TCP/IP stack in SystemVerilog by hypernyan


Modular Ethernet HDL project.


This project's goal is to create a silicon independent, compact, modular, yet easy to implement HDL for Ethenet communications. It is written in SystemVerilog with extensive use of interfaces, structs and other synthesyzable constructs. The code is completely off any licenses and may be used by anyone for any purpose.


  • Functional TCP/IP stack capable of listening and connecting;
  • ARP and ICMP;
  • Configurable number of TCP clients/servers;
  • Generic SystemVerilog;
  • 1-Gbit GMII MAC.


  • No SACK;
  • No compliance to RFC;
  • Limited simulation and hardware tests;
  • Not tested with Xilinx though compiled successfully (Vivado 2019.1).

Known issues

  • Problems with GMII timing constraints on lower speed devices (Cyclone V, Cyclone 10 LP)
  • If TCP_TX_QUEUE_DEPTH is set higher than 14 bits, tx_queue will read 8'h00 regardless of data stored. This is probably due to timing. Observed on both Cyclone V and Cyclone 10 LP. This problem somewhat limits throughput due to low buffer size.

How to use

To use this core, you'll need a 10/100/1000 Mbit GMII/RGMII capable PHY. One popular example is Realtek RTL8211 which doesn't even need any MDIO configuration to work. The top-level entity is eth_vlg described in It is clocked by a single 125MHz clock phy_rx_clk. The phy_tx_clk is a loopback from it. User interface is also clocked by phy_rx_clk.

Configuring the core

The top-level parameters provide flexibility in configuring the core.

Table 1: | Parameter | Description | Default | |:--------------------|:---------------------------------------------------|:--------| |MAC_ADDR |48-bit local MAC |0 | |IPV4_ADDR |32-bit local IPv4 |0 | |N_TCP |Number of TCP cores |1 | |MTU |Maximum Transmission Unit |1400 | |TCP_RETRANSMIT_TICKS |Interval between retransmissions |1000000 | |TCP_RETRANSMIT_TRIES |Tries to retransmit before abort |5 | |TCP_RAM_DEPTH |Address width of 8-bit TX buffer RAM |12 |
|TCP_PACKET_DEPTH |Address width of TX RAM for packets information |8 |
|TCP_WAIT_TICKS |Assemble a packet and send it if no new bytes |100 |

Interfacing the core

The top-level ports provide real-time connection control and monitoring.

Table 2: | Port | in/out |Width | Description | |:-------------------|:------------|:-------|:--------------------------------------------------------| | phy_rx |in |1 |Receive part of GMII interface | | phy.out phy_tx |out |1 |Transmit part of GMII interface | | clk |in |1 |125 MHz clock (same as phy_rx.clk) | | rst |in |1 |Acive-high reset synchronous to clk | | udp_tx |in | |Not used | | udp_rx |out | |Not used | | tcp_din, tcp_vin |in |N_TCP |Outcoming TCP stream. User interface | | tcp_cts |out |N_TCP |Clear-to-send. Must deassert vin 1 tick after it goes '1'| | tcp_dout, tcp_vout |out |N_TCP |Incoming TCP stream. User interface | | rem_ipv4 |in |N_TCP |target 32-bit IPv4 address | | loc_port |in |N_TCP |local port to listen to or establish connecion | | rem_port |in |N_TCP |remote port to establish connection to | | connect |in |N_TCP |connection trigger. assert to try to connect | | connected |out |N_TCP |becomes '1' when connection is established | | listen |in |N_TCP |transfer the core into listen mode |


The logic of the stack is seperated based on the protocol each module handles. Here, handling a protocol means to parse and remove it's header when receiving and attach a new generated header when transmitting. For example, IPv4 handler is responsible to parse the IPv4 header. Each handler module then (if packet received successfully) outputs valid signal and information about the packet to other modules. The protol handler are:

  • MAC (
  • ARP (
  • IPv4 (,
  • ICMP (
  • TCP (

For Transmit path, there are sometimes several handlers interfacing a signle one, for example IPv4 interfaces with ICMP, UDP and TCP. For this, a generic arbiter is used. The arbiter indepenently receives packets from several sources and processes them to a single handler in order.

