University of Berne Institute of Computer Science and Applied Mathematics – IAM/RVS TCP Issues in Mobile IP Networks Ruy de Oliveira December 05, 2001.

University of Berne Institute of Computer Science and Applied Mathematics – IAM/RVS

TCP Issues in Mobile

IP Networks

Ruy de OliveiraDecember 05, 2001

Topics addressed

Brief review on TCP algorithmChallenges for TCP under mobile environmentMain proposed approaches for cellular net.Requirements on mobile multi-hop networksSome proposals for multi-hop environmentConclusions and outlooks

TCP review

TCP has been designed to work on wired networks• Negligible medium loss (low BER)

Under loss it starts probing the net at lower rate by shrinking its congestion window (CWND)

• Slow Start (timeout) exponential back off (RTO)• Congestion Avoidance • Fast retransmit and recover (3 dacks)

Receiver window (RW) limits the maximum rate of the sender

• Upon receiving a RW set to zero, sender enters into “persist mode”

TCP under mobile environment

In mobile networks, pck losses refer to:• Congestion within wired network • Non-negligible wireless losses (high BER)• Disconnection (Handover, fading, etc)

As TCP does not discriminate such losses, it can waste bandwidth by dropping its CWND when

• A pck loss occurs in the wireless link • A fast handover takes place towards a cell with enough

bandwidth

Serial timeouts

Dealing with TCP in mobile IP

The main techniques used to get over TCP behavior in mobile networks include:– To split the e2e connection into two, namely wired and

wireless connection– To push the sender into “persist mode” during handover

by either the• Base Station (BS) (or another intermediate node) or• Mobile Host (MH) (by predicting imminent disconnect.)

– To improve local wireless retransmission– To speed up the TCP recovery after a handover

TCP approaches for cellular network

I-TCPSnoopM-TCPDelayed duplicate acks (dacks)EBSNWTCPFreeze-TCPTCP-probingFast retransmit

Indirect-TCP (I-TCP)

It splits the e2e connection into two parts:The wireless connection can even use another

transport protocol that suits wireless mediumDuring handover pcks from FH are cached at old BS

to be transferred to the new one It’s backward compatible with fixed network

Indirect TCP operation

I-TCP drawbacks

Maintains no e2e TCP semantics• BS acknowledges (ACK) pcks to the sender• It requires cooperation of application layer to provide

reliability

The BS can run out of bufferHigh processing at BSLatency to transfer state information can be

prohibitive

Snoop Protocol

Changes are restricted to BS and optionally to MH as well

E2e TCP semantics is preserved A (snoop) layer is added to the routing code at BS

which keep track of pcks in both directionsPcks meant to MH are buffered at BS and, if

needed, retransmitted in the wireless link It’s robust in dealing with multiple pck losses in a

single transmission window

Snoop Protocol functioning

Snoop Protocol drawbacks

Recovery from handover can be slow due to considerable state information to be handed over

Under long disconnection, sender times outEncrypted traffic cannot be handled

M-TCP Protocol

Also splits the connection into twoUnlike I-TCP, it maintains e2e TCP semanticsUnder long disconnection pushes the sender into

“persist mode” It avoids frequent transferring of state information

during handover It’s appropriated for environment with high cells

switching

M-TCP Protocol operation

M-TCP Protocol disadvantages

When sender transmit occasionally only, it will time out as the SH-agent does not send last ACK

Some retransmission overheadHigh processing at SHConsiderable complexityEncryption is not possibleReliability issues

TCP approaches cellular network

I-TCPSnoopM-TCPDelayed duplicate acksEBSNWTCPFreeze-TCPTCP-probingFast retransmit

Approaches comparison

Events/ feature

I-TCP Snoop M-TCP Delayed dacks

EBSN WTCP Freeze- TCP

TCP- probing

E2e semantics

no yes yes yes yes yes yes yes

Handle high BER

yes yes yes* yes yes yes no yes

Long disconnec.

