CompNet 2

Local DNS

near end hosts who are configured with local server Root: 13 servers globally TLD, Authoritative DNS

Iterative DNS Query

Ask server who to ask next

DNS Cache

records have TTL, can negatively cache names which don't work. Replicate for reliability / load balance. Queries may return additional records which open up cache poisoning vulnerabilities

HTML

Hypertext Markup Language, references objects, formats text, embed hyperlinks

URL Syntax

protocol://hostname[:port]/directorypath/resource

HTTP

Hypertext Transfer Protocol, request-response, needs global namespace, resource metadata, stateless, ASCII

HTTP Request

client initiates TCP connection - SYN, SYNACK, ACK (+request), server responds, client receives, termination exchange HTTP Metadata: info about a resource, separate entity

Cookies

Provide state, stored by client and sent to server with requests, can provide authentication

HTTP Performance

Stop & Wait, Concurrent request in multiple channels may not preserve order

Pipelined

batch request/response, 1 connection, -

Forward Proxy

cache close to clients, reduce network load and latency, ISP or corporate LANs

Content Distribution Networks

forward & reverse, PULL result of clients requests, PUSH except high access rate

Persistent Connections

maintain across multiple requests, reduce overhead, allow TCP to learn RTT & grow window

Hosting

Multiple sites per machine, separate process per site or take full host name in get request. Multiple machines per site, replicate popular sites to reduce load, locate content closer to client.

SIP

Session Initialisation Protocol, gives location, availability, capabilities, session setup, session management

File Distribution

Client Server: dcs = max{NF/us , F/min(di)} P2P: dP2P = max{F/us , F/min(di) , NF/(us + ∑ui) }

BitTorrent

tracker: server tracks peers participating in torrent. torrent: groups of peers exchanging chunks of file. 256KB chunks, register with tracker, get list of peers, connect to subset, upload simultaneously, peers come and go, peers ask each other for list of chunks they have, then send requests for missing chunks, rarest first, send chunks to top 4 highest senders to you, re-evaluate every 10s, every 30s randomly select new peer to start sending chunks to

Distributed Hash Table

distributed P2P database with (k,v) pairs, peers can query and insert into database

Skype

pairs communicate, clients grouped under supernodes, index maps usernames to IPs, separate login server. If both peers behind NATs, outsider peer cannot make call to insider, each peer initiates session with relay & use this

Transport

provide logical communication between app processes on different hosts, run in end systems, break messages into segments pass to network layer, reassemble messages and pass to app layer

Demultiplexing

delivering received segments to correct socket, use IP addresses & port numbers from datagram Multiplexing: gather data from sockets and enveloping data with header

Connectionless Demux

Sockets with port#, UDP Socket: dest IP & port#, datagrams with diff source IP/port directed to same socket Connection-oriented Demux: TCP Socket: source & dest IP, source & dest port#, use all four values to direct socket, support simultaneous sockets identified by 4-tuple, web servers have different socket per client

UDP

'best effort' may be lost/out of order, 'connectionless' no handshaking, segments handled independently. Can be used for streaming media (loss tolerant, rate sensitive), rate not throttled by congestion control; DNS, SNMP, reliability can be added at app layer

Advantages & Disadvantages of UDP

• no connection establishment delay, simple - no state at ends, small header, no congestion control

UDP Header

source & dest port#, length, checksum

UDP Checksum

segments as 16 bit ints, addition (1's complement sum) Eg. add 2 ints, add carryout to result, flip bits

Rdt1.0

reliable transfer over reliable channel

Rdt2.0

channel with bit errors, need error detection (checksum), error recovery ACK &/ NAK - receiver feedback

Rdt2.1

ACK/NAK may corrupt, retransmit, seq# to avoid duplicates

Rdt2.2

NAK-free, ACK for last packet received including seq#, duplicate ACK acts like NAK - retransmit current pkt

Rdt3.0

channel with packet loss, sender times-out if no ACK received, retransmit. Could get duplicate but use seq#

Utilisation

(Stop & Wait) Usender = (L/R) / (RTT + (L/R))

Pipelined

multiple 'in flight' unACKed packets, need more seq#s, buffering at sender/receiver, better utilisation

Selective Repeat

sender has upto N unACKed packets, receiver ACKs individual packets, sender has timer per packet, only retransmit that packet on timeout, buffers out of order packets, reorder later.

