Spring 2006 EE 5304/EETS 7304 Internet Protocols

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Spring 2006

EE 5304/EETS 7304 Internet Protocols

Tom OhDept of Electrical Engineering

[email protected]

Lecture 15

TCP - Part 3

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Administrative Issues

We will have final exam next week (6:30PM May 9, 2006)

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Outline

TCP header options TCP for transactions RED

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TCP Header Options

20-byte TCP 20-byte TCP headerheader

TCP header TCP header optionsoptions DataData

Option kindOption kind Option lengthOption length Option-dependent Option-dependent datadata

1 byte 1 byte Variable

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TCP Header Options (cont)

Maximum segment size (MSS): In first SYN segment, sender can specify largest segment

that it is willing to receive If not specified, default 536 bytes assumed (not including

TCP/IP headers)

Kind = 2Kind = 2 Length = 4Length = 4 MSSMSS

1 byte 1 byte 2 bytes

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Window scale factor [RFC 1323]: Specifies factor to shift advertised window field Shift n means multiply by 2n (max. n = 14 allows window

of 230 = 1 billion bytes) Larger windows are better for routes with large bandwidth-

delay products

Kind = 3Kind = 3 Length = 3Length = 3 Shift countShift count

1 byte 1 byte 1 byte

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Selective acknowledgements (SACK) [RFC 2018]: TCP acknowledgements are cumulative This option allows selective acknowledgement of non-

continuous data, so sender knows exactly what needs to be retransmitted

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TCP for Transactions

Short transactions might consist of only a query and reply (one packet each way)

UDP does not guarantee reliability, but TCP’s 3-way handshake to set up and clear connections will be wasteful

TCP for transactions (T/TCP) tries to keep TCP’s reliability and short-cut connection set-up for single packets

Uses new header option called CC (connection count)

CC is incremented for each new T/TCP connection from a host

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TCP for Transactions (cont)

In best case, only 3 segments used

T/TCP SYN, query, FIN, client CC

T/TCP SYN, reply, ACK, FIN, server CC

Request to open connection with CC, pass query, and close; CC number should be increment of previously used CC numbers from this host

ACK

Request to open connection with CC, pass reply, ACK of FIN, and close; CC number should be increment of previously used CC numbers from this host

ACK of FIN

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TCP for Transactions (cont)

Any packet loss results in time-out and retransmissions

CC allows duplicate packets to be detected If server is not capable of T/TCP, it can resort to

normal TCP 3-way handshake T/TCP is not in common use yet

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RED

Problem: TCP congestion avoidance algorithm will push network into losing a packet, then back off into slow start

TCP sources tend to become synchronized, going into slow start at same time

Router queue

TCP TCP sourcesource

TCP TCP sourcesource

• • •

1. Buffer overflows, dropping packets

2. Sources detect congestion after RTT

3. Buffer overflow continues in meantime, effecting several sources

4. Several sources back off at same time

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RED (cont)

TCP rates

Packet loss

Slow start

Retransmission timeouts

Aggregate source

rate into queue

Packet loss

Packet loss

Severe underutilzation

Severe overutilzation

Sources are syncrhonized

From queueing theory, irregular traffic rate will be bad - large queues and packet loss

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RED (cont)

RED (random early detection) attempts to break the synchronization by making sources back off at different times

No mechanism for explicit notification, so RED relies implicit feedback

Packet will be dropped at random (before buffer overflows), forcing a source to back off before others

Random for fairness De-synchronizing the sources helps to smooth out

aggregate source rate → reduce packet losses and increase throughputs

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RED (cont)

TCP rates

Packet loss

Aggregate source

rate into queue

Out-of-sync sources result in smoother rate - more sources, more smooth

Packet loss

Packet loss

From queueing theory, smooth traffic rate will be good - small queues and packet loss

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RED (cont)

RED keeps track of average queue length = smoothed recent history of queue length (to discount transient changes)

Probability of dropping packet (randomly) depends on average queue length

Pr(drop)

MaxP

MinThresh MaxThresh

1

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RED (cont)

RED has been shown to be very effective with TCP Widely supported in routers

But best choices for parameters (MaxP, MinThresh, MaxThresh) can be difficult to determine