may run out buffer

no yes no no no yes yes

Freq. disconnec

handov. costly

no may be costly

no no no yes yes

Req. interm node TCP

mode

yes yes yes no yes yes no no

Handle encryption

no no no yes no no yes yes

power saving

no no yes no no no yes yes *

Mobile multi-hop (Ad hoc) networks

Mobile multi-hop = mobile Ad hoc = ManetThis wireless framework is “wired infrastructure”

independentEach node is both end-user and router It’s appropriate for environment where wired network

cannot be used or is not desired

TCP challenges in manet networks

All those met in Cellular networks (1-hop)Environment under high route failures

• Frequent routing changes• Partitions

Multi-path routing needs to be consideredPower saving awareness is extremely necessaryCWND may not represents actual available BW

(route dependent)

Manet scheme

Approaches for TCP within manets

To lead the sender into “persist mode” or a similar one fix the RTO under route failuremake use of feedback information rely on cooperation from network and link layers improve link protocol recovery strategy

Some proposals

TCP-FELFN-based approachFixed RTOATCP

TCP-F

Based on feedback schemeSender to distinguish route failure from net. cong.Sender enters snooze state when receives RFN It resumes transmission when receives a RRN

Lack of RFN or RRN makes it performs like std TCP

ELFN-based approach

Employs the concept of Explicit Link Failure Notification (ELFN) techniques

Via ELFN sender is told about link and route failuresELFN carried by routing protocol itself (piggy-back)Upon receiving an ELFN TCP disables cong. control

• Instead it enters a “stand-by” mode timers frozen• Starts probing the network

Retransmission resumes at “full rate”Routing protocol (DSR) staled cache problem

degrades performance significantly

Fixed RTO

The exponential back off algorithm is disable so the sender retransmits at regular intervals

A 2nd RTO happening, indicates route lossThe scheme was evaluated for two on-demand

(AODV, DSR) and one proactive (ADV) routing algorithms

• On-demand ones performed well• Proactive didn’t experience improvement with this

approach

This approach is only feasible for wireless networks

ATCP

Std TCP is not modified InteroperabilityDefines ad hoc layer to work between layers 3 and 4ECN and ICMP “Dest. Unreach.” signaling are used

• ECN congestion• ICMP router failure (partition or re-computation)

ATCP spoofs TCP to obtain the following behavior:• High error Simply retransmit pck from TCP buffer• Route update delay Stop/resume with new CWND• Transient partition idem• Multi-path routing invoke CC

ICMP messages might not reach the sender

ATCP

Based on network feedback atcp puts sender into:• Persist mode• Congestion control mode• Retransmit mode

ATCP State transition at sender

AD Hoc approaches

Event/feature TCP-F ELFN-based Fixed RTO ATCP

Pck loss due to high BER

Not handled Not handled Not handledRetransmit without invoking CC

Route changes or network partition

RRN freezes sender state

ELFN freezes sender state

Upon 2nd timeout RTO is fixed

ICMP message puts sender in PM

Pack reordering

Not handled Not handled Not handledDone by ATCP layer

Congestion Not handled Not handled Not handledVia ECN TCP CC invoked quickly

CWNDOld CWND used

Old CWND used

Old CWND used

Reset for each new route

wired net.interoperation

Req. routing algorithm aware

Requires routing algorithm aware

Not smoothlyRequires ECN use on wired network

Power saving Not handled Not handled Not handled Not handled

Conclusions and outlooks

Any TCP improvements need consider interoperab.Power saving awareness is essentialCooperation among protocol layers seems to be

unavoidableFurther investigation on CWND on resumingBS tends to be part of encryption schemeAd hoc networks (multi-hop)

• TCP performance is Highly dependent on routing pr.» Geographical-based location protocol seems to be useful

• Link layer strategies to play a key role (high BER)• Longer periods of disconnection is highly likely