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NETWORK CODING INTRO AND MODULE DESIGN THINKINGPresenter: Yi-Quan Chen

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OUTLINE

What is Network coding? Main Reference of the Module Design Thinking Module Design Flowchart & Thinking

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WHAT IS NETWORK CODING?

emerged in 2000 Without Network Coding

Independent information are kept separate Intermediate nodes only forward it

With Network Coding Intermediate processes the incoming independent

information flows

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BenefitsBenefits

Throughput Wireless resources Security

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Benefits(1)Benefits(1) Throughput

Y uses all resources Z uses all resources

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Benefits(2)Benefits(2) Wireless resources

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Benefits(2)Benefits(2)

Wireless resourcesEnergy efficiencyDelayWireless bandwidth

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Benefits(3)Benefits(3)

SecurityAssume that adversary can wiretap a single path

Someone can intercept one of them

Can’t decode with single symbols

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ChallengesChallenges

Complexity Security Integration with existing infrastructure

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Challenges(1)Challenges(1)

ComplexityIn wireless example

Node B : 1. additional memory requirements

2. has to perform operations The same with node A and C.

Important questionAssessingTradeoffs

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Challenges(2 & 3)Challenges(2 & 3)

SecurityBanking transactions

Integration with existing infrastructureWithout dramatic changesHow to integrate in current network protocols?

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Main Reference of the Module Design Thinking

“XORs in The Air: Practical Wireless Network Coding”, S. Katti, H. Rahul, W. Hu, D. Katabi, M. Medard, and J. Crowcroft, SIGCOMM’06, Sep. 11-15, 2006

“The importance of being opportunistic: Practical network coding for wireless environments”, S. Katti, D. Katabi, W. Hu, H. S. Rahul, and M. M´edard., 2005.

“Network Coding Fundamentals”, Christina Fragouli and Emina Soljanin, 2007

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COPE Introduction

COPE inserts a coding shim between the IP and MAC layers. To detect coding opportunities and exploits them to forward

multiple packets in a single transmission COPE leads to larger bandwidth savings Is based on two key principles:

Dispose of the p2p abstraction and embraces the broadcast nature of the wireless channel

Employs network coding

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COPE overview

Incorporate three main techniques:

Opportunistic Listening

Opportunistic Coding

Learning Neighbor State

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Opportunistic Listening

Sets the nodes in promiscuous mode Store the overheard packets for a limited period T Each node broadcast reception report to tell its

neighbors which packets it has stored A node that has no data packets to transmit periodically

sends the reception reports in special control packets

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Opportunistic Coding

The key question is what packets to code together to maximize throughput.

It should aim to maximum the number of native packets delivered in a single transmission, while ensuring that each intended nexthop has enough information to decode its native packet

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Opportunistic Coding

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Learning Neighbor State

Reception report may get lost in collisions The node may has already made a suboptimal coding decision Guess ! COPE estimates the probability that a particular neighbor has

a packet as the delivery probability of the link If guess wrong, the relevant native packet is retransmitted,

potentially encoded with a new set of native packets

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Making it work

Packet Coding Algorithm Packet Decoding Pseudo-broadcast Hop-by-Hop ACKs and retransmissions Preventing TCP packet reordering

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Packet Coding Algorithm

Each node has a FIFO queue of packets to be forwarded, which called the output queue.

For each neighbor, the node maintains two per-neighbor virtual queues, one for small packet, and the other for large packets

The node keeps a hash table, packet info, that is keyed on packet-id, the table indicates the probability of each neighbor having that packet

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Packet Decode

Each node maintains a packet pool, in which it keeps a copy of each native packet it has received or sent out.

The packets are stored in a hash table keyed on packet id Is garbage collected every few second

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Pseudo-Broadcast

Piggybacks on 802.11 unicast and benefits from its reliability and backoff mechanism

Promiscuous mode The link-layer destination field is set to the MAC

address of one of the intended recipients XOR-header listing all nexthops of the packet Does not completely solve the reliability problem

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Hop-by-Hop ACKs and Retransmissions

The sender gets syn-ACK only from the link-layer destination of the packet

COPE may optimistically guess that a nexthop has enough information to decode an XOR-ed packet, when it actually does not.

The sender expect the nexthops of an XOR-ed packet to decode it, and ack it in Ta

Nexthop receives an encoded packet decodes it and schedule an acl event

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THE COPE DESIGN IMPLEMENTATION FLOWCHART

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Packet Deque &

Check whether can

Code ?

Add Informati

on to Coding Header

Edit the Mac

XOR Coding

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The Queue In NS2

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THE PACKET IN QUEUE CLASS OF NS2

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5 4 3 2 1

5 4 3 2 1

PacketQueue Class in NS2

I made a copy of each packet

Packet Queue

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THE HEADER CONTENT OF COPE

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THE PACKET STRUCTURE OF NS2

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Insert COPE Header

Coding Header body

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THE TRAFFIC FLOW IMAGINATION UNDER NETWORK CODING

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RTS CTSPacket A RTSCTS Packet BRTSCTSRTS CTS

I have the Coding Chance

Packet A ⊕BPromiscuous Packet A ⊕B

WHAT I HAVE DONE ? 1. 封包在 node 中的資訊，對我比較重要的如：

next_hop, Prev_hop, 以及時 間…等，取出。

2. 寫完 queue.h 內，有關 packetqueue 的註解，並在priqueue.cc 內，把封包的資訊 dump 出來，看看是否能達到我要找的東西。

3. 在 queue.h 內，加入了封包進入 queue 後，封包備份儲存的動作，在此不考慮帶有 routing 資訊、 mac資訊及 ack 的封包。封包可以儲存在自己的 queue 中。

預計加入可以定期清除的機制。

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4. 目前構想的封包 enque 及 deque 程序： enque 的部分：

封包 enque 後， check 是否是 xor 的封包。if (xored...)then (check the coding header to find 自己是否是在 destination 端欄位中。是的話， decode it 。否則就只是單純轉送。 )copy decode 後的 pure packet ，並刪除重複的部分。

deque 的部分：要 deque 前，先用 head_ 封包，與備份的封包做 check ，確保兩個封包的 prev_hop 與 next_hop 不同， ( 這樣代表封包來自不同方向 )if (prev_hop 相同 )then (head_ 不變， next_ 向後移一位，再做 check 。 )else if (next_hop 是否為自己 )如果不是， then coding it 。

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WHAT I HAVE DONE - 2 加入了單層的 Header 。 1. 寫了兩支 cope.cc cope.h ，用來新增 COPE header 這個物件，並成功列印出 hdr_cope class 下

的資料，資料也有填進去， source 欄位填的 OK 。但是目前比較疑惑的是，要如何去實現PAPER 中的 pkt_map 與 ack_map 的部 分。

2. 尚未測試類似 MPLS 中的 push Header 的功能是否可以正確動作，必須另外寫幾個函式，來做到這些事情。

3. ns2 中的 TCP ，做的事情跟實際中的 TCP 不太一樣， one-way TCP 在回 ACK 的時候，就是純的 ACK ，沒有辦法使用 piggyback 的方式，把要送的資料也回填回去給剛剛送資料來的source 。不知道這個部分會不會影響到往後的實現。

4. 目前有修改過的原始程式碼如下： ns-packet.tcl     queue.h     priqueue.cc     cope.h     cope.cc     packet.h

5. 找到一份大陸人寫的講述有關 packet class 的文件，裡面有詳細的講出說在各層中，哪個函式會填入各層對應的資料，我想，填資料的部分，應該可以 SURVEY 看看這份文件。

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Question ?