corporation assigned names and numbers |
| what's the purpose of ICANN | manages root DNS domain |
| what does TLD stand for | top-level domain (type of server) |
| what is the TLD server responsible for | handles ".com/.org/.net..." and other top leve country domains / provides network solutions, called authoritative registry, for .com .net |
| what is an authoritative DNS server | organization's own DNS servers, providing authoritative hostname to IP mapping for organization's named host |
| who can maintain an authoritative DNS server | can be maintained by organization or service provider |
| what are three characteristics of a local DNS name server | does not strictly belong to hierarchy / each ISP has one / when host makes DNS query it'll be sent to its local DNS |
| what's another name for local DNS name server | default name server |
| how do queries work in a local DNS name server | has local cache of recent name-to-address translation pairs / acts as proxy and forwards query into hierarchy |
| how does an iterated query | contacted server replies with name of server to contact (I don't know this name, but ask this server) |
| how does a recursive query work | puts burden of name resolution on contacted name server / heavy load at upper kevels of hierarchy |
| how does cache update DNS records | once any name server learns mapping, it caches mapping (cache entries timeout after some time) |
| where are TLD servers cached | in local name servers (therefore root name servers not often vistsed) |
| what happens when cached entries are out-of-date | if name host changes IP address, may not be known internet-wide until all TTLs expire |
| how are mechanisms notified for proposed IETF standard | RFC 2136 |
| what are DNS records used for | distributed database storing resource records |
| what is the format for RR | name, value, type, ttl |
| what are the different types of DNS records | type A, type CNAME, type NS, type MX |
| how does record type A work | name is hostname and value is IP address |
| how does record type CNAME work | name is alias name for some "canonical"/real name / www.ibm.com = servereast.backup2.ibm.com / value is canonical name |
| how does record type NS work | name is domain (foo.com) / value is hostname of authoritative name server for this domain |
| how does record type MX work | value is name of mailserver associated with name |
| what do DNS query and reply messages have in common | same format |
| what is the format for a message header | identification = (16-bit for both query and reply) > flags = (query or reply/recursion desired/recursion available/reply is authoritative) |
| in DNS protocol messages what is the question space for | name, type of fields for a query |
| in DNS protocol messages what is the answer space for | RRs in response to query |
| in DNS protocol messages what is the authority space for | records for authoritative servers |
| in DNS protocol messages what is the additional info space for | additional info |
| how do you insert records into DNS | register name (networkutopia.com) at DNS registrar, providing names, IP addresses of authoritative name server > create authoritative server locally with IP address |
| if you create an authoritative server locally using type A what would "Network Utopia" look like | www.networkutopia.com |
| if you create an authoritative server locally using type MX what would "Network Utopia" look like | networkutopia.com |
| what is a DDoS attack | bombard root servers with traffic (local DNS servers cache IPs of TLD servers, allowing root server bypass) / bombard TLD servers are potentially more dangerous |
| what is a redirect attacks | man-in-middle (intercept DNS queries) and DNS poisoning (send bogus relies to DNS server, which caches) |
| what does "exploit DNS for DDoS" mean | send queries with spoofed source address (targeting IP) / requires amplification |
| what are the characteristics of a peer-to-peer architecture | no always-on server / arbitrary end systems directly communicate / peers request service from other peers / peers are intermittently connected and change IP addresses (complex management) |
| what is self scalability | new peers bring new service capacity and new service demands |
| what's an example of peer-to-peer architecture | P2P file sharing (BitTorrent), streaming (KanKan, VoIP (Skype)) |
| how does a server transmission work | it must sequentially send (upload) N file copies |
| what is the formula "for time to send one copy" | F/us |
| what is the formula for "time to end N copies" | NF/us |
| how does a "client" work regarding file distribution for client-server | each client must download file copy |
| what does "dmin" mean | min client download rate |
| what does "F/dmin" mean | min client download time |
| what is the formula for "time to distribute F to N client using client-server approach" | Dc-s>max{NF/us, F/dmin} increases linearly in N |
| how does server transmission work for file distribution time for P2P | must upload at least one copy (time to send one copy F/us) |
| how does "client" work for file distribution time for P2P | each client must download file copy (min client download time F/dmin) |
| how do "clients" work for file distribution time for P2P | as aggregate must download NF bits (max upload rate - limiting max download rate) |
| time to distribute F to N clients using P2P approach | Dp2p>max{F/us, F/dmin, NF/(us+Eui)} |
| example on cllient-server vs P2P | slide 78 |
| how does P2P file distribution bittorrent work | file divided into 256kb chunks / peers in torrent send and receive file chunks |
| what's a tracker | tracks peers participating in torrent |
| what's a torrent | group of peers exchanging chunks of a file |
| what is a peer joining torrent | has no chunks but will accumulate them over time from other peers / registers with tracker to get list of peers connects to subset of peers (neighbours) |
| what happens to P2P file distribution while downloading | peer uploads chunks to other peers |
| can peer change peers with who they exchange with | yes |
| what does a churn mean | peers may come and go |
| what happens when a peer has an entire file | it may leave or altruistically remain in torrent |
| what is "requesting chunks" and how does it work | at any given time, different peers have different subsets of file chunks > periodically, Alice asks each peer for list of chunks that they have > Alice requests missing chunks from peers, rarest first |
| what is "sending chunks: tit-for-tat" and how does it work | Alice sends chunks to those four peers currently sending her chunks at highest rate (other peers are choked by Alice because they're receiving chunks from her / re-evaluate top 4 every 10secs) |
| what happens every 30 seconds when "sending chunks: tit-for-tat" | they randomly select another peer, starts sending chunks (optismitically unchoke this peer / newly chosen peer may join top 4) |
| how does a "bittorrent: tit-for-tat" work | alice optismitically unchokes bob > alice becomes one of bobs top-four providers (bob reciprocates) > then bob becomes one of alice's top-four providers |
| END of P2P applications | slide 83 |
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NtokozoN
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