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Presenter: Yi-Quan Chen
CN group, CCU, Chia-Yi, Taiwan
NETWORK CODING INTRO AND
MODULE DESIGN THINKING
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
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With Network Coding
 Intermediate processes the incoming independent
information flows
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Benefits
Throughput
 Wireless resources
 Security
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Benefits(1)

Throughput
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Y uses all resources
Z uses all resources
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Benefits(2)

Wireless resources
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Benefits(2)
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Wireless resources
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Energy efficiency
Delay
Wireless bandwidth
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Benefits(3)
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Security
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Assume that adversary can wiretap a single path
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Someone can intercept
one of them
Can’t decode with single
symbols
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Challenges
Complexity
 Security
 Integration with existing infrastructure
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Challenges(1)

Complexity
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In wireless example
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Important question
Assessing
 Tradeoffs
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Node B : 1. additional memory requirements
2. has to perform operations
 The same with node A and C.
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Challenges(2 & 3)

Security

Integration with existing infrastructure
Without dramatic changes
 How to integrate in current network protocols?
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Banking transactions
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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.

“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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Crowcroft, SIGCOMM’06, Sep. 11-15, 2006
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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:
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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
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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
Edit the Mac
Add
Informati
on to
Coding
Header
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Packet
Deque &
Check
whether can
Code ?
XOR Coding
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The Queue In NS2
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THE PACKET IN QUEUE CLASS OF NS2
Packet Queue
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3
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1
PacketQueue Class in NS2
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I made a copy of each packet
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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
THE TRAFFIC FLOW IMAGINATION UNDER
NETWORK CODING
I have the Coding
Chance
RTS
PacketCTS
RTS
A ⊕B
CTS
CTS
RTS
CTS
RTS
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Promiscuous
Packet A
Packet A ⊕B
Packet B
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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。
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。

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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。
預計加入可以定期清除的機制。
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

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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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。
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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 ?
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