add consistent hash proposal

Signed-off-by: kangmingfa <[email protected]>

docs/proposal/consistent_hash.md

@@ -0,0 +1,158 @@
+---
+title: Consistent hash LB
+authors:
+- "@bfforever" # Authors' GitHub accounts here.
+reviewers:
+- "@supercharge-xsy"
+- "@hzxuzhonghu"
+- "@nlwcy"
+- TBD
+approvers:
+- "@robot"
+- TBD
+
+creation-date: 2024-05-29
+
+---
+
+## Your short, descriptive title
+
+Add Consistent hash based LB in ebpf prog
+
+### Summary
+
+<ol>
+<li>Maglev consistent hash algorithm</li>
+<li>IP tuple info based hash</li>
+<li>L7 http header based hash</li>
+</ol>
+
+#### Goals
+
+<ol>
+<li>Networking load balance</li>
+<li>Guarrant a client conn always send request to same backend</li>
+<li>Minimize remapping</li>
+</ol>
+
+### Proposal
+
+#### maglev
+Algorithm main purpose: map all backends into a fixed look up table, require even mapping, and try to minimize mapping changes when backend change (remove or add a backend), avoid global remapping.
+
+
+#### ebpf implement situation or point
+<ol>
+<li>Cgroup level</li>
+In cgroup socket implement l4 and l7 header based consistent hash.
+<li>Container veth tc level</li>
+In tc level implement l4 based and l7 based consistent hash.
+</ol>
+
+
+### Design Details
+
+![consistent_hash](pics/kmesh_consistent_hash.svg)
+#### maglev 
+main steps:
+
+<ol>
+<li>Map endpoints of Cluster to a fixed table</li>
+Based on Cluster config from istiod to generate a integer table, the value of table is endpoints index;
+
+
+<li>Base l4 or l7 info compute a hash value</li>
+use hash value get an table index, which is <code>index=hash%len(table)</code>;
+<li>Use index to access table</li>
+<code>ep=tables[index]</code>
+</ol>
+
+
+#### L4 implement
+<ol>
+<li>(saddr,daddr,sport,dport,protocol)-->hash</li>
+<li>then execute above second step.</li>
+</ol>
+
+#### L7 implement
+```
+apiVersion: networking.istio.io/v1alpha3
+kind: DestinationRule
+metadata:
+  name: helloworld
+spec:
+  host: helloworld
+  trafficPolicy:
+    loadBalancer:
+      consistentHash: 
+        httpHeaderName: testHeader
+        maglev:
+          tableSize: 
+```
+<ol>
+<li>specify which l7 header is selected to do hash</li>
+<li>extract l7 header info from http header</li>
+<li>compute a hash value base header value</li>
+<li>then execute above second step</li>
+</ol>
+
+##### Risk in l7 header value to compute a hash
+Because in ebpf prog, there is a lack of string-based hash compute.
+<br>
+Below is a simple string-based hash method.Probability we need an effective hash method. 
+```
+static inline __u32 lb_hash_based_header(unsigned char *header_name) {
+    __u32 hash = 0;
+    struct bpf_mem_ptr *msg_header = NULL;
+    char *msg_header_v = NULL;
+    __u32 msg_header_len = 0;
+    __u32 k;
+    __u32 c;
+    char msg_header_cp[KMESH_PER_HASH_POLICY_MSG_HEADER_LEN] = {'\0'};
+
+    if (!header_name)
+        return hash;
+     // when header name is not null, compute a hash value.
+    BPF_LOG(INFO, ROUTER_CONFIG, "Got a header name:%s\n", header_name);
+    msg_header = (struct bpf_mem_ptr *)bpf_get_msg_header_element(header_name);
+    if (!msg_header)
+        return hash;
+
+    msg_header_v = _(msg_header->ptr);
+    msg_header_len = _(msg_header->size);
+    if (!msg_header_len)
+        return hash;
+    
+    BPF_LOG(INFO, ROUTER_CONFIG, "Got a header value len:%u\n", msg_header_len);
+    if (!bpf_strncpy(msg_header_cp, msg_header_len, msg_header_v)) 
+        return hash;
+    BPF_LOG(INFO, ROUTER_CONFIG, "Got a header value:%s\n", msg_header_cp);
+    // Currently a simple hash method
+    hash = 5318;   
+# pragma unroll
+    for (k = 0; k < KMESH_PER_HASH_POLICY_MSG_HEADER_LEN;k++) { 
+        if (!msg_header_v || k >= msg_header_len)
+            break;
+        c = *((unsigned char *)msg_header_cp + k);
+        hash = ((hash << 5) + hash) + c; 
+    }
+    
+    return hash;
+}
+```
+
+
+#### Cgroup level
+Directly througth lb logic, to change the daddr and dport of socket.
+
+#### Tc level
+Do lb algorithm based on network packet or message, to change the daddr and dport of packet.
+<br>
+This way can get full ip address info to do lb.
+
+#### Test Plan
+
+<ol>
+<li>Table mapping is even distribution</li>
+<li>Add or remove a endpoint only affect few connections</li>
+</ol>