Mobile Transport Layer

Wireless Networks and Mobile Computing (COMP 5304)

1

Mobile Transport Layer

Mohammad Reza [email protected] University, Department of SCE , Jan 2009

Introduction

OSI Model vs. TCP/IP Model

Transport Layer Challenges

TCP/IP Protocol Overview

Slow Start & congestion Avoidance

TCP fast retransmit/fast recovery

TCP sender congestion control

TCP Protocol in Wireless Medium

TCP Improvements

TCP Last-HOP Enhancements

Possible Adaptations

Split Connection (Indirect TCP)

Snooping TCP

Explicit Loss Notification (ELN)

Mobile TCP

Forced Fast retransmit/fast recovery

Content

2

Link Level Retransmission

Transmission/time-out freezing

Selective retransmission

Transaction oriented TCP

Comparison of different approaches

TCP Improvements

Cross Layer solutions

Seamless Service Continuity

SCTP Protocol

Multi-homing

SHIM6

Host Identity Protocol

Conclusion

Questions

References

Content

3

•Optimization of transport layer protocols and their performance has big impact on data transmission

•Optimization and performance improvement of transport layer protocols, specially TCP is among highly interesting topics among academic researches.

•Several techniques have been proposed and implemented.

•Traditional TCP is not suitable for wireless medium.

•Joint layer optimization gains more benefits. (Cross Layer Design)

•Transport Layer survivability is important issue for a reliable network .

•SCTP , SHIM6 and HIP are key solutions for a redundant network.

Introduction

4

OSI Model vs. TCP/IP Model

5

Transport Layer defines three main standard transport protocols: TCP implements a reliable data-stream protocol (connection oriented) UDP implements an unreliable data-stream (connectionless) SCTP implements some new features -Both connection oriented and connectionless -Multi-homing

Transport Layer Challenges

6

TCP is optimized for wired networksAssumption

•Packet loss is mostly caused by congestionsCongestion Control and Avoidance

•drastic reduction of packet rate and slow start•larger timeout intervals for retransmissions

Assumption is not valid for wireless networks •Much higher packet error rates•packet losses during handovers and disconnections•Bundled packet losses•Necessary reaction:

•fast error recovery•no reduction of packet rate

Result in case of un-adapted TCP for wireless networks:•Data rates far below possible data rate•Slow error recovery(high response times)

TCP/IP Protocol Overview

7

Provides a reliable, in-order abstraction•Recover lost packets and detect/drop duplicates

•Detect and drop corrupted packets•Full-duplex: bi-directional data flow in same connection

Flow and congestion control•Flow control: sender will not overwhelm receiver

•Congestion control: sender will not overwhelm the network•Sliding window flow control

•Send and receive buffers•Congestion control done via adaptive flow control window size

•TCP/TP Model defined by IETF in RFC 1122

•TCP tries to trade off efficiency and being a “Good Network Citizen” Good Network Citizen: Judged by performance, reliability, availability[1]

•Vast majority of bytes transferred on Internet today are TCP based: Web, Mail, Peer-to-peer (Napster, Gnutella, FreeNet, KaZaa)

TCP protocol

8

Connection Establishment(3-way handshake)

1) Client sends SYN with initial sequence number ( ISN = X)

2) Server responds with its own SYN w/seq number Y and ACK of client ISN with X+1 (next expected byte)

3) Client ACKs server's ISN with Y+1

TCP/IP, Slow Start

9

Sender may not only overrun receiver, but may also overrun intermediate routers:

•No way to explicitly know router buffer occupancy, so infer congestion from packet losses.•Assumption :losses is due to congestion, i.e. intermediate routers have no available buffers

Sender maintains a congestion window:

•Max CW size is un-acknowledged data outstanding.•Successive ACKs from receiver cause CW to grow.