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Spring 2006

EE 5304/EETS 7304 Internet Protocols

Tom OhDept of Electrical Engineering

[email protected]

Lecture 15

Client-server, DNS, WWW, P2P

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Outline

Client-server applications Domain Name System (DNS) World Wide Web (WWW) Peer to peer (P2P) protocols

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Client-Server Applications

Client-server is most common way of organizing processes in a distributed system

Easy to understand, clean organization Computers are clients or servers (or both)

A server is a process running a specific service, eg, file server, web server, name server

A client is a process that makes a request from server and waits for response

RequestReplyClientClient ServerServer

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Client-Server (cont)

Communications can be connectionless, eg, UDP/IP Efficient but problems over WANs If reply is missing, requests can be retransmitted, but reply

might be just late → duplicate request can cause service to be done twice

Hard for client to tell what really happened Reliable TCP/IP is usually used

Cost is 3-way handshake to establish and terminate connections just for 2 data packets

TCP for transactions (T/TCP) is efficient alternative

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3-Tiered Client-Server

3-tiered architectures can have intermediaries that are both client and server

Commonly used in transaction processing (application server = transaction monitor coordinates all transactions across different data servers)

Client

Request operation

Reply result

Applicationserver

Databaseserver

Request data

Reply data

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Horizontal Distribution

Horizontal distribution: a client or server can be physically split into logically equivalent parts, usually for load balancing

Horizontal distribution of servers: example is replicated web servers (identical copies of web contents) for load balancing

Horizontal distribution of clients: peer-to-peer network (actually no server)

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DNS (Domain Name System)

An application used by other applications, not by users

Example of middleware - works between applications and network

Translates between user-friendly host names (www.website.com) and IP addresses

Frees applications to use host names instead of IP addresses

Totally distributed and decentralized for scalability and reliability

http://www.website.com/

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DNS (cont)

In early days, central NIC (Network Info. Center) kept a flat (non-hierarchical) table of name-to-address bindings

New hosts were added manually Updated list was mailed out every few days to be installed

by local administrators Did not scale well

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DNS (cont)

1984 Paul Mockapetris wrote RFCs 882, 883 for DNS and built a reference implementation, JEEVES

1988 Kevin Dunlap wrote an implementation for BSD Unix, BIND (Berkeley Internet Name Domain)

Designed to be a distributed database Local control is allowed over database segments (each

segment holds part of total namespace) Those with administrative responsibility for their domain

names see no outside interference Data from all database segments is accessible

everywhere

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DNS - Name Space

Database info. is robust (resilient against failures) and highly available (responsive)

Replicated for robustness Cached for high availability

All names are elements of a name space Name space can be represented by a labeled,

directed graph

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DNS - Name Space (cont)

3 types of nodes: Leaf node represents a named entity, and has no outgoing

edges Directory node has incoming and outgoing edges, each

labeled with a name• Stores a directory table of outgoing edges = (edge label, node

identifier) Root node (usually single) has outgoing edges, no

incoming edges

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Example

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Each path in graph is referred to by sequence of labels (path name)

Example: n0:<home, steen, mbox> or /home/steen/mbox A path name starting from root node is an absolute

path name A path name starting from another node is a

relative path name

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Common types of naming graphs: Hierarchical: like a tree, every node has exactly one

incoming edge and exactly one absolute path name Directed acyclic graph: nodes can have multiple incoming

edges, but graph cannot have a cycle

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DNS has a hierarchical name space Example: path name = houns54.clearlake.ibm.com

root-level name

top-level name

first-level name

second-level name

third-level name

(root)

edu

dell

clearlake

houns54

gov mil com org net

ibm hp

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DNS hierarchy can be viewed in 3 layers

Global layer: highest level nodes; stable (almost never changing)

Administrative layer: groups belonging to same organization; relatively stable (change occasionally)

Managerial layer: regularly changing, eg, shared files, user-defined directories; could be administered by end users - formally, not part of DNS

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DNS - Servers

Zone is part of name space implemented in a name server

Although domains must be registered centrally, each domain can be arbitrarily subdivided into zones without registration

3 types of DNS servers within a zone: Primary DNS server or master server:

