Email, DNS, and CDN protocols

Email Service

There are three major components:

• User agent
• Mail server
• Mail protocols
  • Simple mail transfer protocol (SMTP)
  • Internet mail access protocol (IMAP)

User agent

• Compose and read mail messages
  • e.g. Outlook
• Outgoing and incoming messages are stored on server

Mail server

• Mailbox (yellow) contains incoming messages for user
• Message queue (green) of outgoiing mail messages
• SMTP protocol is used between mail servers to send messages
  • client: sending mail server
  • server: receiving mail server

SMTP

• Uses TCP to reliably transfer message from client to server
• Three phases of transfer
  1. SMTP handshaking
  2. SMTP transfer of messages
  3. SMTP closure

IMAP

• Mail access protocol: retrieval from server
• IMAP provides retrieval, deletion, folrders of stored messages on server

SMTP versus HTTP

Property	SMTP	HTTP
Main purpose	Transfer e-mail	Transfer web objects
Interaction style	ASCII Command-response	ASCII Request-response
Reliable transport	TCP	TCP
Traditional server port	25	80
Typical direction	Sender pushes message	Client requests/pulls object

HTTP adoption: gmail, outlook, etc. can provide web-based interface on top of SMTP to send and IMAP to retrieve messages

Domain Name System (DNS)

Domain Name System provides services:

• Resolves hostname-to-IP-address translation.
• Host aliasing.
• Replicated Web servers: many IP addresses correspond to one name

It is enabled by:

• Distributed database implemented in hierarchy of many name servers
  • DNS combines hierarchy, distribution, delegation, replication, and caching.
• Applicaiton layer protocol

DNS Hierarchy

1. Root name servers

• 13 logical root name servers worldwide and each server is replicated many times (~200 servers in US)
• Official contract-of-last-resort by name servers that can not resolve name
• ICANN manages root DNS domain

2. Top-level domain servers

• Responsible for .com, .org, .edu, or
• all top-level country domains, e.g.: .cn, .uk, .jp.

3. Authoritative servers

• Organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts
• Can be maintained by organization or service provider (ISP)

4. Local DNS name servers

• When a host needs to resolve a name, it normally sends the request to a local DNS server
• May be operated by an ISP, university, company, etc.
• The local server can:
  • answer from its cache
  • contact other DNS servers to resolve the name
  • cache the result

DNS Records

DNS stores resource records.
Each record can be represented as:

(name, TTL, type, data)

A type record

An A record maps a hostname to an IPv4 address:

example.com.  3600s  A  192.168.1.1

Conceptually:

hostname → IPv4 address

A records gives the final IPv4 address.

NS type record

An NS record maps a domain to an authoritative name server:

example.com.  86400  NS  ns1.example.com.

• example.com identifies a domain or DNS zone
• ns1.example.com is the hostname of a computer running authoritative DNS-server software
• That server stores records belonging to the example.com zone.

CNAME type record

A CNAME record defines an alias for a canonical name.

www.example.com.  3600  CNAME  example.com.

This allows several public names to refer to a canonical hostname. For example:

www.example.com  ─┐
blog.example.com ─┼→ canonical service name
shop.example.com ─┘

Concrete example

When browser needs www.example.com:

1. Which server knows about example.com?
                 ↓ NS
   ns1.example.com

2. Where is ns1.example.com?
                 ↓ A/glue
   192.0.2.53

3. Ask 192.0.2.53:
   What is the address of www.example.com?
                 ↓ CNAME
   www.example.com is app.example.com

4. What is the address of app.example.com?
                 ↓ A
   203.0.113.25

5. Browser connects to:
   203.0.113.25

MX type record

An MX record identifies a mail server responsible for a domain.

example.com.  3600  MX  10 mail.example.com.

Suppose Alice sends mail to bob@example.com:

1. Extract recipient domain: example.com

2. Query DNS:
   What is the MX record for example.com?

3. DNS replies:
   example.com MX 10 mail.example.com

4. Resolve mail.example.com:
   mail.example.com A 192.0.2.25

5. Open a TCP connection:
   192.0.2.25, port 25

6. Transfer the message using SMTP.

7. Bob later accesses the stored message using IMAP.

Summary

Record	Name represents	Data represents	Purpose
A	Hostname	IPv4 address	Locate a host
NS	Domain	Authoritative server name	Delegate DNS responsibility
CNAME	Alias	Canonical hostname	Create an alternate name
MX	Mail domain	Mail-server hostname	Route e-mail

DNS Name Resolution

Iterated query

• contacted server replies with name of server to contact

Recursive query

• puts burden of name resolution on contacted name server
• can cause heavy load at upper levels

DNS Caching Information

Once a DNS server learns a mapping, it can store that result temporarily.
Then, later queries can be served immediately.

Caches may exist in:

• Browsers
• Operating systems
• Local DNS servers
• Other DNS servers

Caching works especially well because DNS receives many more reads than writes.

Video Streaming and Content Delivery Network (CDN)

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