@ndn/lp
Version:
NDNts: NDN Link Protocol
57 lines (56 loc) • 2.15 kB
JavaScript
import { Encoder } from "@ndn/tlv";
import { LpPacket } from "./packet_node.js";
class SeqNumGen {
current = (BigInt(Math.trunc(Math.random() * 0x100000000)) << 32n) |
BigInt(Math.trunc(Math.random() * 0x100000000));
next() {
this.current = BigInt.asUintN(64, this.current + 1n);
return this.current;
}
}
const OVERHEAD = 0 +
1 + 3 + // LpPacket TL
1 + 1 + 8 + // LpSeqNum
1 + 1 + 2 + // FragIndex
1 + 1 + 2 + // FragCount
1 + 3 + // LpPayload TL
0;
/** NDNLPv2 fragmenter. */
export class Fragmenter {
seqNumGen = new SeqNumGen();
/**
* Fragment a packet.
* @param full - LpPacket contains full L3 packet and LpHeaders.
* @param mtu - Transport MTU.
* @returns LpPacket fragments, or empty array if fragmentation fails.
*/
fragment(full, mtu) {
const fragmentRoom = mtu - OVERHEAD;
const sizeofL3Headers = Encoder.encode(full.encodeL3Headers()).length;
const sizeofPayload = full.payload?.byteLength ?? 0;
const sizeofFirstFragment = Math.min(sizeofPayload, fragmentRoom - sizeofL3Headers);
if (sizeofFirstFragment === sizeofPayload) { // no fragmentation necessary
return [full];
}
if (sizeofFirstFragment <= 0) { // MTU is too small for L3 headers, drop the packet
return [];
}
const fragments = [];
const first = new LpPacket();
Object.assign(first, full.l3);
first.fragSeqNum = this.seqNumGen.next();
first.payload = full.payload.subarray(0, sizeofFirstFragment);
fragments.push(first);
for (let offset = sizeofFirstFragment; offset < sizeofPayload; offset += fragmentRoom) {
const fragment = new LpPacket();
fragment.fragSeqNum = this.seqNumGen.next();
fragment.fragIndex = fragments.length;
fragment.payload = full.payload.subarray(offset, offset + fragmentRoom);
fragments.push(fragment);
}
for (const fragment of fragments) {
fragment.fragCount = fragments.length;
}
return fragments;
}
}