Only one primary within each zone Keeps zone data file = snapshot of DNS database, which

is read when DNS service starts up

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DNS - Servers (cont)

Secondary DNS server or slave server: Serve as redundant backup and load balancing Gets its data from primary server in zone Checks for updated info

Caching servers Store recently accessed DNS records from other domains

to avoid performance penalty to make a remote query each time to access a resource outside local domain

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DNS - Servers (cont)

15 root name servers act as top of DNS hierarchy worldwide

Ultimate source for name lookups that cannot be resolved otherwise

Designed to be highly reliable

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DNS - Name Resolution

TCP/IP client can send name query (to resolve an address to name) or address query (to resolve a name to address)

This part of client is a name resolver Must be configured with a local DNS server or learn one

using DHCP Example: query for www.foo.com goes to local DNS

server Unless cached, local DNS server queries a root

server for IP address of .com DNS server

http://www.foo.com/

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DNS -Name Resolution (cont)

Local DNS server queries .com DNS server for IP address of foo.com DNS server, then queries this server for foo.com zone

Query goes to primary server in foo.com zone (or secondary server)

Example of “iterative name resolution”

Root server

Resolver

Client8. DNS Reply

Cache

Local DNSserver

1. DNS QueryTop-level domain server

Second-level domain server

2

34

56

7

37

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World Wide Web

Web is most important example of distributed document-based system

1965 Ted Nelson described idea of “hypertext” (text with hyperlinks)

1989 WWW first proposed by Tim-Berners Lee at CERN for physicists to share hyperlinked data, prototyped on a NeXT cube

1993 first graphical web browser Mosaic released by NCSA

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WWW (cont)

1994 Marc Andreessen founded Mosaic/Netscape, released Netscape Navigator

1994 CERN and MIT joined to form World Wide Web Consortium (w3.org) to coordinate standards

Web traffic now accounts for 70+ percent Internet traffic

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WWW Advantages

Simple graphical user interface instead of complex command line

Allows access to variety of resources (documents, graphics, programs)

Hyperlinks are easier to navigate (eg, compared to gopher)

Easy to use search engines Web pages are easy to create and publish

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WWW Basics: Client-Server

1. DNS query and reply

2. TCP SYN

3. HTTP request4. HTTP response

HTTPTCPIP

HTTPTCPIP

5. TCP FIN

WebWebclientclient

WebWebserverserver

DNSDNSserverserver

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WWW Basics: Client-Server (cont)

Web browser (client) selects URL (uniform resource locator) to request

Browser parses domain name which must be translated into IP address

Sends domain name to one of local DNS servers DNS server responds with IP address Browser establishes TCP connection with web

server Parses URL for document name, sends HTTP request for

document Web server returns document → browser displays

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HTTP Protocol

Exchange consists of an HTTP message pair = client request + server response

Stateless: client and server do not keep memory Each exchange is treated as new Keeps servers simple

By default, TCP is transport layer protocol to relieve HTTP from concerns about lost packets (and connection errors are reported)

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HTTP (cont) HTTP request contains method (operation) + document name +

protocol version Web server applies method in HTTP request and replies with

HTTP response Most common methods:

HTTP request contains method (operation) + document name + protocol version

Web server applies method in HTTP request and replies with HTTP response

Most common methods:Method Description

Head Request to return the header of a document

Get Request to return a document

Put Request to store a document

Post Add data to a collection of documents

Delete Request to delete a document

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HTTP Request Message Format

Mandatory

Optional additional headers, eg, security info for authentication, client’s email address, date/time, acceptable language

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HTTP Reply Message Format

Status codes with explanatory phrase:200 Request honored;400 Bad request403 Forbidden404 Not found

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Web Documents

Every resource is a “document” and usually identified by URL

URL = protocol + host name + (optional port number) + pathname

Example: http://www.cs.vu.nl:80/home/steen/index.html Most documents in HTML (hypertext markup

language) Keywords specify structure of document in different

sections/parts (header, lists, tables, forms,…) Keywords also specify presentation and hyperlinks to

other documents

http://www.cs.vu.nl:80/home/steen/index.html

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Web Documents (cont)

