%Network Subsystem
%Thadeu Cascardo

# Socket Buffers

* include linux/skbuff.h
* struct sk\\_buff
	- struct net\\_device *dev
* alloc\\_skb(len, gfp)
* dev\\_alloc\\_skb(len) - uses ATOMIC, reserves PAD
* netdev\\_alloc\\_skb(netdev, len) - uses ATOMIC, since 2.6.18
* kfree\\_skb, dev\\_kfree\\_skb

# Diagram

* head - start of allocated buffer
* end - end of allocated buffer
* data - start of data, giving space after headroom
* tail - end of used space
* headroom - space between head and data
* tailroom - space between tail and end

# Socket Buffers operations

* skb\\_put(skb, len) - adds data to end of skb
* skb\\_push(skb, len) - adds data to start of skb
* skb\\_pull(skb, len) - allocates headroom
* skb\\_headroom and skb\\_tailroom
* skb\\_reserve - only allowed for empty buffer, reserves headroom
* skb\\_orphan - release it from its socket holder

# Network Device

* include linux/netdevice.h
* struct net\\_device
	- char name[]
	- features
	- stats
	- netdev\\_ops
	- ethtool\\_ops
	- header\\_ops
	- flags
	- mtu
	- type
	- hard\\_header\\_len

# Network Device Setup

* alloc\\_netdev(szpriv, name, setup)
* setup function
* include linux/etherdevice.h
* ether\\_setup
* alloc\\_etherdev
* register\\_netdev
* unregister\\_netdev
* free\\_netdev

# Network Device Operations

* struct net\\_device\\_ops
* ndo\\_init
* ndo\\_open
	- should call netif\\_start\\_queue
* ndo\\_stop
	- should call netif\\_stop\\_queue

# Network Device Address

* struct net\\_device
	- dev\\_addr
* random\\_ether\\_addr
* struct net\\_device\\_ops
	- ndo\\_set\\_mac\\_address
* eth\\_mac\\_addr

# Transmission

* ndo\\_start\\_xmit
* called with a held lock

# Limits on transmission

* When TX buffers are full, xmit may call netif\\_stop\\_queue
* Should arrange to get netif\\_wake\\_queue called after TX buffers are free
  again

# Transmission timeout

# NAPI

# Changes in net device

* Use netdev\\_priv, no priv anymore
* struct net\\_device\\_ops introduced in 2.6.29, with compatibility provided
  for drivers
* Compatibility removed in 2.6.31
* netdev\\_tx\\_t: NETDEV\\_TX\\_OK, NETDEV\\_TX\\_BUSY, NETDEV\\_TX\\_LOCKED

# Other recent changes

* Some members moved to netdev\\_queue to increase cache-line usage
* GRO/GSO
* Multi-queue support
* RPS - Packet Steering