Receive path:          
        +=====>ARP Rx<--->ARP_ABLE
MAC====>|                   +====> ICMP Rx
        +==== =>IPv4 Rx====>|====> UDP Rx
                            +====> TCP Rx ---> TCP_CORE ---> USER LOGIC

Transmit path:          
        |A|<===ARP Tx<--->ARP_ABLE
MAC<====|R|                  |       |A|<=== ICMP Tx
        |B|<==============IPv4 Tx<===|R|<=== UDP Tx
                                     |B|<=== TCP Tx <--- TCP_TX_QUEUE <--- USER LOGIC

<===> handler-to-handler
<---> specific logic
ARB - arbiter (bug_mng)


MAC interfaces PHY outside the FPGA with a GMII interface and with subsequent logic with mac interface defined in mac_vlg and MAC header information. FCS is handled by MAC too.


When receiving a packet, MAC checks for correct preamble and delimiter. After that, if the packet's destination MAC is equal to local MAC set by the MAC_ADDR parameter or if it's broadcast (xFF:...), mac_vlg_rx fills the MAC header fields: destination and source MAC addresses and Ethertype and passes the stripped packet and header information to ARP and IPv4. The data is passed with four signals: data, valid, sof and eof. It is in fact the same packet from PHY but with Preample, Ethernet header and FCS removed.

MAC handler continiously recalculates the 32-bit Frame Check Sequence and compares it with the last four bytes received. If equal, MAC generates eof indicating that packet reception is complete. MAC also passes information extracted from Etherent header of the received packet in a mac_hdr structure. Based on Ethertype, for instance, subesquent handlers are triggered.


The transmit part interfaces ARP and IPv4 through buf_mng arbiter. After writing a packet to buf_mng, it will trigger mac_vlg_tx to start creating a packet of ARP or IPv4 Ethertype. Ethernetheader as well as Preamble, SFD and FCS are appended to a frame and passed to PHY via phy interface.


ARP is responsible for binding IPv4 address with MAC. Knowing target IP address, one can acquire it's MAC with ARP. ARP is composed of several modules:

  • arp_vlg_rx to parse ARP packets;
  • arp_vlg_tx to generate ARP packets;
  • arp_table to store IPv4/MAC pairs and generate ARP requests. ARP will reply to requests automatically, but it's also connected to IPv4. Each time IPv4 is generating a packet, it's logic makes a request to ARP. ARP then scans it's memory for requested IPv4 address and if such an entry isn't found, makes several attempts send ARP requests. If successfull, ARP returns target MAC to IPv4 transmission logic.


ipv4_vlg_top module contains ipv4 related logic including ICPM, UDP and TCP. ipv4_vlg_top directly interfaces user logic with raw TCP streams and TCP control/status ports. ipv4_vlg_top transmission logic is based on buf_mng arbiter module which asynchronously receives packets from ICMP, UDP and TCP and queues them for transmission to MAC.


tcp_vlg contains all logic responsible for TCP operation. It consists of several modules:

  • tcp_vlg_rx Receiver handler. Parses incoming packets
  • tcp_vlg_tx Transmit handler. Composes packets to transmit
  • tcp_vlg_tx_queue is responsible for retransmission logic and payload checksum calculation. It stores raw TCP data from user logic in 8-bit RAM. Information such as length, sequence number and checksum is stored for each packet in a seperate pkt_info RAM. Information stored also includes retransmission-specific information. When a new packet is pending for transmission, space in allocated in transmission queue data RAM and a corrseponding entry is created in info RAM with all necssary information. The latter RAM is constantly scanned through and logic keeps track of unacked packets and tries to retransmit them after time based on TCP_RETRANSMIT_TICKS. tcp_vlg_tx_queue also generates a cts signal. The cts logic allows user to stop data stream 1 tick after ctr goes low to avoid data loss (or wiring cts signal to user logic instead of using a trigger).
  • tcp_vlg_server TCP core. The state machine here handles connection establishment, and termination. The user control interface is implemented here described in Table 2. Depending on run-time signals connect and listen, server can connect by IPv4 or listen for incoming connection.