Other types of documents

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Web Difficulties

TCP connection set-up/close involves overhead for every Web interaction

Delays in reaching web server Network latency depends on congestion Rule of thumb - users will wait 8 sec for a document

Possible web server overload Centralized servers do not scale well to large networks Vulnerable to denial of service attacks

Possible abuse of cookies (state stored in client)

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Web Enhancements: CGI

CGI (common gateway interface) allows a web server to execute a program with client input (usually a form)

Allows dynamic interaction with web server

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Web Enhancements: Servlets and Applets

Slightly different from CGI, a servlet is a program executed within the server (CGI is a separate program)

Applet is similar to servlet but on client side - a Java program sent to client and executed within browser

Browsers include a JVM (Java virtual machine) to interpret Java bytecode and provide a temporary, constrained execution environment

An alternative is Javascript, an interpreted scripting language

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Web Enhancements: Client-Server

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Web Client

Variety of document types → browser can call helper applications for display certain documents

Alternatively, a plug-in is small program that attaches to browser through a standard interface

Plug-ins are dynamically loaded as needed

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Web Client (cont)

Alternatively, a web proxy can act as intermediary between client and server to handle application layer protocols (not HTTP)

Unnecessary today, but web proxies are used for caching purposes

Client Webproxy

HTTP RequestHTTP Reply

FTPserver

FTP RequestFTP Reply

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Client-Side Caching

Cacheproxy

Cache

Enterprisenetwork

Webserver

Webbrowser

Cache

Webbrowser Recently accessed

content is cached;shared by multiple

clients withinenterprise

Recently accessedcontent is cached;

per browser Cache hit ratio turns out less than 50 percent, and only if cache is large

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Client-Side Caching (cont)

If web request is not satisfied by browser’s cache, request is forwarded to cache proxy at edge of enterprise network

Cache proxy may contain recently viewed content for all users in enterprise

If not satisfied, request goes to web server Main problem is cache consistency (out of date

contents) Some proxies send a conditional HTTP get request with a

time, server will respond only if document has changed since that time

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Server Replication

Web server is replicated in a server cluster A front end redirects client requests to one of the servers Protects against server overload

Webserver

Frontend

LAN

HTTP

Webserver

Webserver

Webserver

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Peer-to-Peer Protocols

Client-server separates world neatly into a few publishers (servers) and many consumers (clients)

Peer-to-peer (P2P) is for equal sharing Information is completely decentralized and located at the

edges of network, not centralized (at servers) Communications is more symmetrical than client-server DNS is not used - nodes are too transient

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Peer-to-Peer Protocols

Every computer is a publisher and consumer, searches across scattered collections stored at member nodes which appear to be a single indexed repository

Real-time index tracks all active nodes and files chosen for sharing (usually music, video, games)

Nodes can make direct connections with other nodes to share files

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Peer-to-Peer (cont)

Benefits: Low cost file sharing Easy to build up large networks, dynamically self updating Distributed nature can be resilient to node failures

Disadvantages: Indexing may be inefficient or unreliable Listed information may be cluttered with a lot of noise Most info is music, video, games → used for piracy

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Start of P2P: Napster

1999 Written by 18 year-old Shawn Fanning as a student at Northeastern U to swap MP3 music (Napster was nickname)

June 2002 legally shut down by RIAA (Recording Industry Assoc. of America)

Initially mp3.com allowed users to upload MP3 music onto a server, or download music

But only files released by artists, recording companies, paid for (by user for downloading), or otherwise in public domain

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Napster

Fanning wanted a program to combine 3 functions: Search engine: to find MP3 files easily File sharing: to swap MP3 files directly without a central

server for storage Internet relay chat (IRC): to find and chat with other MP3

users Napster client is installed and runs on each node

A directory is chosen for sharing

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Napster (cont)

Client connects to central Napster index server Server keeps an index of all Napster users currently

online, does not store any MP3 files User requests music by title or artist

Client sends a query to index server Server replies with a list of matches

User can connect directly with any machine in list and download

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2nd Generation P2P: Gnutella

Many P2P file-sharing programs imitated Napster (Aimster, Morpheus, Kazaa, Grokster)

