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Computer Networks

Application layer

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Application Layer

• So far– Socket programming, Network API

• Today– Application layer functions

– Specific applications• DNS

• FTP

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Application layer functions

• Applications– Implement desired functionality within application protocols when no

underlying network service provides support

– Functionality that is common rolled into libraries and “middleware”

• Functions– Security (S/MIME, PGP)

– Delivery semantics (multicast overlays, anycast)

– Reliable data transfer (reliable multicast, reliable UDP)

– Quality of service (QoS overlays, scheduling)

– Congestion control (Non-TCP applications)

– Flow control (Non-TCP applications)

– Naming (DNS)

– Routing (overlays)

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AL: Specific application protocols

• DNS– Background

– Nutshell description

– DNS server design

– DNS client lookups

– DNS protocol messages

• FTP • HTTP• SMTP

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AL: Domain Name System (DNS)

• Internet hosts, routers like to use fixed length addresses (numbers)– IP address (32 bit) - used for addressing datagrams

• Humans like to use names– www.cse.ogi.edu

– keywords

• DNS, keywords, naming protocols– Map from IP addresses to names

– Map from names to IP addresses

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AL: Original Name to Address Mapping

• Flat namespace– /etc/hosts

– SRI kept main copy

– Downloaded regularly

• Problems– Count of hosts was increasing: machine per domain

machine per user• Many more downloads

• Many more updates

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AL: Goals for a new naming system

• Implement a wide area distributed database– Scalability

– Decentralized maintenance

– Robustness, fault-tolerance

– Global scope • Names mean the same thing everywhere

– Don’t need• Atomicity

• Strong consistency

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AL: Goals for a new naming system

Why not centralize DNS?• Single server with all name-to-IP address mappings

– single point of failure

– traffic volume

– distant centralized database (performance)

– maintenance

– doesn’t scale!

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AL: DNS (Domain Name System)

• http://www.rfc-editor.org/rfc/rfc1034.txt

• http://www.rfc-editor.org/rfc/rfc1035.txt

• distributed database implemented in hierarchy of many name servers

• decentralized control and management of data

• application-layer protocol used by hosts, routers, and name servers– communicate to resolve names (address/name translation)

– core Internet function implemented as application-layer protocol• complexity at network’s “edge”

• compare to phone network naming

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AL: DNS nutshell solution

• Hierarchical canonical name space – www.cse.ogi.edu

root

edunetorg ukcom ca

gwu ucb ogi bu mit

cse ece

www

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AL: DNS nutshell solution

• Distributed database stores names hierarchically• Authoritative name servers store parts of the database

– Names assigned to authoritative name servers• For a host, authority stores that host’s IP address, name• Responds to queries for host’s IP address• Perform name/address translation for that host’s name

– Hierarchy organizes authoritative name servers

• DNS hierarchy– Each host has a pointer to a local name server for which to query for

unknown names– Each local name server knows root of hierarchy– Root points to sub-levels, sub-levels point to deeper sub-levels, … ,

deeper sub-levels point to leaf name server representing authority for unknown name

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AL: DNS nutshell figure

Root name servers:• may not know

authoratiative name server

• may know intermediate name server: who to contact to find authoritative name server

• multiple root name servers for fault-tolerance

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

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4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

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8

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AL: DNS server database

• DB contains tuples called resource records (RRs)– RR contains type, class and application data

• Before types added, only one record type (A)

– Classes = Internet (IN), Chaosnet (CH), etc.

– Each class defines types, e.g. for IN:• A = address, NS = name server, CNAME = canonical name (for

aliasing), HINFO = CPU/OS info, MX = mail exchange, PTR = pointer for reverse mapping of address to name

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AL: DNS record types

Resource records (RR) and their types

• Type=NS– name is domain (e.g. foo.com)– value is IP address of

authoritative name server for this domain

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

• Type=A– name is hostname– value is IP address

• Type=CNAME– name is an alias name for

some “cannonical” (the real) name

– value is cannonical name

• Type=MX– value is hostname of mailserver

associated with name

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AL: DNS MX record type

• MX records point to mail exchanger for a name– E.g. mail.acm.org is MX for acm.org

• Addition of MX record type proved to be a challenge– How to get mail programs to lookup MX record for mail

delivery rather than A record?

– Needed critical mass of such mailers

• nslookup example

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AL: DNS server database distribution

• Administrative hierarchy– “.” as separator– Zone = contiguous section of name space

• Zones are created by convincing owner node to create/delegate a subzone– Each zone contains multiple redundant servers– Primary (master) name server updated manually– Secondary (redundant) servers updated by zone transfer of name space– Provides fault-tolerance within zone

• Host name to address section– Top-level domains edu, gov, ca, us, etc.– Sub-domains = subtrees– Human readable name = leaf root path

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AL: DNS client lookups

• Each host has a resolver– Typically a library that applications can link

gethostbyname()– Local name servers hand-configured (e.g. /etc/resolv.conf)

or automatically configured (DHCP)– Host queries local name server for unknown names

• Name servers– Configured with well-known root servers

• Currently {a-m}.root-servers.net

– Local servers• Typically do a recursive lookup of distant host names for local hosts• Typically answer queries about local zone

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AL: Lookup Methods

• Recursive queries– Server goes out and searches for more info on behalf of the

client (recursive)– Only returns final answer or “not found”– Puts burden of name resolution on contacted name server– Heavy load?

