[RFC PATCH 3/5] add doc/README.lwip

[Maxim Uvarov]

Signed-off-by: Maxim Uvarov <maxim.uvarov at linaro.org>
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 doc/README.lwip | 56 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 doc/README.lwip

diff --git a/doc/README.lwip b/doc/README.lwip
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+	RFC LWIP IP stack intergation for U-boot
+        ----------------------------------------
+
+Reliable and bug free IP stack is usually an issue when you are trying to write it
+from scratch. It looks like not, but when addinging new features it will be chelledging.
+This RFC work is a demo to enable lwIP (lightweight IP) which is a widely used open-source 
+TCP/IP stack designed for embedded systems for U-boot. That will allow using already 
+written network applications for microcontrollers.
+
+LwIP  license:
+lwIP is licensed under a BSD-style license: http://lwip.wikia.com/wiki/License.
+
+Main features include:
+- Protocols: IP, IPv6, ICMP, ND, MLD, UDP, TCP, IGMP, ARP, PPPoS, PPPoE
+- DHCP client, DNS client (incl. mDNS hostname resolver), AutoIP/APIPA (Zeroconf), SNMP agent (v1, v2c, v3, private MIB support & MIB compiler)
+- APIs: specialized APIs for enhanced performance, optional Berkeley-alike socket API
+- Extended features: IP forwarding over multiple network interfaces, TCP congestion control, RTT estimation and fast recovery/fast retransmit
+- Addon applications: HTTP(S) server, SNTP client, SMTP(S) client, ping, NetBIOS nameserver, mDNS responder, MQTT client, TFTP server
+
+U-boot implementation details:
+
+1. In general we can build lwIP as .a library and link it against u-boot or compile it in 
+the U-boot tree in the same way as other U-boot files. There are few reasons why I selected 
+the second variant: iwIP is very customizable with defines for features, memory size, types of
+allocation, some internal types and platform specific code. And it was more easy to enable/disable
+ debug which is also done with defines, and is needed periodically.
+
+2. lwIP has 2 APIs - raw mode and sequential (as lwIP names it, or socket API as we name it in Linux).
+  This RFC implements only raw API as the proof of work.
+
+Raw IP means that the call back function for RX path is registered and will be called when packet
+data passes the IP stack and is ready for the application.
+
+This RFC has an unmodified working ping example from lwip sources which registeres this callback.
+  ping_pcb = raw_new(IP_PROTO_ICMP);
+  raw_recv(ping_pcb, ping_recv, NULL); <- ping_recv is app callback.
+  raw_bind(ping_pcb, IP_ADDR_ANY)
+
+Socket API also gives nice advantages due it will be easy to port linux socket applications to u-boot.
+I.e. lwIP sockets compatible with the linux ones. But that will require RX thread running in the background.
+So that means we need some kind of scheduler, locking and threading support or find some other solution.
+
+3.  Input and output
+
+RX packet path is injected to U-boot eth_rx() polling loop and TX patch is in eth_send() accordingly. 
+So we do not touch any drivers code and just eat packets when they are ready.
+
+4. Testing
+
+Unmodified ping example can be used. I did ping from qemu/u-boot tap device on the host:
+
+=> lwip init
+=> lwip ping 192.168.1.2
+ping: recv 3 ms
+tcpdump on the host:
+5:09:10.925951 ARP, Request who-has 192.168.1.200 tell 192.168.1.200, length 28 15:09:12.114643 IP6 fe80::38e2:41ff:fec3:8bda > ip6-allrouters: ICMP6, router solicitation, length 16 15:09:20.370725 ARP, Request who-has 192.168.1.2 tell 192.168.1.200, length 28 15:09:20.370740 ARP, Reply 192.168.1.2 is-at 3a:e2:41:c3:8b:da (oui Unknown), length 28 15:09:20.372789 IP 192.168.1.200 > 192.168.1.2: ICMP echo request, id 44975, seq 1, length 40 15:09:20.372810 IP 192.168.1.2 > 192.168.1.200: ICMP echo reply, id 44975, seq 1, length 40 15:09:25.426636 ARP, Request who-has 192.168.1.200 tell 192.168.1.2, length 28 15:09:25.426870 ARP, Reply 192.168.1.200 is-at f6:11:01:02:03:04 (oui Unknown), length 28
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