Fast L3 Handoff in Fast L3 Handoff in 802.11 Wireless LANs802.11 Wireless LANs

Andrea G. ForteSangho Shin

Henning Schulzrinne

L3 HandoffL3 Handoff

AP AP

Router Router

160.38.x.x 128.59.x.x

Mobile Node

Static Node

DHCP - OverviewDHCP - Overview DHCP Server

Assigns IP addresses to clients that request them via the DHCP protocol. It directly serve clients in its subnet while it needs the Relay Agent in order to server clients in a different subnet than its own.

Relay Agent (RA)We usually have one RA per subnet and usually the RA is located on the router/gateway of that subnet. The RA needs to relay DHCP packets between its network and the DHCP server. The server will know to which subnet a client belongs to (and which IP address to assign) according to which RA the packets came from.

DHCP Procedure - DHCP Procedure - OverviewOverview

MN

DHCP DISCOVER

DHCP REQUEST

DHCP ACK

L2 Handoff Complete

DHCP Server

DHCP OFFER

DAD

Current DHCP Current DHCP PerformancePerformance

0

200

400

600

800

1000

1200

mse

c

1 2 3 4 6 10 11 12 13 14 15

Original DHCP acquisition time

ACK

Offer

Fast L3 HandoffFast L3 Handoff IP address acquisition time is too big

for seamless inter-subnet handoffs. Our mobility scenario VoIP + SIP. Many entities involved in the process:

DHCP server/client Correspondent Node (CN) SIP server

Fast Layer 3 Handoff - Fast Layer 3 Handoff - CacheCache

Spatial locality Cache

We use an extension of the L2 cache:

Current AP (KEY)

Best AP Second best AP

MAC A MAC B MAC C

Channel 1 Channel 11 Channel 6

Gateway D Gateway E Gateway F

+

LEASE FILE

Fast L3 HandoffFast L3 Handoff

We optimize the layer 3 handoff time as follows: Subnet Discovery. IP address acquisition. Multimedia session update (SIP).

Subnet Discovery (1/2)Subnet Discovery (1/2)

Current solutions Router advertisements

Usually with a frequency on the order of several minutes.

DNA working group (IETF) Detecting network attachments in IPv6

networks only.

No solution in IPv4 networks for detecting a subnet change in a timely manner.

Subnet Discovery (2/2)Subnet Discovery (2/2) Our approach

Send bogus DHCP_REQUEST (using loopback address).

DHCP server responds with a DHCP_NAK From the NAK we can extract subnet

information such as default router IP address.

The client saves the default router IP address in cache.

If old AP and new AP have different default router, the subnet has changed.

Fast Address AcquisitionFast Address Acquisition IP address acquisition

This is the most time consuming part of the L3 handoff process DAD takes most of the time.We optimize the IP address acquisition time as follows:

Checking DHCP client lease file for a valid IP. Temporary IP (“Lease miss”) The client “picks” a candidate

IP using particular heuristics. SIP re-invite The CN will update its session with the TEMP_IP. Normal DHCP procedure to acquire the final IP. SIP re-invite The CN will update its session with the final IP.

While acquiring a new IP address via DHCP, we do not have any disruption regardless of how long the DHCP procedure will be. We can use the TEMP_IP as a valid IP for that subnet until the DHCP procedure ends.

Session UpdateSession Update

Multimedia session update (SIP)After a change in IP address, we have to inform the Correspondent Node (CN) about it. This is usually done with a re-Invite. The data stream will be resumed right after the 200 OK has been received.

MN

SIP Re-INVITE

RTP Data

SIP ACK

New IP

CN

SIP OK

Handoff ScenariosHandoff Scenarios Scenario 1

The MN enters in a new subnet for the first time ever.

Scenario 2 The MN enters in a new subnet it has been

before and it has an expired lease for that subnet.

Scenario 3 The MN enters in a new subnet it has been

before and still has a valid lease for that subnet.

TEMP_IP Selection (1/3)TEMP_IP Selection (1/3) Scenario 1

Select random IP address starting from the router’s IP address (first in the pool). MN sends 10 ARP requests in parallel starting from the random IP selected before.

Scenario 2 Same than scenario 1 except that we start

to send ARP requests to 10 IP addresses in parallel, starting from the IP we last used in that subnet.

Scenario 3 We do not need TEMP_IP as we have a valid

lease. We just renew the lease.

TEMP_IP Selection (2/3)TEMP_IP Selection (2/3) Critical factor: time to wait for an ARP

response. Too small higher probability for a duplicate IP. Too big increases total handoff time.

TEMP_IP: for ongoing sessions only Only MN and CN are aware of the TEMP_IP If any of the steps involved in the fast

handoff fail, MNs can always rely on legacy 802.11 mechanisms such as scanning.

TEMP_IP Selection (3/3)TEMP_IP Selection (3/3)

# of IPs used

IP usage rate

0

20

40

60

80

100

120

140

8:00

9:00

10:0

011

:0012

:0013

:0014

:0015

:0016

:0017

:0018

:0019

:0020

:0021

:0022

:00

Time

# o

f IP

s u

sed

0

10

20

30

40

50

60

70

80

90

100

%

Fast Layer 3 - Fast Layer 3 - ImplementationImplementation

SIP client(mca)

Wireless card driver(HostAP driver)

DHCP client

User Space

Kernel Space(version 2.4.20)

Red Hat 9.0

MCA: SIP client for PDAs by SIPquest Inc.

DHCP client by Internet System Consortium (ISC)

HostAP wireless driver

Parameters usedParameters used Consecutive IP addresses in use had a 99th percentile

value of 5. ARP waiting time had a 90th percentile of 130ms and a

99th percentile of 230ms.

Subnet detection time: from L2 assoc response to DHCP_NAK from bogus request.

IP address acquisition time: from the first ARP req. to the expiration of the ARP waiting timer (ARP requests are sent in parallel).

SIP signaling time: from the moment the INVITE is sent to the moment the 200 OK has been received.

Client processing time: gap between components for processing internal signals, etc.

ARP Req.NAK

MN DHCPd

DHCP Req.

ARP Req.

Router

ARP Resp.

CN

SIP INVITE

SIP OK

SIP ACK

RTP packets (TEMP_IP)

138 ms

22 ms

4 ms

4 ms

29 ms

Waiting timeIP acquisition

SIP signaling

L2 handoffcomplete

Detecting subnet change

Processing overhead

Experimental Results (1/2)Experimental Results (1/2)

22

518

829

22

138

829

138

829

829

0

100

200

300

400

500

600

ms

Currentapproach

Scenario 1 Scenario 2 Scenario 3

SIP Signaling

Client processing

IP acquisition

Detection of subnet change

Experimental Results (2/2)Experimental Results (2/2)

Conclusions & Future WorkConclusions & Future Work Modifications in client side only (requirement).

Forced us to introduce some limitations in our approach. Works today, in any network.

Much faster than DHCP although not seamless in some cases.

Scenario 3 obvious but … Windows XP

ARP timeout critical factor SIP presence SIP presence approach (Network support)

Other stations in the new subnet can send ARP requests on behalf of the MN and see if an IP address is used or not. The MN can wait for an ARP response as long as needed since it is still in the old subnet.

Passive DAD (draft-forte-dhc-passive-dad-02.txt)

Thank You!Thank You!

For more information:

Web:

http://www.cs.columbia.edu/~andreaf

http://www.cs.columbia.edu/IRT

E-mail:

andreaf@cs.columbia.edu