• Root server implosion

• Iterative– Server responds with as much as it knows (i.e. name of

server to contact next)– “I don’t know this name, but ask this server”– Client iteratively queries additional servers

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AL: Typical Resolution

• Client does recursive request to local name server

• Local name server does iterative requests to find name

• Local name server has knowledge of root of name space

• Steps for resolving www.ogi.edu– Application calls gethostbyname()

– Resolver contacts local name server (S1)

– S1 queries root server (S2) for (www.ogi.edu)

– S2 returns NS record for ogi.edu (S3)

– S1 queries S3 for www.ogi.edu

– S3 returns A record for www.ogi.edu

• Can return multiple addresses what does this mean?

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AL: DNS Caching

• DNS responses are cached – Quick response for repeated translations– Other queries may reuse some parts of lookup

• NS records for domains

• DNS negative queries are also cached– Don’t have to repeat past mistakes– E.g. misspellings

• Cached data periodically times out– Soft state– Lifetime (TTL) of data controlled by owner of data– TTL passed with every record– TTL affects DNS-based load balancing techniques

• update/notify mechanisms under design by IETF– RFC 2136– http://www.ietf.org/html.charters/dnsind-charter.html

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AL: A word about iterated queries and caching

• Why not do iterative queries from host?– Currently

• Host issues recursive query to local server

• Local server issues iterative queries subsequently

– Win2k client• Does iterative queries from host

– Caching implications?

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AL: DNS Lookup Example

ClientLocal

DNS server

root & edu DNS server

ogi.edu DNS server

www.cse.ogi.edu

NS ogi.eduwww.cse.ogi.edu

NS cs.ogi.edu

www=IPaddr

cse.ogi.eduDNS

server

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AL: Subsequent Lookup Example

ClientLocal

DNS server

root & edu DNS server

ogi.edu DNS server

cse.ogi.eduDNS

server

ftp.cse.ogi.edu

ftp=IPaddr

ftp.cse.ogi.edu

cse.ogi.edu entry cached

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AL: All recursive DNS example

host surf.eurecom.fr wants IP address of gaia.cs.umass.edu

1. Contacts its local DNS server, dns.eurecom.fr

2. dns.eurecom.fr contacts root name server, if necessary

3. root name server contacts authoritative name server, dns.umass.edu, if necessary

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

authorititive name serverdns.umass.edu

local name serverdns.eurecom.fr

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4

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AL: DNS root name servers

• contacted by local name server that can not resolve name

• root name server:– contacts authoritative

name server if name mapping not known

– gets mapping– returns mapping to local

name server• ~ dozen root name servers

worldwide• nslookup, named example

– set iterative– lookup bogus name

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AL: DNS/nslookup example

• Real example using nslookup and named– Shows iterative– Shows root name servers– Shows recursive– Shows negative caching

• nslookup– Set iterative– Lookup bogus name– Get list of root servers– Set recursive– Lookup same bogus name– Get negative response– Set iterative– Lookup bogus name– Get negative response

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AL: DNS protocol, messages

DNS protocol : query and repy messages, both with same message format

msg header• identification: 16 bit # for

query, repy to query uses same #

• flags:

– query or reply

– recursion desired

– recursion available

– reply is authoritative

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AL: DNS protocol, messages

Name, type fields for a query

RRs in reponseto query

records forauthoritative servers

additional “helpful”info that may be used

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AL: DNS issues

• Poor static configuration (root server list)• Lack of exponential backoff• No centralized caching per site

– Each machine runs on caching local server

• UDP used for queries– Need reliability Why not TCP?

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AL: ftp: the file transfer protocol

• transfer file to/from remote host

• client/server model

– client: side that initiates transfer (either to/from remote)

– server: remote host

• ftp: RFC 959

• ftp server: port 21

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

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AL: ftp: separate control, data connections

• ftp client contacts ftp server at port 21, specifying TCP as transport protocol

• two parallel TCP connections opened:

– control: exchange commands, responses between client, server.

“out of band control”

– data: file data to/from server

• ftp server maintains “state”: current directory, earlier authentication

• Allows one ftp client to initiate a transfer between two ftp servers

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

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AL: ftp commands, responses

Sample commands:• sent as ASCII text over

control channel• USER username• PASS password• LIST return list of file in

current directory

• RETR filename retrieves (gets) file

• STOR filename stores (puts) file onto remote host

Sample return codes• status code and phrase (as in

http)• 331 Username OK, password required

• 125 data connection already open; transfer starting

• 425 Can’t open data connection

• 452 Error writing file

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AL: ftp, NAT and the PORT command

• Normal FTP mode– Server has port 20, 21 reserved– Client initiates control connection by connecting to port 21 on server– Client allocates port X for data connection– Client passes the data connection port (X) and its IP address in a PORT

command to server– Server parses PORT command and initiates connection from its own

port 20 to the client on port Y– What if client is behind a NAT device?

• NAT must capture outgoing connections destined for port 21• What if NAT doesn’t parse PORT command correctly?• What if ftp server is running on a different port than 21?

• http://www.practicallynetworked.com/support/linksys_ftp_port.htm

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AL: ftp, NAT, and the PORT command

• Passive (PASV) mode– Client initiates control connection by connecting to port 21

on server– Client enables “Passive” mode– Server responds with PORT command giving client the IP

address and port to use for subsequent data connection (usually port 20, but can be bypassed)

– Client initiates data connection by connecting to specified port on server

– Most web browsers do PASV-mode ftp– What if server is behind a NAT device?

• See client issues