Server logic

After reset, the FSM is in idle state. Asserting listen signal will transition to a state which listens to incoming connection, that is, a packet with SYN flag set targeting local TCP port. Asserting connect signal will generate a TCP packet targeting rem_port TCP port and rem_ipv4 IPv4 address. After either signal is asserted, a structure called Transmission Control Block or TCB is filled with information about connection being created. After successfully performing the three-way handshake, the FSM results is in connected state in which data transfer takes place. TCB rem_ack and rem_seq are being updated with TCP packets received and loc_ack and loc_seq are being updated from internal logic.

Transmission queue and retransmissions

TCP receiver does not need to acknowledge each packet, instead it may cknowledge multiple packets if all were received successfully. This implies that sender stores all unacknowledged data and may retransmit unacknowledged packets. Transmission queue stores user data in an 8-bit RAM which is filled with new user data and freed as remote acknowledge progresses. A seperate pkt_info RAM is also filled with information about each packet when it is formed: sequence number, length and checksum as well as retransmission timer and retransmissions count. After being formed, the packets may only be transmitted as written without the ability to split and/or combine them. This limitation is imposed to avoid recalculating TCP payload checksum. There are several conditions in which the packet is considered complete and is queued for transmission:

  • Data RAM full;
  • MSS reached;
  • No new data for TCP_TX_TIMEOUT ticks (default: 50);
  • Packet forced to be queued without waiting for TCP_TX_TIMEOUT by asserting tcp_snd.

Queue's FSM continiously scans pkt_info RAM for entries with present flag set. This flag indicates that a packet is still stored in data RAM because it is not acked by the receiver yet. After finding and entry with present flag set, the Queue FSM processes the packet in one of the following ways:

  • Packet's sequence number plus packet's length is less or equal than remote acknowledgment number indicates that packet is acked. Present flag will be cleared releasing space for next packets in data RAM and pkt_info RAM.
  • Packet's sequence number plus packet's length higher than remote acknowledgment number and retransmit timer less than timeout value indicates that packet is yet unacknowledged, but there is still time for remote device to acknowledge it before retransmission occurs. Retransmit timer is incremented and next packet is processed.
  • Packet's sequence number plus packet's length higher than remote acknowledgment number and retransmit timer equal to timeout value indicates that packet is unacknowledged and retransmission timeout is reached. Retransmission occurs, number of retransmission tries is incremented and retransmit timer is reset to zero. Note that when a new entry is created, retransmit timer is preloaded with timeout value, so the first time a new packet is transmitted without waiting for retransmission timeout.
  • Packet's retransmission tries reaching retransmission limit will trigger forced connection termination.

Transmission queue is directly wired to user transmission logic with tcp_din, tcp_vin, tcp_cts and tcp_snd.


Simple testbench is provided to verify functionality. Run modelsim.bat. (modelsim.exe location should be in PATH) 2 modules are instantiated, for client and server. After ARP, 3-way handshake is performed and data is streamed in both directions.


The code was compiled with for

  • Cyclone V (5CEBA5) -8 speed grade with Texas Instruments DP83867IR PHY (Quartus 17.1) on a custom PCB.
  • Cyclone 10 LP -8 speed grade with RTL8211 PHY (Quartus 18.0) using QMTech Cyclone 10LP development kit. Both failed timing, although worked as expected. Changing speed grade to -7 fixed timing.

Compiling (Intel)

You can manually specify target FPGA family, part and flash memory used in create_prj.tcl.

set FamilyDev "Cyclone V"
set PartDev "5CEBA5F23C8"
set MemDev "EPCS64"
set FlashLoadDev "5CEBA5"

The pins are modified in pins.tcl.

To compile an example project, run complete_flow.bat. (quartus.exe location should be in PATH). To add code to your desired project

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  • Registered on LibreCores 1 month ago
  • Project started 11 months ago
  • Last commit 6 days ago



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