How CW grows based on which of 2 phases:•Slow-start: initial phase, CW exponentially grows•Continue up to threshold•Congestion avoidance: CW steady grows


10

TCP slow-start algorithm

•Sender calculates a congestion window for a receiver and a congestion threshold (i.e. 65535)

•Start with a congestion CW=1 Segment

•Exponential increase of the CW up to the congestion threshold.•Linear increase of the CW after threshold (Congestion avoidance)

•Missing acknowledgement causes the reduction of the congestion threshold to one half of the current CW •CW =1 segment•Repeat


11

•Sender calculates a congestion window for a receiver and a congestion threshold (i.e. 65535)

•Start with a congestion CW=1

•Exponential increase of the CW up to the congestion threshold.•Linear increase of the CW after threshold (Congestion avoidance)

•Missing acknowledgement causes the reduction of the congestion threshold to one half of the current CW (Additive increase, Multiplicative decrease) •Set CW =1 segment & Repeat

TCP slow-start and Congestion control algorithm


12


•TCP sends an acknowledgement only after receiving a packet.

•If a sender receives triple acknowledgements for the same packet, this is due to a gap in received packets at the receiver however, the receiver got all packets up to the gap and is actually receiving packets.

•Therefore, packet loss is not due to congestion. CW and congestion threshold is set to one half of the current CW, followed by congestion avoidance .(Do not use slow-start)

TCP sender congestion control

13

State Event TCP Sender Action Commentary

Slow Start (SS)

ACK receipt for previously unacked data

CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance”

Resulting in a doubling of CongWin every RTT

CongestionAvoidance (CA)

ACK receipt for previously unacked data

CongWin = CongWin+MSS * (MSS/CongWin)

Additive increase, resulting in increase of CongWin by 1 MSS every RTT

SS or CA Loss event detected by triple duplicate ACK

Threshold = CongWin/2, CongWin = Threshold,Set state to “Congestion Avoidance”

Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.“TCP Reno/SACK”

SS or CA Timeout Threshold = CongWin/2, CongWin = 1 MSS,Set state to “Slow Start”

Enter slow start.“TCP Tahoe”

SS or CA Duplicate ACK

Increment duplicate ACK count for segment being acked

CongWin and Threshold not changed

14

TCP Protocol in Wireless Medium

14

TCP on wireless works inefficiently!

• On wire links, TCP assumes losses if congestion happens.• On wireless links, TCP assumes congestion if packets are dropped

Typically wrong in wireless networks, here we often have packet loss due to transmission errors.

Furthermore, mobility (i.e. handoff) itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible.

However, TCP cannot be changed fundamentally due to the large base of installation in the fixed network.

TCP for mobility has to remain compatible.

TCP Improvements

15

It is difficult to create a “one size fits all” TCP for last-hop wireless networks enhancements upon application area.

B. Sardar and D. Saha, “A survey of TCP Enhancements for last-hop wirelessnetworks”, IEEE Comm. Surveys and Tutorials, 2006.

TCP Last-HOP Enhancements

16

B. Sardar and D. Saha, “A survey of TCP Enhancements for last-hop wirelessnetworks”, IEEE Comm. Surveys and Tutorials, 2006.

• Classification of available solutions for last-hop wireless enhancement

Possible Adaptations

17

Where in the path from sender to receiver to solve problem?

Split connectionOne TCP connection from wired end to base, another from base to wireless endProblem solved locallyBut solved at TCP layer (more semantics)

End-to-end Sender and receiver together addresses problem They address congestion loss They should also address medium loss

Link layer addresses problem Problem occurred in the link and thus should be solved there.


18

Break a single TCP connection from wired end to wireless end into:• TCP connection from wired end to base station• TCP connection from base station to wireless end• The wireless connection performs all the retransmissions in response to wireless losses.• Two TCP stacks in the base station.• Hosts in fixed part of network do not notice the characteristics of the wireless part.


19

Advantages•No changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work.•Transmission errors on the wireless link do not propagate into the fixed network.•Simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host.•A very fast retransmission of packets is possible, the short delay on the mobile hop is known.

Disadvantages•Loss of end-to-end semantics, an acknowledgement to a sender does now not any longer mean that a receiver really got a packet, foreign agents might crash•Higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent

Snooping TCP

20

“Transparent“ extension of TCP within the foreign agent

•Buffering of packets sent to the mobile host•Lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission)The foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs•Changes of TCP only within the foreign agent

Snooping TCP (Cont’)

21

Data transfer to the mobile host•FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out•Fast retransmission possible, transparent for the fixed network

Data transfer from the mobile host•FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH•MH can now retransmit data with only a very short delay

Integration of the MAC layer•MAC layer often has similar mechanisms to those of TCPthus, the MAC layer can already detect duplicated packets due to retransmissions and discard them.

