Anycast by DNS over pure IPv6 network

Minghua Chen & Wei MaoEECS, UC, Berkeley

{minghua, maowei}@eecs

Outline Introduction Anycast Anycast by modifying DNS service

over pure IPv6 Network WTRT server selection Model Conclusions Acknowledgement

Introduction Internet grows dramatically

Exponential growth In 1999, every 10 seconds, a new pc

connects to Internet Distributed information services face a

number of problems of scale excessive server load wasted bandwidth excessive latency

The growth of Internet

0

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1000000

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hosts

460 Million users

Source: Cerf, based on www.nw.com, Jan 2000Source: Cerf, based on www.nw.com, Jan 2000

Server replication & caching Relatively straightforward method to

potentially improve client performance and reduce network load

A key issue in realizing such techniques is how to find the best provider of that service What is difference between the

performance of the best case and the worst case scenario?

Performance variance

What is the relation to our topic? We believe that anycast service is

the right way to do “best” server selection.

Combining anycast service and server replication, the scale problems of distributed information services can be resolved

Outline Introduction Anycast Anycast by modifying DNS service

over pure IPv6 Network WTRT server selection Model Conclusions Acknowledgement

Concept Of Anycast Original definition:a stateless best

effort delivery of an anycast datagram to at least one host, and preferably only one host, which serves the anycast address.

Here, anycast is a communication paradigm service, which connect the client to the “nearest” node in a set of nodes that have some same properties

Anycast Illustration

Typically, client chooses a nearby server.

Anycast realization Network-layer anycast

Routing to the nearest server using routing distance metric

Comments Straightforward idea, hard to

implement Need to modified router Lack of flexibility in selection criteria

Anycast realization (cont.) Application layer anycast

Query with a name and a client address. Returns the unicast address of the “best” server

Comments Does not involve modifying router Flexible selection criteria Need periodically collect information

Why we choose application layer implementation? Implementation Simplicity Flexible selection criteria

e.g. Server load More appropriate selection criteria

Network layer: Hop count only is not a good metric for node selection

Application layer: can use other significant information than hop count as metric

Hop count is a poor predictor of performance

Hop counts

RTT

Problem translation In application-layer anycast

providing anycast service selecting the “best” server given necessary information

These information may include Server load Latency Available bandwidth Client preference

Current server selection schemes Assign server to a client using

round-robin method Balancing server load NSCA WWW servers use it

Geographically server assignment Predicted transfer time (PTT)

model or similar model

available21 BW

sizedocument k RTTk TimeTransfer Predicted

These schemes are not good enough Server load

Server load has impact on response latency

Retransmission cost e.g. Congestion

Traffic segregation according to network topology Keep traffic “local” – principle of scaling

Outline Introduction Anycast Anycast by modifying DNS service

over pure IPv6 Network WTRT server selection Model Conclusions Acknowledgement

Why by modifying DNS One concern of application-layer

anycasting Require bootstrap mechanism

DNS query is almost an essential step to access Internet services By quiz By experience

Is one time query enough for an Internet connection?

Optimal selection changes fairly slowly

The probability of the server’s rank

change > 4 is 0.15

Why over pure IPv6 network We believe that IPv6 will be an

important competitor of the next generation IP protocol

IPv6 adopts anycast IPv6 provides strictly aggregated

address space What will happen in this case?

Outline Introduction Anycast Anycast by modifying DNS service

over pure IPv6 Network WTRT server selection Model Conclusions Acknowledgement

Our work: Propose the server selection criteria

Goal is to select the “nearest” server. Possible distance measures:

lantency, server load, available bandwidth, etc. propose Weighted Total Response Time

(WTRT) TRT is measured from the time the IP request

is sent to the time the whole document is received

The less WTRT, the better Why? Keen to user’s perspective of QoS

Formulation of WTRT Express WTRT as the following:

Key elements: w: segregation weight Latency: time elapsed from request to start of

receiving document E[packets]: expected total number of packets

considering loss and retransmission P_size: average size of TCP packet BW: available bandwidth of the link path K1 and k2: constant coefficients to be

determined

)]_)(

([ 21 BW

sizePpacketsEklatencykwWTRT

Segregation Weight (SW)

?

SW (cont.) Weight the server within the same

subnet less in order to avoid traffic through backbone

Key issue: how to know c/s are in the same subnet???

SW (cont.) In IPv6 addresses are strictly

aggregated Do longest prefix matching using

c/s IP addresses, similar to the algorithm used in CIDR routing

the longer matched prefix, the more “local” the server is to the client

Details of SW

W3 W2

W1W4

W1<W2<W3<W4

Details of SW (cont.) Value of SW should be the same for

the same level of aggregation Who determine the SW?

SW could be determined at the authorized DNS server for a particular domain name

SW could be applied by local DNS server on the query results, in order to realize policy flow balance control

Encourage/discourage outgoing flow

Latency How it affect the TRT

Small latency lead to fast response from server.

When document size is small, this dominates the document retrieve time

What determine latency Current server load Current link load and link characteristics

How to estimate latency? Fei et al proposed a hybrid server

push/probe scheme to estimate the latency from a client to a server

Their simulation results show that the scheme works fairly well

Available Bandwidth How it affect the TRT:

Higher bandwidth leads to faster document transfer

What determines the available bandwidth: The capacity of the c/s path Present traffic on the path

Available Bandwidth (cont.) How to estimate it:

R. L Carter proposed bandwidth probing (BPROBE) algorithm to estimate the total capacity

Also proposed CPROBE algorithm to estimate the current traffic

Their results provide a reasonable ground for other existing researches

Packet Loss & Retransmission Assume:

K original packets need to be transmitted p is the steady-state packet loss rate. Packets lost are retransmitted only once.

Binomial model of the # of retrans. Packets leads to E[packets]=K(1+p).

The total size of the document transmitted becomes E[packets]*P_size.

Average size of TCP packet is 403 bytes.

K1 And K2 Why K1 and K2?

Linear model How to determine K1 and K2

By client preference small document size larger ratio of K1 /

K2 large document size smaller ratio of K1 /

K2 By regression method

Comparison to PTT model WTRT is derived from PTT but

more extensive WTRT

PTT

)]_)(

([ 21 BW

sizePpacketsEklatencykwWTRT

)_

(21 BW

sizedocumentkRTTkPTT

An example

PTT

WTRT

Heavy server load

Modified DNS query procedure

Modified DNS query procedure (cont.) Client IP address and preference are

sent between DNS servers DNS server may return several IP

addresses to client for further selection SW can be determined at authorized

DNS server or local DNS server Periodically, the authorized DNS server

gather information from server and probes to update its database

Outline Introduction Anycast Anycast by modifying DNS service

over pure IPv6 Network WTRT server selection Model Conclusions Acknowledgement

Conclusions Server selection + anycast service =

solution to the problems of scaling of distributed information services

Application layer anycasting has more advantage over network layer anycasting

In application layer anycasting providing anycast service selecting the

“best” server given server and link characteristics

Conclusions (cont.) Anycast by modifying DNS service

provide an easy bootstrap mechanism

In IPv6, strictly aggregated address space can facilitate segregating traffic according to network topology

WTRT model is more reasonable and extensive than PTT model

Acknowledgement Jim Guyton @ Apple Co. for some

discussion on traffic segregation F. Yu and L. Yin for the

development of the initial idea

Thank you very much for your attention !