1 / 22

Checksum ‘offloading’

Checksum ‘offloading’. A look at how the Pro1000 NICs can be programmed to compute and insert TCP/IP checksums. Network efficiency. Last time (in our ‘nictcp.c’ demo) we saw the amount of work a CPU would need to do when setting up an ethernet packet for transmission with TCP/IP protocol format

johnjhall
Download Presentation

Checksum ‘offloading’

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Checksum ‘offloading’ A look at how the Pro1000 NICs can be programmed to compute and insert TCP/IP checksums

  2. Network efficiency • Last time (in our ‘nictcp.c’ demo) we saw the amount of work a CPU would need to do when setting up an ethernet packet for transmission with TCP/IP protocol format • In a busy network this amount of packet- computation becomes a ‘bottleneck’ that degrades overall system performance • But a lot of that work can be ‘offloaded’!

  3. The ‘loops’ are costly • To prepare for a packet-transmission, the device-driver has to execute a few dozen assignment-statements, to set up fields in the packet’s ‘headers’ and in the Transmit Descriptor that will be used by the NIC • Most of these assignments involve simple memory-to-memory copying of parameters • But the ‘checksum’ fields require ‘loops’

  4. Can’t ‘unroll’ checksum-loops • One programming technique for speeding up loop-execution is known as ‘unrolling’, to avoid the ‘test-and-branch’ inefficiency: • But it requires knowing in advance what number of loop-iterations will be needed int sum = 0; sum += wp[0]; sum += wp[1]; sum += wp[2]; … sum += wp[99];

  5. The ‘offload’ solution • Modern network controllers can be built to perform TCP/IP checksum calculations on packet-data as it is being fetched from ram • This relieves a CPU from having to do the most intense portion of packet preparation • But ‘checksum offloading’ is an optional capability that has to be ‘enabled’ – and ‘programmed’ for a specific packet-layout

  6. ‘Context’ descriptors • Intel’s Pro1000 network controllers employ special ‘Context’ Transmit-Descriptors for enabling and configuring the ‘checksum-offloading’ capability • Two kinds of Context Descriptor are used: • An ‘Offload’ Context Descriptor (Type 0) • A ‘Data’ Context Descriptor (Type 1)

  7. Context descriptor (type 0) 63 48 47 40 39 32 31 16 15 8 7 0 TUCSE TUCSO TUCSS IPCSE IPCSO IPCSS MSS HDRLEN RSV STA TUCMD DTYP =0 PAYLEN DEXT=1 (Extended Descriptor) Legend: IPCSS (IP CheckSum Start) TUCSS (TCP/UDP CheckSum Start) IPCSO (IP CheckSum Offset) TUCSO (TCP/UDP CheckSum Offset) IPCSE (IP CheckSum Ending) TUCSE (TCP/UDP CheckSum Ending) PAYLEN (Payload Length) DTYP (Descriptor Type) TUCMD (TCP/UCP Command) STA (TCP/UDP Status) HDRLEN (Header Length) MSS (Maximum Segment Size)

  8. The TUCMD byte 7 6 5 4 3 2 1 0 IDE SNAP DEXT (=1) reserved (=0) RS TSE IP TCP 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

  9. Context descriptor (type 1) 63 48 47 40 39 32 31 16 15 8 7 0 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)

  10. The DCMD byte 7 6 5 4 3 2 1 0 IDE VLE DEXT (=1) reserved (=0) RS TSE IFCS EOP 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

  11. Our usage example • We’ve created a module named ‘offload.c’ which demonstrates the NIC’s checksum-offloading capability for TCP/IP packets • It’s a modification of our earlier ‘nictcp.c’ character-mode device-driver module • We have excerpted the main changes in a class-handout – the full version is online

  12. Data-type definitions // Our type-definition for the ‘Type 0’ Context-Descriptor typedef struct { unsigned char ipcss; unsigned char ipcso; unsigned short ipcse; unsigned char tucss; unsigned char tucso; unsigned short tucse; unsigned int paylen:20; unsigned int dtyp:4; unsigned int tucmd:8; unsigned char status; unsigned char hdrlen; unsigned short mss; } TX_CONTEXT_OFFLOAD;