Problems•Snooping TCP does not isolate the wireless link as good as I-TCP•Snooping might be useless depending on encryption schemes

Explicit Loss Notification (ELN)

22

This is an end-to-end approach.

Adds an Explicit Loss Notification (ELN) option to TCP acknowledgements.When packet dropped over wireless, Subsequent acknowledgements indicate non-congestion related loss occurred.

Upon receiving this information with duplicate acknowledgements, the sender may perform retransmissions without invoking the associated congestion-control procedures.

Potential Problems with End-to-End•Un-necessary duplicate acknowledgements sent all the way to source•Un-necessary retransmissions from source to destination

Mobile TCP- Performance Management

23

Special handling of lengthy and/or frequent disconnectionsM-TCP splits as I-TCP does

The mobile host must discover an interface switch or a handoff and notify the correspondent host. Then both endpoints are aware that subsequent observed losses of the data in transit can be attributed to mobility rather than congestion.

The sender marks the data that it is in its retransmit queue. A lost packet in this range triggers retransmission, but not congestion control mechanism.

Upon an interface switch, the sender resets the congestion window, slow start as though it were initiating a new connection.Advantages

maintains semantics, supports disconnection, no buffer forwardingDisadvantages

loss on wireless link propagated into fixed networkadapted TCP on wireless linkNot a good idea for heavy traffic.

Mobile TCP (Cont’)

24

Movement detection & notification

•When IP endpoints at MH changes, the changing its address is trapped by the kernel device control code.•All the sockets that use the affected interface are notified, and a switch detection routine is invoked. •MH notifies the CH via an option field in the next MTCP packet.•If operating on top of IPv6, MTCP can make use of IPv6 CH notification mechanism instead of using a special option.

Connection managementMTCP preserves end-to-end semantics by dynamically changing end-point address for ongoing connections.

DisadvantageMalicious host to “hijack” an ongoing TCP connection Authorization mechanism is suggested

Forced Fast retransmit/fast recovery

25

Change of foreign agent often results in packet loss TCP reacts with slow-start although there is no congestion

Forced fast retransmit•As soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose

•This forces the fast retransmit mode at the communication partnersadditionally, the TCP on the MH is forced to continue sending with the actual window size and not to go into slow-start after registration

AdvantageSimple changes result in significant higher performance

DisadvantageFurther mix of IP and TCP, no transparent approach

Link Level Retransmission

26

Handle the problem at the link level, that is where the loss occurred. Local retransmission instead of end-to-end retransmission

• Link-level timer much smaller (~20ms)• TCP timers larger (multiples of 500 ms)• Depends on end-to-end delay• Take advantage of the smaller link-level timer; it can be used to retransmit several times before the TCP timer goes off.• Such as HSDPA

Potential problems:

• “Incompatible” timers cause retransmission by both parties.• Competing retransmissions by the link and transport layersoften lead to significant performance degradation.• Packets may reach TCP receiver out-of-order.

Transmission/time-out freezing

27

Mobile hosts can be disconnected for a longer time•No packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or mix with higher priority traffic•TCP disconnects after time-out completely

TCP freezing•MAC layer is often able to detect interruption in advance•MAC can inform TCP layer of upcoming loss of connection•TCP stops sending, but does now not assume a congested link •MAC layer signals again if reconnected

Advantage•Scheme is independent of data

Disadvantage•TCP on mobile host has to be changed, mechanism depends on MAC layer


28

TCP acknowledgements are often cumulative•ACK n acknowledges correct and in-sequence receipt of packets up to nif single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth

Selective retransmission as one solutionRFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gapssender can now retransmit only the missing packets

Advantagemuch higher efficiency

Disadvantagemore complex software in a receiver, more buffer needed at the receiver


29

TCP phases•Connection setup, data transmission, connection release •Using 3-way-handshake needs 3 packets for setup and release, respectively•Even short messages need a minimum of 7 packets!