Sued by music producers, but some have been very difficult to shut down due to total decentralization

2000 Gnutella developed by programmers Justin Frankel and Tom Pepper (previously authored Winamp)

Gnutella is designed as an open protocol, any software can be Gnutella-compatible

Killed by AOL as unauthorized freelancing, but championed by open-source developers

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Gnutella (cont)

Also designed to share any files, without any centralized management

A host needs to only connect to (any) one host in the Gnutella network

Host A joins by sending initialization PING to host B (in network)

B replies with PONG and info about number and size of files it has to share, then B forwards PING to other nodes that it is connected to

Hosts in vicinity of A reply with PONGs to B, which are forwarded to A

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Gnutella (cont)

A makes a request for file (by keyword) Request is forwarded by B to its neighbors, then

propagated neighbor to neighbor Matching replies are forwarded back to A

A chooses a matching reply and connects to that machine

Example of distributed intelligence implemented at application layer: each node interprets and responds to a query in its own way

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Gnutella (cont)

How to initially join? “Host caches” are hosts always up as connecting point

into Gnutella A new host is given a list of other hosts to connect

to (ie, recently seen hosts) But host caches can be swamped by new users

Effectively host caches were causing nodes to be closely clustered into highly interconnected cells, but cells were not well connected

Later enhanced so host caches communicate and distribute new hosts more uniformly

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Gnutella (cont)

Gnutella nodes are not all equal Some nodes are “more equal than others” - have more

capacity and elected as indexing nodes 3rd generation P2P add features for anonymity

Freenet, I2P, GNUnet, Entropy, MUTE, Napshare

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BitTorrent

2002 BitTorrent written by Bram Cohen to use P2P file distribution to aid Web downloads

Now popular method for file distribution Normally, downloading large files (video) causes

bottlenecks at servers BitTorrent breaks files into smaller fragments, eg,

250 kB A downloading host finds fragments of a file from

closest peers, perhaps in random (not sequential) order, while providing an upload connection to the fragments it already has

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BitTorrent - Elements

Web server BitTorrent client: host software for simultaneous

downloading and sharing (uploading) .torrent file: contains information about fragments

of a file (names, sizes, hashes) and address of tracker server

Tracker server: keeps log of which users are downloading and their progress

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BitTorrent - Downloading

BitTorrent client first downloads .torrent file containing tracker address

Tracker server informs client of other peers with fragments that the client wants

Client requests fragments from its peers using BitTorrent P2P protocol (over TCP)

At same time, client allows peers to download fragments from it

• Clients with entire file are “seed nodes” All nodes continually update tracker of their progress

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BitTorrent - Features

Rarest first: Clients first request the fragment that fewest peers have

(rarest fragment) Ensures that peers have fragments which all their peers

want Leech resistance:

BitTorrent gives best download performance to hosts that upload the most, discouraging “leeches” (who only download)

Choking algorithm allows peer to refuse uploading to a perceived leech

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Test #3 Review

MPLS MPLS- FEC, LSR MPLS shim header MPLS concept MPLS traffic engineering

Quality of Service Intserv Guaranteed Service, Resource Reservation, controlled service RSVP Path message Admission control Problem with Intserv

Diffserv Objectives Diffserv concepts EF and AF

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Test #3 Review (Cont)

Transport Layer Connection Oriented or connectionless. Quality of Service Needed by Application? OSI types of Network Service Type A-1 Network service: connection setup, multiplexing, flow control Type A-2 Network service Type B Network service Type C Network Service: Retransmission Strategy, Duplicate Detection, Flow Control. Connection Setup.

UDP concept UDP header UDP Checksum

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TCP Part 1 TCP TCP header: checksum, sequence number, acknowledgement, header length, Flags (Urgent pointer, Push) Windows TCP Retransmissions TCP Adaptive Retransmission Algorithm TCP Duplicate Detection TCP Connection Set-Up TCP Connection Close

TCP Part 2 TCP flow control TCP Congestion control TCP Congestion Avoidance (AIMD) TCP Slow Start Slow Start and Congestion Avoidance Fast retransmit and recovery

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TCP Part -3

TCP Header option TCP Transaction RED

Client-Server DNS