  13. Definitions (continued) // Our type-definition for the ‘Type 1’ Context-Descriptor typedef struct { unsigned long long base_addr; unsigned int dtalen:20; unsigned int dtyp:4; unsigned int dcmd:8; unsigned char status; unsigned char pkt_opts; unsigned short vlan_tag; } TX_CONTEXT_DATA; typedef union { TX_CONTEXT_OFFLOAD off; TX_CONTEXT_DATA dat; } TX_DESCRIPTOR;

  14. Our packets’ layout Ethernet Header (14 bytes) 14 bytes IP Header (20 bytes) HDR CKSUM (no options) 10 bytes TCP Header (20 bytes) TCP CKSUM (no options) 16 bytes Packet-Data (length varies)

  15. How we use contexts • Our ‘offload.c’ driver will send a ‘Type 0’ Context Descriptor within ‘module_init()’ txring[ 0 ].off.ipcss = 14; // IP-header CheckSum Start txring[ 0 ].off.ipcso = 24; // IP-header CheckSum Offset txring[ 0 ].off.ipcse = 34; // IP-header CheckSum Ending txring[ 0 ].off.tucss = 34; // TCP/UDP-segment CheckSum Start txring[ 0 ].off.tucso = 50; // TCP/UDP-segment Checksum Offset txring[ 0 ].off.tucse = 0; // TCP/UDP-segment Checksum Ending txring[ 0 ].dtyp = 0; // Type 0 Context Descriptor txring[ 0 ].tucmd = (1<<5)|(1<<3); // DEXT=1, RS=1 iowrite32( 1, io + E1000_TDT ); // give ownership to NIC

  16. Using contexts (continued) • Our ‘offload.c’ driver will then use a Type 1 context descriptor every time its ‘write()’ function is called to transmit user-data • The network controller ‘remembers’ the checksum-offloading parameters that we sent during module-initialization, and so it continues to apply them to every outgoing packet (we keep our same packet-layout)

  17. Sequence of ‘write()’ steps • Adjust the ‘len’ argument (if necessary) • Copy ‘len’ bytes from the user’s ‘buf’ array • Prepend the packet’s TCP Header • Insert the pseudo-header’s checksum • Prepend the packet’s IP Header • Prepend the packet’s Ethernet Header • Initialize the Data-Context Tx-Descriptor • Give descriptor-ownership to the NIC

  18. The TCP pseudo-header • We do initialize the TCP Checksum field, (but this only needs a short computation) • The one’s complement sum of these six words is placed into ‘TCP Checksum’ Zero Protocol-ID (= 6) TCP Segment-length Source IP-address Destination IP-address

  19. Setting up the Type-1 Context int txtail = ioread32( io + E1000_TDT ); txring[ txtail ].dat.base_addr = tx_desc + (txtail * TX_BUFSIZ); txring[ txtail ].dat.dtalen = 54 + len; txring[ txtail ].dat.dtyp = 1; txring[ txtail ].dat.dcmd = 0; txring[ txtail ].dat.status = 0; txring[ txtail ].dat.pkt_opts = 3; // IXSM=1, TXSM=1 txring[ txtail ].dat.vlan_tag = vlan_id; txring[ txtail ].dat.dcmd |= (1<<0); // EOP (End-Of-Packet) txring[ txtail ].dat.dcmd |= (1<<3); // RS (Report Status) txring[ txtail ].dat.dcmd |= (1<<5); // DEXT (Descriptor Extension) txring[ txtail ].dat.dcmd |= (1<<6); // VLE (VLAN Enable) txtail = (1 + txtail) % N_TX_DESC; iowrite32( txtail, io + E1000_TDT );

  20. In-class demonstration • We can demonstrate checksum-offloading by using our ‘dram.c’ device-driver to look at the packet that is being transmitted from one of our ‘anchor’ machines, and to look at the packet that gets received by another ‘anchor’ machine • The checksum-fields (at offsets 24 and 50) do get modified by the network hardware!

  21. In-class exercise • The NIC can also deal with packets having the UDP protocol-format – but you need to employ different parameters in the Type 0 Context Descriptor and arrange a ‘header’ for the UDP segment that has a different length and arrangement of parameters • Also the UDP protocol-ID is 17 (=0x11)

  22. UDP Header UDP header: Traditional ‘Big-Endian’ representation

More Related