Transaction oriented TCP•RFC1644, T-TCP, describes a TCP version to avoid this overheadconnection setup, data transfer and connection release can be combined•Only 2 or 3 packets are needed

Advantage•Efficiency

Disadvantage•Requires changed TCP•Mobility not longer transparent

Comparison of different approaches for a “mobile” TCP

30

Approach Mechanism Advantages Disadvantages Indirect TCP splits TCP connection

into two connections isolation of wireless link, simple

loss of TCP semantics, higher latency at handover

Snooping TCP “snoops” data and acknowledgements, local retransmission

transparent for end-to-end connection, MAC integration possible

problematic with encryption, bad isolation of wireless link

M-TCP splits TCP connection, chokes sender via window size

Maintains end-to-end semantics, handles long term and frequent disconnections

Bad isolation of wireless link, processing overhead due to bandwidth management

Fast retransmit/ fast recovery

avoids slow-start after roaming

simple and efficient mixed layers, not transparent

Transmission/ time-out freezing

freezes TCP state at disconnect, resumes after reconnection

independent of content or encryption, works for longer interrupts

changes in TCP required, MAC dependant


retransmit only lost data very efficient slightly more complex receiver software, more buffer needed


combine connection setup/release and data transmission

Efficient for certain applications

changes in TCP required, not transparent

TCP Improvements

31

Initial research work•Indirect TCP, Snoop TCP, M-TCP, T/TCP, SACK, Transmission/time-out freezing, …

TCP over 2.5/3G wireless networksLearn to live with

•Data rates: 64 kbit/s up, 115-384 kbit/s down; asymmetry: 3-6, but also up to 1000 (broadcast systems), periodic allocation/release of channels

•High latency, high jitter, packet lossFine tuning today’s TCP, Suggestions

•Large (initial) sending windows, large maximum transfer unit, selective acknowledgement, explicit congestion notification, time stamp, no header compression

Already in use•i-mode running over FOMA•WAP 2.0 (“TCP with wireless profile”)

pRTT

MSSBW

*

*93.0 • max. TCP BandWidth

• Max. Segment Size• Round Trip Time• loss probability

TCP Improvements, Cross Layer Designs

32

Cross-Layer approaches violate the traditional layer architecture-> Creation of new interfaces-> Merging of adjacent layers-> Sharing of variables and parameters among multiple layers“Although layered architectures have served well for wired networks, they are not suitable for wireless networks”

Cross-layer design advantages•Exploits the interactions between layers•Promotes adaptability at all layers based on information exchange between layers

Cross-layer design disadvantages•Hard to characterize the interactions between protocols at different layers•Joint optimization across layers may lead to complex algorithms•Potential to destroy modularity

Cross Layer solutions for TCP improvement

33

Freeze TCP•To implement Freeze-TCP, the network stack needs to be aware of mobility (at least to some extent). In essence, some cross-layer (layers of the protocol stack) efforts and information exchange are needed. [3]

TCP_CL for IEEE 802.11 networks•By making slight modifications to the legacy IEEE 802.11 MAC and TCPprotocols, TCP-CL provides a significant improvement in the performance of TCP in multi-hop wireless environments. [4]

TCP enhancement scheme for OFDM networks •Link recovery scheme combined with Freeze-TCP based on sub-carrier reallocation and AMC can improve the performance of TCP greatly in OFDM networks. [5]

Seamless Service Continuity

34

•The 3GPP defines seamless service continuity as a transition between two wireless networks without user intervention and with minimal service disruptions (i.e., packet loss, etc.).

•This transitioning between wireless networks is what is known as handover.

Requirements for a mobility enabled transport protocol•The transport connection continues while changing the associated IP address (Multi-homing)

•The transport connection automatically switch over to a new IP address when the used path fails.

•IP addresses can be added to or removed from the transport connection without affecting the established association.

SCTP Protocol

35

SCTP (Stream Control Transport Protocol) was developed to provide higher reliability for signaling transport over the Internet.

•Key feature of SCTP is the multi-homing capability allowing immediate switch-over to a alternative path when anything fails.

•Automatic switch-over when a link breaks is exactly the function needed for hand-over.

•Dynamic establishment of new associations (adding new IP addresses - ADDIP) allows unlimited mobility.

