Content

  1. HTTP
    1. Request/Response
    2. Comparison
    3. Web Caches (Proxy Server)
  2. Email Services
    1. Mail Clients & Servers
    2. Protocols
  3. DNS
    1. Hierarchy of DNS Servers
    2. Caching

HTTP

  • Client-server architecture
  • TCP as carrier
  • Request-response interaction
  • Stateless; no information stored between requests
  • Pull protocol

Request/Response

  1. Browser initiates TCP connection to server with port 80
  2. Exchange HTTP messages between browser and server
  3. Close connection (HTTP/1.0)
# Request
|Request line \r\n| -> |Request type| |URL| |HTTP version| \r\n
|Header lines \r\n|
|...          \r\n|
|\r\n             |
|Body             |
# Respond
|Status line  \r\n| -> |HTTP version| |Status code| |Status phrase| \r\n
|Header lines \r\n|
|...          \r\n|
|\r\n             |
|Body             |

Comparison

http/1.0 http/1.1 http/2.0
Text-based Text-based Binary-based
Connection Non-persistent Persistent; pipelining or non-pipelining
Time for fetching 3 objects Non-parallel: 6 RTT + X; parallel: 4 RTT + X Non-pipelining: 4 RTT + X; pipelining: 3 RTT + X
Response message 1 response delivered at a time per connection 1 response delivered at a time per connection Multiplexing of multiple responses in a single connection (1 connection for parallelism)
Other features Compressed header; proactively push resources to client

Web Caches (Proxy Server)

                              (If-Modified-Since:xxx)
Client ---request--> Proxy -------conditional GET------> Server
       <--response--(cache)<----------response----------
                            (HTTP/1.1 304 Not Modified/
                             HTTP/1.1 200 OK)
                                (Last-Modified:xxx)

Email Services

  • Client-server architecture
  • TCP as carrier
  • Command-response interaction
  • Text-based communication

Mail Clients & Servers

  • User agent
    • SMTP to submit outgoing messages to local mail server
    • IMAP or POP to retrieve incoming messages stored on mail server
  • Mail server
    • MTA: mail transfer agent
    • SMTP transfer messages between MTAs
    • Mailbox + queue for outgoing messages
user agent --> sender's outgoing MTA --> receiver's domain MTA --> user agent

Protocols

  • SMTP (Simple Mail Transfer Protocol): PUSH

    • TCP based; port 25

      MTA client ---command--> MTA server  
           <--response--

    • Commands: ASCII text; response: status code & phrase

    • All message content in 7-bit ASCII

  • Mail access protocol: PULL

    • POP (Post Office Protocol)
      • Authorization
      • Download
    • IMAP (Internet Mail Access Protocol)
      • More features
      • Manipulation of stored messages
    • HTTP
      • Webmails

SMTP

1. Start connection
    MTA client --TCP connection--> MTA server
2. Server identifies itself
    MTA client <-220 <domain> service ready/421 service not available-- MTA server
3. Client identifies itself
    MTA client --HELO <server>--> MTA server
4. Server accepts identification
    MTA client <-250 OK-- MTA server
5. Client send 1 or more messages to server (one by one)
    // start of envelope
    MAIL FROM: <mail>
    RCPT TO: <recipients>
    DATA
    [headers] // To: xxx\r\nFrom: xxx ...

    [message content]
    . // end of message
    // end of envelope
6. Client quit
    MTA client --QUIT--> MTA server
7. Server initiates "TCP Close"
    MTA client <-221 closing connection-- MTA server

DNS

  • Client-server interaction
  • UDP as carrier; port 53
  • Information stored in distributed DB; implemented by hierarchy of name servers
  • Services
    • IP to name
    • Name to IP
    • Host aliasing: translate alias name to canonical hostname
    • Mail server aliasing: return canonical names of domain mail servers
    • Load distribution: a set of IPs for 1 host name

Hierarchy of DNS Servers

# Iterative
Client --> Local DNS Server --> Root DNS Server
                            <--
                            --> TLD DNS Server
                            <--
                            --> Authoritative Server
                            <--
# Recursive
Client --> Local DNS Server --> Root DNS Server
                                --> TLD DNS Server
                                    --> Authoritative Server
                                    <--
                                -->
                            <--
  • Root DNS server
    • Known by all local DNS server
    • Contacted by local servers if names not resolved locally
  • TLD (top-level domain) DNS server
    • com, org, net, edu, etc.
    • uk, fr, ca, hk, etc.
  • Authoritative server
    • Providing authoritative hostname to IP mappings for organization’s servers e.g. web, mail
    • Maintained by organization itself or another service provider
  • Local DNS server
    • Or default name server
    • Acts as proxy to answer queries or forward queries

Caching

Preventing root DNS server to become the bottleneck.

  • TTL (time-to-live)
    • Duration (in sec) allowed to cache the data for name servers
    • Tradeoff between performance & consistency

  • Resource records

      RR format: (name, value, type, class, ttl)

  # Types
  Type=A: (hostname, IP)
  Type=NS: (domain, IP of authoritative DNS server)
  Type=PTR: (arpa address, hostname)
  Type=CNAME: (alias name, canonical name)
  Type=MX: (name, name of mailserver)

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