Offloading TCP Segmentation
Using Context Descriptors lets a
driver offload ‘TCP Segmentation’
as well as checksum calculations
What is ‘segmentation’?
MAC
header
IP
header
TCP
header
packet ‘data’ payload
The sender creates an Ethernet packet – but it’s longer than the LAN allows
– so our NIC can be programmed to ‘carve it up’ into smaller Ethernet packets
MAC
header
IP
header
TCP
header
partial payload
MAC
header
IP
header
TCP
header
partial payload
MAC
header
IP
header
TCP
header
partial payload
NIC’s segmentation parameters
•
•
•
•
•
HDRLEN = total length of packet-headers
PAYLEN = total length of data to be sent
DTALEN = length of packet-buffer content
MSS = Maximum Segment Size (w/o hdrs)
POPTS = Packet Options
Context descriptor (type 0)
63
48 47
40 39
TUCSE
TUCSO
MSS
HDRLEN
32 31
TUCSS
RSV
16 15
IPCSE
STA TUCMD
DTYP
=0
8
IPCSO
7
0
IPCSS
PAYLEN
DEXT=1 (Extended Descriptor)
Legend:
IPCSS (IP CheckSum Start)
IPCSO (IP CheckSum Offset)
IPCSE (IP CheckSum Ending)
PAYLEN (Payload Length)
TUCMD (TCP/UCP Command)
HDRLEN (Header Length)
TUCSS (TCP/UDP CheckSum Start)
TUCSO (TCP/UDP CheckSum Offset)
TUCSE (TCP/UDP CheckSum Ending)
DTYP (Descriptor Type)
STA (TCP/UDP Status)
MSS (Maximum Segment Size)
The TUCMD byte
7
IDE
6
5
4
SNAP
DEXT
(=1)
reserved
(=0)
3
RS
2
TSE
Legend:
IDE (Interrupt Delay Enable)
SNAP (Sub-Network Access Protocol)
DEXT (Descriptor Extension)
RS (Report Status)
TSE (TCP-Segmentation Enable)
IP (Internet Protocol)
TCP (Transport Control Protocol)
always valid
valid only when TSE=1
1
IP
0
TCP
Context descriptor (type 1)
63
48 47
40 39
32 31
16 15
8
7
ADDRESS
VLAN
POPTS
RSV
STA DCMD
DTYP
=1
DTALEN
DEXT=1 (Extended Descriptor)
Legend:
DTALEN (Data Length)
DTYP (Descriptor Type)
DCMD (Descriptor Command)
STA (Status)
RSV (Reserved)
POPTS (Packet Options)
VLAN (VLAN tag)
0
The DCMD byte
7
IDE
6
5
4
VLE
DEXT
(=1)
reserved
(=0)
3
RS
2
TSE
Legend:
IDE (Interrupt Delay Enable)
VLE (VLAN Enable)
DEXT (Descriptor Extension)
RS (Report Status)
TSE (TCP-Segmentation Enable)
IFCS (Insert Frame CheckSum)
EOP (End Of Packet))
always valid
valid only when EOP=1
1
IFCS
0
EOP
Context descriptor-pairs
• The sender accompanies each packet with
a Type 0 Context Descriptor and with (one
or more ) Type 1 Context Descriptor(s)
63
48 47
40 39
TUCSE
TUCSO
MSS
HDRLEN
32 31
TUCSS
RSV
16 15
IPCSE
STA TUCMD
DTYP
=0
8
IPCSO
7
0
IPCSS
PAYLEN
BASE ADDRESS
VLAN
POPTS
RSV
STA DCMD
DTYP
=1
DTALEN
Internet Protocol Header
Leave these fields equal to zero
Transport Control Protocol Header
Setup this field with ones-complement sum of words from ‘partial’ pseudo-header
The ‘partial’ pseudo-header
31
0
Source IP-address
Destination IP-address
ZERO
protocol
ID
ZERO
64-bit Statistical Counters
• The NIC’s statistical counters are intended
to operate in a ‘clear-upon-read’ manner,
but this requires reading the entire register
• A few of the NIC’s statistical counters are
64-bit registers (even though a CPU may
have to access them 32-bits at a time)
• Examples: TOTL/TOTH, TORL/TORH
• You must be sure to access both halves!