Computer Networking: Application Layer

• Principles of network applications
  • Internet transport protocols services
• Web and HTTP
  • HTTP: two types
  • Web cache aka proxy server
  • HTTP generation
    • HTTP/1
    • HTTP/2
    • HTTP/3
• DNS
  • DNS name resolution
    • Iterated query
    • Recursive query
  • DNS records
• P2P applications
  • File distribution
    • Real-world example of P2P file distribution: BitTorrent
• Video streaming and CDN
  • Streaming stored video
  • CDN

Principles of network applications

An application-layer protocol defines:

• types of messages exchanged
• message syntax
• message semantics
• rules for when and how to send & response to messages

Internet transport protocols services

TCP:

• reliable transport
• flow control
• congestion control
• connection-oriented

UDP:

• unreliable data transfer
• state-less

Web and HTTP

HTTP: two types

Non-persistent HTTP:

1. TCP connection opened
2. at most one object sent over TCP connection
3. TCP connection closed

Persistent HTTP:

1. TCP connection opened to a server
2. multiple objects can be sent over single TCP
3. TCP connection closed

Maintaining user-server state: cookies

Web cache aka proxy server

performance:

• much lower access link utilization
• much shorter end-end delay

HTTP generation

HTTP/1

HTTP/2

decreased delay in multi-object HTTP requests

Reduce HOL blocking by dividing objects into smaller chunks.

HTTP/3

Adds security, and per object error/congestion control over UDP.

DNS

• distributed database implemented in hierarchy of many name servers
• application-layer protocol: hosts, DNS servers communicate to resolve names (address/name translation)

Services:

• hostname to IP address translation
• host/mail-server aliasing
• load distribution

Architecture: distributed, hierarchical database

DNS name resolution

Iterated query

“I don’t know this name, but ask this server.”

Recursive query

“I don’t know this name, but I will ask it for you.”

DNS records

resource records (RR)

(name, value, type, ttl)

type = A

• name is hostname
• value is IP address

type = NS

• name is domain
• value is hostname of authoritative name server for this domain

type = CNAME

• name is alias name for some “canonical”(the real) name
• www.ibm.com is really servereast.backup2.ibm.com
• value is canonical name

type = MX

• value is name of SMTP mail server associated with name

P2P applications

File distribution

client-server vs P2P

client-server

• server side
  • time to send 1 copy: $\frac{F}{u_s}$
  • time to send N copy: $N\cdot \frac{F}{u_s}$
• client side
  • $d_{min}$: min client download rate
  • min client download time: $\frac{F}{d_{min}}$
• time to distribute F to N clients: $max(\frac{NF}{u_s}, \frac{F}{d_{min}})$ increases linearly in N

P2P

• server side
  • time to send 1 copy: $\frac{F}{u_s}$
• client
  • min client download time: $\frac{F}{d_{min}}$
• clients: aggregate must download NF bits
  • max upload rate: $u_s + \sum{u_i}$
• time to distribute F to N clients: $max(\frac{F}{u_s}, \frac{F}{d_{min}}, \frac{NF}{u_s + \sum{u_i}})$ which does not increases linearly in N

Real-world example of P2P file distribution: BitTorrent

• requesting chunks
• sending chunks

Video streaming and CDN

Streaming stored video

challenges:

• bandwidth vary over time
• packet loss and delay

CDN

• stores copies of content at CDN nodes
• user request content, service provider returns manifest
  • client then retrieves content at highest supportable rate
  • may choose different rate or copy if network environment changed