Multi-homing

36

Objectives• Redundancy• Load sharing • Performance • Policy constraints

How• A host or a site has several network connections through: - Several internet service providers (ISPs) - Multiple network interfaces - Multiple internet address

Multi-homing Types

37

• Site LevelMulti-homing

• Host LevelMulti-homing

Multi-homing Problem & Solution

38

Problem• Network connection dynamically changes• Dynamic changes of the IP address will disturb the live sessions of the

transport-layer protocols.• Transport Layer survivability is not supported

Solution should satisfy the following:• During the dynamic changes, the established sessions should not be

disturbed.• Sessions at transport layer should not be aware of routes changes at

the IP layer• Shim6: Site Multi-homing by IPv6 Intermediation • HIP: Host Identity Protocol

SHIM6

39

Site Multi-homing by IPv6 Intermediation (SHIM6)

• Shim6 is a protocol being developed in the Internet Engineering Task Force (IETF)

• Provides a host-based solution to allow IPv6 sites to multi-home for load sharing, redundancy, and fault tolerance.

• Shim6 uses the initial IP address as an endpoint identity at the transport layer, and multiple locators at the IP layer to perform multi-homing. • The protocol includes components for

securely binding locator sets together to prevent address hijacking

Host Identity Protocol (HIP)

40

Host Identity Protocol (HIP)

• Transport protocols are bound to IP addresses and disconnect when an address changes.

• HIP defines a new global Internet name space.

• HIP uses a defined public key at the transport layer as an endpoint identity, and multiple locators at the IP layer to perform multi-homing.

• With HIP, the transport layer operates on Host Identities instead of using IP addresses as

endpoint names.

• At the same time, the network layer uses IP addresses.

Conclusion

41

•Traditional TCP is not optimized for wireless environment

•Improvement is required on transport layer protocols, mainly TCP to achieve higher performance.•End-to-End, Split connection and Link layer solutions investigated.•Versus layers joint optimization gains more benefits.

•Cross Layer Design exploit dependency between stack layers to achieve some performance gains

•SCTP is a general purpose reliable transport protocol for the Internet, for real-time and non real-time traffic.

•Multi-homing is achieved by Shim6 and Hip.

Questions

42

1) Why TCP on wireless works inefficiently?On wireless links, TCP assumes congestion if packets are dropped due to transmission errors.

Furthermore, mobility (i.e. handoff) itself can cause packet loss. For example if a mobile node roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible.

2) List three methods of TCP optimization solutions.1)Indirect TCP 2)Snooping TCP 3) Mobile-TCP4) Fast retransmit/ fast recovery 5) Transmission/ time-out freezing6) Selective retransmission 7)Transaction oriented TCP

3) What is multi-homing? Explain its two famous solutions.Multi-homing is a technique to increase the reliability of the Internet connection for an IP network.

Multi-homing is generally used to eliminate network connectivity as a potential single point of failure. Multi-homing can be in site or host level.

Multi-homing solutions:-Site Multi-homing by IPv6 Intermediation (Shim6) uses the initial IP address as an endpoint

identity at the transport layer, and multiple locators at the IP layer to perform multi-homing. -HIP (Host Identity Protocol) uses a defined public key at the transport layer as an endpoint

identity, and multiple locators at the IP layer to perform multi-homing.

References

43

[1] Douligeris,N. and N. Serpanos, D., “Network Security: Current Status and Future Directions”, John Wiley and Sons, 2007[2] http://www.jochenschiller.de/ Schiller,J.,“Mobile Communications Chapter 9: Mobile Transport Layer”,[3] Sardar, B. and Saha, D.,“A survey of TCP enhancements for last-hop wireless networks“, IEEE Communications Surveys & Tutorials, Vol. 8, No. 3, 3rd Quarter 2006[4] Cheng, R. and Lin, H.“ A cross-layer design for TCP end-to-end performance improvement in multi-hop wireless networks “,Computer Communications 31 ,3145–3152, 2008[5] Jun, W., Xi, F., Quan,C., Min,J., Ping,Z.,“ A cross-layer wireless TCP enhancement scheme in OFDM network “, TENCON, 2006

Documents

Mobile Transport Layer