Window size

must be ≤ (seq# range / 2), otherwise can't distinguish retransmit on lost ACK from out-of-order

Go-Back-N

sender has upto N unACKed packets, receiver sends cumulative ACKs, sender has timer for oldest unACKed packet, retransmit all on timeout, discard out-of-order packet

Automatic Repeat ReQuest (ARQ)

error-control method for data transmission that uses ACKs & timeouts to achieve reliable data transmission over an unreliable service. Eg Stop & Wait, Go-Back-N, Selective Repeat

TCP

point to point, reliable, inorder byte stream, pipelined, congestion & flow control, pipelined, buffers, connection oriented (handshaking), bi-directional data flow in same connection, maximum segment size (MSS). Doesn't specify how to handle out-of-order segments, this may leave TCP open to out-of-order attack

TCP Segment

source & dest port#, seq#, ack#, receive window, checksum, options, flags and application data

Sample RTT

by measuring time between transmission & ACK receipt, average over several, ignore retransmits

TCP Estimated RTT

(1-α)EstRTT + αSampleRTT, exponentially weighted moving average, 0 ≤ α ≤ 1, eg 0.125

TCP DevRTT

(1-β)DevRTT + β(SampleRTT-EstRTT) Timeout (RTO) = EstRTT + 4*DevRTT, typically β = 0.25

TCP Send

create segment with seq# = # of 1st data byte in segment, start timer for oldest unACKed segment

TCP Receive

If in-order then delay ACK by ½s unless ∃ ACK pending, else send duplicate ACK, if closing gap then ACK

Fast Retransmit

if sender receives 3 ACKs for same data, assumes next segment lost & retransmits before timeout

Silly Window Syndrome

small MSS values may persistent if send immediately, try to fill segment b4 send (timeout)

Flow Control

preventing senders from overrunning the capacity of the receivers

Congestion Control

preventing too much data being injected into network and overloading switches or links

TCP Flow Control

receiver includes rcvWindow in segments = buffer free space, sender unACKed data ≤ rcvWindow

TCP Connection Management (open connection)

Establish: clients sends TCP SYN to server, specifies initial seq#, server replies with SYNACK, allocates buffers, specifies server initial seq#, client replies with ACK which may contain data

Close TCP connection

client sends TCP FIN control segment to server, server replies with ACK, prepares to closes connection, sends FIN. Client replies with ACK, enters timed wait, server receives ACK, connection closed

Causes of congestion

router reaches max achievable throughput, router buffer overflows & drops packets, retransmission worsens problem, wasted upstream capacity for dropped packet

Congestion Control challenges

single flow - bottleneck B, single flow - variable B, multiple flows sharing Bandwidth

End-end congestion control

no feedback from network, inferred by end system by loss, delay

Network-assisted congestion control

routers provide feedback to end systems, flag for congestion or specify rate

TCP Congestion Control

(AIMD) additive increase: CongWin += 1MSS every RTT, on 3 dup ACKs, multiplicative decrease: halve CongWin, Sender limits transmission, # unACKed packets ≤ CongWin, rate ≈ CongWin/RTT

TCP Slow Start

CongWin = 1MSS, double CongWin every RTT (ACK) until loss, if timeout -halve threshold, SS then AI

TCP Throughput

throughput is W/RTT, after loss drops to W/2RTT, average ¾ W/RTT = (MSS/RTT) √(3/2p)

Caching

request(ifmodifiedsince), response(expiry/no cache)

Reverse Proxy

cache close to server, decrease server load, content providers with static content