Week 6 Flashcards
What is a user agent for mail?
allows users to send/read messages.
What are the 3 major components of the Internet Mail System?
- User Agent
- Mail Server
- Simple Mail Transfer Protocol (SMTP)
What is a mail server?
A server that stores a users “mailbox” .
there can be multiple mailboxes in a mail server
What is the SMTP?
Allows to transfer mail from one mail server to another. Has both a client and server side –> for receiving mail from users and other mail servers, and for sending them to other mail servers
What does SMTP use to transfer mail? And what port number?
TCP, Port 25. It uses persistent connections.
Does SMTP use intermediate mail servers for sending mail?
No. It uses Direct Transfer.
What are the 3 phases of a SMTP transfer?
- handshake
- send/receive message
- close
Some restrictions of SMTP format?
- rrequires header and body to be in 7-bit ASCII
- uses persistent connections
How is HTTP and SMTP different?
HTTP has client pull, SMTP has client push
HTTP has each object encapsulated, SMTP has multiple objects
What are some stuff a client sends into SMTP?
- HELO [client’s hostname]
- MAIL FROM:
- RCPT TO:
- DATA
- QUIT
How does the client SMTP indicate the end of the mail?
A single period. “.”
What are the 2 ways to retrieve mail from a mail server?
- HTTP for web-based
- IMAP for mail clients
What is the DNS?
The Domain Name System
- a directory tha translates hostnames –> IP address
What are 2 ways to identify internet hosts?
- Host name
- IP address
What layer is the DNS implemented in?
App-Layer protocol.
hosts and servers communicate to resolve names.
What are 3 other jobs of DNS?
- host aliasing
- mail server aliasing
- load distrubution (many IP addresses correspond to 1 name)
Cons of a centralized DNS
- single point of failure
- too much traffic
- cannot be close to all clients
- unscalable
What are the 3 classes of DNS servers in a Distributed Databse?
Root DNS > Top-Level Domain > Authoritative DNS
How does the client find an IP address? (Order)
Root -> Top-Level Domain -> Authoritative
What is a Local DNS server?
- when a host makes a DNS query it is sent to the local DNS server.
- local DNS server returns a reply answering translations and requests (from its cache)
What is iterated query for DNS name resolution?
- contacted server replies with name of server to contact
- “idk this name but ask this server”
What is recursive query for DNS name resolution?
put the burden of name resolution on the contacted name server (pass the responsibility)
What are resource records of a DNS?
DNS provides RRs (resource records) to provide mappings in response to a query.
What is the RR format?
(Name, Value, Type, Ttl)
Name&Value: depends on type
TTL: time to live
What are the 4 “Types” in RR format?
- A
- NS
- CNAME
- MX
What is the value if Type=A
name = hostname
value = IP address
What is the value if Type=NS
name = domain (foo.com)
value = hostname of DNS server of the domain
What is the value if Type=CNAME
name = alias name
value = CANONICAL hostname
What is the value if Type=MX
value = CANONICAL name of SMTP mail server associated with name
Pull based vs Push based protocols?
Pull based: IMAP
Push based: SMTP
Either: HTTP
