Add BeaconBlocksByRange v3

Author: dapplion

specs/electra/p2p-interface.md

@@ -16,6 +16,9 @@ The specification of these changes continues in the same format as the network s
     - [Global topics](#global-topics)
       - [`beacon_aggregate_and_proof`](#beacon_aggregate_and_proof)
       - [`beacon_attestation_{subnet_id}`](#beacon_attestation_subnet_id)
+- [The Req/Resp domain](#the-reqresp-domain)
+  - [Messages](#messages)
+    - [BeaconBlocksByRange v3](#beaconblocksbyrange-v3)
 
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@@ -57,3 +60,33 @@ The following convenience variables are re-defined
 The following validations are added:
 * [REJECT] `len(committee_indices) == 1`, where `committee_indices = get_committee_indices(attestation)`.
 * [REJECT] `attestation.data.index == 0`
+
+## The Req/Resp domain
+
+### Messages
+
+#### BeaconBlocksByRange v3
+
+**Protocol ID:** `/eth2/beacon_chain/req/beacon_blocks_by_range/3/`
+
+Request Content:
+```
+(
+  block_root: Root
+  start_slot: Slot
+  count: uint64
+)
+```
+
+Response Content:
+```
+(
+  List[SignedBeaconBlock, MAX_REQUEST_BLOCKS]
+)
+```
+
+Extends behaviour of BeaconBlocksByRange v2 as defined in [the altair p2p spec](../altair/p2p-interface.md).
+
+Requests beacon blocks in the slot range `[start_slot, start_slot + count)`, leading up to `block_root`. If the block with `block_root` is unknown the responder MUST respond with `3: ResourceUnavailable`. If the slot of the block with `block_root` is less than `start_slot` the responder MUST respond with `1: InvalidRequest`.
+
+